NIST Special Publication 1000-5 

June 2004 

Progress Report on the 

Federal Building and Fire Safety 

Investigation of the 

World Trade Center Disaster 

Volume 1 
ii

Disclaimer

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a 

procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is 

not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or 

equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or 

negligence by the National Institute of Standards and Technology. 

Disclaimer

The policy of NIST is to use the International System of Units (metric units) in all publications. In this document, 

however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline. 

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the 

National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter 

mentioned in such report (15 USC 281a; as amended by P.L. 107-231). 

National Institute of Standards and Technology Special Publication 1000-5 

Natl. Inst. Stand. Technol. Spec. Publ. 1000-5, 1,054 pages (June 2004) 

CODEN: NSPUE2 

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON: 2004

_________________________________________ 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250 

Mail: Stop SSOP, Washington, DC 20402-0001
vii

VOLUME 1 

TABLE OF CONTENTS

List of Figures.............................................................................................................................................. xi 

List of Tables .............................................................................................................................................xiii 

List of Acronyms and Abbreviations.......................................................................................................... xv 

Metric Conversion Table .........................................................................................................................xxiii 

Preface ....................................................................................................................................................xxvii 

Executive Summary ................................................................................................................................. xxxi 

Chapter 1

Interim Findings and Accomplishments...................................................................................1 

1.1 Introduction...................................................................................................................................... 1 

1.2 Collapse of the WTC Towers .......................................................................................................... 2 

1.3 Collapse of the 47-Story WTC 7 Building..................................................................................... 17 

1.4 Evacuation and Emergency Response ........................................................................................... 19 

1.5 Procedures and Practices................................................................................................................ 29 

1.6 Approach to recommendations ...................................................................................................... 41 

Chapter 2

Progress on the World Trade Center Investigation ...............................................................43 

2.1 Status of Data Collection Efforts ................................................................................................... 43 

2.2 Analysis of Building and Fire Codes and Practices (Project 1)..................................................... 48 

2.2.1 Project Objective ................................................................................................................ 48 

2.2.2 Project Approach................................................................................................................ 49 

2.2.3 Status of Tasks.................................................................................................................... 51 

2.2.4 Building Codes ................................................................................................................... 52 

2.2.5 The 1968 New York City Building Code........................................................................... 57 

2.2.6 WTC 7 Fuel System ........................................................................................................... 65 

2.2.7 Preliminary Findings .......................................................................................................... 68 

2.3 Baseline Structural Performance and Aircraft Impact Damage Analysis (Project 2) .................... 72 

2.3.1 Project Objective ................................................................................................................ 72 

2.3.2 Project Approach................................................................................................................ 72 

2.3.3 Development of Structural Databases for WTC 1 and WTC 2 .......................................... 74 

2.3.4 Development of Reference Structural Models for WTC 1 and WTC 2 ............................. 74 
Volume 1 Table of Contents 

viii

2.3.5 Estimates of Wind Loads on the WTC Towers.................................................................. 75 

2.3.6 Baseline Performance Analysis of the Towers................................................................... 76 

2.3.7 Analysis of Aircraft Impacts into WTC 1 and WTC 2....................................................... 76 

2.3.8 Preliminary Stability Analysis of the WTC Towers........................................................... 77 

2.3.9 Summary and Preliminary Findings................................................................................... 81 

2.4 Metallurgical and Mechanical Analysis of Structural Steel (Project 3)......................................... 83 

2.4.1 Project Objective ................................................................................................................ 83 

2.4.2 Project Approach................................................................................................................ 83 

2.4.3 Physical Evidence............................................................................................................... 84 

2.4.4 Visual Observations............................................................................................................ 86 

2.4.5 Mechanical Properties ........................................................................................................ 87 

2.4.6 Correlation with Engineering Drawings............................................................................. 87 

2.4.7 High-Temperature Excursions............................................................................................ 87

2.4.8 Significant Interim Results ................................................................................................. 87 

2.4.9 Preliminary Findings .......................................................................................................... 88 

2.5 Investigation of Active Fire Protection Systems (Project 4).......................................................... 88 

2.5.1 Project Objectives............................................................................................................... 88 

2.5.2 Project Approach................................................................................................................ 89 

2.5.3 Fire History of WTC 1, 2, and 7......................................................................................... 91 

2.5.4 Fire Sprinkler, Standpipe, and Preconnected Hoses........................................................... 92 

2.5.5 Fire Alarm Systems ............................................................................................................ 93 

2.5.6 Smoke Management ........................................................................................................... 96 

2.6 Reconstruction of Thermal and Tenability Environments (Project 5) ........................................... 97 

2.6.1 Project Objective ................................................................................................................ 97 

2.6.2 Project Approach................................................................................................................ 97 

2.6.3 Collection of Photographic Evidence................................................................................. 99 

2.6.4 Data on the Building Interiors .......................................................................................... 100 

2.6.5 Insulation of Structural Members..................................................................................... 101 

2.6.6 Modeling the Fires............................................................................................................ 103 

2.6.7 Preliminary Findings ........................................................................................................ 105 

2.7 Structural Fire Response and Collapse Analysis (Project 6) ....................................................... 106 

2.7.1 Project Objective .............................................................................................................. 106 

2.7.2 Project Approach.............................................................................................................. 106 

2.7.3 Fireproofing of WTC Towers........................................................................................... 107 

2.7.4 Response of WTC 1 and WTC 2 Floor and Column Systems under Fire Conditions ..... 108 
June 2004 Progress Report 

ix

2.7.5 Standard Fire Endurance Tests of Floor System .............................................................. 121 

2.7.6 WTC 7 .............................................................................................................................. 123 

2.8 Occupant Behavior, Egress, and Emergency Communications (Project 7)................................. 129 

2.8.1 Project Objectives............................................................................................................. 129 

2.8.2 Project Status.................................................................................................................... 131 

2.8.3 Interim Findings and Key Issues ...................................................................................... 132 

2.9 Fire Service Technologies and Guidelines (Project 8)................................................................. 138 

2.9.1 Project Objectives............................................................................................................. 138 

2.9.2 Project Status.................................................................................................................... 139 

2.9.3 Preliminary Results on Emergency Responder Communications .................................... 141 

2.9.4 Preliminary Findings ........................................................................................................ 142 

2.10 References.................................................................................................................................... 143 

Chapter 3

Update on Safety of Threatened Buildings (WTC R&D) Program .....................................147 

3.1 Objectives of Safety of Threatened Buildings (WTC R&D) Program ........................................ 147 

3.2 Background and Desired Outcomes of Safety of Threatened Buildings Program....................... 147 

3.3 Accomplishments of WTC R&D Program .................................................................................. 149 

Chapter 4

Update on the WTC Dissemination and Technical Assistance Program...........................159 

4.1 Objectives of the WTC Dissemination and Technical Assistance Program (DTAP) .................. 159 

4.2 Background and Desired Outcome of DTAP .............................................................................. 159 

4.3 Accomplishments of the DTAP ................................................................................................... 159 
Volume 1 Table of Contents 

x

This page intentionally left blank.
xi

VOLUME 1 

LIST OF FIGURES

Figure 2–1. Locations of emergency generators and fuel tanks in WTC 7. ............................................ 69 

Figure 2–2. Example engine impact subassembly analysis..................................................................... 78 

Figure 2–3. Oblique view of the subassembly engine impact damage.................................................... 79 

Figure 2–4. Model of the 96th floor and columns of WTC 1.................................................................. 79 

Figure 2–5. Model of the core of WTC 1, floors 94–98.......................................................................... 80 

Figure 2–6. Detail of the WTC 2 impact zone exterior column panels................................................... 80 

Figure 2–7. Enhanced precollapse image of the north face of WTC 1.................................................... 86 

Figure 2–8. Monitoring station history tape record for the WTC 7 fire alarm system............................ 94 

Figure 2–9. Features of truss model. .................................................................................................... 109 

Figure 2–10. Truss model........................................................................................................................ 110 

Figure 2–11. Converted ANSYS model of floor 96................................................................................ 111 

Figure 2–12. Exterior connection of the floor truss to the spandrel plate. .............................................. 112 

Figure 2–13. Failure sequence of exterior seats for tensile forces. ......................................................... 112

Figure 2–14. Typical tensile force resistance of exterior seat connection............................................... 113 

Figure 2–15. Finite element model of transversely loaded knuckle........................................................ 114 

Figure 2–16. Finite element solution for floor truss under gravity and temperature loads. ................... 116 

Figure 2–17. Truss seat capacity for vertical forces................................................................................ 117 

Figure 2–18. Truss seat capacity for horizontal forces............................................................................ 118 

Figure 2–19. Compressive stress in longitudinal shear. ......................................................................... 119 

Figure 2–20. Shear force versus displacement for two knuckles under longitudinal shear.................... 119 

Figure 2–21. Floor truss response due to gravity load and uniform heating. .......................................... 120 

Figure 2–22. Fabrication of 35 ft span ASTM E 119 test assembly........................................................ 123 

Figure 2–23. Collapse initiation and vertical progression on the east side of WTC 7. ........................... 127 

Figure 2–24. Transfer trusses and girders between the fifth and seventh floors. .................................... 128 

Figure 2–25. Example of horizontal progression of failure in core columns. ......................................... 128 

Figure 2–26. Horizontal progression to the west side of WTC 7. ........................................................... 129 

Figure 3–1. Ronan Point Collapse in 1968............................................................................................ 150 

Figure 3–2. Insulated steel trusses, steel rod and steel column. ............................................................ 151

Figure 3–3. View of fire compartment from air exhaust outlet............................................................. 152 

Figure 3–4. Stress-strain curves of a conventional structural steel. ...................................................... 152 
Volume 1 List of Figures 

xii

Figure 3–5. High temperature tensile tests to measure performance of structural steel........................ 153 

Figure 3–6. NIST measured the thermal behavior of gypsum/steel wall assemblies. ........................... 154 

Figure 3–7. Exterior flow field as input to CONTAM model of building............................................. 156 
xiii

VOLUME 1 

LIST OF TABLES

Table 2–1. Status of Project 1 tasks. ........................................................................................................ 52 

Table 2–2. Summary of compliance with Local Laws 5/73 and 16/84.................................................... 64 

Table 2–3. Summary of modifications to base emergency power system in WTC 7. ............................. 68 

Table 2–4. Summary of historical fires in WTC 1, 2, and 7. ................................................................... 92

Table 2–5. Floors of visible damage. ..................................................................................................... 100 

Table 2–6. Types and locations of spray-applied fireproofing on fire floors......................................... 102 

Table 2–7. Specified passive fire protection. ......................................................................................... 108 

Table 2–8. Reports of WTC decedents. ................................................................................................. 135 

Table 2–9. Occupancy estimates on September 11, 2001, by tower...................................................... 136 

Table 2–10. WTC fire drills in 12 months prior to September 11, 2001. ................................................ 137 
iii

MASTER TABLE OF CONTENTS

Volume 1 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Preface 

Executive Summary

Chapter 1 

Interim Findings and Accomplishments 

Chapter 2 

Progress on the World Trade Center Investigation 

Chapter 3 

Update on Safety of Threatened Buildings (WTC R&D) Program 

Chapter 4 

Update on WTC Dissemination and Technical Assistance Program 

Volume 2 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Appendix A 

Interim Report on the Analysis of Building and Fire Codes and Practices 

Appendix B 

Interim Report on Development of Structural Databases and Reference Models 

for the WTC Towers 
Master Table of Contents 

iv

Appendix C 

Interim Report on Analysis of Aircraft Impact into the WTC Towers 

Volume 3 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Appendix D 

Interim Report on Preliminary Stability Analysis of the WTC Towers 

Appendix E 

Interim Report on Contemporaneous Structural Steel Specifications 

Appendix F 

Interim Report on Inventory and Identification of Steels Recovered from the 

WTC Buildings 

Volume 4 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Appendix G 

Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to September 11, 2001 

Appendix H 

Interim Report on Evolution of WTC Fires, Smoke, and Damage based on Image Analysis 

Appendix I 

Interim Report on Assessment of Sprayed Fireproofing in the WTC 

Towers—Methodology 
June 2004 Progress Report 

v

Volume 5 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Appendix J 

Interim Report on Experiments to Support Fire Dynamics and Thermal 

Response Modeling 

Appendix K 

Interim Report on Subsystem Structural Analysis of the WTC Towers 

Appendix L 

Interim Report on WTC 7 

Appendix M 

Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

Volume 6 

Table of Contents 

List of Acronyms and Abbreviations 

Metric Conversion Table 

Appendix N 

Interim Report on Analysis of First-Person Accounts from Survivors of the WTC 

Evacuation on September 11, 2001 

Appendix O 

Interim Report on Telephone Interviews 

Appendix P 

Interim Report on Emergency Communications 

Appendix Q 

NIST’s Working Hypothesis for Collapse of the WTC Towers 
xv

LIST OF ACRONYMS AND ABBREVIATIONS

AAPOR American Association of Public Opinion Research 

ABC American Broadcasting Company 

ACI American Concrete Institute 

AISC American Institute of Steel Construction 

AISI American Iron and Steel Institute 

ALE Arbitrary-Lagrangian-Evlerian 

AMCBO Association of Major City/County Building Officials 

ANSI American National Standards Institute 

ANSYS finite element model 

ARA Applied Research Associates, Inc. 

ASCE American Society of Civil Engineers 

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 

ASME American Society of Mechanical Engineers 

ASTM ASTM International 

AWS American Welding Society 

BOCA Building Officials and Code Administrators 

BOCA/BBC BOCA Basic Building Code 

BPAT Building Performance Assessment Team 

BPS Building Performance Study 

BSI British Standards Institution 

C/F cancer free 

CATI computer-assisted telephone interviews 

CBR chemical, biological, and radiological 

CBS Columbia Broadcasting System 

CERF Civil Engineering Research Foundation 

CFD computational fluid dynamics 

CIB International Council for Research and Innovation in Building and Construction 

CII Construction Industry Institute 

CNN Cable News Network 
List of Acronyms and Abbreviations 

xvi

CPP Cermak Peterka Peterson, Inc. 

CPU central processing unit 

CRT cathode-ray tube 

CTB&UH Council on Tall Buildings and Urban Habitat 

CTE coefficients of thermal expansion 

DC/F BlazeShield DC/F fire protective insulation 

DL dead load 

DTAP dissemination and technical assistance program 

EMS Emergency Medical Service 

EMT Emergency Medical Team 

ER&S Emory Roth & Sons 

FBI Federal Bureau of Investigation 

FCA Flux cored arc 

FDNY New York City Fire Department 

FDS Fire Dynamics Simulator 

FE finite element 

FEA finite element analysis 

FEM finite element model 

FEMA Federal Emergency Management Agency 

FMRC Factory Mutual Research Corp. 

FSI Fire-Structure Interface 

FVM Finite Volume Method 

GFI Government Furnished Information 

GG glass over glass 

GHz gigahertz 

GMS, LLP Gilsanz Murray Steficek, LLP 

HAZ heat affected zone 

HNSE Hugo Nue Schnutzer East 

HRR heat release rate 

HVAC heating, ventilating, and air conditioning 

IAQ indoor air quality 

IBC International Building Code 
June 2004 Progress Report 

xvii

ICBO International Conference of Building Officials 

ICC International Code Council 

IMTI Integrated Manufacturing Technology 

JFK John F. Kennedy International Airport 

JIS Japan Industrial Standard 

LERA Leslie E. Robertson Associates 

LES Large Eddy Simulation 

LL live load 

LSTC Livermore Software Technology Corporation 

MBC BOCA National Building Code 

MCC Municipal Code of Chicago 

MPI Message Passing Interface 

NBC National Broadcasting Company

NBFU National Board of Fire Underwriters 

NCSBCS National Conference of States on Building Codes & Standards, Inc. 

NCST National Construction Safety Team 

NEMA National Electrical Manufacturers Association 

NFPA National Fire Protection Association 

NIBS National Institute of Building Sciences 

NIST National Institute of Standards and Technology 

NYC New York City 

NYCBC New York City Building Code 

NYCDOB New York City Department of Buildings 

NYPD New York City Police Department 

NYSBC New York State Building Construction Code 

P.L. Public Law 

PANYNJ Port Authority of New York and New Jersey 

PAPD Port Authority Police Department 

PC&F Pacific Car and Foundry 

PDM Pittsburg-Des Moines 

PONYA Port of New York Authority 

R&D research and development 
List of Acronyms and Abbreviations 

xviii

RWDI Rowan Williams Davis and Irwin, Inc. 

SBCCI Southern Standard Building Code 

SDL superimposed dead load 

SDO standards development organization 

SEAoNY Structural Engineers Association of New York 

SFPE Society of Fire Protection Engineering 

SFRM spray-on fire resistant material or sprayed fire resistive materials 

SHCR Skilling, Helle, Christiansen, & Robertson 

SI metric 

SLB short legs back-to-back 

SMA Shielded Metal Arc 

SOD Special Operations Division 

SOM Skidmore, Ownings & Merrill 

SPH Smoothed Particle Hydrodynamics 

SQL Structured Query Language 

SWMB Skilling, Ward, Magnussen, and Barkshire 

TL Truss Lower Chord 

TM Truss Middle Chord 

TU Truss Upper Chord 

UBC Uniform Building Code 

UL Underwriters’ Laboratories, Inc. 

USC United States Code 

USM United States Mineral Products Co. 

VCBT Virtual Cybernetic Building Testbed 

WABC WABC-TV New York 

WCBS WCBS-TV New York 

WF wide flange (a type of structural steel shape now usually called a W-shape). ASTM A 6 

defines them as “doubly-symmetric, wide-flange shapes with inside flange surfaces that 

are substantially parallel.” 

WNBC NBC4 New York 

WNYW FOX5 New York 

WPIX WPIX-TV New York 

WTC World Trade Center 
June 2004 Progress Report 

xix

WTC 1 World Trade Center Tower 1 

WTC 2 World Trade Center Tower 2 

WTC 7 World Trade Center Building 7 
List of Acronyms and Abbreviations 

xx

Abbreviations 

× by 

± plus or minus 

°C degrees Celsius 

°F degrees Fahrenheit 

μm micrometer 

2D two dimensional 

3D three dimensional 

cm centimeter 

ft foot 

ft2

 square foot 

Fy yield strength (AISC usage) 

g acceleration (gravity) 

g gram 

gal gallon 

h hour 

in. inch 

kg kilogram 

kip a stress unit equal to 1,000 pounds 

kJ kilojoule 

kN kilonewton 

kPa kilopascal 

klb 1,000 pounds 

ksi 1,000 pounds per square inch 

kW kilowatt 

kW/m2

 kilowatts per square meter 

L liter 

lb pound 

m meter 

m2

 square meter 

mm millimeter 

m/s meters per second 
June 2004 Progress Report 

xxi

min minute 

MJ megajoule 

MPa megapascal 

mph miles per hour 

ms microsecond 

Msi millions pounds per square inch 

MW megawatt 

N newton 

Pa pascal 

pcf pounds per cubic foot 

plf pounds per linear foot 

psf pounds per square foot 

psi pounds per square inch 

s second 
xxiii

METRIC CONVERSION TABLE

To convert from to Multiply by 

AREA AND SECOND MOMENT OF AREA 

square foot (ft2) square meter (m2) 9.290 304 E-02 

square inch (in2

) square meter (m2) 6.4516 E-04 

square inch (in2

) square centimeter (cm2) 6.4516 E+00 

square yard (yd2

) square meter (m2) 8.361 274 E-01 

ENERGY (includes WORK) 

kilowatt hour (kW * h) joule (J) 3.6 E+06 

quad (1015 BtuIT) joule (J) 1.055 056 E+18 

therm (U.S.) joule (J) 1.054 804 E+08 

ton of TNT (energy equivalent) joule (J) 4.184 E+09 

watt hour (W * h) joule (J) 3.6 E+03 

watt second (W * s) joule (J) 1.0 E+00 

FORCE 

dyne (dyn) newton (N) 1.0 E-05 

kilogram-force (kgf) newton (N) 9.806 65 E+00 

kilopond (kilogram-force) (kp) newton (N) 9.806 65 E+00 

kip (1 kip=1000 lbf) newton (N) 4.448 222 E+03 

kip (1 kip=1000 lbf) kilonewton (kN) 4.448 222 E+00 

pound-force (lbf) newton (N) 4.448 222 E+00 

FORCE DIVIDED BY LENGTH 

pound-force per foot (lbf/ft) newton per meter (N/m) 1.459 390 E+01 

pound-force per inch (lbf/in) newton per meter (N/m) 1.751 268 E+02 

HEAT FLOW RATE 

calorieth per minute (calth/min) watt (W) 6.973 333 E-02 

calorieth per second (calth/s) watt (W) 4.184 E+00 

kilocalorieth per minute (kcalth/min) watt (W) 6.973 333 E+01 

kilocalorieth per second (kcalth/s) watt (W) 4.184 E+03 
Metric Conversion Table 

xxiv

To convert from to Multiply by 

LENGTH 

foot (ft) meter (m) 3.048 E-01 

inch (in) meter (m) 2.54 E-02 

inch (in) centimeter (cm) 2.54 E+00 

micron (m) meter (m) 1.0 E-06 

yard (yd) meter (m) 9.144 E-01 

MASS and MOMENT OF INERTIA 

kilogram-force second squared 

per meter (kgf * s2

/m) kilogram (kg) 9.806 65 E+00 

pound foot squared (lb * ft2

) kilogram meter squared (kg * m2) 4.214 011 E-02 

pound inch squared (lb * in2

) kilogram meter squared (kg * m2) 2.926 397 E-04 

ton, metric (t) kilogram (kg) 1.0 E+03 

ton, short (2000 lb) kilogram (kg) 9.071 847 E+02 

MASS DIVIDED BY AREA 

pound per square foot (lb/ft2) kilogram per square meter (kg/m2) 4.882 428 E+00 

pound per square inch 

(not pound force) (lb/in2

) kilogram per square meter (kg/m2) 7.030 696 E+02 

MASS DIVIDED BY LENGTH 

pound per foot (lb/ft) kilogram per meter (kg/m) 1.488 164 E+00 

pound per inch (lb/in) kilogram per meter (kg/m) 1.785 797 E+01 

pound per yard (lb/yd) kilogram per meter (kg/m) 4.960 546 E-01 

PRESSURE or STRESS (FORCE DIVIDED BY AREA) 

kilogram-force per square centimeter (kgf/cm2) pascal (Pa) 9.806 65 E+04 

kilogram-force per square meter (kgf/m2

) pascal (Pa) 9.806 65 E+00 

kilogram-force per square millimeter (kgf/mm2

) pascal (Pa) 9.806 65 E+06 

kip per square inch (ksi) (kip/in2

) pascal (Pa) 6.894 757 E+06 

kip per square inch (ksi) (kip/in2

) kilopascal (kPa) 6.894 757 E+03 

pound-force per square foot (lbf/ft2

) pascal (Pa) 4.788 026 E+01 

pound-force per square inch (psi) (lbf/in2

) pascal (Pa) 6.894 757 E+03 

pound-force per square inch (psi) (lbf/in2

) kilopascal (kPa) 6.894 757 E+00 

psi (pound-force per square inch) (lbf/in2

) pascal (Pa) 6.894 757 E+03 

psi (pound-force per square inch) (lbf/in2) kilopascal (kPa) 6.894 757 E+00 
June 2004 Progress Report 

xxv

To convert from to Multiply by 

TEMPERATURE 

degree Celsius (°C) kelvin (K) T/K = t/ °C + 273.15 

degree centigrade degree Celsius (°C) t/ °C ≈ t /deg. cent. 

degree Fahrenheit (°F) degree Celsius (°C) t/ °C = (t/ °F 2 32)/1.8 

degree Fahrenheit (°F) kelvin (K) T/K = (t/ °F + 459.67)/1.8 

kelvin (K) degree Celsius (°C) t /°C = T/ K 2 273.15 

TEMPERATURE INTERVAL 

degree Celsius (°C) kelvin (K) 1.0 E+00 

degree centigrade degree Celsius (°C) 1.0 E+00 

degree Fahrenheit (°F) degree Celsius (°C) 5.555 556 E-01 

degree Fahrenheit (°F) kelvin (K) 5.555 556 E-01 

degree Rankine (°R) kelvin (K) 5.555 556 E-01 

VELOCITY (includes SPEED) 

foot per second (ft/s) meter per second (m/s) 3.048 E-01 

inch per second (in/s) meter per second (m/s) 2.54 E-02 

kilometer per hour (km/h) meter per second (m/s) 2.777 778 E-01 

mile per hour (mi/h) kilometer per hour (km/h) 1.609 344 E+00 

mile per minute (mi/min) meter per second (m/s) 2.682 24 E+01 

VOLUME (includes CAPACITY) 

cubic foot (ft3) cubic meter (m3) 2.831 685 E-02 

cubic inch (in3

 ) cubic meter (m3) 1.638 706 E-05 

cubic yard (yd3) cubic meter (m3) 7.645 549 E-01 

gallon (U.S.) (gal) cubic meter (m3

) 3.785 412 E-03 

gallon (U.S.) (gal) liter (L) 3.785 412 E+00 

liter (L) cubic meter (m3

) 1.0 E-03 

ounce (U.S. fluid) (fl oz) cubic meter (m3

) 2.957 353 E-05 

ounce (U.S. fluid) (fl oz) milliliter (mL) 2.957 353 E+01 
xxvii

PREFACE

The National Institute of Standards and Technology (NIST) initiated the federal building and fire safety 

investigation of the World Trade Center (WTC) disaster on August 21, 2002. This WTC Investigation, 

led by NIST, is being conducted under the authority of the National Construction Safety Team Act 

(Public Law [P.L.] 107-231). 

Goals of the WTC Investigation 

x To investigate the building construction, the materials used, and the technical conditions that 

contributed to the outcome of the WTC disaster. 

x To serve as the basis for: 

 Improvements in the way buildings are designed, constructed, maintained, and used 

 Improved tools and guidance for industry and safety officials 

 Recommended revisions to current codes, standards, and practices 

 Improved public safety 

Objectives of the WTC Investigation 

The objectives of the NIST-led Investigation of the WTC disaster are to: 

1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the 

aircraft and why and how WTC 7 collapsed 

2. Determine why the numbers of injuries and fatalities were so high or low depending on 

location, including technical aspects of fire protection, occupant behavior, evacuation, and 

emergency response 

3. Determine what procedures and practices were used in the design, construction, operation, 

and maintenance of WTC 1, 2, and 7 

4. Identify, as specifically as possible, areas in current national building and fire model codes, 

standards, and practices that warrant revision 

Authorities and Use of Information in Legal Proceedings 

NIST is a nonregulatory agency of the U.S. Department of Commerce. NIST investigations are focused 

on fact finding, not fault finding. No part of any report resulting from a NIST investigation into a 

structural failure or from an investigation under the National Construction Safety Team Act may be used 
Preface

xxviii

in any suit or action for damages arising out of any matter mentioned in such report (15 USC 281a, as 

amended by P.L. 107-231). 

Organization of the WTC Investigation 

The Investigation includes eight interdependent projects that, in combination, meet the objectives. A 

detailed description of each of these eight projects is available at http://wtc.nist.gov. The purpose of each 

project is summarized in Table P–1, and the key interdependencies among the projects are illustrated in 

Figure P–1. 

Table P–1. Federal building and fire safety investigation of the WTC disaster. 

Technical Area Project No. Project Purpose 

Analysis of Building 

and Fire Codes and 

Practices

1 Document and analyze the code provisions, procedures, and practices 

used in the design, construction, operation, and maintenance of the 

structural, passive fire protection, and emergency access and 

evacuation systems of the WTC 1, 2, and 7. 

Baseline Structural 

Performance and 

Aircraft Impact 

Damage Analysis 

2 Analyze the baseline performance of WTC 1 and 2 under design, 

service, and abnormal loads, and aircraft impact damage on the 

structural, fire protection, and egress systems. 

Mechanical and 

Metallurgical Analysis 

of Structural Steel 

3 Determine and analyze the mechanical and metallurgical properties 

and quality of steel, weldments, and connections from steel recovered 

from WTC 1, 2, and 7. 

Investigation of Active 

Fire-Protection 

Systems 

4 Investigate the performance of the active fire protection systems in 

WTC 1, 2, and 7 and their role in fire control, emergency response, 

and fate of occupants and responders. 

Reconstruction of 

Thermal and 

Tenability 

Environment 

5 Reconstruct the time-evolving temperature, thermal environment, and 

smoke movement in WTC 1, 2, and 7 for use in evaluating the 

structural performance of the buildings and behavior and fate of 

occupants and responders. 

Structural Fire 

Response and 

Collapse Analysis 

6 Analyze the response of the WTC towers to fires with and without 

aircraft damage, the response of WTC 7 in fires, the performance of 

open-web steel joists, and determine the most probable structural 

collapse sequence for WTC 1, 2, and 7. 

Occupant Behavior, 

Egress, and 

Emergency 

Communications 

7 Analyze the behavior and fate of occupants and responders, both 

those who survived and those who did not, and the performance of 

the evacuation system. 

Fire Service 

Technologies and 

Guidelines 

8 Building on work done by the Fire Department of New York and 

McKinsey & Company, document what happened during the 

response by the fire services to the WTC attacks until the collapse of 

WTC 7; identify issues that need to be addressed in changes to 

practice, standards, and codes; identify alternative practices and/or 

technologies that may address these issues; and identify research and 

development needs that advance the safety of the fire service in 

responding to massive fires in tall buildings. 
xxix

June 2004 Progress Report

NIST WTC Investigation Projects NIST WTC Investigation Projects

Analysis 

of Steel Structural 

Collapse

Evacuation

Baseline 

Performance

& Impact 

Damage

Analysis of Codes and 

Practices

Fire Service 

Response

Active Fire 

Suppression

Thermal and 

Tenability 

Environment

BPAT RecommendationsGovernment, Industry, Professional, Academic InputsPublic InputsPublic InputsDocumentsVideo/ Photographic RecordsOral History DataEmergency Response RecordsRecovered Structural Steel

Figure P–1. The eight projects in the federal building and fire safety investigation of the WTC disaster. 
Preface

xxx

NIST’s WTC Public-Private Response Plan 

The goal of the WTC Public-Private Response Plan is to develop the technical basis for standards, 

technology, and practices needed for cost-effective improvements to the safety and security of buildings 

and building occupants, including evacuation, emergency response procedures, and threat mitigation. 

The strategy to meet this goal is a three-part NIST-led public-private response program that includes: 

x A federal building and fire safety investigation to study the most probable factors that 

contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7, and 

the associated evacuation and emergency response experience. 

x A research and development (R&D) program to provide a technical foundation that supports 

improvements to building and fire codes, standards, and practices that reduce the impact of 

extreme threats to the safety of buildings, their occupants and emergency responders. 

x A dissemination and technical assistance program (DTAP) to engage leaders of the 

construction and building community in implementing proposed changes to practices, 

standards, and codes. This effort also will provide practical guidance and tools to better 

prepare facility owners, contractors, architects, engineers, emergency responders, and 

regulatory authorities to respond to future disasters. 

The desired outcomes are to make all buildings safer for occupants and first responders and to ensure 

better evacuation systems and emergency response capabilities for future disasters. 
xxxi

EXECUTIVE SUMMARY

Background 

In response to the terrorist attacks of September 11, 2001, the National Institute of Standards and 

Technology (NIST) initiated a formal federal building and fire safety investigation of the World Trade 

Center disaster on August 21, 2002. NIST issued two written updates on its WTC investigation activities 

(December 2002 and December 2003) and a detailed technical progress report in May 2003. 

In addition, NIST held a public meeting in New York City on February 12, 2004 to solicit comments on 

(1) specific technical aspects of the investigation, (2) additional information that NIST might consider in 

the time remaining; and (3) areas that NIST should consider, within the scope of its investigation, in 

making recommendations for specific improvements to building and fire practice, standards, and codes, 

and their timely adoption. 

The present report provides details of the technical progress made since the May 2003 report was 

published. NIST expects to release the draft of the final investigation report for public comment in 

December 2004. NIST’s investigation is still ongoing. Current findings may be revised and additional 

findings will be presented in the December 2004 report. NIST is not making any recommendations at 

this time. All recommendations will be made in the final report.

Status of Progress 

This report includes: 

x A comprehensive summary of interim findings and accomplishments for each of the 

independent investigation objectives. 

x A working hypothesis for the collapse of the WTC towers that identifies the chronological 

sequence of major collapse events and allows for different possible load redistribution paths 

and damage scenarios currently under analysis. The hypothesis will be refined on the basis of 

these analyses to determine the most probable collapse sequence for each building. 

x A working hypothesis for the collapse of the 47-story WTC 7 based on an initiating event, a 

vertical progression at the east side of the building, a subsequent horizontal progression from 

the east to the west side of the building, and global collapse. 

x Key visual observations on the building, fire, and smoke conditions in all three WTC 

buildings (the WTC towers and WTC 7) from analysis of a large collection of photographic 

and videographic images. 

x A summary of major progress in building comprehensive models for analyzing the most 

probable collapse sequence, from aircraft impact to collapse initiation, and simplified 

analytical models with results to supplement those from detailed models. 
Executive Summary 

xxxii

x Results from experimental work to (1) analyze the recovered WTC structural steel, (2) 

support the fire dynamics and thermal modeling, and (3) conduct fire endurance testing of 

typical floor systems of the WTC towers based on ASTM E 119. 

x Reports on the inventory and identification of the steels recovered from the WTC buildings 

and on the contemporaneous (1960s era) structural steel and welding specifications used to 

construct the WTC towers. 

x First-person interviews of nearly 1,200 WTC occupants, first responders, and families of 

victims to collect data on occupant behavior, evacuation, and emergency response with some 

early results from analysis of that data. 

x Review of the New York City 911 tapes and logs and the transcripts of about 500 interviews 

with Fire Department of New York (FDNY) employees involved in WTC emergency 

response activities with analysis still in progress. 

x Preliminary analysis of emergency responder communication tapes recorded by the Port 

Authority, including the high-rise radio repeater, and by the New York Police Department 

(NYPD), including internal department operations. 

x Analysis of building and fire codes and practices, including: a review of available documents 

related to the design, construction, operation, maintenance, and modifications to the three 

WTC buildings; and a comparison of selected building regulatory and code requirements. 

x Analysis of the design, capabilities, and performance of the installed active fire protection 

systems for all three WTC buildings (i.e., fire alarm, sprinkler, and smoke management 

systems) with documentation of the fire history of the WTC towers. 

x Progress on both the research and development and the dissemination and technical 

assistance programs related to the WTC Investigation. 

x Seventeen appendices with detailed interim reports on specific technical tasks within the 

eight investigation projects where significant progress has been made. 

NIST has received large amounts of data and information related to the design, construction, operation, 

inspection, maintenance, repair, alterations, emergency response, and evacuation of the WTC complex. 

NIST has received considerable cooperation from the Port Authority of New York and New Jersey 

(PANYNJ or Port Authority), the City of New York, the National Commission on Terrorist Attacks Upon 

the United States (9-11 Commission), designers, leaseholders, contractors, suppliers, insurers, news 

media, tenants, first responders, survivors, and families of victims. 

NIST has received all of the essential information it needs for the WTC Investigation. NIST has made a 

few requests for materials that are lost, currently pending, or not yet located; NIST is making efforts to 

assemble this information from various sources since much of it was lost when the buildings collapsed.

NIST continues to pursue other materials that can further clarify some aspects of the Investigation. 
June 2004 Progress Report 

xxxiii

The Web site http://wtc.nist.gov provides comprehensive information on the WTC investigation and 

related work to improve the safety of buildings, their occupants, and first responders. 

Investigation Objectives and Key Questions 

The key interim findings are summarized below in subsections that relate to the investigation objectives 

contained in the NIST investigation plan (see Chapter 1 for a comprehensive discussion of all interim 

findings). The investigation objectives are: 

1. To determine (a) why and how the WTC 1 and WTC 2 collapsed following the initial impact 

of the aircraft, and (b) why and how the 47-story WTC 7 collapsed. 

2. To determine why the loss of life and injuries were so low or so high depending on location, 

including technical aspects of fire protection, occupant behavior, evacuation, and emergency 

response.

3. To determine the procedures and practices which were used in the design, construction, 

operation, and maintenance of the WTC buildings. 

4. To identify, as specifically as possible, areas in national building and fire codes, standards, 

and practices that warrant revision. 

Among the specific questions that NIST is investigating within the above four objectives are the 

following:

x How and why did WTC 1 stand nearly twice as long as WTC 2 before collapsing (103 min 

versus 56 min), though they were hit by virtually identical aircraft? 

x What factors related to normal building and fire safety considerations not unique to the 

terrorist attacks of September 11, 2001, if any, could have delayed or prevented the collapse 

of the WTC towers? 

x Would the undamaged WTC towers have remained standing in a normal major building fire? 

x What factors related to normal building and fire safety considerations, if any, could have 

saved additional WTC occupant lives or could have minimized the loss of life among the 

ranks of first responders on September 11, 2001? 

x How well did the procedures and practices used in the design, construction, operation, and 

maintenance of the WTC buildings conform to accepted national practices, standards, and 

codes? 
Executive Summary 

xxxiv

Context for Findings 

When reviewing these interim findings, the following should be considered: 

x Buildings are not specifically designed to withstand the impact of fuel-laden commercial 

airliners. While documents from the PANYNJ indicate that the impact of a Boeing 707 

flying at 600 mph, possibly crashing into the 80th floor, was analyzed during the design of 

the WTC towers in February/March 1964, the effect of the subsequent fires was not 

considered. Building codes do not require building designs to consider aircraft impact. 

x Buildings are not designed for fire protection and evacuation under the magnitude and scale 

of conditions similar to those caused by the terrorist attacks of September 11, 2001. 

x The load conditions induced by aircraft impact and the extensive fires on September 11, 

2001, which triggered the collapse of the WTC towers, fall outside the norm of design loads 

considered in building codes.

x Prior evacuation and emergency response experience in major events did not include the total 

collapse of tall buildings such as the WTC towers and WTC 7 that were occupied and in 

everyday use; instead, that experience suggested that major tall building fires result in 

burnout conditions, not global building collapse. 

x The PANYNJ was created as an interstate entity, under a clause of the U.S. Constitution 

permitting compacts between states, and is not bound by the authority of any local, state, or 

federal jurisdiction, including local building and fire codes. The PANYNJ’s longstanding 

policy is to meet and, where appropriate, exceed the requirements of local building and fire 

codes.

Collapse of the WTC Towers 

Working Hypothesis. The following chronological sequence of major events led to the eventual collapse 

of the towers; specific load redistribution paths and damage scenarios are currently under analysis to 

determine the most probable collapse sequence for each building: 

x Aircraft impact damage to perimeter columns, resulting in redistribution of column loads to 

adjacent perimeter columns and to the core columns via the hat truss; 

x After breaching the building’s exterior, the aircraft continued to penetrate into the buildings, 

damaging core columns with redistribution of column loads to other intact core and perimeter 

columns via the hat truss and floor systems; 

x The subsequent fires, influenced by the post-impact condition of the fireproofing, weakened 

columns and floor systems (including those that had been damaged by aircraft impact), 

triggering additional local failures that ultimately led to column instability; and 

x Initiation and horizontal progression of column instability resulted when redistributing loads 

could not be accommodated any further. The collapses then ensued. 
June 2004 Progress Report 

xxxv

The working hypothesis (see Chapter 1 and Appendix Q for a detailed description) is consistent with all 

evidence currently held by NIST, including photographs and videos, eyewitness accounts, and emergency 

communication records. In addition to evidence of hanging floor slabs on the east and north faces of 

WTC 2 that were reported previously, new evidence has been found showing inward bowing of perimeter 

columns several minutes (less than 10 min) prior to collapse in both WTC towers. Inward bowing of 

about a quarter to a third of the perimeter columns was observed in photographs on the south face of 

WTC 1 and the east face of WTC 2 in regions that contained active fires. Further, initiation of global 

collapse was first observed by the tilting of building sections above the impact regions of both WTC 

towers. WTC 1 tilted to the south (observed via antenna tilting in a video recording) and WTC 2 tilted to 

the east and south and twisted in a counterclockwise motion. 

Discussed below are interim findings for several factors relevant to the condition and collapse of the 

WTC towers, including the innovative structural system, aircraft impact and the ensuing fires, post-impact 

condition of the fireproofing, and the quality and properties of the structural steel used in the WTC 

towers. In addition to the role played by these factors in the collapse of the WTC towers, NIST continues 

to investigate the relative roles of the perimeter and core columns and the composite floor system, 

including connections. 

Innovative Structural System. The WTC tower structures represented an innovative structural system 

when they were built, incorporating many new and unusual features. Among them, two features require 

additional consideration: the composite floor system, using open-web bar joist elements, to provide 

lateral stability and diaphragm action, and the use of wind tunnel testing to estimate lateral wind loads—

which were a major governing factor in the design of the WTC tower structures. 

The performance of the former under fire conditions, which is relevant to evaluating the collapse of the 

WTC towers, was of some concern to the building owner and designers throughout the life of the 

buildings. The concern stemming from the latter, identified by the leaseholder and insurers in litigation 

after September 11, 2001, is representative of the still evolving state-of-knowledge in the field of wind 

engineering and is relevant to establishing the baseline performance of the WTC towers and to assessing 

the practices and procedures used in design. 

The fire protection of a truss-supported floor system by directly applying spray-on fireproofing to the 

steel trusses was innovative and not consistent with prevailing practice at the time the WTC towers were 

designed and constructed. The fireproofing thickness required to meet the 2 h fire rating evolved from the 

specified 1/2 in. when the WTC towers were built to 1-1/2 in. for use in upgrading the fireproofing some 

years prior to September 11, 2001. Unrelated to the WTC buildings, a model code evaluation service 

recommended in June 2001 a minimum thickness of 2 in. for a similar floor system. This three to four 

fold difference in specifying the fireproofing thickness to meet the required fire rating for a structural 

assembly is extraordinarily large and confirms the lack of technical basis in the selection of thickness. 

While the benefits of conducting a full-scale fire endurance test to determine the required fireproofing 

thickness were recognized by the building designers, no tests were conducted on the floor system used in 

the WTC towers to establish a fire endurance rating. NIST has awarded a contract to Underwriters 

Laboratories (UL) to determine the fire resistance rating of typical WTC floor systems under both 

as-specified and as-built conditions. The tests, expected to be conducted in August 2004, are also 

designed to evaluate the effects of test scale, fireproofing thickness, and thermal restraint. 
Executive Summary 

xxxvi

Further, use of the “structural frame” approach, in conjunction with the prescriptive fire rating, would 

have required the floor system trusses, the core floor framing, and perimeter spandrels in the WTC 

towers—essential to the stability of the building as a whole—to be 3 h fire-rated as the columns were 

required to be rated by the 1968 New York City (NYC) Building Code. This approach, which appeared 

in the Uniform Building Code (a model building code) as early as 1953, was carried into the 2000 

International Building Code (one of two current national model codes). Neither the 1968 edition of the 

NYC Building Code which was used in the design of the WTC towers, nor the 2001 edition of the code, 

adopted the “structural frame” requirement. 

Results of two sets of wind tunnel tests conducted for the WTC towers in 2002 by independent 

laboratories, and voluntarily provided to NIST by the parties to an insurance litigation, show large 

differences, of as much as about 40 percent, in resultant forces on the structures, i.e., overturning 

moments and base shears. In addition, the wind loads estimated from these tests are about 20 percent to 

60 percent higher than those apparently used in the original design of the WTC towers, also obtained from 

wind tunnel testing. NIST is conducting an independent analysis to establish the baseline performance of 

the WTC towers under the original design wind loads and will compare those wind load estimates with 

then-prevailing code requirements. Wind loads were a major governing factor in the design of structural 

components that made up the frame-tube steel framing system. 

Relative Roles of Aircraft Impact and Fires. The two WTC towers withstood the initial impact of 

virtually identical aircraft (Boeing 767-200ER) during the terrorist attacks of September 11, 2001. The 

robustness of the perimeter frame-tube system and large dimensional size of the WTC towers helped the 

buildings withstand the aircraft impact. The WTC towers displayed significant reserve capacity, vibrating 

immediately following impact with amplitudes that were about half the amplitudes for design wind 

conditions expected by the building designers and an oscillation period nearly equal to that measured for 

the undamaged building. 

Preliminary aircraft impact damage analysis indicates that the impact of a fuel-filled wing section results 

in extensive damage to the exterior wall panel, including complete failure of the perimeter columns. A 

normal impact of the exterior wall by an empty wing segment produces significant damage to the 

perimeter columns, not necessarily complete failure. Also, engine impact against an exterior wall panel 

results in a penetration of the exterior wall and failure of impacted perimeter columns. The residual 

velocity and mass of the engine after penetration of the exterior wall is sufficient to fail a core column in 

the event of a direct impact. 

Fires played a major role in further reducing the structural capacity of the buildings, initiating collapse. 

While aircraft impact damage did not, by itself, initiate building collapse, it contributed greatly to the 

subsequent fires by: 

x Compromising the sprinkler and water supply systems; 

x Dispersing jet fuel and igniting building contents over large areas; 

x Creating large accumulations of combustible matter containing aircraft and building contents; 
June 2004 Progress Report 

xxxvii

x Increasing the air supply into the damaged buildings that permitted significantly higher 

energy release rates than would normally be seen in ventilation limited building fires, 

allowing the fires to spread rapidly within and between floors; and 

x Damaging ceilings that enabled “unabated” heat transport over the floor-to-ceiling partition 

walls and to structural components. 

The jet fuel, which ignited the fires, was mostly consumed within the first few minutes after impact. The 

fires that burned for almost the entire time that the buildings remained standing were due mainly to 

burning building contents and, to a lesser extent, aircraft contents, not jet fuel. 

By contrast, typical office furnishings were able to sustain intense fires for at least an hour on a given 

WTC floor. The typical WTC office floor using modern workstation furnishings had on average about 

4 pounds per square foot (psf) of combustible materials on floors without unusual file rooms, film 

storage, etc. Further, the mass of aircraft solid combustibles was significant relative to the building 

combustibles in the immediate impact region of both WTC towers. 

Consistent with available photographic and videographic evidence, computer simulations conducted by 

NIST have been able to capture the broad patterns of fire movement around the floors, with the flames in 

a given location lasting for about 20 min before spreading to adjacent, yet unburned combustibles. This 

spread is generally continuous due to the relatively even distribution of combustibles and the paucity of 

interior partitions. There are some observed instances where fires persisted over longer durations in 

regions with accumulated combustible debris and other instances of sudden or interrupted fire spread. 

Applying the 1968 NYC Building Code, the WTC towers were required to have 1 h fire-rated tenant 

separations, but the code did not impose any minimum compartmentation requirements (e.g., 7,500 ft2

) to 

mitigate the horizontal spread of fire in buildings with large open floor plans. The sprinkler option was 

chosen for the WTC towers in preference to the compartmentation option in meeting the subsequent 

requirements of Local Law 5, adopted by New York City in 1973. The affected floors in the WTC towers 

were mostly open—with a modest number of perimeter offices and conference rooms and an occasional 

special purpose area. Some floors had two tenants, and those spaces, like the core areas, were partitioned 

(slab to slab). Photographic and videographic evidence confirms that even non-tenant space partitions 

(such as those that divided spaces to provide corner conference rooms) provided substantial resistance to 

fire spread in the affected floors. For the duration of about 50 min to 100 min prior to building collapse 

that the fires were active, the presence of undamaged 1 h fire-rated compartments may have assisted in 

mitigating fire spread and consequent thermal weakening of structural components. 

Role of Fireproofing Conditions. NIST has developed a rigorous technical approach (see Appendix I for 

details) to evaluate the role fireproofing conditions may have played in the collapse of the WTC towers. 

The approach considers both the thickness and variability of fireproofing and the extent to which 

fireproofing may have been damaged due to aircraft impact. 

In general, the floor systems in WTC 1 subject to aircraft impact and subsequent fires on September 11, 

2001 had upgraded or thicker fireproofing (1.5 in. specified), while the affected floors in WTC 2 had the 

original fireproofing (0.5 in. specified). 
Executive Summary 

xxxviii

The response of a structural component to fires is sensitive to variability in fireproofing thickness along 

its length. For the original fireproofing in the WTC towers, the as-applied fireproofing thickness on the 

floor trusses (0.75 in. average and 0.4 coefficient of variation) was found to be thermally equivalent to a 

uniform thickness of 0.6 in., which is greater than the specified minimum thickness of 0.5 in. For the 

upgraded fireproofing in some floors of the WTC towers, the as-applied upgraded fireproofing thickness 

(2.5 in. average and 0.24 coefficient of variation) was found to be thermally equivalent to a uniform 

thickness of 2.2 in., which is greater than the specified minimum thickness of 1.5 in. An alternative 

criterion for determining the equivalent thickness is currently being examined to confirm these findings. 

Based on simplified analytical models, it was found that acceleration of a structural element, on the order 

of 100 to 150 times the acceleration due to gravity (or 100 g to 150 g), would be required to dislodge 

fireproofing similar to that used in the WTC towers with a typical thickness of about 1 in. from the 

structural component. Experiments are underway to verify the results of these simplified analyses. 

Similarly, analytical studies are underway to estimate the magnitude of accelerations of the structural 

members due to aircraft impact, from which the regions where fireproofing may have been dislodged will 

be identified. Those results are being used to analyze the role of the post-impact condition of the 

fireproofing, including its thickness, on the collapse of the WTC towers. 

Analysis of Recovered WTC Steel. NIST has 236 pieces of steel in its possession; this collection of steel 

is adequate for purposes of determining the quality and properties of steel for the investigation. The 

regions of impact and fire damage were emphasized in the selection of steel pieces for the investigation.

As a result, pieces of all 14 specified steel grades for exterior panels in the WTC towers are available, as 

well as the two specified grades that represent 99 percent of the core columns and both specified grades 

for the steel trusses that comprised the composite floor truss system. 

Analysis of steel recovered from the WTC towers, based on stampings on the steel and mechanical tests, 

indicates that the correct specified materials were provided for the specified elements. When these data 

were combined with pre-collapse photographic images of damaged steel, it was found that aircraft 

impacted pieces of steel recovered from WTC 1 were in the precise locations as specified in the design 

drawings. Metallography and mechanical property tests indicate that the strength and quality of the steel 

used in the towers was as specified, typical of the era, and likely met all qualifying test requirements. 

The room-temperature strength of the steel used in the towers met the relevant standards and, in many 

instances, exceeded the requirements by 5 percent to 10 percent. Work is ongoing to analyze the 

performance of the steel building components under impact and fire conditions up to initiation of global 

building collapse. 

Collapse of the 47-Story WTC 7 Building 

Working Hypothesis. The working hypothesis for the collapse of the 47-story WTC 7 building, if it 

remains viable upon further analysis, suggests that it was a classic progressive collapse including: an 

initiating event, a vertical progression at the east side of the building, a subsequent horizontal progression 
June 2004 Progress Report 

xxxix

from the east to the west side of the building, and global collapse. The chronological sequence of major 

events under analysis is: 

x An initial local failure at the lower floors (below Floor 13) of the building due to fire and/or 

debris induced structural damage of a critical column (the initiating event), which supported a 

large span floor bay with an area of about 2,000 ft2

;

x Vertical progression of the initial local failure up to the east penthouse, as large floor bays 

were unable to redistribute the loads, bringing down the interior structure below the east 

penthouse; and 

x Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7, 

that were much thicker than the rest of the floors), triggered by damage due to the vertical 

failure, resulting in disproportionate collapse of the entire structure.

Visual Observations. The working hypothesis (see Chapter 1 and Appendix L for a detailed description) 

is consistent with all evidence currently held by NIST, including photographs and videos, eyewitness 

accounts, and emergency communication records. Specifically, the evidence indicates: 

x The sequence of failures associated with the sinking of the east penthouse roof structure into 

the building, the near simultaneous window breakage along the east side of the north face, the 

sinking of the other roof structures, the near simultaneous breakage of a second set of 

windows along the west side of the north face, and the entire north façade above the 

13th floor appearing to drop as an intact unit. 

x Structural damage on the south face and southwest corner from WTC 1 debris that included 

(1) a multi-story gash across approximately a quarter to a third of the lower portion of the 

south face and extending inwards to the core, (2) approximately two columns in the 

southwest corner and related floor areas missing from Floors 8 to 18, and (3) severed 

spandrels between exterior columns near the southwest corner from the roof level for at least 

5 to 10 floors. 

x The sequence of fires in WTC 7—which began soon after WTC 1 collapsed—was observed 

(1) on the south face and near the southwest corner on Floors 22, 29, and 30, (2) across Floors 

11 and 12 on the east face, from the south to the north, (3) on Floors 7 and 12 along the north 

face, (4) on Floors 8 and 13, with the fire on Floor 8 moving from west to east and the fire on 

Floor 13 moving from east to west, and (5) finally, on Floors 7, 8, 9, and 11 near the middle 

about half an hour before collapse; Floor 12 was burned out by this time. Interview responses 

indicate that there was no water in the standpipe system supplying the sprinklers in WTC 7. 

Fuel System for Emergency Power. Based on a review of the fuel system for emergency power in 

WTC 7, Floor 5—which did not have any exterior windows and contained the only pressurized fuel 

distribution system on the south, west and north floor areas—is considered a possible fire initiation 

location, subject to further data and/or analysis that improve knowledge of fire conditions in this area. 
Executive Summary 

xl

Evacuation and Emergency Response 

Building Population Characteristics. Based on information and data gathered during the first-person 

interviews of WTC surviving occupants: 

x It is estimated that 17,400 occupants (± 1,200) were present in the WTC towers on the 

morning of September 11, 2001. The initial population of each tower was similar: 8,900 

(± 750) in WTC 1 and 8,500 (± 900) in WTC 2. Of those present on September 11, 2001, 

16 percent were also present during the 1993 bombing. 

x About 6 percent of the surviving occupants reported a pre-existing limitation to their 

mobility. These limitations included obesity, heart condition, needing assistance to walk, 

pregnancy, asthma, being elderly, chronic condition, recent surgery or injury, and other. 

x About 7 percent of the surviving occupants reported having special knowledge about the 

building. These included fire safety staff, floor wardens, searchers, building maintenance, 

and security staff. Searchers assist the floor wardens in facilitating evacuation. 

Evacuation. Two-thirds of surviving occupants reported having participated in a fire drill in the 

12 months prior to September 11, 2001, while 17 percent reported that they received no training during 

that same period. Of those participating in fire drills, 93 percent were instructed about the location of the 

nearest stairwell. Overall, slightly over half of the survivors, however, had never used a stairwell at the 

WTC prior to September 11, 2001. 

Approximately 87 percent of the WTC tower occupants, including more than 99 percent of those below 

the floors of impact, were able to evacuate successfully. Two-thousand one-hundred fifty-nine building 

occupants (1,560 in WTC 1 and 599 in WTC 2) and an additional 433 first responders, including security 

guards but not aircraft passengers and crew or bystanders, were reported to have lost their lives on 

September 11, 2001. 

Rough initial estimates indicate that about 20 percent or more of those who were in the WTC towers and 

lost their lives may have been alive in the buildings just prior to their collapse. This estimate, which will 

be refined as data analysis is completed, assumes that nearly all of the first responders and 76 building 

occupants below the floors of impact, but none of the people at or above the floors of impact, may have 

been alive. It is estimated that there were a total of 2,592 building occupants and first responders who 

were in the WTC towers and lost their lives. 

Overall, about 7,900 survivors evacuated WTC 2 in 73 min (i.e., from the instant the WTC 1 was struck 

by aircraft until WTC 2 collapsed) while about 7,500 survivors evacuated WTC 1 in 103 min. Thus, the 

overall evacuation rate in WTC 2 (108 survivors per min) was about 50 percent faster than that in WTC 1 

(73 survivors per min). Functioning elevators allowed many survivors to evacuate WTC 2 prior to 

aircraft impact. Most of the elevators in WTC 1 were not functioning, and survivors could only use the 

stairways. The stairwells, with partition wall enclosures that provided a 2 h fire-rating but little structural 

integrity, were damaged in the region of the aircraft impacted floors. 

x After the first airplane struck WTC 1 and before the second airplane struck WTC 2, the 

survivors in WTC 2 were twice as likely as those in WTC 1 to have already exited the 
June 2004 Progress Report 

xli

building (41 percent versus 21 percent). The rate of evacuation completion in WTC 2 was 

twice the rate in WTC 1 during that same period. 

x Soon after WTC 2 was struck by the airplane until about 20 min before each building 

collapsed, the survivors in WTC 2 and WTC 1 had exited at about the same rate (the prior 

evacuation rate of WTC 1). 

x During the last 20 min before each building collapsed, the evacuation rate in both buildings 

had slowed to about one-fifth the immediately prior evacuation rate. This suggests that for 

those seeking and able to reach and use undamaged exits and stairways, the egress capacity 

(number and width of exits and stairways) was adequate to accommodate survivors. 

Preliminary results from application of existing computer egress models for a full capacity evacuation of 

a single WTC tower with 25,000 occupants and visitors indicate a movement time of 2 h and 15 min. 

This is a minimum time estimate; the simulation assumed that there was no survivor delay, continual 

movement on the stairs, and no damage to the egress system. It was also assumed that elevators were not 

available. The egress model estimate for a September 11, 2001 capacity evacuation under the same 

assumptions is about 50 min, which is 2.5 times less than the time estimate for evacuating 25,000 people. 

Given that the actual evacuation time on September 11, 2001 was about 100 min without elevator use, a 

full capacity evacuation of each WTC tower with 25,000 people would have required about 4 h (2.5 times 

100 min). To achieve a significantly faster total evacuation at full capacity would have required increases 

in egress capacity (number and width of exits and stairways). 

In addition to the full evacuation of the WTC towers on September 11, 2001, a full evacuation was 

ordered during the 1993 bombing at the WTC site and during a 1977 terrorist threat associated with 

bombings in two remote midtown Manhattan buildings. Sufficient data do not exist on the frequency 

with which full evacuations are conducted in buildings not at risk for terrorist attacks and whether this 

frequency has increased since September 11, 2001 among the general population that did not directly 

experience the events on that day. 

Roof Evacuation. A preliminary evaluation indicates that the PANYNJ’s standard occupant evacuation 

procedures and drills required the use of stairways to exit at the bottom of the WTC towers. The standard 

procedures were to keep the doors to the roof locked with a key being required to gain roof access. The 

PANYNJ reports that it never advised tenants to evacuate upward. 

There were at least two decedents who had tried to get to the roof and found the roof access locked to 

both the WTC towers. In addition, a PANYNJ employee trapped on Floor 105 of WTC 2 was unable to 

walk down the stairs, or go to the roof as instructed on radio by another PANYNJ employee. 

The NYPD aviation unit arrived at the WTC site soon after WTC 1 was attacked. Despite repeated 

attempts to examine the possibility of roof rescue, smoke and heat conditions at the top of the WTC 

towers prevented the conduct of safe roof evacuation operations.

Considering the capacity of typical helicopters and travel times, it is not clear what fraction of the large 

number of occupants could have been evacuated from the WTC towers prior to their collapse had roof 

rescue been possible on September 11, 2001. 
Executive Summary 

xlii

Emergency Communication Systems. A partial analysis of emergency responder communications (see 

Appendix P for details) has been completed, including: 

x Audio communications tapes recorded by the PANYNJ, including a recording of the FDNY’s 

city-wide high-rise Channel 7 (Port Authority Police Department’s [PAPD] Channel 30) 

radio repeater that was located at the WTC. 

x Audio tapes copied from original NYPD communications tapes, including NYPD internal 

department operations. 

FDNY communications recordings from the WTC location on September 11, 2001, are not available 

because the primary field communication truck was in the shop for repairs. A back-up fieldcommunications van used in its place—which did not have a recording capability—was destroyed when 

the WTC towers collapsed. 

The best record of radio communications reflecting fire department operations came from the FDNY 

Channel 7/PAPD Channel 30 and first person accounts provided by FDNY personnel during their 

interviews. The PANYNJ installed the radio repeater system for use by FDNY after the 1993 bombing. 

The analysis of the emergency responder communication tapes indicates that: 

x After the first aircraft struck WTC 1, there was an approximate factor of 5 peak increase in 

traffic level over the normal level of emergency responder radio communications, followed 

by an approximate factor of 3 steady increase in the level of subsequent traffic. 

x A surge in communications traffic volume made it more difficult to handle the flow of 

communications and delivery of information. 

x Roughly a third to a half of the radio messages transmitted during these radio traffic surge 

conditions were not complete messages or understandable. 

x FDNY’s city-wide high-rise Channel 7 (PAPD Channel 30) radio repeater at the WTC site 

was operating.

x NYPD aviation unit personnel reported critical information about the impending collapse of 

the WTC towers several minutes prior to their collapse. No evidence has been found to 

suggest that the information was further communicated to all emergency responders at the 

scene.

Several FDNY personnel at the incident site did not think that the high-rise radio repeater was working. 

This is based on radio communications tests that were conducted by two chief officers working inside 

WTC 1 when the first command post was being set up in that lobby. Following this radio test, a chief 

officer involved in the test chose to use different channels for command and tactical communications 

during the incident. However, as FDNY operations increased in WTC 2, it was determined by FDNY 

members that the high-rise repeater was functioning, and use of the channel developed. 

While the preliminary analysis indicates that the repeater was operating, there also appears to have been 

some type of malfunction with the communications equipment that was detected, but not identified, by 
June 2004 Progress Report 

xliii

FDNY officers during the initial test. NIST continues to evaluate the repeater system and its operations, 

as well as the handheld radios, which were used on September 11, 2001. These findings will be updated 

and additional findings will be documented when the investigation is complete. 

Command and Control. Based on face-to-face interviews, NIST has determined that first responders— 

including key incident commanders—did not have adequate information (voice, video, and data) on, nor 

an overall perspective of, the conditions in the WTC buildings and what was happening elsewhere at the 

WTC site. Interagency information sharing was inadequate. 

The three FDNY suitcase-based, magnetic Command Boards that were set up at the incident site—on 

which a record is kept of the identification of the units on site, their assignment, location, and activities—

were damaged and lost with the collapse WTC 2. Since there was no back-up capability for the 

Command Boards, all information related to command, control, and accountability was lost. 

Active Fire Protection Systems. Investigation of the design, capabilities, and performance of the active 

fire protection systems in the WTC towers and WTC 7 indicates that: 

x The smoke management systems in the WTC towers were not activated during the fires on 

September 11, 2001. It was determined that the likelihood of these systems being functional 

was very low due to the damage inflicted by the aircraft impacts. 

x The analysis of smoke flow in WTC 1 and WTC 2 on September 11, 2001 shows that HVAC 

(heating, ventilation, and air-conditioning) ductwork was a major path for vertical smoke 

spread in the buildings. Fire dampers were installed in the systems, but not smoke dampers 

that could have provided a barrier to hot gas and smoke penetration into the vertical HVAC 

shafts. However, smoke dampers were not available when the towers were built. 

x Modeling results show that stair pressurization systems would have provided minimal 

resistance to the passage of smoke in WTC 1 and WTC 2 had they been installed on 

September 11, 2001. While the existence of such systems was known when the WTC towers 

were built, the alternative smoke purge system used in the WTC towers was considered to be 

equivalent.

x The fire alarm system in WTC 7 sent only one signal (at 10:00:52 a.m. shortly after the 

collapse of WTC 2) to the monitoring company indicating a fire condition. The signal did not 

contain any specific information about the location of the fire within the building. Since the 

system was placed on TEST for a period of 8 h beginning at 6:47:03 a.m. on September 11, 

2001, alarm signals would not have been shown on the operator’s display; instead, they 

would have to be recorded into the history file. 

x The resistance to failure of the fire alarm system communications paths between the fire 

command station and occupied WTC tower floors could have been enhanced if fiber optic 

communications cable had been used instead of copper lines. Fiber optic cable is not 

susceptible to electric short-circuits and would have provided full communications with fire 

alarm system components, including voice communications systems, to the point where the 

cable was severed. Electric shorts in the voice communications disable that communication 

system over the entire cable length affected by the electric short-circuit. During initial design 
Executive Summary 

xliv

of the system, the PANYNJ requested, but did not receive, approval of the City of New York 

for use of fiber optic communication cable in the system. The NYC code required copper 

wiring.

x There was adequate multiple point redundancy in the water supply to the sprinkler system, 

and the water flow rate exceeded the minimum requirement by a considerable margin. 

However, the potential for single point failure of the water supply to the fire sprinklers 

existed at each floor due to lack of redundancy in the sprinkler riser system that provided 

only one water supply connection on each floor. While this lack of redundancy may not have 

had an impact on September 11, 2001 because the sprinkler system was damaged by aircraft 

impact, it could have made a difference in other building emergencies. 

Procedures and Practices 

The 110-story WTC towers were among the world’s tallest buildings, while the 47-story WTC 7 

represented a more typical tall building. These buildings provide case studies to document, review, and, 

if needed, improve the procedures and practices used in the design, construction, operation, and 

maintenance of tall buildings. This investigation objective is independent of other objectives which are 

focused specifically on the consequences of the attack on September 11, 2001, viz., the building 

collapses, evacuation, and emergency response. While some findings under this objective are directly 

relevant to the events of September 11, 2001, others are concerned with general building and fire safety 

procedures and practices. 

Applicable Building Codes. Although not required to conform to NYC codes, the PANYNJ adopted the 

provisions of the proposed 1968 edition of the NYC Building Code, more than three years before it went 

into effect. The 1968 edition allowed the PANYNJ to take economic advantage of less restrictive 

provisions compared with the 1938 edition that was in effect when design began for the WTC towers in 

1962. The 1968 code: 

x Eliminated a fire tower1

 as a required means of egress; 

x Reduced the number of required stairwells from 6 to 3 and the size of doors leading to the 

stairs from 44 in. to 36 in.; 

x Reduced the fire rating of the shaft walls in the building core from 3 h to 2 h; 

x Changed partition loads from 20 psf to one based on weight of partitions per unit length (that 

reduced such loads for many buildings including the WTC buildings); and 

x Permitted a 1 h reduction in fire rating for all structural components (columns from 4 h to 3 h 

and floor framing members from 3 h to 2 h). 

 

1 A fire tower (also called a smoke-proof stair) is a stairway that is accessed through an enclosed vestibule that is open to the

outside or to an open ventilation shaft providing natural ventilation that prevents any accumulation of smoke without the need for 

mechanical pressurization.
June 2004 Progress Report 

xlv

The NYC Department of Buildings reviewed the WTC tower drawings in 1968 and provided comments 

to the PANYNJ concerning the plans in relation to the 1938 NYC Building Code. The architect-of-record 

submitted to the PANYNJ responses to those comments, noting how the drawings conformed to the 1968 

NYC Building Code.

In 1993, the PANYNJ and the NYC Department of Buildings entered into a memorandum of 

understanding that restated the PANYNJ’s longstanding policy to assure that its facilities in the City of 

New York meet and, where appropriate, exceed the requirements of the NYC Building Code. The 

agreement also provided specific commitments to the NYC Department of Buildings regarding 

procedures to be undertaken by the PANYNJ to assure that buildings owned or operated by the PANYNJ 

are in conformance with the Building Standards contained in the NYC Building Code. 

In 1993, the PANYNJ adopted a policy providing for implementation of fire safety recommendations 

made by local government fire departments after a fire safety inspection of a PANYNJ facility and for the 

prior review by local fire safety agencies of fire safety systems to be introduced or added to a facility. 

Later that year, the PANYNJ entered into an agreement with FDNY which reiterated the policy adopted 

by the PANYNJ and set forth procedures to assure that new or modified fire safety systems are in 

compliance with local codes and regulations. 

Standards, Codes, and Regulations. NIST has reviewed the then-prevailing and current standards, 

codes, and regulations relevant to assessing the procedures and practices used in the design, construction, 

operation, and maintenance of the WTC buildings. That review raises the following issues that merit 

further consideration (see Chapter 1 for a discussion and WTC-related rationale): 

x Code provisions that would detail procedures to analyze and evaluate data from fire 

resistance tests of other building components and assemblies to qualify an untested building 

element. 

x Code provisions that would require the conduct of a fire resistance test if adequate data do not 

exist from other building components and assemblies to qualify an untested building element. 

x Regulations that would adopt code provisions using the “structural frame” approach to fire 

resistance ratings which requires structural members, other than columns, that are essential to 

the stability of the building as a whole to be fire protected to the same rating as columns. 

x Code provisions that would ensure that structural connections are provided the same degree 

of fire protection as the more restrictive protection of the connected elements. 

x Code provisions and standards that would establish whether the minimum mechanical and 

durability related properties of spray-applied fire resistive materials (SFRM) are sufficient to 

ensure acceptable in-service performance in buildings. While minimum bond strength 

requirements exist, there are no serviceability requirements for such materials to withstand 

typical shock, impact, vibration, or abrasion effects over the life of a building. 

x Rigorous field application and inspection provisions and regulatory requirements that would 

assure that the as-built condition of the passive fire protection, such as SFRM, conforms to 

conditions found in fire resistance tests of building components and assemblies. 
Executive Summary 

xlvi

x Rigorous provisions and regulatory requirements for in-service inspections of passive fire 

protection during the life of the building. 

x Early installation of sprinklers in existing buildings, not as an option in lieu of 

compartmentation. 

x Standards and code provisions that would provide minimum structural integrity for the means 

of egress (stairwells and elevator shafts) in the building core which are critical to life safety. 

x Standards and code provisions that would permit the installation of fire-protected elevators 

and their use for routine emergency access by first responders or as a secondary method (after 

stairwells) for emergency evacuation of building occupants. 

x Explicit standards and code provisions for structural integrity that would mitigate progressive 

collapse.

x Standards and code provisions for conducting wind tunnel tests and for the methods used in 

practice to estimate design wind loads from test results. 

x Regulatory requirements for retention of documents related to the design, construction, 

operation, maintenance, and modifications of buildings, including retention off-site. For 

example, there are few, if any, requirements for retention of documents throughout the 

service life of a building. 

Fire Safety and Egress Design Methods. Historical fire loss data over more than half a century, for 

different high-rise building occupancies, suggests that prescriptive requirements in standards and codes 

have considerable built-in conservatism to adequately protect building occupants. As a result, there has 

been a trend in recent decades to reduce fire rating and egress requirements, sometimes in conjunction 

with addition of other new and complementary fire protection requirements (e.g., detectors and 

sprinklers). The lower fire rating requirements when combined with the considerable increases in 

building design efficiency that have been achieved, have also led to reductions in the thermal mass of 

buildings—an indicator of how much heat energy a building can absorb passively without damage. 

The empirical rules and test methods used in prescriptive design, which have evolved with experience 

over the years, do not lend themselves readily to evaluate whether the performance of building fire safety 

and egress systems is risk-consistent, considering both the hazards and the consequences of the hazards.

Performance-based methods that explicitly define the design objectives and specific design-basis fire 

hazards or evacuation events are better suited to risk analysis, enabling appropriate protection to be 

provided where it is needed.

The increasing use of performance-based methods, as an alternative to prescriptive design, in fire safety 

and egress design, raises the following issues that merit further consideration (see Chapter 1 for a detailed 

discussion and rationale related to the WTC investigation): 

x Considering fire as a design condition in structural design, including evaluation of the fire 

performance of the structure as a whole system. This design approach is already being used 

in building design practice for earthquake and wind hazards (e.g., a two-level design that 
June 2004 Progress Report 

xlvii

includes an operational event with a 10 percent probability of occurrence in 50 years and a 

life safety event with a 2 percent probability of occurrence in 50 years). 

x Detailed procedures to select appropriate design-basis fire scenarios for performance-based 

design of the sprinkler system (e.g., a frequent but low severity fire), compartmentation (e.g., 

a moderate severity but less frequent fire), and passive protection of the structure (e.g., a 

maximum credible fire). 

x Validated and verified tools for use in performance-based design practice to analyze the 

dynamics of building fires and their effects on the structural system that would allow 

engineers to evaluate structural performance under alternative fire scenarios and fire 

protection strategies. While considerable progress has been made in recent years in 

advancing the tools that will help to improve the fire-safe design of new structures and 

analyze conditions of existing structures, significant work remains to be done before adequate 

tools are available for use in routine practice. 

x The technical basis to establish whether the construction classification and fire rating 

requirements are risk-consistent. Specifically, it is not apparent how the current height and 

area tables in building codes consider the technical basis for the progressively increasing risk 

to an occupant on the upper floors of tall buildings that are much greater than 200 ft in height. 

The maximum fire rating in current codes applies to any building more than about 12 stories 

in height. 

x Sprinklers improve safety in most common building fires and prevent them from becoming 

large fires. The technical basis to establish the “sprinkler trade-off” in current codes, 

considering fire safety risk factors such as: (1) the complementary functions of sprinklers and 

fire-protected structural elements, (2) the different design-basis fire scenarios for which each 

system is designed to provide protection, and (3) the need for redundancy should one system 

fail to function as intended is not available. The sprinkler trade-off provides an economic 

incentive to encourage installation of sprinklers by allowing a lower fire rating for sprinklered 

buildings. 

x The design of egress systems to achieve a target performance (e.g., evacuation rate or time) 

for a given occupant population by adequately considering travel distance, remoteness 

requirements, and human factors such as occupant size, stairwell environmental conditions, 

visibility, and congestion. 

Building Practices. While the PANYNJ entered into agreements with the NYC Department of Buildings 

in the 1990s with regard to conformance of PANYNJ buildings constructed in New York City to the NYC 

Building Code, the PANYNJ did not yield jurisdictional authority for regulatory and enforcement 

oversight to the NYC Department of Buildings. The PANYNJ was created as an interstate entity, under a 

clause of the U.S. Constitution permitting compacts between states, and is not bound by the authority of 

any local, state, or federal jurisdiction. 

The architect is responsible for specifying the fire protection in current building practice. The structural 

engineer is not required to evaluate and certify that the passive fire protection is adequate to protect the 

structural system. In accordance with established practice, the structural engineer was not responsible for 
Executive Summary 

xlviii

the passive fire protection in the design of the WTC tower structures. In addition, there is no requirement 

to involve a fire protection engineer in the design and evaluation of a building’s fire protection system. In 

some cases, architects retain fire protection engineers to assist with the fire protection design for a 

building. There are only a few academic degree programs or continuing education programs that qualify 

engineers (or architects) to evaluate the fire performance of structures. The current state-of-practice is not 

sufficiently advanced for engineers to routinely analyze the performance of a whole structural system 

under a prescribed design-basis fire scenario. 

Approach to Recommendations 

In the United States, state and local governments are responsible for promulgating and enforcing building 

and fire regulations. With some exceptions, the state and local regulations are based on national model 

building and fire codes developed by private sector organizations. The model codes, in turn, reference 

voluntary consensus standards developed by a large number of private sector standards development 

organizations (SDOs) accredited by the American National Standards Institute (ANSI). 

NIST is a non-regulatory agency of the U.S. Department of Commerce. NIST does not set building codes 

and standards, but provides technical support to the private sector and other government agencies in the 

development of U.S. building and fire practices, standards, and codes. NIST recommendations are given 

serious consideration by private sector organizations that develop national standards and model codes – 

which provide minimum requirements for public welfare and safety. 

The NIST building and fire safety investigation of the WTC disaster has not yet formulated 

recommendations. However, in formulating its recommendations, NIST will consider the following: 

x Findings from the first three independent investigation objectives related to building 

performance, evacuation and emergency response, and procedures and practices. 

x Whether findings relate to the unique circumstances surrounding the terrorist attacks of 

September 11, 2001, or to normal building and fire safety considerations, including 

evacuation and emergency response. 

x What technical solutions are needed, if any, to address potential risks to buildings, occupants, 

and first responders, considering both identifiable hazards and the consequences of those 

hazards? 

x Whether the risk is in all buildings or limited to certain building types (e.g., height and area, 

structural system), buildings that contain specific design features, iconic/signature buildings, 

or buildings that house critical functions. 

NIST urges organizations responsible for building and fire safety at all levels to carefully consider the 

interim findings contained in this report. NIST welcomes comments from technical experts and the 

public on the interim findings presented herein. Comments can be sent by e-mail to wtc@nist.gov, 

facsimile to (301) 975-6122, or regular mail to WTC Technical Information Repository, Stop 8610, 

100 Bureau Drive, Gaithersburg, MD 20899-8610. 
June 2004 Progress Report 

xlix

In its final report, a draft which is expected to be released in December 2004, NIST will recommend 

appropriate improvements in the way buildings are designed, constructed, maintained and used. It will be 

important for those recommendations to be thoroughly and promptly considered by the many 

organizations responsible for building and fire safety. As part of NIST’s overall WTC response plan, the 

Institute has begun to reach out to these organizations to pave the way for timely, expedited consideration 

of recommendations stemming from this investigation. NIST also has expanded its research in areas of 

high priority need. 
1

Chapter 1

INTERIM FINDINGS AND ACCOMPLISHMENTS

1.1 INTRODUCTION 

In response to the terrorist attacks of September 11, 2001, the National Institute of Standards and 

Technology (NIST) initiated a formal federal building and fire safety investigation of the World Trade 

Center (WTC) disaster on August 21, 2002. At the same time, NIST also released the final plan for its 

investigation. 

NIST has received large amounts of data and information related to the design, construction, operation, 

inspection, maintenance, repair, alterations, emergency response, and evacuation of the WTC complex. A 

summary is included in Section 2.1. NIST has received considerable cooperation from the Port Authority 

of New York and New Jersey (PANYNJ or Port Authority), the City of New York, the National 

Commission on Terrorist Attacks Upon the United States (9-11 Commission), designers, leaseholders, 

contractors, suppliers, insurers, news media, tenants, first responders, survivors, and families of victims. 

NIST has received all of the essential information it needs for the WTC Investigation. NIST has a few 

requests for materials that are lost, currently pending, or not yet located; NIST is making efforts to 

re-create this information from various sources since much of it was lost when the buildings collapsed. 

NIST continues to pursue other materials that can clarify some aspects of the Investigation. 

The interim findings summarized in this progress report are based on information contained in public 

updates issued by NIST in December 2002 and December 2003, a previous progress report issued in 

May 2003, and work completed since the last progress report. (Previous updates and progress reports 

may be found at http://wtc.nist.gov.) 

NIST expects to release the draft of the final investigation report for public comment in December 2004. 

NIST is not making any recommendations at this time. All recommendations will be made in the 

final report. These interim findings and the working hypothesis for the collapse of the WTC towers and 

WTC 7 are subject to refinement or change as further information becomes available prior to release of 

the final investigation report. The final report will include any recommendations that NIST considers 

appropriate based on these and other findings yet to be made. 

NIST welcomes comments from technical experts and the public on the interim findings presented herein. 

Comments may be sent by e-mail to wtc@nist.gov, facsimile to (301) 975-6122, or regular mail to 

WTC Technical Information Repository, Stop 8610, 100 Bureau Drive, Gaithersburg, MD 20899-8610. 

The interim findings are presented in four subsections that relate to the investigation objectives listed 

below. They are then subdivided according to the technical areas to which they relate. NIST findings are 

presented in italics to distinguish them from narrative text throughout this section. 
Chapter 1 

2

The stated objectives contained in the NIST investigation plan are: 

1. To determine (a) why and how the WTC 1 and WTC 2 collapsed following the initial impact 

of the aircraft, and (b) why and how the 47-story WTC 7 collapsed. 

2. To determine why the loss of life and injuries were so low or so high depending on location, 

including technical aspects of fire protection, occupant behavior, evacuation, and emergency 

response. 

3. To determine the procedures and practices which were used in the design, construction, 

operation, and maintenance of the WTC buildings. 

4. To identify, as specifically as possible, areas in national building and fire codes, standards, 

and practices that warrant revision. 

1.2 COLLAPSE OF THE WTC TOWERS 

Working Hypothesis 

NIST is investigating possible collapse scenarios to establish the sequence of events that led to the 

collapse of the WTC towers following the initial impact of the aircraft. The objectives of the NIST 

analysis are to determine the most probable sequence of events from the moment of aircraft impact until 

the initiation of global building collapse and to identify the factors that have the strongest influence on the 

most probable sequence. 

NIST has developed a working hypothesis to explain the collapse initiation of the WTC towers. The 

working hypothesis (summarized below and in Appendix Q) identifies the chronological sequence of 

major events as the WTC tower structures redistributed loads from structural element to structural 

element to accommodate the aircraft impact and subsequent fire damage until no further load 

redistribution was possible to maintain overall stability, thus, leading to collapse. The hypothesis: 

• Is based on analysis of the available evidence and data, consideration of a range of 

hypotheses (including those postulated publicly by experts), and a newly enhanced 

understanding of structural and fire behavior. 

• Is consistent with all evidence currently held by NIST, including photos and videos, 

eyewitness accounts, and emergency communication records. 

• Allows for multiple load redistribution paths and damage scenarios for each building, 

currently under analysis. 

• Will be further refined based on the results of NIST’s continuing analyses to identify specific 

load redistribution paths and damage scenarios that are possible for each building, from 

which the most probable collapse sequence will be identified. 
Interim Findings and Accomplishments 

3

NIST welcomes comments from technical experts and the public on the working hypothesis. Among the 

key questions that NIST continues to investigate within the framework of the working hypothesis are the 

following: 

• How and why did WTC 1 stand nearly twice as long as WTC 2 before collapsing (103 min 

versus 56 min), though they were hit by virtually identical aircraft? 

• What were the relative roles of the perimeter and core columns1

 and the composite floor 

system,2

 including connections, in initiating the collapses? 

• What was the post-impact condition of the fireproofing, especially the extent to which 

fireproofing may have been damaged due to aircraft impact? 

• What factors related to normal building and fire safety considerations not unique to the 

terrorist attacks of September 11, 2001, if any, could have delayed or prevented the collapse 

of the WTC towers? 

In evaluating the working hypothesis for the collapse of the WTC towers, NIST is also considering the 

following factors: 

• The relative contributions of aircraft impact damage and subsequent fires; 

• How safe each building was immediately after aircraft impact but before fire weakened the 

structures, i.e., to what extent the capacity of the buildings to carry design loads3

 was 

reduced; 

• Whether the undamaged towers would have remained standing in a “maximum credible 

fire”;4

 and 

• The role compartmentation (i.e. areas divided by fire-rated walls) may have played, i.e., what 

would have happened if the floors had been separated into 7,500 or 10,000 ft2

 compartments 

with 1 h fire-rated partition walls or separations. 

 

1

 The perimeter columns were designed to carry both gravity and wind forces and acted together as a framed-tube system. The 

core columns were designed to carry only gravity loads and were not required to provide frame action. 2

 The composite floor truss system, which included long-span open-web bar joist elements, was designed to carry floor loads to 

the supporting core and perimeter columns. It also acted as a diaphragm that distributed wind forces to the perimeter columns 

of the framed-tube system and provided lateral stability to the perimeter columns. 3

 The design of the WTC towers was governed by gravity and lateral wind loads. 

4

 A maximum credible fire for the WTC towers is assumed to have the following characteristics: the sprinkler system is 

compromised, overwhelmed, or not present; there is no active firefighting; combustible building contents averaging 10 psf (in 

the range of about 5 psf to 20 psf for conventional office buildings); floor-to-floor fire spread to next upper floor at 30 min or 

60 min; and ventilation from windows broken by fire and a total of 50 ft2

 of air leakage between floors. 
Chapter 1 

4

Role of the Hat Truss System

The purpose of the hat truss was to support 

gravity and wind loads on the antenna. It 

was not designed to resist lateral forces on 

the towers, and, in an undamaged state, it 

did not have a significant role in carrying 

gravity loads. Lateral loads due to wind were 

distributed to the framed-tube system via 

diaphragm action of the floor system. The 

hat truss was connected to each perimeter 

face at only four points, all at the same level 

(at the 108th floor just below the concrete 

floor slab). The 47 core columns were 

connected to diagonal elements, heavier 

transfer beams, or smaller beam elements of 

the hat truss. Most of the core columns 

extended to the roof level, but four core 

columns, which were only minimally 

connected to the hat truss, terminated at 

Floor 110. The hat truss provided minimal 

redistribution of loads (less than 10 percent) 

from perimeter columns to core columns. 

Most of the load redistributed due to aircraft 

impact damage occurred on the external face 

through vierendeel action. 

Finding 1a.1: The following chronological sequence of major events led to the eventual collapse of the 

towers; specific load redistribution paths and damage scenarios for each building continue to be refined: 

• Aircraft impact damage to perimeter columns, resulting in redistribution of column loads to 

adjacent perimeter columns and to the core columns via the hat truss; 

• After breaching the building’s exterior, the aircraft continued to penetrate into the buildings, 

damaging core columns with redistribution of column loads to other intact core and perimeter 

columns via the hat truss and floor systems; 

• The subsequent fires, influenced by the post-impact condition of the fireproofing, weakened 

columns and floor systems (including those that had been damaged by aircraft impact), 

triggering additional local failures that ultimately led to column instability; and 

• Initiation and horizontal progression of column instability resulted when redistributing loads 

could not be accommodated any further. The collapses then ensued. 

Aircraft Impact. Buildings are not specifically designed to 

withstand the impact of fuel-laden commercial airliners. 

However, PANYNJ documents indicate that the impact of a 

Boeing 707 flying at 600 mph, possibly crashing into the 80th 

floor, was analyzed during the design of the WTC towers in 

February/March 1964. While NIST has not found detailed 

evidence of the analysis, the documents in NIST’s possession 

state that the postulated aircraft collision would have resulted 

in only local damage that would not cause collapse or 

substantial damage to the WTC towers. The effect of the fires 

due to jet fuel dispersion and ignition of building contents was 

not considered. The loss of life in the immediate area of 

aircraft impact was recognized, but the loss of life due to the 

growth and spread of fires and smoke was not considered. 

Building codes do not require building designs to consider 

aircraft impact. 

Finding 1a.2: The two WTC towers withstood the initial 

impact of virtually identical aircraft (Boeing 767-200ER) 

during the terrorist attacks of September 11, 2001. The 

robustness of the perimeter frame-tube system and large 

dimensional size of the WTC towers helped the buildings withstand the aircraft impact. The WTC towers 

displayed significant reserve capacity, vibrating immediately following impact with amplitudes that were 

about half the amplitudes for design wind conditions expected by the building designers. WTC 2, which 

collapsed first and in about half the time as WTC 1, vibrated for over 4 min at an oscillation period 

nearly equal to that measured for the undamaged building. The lightly damped (about 1.2 percent of 

critical damping) oscillation had an initial amplitude of approximately 20 in at the roof level, where 

expected sway was about 3 ft to 4 ft under design wind conditions. 
Interim Findings and Accomplishments 

5

Aircraft Impact Damage to Perimeter Columns. Based on the above information, structural damage to 

perimeter columns as a result of aircraft impact of the framed-tube system appears to have played a 

minimal role in initiating the collapse. Perimeter column bowing prior to collapse occurred on other faces 

(i.e., fire floors on the south face of WTC 1 and east face of WTC 2) that were not severed by the aircraft. 

Aircraft Impact Damage to Core Columns. The core columns were designed to carry only gravity loads 

and not required to provide frame action. The aircraft trajectory at impact suggests damage to the core 

columns occurred as follows: 

WTC 1—The aircraft was traveling about 450 mph and hit the tower near the center of the north face 

damaging Floors 93 to 99. The aircraft fully entered the core area and severed or damaged central core 

columns in the north-south direction. Aircraft and building debris accumulated in the remaining core area 

and south-side floor areas as contents were displaced from the point of impact. 

WTC 2—The aircraft was traveling about 550 mph and hit the tower near the southeast corner of the 

building damaging Floors 77 to 85. Core columns to the south and east were severed or damaged. 

Aircraft and building debris accumulated in the core area and floor areas to the east and north. 

Severed core columns redistributed their loads in three ways, depending on how many and which core 

columns were severed. 

1. Isolated core columns were severed. Severed column and tributary floor loads, at and 

above the point of impact, were redistributed locally at each floor to adjacent intact core 

columns via core floor framing. This was limited by shear/bending capacity of 

floor-framing connections to adjacent columns. 

2. Critical (e.g., corner) core columns and/or several other core columns were severed. The 

severed column and tributary floor loads, at and above impact floors, redistributed to intact 

core columns via the hat truss. Significant hat truss deflections may have occurred if there 

was adequate connection capacity since the severed core columns and the associated floors 

were hanging from the hat truss which was not designed to carry such loads. This was 

limited by the tensile capacity of bolted splices in the severed core columns, 

tensile/compression capacity of hat truss members, and tensile capacity of column 

connections to the hat truss. 

3. Extent of core column failures precluded redistribution through the hat truss and/or 

exceeded redistribution capacity of the hat truss. Severed column and associated floor 

loads, at and above floors of impact, redistributed to intact core and perimeter columns via 

the core and composite truss floor system. Floors were subjected to combined bending and 

diaphragm action (e.g., consider the scenario of no core columns in the floor span direction 

to visualize this action). The overall capacity of the floors was limited by shear capacity of 

floor-to-column connections (including perimeter columns) and tensile/bending capacity of 

composite truss floor connections to core or perimeter columns. Significant sagging of the 

hat truss system may have occurred if its capacity was exceeded. 
Chapter 1 

6

Role of Fireproofing 

The post-impact condition of the fireproofing 

played a key role in the structural response to 

fires. The post-impact condition of the 

fireproofing depends on the condition of the 

fireproofing prior to aircraft impact and the 

extent to which fireproofing was damaged 

due to aircraft impact. The fire-affected floors 

in WTC 1 had, in general, upgraded or thicker 

fireproofing (1.5 in. specified) while, in 

general, those in WTC 2 did not have 

upgraded fireproofing (0.5 in. specified).

Relative Roles of Fires and Aircraft Impact 

Finding 1a.3: Fires played a major role in further reducing the structural capacity of the buildings, 

initiating collapse. The tower structures withstood the initial aircraft impacts and remained stable. 

While aircraft impact damage did not, by itself, initiate building collapse, it contributed greatly to the 

subsequent fires by: 

• Compromising the sprinkler and water supply systems; 

• Dispersing jet fuel and igniting building contents over large areas; 

• Creating large accumulations of combustible matter containing aircraft and building contents; 

• Increasing air supply into the buildings (through broken windows and holes in the sides of the 

buildings, and between floors due to damaged floors, vertical shafts, and columns) that 

permitted significantly higher energy release rates than would normally be seen in 

ventilation-limited building fires, allowing the fires to spread rapidly within and between 

floors; and 

• Damaging ceilings that enabled “unabated” heat transport over the floor-to-ceiling partition 

walls and to the floor trusses, spandrels, and tops of columns. 

Finding 1a.4: The jet fuel, which ignited the fires, was mostly consumed within the first few minutes after 

impact. The fires that burned for almost the entire time that the buildings remained standing were due 

mainly to burning building contents and, to a lesser extent, aircraft contents, not jet fuel. 

Thermal Effects on Columns and Floors. Some floors in 

WTC 2 experienced partial collapse due to aircraft impact. 

For example, partially collapsed floor slabs were visible on 

the east and north faces. This included failures at the edges 

with perimeter columns causing floor edge sagging. Based 

on photographs or videographs there is no visible evidence 

of partially collapsed floors in WTC 1. 

Fires may have had the following thermal effects: 

• Core columns and core floors may have been 

further weakened, with reduced ability to carry and/or redistribute load, causing such loads to 

be redistributed to other core and perimeter columns consistent with the residual reserve 

capacities of these columns and the transfer mechanisms (i.e., hat truss and floor system). 

• The floor system may have been further weakened, either along the span of the floor system 

or localized at connections with columns. The weakening floor system may have pulled the 

perimeter columns inward (observed on the south face of WTC 1 and the east face of WTC 2 

minutes prior to building collapse) and then initiated connection failures at perimeter or core 

columns. 
Interim Findings and Accomplishments 

7

• Perimeter columns may have been further weakened, with reduced ability to carry loads. 

Thermal effects could also cause inward bowing of perimeter columns due to differential 

temperatures between the inner and outer faces of the columns. The loads that could no 

longer be carried by the weakened columns would have been redistributed to adjacent 

perimeter columns. 

Column Instability and Collapse Initiation. The perimeter columns were designed as part of a framedtube system to carry both gravity and wind forces. Instability of perimeter columns resulted from a 

combination of (1) redistributed loads from the core columns via the floor system and possibly the hat 

truss, (2) inward bowing due to thermally-weakened and sagging floors, (3) increased unsupported length 

due to failed floors, and (4) thermal effects directly on the perimeter columns. 

The instability of a few perimeter columns was observed to propagate across the entire face and around 

the corners just before or during collapse initiation. The initiation or spread of perimeter column 

instability also may have been facilitated by the hoop stress demand on the framed-tube system exceeding 

the capacity of the spandrels (horizontal steel plates) that tied the perimeter columns together (e.g., at the 

northeast corner of WTC 2). 

The initiation of global collapse for both towers was first observed by the tilting of the sections above the 

impact regions of both WTC towers. The tilting and the propagation of column instability are 

synchronous processes that initiated global collapse. The tilting may have caused forces such as shear 

and torsion to spread the column instability laterally. 

Issues Being Investigated. Over the next few months, NIST will continue to investigate the following 

technical issues and modify its working hypothesis as needed. Findings on these issues will be included 

in the final report. 

• Aircraft impact damage to structural components, fireproofing, and hat truss connections. 

• Distribution of aircraft/building contents. 

• Thermal effects on core columns and core floors, especially extent of fires and growth 

history. 

• Thermal effects on welded perimeter columns, especially temperature gradients on columns. 

• Extent of load redistribution to intact core columns and their reserve capacity to 

accommodate thermal loads. 

• Capacity of hat truss connections to perimeter columns, especially to meet the demands of 

aircraft impact and any torsional effects. 

• Capacity of hat truss to accommodate the load redistribution from severed columns. 

• Capacity of bolted splices in the severed core columns to carry loads to the hat truss. 

• Relative magnitude of the load redistribution provided by the local core floor, hat truss, and 

the core-truss floor system for each tower. 
Chapter 1 

8

• Axial/shear/bending capacity of floor connections to core and perimeter columns. 

• Effect of localized fires on floor truss connections. 

• Mechanisms to propagate instability laterally in the perimeter columns (e.g., shear and torsion 

forces induced by a rigid body movement) 

• Capacity of spandrels, including splices, to carry shear transfer in the framed-tube system, 

especially at the corners. 

• Role of bolted splices on instability of perimeter columns. 

• Outward bowing of perimeter columns due to thermal expansion of floors. 

• Effect of uneven floor thermal expansion on perimeter column instability due to potential 

biaxial bending. 

• Comparison and reconciliation of working hypothesis with observed facts (photographs and 

videos, eyewitness accounts, emergency communication records). 

• Examination of other possible or probable hypotheses. 

Visual Observations 

Photographic and video images of damage and fires in the WTC buildings are providing critical guidance 

for the investigation. NIST has collected and assembled the visual materials into a searchable 

computerized database. The database now contains well in excess of 6,000 photographs representing the 

work of more than 185 photographers, and 150 hours of video from most of the major media outlets and 

more than 20 individuals. 

Based on an analysis of this visual evidence, NIST has identified significant events for WTC 1 and 

WTC 2 related to aircraft impact, fire development, and building damage, and work is progressing on 

WTC 7. As part of this analysis, NIST has developed detailed mappings and time-dependent 

visualizations for fire, smoke, and window conditions of the WTC towers, and similar efforts are nearing 

completion for WTC 7. 

Finding 1a.5: On the east face of WTC 2, what appears to be the 83rd floor slab was seen hanging 

across window openings over a large fraction of the 82nd floor. The object was observed in a number of 

photographs and videos very shortly after the plane strike and found to sag further prior to collapse. On 

the north face of WTC 2, shorter lengths of what appear to be the 81st, 82nd, and 83rd floor slabs were 

seen hanging through the windows in the floors below. There was no visible evidence of floors sagging in 

WTC 1. NIST continues to investigate what role aircraft impact and subsequent fires had on causing 

such floor failures. 

Finding 1a.6: Several minutes (less than 10 min) prior to building collapse, inward bowing of about a 

quarter to a third of the perimeter columns was observed in photographs on the south face of WTC 1 and 

the east face of WTC 2 in regions that contained active fires. 
Interim Findings and Accomplishments 

9

Finding 1a.7: At 9:36 a.m. an occupant of WTC 2 called the New York City 911 telephone operator and 

reported that a floor in the 90’s had collapsed underneath them and that they were now on the 

105th floor. The NYPD aviation unit observed the following events with respect to WTC 2. At 9:49 a.m., 

they reported “large pieces” falling from WTC 2. At 9:58 a.m., it was reported that the south tower was 

coming down. With respect to WTC 1, at 10:06 a.m., an advisory was transmitted by the aviation unit 

that it wasn’t going to take much longer before the north tower comes down and to pull emergency 

vehicles back from the building. At 10:20 a.m., the unit reported that the top of the (north) tower might 

be leaning. At 10:21 a.m., they reported that the north tower was buckling on the southwest corner and 

leaning to the south. At 10:27 a.m., a report was made that the roof was going to come down very shortly. 

Finding 1a.8: The initiation of global collapse was first observed by the tilting of building sections above 

the impact regions of both WTC towers. WTC 1 tilted to the south (observed via antenna tilting in a video 

recording), and WTC 2 tilted to the east and south and twisted in a counterclockwise motion. The 

primary direction of tilt was around the weak axis of the core (north-south for WTC 1 and east-west for 

WTC 2). An earlier building performance study, performed by a private-public sector team with funding 

support from the Federal Emergency Management Agency (FEMA), concluded that the core failed first in 

WTC 1 based on vertical movement of the antenna observed in a video recording from due north that did 

not capture the antenna tilt due to the angle from which the video was shot. NIST is reevaluating this 

conclusion based on new visual information available from a different angle. 

Analysis of Recovered WTC Steel 

NIST believes the collection of steel in its possession is adequate for purposes of analyzing the quality 

and properties of steel for the Investigation. NIST has 236 pieces of steel in its possession. The regions 

of impact and fire damage were emphasized in the selection of steel pieces for the investigation. As a 

result, NIST has all 14 specified steel grades for the exterior panels in the WTC towers, 2 specified grades 

that represent 99 percent of the core columns, and both specified grades for the steel trusses that 

comprised the composite floor truss system. 

Finding 1a.9: Analysis of steel recovered from the WTC towers, based on stampings on the steel and 

mechanical tests, indicates that the correct specified materials were provided for the specified 

elements. When these data were combined with pre-collapse photographic images of damaged steel, it 

was found that aircraft impacted pieces of steel recovered from WTC 1 were in the precise locations as 

specified in the design drawings. 

Finding 1a.10: Metallography and mechanical property tests indicate that the strength and quality of the 

steel used in the towers was as specified, typical of the era, and likely met all qualifying test requirements. 

Further metallographic and fractographic analyses on the pieces from the impact zone are being 

considered. 

Finding 1a.11: The room-temperature strength of the steel used in the towers met the relevant standards 

and, in many instances, exceeded the requirements by 5 percent to 10 percent. Ten different steel 

companies fabricated structural elements for the WTC towers, using steel supplied by at least eight 

different suppliers; four fabricators supplied the major structural elements from the 9th to the 

107th floors. Work is ongoing to analyze the performance of the steel building components under impact 

and fire conditions up to initiation of global building collapse. 
Chapter 1 

10

Reconstruction of the Fires 

NIST has completed three series of large-fire tests to enhance and validate the fire dynamics and thermal 

modeling tools being used in the WTC investigation, including: 

• Single office workstation cubicle fire tests, based on descriptions of furnishings used in the 

WTC 1 office space, to generate a database on the thermo-physical properties of the materials 

for input to the fire dynamics simulation tool. The effects of debris and jet fuel on these fires 

were also investigated. 

• Fire tests of multiple WTC workstation cubicles to validate the model predictions of the 

sensitivity of fire intensity, duration, and spread to the distribution and nature of the 

combustibles, effect of ventilation on the fire, and the effects of debris and jet fuel. 

• Fire tests to measure the thermal environment (heat release and transfer rate to compartment 

gases) in a burning compartment and to establish a data set to validate the prediction of the 

temperature rise of structural steel components (similar to WTC steel trusses and columns, 

with and without fireproofing). 

In addition, NIST has completed a series of experiments on ceiling tile systems similar to those present in 

the WTC towers. Shake table experiments were conducted to determine the magnitude of impulses that 

could result in damage to the ceiling tile systems, increasing the accessibility of the fire energy to the 

ceiling/floor membranes. 

NIST has significantly enhanced the capabilities of computational tools for fire dynamics and thermal 

modeling and validated their predictions during the investigation. The tools can now be applied with 

greater confidence to recreate possible fires from the complex arrays of combustibles that existed in the 

WTC buildings, given initial damage conditions and descriptions of combustibles that have been collected 

by NIST. Key enhancements include: underventilated fire scenarios, charring materials such as those 

comprising much of the office furniture, fire spread, dimensional resolution in the vicinity of structural 

components, time-efficient computations for large simulations, and visualization of large data sets. These 

tools will help to improve the fire-safe design of new structures and analyze conditions of existing 

structures. 

Finding 1a.12: Unlike the jet fuel that was mostly consumed in only a few minutes, typical office 

furnishings were able to sustain intense fires for at least an hour on a given WTC floor. NIST has 

obtained generic and, in some cases, specific information about the furnishings in the WTC offices. In 

addition, NIST has obtained descriptions of the combustible contents of the aircraft. This includes cabin 

materials, aircraft components, and cargo bay contents. Based on a review of this information, NIST has 

found that: 

• The typical WTC office floor, using modern workstation furnishings, had on average about 

4 pounds per square foot (psf) of combustible materials on floors without unusual file rooms, 

film storage, etc. For conventional office buildings, the weight of combustibles varies from a 

combustible load commensurate with that in the WTC to about 10 psf to 20 psf when there 

are extensive bookcases, file storage, etc. 
Interim Findings and Accomplishments 

11

• The mass of the aircraft solid combustibles was significant relative to the mass of the 

building combustibles in the immediate impact region of each of the WTC towers. 

Following the experimental validation of the NIST Fire Dynamics Simulator for the WTC application, an 

extensive series of simulations of the fire floors in all three buildings is well underway. Input to the 

simulations includes the information gathered on the layouts of the floors, ventilation through the broken 

windows (as observed in the visual collection), the nature and loading of the combustible contents of the 

buildings, and preliminary (visual) estimates of damage to the towers from the aircraft impact. 

Alternative estimates of possible damage conditions in the interiors of the buildings are examined 

parametrically. 

Consistent with available photographic and videographic evidence, these simulations have been able to 

capture the broad patterns of fire movement around the floors, with the flames in a given location lasting 

for about 20 min before spreading to adjacent, yet unburned combustibles. This spread is generally 

continuous due to the relatively even distribution of combustibles and the paucity of interior partitions 

and tends to be controlled by the air supply available through broken windows. There are some observed 

instances where fires persisted over longer durations in regions with accumulated combustible debris and 

other instances of sudden or interrupted fire spread. To the extent that these fires are locally of high 

intensity and duration and occur in a vicinity where they could contribute to structural weakening, they 

are being examined further via additional simulations. 

Applying the 1968 NYC Building Code, the WTC towers were required to have 1 h fire-rated tenant 

separations, but the code did not impose any minimum compartmentation requirements (e.g., 7,500 ft2

) to 

mitigate the horizontal spread of fire in buildings with large open floor plans. The affected floors in the 

WTC towers were mostly open—with a modest number of perimeter offices and conference rooms and an 

occasional special purpose area. Some floors had two tenants, and those spaces, like the core areas, were 

partitioned (slab to slab). Photographic and videographic evidence confirms that even non-tenant space 

partitions (such as those that divided spaces to provide corner conference rooms) provided substantial 

resistance to fire spread in the affected floors. Exit access corridors had a 2 h fire-rating with 1 1/2 h firerated doors. Enclosures for vertical exits, exit passageways, hoistways, and shafts were required to be 

2 h. 

Finding 1a.13: Laboratory experiments show that impulses like those estimated for the aircraft impact 

caused serious damage to the ceilings in the WTC towers. This is consistent with the accounts of 

survivors from floors below the impact region. This damage enables “unabated” heat transport over the 

floor-to-ceiling partition walls and to the steel trusses, spandrels, and tops of columns. 

Structural Response and Collapse Analysis 

NIST has developed and adopted a comprehensive approach to identify the most probable of technically 

possible collapse sequences, from aircraft impact to collapse initiation. The approach: 

• Combines mathematical modeling, well-established statistical and probability-based analysis 

methods, laboratory experiments, and analysis of photographic and videographic evidence. 

• Allows for evaluation and comparison of possible collapse sequences based on different 

damage states, fire paths, and structural responses. 
Chapter 1 

12

• Accounts for variations in models, input parameters, analyses, and observed events. 

NIST has defined detailed requirements for this complex series of analyses, formulated detailed modeling 

approaches to capture important structural behavior, made significant progress in developing and 

evaluating the adequacy of the models, and obtained results from preliminary analyses using the models. 

The objective of the analyses is to simulate highly-complex failure modes at the component level, 

subsystem level, and over the entire structure due to aircraft impact and the subsequent fires. In many 

instances, NIST is testing the limits of current engineering software. Most such systems are used in 

general practice for design purposes, not for high-fidelity modeling and failure analysis of complex 

systems. 

The computational models developed by NIST include: 

• A detailed model of a typical truss-framed floor of the WTC towers with over 

40,000 elements and 166,000 degrees of freedom. 

• A detailed model of a typical beam-framed floor of WTC towers with over 12,000 elements 

and 35,000 degrees of freedom. 

• A detailed global model of WTC 1 with over 80,000 elements and 218,000 degrees of 

freedom (with 17 flexible and other rigid diaphragm floors). 

• A similar detailed global model of WTC 2 with over 78,000 elements and 200,000 degrees of 

freedom. 

• A model of a typical turbofan engine of the Boeing 767-200ER aircraft with over 

60,000 elements and 100,000 nodes. 

• A comprehensive model of the Boeing 767-200ER aircraft, including engines, airframe, 

landing gear, fuel tanks, passenger cabin, and cargo bay, with over 530,000 elements and 

740,000 nodes. 

The first four models described above are being used to evaluate the baseline performance of the WTC 

towers under design gravity and wind loads. They also serve as reference models for other phases of the 

investigation involving analyses of aircraft impact damage and response of the thermally-insulated 

WTC structures to the subsequent fires. 

Finding 1a.14: The WTC tower structures represented an innovative structural system when they were 

built. The structural system incorporated many new and unusual features, including: 

• First frame-tube framing system for a high-rise steel building. 

• Composite floor system, using open-web bar joist elements, to provide lateral stability and 

diaphragm action. 

• First extensive use of prefabricated perimeter panels (3 columns wide by 3 stories high) in 

steel construction with bolted butt-plate column splices. 
Interim Findings and Accomplishments 

13

• Uniform perimeter column geometry (14 in x 14 in cross-section) over most of the height of 

the 110-story buildings. 

• First use of more than 14 different grades of specified steel in a tall building, with 14 grades 

specified for the uniform perimeter column geometry. 

• Use of deep spandrel plates as beam elements connecting perimeter columns, enabling 

frame-tube action by providing lateral bracing around the structure. 

• First use of wind tunnel testing to estimate the lateral wind loads in the design of a super tall 

building. 

• First use of structural dampers to control dynamic motion in tall buildings, especially those 

due to winds (10,000 viscoelastic dampers were installed in each building connecting the 

floor trusses to the perimeter frame-tube system). 

• Use of specially designed prefabricated panels to transfer forces at the chamfered corners of 

the frame-tube system. 

Finding 1a.15: NIST has completed a preliminary stability analysis of the WTC towers. The findings 

from the preliminary analysis, if they remain viable upon further more detailed analysis, suggest that: 

• For global instability of the WTC tower to occur under service loading conditions, five floors 

must have separated completely from all columns if the columns are at room temperature or 

four full floors must separate if the columns are uniformly heated to 600 oC. Linear stability 

analysis indicates that some individual core columns begin buckling with fewer “failed” 

floors at both temperatures without significantly affecting global stability. 

• For typical truss-framed floors under service loading conditions, if fifteen core columns are 

assumed severed due to aircraft impact, tension is induced in those columns by the floor 

immediately above the failure location of the columns. The tension force increases as more 

floor loads are picked up by the columns as they approach the hat truss at the roof level. The 

increase in tension load is limited by the tensile capacity of the column splice. When the 

tensile load exceeds the column splice capacity at a certain floor level, the splice fails, and 

all floors below the failed splices must redistribute their own loads directly to neighboring 

undamaged core columns. When fewer (only eight) core columns are assumed severed, the 

tension forces in the core columns are smaller due to the larger stiffness of the damaged floor 

area for eight severed columns, relative to that for 15 severed columns. The stiffer floor area 

redistributed more of the floor loads directly to neighboring undamaged core columns. The 

extent to which the severed core columns assist in transferring loads via the hat truss without 

failure of the column splices is sensitive to the relative magnitudes of the floor loads, column 

tension force, and column splice capacity. 

• WTC 1 maintained stability after aircraft impact, with the highest stressed elements being the 

perimeter columns next to the region where columns and spandrels were severed on the north 

face of the tower. The analysis assumed eight columns in the core were severed due to 

aircraft impact. A “pushdown” analysis was used for evaluating structural stability accounts 
Chapter 1 

14

for geometric and material nonlinearities with plastic hinges. WTC 1 also maintained 

stability with remaining residual reserve capacity when additional perimeter columns were 

removed on the south face to represent the inward bowing observed in videos a few minutes 

prior to collapse. However, loss or weakening of additional core columns, weakening of 

additional perimeter columns, or loss of additional floors would be needed for global 

collapse of the tower to occur. 

NIST has completed a series of preliminary aircraft impact analyses using component-level models of 

tower perimeter and core columns with wing section and engine component models as impactors. These 

models were used to develop the simulation techniques required for the global analysis of the aircraft 

impacts into the WTC towers. 

Finding 1a.16: A 500 mph engine impact against an exterior wall panel results in a penetration of the 

exterior wall and failure of impacted perimeter columns. If the engine does not impact a floor slab, the 

majority of the engine core remains intact through the exterior wall penetration with a reduction in 

velocity between 10 percent and 20 percent. The residual velocity and mass of the engine after 

penetration of the exterior wall is sufficient to fail a core column in the event of a direct impact. 

Interaction with additional interior building contents prior to impact, or an indirect impact against the 

core column, could change this result. 

Finding 1a.17: A normal impact of the exterior wall by an empty wing segment (toward the wing tip 

region) will produce significant damage to the perimeter columns, but not necessarily complete failure. 

This is consistent with photographs showing the exterior damage to the towers immediately after impact. 

Specific details of the damage depend on details of the impact orientation and locations of internal wing 

components such as control surface actuators and arms. 

Finding 1a.18: Impact by a fuel-filled wing section (away from the wing tip toward the fuselage) results 

in extensive damage to the exterior wall panel, including complete failure of the perimeter columns. This 

is also consistent with photographs of the exterior damage. The resulting debris propagating into the 

building maintains the majority of the initial momentum of the wing prior to impact. 

NIST has completed detailed preliminary analyses of the response of a single floor truss assembly if it 

were subjected to a severe fire. These include the response of the truss and its seat connections (to 

columns) and knuckles (to provide composite action with the concrete floor slab) to service load 

conditions, uniformly increasing elevated temperatures in the steel, and increasing the temperature 

gradient in the concrete slab. The truss model includes all potential failure modes that may occur under 

loading and thermal conditions, though the actual sequence of failure may differ under other fire 

conditions. 

Finding 1a.19: NIST’s preliminary analyses of a single floor truss assembly if it were subjected to a 

severe fire suggest the following sequence of events: 

• The floor truss first experiences increasing vertical deflections at mid-span as it pushes 

(expands) outward and exerts a compressive lateral load on the exterior column. The 

exterior column begins to displace outward at the floor connection. 
Interim Findings and Accomplishments 

15

• Web diagonals begin to buckle at 340 oC, the mid-span deflection continues to increase, but 

the horizontal displacement of the exterior column begins to decrease. The maximum 

horizontal displacement of the exterior column is approximately 0.7 in. when the diagonals 

begin to buckle. (The interior column is assumed to have no lateral displacements at the 

floor level, as it is braced by the core framing.) 

• The shear connectors (steel-knuckle-to-concrete slab connections referred to as knuckles) at 

each end of the truss begin to fail as the steel and bottom surface of the slab reach 400 oC, 

with such failures moving progressively inward from the truss ends. The failure of web 

diagonals and knuckles at the ends of the truss reduce the bending rigidity of the floor truss 

at the ends, further increasing the floor sag and decreasing the lateral outward force exerted 

on the columns. 

• The truss bearing angle slips until the bolt is bearing against the edge of the slotted hole. 

The bolt shears off at the interior seat connection at approximately 500 oC. The floor truss 

sag increases to 20 in. when the bolt fails. 

• The interior end of the reinforced slab continues to carry vertical loads as the truss bearing 

angle continues to slip. At 560 oC, the exterior column begins to be displaced inward as the 

floor truss continues to sag and exert vertical and horizontal tensile loads. 

• At 650 oC, the truss slides off the interior seat, followed by the gusset plate fracture at the 

exterior connection at 660 oC. 

NIST has developed a rigorous technical approach to evaluate the role fireproofing conditions may have 

played in the collapse of the WTC towers, considering: 

• Specified spray-applied fire resistive material (SFRM) and thicknesses for the various 

structural components. 

• The as-built condition of the fireproofing prior to September 11, 2001, including the average 

SFRM thicknesses applied to different structural components, and variability in the thickness 

along the length of components. 

• The mechanical and thermal properties of the fireproofing materials, including adhesive and 

cohesive bond strengths, and temperature-dependent heat capacity and thermal conductivity. 

• The extent to which fireproofing may have been damaged due to aircraft impact via debris 

impact and local deformation/acceleration of structural components. 

Finding 1a.20: Available records suggest that the fireproofing of the columns, beams, and spandrels of 

the WTC towers was not a subject of concern to the building owner and designers, while fireproofing of 

the floor trusses was the focus of continuous reassessment and revision. 

• The WTC towers were identified as Occupancy Group E – Business, and classified as 

Construction Class IB in accordance with the 1968 New York City Building Code. This 
Chapter 1 

16

classification required that the columns and floor systems of the towers have a 3 h and 2 h 

fire endurance, respectively. 

• The steel trusses that supported the floors of WTC 1 and WTC 2 were specified to be 

fireproofed with 1/2 in. of SFRM, although the technical basis for the selection of 

fireproofing material and its thickness are not known. 

• In 1999, a decision was made to begin upgrading the fireproofing to a specified 1.5 in. 

thickness as tenant spaces became unoccupied. In general, the floor systems in WTC 1 

subject to aircraft impact and subsequent fires had been upgraded by September 11, 2001; 

the affected floors in WTC 2 had not. 

• The fire protection of a truss-supported floor system by directly applying spray-on 

fireproofing to the steel trusses was innovative and not consistent with prevailing practice at 

the time the WTC towers were designed and constructed. While the benefits of conducting a 

full-scale fire endurance test were recognized by the building designers, no tests were 

conducted on the floor system used in the WTC towers to establish a fire endurance rating. 

Finding 1a.21: The response of a structural component to fires is sensitive to variability in fireproofing 

thickness along its length. Such variations can be random in nature or in some instances as stark as bare 

spots. In the case of random variations, given an average fireproofing thickness and a coefficient of 

variation, it is possible to identify an equivalent uniform thickness without variation that gives nearly the 

same time history of temperature rise and component structural response under thermal loads. 

• For the original fireproofing in the WTC towers, the as-applied fireproofing thickness 

(0.75 in. average and 0.4 coefficient of variation) on the floor trusses is thermally equivalent 

to a uniform thickness of 0.6 in. with no variation. This uniform thickness is greater than the 

specified minimum thickness of 0.5 in. 

• For the upgraded fireproofing in some floors of the WTC towers, the as-applied upgraded 

fireproofing thickness (2.5 in. average and 0.24 coefficient of variation) is thermally 

equivalent to a uniform thickness of 2.2 in. with no variation. This uniform thickness is 

greater than the specified minimum thickness of 1.5 in. 

• Thus, it is possible to evaluate if the as-applied average fireproofing thickness and variation 

on a component is thermally equivalent to a uniform thickness that is at least equal to the 

minimum fireproofing thickness specified on that component. 

• For a specified fireproofing thickness, it is also possible to recommend an as-applied average 

thickness, given the expected variability (coefficient of variation) in quality of fireproofing 

application. 

NIST is currently examining an alternative performance criterion for determining the equivalent thickness 

based on restrained conditions to confirm the above finding. 

Finding 1a.22: Based on simplified analytical models, it was found that acceleration of a structural 

element, on the order of 100 times the acceleration due to gravity (or 100 g), would be required to 
Interim Findings and Accomplishments 

17

dislodge 1 in thick SFRM from a planar surface. Acceleration on the order of 150 g would be required to 

dislodge a similar thickness of SFRM from a 1 in. diameter bar. In both cases, SFRM cohesive and 

adhesive strength properties and densities were typical of those used in the WTC towers. Experiments 

are underway to verify the results of these simplified analyses. In addition, analytical studies are 

underway to estimate the magnitude of accelerations of the structural members due to aircraft impact, 

from which the regions where fireproofing may have been dislodged will be identified.

1.3 COLLAPSE OF THE 47-STORY WTC 7 BUILDING 

Working Hypothesis 

A working hypothesis has been developed around four phases of the collapse of WTC 7 that were 

observed in photographic and videographic records: an initiating event, a vertical progression at the east 

side of the building, a subsequent horizontal progression from the east to the west side of the building, 

and global collapse. The working hypothesis will be revised and updated as results of ongoing, more 

comprehensive analyses become available. NIST welcomes comments from technical experts and the 

public on this working hypothesis. 

Finding 1b.1: The working hypothesis for the collapse of the 47-story WTC 7, if it remains viable upon 

further analysis, suggests that it was a classic progressive collapse that included: 

• An initial local failure at the lower floors (below floor 13) of the building due to fire and/or 

debris induced structural damage of a critical column (the initiating event), which supported 

a large span floor bay with an area of about 2,000 ft2

; 

• Vertical progression of the initial local failure up to the east penthouse, as large floor bays 

were unable to redistribute the loads, bringing down the interior structure below the east 

penthouse; and 

• Horizontal progression of the failure across the lower floors (in the region of floors 5 and 7, 

that were much thicker than the rest of the floors), triggered by damage due to the vertical 

failure, resulting in disproportionate collapse of the entire structure. 

WTC 7 Steel 

No steel from WTC 7 has been identified from the pieces of recovered WTC steel in NIST’s possession. 

WTC 7 had two specified grades of steel for columns and beams and four grades of steel for cover plates 

used in built-up columns. The specified grades (ASTM A36, A572 Grades 42 and 50, and A588 Grades 

42 and 50) of steel are readily available. Properties were estimated from available test data in the 

literature. 

Visual Observations 

Finding 1b.2: The first exterior sign of structural failure in WTC 7 was the sinking of the east penthouse 

roof structure into the building. Photographic and videographic records taken from the north have 

provided information about the sequence of failure events and their relative times. Other key observations 
Chapter 1 

18

include window breakage along the east side of the north face, occurring almost simultaneously with the 

sinking of the east penthouse structure, an approximate 5 s delay before the other roof structures also 

sank into the building core, a second set of window breakage along the west side of the north face 

occurring simultaneously with the other roof structure movements, and the appearance of the entire north 

façade above the 13th floor dropping as an intact unit 8 s after the east penthouse movement was first 

detected. 

Finding 1b.3: Witnesses reported structural damage to WTC 7 on its south face and southwest corner 

from WTC 1 debris. A multi-story gash that extended across approximately a quarter to a third of the 

south face, in the lower portion of the face, was reported by a number of individuals, though details vary. 

This damage extended to the core area as two elevator cars were reportedly ejected from the elevator 

shaft at floor 8 or 9. Reported damage to the southwest corner was confirmed visually in photographic 

records, which show approximately two columns and related floor areas missing from floors 8 to 18. 

Multiple photographic and videographic records also appear to show damage on the south face that 

started at the roof level and severed spandrels between exterior columns near the southwest corner for at 

least 5 to 10 floors. However, the extent and details of this damage have not yet been discerned, as smoke 

is present in the photographs. 

Finding 1b.4: Fires were first observed in WTC 7 after WTC 1 collapsed. Fires, or evidence of fires, 

were observed initially on the south face and near the southwest corner on Floors 22, 29, and 30. Many 

of these fires appeared to burn out before 2:00 p.m. Around 2:00 p.m., fires were observed in 

photographic and videographic records to be burning across Floors 11 and 12 on the east face, from the 

south to the north. Around 3:00 p.m., fires were observed on Floors 7 and 12 along the north face. The 

fire on Floor 12 appeared to bypass the northeast corner and was first observed at a point approximately 

one third of the width of the building from the northeast corner, and then spread both east and west 

across the north face. Sometime later, fires were observed on Floors 8 and 13 with the fire on floor 8 

moving from west to east and the fire on Floor 13 moving from east to west. At this time, the fire on 

Floor 7 appeared to have stopped progressing near the middle of the north face. The fire on Floor 8 

continued to move east on the north face, eventually reaching the northeast corner and moving to the east 

face. Around 4:45 p.m., a photograph showed fires on Floors 7, 8, 9, and 11 near the middle; Floor 12 

was burned out by this time. Interview responses indicate that there was no water in the standpipe system 

supplying the sprinklers in WTC 7. 

Fuel System for Emergency Power 

NIST has reviewed the fuel system for emergency power in WTC 7. There were two 12,000 gal fuel 

tanks below the first floor loading dock and one 6,000 gal above ground tank on the first floor. These 

tanks supplied fuel to 275 gal day tanks on floors 5, 7, and 8, and a 50 gal day tank on floor 9. In 

addition, there were two 6,000–gal tanks located below the first floor loading dock with pressurized pipes 

leading to floor 5. 

Floor 5 did not have any exterior windows but it did have exhaust vents for generators near the south and 

north corners of the building. Any fires that may have burned on this floor would not have been visible in 

photographic or videographic records, except for smoke at the exhaust vents, which was not observed. 

The large opening created by the reported gash in the south face may have so altered the air flow on 

Floor 5 as to vent any smoke generated on this floor out the south face of the building, where overall 
Interim Findings and Accomplishments 

19

smoke conditions prevented photographs or other observations. However, there was a pressurized fuel 

distribution system on the south, west and north floor areas. Given the variability of damage descriptions 

for the south face from WTC 1 debris impact, Floor 5 is considered a possible fire initiation location, 

subject to further data and/or analysis on building conditions that improve knowledge of fire conditions in 

this area. 

Finding 1b.5: The owner of the two 6,000 gal tanks supplying 5th floor generators through a pressurized 

piping system contracted with an environmental mitigation firm to recover any remaining fuel and to 

determine the extent of any contamination from fuel leakage from these tanks several months after the 

collapse. They reported that the tanks had been damaged by debris and were empty. No residual 

petroleum product or sludge was found in the tanks or piping. Examination of the gravel below the tanks 

and the sand below the slab on which the tanks were mounted showed some fuel contamination, but none 

was found in the organic marine silt/clay layer below. Witnesses also reported that the two 6,000 gal fuel 

tanks were always kept full for emergencies and were full that day. This finding allows for the possibility, 

though not conclusively, that the fuel may have contributed to a fire on Floor 5. 

1.4 EVACUATION AND EMERGENCY RESPONSE 

Buildings are not designed for fire protection and evacuation under magnitude and scale of conditions 

similar to those caused by the terrorist attacks of September 11, 2001. Prior evacuation and emergency 

response experience in major events did not include the total collapse of tall buildings such as the WTC 

towers and WTC 7 that were occupied and in everyday use. Recent experience with major tall building 

fires suggests that they typically result in burnout conditions, not global building collapse. The intent of 

building codes is for buildings to withstand design loads without local structural collapse until the 

occupants can escape and the fire service can complete search and rescue operations. The load conditions 

induced by aircraft impact and the extensive fires on September 11, 2001, which triggered the collapse of 

the WTC towers, fall outside the norm of design loads considered in building codes. 

NIST is interested in determining what factors related to normal building and fire safety considerations, if 

any, could have saved additional WTC occupant lives on September 11, 2001, or could have minimized 

the loss of life among the ranks of first responders. This is being accomplished by addressing the 

following key questions related to occupant behavior, evacuation, and emergency response: 

• How did the evacuation technologies and practices affect the resulting fatalities and injuries? 

• How did the first responder technologies and practices affect the resulting fatalities and 

injuries? 

• How did the command, control, and communication systems support the activities of the first 

responders? 

• What were the design, capabilities, and performance of the installed active fire protection 

systems (i.e., fire alarm, sprinkler, and smoke management systems)? 

• What were the physical conditions within the buildings associated with occupant safety, 

tenability, and emergency responder operations? 
Chapter 1 

20

• How did building design features affect egress and emergency access? 

NIST is using multiple sources of data to investigate occupant behavior, evacuation, and emergency 

response. These sources include: 

• Existing published first-person accounts of WTC evacuation; over 725 accounts collected and 

analyzed. 

• Communication tapes from the PANYNJ and NYPD; 1,000-plus hours of taped recordings. 

• Filings with the Occupational Safety and Health Administration by survivors and families of 

victims; about 60 written statements. 

• Documents from the PANYNJ, FDNY, NYPD, and others on design of egress and 

emergency communication systems; WTC evacuation history; WTC evacuation planning and 

drills; emergency response preparedness and operational data. 

• Photographic and videographic data on occupant behavior, evacuation, and emergency 

response. 

• First-person data collection from WTC survivors, current and retired first responders, and 

families of victims. 

• New York City 911 tapes and logs, and transcripts of about 500 interviews with FDNY 

employees involved in WTC emergency response activities. 

Occupant Behavior and Evacuation 

NIST is documenting occupant behavior and evacuation efforts by gathering and analyzing information 

about: 

• Evacuation systems, emergency communications, and human factors; 

• Occupant location, evacuation experience, and observed building conditions; and 

• Interaction between occupants, first responders, and the buildings. 

NIST has completed first-person interviews of nearly 1,200 WTC occupants and first responders to 

collect data on occupant behavior, evacuation, and emergency response, including: 

• 803 telephone interviews with occupants of WTC 1 and WTC 2; 

• 228 face-to-face interviews of WTC occupants and families of victims, including 28 near the 

floors of impact in both WTC towers, 33 persons with responsibility within the buildings, 

15 who were in elevators, 13 who had a disability, 8 family members of victims, and 

7 occupants from WTC 7; further, 8 interviews were conducted with key personnel present 

inside WTC 7; 
Interim Findings and Accomplishments 

21

• 108 face-to-face interviews of first responders, including 68 from FDNY, 24 from NYPD, 

13 from the Port Authority Police Department (PAPD) and other PANYNJ safety and 

communications personnel, and 3 other building security and fire safety personnel; 

• Six focus groups, including a group each from WTC 1 and WTC 2, maintenance and security 

personnel, floor wardens, people near/above the floor of impact, and mobility-challenged 

survivors. 

Based on information and data gathered during these interviews with surviving occupants:: 

Finding 2.1: It is estimated that 17,400 occupants (± 1,200) were present in the WTC towers on the 

morning of September 11, 2001. The initial population of each tower was similar: 8,900 (± 750) in 

WTC 1 and 8,500 (± 900) in WTC 2. Of those present on September 11, 2001, 16 percent were also 

present during the 1993 bombing. 

Finding 2.2: The average age of surviving occupants of the WTC towers was mid-forties, with a range of 

ages from their early twenties to mid-seventies. Occupants were twice as likely to be male (65 percent for 

WTC 1 and 69 percent for WTC 2) as female. 

Finding 2.3: Two-thirds of WTC 1 surviving occupants had started working in the building during the 

previous four years (1998-2001), while half of WTC 2 occupants had begun working there during the 

same time period. The median residence time in WTC 1 was 2 years, while the median in WTC 2 was 3 

years. In WTC 1, 4 percent of the occupants had worked in the building since 1975, while there was only 

one such respondent in WTC 2. 

Finding 2.4: About 6 percent of the surviving occupants reported a pre-existing limitation to their 

mobility. These limitations included obesity, heart condition, needing assistance to walk, pregnancy, 

asthma, being elderly, chronic condition, recent surgery or injury, and other. 

Finding 2.5: Overall, 7 percent of the surviving occupants reported having special knowledge about the 

building. These included fire safety staff, floor wardens, searchers, building maintenance, and security 

staff. Searchers assist the floor wardens in facilitating evacuation. 

Finding 2.6: Approximately 87 percent of the WTC tower occupants, including more than 99 percent of 

those below the floors of impact, were able to evacuate successfully. Two-thousand one- hundred 

fifty-nine building occupants (1,560 in WTC 1 and 599 in WTC 2) and an additional 433 first responders, 

including security guards, were reported to have lost their lives that day. This does not include aircraft 

passengers and crew or bystanders. 

Rough initial estimates suggest that about 20 percent or more of those who were in the WTC towers and 

lost their lives may have been alive in the buildings just prior to their collapse. This estimate—which will 

be refined as analysis of the data is completed—assumes that nearly all of the first responders and 76 

building occupants below the floors of impact but none of the people at or above the floors of impact who 

may have been alive. Below the floors of impact, there were 72 fatalities reported in WTC 1 and four 

fatalities reported in WTC 2, not including first responders. It is estimated that were a total of 2,592 

building occupants and first responders who were in the WTC towers and lost their lives. 
Chapter 1 

22

Finding 2.7: Two-thirds of the surviving occupants reported having participated in a fire drill in the 12 

months prior to September 11, 2001, while 17 percent reported that they received no training during that 

same period. Of those participating in fire drills, 93 percent were instructed about the location of the 

nearest stairwell. Overall, slightly over half of the survivors, however, had never used a stairwell at the 

WTC prior to September 11, 2001. 

Finding 2.8: Overall, about 7,900 surviving occupants evacuated WTC 2 in 73 min (i.e., from the instant 

the WTC 1 was struck by aircraft until WTC 2 collapsed) while about 7,500 survivors evacuated WTC 1 

in 103 min. Thus, the overall evacuation rate in WTC 2 (108 survivors per min) was about 50 percent 

faster than that in WTC 1 (73 survivors per min). Functioning elevators allowed many survivors to 

evacuate WTC 2 prior to aircraft impact. 

• After the first airplane struck WTC 1 and before the second airplane struck WTC 2, the 

survivors in WTC 2 were twice as likely as those in WTC 1 to have already exited the 

building (41 percent versus 21 percent). The rate of evacuation completion in WTC 2 was 

twice the rate in WTC 1 during that same period. The elevators in WTC 2 were functioning 

at this time, while most of those in WTC 1 were not and survivors could only use the 

stairways. The stairwells, with partition wall enclosures that provided a 2 h fire-rating but 

little structural integrity, were damaged in the region of the aircraft impacted floors. 

• Soon after WTC 2 was struck by the airplane until about 20 min before each building 

collapsed, the survivors in WTC 2 and WTC 1 had exited at about the same rate (the prior 

evacuation rate of WTC 1). Most of the elevators in both towers were not functioning at this 

time, and survivors could only use the undamaged stairways. 

• During the last 20 min before each building collapsed, the evacuation rate in both buildings 

had slowed considerably to about one-fifth the immediately prior evacuation rate. This 

suggests that for those seeking and able to reach and use undamaged exits and stairways, the 

egress capacity (number and width of exits and stairways) was adequate to accommodate 

survivors.

NIST has utilized existing computer egress models to better understand the evacuation experience on 

September 11, 2001. While these models were developed using data not indicative of the WTC buildings 

or events, they can provide some perspective into the relative magnitudes of evacuation times for phased 

evacuation (as the buildings are designed) and full evacuation of occupants. Three full evacuation 

scenarios are considered: a typical full capacity building evacuation assuming the WTC tower is fully 

occupied—with one case considering only tenants and another case considering both tenants and visitors; 

a full capacity building evacuation of each WTC tower with aircraft impact damage; and a September 11, 

2001, capacity evacuation from a WTC tower. 

NIST is using two classes of egress models in order to frame the evacuation questions: (1) partial 

behavior: simulates occupant movement and limited behavioral rules by including delay times, smoke 

effects, and occupant characteristics; and (2) behavioral: simulates movement and more comprehensive 

evacuation decisions and activities. 

Finding 2.9: Preliminary results from application of existing computer egress models for a full capacity 

evacuation of a single WTC tower with 25,000 occupants and visitors indicate a movement time of 2 h 
Interim Findings and Accomplishments 

23

and 15 min. This is a minimum time estimate since the simulation assumed that there was no survivor 

delay, continual movement on the stairs, and no damage to the egress system. It was also assumed that 

elevators were not available. The egress model estimate for a September 11, 2001 capacity evacuation 

under the same assumptions is about 50 min, which is 2.5 times less than the time estimate for evacuating 

25,000 people. 

Finding 2.10: Given that the actual evacuation time on September 11, 2001 was about 100 min without 

elevator use, a full capacity evacuation of each WTC tower with 25,000 people would have required 

about 4 h (or 2.5 times 100 min). To achieve a significantly faster total evacuation at full capacity would 

have required increases in egress capacity (number and width of exits and stairways). 

Finding 2.11: Ingress/egress was a tremendous physical challenge for first responders and many 

occupants; inadequate footwear presented a mobility challenge, particularly for many women. Many 

people were left shoeless, and their discarded shoes often littered the stairwells. 

NIST also has studied the possibility of roof evacuations from the WTC towers. 

Finding 2.12: A preliminary evaluation indicates that the PANYNJ’s evacuation procedures did not 

include a plan to provide roof rescue for occupants trapped in a building incident at the WTC site. The 

standard policy was to keep the doors to the roof locked with a key being required to gain roof access. 

No fire safety procedures explicitly called for opening these doors, including anyone on a “key run.” 

Instead, the standard occupant evacuation procedures and drills required the use of stairwells to exit at 

the bottom of the WTC towers. The PANYNJ reports that it never advised tenants to evacuate upward.

Finding 2.13: There were at least two instances reported on September 11, 2001, where roof access was 

found to be locked in both WTC towers. In the case of WTC 1, a decedent had called and informed a 

parent that they had tried to get to the roof and found the door locked. In the case of WTC 2, a decedent 

had called and informed a spouse that he had tried to get to the roof and found the door locked. 

Finding 2.14: At least one case was reported on September 11, 2001, where a PANYNJ employee, 

trapped on Floor 105 of WTC 2, was instructed on the radio (PANYNJ Channel Y) by another PANYNJ 

employee to go to the roof of the building. The trapped occupant was unable to walk down the stairs, or 

go to the roof as instructed. 

Finding 2.15: The NYPD aviation unit arrived at the WTC site soon after WTC 1 was attacked. Despite 

repeated attempts to examine the possibility of roof rescue, smoke and heat conditions at the top of the 

WTC towers prevented the conduct of safe roof evacuation operations. A helicopter, attempting to 

inspect the roof condition and determine if occupants were on the roof, experienced engine temperature 

increase as it approached WTC 1. 

Finding 2.16: NYPD has an aviation-training manual to guide roof rescue in high-rise emergencies. The 

manual is not specific to the WTC. Considering the capacity of typical helicopters and travel times, it is 

not clear what fraction of the large number of occupants could have been evacuated from the WTC 

towers prior to their collapse had roof rescue been possible on September 11, 2001. 

The analysis of the first-person accounts collected from occupants by NIST and the evacuation modeling 

work is ongoing, and NIST will report its additional findings on occupant behavior and evacuation at a 
Chapter 1 

24

later date. That report will incorporate results from the analysis of previously published first-person 

accounts provided to the media by survivors and information released to the public by the 9-11 

Commission. 

Emergency Response and Communications 

NIST’s investigation of fire service technologies and guidelines, and more broadly the emergency 

response related to firefighting and evacuation on September 11, 2001, seeks to: 

1. Document what happened during the emergency response to the attacks on the WTC up 

until the collapse of WTC 7; 

2. Identify issues that need to be addressed in practices, standards, and codes; 

3. Identify alternative practices and/or technologies that may address these issues; and 

4. Identify technologies and guidelines to advance the safety of first responders in tall 

building emergencies. 

Three FDNY suitcase-based, magnetic Command Boards were set up at the incident site. The unit 

identification and assignment for each unit that arrives at the scene is written on a magnetic chip and 

placed on the board. Information related to the location and activities of the units once they are on site 

are also recorded. One Command Board was set up at the original Incident Command Post in the lobby 

of WTC 1. The WTC 1 lobby Command Post became an Operations Post when the Incident Command 

Post moved outside. However, the original Command Board remained in place inside WTC 1 when the 

Incident Command Post was moved outside. The second Command Board was set up at the fire 

department’s Operations Post in the lobby of WTC 2, and the third Command Board was set up at the 

new Incident Command Post established by the Chief of Department on West Street in front of the World 

Financial Center 2 building. 

Finding 2.17: The FDNY suitcase-based, magnetic Command Board system that was generally adequate 

for normal fire and rescue operations was not adequate for handling the massive operations that were 

necessary as a result of the terrorist attacks on the WTC. Interviews with FDNY personnel indicated that 

some FDNY personnel and others entered the towers and were not recorded on the Command Boards. 

This resulted in the lack of important command information. In addition, each of these Command Boards 

was damaged, and all were lost with the collapse WTC 2. There was no back-up capability for the 

Command Boards, and all information related to command, control, and accountability was lost. 

NIST has completed a partial analysis of emergency responder communications including: 

• Digital copies of the audio communications tapes recorded by the PANYNJ, including 

communications from emergency response personnel, maintenance personnel, PAPD 

personnel, and a recording of the FDNY’s city-wide high-rise Channel 7 (PAPD’s Channel 

30) radio repeater that was located at the WTC; and 

• Audio tapes copied from original NYPD communications tapes, including NYPD internal 

department operations. 
Interim Findings and Accomplishments 

25

FDNY communications recordings from the WTC location on September 11, 2001, are not available 

because the primary field communication truck was in the shop for repairs, and a backup field 

communications van was used in its place. The backup van did not have the capability to record the 

on-scene incident command or tactical communications and was destroyed when the WTC towers 

collapsed. 

The best record of radio communications reflecting fire department operations available to NIST came 

from the FDNY Channel 7/PAPD Channel 30 tape and first person accounts provided by FDNY 

personnel during their interviews. The tape provides limited information on FDNY communications and 

operations at the incident, but it does provide insight into FDNY operations inside WTC 2. FDNY 

Channel 7/PAPD Channel 30 was a city-wide channel designated by FDNY for use in high-rise building 

operations. The PANYNJ installed the radio repeater system at the WTC for use by FDNY after the 1993 

bombing. 

Finding 2.18: The following findings have been drawn from the first-person interviews with emergency 

responders regarding telephone communications: 

• Before the attack occurred, both the landline and cellular systems appeared to be working 

normally. 

• Only moments after the first aircraft impacted WTC 1, the landline and cellular telephone 

systems were stressed by increased caller volume that made it difficult to get messages 

through. This condition continued for many hours following the attack. 

• Telephone calls from the WTC to the 911 emergency operators, and statements from various 

individuals interviewed, show that even though WTC 1 and WTC 2 were severely damaged by 

aircraft impact and fires, many of the landline telephones in the buildings continued to work 

up until the collapse of WTC 2. 

• After the collapse of WTC 2, a number of cellular phone systems were not functional in the 

area of lower Manhattan. 

• After the collapse of WTC 2, there were still some landline telephones working within the city 

block areas adjacent to the WTC site. 

NIST has developed a preliminary chronology, based on analysis of selected communications messages, 

to provide information concerning (1) dispatch and arrival of emergency response units, (2) evacuation, 

(3) emergency response operations, (4) emergency response communications, and (5) observations of 

building conditions. 

Finding 2.19: The following findings have been drawn from the analysis of the emergency responder 

communication tapes: 

• After the first aircraft struck WTC 1, there was an approximate factor of 5 peak increase in 

traffic level over the normal level of emergency responder radio communications, followed by 

an approximate factor of 3 steady increase in the level of subsequent traffic. 
Chapter 1 

26

• A surge in communications traffic volume made it more difficult to handle the flow of 

communications and delivery of information. 

• Roughly a third to a half of the radio messages transmitted during these radio traffic surge 

conditions were not complete messages or understandable. 

• Preliminary analysis of the FDNY city-wide high-rise Channel 7 (PAPD Channel 30) 

recording indicates that the WTC site repeater was operating. 

• Communications records and interviews with aviation unit personnel indicate that smoke and 

heat conditions on the top of the WTC towers prevented NYPD helicopters from conducting 

safe roof evacuation operations. 

• NYPD aviation unit personnel reported critical information about the impending collapse of 

the WTC towers several minutes prior to their collapse. No evidence has been found to 

suggest that the information was communicated to all emergency responders at the scene. 

Finding 2.20: Several FDNY personnel at the incident site did not think that the high-rise radio repeater 

was working. This is based on radio communications tests that were conducted by two chief officers 

working inside WTC 1 when the first command post was being set up in that lobby. This radio 

communications test was recorded on the FDNY Channel 7/PAPD Channel 30 tape. Following this radio 

test, a chief officer involved in the test chose to use different channels for command and tactical 

communications during the disaster. However, as FDNY operations increased in WTC 2, it was 

determined by FDNY members that the high-rise repeater was functioning, and use of the channel 

developed. 

Finding 2.21: While the preliminary NIST analysis indicates that the repeater was operating, there also 

appears to have been some type of malfunction with the communications equipment that was detected, but 

not identified, by FDNY officers during the initial test. Three hypotheses are being investigated related to 

this malfunction: (1) damage to the repeater antenna system located in WTC 5, e.g., changing its 

direction, (2) failure of the radio handset located at the fire command desk in the lobby of WTC 1, and 

(3) the volume of the radio hand set not being turned up. 

NIST continues to evaluate the repeater system and its operations, as well as the handheld radios, which 

were used on September 11, 2001. The findings listed above will be updated and additional findings will 

be documented when the investigation is complete. 

NIST has completed its review of the NYC 9-1-1 tapes and logs and the transcripts of about 500 

interviews with FDNY employees involved in WTC emergency response activities. Analysis of this and 

other information is ongoing. 

Finding 2.22: Based on face-to-face interviews, NIST has determined that first responders— including 

key incident commanders—did not have adequate information (voice, video, and data) on and an overall 

perspective of the conditions in the WTC buildings and what was happening elsewhere at the WTC site; 

interagency information sharing was inadequate. 
Interim Findings and Accomplishments 

27

NIST continues to analyze all of the data sources (documents, visual images, first-person accounts, and 

communications records) related to the emergency response activities at the WTC site, including 

deployment, evacuation, operations, and communication systems and protocols. A future report will 

document the findings from that analysis. 

Fire History of the WTC Towers 

NIST has completed a review of the history of post-occupancy fire incidents and identified significant fire 

incidents—those that exercised the fire suppression systems, specifically multiple sprinklers or one or 

more standpipes (with or without activation of at least one sprinkler). 

The FDNY fire reports and fire investigation records indicate that in areas protected by automatic 

sprinklers, no fire activated more than three sprinklers. The design area for three sprinklers is a floor area 

of 63 m2

 (675 ft2) in light hazard occupancy, such as a high-rise office building as specified in the 

National Fire Protection Association Standard for the Installation of Sprinkler Systems (NFPA 13). 

Many of the fires that occurred were recorded as suspicious or unknown in cause, occurred during 

off-peak work hours, and involved materials such as trash or paper-based supplies. In cases where 

sprinklers were activated, the fire department records indicated that the sprinklers either extinguished the 

fire completely or aided in controlling the spread. In summary: 

• 16 significant fires occurred in WTC 1, 2, and 7, with 12 in WTC 1, three in WTC 2, and one 

in WTC 7. Twelve of the 16 fires occurred between 6 p.m. and 4 a.m. when the number of 

occupants in the buildings was likely to be small. 

• Of the 16 fires and their causes, five were labeled as unlisted or unclassified, six as suspicious 

or incendiary, two as discarded material, and three as an electrical failure or mechanical 

failure. 

• Of the 16 fires, four were concentrated above the 100th floors and six fires were located in 

the basements. The other six were distributed throughout the rest of the buildings. 

• 31 additional fires occurred in WTC 1 and WTC 2, which involved the use of one standpipe, 

with 23 in WTC 1 and eight in WTC 2. 

• There is no known loss of life as a result of any of these fires (not including the 1993 

bombing incident and September 11, 2001, terrorist attacks). 

The following significant fires (not including the 1993 bombing incident and September 11, 2001, 

terrorist attacks) are noteworthy: 

• February 14, 1975: Fire started on the 11th floor of WTC 1. Fire damage occurred on the 

10th through the 19th floors. Approximately 9,000 ft2

 of 11th floor contents on the southeast 

corner was destroyed or damaged. 

• April 19, 1980: Fire started on the 106th floor of WTC 1. Approximately 300 occupants 

from the Windows on the World restaurant on the 107th floor were evacuated. 
Chapter 1 

28

• April 17, 1981: Fire started on the 7th floor of WTC 1. Approximately 1,500 occupants were 

evacuated from Floors 9 through 23. 

Active Fire Protection Systems – Sprinkler Systems 

Finding 2.23: In WTC 1, 2, and 7, primary and secondary water supplies, fire pump size and locations, 

water storage tanks, and fire department connections provided multiple points of water supply 

redundancy. The potential for single point failure of the water supply to the fire sprinklers existed at each 

floor due to lack of redundancy in the sprinkler riser system that provided only one supply connection on each 

floor. As a result, the water supply to the sprinkler systems or a standpipe serving pre-connected hoses 

could be interrupted by routine maintenance needs (i.e., shutdown of the riser or standpipe) or by 

impairment due to deliberate acts to damage the sprinkler riser or standpipe systems. While this lack of 

redundancy may not have had an impact on September 11, 2001 because the sprinkler system was damaged by 

aircraft impact, it could have made a difference in other building emergencies. 

Finding 2.24: Aided by the results of hydraulic modeling of a sprinkler system in WTC 1 and WTC 2—

undamaged by aircraft impact and fully operational—the delivered water flow rate available from the 

automatic sprinkler systems was found to generally exceed the minimum requirements (by a considerable 

margin) for a high-rise office hazard classification in accordance with NFPA 13. In a number of cases, 

the amount of available water flow from sprinklers on specific floors was capable of protecting higher 

fire hazard classes than those associated with light hazard office buildings. 

Active Fire Protection Systems—Fire Alarm Systems 

Finding 2.25: The fire alarm system that was monitoring WTC 7 sent to the monitoring company only 

one signal (at 10:00:52 a.m. shortly after the collapse of WTC 2) indicating a fire condition in the 

building on September 11, 2001. This signal did not contain any specific information about the location 

of the fire within the building. From the alarm system monitor service view, the building had only one 

zone, “AREA 1.” The building fire alarm system was placed on TEST for a period of 8 h beginning at 

6:47:03 a.m. on September 11, 2001. Ordinarily, this is requested when maintenance or other testing is 

being performed on the system, so that any alarms that are received from the system are considered the 

result of the maintenance or testing and are ignored. NIST was told by the monitoring company that for 

systems placed in the TEST condition, alarm signals are not shown on the operator’s display, but records 

of the alarm are recorded into the history file. 

Finding 2.26: The resistance to failure of the fire alarm system communications paths between the fire 

command station and occupied WTC tower floors could have been enhanced if fiber optic 

communications cable had been used instead of copper lines. Extensive damage to the towers upon 

aircraft impact is likely to have cut and short-circuited the wiring of the alarm system network cables. If 

that occurred, communications between the distributed fire alarm panels, which are components of the 

integrated fire alarm system, would have been degraded and lost to certain panels depending on the 

location of those panels. Fiber optic cable is not susceptible to electric short-circuits and would have 

provided full communications with fire alarm system components, including voice communications 

systems, to the point where the cable was severed. Electric shorts in the voice communications disable 

that communication system over the entire cable length affected by the electric short-circuit. During 

initial engineering design for the fire alarm system in WTC 1 and WTC 2, the PANYNJ requested, but did 
Interim Findings and Accomplishments 

29

not receive, approval of the City of New York for use of fiber optic communications cable in the system. 

The NYC code required copper wiring. As a result, ordinary copper wire communication cable was 

specified. 

A dedicated communications system for emergency responders, known as the “standpipe telephone” 

system, was installed in the stairwells of WTC 1 and WTC 2. To use the system, a compatible telephone 

handset was needed. Some firefighters that received handsets at the command post in the lobby of 

WTC 1 were interviewed as part of the Investigation. Every one of the firefighters interviewed indicated 

that they did not use the standpipe telephone communication system on September 11, 2001. Due to the 

loss of firefighters in WTC 2, there is no information about the use of the system in WTC 2. 

Active Fire Protection Systems—Smoke Management System 

The smoke management system in the WTC towers as designed and documented in the operation manuals 

consisted of a smoke purge mode using the components of the main HVAC (heating, ventilation, and 

air-conditioning) system. This system was intended to remove smoke and other gaseous combustion 

products from the fire area after a fire had been extinguished. This system was to be activated 

“manually” at the direction of FDNY. 

Finding 2.27: Based on the information reviewed, the smoke management systems were not activated 

during the fires on September 11, 2001. It was determined that the likelihood of these systems being 

functional in WTC 1 and WTC 2 was very low due to the damage inflicted by the aircraft impacts. In 

addition to the significant openings created in the building envelopes, the aircraft impacts are likely to 

have severed major vertical shafts through which ran electrical power supply and duct risers of the 

HVAC system, thereby causing the loss of power to the smoke management system air handlers and 

damage to the vertical HVAC duct risers used to provide smoke management (smoke purge). 

Finding 2.28: The analysis of smoke flow in WTC 1 and WTC 2 on September 11, 2001, shows that 

HVAC ductwork was a major path for vertical smoke spread in the buildings. Fire dampers were 

installed in the systems, but not smoke dampers. Operational combined fire/smoke dampers in the HVAC 

ductwork on each floor would have provided a barrier to hot gas and smoke penetration into the vertical 

HVAC shafts in WTC 1 and WTC 2. However, smoke dampers were not available when the towers were 

built. 

Finding 2.29: Modeling results show that in WTC 1 and WTC 2 stair pressurization systems would have 

provided minimal resistance to the passage of smoke had they been installed on September 11, 2001. 

While the existence of such systems was known when the WTC towers were built, the alternative smoke 

purge system used in the WTC towers was considered to be equivalent. Multiple stair doors being open 

for substantial periods of time due to occupant egress and stairway walls damaged by aircraft impact 

would have resulted in an inability to prevent smoke from entering stairwells. 

1.5 PROCEDURES AND PRACTICES 

The 110-story WTC towers were among the world’s tallest buildings, while the 47-story WTC 7 

represented a more typical tall building. These buildings provide case studies to document, review, and, 

if needed, improve the procedures and practices used in the design, construction, operation, and 
Chapter 1 

30

maintenance of tall buildings. This investigation objective is independent of other objectives which are 

focused specifically on the consequences of the attack on September 11, 2001, viz., the building 

collapses, evacuation, and emergency response. While some findings under this objective are directly 

relevant to the events of September 11, 2001, others are concerned with general building and fire safety 

procedures and practices. 

NIST seeks to determine the building and fire safety procedures and practices that were used over the life 

of the WTC buildings and how well those procedures and practices conformed to accepted national 

building and fire safety practices, standards, and codes. The procedures and practices of interest to the 

investigation include those related to: 

• Design and construction, 

• New and innovative design features, 

• New and innovative technologies and materials, 

• Passive and active fire safety systems, 

• Emergency access and egress systems, and 

• Structural modifications, inspection, and maintenance. 

To provide context for studying the specifications and criteria used for the WTC buildings, NIST has 

completed a preliminary comparison of building regulatory and code requirements. For the structural 

system, this comparison included the following building codes: 

• New York City Building Code - 1968 edition 

• New York State Building Code - 1964 edition 

• Chicago Building Code - 1967 edition 

• BOCA Basic Building Code - 1965 edition (a national model building code) 

• New York City Building Code - 2001 edition 

For the fire protection and egress system, the comparison included the five codes listed above plus the 

1966 edition of the National Fire Protection Association Life Safety Code (NFPA 101). 

Applicable Building Codes 

Finding 3.1: Although not required to conform to NYC codes, the PANYNJ adopted the provisions of the 

proposed 1968 edition of the NYC Building Code, more than three years before it went into effect. The 

1968 edition allowed the PANYNJ to take economic advantage of less restrictive provisions compared 
Interim Findings and Accomplishments 

31

with the 1938 edition that was in effect when design began for the WTC towers in 1962. The 1968 code: 

• Eliminated a fire tower5

 as a required means of egress; 

• Reduced the number of required stairwells from 6 to 3 and the size of doors leading to the 

stairs from 44 in. to 36 in.; 

• Reduced the fire rating of the shaft walls in the building core from 3 h to 2 h; 

• Changed partition loads from 20 psf to one based on weight of partitions per unit length (that 

reduced such loads for many buildings including the WTC buildings); and 

• Permitted a 1 h reduction in fire rating for all structural components (columns from 4 h to 3 h 

and floor framing members from 3 h to 2 h) by allowing the owner/architect to select Class 

1B construction for business occupancy and unlimited building height. 

Many of these newer requirements, instituted in the 1968 NYC Building Code, are contained in current 

codes. 

Finding 3.2: The NYC Department of Buildings reviewed the WTC tower drawings in 1968 and provided 

comments to the PANYNJ concerning the plans in relation to the 1938 NYC Building Code. The 

architect-of-record submitted to the PANYNJ responses to those comments, noting how the drawings 

conformed to the 1968 NYC Building Code. NIST continues reviewing documents to determine the level 

of review conducted by the NYC Department of Buildings and the six specific items identified in that 

review. 

Finding 3.3: In 1993, the PANYNJ and the NYC Department of Buildings entered into a memorandum of 

understanding that restated the PANYNJ’s longstanding policy to assure that its facilities in the City of 

New York meet and, where appropriate, exceed the requirements of the New York City Building Code. 

The agreement also provided specific commitments to the NYC Department of Buildings regarding 

procedures to be undertaken by the PANYNJ to assure that buildings owned or operated by the PANYNJ 

are in conformance with the Building Standards contained in the NYC Building Code. Some salient 

points included in this agreement and the 1995 enhancement to the agreement are: 

• Each project would be reviewed and examined for compliance with the Code; 

• All plans would be prepared, sealed, and reviewed by New York State licensed professional 

engineers or architects; 

• The PANYNJ engineer or architect approving the plans would be licensed in the State of New 

York and would not have assisted in the preparation of the plans; and 

 

5

 A fire tower (also called a smoke-proof stair) is a stairway that is accessed through an enclosed vestibule that is open to the 

outside or to an open ventilation shaft providing natural ventilation that prevents any accumulation of smoke without the need 

for mechanical pressurization. 
Chapter 1 

32

• The person or firm performing the review and certification of plans for WTC tenants should 

not be the same person or firm providing certification that the project had been constructed 

in accordance with the plans and specifications. 

• Variances from the Code, acceptable to the PANYNJ, would be submitted to the NYC 

Department of Buildings for review and concurrence. 

Finding 3.4: In 1993, the PANYNJ adopted a policy providing for implementation of fire safety 

recommendations made by local government fire departments after a fire safety inspection of a PANYNJ 

facility and for the prior review by local fire safety agencies of fire safety systems to be introduced or 

added to a facility. Later that year, the PANYNJ entered into an agreement with FDNY which reiterated 

the policy adopted by the PANYNJ, recognized the right of FDNY to conduct fire safety inspections of 

PANYNJ properties in the City of New York, provided guidelines for FDNY to communicate needed 

corrective actions to the PANYNJ, assured that new or modified fire safety systems are in compliance 

with local codes and regulations, and required third-party review of such systems by a New York State 

licensed architect or engineer. 

Standard Fire-Resistance Tests 

Finding 3.5: Availability of code provisions with detailed procedures to analyze and evaluate data from 

fire resistance tests of other building components and assemblies to qualify an untested building element. 

Based on available data and records, no technical basis has been found for selecting the spray-applied 

fire resistive material (SFRM) used (two competing materials were under evaluation) or its thickness for 

the large-span open-web floor trusses of the WTC towers. The assessment of the fireproofing thickness 

needed to meet the 2 h fire rating requirement for the untested WTC floor system evolved over time: 

• In October 1969, the PANYNJ directed the fireproofing contractor to apply 1/2 in. of 

fireproofing to the floor trusses. 

• In 1999, the PANYNJ issued guidelines requiring that fireproofing be upgraded to 1-1/2 in. 

for full floors undergoing alterations. 

• Unrelated to the WTC buildings, an International Conference of Building Officials (ICBO) 

Evaluation Service report (ER-1244), re-issued June 1, 2001, using the same SFRM 

recommends a minimum thickness of 2 in. for “unrestrained steel joists” with “lightweight 

concrete” slab. 

Finding 3.6: Availability of code provisions that require the conduct of a fire resistance test if adequate 

data do not exist from other building components and assemblies to qualify an untested building element. 

Instead, several alternate methods based on other fire-resistance designs or calculations or alternative 

protection methods are permitted with limited guidance on detailed procedures to be followed. Both the 

architect-of-record (in 1966) and the structural-engineer-of-record (in 1975) stated that the fire rating of 

the floor system of the WTC towers could not be determined without testing. NIST has not found evidence 

indicating that such a test was conducted to determine the fire rating of the WTC floor system. The 

PANYNJ has informed NIST that there are no such test records in its files. 
Interim Findings and Accomplishments 

33

NIST has awarded a contract to Underwriters Laboratories (UL) to determine the fire resistance rating of 

the WTC floor system under both as-specified and as-built conditions. The tests, which are expected to 

be conducted in August 2004, will provide the fire endurance ratings of typical WTC floor construction to 

evaluate three primary factors: (1) test scale, (2) fireproofing thickness, and (3) thermal restraint. The 

four tests will be performed as follows: 

• 17 ft span assembly, thermally restrained, SFRM thickness of 1/2 in. 

• 17 ft span assembly, thermally restrained, SFRM thickness of 3/4 in. 

• 35 ft span assembly, thermally restrained, SFRM thickness of 3/4 in. 

• 35 ft span assembly, thermally unrestrained, SFRM thickness of 3/4 in. 

The first test represents current U.S. practice for establishing a fire endurance rating of a building 

assembly. The test assembly, fabricated to meet the design of the WTC steel joist-supported floor system, 

has a span of 17 ft. This span is typical of the floor assembly test furnaces used by the U.S. testing 

laboratories that routinely conduct the ASTM E119 test for the construction industry. As is common 

practice, the floor assembly will be tested in the thermally restrained condition. This test will be 

conducted at UL’s Northbrook, Illinois, fire test facility. A second test will be identical except for the 

thickness of SFRM. The third and forth tests will be at twice the scale of the first two tests, with a span of 

35 ft. This span represents a full-scale assembly of the 35 ft floor panel of the WTC floor system. The 

floor assembly for the third test will be thermally restrained as in the first two tests thereby allowing 

direct comparison for the determination of the effect of test scale on fire endurance rating. The fourth test 

will be conducted in the thermally unrestrained support condition which will allow direct comparison of 

the effect of thermal restraint on the fire endurance rating. The third and fourth tests will be conducted at 

the UL Canada fire test facility near Toronto. 

Building Classification and Fire Rating 

Finding 3.7: Use of the “structural frame” approach, in conjunction with the prescriptive fire rating, 

would have required the floor trusses, the core floor framing, and perimeter spandrels in the WTC towers 

to be 3 h fire-rated, like the columns for Class 1B construction in the 1968 NYC Building Code. Neither 

the 1968 edition of the NYC Building Code which was used in the design of the WTC towers, nor the 2001 

edition of the code, adopted the “structural frame” requirement. The “structural frame” approach to 

fire resistance ratings requires structural members, other than columns, that are essential to the stability 

of the building as a whole to be fire protected to the same rating as columns. This approach, which 

appeared in the Uniform Building Code (a model building code) as early as 1953, was carried into the 

2000 International Building Code (one of two current model codes) which states: “The structural frame 

shall be considered to be the columns and the girders, beams, trusses and spandrels having direct 

connections to the columns and bracing members designed to carry gravity loads.” The WTC floor 

system was essential to the stability of the building as a whole since it provided lateral stability to the 

perimeter columns and diaphragm action to distribute wind loads to the perimeter columns. 

Finding 3.8: Availability of technical basis to establish whether the construction classification and fire 

rating requirements in modern building codes are risk-consistent with respect to the design-basis hazard 

and the consequences of that hazard. The fire rating requirements, which were originally developed 
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34

based on experience with buildings less than about 20 stories in height, have generally decreased over 

the past 80 years since historical fire data for buildings suggested considerable conservatism in those 

requirements. However, for tall buildings, the likely consequences of a given threat to an occupant on the 

upper floors are more severe than the consequences to an occupant, say, on the first floor. It is not 

apparent how the current height and area tables in building codes consider the technical basis for the 

progressively increasing risk to an occupant on the upper floors of tall buildings that are much greater 

than 200 ft in height. The maximum required fire rating in current codes applies to any building more 

than about 12 stories in height. There are no additional categories for buildings above, for example, 

40 stories and 80 stories, where different building classification and fire ratings requirements may be 

appropriate, recognizing factors such as the time required for stairwell evacuation without functioning 

elevators (e.g., due to power failure or major water leakage), the time required for first responder access 

without functioning elevators, the presence of sky lobbies and/or refuge floors, and limitations on the 

height of elevator shafts. The 110-story WTC towers, initially classified as Class IA based on the 

1938 NYC Building Code, were classified as Class 1B before being built to take advantage of the 

provisions in the 1968 edition of the code. This re-classification permitted a reduction of 1 h in the fire 

rating of the components (columns from 4 h to 3 h and floor framing members from 3 h to 2 h). 

Fire Performance of Structures 

Finding 3.9: Structural design does not consider fire as a design condition, as it does the effects of dead 

loads, live loads, wind loads, and earthquake loads. Current prescriptive code provisions for 

determining fire resistance of structures—used in the design of the WTC towers and WTC 7— are based 

on tests using a standard fire that may be adequate for many simple structures and for comparing the 

relative performance of structural components in more complex structures. A building system with 3 h 

rated columns and 2 h rated girders and floors could last longer than 3 h or shorter than 2 h depending 

upon the performance of the structure as a 3-dimensional system in a real fire. The standard tests cannot 

be used to evaluate the actual performance (i.e., load carrying capacity) in a real fire of the structural 

component, or the structure as a whole system, including the connections between components. 

Performance-based code provisions and standards are not available for use by engineers, as an 

alternative to the current prescriptive fire rating approach, to (1) evaluate the system performance of 

tall-building structures under real fire scenarios, and (2) enable risk consistent design with appropriate 

thickness of passive protection being provided where it is needed on the structure. Standards 

development organizations, including the American Institute of Steel Construction, have initiated 

development of performance-based provisions to consider fire effects in structural design. 

Finding 3.10: Availability of detailed procedures to select appropriate design-basis fire scenarios to be 

considered in the performance-based design of the sprinkler system, compartmentation, and passive 

protection of the structure. The standard fire in current prescriptive fire resistance tests is not adequate 

for use in performance-based design. While the NFPA 5000 model building code contains general 

guidance on design fire scenarios (the IBC Performance Code contains no such guidance), the details of 

the scenarios are left to the fire engineer and regulatory official. The three major scenarios that are not 

considered adequately are: frequent but low severity events (for design of sprinkler system), moderate but 

less frequent events (for design of compartmentation), and a maximum credible fire (for design of passive 

fire protection on the structure). The maximum credible fire scenario for passive protection of structures 

would assume that the sprinkler system is compromised or overwhelmed and that there is no active 

firefighting. These building-specific representative fire scenarios are similar in concept, though not 
Interim Findings and Accomplishments 

35

identical, to the approach used in building design where the performance objectives and design-basis of 

the hazard are better defined (e.g., a two-level design that includes an operational event with a 

10 percent probability of occurrence in 50 years and a life safety event with a 2 percent probability of 

occurrence in 50 years). The design-basis fire hazards for the WTC towers and WTC 7 are unknown, and 

it is difficult to evaluate the performance of the fire protection systems in these buildings under specific 

fire scenarios. 

Finding 3.11: Availability of code provisions to ensure that structural connections are provided the same 

degree of fire protection as the more restrictive protection of the connected elements. The provisions that 

were used for the WTC towers and WTC 7 did not require specification of a fire-rating requirement for 

connections separate from those for the connected elements. It is not clear what the fire rating of the 

connections were when the connecting elements had different fire ratings. 

Finding 3.12: Availability of technical basis to establish whether the minimum mechanical and durability 

related properties of SFRM are sufficient to ensure acceptable in-service performance in buildings. 

While minimum bond strength requirements exist, there are no serviceability requirements for such 

materials to withstand typical shock, impact, vibration, or abrasion effects over the life of a building. 

There are existing testing standards for determining many of these properties, but the technical basis is 

insufficient to establish serviceability requirements. Knowledge of such serviceability requirements is 

relevant to determine the post-impact fireproofing condition of the WTC towers. 

Finding 3.13: Availability of validated and verified tools for use in performance-based design practice to 

analyze the dynamics of building fires and their effects on the structural system that would allow 

engineers to evaluate structural performance under alternative fire scenarios and fire protection 

strategies. Existing tools are either too simplified to adequately capture the performance of interest or 

too complex and computationally demanding, and lack adequate validation. While considerable progress 

has been made in recent years, significant work remains to be done before adequate tools are available 

for use in routine practice. NIST has had to further develop and validate existing tools to investigate the 

fire performance of the WTC towers and WTC 7. 

Structural Integrity 

Finding 3.14: Availability of explicit structural integrity provisions to mitigate progressive collapse. 

Federal agencies have developed guidelines to mitigate progressive collapse and routinely incorporate 

such requirements in the construction of new federal buildings. The United Kingdom incorporates such 

code requirements for all buildings. New York City adopted by rule in 1973 a requirement for buildings 

to resist progressive collapse under extreme local loads. The rules, which were adopted after the WTC 

towers were built but before WTC 7 was built, applied specifically to buildings that used precast concrete 

wall panels and not to other types of buildings. As stated in Finding 1b.1, the current working hypothesis 

for the collapse of the 47-story WTC 7, if it remains viable upon further analysis, would suggest that it 

was a classic progressive collapse. 

Finding 3.15: Availability of minimum structural integrity provisions for the means of egress (stairwells 

and elevator shafts) in the building core that are critical to life safety. In most tall buildings the core is 

designed to be part of the vertical gravity load carrying system of the structure. However, in many of 

those buildings, especially in regions where earthquakes are not dominant, the core may not be part of 
Chapter 1 

36

the lateral load carrying system of the structure. Thus, the core may be designed to carry only vertical 

gravity loads with no capacity to resist lateral loads, i.e., overturning moment and shear loads. In such 

situations, the structural designer may prefer the use of partition walls over structural walls in the core 

area to reduce building weight. The decision to have the core carry a specified fraction of the lateral 

design loads or be made part of a dual system to carry lateral loads, each of which would enhance the 

structural integrity of the core if structural walls were used, is left to the discretion of the structural 

engineer. Alternatively, stairway/elevator cores built with concrete or reinforced concrete block, which 

are not part of the lateral load carrying system, may be able to provide sufficient structural integrity if 

they meet, for example, the hose-stream impact test already required by ASTM E 119, or other more 

appropriate test. In the case of the WTC towers, the core had 2 h fire-rated partition walls with little 

structural integrity and the core framing was required to carry only gravity loads. Had there been a 

minimum structural integrity requirement to satisfy normal building and fire safety considerations, it is 

conceivable that the damage to stairways, especially above the floors of impact, may have been less 

extensive. 

Finding 3.16: Availability of standards and code provisions for conducting wind tunnel tests and for the 

methods used in practice to estimate design wind loads from test results. Building codes allow the 

determination of wind pressures from wind tunnel tests for use in design. Such tests are frequently used 

in the design of tall buildings. Results of two sets of wind tunnel tests conducted for the WTC towers in 

2002 by independent commercial laboratories as part of insurance litigation, and voluntarily provided to 

NIST by the parties to the litigation, show large differences, of as much as about 40 percent, in resultant 

forces on the structures, i.e., overturning moments and base shears. Independent reviews by a NIST 

expert on wind effects on structures and a leading engineering design firm contracted by NIST indicated 

that the documentation of the test results did not provide sufficient basis to reconcile the differences. In 

addition, the wind loads estimated from these tests are about 20 percent to 60 percent higher than those 

apparently used in the original design of the WTC towers, also obtained from wind tunnel testing. NIST 

is conducting an independent analysis to establish the baseline performance of the WTC towers under the 

original design wind loads and will compare those wind load estimates with then-prevailing code 

requirements. Wind loads were a major governing factor in the design of structural components that 

made up the frame-tube steel framing system. 

Compartmentation and Sprinklers 

Building fire protection is based on a four-level hierarchical strategy comprising detection, suppression 

(sprinklers and firefighting), compartmentation, and passive protection of the structure. 

• Detectors are typically used to activate fire alarms and notify building occupants and 

emergency services. 

• Sprinklers are designed to control small and medium fires and to prevent fire spread beyond 

the typical water supply design area of about 1,500 ft2

. 

• Compartmentation mitigates the horizontal spread of more severe but less frequent fires and 

typically requires fire-rated partitions for areas of about 7,500 ft2

. Active firefighting also 

covers up to about 5,000 ft2

 to 7,500 ft2. 
Interim Findings and Accomplishments 

37

• Passive protection of the structure seeks to ensure that a maximum credible fire scenario, with 

sprinklers compromised or overwhelmed and no active firefighting, results in burnout, not 

overall building collapse. The intent of building codes is also for the building to withstand 

local structural collapse until occupants can escape and the fire service can complete search 

and rescue operations. 

Compartmentation of spaces has long been a cornerstone to building fire safety to limit fire spread. The 

WTC towers initially had 1 h fire-rated partitions separating tenants (demising walls) that extended from 

the floor to the suspended ceiling, not the floor above (the ceiling tiles were not fire rated). Over the 

years, these partitions were replaced with partitions that were continuous from floor to floor (separation 

wall) as required by the 1968 NYC Building Code. Some partitions had not been upgraded by 1997, and 

a consultant recommended to the PANYNJ that it develop and implement a survey program to assure that 

the remediation process occurred as quickly as possible. It appears that with few exceptions, nearly all of 

the floors not upgraded were occupied by a single tenant, and it is not clear whether separation walls 

would have mattered in terms of meeting the 1968 code. The PANYNJ adopted guidelines in 1998 that 

required such partitions to provide a continuous fire barrier from top of floor to underside of slab. 

Finding 3.17: Building codes typically require 1 h fire-rated tenant separations but do not impose 

minimum compartmentation requirements (e.g., 7,500 ft2

) for buildings with large open floor plans to 

mitigate the horizontal spread of fire. This is the case with both the 1968 NYC Building Code, which did 

not require above-ground sprinklers, and the 2001 NYC Building Code, which requires sprinklers. The 

sprinkler option was chosen for the WTC towers in preference to the compartmentation option in meeting 

the subsequent requirements of Local Law 5 adopted by New York City in 1973. Thus, if there was only 

one tenant on a WTC floor there would be no horizontal compartmentation requirement. Conversely, if 

there were a large number of tenants on a WTC floor, it would be highly compartmented with separation 

walls. The affected floors in the WTC towers were mostly open—with a modest number of perimeter 

offices and conference rooms and an occasional special purpose area. Some floors had two tenants and 

those spaces, like the core areas, were partitioned (slab to slab). Photographic and videographic 

evidence confirms that even non-tenant space partitions (such as those that divided spaces to provide 

corner conference rooms) provided substantial resistance to fire spread in the affected floors. For the 

duration of about 50 to 100 min prior to collapse of the WTC towers that the fires were active, the 

presence of undamaged 1 h fire-rated compartments may have assisted in mitigating fire spread and 

consequent thermal weakening of structural components. 

Finding 3.18: Availability of state and local building regulations for early installation of sprinklers in 

existing buildings, not as an option in lieu of compartmentation. Functioning sprinklers can provide 

significant improvement in safety for most common building fires and prevent them from becoming large 

fires. NYC promulgated local laws in 1973 and 1984 to encourage installation of sprinklers in new 

buildings, and is now considering a law to require sprinklers in existing buildings. The WTC towers were 

fully sprinklered by 2001, about 30 years after their construction. Sprinklering of the tenant floors in the 

WTC towers was completed by October 1999, while sprinklering of the sky lobbies was still underway at 

that time. The sprinkler system was installed in three phases. Phase 1 was completed during initial 

building construction and included the sub-grade areas. Phase 2 was done in 1976, in compliance with 

Local Law 5, and included sprinklering the corridors, storage rooms, lobbies, and certain tenant spaces. 

Phase 3 was begun in 1983 and completed in 2001 and resulted in fully sprinklering the complex. 
Chapter 1 

38

Finding 3.19: Modern building codes allow a lower fire rating for structural elements when a building is 

sprinklered. This trade-off provides an economic incentive to encourage installation of sprinklers. 

Sprinklers provide better intervention against small and medium fires, fires which are more likely to 

occur than a WTC disaster, as long as the water supply is not compromised and there is redundant 

technology in place. The required technical basis is not available to establish whether the “sprinkler 

trade-off” in current codes adequately considers fire safety risk factors such as: (1) the complementary 

functions of sprinklers and fire-protected structural elements, (2) the different fire scenarios for which 

each system is designed to provide protection, and (3) the need for redundancy should one system fail. It 

is noteworthy that the British Standards Institution (BSI) has established a group to review all the 

sprinkler trade-offs contained in their standards. While the classification and fire rating of the WTC 

towers did not take advantage of the sprinkler-tradeoff since such provisions were not contained in the 

1968 NYC Building Code, had such provisions existed, they would have required a lower fire rating for 

many WTC building elements. 

Occupant Behavior and Evacuation 

The capacity of egress systems in very tall buildings is based on the phased evacuation concept, where 

occupants are evacuated first from the three floors closest to the emergency (e.g., fire), while others wait 

their turn. Such systems require a voice communication system to manage the process from a fire 

command center and, e.g., in New York City, fire wardens on each floor directing the flow. These 

systems are not designed to accommodate a total or full emergency evacuation of the building. 

There were at least three instances where a full evacuation of the WTC towers was ordered; a 

1977 terrorist threat associated with bombings in two midtown Manhattan buildings, during the 

1993 bombing, and on September 11, 2001. During the 1977 event, a full evacuation of the 

WTC complex was ordered at 11:45 a.m., and fire safety teams searched and evacuated 35,000 occupants. 

The evacuation was orderly, and no one was injured. The WTC complex was reopened to the public at 

3:00 p.m. In addition, about 1,500 people were evacuated from 15 floors of WTC 1 during a fire on the 

morning of April 17, 1981. 

Sufficient data do not exist on the frequency with which full evacuations are conducted in buildings not at 

risk for terrorist attacks and whether this frequency has increased since September 11, 2001. In one of the 

three instances, the WTC towers had to be fully evacuated even though the terrorist threat was to other 

remote buildings in the city. Based on NIST interviews of WTC survivors and anecdotal data reported to 

the 9-11 Commission, it appears that since September 11, 2001, building occupants may be more likely to 

evacuate even when the safety risk at their location may not be sufficient to require them to evacuate. It 

is not clear how widespread this sentiment is among the general population that did not experience the 

events of September 11, 2001 directly. 

Finding 3.20: Availability of technical basis for the design of egress systems. Current prescriptive 

methods (e.g., unit width or inches per person) for minimum stair width or number of stairs do not 

provide information on the target performance to be achieved. For example, what would be the 

evacuation rate or time, for a given occupant population, considering travel distance, remoteness 

requirements, and human factors such as occupant size (reflecting current population data), stairwell 

environmental conditions, visibility, and congestion. Also, the technical basis for the “sprinkler 

trade-off”) in egress specifications (generally a doubling of allowed travel distance is not available). 
Interim Findings and Accomplishments 

39

Further, proposals for increases in stair width do not consider the effect of doors in limiting the flow. 

Also, proposals for increases in the number of stairs do not balance the need to meet remoteness 

requirements (physical separation of stairways that are located in separate enclosures) while possibly 

permitting scissor stairs (two separate stairways within the same enclosure and separated by a fire rated 

partition). Scissor stairs are credited as a single stair but provide additional capacity and additional 

doors that achieve real increases in evacuation rate with only minor impact on leasable space. The 

egress capacity in the WTC towers was based on the unit-width method contained in the 1968 NYC 

Building Code; it is not possible to assess the adequacy of the resulting egress capacity to achieve a 

target performance (e.g., evacuation rate or time) under a design-basis evacuation event. Further, 

although the NYC code permitted scissor stairs—which are prohibited in most other codes—none were 

used in the WTC towers or WTC 7. 

Finding 3.21: Consideration of counterflow, e.g., due to first responder emergency access, in the design 

of egress systems. For typical short height buildings, the occupants are evacuated from the affected 

floors within a matter of several minutes, before first responders arrive and climb up the stairs. While 

such evacuation still would be ongoing after arrival of first responders in taller buildings, NIST 

interviews with WTC occupants suggest that on September 11, 2001, with about one-third of building 

occupants present, there were only occasional encounters with first responders, and counterflow was not 

a significant issue. This finding is consistent with the Finding 2.8—based on occupant first-person 

interviews—related to adequacy of the egress capacity in the WTC towers on September 11, 2001. 

Use of Elevators in Emergencies 

Finding 3.22: With a few special exceptions, building codes in the United States do not permit the use of 

fire-protected elevators for routine emergency access by first responders or as a secondary method (after 

stairwells) for emergency evacuation of building occupants. The use of elevators by first responders 

would additionally mitigate counterflow problems in stairwells. While the United States conducted 

research on specially protected elevators in the late 1970s, the United Kingdom has required such 

“firefighter lifts,” located in protected shafts, for a number of years. Without functioning elevators (e.g., 

due to a power failure or major water leakage), first responders carrying gear typically require about a 

minute per floor to reach an incident using the stairs. While it is difficult to maintain this pace for more 

than about the first 20 stories, it would take a first responder about an hour to reach, for example, the 

60th floor of a tall building if that pace could be maintained. Such a delay, combined with the resulting 

fatigue and physical effects on first responders that were reported on September 11, 2001, would make 

firefighting and rescue efforts difficult even in tall building emergencies not involving a terrorist attack. 

Each of the WTC towers had 100 elevators, and WTC 7 had 38 elevators. By code, the elevators could not 

be used for fire service access or occupant egress during an emergency since they were not fireprotected, nor were they located in protected shafts. The elevators were equipped through normal 

modernization with fire service recall. Most were damaged by the aircraft impacts; though prior to the 

impact in WTC 2 the elevators were functioning and contributed greatly to the much faster initial 

evacuation rate in WTC 2 as stated in Finding 2.8. 

Building Practices 

Finding 3.33: While the PANYNJ entered into agreements with the NYC Department of Buildings in the 

1990s (with regard to conformance of PANYNJ buildings constructed in New York City to the NYC 
Chapter 1 

40

Building Code), the PANYNJ did not yield jurisdictional authority for regulatory and enforcement 

oversight to the New York City Department of Buildings. The PANYNJ was created as an interstate 

entity, under a clause of the U.S. Constitution permitting compacts between states, and is not bound by 

the authority of any local, state, or federal jurisdiction. 

Finding 3.34: Availability of rigorous field application and inspection provisions and regulatory 

requirements to assure that the as-built condition of the passive fire protection, such as SFRM, conforms 

to conditions found in fire resistance tests of building components and assemblies. For example, 

provisions are not available to ensure that the as-applied average fireproofing thickness and variability 

(reflecting the quality of application) is thermally equivalent to the specified minimum fire proofing 

thickness. In addition, requirements are not available for in-service inspections of passive fire protection 

during the life of the building. The adequacy of the fireproofing of the WTC towers posed an issue of 

some concern to the PANYNJ over the life of the buildings, and the availability of accepted requirements 

and procedures for conducting in-service inspections would have provided useful guidance. 

Finding 3.35: State and local jurisdictions do not require retention of documents related to the design, 

construction, operation, maintenance, and modifications of buildings, with few exceptions. These 

documents are in the possession of building owners, contractors, architects, engineers, and consultants. 

Such documents are not archived for more than about 6 to 7 years and there are no requirements that 

they be kept in safe custody physically remote from the building throughout its service life. In the case of 

the WTC towers, the PANYNJ and its contractors and consultants maintained an unusually 

comprehensive set of documents, a significant portion of which had not been destroyed in the collapse of 

the buildings but could be assembled and provided to the investigation. In the case of WTC 7, the 

situation was more typical of current practice, and a significant portion of the documents could not be 

assembled since they were lost in the collapse of the building. However, NIST has adequate information 

for its investigation. Neither the original general contractor nor the architects was able to supply more 

than a few documents. 

Finding 3.36: The architect is responsible for specifying the fire protection and designing the evacuation 

system in current building practice. The structural engineer is not required to evaluate and certify that 

the passive fire protection is adequate to protect the structural system. In accordance with established 

practice, the structural engineer was not responsible for the passive fire protection in the design of the 

WTC tower structures. In addition, there is no requirement for a fire protection engineer to be part of the 

team designing the overall fire protection (including detection, suppression, compartmentation, and 

passive fire protection) and evacuation systems for the building. A change in this respect is already 

underway for signature/iconic buildings, where it is becoming more common for a fire protection 

engineer to be included in the design team. In the case of the WTC towers, the building owner played a 

significant role in specifying the fire protection and evacuation systems; a fire protection engineer was 

not part of the original design team. There are only a few academic degree programs or continuing 

education programs that qualify engineers (or architects) to evaluate the fire performance of structures. 

The current state-of-practice is not sufficiently advanced for engineers to routinely analyze the 

performance of a whole structural system under a prescribed design-basis fire scenario.
Interim Findings and Accomplishments 

41

1.6 APPROACH TO RECOMMENDATIONS 

In the United States, state and local governments are responsible for promulgating and enforcing building 

and fire regulations. While states are increasingly adopting a single, uniform set of statewide regulations, 

in many instances major cities and counties adopt their own regulations. The national organization 

representing state building officials is the National Conference of States on Building Codes and Standards 

(NCSBCS)—a body of the National Governors Association—and that representing building officials of 

major local jurisdictions is the Association of Major City/County Building Officials (AMCBO). 

With some exceptions, the state and local regulations are based on national model building and fire codes 

developed by private sector organizations, the International Code Council (ICC) and the NFPA. The 

model codes, in turn, reference voluntary consensus standards developed by a large number of private 

sector standards development organizations (SDOs). They include organizations such as NFPA, ASTM 

International, the American Society of Civil Engineers (ASCE), the American Institute of Steel 

Construction (AISC), and the American Concrete Institute (ACI). The SDOs are accredited by the 

American National Standards Institute (ANSI). 

Other key stakeholder groups involved in the design, construction, operation, and maintenance of 

buildings include organizations representing building owners and managers, real estate developers, 

contractors, architects, engineers, suppliers, and insurers. 

NIST is a non-regulatory agency of the U.S. Department of Commerce. NIST does not set building codes 

and standards, but provides technical support to the private sector and other government agencies in the 

development of U.S. building and fire practices, standards, and codes. NIST recommendations are given 

serious consideration by private sector organizations that develop national standards and model codes – 

which provide minimum requirements for public welfare and safety. The model codes become regulation 

when adopted by state and local governments. 

The NIST building and fire safety investigation of the WTC disaster has not yet formulated 

recommendations under this objective. However, in formulating its recommendations, NIST will 

consider the following: 

• Findings from the first three independent investigation objectives related to building 

performance, evacuation and emergency response, and procedures and practices. 

• Whether findings relate to the unique circumstances surrounding the terrorist attacks of 

September 11, 2001, or to normal building and fire safety considerations, including 

evacuation and emergency response? 

• What technical solutions are needed, if any, to address potential risks to buildings, occupants, 

and first responders, considering both identifiable hazards and the consequences of those 

hazards? 

• Whether the risk is in all buildings or limited to certain building types (e.g., height and area, 

structural system), buildings that contain specific design features, iconic/signature buildings, 

or buildings that house critical functions? 
Chapter 1 

42

NIST urges organizations responsible for building and fire safety at all levels to carefully consider the 

interim findings contained in this report. NIST welcomes comments from technical experts and the 

public on the interim findings presented herein. Comments can be sent by e-mail to wtc@nist.gov, 

facsimile to 301-975-6122, or regular mail to WTC Technical Information Repository, Stop 8610, 

100 Bureau Drive, Gaithersburg, MD 20899-8610. 

In its final report, a draft which is expected to be released in December 2004, NIST will recommend 

appropriate improvements in the way buildings are designed, constructed, maintained and used. It will be 

important for those recommendations to be thoroughly and promptly considered by the many 

organizations responsible for building and fire safety. As a part of NIST’s overall WTC response plan, 

the Institute has begun to reach out to those organizations to pave the way for a timely, expedited 

consideration of recommendations stemming from this investigation. NIST also already has expanded its 

research in areas of high priority need. 
43

Chapter 2

PROGRESS ON THE WORLD TRADE CENTER INVESTIGATION

2.1 STATUS OF DATA COLLECTION EFFORTS 

The National Institute of Standards and Technology (NIST) is basing its review, analysis, modeling, and 

testing work for the World Trade Center (WTC) Investigation on a solid foundation of technical evidence. 

This requires access to critical data such as building documents, videographic and photographic records, 

emergency response records, and oral histories, in addition to the samples of steel that have been 

recovered. 

NIST has received considerable cooperation and large volumes of information from a variety of 

organizations and agencies representing the building designers, owners, leaseholders, suppliers, tenants, 

first responders, contractors, insurers, news media, survivors, and families of victims. In addition, NIST 

has received and is grateful for cooperation from The National Commission on Terrorist Attacks Upon 

the United States (9-11 Commission). The documents and other information relate to the design, 

construction, operation, inspection, maintenance, repair, alterations, emergency response, and evacuation 

of the WTC complex. 

Local authorities providing information include the Port Authority of New York and New Jersey 

(PANYNJ or Port Authority) and its consultants and contractors; the New York City Fire Department 

(FDNY); the New York City Police Department (NYPD); the New York City (NYC) Law Department; 

the NYC Department of Design and Construction; the NYC Department of Buildings; and the NYC 

Office of Emergency Management. In addition, the Occupational Safety and Health Administration 

provided correspondence sent to it regarding the evacuation experience of WTC occupants on 

September 11, 2001. 

NIST also has received information from Silverstein Properties and its consultants and contractors; the 

group of companies that insured the WTC towers and its technical experts; Nippon Steel; Laclede Steel; 

U.S. Mineral Products Co. and Isolatek International; Morse Zehntner Associates; W.R. Grace & Co.; 

Citigroup, formerly Salomon Smith Barney; United Airlines; American Airlines; and Boeing. NIST also 

received information on floor plans, furnishings, and contents from tenants of all three buildings. 

The information from Silverstein and the insurance companies includes the large body of technical work 

completed by both parties as part of the insurance litigation involving the WTC towers, such as reports on 

the structural collapse, fire spread and severity, and wind tunnel test results for the WTC towers. In 

addition, technical experts for both parties independently provided extensive briefings to the WTC 

investigation team and discussed the tenability environment and the evacuation procedures in the 

buildings. 

NIST has received all of the essential information it needs for the WTC investigation. That 

information includes NYC 9-1-1 tapes, the transcripts of approximately 500 interviews of employees of 

the FDNY who were involved in WTC emergency response activities, and supporting documents for 

McKinsey & Company’s FDNY study. 
Chapter 2 

44

The following is the list of documentary information received or inspected by NIST. 

December 2002

• The original design drawings (structural, architectural, mechanical, electrical, plumbing) and 

the original fabrication and construction drawings for the WTC towers 

• Tenant alteration application reports, including drawings and specifications, for the WTC 

towers and WTC 7, and associated construction audit reports 

• Tenant design standards manuals for structural; architectural; heating, ventilating, and airconditioning (HVAC); fire protection; plumbing; electrical; fire alarm; and construction 

review 

• Emergency evacuation procedures manuals, including fire safety guide 

• Operations manuals for the fire protection system, including sprinklers, standpipes, alarm 

system and communication protocols, and water and power supply 

• Operations manuals for the HVAC systems 

• Reports on facility condition surveys and structural integrity inspections for the WTC towers 

and WTC 7 

• Recent inspection and maintenance reports for the elevators and escalators in the WTC 

towers; elevator numbering system 

• Reports on pre-design tests of structural components, including dampers for the WTC towers 

• Reports on wind tunnel tests of the WTC towers and wind speed measurements near the 

WTC site 

• Reports on the 1993 bombing damage assessment and repairs, and documentation of changes 

made to the evacuation system after 1993 

• Documents related to the location, approval, and inspection of fuel tanks in WTC 7 

• Documents related to fire rating and fireproofing of structural steel members in the WTC 

towers 

• Documents related to PANYNJ building and fire code requirements and practices 

• Correspondence sent to the Occupational Safety and Health Administration regarding the 

evacuation experience of WTC occupants on September 11, 2001 

• Documents related to the lease of the WTC towers by Silverstein Properties 

• Reports prepared by McKinsey & Company for FDNY and NYPD 
Progress on the World Trade Center Investigation 

45

• Basic FDNY dispatch data, including time of dispatch and unit identification 

• Firefighter fatality and injury data from FDNY 

May 2003

• More than 1,000 hours of recordings made by PANYNJ on September 11, 2001 (from 0705 

through 1900 hours) of telephone calls, as well as police, fire, operations, maintenance, 

security, and other radio transmissions from four distinct locations 

• Personal injury data from FDNY and Port Authority of New York and New Jersey Police 

Department (PAPD) 

• Handwritten notes on the events of September 11, 2001, by PAPD staff 

• Emergency responder fatality data for FDNY, NYPD, and PAPD 

• WTC list of tenants with contact information from PANYNJ and Silverstein 

• WTC list of occupants issued security badges by PANYNJ 

• Report on WTC smoke management system by Hughes Associates, Inc. 

• Phase I and final reports on fire engineering of WTC steelwork by Buro Happold 

• Transcripts of depositions by two PANYNJ staff in the WTC insurance litigation 

• Documents, videos, and photographs related to the fireproofing of the WTC tower structures 

• WTC floor plan for the fire alarm system and drawings of WTC subgrade plumbing and city 

water main 

• Information regarding building contents such as partitions and furnishings from a key WTC 

tower tenant, to characterize the types of combustibles and estimates of the mass loading in 

the region of the fires 

• FDNY WTC incident summary, September 20, 2001 

• FDNY reports on the fire history of WTC 1, 2, and 7 from 1970 to 2001 

• FDNY reports related to inspections of WTC 1, 2, and 7 from 1999 to 2001 

• FDNY policies and practices on operations specific to the WTC buildings and on 

accountability of firefighters at incidents 

• FDNY information on dispatched units, apparatus, command posts, and staging areas 

• FDNY information on number of command and company officers and firefighters operating 

in and around WTC 1, 2, and 7 with number of surviving personnel 
Chapter 2 

46

• Detailed briefing on the NYPD communications system, including 9-1-1 system and radio 

networks 

August 2003

• Design and structural calculations from Leslie E. Robertson Associates (LERA) for the WTC 

towers, including TV antenna, beams, and beam girders, as well as wind analysis and 

calculations 

• Correspondence from LERA during the time of construction 

• Laclede floor truss shop drawings (1,364 sheets) and other documents on steel and joints 

• Information on steel from Nippon 

• List of WTC drawings in possession of Yamasaki and Associates 

• Information on the flammable contents of the American Airlines B-767 aircraft 

• Information regarding building contents and floor layouts from some WTC tower and 

WTC 7 tenants 

• Mechanical and electrical specifications for WTC 7 

• Asbestos litigation documents from PANYNJ 

• Underwriters’ Laboratories, Inc. (UL) test reports regarding spray-on fireproofing from 

supplier (Isolatek) 

• Correspondence on the selection of WR Grace fireproofing products, test data, and UL design 

listings (WR Grace) 

• Data on the WTC internal radio system and FDNY radio repeater from PANYNJ 

• Some FDNY training practices for operations in high-rise buildings 

• Global positioning system coordinates and map where human remains and equipment were 

located from FDNY 

• Information on FDNY personnel killed on September 11, 2001, and map of fire and 

Emergency Management Services Command Post Locations 

• NYPD internal communications concerning the terrorist attacks on WTC (43 cassette tapes) 

• Disaster Response Plan, Patrol Guide Procedures, and other guides and manuals from NYPD, 

including the Unusual Occurrence Report on the 1993 WTC bombing 

• A large portion of NYPD and FDNY extensive photographic and videographic collection 
Progress on the World Trade Center Investigation 

47

• Updated badge list of WTC occupants maintained by PANYNJ 

• WTC fire safety and PA/FDNY WTC training videos and pre-September 11, 2001 

WTC photographs 

September 2003

• Information on the flammable contents of the United Airlines B-767 aircraft 

• Documents from PANYNJ on accessibility for disabled persons, active fire protection 

systems, and adoption of revisions to NYC Building Code 

• Elevator and escalator contract information from PANYNJ 

• Status of changes to WTC towers (March 1973) from PANYNJ 

• Transcripts from September 11 PAPD audiotapes, police reports, and PAPD special awards 

ceremony documents for September 11, 2001 

• Additional documents from PANYNJ on asbestos litigation 

October 2003

• Supporting documents for McKinsey & Company’s FDNY and NYPD studies 

• Review of UL test reports regarding spray-on fireproofing from supplier (W.R. Grace) 

• Information from Boeing on flammable contents of aircraft that contributed to fires 

May 2004

• Review of NYC 9-1-1 tapes and logs, transcripts of about 500 first responder interviews with 

employees of the FDNY who were involved in WTC emergency response activities 

• General description of WTC building systems and capital program 

• WTC documents presented as exhibits in asbestos litigation 

• Additional documents on WTC maintenance services, accessibility, elevators, code 

compliance, fire rating, fire detection system, fire alarm system, etc. 

• Photographs of WTC 7 construction project 

• Architectural and HVAC drawings for WTC 7, including modifications 

• Well in excess of 6,000 photographs representing more than 185 professional and amateur 

photographers. Organizations that have provided materials include FDNY, NYPD, 

Associated Press, Corbis, Reuters, The New York Times, The New York Daily News, and the 
Chapter 2 

48

Star Ledger. Many organizations have provided both published and unpublished 

photographs. 

• In excess of 150 hours of videotapes from news media (NBC, CBS, ABC, CNN, and local 

New York stations WABC, WCBS, WNBC, WPIX, WNYW, and New York One), FDNY, 

NYPD, and more than 20 individuals. In many cases, the videos provide not only broadcast 

material (known as air checks), but also material that was recorded but not broadcast (known 

as outtakes). 

The few NIST requests for materials that are lost, currently pending, or not yet located include: 

• Original contract specifications for WTC towers (lost in the collapse of the buildings) 

• Construction and maintenance logs for WTC 1, 2, and 7 (lost in the collapse of the buildings) 

• Calculations and analyses that supported the original aircraft impact studies (lost in the 

collapse of the buildings) 

• Descriptions of partitions and furnishings in most of the tenant spaces of WTC 2 and WTC 7 

in the fire and impact zones 

• Shop drawings showing connection details of WTC 7 

NIST is making efforts to assemble this information from various sources because much of it was lost 

when the buildings collapsed. NIST continues to pursue other materials that can further clarify some 

aspects of the Investigation. 

2.2 ANALYSIS OF BUILDING AND FIRE CODES AND PRACTICES 

(PROJECT 1) 

2.2.1 Project Objective 

One of the four primary objectives of the NIST Investigation of the WTC disaster is to determine the 

procedures and practices that were used in the design, construction, operation, and maintenance of the 

WTC towers and WTC 7. A key focus is on acceptance procedures and practices for innovative systems, 

technologies, and materials, and for variances from requirements of building and fire code provisions. 

This documentation of historical information is expected to be of value to the professional community in 

identifying and adopting changes to procedures and practices that may be warranted. 

For most buildings constructed in the United States, building codes adopted by local jurisdictions 

establish minimum requirements for design and construction. However, because PANYNJ is an interstate 

agency, its construction projects are not required to comply with any local or national model building 

code. Thus, to determine the criteria, procedures, and practices that were used in the design, construction, 

operation, and maintenance of WTC 1, 2, and 7, Project 1, Analysis of Building and Fire Codes and 

Practices, has the following objectives: 

• Document the requirements that governed the design and construction of WTC 1, 2, and 7 
Progress on the World Trade Center Investigation 

49

• Document any differences between the Port Authority requirements used for design and the 

then current building code requirements of other jurisdictions and the appropriate model 

building code 

• Document the procedures used by the Port Authority to accept new and innovative design 

features that deviated from the Port Authority building design requirements 

• Document the procedures used to accept new technologies and materials that were not 

recognized by then-current standards 

• Document passive and active fire safety, emergency access and egress provisions that were 

incorporated in the original design and subsequent modifications during occupancy 

• Document major modifications made to structural, fire protection, and egress systems of 

WTC 1, 2, and 7 

• Document the inspection and maintenance procedures used for WTC 1, 2, and 7. 

2.2.2 Project Approach 

The design and construction documents of the WTC buildings that were kept centrally at the Port 

Authority office in WTC 1 were destroyed when the tower collapsed. Thus, existing copies of design and 

construction documents of WTC 1, 2, and 7 had to be assembled from various sources that were 

associated with these projects. Documents were obtained principally from: 

• The Port Authority, and 

• Architectural and engineering firms who designed and inspected the WTC buildings. 

In addition, information was obtained from others who were associated with WTC 1, 2, and 7 

construction projects. 

The information collected will enable the NIST investigators to accomplish the five tasks of Project 1: 

• Task 1. Document the design and construction of structural systems to determine: 

− Provisions used to design and construct the buildings. This will include the Port 

Authority building design and construction requirements, the building code used, 

standards referenced, and Port Authority policies and agreements with the NYC 

Department of Buildings regarding building code requirements. 

− Tests performed to support the design, such as wind tunnel tests and tests of structural 

assemblies. 

− Criteria used to proportion structural members and other components of the buildings 

including structural connections. 
Chapter 2 

50

− Innovative systems, technologies, and materials that were used, and the acceptance 

procedures used by the Port Authority. 

− Variances granted by the Port Authority, including the justification for those variances. 

− Special fabrication and inspection requirements. 

− Inspection protocols used during construction. 

− Technical problems that occurred during construction of the buildings and their 

resolution. 

• Task 2. Document the design and construction of the fire protection and egress systems to 

determine: 

− Provisions used to design and construct the fire protection (passive and active) and egress 

systems of the buildings. This will include the Port Authority building and fire regulatory 

requirements, the building and fire code used, and standards referenced. 

− Building regulations adopted after the issuance of the certificates of occupancy, or 

equivalent, that were applied to the buildings through retroactivity (including any 

provisions of NYC Local Laws), and any permits issued or special inspections required 

resulting from the installation of special hazards or equipment in the buildings. 

• Task 3. Document the fuel system for emergency power in WTC 7 to determine: 

− Locations of emergency power generating systems; 

− Size and locations of the fuel storage tanks and distribution systems; 

− Specific fire protection systems used for the fuel storage and distribution systems; 

− Normal and emergency operating procedures; and 

− Maintenance history. 

• Task 4. Compare building regulatory and code requirements to document: 

− Port Authority building regulatory requirements. 

− Differences among the Port Authority building regulatory requirements and the thencurrent NYC, New York State, Chicago, and Building Officials Conference of America 

(now known as the Building Officials and Code Administrators [BOCA]) building code 

provisions. 

− Differences between the Port Authority building regulatory requirements and the current 

(2001) NYC Building Code provisions. 
Progress on the World Trade Center Investigation 

51

− Evolution of the life safety provisions in the NYC Building Code since the design of 

WTC 1 and WTC 2. 

• Task 5. Document maintenance of and modifications to the structural, fire protection, and 

egress systems to determine: 

− Guidelines used by the Port Authority for inspection, repair, and modifications to 

structural, fire protection, and egress systems. 

− Structural integrity inspection programs during the occupancy of the buildings. 

− Any significant modifications and/or repairs of the original structural framing system by 

the owner or tenants during original construction and occupancy. 

− Any repairs and modifications made to the passive and active fire protection systems 

from initial occupancy to September 11, 2001. 

2.2.3 Status of Tasks 

Except for the task of comparing building regulatory and code requirements (Task 4), the tasks depend 

upon the availability of design, construction, and maintenance documentation related to WTC 1, 2, and 7. 

Efforts by NIST to obtain needed documents are described briefly, and the current status of each of the 

five tasks is stated below. 

Sections 2.2.4 and 2.2.5 present salient points related to Tasks 1 and 2, particularly how changes in NYC 

Building Code provisions from the 1938 edition to the 1968 edition, and subsequent promulgation of 

NYC Local Laws, affected the design, construction and maintenance of WTC 1, 2, and 7. Section 2.2.6 

describes the fuel system that powered the emergency generators in WTC 7 (Task 3). Comparison of the 

requirements of several building codes (Task 4) pertaining to structural and fire safety is presented in 

Appendix A, Interim Report on the Analysis of Building and Fire Codes and Practices, of this report. 

Task 5, which deals with maintenance and modifications to the structural, fire protection, and egress 

systems of WTC 1, 2 and 7, is near completion. 

Collection of Design and Construction Data 

NIST requested that the Port Authority and the design and construction firms for WTC 1, 2, and 7 provide 

design, construction, and maintenance documents. NIST obtained a considerable amount of information 

(design drawings, shop drawings, specifications, project correspondence, and inspection reports) related 

to WTC 1 and WTC 2 from the structural engineers who were involved in the original design and 

subsequent modifications to the towers. The Port Authority provided construction related files for 

WTC 1, 2, and 7, mostly pertaining to tenant alteration projects, wherein tenants modified parts of the 

buildings to meet their needs. No document was obtained from the general contractor of WTC 1, 2, and 7. 

The general contractor retains construction documents for about 7 years. As a result, few records are 

available related to changes to the structural and fire safety systems that were made during construction of 

these buildings. However, documents obtained from the structural engineer included revisions to 

structural modifications for tenant renovations. 
Chapter 2 

52

It should be pointed out that there is no mandated requirement by NYC for design and construction firms 

to retain their documents for a specific duration. Currently, no municipalities in the United States have 

document retention policies that require design and construction firms to retain their documents. 

Collected documents have been examined by NIST, and pertinent documents have been organized into a 

searchable database. Using keywords, the user of the database can retrieve relevant documents. NIST 

has engaged a contractor to assist in reviewing the vast amount of documentation that has been collected. 

Task Status and Report Preparation 

Under NIST guidance and direction, a team of NIST contractors, led by Rolf Jensen and Associates, Inc., 

has made an in-depth review of the relevant documents and is in the process of preparing draft reports that 

address the Project 1 tasks. Independent examination of documents by NIST engineers together with the 

contractor’s reports will be the basis for the final report of this project. Table 2–1 indicates the status of 

each of the tasks. 

Table 2–1. Status of Project 1 tasks. 

Task Status 

1 Documented code provisions used to design and construct WTC 1, 2 and 7. 

Documented criteria used to design WTC 1 and WTC 2. 

Contractor submitted final draft report to NIST. 

2 Documented code provisions used to design and construct the passive and 

active fire protection systems for WTC 1, 2, and 7. Documented adoption of 

Local Laws that modified and/or amended the fire safety provisions of the 

1968 NYC Building Code. 

Contractor submitted second draft report to NIST. 

3 Documented all emergency power generating systems and the size and 

locations of the fuel storage tanks and distribution system in WTC 7. 

Contractor submitted final draft report to NIST. 

4 Documented line-by-line comparison of structural and life safety provisions 

of the 1968 NYC Building Code vs. three other contemporaneous building 

codes and the 2001 NYC Building Code. 

Contractor submitted final draft report to NIST. 

5 Documented repairs, modifications to the structural and fire protection 

systems, and emergency access and egress systems. 

Contractor submitted final draft to NIST. 

2.2.4 Building Codes 

As discussed in Appendix A, the Port Authority adopted the 1968 NYC Building Code (NYCBC 1968) 

for the final design of the WTC buildings. Therefore, this code served as the basis for the code 

comparison. NIST examined the structural and fire safety provisions in several contemporaneous codes, 

including the 1964 New York State Building Construction Code (NYSBC 1964), the 1965 BOCA model 

building code (Basic Building Code), the 1967 Municipal Code of Chicago (MCC 1967), and the 1966 

National Fire Protection Association (NFPA) 101 Life Safety Code (NFPA 1966) egress requirements. A 

comparison was also made between the 1968 NYC Building Code and the current (2001) NYC Building 
Progress on the World Trade Center Investigation 

53

Code. The current NYC Building Code (NYCBC 2001) is basically the document adopted in 1968 with 

modifications made over the years by adoption of Local Laws and rules. 

Because fire protection and fire safety provisions are of major importance to this Investigation, this 

section provides background information on building codes, focusing on matters related to fire protection. 

Appendix A provides additional background information and provides a summary of the code 

comparison. 

Code Provisions on Fire Safety 

The fire safety provisions in building codes can be confusing to those who are not familiar with the code 

provisions that have evolved over the past century. The following provides basic concepts related to fire 

protection and fire safety. 

Fire Rating 

This is a time expressed in hours or minutes. It represents fire resistance assigned to a building element 

on the basis of a test. Fire rating is the time that a test assembly is able to withstand the furnace 

temperature exposure specified in American Society for Testing and Materials (ASTM) E 119 

(ASTM 2003) without exceeding one of the “failure conditions.” Thus, a fire rating of 2 h for a structural 

member indicates that when the member was tested in accordance with ASTM E 119, it performed 

successfully for at least 2 h. It is not necessary to conduct a test for all materials to be used in a building 

if data are available from past standard tests showing acceptable performance, and the same materials and 

application methods will be used. The fire rating of a member or assembly does not indicate for how long 

a similar member or assembly in the building would perform under a real fire because the actual fire 

exposure and structural configuration are never the same as during the ASTM E 119 test. 

Occupancy Group 

Buildings and spaces are classified according to how the buildings and spaces will be used. The concept 

of “occupancy group” is used to define different types of occupancy or use such as storage, industrial 

buildings, general assembly, business, and so forth. A given occupancy group is associated with a 

different level of fire risk. Factors such as amount of combustible material, ignition sources, and 

characteristics of the occupants are considered in developing these groupings. In some codes, such as the 

1968 NYC Building Code, occupancy groups are listed in a hierarchal sequence (highest to lowest 

hazard) and assigned an overall “fire index” rating in hours. For example, “high hazard” occupancy is 

assigned a fire index of 4 h, while “business” occupancy is assigned a fire index of 2 h. 

Construction Classification 

The nature of the materials used in constructing exterior walls and interior building elements define the 

construction type. In general, building codes characterize construction materials as “combustible” and 

“noncombustible” materials. For example, the 1968 NYC Building Code uses the designations “Group I” 

and “Group II” for noncombustible and combustible construction materials, respectively. Other codes, 

however, also consider whether these types of materials are used in exterior walls, or interior elements, or 

both. For example, International Building Code (IBC) 2000 (ICC 2000) lists Type I through Type V. 

These types, or groups, are further divided into different subclasses that are designated with letters, such 
Chapter 2 

54

as Class IA, Class IB, Class IIA, Class IIB, or in some codes as Type IA, Type IB, Type IIA, Type IIIA, 

and so forth. Building codes typically include tables that specify fire ratings for different structural 

elements (columns, walls, beams) and for different construction classifications (Type IA, Type IB, and so 

forth). 

Height and Area Limits 

Tall buildings and buildings with large floor areas pose greater risks in the event of fire. Building codes, 

therefore, place limitations on the heights and floor areas of buildings based on the construction 

classification, the occupancy group, and whether or not the building has sprinklers. 

Partitions 

Partitions, in general, are walls that provide separations between spaces within the story of a building. 

Partitions may or may not require minimum fire ratings, depending on the spaces being separated. In 

general, three types of partitions require fire ratings: 

• Walls that provide separations between different occupancy groups 

• Walls that provide separations between tenants (often called demising partitions) 

• Walls that separate large floor areas into smaller compartments 

Buildings codes specify different minimum fire ratings based on the type of partition and the types of 

occupancies in the spaces being separated. 

Variances 

All building codes make allowances for obtaining approval of materials and methods not strictly in 

compliance with code provisions, but which are judged to be equivalent. Normally, the regulatory 

authority makes this equivalence determination during the plan approval process, and a variance is issued. 

Compliance with building code provisions is verified by controlled inspections by building (and fire) 

inspectors at specified points in the construction process using sets of approved plans showing all such 

variances. 

Evolution of Fire Safety Provisions in Model Building Codes 

Model building codes are documents prepared by qualified nongovernmental organizations. When 

adopted by local jurisdictions, the code provisions become law. The following provides a short review of 

the evolution of provisions in model codes related to types of construction and fire resistance 

requirements of structural elements.6

The 1927 edition of the Uniform Building Code (ICBO 1928) placed office occupancies in Group F 

Division 1 and allowed only Type 1 construction for such buildings. This required a 4 h fire rating for 

 

6

 This historical summary was provided by Joseph Messersmith of the Portland Cement Association with a letter of July 31, 

2002. See also Messersmith (2002). 
Progress on the World Trade Center Investigation 

55

columns, beams, and girders and a 3 h rating for floors for buildings (steel and reinforced concrete) over 

eight stories or 85 ft in height. 

The 1934 (5th) edition of the National Building Code (NBFU 1934) required business buildings over 

75 ft in height to be “fireproof” construction, defined as having a 4 h fire rating for bearing walls, 

firewalls, party walls, piers and columns, and a 3 h rating for other walls, girders, beams, and floors. 

The 1946-1947 edition of the Southern Standard Building Code (SBCC 1946) required Type 1 

construction for business occupancies over 80 ft in height. Type 1 was defined as a 4 h fire rating for 

columns, bearing walls, trusses or girders supporting masonry or bearing walls, columns or girders, and 

beams, and a 2 1/2 h rating for floors. There is, however, a note under Type 2 construction that allows 

residential and business occupancies of unlimited height with a rating of 3 h for columns and a rating of at 

least 2 h for other structural members including floors. 

The 1950 Basic Building Code (BOCA 1950) permits Group E business occupancies of unlimited height 

to be Type 1A or 1B. Type 1A requires a 4 h fire rating for bearing walls and for columns supporting 

more than one floor, and 3 h rating for floors including beams. Type 1B reduces those to 3 h and 2 h, 

respectively. Because this is the model code used in the Northeastern United States, this may have 

provided the basis for the changes from the 1938 to the 1968 NYC Building Code (see next section). 

However, because records of technical substantiations for code changes were not kept in this era, 

conclusive evidence does not exist. 

Evolution of the NYC Building Code 

Historically, NYC has developed and promulgated its own building code, in contrast to most jurisdictions 

that adopt (locally modified) versions of one of the model building codes. At the time the WTC project 

was begun (early 1960s), the 1938 NYC Building Code, which was first adopted January 1, 1938, was in 

effect and enforced throughout the five boroughs. 

In the late 1950s, it was noted that “great changes have occurred in all facets of the building industry” and 

that “As a result of these developments, and the failure in many instances, of the Code to keep pace, there 

had been a growing dissatisfaction with it” (Schaffner 1964). Thus in 1960, the Building Commissioner 

requested the New York Building Congress to form a working committee to study the problem. The 

committee recommended that the Code should not be rewritten by a group of volunteers and that a local 

educational institution should conduct a study to develop an approach to solve the problem. The 

Polytechnic Institute of Brooklyn conducted the study, and in July 1961, the Institute made the following 

recommendations (Schaffner 1964): 

1. The NYC Building Code be completely rewritten. The new Code 

should provide for frequent periodic revision through a committee or 

board appointed solely for this purpose. 

2. The new Code be a combination of performance and specification 

types with heavy emphasis on performance, wherever possible, and 

with liberal reference to accepted national standards. 

3. The BOCA Basic Building Code be used as a guide for the 

development of the NYC Building Code. 
Chapter 2 

56

4. The Code be rewritten by a private professional group such as an 

engineering company, architectural firm, educational institution, or 

any combination of the three. Those rewriting the Code should work 

closely with the NYC Building Department. They should be 

supported, for review purposes, by volunteer committees composed 

of representatives of professional, trade, and industry associations.” 

In April 1962, NYC signed an agreement with the Polytechnic Institute of Brooklyn for the writing of a 

new Code to be completed in 3 years. The first draft was completed in 1964. A public relations 

document highlighted the “major advantages to be gained from recommendations in the proposed new 

Building Code” (Bell and Stanton 1964). One of these related to the “area and height limitations,” and it 

was stated that: 

Area and height limitations will be liberalized and present unrealistically 

high construction requirements for fire protection in structures of low 

combustible content such as auditorium, halls, schools, institutions and 

residences will be significantly reduced and considerable economy will 

result. 

On December 6, 1968, Local Law 76 repealed the 1938 code and replaced it with the 1968 code, which 

itself was subsequently amended by Local Laws. As is the general custom with changes to building 

codes, the new provisions generally are not applied to existing buildings (those approved under the prior 

code) provided they do not represent a danger to public safety and welfare. 

There were 79 Local Laws adopted between 1969 and 2002 that modified the 1968 code. Of particular 

importance with regard to fire protection and life safety are Local Law 5, adopted in 1973, and Local 

Law 16, adopted in 1984. Local Law 5, among other things, added requirements on compartmentation of 

large floor areas, and Local Law 16 added requirements for sprinklers in high-rise buildings (greater than 

100 ft). Local Law 5 is particularly significant because its provisions, which are reviewed in a subsequent 

section, applied retroactively to existing office buildings. Local Law 84, which was passed in 1979, 

revised the compliance dates of Local Law 5 so that full compliance was required by February 7, 1988. 

As discussed in Appendix A, the 1968 NYC Building Code contained a number of provisions not 

addressed in the other codes of the time, but which were added to these other codes at later times. 

Selection of Construction Type for WTC Towers 

The 1938 NYC Building Code recognized one construction type for buildings of unlimited height and 

area, namely Class 1—Fireproof Structures, which required a 4 h fire rating for columns and a 3 h rating 

for floors. In the 1968 code, Group I (Noncombustible) construction was subdivided into 

“Class 1A—4-hr protected” and “Class 1B—3-hr protected” construction. Class 1A specifies similar 

protection as the previous Class 1, and Class 1B specifies a 3 h rating for columns and girders supporting 

more than one floor and a 2 h rating for floors including beams. Both Class 1A and Class 1B construction 

permit unlimited height and area for unsprinklered business occupancy. 

If a building qualifies for more than one construction classification, such as Class 1A or Class 1B, codes 

are silent on which classification should be used. In such situations, the classification selected for 

construction is at the discretion of the owner/architect. To date, no contemporaneous documentation has 
Progress on the World Trade Center Investigation 

57

been found that provides the rationale for the decision to select Class 1B for the WTC towers. This 

decision, however, appears to have been made by the architect-of-record on the basis of economics. In a 

1987 memorandum on the subject of fire rating of the WTC buildings, the following statement was 

included (Feld 1987): 

For office buildings there is no economic advantage in using Class 1A 

Construction, and ER&S [architect-of-record] used Class 1B 

Construction for the WTC Towers and Plaza Buildings which are 

Occupancy Group “E” (Business) with a fire index of 2 hours. 

An interoffice memorandum between staff of the general contractor written in 1969 is the only 

contemporaneous document found to date that refers to the classification of the WTC towers 

(Bracco 1969). The following statement is included in that memorandum: 

The WTC towers would be classified, by our interpretation of the code, 

as occupancy Group E, Business; Construction Group 1, Noncombustible; and Construction Classification 1-B, (since there are no 

area or height limitations applicable). 

2.2.5 The 1968 New York City Building Code 

Applicability to Port Authority Properties 

Established in 1921, the Port Authority is a self-supporting, public interstate agency and is not subject to 

the local laws of jurisdictions where its properties are constructed. This means that for the construction of 

the WTC buildings, the Port Authority was not bound by the NYC Building Code or any regulations 

requiring inspection or approval of the building construction or operation. The Port Authority could 

establish its own requirements, conduct its own inspections, and enforce its own rules without 

independent oversight. 

According to a joint report written by the Fire Commissioner and Commissioner of Buildings on 

March 15, 1993 (after the 1993 bombing), in 1975 the NYC Council submitted a resolution to the New 

York State Legislature to require the Port Authority to comply with the NYC Building Code when 

building within the City (Rivera and Rinaldi 1993). The 1993 report includes the following statements 

with respect to jurisdiction over the WTC complex: 

After several major fires in the 1970s, the Fire Department in 1975 

testified at the City Council for the need to have jurisdiction over this 

complex as well as other buildings owned by public benefit corporations, 

again particularly for Local Law 5 compliance. As a result, the City 

Council forwarded a Resolution dated August 29, 1975, to the State 

legislature. ... Proposed legislation which would have granted City 

agencies jurisdiction was introduced in the State legislature over the 

years; the State has not enacted such legislation. 
Chapter 2 

58

It appears that there was friction between the FDNY and the Port Authority as evidenced by the following 

statements in the same joint report (Rivera and Rinaldi 1993): 

Prior to the February 26, 1993, explosion, the Fire Department acted 

pursuant to the joint protocol for inspectional activity at the WTC which 

was signed in 1986. The Port Authority’s policy was to voluntarily 

cooperate with the Fire Department ‘to the fullest extent practicable.’ 

Fire Department representatives met continually with Port Authority 

officials to discuss problems with the WTC’s emergency procedures and 

fire safety equipment. Generally, the Port Authority was cooperative and 

verbally informed the Fire Department that it was their intent to fully 

comply with Local Law 5. However, since its compliance with fire code 

requirements was dependent upon economic and design feasibility, the 

PA agreed to comply with selected provisions of the code, but has not 

fully done so. Moreover, it was difficult for the Fire Department to 

monitor code compliance by the WTC because the WTC consistently 

asserted its legal exemption from local law. Fire officials relied on 

persuasion and negotiation to gain compliance. The extent of these 

negotiations is reflected in the voluminous WTC files maintained at the 

Fire Department. Code compliance at the WTC has been dealt with by 

every Fire Commissioner and Chief of the Department over the last 

twenty-five years. 

It was not until 1993 that a formal agreement was reached between the Port Authority and the NYC 

Department of Buildings with regard to code conformance for Port Authority buildings constructed in 

NYC (PANYNJ and NYCDOB 1993). The introduction of the memorandum of understanding contained 

the following statements: 

While the facilities of the Port Authority, an agency of the States of New 

York and New Jersey, are not technically subject to the requirements of 

local building codes, the long-standing policy of the Port Authority has 

been to assure that its facilities meet and, where appropriate, exceed 

Code requirements. 

The purpose of this Memorandum is not only to restate that longstanding 

policy as part of an understanding with the City but to provide specific 

commitments to the Department, as the agency of the City responsible 

for assuring compliance with the Code, regarding procedures to be 

undertaken by the Port Authority for any Project at its facilities in the 

City to assure that the buildings owned or operated by the Port Authority 

within the City are in conformance with the Building Standards 

contained in the Code. 

Some salient points included in this agreement are: 

• Each project would be reviewed and examined for compliance with the Code; 

• All plans would be prepared, sealed, and reviewed by New York State licensed professional 

engineers or architects; and 
Progress on the World Trade Center Investigation 

59

• The Port Authority engineer or architect approving the plans would be licensed in the State of 

New York and would not have assisted in the preparation of the plans. 

This agreement was enhanced in 1995 by the approval of a supplement to the 1993 memorandum of 

understanding (PANYNJ and NYCDOB 1995). The supplement added that: 

• The person or firm performing the review and certification of plans for WTC tenants should 

not be the same person or firm providing certification that the project had been constructed in 

accordance with the plans and specifications. 

In 1993, the Port Authority entered also into an agreement with the FDNY related to fire safety 

inspections (PANYNJ and FDNY 1993). The introduction to the memorandum of understanding contains 

the following statements: 

On April 15, 1993, the Port Authority, in order to maintain and enhance 

the safety of Port Authority facilities, adopted a policy providing for the 

implementation of fire safety recommendations made by local 

government fire departments after a fire safety inspection of a Port 

Authority facility and for the prior review by local fire safety agencies of 

fire safety systems to be introduced or added to a facility. 

The purpose of this Memorandum of Understanding is to reiterate the 

Port Authority’s commitment to this policy and to set forth certain 

procedures to facilitate the implementation of this policy for buildings at 

Port Authority facilities located in New York City. 

The agreement recognized the right of the FDNY to conduct fire safety inspections of Port Authority 

properties in NYC and provided guidelines related to corrective actions. 

Port Authority’s Transition from the 1938 to the 1968 Code 

As discussed in Appendix A, in 1963 the Port Authority instructed the designers of the WTC to follow the 

then current 1938 NYC Building Code. During this time, the code was in the process of being revised (as 

noted above), and in 1965, the Port Authority directed its designers to adopt the draft version of the new 

code for their final designs. Some of the advantages of the new draft code were noted to be the following 

(Levy 1965): 

• Fire towers7

 could be eliminated; 

• Provisions for exit stairs were more “lenient;” and 

• Criteria for partition weights were more “realistic.” 

It was not certain whether all the changes being proposed to the 1938 code would be incorporated into the 

final version of the new code. Thus in 1966, the Chief Engineer of the Port Authority suggested that the 

 

7

 A “fire tower” is a stair tower enclosed within a 4 h fire rated shaft that is entered through a naturally ventilated vestibule. The 

1938 Code stipulated that one of the required exits in most buildings over 75 ft in height be a fire tower. 
Chapter 2 

60

“architect/engineers prepare a listing of the elements of the design which do not conform to old code 

requirements, but are acceptable under the new. With this list in hand, we could initiate discussions, at 

top level in the Building Department, to see if we can secure agreement to go along with our design 

(Kyle 1966).” 

A one-page document,8

 dated “2/15/67”, with the initials “CKP” listed the following items: 

1. Fire tower corridors [sic] eliminated. 

2. Number of stairs reduced from 6 to 3. (Old plans had 5 stairs at 

3’-8” and 1 stair at 4’-8” for a total population of 390. New plans 

have 2 stairs at 3’-8” and 1 stair at 4’-8” allowing a population 

of 390.) 

3. The size of doors leading to the stairs are [sic] changed from 

3’-8” to 3’-0”. 

4. All stairs exit through a lobby. Old plans had fire tower stair 

exiting through a fire enclosed corridor. 

5. Shaft walls are changed from a 3-hour rating to a 2-hour rating. 

6. Corridors are limited to a 100’ dead end and with a 2-hour 

rating. 

7. Additional [word(s) missing] changed from 20 pounds per 

square foot to 6 pounds per square foot (based on partition 

weight of 50 pounds to 100 pounds per linear foot). 

Apparently, the above list represents elements of the WTC design that would not have satisfied the 

1938 code, but did satisfy the then-current draft version of the new code. 

A letter dated February 18, 1975, from the architect-of-record to the Port Authority discusses compliance 

with the 1968 NYC Building Code (Solomon 1975). This letter begins with the following paragraph: 

In accordance with the instructions issued by the Port Authority at the 

start of the project, construction drawings for the World Trade Center 

were to conform with requirements of the Building Code of New York 

City, and any variations therefrom were to be called to the attention of 

the Port Authority for final decision and authorization. This procedure 

has been followed in the production of the contract drawings and, with 

the exceptions authorized by the Port Authority noted below, the 

drawings are in accordance with the new Building Code adopted in 

December, 1968. The Building Department reviewed the tower 

drawings in 1968 and made six comments concerning the plans in 

relation to the old code. Specific answers noting how the drawings 

conformed to the new code with regard to these points were submitted to 

the Port Authority on March 21, 1968. 

 

8

 “Changes to Building to Conform to New New York City Building Code,” dated 2/15/67. 
Progress on the World Trade Center Investigation 

61

The same letter continues with a list of four items that the architect was “instructed by the Port Authority 

to deviate from code (Solomon 1975).” The four items are: 

• Omission of vents from closed shafts. 

• Demising partitions to stop at suspended ceiling or bottom of truss 

instead of running from slab to slab. 

• Omission of fire protected openings on exterior walls with separation 

of less than 30 ft. 

• Treatment of the concourse level as Underground Street. 

Section C26-504.3(a) of the 1968 NYC Building Code required that tenant spaces be separated “by fire 

separations having at least the fire resistance rating prescribed in table 5-1, but in no case less than 1 hr, 

and shall continue through any concealed spaces of the floor or roof construction above.” The Port 

Authority chose to stop tenant (demising) partitions at the bottom of the suspended ceiling and use 10 ft 

strips of 1 h rated ceiling on either side of the partition (Solomon 1969). The general contractor stated in 

a letter to the Port Authority “...we have been unable to find any precedent for the fire rated ceiling 10’ 

on either side of the demising partitions beyond the one you described from your construction experience 

on Port Authority hangers [sic] (Endler 1969).” 

In a code compliance evaluation report written in 1997, it was stated “Tenant demising partitions, 

including separations from the public corridor, do not in all cases meet the requirement of being built to 

the slab above (Coty 1997).” The author of the report recommended that: “Generally, this condition has 

been and will continue to be remediated as a requirement of new tenant alterations. However, it is 

recommended that the Port Authority develop and implement a survey program to assure that this 

remediation process occurs as quickly as possible.” 

The tenant alteration guidelines issued in 1998 require that tenant partitions have a 1 h fire rating, and the 

standard details for fire rated partitions indicate a continuous fire barrier from top of floor to bottom of 

slab (PANYNJ 1998). 

Compartmentation and Sprinklers 

Neither the 1968 NYC Building Code nor any of the other contemporaneous codes that were examined 

required sprinklers in tall buildings except for underground spaces. Thus, only the parking garage under 

WTC 1 and WTC 2 was originally sprinklered. Although Local Law 16, adopted in 1984, required 

sprinklers in new office occupancies, it was not retroactive. The incentive to retrofit for sprinklers (as 

explained below) was the passage of Local Law 5 in 1973, which was retroactive. 

In the 1968 NYC Building Code, Class 1B construction for business occupancies had no limit on floor 

area. Local Law 5 required compartmentation of large floor areas in existing business occupancies over 

100 ft in height by the installation of fire rated partitions in accordance with the following: 

• Compartmentation to 7,500 ft2

 with 1 h partitions; or 

• Compartmentation to 10,000 ft2

 with 2 h partitions; or 
Chapter 2 

62

• Compartmentation to 15,000 ft2

 with 2 h partitions and smoke detectors. 

Compartmentation was not required, however, if “complete sprinkler protection” were provided. 

Compliance dates for these provisions were revised in 1979 by Local Law 84 so that one-third of the total 

area of buildings had to be in compliance by December 13, 1981, two-third of the total area had to 

comply by August 7, 1984, and full compliance was required by February 7, 1988. 

Following the February 13, 1975, fire in the lower stories of WTC 1 (Powers 1975), an independent 

consultant was retained to review WTC life-safety provisions, including response to Local Law 5. It is 

reported that the “consultant concluded that the existing structural fire retardants of the building are 

sufficient to make the probability of serious structural damage extremely remote and the degree of 

vertical compartmentation provided sufficiently limits the spread of fire in the structures but that the 

spread of smoke requires attention from a life safety standpoint (PONYA 1976).” The consultant 

reported that “...either of the two fire protection options provided for under Local Law 5 would provide a 

good level of occupant life safety within the World Trade Center complex, provided that whichever is 

selected is supplemented by certain additional measures.” The consultant provided a series of 

recommendations to supplement either the compartmentation option or the sprinklering option. 

The Port Authority initially decided to adopt the compartmentation option in response to Local Law 5. 

The summary of the January 1976 report on the Fire Safety of the World Trade Center lists the following 

actions to be implemented to enhance the fire safety of the WTC towers (PONYA 1976): 

1. The openings between floors of telephone closets, which was a 

source of fire spread during the February 13, 1975, fire should be 

closed. This work has been accomplished to prevent any 

reoccurrences of a similar condition. 

2. In addition, the Port Authority will proceed with the 

compartmentation option of Local Law 5, including all of its 

requirements for fire alarm, communications, and stairway 

pressurization. 

3. Sprinklering of all storage rooms, janitor closets, mail rooms and file 

rooms in the central core of each floor. 

4. Building additional sprinkler capacity and provisions for extension 

of a sprinkler system to any area of such usage requiring it in the 

event of an occupancy change. 

5. Equipping those doors which are normally kept open to the corridor 

system, such as doors at consumer service areas, with 

electromagnetic ‘hold open’ devices which would be activated by 

smoke detectors to close the doors. 

6. Providing fail-safe automatic door closers, arranged to close upon 

activation by smoke detectors, for the overhead rolling fire doors 

separating the below-grade truck dock from the elevator lobby. 
Progress on the World Trade Center Investigation 

63

7. Developing an optimum mode of operation of the building airconditioning system to remove smoke from the central core 

compartments without contaminating adjacent areas. 

Thus, while the Port Authority initially chose to implement the compartmentation option, it also chose to 

provide “for extension of sprinkler system to any area of such usage requiring it.” According to the 

1993 joint report written by the NYC Fire Commissioner and Commissioner of Buildings, in the 1980s 

the Port Authority began “a program to fully sprinkler the Tower buildings (Rivera and Rinaldi 1993).” 

The report goes on to state that by March 1993 sprinklering was “ nearly complete in Tower 2 and 

85 percent complete in Tower 1.” The report also included a table that summarized “the major system 

requirements of Local Laws 5/73 and 16/84 with conditions in place when the1993 explosion occurred.” 

The content of that table is reproduced here as Table 2–2. 

The tenant alteration guidelines issued in 1998, contained the following requirement and information 

(PANYNJ 1998): 

All tenant spaces shall be sprinklered. Except for a few areas, most 

tenant floors in The World Trade Center are provided with wet-pipe 

sprinkler systems. New tenants normally require a new sprinkler system. 

For renovations of existing spaces, modifications to the existing system 

are normally needed to comply with any new partition configuration. 

Because Local Law 16 required that business occupancies taller than 100 ft be 

sprinklered, WTC 7 was sprinklered during the original construction. 

Emergency Egress 

The 1968 NYC Building Code has requirements for the number and capacity of stairs and for the assumed 

occupant load that are similar to requirements in the other contemporaneous codes (see Appendix A). 

Codes of the time required that multiple stairs be located “as remote from each other as practicable.” 

NYC permits scissor stairs,9

 and the code requires the exit doors to be at least 15 ft apart. Local Law 16 

(1984) first imposed a remoteness requirement of 30 ft or one-third the maximum travel distance of the 

floor (whichever is greater), which was not retroactive, so it did not apply to WTC 1 and WTC 2 but did 

apply to WTC 7. 

The 1968 NYC Building Code also has a requirement that, “ ...vertical exits should extend in a 

continuous enclosure to discharge directly to an exterior space or at a yard, court, exit passageway or 

street floor lobby ...” (C26-602.4). Similar requirements are found in the 1965 BOCA Basic Building 

Code and in 1966 NFPA 101, but not in the 1964 New York State Building Construction Code or the 

1966 Municipal Code of Chicago. Current code language (2003 IBC, section 1003.6) defines continuous 

as: not “ ... interrupted by any building element other than a means of egress component.” 

This requirement was the subject of ongoing discussion with respect to the stairs in WTC 1 and WTC 2 

discharging onto the mezzanine level, which was not at street level but rather at the Plaza level. It was the 

 

9

 Scissor stairs refers to two separate interior stairways contained within the same enclosure and separated by a fire rated 

partition. 
Chapter 2 

64

position of the Port Authority that the Plaza was like a street, and the arrangement met the intent of the 

Code. 

Table 2–2. Summary of compliance with Local Laws 5/73 and 16/84 provided in 

March 1993 report by Fire Commissioner and Commissioner of Buildings 

(Rivera and Rinaldi 1993). 

Type of Work 

{Code Section} Compliance 

1 Compartmentation 

{504.1(c)} 

Not required in sprinklered buildings 

2 Smoke shaft of stair 

pressurization 

{504.15(c)} 

Not required in sprinklered buildings. However, smoke purge and 

pressurization of corridors with 100% fresh air is provided. 

3 Emergency power exit lights 

{605.2(b)} 

Exceeds requirement. 

Required — On separate circuit ahead of main switch 

Provided — Separate feeders and emergency generators (NOTE A) 

4 Emergency power exit signs 

{606.2(b)} 

Exceeds requirement. 

Required — On separate circuit ahead of main switch 

Provided — Separate feeders and emergency generators (NOTE A) 

5 Stair and elevator signs 

{608.0} 

Yes 

6 Emergency power 

{610.0} 

Exceeds requirement. 

Required — None 

Provided — See NOTE A above 

7 Sprinklers 

{1703.1} 

Yes 

95% completed for one tower [WTC 2] 

85% completed for other tower [WTC 1] 

8 Class “E” fire alarm signal 

system 

{1704.5(f)} 

Yes — But air supply and exhaust air to fire floor are not closed off when 

sprinklers are activated. 

Note: equivalent system provided by item #2 above and smoke detectors 

at fans, which stop fans. 

9 Fire command and 

communication 

{1704.8} 

Yes — except that each building does not have its own fire command 

station 

10 Elevator in readiness 

{1800.8(b)} 

Yes — See NOTE A above 

11 Removal of locks on elevators 

and hoistway doors 

{1801.4} 

Yes 

12 Firemen’s service operation 

{1801.5} 

Yes — See NOTE A above 
Progress on the World Trade Center Investigation 

65

Fire Alarm Systems 

Consistent with practice at the time, the original fire alarm system in WTC 1 and WTC 2 was a manual 

system with four smoke detectors on each tenant floor, positioned to monitor for smoke entering the 

HVAC returns and arranged to stop the fans to prevent smoke circulation to non-fire areas. Local Law 5 

(1973) included retroactive requirements for fire alarm systems and emergency voice communication 

systems in business occupancies over 100 ft in height. Subsequently, such systems were installed in 

WTC 1 and WTC 2 with the required fire command center located in the underground parking garage. 

Following the 1993 bombing, the fire command stations were relocated to the tower building lobbies with 

a third monitoring location in the Port Authority offices. There are no code requirements for off-site 

monitoring of fire alarm systems in this occupancy. 

Elevators 

Local Law 5 requires that elevators be provided with an emergency recall system. This requirement was 

incorporated subsequently into the American Society of Mechanical Engineers (ASME) A17.1, Safety 

Code for Elevators and Escalators, that governs elevator design and operation in all the building codes. 

The ASME Code requires that: 

• All passenger elevators be marked with signs stating that they cannot be used during a fire; 

• Fire detectors installed in every elevator lobby and machine room be arranged to initiate a 

recall of the elevators to the ground floor where the doors open and the elevator is taken out 

of service; and 

• Fire service personnel can use a special key to operate any individual car in a manual mode as 

long as they feel it is safe to do so. 

The elevator and building codes require that at least one elevator serving every floor be connected to 

emergency power. Refer to Table 2–2 for elevator status in WTC 1 and WTC 2 in 1993. 

Structural Stability 

As discussed in Appendix A, provisions related to structural stability in the 1968 NYC Building Code 

were in general agreement with those of the contemporaneous codes that were compared. There were, 

however, a number of provisions in the NYC code that were not included in the other codes such as 

uniform partition dead loads based on the weight of the partitions, consideration of loads due to thermal 

expansion/contraction and shrinkage of concrete, minimum strength requirements for bracing of 

compression members, and allowance for design wind loads based on wind tunnel tests. The NYC code, 

however, does not provide a standard protocol for wind tunnel testing to establish design wind loads. 

2.2.6 WTC 7 Fuel System 

WTC 7 was constructed and owned by Silverstein Properties on land owned by the Port Authority. It was 

built and operated by Silverstein as a Port Authority tenant alteration (see Appendix A.1). Many of the 

tenants conducted critical business operations in the building and required uninterruptible power to 
Chapter 2 

66

prevent the loss of information or operational continuity in the event of a power failure. This backup 

power was provided by diesel generators located in the mechanical spaces of the building. These 

generators were designed to start automatically in the event of an interruption of the utility supply. The 

total generator capacity and quantity of fuel stored in the building was sized to tenant needs. 

Code Requirements 

Design and installation of the WTC 7 emergency power and associated fuel systems were to follow the 

NYC Building Code. The base system was installed in 1987 with modifications occurring in 1990, 1994, 

and 1999. Over the period 1987 to 1999, the NYC Building Code provisions discussed below were not 

changed, so all systems were installed to the same requirements. Some of the key code provisions for the 

construction and location of fuel storage tanks, piping, and controls are discussed here, and additional 

details will be published in a separate report. 

Tanks [27-828 and 27-829] 10

All tanks must be fabricated of steel and coated to prevent corrosion. Minimum thicknesses are specified 

by tank diameter for storage tanks and for so-called “day tanks” (60 gal or 275 gal). Large storage tanks 

(up to 20,000 gal) may be buried inside or outside the building or on the lowers floor of the building with 

protection related to the tank capacity. For example, tanks from 550 gal to 1,100 gal must be enclosed in 

2 h fire rated, noncombustible construction and tanks larger than 1,100 gal in 3 h construction. 

Tanks on floors above the lowest floor are limited to 275 gal and one such tank per story. These “day 

tanks” must be surrounded by a concrete curb or steel pan with the capacity to hold twice the volume of 

the tank in the event of a leak. The curb or pan must be provided with a float switch to sound an alarm 

and shut off the transfer pump in case of tank failure. Appropriate controls (generally a float switch in the 

day tank) are provided to transfer fuel from the storage tanks to the day tank through a transfer pump and 

piping, with only one such transfer pump and piping network per day tank. 

Piping [27-830] 

Piping from transfer pumps to day tanks is required to be enclosed in a shaft of 4 in. thick concrete or 

masonry with a 4 in. clearance to the fuel pipe. Horizontal offsets may be enclosed in a steel sleeve two 

(pipe) sizes larger and enclosed in 2 h fire rated construction. The spaces between the fuel pipe and 

sleeve or shaft must lead to an open sight drain or an open sump so leaks can be detected. 

Power Systems Designs 

NIST located and reviewed specifications and drawings for each of the emergency power systems. It was 

noted that some of the fuel risers were installed in existing shafts containing other utilities. The NYC 

Building Code requires that pipe shafts containing piping from the transfer pump to storage tanks above 

the lower floors not be penetrated by or contain other piping or ducts [27-830(f)(5)]. Correspondence 

relating to the system for the Mayor’s Office of Emergency Management shows that this system was 

 

10 Sections of the New York City Building Code in which these requirements are found. These provisions are found in the 

subchapter on “Heating and Combustion Equipment.” 
Progress on the World Trade Center Investigation 

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reviewed and inspected by the FDNY, a list of needed corrections was produced, and each item was 

initialed as the corrections were verified. 

Base Building System 

The initial base emergency power system was installed in 1987, and consisted of two 900 kW generators 

and a 275 gal day tank located on floor 5. Main fuel storage was in two 12,000 gal tanks buried under the 

loading dock on the south side of the building. The tanks were double wall fiberglass11 with leak 

detectors between the walls. 

Fuel was transferred by one of the two pumps through a 2 in. supply line in an existing shaft containing 

other utilities, near the west bank of passenger elevators. The transfer pump was controlled by a float 

switch in the day tank with a low (pump on) and high (pump off) position. An alarm would be sounded if 

the fuel level in the day tank fell below the low level or went above the high level. The day tank was 

located within a 550 gal pan fitted with an alarm and another pump cutoff. The vent for the day tank 

terminated outside the south wall. 

The 2 in. fuel lines were encased in a second pipe covered with 2 in. of calcium silicate to provide the 

required 2 h fire rating. Pipe supports were located approximately 10 ft apart, and inspection plugs were 

provided approximately 50 ft apart. Mechanical equipment rooms were sprinklered (ordinary hazard 

group I), and the fuel pump room was sprinklered (ordinary hazard group III). The generator area on 

floor 5 was not sprinklered. 

Modifications to System 

From 1990 to 1999, four major modifications (additions) were made to the base emergency power 

system. These modifications are summarized in Table 2–3. Of significance are the 1990 modification 

(Salomon Brothers) that required a pressurized fuel supply system, because a day tank already existed on 

floor 5, and the 1999 modification (Mayors’ Office of Emergency Management) that required a separate 

6,000 gal tank on the first floor. Figure 2–1 is a schematic of the locations of the various components of 

the base system and the four major modifications. 

For the Salomon Brothers system, the transfer pumps were powered from the output of the generators. In 

the event of a failure of utility power, all nine generators were started automatically to ensure that if any 

of the nine did not start there would be enough power. Once the generators were up to speed, the control 

system would shut down those that were not needed, but these could be restarted later if power demand 

increased. There was enough fuel and residual pressure in the lines to start the generators and to run them 

for a few minutes, but once running, the fuel pumps were powered to supply fuel. As long as any one 

generator was running, the pumps ran at full capacity. 

 

11 While the NYCBC requires steel tanks, effective in November of 1985 the U.S. Environmental Protection Agency required 

(40CFR280) that all new underground fuel storage tanks be double wall fiberglass and that any steel tanks older than 20 years 

be replaced by double wall fiberglass. 
Chapter 2 

68

Table 2–3. Summary of modifications to base emergency power system in WTC 7. 

Year Day Tank/Generator Storage Tank Piping Comments 

1990 No day tank permitted since 

base design included one on 

floor 5/9 generators on 

floor 5, 1750 kW combined 

capacity 

Two 6,000 gal next to 

base tanks. 

Two 2 1/2 in. pipes 

in separate rated 

shaft 

50 psi pressurized 

fuel system 

1994 50 gal/125 kW on floor 9; 

generator room sprinklered 

Used existing base 

tanks 

1 1/4 in. in new 2 h 

rated dedicated shaft 

New transfer pump 

connected to existing 

storage tanks 

1994 275 gal/350 kW on floor 8; 

generator room sprinklered 

Used existing base 

tanks 

2 in. in same 

dedicated shaft as 

above 

New transfer pump 

connected to existing 

storage tanks 

1999 275 gal/three 500 kW on 

floor 7; smoke detectors in 

generator room 

6,000 gal on floor 1, in 

4 h rated enclosure; 

gaseous (clean) fire 

suppression system; 

space below tank 

sprinklered 

10 gauge conduit in 

2 h rated enclosure 

Storage tank kept 

filled from base 

storage tanks. 

2.2.7 Preliminary Findings 

The following preliminary findings are based on (1) review of the design and construction documents of 

WTC 1, 2, and 7; (2) review of the 1968 NYC Building Code, the 1964 New York State Building Code, 

the 1967 Municipal Code of Chicago, the 1965 BOCA Basic Building Code, and the 2001 NYC Building 

Code; and (3) correspondence of the Port Authority, design consultants, and general contractor. 

1. Building code used for design of WTC 1, 2, and 7 

When the design of WTC 1 and WTC 2 began in 1962, the governing building code in NYC 

was the 1938 edition. In September 1965, the Port Authority instructed its consultants to 

revise their design for WTC 1 and WTC 2 to comply with the second and third drafts of the 

new building code of NYC that was under development (Levy 1965). The new building code 

was adopted on December 6, 1968. 

The Port Authority took advantage of some of the less restrictive provisions of the 1968 Code 

compared with the outdated 1938 Code. Some of these new provisions included: 

a. Elimination of a fire tower as a required means of egress; 

b. Reduction in the number of required stairs; 

c. Reduction in fire rating of shaft walls from 3 h to 2 h; 

d. Use of uniform partition load that depends on weight of partition per unit length; and 
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5TH FLOOR

4TH FLOOR

3RD FLOOR

2ND FLOOR

1ST FLOOR

DISTRIBUTION SYSTEM

BASE BULDING

9TH FLOOR

8TH FLOOR

6TH FLOOR

7TH FLOOR

AMERICAN EXPRESS

DISTRIBUTION SYSTEM

DISTRIBUTION SYSTEM

AMBASSADOR CONSTRUCTION

SALOMON BROTHERS

DISTRIBUTION SYSTEM

MAYOR'S OFFICE

DISTRIBUTION SYSTEM

(QTY. OF 2)

(QTY. OF 9)

(QTY. OF 3)

DAY TANK

STORAGE TANK

EMERGENCY GENERATOR

PUMP SET

LEGEND

Figure 2–1. Locations of emergency generators and fuel tanks in WTC 7. 
Chapter 2 

70

e. Allowance of Class 1B construction for business occupancy and unlimited building 

height. 

One of the reviewed documents noted that “For office buildings there is no economic 

advantage in using Class 1A Construction, and ER&S [architect-of-record] used Class 1B 

Construction for the WTC Towers and Plaza Buildings....” (Feld 1987). 

The design of WTC 7 followed the 1968 code as amended by Local Laws, which includes 

Local Law 16, requiring sprinklers in buildings taller than 100 ft. 

2. Review of design documents by the Department of Buildings of NYC 

In 1963, the Port Authority instructed its consultants that the design of the WTC should 

comply with NYC Building Code. The Port Authority also stated that: “When preliminary 

designs have been completed, the Chief Engineer will review all design concepts with the 

appropriate municipal agencies before the consultants proceed with the final design 

(Levy 1963).” This implies that the NYC Department of Buildings would be involved in 

reviews of the design. A letter in 1975 from the architect-of-record to the Port Authority 

indicates that in 1968 the NYC Department of Buildings reviewed the tower drawings and 

“made six comments concerning the plans in relation to the old code (Solomon 1975).” It 

was stated further that on March 21, 1968, the architect submitted to the Port Authority 

responses to these comments “noting how the drawings conformed to the new code.” NIST 

is attempting to locate a copy of this correspondence to determine, if possible, the level of 

review conducted by the Department of Buildings and the specific six items identified in that 

review. 

3. The 1968 NYC Building Code compared with contemporaneous codes 

The 1968 NYC Building Code was more comprehensive in the coverage of provisions in 

certain areas compared with the other contemporaneous codes that were reviewed. For 

example, the 1968 NYC Building Code requires special design consideration for expansion 

and contraction due to temperature and shrinkage of concrete, and it permits determination of 

design wind loads from tests. In general, except for permitted construction classifications, 

provisions for structural stability, fire safety and egress were similar among the four 

contemporaneous codes that were compared (see Appendix A.5.1 and A.5.2 for details). 

There were, however, differences in permitted live load reduction, design wind pressures, and 

the treatment of partition loads. Table A–8 in Appendix A provides a summary of fire safety 

provisions for high-rise buildings used for business. 

In the 1993 joint report, the Fire Commissioner and Commissioner of Buildings made this 

comment about the NYC Building Code (Rivera and Rinaldi 1993): 

We pride ourselves that our codes are among the most stringent in the 

nation, and we have been in the forefront in applying technological 

advances to assure fire and structural safety in buildings. 
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4. Permitted construction classification for high-rise buildings 

The 1938 NYC Building Code permitted only Class 1 (fireproof) construction for 

unsprinklered office buildings of unlimited height and area, which required a 4 h fire rating 

for columns and a 3 h rating for floor framing members (4-3 rated construction). The 1968 

New York Building Code subdivided Group 1 (noncombustible) construction into Class 1A 

and 1B. Class 1B required a 3 h fire rating for columns and a 2 h rating for floor framing 

members (3-2 rated construction). There was precedence in earlier model building codes for 

permitting 3-2 fire rated construction for unsprinklered high-rise office buildings (BOCA 

1950 and SBCC 1946). Of the contemporaneous codes that were reviewed, the 1964 New 

York State Building Code and the 1965 BOCA Basic Building Code permitted 3-2 rated 

construction for unsprinklered office buildings of unlimited height and area. The 1967 

Chicago Municipal Building Code permitted only 4-3 rated construction for high-rise office 

occupancy. (See Appendix A.4.4 for more detail.) 

5. Deviations from NYC Building Code 

In 1975, the architect-of-record wrote a letter to the Port Authority pointing out that the Port 

Authority had provided instructions to deviate from the NYC Building Code with respect to 

four items (Solomon 1975). One of these was a deviation from the requirement that tenant 

fire-rated partitions be continuous from floor to floor. Over the years, these partitions were 

replaced with partitions that were continuous as required by the 1968 Code. In the 1997 

report on code compliance, it was noted that some partitions did not meet the requirement 

(Coty 1997). The 1998 tenant alteration guidelines require that core walls have a 2 h fire 

rating and walls separating tenants (demising walls) have a 1 h rating (PANYNJ 1998). The 

standard details for 2 h and 1 h rated partitions show that the partitions provide a continuous 

fire barrier from top of floor to underside of slab. 

6. Compartmentation and sprinklering 

Following the passage of Local Law 5 in 1973, the Port Authority implemented a program to 

proceed with the compartmentation option of Local Law 5 and to provide for the extension of 

the sprinklering system beyond the below grade spaces installed during original construction. 

In addition, sprinklers were installed in storage rooms, janitor closets, mailrooms and other 

spaces in the core area of each floor, and outside the core for tenants not selecting the 

compartmentation option. In the 1980s, the Port Authority began a program to sprinkler the 

remaining tenant spaces, initially as tenants changed, and later on negotiated schedules. 

According to Local Law 86, passed in 1979, full compliance with Local Law 5 was required 

by February 7, 1988. A report in 1997 states that there were four floors and the sky lobbies 

(all in WTC 1) that remained to be sprinklered, and that installation of sprinklers on these 

floors was in progress (Coty 1997). In the October 1999 report on code compliance, it is 

stated that sprinklering of the tenant floors was completed and sprinklering of the sky lobbies 

was “currently under way” (PANYNJ 1999). The tenant design guidelines in 1998 require 

that all tenant spaces be sprinklered (PANYNJ 1998). 
Chapter 2 

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7. Fuel system for emergency generators in WTC 7 

The generators and associated fuel distribution system installed in WTC 7 followed the 

requirements of the NYC Building Code with two exceptions. First, the underground storage 

tanks were fiberglass and not steel, but this is consistent with federal requirements 

promulgated by the U.S. Environmental Protection Agency, which preempt the City 

requirements. The second deviation was the installation of the fuel risers for the base system 

in an existing shaft with other utilities. All of the subsequent sets of fuel risers were installed 

in a separate 4 h rated shaft. 

The modification in 1990 included a pressurized fuel system, and the generators powered the 

fuel pumps. As long as one generator was running, the pumps ran at full capacity. 

2.3 BASELINE STRUCTURAL PERFORMANCE AND AIRCRAFT IMPACT 

DAMAGE ANALYSIS (PROJECT 2) 

2.3.1 Project Objective 

Project 2 of the NIST investigation into the collapse of the WTC towers focuses on (1) establishing the 

baseline structural performance of each of the two towers under design gravity and wind loads and 

(2) analyzing the aircraft impacts into each of the two towers to estimate the damage to the towers and 

establish the initial conditions for the fire dynamics modeling in Project 5 and thermal-structural response 

and collapse initiation analysis in Project 6. The objective of the project is to evaluate the role of the 

structural system and the abnormal loads from aircraft impact on the collapse of the WTC towers by 

(1) developing reference structural models of the WTC towers that serve as reference for more detailed 

models to be developed for Projects 2 and 6, (2) using these models to establish the baseline performance 

of each of the towers under design loading conditions, (3) estimating probable damage to the structural, 

mechanical, and architectural systems of the towers due to aircraft impacts, (4) evaluating the role of floor 

diaphragms and hat trusses on the structural integrity of the towers, and (5) estimating the structural 

reserve capacities of the towers under service loading conditions after losing a number of exterior and 

core columns and floor segments due to aircraft impact. 

2.3.2 Project Approach 

This project is divided into two primary focus areas. The first, related to establishing the baseline 

performance of the towers under design loading conditions, is divided into the following tasks: 

• Develop Structural Databases of the Primary Components of WTC 1 and WTC 2. To 

develop electronic databases for the major structural components of the WTC towers from 

original computer printouts of the structural design documents and modifications made after 

construction. This task will also estimate all cross-sectional properties and link the databases 

into a format suitable for the development of the reference structural models of the towers. 

• Develop Reference Structural Analysis Models of WTC 1 and WTC 2. To use the 

structural databases to develop finite element structural models of WTC 1 and WTC 2 that 

capture the intended behavior of the towers. The models include typical floor models and 
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73

global models of the towers. The models are used to establish the baseline performance of 

each of the towers under gravity and wind loads and serve as reference for more detailed 

structural models to be used for other phases of the investigation. 

• Estimate Wind Loading Criteria on the WTC Towers Based on the State of the Art. To 

develop wind loads on the towers based on currently available aerodynamic information 

(from two wind tunnel tests conducted recently by parties to an insurance litigation 

concerning the towers) and on extreme climatological information from available data and 

applicable standards. 

• Establish the Baseline Performance of WTC 1 and WTC 2 Under Design Loading 

Conditions. To use the reference structural models to analyze the two towers to estimate 

stresses, deflections, and member utilization ratios under the following loads: 

− Gravity loads considering the following cases: (1) dead loads, (2) live loads used in the 

original design of the towers, and (3) live loads according to the current American 

Society of Civil Engineers (ASCE) 7 Standard. 

− Lateral wind loads considering the following cases: (1) wind loads used in the original 

design of the towers, and (2) wind loads based on the state of the art. 

The second focus area, related to analyzing the aircraft impacts into each of the two towers, is divided 

into the following tasks: 

• Analyze the Aircraft Impacts into WTC 1 and WTC 2. To analyze the aircraft impact into 

each of the two towers to provide the following: (1) estimates of the damage to structural 

systems due to aircraft impact – including exterior walls, floor systems, and interior core 

columns; (2) estimates of the aircraft fuel dispersal during the impact; (3) estimates of 

accelerations and deformations in each of the two towers due to aircraft impact to be used for 

estimating damage to fire proofing; and (4) a database of the major fragments of the aircraft 

and destroyed structural components of the towers to be used for estimating damage to the 

mechanical and architectural systems inside the towers. The impact analyses are conducted 

at various levels including: (1) the component level, (2) the subassembly level, and (3) the 

global level to estimate the probable damage to the towers due to aircraft impact. The 

analyses also include simplified and approximate methods. This task will include the 

development of detailed models of the aircraft and the towers at the impact zone. 

• Analyze the Post-Impact Stability of WTC 1 and WTC 2. To examine the stability of 

each of the two towers and determine the reserve capacity after losing columns and floor 

segments due to aircraft impact and show that the towers did not collapse immediately after 

impact. The analyses will help understand the mechanism by which the towers remained 

standing after impact, including the load redistribution provided by the hat truss system, and 

determine how close to collapse were each of the towers immediately after impact. 

• Perform Sensitivity and Probabilistic Analysis of Aircraft Impact. To (1) conduct a 

sensitivity analysis to assess the effects of variability associated with various input 

parameters and identify the most influential parameters that affect the damage estimates and 
Chapter 2 

74

(2) perform probabilistic analysis to determine the probabilities associated with different 

damage estimates. 

Work completed to date on the above tasks is summarized in the following sections. 

2.3.3 Development of Structural Databases for WTC 1 and WTC 2 

The development of structural databases of the primary components of the towers has been completed 

under a contract from NIST by the firm of LERA, the firm responsible for the structural engineering of 

the WTC towers. The work included digitization of the original drawing books with tabulated 

information, a quality control procedure to ensure consistency of the generated databases with original 

design documents, cross sectional property calculations, and developing relational databases to link the 

database files into a format suitable for models development. The developed databases include 

modifications made after construction. 

NIST has implemented a rigorous review procedure to mitigate potential conflicts of interest and to 

ensure the integrity and objectivity of the deliverables. The review procedure includes an in-house NIST 

review and a third-party review by the firm of Skidmore, Owings, & Merrill (SOM) also under a contract 

from NIST. The third-party review by SOM included random checks of the digitized structural databases 

and cross section property calculations. The review indicated no discrepancies between the developed 

databases and the original drawing books. The in-house NIST review included: (1) line-by-line review of 

all database files, (2) random checks on the developed databases by project leader, and (3) calculation of 

all cross section properties and comparing with those in the developed databases. The review indicated 

minor discrepancies between the developed databases and the original drawing books. These 

discrepancies were reported to LERA, who implemented the changes and modified the databases 

accordingly. Consequently, the structural databases have been approved by NIST and are being made 

available for other phases of the NIST investigation. 

Additional details on the development of the structural databases appear in Appendix B. 

2.3.4 Development of Reference Structural Models for WTC 1 and WTC 2 

The development of the reference structural models for the towers has been completed by LERA. These 

are three-dimensional, linear, finite element models (FEMs) of the towers developed using SAP2000 

software. The models include: 

• Typical truss-framed floor model (floor 96 of WTC 1): The model contains all primary 

structural members of the floor system, including primary and bridging trusses, beams in the 

core, strap anchors, viscoelastic dampers, exterior and core columns above and below floor 

level, spandrel beams, and concrete slabs. Initial verification of the model has also been 

performed. 

• Typical beam-framed (mechanical) floor model (floor 75 of WTC 2): The model contains all 

primary structural members of the floor system, including composite beams, horizontal 

trusses, viscoelastic dampers, exterior and core columns above and below floor level, 

spandrel beams, and concrete slabs. Initial verification of the model has also been performed. 
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• Global models of each of the two towers: These are models of the 110-story above grade and 

6-story below grade structure for each of the two towers and include the following six main 

parts: core columns, exterior wall (foundation to floor 7), exterior wall trees (floors 7 to 9), 

exterior wall (floors 9 to 106), exterior wall (floors 107 to 110), and hat trusses. These 

models were developed separately and then assembled into a unified model. Rigid and 

flexible diaphragms representing the floor systems, core bracing, and loads were then added 

to the unified model. Parametric studies were undertaken to establish the idealizations used 

in the global models. These studies included detailed shell element and simplified beam 

element models for typical exterior wall panels and exterior corner panels. The parametric 

studies also included development of a simplified flexible floor diaphragms calibrated against 

the detailed floor models. Initial verification of the global models has also been performed. 

Similar to the structural databases, the developed reference models were thoroughly reviewed. As part of 

the review process, NIST conducted a workshop for NIST investigators, outside experts, and contractors 

to review the reference structural models developed by LERA. The purpose of the workshop was to 

discuss the methodology, assumptions, and details of the developed reference models. The feedback from 

individual workshop participants was included in the final review of the models. 

The in-house NIST review and the third-party review by SOM included: (1) checks on the consistency of 

the developed reference models with the original structural drawings and drawing books, and (2) 

verification and validation of the models, including reviewing assumptions and level of detail and 

performing analyses using various loading conditions to assess the accuracy of the models. The reviews 

indicated minor discrepancies between the developed reference models and the original design 

documents. The reviews also indicated that, in general, the modeling assumptions and level of detail in 

the models were accurate and suitable for the purpose of the project. The reviews identified two areas 

where the models need to be modified. The first is the effect of additional vertical stiffness of the exterior 

wall panels due to the presence of the spandrel beams. The second area is the modeling of the 

connections of the floor slab to the exterior columns of the typical beam-framed floor model, where this 

connection appeared to be fixed while the connection should be modeled as pinned. The minor 

discrepancies and the areas identified for modification were reported to LERA, who implemented the 

changes and modified the models accordingly. Consequently, the reference structural models have been 

approved by NIST and are being made available for other phases of the NIST investigation. 

More details on the development of the reference structural models appear in Appendix B. 

2.3.5 Estimates of Wind Loads on the WTC Towers 

The development of estimates of wind loads on the WTC towers has been completed by NIST on the 

basis of the current state of the art in wind engineering. The estimates make use of wind tunnel test 

results and extreme wind climatological estimates obtained by Rowan Williams Davis and Irwin, Inc. 

(RWDI) and by Cermak Peterka Peterson, Inc. (CPP) as part of insurance litigation concerning the WTC 

towers. In addition, the estimates of wind-induced forces and moments on the WTC towers make use of 

independent extreme wind climatological estimates performed by NIST, based on airport wind speed data 

obtained from the National Climatic Data Center, National Oceanic and Atmospheric Administration, and 

on the NIST hurricane wind speed database. 
Chapter 2 

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A comparison of estimates by CPP and RWDI of wind-induced maximum base moments on WTC 2 

indicates a difference of about 40 percent between the two estimates. NIST studied the two wind tunnel 

reports and attempted to identify the sources of disagreement between them in order to develop the wind 

loading on the towers. The NIST study included: estimates of the wind speeds for the direction that 

corresponds in the CPP and RWDI reports to the peak wind-induced base moment, and a critique of wind 

profiles used in estimation of wind loads by RWDI and methods used to integrate aerodynamic and 

extreme wind climatological data (the sector-by-sector approach in the CPP report and the up-crossing 

method in the RWDI report). 

The wind load estimates are currently being reviewed by SOM. Upon completion of the third-party 

review, the loads will be applied to the global models of the towers as part of the baseline analysis. 

2.3.6 Baseline Performance Analysis of the Towers 

Work is under way to complete this portion of the study. Significant progress has been made in using the 

reference models subject to gravity loads (dead loads and live loads used in the original design and 

according to ASCE 7-02 Standard) and wind loads used in the original design of the towers. Upon 

completion of loads application into the models, the models will be analyzed to establish the baseline 

performance of the towers. The results of the analysis will be reported at a later date. 

2.3.7 Analysis of Aircraft Impacts into WTC 1 and WTC 2 

The objective of the analysis of aircraft impacts into the WTC towers is to estimate the impact response of 

the towers, including damage to structural systems, acceleration environment, and fuel and debris 

dispersion. The analysis is being conducted at various levels including: (1) the component level, (2) the 

subassembly level, and (3) the global level to estimate the probable damage to the towers due to aircraft 

impact. The analyses also include simplified and approximate methods. NIST is working with experts 

from Applied Research Associates, Inc. (ARA) under a contract from NIST to conduct these analyses. 

The commercially available finite element analysis (FEA) software, LS-DYNA is being used for most 

impact analyses in this project. 

The development of constitutive models describing the actual behavior of the structure under the dynamic 

impact conditions of the aircraft is an important step prior to conducting the impact analyses. Significant 

progress has been made to identify the proper constitutive relationships, including high strain-rate effects 

and failure criteria for the various materials included in the analysis of aircraft impacts into the WTC 

towers. These materials include the various grades of steels used in the exterior walls and core columns 

of the towers, weldment, bolts, reinforced concrete, and aircraft materials. Details on the development of 

the materials constitutive models appear in Appendix C. 

Another important step prior to conducting the various impact analyses is the development of an aircraft 

model to be used in the component, subassembly, and global analyses. The model is developed based on 

information gathered from documentary aircraft structural information, and data from measurements on a 

Boeing 767 aircraft. The development of the Boeing 767 aircraft model for impact analysis is nearing 

completion. The engine and wing models have been completed and are being used in the component and 

subassembly analyses. Also completed is the empennage and landing gears. Work is under way to 
Progress on the World Trade Center Investigation 

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finalize the model of the fuselage, nose, and nonstructural components of the aircraft. Details on the 

development of the aircraft model appear in Appendix C. 

The WTC towers and Boeing 767 aircraft are complex structural systems, and a large database of detailed 

structural information has been collected on them. In the model development process, the objective was 

to include all of the primary structural components and details of both the aircraft and towers. This 

approach, however, results in very large models. The component and subassembly analyses were used to 

determine model simplifications that can reduce the overall model size while maintaining fidelity in the 

analysis. Therefore, a series of component impact analyses were performed. The primary objectives of 

component modeling are to (1) develop understanding of the interactive failure phenomenon of the 

aircraft and tower components and (2) develop the simulation techniques required for the global analysis 

of the aircraft impacts into the WTC towers, including variations in mesh density and numerical tools for 

modeling fluid-structure interaction for fuel impact and dispersion. The approach taken for component 

modeling is to start with finely meshed, brick element models of key components of the tower structure 

and progress to relatively coarsely meshed beam and shell element representations that will be used for 

the global models. Much progress has been made on the component level analyses using models of tower 

exterior and core columns with column end bolted connections and spandrel bolted connections, as well 

as floor segments impacted separately with an engine or a wing section with and without fuel. This 

analysis is nearing completion, and details on the analysis methodology and results appear in Appendix C. 

Not reported in Appendix C is progress made on the subassembly analysis. This work is under way. 

Preliminary subassembly engine impact analyses into a strip from the exterior wall to the core of WTC 1 

have been performed. An example analysis, shown in Fig. 2–2, is for a 500 mph engine impact centered 

on the spandrel for exterior panel 121A at floor 96 and includes core columns 503A and 603A between 

floors 94 and 98. This model includes a single width exterior panel and floor assembly of the same width. 

The concrete slab is modeled with brick elements, and the diagonal round bar members in the floor 

trusses are modeled with beam elements. The remainder of the structures, including the columns, metal 

decking, and truss upper and lower chord components, are modeled with shell elements. An alternate 

view of the impact damage at 0.25 s is shown in Fig. 2–3. Current work focuses on expanding the size of 

the model in width (larger number of exterior panels), height (larger number of floors), and depth 

(extension all the way through the core) to minimize the effect of boundary conditions on the model 

response. Details of further work on the subassembly analysis will be reported at a later date. 

Also not reported in Appendix C is progress made on the development of the models of the towers in the 

impact zone to be used for the global impact analysis. This work is ongoing. Examples include single 

floor models in the core (Fig. 2–4), multiple floor models (Fig. 2–5), and exterior wall models (Fig. 2–6). 

Details of further work on the development of the global models will be reported at a later date. 

2.3.8 Preliminary Stability Analysis of the WTC Towers 

Preliminary system stability analyses of the WTC towers have been performed to: (1) examine the 

overall stability of the undamaged tower upon removal of floors, (2) study possible load redistribution 

mechanisms upon losing columns in the core due to aircraft impact, and (3) study the response of WTC 1 

when columns in the exterior walls and the core are assumed destroyed due to aircraft impact and 

columns in the exterior are damaged due to subsequent fire effects. 
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(a) Initial configuration 

(b) Impact response at 0.25 s 

Figure 2–2. Example engine impact subassembly analysis. 
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Figure 2–3. Oblique view of the subassembly engine impact damage. 

Figure 2–4. Model of the 96th floor and columns of WTC 1. 
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Figure 2–5. Model of the core of WTC 1, floors 94–98. 

Type 300 med. res.

Type 200 coarse res.

Type 100 coarse res.

Type 400 coarse res.

Figure 2–6. Detail of the WTC 2 impact zone exterior column panels. 
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The analyses used the typical truss-framed floor model and a reduced version of the global reference 

model of WTC 1 (see Section 2.3.4) with proper modifications. Modifications included adding vertical 

springs at the bottom of the reduced models to account for the removed lower portion of the towers, and 

using actual steel properties and actual loads on the towers. The analyses used staged construction 

technique to account for the sequential construction of the towers, especially in the zone of the hat trusses. 

Linear buckling analysis and nonlinear analysis with plastic hinges were used to study the effects of 

removal of floors and loss of exterior and core columns, respectively. In addition, analysis of the floor 

system, where severed core columns were replaced by equivalent springs representing the stiffness of the 

hat trusses and columns between the floors and hat trusses, was conducted to study the mechanism by 

which the floor loads were redistributed when the core columns were severed by aircraft impact. 

Details on the preliminary stability analyses appear in Appendix D. 

2.3.9 Summary and Preliminary Findings 

Significant progress has been made on the first focus area of this project dealing with the baseline 

performance of the WTC towers. This includes the completion, review, and, final approval by NIST of 

the structural databases and reference structural models of the towers. Also completed are the NIST 

estimates of the wind loading on the towers based on the state-of-the-art, which is currently under review. 

Progress has been made on performing the baseline analysis. 

For the second focus area, dealing with aircraft impact into the towers, work is nearing completion on the 

development of materials constitutive modeling, aircraft model, and component level analyses. Progress 

has been made on the subassembly and global models development. In addition, preliminary stability 

analyses of the towers under damage from aircraft impact have been performed. 

The following presents some preliminary findings obtained from the component impact analyses (see 

Appendix C): 

• A 500 mph engine impact against an exterior wall panel results in a penetration of the 

exterior wall and failure of impacted exterior columns. If the engine does not impact a floor 

slab, the majority of the engine core will remain intact through the exterior wall penetration 

with a reduction in velocity of about 10 percent and 20 percent. The residual velocity and 

mass of the engine after penetration of the exterior wall is sufficient to fail a core column in a 

direct impact condition. Interaction with additional interior building contents prior to impact 

or a misaligned impact against the core column could change this result. 

• A normal impact of the exterior wall by an empty wing segment from the wing tip region will 

produce significant damage to the exterior columns, but not necessarily complete failure. A 

fuel-filled wing section impact results in extensive damage to the exterior wall, including 

complete failure of the exterior columns. This is consistent with photographs showing the 

exterior damage to the towers due to impact. 

• Three different numerical techniques were investigated for modeling impact effects and 

dispersion of fuel: (1) standard Lagrangian FEA with erosion, (2) Smoothed Particle 

Hydrodynamics (SPH) analysis, and (3) Arbitrary-Lagrangian-Eulerian (ALE) analysis. Of 

these approaches, SPH analyses appear to offer the greatest potential for modeling fuel in the 
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global impact analysis due to the combination of both computational efficiency and modeling 

fidelity. 

The following presents some preliminary findings obtained from the preliminary stability analyses under 

service live loads and subject to the assumptions and the limitations of these models (see Appendix D): 

• Linear stability analysis was used to examine the stability of the undamaged WTC 1 under 

service loads through increased unbraced column lengths (floor removal). The tower was 

stable when two floors were removed. Two core columns buckled when three floors were 

removed, but the tower maintained its overall stability. The tower also maintained its 

stability when four columns buckled with four floors removed. The analysis suggested that 

global instability of the tower occurred when five floors were removed from the model. 

Assuming that all columns at the region of the removed floors reached a temperature of 

600 °C (reduced modulus of elasticity), the analysis indicates that removal of four floors 

would induce global instability. 

• Analysis of the typical truss-framed floor model with fifteen severed core columns indicated 

that, under service loads, the floors first attempted to redistribute their loads to the hat trusses 

through tension in the columns above the damage. The load followed this path due to the 

relatively large stiffness of the hat trusses-column system compared to the flexural stiffness 

of the floors. At a certain floor level, column splices fail due to the large tensile forces and 

the floors below the failed splices must redistribute their loads directly to neighboring 

undamaged core columns. When only eight core columns were assumed severed, the analysis 

indicated that the tensile forces in the columns were smaller, due to the relatively larger 

stiffness of the floor. These forces may still have failed the columns at the splices. 

• Nonlinear analysis that included geometric nonlinearities and material nonlinearities using 

plastic hinges was conducted on the reduced global model of WTC 1. The model assumed 

the following damage to the tower: (1) due to aircraft impact, loss of columns and spandrels 

in the north face, and an exterior panel in the south face of the tower, as well as eight columns 

in the core; and (2) due to fire, loss of columns in the south face, which were shown in videos 

to be bowing inward a few minutes prior to collapse. The analysis indicated that after aircraft 

impact, the tower maintained its stability, where the highest stressed elements were the 

exterior columns next to the damaged area on the north face of the tower. The tower also 

maintained its stability after losing columns in the south wall due to fire effects with some 

reserve capacity left, indicating that additional loss or weakening of columns in the core, 

weakening of additional columns in the exterior, or additional loss of floors is needed to 

collapse the tower. More detailed models will account for local bucking of columns, and the 

failure and role of the floor system in redistributing the loads; factors that are not considered 

in this analysis. 
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2.4 METALLURGICAL AND MECHANICAL ANALYSIS OF STRUCTURAL 

STEEL (PROJECT 3) 

2.4.1 Project Objective 

Structural steel recovered from the WTC site provides information essential to understanding the events 

of September 11, 2001. Important data available from analysis of the steel include failure modes of the 

steel that provide clues to the interaction of the aircraft with the buildings and mechanical properties of 

the steel that assist in modeling of the buildings during impact and under the high temperatures 

concomitant with the fires. The steel may provide additional clues, such as information on the extent of 

high temperature exposure of the steel in the fires. 

Thus, the objective of Project 3 is to analyze structural steel available from WTC 1, 2, and 7 for 

determining the metallurgical and mechanical properties and quality of the metal, weldments, and 

connections, and providing essential data to other investigation projects. 

2.4.2 Project Approach 

This project is divided into five substantive tasks as follows: 

• Task 1–Physical Evidence. Collect and catalog the physical evidence (structural steel 

components and connections) and other available data, such as specifications for the steel, the 

location of the steel pieces within the buildings, and the specified steel properties. 

• Task 2–Visual Observations. Document failure mechanisms and damage based on visual 

observations of recovered steel, especially for available columns, connections, and floor 

trusses. Photographs taken before collapse will be used to determine damage occurring to the 

recovered steel before collapse. 

• Task 3–Mechanical Properties. Determine the metallurgical and mechanical properties of 

the steel, weldments, and connections, including temperature dependence of properties. The 

grades of steel will be identified in the columns, welds, spandrels, trusses, truss seats, and 

fasteners. The identification will include composition, microstructure, mechanical, and 

impact properties. This task will provide steel property data, including models of elevated 

temperature behavior for relevant steels, to estimate damage to the structural steel members 

from aircraft impact, evaluate structural fire response, and study the initiation of structural 

collapse in Project 6, Structural Fire Response and Collapse Analysis. 

• Task 4–Correlation with Engineering Drawings. Correlate determined steel properties 

with the specified properties for construction of the buildings. The quality of the steel used in 

the buildings will be compared with that specified. 

• Task 5–High Temperature Excursions. Analyze the steel metallographically to estimate 

maximum temperatures reached. It is recognized that high temperature exposure before the 

collapse may be difficult to distinguish from exposure during post-collapse fires. 
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2.4.3 Physical Evidence 

NIST has studied steel elements from the WTC buildings and collected and analyzed documents on steel 

and welding specifications from the 1960s applicable to the WTC towers. This analysis has resulted in 

the documents described below. 

Catalog of Structural Steel 

NIST has catalogued the 236 structural steel elements from the WTC buildings recovered for the 

investigation. These pieces represent a small fraction of the enormous amount of steel examined at the 

various recovery yards where the debris was sent as the WTC site was cleared. Components include full 

exterior column panels, core columns, portions of the floor truss members, channels used to attach the 

floor trusses to the interior columns, and other smaller structural components (e.g., bolts, diagonal bracing 

straps, aluminum facade). 

NIST catalogued and documented the steel pieces, and when possible, identified markings on the steel 

which pinpoint the intended as-built location within the buildings. Roughly 0.25 percent to 0.5 percent of 

the 200,000 tons of steel used in the construction of the two towers was recovered. The recovered steel 

includes portions of: 

• 90 exterior column panels; the as-built location of 41 distinct sections has been 

unambiguously identified within WTC 1 and WTC 2: 

− 26 panels from WTC 1: 22 from near the impact floors, 4 hit directly by the airplane 

− 15 panels from WTC 2: 4 from near the impact floors. 

• 55 wide flange sections and built-up box sections; 12 core columns have been positively 

identified from WTC 1 and WTC 2, including 1 column from the impact zone of WTC 1 and 

2 columns from the impact zone of WTC 2. 

• 23 pieces of floor truss material from WTC 1 and WTC 2; however, the as-built location of 

the trusses within the buildings could not be identified. 

• 25 pieces of channel sections that connected the floor trusses to the core columns in WTC 1 

and WTC 2; however, the as-built location of the channels could not be identified. 

The design drawings for WTC 1 and WTC 2 designate 14 different grades (or strengths) of steel for the 

exterior panels, four grades for the core columns, and two grades for the floor trusses. Stampings on 

identified perimeter and core columns indicate that the steel supplied was the appropriate strength as 

indicated on the design drawings, with the exception that 100 ksi plate was used for the 85 ksi and 90 ksi 

material called for in the design, leading to a total of 12 grades of steel in the buildings. The recovered 

structural elements have yielded representative samples of the following: 

• All 12 grades of exterior panel material; 
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• Two grades of core column steel (representing 99 percent, by total number, of the columns); 

and 

• Both grades for the floor truss material. 

The collection of steel from the WTC towers is adequate for purposes of NIST’s investigation (i.e., 

chemical, metallurgical, and mechanical property analyses as well as a substantial damage assessment and 

failure mode examination) to examine why and how WTC 1 and WTC 2 collapsed following the impact 

of the aircraft and ensuing fires. 

More detail on the recovered steel appears in Appendix F, Inventory and Identification of Steels 

Recovered from WTC Buildings. 

Contemporaneous Specifications and Other Documents 

As part of an analysis of contemporaneous (1960s era) documents, NIST has studied the building 

drawings to ascertain the major structural elements and grades of steel in the towers relevant to the 

investigation. Also, 1960s era steel and welding specifications used to construct the WTC towers have 

been located and analyzed. The many steels (combinations of strengths and manufacturers) that were 

used have been characterized based on structural engineering specifications for the buildings and 

manufacturer documents of the era. Appendix E, Contemporaneous Structural Steel Specifications, also 

describes the major structural elements in the towers relevant to the investigation. 

Ten steel companies fabricated structural elements for the two towers. The floors involved in the aircraft 

impact and major fires contained steel from four of these companies. Laclede Steel (St. Louis, Missouri) 

fabricated the trusses for the floor panels that spanned the opening between the core and the perimeter 

columns. They used steels conforming to ASTM A36 and A242, which they made and rolled in their 

own mill. NIST chemical analyses and strength tests, as well as contemporaneous mill reports indicate 

that many of the floor truss components specified as ASTM A36 were actually fabricated with a microalloyed steel of considerably higher yield strength. 

Pacific Car and Foundry (Seattle, Washington) fabricated the perimeter box column panels (generally 

3 columns wide by 3 stories tall) above Floor 9. Although 14 grades of steel (36 ksi to 100 ksi yield 

strength) were specified in the structural steel drawings, only 12 grades were supplied due to an 

upgrading of two of the specified steels. Most of the steel came from Yawata Iron and Steel (now Nippon 

Steel) and Kawasaki Steel, although about 10 percent of the plate was produced domestically, primarily 

by Bethlehem Steel. Many of these steels were relatively new proprietary steels and were not covered by 

ASTM standards of the time. In the impact zones of the towers, the perimeter columns damaged by the 

aircraft were largely of three specified grades: 55 ksi, 60 ksi, and 65 ksi steels. 

Stanray Pacific (Los Angeles, California) fabricated the welded core box columns (rectangular columns 

assembled from four steel plates) above Floor 7, primarily using steels conforming to ASTM A36. The 

thicker plates came from Colvilles, Ltd. (Motherwell, Scotland, now Corus Steel), while the thinner plates 

came from Fuji Steel (now Nippon Steel). 

Montague-Betts (Lynchburg, Virginia) fabricated the rolled wide-flange core columns and beams above 

the Floor 9. Much of the steel for the wide-flange columns came from Yawata Iron and Steel. The rest 
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Montague-Betts purchased from numerous domestic suppliers. For WTC 1, the core columns damaged 

by impact and fire were mostly wide-flange shapes, since the highest floors of the buildings contained 

few box columns. In WTC 2, the damaged columns were a roughly equal mix of welded box columns 

and wide-flange shapes. 

Details on the 1960s era documents appear in Appendix E. 

2.4.4 Visual Observations

Visual analysis includes analysis of both photographic evidence just before the collapse and analysis of 

the recovered steel for clues to the performance of the steel structure throughout the event. Airplane 

impact damage to the towers has been characterized from enhanced precollapse photographs (Fig. 2–7). 

Such analyses provide input for validation of airplane impact models. In addition, these images allow 

investigators to determine whether damage observed in the recovered steel occurred before or after the 

collapse, greatly aiding the failure analyses of these pieces. 

Figure 2–7. Enhanced precollapse image of the north face of WTC 1 with superimposed 

outline of the Boeing 767. Likely failure modes of damaged columns are indicated. 

The recovered steel has been studied to determine failure modes of the various components and 

connections. In addition, a NIST contractor, Wiss, Janney, Elstner, has surveyed the recovered steel to 

characterize failure behavior. 

Photo © 2001. Roberto Rabanne/CORBIS
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2.4.5 Mechanical Properties 

Mechanical tests of steel at room temperature (for baseline performance), high temperature (for strength 

and deformation behavior in fire conditions) and high rates of deformation (to calibrate strength 

enhancements occurring during airplane impact) are completed and are being analyzed. Preliminary 

analysis of the perimeter column and truss steel indicates that the quality and strength (relative to required 

minimum values) of the steel is as expected for steel of the period. Results for the core columns and 

connections are still being analyzed. Data are being provided to the impact damage and fire modeling 

teams. 

Mathematical models of the stress-strain behavior of 21 steels (various grades and manufacturers) in the 

WTC towers have been developed and provided to contractors for use in computer models of the behavior 

of the buildings. The models of the steel behavior are based on the conventional room temperature tests, 

high strain rate tests, and high temperature characterization. 

2.4.6 Correlation with Engineering Drawings 

In order to determine if the great variety of steels were in the proper positions in the buildings, NIST has 

correlated stamped yield strength values (generally stamped on each plate in the perimeter panels) and the 

measured mechanical properties with the yield strengths on the design drawings. This correlation is 

largely complete, and there are no indications that any inappropriate steel was in place in the buildings. 

2.4.7 High-Temperature Excursions 

The structural steel from the impact area of the towers is being characterized to determine maximum 

exposure temperatures for input to Project 5. After surveying a number of possibilities, NIST developed a 

technique to map thermal exposure of the relevant pieces by characterization of paint condition. By this 

means, sections with no damage (i.e., no “mud-crack” patterns) to the paint are known to have remained 

below approximately 250 °C, and paint with mud-cracks, but remaining relatively intact, remained below 

approximately 750 °C. Above 750 °C the paint becomes powdery and flakes off. Mapping of the steel is 

nearly complete, and data will be supplied to Project 5. 

2.4.8 Significant Interim Results 

1. NIST has cataloged the 236 pieces of recovered steel (Appendix F). 

2. Material and construction specifications of the construction period, as well as steel fabrication 

documents, have been located, analyzed, and documented (Appendix E). 

3. Mechanical tests at room temperature (for baseline performance), high temperature (for strength 

and deformation behavior in fire conditions) and high rates of deformation (to calibrate strength 

enhancements occurring during airplane impact) are completed and are being analyzed. 

4. Mathematical models of the stress-strain behavior of 29 steels (various grades and manufacturers) 

in the WTC towers have been developed for use in computer models of the behavior of the 

building during the airplane impact and during the resulting fires. The models of the steel 
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behavior are based on the conventional room temperature tests, high strain rate tests, high 

temperature characterization and literature on properties of steel produced in the era in which the 

WTC towers were constructed. 

5. Airplane impact damage has been characterized from enhanced precollapse photographs and 

correlated with recovered steel. 

6. Ten different steel companies fabricated structural elements for the towers, using steel supplied 

from at least eight different suppliers; four fabricators supplied the major structural elements of 

floors 9 to 107. 

7. Documents from the era of WTC tower construction, from the steel suppliers and others, were 

used to estimate average yield strengths for each of the supplied steels. These strengths typically 

exceed the specified minimum strengths given in the engineering drawings by 5 percent to 

10 percent. 

8. In contrast to the above, extensive studies of steel from the construction period show that due to 

statistical variation expected in steel products, a fraction of mechanical tests would be expected to 

fall below specified minimums. 

9. Although ASTM structural steel standards have evolved since the construction of the towers, 

changes have been minor and do not represent changes to the basic mechanical properties of the 

steels.

2.4.9 Preliminary Findings 

1. Analysis of recovered samples of the many grades of steel in the towers indicates that, based on 

stampings on the steel and mechanical tests, the correct specified grades of steel were provided 

for the specific fabricated elements. Furthermore, when this data is combined with pre-collapse 

photographic images of the five recovered WTC 1 panels in NIST’s possession that were 

damaged by the aircraft impact, it has been shown that these particular elements contained proper 

steel in the precise locations as specified in the design drawings. 

2. Metallography and mechanical property tests indicate that the strength and quality of steel in the 

towers was adequate, typical of the era, and likely met all qualifying test requirements.

2.5 INVESTIGATION OF ACTIVE FIRE PROTECTION SYSTEMS (PROJECT 4) 

2.5.1 Project Objectives 

The active fire protection systems studied in this investigation are the automatic fire sprinklers, fire 

detection and alarms, smoke purging, and preconnected hose lines. The automatic fire sprinkler system is 

the first line of defense against fires in these buildings. Water stored in the building, from public sources 

and even pumped from fire apparatus can be supplied through dedicated piping to the area of the fire. 

Also present in the buildings were preconnected hose lines connected to a water supply through 

standpipes located in the stairwells and other utility shafts. The standpipes provided hose connections at 
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each floor for the FDNY. In addition, standpipe preconnected hoses were installed for trained occupants 

to manually suppress fires. The heart of the fire detection system is the automatic fire alarm and 

emergency notification system. Occupants in the building depend on this system to detect fires and 

provide information for emergency evacuation. Capabilities were also designed for the ventilation system 

to operate in a way to purge smoke produced by fires from the building. Smoke purge was intended to be 

used for post-fire clean-up, but could be used during a fire event at the discretion of the FDNY. 

All of the active fire protection systems provide capabilities that are important for fire control, providing 

information for occupants and first responders, and limiting the effects of the fire on the building and its 

occupants. Therefore, this project has the objectives of documenting and evaluating the performance of 

the installed active fire protection systems in WTC 1, 2, and 7 and assessing their role in fire control, 

emergency response, and the fate of occupants and responders. 

2.5.2 Project Approach 

The tasks of this project are to (1) document fire protection systems design and installation, and 

(2) evaluate performance without the benefit of any physical evidence from the collapsed buildings. The 

need to document facts associated with the installed fire protection systems was made difficult because 

many of the relevant documents for WTC 1, 2, and 7 were lost in the collapse of those buildings. 

With the cooperation of the PANYNJ and Silverstein Properties Inc., information was obtained from 

other locations and from contractors, consultants, and operators. As an example, some information was 

obtained from the engineering offices of PANYNJ in Newark. Other written materials describing the 

design and operation of active fire protection systems were obtained through files maintained by 

contractors. Lastly, information from engineers and system operators was helpful in clarifying details of 

the installation and operation. 

NIST investigators led three groups of fire protection systems contract experts. Each group specialized in 

one of the fire protection systems being investigated – fire sprinkler, fire alarm, and smoke management. 

The group examining the sprinkler system was also tasked with investigation of the other water-based fire 

suppression systems—the standpipe and preconnected hoses. 

Technical assistance to NIST in the investigation of the sprinklers, standpipes and preconnected hoses 

was provided by Hughes Associates Inc. of Baltimore, Maryland. This group was tasked with: 

1. Documenting the design and installation of the systems; 

2. Documenting the design and capacity of the water supply including provisions for redundancy; 

3. Identifying differences in the designs used in WTC 1, 2, and 7; 

4. Documenting the normal operation and effect of the fully functional systems for fire control; 

5. Assessing the probable performance of the systems in WTC 1 and WTC 2 on 

September 11, 2001; and 

6. Assessing the installed systems with respect to present best practices. 
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The amount of water that the water supply and sprinkler systems were capable of delivering for a series of 

fire scenarios was determined using a hydraulic model of the sprinkler system and the associated water 

supply. 

Technical assistance to NIST in the investigation of the fire alarms was provided by Rolf Jensen and 

Associates, Inc., of Fairfax, Virginia. This group was tasked with: 

• Documenting the design and installation of the system; 

• Documenting the normal operation and effect of the fully functional systems, including 

provisions for redundancy; 

• Documenting modifications made to the fire alarm systems in WTC 1 and WTC 2 after the 

1993 bombing; 

• Assessing the probable performance of the systems in WTC 1 and WTC 2 on 

September 11, 2001; and 

• Assessing the installed systems with respect to present best practices 

Technical assistance to NIST in the investigation of the smoke management systems was provided by 

Hughes Associates, Inc., of Baltimore, Maryland. This group was tasked with: 

• Documenting the design and installation of the systems; 

• Describing the normal operation in fire emergencies; 

• Assessing the probable performance of the systems in WTC 1 and WTC 2 on 

September 11, 2001; and 

• Assessing the installed systems with respect to present best practices. 

The NIST building airflow and contamination dispersal computer model, CONTAM, was used to 

evaluate the performance of several smoke management system configurations in WTC 1 and WTC 2 

under specific fire scenarios. 

Significant fires in WTC 1, 2, and 7 prior to September 11, 2001, were of interest to understand, in 

particular, how the fires were suppressed. Information was sought on all fires that activated multiple 

sprinklers or where hose lines were used to suppress the fires. Because the records of fire events in the 

buildings maintained by the PANYNJ were destroyed in the fire and collapse of WTC 1, information was 

collected from FDNY fire reports. 
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2.5.3 Fire History of WTC 1, 2, and 7 

Fires occurred in WTC 1, 2, and 7 prior to September 11, 2001. The facts related to the performance of 

automatic sprinkler, manual suppression, fire detection and smoke purge systems during significant fires 

in the buildings after first occupancy were documented. 

Extensive records of fire incidents kept in the WTC 1 offices of the PANYNJ were lost in the collapse of 

the building; however, FDNY maintains records of the responses to all fires. These records consist of 

standardized forms on which fire events are described using codes from a predefined list of descriptive 

phrases and categories. 

The FDNY provided 397 Bureau of Operations Fire Reports and 112 Bureau of Fire Investigation 

Records (Fire Marshals’ Reports) that served as the basis for this summary of the fire history in the WTC 

1, 2, and 7. NIST reviewed these reports of fires for the period of 1970 to 2001 and fire investigation 

records between 1977 and 2001 for WTC 1, 2, and 7. All of these records consist of standardized forms 

that may be supplemented with other materials. Many were for minor fire events, such as fires that were 

extinguished by occupants before FDNY arrival. These were not of interest for this investigation. The 

records of significant fires were identified. Significant fire incidents were those involving the discharge 

of multiple sprinklers, use of a standpipe connected hose, or the combination of a single sprinkler 

discharge and a hose. As an aside, the majority of fire records for significant fires documented the 

performance of the detectors and sprinkler systems, but almost all reports lacked information about the 

performance of the smoke purge system. 

Table 2–4 contains the categorization of all structural fire incidents contained in the FDNY records for 

WTC 1, 2, and 7 available to this Investigation. This information was obtained from 345 of the 397 

Bureau of Operations Fire Reports that reported structural fire incidents. The table contains information 

on the category of fire incident, the time period over which the fire occurred, the number of records in 

that category, and a descriptive statement about the category. 

Forty-seven of these cases were considered significant fires based on information about the number of 

sprinklers activated, and/or hose lines used to suppress the fire. Sixteen fire incidents exercised multiple 

sprinklers or multiple standpipes (with or without the activation of at least one sprinkler). Thirty-one fires 

involved the use of one standpipe line or one standpipe line and discharge of one sprinkler. These 

incidents are documented further in Appendix G. In addition, the appendix contains information from 

publicly available investigation reports of the 1975 office fire in WTC 1 and the 1993 bombing incident. 

The FDNY fire reports and fire investigation records indicate that in areas protected by automatic 

sprinklers, no fire activated more than three sprinklers. The design area for three sprinklers is a floor area 

of 63 m2

 (675 ft2) in a light hazard occupancy, such as a high-rise office building as specified in the 

NFPA Standard for the Installation of Sprinkler Systems (NFPA 13). 

Many of the fires that occurred were recorded as suspicious or unknown in cause, occurred during offpeak work hours, and involved materials such as trash or paper-based supplies. In cases where sprinklers 

were activated, the FDNY records indicated that the sprinklers either extinguished the fire completely or 

aided in controlling the spread. 
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Table 2–4. Summary of historical fires in WTC 1, 2, and 7. 

WTC 1 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

1980–2001 66 Unattended food/appliances, overheated elevator 

equipment, discarded material, welding 

operations, electrical failure and suspicious fires 

No detection 

information and no 

sprinklers 

1970–1979 79 Trash can fires, discarded material, food on stove, 

electrical failure, overheated equipment 

Detection, no sprinklers 1980–2000 57 Unattended food/appliances, overheated elevator 

equipment, discarded material, welding 

operations, electrical failure 

Detection and sprinklers 1977–1999 18 Suspicious, electrical failure, discarded material 

WTC 2 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

1980–1999 37 Discarded material, welding too close, overheated 

equipment, suspicious, elevator motor 

No detection 

information and no 

sprinklers 

1975–1979 40 Discarded material, fire in office furniture, trash 

can fires 

Detection, no sprinklers 1981–1999 40 Food on stove, small elevator fire, electrical 

failure, suspicious, overheated equipment 

Detection and sprinklers 1977–2000 5 Mechanical failure, suspicious 

WTC 7 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

2000 1 Trash can fire/discarded material 

Detection, no sprinklers 1990 1 Electrical switch on floor – explosion 

Detection and sprinklers 1988 1 Suspicious 

2.5.4 Fire Sprinkler, Standpipe, and Preconnected Hoses 

Resources used in the investigation of the sprinkler, standpipe, and preconnected hoses in WTC 1, 2, and 

7 are being documented by NIST investigators and subject experts at Hughes Associates, Inc. This 

information will be included in the final report 

The design and installation of the fire sprinkler, standpipe, and preconnected hoses are described in a 

report being prepared for NIST by Hughes Associates, Inc. This report will provide an analysis of the 

performance capabilities of the suppression systems based on hydraulic modeling of the water supply and 

sprinkler distribution system. 

Preliminary Findings 

1. Sprinkler Risers and Standpipes. In WTC 1, 2, and 7, primary and secondary water supplies, 

fire pump size and locations, water storage tanks, and FDNY connections provided multiple 
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points of water supply redundancy. The potential for single point failure of the water supply to the 

fire sprinklers existed at each floor due to lack of redundancy in the sprinkler riser system that 

provided only one supply connection on each floor. As a result, the water supply to the sprinkler 

systems or a standpipe serving preconnected hoses could be interrupted by routine maintenance 

needs (i.e., shutdown of the riser or standpipe) or by impairment due to deliberate acts to damage 

the sprinkler riser or standpipe systems. While this lack of redundancy may not have had an impact 

on September 11, 2001 because the sprinkler system was damaged by aircraft impact, it could have 

made a difference in other building emergencies. 

2. Water Flow Rate to Sprinklers. Aided by the results of hydraulic modeling of the sprinkler 

system in WTC 1 and WTC 2—undamaged by aircraft impact and fully operational—the 

delivered water flow rate available from the automatic sprinkler systems was found to generally 

exceed the minimum requirements (by a considerable margin) for a high-rise office hazard 

classification in accordance with NFPA 13. In a number of cases, the amount of available water 

flow from sprinklers on specific floors was capable of protecting higher fire hazard classes than 

those associated with light hazard office buildings. 

2.5.5 Fire Alarm Systems 

The WTC 7 fire alarm system was monitored by AFA Protective Systems, Inc., at a location remote from 

the WTC site. AFA Protective Systems furnished the record from the fire alarm system history tape to 

NIST for use in the Investigation. 

Other resources used in the investigation of the fire alarm systems in WTC 1, 2, and 7 are being 

documented by NIST investigators and subject experts at Rolf Jensen and Associates, Inc. This 

information will be included in the final report. 

The design and installation of the fire alarm systems will be described in a report being prepared for NIST 

by Rolf Jensen and Associates, Inc. This report will provide the analysis of performance based on the 

design and programming of the systems. 

WTC 7 Alarm System Monitoring Record 

Although a great amount of information is normally collected and stored by any fire alarm system from 

fire detectors installed throughout a building, typically, and in the case of WTC 7, no specific fire 

information is sent to the monitoring site beyond the fact that a fire condition has been detected. 

The information from the WTC 7 alarm system monitoring record for September 11, 2001, is shown in 

Fig. 2–8. 
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Figure 2–8. Monitoring station history tape record for the WTC 7 fire alarm 

system on September 11, 2001. 

The fire alarm history tape record is read from the bottom to the top. Some entries occur as the result of 

normal operations, and others are the result of actions taken by operators. The bottom line of the record 

shows that at 6:05:01 a.m. on September 11, 2001, the fire alarm system completed a normal 

communications check with the central monitoring station. This check is made every day. 

At 6:47:02 a.m., AFA placed WTC 7 in a “TEST: ALL” condition. This is normally done in response to 

a request from the building manager. Ordinarily, it is requested when maintenance or other testing is 

being performed on the system, so that any alarms that are received from the system are considered the 

result of the maintenance or testing and are ignored. NIST was told by AFA that for systems placed in 

the TEST condition, alarm signals are not shown on the operator’s display, but records of the alarm are 

recorded into the history file. 

At 6:47:03 a.m., the record includes an explanation of the request to put the system in the TEST 

condition. Continuing to read from bottom to top, the date and time the system was placed in TEST is 

recorded. In this case it is 091101 647 (6:47 a.m., September 11, 2001), and the system will 

automatically go back to normal monitoring after 8 hr, a system default value, at 091101 1447 

(2:47 p.m., September 11, 2001). On the next line above, “RIC” identifies the AFA operator; 4210 is a 

code number for the “PLACE ON TEST” message. CAT:11 indicates the authority of the person 

requesting the action. On the next line above, the comment entered by RIC identifies that the person who 

requested that the system be placed on TEST was Williams. This action appears to be common for the 

building alarm system. Records show that the system was placed on test condition every morning for the 

7 days preceding September 11, 2001. 

At 10:00:52 a.m., a fire condition [1 1510 CO TO CLASS E] was indicated in WTC 7 by sensing 

performed by the fire alarm system. The *T at the right end of that record indicates that the system was in 

TEST at the time. The alarm record also shows that the fire condition is in AREA 1. NIST has been told 

by AFA that AREA 1 is not a specific area within the building, but a reference to a zone consisting of the 

entire building. That is to say, fires detected in any fire alarm zone in the building by the fire alarm 

system would result in the same AREA 1 identification at the monitoring station. The time 10:00:52 a.m. 

09/11/01 14:48:22 DYJ 4612 **** FULL CLEAR **** 

09/11/01 14:47:22 LATE 3923 SYSTEM TEST OVER 

09/11/01 14:47:22 COMMENT: TEST: ALL 

09/11/01 14:47:21 COMMENT: LAST SET: 091101 64742 

09/11/01 10:00:52 1 1510 CO TO CLASS E AREA:1 *T

09/11/01 06;47:43 COMMENT: RIC: WILLIAMS 

09/11/01 06:47:03 RIC 4210 PLACE ON TEST CAT:11 

09/11/01 06:47:03 COMMENT: 091101 647 091101 1447 

09/11/01 06:47:02 COMMENT: TEST: ALL 

09/11/01 06:05:01 RP 20 TIMER TEST 
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is shortly after the collapse of WTC 2. It is unknown if this fire alarm was triggered by smoke from a fire 

or dust entering smoke detectors. 

At 2:47:21 p.m. and 2:47:22 p.m. (14:47:21 hr and 14:47:22 hr), at the time the 8 hour “TEST: ALL” 

condition was set to expire, additional actions are recorded that end in an operator (DYJ) entry to “FULL 

CLEAR.” 

Alarm System Network Communications Paths 

During initial engineering design for the fire alarm system in WTC 1 and WTC 2, PANYNJ requested 

approval of the City of New York for use of fiber optic communications cable in the system. NYC 

Building Authorities denied the use of fiber optic cable. As a result, ordinary copper wire communication 

cable was specified. The copper communications cable is susceptible to electrical shorts that can prevent 

any communication between all of the distributed control units of the system. Fiber optic cable is not 

susceptible to electrical shorts. If fiber optic communications cable had been used, communications 

between panels where the fiber optic cable had not been severed would continue to be able to 

communicate with each other. 

Severing either type of data communications cabling without electrical shorts would have produced the 

same effect on the system. The system was designed with redundant communication paths to provide 

Class A signaling circuits. If one communication path is served without electrical shorts, a trouble 

condition would be annunciated, but communications would not be impaired. 

“Standpipe Telephone” System 

A dedicated communications system for emergency responders was installed in the stairwells of WTC 1 

and WTC 2. To use the system, a compatible telephone handset was needed. This system was known as 

the “standpipe telephone” system. 

Preliminary Findings 

1. Fire Alarm Monitoring System. The fire alarm system monitoring WTC 7 sent to the 

monitoring company only one signal indicating a fire condition in the building on September 11, 

2001. This signal did not contain any specific information about the location of the fire within 

the building. From the alarm system monitor service view, the building had only one zone, 

“AREA 1.” 

2. Alarm System Communications Paths. The resistance to failure of the fire alarm system 

communications paths between the fire command station and occupied floors may have been 

enhanced if fiber optic communications cable had been used instead of copper lines. Extensive 

damage to the towers upon aircraft impact is likely to have cut and shorted the wiring of the alarm 

system network cables. If that occurred, communications between the distributed fire alarm 

panels, which are components of the integrated fire alarm system, would have been degraded and 

lost to certain panels depending on the location of those panels. Fiber optic cable is not 

susceptible to electric short-circuits and would have provided full communications with fire alarm 

system components, including voice communications systems, to the point where the cable was 
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96

severed. Electric shorts in the voice communications disable that communication system over the 

entire cable length affected by the electric short-circuit. During initial engineering design for the 

fire alarm system in WTC 1 and WTC 2, the PANYNJ requested, but did not receive, approval of 

the City of New York for use of fiber optic communications cable in the system. The NYC code 

required copper wiring. As a result, ordinary copper wire communication cable was specified. 

3. “Standpipe Telephone” System. Some firefighters that received handsets at the command post 

in the lobby at WTC 1 were interviewed as part of the investigation. Every one of the firefighters 

interviewed indicated that they did not use the standpipe telephone communication system on 

September 11, 2001. Due to the loss of firefighters in WTC 2, there is no information about the 

use of the system in WTC 2. 

2.5.6 Smoke Management 

Resources used in the investigation of the fire alarm systems in WTC 1, 2, and 7 are being documented by 

NIST investigators and subject experts at NIST contractor, Hughes Associates, Inc. This information will 

be included in the final report. 

The design and installation of the smoke management systems will be described in a report being 

prepared for NIST by Hughes Associates, Inc. This report will provide the analysis of performance based 

on the design and programming of the systems. 

The smoke management systems as designed and documented in the operation manuals consisted of a 

smoke purge mode using the components of the main HVAC systems. The systems were intended to 

remove smoke and other gaseous combustion products from the fire area after a fire was extinguished. 

This system was to be activated “manually” at the direction of FDNY. 

Preliminary Findings 

1. Smoke Management Systems Performance on September 11, 2001. Based on the information 

reviewed, the smoke management systems were not activated during the fires on September 11, 

2001. It was determined that the likelihood of these systems being functional in WTC 1 and 

WTC 2 was very low due to the damage inflicted by the aircraft impacts. In addition to the 

significant openings created in the building envelopes, the aircraft impacts are likely to have 

severed major vertical shafts through which ran electrical power supply and HVAC system duct 

risers, thereby causing the loss of power to the smoke management system air handlers and 

damage to the vertical HVAC duct risers used to provide smoke management (smoke purge). 

2. Fire/Smoke Dampers. The analysis of smoke flow in WTC 1 and WTC 2 on September 11, 

2001, shows that HVAC ductwork was a major path for vertical smoke spread in the buildings. 

Fire dampers were installed in the systems, but not smoke dampers. Operational combined 

fire/smoke dampers in the HVAC ductwork on each floor would have provided a barrier to hot 

gas and smoke penetration into the vertical HVAC shafts in WTC 1 and WTC 2. However, 

smoke dampers were not available when the towers were built. 
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3. Stair Pressurization Systems. Modeling results showed that in WTC 1 and WTC 2 stair 

pressurization systems would have provided minimal resistance to the passage of smoke had they 

been installed on September 11, 2001. While the existence of such systems was known when the 

WTC towers were built, the alternative smoke purge system used in the WTC towers was 

considered to be equivalent. Multiple stair doors being open for substantial periods of time due to 

occupant egress, and stairway walls damaged by aircraft impact, would result in an inability to 

prevent smoke from entering stairwells. 

2.6 RECONSTRUCTION OF THERMAL AND TENABILITY ENVIRONMENTS 

(PROJECT 5) 

2.6.1 Project Objective 

The collapse of the WTC towers resulted from a combination of aircraft impact damage and the ensuing 

fires. However, both the relative importance of these two factors and their interaction leading to the 

observed total collapse is at present unclear. It is also unresolved: 

• Which structural features of the buildings were affected, and thus what location, magnitude, 

and duration of fire brought about the collapse, and 

• Whether the nature of the fires is typical of what might be expected in common occupancies, 

or whether there were special features that made these fires especially severe. 

These facets are even more pivotal for WTC 7, where the fires that led to the unexpected collapse 

followed an unknown ignition in an unknown location. 

In addition to the flames and heat, the smoke from the fires plays multiple roles, for example: 

• It serves as a telltale for the locating of fires, although the determination of location also 

requires a knowledge of smoke movement within the buildings. 

• Its visual obscuration and perhaps its toxicity may have affected choices made by people as 

they decided direction of movement, whether to wait for rescue, etc. 

Thus, Project 5 has as its objective to reconstruct, with assessed uncertainty limits, the time-evolving 

temperature, thermal radiation, and smoke fields in WTC 1, 2, and 7 for use in understanding the behavior 

and fate of occupants and responders and the structural performance of the buildings. 

2.6.2 Project Approach 

Due to the near absence of physical evidence, the recreation of the fires depends on computer modeling. 

NIST is redefining the state-of-the-art in fire and thermostructural modeling, since this type of 

reconstruction has never been done before. Fire experiments are being used to guide adaptation of 

existing models and develop new algorithms for them; additional experiments form the basis for 

validating the models. 
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The models will then be exercised for a range of possible initial conditions. Those simulations that agree 

with the photographic evidence and the eyewitness information will be accorded a higher likelihood of 

being correct. To the extent that simulations contradict the evidence, they will be deemed less plausible. 

There is essential input that will arise from other projects, for example: 

• Specifications of the liquid fuel storage systems in WTC 7 – Project 1 

• Degree and nature of aircraft impact damage – Project 2 

• Maximum temperatures experienced by the structural steel – Project 3 

• Performance specifications for the smoke handling system – Project 4 

• Extent and location of structural weakening needed for collapse to be initiated – Project 6 

• Eyewitness accounts of building damage and fire locations – Projects 7 and 8 

The interdependence with Projects 2, 6, and 7 is particularly broad, with Project 5 providing, for example: 

• Establishment of the nature and precision of the information needed from the aircraft input 

modeling – Project 2 

• Descriptions of the duration and intensity of likely fires for use in assessing the possible 

locations of collapse initiation – Project 6 

• Description of fire and damage information to be requested of survivors – Project 7 

Project 5 is divided into the following eight tasks: 

• Visual Collection and Time Line Development for WTC 1, 2, and 7. To acquire and use 

photographs, videos, and other relevant information to develop detailed time lines for the 

spread and growth of fires at the peripheries of WTC 1, 2, and 7 and to organize the 

information such that it can be utilized by other investigation team members. The cataloging 

and analysis will provide guidance on the initial conditions for modeling the fires, the rates of 

spread of the fires, the floors on which the structural collapses appear to have begun, etc. 

• Characterization of Combustibles. To gather data on and characterize the types, mass and 

distribution of combustibles in the pertinent floors of WTC 1, 2, and 7 at the time of the 

September 11, 2001, disaster. The results are to serve as input to the overall Project 5 effort 

to reconstruct the thermal and tenability environment within the three buildings. 

• Characterization of Partitions. To identify the location of and characterize the fire 

endurance properties of the internal partitions (floors, walls, and ceilings) in the pertinent 

floors of WTC 1, 2, and 7 at the time of the September 11, 2001, disaster. This entails 

obtaining existing data on the fire performance of floor, wall, ceiling systems, and 

complementing this with additional measurements as needed. The results will help in 
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determining the potential and rates of intercompartment fire spread and also the degree to 

which the interior of a building was visible in the photographs and videos. 

• Characterization of Structural Insulation. To determine the effective thermal properties of 

the structural fireproofing systems, the effect of vibration, impact, and shock on their thermal 

insulation performance, and whether chemical interaction between the fireproofing materials 

and the steel at elevated temperatures could degrade the steel and fireproofing performance 

during thermal insult. This will enable simulation of the temperature rise within the structural 

elements as a result of the changing thermal environment. 

• Model Development. To upgrade the NIST computational fluid dynamic (CFD) Fire 

Dynamics Simulator (FDS) for its application to the reconstruction of the fires in 

WTC 1, 2, and 7. This will affect a pragmatic fire growth routine and also improve the 

efficiency of the model, enabling more extensive simulations during the timeframe of the 

Investigation. In addition, this task will develop a computational method for relating the 

turbulent fire environment to the transport of heat to and through the insulating layer to the 

underlying structural steel. This will enable simulation of the temperature rise and resulting 

loss of structural capability of the steel. 

• Experiments for Model Development. To provide input parameters and guidance for the 

FDS combustion submodel. 

• Fire Reconstruction. To reconstruct the gaseous thermal environment (radiation and 

temperature fields) surrounding the structural elements and in the inhabitable spaces within 

WTC 1, 2, and 7. Using such input information as the estimated aircraft damage from 

Project 2, the contents and layout of the building from the above tasks, NIST will use FDS to 

simulate fully involved fires in the three buildings, with and without the initial damage from 

the aircraft or incident debris, enabling addressing the extent to which that damage affected 

the thermal environment felt by the structure. Parameters in the re-creation of the fires will 

enable estimating the roles of jet fuel and building contents, ventilation system, compartment 

damage, pressurized core, and fire protection system on the growth and spread of fire. The 

use of statistical design for the sets of simulations will lead to identification of those input 

conditions to which the results are the most sensitive and those combinations of input 

conditions that lead to the best agreement with the photographic evidence. 

• Reconstruction Validation. To generate and use experimental data for assessing the 

accuracy of the fire model prediction of thermal insult on structural members such as 

columns, trusses, beams and other support structures like those in WTC 1, 2, and 7. 

Comparison of the data from large-compartment tests of fire growth and heat transfer to steel 

specimens will establish the accuracy of FDS in simulating heat transfer and complex burning 

at a realistic scale. 

2.6.3 Collection of Photographic Evidence 

NIST has compiled an extraordinary collection of still and video images of the three buildings. These 

have been digitized and organized into a searchable database. The user can organize a search to view 
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each of the two towers from all four sides from the time of the airplane strike to the time of collapse, 

although there are still significant gaps in time and vantage point. The collection is less definitive for 

WTC 7, due mainly to the high hazard level in the vicinity following the collapses of the two towers and 

to obscuration of the building by other structures and the smoke cloud from the tower collapses. 

To facilitate comparison of the predictions of the fire modeling (see below) with the photographic 

evidence, NIST has created animations of the building facades that depict the evolving breaking of 

windows, the emanation of smoke from the windows, the appearance of fire through the windows, and the 

emanation of flames out the windows. 

More detail on the collection appears as Appendix H. 

2.6.4 Data on the Building Interiors 

The solicitation of information on the tenant spaces was focused on those floors of the three buildings in 

which physical damage was observed and those floors where fires were observed or might have existed 

unobserved (as shown in Table 2–5). 

Table 2–5. Floors of visible damage. 

Building Aircraft Impact Damage Observed Fires 

1 93–99 92–99, 100, 104 

2 77–85 78–83 

7 – 7, 8, 9, 11, 12, 13, 22, 29, 30 

Floor Plans 

Examination of documents combined with discussions with architects, product manufacturers, occupants, 

and building managers indicate a general picture of the building interior. The floor slab was generally 

carpeted; there are some cases of raised floors and of wood- or stone-covered areas. There were glass 

walls at the entrances to some of the suites. For multi-tenant floors, the demising walls between the 

tenants were of gypsum board over steel studs and ran from the floor slab to the bottom of the slab above. 

Tenant space interior walls were of similar construction, but ran from the floor slab to just above the drop 

ceiling. Thus, the joist space was often open across a whole story or large fractions of a story. The drop 

ceiling systems, one for the tenant spaces and one for the core areas, were designed for the WTC. 

NIST requested that tenant companies provide architectural drawings of the most recent renovation of 

their space. The following features are of particular importance: 

• Location of walls. 

− These can act as a barrier to fire spread. In the towers, the fire resistance time of the 

demising and interior walls may have been comparable to the time between aircraft 

impact and building collapse. Even though the overall duration of the fires in WTC 7 

was much longer, the walls could have limited the rate of fire spread. 
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− When there were walled offices at the perimeter of a floor, the interior walls block the 

view of the interior from the exterior. Thus, the nonobservation of fire through a 

particular window could indicate either the absence of a fire on that floor or the presence 

of a vision-obstructing wall. 

• Air flow between floors. Fires need both fuel and air. To the extent that the flow of air to the 

fire is limited, so is the size of the fire. The direction of air access will also determine the 

direction(s) of fire spread. Thus, it is important to know where there were perforations, e.g., 

interior stairwells, air ducts, to have the most realistic sets of input conditions for the fire 

simulations. 

NIST has obtained floor plans for a large fraction of the floors of interest in the three buildings. For the 

few that are missing, NIST is working with the design drawings and is estimating their similarity to the 

layouts on September 11, 2001, from eyewitness, tenant, and manager accounts. 

Combustibles 

While much of the public attention has been focused on the jet fuel, this was fully combusted in only a 

few minutes. By contrast, typical office furnishings can sustain intense fires of at least an hour's duration 

on a given floor. NIST has obtained generic information about the furnishings in many of the suites and 

specific details for a few. This information has already been of use in the design of workstation fire tests 

(see below). NIST is checking to identify the location and size of any unusual fuel loads, such as file 

rooms, film storage, etc. 

In addition, NIST has obtained descriptions from the airlines of the combustible contents of the airplanes 

on that day. This includes the cabin materials (both installed and carried on by the passengers), aircraft 

components (e.g., wire insulation, flammable fluids other than the jet fuel), and the cargo bay contents. A 

preliminary estimate indicates that the combined mass of aircraft-borne combustibles is a considerable 

fraction of the building combustibles in the impact zone. 

For WTC 7, NIST is aware of two special sources of combustible fluids. Rolf Jensen and Associates, 

Inc., a NIST contractor, is gathering data on the fuel tanks and distribution lines for the emergency 

generators in WTC 7. NIST has obtained information on the magnitude of the volume of the transformer 

fluids located in the power substation in WTC 7. 

2.6.5 Insulation of Structural Members 

The required fire resistance ratings of the structural members in the three buildings were obtained by 

gypsum framing of some columns or the use of spray-applied fireproofing on other columns and web 

joists. The architectural drawings provide definition of the former. Working from documents and 

discussions with engineers, NIST has identified the various spray-applied fireproofing materials and 

where each was used, as shown in Table 2–6. For the exterior columns in the towers, both Vermiculite 

plaster and BlazeShield DC/F were used. NIST is still investigating the specific locations for each. 
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Table 2–6. Types and locations of spray-applied fireproofing on fire floors. 

Locations 

Building Fireproofing Material Interior Columns Floor Systems Exterior Columns 

BlazeShield II Floors 92 to 100, 102 

BlazeShield DC/F Yes Remaining floors WTC 1 

Vermiculite plaster and 

BlazeShield DC/F 

 Yes 

WTC 2 Vermiculite plaster and 

BlazeShield DC/F 

Yes Yes Yes 

WTC 7 Monokote ML-5 Yes Yes Yes 

The ability of the fireproofing to delay the rise of temperature in the protected structural steel depends on: 

• The thermophysical properties of the insulation material. NIST has obtained samples of 

the three types of spray-applied insulation and four types of gypsum wallboard and has sent 

them to testing laboratories for determination of their thermal conductivity, density, and heat 

capacity, all as a function of temperature from ambient. The spray-applied material data will 

be from 25 °C to 1,200 °C; the wallboard data will be from 25 °C to 600 °C. 

• The thickness of the insulation. The gypsum wallboard thickness is described in the 

architectural drawings. As documented in the May 2003 Progress Report, NIST has traced 

the evolution of the intended thickness of the spray-applied material. NIST has evaluated the 

variation in the actual thickness of the fireproofing in the WTC towers. Appendix I presents 

the results of that evaluation. 

• Any damage to the layer during construction or refitting of the building.Damage from 

the impact or shock of the incident airplane (WTC 1 and WTC 2). NIST will obtain 

estimates of the impact intensity from ARA, which is the NIST contractor modeling the 

aircraft impact under Project 2. Using both standard and custom measurement methods, 

NIST is determining the cohesive properties (shear strength and tensile strength) to steel of 

the two types of spray-applied insulation used in WTC 1 and WTC 2. NIST is also 

developing models to predict dislodgement of the insulation.

• Damage from distortion of the WTC 1 and WTC 2 structures from the impact or the 

fires. 

• Damage to the insulation in WTC 7 from incident debris from the collapse of the 

towers. 

Prior to this Investigation, there was no computational method for modeling in three dimensions the effect 

of a fire on structural assemblies, that is, modeling the absorbance of incident heat from a turbulent fire by 

an insulating surface, the transport of heat through the insulating layer, and the distribution of heat 

throughout the steel structure. This is in large part because the turbulent fire is characterized by short 

time steps and large computational cell size, while the structural member is characterized by longer time 

steps (slower changes in temperature) and smaller computational cell size. 
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NIST has developed a tool to do just this, the Fire-Structure Interface (FSI), for the first time linking FDS 

with ANSYS (a finite element, FE, thermostructural model). Of particular importance is the relationship 

between the thickness of the insulation and the time for the underlying steel to reach a temperature at 

which the steel strength is compromised. Variations in thickness could be random in nature or as stark as 

bare spots. 

The first computational runs were for a column as heavy as the thickest core column on the floors of 

impact in WTC 1. For each of three cases, a sequence of portraits of the temperature distribution in the 

steel was generated throughout an exposure to a uniform external temperature of 1,100 °C. From these 

depictions, the following times were determined for when the steel temperature reached 600 °C, a 

temperature near which significant compromising of the steel's structural properties would ensue: 

• Insulation with the (estimated) properties of BlazeShield applied to a thickness of 13 mm. 

The time to reach 600 °C was over 10 h. 

• A 20 percent reduction in the total mass of insulation, with the loss of thickness being varied 

randomly. The time to reach 600 °C was about 6 h. 

• All insulation removed from one face of the column. The time to reach 600 °C was about 

12 min. 

In a second set of calculations, the same material was applied to a bar 25 mm × 25 mm × 1,500 mm. A 

25 mm notch (to bare metal) of insulation was removed from the midpoint of the bar. Upon exposure to 

the same thermal environment, the temperature of the steel reached 600 °C along its full length in a matter 

of minutes. 

A further description of the interface and sample calculations appears in Appendix J. The preliminary 

indications are that, in the future predictions of the impact of the WTC fires on structural members, NIST 

should expect the results to be sensitive to small gaps in the thickness of the insulation. In other words, 

small areas of thin or missing insulation can lead to accelerated heating, and this effect could be felt well 

away from the susceptible sites. 

2.6.6 Modeling the Fires 

In simulating the fires, NIST will be examining the effects of uncertainty in knowing the initial conditions 

of the fire and the building. Thus, it is critical that the accuracy of the fire model itself be established so 

that the uncertainty in the model's predictions is small compared to the effects caused by the differences 

in the initial conditions. 

As a first step, certain enhancements to FDS were implemented: 

• Realistic state relation curves for underventilated fire scenarios. The prior computational 

code used ideal state relations for its combustion routine, i.e., user-prescribed values for the 

combustion efficiency that did not change as the ventilation within the fire compartments 

evolved. 
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• The combustion module was enhanced to enable the inclusion of charring materials, such as 

those that comprise much of the office furniture. 

• Computational code enhancements to enable time-efficient computations. Multiblock 

gridding now enables needed dimensional resolution in the vicinity of the structural 

components. The FDS code was also re-written for parallel processing. 

• Fire spread. Each combustible is characterized by a heat of gasification, with the surface 

irradiance calculated from the thermal radiation field generated by existing fire. When the 

mixture fractions in two adjacent computational cells straddle the stoichiometric value, the 

flame then extends to the interface between the cells. 

• Enhanced visualization. Smokeview has been modified to handle the extremely large data 

sets that will be generated in these simulations. 

A first set of experiments was conducted in the NIST Large-scale Fire Laboratory to assess the accuracy 

with which FDS predicts the thermal environment in a burning compartment and to establish a data set to 

validate the prediction of the temperature rise of structural steel elements using FSI. Within a large test 

compartment, assorted steel members were exposed to controlled fires of varying heat release rate and 

radiative intensity. The steel members were bare or coated with spray-applied fireproofing of two 

thicknesses. The thermal profile of the fire was measured at multiple locations within the compartment. 

Temperatures were also recorded at multiple locations on the surfaces of the steel, the insulation, and the 

compartment. Prior to each test, a prediction of the thermal environment in the compartment was 

determined using FDS. Following the tests, the prediction and experimental results were compared. 

Much of the combustible material on the fire floors of the WTC buildings consisted of employee 

workstations. Each such space was a combination of desk space, generally made of fiberboard with a 

laminated finish; file cabinets; carpet; chair; computer; paper; etc. NIST conducted a set of fire tests of a 

generic workstation in our Large Fire Laboratory. A single unit was burned under a large hood with a 

soffitted ceiling. Ignition was by a 2 MW spray burner, simulating an already burning adjacent 

workstation. Test variables included the combustible mass, the presence of jet fuel, and the presence of 

inert material (simulating fallen ceiling tiles or wall fragments). Gasification data for the combustible 

was generated using a Cone Calorimeter. These data plus the geometry of the workstation were used as 

input to the fire model. The intent was to use the experiments to identify any needed changes in the 

combustion algorithms. In fact, little adjustment was needed. These tests and their analyses are detailed 

in Appendix J. 

A third set of tests was conducted to determine the accuracy of the FDS under conditions simulating a 

portion of a representative floor of the WTC towers. Thus, the predictions of the outcome of the tests 

were performed prior to the experiments. Three workstations were situated in a large compartment. Two 

were contiguous, the third was across an aisle. The open end of the compartment had windows of aspect 

ratio similar to those in the towers. The test variables included the presence of jet fuel, the presence of 

inert material, the location of the ignition burner (at or away from the windows), and the extent to which 

the workstations had been reduced to rubble. The analysis of the results is under way, but preliminary 

indications are that the model predictions closely resembled the test results. 
Progress on the World Trade Center Investigation 

105

Additional information on the three sets of experiments appears in Appendix J. Full reports on the first 

two sets of experiments are expected in the coming months. 

2.6.7 Preliminary Findings 

To date, the calculations and analyses have led to several interim findings that will guide the 

reconstruction of the fires in the three WTC buildings: 

Observed Fires 

1. Despite the airplane striking the center of the north face of WTC 1, the resulting fires are not 

symmetric about the centerline of the building. After the initial fireballs, the flames damped 

considerably. The early fire growth was on the north face, the center of the east face and the west 

side of the south face. On some floors, there was continuous spread; in some instances sudden, 

noncontiguous fires appeared. 

2. The damage and initial fires in WTC 2 were highly asymmetric, as the airplane struck off center 

to the east. Burning debris piles of long duration were observed at the northeast corner of some 

floors. In general, the fires spread less actively than in WTC 1, but there were sudden fires here 

as well. There was visual evidence of collapsed floors. 

Building Interiors 

1. The view through many windows was blocked by interior walls. 

2. The mass of aircraft solid combustibles was significant relative to the mass of the building 

combustibles in the impact zone. 

3. In laboratory experiments, impulses like those estimated from the aircraft caused serious damage 

to the ceilings. This is consistent with the accounts of survivors from floors below the impact 

zone. This damage enabled “unabated” heat transport over the walls and to the joists.Small areas 

of thin or missing insulation can lead to accelerated heating over much larger lengths of steel. 

Combustion Modeling 

1. FDS is a useful tool to recreate the burning of the complex arrays of combustibles that existed in 

the WTC buildings, provided that the initial damage conditions and combustible descriptions are 

accurate. 

2. FSI is a tractable construct for linking the output of a computational fluid dynamic model of the 

fire-generated thermal environment in the building compartments to a FEM of the building 

structure. 
Chapter 2 

106

2.7 STRUCTURAL FIRE RESPONSE AND COLLAPSE ANALYSIS 

(PROJECT 6) 

2.7.1 Project Objective 

Both the north and south towers, WTC 1 and WTC 2, were severely damaged by the impact of 

Boeing 767 aircraft, yet they remained standing for some time. The ensuing fires were observed to move 

through both buildings until their eventual collapse. The extent and relative importance of the structural 

damage caused by the aircraft impact, and subsequent weakening due to the fires, is still being 

investigated. WTC 7 was reported to be damaged by falling debris from the collapse of WTC 1. The 

fires in WTC 7 that burned for much of the day appeared to play a key role in the building collapse. 

Project 6 addresses the first primary objective of the NIST-led technical investigation of the WTC 

disaster: to determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the 

aircraft and why and how WTC 7 collapsed. Specifically, the objective of this Project is to determine the 

response of structural components and systems to the impact damage and fire environment in 

WTC 1, 2, and 7, and to identify probable structural collapse mechanisms. 

2.7.2 Project Approach 

Three steps are required to determine the response of structural components and systems to fire 

conditions. First, the thermal environment (radiation flux and temperature fields) for the floors involved 

in fire are determined using computational fluid dynamics calculations. The predicted upper layer gas 

temperatures vary both spatially and temporally. Next, transient thermal analysis is used to predict the 

time-temperature relationship for the structural components and systems for bare and fireproofed steel 

conditions. Finally, the time-dependent structural response of the components and systems to the 

estimated service loads and elevated temperatures is computed using thermal-mechanical FEA. 

Project 6 relies heavily on information provided by other projects, specifically: 

• Reference structural models of typical floor and exterior wall subsystems of each WTC 1 and 

WTC 2 tower (Project 2) 

• Extent of aircraft damage to WTC 1 and WTC 2 (Project 2) 

• Mechanical properties of the steels, welds, and bolts used in the construction of the towers, 

including elastic, plastic, and creep properties from 20 oC to 700 oC (Project 3) 

• Thermal properties of spray-on fire resistant materials (SFRM) (Project 5) 

• Temperature time-histories for various components, sub-systems and systems for both 

standard fires (e.g., ASTM E 119) and real fires based on fire dynamics simulations 

(Project 5). 

This project is divided into several tasks as follows: 

• Evaluate the structural response of floor and column subsystems under fire conditions. 
Progress on the World Trade Center Investigation 

107

• Evaluate the response of the WTC towers under fire conditions, both with and without 

aircraft impact damage. 

• Identify and evaluate candidate hypotheses for initiation and propagation of collapse, and 

estimate the uncertainty for probable collapse initiation and propagation mechanisms. 

• Conduct tests of structural components and systems under fire conditions. 

• Report on the performance of open-web steel trussed joist systems in fire. 

• Analyze the response of WTC Building 7 under fire conditions. 

Work completed to date on the above tasks is summarized in the following sections. 

2.7.3 Fireproofing of WTC Towers 

In May 2003, NIST issued an interim report on the Procedures and Practices Used for Passive Fire 

Protection of the Floor System of the World Trade Center Tower Structures as Section 3.3 of the 

May 2003 Progress Report. The report summarized factual data contained in documents provided to 

NIST by the PANYNJ and its contractors and consultants; by Laclede Steel Company, the firm that 

supplied the floor trusses for the WTC towers; and by United States Mineral Products Co. (USM) doing 

business as Isolatek International, the manufacturer of the fireproofing material. 

The report discusses the applicable building codes and building classification system, which dictates the 

fire rating required for structural members and assemblies. The structural system for the WTC towers 

was constructed predominantly with steel, which, in general, requires protection from fire to maintain its 

strength and stiffness. Available information on the spray-on fireproofing and the procedures and 

practices used in its selection and application is presented. Additionally, the report discusses the 

procedures and practices used to determine whether tests were needed to evaluate the fire endurance of 

the structural elements, and it presents the results from one such test. 

In May 1963, the Port Authority instructed its consulting engineers and architects to comply with the 

NYC Building Code for the design and construction of the WTC towers. Because the NYC Building 

Code was being revised during this period, the plans for fire protection of the structural steel underwent 

concurrent modification. While available records suggest that the fireproofing of the columns, beams, 

and spandrels was not a subject of concern, fireproofing of the floor bar joists was the focus of continuous 

reassessment and revision. 

A few of the more significant interim findings are: 

• The WTC towers were identified as Occupancy Group E – Business, and classified as 

Construction Class IB in accordance with the 1968 NYC Building Code. This classification 

required that the columns and floor systems of the towers have a 3 h and 2 h fire endurance, 

respectively. 
Chapter 2 

108

• The steel trusses that supported the floors of WTC 1 and WTC 2 were fireproofed with 

specified 1/2 in. of spray-on fire-protection although the technical basis for the selection of 

fireproofing material and its thickness are not known. 

• In 1999, a decision was made to begin upgrading the fireproofing to a specified 1 1/2 in. 

thickness as tenant spaces became unoccupied. In general, the floor systems in WTC 1 

subject to impact and fire conditions had been upgraded; the floors in WTC 2 subject to 

impact and fire conditions had not been upgraded. 

• The fire protection of a truss-supported floor system by directly applying spray-on 

fireproofing to the steel trusses was innovative at the time the WTC towers were designed 

and constructed and, while the benefits of conducting a full-scale fire endurance test were 

recognized by the building designers, no tests were conducted on the floor system used in the 

WTC towers to establish a fire endurance rating. 

The specified material and thickness of SFRM at the time of construction are as given in Table 2–7. 

Table 2–7. Specified passive fire protection. 

Structural 

Component 

Member 

Size Location Material 

Thickness 

(in.) 

Floor trusses All All Cafco DC/F 1/2 

Interior columns < 14WF228 All Cafco DC/F 2 3/16 

≥ 14WF228 All Cafco DC/F 1 3/16 

“Heavy” Exterior 

faces 

Exterior columns Cafco DC/F 1 3/16 

“Heavy” Interior 

faces 

Vermiculite 

aggregate 

7/8 

All Exterior 

face 

Spandrel beams Cafco DC/F 1/2 

All Interior face Vermiculite 

aggregate 

1/2 

2.7.4 Response of WTC 1 and WTC 2 Floor and Column Systems under Fire 

Conditions 

The detailed component and subsystem models will provide guidance for the analysis of the larger, global 

analysis of each WTC tower under damage and fire conditions. To determine the structural response of 

components and subsystems under fire conditions requires the development of nonlinear structural models 

that account for gravity (service) and thermal loads, temperature dependent material properties, and 

nonlinear structural behavior, such as plastification and large deflection effects, including instability. 

These nonlinear structural models are then subjected to thermal-mechanical analysis to determine the 

time-dependent structural response to the estimated service and fire loads. The commercially available 

FEA code, ANSYS (version 8.0), is being used for the thermal and mechanical analyses. 
Progress on the World Trade Center Investigation 

109

bottom chord 

web diagonal 

knuckle top chord

The analytical work is being conducted with the assistance of Simpson Gumpertz & Heger Inc. under a 

contract from NIST and includes the following tasks: 

• Task 1. Component, Connection, and Subsystem Structural Analysis 

• Task 2. Global Analysis of the WTC Towers Response to Fire Without Impact Damage 

• Task 3. Global Analysis of the WTC Towers Response to Fire With Impact Damage 

The scope of work under Task 1 includes: (1) the development and validation of ANSYS models of the 

full floor and exterior wall subsystems, (2) evaluation of structural responses under dead and live loads 

and elevated structural temperatures, (3) identification of failure modes and failure sequences, and the 

associated temperatures and times-to-failure, and (4) identification of simplifications for the global 

models and analyses. 

Selected technical results and findings for the typical floor system and its components are presented in the 

following sections. More detailed coverage is given in Appendix K, Interim Report on Structural Fire 

Response and Collapse Analysis. A simplified approach to the analysis of the WTC truss-framed floor 

system response to fire is presented in Appendix M, Interim Report on 2-D Analysis of WTC Towers 

Under Gravity Load and Fire. 

Structural Models and Analyses 

Structural FEMs of components and subsystems have been 

developed for the following: 

• Shear connector between the truss and concrete slab, 

referred to as a knuckle, shown in Figure 2–9 

• Truss-to-column bearing seats 

• Truss section, including composite floor slab, 

knuckles, and truss seat connections to columns 

• Single-story exterior column for a 9-story height 

• Exterior wall subsystem consisting of a 3-by-3 panel section of the exterior wall, where a 

panel is 3 columns wide and 3 stories long 

• Full floor subsystem including the concrete slab, truss seat connections to columns, and core 

floor area 

The truss section model (see Figs. 2–9 and 2–10) includes the following: 

• Temperature-dependent elastic material properties for both steel and concrete 

• Temperature-dependent steel plasticity 

Figure 2–9. Features of 

truss model. 
Chapter 2 

110

Figure 2–10. Truss model. 

• Buckling of truss members 

• Failure of knuckle leading to loss of composite action 

• Failure of studs on the strap 

• Failure of stud between the spandrel and the concrete slab 

• Failure of truss resistance welds between the web diagonals and the chords 

• Failure of the exterior and interior truss seats 

The full floor subsystem model includes the following: 

• The main trusses and bridging trusses 

• Concrete slab with metal deck 

• Strap and seated connections to columns 

Plan View 

40 in.

Stud

Stra

Main 

Trusse

s

Spandre

Exterior 

Column

Y 

 X 

Elevation View

Span = 713 in 

Floor Height = 288 in 

Span = 713 in. 

Floor Height = 288 in. 
Progress on the World Trade Center Investigation 

111

• Restraint provided by interior and exterior columns 

The model was developed by translating a SAP2000 full floor model developed under Project 2 into 

ANSYS format and validating the analysis results against the SAP2000 model for design loads. A corner 

of the full floor model is shown in Fig. 2–11. 

Figure 2–11. Converted ANSYS model of floor 96. 

Summary of Technical Results 

Capacity of Truss-to-Column Connections 

The horizontal and vertical load capacities of the twelve truss-to-column connection configurations on 

floor 96 have been calculated. These calculated capacities were used to develop simplified models of the 

connection behavior for use in the floor and exterior wall subsystem analyses. As an illustration, 

Fig. 2–12 shows a seated connection of the floor truss to the exterior wall of the tower (spandrel plate). 

The connection is designed to carry vertical floor loads and horizontal loads that are at least 2 percent of 

the column design load. These connections may be subjected to large horizontal forces, and the capacity 

of the exterior truss connection under such circumstances must be ascertained. 
Chapter 2 

112

Stand-off

Seat angle

5/8 in. diameter bolt

5/8 in. diameter bolt

Bearing angle

Truss top chord

Strut

Truss top chord

Gusset plate

Strut

Column/spandrel

Bearing angle

Stand-off

Seat angle

Gusset plate

Column/spandrel

Figure 2–12. Exterior connection of the floor truss to the spandrel plate. 

The failure modes considered for the truss-to-column connections are: (1) failure of the groove weld 

between gusset plate and spandrel, (2) failure of the fillet weld between the gusset plate and the truss top 

chord, (3) tensile failure of the gusset plate, (4) bolt shearing off, (5) bolt bearing, (6) bolt tear-out, and 

(7) block shear failure. Possible failure sequences are illustrated in Fig. 2–13. Of the seven exterior 

connections types that were analyzed, path A is the failure sequence most frequently followed, which is 

described as follows: first the gusset plate yields across its section and then fractures, followed by truss 

sagging and deformation and the bolts slipping until they bear against the edge of the slotted hole, then 

the bolt shears off, and finally the truss walks off the seat. The travel distance for the truss to walk-off of 

the seat is 4 5/8 in. Sequence (A) and typical tensile force resistance for an exterior seat is shown in 

Fig. 2–14. 

Figure 2–13. Failure sequence of exterior seats for tensile forces. 

Truss walkoff seat

(C) 

(B) 

(A) 

Gusset plate 

Yields

Gusset plate 

Fractures

Fillet weld 

Fractures

Bolt 

shears off

Bolt 

shears off

Fillet weld 

Fractures
Progress on the World Trade Center Investigation 

113

0

20

40

60

80

100

120

140

160

0 0.2 0.4 0.6 0.8

20oC - 200oC

300oC

600oC

400oC

500oC

700oC

Truss travel distance (in.)

Tensile force resistance (kip)

600oC

500oC

400oC

300oC

20oC

50o

C

200oC

100oC

700oC

Gusset plate yields Gusset plate fractures

Bolt shears off

Slip resistance from 

bolt connection

At travel distance 4-5/8 in. 

truss walks off support 

Figure 2–14. Typical tensile force resistance of exterior seat connection. 

Knuckle Analysis 

The knuckle is a shear connector formed by the extension of the truss diagonals into the concrete slab. 

Composite action is developed due to the shear transfer between the knuckle and the concrete slab in both 

the longitudinal and transverse truss directions. The objective of the knuckle analysis is to predict its 

shear capacity when the truss and concrete deck act compositely and to develop a simplified model of the 

knuckle behavior for the full floor subsystem model. FEAs have been conducted and calibrated against 

tests of both longitudinal and transverse loading conditions that were conducted by Laclede Steel in 1967. 

Figure 2–15 shows the FEM of the longitudinally loaded knuckle, representing one quarter of the knuckle 

test specimen. The ANSYS LS-DYNA program, which is part of the ANSYS software package for 

explicit nonlinear structural analysis, was used for the analysis of the knuckle tests as it had a concrete 

material model for nonlinear behavior. Solid steel elements were used for the knuckle and channel 

members and the Psuedo Tensor material model in LS-DYNA was used for the concrete. The knuckle-toconcrete interface was modeled as a bonded or no-friction contact. The finite-element analysis results of 

the knuckle capacity depended on the steel-concrete interface assumption of bonded or no-friction 

contact. 
Chapter 2 

114

UXmax = -0.10

Symmetry

UX = 0

UZ = 0 

UZ = 0 

(2.1 in. region)

UZ = 0 

(2.1 in. region)

UXmax = -0.10

Symmetry

UX = 0

UZ = 0 

UZ = 0 

(2.1 in. region)

UZ = 0 

(2.1 in. region)

Figure 2–15. Finite element model of transversely loaded knuckle. 

Truss Analysis 

A typical long-span truss, designated as C32T1, is modeled to study its response to failure when subjected 

to dead and live loads and thermal loads. The model includes the following: 

• One truss of the pair of trusses at column line 143 in floor 96 of WTC1, 

• Two exterior columns (columns 143 and 144) with half the area and bending properties (see 

plan view of Fig. 2–10) , and a length of 24 ft (12 ft above and below the floor level), 

• The portion of the spandrel between the two exterior columns, 

• The portion of the slab (40 in. wide) between the two exterior columns, 

• One strap anchor that is attached to the truss top chord, concrete slab and the adjacent exterior 

column (Column 144), and 

• Exterior and interior seats, and the top plate at the exterior end. 

A typical slab section consists of 4 in. thick lightweight concrete on 22 gauge metal deck and has two 

layers of welded wire fabric. An equivalent thickness of 4.35 in. is used as the slab thickness to account 

for the fluted metal deck profile. The metal deck and the welded wire fabric are not included in the truss 

model. Steel bar joist trusses support the concrete slab and act compositely with it. The chords of the 

trusses consist of double angles while the web members are round bars. 

The truss and the columns are modeled with temperature-dependent elastic and plastic material properties. 

The concrete slab is modeled with shell elements. The nodes of the concrete slab are located at the 

neutral plane of the concrete slab with an offset relative to the nodes of the top chords. A low tensile 

yield stress is used to simulate concrete cracking. At knuckle locations, the top chord elements and the 

elements representing the concrete slab are connected by control elements with capacities determined 
Progress on the World Trade Center Investigation 

115

from the knuckle analysis. Studs on the strap between the top chord and column are also modeled by 

control elements that connect the strap to the slab. The exterior and core truss seats are modeled by a 

combination of control elements and link elements, which can have temperature-dependent capacities 

determined from the truss seat analysis. The interior column is modeled as a fixed support for the interior 

truss connection, allowing no lateral displacements at the floor level, as the column was braced by the 

core framing. 

Loading consists of gravity dead and live loads and temperature time-histories for all steel members, 

including the truss seats. The gravity loads include weight of the structure, superimposed dead load 

(including nonstructural dead loads due to architectural items and fixed service equipment), and a service 

live load equal to 25 percent of design live load. The steel and concrete temperatures were subjected to a 

uniform heating condition by ramping to a maximum temperature over 1,800 s and then holding the 

maximum temperatures for another 1,800 s. The steel temperature increased from 20 °C to 700 °C; the 

bottom surface of the slab increased from 20 °C to 700 °C, and the top surface of the slab increased from 

20 °C to 300 °C. This thermal load creates a linear temperature gradient through the slab from 300 °C at 

the top surface to 700 °C at the bottom surface of the slab. Elevated temperatures are not applied to the 

columns. 

The truss model can capture the following: 

• Temperature-dependent elastic material properties for both steel and concrete 

• Temperature-dependent steel plasticity 

• Buckling of truss members 

• Failure of knuckle – loss of composite action 

• Failure of knuckle causing loss of composite action 

• Failure of studs on the strap 

• Failure of stud between the spandrel and the concrete slab 

• Failure of the exterior and interior truss seats 

Figure 2–16 shows that the top chords of the truss yield in compression beyond 300 oC (for clarity, the 

concrete floor slab is not shown). This is due to a significant difference of coefficients of thermal 

expansion (CTE) between concrete and steel. At 500 oC, the CTE of steel is twice that of light-weight 

concrete. Bottom chords remain in the elastic range throughout the thermal loading. Web diagonal 

buckling starts around 350 o

C and, as seen in the figure, some diagonals are bent significantly in the plane 

of the truss by high axial force and end moments. 
Chapter 2 

116

MX

-23549

-18398

-13246

-8095

-2944

2207

7359

12510

17661

22812

Figure 2–16. Finite element solution for floor truss under 

gravity and temperature loads. 

Preliminary Findings 

Preliminary findings are described here for floor truss connection capacities, knuckle capacities, and floor 

truss response to the uniform heating condition. 

Floor Truss Connection Capacities 

Capacities of the interior and exterior floor truss connections, for both vertical (gravity) and horizontal 

forces (tension and compression), have been computed for the variety of connection types found on floor 

96 of WTC 1. In all cases, the sequences of failures of the connection components have been taken into 

account, as illustrated in Fig. 2–13, for the exterior seated connection under horizontal (tension) force. 

Capacities have been computed as a function of temperature for the applicable plate, weld, or bolt 

properties. It should be noted that while the computed vertical and horizontal capacities are primarily due 

to the loads they must support, construction-related decisions may have increased the capacity. For 

example, an available bolt or steel section size or a minimum allowable weld thickness may provide 

greater capacity than that required for design loads. 

Failure mode of the interior truss seat for vertical force is the fracture of the fillet welds at the seat-tochannel beam connection. Failure mode of the exterior truss seat for vertical force is fracture of the fillet 

welds at the stand-off-to-spandrel connection. Preliminary findings of connection capacities for vertical 

bearing forces at room temperature (20 o

C), 400 oC, 600 oC, and 700 oC are summarized in Fig. 2–17. 

Similarly, connection capacities for horizontal tensile forces at the same temperatures are shown in 

Fig. 2–18. For interior truss seat connections, the shear strength of the two bolts controls the horizontal 

tensile capacity. The connection capacity of exterior truss seats that follow failure sequence (A), as shown 

in Fig. 2–13, equals the failure load for the tensile capacity of the gusset plate. Note that the strength of 

the truss seat #1013 increases by approximately 38 percent at a temperature of 100 °C. For temperatures 

less than 100 °C, the horizontal capacity is controlled by the gusset fillet weld strength, and for 

temperatures above 100 °C, the bolt bears against the edge of the slotted hole and increases the capacity 

of the connection. 
Progress on the World Trade Center Investigation 

117

207 184

90

41

20C 400C 600C 700C

1411 1411 1411 1411 1411 1411 1411 1411 1411 1511 1411 1411 1311 1311 1313 

1013 

1212 

1212 

1212 

1611 

1111 

1111 

1111 

1111 

1111 

1111 

1111

23 23 23 23 22 22 21 21 20 20 

226A, 

17 

17 

17 

17 

17 

15 

15

#1411 #1511 #1311

#1013 and #1313

#1212

#1611

#1111

#20 #226A #15 and #17

#21 

#22 and #23 

385

343

167

76

20C 400C 600C 700C

226 201

98

45

20C 400C 600C 700C

94 84 60

27

20C 400C 600C 700C

187 167

82

37

20C 400C 600C 700C

94 84

45 29

20C 400C 600C 700C

193 172

84

38

20C 400C 600C 700C

221 197

96

44

20C 400C 600C 700C

140 126

78

35

20C 400C 600C 700C

265

236

116

53

20C 400C 600C 700C

94 84 58

26

20C 400C 600C 700C

111 100

53 34

20C 400C 600C 700C

Figure 2–17. Truss seat capacity for vertical forces. 
Chapter 2 

118

182 156

67

32

20C 400C 600C 700C

100 115

42 20

20C 400C 600C 700C

134 120

62

31

20C 400C 600C 700C

104 93

49 25

20C 400C 600C 700C

1411 1411 1411 1411 1411 1411 1411 1411 1411 1511 1411 1411 1311 1311 1313 

1013 

1212 

1212 

1212 

1611 

1111 

1111 

1111 

1111 

1111 

1111 

1111

23 23 23 23 22 22 21 21 20 20 

226A, 

17 

17 

17 

17 

17 

15 

15

#1311, #1411, and #1511

#1013

#1212

#1611

#1111

For all interior seats 

44 34 9 4

20C 400C 600C 700C

134 120

62

31

20C 400C 600C 700C

#1313 

182 156

67

32

20C 400C 600C 700C

Figure 2–18. Truss seat capacity for horizontal forces. 

Knuckle Analysis 

FEA results of the knuckle under longitudinal loading are shown in Fig. 2–19. Displacement results 

shown in Fig. 2–20 indicate a significant dependence on the characteristic of the interface between the 

steel and concrete. Results show that each knuckle has a capacity in the range of 15 klb to 35 klb, 

depending on the steel-to-concrete interface assumption. Results of longitudinal shear tests conducted by 

Laclede Steel in 1967, using normal weight concrete with an average compressive strength of 3,707 psi, 

indicate an average shear capacity of approximately 28.3 klb per knuckle. After adjusting for the strength 

of in-place light-weight concrete of 4,100 psi (i.e., multiplying 28.3 klb by the ratio of 4,100 to 3,707 psi), 

the longitudinal shear capacity of the knuckle is approximately 31 klb per knuckle. This is consistent 

with the finite element solution for the fully bonded case shown in Figure 2–21 (note results are for two 

knuckles). A shear capacity of approximately 30 klb per knuckle is used for subsequent analyses of the 

floor truss. 
Progress on the World Trade Center Investigation 

119

Figure 2–19. Compressive stress in 

longitudinal shear. 

0

10000

20000

30000

40000

50000

60000

70000

80000

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Displacement (in.)

Force (lbs.)

0 Friction Contact

Fully Bonded

Figure 2–20. Shear force versus displacement for 

two knuckles under longitudinal shear. 

Truss Analysis 

The floor truss analysis was carried out dynamically with 5 percent Rayleigh damping and a temperature 

ramp set to 1.0 s. The floor slab had only gravity loads applied; no other loads related to floor diaphragm 

action were included. The analysis of truss behavior under the gravity plus thermal loading proceeded to 

a temperature of T=663 oC. Figure 2–20 shows the horizontal displacement of the column and the vertical 

midspan displacement of the truss. A positive horizontal displacement indicates that the exterior columns 

are pushed out, and a negative vertical displacement indicates that the truss is deflected downward. 
Chapter 2 

120

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 200 400 600 800

Temperature (°C)

Displacement (in)

Node 3908

diagonal bucklingknuckle failurebolt shear failure

seat walk-off

-35

-30

-25

-20

-15

-10

-5

0

0 200 400 600 800

Temperature (°C)

Displacement (in)

Node 540

diagonal bucklingknuckle failurebolt shear failure

seat walk-off

 (a) Horizontal displacement at Column 143 (b) Vertical displacement at midspan 

Figure 2–21. Floor truss response due to gravity load and uniform heating. 

As the truss and floor slab heat up, the column is pushed outward by thermal expansion. The trust top 

chord begins to yield in compression around 300 o

C due to the difference in coefficient of thermal 

expansion between steel and lightweight concrete. At approximately 340 o

C, web diagonals begin to 

buckle and the horizontal displacement at the exterior column reverses and begins to decrease. At 400 oC, 

knuckles start to fail sequentially from both interior and exterior supports toward the center. With the loss 

of composite action, the floor begins to sag at an increasing rate. Eventually, at about 500 oC, with the 

truss sagging almost 20 in., the bolts at the interior connection are found to shear. At 560 o

C, the exterior 

columns begin to displace inward, and the truss begins to act as a catenary. At 650 oC, the truss walks off 

the interior seat while the interior end of slab remains intact and continues to carry vertical load. For the 

truss to walk off the interior seat, the truss must shorten by 4 in. This shortening is caused mainly by the 

significant plastic deformation of the top chord of the truss (Fig. 2–14), resulting from differences in the 

thermal expansion of the top chord relative to the slab and the failure of the first two knuckles near the 

interior seat. At roughly 660 o

C, the gusset plate fracture at the exterior end which precipitates vertical 

failure of the exterior seated connection. 

The results for the additional debris weight show that the knuckles start failing when 2.4 times dead load 

is applied. Most knuckles fail before 3.0 times dead load. After the knuckle failure, the truss loses 

composite action between the truss and the concrete slab, and the vertical displacement increases 

significantly. As a result, the horizontal reaction force increases. 

Models of the truss, including knuckles with temperature-dependent capacities, diagonal weld failure, 

steel creep strains, and concrete cracking and crushing, are under study. 

Summary of Preliminary Findings 

Preliminary findings for a single truss and its seat connections and knuckles subject to service load 

conditions, uniformly increasing elevated temperatures in the steel, and an increasing temperature 

gradient in the concrete slab, can be summarized as follows: 
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• The floor truss first experiences increasing vertical deflections at midspan as it pushes 

outward and exerts a compressive lateral load on the exterior column. The exterior column 

begins to displace outward at the floor connection. 

• As web diagonals begin to buckle at 340 o

C, the midspan deflection continue to increase but 

the horizontal displacement of the exterior column begins to decrease. The maximum 

horizontal displacement of the exterior column is approximately 0.7 in. when the diagonals 

begin to buckle. The interior column is assumed to have no lateral displacements at the floor 

level, as it was braced by the core framing. 

• Knuckles at each end of the truss begin to fail as the steel and bottom surface of the slab 

temperatures reach 400 o

C, with knuckle failures moving progressively inward from the truss 

ends. The failure of web diagonals and knuckles at the ends of the truss reduce the flexural 

rigidity of the floor truss at the ends, further increasing the floor sag and decreasing the lateral 

outward force exerted on the columns. 

• The truss bearing angle slips until the bolt is bearing against the edge of the slotted hole. The 

bolt shears off at the interior seat connection at approximately 500 o

C. The floor truss sag 

increases to 20 in. when the bolt fails. 

• The interior end of the reinforced slab continues to carry vertical loads as the truss bearing 

angle continues to slip. At 560 o

C, the exterior column begins to be displaced inward as the 

floor truss continues to sag and exert vertical and horizontal tensile loads. 

• At 650 o

C, the truss begins to walk off the interior seat, followed by fracture of the gusset 

plate at the exterior connection at 660 o

C. Fracture of the gusset plate precipitates weld 

failure of the exterior seat connection resulting in complete loss of vertical support of the 

truss. 

• The truss model, with knuckle and seat connections, includes all potential failure modes that 

may occur under loading and thermal conditions, though the actual sequence of failure may 

differ under other loading and fire conditions. 

2.7.5 Standard Fire Endurance Tests of Floor System 

Standard Fire Tests of the steel truss-supported concrete slab floor system used in the WTC towers are 

being conducted by UL. The results of the testing will provide the fire endurance ratings of typical floor 

construction to evaluate three primary factors: (1) test scale, (2) fireproofing thickness, and (3) thermal 

restraint. Four ASTM E 119 Standard Fire Tests of the WTC floor construction will be performed as 

follows: 

• 17 ft span assembly, thermally restrained, SFRM thickness of 1/2 in. 

• 17 ft span assembly, thermally restrained, SFRM thickness of 3/4 in. 

• 35 ft span assembly, thermally restrained, SFRM thickness of 3/4 in. 
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122

• 35 ft span assembly, thermally unrestrained, SFRM thickness of 3/4 in. 

The first test represents current U.S. practice for establishing a fire endurance rating of a building 

construction. The test assembly, fabricated to meet the design of the World Trade Center steel joistsupported floor system, has a span of 17 ft. This span is typical of the floor assembly test furnaces used 

by the U.S. testing laboratories that routinely conduct the ASTM E 119 test for the construction industry. 

As is common practice, the floor assembly will be tested in the thermally restrained condition. This test 

will be conducted at UL’s Northbrook, Illinois, fire test facility. A second test will be identical except for 

the thickness of SFRM. 

The third and forth tests will be at twice the scale of the first two tests, with a span of 35 ft. This span 

represents a full-scale assembly of the 35 ft floor panel of the WTC floor system. The floor assembly for 

the third test will be thermally restrained as in the first two tests, thereby allowing direct comparison for 

the determination of the effect of test scale on fire endurance rating. The fourth test will be conducted in 

the thermally unrestrained support condition, which will allow direct comparison of the effect of thermal 

restraint on the fire endurance rating. The third and fourth tests will be conducted at the UL Canada fire 

test facility near Toronto. 

In all tests, individual structural members of the steel trusses with varying thickness of SFRM will be 

exposed to the standard fire environment, and temperatures will be recorded. This will allow comparison 

of results for various amounts of fireproofing based on the end point criteria for steel temperatures. 

The test specimens have been designed and fabricated to duplicate as closely as possible the actual floor 

system in the WTC towers. Laclede Steel shop drawings were used to ensure the specimens were 

dimensionally accurate. Properties of the constituent materials and components have been duplicated as 

closely as possible, including the steel angles and rods that make up the floor trusses, concrete 

(lightweight aggregate, air entrainment, etc.), metal deck, welded wire fabric, reinforcing steel, shop 

primer, and Cafco DC/F SFRM. NIST has overseen fabrication of the steel trusses, assembly of test 

specimens, and casting of the concrete slab and test cylinders. NIST will continue oversight of the 

installation of instrumentation and application of SFRM. 

The test assemblies have been fabricated as shown in Fig. 2–22, and at this time, the concrete floor slabs 

are drying to the ASTM E 119 specified moisture condition under controlled conditions to obtain concrete 

design strength. The specimens are currently drying in a temperature/humidity controlled environment to 

achieve the ASTM E 119 prescribed moisture equilibrium. 
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Figure 2–22. Fabrication of 35 ft span ASTM E 119 test assembly. 

2.7.6 WTC 7 

The structural response of WTC 7 to damage from debris and fires is being evaluated to identify possible 

collapse sequences and critical components that are consistent with the videographic and photographic 

records, interview accounts by individuals that were in or around WTC 7, and other available data. 

The analytical work is being conducted with the assistance of Gilsanz Murray Steficek LLP under a 

contract from NIST and includes the following tasks: 

• Task 1. Structural response analysis to identify critical components 

• Task 2. Structural analysis of possible collapse initiation hypotheses 

The scope of work under Task 1 includes (a) develop a nonlinear global structural model of WTC 7 and 

evaluate its performance under design gravity loads, (b) identify credible failure sequences for the 

structural model with service loads and initial structural damage by analyzing the effect of component 

failures (that may have occurred directly or indirectly from fires) on the structural system stability, 

(c) identify dominant failure modes for critical components and subsystems determined in (b) for service 

loads and elevated structural temperatures, (d) conduct parametric studies of critical subsystems to 

identify influential parameters, and (e) develop approaches to simplify structural analyses for global 

modeling and analyses. 

Selected technical results and finding for progress on Task 1 (a), (b), and (c), data collection of building 

conditions, working collapse hypotheses, and supporting analyses are presented in the following sections. 

Appendix L presents more detailed information about the WTC 7 structural design, observations about 

damage and fires, a timeline and description of the collapse sequence from videographic records, and 

working collapse hypotheses developed to date. Detailed thermal-structural analyses of selected collapse 

sequences are planned to refine the working hypotheses presented here and identify probable collapse 

hypotheses. 
Chapter 2 

124

Summary of Data Collection for Building Conditions 

Data that have been obtained about building conditions from photographic and videographic records and 

interview accounts include: 

• Structural damage to the south face and to the southwest corner from WTC 1 debris was 

reported by witnesses. A multi-story gash that extended across approximately a quarter to a 

third of the south face, in the lower portion of the face, was reported by a number of 

individuals, though details vary. This damage extended to the core area as two elevator cars 

were reported to be ejected from the elevator shaft at floor 8 or 9. Reported damage to the 

southwest corner was also seen in photographic records, which show approximately 

2 columns and related floor areas missing from floors 8 to 18. Multiple photographic and 

videographic records also appear to show damage on the south face that started at the roof 

level and severed the spandrels between exterior columns near the southwest corner for at 

least 5 to 10 floors. However, the extent and details of this damage have not yet been 

discerned, as smoke is present. 

• The south face was covered by smoke the entire day, following the collapse of WTC 1. This 

smoke appeared to be emanating from fires in WTC 5, 6, and 7, though the contribution from 

each building cannot be discerned. The smoke was dense enough that no information about 

structural damage to the south face has been seen in photographic or videographic records. 

• No fires were observed in WTC 7 after WTC 2 collapsed, but fires were observed after 

WTC 1 collapsed. Fires, or evidence of fires, were observed initially on the south face and 

near the southwest corner. Many of these fires appeared to burn out before noon to 2 p.m. 

Around 2 p.m., fires were observed in photographic and videographic records to be burning 

across floors 11 and 12 on the east face, from the south to the north. Around 3 p.m., fires 

were observed on floors 7 and 12 along the north face. The fire on floor 12 appeared to 

bypass the northeast corner and was first observed at a point approximately one third of the 

width from the northeast corner, and then spread both east and west across the north face. 

Some time later, fires were observed on floors 8 and 13, with the fire on floor 8 moving from 

west to east and the fire on floor 13 moving from east to west. At this time, the fire on floor 7 

appeared to have stopped progressing near the middle of the north face. The fire on floor 8 

continued to move east on the north face, eventually reaching the northeast corner and 

moving to the east face. Around 4:45 p.m., a photograph showed fires floors 7, 8, 9, and 11 

near the middle; floor 12 was burned out by this time. 

• Floor 5 did not have any exterior windows, and any fires that may have burned on this floor 

would not have been visible in photographic or videographic records. However, there was a 

fuel distribution system on the south, west and north floor areas. Given the variability of 

damage descriptions for the south face from WTC 1 debris impact, fires on floor 5 will be 

considered as a possible fire location, subject to further data and/or analysis on building 

conditions that improve knowledge of fire conditions in this area. 

• The first exterior sign of structural failure in WTC 7 was the sinking of the east penthouse 

roof structure into the building. Photographic and videographic records taken from the north 

have provided information about the sequence of failure events and their relative times. 
Progress on the World Trade Center Investigation 

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Other key observations include window breakage along the east side of the north face, 

occurring almost simultaneously with the sinking of the east penthouse structure, an 

approximate 5 s delay before the other roof structures also sink into the building core, a 

second set of window breakage along the west side of the north face occurring 

simultaneously with the other roof structure movements, and the appearance of the entire 

north façade above floor 13 appearing to drop as an intact unit 8 s after the east penthouse 

movement was first detected. 

Structural Models and Analysis 

Analyses have been conducted to assess proposed collapse hypotheses that are based upon available 

information about the building conditions and sequence of events prior to the global collapse. Models 

and analyses to date have included the following: 

• Structural analysis of WTC 7, as built in 1985, for design gravity and wind loads on a global 

structural model. Development of a reference model for understanding global behavior of the 

structure, and providing a foundation for other models. 

• The global structural model was modified for reported structural damage and estimated 

service loads for analysis of the structural system response to building condition after debris 

impact. 

• A kinematic structural analysis assisted with identifying possible failure sequences following 

an initiating event, such as a column or group of columns becoming unstable as steel 

temperatures reach critical levels. 

• Typical tenant floors were analyzed to identify the sequence of floor system load 

redistribution and component failure for initiating events, such as failure of a support column. 

• The global structural model was used to develop a submodel of the lower 10 floors of WTC 7 

to evaluate the effect of component failure and load redistribution within this portion of the 

structural system. 

• Thermal-structural analysis of critical columns to fire scenarios for proposed collapse 

hypotheses have been conducted to evaluate the effect of component response to fires, 

including time to reach critical uniform elevated temperatures and temperature gradients 

across component cross-section and length. 

• These models were used to consider a range of possible failure scenarios, based upon 

knowledge of the reported damage, observed or possible fire scenarios, and the exterior 

failure sequence recorded on videos. 
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126

Summary of Technical Results 

Some of the more important technical results developed to date, based upon known building conditions 

and analyses, include the following: 

• The perimeter moment frame was highly redundant and was able to redistribute the loads 

around the reported damage areas without over-stressing or failing surrounding members. 

• The working hypothesis has been developed around four phases of the collapse that were 

observed in photographic and video records: the initiating event, a vertical progression at the 

northeast corner of the building, and horizontal progression from the east to west side of the 

building, and global collapse. 

• The first exterior sign of failure in WTC 7 was the displacement at the center of the east 

penthouse roofline, which appeared to be a kink in the roof line. This kink aligns with 

columns 79, 80, and 81. This observation has led to postulating initiating event failure 

sequences that lead to the failure of one of these columns. 

• Thermal-structural analyses have been used to evaluate components in postulated initiating 

event failure sequences. Analyses to date have included single and multi-story columns 

subject to severe fires (gas temperatures of 1,100 oC), intact and damaged fireproofing, intact 

and missing lateral support conditions, and temperature-dependent material properties (yield 

strength, ultimate tensile strength, thermal expansion, and creep strains) to evaluate the 

structural response to thermal softening, axial expansion, and bowing from thermal gradients. 

These types of analyses continue to be developed and refined. Simpler initiating events that 

have been analyzed appear unlikely as initiating events, such as the direct failure of columns 

79, 80, or 81 in the lower portion of the building for intact fireproofing and the fires observed 

in the photographic and videographic records. Other initiating event sequences continue to 

be postulated and analyzed. Possible initiating events include consideration of interior 

columns 69, 72, 75, 78, and 78A, the east transfer girder (which supports column 78A and 

frames into transfer truss #2), and adjacent framing and floor systems and their response to 

possible structural and fireproofing damage from debris impact and subsequent fire growth 

and progression. See Appendix L for component locations. 

• Interior columns 79, 80, and 81, were located directly below the east penthouse on the roof 

and supported large tributary areas. The tenant floor areas on the east side of the building had 

spans of approximately 50 ft between columns. Their failure would likely result in failure of 

the tributary floor system, as analysis indicates that the floors would not be able to 

redistribute their loads. This failure mechanism would progress vertically upward within the 

failed bay to the roof level, and would not be visible from the exterior until the east penthouse 

lost support, as shown in Fig. 2–23. Available information on the floor-to-column 

connections indicate that the connections would fail under this scenario without significantly 

damaging the perimeter or interior columns. 
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Figure 2–23. Collapse initiation and vertical 

progression on the east side of WTC 7. 

• The debris pile from a vertical failure progression on the east side of the building would 

damage or sever transfer girders and trusses between the fifth and seventh floors, shown in 

Fig. 2–24. Two system responses to this secondary damage have been postulated and are 

being further investigated. (1) The columns supported by these transfer components would 

become unstable, and their loads would transfer to adjacent core columns. If the columns 

could not support the transferred loads, the column instability would progress sequentially to 

adjacent core columns. (2) The floor-to-column connections in the fifth and seventh floors 

are strong enough to impose lateral displacements upon the other core columns, particularly 

in the center of core where there were elevator areas without reinforced concrete slab. Such a 

horizontal pull would fail the columns at their connection near the seventh floor, as shown in 

Figs. 2–25 and 2–26. 

• The core columns failed sequentially and redistributed loads until the building loads could no 

longer be supported, and the global collapse occurred with few external signs prior to the 

system failure, as illustrated in Fig. 2–26. 

This working hypothesis has been developed to date for the data and analyses described. Continued 

analyses and evaluation continue toward determining probable collapse sequences. 

East 

Penthouse

West 

Penthouse

Substation

48th Fl

40th Fl

30th Fl

13th Fl

Collapse 

Initiation

5th Fl

7th Fl

Column 

above 

Initiation

Penthouse 

Kink

Screenwall East Penthouse Screenwall West 

Penthouse

Window 

Breakage 

 around 

Substation

48th Fl

40th Fl

30th Fl

13th Fl

Vertical Collapse 

Progression

5th Fl

7th Fl

Failed 

Column
Chapter 2 

128

Figure 2–24. Transfer trusses and girders between the fifth and seventh floors. 

Figure 2–25. Example of horizontal progression of failure in core columns following 

damage to transfer components on the east side of the building. 

Truss#1 

Truss#2

Cantilever Girders

Truss#3

Girders 

Con-Edison Substation 

80 77 74 71 68 65

5th

7th

9th 

Truss #2

Splices Fail in 

Bending

Column Fails 

in Bending E
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Figure 2–26. Horizontal progression to the west side of WTC 7. 

2.8 OCCUPANT BEHAVIOR, EGRESS, AND EMERGENCY 

COMMUNICATIONS (PROJECT 7) 

The purpose of this project is to determine the behavior and fate of occupants and responders - both those 

who survived and those who did not - by collecting and analyzing information on occupant behavior, 

human factors, egress, and emergency communications in WTC 1, 2, and 7, and evaluating the 

performance of the evacuation system on September 11, 2001. 

2.8.1 Project Objectives 

This project is divided into six tasks as follows: 

• Task 1. Gather baseline information on the evacuation of the WTC buildings on 

September 11, 2001 through a comprehensive, systems-oriented, and interdisciplinary data 

collection effort focused on occupant behavior, human factors, egress, and emergency 

communications (including instructions given, interpretation of instructions, and response to 

instructions). This involves the collection of new data from people affected by the WTC 

attacks (e.g., building occupants, building operators, and first responders via direct accounts 

from survivors and families of victims), especially those who had to evacuate the buildings. 

Experts in human behavior and statistical sampling were used develop a data acquisition 

strategy that considers various data collection methods such as interviews and questionnaires. 

Inputs and suggestions were obtained from organizations with an interest in the content of the 

data collection effort. Additionally, written accounts, transcripts of (emergency) 

Substatio Column

Horizontal 

Collapse 

Progressio

Debris Impacts

Strong Floor /

Truss Below

East Penthouse 

Sinks into Building

Substatio

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Sky 

Horizontal 

Progression 

Initiates 

Failure in 

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Global 

Collaps

Window 

Breakag

Remainder of 

Building Sinks
Chapter 2 

130

communications, published accounts, and other sources of egress related information were 

obtained, in coordination with other data collection efforts for the investigation. 

• Task 2. Collect archival records from prior WTC evacuation incidents (e.g., 1975 fire, 

1977 blackout, 1980 bomb scare, 1990 power outage, and 1993 bombing) and practice 

evacuations, including oral history data from floor wardens and fire safety directors. These 

records are compared and contrasted with the September 11, 2001, incident evacuation. 

Changes made to the evacuation procedures following the earlier incidents and in recent years 

will be evaluated in the context of the experience on September 11, 2001. 

• Task 3. Document pre-event data for WTC Buildings 1, 2, and 7. This information includes, 

but is not limited to, physical aspects of building egress components, such as stairs (width, 

number, location, vertical continuity), evacuation lighting, back-up power, elevators (number, 

operational before and after impact, role in evacuation), and active fire protection systems 

(sprinklers, manual suppression, fire alarms, smoke control). Building plans, emergency 

plans, type and frequency of evacuation drills, occupancy level and distribution on the 

morning of September 11, 2001, and communications also constitute pre-event data. This 

information provides a baseline for evaluating the performance of the egress system. 

• Task 4. Store the information collected in Task 1 in a database. Additionally, information 

from third-party sources, such as television interviews and newspaper articles, as well as 

other relevant published material, will be analyzed, examined, and assembled in the database. 

• Task 5. Analyze the data to study the movement of people during the evacuations, decisionmaking and situation awareness, and issues concerning persons with disabilities. A timeline 

of the evacuation will be developed using the results of these analyses together with other 

data sources. This timeline will be compared with the timeline of the structural response, the 

development of the interior conditions (fire and smoke), as well as activation of the active fire 

protection systems. The characteristics of the WTC evacuation designs and protocols will be 

evaluated, including the performance of stairs and elevators, emergency communications, and 

the temperature and smoke conditions. The designs will also be compared with building code 

requirements and practices for tall buildings in other major cities worldwide. The observed 

evacuation data will be compared with results obtained using alternate egress models to better 

understand occupant behavior and identify needed improvements to existing egress models. 

In addition, the evacuation experience will be compared with previous evacuation incidents in 

these buildings. The results of the analyses will be reviewed in the context of occupant 

protection practices for tall buildings, including the consideration of full evacuation and 

phased evacuation strategies. 

• Task 6. Report preparation. The results of this project will be synthesized into a chapter to 

describe the occupant behavior, egress, and emergency communications in WTC 1, 2, and 7, 

and the performance of the evacuation system. The project staff will contribute to drafting 

the final investigation report for review by the National Construction Safety Team (NCST) 

Advisory Committee. 
Progress on the World Trade Center Investigation 

131

2.8.2 Project Status 

Project 7 has made significant progress since the last interim report. Data collection is substantially 

complete, including interviews and focus groups with survivors and family members. Significant 

portions of the data analysis are complete, and egress modeling is under way. 

Face-to-Face Interviews, Telephone Interviews, and Focus Groups 

Over 220 face-to-face interviews have been completed with occupants of WTC 1, 2, and 7, and family 

members of victims. These numbers do not reflect interviews conducted with first responders or other 

key building personnel. A preliminary analysis of the interview data indicates that the topic areas of 

interest and physical building locations of the occupants have resulted in an adequate number of 

interviews to support analysis. While additional face-to-face interviews may occur in the near future 

should conditions necessitate, no further interviews are scheduled. Face-to-face interviews typically 

required two hours, with some requiring significantly more or less depending upon the particular 

experiences of the respondent. Data were collected using a cooperative, electronic format, previously 

described as the cue-action-reason technique. The technique formalized a logical, chronological data 

collection, enabling detailed recall, and diminishing gaps in continuity of actions. 

Egress Simulation 

The purpose of the modeling project is to obtain evacuation times for two different evacuation procedures 

for the WTC towers, phased evacuation and total evacuation of the occupants from the buildings. The 

first objective is to simulate a phased evacuation of WTC 1 or WTC 2, which involves the evacuation of 

the occupants on the fire floor, the floor above, and the floor below to a specific floor of the building. For 

this simulation, a representative fire floor was chosen within one of the towers, and the simulation 

consisted of occupants from the fire floor, the floor below, and the floor above evacuated to two floors 

below the fire floor. The purpose of the phased evacuation simulation is to obtain evacuation results 

(time) on how the evacuation from a fire emergency was supposed to work. 

The second objective is to simulate total evacuation of the building, and involves three different 

scenarios. The first total evacuation scenario is the simulation of a full capacity building evacuation 

(without damage) involving all occupied floors of a WTC tower. The second scenario involves a full 

capacity building evacuation of WTC 1 with plane damage blocking floors 91 to 110 and a full capacity 

building evacuation of WTC 2 with plane damage blocking floors 78 to 110. The second scenario will be 

used to show how long it would have taken occupants in a scenario resembling the September 11, 2001, 

emergency to evacuate if each tower was fully occupied. Finally, the third scenario is a simulation of a 

September 11, 2001, capacity building evacuation from a WTC tower. The results from the simulation of 

the third full evacuation scenario will be compared with evacuation time results obtained from the 

telephone interviews in an attempt to verify the accuracy of the model. None of the simulations run for 

this project involves the simulation of the fire environment. 

Three evacuation models will be used to perform the modeling objectives outlined in the previous 

paragraphs. These models are Simulex (IES 2000 and IES 2001), EXIT89 (Fahy 1999), and 

buildingEXODUS (Galea et al. 2001). Simulex will be used in a limited capacity to simulate the phased 
Chapter 2 

132

evacuation and observe occupant movement on specific floors. EXIT89 and buildingEXODUS will be 

used in all of the objectives stated above. 

Other September 11, 2001, Data Collection 

A systematic review of all 9-1-1 emergency calls related to the WTC attacks between 8:46 a.m. and 

10:28 a.m. was completed. Occupants from inside the WTC called 9-1-1, sometimes repeatedly, in order 

to report where they were trapped, the conditions on the floor, and how many other people they were 

with. The callers often requested advice, guidance, information about the attack, and the progress of the 

rescue efforts. The information relayed to the 9-1-1 system was unique and invaluable in specificity and 

timing. Information related to building damage, fire and smoke spread, and occupant mobility has been 

integrated with other aspects of the NIST investigation. 

Seven-hundred forty-five media accounts from were compiled and analyzed. Finally, information about 

significant previous building fires and evacuations has been compiled as background. An overview of the 

preliminary results of both analyses is included in the next section, while more detailed analyses are 

contained in Appendix N and Appendix O. The database can be obtained electronically at 

http://wtc.nist.gov. 

Collection of documents related to the design and maintenance of the egress and emergency 

communication systems is complete. This collection was coordinated with Project 1, Analysis of Codes, 

Standards, and Practices. 

Data Analysis 

Every data channel is being analyzed and synthesized to form a complete understanding of the evacuation 

of WTC 1, 2, and 7 on September 11, 2001. There are two primary analysis modes, proceeding in 

parallel: quantitative and qualitative. Quantitative analysis techniques are being implemented with the 

telephone interview data. An example of the quantitative data analysis can be found in Appendix N. The 

SPSS 12.0 data analysis package is being used to analyze the telephone interview data. The qualitative 

analysis is being consolidated with the ATLAS.ti 4.2 program. All face-to-face interviews, 9-1-1 

emergency call records, focus group notes, emergency communications, and formal complaints will be 

simultaneously analyzed using over 100 coding variables. The coding variables encompass the entire 

scope of Project 7 objectives and are coordinated with the analyses conducted by other projects within the 

investigation, particularly Project 8, First Responder Technologies and Guidelines. 

2.8.3 Interim Findings and Key Issues 

Significant Historical Building Incidents 

Although historically the WTC attack, subsequent fires, and building collapses are arguably the most 

significant fire event where building egress played a critical role, concern for fires in large buildings is 

hardly new. In many cases, provisions in current building codes evolved as a result of high-rise fires as 

early as 1911 where egress or collapse was an important issue. These provisions include remoteness and 

protection of egress stairways, the need for control of combustible contents of buildings, the need to 
Progress on the World Trade Center Investigation 

133

provide sprinklers or other alternatives for high-rise buildings, and concern that sprayed-on fireproofing 

may not adhere properly to surfaces or may be dislodged. 

Analysis of Published Accounts 

NIST contracted with the NFPA to collect first-person accounts from newspapers, radio and television 

programs, e-mail exchanges, and a variety of websites. Over a period of 18 months, a total of 

745 first-person accounts were collected. These accounts had been published up to 14 months after the 

event. Although media accounts do not provide the scientific rigor of a proper study, they do present 

important insights into the events of the day. The objectives of the analysis of the first-person accounts 

were to gain insight into the variability of human behavior and response time displayed during the 

evacuation, with the findings to be used as a guide for additional investigation. It should be 

acknowledged that content analysis of first-person accounts has important limitations: the questions asked 

by journalists are usually unknown, and, some details might be left unreported and the most dramatic 

stories over represented. Consequently, the results cannot be generalized to the overall population of the 

WTC towers. 

To analyze the content of the first-person accounts, a questionnaire tool was developed and used to 

“interview” each account. The questionnaire had 33 questions such as: “On what floor was the person?,” 

“What was the first cue of the event?,” “Was the person injured?,” or “What were the conditions in the 

stairs?” Not every account provided answers for all 33 questions, since some accounts lacked certain 

details, but this is similar to a respondent who did not answer some questions in a survey. Once the 

745 first-person accounts were summarized, multiple accounts from the same person were merged into 

one, which provided accounts for 465 individuals. (Some survivors provided multiple accounts through 

different sources.) Before any analysis began, the database was further limited to the 435 civilian 

building occupants who were in either WTC 1 or WTC 2 on that day. 

In summary, the accounts analyzed were from 435 individuals; 251 occupants of WTC 1 and 

184 occupants of WTC 2. They represented the three different floor strata of the two towers. The 

accounts were mainly from men (314 versus 118) and from people varying in age from 20 to 89 years old. 

Among the interesting results found was the means of egress used that morning. Out of 158 people who 

mentioned their means of egress in WTC 2, 18 used the elevators, and 26 used a combination of stairs and 

elevators to leave the tower. It was found that the higher the person was located in the tower initially, the 

more likely it was that this person used an elevator to evacuate. In WTC 1, out of 202 people who 

mentioned their means of egress, 198 used the stairs, 1 used an elevator, and 3 used a combination of 

stairs and elevator. This does not include the 22 people who were stuck in elevators when WTC 1 was 

hit. The most common adverse floor condition mentioned by people in WTC 1 was the presence of 

smoke (mentioned by 74 people), debris or collapsed walls, ceilings, or floors (72 people), and fires 

(41 people). In WTC 2, 37 people reported debris or collapsed walls, ceilings or floors on their floor, and 

25 people saw smoke. 

The most prevalent condition reported for the stairwell was that it was crowded and hot (mentioned by 

106 people). A particular condition mentioned for the stairs in both towers was the presence of smoke, 

mentioned by 78 people in WTC 1 and 29 in WTC 2. The presence of water, usually on the lower 

stairwell floors, was mentioned by 49 people in WTC 1 and four people in WTC 2. Jammed or locked 

doors were mentioned by 20 people in WTC 1 and two people in WTC 2. 
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In WTC 2, 96 people mentioned hearing a message over the communication system to “stay in or return 

to their office.” The majority of them, 69 people, decided to disregard the instructions and continued their 

evacuation. The 16 people who decided to remain in their offices or decided to turn back didn’t have time 

to travel very far before the second plane hit; at that point they all resumed their evacuation downward. 

Among the accounts analyzed, 27 people reported having a disability, and 47 were injured that morning. 

All these people were supported in their evacuation by coworkers. Half of them stated that they started 

their evacuation immediately, and one-third mentioned some delay to get organized and seek first-aid. 

Several people who were disabled or injured evacuated the towers swiftly as occupants formed a single 

line to let them through rapidly down the stairwell. Many people (143 in WTC 1 and 26 in WTC 2) 

mentioned being reassured and feeling safe when meeting firefighters in the building. Although the 

emergency crews disrupted the evacuation in the stairwell by going against traffic, the people 

appreciatively cheered them on. Phone calls were made by 151 survivors to family and friends to give 

and obtain information; 20 people called their bosses or colleagues; and another 12 people made calls to 

authorities. Another 14 people used e-mail wireless technology and pagers to exchange information, 

which seems to be the only reliable devices used from inside the stairwells. 

Telephone Interviews 

The survey objectives of the telephone interviews called for collecting 800 computer-assisted telephone 

interviews (CATI) of persons occupying either of the two WTC towers at the time of the terrorist attacks 

on September 11, 2001. Attempts were made to equally divide the respondents among WTC 1 and 

WTC 2 occupants (i.e., n = 400 occupant interviews from each tower). Within each of the WTC 

buildings, independent, proportionate, stratified samples of survivors were drawn. Eight-hundred three 

telephone interviews were completed, with 440 from WTC 1 and 363 from WTC 2. Additional 

discussion of the sampling methodology and disposition can be found in Appendix O. 

A response rate analysis indicated differential nonresponse, more noticeably near the impact floors in 

WTC 1. In other words, respondents were less likely to complete a telephone interview if they had been 

near the impact floor than respondents who had been lower in the building. Thus, percentages presented 

in this summary are weighted, unless otherwise indicated. Weighting preserves the ability to accurately 

generalize the results. 

Population of WTC 1 and WTC 2 on September 11, 2001 

The total building population is the sum of survivors and decedents. At the time of this report, the City of 

New York has officially determined that 2,749 people were killed at the WTC on September 11, 2001; no 

official breakdown of where people were killed presently exists. While an analysis of this issue by 

Dennis Cauchon, a reporter for USA Today, in the months immediately following September 11, 2001, 

was remarkably complete (Cauchon 2001), differences exist between his projections and the official 

numbers from the City of New York and other official sources. These differences are shown in 

Table 2–8. For example, the number of first responders depends on the definition of first responder. The 

City of New York published an occupational analysis of WTC decedents based on a Census of Fatal 

Occupational Injuries (U.S. Department of Labor, Bureau of Labor Statistics, in cooperation with the 

NYC Department of Health and Mental Hygiene and State and Federal agencies). Four hundred and 

thirty-three decedents’ occupations were listed as firefighting, police, or security. This number exceeds 
Progress on the World Trade Center Investigation 

135

by 30 the number of FDNY, NYPD, and PAPD reported killed. This may be attributable to private 

security forces present inside the towers on September 11 and/or first responders not employed by New 

York City or PANYNJ. NIST is attempting to resolve these differences in order to fully understand the 

initial building population. 

Table 2–8. Reports of WTC decedents. 

Decedent 

Official 

Numbers 

USA 

Todaya

WTC 1 occupants 1,434 

At or above impact 1,360 

Below impact 72 

WTC 2 occupants 599 

At or above impact 595 

Below impact 4 

First responders (total) 433b,c 479 

FDNY 343e 

NYPD 23f 

PAPD 

403d

37g 

UA 175 and AA 11 157d 157 

Uncertain location in towers 147 

Bystanders 10 

Total number of decedents 2,749b,h 2,826 

a. Cauchon, Dennis. ‘For many on September, 11, 2001, survival was no 

accident.’ USA Today, December 20, 2001. 

b. Summary of Vital Statistics 2002: The City of New York. Bureau of 

Vital Statistics, NYC Department of Health and Mental Hygiene. 

December 2003. 

c. Table WTC 8: Occupation of Decedents. All decedents classified as 

‘protective service’ occupations, which includes firefighting, police, and 

guards. 

d. World Trade Center Building Performance Study. FEMA 403. May 

2002. Includes 10 hijackers as passengers. 

e. Increasing FDNY’s Preparedness (McKinsey Report). Available at: 

http://www.ci.nyc.ny.us/html/fdny/html/mck_report/index.shtml 

f. Available at http://www.ci.nyc.ny.us/html/nypd/html/memorial_ 

01.html 

g. Available at http://www.panynj.gov/AboutthePortAuthority/ 

PortAuthorityPolice/InMemorium/ 

h. Does not include 10 airplane hijackers for whom the City has not issued 

death certificates. 

Using the known eligibility rates allows for a projection of the survivors of WTC 1 and WTC 2 present in 

the building at 8:46 a.m. on September 11, 2001. The analysisindicates that WTC 1 had approximately 

7,500 ± 750 surviving occupants, while WTC 2 had approximately 7,900 ± 900 surviving occupants. 

Thus, the total population of survivors from both towers was 15,400 ± 1,200. Table 2–9 summarizes the 

projection of population of WTC 1 and WTC 2 on September 11, 2001. Pending resolution of decedent 

locations, the total building population at the time of the first airplane impact was 17,400 ± 1,200, 

calculated using the building decedent locations reported by Cauchon. 
Chapter 2 

136

Table 2–9. Occupancy estimates on September 11, 2001, by tower. 

WTC 1 WTC 2 Total 

Estimated total population 

of survivors 

7,500 7,900 15,400 

Statistical Precision Calculations 

Sample n 427 376 803 

Standard error (p) 1.90 % 1.92 % 1.36 % 

Standard error (total) 750 900 1,200 

Confidence limits at 5% ±1,470 ±1,790 ±2,320 

Number of Occupant Decedents 

Decedents 1,434a 599a 2,033a – 2,236b

Total Building Population 

Total population 8,900 8,500 17,400 

a. Calculated as 2,749 – 343 FDNY – 23 NYPD – 147 airplane passengers (not including hijackers). 

b. Calculated as 2,749 – 403 First Responders – 157 airline passengers (not including hijackers). 

Previous Evacuation Experience 

Whether an occupant had a previous evacuation experience may have affected the decisions an individual 

made during the September 11, 2001, evacuation. NIST will conduct further analysis to develop this 

hypothesis. Of the WTC 1 occupants present on September 11, 2001, 16 percent (n = 64) were also 

present during the 1993 bombing. Sixty percent (n = 38) of WTC 1 evacuees in 1993 reported that they 

evacuated immediately, 30 percent (n = 20) reported that they waited to evacuate, and 9 percent (n = 6) 

did not recall. Most (95 percent [n = 53]) who were able to recall their evacuation decision felt that they 

made the right decision, while 5 percent (n = 3) did not believe they made the right decision. 

Similarly, 16 percent (n = 59) of WTC 2 evacuees on September 11, 2001, also evacuated in 1993. In 

WTC 2, however, only 75 percent (n = 42) felt that they made the right decision in 1993, possibly due to 

the fact that many more waited to evacuate in 1993 in WTC 2 (69 percent [n = 39]) than did so in WTC 1. 

Only 31 percent (n = 17) who reported their decision evacuated immediately from WTC 2 in 1993, 

keeping in mind that the bomb had a more significant impact on WTC 1 in 1993. 

Preparedness and Training 

Long a cornerstone of public policy on the emergency preparedness of office workers around the country, 

the Port Authority required tenants to conduct fire drills every 6 months and appoint employee floor 

wardens and searchers. Sixty-six percent (n = 529) of WTC 1 and WTC 2 occupants reported 

participation in at least one fire drill in the 12 months immediately prior to September 11, 2001. 

Seventeen percent (n = 139) reported that they did not participate in any fire drills in the 12 months prior 

to September 11, and 17 percent (n = 135) did not know. Fire drill participation rates were similar 

between the two towers (as shown in Table 2–10). 
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137

Table 2–10. WTC fire drills in 12 months 

prior to September 11, 2001.a

Number of 

Drills WTC 1 WTC 2 

None 18 % (n = 78) 17 % (n = 61) 

1 13 % (n = 57) 8 % (n = 29) 

2 21 % (n = 90) 24 % (n = 88) 

3 11 % (n = 47) 15 % (n = 53) 

4 10 % (n = 44) 9 % (n = 32) 

5–11 7 % (n = 31) 9 % (n = 32) 

12 or more 3 % (n = 13) 4 % (n = 13) 

Unknown 18 % (n = 80) 15 % (n = 55) 

a. Percentages are weighted, n values unweighted. 

One of the primary goals of fire drill training is to make occupants aware of the location of the emergency 

exits. Ninety-three percent (n = 490) of respondents who reported participation in a fire drill were 

instructed about the location of the nearest stairwell as part of the drill. However, of the respondents who 

reported being shown a stairwell, 82 percent (n = 432) did not enter or use the stairwell. Seventeen 

percent (n = 92) reported that they did use the stairs during a drill, while approximately 1 percent (n = 5) 

reported not knowing. Overall, more than half (51 percent [n = 415]) of the occupants had never used a 

stairwell in WTC 1 or WTC 2 prior to September 11, while 48 percent (n = 386) had used a stairwell. 

Two persons reported not knowing whether they had used the stairs previously. 

Another goal of the fire drills was to introduce the floor warden system and evacuation procedures. 

Eighty-two percent (n = 528) of the occupants with fire drill training were aware that there was a floor 

warden for their floor. Approximately 70 percent (n = 557) of all occupants reported that they were 

aware of the evacuation procedures. When asked what those evacuation procedures comprised, however, 

answers varied significantly, including: wait in hallway for further instructions; do not use elevators, use 

stairs; meet at a designated site outside the building for a head count; or proceed down (varied number of) 

flights of stairs and wait. Further analysis of the understanding and implementation of the emergency 

procedures is under way. 

Future Work 

Significant additional analysis is presently under way. It is particularly important that results of questions 

related to the events, observations, and activities within the towers on September 11, 2001, be analyzed 

within the context of the findings coming from face-to-face interviews, focus groups, and other data 

collection activities. As with the work presented in this appendix, ongoing analysis of the telephone 

interviews will form the statistical basis for many significant recommendations, and there will be 

continued analysis of the information collected during the face-to-face interviews and focus groups. 

NIST is utilizing existing computer egress models to better understand the evacuation experience on 

September 11, 2001. Three full evacuation scenarios are being considered: a typical full capacity 

building evacuation assuming the WTC tower is fully occupied—with one case considering only tenants 

and another case considering both tenants and visitors; a full capacity building evacuation of each WTC 
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138

tower with aircraft impact damage; and a September 11, 2001, capacity evacuation from a WTC tower. 

NIST is using two classes of egress models in order to frame the evacuation questions: (1) partial 

behavior: simulates occupant movement and limited behavioral rules by including delay times, smoke 

effects, and occupant characteristics; and (2) behavioral: simulates movement and more comprehensive 

evacuation decisions and activities. 

2.9 FIRE SERVICE TECHNOLOGIES AND GUIDELINES (PROJECT 8) 

The purpose of this project is to build on work already done by the FDNY and McKinsey & Company by 

(1) fully documenting what happened during the response by the fire services to the attacks on the WTC 

up to the time of collapse of WTC 7; (2) identifying issues that need to be addressed in changes to 

practices, standards, and codes; (3) identifying alternative practices and/or technologies that may address 

these issues; and (4) identifying research and development (R&D) needs that advance the safety of the 

fire service in responding to massive fires in tall buildings. 

2.9.1 Project Objectives 

Project 8, Fire Service Technologies and Guidelines, has four tasks: 

• Task 1. Collect emergency response data in cooperation with FDNY to document first 

responder fatalities, command and control procedures, and equipment performance. Records 

of interest include dispatch logs, recorded radio communications, run logs from surviving 

responding units, 9-1-1 records, data recorded by the FDNY, PANYNJ operations, and the 

NYPD, and fireground positioning of emergency apparatus. Information will also be sought 

on operations and function of communications systems, on-site emergency information 

systems, fire alarm panels, elevator control panels, standpipes and fire hoses, and other prepositioned emergency equipment. In coordination with Project 7, Occupant Behavior, 

Egress, and Emergency Communications, oral history data will be collected from witnesses, 

those in control of emergency operations, and surviving first responders to the extent their 

oral history has not already been documented. Technical experts will review and conduct a 

fact-based analysis of the data. 

• Task 2. Interpret the factual analysis to determine the effect on responder successes of 

factors such as: 

− The influence of building design (e.g., height, stairways, elevators, smoke control 

systems) on fire service command and control procedures, life saving operations, and 

safety of rescue personnel; 

− The influence of aircraft impact damage and fuel run-off on fire service command and 

control procedures, life saving operations, and safety of rescue personnel; 

− The impact of systems failures (e.g., communications systems, water supply, sprinklers, 

standpipes) on fire service command and control procedures, life saving operations, and 

safety of rescue personnel; 
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139

− Building occupant egress as related to fire service operations; 

− The ability to fight large fires on the upper floors of tall buildings; 

− The impact that the 1993 bombing of the WTC had on codes, standards, and procedures 

that affected first responders in tall buildings; 

− Preplanning, training, and standard operating procedures (including command and 

control) at the time of the incident; 

− Firefighter accountability, location, and tracking; 

− Fire and emergency response protocols for tall buildings; 

− The resources available for initial situation assessment and incident management, and 

practices for determining the possibility of structural collapse; and 

− Communications and coordination of response activities with other authorities at the 

incident. 

• Task 3. Identify alternative emergency response practices and technologies that may 

advance the safety and effectiveness of first responders, such as: knowledge/information 

systems for command and control decisions; elevator use by firefighters; firefighter tracking 

systems; interoperability of communication systems (occupants, firefighters, police, 

emergency management services); fire growth and smoke hazard prediction; structural safety 

monitoring, assessment and prediction; and simulation tools for training. 

• Task 4. Report preparation. The results of this project will be synthesized into a report to 

describe the actions of the fire service and performance of their equipment; identify available 

alternatives related to fire service technology, training, and operational procedures; and 

identify R&D needs in support of their capability to protect the public, themselves, and vital 

physical infrastructure during extreme events. 

2.9.2 Project Status 

Task 1, Data Collection 

Project 8 has collected or reviewed a large volume of data from the three primary departments that sent 

first responders to the WTC, FDNY, NYPD, and PANYNJ. NYC submitted data directly to NIST for 

analysis and also provided NIST with the opportunity to review other data related to the incident in their 

offices. The data collection and review process is largely completed. Data collected and review includes: 

documentary data, electronic data, and first-person face-to-face interviews. 

Documentary data collected or reviewed included the following: 

• Official lists of first responder fatalities 

• FDNY and NYPD McKinsey & Company reports 
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140

• FDNY dispatch logs 

• FDNY run logs of surviving units (CD12/CD15) 

• FDNY standard operating procedures and policies 

• Documents describing radio communications equipment and operations 

• Review of approximately 500 interviews conducted by FDNY 

The following electronic data have been collected or reviewed: 

• Approximately 1,000 hours of radio and telephone communications for PAPD and other 

departments within the Port Authority 

• NYPD Special Operations Division and Division 1 tape recordings 

• NYC 9-1-1 Emergency Telephone Operator tapes 

• FDNY Fire Dispatcher tapes 

First-person interviews—To date, 108 face-to-face emergency responder interviews have been completed: 

• 68 FDNY personnel including Commissioners, Chief Officers, Company Officers, 

Firefighters, and Emergency Medical Service personnel 

• 24 NYPD personnel including Commissioners, Chief Officers, Aviation Officers, Emergency 

Service Unit personnel, and Police Officers 

• 13 PANYNJ personnel including managers, directors, communications personnel, vertical 

transportation personnel, fire safety personnel, security personnel, PAPD Chief Officers, and 

Police Officers 

• 3 other emergency responders including security personnel, fire safety personnel, and 

communications personnel 

Task 2, Factual Analysis of the Collected Data 

As documentary data were collected they were reviewed for information critical to understanding the 

emergency response at the WTC. This information has been filed relative to the source of information 

and the type of information. The documentary data are being compared with other data gained through 

communications records and recordings and first-person face-to-face interviews. The data are in the 

process of being coded for analysis using Atlas.ti software. 

In addition, Appendix P contains results from a preliminary analysis of emergency responder 

communications. The objective of this analysis is to develop a better understanding of the role that 

emergency communications played during the WTC attack, and to quantify information related to 

communications effectiveness. Although there have been numerous reports of radio equipment failures 
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141

during the emergency response at the WTC, and these are all being examined as part of the overall 

project, the only radio equipment system examined in this preliminary report is that of the FDNY WTC 

site high-rise repeater that was installed by the Port Authority. Many factors are associated with the 

ability of emergency communications to be successful. The following objectives were set in this report: 

• To document radio and telephone communications operations 

• To document radio communications readability or understandability 

• To quantify radio communications traffic volume 

• To understand the impact of traffic volume on communications readability and the transfer of 

information 

• To identify communications associated with dispatch and arrival of responders 

• To identify communications related to evacuation and emergency response operations 

• To identify communications related to building conditions at the WTC and the impact of this 

information on the emergency response. 

Results from this analysis are presented in Sections 2.9.3, Preliminary Results on Emergency Responder 

Communications; 2.9.4, Preliminary Findings; and Appendix P, Interim Report on Emergency 

Communications. 

Appendix P also contains a detailed description of methods used for documenting and analyzing the 

communications data obtained from the various sources. 

Task 3, Alternative Emergency Response Practices and Technologies 

As the data analysis has progressed, issues critical to emergency responder operations are being 

identified. Most of the critical practices and technologies identified relate to Command and Control, 

Communications, and significant issues related to emergency responder high-rise building operations. 

This work is still in progress, and details concerning alternative practices and technologies will be 

addressed in the final report. 

Task 4, Report Preparation 

The efforts described above in Tasks 1, 2, and 3 continue. Data analysis necessary for final report 

preparations is ongoing. The final report outline has been drafted, and report preparation has been 

initiated. 

2.9.3 Preliminary Results on Emergency Responder Communications 

A more detailed analysis of radio equipment and systems operations is in progress, and it will be 

addressed in the final report. However, the following preliminary results on emergency communications 
Chapter 2 

142

provide some insight into the challenges associated with communications systems following the attack on 

the WTC. 

All radio systems analyzed appeared to work well during the normal operations period just before the 

attack on the WTC. It was noted that Channel W of the PAPD was experiencing some difficulty with a 

handie-talkie radio transmitting a carrier wave from an open or keyed microphone. This disrupted 

communications on that channel. PAPD personnel were trying to correct the problem just before the first 

aircraft struck WTC 1. The problem continued after the attack occurred. NYPD also had a problem with 

an open microphone after the incident began. This occurred on the Special Operations Division (SOD) 

channel. Efforts made by NYPD personnel corrected the problem. 

All radio communications evaluated experienced traffic volume surge load conditions as a result of the 

attack. Coupled with the increase of traffic load was a significant increase in a process known as 

“Doubling” or “Crossing” of radio signals. This condition occurs when more that one person attempts to 

transmit a message at the same time on the same radio frequency. Doubling results in the mixing of radio 

signals that seriously degrades the signal readability. This condition will often block all clear 

communications from getting through, except possibly from a high power base station. 

2.9.4 Preliminary Findings 

The following are preliminary findings based on the current status of emergency responder 

communications analysis: 

• After the first aircraft struck WTC 1, there was an approximate factor of 5 peak increase in 

traffic level over the normal level of emergency responder radio communications, followed 

by an approximate factor of 3 steady increase in the level of subsequent traffic. 

• A surge in communications traffic volume made it more difficult to handle the flow of 

communications and delivery of information. 

• Analysis of the radio communications records received by NIST indicates that roughly onethird to one-half of the radio messages transmitted during these radio traffic surge conditions 

were not complete messages nor understandable. 

• Preliminary analysis of the FDNY City-wide, high-rise Channel 7 (PAPD Channel 30) 

recording indicates that the WTC site repeater was operating. 

• Communications records and interviews indicate that smoke and heat conditions on the top of 

the two WTC buildings prevented the NYPD helicopters from conducting safe roof 

evacuation operations. 

• NYPD aviation unit personnel reported critical information about the impending collapse of 

the WTC towers several minutes prior to their collapse. No evidence has been found to 

suggest that the information was further communicated to all emergency responders at the 

scene. 
Progress on the World Trade Center Investigation 

143

Analysis of communications records and face-to-face interviews with emergency responders indicate that 

radio and telephone communications were a critical part of the emergency response operations. It is also 

clear from the evaluation that communications technology was not operating at a performance level 

adequate for handling emergency responder requirements at the incident. 

2.10 REFERENCES 

ASME (American Society of Mechanical Engineers). 2000. Safety Code for Elevators and Escalators, 

ASME 17.1, New York, NY. 

ASTM (ASTM International). 2003. Test Methods for Fire Tests of Building Construction and Materials, 

ASTM E 119-00a, 2003 Annual Book of ASTM Standards, Vol. 04.07, West Conshohocken, PA. 

Bell and Stanton, Public Relations. 1964. Polytechnic Institute of Brooklyn, 757 Third Ave, New York, 

NY. 

BOCA (Building Officials Conference of America). 1950. Basic Building Code, 1950 Edition, New 

York, NY. 

BOCA (Building Officials Conference of America). 1965. Basic Building Code, Fourth Edition, Building 

Officials Conference of America, Chicago, IL. 

Bracco, W.M. (Tishman Research Corporation). 1969. Office Correspondence to James R. Endler (Vice 

President, Tishman Realty & Construction Co.), May 26. 

Cauchon, Dennis. 2001. For Many on Sept.11, Survival Was No Accident, USA Today, December 20. 

Coty, Eric H. (Rolf Jensen and Associates, Inc.). 1997. Included as Attachment A to World Trade 

Center – Code Compliance Evaluation compiled by PANYNJ October 15, 1999, March 14. 

Endler, James R. (Vice President, Tishman Realty & Construction Co., Inc.). 1969. Letter to Malcolm P. 

Levy (General Manager, World Trade Center Operations), July 9. 

Fahy, R.F. 1999. User’s Manual, EXIT89 v 1.01, An Evacuation Model for High-Rise Buildings, 

National Fire Protection Association, Quincy, MA., December. 

Feld, Lester S. (Chief Structural Engineer, World Trade Department). 1987. Memorandum to Robert J. 

Linn (Deputy Director for Physical Facilities, World Trade Department), January 15. 

Galea, E.R., S. Gwynne, P.J. Lawrence, L. Fillippidis, and D. Blackshields. 2001. buildingEXODUS 

V3.01 User Guide and Technical Manual, Revision 3.0, University of Greenwich, London, UK, 

February. 

ICBO (International Conference of Building Officials). 1928. Uniform Building Code 1927 Edition, Long 

Beach, CA. 

ICC (International Code Council). 2000. International Building Code, IBC 2000, Falls Church, VA. 
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ICC (International Code Council). 2003. International Building Code, IBC 2003, Falls Church, VA. 

IES (Integrated Environmental Solutions, Inc.). 2000. Simulex Technical Reference, Evacuation 

Modeling Software, November. 

IES (Integrated Environmental Solutions, Inc.). 2001. Simulex User Manual, Evacuation Modeling 

Software, March. 

Kyle, John M. (Chief Engineer, Port Authority). 1966. Memorandum to Guy F. Tozzoli (Director, World 

Trade Department), June 2. 

Levy, Malcolm. 1963. Port of New York Authority Letter to Minoru Yamasaki (Architect, Minoru 

Yamasaki & Associates), May 15. 

Levy, Malcolm. 1965. Port of New York Authority Letter to Minoru Yamasaki (Architect, Minoru 

Yamasaki & Associates), September 29. 

MCC (Municipal Code of Chicago). 1967. Municipal Code of Chicago Relating to Buildings (As 

amended to and including January 1, 1967), Index Publishing Corp., Chicago, IL. 

Messersmith, J.J. 2002. Unwarranted Relaxation, Concrete Products, Sept. 1. 

http://concreteproducts.com/mag/concrete_unwarranted_relaxation/index.html 

NBFU (National Board of Fire Underwriters). 1934. National Building Code, Fifth Edition, Revised 

Reprint of 1931 Edition, New York, NY. 

NFPA (National Fire Protection Association). 1966. Life Safety Code, NFPA 101, Quincy, Massachusetts 

NFPA (National Fire Protection Association). 2003. Building Construction and Safety Code, NFPA 5000, 

Quincy, MA. 

NYCBC 1968. Building Code—Local Law No. 76 of the City of New York, New York, NY. 

NYCBC 2001. Building Code of the City of New York, 2001 Edition, Gould Publications, 

Binghamton, NY. 

NYSBC 1964. State Building Construction Code Applicable to General Building Construction, Building 

Code Bureau, State of New York, New York, NY, December 1. 

PANYNJ (Port Authority of New York and New Jersey). 1998. World Trade Center Design Guidelines, 

Specifications, and Standard Details, February 27, revised May 11. 

PANYNJ (Port Authority of New York and New Jersey). 1999. World Trade Center – Code Compliance 

Evaluation, October 15. 

PANYNJ and FDNY. 1993. Memorandum of Understanding Between the Fire Department of the City of 

New York and the Port Authority of New York and New Jersey, signed by Stanley Brezenhoff, 

Executive Director, PANYNJ, and William Feehan, Fire Commissioner, FDNY, December 30. 
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PANYNJ and NYCDOB. 1993. Memorandum of Understanding between the Port Authority of New York 

& New Jersey and New York City Department of Buildings, signed by Stanley Brezenhoff, Executive 

Director, PANYNJ, November 3, and Stewart D. O’Brien, Acting Commissioner, NYCDOB, 

October 28. 

PANYNJ and NYCDOB. 1995. Supplement to Memorandum of Understanding Between the New York 

City Department of Buildings and the Port Authority of New York and New Jersey, signed June 1 

and 7, 1995, with change dated September 15. 

PONYA (Port of New York Authority). 1976. Report on Fire Safety of the World Trade Center, January. 

Powers, W. Robert (New York Board of Fire Underwriters). 1975. One World Trade Center Fire, Bureau 

of Fire Prevention and Public Relations, New York, NY, February 13. 

Rivera, Carlos M. and Rudolf J. Rinaldi. 1993, Fire and Building Codes and the Jurisdiction and 

Compliance of Federal, State, State Authorities and Foreign Government Buildings, March 25. 

SBCC (Southern Building Code Congress). 1946. Southern Standard Building Code, 1946-47 Edition, 

Birmingham, AL. 

Schaffner, Charles E. 1964. The new building code for New York City—Performance or specification, 

The Magazine of Standards, October. 

Solomon, Joseph H. (Emery Roth & Sons). 1969. Letter to Malcolm P. Levy (Chief, WTC Planning 

Division Port Authority), June 6. 

Solomon, Joseph H. (Emery Roth & Sons). 1975. Letter to Malcolm P. Levy (General Manager, World 

Trade Center Operations), February 18. 
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Chapter 3

UPDATE ON SAFETY OF THREATENED BUILDINGS 

(WTC R&D) PROGRAM

3.1 OBJECTIVES OF SAFETY OF THREATENED BUILDINGS (WTC R&D) 

PROGRAM 

This program is designed to (1) facilitate the implementation of recommendations resulting from the 

World Trade Center (WTC) investigation, and (2) provide the technical basis for cost-effective 

improvements to national building and fire codes, standards, and practices. Under the program National 

Institute of Standards and Technology (NIST) will develop guidance and tools to assess and reduce 

building vulnerabilities and will support private sector organizations that develop building and fire codes 

and standards in the United States. Implementation of the results will better protect building occupants 

and property in the future, will enhance the safety of fire and emergency responders, and will increase 

confidence in the safety of commercial and public buildings. 

Four general areas of research are targeted to support near- and long-term improvements to reduce the 

vulnerability of the structure, building occupants, and first responders to potential threats: 

• Increased Structural Integrity 

• Enhanced Fire Resistance 

• Improved Emergency Egress and Access 

• Building and Emergency Equipment Standards and Guidelines 

3.2 BACKGROUND AND DESIRED OUTCOMES OF SAFETY OF 

THREATENED BUILDINGS (WTC R&D) PROGRAM 

Building and fire codes in the United States exist, among other reasons, to ensure the safety of occupants 

in the event of anticipated excessive loads due to wind, earthquake, and snow, and the potential for severe 

fires. The tragic collapse of the WTC buildings in 2001 (along with the terrorist attacks on the Pentagon, 

Hart Senate Office Building, and the Murrah Federal Building) has focused the general public, 

governments at all levels, and the construction and building products industries on the need to understand 

the possible impacts of terrorist acts on building operations, structural integrity, and emergency response 

procedures, and on the need to develop economically justifiable strategies to mitigate the potential loss of 

life from possible future threats. 

The standard test methods and building practices upon which current building and fire codes are based 

rank the performance of one material, component, or system against alternative designs, with the 

expectation that some minimum rating translates into a sufficient level of safety of the material, 

component, or system when installed in a building. Safety factors are used to account for our ignorance 
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abut the magnitude of actual loads, and of the uncertainty in response of the complex building frame to 

these loads. 

The prediction of failure modes in a closely-coupled building system is beyond our current capability, and 

standard test methods provide little information on the expected performance of the building should the 

mechanical or thermal load exceed a prescribed value. 

In addition, building designers, operators, occupants, and first responders are faced with chemical and 

biological threats unforeseen as little as two years ago. How should heating, ventilation, and airconditioning (HVAC) systems be designed and operated to contain a poisonous aerosol or gas? How has 

the behavior of occupants changed since September 11, 2001, in responding to an emergency? Should the 

same emergency egress and fire service access techniques and strategies be used in the case of a 

biological threat as for a fire? Can new technologies be developed, or design practices adapted, to 

increase the safety of the building occupants without undue economic burden on the owners/operators? 

Additional research and development is being conducted in this program to answer questions like these, to 

provide guidance and tools to assess and reduce future vulnerabilities, and to better prepare facility 

owners, contractors, designers, and emergency personnel to respond to future disasters, naturally or 

intentionally initiated. 

Increasing Structural Integrity—Structural integrity will be increased through the development and 

implementation of performance criteria for codes and standards, tools and practical guidance for 

prevention of progressive structural collapse. System design concepts, retarded collapse mechanisms, 

built in redundancy, and hardening structures though retrofit are being considered. Performance criteria 

for fire safety design and retrofit of structures is being developed through examination of five key factors: 

the suitability of standard fire resistance test methods; the role of structural connections, diaphragms, and 

redundancy in enabling load transfer and maintaining overall structural integrity; the effectiveness of 

alternative retrofit, design and fire protection strategies to enhance structural fire endurance; the fire 

behavior of structures built with innovative materials; and models to predict the fire hazard to structures 

from internal and external fires. Guidance on methods to enhance fire resistance of steel and concrete 

structures based upon our current state of knowledge is being developed as well. 

Enhancing Fire Resistance—Fire resistant steels exist and are in use elsewhere in the world. More 

efficient and accurate tests for performance of steels under building fire conditions are needed and are 

being developed to help industry incorporate fire resistant steels into U.S. construction practice. 

Fundamental mechanical and thermal properties of fire protective materials are being measured. This 

requires the development of new test methods and instrumentation, and a data base that spans the full 

range of expected temperatures and mechanical loads. These data will supplement, or may even supplant 

the need for, the ASTM E 119 test in certain situations, and in any case are essential to the 

implementation of meaningful performance codes and design criteria. 

Facilities do not yet exist that are suitable for demonstrating in a quantitative manner the improved 

performance of new materials, systems, and processes in their end-use within a building under actual fire 

conditions. Hence, simulations are required to bridge the fundamental data and the results of bench- and 

pilot-scale tests to the environment in which they would be exposed during severe fire conditions. The 

severity of a fire is dependent upon many parameters that are beyond the control of the building designer, 

especially when one considers the range of terrorist threats that are possible. The performance in a fire of 
Update on Safety of Threatened Buildings (WTC R&D) Program 

149

non-structural elements such as walls and ceilings is directly linked to the structural integrity of the 

building because a collapsed wall, ceiling, or floor exposes more areas of the building to the fire while 

providing additional fuel and air upon which the fire can feed. The technical basis for accurate 

measurement methodology and simulation tools for the inclusion of fire resistant properties of walls and 

ceilings in performance-based fire safety design is being developed under this program. 

Improving Emergency Egress and Access—By working with the primary stakeholders (elevator and 

construction industries, fire services, professional societies, and code making bodies), the role of elevators 

in providing access by the fire service to a fire in a high rise building is being greatly enhanced over 

current practice. The development of hardened fire service elevators and new emergency operation 

procedures/controls will also lead to improved egress capabilities from tall buildings, especially for 

mobility-impaired or injured occupants. However, the behavior of people in an emergency situation has 

been altered in unpredictable ways by the events of 9/11. Current egress models may be inappropriate 

and/or insufficient for the design and placement of doors and stairways and the control of elevator 

movement. Behavioral and engineering studies are being conducted, drawing on experts in academia and 

elsewhere, to enable the development of simulation tools that better capture the movement of people 

within a building under fire and other emergency situations. 

Developing Building and Emergency Equipment Standards and Guidelines—Partnering with other federal 

agencies and American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 

(ASHRAE), NIST-developed indoor air quality (IAQ) simulation tools are being extended to analyze and 

guide the assessment and subsequent reductions in the vulnerability of buildings to chemical, biological, 

and radiological aerosols. Standard building information models that facilitate the simulation of building 

system behavior during adverse events are being developed to allow communication among IAQ controls 

and other building controls associated with, for example, security, transportation, energy, and fire alarm 

systems. A user-friendly tool is being developed for building owners and managers to aid in the selection 

of cost-effective strategies for the management of terrorist and environmental risks. Also, facilities are 

being established for science-based exposures for measurement of firefighter equipment performance 

attributes essential to support informed fire service procurement decisions.

3.3 ACCOMPLISHMENTS OF WTC R&D PROGRAM 

Prevention of Progressive Collapse 

Buildings that are designed according to modern building codes are not expected to collapse during their 

service life period. They are designed typically to resist traditional governing vertical loads and lateral 

loads such as wind and earthquake. This situation is changing, however, due to an increase in deliberate 

terrorist attacks. In general, a terrorist attack may lead to failure of a small part of a building. When an 

initial local failure causes the loss of gravity load capacity in the structural frame, the failure spreads from 

story to story, which may lead to the total collapse of an entire building or a disproportionately large part 

of the building. This type of collapse is defined as “progressive collapse” (see Fig. 3–1). At present, U.S. 

building codes do not provide explicit provisions to enhance the resistance to progressive collapse. In 

terms of magnitude and probability of occurrence, the traditional vertical and lateral design loads are 

quantifiable. In contrast, terrorist loads are difficult to quantify as to size, location, and the nature of the 

loads. Terrorist attacks may include thermal, impact, or blast loads. Thus, in order to improve resistance 

to progressive collapse, U.S. building code developers have attempted to incorporate into the codes 
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structural redundancies by introducing prescriptive requirements for “structural integrity.” Changes are 

needed in the way buildings are designed and constructed so that resistance to progressive collapse is 

provided explicitly. Following the 2002 National Workshop, NIST is working jointly with the 

Multihazard Mitigation Council of the National Institute of Building Sciences and industry experts to 

produce a “Best Practices Guide” for mitigating progressive collapse of buildings for design 

professionals. This document will be published in FY2004. Subsequent research efforts will focus on the 

development of tools to assist design professionals in the design of new buildings against progressive 

collapse and methods to enhance the resistance of existing buildings to progressive collapse. 

Figure 3–1. Ronan Point Collapse in 1968. 
Update on Safety of Threatened Buildings (WTC R&D) Program 

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Fire Safety Design and Retrofit of Structures 

Current building design practice does not consider fire as a design condition to predict and evaluate 

structural performance in the presence of an uncontrolled fire. Instead, fire endurance ratings of building 

members, derived from standard fire endurance tests, are specified in building codes. At the present time, 

there is no accepted science-based set of verified tools to evaluate the fire performance of entire structures 

under realistic fire conditions. Thus, there is an urgent and critical need to develop and implement 

verified and improved standards, technology, and practices that explicitly consider structural fire loads in 

the design of new structures and the retrofit of existing structures. A workshop was held recently in 

cooperation with the Society of Fire Protection Engineers to assess current fire safety practice and 

existing codes and standards, and to identify research gaps for an improved fire safety design and retrofit 

approach. The workshop was attended by national and international fire safety experts and provided the 

technical basis for the development of a national R&D roadmap for Fire Safety Design and Retrofit of 

Structures. 

In addition, an evaluation has been performed of state-of-the-art numerical tools, including ANSYS and 

SAFIR, to assess their suitability for use in analyzing performance of structures under the combined fire 

and mechanical loadings. The evaluation process of these analytical platforms, which are rarely used in 

practice for fire safety design, is ongoing and necessary due to the complexity of structural systems, 

loading conditions, boundary conditions, and the highly nonlinear nature of material and structural 

behaviors. The effect of high thermal loading on structural performance of tested concrete columns, 

WTC steel connections, members, and subassemblies was examined. To better inform the modeling 

effort, a series of large-scale tests were conducted of steel components in a fire environment (see Fig. 3–2 

and Fig. 3–3). The tested components included steel rods, columns, and open-web steel joists that were 

either left bare or had sprayed-on fire protective insulation material of varying thickness. Test fires were 

generated using liquid hydrocarbon fuels to produce medium-soot fires and high-soot fires, and the tests 

were continued until the temperature at any steel surface approached approximately 600 °C. 

Figure 3–2. Insulated steel trusses, steel rod 

and steel column inside the NIST large-scale 

fire laboratory. 
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Figure 3–3. View of fire compartment from air 

exhaust outlet several minutes after the start of a 

fire test. Note: the flame impingement on the 

steel trusses and bar. 

Fire Resistant Steel 

Structural steel loses strength at building fire temperatures, leading to the need for fireproofing (see 

Fig. 3–4). Fireproofing adds costs and can be damaged or removed from the structure in a blast situation 

or unanticipated impact. In contrast, a new class of fire-resistant steels is specifically designed to retain 

more of the design strength at high temperature. These steels are being produced in Japan and Europe, 

and are now in use, either with or without additional fire protection. The use of fire-resistant steels leads 

to cost savings and schedule benefits during construction when application of fire protection can be 

avoided, and enhanced performance when protection is applied in the case of damage to the insulation. 

Unfortunately, the benefits of fire-resistant steel are not adequately tested under the standard U.S. 

structural fire standards (ASTM E 119), and thus, are not currently used in the U.S. 

Figure 3–4. Stress-strain curves of a conventional 

structural steel, showing degradation of 

properties at high temperature. 
Update on Safety of Threatened Buildings (WTC R&D) Program 

153

A project has been initiated to ascertain which properties of steel are critical for efficient use of fireresistant steels, such as high temperature strength and creep (see Fig. 3–5). Conventional steels and fireresistant steels are being characterized to determine these critical properties. Our goals include both 

provision of accurate data on fire-resistant steels and development of quick and accurate tests for 

measuring relevant high temperature properties. 

Figure 3–5. High temperature tensile tests to measure performance 

of structural steel at temperatures found in building fires. 

Methodology for Fire Resistance Determination 

Compartmentation is the cornerstone of limiting room-to-room and building-to-building fire spread. 

Standard fire resistance testing of wall/floor/ceiling assemblies provides an indicator of fire resistance and 

has proven valuable over time. However, these procedures have significant limitations that restrict their 

value for performance-based design and especially for high-risk occupancies. These limitations include: 

(a) standard time-temperature curves that may not be sufficient for all threats; (b) uniform heating, while 

many fires produce hot spots that may make the tests non-conservative; (c) single point thermal 

measurements; (d) pass/fail criteria, which make adaptation to other fire scenarios difficult; 

(e) documentation of the initial failure mode, but not the relative time to any successive modes, and 

(f) relative ratings, not absolute values. 

Compartmentation is especially important in tall buildings, where the egress of numerous occupants can 

be a complex process, and barriers to the spread of flame keep the egress paths open, extend the time 

available for escape, and increase the safe time in places of refuge. For all these functions, it is necessary 

to know, in terms of real time, how long the interior partitions in a building will contain flames and 

smoke. 

NIST has embarked on a course to provide such a methodology for inclusion in performance-based 

design of buildings. The research involves obtaining real-scale experimental data, modeling the behavior 

of partitions as they are driven to failure by fire, and developing recommendations for obtaining the input 

parameters from modifications of standard fire resistance tests such as ASTM E 119 and ISO 834. The 

initial work will focus on non-loadbearing walls of gypsum panels and steel studs, the most common 

interior construction in tall buildings. A continuing effort will extend the research to glass-panel walls. 
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The modeling effort will be done in three steps. First is a simple model for failure, beginning with crack 

initiation and propagation, continuing to the supporting structure, and finally to the fasteners and their 

failure points. This is now underway. The second component will be development of a detailed model of 

the partition materials to ascertain what additional data need to be obtained from the test method. The 

third component is development of a detailed model of a partition assembly for use by building design 

and engineering firms. 

A series of real-scale compartment tests is providing information on the phenomenology of partition 

response and failure and also quantitative information to guide the model development (see Fig. 3–6). 

Various wall assemblies 2.44 m x 2.44 m were exposed to intense fires from the time of ignition to 

beyond flashover. Flux meters provided time histories of the energy incident on the walls. 

Thermocouples and infrared videos provided data on the transport of heat through the walls and on the 

progress toward perforation. 

Figure 3–6. NIST measured the thermal behavior of gypsum/steel 

wall assemblies subjected to severe fire conditions. 

Emergency Use of Elevators 

This project is aimed at the development and implementation of protected elevators that can be used for 

fire department access and occupant egress during emergencies in tall buildings. The general strategy is 

to first incorporate into U.S. codes and standards a protected elevator system for fire department access. 

These are known in other countries as firefighter lifts, and there are existing requirements for these in at 

least 12 countries (as identified in a report by ISO TC178 on Elevators and Escalators). Once the U.S. 

fire services are satisfied that elevators are safe and reliable during fires, codes and standards would be 

changed to recognize protected elevators for occupant egress, secondary to, or integrated with, stairs. 

The key technological advancement offered in the NIST strategy is the (new) concept of remote manual 

control. Here, the elevator system safety is monitored in real time by the fire alarm system and displayed 

on a standardized fire service interface developed jointly by NIST and the fire alarm industry, through the 

National Electrical Manufacturers Association (NEMA) and implemented in the 2002 edition of the 
Update on Safety of Threatened Buildings (WTC R&D) Program 

155

National Fire Alarm Code (NFPA72). This system addresses residual concerns held by the fire service 

and elevator industry even where such systems are utilized under existing codes and standards. The 

system might be further specified for accessible elevators required in U.S. and other building codes for 

access by people with disabilities but where the safety for use in egress during fires is still considered 

questionable. 

Several technical papers have been written by NIST and presented at a recent international conference on 

Tall Buildings organized by International Council for Research and Innovation in Building and 

Construction (CIB) and Council on Tall Buildings and Urban Habitat (CTB&UH). NIST organized and 

chaired the speakers’ session on emergency use of elevators, which included papers by two of the largest 

U.S. elevator companies (Otis and Kone). NIST also co-sponsored and presented papers at a workshop 

in March 2004, organized by American Society of Mechanical Engineers (ASME) and their A17 

committee, who are responsible for the standard convening the safe use of elevators referenced in all U.S. 

building codes. NIST is working with the key representatives of the elevator industry and regulators 

represented on the A17 committee and with the product development engineers at Otis and Kone to 

implement the required technology and interfaces into their elevator controls, and on a novel approach to 

work out changes to the elevator control software for emergency operations protocols during fires. This 

approach would utilize NIST Virtual Cybernetic Building Testbed (VCBT) to allow numerous 

simulations of building fires to test the ability of the control software to adapt to conditions and to 

maintain safe operations. 

Workshop on Building Occupant Movement During Fire Emergencies 

NIST, in cooperation with the United Technologies Research Center, hosted a 2-day workshop focusing 

on the needed research on occupant behavior and movement during building emergencies. This 

workshop was motivated by a renewed interest in how buildings should be evacuated during fire 

emergencies and by the desire to provide a forum for the exchange of experiences among the fire and 

non-fire communities working on emergency egress. Sessions were held on codes and standards 

requirements for building evacuation, data needs for predictive building movement models, building 

movement strategies, and a roundtable discussion among selected government agencies. Participants 

included national and international experts in building occupant movement, representing the academic, 

consulting engineering, building products, and codes and standards communities. An outcome of the 

workshop will be the identification of areas where research is needed most to aid government agencies, 

industry and academic researchers in prioritization of their resource investments. 

Guidelines and Technologies for Mitigation of Chemical, Biological and Radiological 

Aerosol Attacks 

The increased attention to the potential vulnerability of buildings to airborne chemical, biological and 

radiological (CBR) agents has led to the need for better simulation tools to evaluate the transport and fate 

of such agents in buildings. NIST’s longstanding expertise in airflow and contaminant transport 

modeling in buildings systems has been employed in many such analyses, and recently these capabilities 

have been extended via the release of version 2.1 of the CONTAM software. Among other 

enhancements, CONTAM is now able to use the output of exterior plume models as an input, such that 

outdoor contaminant concentrations from an exterior agent release can vary as a function of opening 

location on the building façade and time. This new capability allows users to link their exterior transport 
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models to CONTAM and allow detailed analyses of the impact of an exterior release on indoor 

concentrations. In addition, CONTAM version 2.1 has improved models of particulate contaminants and 

has added fan and damper transients to the ability to simulate controls. The updated CONTAM model is 

now being used by an increasing number of researchers and practitioners in their evaluation of specific 

buildings and of technologies with the potential to increase building protection (see Fig. 3–7). 

Figure 3–7. Exterior flow field as input to CONTAM model of building. 

Cost-Effectiveness Tool for Managing Terrorist Risks in Constructed Facilities 

Owners and managers of constructed facilities are faced with the task of responding to the potential for 

future terrorist attacks in a financially responsible manner. An economic tool is needed to direct limited 

resources to investments in mitigation strategies that will provide the most cost-effective reduction in 

personal injuries, financial losses, and damages to buildings, industrial facilities, and infrastructure. 

The economic tool under development by NIST is a decision methodology, embedded in user-friendly, 

decision-support software, that helps building/facility owners and managers choose the most costeffective mix of mitigation strategies. Three mitigation strategies are considered: (1) engineering 

alternatives; (2) management practices; and (3) financial mechanisms. The economic tool will provide 

decision makers with the basis for generating a risk mitigation plan that responds to the potential for 

future terrorist attacks in a financially responsible manner. 

Early in 2002, NIST prepared a white paper outlining the tool development effort. NIST used the white 

paper to solicit stakeholder inputs, create opportunities for collaborative efforts, and form a technical 

working group of individual external subject matter experts. This has resulted in collaborative efforts 
Update on Safety of Threatened Buildings (WTC R&D) Program 

157

with the Wharton Risk Management and Decision Processes Center, the Construction Industry Institute, 

ASTM International, and the U.S. Environmental Protection Agency. Safe Buildings Program. An 

expanded version of the white paper entitled “Economic Approaches to Homeland Security for 

Constructed Facilities” was delivered at the September 2002 CIB Meeting in Cincinnati as the invited 

Keynote Address. 

Significant recent products include a prototype version of the software and a NIST Internal Report 

illustrating the methodology via a case study building. The prototype version of the software was 

completed and presented to the Steering Committee in September 2003. The prototype includes the 

software’s graphical user interface and linkage to database files and key reports. The beta version of the 

software is planned for completion in 2004; it will facilitate a variety of user-specified analyses. All 

analyses employed in the software will be consistent with ASTM standard practices. A case study report 

illustrating how to apply the life-cycle cost method (ASTM E 917) to a prototypical commercial building 

renovation project was published in NIST IR 7025. A subsequent technical report documenting the 

decision methodology is planned for publication in 2004. 
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Chapter 4

UPDATE ON THE WTC DISSEMINATION AND TECHNICAL 

ASSISTANCE PROGRAM

4.1 OBJECTIVES OF THE WTC DISSEMINATION AND TECHNICAL 

ASSISTANCE PROGRAM (DTAP) 

An industry-led dissemination and technical assistance program (DTAP) is the third part of the National 

Institute of Standards and Technology (NIST) response plan. The DTAP is designed to engage leaders of 

the construction and building community in assuring timely implementation of proposed changes to 

practices, standards, and codes. It also will provide practical guidance and tools to better prepare facility 

owners, contractors, architects, engineers, emergency responders, and regulatory authorities to respond to 

future disasters. The DTAP is an important component of the World Trade Center (WTC) Response Plan 

because it will facilitate the timely adoption and widespread use of proposed changes to practice, 

standards, and codes resulting from the WTC Investigation and the research and development (R&D) 

program. 

4.2 BACKGROUND AND DESIRED OUTCOME OF DTAP 

NIST is working closely with other government agencies; with the engineering and architecture 

professions; with the construction and manufacturing industries; with authorities having jurisdiction over 

building and fire code enforcement; and with national code-making organizations to target building safety 

issues (including those identified in the Federal Emergency Management Agency (FEMA) Building 

Performance Study) that (1) could be addressed by expeditious revision to national building codes, 

standards, and practices based upon current knowledge, and (2) would benefit from additional research. 

The mechanism by which this is being done is through contracts to the private sector. About $1 M has 

been spent to date since the start of the WTC Investigation. The objectives of each contract are described 

in the following section. Note that these contracts are over and above those in direct support of the 

Investigation and that the DTAP will continue beyond the completion of the WTC Investigation. 

4.3 ACCOMPLISHMENTS OF THE DTAP 

Contracts have been issued to the following organizations: the Civil Engineering Research Foundation 

(CERF), the National Institute of Building Sciences (NIBS), the National Conference of States on 

Building Codes & Standards, Inc. (NCSBCS), the Construction Industry Institute (CII), Integrated 

Manufacturing Technology (IMTI), the Society of Fire Protection Engineering (SFPE), and the Wharton 

School of Business. 

National Workshop on Progressive Collapse (NIBS) 

The Multihazard Mitigation Council of NIBS conducted a workshop in February 2004 that brought 

together national experts who have an interest in mitigating the threat of progressive collapse. The two-
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day workshop included presentation of ten white papers to frame the issues associated with mitigation of 

progressive collapse with respect to guidelines, codes, design of new buildings, and retrofit of existing 

buildings. The workshop also included breakout sessions on the topics of development of codes and 

guidelines, structural systems and analytical methods, and existing buildings. The breakout sessions 

resulted in identification of research needs and included estimation of costs to address each of those 

needs. The workshop also provided an opportunity for industry to review and comment on draft 

guidelines for retrofit of existing buildings and design of new buildings to resist progressive collapse. 

Information Technology in the Building Regulatory Process (NCSBCS) 

The National Alliance for Building Regulatory Reform in the Digital Age, a public-private partnership, 

will continue action on its agenda to stimulate economic recovery, enhance public safety, and increase the 

security of buildings. The agenda focuses on the use of information technology and the development and 

use by state and local jurisdictions of products, guidelines, model processes and procedures that enable 

jurisdictions to better respond to natural and manmade disasters and reduce the regulatory cost of 

construction by up to 60 percent. This effort includes support for the development and initial testing of a 

prototype secure database for first responders of as-built designs, evacuation plans, and other contact 

information. 

National Alliance Interoperability Project (NCSBCS) 

The objective of this project is to speed the development of technologies and requirements needed to 

advance the creation of a state-of-the-art integrated and interoperable building regulatory system by 

developing, in collaboration with New York City, an interoperability statement. This statement is to be 

included in their fall 2003 request for proposals for permitting software services and holding a national 

summit workshop with the software industry to identify common data requirements for currently 

available software component systems, common data/information formats, and recognized standards and 

best practices. The workshop will also identify actions that the industry and the National Alliance and 

state and local jurisdictions can take together to generate national standards for interoperable hardware 

and software for use in the building codes adoption, administration, and enforcement processes. 

Strategies, Candidate Liaison Teams, and Actions to Conduct and Implement the NIST 

Response Plan (NIBS) 

The NIST response plan and R&D program and the NIST outputs will be reviewed. Strategies will be 

suggested for achieving the private sector and state and local involvement needed to assure the likelihood 

of implementing each of the final products and for assessing their impacts in use. Synergistic benefits 

will be identified beyond those relevant to homeland security for which the outputs and strategies hold 

promise. The contractor will identify potential liaison teams and develop action plans for implementation 

of each product, or group of products. The liaison teams will include potential advocates, and those with 

serious concerns from public and private sector organizations most likely to be affected by the product. 

Capital Projects Technology Roadmap (IMTI) 

The baseline Capital Projects Technology Roadmap is being updated to address technological issues and 

solution paths related to homeland security and economic development. An outcome of this effort will be 
Update on the WTC Dissemination and Technical Assistance Program 

161

a detailed plan for the necessary R&D to support the deployment of technological solutions. Specific 

tasks include a workshop with FIATECH, follow-on meetings with technical experts, and review and 

teambuilding with top-level executives from the construction/capital facilities industry. 

Benchmarking Homeland Security Construction Practices (CII) 

The goal of this effort is to collect information on 9/11-driven security initiatives from industry leaders in 

the areas of chemical manufacturing, oil production and refining, natural gas processing and distribution, 

water treatment, and other critical industries needed to support the Nation's infrastructure. Information 

collected as part of a series of regional workshops and field site visits shall establish a basis for 

identifying best practices related to the security of capital facilities projects, and provide the basis for 

assessing the impacts of these practices on the key project outcomes of cost, schedule, and safety. 

Best Practices Guidelines for the Mitigation of Progressive Collapse of Buildings (NIBS) 

NIBS has begun to formulate a course of action that will increase the design and construction 

community's understanding of progressive collapse and provide practitioners with appropriate guidance. 

Draft guidelines have been completed and were reviewed by national experts during a February 2004 

workshop. The draft is currently being revised, and a complete draft is planned for publication in 

September 2004. Following publication of the draft guidelines, a series of regional seminars is planned to 

educate practitioners and gather additional input that will be used to finalize the guidelines. 

Workshop on Structural Fire Resistance (SFPE) 

An international group of experts was convened in early October, 2003, to examine different technical 

aspects associated with structural fire resistance and to develop a detailed roadmap identifying research 

gaps to be filled to meet industry needs. Ten detailed white papers were presented by leading national 

and international experts on topics relevant to fire resistance of building structures. Workshop attendees 

supported the development of a best-practices manual for structural fire protection, including design and 

analysis tools, to add to the knowledge base and aid building officials in evaluating the adequacy of 

performance-based rather than “prescriptive” designs. The participants also developed a list of 

actions/research needs having the highest priority in developing best practices for fire design and retrofit 

of structures. 

Accelerating Technologies/Systems for Fire Protection of Structural Steel in High Rise 

Buildings (CERF) 

There are a number of objectives for this study. The first is to conduct a brief review of the types of 

materials and systems that are in use, in development or proposed for fire safety protection of structural 

steel in high-rise buildings. A second objective is to identify the performance requirements for such 

systems, including fire resistance, durability, impact and/or vibration resistance. A workshop was held in 

February 2004. In preparation for the workshop, white papers were developed on the topics cited above 

to frame the issues. Participants developed a prioritized set of recommendations to address technical and 

procedural/organizational issues. 
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Cost-Effective Risk Management (Wharton, CII) 

The Wharton Risk Management and Decision Processes Center is expected to deliver in October 2004 a 

draft report covering (1) economic incentives for mitigation of consequences of extreme events (e.g., 

natural disasters and terrorism) and (2) procedures for estimating potential benefits and costs of 

alternative mitigation measures. The CII has developed a security rating index for industrial facilities. It 

is planned to be published by October 2004 in a report entitled Best Practices for Project Security. 
A–i

TABLE OF CONTENTS - Appendix A

List of Figures.........................................................................................................................................A–iii

List of Tables .......................................................................................................................................... A–iv

Appendix A

Interim Report on the Analysis of Building and Fire Codes and Practices...................... A–1

A.1 Introduction................................................................................................................................. A–1

A.1.1 The New York City Building Code................................................................................. A–3

A.1.2 Scope of Appendix .......................................................................................................... A–3

A.2 Loads........................................................................................................................................... A–4

A.2.1 Dead Loads...................................................................................................................... A–4

A.2.2 Live Loads....................................................................................................................... A–6

A.2.3 Wind Load....................................................................................................................... A–7

A.2.4 Earthquake Load............................................................................................................ A–10

A.2.5 Other Loads ................................................................................................................... A–12

A.2.6 Distribution of Loads..................................................................................................... A–12

A.3 Materials, Design, and Construction......................................................................................... A–13

A.3.1 Design Standards........................................................................................................... A–14

A.3.2 Load Combinations ....................................................................................................... A–14

A.3.3 Alteration of Existing Buildings.................................................................................... A–16

A.3.4 Materials and Methods of Construction ........................................................................ A–18

A.3.5 Stability, Bracing, and Secondary Stresses ................................................................... A–20

A.3.6 Deflection Limitations................................................................................................... A–20

A.3.7 Load Tests ..................................................................................................................... A–20

A.4 Fire Safety................................................................................................................................. A–21

A.4.1 Fire Protection ............................................................................................................... A–21

A.4.2 General Code Provisions for Fire Safety....................................................................... A–21

A.4.3 Occupancy Classification .............................................................................................. A–22

A.4.4 Construction Classification ........................................................................................... A–23

A.4.5 Fire Resistance of Structural Elements.......................................................................... A–24

A.4.6 Means of Egress ............................................................................................................ A–26

A.5 Preliminary Findings................................................................................................................. A–29
Appendix A 

A–ii

A.5.1 Provisions Related to Structural Stability...................................................................... A–30

A.5.2 Provisions Related to Fire Safety .................................................................................. A–31

A.5.3 Provisions Related to Egress ......................................................................................... A–32

A.6 References................................................................................................................................. A–32
A–iii

LIST OF FIGURES

Figure A–1. Wind load pressure elevation.............................................................................................. A–8

Figure A–2. Wind load pressure versus elevation................................................................................. A–10
A–iv

LIST OF TABLES

Table A–1. Examples of dead loads given in NYC Building Code and BOCA Code. .......................... A–5

Table A–2. Examples of minimum uniformly distributed live loads ..................................................... A–6

Table A–3. Reduced live load for beams and girders............................................................................. A–9

Table A–4. Design standards for concrete and steel............................................................................. A–14

Table A–5. Compliance requirements for alterations........................................................................... A–17

Table A–6. Excerpts of inspection requirements for materials and assemblies in 1968 New York 

City Building Code............................................................................................................ A–19

Table A–7. Excerpts of inspection requirements for methods of construction in 1968 New York 

City Building Code............................................................................................................ A–19

Table A–8. Summary of fire safety provisions for business occupancy in high-rise 

buildings (> 75 ft). ............................................................................................................. A–26
A–1

Appendix A

INTERIM REPORT ON THE ANALYSIS OF BUILDING AND FIRE CODES 

AND PRACTICES

A.1 INTRODUCTION 

As stated in Chapter 2, one of the objectives of the National Institute of Standards and Technology 

(NIST) Investigation of the World Trade Center (WTC) disaster is to determine the procedures and 

practices that were used in the design, construction, operation, and maintenance of the WTC buildings. 

(For other objectives refer to http://wtc.nist.gov.) Since WTC 1, 2, and 7 were designed according to the 

New York City Building Code, it is important to understand how this code compared with 

contemporaneous building codes. This appendix summarizes the structural and fire provisions of the 

1968 New York City Building Code and compares them with a number of building codes that existed 

when WTC 1, 2, and 7 were designed, as well as the 2001 edition of the New York City Building Code. 

As a result of devastating fires in major cities of the U.S. such as Boston, New York, Chicago, and 

Baltimore in the late 1800s, the first model building code was developed by the fire insurance industry to 

minimize future fire losses. The National Board of Fire Underwriters (predecessor of the American 

Insurance Association) published the National Building Code in 1905. Subsequently, the Pacific Coast 

Building Officials Conference (predecessor of the International Conference of Building Officials) issued 

the Uniform Building Code (UBC) in 1927, the Southern Building Code Congress International Inc. 

published its Southern Standard Building Code (SBCCI) in 1946, and the Building Officials and Code 

Administrators, Inc. (BOCA) published the Basic Building Code in 1950. In the mid-1980s, the Basic 

Building Code was changed to the BOCA National Building Code (NBC). These regionally used model 

building codes were revised annually to incorporate developments in new materials and construction 

methods, and new additions were published every three years. 

It should be pointed out that provisions in these model building codes establish minimum requirements to 

safeguard life, health and property and public welfare by means of regulations pertaining to the design, 

construction, and quality of materials, use and occupancy, and maintenance of buildings. When buildings 

are designed, constructed, and maintained according to building code requirements, they are considered to 

have met minimum requirements. While building code regulations address a number of objectives 

demanded by society, the primary objectives of building codes are structural stability and fire safety. 

Before the issuance of the International Building Code (IBC) in 2000, which was published by the 

International Code Council (an amalgamation of the three regional code organizations), most local and 

state building codes in the U.S. were patterned after one of the three model building codes, UBC, SBCCI, 

or NBC. These model codes evolved in the mid-20th century to incorporate regional differences in 

construction materials and practices. When adopted by local jurisdictions, the model building code 

becomes a legal document and code provisions become mandatory laws. A number of major cities in the 

U.S. have developed their own building codes to meet their specific needs, such as San Francisco for 

earthquake resistant design and New York City for high-rise buildings design. 
Appendix A 

A–2

The National Fire Protection Association (NFPA) in the early 1900s initiated the development of a “life 

safety code” for safety of building occupants. This code, while not a building code, is frequently used as 

a supplement to the building codes. In 2002, NFPA also published a model building code known as the 

NFPA Building Construction and Safety Code (NFPA 5000). 

The Port of New York Authority (PONYA) (whose name was changed to the Port Authority of New York 

and New Jersey [PANYNJ] in 1972) is an interstate agency that was established in 1921under a clause in 

the U.S. constitution that permits compacts between states. As an interstate agency, construction projects 

of the Port Authority are not required to comply with any building code. With respect to design and 

construction of the WTC towers, however, in 1963 the Port Authority instructed its consultants to prepare 

their designs of WTC 1 and WTC 2 to comply with the New York City Building Code (Levy 1963). 

Although it was not specifically stated in the letter to the architect, the 1938 edition of the Code was in 

effect at that time. In areas where the Code was not explicit or where technological advances made 

portions of the 1938 Code obsolete, the Port Authority directed the architect and consulting engineers to 

propose designs “based on acceptable engineering practice.” The Port Authority also required the design 

professionals to inform the Planning Division of the WTC when such situations occurred. The Port 

Authority established a special WTC office that reviewed and approved plans, issued variances, and 

conducted inspections during construction instead of the city agencies and employees that would 

normally perform these duties. 

The Port Authority further stated that all design concepts would be reviewed before the final design by 

the Chief Engineer of the Port Authority and by the appropriate New York City agencies. According to 

correspondence in 1975 from the architect-of-record for the WTC project, the New York City Building 

Department reviewed the design drawings of WTC 1 and WTC 2 in 1968 (Solomon 1975). 

In 1965, the Port Authority instructed the architect and consulting engineers to revise their designs for 

WTC 1 and WTC 2 to comply with the second and third drafts of the New York City Building Code then 

being finalized and to undertake any design modifications necessary to comply with the new code 

provisions (Levy 1965). The new New York City Building Code (NYC BC 1968) was enacted by the 

City Council on October 22, 1968, approved by the Mayor on November 6, 1968, and became effective 

on December 6, 1968. 

The Port Authority intended to lease space in WTC 1 and WTC 2 to tenants who would adapt their spaces 

to their own needs through a tenant alteration process. To maintain structural integrity and fire safety, the 

Port Authority issued a set of requirements for the alteration process. The first edition of the Tenant 

Construction Review Manual was issued in 1971, shortly after the first tenants occupied WTC 1 in 

December 1970 and before initial occupancy of WTC 2 in 1972. The manual contained the technical 

criteria to be used in planning alterations (architectural, mechanical, electrical, fire protection, and so 

forth) to Port Authority facilities. Included were applicable standards to be used by tenants and their 

agents and review criteria to be used by the Engineering Department of the Port Authority. Alteration 

designs were to be completed by registered design professionals, and at the completion of the work, asbuilt drawings were to be submitted to the Port Authority. The 1968 New York City Building Code was 

referenced, and specific code provisions were referenced in various checklists. The review manual was 

updated in 1979, 1984, 1990, and 1997, at which times changes that had been made to the New York City 

Building Code were incorporated. In 1998, the manual was replaced by the Architectural and Structural 

Design Guidelines, Specifications, and Standard Details, which dealt specifically with alterations to 

WTC 1 and WTC 2. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–3

Unlike WTC 1 and WTC 2, which were developed and owned by the Port Authority, WTC 7 was 

developed on land owned by the Port Authority, but the building was owned by Seven World Trade 

Company and Silverstein Development Corporation, General Partner. It was designed and constructed as 

a “Tenant Alteration” project of the Port Authority. When WTC 7 was designed in the mid-1980s, the 

1968 New York City Building Code with amendments was in effect. The Project Specifications for 

WTC 7 issued in 1984 required that the structural steel be designed in accordance with the then current 

New York City Building Code. 

A.1.1 The New York City Building Code 

The New York City Building Code is part of the Administrative Code of New York City. It is amended 

from time to time by Local Laws to improve safety requirements or to incorporate technological 

advances. New York City Council Members, at the request of any person or group, can introduce a bill to 

the Council for the purpose of amending the Building Code requirements. When passed by the Council 

and approved by the Mayor, the bill becomes a Local Law. Seventy-nine Local Laws were adopted 

between 1969 and 2002 that modified the 1968 Building Code. For example, Local Laws 5, 16, and 86 

made important modifications to fire protection and life safety features of the 1968 Building Code. 

To aid the implementation of and to clarify building code requirements, New York City issues “rules.” 

These rules are initiated typically by City Government offices such as the Department of Buildings and 

the Department of Environment. These rules do not require enactment by the City Council, and new rules 

issued by the Building Commissioner can be put into effect expeditiously. The rules are part of the 

Building Code, and are required to be complied with for design, construction, and maintenance of 

buildings. 

The 1968 New York City Building Code includes “Reference Standards.” These include standard test 

methods published by the ASTM International, or design standards published by other consensus-based 

organizations. These reference standards may include modifications to the provisions in the published 

standards, or they may be stand-alone requirements developed by New York City. 

A.1.2 Scope of Appendix 

The 1968 New York City Building Code (NYCBC 1968) is compared with four other codes. They are: 

the 1964 New York State Building Construction Code (NYSBC 1964); the 1965 BOCA Basic Building 

Code (BOCA/BBC 1965); the 1967 Municipal Code of Chicago Relating to Buildings (MCC 1967); and 

the 2001 edition of the New York City Building Code (NYCBC 2001) 

The 1964 New York State Building Construction Code was selected for comparison, as it would have 

been a governing building code outside the New York City limits. The 1965 BOCA Basic Building Code 

was selected, as it was typically adopted by local jurisdictions in the northeastern region of the U.S. The 

1968 New York City Building Code is compared with the 1967 Municipal Code of Chicago to see 

whether there are any substantial differences in the structural and fire safety requirements of the two 

codes. In the late 1960s and early 1970s, several tall buildings were built in Chicago including the Sears 

Tower (110 stories) and the John Hancock Tower (100 stories). The 2001 edition of the New York City 

Building Code is compared with the 1968 version to examine the extent to which Local Laws have 
Appendix A 

A–4

modified the code provisions, and in most cases, is only addressed in areas where changes have occurred 

between the two versions. 

A provision by provision comparison was made between the 1968 New York City Building Code and 

these four codes. The code provisions that were compared are limited to the requirements related to 

structural stability, active and passive fire safety, and emergency egress. This appendix presents a 

summary of substantial differences noted in the comparison. This summary focuses on the following 

topics: 

• Loads to be considered in the design of buildings; 

• Requirements for materials, design, and construction; 

• Fire protection requirements; and 

• Egress requirements. 

With respect to structural stability, no Local Law other than Local Law 17 (seismic provisions for new 

construction) has been adopted that modified the structural requirements of the 1968 New York City 

Building Code. Hence comparison between the structural requirements of the 1968 and 2001 New York 

City Building Code is not discussed here, with the exception of earthquake loads. 

A.2 LOADS 

A key aspect of any structural design is the loading that the structure is intended to support. Building 

codes provide minimum values for the different types of loads that are considered in typical building 

designs. The designer is permitted to use larger values for these loads, but is not permitted to use smaller 

values without approval by the building official. This section compares the specified loads in the codes 

that were compared. Similarities and differences are noted. 

A.2.1 Dead Loads 

Dead loads refer to loads that are permanently present in a building. They include, for example, the 

weight of the structural components, the weights of permanent partitions, the weights of floor and wall 

finishes, and the weights of service equipment that is part of the building (elevator equipment, plumbing, 

electrical, heating, air conditioning, and ventilation systems). Weights of the structural components are 

computed from the sizes of the members and the densities of the material, and codes typically provide 

default density values for different materials. The dead loads of partitions and walls are typically 

prescribed in terms of weight per unit area of wall, and the weight per unit length of wall or partition is 

determined from these prescribed values and the heights of the partitions or walls. Floor finishes and 

ceilings are typically specified in terms of a uniform load per unit area of floor or ceiling. Table A–1 

gives examples of the minimum values of dead load prescribed in Referenced Standard RS 9-1 in the 

1968 New York City Building Code and in Appendix J of the 1965 BOCA Basic Building Code. There 

are no corresponding provisions in the 1964 New York State Building Construction Code or the 

1967 Municipal Code of Chicago. Typically, the designer is permitted to use weights based on available 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–5

data that are greater than the specified minimum values, but the designer is not permitted to use lower 

values without approval of the Code Official. 

Table A–1. Examples of dead loads given in NYC Building Code and BOCA Code. 

 NYC BOCA 

Walls and Partitions 

Hollow concrete block – 8 in. thick 

Clay tile, nonload bearing – 8 in. thick 

Plaster partition, metal studs & lath, gypsum plaster both sides 

53 psf 

34 psf 

18 psf 

50 psf 

36 psf 

18 psf 

Floor Finishes 

Resilient flooring 

Hardwood flooring 7/8 in. thick (1 in. for BOCA) 

Cement, 1 in. thick 

2 psf 

4 psf 

12 psf 

2 psf 

4 psf 

12 psf 

Ceilings 

Suspended acoustical tile 

Suspended metal lath and gypsum plaster 

2 psf 

9 psf 

– 

10 psf 

Miscellaneous Materials 

Marble 

Concrete (normal density stone or gravel) 

Reinforced concrete (normal density) 

168 pcfa

144 pcf 

150 pcf 

168 pcf 

144 pcf 

150 pcf 

a. Note that the units in the 1968 New York City Building Code are given incorrectly as “psf.” 

According to the 1968 New York City Building Code, weights from service equipment (plumbing stacks, 

piping, heating, ventilating, and air conditioning (HVAC), etc.) are to be included in the dead load 

(C26-901.2)1

. The weight of equipment that is part of the occupancy of a given area is to be considered 

as live load (see next section). The 1964 New York State Building Construction Code and the 1967 

Municipal Code of Chicago do not have a provision in this regard. The 1965 BOCA Basic Building Code 

has a similar provision but without citing specific types of service equipment as in the New York City 

Code. 

The 1968 New York City Building Code requires that weights of partitions be considered in two ways: 

(1) using line loads at locations shown on plans or (2) using the equivalent uniform load given in 

Reference Standard RS 9-1. The stipulated equivalent uniform load depends on the partition weight, for 

example, if a partition weighs 201 plf to 350 plf, it may be taken into account by designing for a uniform 

load of 20 psf. The uniform loading approach, however, is not permitted in certain situations for which 

actual partition weights must be used. Equivalent uniform loads must be used in areas where the 

locations of partitions are not shown on plans, or in areas where partitions can be relocated. The 1964 

New York State Building Construction Code does not have a specific provision in this regard. The 1967 

Municipal Code of Chicago prescribes a minimum partition load of 20 psf. The BOCA Basic Building 

Code requires consideration of the actual weight of the partitions or an equivalent uniform load of at least 

20 psf. 

 

1

 Refers to section number in the 1968 New York City Building Code. 
Appendix A 

A–6

A.2.2 Live Loads 

Live loads are those resulting from the use and occupancy of the building, and include loads such as 

weights of occupants, furniture, filing cabinets, safes, mechanical equipment, and other items that the 

structure is called upon to support. Live loads are specified in terms of weight per unit of floor (or roof) 

area or in terms of concentrated loads. The values specified in codes are based largely on load survey 

data, experience, and judgment. 

Floor Live Loads 

In general, values of minimum uniformly distributed live loads specified in codes are organized on the 

basis of use or occupancy of spaces and there is no consistency in the names of these use categories. 

Thus comparison between codes is not straightforward. Table A–2 gives some examples of minimum 

uniformly distributed live loads for floors. It is seen that there is general agreement in the values of these 

selected minimum uniform live loads specified by the four codes. 

Table A–2. Examples of minimum uniformly distributed live loads 

1968 NYC 1964 NYS 1967 Chicago 1965 BOCA 

Office space 50 psf 50 psf 50 psf 50 psf 

Restaurant 100 psf 100 psf – 100 psf 

Lobbies 100 psf 100 psf 100 psf 100 psf 

Stairways 100 psf 100 psf 75–100 psfa 100 psf 

Rest rooms 40 psf 60 psf – – 

Hospital operating room 60 psf 60 psf 40 psf 60 psf 

School classroom 40 psf 60 psf 40 psf – 

a. Depends on occupancy, for example, 75 psf for business, 100 psf for schools. 

The codes also specify concentrated live loads placed so as to result in maximum stresses. 

Live Load Reduction 

There is a low likelihood that the full design floor live loads will be present on all floors of a building at 

the same time. In addition, the likelihood that the complete area on any one floor is loaded with the 

design load decreases as the floor area increases. To account for these factors, building codes permit 

“live-load reductions” in calculating the design loads for primary members (columns and girders) that 

support the roof and floors. The codes use several methods for live-load reduction (CTB&UH 1980): 

1. Percentage Method—In this method, the live load reduction increases by a certain percentage 

with increasing numbers of floors, with a limit on the maximum value of reduction (typically 

50 percent). 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–7

2. Tributary Area Method—The live load is reduced as the accumulated tributary area that is 

supported by a member is increased. The limiting value depends on the ratio of live load to 

dead load. The type of occupancy affects whether a reduction is permitted. 

3. Live Load to Dead Load Ratio—The permitted reduction depends on the ratio of live load to 

dead load, provided that the dead load is greater than the live load. 

The 1968 New York City Building Code uses the tributary area method and permits the percentage 

method as an alternative for columns, piers, and walls. The 1964 New York State Building Construction 

Code and the 1967 Municipal Code of Chicago use the tributary area method for beams and girders and 

the percentage method for columns and walls. The 1965 BOCA Basic Building Code uses a tributary 

method that is similar to the New York State Code. 

Figure A–1 compares the reduced live load for columns, walls, and piers on the basis of the percentage 

method for three of the codes. It is seen that the permitted reductions are similar with the exception of the 

roof and top floor, where the 1968 New York City Building Code and the 1967 Municipal Code of 

Chicago are more conservative (less reduction permitted) than the 1964 New York State Building 

Construction Code. 

Table A–3 compares the reduced live loads for beams and girders for the different codes. For the 1968 

New York City Building Code, the reduced value of live load for a given contributory area depends on 

the live load to dead load ratio, with lower values permitted for lower live load to dead load ratios. For 

the 1964 New York State Building Construction Code and the 1965 BOCA Basic Building Code, the 

values shown in the table are based on a reduction factor of 0.08 %/ft2

. The lowest reduced value, 

however, is limited to 40 percent or 

D

L

D

L

33.4

133.3

%100

−

 (A.1) 

whichever is larger, where L/D is the live load to dead load ratio. As the ratio of live load to dead load 

increases, less live load reduction is permitted. A comparison of the values in Table A–3 shows that the 

1967 Municipal Code of Chicago did not permit as large a reduction in live load for the same contributory 

area as the other codes. 

A.2.3 Wind Load 

The effect of wind on buildings is taken into account by the building codes by specifying a uniform 

pressure to be applied horizontally to a building. These pressures are to be applied in any direction so as 

to obtain the most critical loading condition. 
Appendix A 

A–8

Figure A–1. Wind load pressure elevation. 

1968 NYC Building Code 

(Alternative Method) 

1967 Chicago Municipal Code 

1964 New York State Building 

Construction Code 

Roof

1st Floor Below

2nd Floor Below

3rd Floor Below

4th Floor Below

5th Floor Below

6th Floor Below

7th Floor Below

8th and Subsequent 

Floor Below 

100 %

85 %

80 %

75 %

70 %

65 %

60 %

55 %

50 %

80 % 

80 % 

80 % 

75 % 

70 % 

65 % 

60 % 

55 % 

50 % 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–9

Table A–3. Reduced live load for beams and girders. 

Contributary Area 

(ft2

) 

1968 NYC 

Building Code 

1967 Chicago 

Municipal Code 

1956 NY State and 

1965 BOCA Codes 

100 or less 100 % 100 % 100 % 

100–149 100 % 95 % 100 % 

150–199 80 % to 85 %a 95 % 84 % to 88 %b

200–299 80 % to 85 %a 90 % 76 % to 84 %b

300–449 60 % to 75 %a 85 % 64 % to 76 %b

450–599 50 % to 70 %a 85 % 52 % to 64 %b

600 and more 40 % to 65 %a 85 % 40 % to 52 %b

a. Permitted value depends on live load to dead load ratio; less reduction permitted with higher ratio. 

b. The lowest value is limited to 40 percent or 100 percent (3.33 L/D –1)/(4.33 L/D), whichever is greater. 

The pressure due to wind varies with the square of the wind speed, and wind speed increases with height. 

Thus building codes specify minimum design wind pressures that increase with elevation. The variations 

of pressure with height, however, are not the same among the building codes compared. Figure A–2 

compares the specified wind pressure versus height relationships for the four codes that were compared. 

Several observations are noted: 

• For buildings up to 600 ft in height, the 1964 New York State Building Construction Code 

prescribes the largest wind pressures. 

• The 1967 Municipal Code of Chicago prescribes the lowest wind pressures for buildings up 

to 900 ft in height. 

• The 1968 New York City Building Code and the 1965 BOCA Basic Building Code provide 

similar wind pressures for buildings up to 700 ft in height; for taller buildings the BOCA 

Code specifies larger pressures. 

For a building height of 1,370 ft (the approximate heights of WTC 1 and WTC 2), the wind pressure 

distribution specified by the 1965 BOCA Basic Building Code would result in the largest shear force and 

overturning moment at the base of the building. 

The 1968 New York City Building Code permits the designer to use wind pressure values, other than 

specified minimums, on the basis of wind tunnel tests and with approval of the building official. The 

following wording is provided in Section 6 of Reference Standard RS 9-5, “Minimum Design Wind 

Pressures.” 

In lieu of the design wind pressures established in sections 1 and 2 of this 

reference standard, and subject to review and approval of the commissioner, 

design wind pressures may be approximated from suitably conducted model 

tests. The tests shall be predicated on a basic wind velocity of 80 mph at the 

30 ft level, and shall simulate and include all factors involved in considerations 

of wind pressure, including pressure and suction effects, shape factors, functional 

effects, gusts, and internal pressures and suctions. 
Appendix A 

A–10

Figure A–2. Wind load pressure versus elevation. 

Thus the 1968 New York City Building Code presumes a wind with a speed of 80 mph measured 30 ft 

above the ground. The 1964 New York State Building Construction Code, on the other hand, states that 

the prescribed wind loads “are based on a design wind speed of 75 mph at a height of 30 ft above grade 

level.” Both the 1965 BOCA Basic Building Code and the 1967 Municipal Code of Chicago do not 

specify the design wind speed. 

A.2.4 Earthquake Load 

The 1968 New York City Building Code does not have provisions for earthquake loads. Among the 

contemporaneous codes that were compared, only the 1965 BOCA Basic Building Code has earthquake 

0

200

400

600

800

1000

1200

0 10 20 30 40 50

1968 NYC Code

1964 NYS Code

1967 Chicago 

Code

1965 BOCA/BBC

Height, ft

Pressure, psf
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–11

load provisions. These are contained in Appendix K-11 of that Code and were adapted from the 1962 

edition of the Uniform Building Code. 

The 2001 edition of the New York City Building Code contains seismic design provisions from the 1988 

edition of the Uniform Building Code (UBC 1988), including the 1990 Accumulative Supplement. These 

provisions were put into effect in 1996 as a result of Local Law17 (passed in 1995). Significant 

modifications to the 1988 Uniform Building Code were made, and described in Reference Standard 

RS 9-6. 

One modification is to the paragraph on “Minimum Seismic Design,” which is modified to read: 

The following types of construction shall, at a minimum, be designed and 

constructed to resist the effects of seismic ground motions as provided in this 

section: 

new structures on new foundations; 

new structures on existing foundations; and 

enlargements in and of themselves on new foundations. 

Buildings classified in New York City occupancy group J-3 and not more than 

three stories in height need not conform to the provisions of this section. The 

Commissioner may require that the following types of construction be designed 

and constructed to incorporate safety measures as necessary to provide safety 

against the effects of seismic ground motions at least equivalent to that provided 

in a structure to which the provisions of the section are applicable: 

new buildings classified in occupancy group J-3 and which are three 

stories or less in height; and 

enlargements in and of themselves where the costs of such enlargement 

exceeds sixty percent of the value of the building. 

In the subdivision on “Criteria Selection” the following paragraph was added: 

Seismic Zone. The seismic zone factor, Z, for buildings, structures and portions 

thereof in New York City shall be 0.15. The seismic zone factor is the effective 

zero period acceleration for S1 type rock. 

Other significant amendments include consideration of soil liquefaction that is not to be found in the 

Uniform Building Code. 
Appendix A 

A–12

A.2.5 Other Loads 

The 1968 New York City Building Code has provisions dealing with types of loadings not considered in 

the other codes that were compared. Two examples are “thermal forces” and “shrinkage.” Paragraph 

C26-905.7 deals with thermal forces and includes the following requirement: 

...For exterior exposed frames, arches, or shells regardless of plan dimensions, 

the design shall provide for the forces and/or movements resulting from an 

assumed expansion and contraction corresponding to an increase or decrease in 

temperature of forty degrees F for concrete or masonry construction and sixty 

degrees F for metal construction.... 

Paragraph C26-905.8 on shrinkage includes the following requirement: 

The design of reinforced concrete components shall provide for the forces and/or 

movements resulting from shrinkage of the concrete in the amount of 0.0002 

times the length between contraction joints for standard weight concrete, and 

0.0003 times the length between contraction joints for lightweight concrete.... 

A.2.6 Distribution of Loads 

Another topic that is contained only in the 1968 (and 2001) New York City Building Code is the 

distribution of loads, which is covered in Article 7 of Sub-chapter 9. Section C26-906.1 deals with 

vertical loads and states: 

“Distribution of vertical loads to supporting members shall be determined on the 

basis of a recognized method of elastic analysis or system of coefficients of 

approximation. Elastic or inelastic displacements of supports shall be considered 

and, for the distribution of dead loads, the modulus of elasticity of concrete or 

composition [composite] sections shall be reduced to consider plastic flow. 

Secondary effects, due to warping of the floors shall be considered.” 

Section C26-906.2 deals with distribution of horizontal forces. Because this section provides important 

information in the design assumptions to be used in the design of high-rise buildings, several key sections 

are repeated here, 

The following provisions shall apply to superstructure framing only, and shall not 

apply to structures wherein horizontal loads are transmitted to the foundation by 

staycables, arches, non-rectangular frames, or by frames, trusses, or shear walls 

not oriented in vertical planes. 

(a) Distribution of horizontal loads to vertical frames, trusses and shear 

walls. - Horizontal loads on the superstructure shall be assumed to be distributed 

to vertical frames, trusses, and shear walls by floor and roof systems acting as 

horizontal diaphragms. The proportion of the total horizontal load to be resisted 

by any given vertical frame, truss, or shear wall shall be determined on the basis 

of relative rigidity, considering the eccentricity of the applied load with respect to 

the center of resistance of the frames, trusses, or shear walls. For vertical trusses, 

web deformations shall be considered in evaluating the rigidity. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–13

(b) Distribution of horizontal loads within rigid frames of tier buildings. - 

(1) ASSUMPTIONS. - The distribution of horizontal loads within rigid frames 

of tier buildings may be determined on the basis of a recognized method of 

elastic analysis or, subject to limitations in paragraph two of this subdivision, 

may be predicated on one or more of the following simplifying assumptions: 

a. Points of inflection in beams or columns are at their midspan and 

midheight, respectively. The story shear is distributed to the columns in 

proportion to their stiffnesses. 

b. The change in length of columns due to axial effects of the horizontal 

loads may be neglected. 

c. Vertical column loads due to horizontal forces are taken by the exterior 

columns only, or are resisted by the columns in proportion to the column 

distances from the neutral axis of the bent. 

(2) LIMITATIONS. - 

a. For buildings over 300 ft in height, the change in length of the columns, 

due to the effects of the horizontal loads, shall be evaluated or the framing 

proportioned to produce regular movements of the successive joints at each 

floor so that warping of the floor system may be neglected. 

b. Simplifying assumptions used in design shall be subject to approval by 

the commissioner for any of the following conditions or circumstances: 

1. For buildings over 300 ft in height or for buildings with a heightwidth ratio greater than five. 

2. At two-story entrances or intermediate floors. 

3. Where offsets in the building occur. 

4. Where transfer columns occur. 

5. In any similar circumstances of irregularities or discontinuities in 

the framing. 

A.3 MATERIALS, DESIGN, AND CONSTRUCTION 

Subchapter 10 of the 1968 New York City Building Code is entitled “Structural Work” and it provides 

minimum requirements for materials, design, and construction of all structural elements in buildings. 

This summary reviews design standards, materials, load combinations, and load tests. 
Appendix A 

A–14

A.3.1 Design Standards 

Design standards refer to those documents that are used to proportion the structural elements and their 

connections. The principal structural materials in the WTC were concrete and steel, and the design 

standards are those produced by the American Concrete Institute (ACI) and the American Institute of 

Steel Construction (AISC). The ACI produces the standard known as ACI 318, Building Code 

Requirements for Reinforced Concrete,2

 and the AISC produced the following: 

• Specification for the Design, Fabrication and Erection of Structural Steel for Buildings 

(AISC 1963) 

• Specifications for Structural Steel Buildings–ASD and Plastic Design (AISC 1989) 

• Load and Resistance Factor Design Specifications for Structural Steel Buildings (AISC 1993) 

Table A–4 summarizes the concrete and steel design standards adopted by the codes that were compared. 

The 1964 New York State Building Construction Code is a performance standard and does not adopt 

design standards by reference. Thus at the time the WTC Towers were being designed, the other two 

codes (Chicago and BOCA) referenced the same concrete and steel design standards as the New York 

City code. 

Table A–4. Design standards for concrete and steel. 

1968 NYC Code 2001 NYC Code 

1967 Chicago 

Code 

1965 BOCA 

Code 

Concrete ACI 318-63 ACI 318-89 ACI 318-63 ACI 318-63 

Steel 

AISC 1963 AISC 1989 

AISC 1993 

AISC 1963 AISC 1963 

The 1963 edition of ACI 318 permits reinforced concrete members to be designed by either the working 

stress (or allowable stress) method or by the ultimate strength method. The 1963 AISC specification, on 

the other hand, is based on allowable stress design. The design method affects the loads used in the 

design calculations. 

A.3.2 Load Combinations 

The loads prescribed by the codes are used in different combinations to assess the governing design 

condition. The codes distinguish between sustained loads and loads of short duration or infrequent 

occurrence. For allowable stress design, two approaches are used for dealing with these two categories of 

loads, as will be discussed. For ultimate strength design, the prescribed loads are multiplied by specified 

load factors. In either case, the designer considers all applicable load combinations and determines the 

most critical condition, which becomes the design basis for a particular element. 

 

2

 In 1999, the title was changed to Building Code Requirements for Structural Concrete.
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–15

Allowable Stress Design 

The 1968 New York City Building Code defines two categories of loads: 

1. Basic loads, which include dead load, live load, and reduced live load where applicable; and 

2. Loads of infrequent occurrence, which include wind load, thermally induced load, shrinkage 

induced load, and unreduced live load where live load reduction is permitted. 

Stresses in structural elements cannot exceed the allowable values specified in the referenced design 

standards under the following load combinations: 

• The sum of the basic loads multiplied by a factor equal to 1; 

• The factored sum of one or more basic loads and one load of infrequent occurrence, where 

the load factor equals 0.75; 

• The factored sum of one or more basic loads plus two or more loads of infrequent occurrence, 

where the load factor equals 0.6. 

The 2001 New York City Building Code is similar with the exception that it includes earthquake load as 

another load of infrequent occurrence. 

The other Codes that were compared use a different approach for dealing with loads of infrequent 

occurrence. The 1964 New York State Building Construction Code states that stress due to wind load 

may be ignored if it less than 1/3 of the stress due to dead load plus imposed load excluding wind load. If 

the stress due to wind load exceeds this limit, the allowable stress for the material is permitted to be 

increased by 1/3. 

The 1967 Municipal Code of Chicago uses a similar approach and states: “For combined stresses due to 

dead, live, and wind load, the allowable stresses in materials may be increased 1/3, provided the section 

thus determined is at least as strong as that required for dead and live load alone. Snow load shall be 

considered a live load.” 

The 1965 BOCA Basic Building Code is similar except that wind load or earthquake load is considered 

along with dead load and live load (including snow load). The same 1/3 increase in allowable stress is 

permitted under wind or earthquake load. The BOCA Code also explicitly states that wind load is 

permitted to be neglected if it results in stress less than 1/3 the stress due to dead load plus live load. 

Ultimate Strength Design 

In the 1960s, ultimate strength design was standardized only for reinforced concrete. As shown in 

Table A–4, the three codes from the 1960s referenced ACI 318-63, which include the following load 

combinations to establish the design loads (U) for structural members: 

1. For structures where wind and earthquake loads may be neglected, U = 1.5 D + 1.8 L. 
Appendix A 

A–16

2. For structures where wind load must be included, U = 1.25 (D + L) or U = 0.9 D+ 1.1 W, 

whichever produces the most unfavorable condition for the member. 

3. For structures where earthquake loading is included, E shall be substituted for W in 

condition 2. 

4. In structures where effects of shrinkage and temperature are included, the effects of such 

items shall be considered on the same basis as the effects of dead load. 

The 2001 New York City Building Code refers to ACI 318-99, which includes many more load 

combinations to be considered. These are as follows: 

1. For all structures, U = 1.4 D + 1.7 L. 

2. For structures where wind load must be included, U = 0.75[1.4 D + 1.7 L + 1.7 W)] or 

U = 0.9 D + 1.3 W, whichever produces the most unfavorable condition for the member. 

3. For structures where resistance to earthquakes must be included, the load combinations of 

condition 2 are used with 1.1 E substituted for W. 

4. For structures where resistance to earth pressure (H) must be included, 

U = 1.4 D + 1.7 L + 1.7 H or 0.9 D + 1.7 H, whichever produces the most unfavorable 

condition. 

5. For structures where resistance to fluid pressure (F) must be included, 

U = 1.4 D + 1.7 L + 1.4 F or 0.9 D + 1.7 F, whichever produces the most unfavorable 

condition. 

6. For structures where resistance shrinkage and temperature (T) must be included, 

U = 0.75 (1.4 D + 1.4 T + 1.7 L) > 1.4 (D + T). 

7. For structures where resistance to impact must be taken into account, such effects shall be 

included with live load L. 

A.3.3 Alteration of Existing Buildings 

The compared codes have provisions to address code compliance when existing buildings are altered. 

The provisions of all codes, other than the 1964 New York State Building Construction Code, are broadly 

similar. In general, whether the altered building or only the alternations need to comply with code 

requirements depends on the ratio of alterations to the total building expressed either in terms of cost or 

dimensions. When the ratio is low, even the alterations may not have to be in compliance with the code, 

provided stipulated conditions are met. The 1964 New York State Building Construction Code, however, 

requires that any addition or alteration regardless of building value shall be made in conformity with that 

code. It is silent as to the structure being altered. Table A–5 summarizes code provisions related to 

alterations. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–17

Table A–5. Compliance requirements for alterations. 

Code Provisions 

1968 New York City 

Building Code 

Alterations exceeding 60 percent of building value (in any 12-month period): The 

entire building shall be made to comply with the requirement of the code. 

Alterations between 30 percent and 60 percent of building value: Only those 

portions of the building altered shall be made to comply with the requirements of the 

code. 

Alteration under 30 percent of building value: Those portions altered may, at the 

option of the owner, be altered in accordance with the requirement of the code, or 

altered in compliance with their previously required condition and with the same or 

equivalent materials and equipment, provided the general safety and public welfare are 

not thereby endangered. 

2001 New York City 

Building Code 

Same as 1968 Code, except that wording for alterations less than 30 percent of 

building values was changed to: “those portions of the building altered may, at the 

option of the owner, be altered in accordance with the requirements of this code, or 

altered in compliance with the applicable laws in existence prior to December sixth, 

nineteen hundred sixty-eight, provided the general safety and public welfare are not 

thereby endangered.” 

In addition, certain alterations are required to conform to the code regardless of 

magnitude or cost. These include, among others: 

Alterations to standpipes, sprinklers, or interior fire alarm and signal systems; 

Alterations to equipment for heating or storing water; 

Sprinkler, alarm protection, and emergency lighting requirements for places of 

assembly. 

1964 New York State 

Building Construction 

Code 

Addition or alteration: Any addition or alteration, regardless of cost, made to a 

building shall be made in conformity with applicable regulations of the code. 

1967 Municipal Code 

of Chicago 

More than 50 percent: Such buildings and structures shall be made to conform to all 

requirements of the code that are applicable to new buildings and structures. 

25 percent to 50 percent: All new constructions shall conform to the requirements of 

the code for new buildings or structures of like area, height and occupancy. 

25 percent or less: Certain exceptions can be made that allow the use of materials that 

conform to the strength and fire resistance for the materials with which the building is 

constructed. Otherwise, all new construction shall conform to the requirements of this 

code for a new building. 

1965 BOCA Basic 

Building Code 

“In the reconstruction, repair, extension or alteration of existing buildings, the 

allowable working stresses used in design shall be as follows: 

1. Building extended: If altered by an extension in height or area, all existing 

structural parts affected by the addition shall be strengthened where necessary and all 

new structural parts shall be designed to meet the requirements for buildings hereafter 

erected. 

2. Building repaired: When the uncovered structural parts are found unsound, such 

parts shall be made to conform to the requirements for buildings hereafter erected. 

3. Existing live load: When an existing building heretofore approved is altered or 

repaired within the limitation prescribed in Section 106.3 (alteration under 50 percent) 

and 106.4 (alteration under 25 percent), the structure may be designed for the loads and 

stresses applicable at the time of erection, provided that public safety is not 

endangered. 

4. Posted live load: May be posted for original approved live loads.” 
Appendix A 

A–18

A.3.4 Materials and Methods of Construction 

The compared codes have requirements for the materials and construction methods. Each code makes 

distinctions in materials and methods that depend on the nature of inspection and conformance with 

standards. 

The 1968 New York City Building Code prescribes testing and inspection requirements for all materials, 

assemblies, forms, and methods of construction. A distinction is made between materials and methods 

subject to “controlled inspection” and those that are not subject to controlled inspection. Materials and 

methods subject to controlled inspections “shall be inspected and/or tested to verify compliance with code 

requirements.” In general, activities related to controlled inspections “shall be made and witnessed by or 

under the direct supervision of an architect or engineer retained by or on behalf of the owner or lessee, 

who shall be, or shall be acceptable to, the architect or engineer who prepared or supervised the 

preparation of the plans.” On the other hand, materials and methods not designated for controlled 

inspection “shall be inspected and/or tested to verify compliance with code requirements by the person 

superintending the use of the material or its incorporation into the work...” 

The 1968 New York City Building Code provides tables to indicate which materials and methods are 

subject to controlled inspections and which are not. Table A–6 includes excerpts from the requirements 

for inspection of materials and assemblies. A footnote to the table in the code states that “All structural 

materials and assemblies subject to controlled inspection shall be tested and/or inspected at their place of 

manufacture and evidence of compliance with the provisions of this subchapter shall be provided as 

stipulated in sub-articles 1003.0 through 1011.0.” Table A–7 is an excerpt of the inspection requirements 

for methods of construction. A footnote to the companion table in the code states that “All construction 

operations designated for controlled inspection shall be inspected by the architect or engineer designated 

for controlled inspection during the performance of such operation.” 

The 1968 New York City Building Code required that the installation of “sprayed-on fire protection” of 

structural members (except those encased in concrete) be subjected to controlled inspection requirements, 

as defined above. There were, however, no specific provisions on what testing was required. 

The 1964 New York State Building Construction Code and the 1965 BOCA Basic Building Code make 

distinctions between “controlled” and “ordinary” materials in reference to establishing allowable stresses. 

For example BOCA defines “controlled materials” as those that are “certified by an accredited 

authoritative agency as meeting accepted engineering standards for quality.” Ordinary materials are those 

that do not conform to the requirements for controlled materials. 

The 1967 Municipal Code of Chicago specifies that all materials and methods used in the design and 

construction of buildings shall be classified as “controlled materials” or “ordinary materials.” According 

to the Chicago Code, “controlled materials” means a building, structure, or part thereof, which has been 

designed or constructed under the following conditions: (a) All controlled materials must be selected or 

tested to meet the special strength, durability and fire resistance requirements upon which the design is 

based. (b) The design, preparation of working drawings, including details and connections, the checking 

and approval of all shop and field details and the inspection of the work during construction shall be 

under the supervision of a registered architect or structural engineer (Section 69-3.1). 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–19

Table A–6. Excerpts of inspection requirements for materials and assemblies in 

1968 New York City Building Code. 

Material Elements Subject to Controlled Inspection 

Elements Not Subject to Controlled 

Inspection 

Steel None All structural elements and connections 

Concrete Materials for all structural elements 

proportioned on the basis of calculated 

stresses 70 percent or greater, of basic 

allowable stresses. See Section 1004.0 for 

specific requirements relating to “quality 

control of materials and batching.” 

(1) All materials for all structural elements 

proportioned on the basis of calculated stresses 

less than 70 percent or greater of basic allowable 

values. 

(2) Concrete materials for: 

(a) Short span floor and roof construction 

proportioned as per section 1004.8. 

(b) Walls and footings for buildings in 

occupancy group J-3. 

(3) Metal reinforcement. 

Table A–7. Excerpts of inspection requirements for methods of construction in 

1968 New York City Building Code. 

Material 

Operations Subject to Controlled 

Inspection 

Operations Not Subject to Controlled 

Inspection 

Steel (1) Welding operations and the tensioning of 

high strength bolts in connections where the 

calculated stresses in the welds or bolts are 50 

percent or more of basic allowable values. 

(2) Connection of fittings to wire cables for 

suspended structures, except where cables 

together with their attached fittings are proofloaded to not less than 50 percent of ultimate 

capacity. 

(1) Welding operations and the tensioning of 

high strength bolts in connections where the 

calculated stresses in the welds or bolts are less 

than 50 percent of basic allowable values. 

(2) All other fabrication and erection operations 

not designated for controlled inspection. 

Concrete Except for those operations specifically 

designated in this table as not subject to 

controlled inspection, for all concrete, the 

operations described in section 1004.5(a) shall 

be subject to controlled inspection.” 

(1) All operations relating to the constriction of 

members and assemblies (other than prestressed 

concrete) which involve the placement of a total 

of less than 50 cubic yards of concrete and 

wherein said concrete is used at levels of 

calculated stress 70 percent or less of basic 

allowable values. 

(2) placing and curing of concrete for all: 

(a) short span floor and roof construction as per 

section 1004.8. 

(b) Walls and footings for buildings in 

occupancy group J-3. 

(3) Size and location of reinforcement for walls 

and footings in occupancy group J-3. 

(4) All other operations not described in 

Sections C26-1004.5(a). 
Appendix A 

A–20

A.3.5 Stability, Bracing, and Secondary Stresses 

The 1968 and 2001 New York City Building Code are the only codes of those compared that include 

provisions for stability, bracing, and secondary stresses. The provisions are the same in the two editions 

of the code. Stability, in this case, refers to resistance to sliding or overturning of the building on its 

foundation. The New York City Building Code requires a factor of safety of 1.5 against failure by sliding 

or overturning. The required stability is to be provided solely by the dead load plus any permanent 

anchorage that is provided. Bracing refers to lateral support to prevent buckling of compression members 

(columns and walls). The New York City Building Code requires that the bracing be proportioned to 

resist a load of at least 2 percent of the total design compression load in the braced member plus any 

transverse shear load on the bracing member. Secondary stresses refer to stresses associated with 

transverse deflection of a member. In trusses, for example, secondary stresses arise because joints are not 

true pins and some bending is introduced, which results in transverse displacements of the individual 

elements. The New York City Building Code requires that secondary stresses in trusses be considered in 

designing the size of the individual elements. 

A.3.6 Deflection Limitations 

All five codes contain limits on vertical deflections of floor and roof assemblies. Except for the New 

York City Building Codes (both the 1968 and 2001 versions), the deflection limits relate to crack 

formation of plastered building components. The deflection is limited 1/360 of the span for plastered 

members and 1/240 of the span for non-plastered members. The New York City Building Codes refer to 

the reference standards for deflection limits in addition to the 1/360 of the span limit. For concrete 

members, ACI 318-63 specifies limits for both short- and long-term deflections of beams and one-way 

slabs. For steel members, the 1963 AISC Specification specifies deflection limits to avoid damage to 

plastered ceilings and to limit deflections of flat roofs. 

A.3.7 Load Tests 

Building codes generally allow load tests to ascertain the adequacy of load carrying capacity of structural 

members. Specifically building codes allow load tests or tests of in-place materials: 

• To verify adequacy of structural design for a member or an assembly, 

• To verify adequacy of partially completed construction, 

• To prequalify structural members or assemblies before used in service, 

• To verify adequacy of questionable completed structure, and 

• To determine concrete strength by means of core tests. 

The New York City Building Codes have provisions to cover all five categories. The New York State 

Code has provisions for (1) and (4). The Chicago Municipal Code has provisions for (1), (4) and (5). 

The BOCA/Basic Building Code has provisions for (1) and (2). 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–21

A.4 FIRE SAFETY 

A.4.1 Fire Protection 

As mentioned in A.1, in 1965, the Port Authority directed that the design of the WTC towers be updated 

to conform to the second and third drafts of the 1968 New York City Building Code then under 

development. However, since it was not known which proposals in the second and third drafts would be 

adopted into the final 1968 code, the strategy was to identify any proposed design provision that differed 

from the 1938 code requirement as a variance to be specifically approved by the Port Authority 

(Kyle 1966a and Kyle 1966b). The Port Authority established the World Trade Department, a special 

Port Authority office, to review and approve plans, to issue variances, and to conduct inspections during 

construction. 

The 1968 New York City Building Code contained detailed fire safety provisions on a number of topics 

not addressed in the other codes of the time, most of which appeared in these other codes at later times. 

For example, while the contemporaneous codes contained requirements for flame spread ratings for 

interior finish materials, the 1968 New York City Building Code also included requirements for “smoke 

developed" ratings that did not appear in other codes until later. 

As stated in Section A.1.1, the New York City Building Code is amended by “Local Laws” (LL) and 

refined or interpreted by administrative orders issued by the Building Commissioner. While there were 

some 79 Local Laws adopted between 1969 and 2002 that modified the 1968 code, those that contained 

significant modifications to fire protection and life safety requirements include LL54/1970, LL5/1973, 

LL26/1975, LL55/1976, LL33/1978, LL41/1978, LL84/1979, LL86/1979, LL16/1984 and LL16/1987. 

Of particular importance in this group are LL5/1973 (and LL86/1979 that changed the compliance dates 

for LL5/1973) and LL16/1984 because some of their provisions applied retroactively to existing office 

buildings. 

A.4.2 General Code Provisions for Fire Safety 

Fire safety of building construction is generally regulated through limits placed on the height and the area 

per floor as a function of the type and degree of fire resistance of materials used in the structural elements. 

These material characteristics are categorized as types of construction, e.g., Type I through V, and the 

associated limits are contained in “heights and areas” tables that are a cornerstone of most (prescriptive) 

building codes, worldwide. 

The intent of building height limits is to restrict taller buildings to non-combustible structural members 

with the greatest fire resistance (as measured in the ASTM E 119 test method). The primary concern with 

combustible structural members is that they can become ignited by an exposing fire and can continue to 

burn (often in concealed spaces) even after the exposing fire has been extinguished, leading to collapse. 

The other important height factor is the definition of a high-rise building. This is generally based on the 

height above which fire department ladders will not reach, requiring that fires be fought from inside. An 

interior attack is limited to hand-held hoses supplied from standpipes and working from interior 

stairways. Traditionally, high-rise buildings have been defined as those that exceed 75 ft or 6 stories 
Appendix A 

A–22

above grade in height, but some newer codes increase this height to 100 ft, as modern fire department 

ladders are longer. 

The intent of area limits is generally to limit property risk and to limit the size (area involved on any 

floor) of the fire to that which can be dealt with by the fire department with the number of people and 

equipment typical of an initial response. 

A.4.3 Occupancy Classification 

The building codes define categories of occupancy (which may have more than one sub-class). The 

group designations vary in different codes, the ones presented here are those used in the New York City 

Building Code. These are: 

• High Hazard (Group A) 

• Storage (Groups B-1 and B-2) 

• Mercantile (Group C) 

• Industrial (Group D-1 and D-2) 

• Business (Group E) 

• General Assembly (Group F-1 through F-4) 

• Educational (Group G) 

• Institutional (Groups H-1 and H-2) 

• Residential (Groups J-1 through J-3) 

• Miscellaneous (Group K) 

Building codes use occupancy as a surrogate for risk factors that determine the level of performance 

needed. For example, occupancy is determined by a combination of factors such as types and quantity of 

combustible contents, common ignition sources, and typical occupant characteristics. Business 

occupancies (which includes office buildings) are considered among the lowest risk because they 

typically contain grades of furniture that constitute relatively low combustible loads, few ignition sources, 

and a population that is predominately adult, in good physical and mental condition (e.g., not using 

alcohol), and not sleeping. The most risky occupancies are High Hazard, in which are found highly 

flammable, toxic, or explosive materials, and Institutional (e.g., hospitals and prisons) in which occupants 

are likely to be incapable of unassisted egress. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–23

A.4.4 Construction Classification 

The model building codes classify building constructions into different “Types.” Although there are some 

variations in categories, they are reasonably consistent.3

 The main categories are Type 1 (fire resistive), 

Type 2 (non-combustible), Type 3 (combustible), Type 4 (heavy timber) and Type 5 (ordinary). 

Types 1 and 2 are constructed with non-combustible exterior and interior bearing walls and columns. Fire 

resistance ratings (see A.4.5) are greatest for Type 1, and Type 2 is any (non-combustible) construction 

not meeting Type I requirements. Type 3 is where exterior bearing walls are non-combustible and 

interior bearing walls and some columns may employ approved combustible materials. Type 4 is known 

as heavy timber, which utilizes large, solid cross section wooden members such as in post and beam 

construction. Type 5 is traditional wood frame construction. Common non-combustible structural 

elements use steel or reinforced concrete. Combustible structural elements are usually made of solid- or 

engineered-wood and laminates. 

Combustibility of the materials in a structural element is determined in ASTM E 136 in which the 

material is placed in a furnace at 750 oC (1,380 °F), which is a “typical” fire temperature. Some minor 

surface burning (e.g., from paint or coatings) is allowed in the first 30 s but there cannot be any 

significant energy release as indicated by more than 30 oC (54 oF) increase in the furnace temperature, and 

the test specimen cannot lose more than half its initial mass. Materials that pass are designated noncombustible and the rest are combustible. 

Within each construction type, there are several sub-categories determined by the fire resistance ratings of 

the columns, beams, and floor supports. In some codes these sub-categories are identified by letters 

following the type (e.g., 1B or 3A) or by a set of three numbers that represent the fire resistance required 

(in hours) of the columns, beams, and floors, respectively (e.g., Type 1 [3,3,2]). 

For unsprinklered office buildings, the following construction classes are permitted in the five building 

codes reviewed. 

• Type 1A and 1B—NYC BC 68, NYS BC 64, BOCA/BBC 65 (Unlimited height) 

• Type 1A, 1B, 1C, 1D—NYC BC 01 (Height limited to 75 ft) 

• Type 1A only—Chicago BC 67 (Unlimited height) 

It is noted that the 1938 New York City Building Code did not include Type 1B construction for office 

occupancies. The reasons for the inclusion of Type 1B construction for office occupancies into the 1968 

New York City Building Code are not recorded (recordkeeping in the codes and standards development 

process was very poor prior to the Hydrolevel vs. ASME Supreme Court decision in 1982). The codes 

then and now tend to follow each other as champions of changes to one code usually try to change all of 

the codes. The 1950 edition of the Basic Building Code (BOCA) included a Type 1B construction class 

with unlimited height and area for business and low hazard storage occupancies without sprinklers. 

Among other model codes, the Standard Building Code (1946-47 edition, SBCCI) had a Type 2 

 

3

 Construction type definitions varied among the model codes until an effort was expended in the 1970s by the Board for the 

Coordination of the Model Codes (BCMC) to eliminate unnecessary differences. 
Appendix A 

A–24

construction similar to Type 1B for business occupancies and buildings more than 80 ft in height, the 

National Building Code (1934 edition, NBFU) had a semi-fireproof similar to Type 1B for buildings 

above 75 ft, and the Uniform Building Code (1927 edition, ICBO) had a Type 2 similar to Type 1B for 

buildings above 75 ft. 

The Basic Building Code (BOCA) would be expected to have the strongest influence on New York City 

since BOCA was the regional code used in the Northeast U.S. This may be why Type 1B construction 

was included in the 1968 New York City Building Code. 

Mandatory sprinkler requirement for new high-rise buildings was first introduced in the New York City 

Building Code in 1984 (by Local Law 16), in BOCA in 1984, and in the Chicago building code (which 

allows a compartmentation alternative) in 1975. Before Local Law 16 was adopted, the 1968 New York 

City Building Code permitted Type 1A, 1B, 1C, and 1D construction for sprinklered office buildings of 

unlimited height. In the 2001 New York City Building Code, the minimum permitted construction 

classification for office buildings of unlimited height is Type 1C. 

A.4.5 Fire Resistance of Structural Elements 

The structural elements of a building are protected against failure in fire for a specified period as 

determined in the ASTM E 119 test. The intent of the fire rating requirements is for the structure as a 

minimum to withstand design loads (including fire) without local structural collapse until occupants can 

escape and the fire service can complete search and rescue operations. 

Fire resistance requirements in the building codes are greatest for structural members that are essential to 

the stability of the building as a whole. These include columns and other major gravity load carrying 

members that connect directly to columns such as girders and trusses. 

For various construction classes, the building codes specify different fire resistance ratings. The building 

codes reviewed specify fire resistance ratings for high-rise office occupancies as follows: 

• Type 1A 

− Columns: 4 h (supporting more than one floor) 

− Beams: 3 h (floor construction) 

• Type 1B 

− Columns: 3 h (supporting more than one floor) 

− Beams: 2 h (floor construction). 

• Type 1C (for sprinklered buildings only) 

− Columns: 2 h (supporting more than one floor) 

− Beams: 1 1⁄2 h (floor construction). 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–25

The choice among permitted construction classes for a particular building is made by the architect and/or 

the owner. Thus, an unsprinklered high-rise office building that was designed according to the 

1968 version of the New York City Building Code could follow either Type 1A or 1B, and if designed 

subsequent to the passage of Local Law 16/1984 a high-rise office building would have to be sprinklered 

but it could follow Type 1C as a minimum classification. Similar reductions in the minimum required 

fire resistance ratings for sprinklered buildings are found in all national model building codes over this 

period as requirements for fire sprinklers, especially in high-rise buildings, have become common. 

Type 1B, and eventually Type 1C, construction was permitted for high-rise office occupancies because 

this occupancy is considered low risk. Most other use groups in high-rise buildings were restricted to 

Type 1A, which is the construction type with the maximum structural fire protection defined in these 

codes. 

Compartmentation and Sprinklers 

Section 6 of Local Law 5 adopted by New York City in 1973, required the subdivision of unsprinklered 

space in new office occupancies and in existing offices over 100 ft in height by fire rated partitions. 

Local Law 5 was challenged in the courts and was eventually upheld, although the original compliance 

dates were amended by Local Law 86 (1979) so that full compliance was required by February 7, 1988. 

After the passage of Local Law 5, the Port Authority implemented a program to retrofit sprinklers and to 

offer tenants the option of sprinklering or compartmentation consistent with Local Law 5 provisions. 

Sprinklering of WTC 1 and WTC 2 was undertaken in three phases: Phase 1 was the sprinklering of 

below grade spaces completed with the original construction. Phase 2 was begun after Local Law 5 was 

adopted and included the installation of sprinkler risers and other infrastructure, and the installation of 

sprinklers in corridors, storage rooms, lobbies, and smaller tenant spaces for tenants not selecting the 

compartmentation option. Phase 3 involved sprinklering the remaining tenant spaces, initially as tenants 

changed, and later on negotiated schedules. This process was underway when in 1984 Local Law 16 was 

adopted, which required sprinklers in new high-rise buildings including offices. Thus all floor spaces by 

February 8, 1988, had to either be subdivided in accordance with the compartmentation requirement or 

sprinklered. A 1997 report states that there were four floors and the sky lobbies (all in WTC 1) left to be 

sprinklered, and that the installation of sprinklers at these floors was underway (Coty 1997). In an 

October 1999 report, it is stated that sprinklering of the tenant floors was completed and sprinklering of 

the sky lobbies was “currently underway” (PANYNJ 1999). 

Summary 

Table A–8 summarizes key fire safety requirements for business occupancy in high-rise buildings (greater 

than 75 ft) as stipulated in the codes that were compared. In addition, the provisions from the 1938 New 

York City Building Code are provided for comparison. It is seen that, overall, the 1968 New York City 

Building Code was in accord with contemporaneous codes. Exceptions are the permitted construction 

classes and minimum fire ratings, which were more restrictive in the 1967 Municipal Code of Chicago. 
Appendix A 

A–26

Table A–8. Summary of fire safety provisions for business occupancy in 

 high-rise buildings (> 75 ft). 

 1938 NYC 1968 NYC 2001 NYC 1964 NYS 1967 MCC 1965 BOCA 

Detection No 

requirement 

Smoke 

detectors to 

shut down 

HVAC fans to 

prevent 

smoke 

recirculation 

Class E fire 

alarm system 

with voice 

communication 

Fire alarm 

system 

required; fire 

detection or 

sprinkler 

system is 

alternative 

Not required Fire alarm 

system 

required if not 

sprinklered 

Suppression 

(Sprinklers) 

No 

Requirement 

Below grade 

only 

Required if 

gross floor area 

>100,000 ft2

Below grade 

only 

Not required Not required 

until 1984 

Permitted 

Construction 

Class 

1A 1A, 1B 1A, 1B, 1C 

with 

sprinklers 

1A, 1B 1A 1A, 1B 

Fire 

Separation 

(Compartmentation) 

3 h shaft 

enclosures; 

1 h tenant 

separations 

(demising 

walls) 

2 h shaft 

enclosures; 

1 h tenant 

separations 

2 h shaft 

enclosures; 

1 h tenant 

separations; 

unsprinklered 

requires 

compartmentation if 

>7,500 ft2

 or 

>15,000 ft2

with smoke 

detectors 

2 h shaft 

enclosures; 

1 hr tenant 

separations 

2 h hoistways; 

1 h shaft 

enclosures; 

2 h tenant 

separations 

every 

10,000 ft2

2 h shaft 

enclosures; 

3/4 h tenant 

separations 

Minimum fire 

resistance 

ratings: 

Ext. and int. 

bearing walls 

and columns 

supporting >1 

floor 

Floors 

including 

beams 

4 h 

3 h 

3 h 

2 h 

2 h 

1 1⁄2 h 

3 h 

2 h 

4 h 

3 h 

3 h 

2 h 

A.4.6 Means of Egress 

The basic concept of occupant egress implemented in building codes involves the provision of a properly 

designed means of egress that is continuous and unobstructed from any point in the building to the 

outside. Proper design includes the width of the spaces and doors, direction of door swing, lighting and 

marking, protection from the fire and its effects, and geometry of stairs or ramps. Limits on travel 

distances to reach a means of egress and on common paths of travel, dead ends, and the provision of 

alternative means of egress if the primary path is blocked by fire are also basic concepts of egress design. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–27

The means of egress described in building codes consists of three parts. The exit access is a corridor, 

aisle, balcony, gallery, room, porch, or portion of a roof over which an occupant must travel to reach the 

exit. The exit is a door leading to the outside or through a protected passageway to the outside, a smokeproof tower, protected stairway, exit passageway, enclosed ramp, escalator, or moving walkway within a 

building. The exit discharge is the door to the outside, although some codes allow not more than half the 

exits to discharge onto a floor with an unobstructed path to the outside, and which is protected by 

sprinklers and a 2-h fire resistance separation from floors below. 

Another concept found in the context of Means of Egress systems is that of defense in place. This is 

normally associated with occupancies where the occupants cannot escape such as hospitals or prisons, but 

may also be applied to refuge areas (sometimes called areas of rescue assistance) where people with 

disabilities await assistance or to areas in which occupants are being held temporarily while they await 

their turn to evacuate in a phased evacuation. Defense in place usually involves providing some 

protection against exposure to fire, heat, and smoke for the time needed to move these people to a safe 

place. 

Egress System Design 

The objective of the egress system design is to allow unimpeded evacuation of the building population 

without exposure to fire or smoke. Prescriptive building code regulations address this by specifying a 

population density (people per unit floor area) for each building use group, called the “occupant load 

factor.” When multiplied by the floor area, the occupant load is obtained on which the egress system 

design is based (unless there is reason to believe that the actual load will be greater or the owner desires a 

greater allowance). 

The means of egress is then designed to accommodate that occupant load on the basis of an egress width 

per occupant served, also specified in the building code. Values are specified for stairs and for other 

egress components, sprinklered and unsprinklered, and with special values for high hazard and 

institutional building occupancies to allow for higher egress speeds (high hazard) and greater number of 

wheelchairs or evacuation in patient beds (institutional), respectively. 

The width of the egress system at each floor is sized to accommodate the number of occupants on that 

floor only. There is an additional requirement that the egress system width cannot become narrower in 

the direction of egress travel and beyond any convergence of two or more egress systems the capacity 

cannot be less than the sum of the capacities. These requirements are intended to account for the 

accumulation of flows from multiple floors. 

For very tall buildings, it was recognized that the accumulating flow from a large number of floors would 

result in congestion in the stairways and a reduction of flow speeds. Widening stairways to increase the 

capacity has an economic consequence. Thus the concept of phased evacuation was developed where 

occupants are evacuated first from the three floors closest to the fire, while others wait their turn. Such 

systems require a voice communication system to manage the process by voice messages from a fire 

command center staffed by the fire service, and (e.g., in New York City) fire wardens on each floor 

directing the flow. 
Appendix A 

A–28

Prescriptive Egress Specifications 

Traditional (prescriptive) building codes specify the design of egress systems by first, estimating the 

number of occupants in an area to be evacuated, second, determining the (combined) width of the exit 

system needed for that number of occupants, and third, dividing that width among the number of exits 

needed to achieve the travel distance limits. These codes further establish some minimum requirements 

for the number and width of exits and exit components. These minimum requirements seem to derive 

from a 1935 National Bureau of Standards (now NIST) publication titled “Design and Construction of 

Building Exits” (NBS 1935). A survey of buildings showed that the nominal 44-in. (2 units of width) exit 

stair was in common use, and a majority of buildings provided two such stairs. This report recommended 

that these be adopted as minimum requirements in building codes. This report also recommended the 

values for occupant load and capacity allowances discussed below. 

For the most part, the egress design factors found in all of the model building codes are identical. In the 

IBC and NFPA 5000 model codes, design occupant densities (loads) range from 500 ft2

 (gross) per 

occupant (aircraft hangers, warehouses) to 5 ft2

 (net) per occupant (assembly, standing space). Common 

values are 100 ft2

 (gross) per occupant (business, industrial) or 200 ft2 (gross) per occupant (residential). 

By multiplying occupant loads by estimated floor area, the number of people to be evacuated is obtained. 

These same values have appeared in the building codes for a long time. 

The means of egress provisions in the (1968 through 2001) New York City Building Code use the “unit 

width” method of computing egress capacity. This method was previously found in all the model codes 

but was changed to the “inches per person” method in the late 1980s (e.g., the NFPA 101 Life Safety 

Code changed to the “inches per person” method in its 1988 edition). The major reason for the change 

was that the capacity of elements of the egress system was measured in 22 in. increments (called a unit of 

exit width) with remaining fractions only credited in half units (12 in. or more). Thus the “inches per 

person” method allows egress credit for exit elements that may have non-standard dimensions. The New 

York City Building Code (1968 through 2001) requires at least two exits per floor for business occupancy 

(Group E) in buildings taller than 60 ft and it permits 80 people per (22 in.) unit of exit width and 60 

people per unit width on stairs. These are identical to the values found in the contemporaneous codes 

(1965 BOCA and 1968 NFPA 101) reviewed. For example, for a floor space of 30,000 ft2

, 5 unit widths 

of stairs would be needed. Thus three 44 in. wide stairs (the minimum permitted width) would meet the 

New York City Building Code requirement. New York City further permits scissor stairs (an 

arrangement where two stairs are intertwined in a single shaft) with openings located at least 15 ft apart 

(C26.602.3), which are prohibited in most other building codes. 

Both the IBC 2000 and NFPA 5000 model codes, as well as most building codes based on them (but not 

2001 NYC), now determine the capacity of the egress system by specifying the width per person for 

egress system components. With the exception of hazardous and health care occupancies, both the IBC 

(without sprinkler protection) and the NFPA 5000 Code (sprinklered or not) specify the same egress 

system width of 0.3 in. per occupant in stairways and 0.2 in. per occupant elsewhere. The IBC reduces 

egress capacity where sprinklered to 0.2 in. per occupant in stairs and 0.15 in. elsewhere. The egress 

capacity of the exit system is the smallest capacity of any component. For example, a 34 in. (clear width) 

door leading into a 44 in. (clear width) stair have capacities of 170 and 147 occupants, respectively. Thus 

the exit capacity is the smaller of the two, or 147 occupants. The minimum number of exits specified in 

both model codes is two for populations up to 500, three from 501 to 1,000, and four if over 1,000. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–29

Finally, building codes specify maximum travel distances to an exit by occupancy and presence of 

sprinklers, and the same values are found in all the codes, including New York City. For example, in the 

1968 New York City Building Code, the maximum travel to an exit for business occupancy is 200 ft for 

unsprinklered construction and 300 ft for sprinklered construction. The IBC 2000 code specifies 200 ft 

(unsprinklered) and 250 ft (sprinklered) for most occupancies, except for business which is allowed 300 ft 

if sprinklered. The NFPA 5000 code specifies travel distances without sprinklers of 100 ft (for hotels, 

apartments, and mercantile), 50 ft (for health care and educational), or 200 ft (for business, industrial, and 

assembly). When fully sprinklered, these increase to 200 ft (for hotel, apartments, educational), 250 ft 

(for mercantile, industrial, assembly) and 300 ft (business). While most buildings will require two or 

more exits, the travel distance requirement only applies to the distance from any point to the closest 

(single) exit. The distance to any other exit(s) is unregulated. 

Elevators 

Currently there are no U.S. building codes that permit elevators to be used as a means of occupant egress 

in emergencies, and ASME A17.1 (ASME 2000) requires signs at all elevators warning that they shall not 

be used in fires. There are some recent exceptions to this, but these are limited to special cases. For 

example, NFPA 5000 permits protected elevators as a secondary means of egress for air traffic control 

towers and the City of Las Vegas accepted elevators as a primary means of occupant egress from 

Stratosphere Tower based on a performance-based design (Bukowski 2003). 

U.S. building codes (including New York City) require accessible elevators as part of a means of egress 

that may be used by the fire service to evacuate people with disabilities. These elevators must comply 

with the emergency operation requirements of ASME A17.1 (Phase II emergency operation by the fire 

service), be provided with emergency power, be accessible from an area of refuge or a horizontal exit 

(unless the building is fully sprinklered), and operate in a smoke protected hoistway. Phase II operation 

involves the use of an elevator by a firefighter for fire service access or for rescue of people with 

disabilities performed under manual control (with the use of a special key). 

A.5 PRELIMINARY FINDINGS 

The 1968 New York City Building Code was compared with contemporaneous building codes to 

determine whether there were significant differences in code provisions. In addition, the 2001 New York 

City Building Code was also compared to examine the changes resulting from the adoption of Local Laws 

and rules. The comparison was limited to provisions related to structural stability, fire safety, and egress. 

In general, it was found that the majority of the provisions were similar among the four codes that were 

compared. It was also found that the New York City Building Code was more advanced in certain areas 

than the other three contemporaneous codes (1964 New York State Building Construction Code, 1967 

Municipal Code of Chicago, and 1965 BOCA Basic Building Code). The following summarizes the 

major findings of the code comparison. 
Appendix A 

A–30

A.5.1 Provisions Related to Structural Stability 

Dead Load 

The New York City Building Codes (both 1968 and 2001 editions) allow lower than 20 psf for uniformly 

distributed partition loads for partitions that weigh less than 200 plf, while the other codes compared 

prescribed a minimum uniform partition load of 20 psf. As a reference, a 10 ft high partition made of 

1/2 in. thick gypsum wallboard on both sides of 2 by 4 wood studs spaced at 16 in. on center weighs 

about 60 plf. Except for the partition load provision, other dead load provisions in the five codes 

compared are similar. 

Live Load 

The five codes compared had similar minimum live load provisions. They all permit live load reduction 

for the design of columns and floor framing members. The amount of reduction allowed varies among 

the codes. While the 1967 Chicago Municipal Code allowed only a maximum of 15 percent reduction for 

floor-framing members (beams and girders), other codes allowed as much as 60 percent reduction (see 

Table A–3). The amount of reduction allowed for columns is about the same for all five codes (see 

Fig. A–1). 

Wind Load 

The general trends of the specified wind pressure distributions along the height of a building in the five 

codes compared are similar. However, specific design wind pressure values vary as much as 10 psf at 

specific heights. For tall buildings, like the WTC towers, the 1965 BOCA Basic Building Code would 

produce the largest shear force and bending moment at the base of the building. 

The 1968 New York City Building Code allows determination of design wind pressure based on wind 

tunnel tests with approval by the Building Commissioner. 

Earthquake Load 

In the 1960s, only the BOCA Basic Building Code had the earthquake design provisions, which were 

based on the Uniform Building Code. The 2001 New York City Building Code contains the earthquake 

design provisions that are based on the 1997 Uniform Building Code. 

Others Loads 

The 1968 New York City Building Code is the only code among the contemporaneous codes compared 

that has provisions to consider loads due to thermal expansion/contraction of structural members and 

shrinkage of reinforced concrete members. The New York City Building Code also included language on 

the distribution of loads among structural members in a building. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–31

Referenced Design Standards 

The 1968 New York City Building Code and the other contemporaneous codes compared, except the 

1964 New York State Building Construction Code, make reference to the same national design standards 

for steel and reinforced concrete. 

Building Alterations 

All codes compared, with the exception of the 1964 New York State Building Construction Code, had 

similar provisions on whether alterations were required to comply with the current building code. In 

general, alterations need to comply with the current code when the alteration is a significant portion of the 

building size or value (see Table A–5). The New York State code, on the other hand, required any 

alteration to comply with the code. 

Construction Inspection 

The codes that were compared include provisions related to inspection of materials and methods during 

construction. The specific requirements, however, are not similar. The 1968 New York City Building 

Code distinguishes between “controlled inspection” that require supervision by a design professional and 

those not so designated as controlled, which can be tested by the person superintending the use of the 

material. The New York City Building Code required controlled inspection of sprayed-on fire protection, 

but it did not specify required tests. 

Bracing 

Only the New York City Building Code among the codes compared included provisions on bracing of 

compression members. The New York code requires that members designed to brace compression 

members (for example, columns), be proportioned to resist 2 percent of the design compression load in 

the member being braced. 

A.5.2 Provisions Related to Fire Safety 

Construction Types 

The building codes compared classify building constructions into different types depending on the 

combustibility of the construction materials. The New York City Code uses two types, noncombustible 

and combustible, whereas modern model codes have adopted five types: fire resistive (Type 1); noncombustible (Type 2); combustible (Type 3); heavy timber (Type 4); and ordinary (Type 5). These types 

are typically sub-divided into classes such as Type 1A and Type 1B. 

The cornerstone of fire safety of construction is the “height and area” table that defines the limiting floor 

and area and building height for different construction classifications (1A, 1B, and so forth) and 
Appendix A 

A–32

occupancy groups. In the case where a building could be assigned to more than one construction 

classification, the codes are silent on which classification should be used, and the selection of the building 

classification is at the discretion of the owner/architect. 

Compartmentation 

In 1973, Local Law 5 was adopted by New York City, which required that large, open floors in existing 

office building over 100 ft in height be subdivided by 1-hour separations into areas not greater than 

7,500 ft2

. Areas could be increased to 15,000 ft2 with 2-hour separations and smoke detectors. Unlimited 

floor areas were permitted where fully sprinklered. 

Sprinklers 

In 1985, Local Law 16 was adopted in New York City, which limited Type 1B construction to buildings 

of 75 ft or less unless they were fully sprinklered, for which there were no height or floor area limitations. 

A.5.3 Provisions Related to Egress 

The 1968 New York City Building Code contained similar requirements to the other contemporaneous 

codes, which were compared, for the number and capacity of exits and stairs and for the design occupant 

load. The New York City Code permitted scissor stairs (two stairs in one shaft separated by a fire-rated 

partition) with doors located at least 15 ft apart, whereas other building codes prohibited scissor stairs. 

A.6 REFERENCES 

ASME 2000. Safety Code for Elevators and Escalators ASME 17.1, American Society of Mechanical 

Engineers, New York, NY. 

BOCA/BBC 1965. BOCA Basic Building Code, Fourth Edition, Building Officials Conference of 

America, Chicago, IL. 

Bukowski, R.W. 2003. Protective Elevators for Egress and Access During Fires in Tall Buildings, 

Proceedings of the CIB-CTBUH Conference on Tall Buildings, Kuala Lumpur, Malaysia. 

October 20-23. 

Coty, Eric H. (Rolf Jensen & Associates). 1997. Included as Attachment A to World Trade Center – 

Code Compliance Evaluation compiled by PANYNJ October 15, 1999, March 14. 

CTB&UH (Council on Tall Buildings and Urban Habitat). 1980. Monograph on Planning and Design of 

Tall Buildings, Volume CL-Tall Building Criteria and Loading, American Society of Civil Engineers, 

Reston, VA. 

IBC 2000. International Building Code, International Code Council, Falls Church, VA. 
Interim Report on the Analysis of Building and Fire Codes and Practices 

A–33

Kyle, John M. (Chief Engineer, Port Authority). 1966a. Memorandum to Guy F. Tozzoli (Director, 

World Trade Department), May 9. 

Kyle, John M. (Chief Engineer, Port Authority). 1966b. Memorandum to Guy F. Tozzoli (Director, 

World Trade Department), June 2. 

Levy, Malcolm. 1963. Port of New York Authority Letter to Minoru Yamasaki (Architect, Minoru 

Yamasaki & Associates), May 15. 

Levy, Malcolm. 1965. Port of New York Authority Letter to Minoru Yamasaki (Architect, Minoru 

Yamasaki & Associates), September 29. 

MCC (Municipal Code of Chicago). 1967. Municipal Code of Chicago Relating to Buildings (As 

Amended to and including January 1, 1967), Index Publishing Corp., Chicago, IL. 

NBS 1935. Design and Construction of Building Exits, National Bureau of Standards Miscellaneous 

Publication M151, U.S. Government Printing Office, Washington, DC. 

NFPA (National Fire Protection Association). 1966. Life Safety Code, NFPA 101, Quincy, MA. 

NFPA (National Fire Protection Association). 2003. Building Construction and Safety Code, 

NFPA 5000, Quincy, MA. 

NYCBC 1968. Building Code—Local Law No. 76 of the City of New York, New York, NY 

NYCBC 2001. Building Code of the City of New York, 2001 Edition, Gould Publications, 

Binghamton, NY. 

NYSBC 1964. State Building Construction Code Applicable to General Building Construction, Building 

Code Bureau, State of New York, New York, NY, December 1. 

PANYNJ (Port Authority of New York and New Jersey). 1999. World Trade Center – Code Compliance 

Evaluation, October 15. 

Solomon, Joseph H. (Emery Roth & Sons). 1975. Letter to Malcolm P. Levy (General Manager, World 

Trade Center Operations), February 18. 

UBC 1988. Uniform Building Code, International Conference of Building Officials, Whittier, CA. 
Appendix A 

A–34

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B–i

TABLE OF CONTENTS - Appendix B

List of Figures.......................................................................................................................................... B–ii

List of Tables .......................................................................................................................................... B–iv

Appendix B

Interim Report on Development of Structural Databases and Reference Models for 

the WTC Towers..................................................................................................................... B–1

B.1 Introduction..................................................................................................................................B–1

B.1.1 Description of WTC Structural System............................................................................B–1

B.2 Development of Structural Databases for the Towers .................................................................B–5

B.2.1 Description of WTC Structural Documents .....................................................................B–7

B.2.2 Overview of the WTC Structural Database (WTC-DB)...................................................B–9

B.2.3 Methodology for the WTC-DB Development................................................................B–10

B.2.4 Modifications to Database Elements ..............................................................................B–12

B.2.5 Section Property Calculations ........................................................................................B–16

B.3 Development of Reference Structural Analysis Models for the Towers ...................................B–22

B.3.1 Global Models of the Towers .........................................................................................B–22

B.3.2 Typical Truss-Framed Floor Model—Floor 96A...........................................................B–38

B.3.3 Typical Beam-Framed Floor Model—Floor 75B...........................................................B–44

B.3.4 Parametric Studies..........................................................................................................B–47

B.4 Review of the Structural Databases and Reference Models of the Towers ...............................B–53

B.4.1 Review of the Structural Databases................................................................................B–55

B.4.2 Review of the Reference Structural Models...................................................................B–56
B–ii

LIST OF FIGURES

Figure B–1. Typical WTC tower architectural floor plan ....................................................................... B–2

Figure B–2. Typical WTC exterior wall, foundation to floor 9............................................................... B–3

Figure B–3. Typical WTC tower hat truss elevation ............................................................................... B–4

Figure B–4. Typical WTC floor truss framing zones. ............................................................................. B–5

Figure B–5. Part plan of floor 96 of WTC 1, components of typical truss framing system. ................... B–6

Figure B–6. Part section typical truss floor panel.................................................................................... B–7

Figure B–7. Drawing Book 3 flowchart: WTC 1 and WTC 2 core columns ....................................... B–10

Figure B–8. Core column reinforcement. .............................................................................................. B–13

Figure B–9. Column section at original column strap detail ................................................................. B–15

Figure B–10. Core column series 300. .................................................................................................... B–16

Figure B–11. Exterior column type 300, floor 9 to floor 106.................................................................. B–19

Figure B–12. Column type 6000 with tapered spandrel .......................................................................... B–20

Figure B–13. Box section and a built-up column. ................................................................................... B–20

Figure B–14. Global model coordinate axis location. ............................................................................. B–24

Figure B–15. Typical exterior panel nomenclature. ................................................................................ B–25

Figure B–16. Rendered isometric views of the WTC 1 model................................................................ B–28

Figure B–17. Frame view of the WTC 2 model: exterior wall elevation and interior section ................ B–29

Figure B–18. Frame view and rendered view of the WTC 1 model........................................................ B–31

Figure B–19. Exterior wall tree panel...................................................................................................... B–32

Figure B–20. Exterior wall tree: as-built cross sections for level E ........................................................ B–34

Figure B–21. Frame view and rendered view of an exterior wall tree. ................................................... B–34

Figure B–22. Typical WTC tower exterior wall panel. ........................................................................... B–35

Figure B–23. As-modeled plan of the WTC 1 hat truss. ......................................................................... B–37

Figure B–24. Rendered 3–D model of the WTC 1 hat truss.................................................................... B–37

Figure B–25. Typical truss-framed floor panels arrangement................................................................. B–39

Figure B–26. Typical truss-framed floor model (floor 96A)................................................................... B–40

Figure B–27. Typical primary truss cross-section................................................................................... B–41

Figure B–28. Typical bridging truss cross–section ................................................................................. B–42

Figure B–29. Strap anchors modeling ..................................................................................................... B–43

Figure B–30. Typical beam-framed floor arrangement. .......................................................................... B–45

Figure B–31. Typical beam-framed floor model (floor 75B).................................................................. B–45
Appendix B 

B–iii

Figure B–32. Horizontal truss modeling, slab not shown........................................................................ B–46

Figure B–33. Shell element and frame models of typical exterior wall panel......................................... B–48

Figure B–34. Column and spandrel rigidity of typical exterior wall panel. ............................................ B–49

Figure B–35. Shell element and frame models of typical exterior wall corner panel.............................. B–51

Figure B–36. Column and spandrel rigidity of typical exterior wall corner panel. ................................. B–51

Figure B–37. Detailed and simplified model of the exterior wall corner panel....................................... B–52

Figure B–38. Deflection of typical truss-framed floor model due to lateral loading............................... B–54

Figure B–39. Deflection of equivalent floor model due to lateral loading.............................................. B–54

Figure B–40. Deflections of the north and south faces of the floor for the detailed and equivalent 

floor models....................................................................................................................... B–55
B–iv

LIST OF TABLES

Table B–1. Modifications to members of the WTC database (WTC-DB)............................................. B–9

Table B–2. Approximate size of the reference structural models (rounded)........................................ B–30

Table B–3. Lateral displacement for the shell and frame models of typical exterior wall panel 

with varied column and spandrel rigidities........................................................................ B–50

Table B–4. Lateral displacement for the shell and frame models of typical exterior wall corner 

panel with varied column and spandrel rigidities. ............................................................. B–52
B–1

Appendix B

INTERIM REPORT ON DEVELOPMENT OF STRUCTURAL DATABASES 

AND REFERENCE MODELS FOR THE WTC TOWERS

B.1 INTRODUCTION 

The objectives of this project are to (1) develop structural databases for the primary structural components 

of the World Trade Center towers, (WTC 1 and WTC 2), (2) use the databases to develop reference 

structural analysis models that capture the intended behavior of each of the two towers, and (3) perform 

linear, static structural analyses to establish the baseline performance of each of the two towers under 

design gravity and wind loads. This appendix focuses on the tasks related to the first two objectives. The 

appendix reports on the work conducted by the firm of Leslie E. Robertson Associates (LERA), the firm 

responsible for the original structural engineering of the WTC towers, for the development of the 

structural databases and reference models. It also outlines the comprehensive review process for the 

structural databases and reference models that includes the rigorous in-house NIST review and third-party 

review by the firm of Skidmore, Owings, and Merrill (SOM). 

Section B.1 presents an introduction and a brief description of the structural system of the towers. 

Section B.2 presents an outline and methodology used for the development of the structural databases for 

both towers from the original computer printouts of the structural documents, along with the relational 

databases that are used for the development of the reference structural models. Section B.3 presents the 

development of the reference structural analysis models for WTC 1 and WTC 2, including global tower 

models, typical floor models, and parametric studies needed for the development of the global models. 

Finally, Section B.4 outlines the in-house and third-party review process for the structural databases and 

reference models. 

B.1.1 Description of WTC Structural System 

Global Structural System 

WTC 1 and WTC 2 each consisted of a 110-story above grade structure and 6-story below grade 

structure. The buildings, which were each approximately 207 ft by 207 ft square in plan and with story 

heights of typically 12 ft, rose to heights of 1,368 ft (WTC 1) and 1,362 ft (WTC 2) above ground. The 

exterior walls of the towers supported gravity loads and all lateral loads, and were constructed of steel 

closely spaced, built-up columns and deep spandrels. The core contained columns that supported the 

remainder of the gravity loads of the towers. The core area was approximately 135 ft by 87 ft in plan 

(refer to Fig. B–1). The distances between the rectangular core and the square exterior wall were 

approximately 36 ft and 60 ft. The areas outside of the core were free of columns and the floors were 

supported by truss-framing in the tenant areas and beam-framing in the mechanical rooms and other areas. 

The primary structural systems for the towers included exterior columns, spandrel beams, and bracing in 

the basement floors, core columns, core bracing at the mechanical floors, core bracing at the main lobby 

atrium levels, hat trusses, and the floor systems.
Appendix B 

B–2

Drawing used with permission from PANYNJ. 

Figure B–1. Typical WTC tower architectural floor plan (floor 26, WTC 2). 

The exterior wall columns from the foundation level up to Elevation 363 ft were spaced 10 ft 0 in. on 

center, built-up of steels plates, and connected by spandrels. Bracing existed in the plane of the exterior 

wall between the Concourse Level and the foundation, (see Fig. B–2). Between Elevation 363 ft and 

floor 7, the single exterior wall columns spaced 10 ft 0 in. on center transitioned to three columns spaced 

at 3 ft 4 in. on center as shown in Fig. B–2 (see also Fig. B–19). 

The exterior wall columns above floor 7 that were spaced 3 ft 4 in. on center, were built-up of steel plates, 

and were connected to each other by spandrel plates, typically 52 in. deep. The exterior columns and 

spandrels were pre-assembled into exterior wall panels, typically 3-columns wide by 3-stories tall (refer 

to Fig. B–22). 

The core columns were typically built-up box members at the lower floors and transitioned into rolled 

structural steel shapes at the upper floors. The core columns were typically spliced at 3-story intervals at 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–3

Drawing used with permission from PANYNJ. 

Figure B–2. Typical WTC exterior wall, foundation to floor 9. 

3 ft above floor level. Diagonal bracing of the core columns existed at the lobby atrium levels, the 

mechanical levels, and in the area of the hat truss. 

At the top of each tower, hat trusses interconnected the core columns with the exterior wall panels and 

provided a base for the antennae. The vertical members of the hat trusses were wide flange core columns. 

The diagonals were primarily wide flange rolled sections with the exception of the end diagonals 

interconnecting the core to the exterior walls which were built-up box sections. The majority of the 

horizontal members in the hat truss system were wide flange and built-up box section floor beams. The 

members of the hat trusses were shown in the SA/B-400 series elevations (refer to Fig. B–3). 
Appendix B 

B–4

Drawing used with permission from PANYNJ. 

Figure B–3. Typical WTC tower hat truss elevation (Drawing SA 401). 

Floor Structural System 

In the typical WTC tower floor plan, the area inside the core was framed with rolled structural steel 

shapes acting compositely with formed concrete slabs. The area outside the core was framed in either 

trusses (typical on tenant floors) or in rolled structural steel shapes (typical on mechanical floors). 

Truss-Framed Floors––The majority of the floors of the WTC towers were tenant floors where the areas 

outside of the core were constructed of steel trusses acting compositely with concrete slabs cast over 

metal deck. The trusses consisted of double angle top and bottom chords with round bar webs and were 

designed to act compositely with the concrete slab. Composite action was achieved by the shear 

connection provided by the web bar extending above the top chord and into the slab. Two trusses were 

placed at every other exterior column line, resulting in a 6 ft 8 in. spacing between truss pairs. The 

typical floor consisted of three truss zones: a long span zone, a short span zone, and a two-way zone, (see 

Fig. B–4). 

The floor trusses were pre-assembled into floor panels as defined in the contract drawings. The span of 

the trusses was about 36 ft in the short direction and 60 ft in the long direction. The floor panels included 

primary trusses, bridging trusses, deck support angles, metal deck, and strap anchors, all of which were 

defined by the contract drawings and specifications. 

The floor truss panel types are indicated in the structural plans (see Fig. B–5) and the plans refer in turn to 

Drawing Book 7 for information regarding the components of the floor truss panels and to Drawing 

Book D for damper information. Drawing Book 7 provided panel by panel layout plans and elevations of 

each referenced truss. The section through a floor panel after the concrete was placed is illustrated in 

Fig. B–6. 

Beam-Framed Floors––The typical locations of the beam-framed floors were the mechanical floors, the 

mechanical mezzanines, and the floors above the mezzanines (e.g., floors 41, 42, and 43). These floors 

were constructed using rolled structural steel shapes. The beam framing for the typical floor system was 

W27 beams in the long span region and W16 beams in the short span region. Typically, beam spacing 

was 6 ft 8 in. The steel beams acted compositely with the normal weight concrete slab on metal deck. 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–5

Figure B–4. Typical WTC floor truss framing zones. 

The deck spanned in the direction of the primary beams and was supported typically at 6 ft 8 in. intervals 

by a 4C5.4 deck support channel. A 2 in. concrete topping slab was placed on top of the structural slab. 

The core area was framed similar to the core of the truss-framed floors, but the steel beams were typically 

larger, and the concrete slab was 6 in. deep. The beam-framed floors above the mechanical mezzanine 

had a 7 3/4 in. normal weight concrete slab on 1 1/2 in. metal deck, while the core slab was 8 in. normal 

weight concrete. 

Beam-framing was added to truss-framed floors at levels which supported escalators or stairs in the areas 

outside of the core. The escalator floors occurred typically in the two levels directly above the 

mechanical rooms. 

B.2 DEVELOPMENT OF STRUCTURAL DATABASES FOR THE TOWERS 

This section outlines the development of the electronic databases for the major structural components of 

the WTC towers from original computer printouts of the structural documents. The structural databases 

are used to develop the reference structural models of the towers as outlined in Section B.3. Included in 

this section are an overview of the WTC towers’ structural design documents, a description of the 

structural database contents, methodology for the development of the database, and a description of the 

relational database. 
Appendix B 

B–6

Original drawing used with permission from PANYNJ. 

Figure B–5. Part plan of floor 96 of WTC 1 (Drawing SA-104), components of typical truss 

framing system. 

Bridging Truss One 

every forth column line, 

max. 

Deck Support Angle One 

every forth column line, 

max. 

Primary Long 

Span Truss 

Two every other 

column line 

Strap Anchors 

Every other column line, 

max.

Primary Short Span 

Truss 

Two every other column 

line.

Bridging Truss One 

every forth column 

line, max. 

Deck Support Angle

One every forth column 

line, max.
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–7

Original drawing used with permission from PANYNJ. 

Figure B–6. Part section typical truss floor panel. 

B.2.1 Description of WTC Structural Documents 

The WTC structural drawings were issued in two main formats: large-size sheets containing plan and 

elevation information and smaller book-sized drawings containing details and tabulated information. 

Throughout the WTC drawings, Tower A or WTCA denotes WTC 1 (north tower) and Tower B or 

WTCB denotes WTC 2 (south tower). The large size drawings always make reference to the structural 

drawing books through their notes, sections, and detail references. The structural drawing books for 

WTC 1 and WTC 2 include the following: 

• Book 1 contains exterior wall information to elevation 363 ft. (Dates: 02/1967 to 12/1968, 

Approx. 213 pages). 

• Book 2 contains exterior wall information elevation 363 ft to floor 9. (Dates: 04/1967 to 

12/1967, Approx. 62 pages). 

• Book 3 contains core column information. (Dates: 03/1967 to 09/1969, Approx. 137 pages). 

• Book 4 contains exterior wall information floor 9 to floor 110. (Dates: 04/1967 to 10/1972, 

Approx. 1,080 pages). 

• Book 5 contains the beam schedule. (Dates: 05/1967 to 08/1969, Approx. 292 pages). 

• Book 6 contains connection details and core bracing. (Dates: 08/1967 to 05/1969, Approx. 

1,060 pages). 

Primary Truss Members Bridging Truss 

4 in. Slab on 1-1/2 in. Metal 

Deck
Appendix B 

B–8

• Book 7 contains truss floor panel information. (Dates: 10/1967 to 07/1969, Approx. 345 

pages). 

• Book 8 contains concrete notes and details. (Dates: 03/1968 to 07/1974, Approx. 926 pages). 

• Book 9 contains roof area column splice details. (Dates: 05/1970 to 04/1971, Approx. 

440 pages). 

• Book 18 contains strap anchor and core truss seat information. (Dates: 10/1968 to 11/1969, 

Approx. 219 pages). 

• Book 19 contains revisions after fabrication. (Dates: 08/1968 to 05/1975, Approx. 374 

pages). 

• Book 20 contains structural steel details. (Dates: 07/1968 to 03/1971, Approx. 41 pages). 

• Book D contains damper details. (Dates: 03/1969 to 09/1971, Approx. 43 pages). 

The remaining number books (Books 10, 11, 12, and 13) contain information about the sub-grade 

structure. Books 14, 15, 16, and 17 were never used. 

Until fabrication was begun, the above drawings and drawing books (with the exception of Book 19) for 

the project were modified in keeping with the requests for changes by contractor(s) and early tenant 

modifications. The drawings were modified up until such time as the fabrication of elements 

commenced. At that time, Book 19 was introduced. It contained the information regarding ‘revisions 

after fabrication’. 

LERA believes that the original structural drawings represent significantly accurate ‘as-built’ drawings 

for the towers. As tenant modification requests became large in scope, they became separate projects 

(e.g., the Fiduciary Trust Vault Project, see Section B.2.4). Tenant structural modifications designed by 

LERA were then documented in a single book of quarter-size plans referred to as the ‘WTC Tenant 

Structural Modifications Book’. Later tenant modifications were mostly archived on a job-by-job basis 

without a central accounting for all the changes. NIST has in its possession complete copies of all the 

drawings, drawing books, and modifications to the towers performed by LERA. In some instances 

modifications were made by the Port Authority of New York and New Jersey (PANYNJ) Engineering, 

such as additions to the mechanical levels. In other instances, tenant modifications were performed by 

other engineers. For these instances, LERA does not have record of the work completed. According to 

the PANYNJ, no record of structural work could be found so far for the additions to the mechanical floors 

made by the PANYNJ Engineering, and it is likely that they were lost with the collapse of the towers. 

Modifications made by other engineering firms include openings or closings of floor slabs and local 

reinforcement of floor segments to accommodate new loads. For these modifications, NIST has access to 

the documents related to the work. 

The few modifications made by LERA to the components compiled in the WTC structural databases that 

will have an effect the global behavior of the towers are listed in Table B–1. 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–9

Table B–1. Modifications to members of the WTC database (WTC-DB). 

Item Description Tower Element Floor Element Effected 

WTC-DB 

Modified Archived 

1 

 Core column 

 reinforcing 

WTC 1 

and 

WTC 2 Numerous 98–106 Core columns Book 3 Book 19 

2 

 Fiduciary Bank 

 Vault 

WTC 2 Col. 508B and 

 Col. 1008B 45–97 Core columns Book 3 LERA P209 

3 

 Bombing of 26 

 February 1993 

 repair 

WTC 1 Col. 324, 

 bracing 

 G313A and 

 G304A B-2 level 

 Perimeter 

 column and 

 bracing NA 

 LERA 

 P1003118 

4 EXCO stair 

WTC 1 

 Col. 901A 26 Core column NA 

 LERA 

 P1003249 

B.2.2 Overview of the WTC Structural Database (WTC-DB) 

The original WTC design documents used the concept of limiting the need for repetition in documenting 

the data shared between different elements with similar characteristics. The drawing book schedules refer 

to subsequent tables for information common to several lines of the same schedule. In an effort to 

minimize the amount of repeated information and thereby the data checking of the digital WTC-DB, the 

drawing book data within the databases created for this project were linked in a similar manner. In order 

to accurately follow the original flow of the drawing book links, flowcharts of the drawing books to be 

digitized were developed for this project. These flowcharts were used to organize the links of the 

digitized data within the relational database. An example of such flowcharts for Drawing Book 3 (core 

columns) is illustrated in Fig. B–7. 

The WTC-DB contains the computer and hand-tabulated data for the major structural components from 

the original Drawing Books 1 through 5, including exterior walls, core columns, and beam schedule for 

the towers. Where information from Drawing Books 1 through 5 was modified by Drawing Book 19 and 

would affect the towers’ modeling, the information is included in the database. In addition, some 

information from Drawing Books 6 (core bracing schedule) and 9 (beams in the hat truss region) has been 

included in the database files as it was utilized in the finite element modeling of the towers. 

The drawing book tables are first digitized and stored in Microsoft Excel format files. The Excel files 

include several worksheets that describe the evolution of the data from the drawing book to the final 

database format, as well as additional information and notes for interpreting the data. 

The WTC relational database links the Excel files and allows users to view and select data through query 

commands. The primary benefit of the relational database format is the ability to programmatically query 

the database for data required in assembling the structural models of the towers. The query routine allows 

multiple users the ability to review, extract, and export the basic data in any required form. The data can 

be manipulated using Structured Query Language (SQL) according to the desired output, for example the 

structure of the user’s finite element model input file. 
Appendix B 

B–10

Core Column Number/Location 

and Elevation

Core Column Schedule 

3-A1-2 > 48 [47TC] 

3-B1-2 > 48 [47TC] 

WTCA-Bk3-CoreColmAdata.xls (A_CoreCol) 

WTCB-Bk3-CoreColmBdata.xls (B_CoreCol)

Base Details

((15)) 

3-AB2-20, 21 

[2D] 

Splice Details

((15)) 

3-AB2-4,7,8>13, 15>16 [10TH] 

3-AB2-3.1>19 [18D] 

Floor 106 Splice Details

((15)) 

3-AB2-3 [1D] 

Splice Location

((1)) 

3-AB2-22 [1D] 

Reference Floor Elevation

((2, 3)) 

3-A2-23 [1TC] 

3-B2-23 [1TC] 

WTCA-Bk3-CoreColmAfloorelev.xls (A_RefElevUPR/LWR) 

WTCB-Bk3-CoreColmBfloorelev.xls (B_RefElevUPR/LWR)

{B, C}

Shape Property 

Table

Shape Property Table.xls 

(AB_ShapeProp)

{G}

NOTES: 

1. 4-AB-* Denotes Drawing Book 4, Tower A , B or AB, and page number 

2. [*TC] Number of pages and type, where page types include: 

TC - Computer generated tables; TH - Hand written tables; D - Diagrams 

3. *.xls Excel spreadsheet file name; (AB_***) database heading 

4. ((*)) Reference note key from original Drawing Book information table. 

5. {*} Relational database link (i.e. Excel column number) from previous *.xls file. 

6. Figures of columns and panels are shown from inside of building looking out, unless otherwise noted. 

Figure B–7. Drawing Book 3 flowchart: WTC 1 and WTC 2 core columns, foundation to 

floor 106. 

B.2.3 Methodology for the WTC-DB Development 

Data Entry 

The tabulated portions of WTC Drawing Books 1, 2, 3, 4, 5, 6, and 9 were first scanned and stored in 

TIFF image format files. The image files containing the tabulated information were then opened in an 

Optical Character Recognition (OCR) program that converted the information into a text file. The OCR 

program was modified to allow for the filtration of unnecessary characters during the documents 

conversion process. In other words, the user could direct the program to block specific characters that are 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–11

not on the actual page. As an example, if after reviewing a table, one recognizes that it uniformly 

contains numbers and only the characters “A, B, -”, and “/” then the remaining characters can be frozen 

out by the software. This reduced the misinterpretation, as an example, of a ‘Z’ for a ‘2’, or an ‘O’ for a 

‘zero’. 

The raw text file was then opened in a word processing program, where it was compared with the original 

hardcopy drawing books. As needed, data columns were adjusted, and obvious errors were individually 

corrected. The ‘cleaned’ text file was then imported into a Microsoft Excel spreadsheet with column 

headings and proper alignment. When importing into Excel, “text” was the cell format used for handling 

of the scanned information to avoid the misinterpretation of fractions as dates (e.g., 3/8 as March 8). An 

Excel macro was written at this stage to convert the text fractions into number fractions. The final 

product of this stage was an Excel file that contains the information from the drawing book table. 

Quality Control 

Checking began during the OCR data entry process, where the files being entered and the OCR software 

interpretation were viewed simultaneously. This was considered a first check. Once the Excel file was 

complete, the file entered the ‘second check’ process. 

The ‘Second Check’––An engineer not involved during the OCR process performed a second, sample 

checking of the database in a random but methodical manner. For approximately once in four pages, 

every cell of data in the page was compared with the original drawing books. Discrepancies of the files 

were then either re-entered using OCR or were individually corrected to agree with the original books. 

The ‘Cross-Check Rectify’ Check––After completing the ‘second check,’ the files were compared with 

the database provided by a consultant for the leaseholder of the WTC towers as part of an insurance 

litigation concerning the towers (provided to LERA as Government Furnished Information [GFI]) using a 

cross-check macro formula worksheet. Once compared, conflicting information appeared in a yellow 

highlighted cell displaying both sets of compared information in the “Calculation” macro formula 

worksheet. The cell was then reviewed and confirmed with the WTC drawing books. If errors were from 

the developed worksheet, data was rectified, and the yellow highlight in the ‘Calculation’ worksheet was 

then removed by comparing the files again. If errors were from the GFI worksheet, raw GFI data was not 

modified, but the cell was highlighted in blue to note that it has been reviewed. The files were then 

compared again, and the cell color in the ‘Calculation’ worksheet changed to blue. The process was 

repeated to remove all the yellow cells so that only blue highlighted cells remain. The worksheet 

‘ComparisonORIGINAL’ was retained for the record of the original comparison, and the updated 

worksheet ‘ComparisonFINAL’ was retained for the record of the final comparison. 

Final Review––Finally, the files were reviewed for completeness, formatting, and data units. A final 

check was made to find any numbers that may have been input as text letters. Following this review, the 

worksheet was used to develop the member section properties. 

Cross Section Property Calculations 

The next step was to calculate the cross section properties for the members included in the database. The 

section properties calculated included cross sectional area (A), moment of inertia (I), section modulus (S), 
Appendix B 

B–12

plastic section modulus (Z), radius of gyration (r), and torsional constant (J) for both the major and minor 

axes (where applicable). The Section Designer function of SAP2000 Version 8 was used to calculate the 

cross section properties since it enables the program to perform more precise code checks, as the 

dimensions of each plate element that is part of the section would be input into the finite element model. 

The current rolled shape database in SAP2000 represents the modern day rolling practices. The rolled 

shapes used in the construction of the WTC towers were from a different era and thus, had different 

properties in comparison to present day shapes. Therefore, a rolled shape database consistent with the 

time of construction was developed in this project. See Section B.2.5 for further discussion about the 

rolled shape database. 

Relational Database Development 

As discussed earlier, the original WTC drawing books were designed to avoid repeating identical 

information. The drawing book schedules, therefore, refer to other tables for information common to 

several lines of the same schedule. In keeping with the nature of the original drawing books and to 

minimize the data in the digital WTC-DB, the drawing book and section property data were linked using 

Microsoft Access. 

The assembly of the relational database began with the mapping of the original WTC drawing book into 

flowcharts (see, e.g., Fig. B–7). The digitized drawing book data with the corresponding cross sectional 

member properties from the Excel-format files were then imported into the Microsoft Access database 

program and partitioned into tables. The tables were then joined using the links cataloged in the 

flowcharts. These tables were developed to provide the input files for the finite element modeling of the 

towers as illustrated in Section B.3.1. 

B.2.4 Modifications to Database Elements 

The majority of the original members and elements defined within the WTC-DB could be fully defined by 

the original data in the drawing books. As outlined in Table B–1, however, some modifications were 

made that are described in the following sections. Of the items outlined in Table B–1, items 1 and 2 have 

been included within the database. 

Core Column Reinforcing at Floors 98 to 106 

A number of core columns in both WTC 1 and WTC 2 were reinforced at floors 98 to 106. Book 19, 

pages 19–AB–974.1 through 4, shows that core columns 501, 508, 703, 803, 904, 1002, 1006, and 1007 

from floors 98 to 106 in both towers were reinforced with steel plates. Three methods were used to attach 

the reinforcing plates to the wide flange columns: (1) the plates were welded to the flanges; (2) the plates 

were welded to the webs; and (3) the plates, which were parallel to the web, were welded to the flange 

ends. The plate information (width, thickness, length, and yield strength) was incorporated into the 

database tables of Book 3. Since the plates varied from floor to floor, the original column (defined over a 

three-story height) was split into typically three floor-by-floor sections and the designation of the column 

was appended to include either U(upper), M(middle), or L(lower) designation (refer to Fig. B–8). For 

floors 104 and 106, the columns are two-story columns. Hence, the columns at these floors had only 

U and L designations. The section property calculations included the contributions of the reinforcing 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–13

plate at each level. For the built-up section property data, the reinforcing plate was considered to be 

applied to the column for its floor-to-floor height. 

FL 101 

FL 100 

FL 99 

FL 98

Reinforcing Plate 

CC703A101L

CC703A101M 

CC703A101U 

Col. Splice 

Col. Splice 

Figure B–8. Core column reinforcement. 

Core Column Reinforcing Due to Construction of Fiduciary Trust Vault 

The Fiduciary Trust Company added a concrete vault at floor 97 of WTC 2, which required reinforcing 

two corner core columns at the north end of the core. This work is included in the WTC Towers A and B 

Structural Renovation Drawings Reference Manual. The Fiduciary Trust Structural Drawing 765–S–A–4 

shows that WTC 2 core columns 508 and 1008 were reinforced with steel plates from floors 45 to 97. 

The reinforcement consisted of plates welded to the flanges of the built-up box columns (floors 45 to 83) 

and the flanges of the rolled shape columns (floors 83 to 97). These reinforcing plate modifications and 

the reinforcing plates yield strength, FY, were added to the original Book 3 data contained in the 

WTC-DB. The database included plates that extended long enough such that they substantially affect the 

member properties of the column, e.g., the added plates increase the capacity of the columns. Where the 

plates appeared to reinforce only the column splice, they were not included in the database. 

The reinforcing plate data were tabulated and incorporated into the database in the same manner as the 

plates discussed in the previous section, except that a second length designation was added to differentiate 

the length of the plates on the north and south faces of the columns, i.e., the column designation “LN” 

refers to the length of the plates on the north face of the column. Again, when calculating the built-up 
Appendix B 

B–14

column properties, the plate was assumed to be continuous along the floor-to-floor height of the column. 

When the length of the reinforcing plate shown in the drawing was greater than the floor height, the plate 

was attributed to the two column segments. Where the plate extended over the entire height of the 

segment, the length was tabulated as the height of the column segment. The remaining length of plate 

was attributed to the other column segment. 

Repair Due to the Bombing of February 26, 1993 

The 1993 bombing resulted in structural damage to WTC 1, centered at exterior column 324 (south wall), 

B-2 level. The face of the column towards the explosion was slightly bowed, and the splice in the column 

developed a hairline crack. The column was reinforced locally to account for the loss of steel area. The 

bracing on either side was replaced with equivalent sections and attached in a similar manner as the 

originals. No modification to the WTC-DB has been made for this repair. 

Tenant Alteration for an Interoffice Stair 

A tenant alteration was provided for an interoffice stair between floors 25 and 26 in WTC 1. This work, 

adjacent to core column 901A, was performed by an engineering firm (other than LERA) and 

unknowingly resulted in the loss of a core column bracing strap (refer to Fig. B–9), leaving the column 

unbraced about its minor axis for two stories. The PANYNJ alerted LERA to the issue and asked LERA 

to review. The situation was reviewed by LERA, and the column stability was found to be adequate. No 

modification to the WTC-DB has been made for this modification. The effect of removing the strap is 

accounted for in the global model of WTC 1, see Section B.3.1. 

Drawing Book Data Discrepancies 

In the original WTC drawing book data, the following discrepancies were discovered by LERA: 

• Book 1 page 1–B–15. For member number G311A, the inch portion of the length is listed as 

3-1/18. Based on the comparison to similar bracing types in the area, this dimension was 

modified to be 3-1/8 in. in the WTC-DB. 

• Book 3 page 3–A1–10. For core column 601A between floors 86 to 89 and 89 to 92, the 

column type is listed as 213. Type 213 is a column type which by definition has reinforcing 

plates, but for this location, no plate data was provided in the schedule. This, in combination 

with comparisons to similar columns in plan, led to modifying the column type to 111. This 

also applies to column 601B, page 3–B1–10 between floors 86 to 89 and 89 to 92. 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–15

Figure B–9. Column section at original column strap detail (taken from drawing 

book 18, page 18–AB2–12). 

• Book 3 page 3–B1–48. For column 1008B between floors 63 to 66, the yield strength, FY, is 

listed as 6 ksi in the table. Based on the yield strength of the columns above and below these 

floors, the yield strength was modified to be 36 ksi. For the same column number and floor 

segments, the lower splice detail number is listed as “ 01G.” Based on the lower splice detail 

number of the columns above and below these floors, the number was modified to be 

“301G.” 

• Book 3 page 3–B1–9. For core column 508B between floors 21 to 24, the length of plate 1, 

W1, is tabulated as 11.25 in. However, length B for this column is 22 in. and thickness t2 is 

5.5 in. W1 equals B minus two times t2 (see Fig. B–10). Hence, assuming t2 was listed 

correctly in the table, W1 was modified to be 11 in. 

• Book 1 page 1–B–23 and 1–B2–19. The details for column types 1024, 1025, 5024, and 

6025 listed in the tables are not explicitly shown in the drawing book. For these members, 

column shapes are assumed to be as shown in the typical details in page 1–B–19 for the 

1000 series columns, 1–B–24 for the 5000 series column, and the 1–B–27 for the 6000 series 

column. 

• Book 3 page 3–AB2–6. The column type 216 does not appear to be assigned to any member 

in the drawing book. 
Appendix B 

B–16

Drawing used with permission from PANYNJ. 

Figure B–10. Core column series 300. 

B.2.5 Section Property Calculations 

SAP2000 Section Designer was typically used to calculate section properties for built-up sections. The 

sections were "built-up" within SAP2000 by defining plate dimensions and offsets from 0-0 location. 

Section orientations were defined with the X-X axis horizontal to the bottom of the original drawing book 

page as the detail is shown in the drawing book. 

During the process of calculating properties there was an exception to this orientation rule. Core column 

members CC1007A104L, CC1002A104L, CC703A106L, CC1007B104L, CC1002B104L, and 

CC703B106L consist of a wide flange shape and web reinforcing plates. These members were input into 

SAP2000 rotated 90 degrees from the orientation shown in the details to utilize the default orientation of 

the wide flange section in Section Designer. Once the properties were calculated, the sections were 

placed in the WTC-DB following the orientation of the detail (i.e., the axis was shifted back 90 degrees). 

When rolled shapes were used to create built-up sections, the rolled shapes database developed for this 

project was used to build the sections in SAP Section Designer as explained later in this section. The 200 

series core columns (wide flange rolled columns reinforced with plates) are examples of members whose 

properties were calculated in this manner. 

Member Designations 

For member section property calculations and assembly of the finite element models, the members were 

named using the following general member designations. The member designations are listed in the 

Microsoft Excel files. 

First character: 

• Book 1—below tree–B 

• Book 2—exterior wall tree–T 

• Book 3—core columns–C 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–17

• Book 4—exterior columns and spandrels–E 

Second character: 

• C—column 

• S—spandrel and below grade exterior wall spandrel, strut, or bracing 

Third to fifth character: (third to sixth character for 4 digit column e.g., 1004) 

• Column number 

Sixth character: 

• A—WTC 1 

• B—WTC 2 

Seventh character and above: 

• Upper splice level—for core columns 

• U(upper), M(middle), or L(lower)—column segment where reinforcing plates are added 

• T or B—top or bottom of nonprismatic columns 

• Detail letter (lowercase)—(where more than one section is calculated) 

• F or C—face or center of nonprismatic spandrel 

• Elevation—below tree spandrel elevations 

Column Member Multiple Section Property Calculation 

In the database, the following three types of column members had different cross sections along the 

length of the members: 

• Exterior wall tree at level C in Drawing Book 2 (two different cross sections) 

• Exterior wall tree at level E in Drawing Book 2 (three different cross sections) 

• Exterior column type 300 (floor 9 to 106) in Drawing Book 4 (two different cross sections) 

For these three member types, the section properties of the different cross sections were calculated and 

listed in the database tables. In an effort to minimize repeated information, the raw input data for all 

sections were only shown in the rows that correspond to the first cross section. For the second and third 

(if any) cross sections, the calculated data followed in the rows below. The constant raw data such as the 

column number were not repeated in these rows of the table, and thus the corresponding cells were left 

blank. Since the column number was used as a link for the development of the relational database, only 
Appendix B 

B–18

the row containing the raw input data and the first cross section properties was returned in a query, and 

thus, the user must refer back to the Microsoft Access ‘Tables’ for the remaining section property 

information. The section names of the different cross sections along the member length were 

distinguished by the last one to two characters, which identified the cross sections where the section 

properties were calculated. 

For example, exterior column EC339 (mechanical floors) tapers over a portion of the length of the 

member (refer to Fig. B–11). The section properties above and below the spandrel were calculated. The 

column section above the spandrel was called EC339, while the column section below the spandrel was 

called EC339cc. The suffix ‘cc’ denoted the section below the spandrel. Note that the raw dimensional 

data for EC339cc were not shown in the table, as the information was the same as for EC339. 

Spandrel Member Multiple Section Property Calculation 

In the database, the exterior columns below elevation 363 ft in column series 5000, 6000, and 7000 in 

Drawing Book 1 had corresponding spandrels shown in the details in Book 1. There were two types of 

spandrels for these members, tapered built-up box shapes and built-up I shapes. For the tapered built-up 

box shapes, the section properties of the different cross sections were calculated and listed in the database 

tables. The data were listed in the database files as described for columns with multiple cross sections. 

The section names of the different spandrel cross sections along the member length were distinguished by 

the last three to four characters, which identified the cross sections where the section properties were 

calculated. 

For these exterior columns, there are spandrels at two elevations, 332 ft and 350 ft. At elevation 350 ft, 

the spandrels tapered, and as a result two cross section properties were calculated. The first section was at 

the face of the exterior column, and the corresponding section name had a Suffix F (face). The second 

section was at the center of the spandrel in between two exterior columns, and the corresponding section 

name had a Suffix C (center). The elevations and locations of the cross sections of the spandrels were 

shown in the figures in the “Cross Section” worksheets in the database Excel files. 

For example, four different section properties were calculated for exterior column 6009 in WTC 1. The 

first section was the exterior column itself, and the section name was BC6009A. The other three sections, 

BS6009AB332, BS6009AT350C, and BS6009AT350F were for the spandrel sections. The suffix B332 

in BS6009AB332 denoted the bottom spandrel at elevation 332 ft. Suffixes T350C and T350F in 

BS6009AT350C and BS6009AT350F, respectively denoted the top spandrel at elevation 350 ft, and the 

“C” or “F” identified the locations where the section properties were calculated, see Fig. B–12. 

Section Property Calculation Comparisons 

For all the members whose section properties were included in the GFI database, the cross sectional 

properties in the GFI data were compared with the data contained within the WTC-DB. Most section 

property results compared with good accuracy between GFI and the WTC-DB (within 1 percent). It was 

found that results from the calculations of the torsional constant, J, however, did vary. LERA in-house 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–19

Figure B–11. Exterior column type 300, floor 9 to floor 106 (taken from 

drawing book 4, page 4–AB2–18). 

programs were then used to confirm the accuracy of the J calculation. For core columns in WTC 1, 

SAP2000 generated values used in the WTC-DB were on average 8 percent larger than J values 

calculated using a LERA in-house program, while the results provided by the GFI database were on 

average 13 percent greater than LERA in-house program J calculations. 

It was found that for box sections, J values calculated by the above equation matched the J values given 

by SAP Tube Section. However, for the same tube section, the J values given by SAP Section Designer

were greater than J given by SAP Tube Section, even while all other properties were equivalent. 

According to Computers and Structures, Inc., the developer of SAP2000; the J values given by SAP 

Section Designer are more accurate as SAP Section Designer uses a finite element method to calculate the 

J values while an approximate equation is used in SAP Tube Section. 
Appendix B 

B–20

Figure B–12. Column type 6000 with tapered spandrel (taken from drawing book 1, 

pages 1–A2–27 and 28). 

The approximate equation used to calculate J values by the LERA in-house program for a built-up 

column or box section as shown in Fig. B–13 is as follows: 

( )

wf t

h

t

b

bh J

+

=

2 2

 (a) (b) 

Figure B–13. Box section and a built-up column. 
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B–21

In order to minimize the complexity of the model, where the member cross-section was of the type 

illustrated in Fig. B–13 (a), box column members were defined in SAP Tube Section. The remaining 

built-up box columns (similar to Fig. B–13 b) were defined in SAP Section Designer. 

For members whose properties are not given in the GFI database, hand calculations or calculations by 

LERA in-house program were carried out to verify the results from SAP2000 Section Designer for at least 

one section for each member type. 

In summary, it was found that SAP2000 Section Designer provided section properties in close agreement 

to LERA calculated properties. In most cases these properties also closely matched with the properties 

listed in the GFI database. In the cases where SAP2000 results disagreed with the GFI database, the 

results were reviewed and it was concluded that the SAP2000 calculation provided the correct properties. 

Therefore the section property results calculated using SAP2000 were used in the WTC-DB and the 

development of the finite element models of the towers. 

Rolled Shape Database 

While the majority of the primary members of the WTC towers’ super-structure were built-up members, 

rolled shapes were also used. The rolled shapes specified in the drawings in a number of cases are no 

longer produced and therefore, are not included in the rolled shape database embedded within SAP2000. 

Therefore, a rolled shape database was developed using the old nomenclature and section properties. The 

result was a file named ‘Shape Property Table.xls’ and it contains three worksheets, ‘Database’, ‘Excel 

Format’, and ‘WF Shape Properties from SAP’. The following is a discussion of their contents. 

Data contained in ‘Database’ and ‘Excel Format’––Drawing Books 3, 4, and 5 include reference to 

specific rolled shapes. The referenced shape names were extracted from the above books and assembled 

into a single reference database for rolled shapes. Most of the section properties were obtained from the 

Manual of Steel Construction, American Institute of Steel Construction (AISC), Sixth Edition, 1963 

(AISC 6th Edition) with few exceptions where cross sections were not included in this edition. Examples 

of these exceptions include the following: 

• Section properties of 14WF455 to 14WF730 were obtained from the Manual of Steel 

Construction–Load and Resistance Factor Design, American Institute of Steel Construction, 

Third Edition, 2001 (AISC-LRFD 3rd Edition). 

• Section properties of 6CH12, 6CH15.1, 12CH40, 12CH45, and 12CH50 were obtained from 

the MC-shapes table in the AISC-LRFD 3rd Edition. 

• Section properties of 18WF69 were obtained from the Iron and Steel Beams 1873 to 1952, 

the American Institute of Steel Construction, 1968. 16WF342 was assumed to have the same 

section properties of 16H342 tabulated in Iron and Steel Beams 1873 to 1952, the American 

Institute of Steel Construction, 1968. 

• For 7x5 tube, Zx, Zy, and J were obtained from the AISC- Allowable Stress Design (ASD), 

1989, 9th Edition. 
Appendix B 

B–22

• For 2L 3 1/2 × 3 × 1/2in. long leg back to back, the combined properties were taken from 

SAP’s embedded rolled shape database. 

Data contained in ‘WF Shape Properties from SAP’––For the rolled wide flange shapes, an additional 

database was created in SAP2000 based on the tabulated shape dimensions from the AISC Manuals as 

discussed above. Computers and Structures, Inc. provided an MS Excel file named ‘Proper.xls’ with a 

macro that allowed the accurate calculation of the section properties for use within SAP2000. This 

information was then used by SAP2000 Section Designer to calculate section properties for built-up 

members comprised of wide flange sections and added plates. 

For calculation of the properties with ‘Proper.xls’, dimensions of the webs and flanges, as well as the size 

of the fillet, were input into the spreadsheet. The macro then calculated the section properties based on 

the input information. The results were shown to be in good agreement with the original tabulated 

properties. 

B.3 DEVELOPMENT OF REFERENCE STRUCTURAL ANALYSIS MODELS 

FOR THE TOWERS 

This section outlines the development of the reference structural analysis models for each of the two 

towers. Included in this section are descriptions of the structural models, modeling techniques, 

parametric studies utilized in the development of the models, and a description of the methodology used 

in exporting data from the relational database presented in Section B.2 into the global models. 

The main types of the models developed are as follows: 

• Two global models of the major structural components and systems for the towers, one each 

for WTC 1 and WTC 2; and 

• One model each of the typical truss-framed floor and typical beam-framed floor (mechanical 

level) within the impact and fire regions. 

The models are all linear elastic, three dimensional structural analysis models developed using Computers 

and Structure, Inc.’s SAP2000 Software, Version 8. The models will be used to establish the baseline 

performance of each of the two towers under gravity and wind loads in the third phase of this project. In 

addition, these models serve as a reference for significantly more detailed models to be developed 

independently in other parts of the NIST investigation for the aircraft impact analysis (Project 2) and 

thermal-structural response and collapse initiation analyses (Project 6). 

B.3.1 Global Models of the Towers 

Three-dimensional structural analysis computer models of the 110-story above grade structure and 6-story 

below grade structure for each of the two towers were developed. The global models for the towers 

consist of the major structural components and systems required to establish the baseline performance of 

the towers under gravity and wind loads. 

In establishing the modeling techniques for the global models, parametric studies were performed to 

evaluate the behavior of typical portions of the structure (Section B.3.4). In addition, once the models 
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B–23

were completed, order-of-magnitude checks were performed for gravity load, wind load, and eigenvalue 

results to check the accuracy of the models. More refined checks will be done in the third phase of this 

project on baseline performance analysis. 

Components and Systems in the Towers Global Models 

The models included all primary structural elements in the towers including exterior columns, interior 

(core) columns, exterior wall bracing in the basement floors, core bracing at the mechanical floors, core 

bracing at the main lobby atrium levels, spandrel beams, hat trusses, and rigid and flexible diaphragms 

representing the floor systems as developed in Section B.3.4 of this report. 

Coordinate System, Nomenclature, and Models Assembly Overview 

The extent of the data required to assemble the tower models dictated that the relational database 

capability of the WTC-DB be used (see Section B.2.3). The methodology for the development of the 

models using the relational database is described in this section. 

Coordinate System––The coordinate system for the model geometry was based on the column layout 

from the original drawings. Figure B–14 shows the location of the X and Y axes for the global models 

and the floor models. The Z coordinates were based on actual elevations of the towers. The original 

column numbers were used throughout the models for member identification. 

Nomenclature––A standard nomenclature for joints, frame names, and section names for use in the 

models was established. The nomenclature enables the user to know quickly where in the building a 

section is located by viewing any given piece of the model. Joint names generally included the column 

number, tower letter, and floor level. Frame element names generally included the joint name at the ‘j’ 

end (second node). Section names were based on the section as described in the drawing book and were 

repeated for each steel yield strength assigned for that section. Alternatively, where the section was 

unique to a particular member in the building, sections were named based on the frame member. 

As an example, most nodes (or joints) in the tower models were named according to the following format: 

• Column number 

• Tower letter (A for WTC 1 and B for WTC 2) 

• Floor level 

• S for column splice nodes only 

• J for spandrel splice nodes only 
Appendix B 

B–24

Original drawing used with permission from PANYNJ. 

Figure B–14. Global model coordinate axis location. 

Figure B–15 illustrates the detailed frame and joint nomenclature for a typical exterior wall panel. 

Model Assembly Overview––An overview of the assembly of the data into the tower model is described 

herein along with an expanded section on the programmatic assembly of the models. 

Following a basic study of modeling techniques and testing of SAP2000, Version 8 input format and 

capabilities, it was determined that the best approach was to divide the model into six main parts and 

thenassemble them into a unified model. Manipulation of these individual parts was more efficient than 

attempting to build the whole model simultaneously. 

X

Y 

(0,0)
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B–25

TYPICAL PANEL NOMENCLATURE 

COL 1 COL 3

COL1+USL COL2+USL COL3+USL 

COL1+USL+S COL2+USL+S COL3+USL+S

COL3+USL+J

COL1+(USL-1) COL2+(USL-1) COL3+(USL-1) 

COL3+(USL-1)+J 

COL1+(USL-2) COL2+(USL-2) COL3+(USL-2) 

COL3+(USL-2)+J 

USL

USL-1

USL-2

COL 2

PANEL 

USL: Upper Splice Level 

COL1,2,3+USL,(-1),(-2): Joint name

EC+JOINT ABOVE: Column name 

ES+JOINT RIGHT: Spandrel name 

***: Defined by adjacent panel 

(***) 

(***) 

(***) 

(***) (***) (***) 

Figure B–15. Typical exterior panel nomenclature. 
Appendix B 

B–26

The six initial models were: 

• Core columns 

• Exterior wall, foundation to floor 7 

• Exterior wall trees (floors 7 to 9) 

• Exterior wall, floors 9 to 106 

• Exterior wall, floors 107 to 110 

• Hat truss 

For the core columns and exterior wall at floors 9 to 106; most of the analysis input files were generated 

from queries of the WTC-DB. The other four parts of the model were assembled primarily in a more 

conventional manner. 

Core columns and exterior wall panels (floors 9 to 106) were the greatest data-intensive challenges in the 

model development. Both areas included a large number of frame members and section and material 

property variations. The query files were used to gather the necessary data, and then simple computer 

programming was used to convert the data into the SAP input file format. Four main input tables for the 

SAP input file were developed programmatically: 

• Joint coordinates table 

• Connectivity-frame/cable table 

• Frame section properties tables 

− Frame section properties 1—general 

− Frame section properties 5—nonprismatic 

− Section designer properties 04—shape I/wide flange 

− Section designer properties 05—shape channel 

− Section designer properties 11—shape plate 

• Frame assignments table 

The remaining data is added directly in the SAP model: 

• Material properties 

• Frame local axis 

• Joint restraint 
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B–27

• Insertion point 

• Constraint 

• Gravity and wind load assignments 

After the joint coordinates, connectivity, frame section properties, and frame assignments were complete 

for the six parts, the individual models were combined into a unified model. Rigid diaphragms, flexible 

diaphragms, core bracings, gravity loads, wind loads, and masses were then added to the unified model. 

After assembly of the model, the assignment of properties for selected model elements was spot-checked 

and the model was executed to verify its performance. 

The development of the WTC 1 and WTC 2 models has been separate and consecutive endeavors. The 

lessons learned in the assembly of the WTC 1 model were applied to the development of the WTC 2 

model. While there were only minor differences in the basic structural systems of the two towers, there 

were significant differences in section properties, material properties, and additional column transfers at 

the lower levels in WTC 2. 

Isometric views of the complete WTC 1 model are illustrated in Fig. B–16. Elevations of the complete 

WTC 2 model are illustrated in Fig. B–17. A summary of the size of the global models of WTC 1 and 

WTC 2 is presented in Table B–2. The following presents the details of each of the six parts used in the 

development of the unified global models for WTC 1 and WTC 2. 

Core Columns Modeling 

Core column coordinates were tabulated based on the structural drawings. Column locations were 

typically referenced at their centerlines. Columns on lines 500 and 1000, however, were located in plan 

drawings along most of their height according to the face of the column to which the floor trusses frame 

(i.e., WTC 1 north face for 500 columns and south face for 1000 columns). The centerline of these 

columns was based on their dimensions given in the drawing books. Where these column centerlines 

varied along the height of the towers (typically 1 1/2 in. between three-story pieces), a representative 

location was chosen to define the column node. Thus, the column coordinate at floor 106 was used as a 

constant along the tower height because at this level, these columns align with the hat truss above. 

The spandrel centerline elevation was selected as the representative floor elevation for exterior columns 

and used also for core columns. If there were no spandrels in exterior panels, reference elevations were 

used for the core columns. 

There were over 5000 nodes in the core column model. This amount of data required that the Interactive 

Database input table be set up using a macro. These data were converted to text file format and later 

imported into SAP. Built-up sections were defined as Section Designer sections, and wide flange shapes 

were defined directly from “SectionWF1.pro” file (see Section B.2.5). All section names were identical 

to those in the database. Around 1280 Section Designer sections were defined in this model and imported 

through Interactive Database function of SAP2000 to the model. 
Appendix B 

B–28

Figure B–16. Rendered isometric views of the WTC 1 model. 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–29

 

Figure B–17. Frame view of the WTC 2 model: exterior wall elevation and interior 

section illustrating the core columns, core bracing, and hat truss. 

Core 

bracings 
Appendix B 

B–30

Table B–2. Approximate size of the reference structural models (rounded). 

Model 

Number of 

Joints 

Degrees of 

Freedom 

Number of 

Frame Elements

Number of 

Shell Elements 

Total Number 

of Elements 

WTC 1 global modela 53,700 218,700 73,900 10,000 83,900 

WTC 2 global modela 51,200 200,000 73,700 4,800 78,500 

Typical truss-framed model 28,100 166,000 27,700 14,800 42,500 

Typical beam-framed model 6,500 35,700 7,500 4,600 12,100 

a. Model does not include floors except for flexible diaphragms at 17 floors as explained later. 

The core columns were defined as frame members spanning from node to node at the representative floor 

elevations. Splices in core columns occurred typically 3 ft above the floor level. In the models, however, 

the splice was considered to occur at the floor level, and nodes were only defined at these levels (i.e., 

typically at spandrel centerlines). Most three-story column pieces are unique, as tabulated in WTC-DB 

(Drawing Book 3). A section for each three-story piece was defined and then assigned to each of the 

three frame members that make up that column. Using the SAP shading feature to graphically show the 

section on the model, each frame was rotated to its proper orientation based on the structural drawings. 

In the as-designed drawings, there were strap anchors connecting the core columns to the concrete floor 

slab to provide lateral bracing for the column. At floor 26 of WTC 1 the straps at column 901 were 

removed during a renovation project that was engineered by a firm other than LERA (see Section B.2.4). 

The loss of the straps at this location has been included in the model by releasing the column from the 

diaphragm in the direction of the straps. 

Exterior Wall, Foundation to Floor 7 Modeling 

The models of the exterior wall up to elevation 363 ft were developed manually, assigning joints and 

members connectivity as shown in the drawings. The elevation drawings show that below elevation 

363 ft, columns were typically spaced at 10 ft and braced with spandrels and diagonals. Joints were 

defined at all locations where diagonals braced the columns. However, when coordinates were not given 

in the drawings, joint coordinates were determined based on the geometry of the diagonal. Details in 

WTC Drawing Book 1 show that the column-diagonal intersections had continuity. Joints at elevation 

253 ft (level B-5) were defined only where the diagonals connect to the columns, since the tower floor did 

not frame into the exterior spandrels at that floor. 

Where noted in elevation drawings, spandrel centerline elevations were used to define joint coordinates. 

Additionally, joints were defined at the spandrel splice midway between two columns at elevation 350 ft 

3 in. (floor 3) and at elevation 329 ft 3 in. (floor 2) to allow for section type transitions. 

The majority of the elements at these levels were defined as Section Designer sections, except for box 

shapes which were defined as “Box/Tube”. Channel shapes were defined directly from 

“SectionWF1.pro” file (see Section B.2.5). All section names were identical to those in the database. 

Around 200 sections were defined in this model using the Interactive Database function of SAP2000, 

which was used to import data into SAP2000. 

Typical columns were connected from bottom to top and typical spandrels were connected from left to 

right. Frame names followed the nomenclature description presented earlier. The SAP2000 program 
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B–31

allows assignment of rigid zone factors to frame end offsets to account for the overlap of cross sections. 

At the intersection of columns and spandrels, 100 percent rigidity for the column and the spandrels were 

assigned due to the large size of both columns and spandrels. Using the SAP shading feature to 

graphically show the section on the model, each frame was rotated to its proper orientation based on the 

structural drawings. 

Refer to Fig. B–18 for a frame view and rendered view of the exterior wall (foundation to floor 9) of the 

WTC 1 model. The figure also shows the core columns and core bracings. 

Exterior Wall Trees (Floor 7 to 9) Modeling 

The panels of the exterior wall between elevation 363 ft and elevation 418 ft 11 1/2 in. are called exterior 

wall trees. At the exterior wall trees, the typical exterior wall columns transitioned from a spacing of 

10 ft to a spacing of 3 ft 4 in. A typical exterior wall tree panel is shown in Fig. B–19. Each panel was 

divided into five different levels; level B, C, D, E, and F. For each panel in the model, the three exterior 

columns from above elevation 418 ft 11 1/2 in. continued down to level D. At that level, the three 

columns were connected by a horizontal rigid element to become one member, which extended down to 

elevation 363 ft. 

 

Figure B–18. Frame view and rendered view of the WTC 1 model (foundation to floor 9). 

In the model, the tree was also the location where the column insertion point transitioned from the inside 

face (at the spandrel) of the upper column to the centerline of the lower column. Between levels B and D 

(see Fig. B–19), the location of the spandrel transitioned from 6 1/2 in. offset from the exterior column 

reference line to the center of this reference line. Within the floor 9 spandrel, the exterior columns taper; 

however, in the model, the tapering of the columns was not included because frame end length offsets 

were assigned to the columns to account for the rigidity of the spandrels. 

Through the height of level C, the box-shaped columns tapered (Fig. B–19). In the model, non-prismatic 

members were used to model the tapering columns. The columns start to taper at the bottom of the 
Appendix B 

B–32

spandrel at level B, and cease to taper at the top of the spandrel at level D. The dimensions of the 

columns at the spandrel edges were defined in the drawing book. In the model, the column extended from 

the centerline of the spandrel at level B to 1 ft below the top of the spandrel at level D (see discussion for 

level D below). Therefore in order to obtain the correct section properties along the length, the 

dimensions of the section at the joints were interpolated based on the dimensions of the section at the 

spandrel edges shown in the drawing book. The section properties of the tapering column were assumed 

to vary linearly between the two sections. Frame end length offsets were assigned to the columns to 

account for the rigidity of the spandrel at level B and the one foot dimension at level D. 

x

X Joint offset

Joint

6 ft

1 ft

6.5 in.

x

X Joint offset

Joint

6 ft

1 ft

6.5 in.

Original drawing used with permission from PANYNJ. 

Figure B–19. Exterior wall tree panel (taken from Drawing Book 2, page 2-AB2-2). 

At level D, two transitions occurred in the model. The first transition was for the exterior columns, where 

the three columns coming down from level C were connected by a horizontal rigid element to become one 

member at the bottom of the tree. This frame member consisted of the three exterior columns and the 

spandrel plate. Another horizontal member of the same section properties with the spandrel plate was 

also defined and connected between the neighboring exterior wall trees. This member connected the 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–33

neighboring exterior wall trees and provided lateral bracing for the columns. Frame end length offsets 

were assigned to the spandrel to account for the overlap of the spandrel plate with the frame member, 

which also included the spandrel plate. The transition of the three members into one member was 

assumed to occur at one foot below the top of the spandrel at level D to account for the fact that the 

spandrel becomes engaged with the exterior columns after being connected to the exterior columns for a 

certain distance. Hence, the joints were defined at one foot below the top of the spandrel at level D. 

There was a second transition at level D (Fig. B–19). The nodes for the exterior wall columns were 

typically defined at 6 1/2 in. offset from the exterior column reference line. But for the joints at and 

below level D in the exterior wall tree, the joint coordinates were defined along the exterior column 

reference line. As a result, for the column member that framed between the nodes at levels B and D, a 

joint offset of 6 1/2 in. was assigned at the top of the member, while no offset was assigned at the bottom. 

The column therefore remained a vertically straight element while being connected to nodes that were not 

aligned vertically. 

At level E, the exterior columns tapered and had two different types of cross section (Fig. B–20). For 

each panel, the exterior column transitioned from Section b–b in Fig. B–20 into a box-shaped column 

(Section c–c in Fig. B–20). The location of the transition between the different types of cross section 

varied for different column types from 5 ft 8 in. to 6 ft 4 in. measured from the bottom of level E. In the 

model, the transition was assumed to be at 6 ft measured from the bottom of level E. For each panel, the 

exterior column at level E was modeled as two nonprismatic members. The top section of the first nonprismatic member consisted of three box-shaped columns and a middle plate, while the bottom section 

was a box-shaped column (Section c–c in Fig. B–20). The properties were assumed to vary linearly 

between the two sections. The second nonprismatic member was a tapering box shaped column 

(Section c-c in Fig. B–20), and again, the properties were assumed to vary linearly between the two 

sections. At level F, the exterior wall tree columns were prismatic box-shaped columns. 

The final model of a typical tree is illustrated in Fig. B–21. 

Exterior Wall (Floor 9 to 106) Modeling 

In plan, column and spandrel members connected at nodes located at the outside face of the spandrel, 

6 1/2 in. from the exterior column reference line (see Fig. B–22). The columns were offset horizontally, 

or ‘inserted’ at this node, using an insertion point located at the centerline of plate T3. Insertion points 

were not adjusted for spandrel thickness. With this modeling, gravity and wind loads can be applied at 

the spandrel location. 

In elevation, the columns and spandrel members connected at the spandrel centerline, typically 12 1/2 in. 

below the reference floor elevation (Fig. B–22). The spandrels were then located correctly without the 

need for offsets to be defined. The effect of applying loads at both the spandrel centerlines and the 

reference floor elevations was studied, and it was found that it has a negligible difference in spandrel 

stresses. 

For typical exterior wall panels (i.e., three columns wide by three stories high), nodes at five elevations 

were defined. The models included nodes at the three representative floor levels (defined at the spandrel 

centerlines) as well as the upper and lower column splices. Diaphragms were assigned to all nodes at 

floor levels where concrete slabs exist, to represent the high in-plane stiffness of the concrete floor slabs. 
Appendix B 

B–34

Original drawing used with permission from PANYNJ. 

Figure B–20. Exterior wall tree: as-built cross sections for level E (taken from 

Drawing Book 2, page 2-AB2-13). 

 Original drawing used with permission from PANYNJ. 

Figure B–21. Frame view and rendered view of an exterior wall tree. 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–35

Spandrel

Column

Original drawing used with permission from PANYNJ. 

Figure B–22. Typical WTC tower exterior wall panel. 

The SAP2000 program allows assignment of rigid zone factors to frame end offsets to account for the 

overlap of cross-sections. In the global model, 50 percent rigidity for the column and 100 percent rigidity 

for the spandrels were assigned for the typical exterior wall panels to match the lateral deflection of the 

detailed shell model of the panel based on the parametric study results (see Section B.3.4). It was also 

found that, due to the relatively large depth of the spandrels and the close spacing between the columns, 

the spandrels contribute to the axial stiffness of the columns in the panels. This contribution was 

estimated to range from 20 percent to 28 percent increase in the vertical stiffness of the panels. 

Therefore, a frame property multiplier for the exterior wall column’s cross-sectional area was used to 

provide a 25 percent increase in columns’ axial stiffness (see Section B.3.4). 
Appendix B 

B–36

For exterior wall corner panels, 25 percent rigidity for the column and 50 percent rigidity for the 

spandrels were assigned based on the parametric study results (see Section B.3.4). Also, an area modifier 

was used to provide a 25 percent increase in the axial stiffness of the two continuous columns of the 

corner panels (Section B.3.4). No modifier was used for the 100, 200, 300, and 400 series intermittent 

columns. 

Exterior column types were defined in Drawing Book 4. A few types (100 series typical, 300 series at 

mechanical floors, and 400 to 500 series at corners) repeated extensively throughout the building, with 

steel yield strengths that vary from 36 ksi to 100 ksi. Since SAP does not allow for the assignment of 

material properties at the member assignment stage, the number of different steel strengths was 

determined for each exterior column type, and sections were defined for each. The section name included 

the section number and the yield strength as tabulated in the drawing books. 

Typical spandrels and corner panels were defined as rectangular shape and Section Designer section with 

stiffener, respectively. The top and bottom stiffener of each corner spandrel were included in both the 

parametric study and the global models. The detail shows that the stiffeners were 6 in. plates of thickness 

matching plate T2 in the corner column. 

Exterior Wall (Floor 107 to 110) Modeling 

Spandrel depths varied at floors 108 and 110. A weighted average of spandrel depth was determined in 

order to define the average centerline elevation of the spandrels and, therefore, the node elevation for the 

entire floor. 

For the 7 × 5 structural tube sections that were used in these floors, sections from the current AISC 

Manual were assigned, and modification factors of 1.04 were applied to the section properties. The 

modifiers were used to match the section properties from the 6th Edition AISC Manual. 

The exterior wall members from floors 107 to 110 were typically rolled shapes with Fy =42 ksi or 

Fy =50 ksi. Where not shown in the drawings as Fy =50 ksi, Fy =42 ksi was used. 

Hat Truss Modeling 

In both WTC 1 and WTC 2, a truss system referred to as a ‘hat truss’ was constructed between floor 107 

and the roof. The hat truss system was intended to support the load of the antenna on top of the tower and 

to interconnect the exterior walls to the core. The hat truss was made up of eight trusses spanning 

perpendicular to the long-direction of the core and six trusses spanning perpendicular to the shortdirection of the core (refer to Figs. B–23 and B–24). 

Frame members between floors 107 and 110 were assigned to the model according to plan and elevation 

drawings of the hat truss. Node locations were set to coincide with the centerline of spandrels at the 

exterior wall. Columns, diagonals, and beams were included in the model. All columns and diagonals 

shown in drawings SA/B–400 through SA/B-404 were included in the model. Floor beams that did not 

participate in the hat truss system were not included in the model, unless they were used to transfer truss 

chords or core columns. Flexible floor diaphragms were used in this area. 
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B–37

Coordinates were generally not given at floor 109, as this level does not contain a complete concrete floor 

slab. The geometry of the diagonals, columns, and beams was used to determine the location of the node 

where the diagonal would intersect floor 109. Unless otherwise noted in the drawings, diagonals and 

Figure B–23. As-modeled plan of the WTC 1 hat truss. 

Figure B–24. Rendered 3–D model of the WTC 1 hat truss (prior to assembly in the 

unified model). 
Appendix B 

B–38

columns were assumed to be non-composite and floor beams were assumed to be composite. Hat truss 

diagonals, main chords, and main columns were modeled with continuous joints. Hat truss beams, 

however, had pinned ends. 

Flexible and Rigid Floor Diaphragm Modeling 

For most floors, rigid diaphragms provide for a sufficiently accurate representation of the flow of forces 

and deformations for global structural response. This is a customary engineering practice. In cases where 

the flow of forces and deformations would be affected significantly by the use of rigid diaphragms, the 

floors were modeled as flexible diaphragms. 

The floor models described in Sections B.3.2 and B.3.3 were used to develop the flexible diaphragm 

stiffness utilized within the global models. Section B.3.4 outlines the study for the determination of the 

in-plane diaphragm stiffness of the detailed floor models, using that in-plane stiffness to arrive at an 

equivalent shell element floor model. The equivalent shell element floor was used to represent the inplane floor stiffness in the global model. The shell elements attached to all exterior wall columns and 

core columns. 

Flexible diaphragms were used at the floors of the towers in the core of the atrium area, in the mechanical 

floors, and in the floors of the hat trusses. The floors modeled using flexible diaphragms are floors 3, 4, 

5, 6, 7, 9 (atrium levels); 41, 42, 43, 75, 76, 77 (mechanical levels); 107, 108, 109, 110, and roof (hat truss 

region) of both towers. 

Initial Verification of Global Models 

Several steps were taken to verify the model input. SAP2000 Version 8 offers a ‘shading’ option once a 

model has been built with frame section assignments. This allows the user to view the members as the 

program has interpreted their input. The shading option was helpful for using section-designed shapes, 

and for verifying the orientation (i.e., local axes) of members. Note that shading is not correct when two 

Section Designer sections are used in non-prismatic members, so orientations for these sections were 

verified by reviewing their local axis member properties. The work was independently reviewed by 

engineers not associated with the initial model development. 

Once the models were completed, checks for gravity loads, wind loads, and eigenvalue results were 

performed. The overall performance of the tower models under these loads was found to be reasonable 

by checking deformations, stresses, reactions, etc. More refined checks will be done as part of the third 

phase of this project on baseline performance analysis. The natural periods will be calculated using mass 

properties estimated from realistic loads on the towers as part of the baseline analysis. Calculated natural 

periods will be compared with the measured periods of WTC 1. 

B.3.2 Typical Truss-Framed Floor Model—Floor 96A 

In order to select the typical truss-framed floor within the expanded impact and fire zones of both towers, 

the drawings for floors 80 to 100 were reviewed to identify structural similarities. It was found that 

floor 96 of WTC 1 (96A) represented the typical truss-framed floor in the expanded region for WTC 1 
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B–39

(floors 89A to 103A). The lone exception in this region of WTC 1 was floor 92 which had an increased 

dead load capacity required for the support of secondary water lines. 

Floor 96A was also representative of the typical truss-framed floor in the expanded region for WTC 2 

(floors 74B–88B). Specifically, floor 96A was similar to the truss framing at floor 74B and floors 84B 

through 88B. Floors 78B and 79B were sky lobby and upper escalator floors, respectively. Both 

contained long span trusses which were similar to floor 96A, but also contained beam-framed floor 

construction in the entire short span area (where the escalators were located). Floors 80B through 83B 

had beam framing in place of a single truss panel in the short span area, while the remaining area 

contained trusses which were similar to floor 96A. 

Based on the above discussion, floor 96 of WTC 1 was selected as the overall representative truss-framed 

floor for the majority of the expanded impact and fire zone in both towers and is described in the 

following sections (see Fig. B–25). An isometric view of the typical truss-framed floor model is 

illustrated in Fig. B–26. Table B–2 includes a summary of the size of the 96A floor model. The 

following presents the major structural systems and components of the truss-framed floor model. 

Type 1 - WTC Typical Truss Floor Panel Plan

Tower A Floors: 10 - 24 60 - 66 Tower B Floors: 14 - 24 60 - 74

26 - 40 68 - 74 26 - 40 84 - 91

50 - 58 84 - 91 50 - 58 93 - 106

93 - 105

Note: All panel types within 1" length tolerance, except floors 10,11, 39, 40, 70, & 71 which are within 6"-10". Floors 72-74 vary 18"-26".

FR1 ER1 DR1 CR1 BR1 A1 B1 C1 D1 E1 F1

Note: H1 = H6

Note: J1 =K1 = KR1

Note: HR1 = HR6 Note: H1 =M1 = MR1

(all C32T5 Trusses)

F1 E1 D1 C1 B1 A1 BR1 CR1 DR1 ER1 FR1

G1

G1

CORE

GR1

HI HR1

J1 J1

HR1 H1

GR1

Figure B–25. Typical truss-framed floor panels arrangement. 
Appendix B 

B–40

Figure B–26. Typical truss-framed floor model (floor 96A), slab not shown. 

Primary Trusses 

The primary trusses consisted of double angle top and bottom chords which were 29 in. out-to-out of the 

chords. The trusses acted compositely with a 4 in. concrete slab on 1 1/2 in. metal deck. For a typical 

long-span truss, C32T1, the top chord consisted of two angles 2 by 1.5 by 0.25 in., short legs back-toback (SLB), and the bottom chord consisted of two angles 3 by 2 by 0.37 in., SLB. The distance between 

the centroid of the two chords was calculated to be 28.05 in. The distance from the centroid of the top 

chord to the neutral axis of the transformed composite slab with top chord was calculated to be 1.93 in. 

The sum of (28.05 + 1.93) is 29.98 in. (Fig. B–27). In the model, therefore, 30.0 in. was taken as the 

typical distance between the top and bottom chords for both short- and long-span primary trusses. 

In the long-span truss zone, the two individual primary trusses, which were part of the same floor panel 

and attached to the same column, were separated (typically) by a distance of 7 1/8 in. At the joint 

between panels, the distance between the abutting long-span trusses was 7 1/2 in. Therefore in the model, 

7 1/2 in. was used as the spacing between all long span primary trusses. In the short-span truss zone, 

two individual trusses which attached to the same column were separated by a distance that varied 

between 4 7/8 in., 5 in., and 5 1/4 in. In the model, the typical spacing between all short-span double 

trusses was 5 in. The long span trusses in the two-way zone had an as-modeled length of 58 ft 10 in. 

while the long span trusses in the one-way zone had an as-modeled length of 59 ft 8 in. 
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B–41

28.05”

0.537”

2”

4” Slab

0.414”

L3 x 2 x 0.37

L2 x 1.5 x 0.25

2.48”

29.98” 

≈ 30”

C32T1 (Primary Truss Section)

(Note: 2nd Truss of Pair Not Shown)

ACTUAL MODEL

N.A. Combined Slab + 

Primary Truss Double L

Web member extension 

into slab

28.05”

0.537”

2”

4” Slab

0.414”

L3 x 2 x 0.37

L2 x 1.5 x 0.25

2.48”

29.98” 

≈ 30”

C32T1 (Primary Truss Section)

(Note: 2nd Truss of Pair Not Shown)

ACTUAL MODEL

N.A. Combined Slab + 

Primary Truss Double L

Web member extension 

into slab

Figure B–27. Typical primary truss cross-section, as-built and as-modeled 

transformed truss work points. 

The diagonal web bars for the primary trusses were most often 1.09 in. diameter bars. Therefore, for 

double angle shapes in the primary trusses, 1.09 in. is taken as the distance between the two angles. This 

holds true for primary trusses where bar diameters varied between 0.92 in. and 1.14 in. 

The as-built truss diagonals had end fixity, but were considered pinned for the analysis. Pinning the 

diagonals is conservative and provides an upper bound of the gravity load stresses. To mitigate the effect 

of the pinned member approach, end length offsets were used for the truss diagonals to compensate for 

the difference in the as-built diagonal unbraced length and the model unbraced length. The as-built 

unbraced length for a typical diagonal in a primary truss was 32.4 in., while the modeled member length 

was 36.05 in., and therefore, an end offset of 1.8 in. was used at both ends. Similarly, for the bridging 

trusses, the actual unbraced length for a typical diagonal of a bridging truss was 29 in., while the modeled 

length was 30.66 in. Therefore, an end offset of 0.83 in. was used at both ends. A rigid zone factor of 

100 percent is used for all offset zones. 

In the model, the deck support angles, typically 3 by 2 by 0.75 in. were located in the same plane as the 

combined truss top chord and composite slab centroid. 

Bridging Trusses 

The bridging trusses were 24 in. deep, edge-to-edge, with double angle chords. For a typical bridging 

truss, 24T11, the top and bottom chords consisted of two angles 1.5 by 1.25 by 0.23 in., SLB. The 

distance between the centroid of the two chords was 23.26 in. The distance used as the offset between the 

top and bottom chords for all bridging trusses was taken as 23.25 in. (Fig. B–28). The distance between 

the work points of the top chord of the bridging truss and the top chord of the primary trusses and 

equivalent slab plate for 24T11 was calculated to be 3.39 in. This distance was selected for all bridging 

trusses to be 3.375 in. As in the as-built structure, the bridging truss was not connected along its length to 

the slab shell elements in the model. At the intersection of the top chords of the primary and the bridging 
Appendix B 

B–42

trusses, the intersection was modeled using vertical rigid links, connected in turn to the slab shell 

elements representing the concrete slab. 

23.26”

0.368”

4” Slab 1.5”

0.368”

L1.5 x 1.25 x 0.23

2.48”

≈ 23.25” 

24T11 (Bridging Truss Section)

T.O.S.

5.868”

Rigid Link to Slab at 

Primary Truss and 

Bridging Truss 

Intersection

3.375”

N.A. Combined Slab + 

Primary Truss Double L

ACTUAL MODEL

23.26”

0.368”

4” Slab 1.5”

0.368”

L1.5 x 1.25 x 0.23

2.48”

≈ 23.25” 

24T11 (Bridging Truss Section)

T.O.S.

5.868”

Rigid Link to Slab at 

Primary Truss and 

Bridging Truss 

Intersection

3.375”

N.A. Combined Slab + 

Primary Truss Double L

ACTUAL MODEL

Figure B–28. Typical bridging truss cross–section, as-built and as-modeled 

transformed truss work points. 

The bottom chord of the primary trusses was connected to the bottom chord of the bridging trusses along 

the length of the primary trusses only on column lines 111, 149, 311, and 349. The connection consisted 

of double angles 2 by 1 1/2 by 0.25 in. These connection angles were included within the model. 

For bridging trusses in the model, a 0.75 in. angle gap was used for trusses with web bar diameters that 

varied between 0.75 in. and 0.98 in. 

Truss Member Cover Plates 

In 30 percent of the floor area, truss members were supplemented with cover plates. The members with 

additional plates included top chords, web members, and most typically bottom chords. Section 

properties were calculated with SAP Section Designer. The primary truss top chords were reinforced 

with an additional set of double angles at truss end connections. At these locations, the work points for 

the section were located at the centroid of the composite double angle and concrete slab. 

The Laclede shop drawings indicated plates 3/8 in. by 3 in. connecting the bottom chord of the primary 

truss pairs together at each end and where intersected by a bridging truss. These plates were included in 

the model. 

Viscoelastic Dampers 

Viscoelastic dampers were located where the bottom chords of the long span, short span, and bridging 

trusses intersected the exterior columns. The dampers were defined in Drawing Book D. The dampers 
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B–43

resisted static and quasi-static loads (such as gravity loads) at the time of load application. Immediately 

following load application, the dampers shed load until the stress in the dampers was dissipated. A 

placeholder element was located in the model at the damper location. 

Strap Anchors 

Exterior columns not supporting a truss or truss pair were anchored to the floor diaphragm by strap 

anchors. These strap anchors were connected to the columns by complete penetration welds. The strap 

anchors were then connected to the slab with shear stud connectors and to the top chords of the trusses by 

fillet welds. The straps were included in the model and located in the plane of the centroid of the 

composite top chord. Also, in the model the work points intersected with the centerline of the column and 

used a rigid link to attach back to the spandrel (see Fig. B–29). 

Straps

Trusses

Columns

Rigid Link

Frame Elements

Attachment 

PL to Slab

Attachment 

PL to Slab Spandrel PL

Deck Support 

Angles

Straps

Trusses

Columns

Rigid Link

Frame Elements

Attachment 

PL to Slab

Attachment 

PL to Slab Spandrel PL

Deck Support 

Angles

Figure B–29. Strap anchors modeling, slab not shown. 

Concrete Slab and Metal Deck 

Outside the core, the primary trusses acted compositely with the 4 in. concrete slab on 1 1/2 in. metal 

deck. In the model, the average depth of the slab plus deck was modeled as 4.35 in. The concrete slab 

consisted of lightweight concrete with a self-weight of 100 pcf and a design compressive strength, 

f’c= 3,000 psi. The concrete modulus of elasticity, Ec, used for modeling is 1,810 ksi, and the calculated 

modular ratio, n=Es/Ec, is taken as 16, where Es is the steel modulus of elasticity. These values are 

consistent with those included within the WTC Structural Design Criteria Book. 

Typically, inside the core, the beams acted compositely with a 4 1/2 in. formed concrete slab. The 

concrete slab consisted of normal weight concrete with a self-weight of 150 pcf and a design compressive 
Appendix B 

B–44

strength, f’c= 3000 psi. The concrete modulus of elasticity, Ec, used for modeling was 3,320 ksi and the 

calculated n ratio, Es/Ec, was taken as 8.7. 

The floors of the WTC towers had an in-floor electrical distribution system of electrified metal deck and 

trench headers. The effects of the in-slab trench headers were accommodated by reducing the slab shell 

element thickness. A 1 ft 8 in. wide shell panel (the typical truss-floor shell mesh size) was reduced in 

thickness from 4.35 in. to 2.35 in. or 1.35 in. at the trench header locations per drawing SCA–109 

(Floor 96A Structural Concrete Floor Plan). 

Initial Verification of the 96th Floor Model 

Several steps were taken to verify the model input. SAP2000 Version 8 offers a ‘shading’ option once a 

model has been built with frame section assignments. This allows the user to view the members as the 

program has interpreted their input. The shading option was helpful for using section designed shapes, 

and for verifying the orientation (i.e., local axes) of members. The work was independently reviewed by 

engineers not associated with the initial model development. 

Once the model was completed, checks were performed for gravity loads. All superimposed dead loads 

and live loads included in the model were based on WTC Design Criteria; self weight is accounted for by 

SAP2000. To justify the modeling assumptions, several studies were performed to compare stress results 

to hand calculations for representative composite sections. Hand calculations estimate deflections and 

member stresses for a simply supported composite truss under gravity loading. For the composite truss 

sections, the steel stress results were within 4 percent of those calculated by SAP2000 for the long-span 

truss and 3 percent for the short-span truss. Deflections for the beams and trusses matched hand 

calculations within 5 to 15 percent. 

B.3.3 Typical Beam-Framed Floor Model—Floor 75B 

As described in Section B.3.2 for truss-framed floors, the structural drawings were reviewed to identify 

structural similarities between the beam-framed floors within the expanded impact and fire zones of both 

towers. It was found that floor 75 of WTC 2 (75B) represents the typical beam-framed floor in the 

expanded impact zone for WTC 2 (floors 74B to 88B). There were no beam-framed floors within the 

expanded impact zone of WTC 1. 

Floors 75 and 76 of WTC 2, lower and upper mechanical equipment (MER) floors, respectively, were 

typical of the lower three mechanical equipment floor pairs in both towers (floors 7 and 8, 41 and 42, and 

75 and 76 for both WTC 1 and WTC 2). Floor 77 of WTC 2, a lower escalator floor, was a beam-framed 

floor similar to the lower floor of the mechanical equipment floor pairs, i.e., floor 75B. 

Based on the above discussion, floor 75 of WTC 2 was selected as the overall representative beam-framed 

floor for the expanded impact and zone in both towers and is described in the following sections (see 

Fig. B–30). An isometric view of the typical beam-framed floor model is illustrated in Fig. B–31. 

Table B–2 includes a summary of the size of the 75B floor model. The following presents the major 

structural systems and components of the beam-framed floor model. 
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B–45

Type 12 - WTC Beam Framed Floor Floor Plan

Towers A & B MER Floors: 7,41,75,108

 Towers A & B Near MER Floors: 9,43,77,107,110,Roof Beams Beams

Beams CORE Beams

Figure B–30. Typical beam-framed floor arrangement. 

Figure B–31. Typical beam-framed floor model (floor 75B). 
Appendix B 

B–46

Composite Beams 

The beams in the model were located at the elevation of the centerline of the concrete slab. The insertion 

point for the beams was set at the beam top flange, and then the beam was offset down by one-half the 

thickness of the slab. The beam was rigidly linked with the slab to simulate the composite action. This 

option provided for accurate estimation of the composite stiffness of the floor. 

For beams with cover plates, the properties were calculated by SAP Section Designer, and the slab, beam, 

and reinforcing plates were rigidly linked. 

Horizontal Trusses 

Exterior columns which did not support a beam were connected to the floor for bracing purposes by 

horizontal trusses. These exterior horizontal trusses were anchored to the columns with complete joint 

penetration welds. The horizontal trusses were then connected with shear stud connectors to the slab. 

The truss angles (typically 4 by 4 by 5/16 in.) were then connected to the top flange of the beams. In the 

model, the work points intersected with the centerline of the column and used a rigid link to attach back to 

the spandrel. The truss members were located in the plane of the centroid of the composite top chord (see 

Fig. B–32). 

Horizontal 

truss

Beams +

Cover 

Plates

Columns Rigid Link

Frame 

Elements

Spandrel Plate

Horizontal 

truss

Beams +

Cover 

Plates

Columns Rigid Link

Frame 

Elements

Spandrel Plate

Figure B–32. Horizontal truss modeling, slab not shown. 

Concrete Slab and Metal Deck 

Outside the core on the mechanical floors, the beams acted compositely with a 5 3/4 in. concrete slab on 

1 1/2 in. metal deck. The average cross-sectional depth of the slab in the model was taken as 6.1 in. The 
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B–47

concrete slab consisted of normal weight concrete with a self-weight of 150 pcf and a design compressive 

strength of typically f’c= 3,000 psi. The concrete modulus of elasticity, Ec, used for modeling is 3,320 ksi 

and the calculated modular ratio, n, is taken as 8.7. 

Typically, inside the core, the beams acted compositely with a 6 in. formed concrete slab. The concrete 

slab consisted of normal weight concrete with the same properties as concrete outside the core. 

The mechanical floors had a 2 in. maximum depth topping slab both inside and outside the core. The 

topping slab stiffness was not included in the models, but the weight will be accounted for in the baseline 

analysis. 

Viscoelastic Dampers 

Viscoelastic dampers were located below the bottom flange of the beams where the beams intersected the 

exterior columns. Similar to the 96 floor model, a placeholder element was located in the model at the 

damper location. 

Initial Verification of the 75 Floor Model 

Similar to the 96 floor model, the ‘shading’ option in SAP2000 was used to view the members as the 

program has interpreted their input. The shading option was helpful for using section designed shapes, 

and for verifying the orientation (i.e., local axes) of members. The work was independently reviewed by 

engineers not associated with the initial model development. 

Once the model was completed, checks were performed for gravity loads. All superimposed dead loads 

and live loads included in the model are based on WTC Design Criteria; self weight is accounted for by 

SAP2000. To justify the modeling assumptions, several studies were performed to compare stress results 

to hand calculations for representative composite sections. Hand calculations estimate deflections and 

member stresses for a simply supported composite beam under gravity loading. The model yielded 

accurate steel stress results compared to hand calculations—around 1 percent for both short and long span 

beams. Where the beams were built-up with reinforcing plates, it was found that SAP Section Designer

shapes were not calculating the stresses correctly, so instead, separate beam and plate elements drawn 

over each other were inserted. This method yielded very accurate steel stress results—between 1 percent 

and 2 percent for both short and long span beams. 

B.3.4 Parametric Studies 

Modeling techniques employed in the development of the global models of WTC 1 and WTC 2 are 

consistent with, but often more advanced than, the techniques typically employed in the analysis and 

design of high-rise buildings. As such, building components were idealized so that overall performance 

was replicated while appropriately reducing the computational requirements. The following describes the 

studies undertaken to establish the idealizations used in the models including typical exterior wall panels, 

exterior corner panels, and flexible floor diaphragms. 
Appendix B 

B–48

Exterior Wall Columns/Spandrel Typical Panels (Floors 9 to 106) 

A parametric study of typical three-column, three-spandrel exterior wall panels from the face of the 

towers (floors 9 to 106) was performed using two modeling methods (see Fig. B–33). The first model 

was a detailed shell model where each plate of each column or spandrel was specifically modeled, and the 

second was a simplified frame model. The parametric study assumes that the shell model best represents 

the as-built panel performance, and therefore, it was used to tune the performance of the frame model 

which was used throughout the global model (see Section B.3.1). The objectives of the study were to 

(1) match the axial stiffness of the frame model with the detailed shell model under gravity load and 

(2) match the inter-story drift of the two models by modifying the rigidity of the column/spandrel 

intersections in the frame model. 

Figure B–33. Shell element and frame models of typical exterior wall panel. 
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B–49

For the axial stiffness of the simplified frame model of the panel versus the detailed shell model, results 

of loading both models vertically indicated that the shell model was stiffer than the equivalent beam 

model due to the contribution of the spandrel beams to the columns’ axial stiffness. This is due to the 

rigidity of the spandrel beams and the proximity between the columns. The parametric study on a wide 

range of panels over the height of the towers showed that the vertical stiffness of the columns in the 

bottom third of the towers should be increased by a factor in the range of 25 percent to 28 percent, and the 

columns in the middle and upper thirds of towers should be increased by a factor in the range of 

20 percent to 28 percent. Based on these figures, 25 percent increase of axial stiffness of exterior 

columns was selected as a reasonable representation for the panel vertical stiffness over the height of the 

towers between floors 9 and 106 (see Section B.3.1).

For studying the lateral deformation of the exterior panels, panel properties were taken from three 

different areas of the building. These include floors 79 to 82, 53 to 56, and 23 to 26. Internal column 

stiffeners were included in the shell model. The deformations at points A, B, I, and II (see Fig. B–34) 

were studied for three different panel locations and their respective spandrel and column thickness. The 

top most columns were connected via a rigid link and loaded in the plane of the panel and perpendicular 

to the column with a 100 kip load. 

Column

Spandrel

C

S

A

B

I II

Figure B–34. Column and spandrel rigidity of typical exterior wall panel. 

The lateral displacements found for the shell and frame models of typical exterior wall panels with varied 

column and spandrel intersection rigidities are reported in Table B–3. The study found that 50 percent 

column rigidity and 100 percent spandrel rigidity in the frame model produced deflection results 

consistent with the shell model. 
Appendix B 

B–50

Table B–3. Lateral displacement (in.) for the shell and frame models of typical 

exterior wall panel with varied column and spandrel rigidities. 

No rigidity C:50%, S:100% C:100%, S:100%

A 0.60 1.04 0.59 0.35

B 0.28 0.52 0.29 0.18

I 0.45 0.78 0.44 0.26

II 0.45 0.78 0.44 0.26

No rigidity C:50%, S:100% C:100%, S:100%

A 0.26 0.43 0.27 0.18

B 0.12 0.22 0.14 0.11

I 0.19 0.32 0.2 0.15

II 0.19 0.32 0.2 0.15

No rigidity C:50%, S:100% C:100%, S:100%

A 0.21 0.37 0.21 0.12

B 0.1 0.18 0.1 0.06

I 0.16 0.28 0.16 0.09

II 0.16 0.28 0.16 0.09

Frame model (Rigidity)

Floor 23-26

Frame model (Rigidity)

Floor 53-56

Frame model (Rigidity)

Floor 79-82

Lateral displacement (in)

Shell model

Shell model

Shell model

Exterior Wall Columns/Spandrel Corner Panels (Floors 9 to 106) 

A parametric study was performed of an exterior wall corner panel typical over each corner of the towers 

from floors 9 to 106. Similar to the exterior typical panels, to account for the contribution of the 

spandrels into the axial stiffness of the columns, it was found that an area modifier to provide a 25 percent 

increase in the axial stiffness of the two continuous columns of the corner panels is suitable for modeling 

the columns’ axial stiffness. No modifiers were needed for the 100, 200, 300, and 400 series intermittent 

columns. 

The panel from floor 53 to 56 was selected to be representative with two additional columns attached on 

either side. The objective of the study was to match the inter-story drift of a detailed shell model and a 

simplified frame model of the corner panel by modifying the rigidity of the column/spandrel intersections 

in the frame model. For this parametric study, the panel was straightened to simplify the study and to 

isolate the behavior of interest (see Fig. B–35). The deformations at points T1, T2, B1, B2, and M2 

(Fig. B–36) were studied for representative column and spandrel plate dimensions. The top most columns 

were connected via a rigid link and loaded in the plane of the panel and perpendicular to the column with 

a 100 kip load. 

The lateral displacements calculated for the shell and frame models of the typical exterior wall corner 

panel with varied column and spandrel rigidities are reported in Table B–4. The study indicated that 

25 percent column rigidity and 50 percent spandrel rigidity in the frame model produced deflection results 

consistent with the shell model. 
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B–51

Figure B–35. Shell element and frame models of typical exterior wall corner panel. 

Typcial exterior column

C:50%, S:100%

Corner panel

Rigidity

T1 T2

B1 B2

M1

Column

Spandrel

C

S

Figure B–36. Column and spandrel rigidity of typical exterior wall corner panel. 
Appendix B 

B–52

Table B–4. Lateral displacement (in.) for the shell and frame models of typical exterior 

wall corner panel with varied column and spandrel rigidities. 

No rigidity C:25%, S:50% C:100%, S:100%

T1 0.227 0.236 0.222 0.152

T2 0.227 0.236 0.222 0.152

M1 0.149 0.154 0.149 0.102

B1 0.084 0.072 0.077 0.053

B2 0.084 0.072 0.077 0.053

Shell model Corner panel rigidity

Floor 53-56

As part of the in-house NIST review of the reference structural models (see Section B.4), a detailed shell 

element model of original corner panel (not straightened) was analyzed under lateral loads to test the 

accuracy of the simplified frame model with 25 percent column rigidity and 50 percent spandrel rigidity 

calculated above. Both the detailed and simplified models were loaded as shown in Fig. B–37. The 

deflections calculated from the frame model were consistent with those estimated from the shell model, 

indicating that the rigidities estimated using the straight model (Fig. B–35) accurately represent the actual 

corner panel behavior. 

Figure B–37. Detailed and simplified model of the exterior wall corner panel. 

Flexible Floor Diaphragm 

The floor models developed in Sections B.3.2 and B.3.3 were used to develop the flexible diaphragm 

stiffness used within the WTC 1 and WTC 2 global models. The in-plane diaphragm stiffness of the 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–53

detailed floor models was determined and used to arrive at an equivalent shell element floor model. This 

flexible shell element floor model is then inserted in the global models at specific floors to capture the inplane flow of forces and deformations. These flexible diaphragms were not used throughout, as the rigid 

diaphragms in the majority of floors provided for a sufficiently accurate representation of the flow of 

forces and deformations while keeping manageable the model’s computational requirements. In the 

global models, flexible diaphragms were used at the beam-framed floors 3, 4, 5, 6, 7, 9, 41, 42, 43, 75, 76, 

77, 107, 108, 109, 110, and roof of both towers. 

Parametric studies were performed to compare the diaphragm stiffness of two different floor models for 

both the typical truss-framed floor and the beam-framed floor. The typical floor models were compared 

with the simplified equivalent models that duplicate the representation of the exterior wall columns, 

exterior wall spandrels, core columns, and their boundary conditions. The floor framing, both inside and 

outside the core was replaced by shell elements. The material properties of the shell model matched the 

properties of the concrete floor outside the core in the respective floor model. 

The comparative floor models were loaded in the plane of the floors with a lateral load of 180 lb/ft. 

(equivalent to 15 psf over the 12 ft story height) on both the windward and leeward faces. The column 

base supports were released for the exterior wall columns along the loaded faces and for all core columns 

to allow lateral translation only in the direction of loading. 

The comparative models were executed to assess the horizontal deflection of the floor on both the 

windward and leeward sides of the model and for the case where the lateral loads were applied nonconcurrently along the 100 face and 200 face of the tower. Both the total horizontal deflection of the slab 

and the relative displacement between the windward and leeward sides were compared between the 

models. The shell thickness was modified to match the in-plane stiffness determined by the detailed floor 

models. 

The deformations from the lateral load case using the 96 floor model of WTC 1 are illustrated in 

Fig. B–38, while Fig. B–39 shows the deformations of the simplified floor model. Fig. B–40 shows the 

lateral deflection of the north and south sides of the floor model under lateral load applied in the north 

direction using the detailed and equivalent floor models. 

B.4 REVIEW OF THE STRUCTURAL DATABASES AND REFERENCE 

MODELS OF THE TOWERS 

NIST has implemented a rigorous and comprehensive review procedure to mitigate potential conflicts of 

interest and to ensure the integrity and objectivity of the deliverables of this project, including the 

structural databases and reference models. The review procedure includes an in-house NIST review as 

well as a third-party review by the firm of SOM. The following summarizes the results of these reviews 

for the developed structural databases and reference models. 
Appendix B 

B–54

Figure B–38. Deflection of typical truss-framed floor model due to lateral loading 

(exaggerated scale). 

Figure B–39. Deflection of equivalent floor model due to lateral loading (exaggerated 

scale). 
Interim Report on Development of Structural Databases and Reference Models for the WTC Towers 

B–55

0.002

0.003

0.004

0.005

0.006

0.007

0.008

Lateral Displacement (in.)

North Face, Detailed Model

South Face, Detailed Model

North Face, Equivalent Model

South Face, Equivalent Model

East Corner West Corner

Figure B–40. Deflections of the north and south faces of the floor for the detailed and 

equivalent floor models. 

B.4.1 Review of the Structural Databases 

The third-party review by SOM included random checks of the digitized structural databases and cross 

section property calculations. The review indicated no discrepancies between the developed databases 

and the original drawing books. Also for cross section property calculations, the review indicated a good 

agreement (within 1 percent) between the properties in the developed databases and those estimated by 

SOM. Special attention was given to the calculation of the torsional constant, J (see Section B.2.5). It 

was found that using the software ShapeBuilder, version 3.0 which uses a finite element approach for the 

calculation of J a good agreement was obtained between the J values in the developed databases and 

those estimated by SOM. 

The in-house NIST review included the following steps: (1) line-by-line review of all database files, 

(2) random checks on the developed databases by the project leader, and (3) calculation of all cross 

section properties and comparison with those in the developed databases. The review indicated minor 

discrepancies between the developed databases and the original drawing books. For cross section 

property calculations, good agreement was obtained between the properties in the developed databases 

and those estimated by NIST. The discrepancies between the developed databases and the original 

drawing books were reported to LERA, who implemented the changes and modified the databases 

accordingly. Consequently, the structural databases have been approved by NIST and are being made 

available for other phases of the NIST investigation. 
Appendix B 

B–56

B.4.2 Review of the Reference Structural Models 

The third-party review by SOM included: (1) random checks of the consistency of the developed 

reference models with the original structural drawings and drawing books, and (2) verification and 

validation of the models, including reviewing assumptions and level of detail and performing analyses 

using various loading conditions to test the accuracy of the models. The review indicated that the 

developed models are consistent with the original design documents. The review indicated that, in 

general, the modeling assumptions and level of detail in the models are accurate and suitable for the 

purpose of the project. The SOM review identified two areas where the models need to be modified. The 

first is the effect of additional vertical stiffness of the exterior wall panels due to the presence of the 

spandrel beams (see Sections B.3.1 and B.3.4). The second area is the modeling of the connections of the 

floor slab to the exterior columns of the 75B floor model (Section B.3.3), where this connection appeared 

to be fixed while the connection should be modeled as pinned. 

The in-house NIST review included: (1) checks on the consistency of the developed reference models 

with the original structural drawings and drawing books, and (2) verification and validation of the models, 

including reviewing assumptions and level of detail and performing analyses using various loading 

conditions to test the accuracy of the models. The review indicated minor discrepancies between the 

developed reference models and the original design documents. Similar to the third-party review, the inhouse NIST review identified the proper modeling of the vertical stiffness of the exterior wall panels and 

the accurate modeling of the floor slab connections to the exterior columns in the 75B floor model as 

areas that need to be modified in the models. 

In addition, NIST conducted a workshop for NIST investigators and contractors to review the reference 

structural models developed by LERA. The workshop attendees included experts from LERA (two 

experts); SOM (two experts); Teng and Associates (one expert, outside experts on probable structural 

collapse); Professor Kasper Willam (outside expert on thermal-structural analysis); Professor David M. 

Parks (outside expert on computational mechanics for aircraft impact analysis); Applied Research 

Associates (two experts, contractor on analysis of aircraft impact into the WTC towers) as well as all key 

investigators from NIST (17 experts). The purpose of the workshop was to discuss the methodology, 

assumptions, and details of the developed reference models. The minutes of the workshop are being 

prepared and will be made public. The feedback from the workshop was included in the final review of 

the models. 

The discrepancies between the developed models and the original design documents, as well as the areas 

identified by both the third-party and in-house review for modification, were reported to LERA, who 

implemented the changes and modified the models accordingly. Consequently, the reference structural 

models have been approved by NIST and are being made available for other phases of the NIST 

investigation. 
C–i

TABLE OF CONTENTS - Appendix C

List of Figures.......................................................................................................................................... C–ii

List of Tables ............................................................................................................................................C–v

Appendix C 

Interim Report on Analysis of Aircraft Impact into the WTC Towers................................ C–1

C.1 Introduction..................................................................................................................................C–1

C.2 Material Constitutive Modeling...................................................................................................C–1

C.2.1 WTC Tower Steel Constitutive Models ...........................................................................C–2

C.2.2 Failure Models................................................................................................................C–12

C.2.3 Concrete Constitutive Models ........................................................................................C–31

C.2.4 Aircraft Constitutive Models..........................................................................................C–32

C.3 Development of Aircraft Model.................................................................................................C–33

C.3.1 Aircraft Data Collection .................................................................................................C–33

C.3.2 Description of the Aircraft Model ..................................................................................C–33

C.3.3 Analysis of Fuel Distribution at Impact .........................................................................C–40

C.4 Component-Level Analyses.......................................................................................................C–47

C.4.1 Analysis Methodology....................................................................................................C–49

C.4.2 Exterior Column Impact Analyses .................................................................................C–50

C.4.3 Core Column Impact Analyses.......................................................................................C–63

C.4.4 Combined Engine Impact Analyses ...............................................................................C–63

C.4.5 Floor Assembly Component Analyses ...........................................................................C–67

C.5 Summary and Preliminary Findings ..........................................................................................C–68

C.6 References..................................................................................................................................C–71
C–ii

LIST OF FIGURES

Figure C–1. Example finite element models of the ASTM 370 rectangular tensile specimen. .......... C–3

Figure C–2. Example finite element models of the ASTM 370 round bar tensile specimen.............. C–4

Figure C–3. Test data and true stres-strain conversion for the 75 ksi steel.......................................... C–6

Figure C–4. Tabular true stress-strain constitutive model curve for the 75 ksi steel. .......................... C–6

Figure C–5. Calculated tensile test response with necking for the 75 ksi steel.................................... C–7

Figure C–6. Comparison of measured and calculated engineering stress-strain curves for the 

75 ksi steel........................................................................................................................ C–7

Figure C–7. Comparison of the constitutive model and synthetic steel behaviors for the 75 ksi 

steel. ................................................................................................................................. C–8

Figure C–8. Tabular true stress-strain constitutive model curves. ....................................................... C–9

Figure C–9. Comparison of the constitutive model and synthetic steel behaviors for the higher 

strength perimeter steel curves......................................................................................... C–9

Figure C–10. Comparison of the constitutive model and synthetic steel behaviors for the lower 

strength perimeter steel curves....................................................................................... C–10

Figure C–11. Comparison of the constitutive model and synthetic steel behaviors for the core 

steel curves. .................................................................................................................... C–10

Figure C–12. Comparison of rate effects model and test data. ............................................................ C–12

Figure C–13. Calculated necking response in the 75 ksi tensile specimen. ......................................... C–14

Figure C–14. Coarse shell element mesh for the 75 ksi tensile specimen............................................ C–14

Figure C–15. Mesh refinement effects in the calculated 75 ksi tensile test. ........................................ C–15

Figure C–16. Notched round bar specimen dimensions....................................................................... C–17

Figure C–17. Notched round bar tensile specimen models.................................................................. C–18

Figure C–18. Calculated notched round bar stress triaxiality. ............................................................. C–18

Figure C–19. Calculated critical plastic strain as a function of notched stress triaxiality.................... C–19

Figure C–20. Simulation of the notched and smooth round bar tests. ................................................. C–20

Figure C–21. Photograph of an exterior column with weld zone fractures.......................................... C–21

Figure C–22. Micrograph of an exterior column weld geometry......................................................... C–21

Figure C–23. Microhardness characterization of the weld and HAZ materials. .................................. C–22

Figure C–24. 2-D weldment model developed for analysis of failure behavior. ................................. C–23

Figure C–25. Simulation of the 55 ksi steel tensile test. ...................................................................... C–24

Figure C–26. Mesh resolutions used in the 2-D weldment failure model............................................ C–24

Figure C–27. Calculated weldment deformations and failure.............................................................. C–25

Figure C–28. Detail of the calculated weldment failure behavior........................................................ C–25
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–iii

Figure C–29. Mesh refinement effects on calculated weldment failure behavior................................ C–26

Figure C–30. Calculated energy balance for the 2-D weldment models.............................................. C–27

Figure C–31. Drop test model for column weld fracture analysis. ...................................................... C–27

Figure C–32. Models developed for column weld fracture analysis.................................................... C–28

Figure C–33. Calculated strain profile for the weld zone. ................................................................... C–29

Figure C–34. Calculated drop test fracture behavior............................................................................ C–29

Figure C–35. Measured bolt load-displacement behavior.................................................................... C–30

Figure C–36. Comparison of the measured and calculated bolt behavior............................................ C–31

Figure C–37. Finite element analysis of the unconfined compression test. ......................................... C–32

Figure C–38. Comparison of the calculated unconfined compression behavior with concrete 

compression test data. .................................................................................................... C–33

Figure C–39. Digitized engineering stress-strain curves for various 2024 aluminum alloys............... C–34

Figure C–40. Digitized engineering stress-strain curves for various 7075 aluminum alloys.............. C–34

Figure C–41. True stress-strain curves developed for various aircraft aluminum alloys.................... C–35

Figure C–42. Tabular true stress-strain curves developed for various aircraft aluminum alloys........ C–35

Figure C–43. Pratt and Whitney PW4000 turbofan engine.................................................................. C–36

Figure C–44. PW4000 engine cross sectional geometry and simplification........................................ C–37

Figure C–45. Pratt and Whitney PW4000 turbofan engine model....................................................... C–37

Figure C–46. Finite element model of the Boeing 767-200ER............................................................ C–40

Figure C–47. Complete empennage for the 767 aircraft model. .......................................................... C–41

Figure C–48. Retracted landing gear components for the 767 aircraft model. .................................... C–41

Figure C–49. Underside of the 767 airframe model (skin removed) showing the position of 

retracted main landing gear............................................................................................ C–42

Figure C–50. Complete wing structures for the 767 aircraft model..................................................... C–42

Figure C–51. Small wing section model for component-level analyses. ............................................. C–43

Figure C–52. Wing section model developed for the subassembly impact simulations. .................... C–43

Figure C–53. Flammable material locations in a Boeing 767 aircraft.................................................. C–44

Figure C–54. Layout of fuel tanks in the Boeing 767 wing. ................................................................ C–45

Figure C–55. Approximate fuel tank dimensions ................................................................................ C–46

Figure C–56. Wing cross sections at various rib locations. ................................................................. C–46

Figure C–57. Fuel tank capacity. ......................................................................................................... C–47

Figure C–58. Approximate fuel locations for smooth and level flight................................................. C–48

Figure C–59. Exterior column response comparison, showing contours of the displacement 

magnitude....................................................................................................................... C–51

Figure C–60. Calculated impact damage with the empty short wing section component model......... C–52

Figure C–61. Damage produced by the empty wing section impact.................................................... C–52
Appendix C 

C–iv

Figure C–62. SPH fuel in the small wing segment. ............................................................................. C–53

Figure C–63. Exterior panels impact behavior for a wing segment with fuel (SPH fuel model)........ C–54

Figure C–64. SPH Analysis of structural damage and fuel dispersion (top view)............................... C–55

Figure C–65. ALE analysis of structural damage and fuel dispersion (top view)................................ C–55

Figure C–66. SPH analysis of structural damage and fuel dispersion (side view)............................... C–56

Figure C–67. ALE analysis of structural damage and fuel dispersion (side view). ............................. C–56

Figure C–68. Exterior column end connection modeling. ................................................................... C–58

Figure C–69. Typical bolt bearing shear failures of spandrel connections. ......................................... C–58

Figure C–70. Bolted spandrel connection modeling. ........................................................................... C–59

Figure C–71. Example engine impact analysis with exterior columns. ............................................... C–60

Figure C–72. Engine velocity history for the exterior wall impact...................................................... C–61

Figure C–73. Example engine impact analysis with different impact locations. ................................. C–61

Figure C–74. Example engine impact analysis with different spandrel connection treatments........... C–62

Figure C–75. Engine velocity history for the exterior wall impact...................................................... C–62

Figure C–76. Wide flange core column response comparison, showing contours of the 

displacement magnitude................................................................................................. C–64

Figure C–77. Displacement and kinetic energy comparison for wide flange core column wing 

impact analysis. .............................................................................................................. C–64

Figure C–78. Box core column response comparison, showing contours of the displacement 

magnitude....................................................................................................................... C–65

Figure C–79. Displacement and kinetic energy comparison for box core column wing impact 

analysis........................................................................................................................... C–65

Figure C–80. Example engine impact analysis with interior and exterior columns............................. C–66

Figure C–81. Example engine impact analysis with interior and exterior columns............................. C–67

Figure C–82. Floor assembly impact analysis with brick element concrete slab................................. C–69

Figure C–83. Floor assembly impact analysis with shell element concrete slab. ................................ C–70
C–v

LIST OF TABLES

Table C–1. Material constitutive parameter table–WTC tower steels. ............................................. C–11

Table C–2. Summary of notched round bar tensile tests. ................................................................. C–17

Table C–3. 2-D weldment model comparison. ................................................................................. C–26

Table C–4. Engine model parameters. .............................................................................................. C–38

Table C–5. Boeing 767 engine comparison...................................................................................... C–38

Table C–6. Summary of Boeing 767-200 aircraft model size (under construction). ........................ C–42

Table C–7. Truss floor assembly component analyses comparison. ................................................ C–68
Appendix C 

C–vi

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C–1

Appendix C

INTERIM REPORT ON ANALYSIS OF AIRCRAFT IMPACT INTO THE 

WTC TOWERS

C.1 INTRODUCTION 

One of the objectives of Project 2, Baseline Structural Performance and Aircraft Impact Damage 

Analysis, of the National Institute of Standards and Technology (NIST)-led investigation into the collapse 

of the World Trade Center (WTC) towers is to analyze the aircraft impact into each of the two towers. 

The purpose of this analysis is to provide the following: (1) estimates of the damage to structural systems 

due to aircraft impact––including exterior walls, floor systems, and interior core columns; (2) estimates of 

the aircraft fuel dispersal during impact; (3) estimates of accelerations and deformations as a function of 

time in each of the two towers due to aircraft impact to be used for estimating damage to fire proofing; 

and (4) a database of the major fragments of the aircraft and destroyed structural components of the 

towers to be used for estimating damage to the mechanical and architectural systems inside the towers. 

The analyses, thus, establish the initial conditions for the fire dynamics modeling in Project 5, 

Reconstruction of Thermal and Tenability Environment, and the thermal-structural response and collapse 

initiation analysis in Project 6, Structural Fire Response and Collapse Analysis. 

The impact analyses are conducted at various levels including: (1) the component level, (2) the 

subassembly level, and (3) the global level to estimate the probable damage to the towers due to aircraft 

impact. The analyses also include simplified and approximate methods. Analysis of uncertainties using 

the component, subassembly, global, and simplified analyses will also be conducted to assess the effect of 

variability associated with various parameters on the damage estimates. NIST is working with experts 

from Applied Research Associates (ARA), Inc., to conduct the impact analyses. This appendix 

summarizes the progress made to date on this project. Emphasis is on the models development and 

component level analyses. 

Section C.2 outlines the development of constitutive models describing the actual behavior of the various 

materials included in the aircraft impact analysis. The materials in this section include WTC steels, 

reinforced concrete, and aircraft materials. Special emphasis is also placed on the modeling of weldments 

and bolts. Section C.3 presents the status of the development of the aircraft model, including the engine 

and airframe. This section also provides an analysis of the fuel distribution at the time of aircraft impact. 

Section C.4 provides details on the component level analyses performed to date, including exterior and 

core columns, column and spandrel connections, and floor segments under the impact of an aircraft 

engine or a segment of a wing. A summary and preliminary findings are provided in Section C.5. 

C.2 MATERIAL CONSTITUTIVE MODELING 

An important requirement for high fidelity simulation of the aircraft impact damage is the development of 

constitutive models to represent the actual behavior of the structure under the dynamic impact conditions 

of the aircraft. The primary materials that need to be considered for the component analyses are the 

several grades of steel used in the columns, spandrels, and floor trusses and beams of the WTC towers, 
Appendix C 

C–2

the concrete floor slabs, and the aluminum airframe structure of the Boeing 767 aircraft. All of these 

materials play a significant role in the aircraft impact damage analyses. These materials also display 

significant nonlinear rate-dependent deformation and failure behavior. 

The analysis of the aircraft impact damage is being performed with the LS-DYNA finite element code 

(LS-DYNA 2003). LS-DYNA has an extensive library of more that 130 different constitutive models and 

is capable of accurately reproducing the important material behaviors required in this analysis. Material 

models currently available in LS-DYNA have been utilized for the analyses using material data from tests 

conducted by NIST or available in the public domain. 

Secondary materials of interest include the nonstructural aircraft components and masses such as fuel, 

seats, interior trim, cargo, and luggage. Furthermore, a significant part of the mass of the WTC towers is 

material not included in the primary structural steel frame that includes windows, nonstructural walls, 

partitions, furniture and other building contents, flooring, mechanical equipment, and insulation. 

Selection of the constitutive modeling for these secondary material behaviors has not yet been performed. 

However, the strength of these materials is relatively small compared to the structural materials and 

simple description of their constitutive behavior will be adequate. 

C.2.1 WTC Tower Steel Constitutive Models 

Various constitutive models are available in LS-DYNA that can capture the nonlinear behavior of the 

steel under impact conditions including strain rates effects and failure. The primary constitutive model 

applied to date is the Piecewise Linear Plasticity model. This model is sufficient to model the nonlinear 

dynamic deformation and failure of the steel structures. A tabular effective stress versus effective strain 

curve can be used in this model with various definitions of strain rate dependency. 

The constitutive model parameters were developed for each grade of steel used in the construction of the 

WTC towers based on engineering stress-strain data provided by Project 3 of the NIST-led investigation 

on Mechanical and Metallurgical Analysis of Structural Steel. The approach to developing the 

constitutive parameters for each grade of steel was: 

• Convert the engineering stress-strain curve to a true stress versus true strain curve. The 

conversion process is described below and is valid up to the onset of necking in the specimen. 

• Extrapolate the true-stress-strain curve beyond the point of necking onset. 

• Perform iterative finite element analyses of the tensile test and adjust the true stress-strain 

curve extrapolation and failure strain until the necking behavior and failure point are 

accurately captured. The primary criterion is the quantitative agreement of the measured and 

calculated engineering stress-strain behavior in the softening region beyond maximum stress. 

These analyses require a fine mesh resolution in the specimen to accurately model the large 

strain deformation response during necking. 

• Perform a final finite element analysis of the material test using a coarser mesh resolution 

(medium mesh corresponding to the mesh resolution applied in the component analyses). 

Adjust the failure criteria (strain at failure) to obtain failure at the same engineering strain 

level. 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–3

The advantage of this approach is that the measured nonlinear material behavior up to failure is accurately 

captured in the constitutive model. In addition, the simulation of the material testing provides a validation 

that the constitutive model parameters are defined accurately and that the model can reproduce the 

measured response for the test conditions. 

The tensile tests performed by NIST applied the ASTM 370 test standard (ASTM Designation 

A 370-03a 2003). Example finite element models of a rectangular test specimen with the fine and 

medium mesh resolutions are shown in Fig. C–1. This specimen type was used for all of the tests on 

WTC exterior column materials. A similar figure of the round bar specimen models used for testing of 

the WTC core column steels is shown in Fig. C–2. The typical element length used in the gage section 

for the fine meshes is approximately 0.015 in. and for the medium meshes is approximately 0.10 in. The 

use of the specimen models to develop the constitutive model parameters is described in the following 

subsections. 

Figure C–1. Example finite element models of the ASTM 370 rectangular 

tensile specimen. 

True Stress and True Strain Corrections 

In most tensile tests, a plot is generated of the load measured at the cross head of the testing machine 

against the displacement of the specimen. A plot of engineering stress versus strain can be generated 

from this plot by dividing the load by the original cross sectional area of the specimen and the 

displacement by the original length of the specimen. If the changes in area and length are small during 

the test, these measures give a good indication of material behavior. However, in reality, the cross section 

changes (shrinks) significantly during the test and the engineering stress does not yield the “true” stress in 

the cross section. Similarly, the engineering strain is not representative of the material behavior, 

especially when a general 3-D state of strain exists. As a result, the engineering stress decreases as some 

materials approach failure, implying a weakening of the material. In reality, the stress in the cross section 

is increasing due to the reduction in the cross sectional area (i.e., necking). 

Test Sample

Fine Mesh

Medium Mesh

Grip
Appendix C 

C–4

Figure C–2. Example finite element models of the ASTM 370 round bar 

tensile specimen. 

There are several different ways to measure stress and strain based on the coordinate system used. Some 

are based on material (Lagrangian) coordinates and some on spatial (Eulerian) coordinates. These give 

rise to terms such as “Green” and “Almansi” strain tensors. These are important in writing a computer 

code to solve large strain problems. An alternate approach is to define a “true” or “natural” stress and 

strain. The true stress is based on the load divided by the actual cross sectional area of the specimen and 

is equal to the engineering stress multiplied by a term to correct for the change in cross section. 

e)1( σ T = σ eng + (1)

where: 

σ T = true stress 

σ eng = engineering stress 

e = engineering strain 

The natural or true strain is defined as 

e)1ln()ln(l

l

o

ε T +== (2)

where: 

T ε = true strain 

Fine Mesh

Medium Mesh

Grip
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–5

This definition comes about from defining the incremental true or “natural” strain as the current “change 

in length” divided by the current length, or 

l

dl dε T = (3)

This is in contrast with the definition of engineering strain that references the change in length, ∆l ,

divided by the original length, 0l , or 

0l

l

e

∆ = (4)

Development of Steel Constitutive Properties 

In this section, examples are provided to illustrate the methodology for the development of the steel 

constitutive models and typical results. Figure C–3 shows an example of the measured engineering 

stress-strain behavior for the 75 ksi perimeter column steel. Four tests were performed by Project 3, and 

the data clearly indicates anisotropy in the behavior introduced by the roll forming process (longitudinal 

tests L1 and L2 versus transverse tests T1 and T2 in the figure). This particular grade of steel had a larger 

anisotropy than seen in most of the other steel grades. Whenever anisotropy was observed, the material 

had greater ductility for specimens aligned with the rolling direction. In addition, the largest effects of the 

anisotropy were seen in the behavior after the onset of necking. 

The first step in the constitutive model development process is to obtain a true stress-true strain curve. 

The typical approach is to select a representative test and perform the data conversion process described 

earlier. In this example, the data from test L1 was used to create the true stress-strain curve shown in 

Fig. C–3. This true stress-strain curve is then approximated by a piecewise linear curve in tabular form as 

shown in Fig. C–4. This tabular curve is the input used to specify the mechanical behavior in the 

constitutive model. 

The final step is to apply the tabular true stress-strain behavior in the constitutive model to simulate the 

tensile test as shown in Fig. C–5. If necessary, the extrapolation of the true stress-strain behavior is 

adjusted until the simulation matches the measured engineering stress-strain response including necking 

and failure (the portion of the stress-strain curve beyond the maximum engineering stress). A comparison 

of the calculated and measured tensile behavior for the 75 ksi perimeter steel is shown in Fig. C–6. In this 

example, the constitutive model was developed as an average between the longitudinal and transverse 

properties. Test results conducted by Project 3 on the tower steels indicated that the stress-strain behavior 

is very similar in the longitudinal and transverse directions up to the onset of necking. The difference 

between the longitudinal and transverse properties is in the ductility, where the average ratio of the 

longitudinal to transverse strain to failure in the exterior column steels is about 1.22:1. The approach 

used in the constitutive modeling is to use an average between the longitudinal and transverse properties 

and ignore the orthotropic characteristics of the material in the impact analyses. The effects produced by 

the variation of ductility between the longitudinal and transverse directions will be assessed as part of the 

uncertainty analyses. 
Appendix C 

C–6

Figure C–3. Test data and true stress-strain conversion for the 75 ksi steel. 

Figure C–4. Tabular true stress-strain constitutive model curve for the 75 ksi steel. 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–7

Figure C–5. Calculated tensile test response with necking for the 75 ksi steel. 

Figure C–6. Comparison of measured and calculated engineering 

stress-strain curves for the 75 ksi steel. 

The resulting true stress-true strain behavior incorporated into the constitutive model is representative of 

the steels tested in this project. However, there were multiple sources (suppliers) of steel used in the 

construction of the WTC towers. Project 3 developed synthetic stress-strain curves for each grade of steel 

based on several data sources. A comparison of this synthetic curve with the constitutive model behavior 

is shown in Fig. C–7. There are observable differences in the curves such as the representation of a yield 

point behavior in the constitutive model. However, the differences are not sufficiently large to produce a 

large variation in the calculated structural impact behavior. In addition, the effect of material strength 

variation on impact damage will be assessed in the uncertainty analyses. 

The above procedure was applied to develop constitutive models for all of the WTC tower steels for 

which test data was provided by Project 3. 
Appendix C 

C–8

Figure C–7. Comparison of the constitutive model and synthetic 

steel behaviors for the 75 ksi steel. 

Summary of Steel Constitutive Properties 

A summary of the true stress-strain curves used in the constitutive models for the various WTC tower 

steels are summarized in Fig. C–8. Similarly a comparison of the true stress-strain constitutive curves 

with the synthetic stress-strain curves developed by Project 3 for the various exterior column steels is 

shown in Figures C–9 and C–10, and for core column steels in Fig. C–11. In general, the true stressstrain curves developed from the test data and the synthetic curves developed from multiple sources agree 

to within approximately 10 percent. This variation in measured and expected material strengths will be 

used as part of the future uncertainty analyses. 

Strain Rate Effects in Steel Constitutive Models 

Strain rate effects on the steel yield strength are included in the constitutive model for tower steels with 

the Cowper and Symonds rate effect model. The functional form for the rate effects on strength is 

governed by the equation: 









































 +=

 





•

•

p

y y C

/1

1 ε

σεσ (5)

Where σ y is the yield strength, ε& is the strain rate, and C and p are the Cowper and Symonds 

parameters. 
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C–9

Figure C–8. Tabular true stress-strain constitutive model curves. 

Figure C–9. Comparison of the constitutive model and synthetic steel 

behaviors for the higher strength perimeter steel curves. 
Appendix C 

C–10

Figure C–10. Comparison of the constitutive model and synthetic steel behaviors 

for the lower strength perimeter steel curves. 

Figure C–11. Comparison of the constitutive model and synthetic steel behaviors 

for the core steel curves. 
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C–11

A series of high-rate characterization tests was performed on tower steels by Project 3. In addition to 

quasi-static tests (performed at a rate below 0.001 s-1), a series of high rate tests was performed primarily 

at strain rates between 100 s-1 and 1000 s-1. The Cowper and Symonds model parameters C and p were 

then fit to the test data and provided in the following functional form for a strain rate in s-1 and a yield 

strength in ksi: 

• Log(C) = -7.55 + 0.324σy-0.00153( σy)

2

• p = 6.7824 

The specific values used for each of the different tower steels are summarized in Table C–1. The 36 ksi 

and 42 ksi steels are materials used in the core columns and the remaining steels are used in the exterior 

columns. The resulting rate effects used in the constitutive modeling of tower steel based on Equation (5) 

are compared to the measured high-rate test data for the 50 ksi, 75 ksi, and 100 ksi tower steels in 

Fig. C–12. The comparison shows that the Cowper and Symonds model is capable of reproducing the 

rate effects for the range of data available. 

Table C–1. Material constitutive parameter table–WTC tower steels. 

Material 

Yield 

Specification 

Young’s 

Modulus Poisson’s Ratio 

Strain-Rate 

Coefficient (C) 

Strain-Rate 

Exponent (p) 

Brick Element 

Failure Strain 

36 ksi 29,700 ksi 0.288 7.900e+01 6.782 0.32 

42 ksi 29,700 ksi 0.288 1.360e+05 6.782 0.32 

50 ksi 29,700 ksi 0.288 4.220e+06 6.782 0.50 

55 ksi 29,700 ksi 0.288 6.700e+06 6.782 0.64 

60 ksi 29,700 ksi 0.288 3.950e+07 6.782 0.56 

65 ksi 29,700 ksi 0.288 1.270e+08 6.782 0.51 

70 ksi 29,700 ksi 0.288 1.270e+08 6.782 0.62 

75 ksi 29,700 ksi 0.288 6.710e+08 6.782 0.56 

80 ksi 29,700 ksi 0.288 2.440e+09 6.782 0.49 

100 ksi 29,700 ksi 0.288 3.430e+09 6.782 0.53 

Test results indicated that the influence of strain rate on the ductility of the tower steels did not follow a 

consistent trend. Several grades of steel had an increased ductility at high rates (more common for low 

strength steels), some had a reduced ductility at high rates (more common for high strength steels), and 

some showed no significant effect of strain rate on ductility. The approach used in the constitutive 

modeling is to ignore the changes in ductility produced by elevated strain rates. The effects of the 

variation of ductility over the expected range of strain rates will be assessed as part of the uncertainty 

analyses. 
Appendix C 

C–12

Figure C–12. Comparison of rate effects model and test data. 

C.2.2 Failure Models 

A challenge for calculating the aircraft impact response and damage to the WTC towers is the wide range 

of failure mechanisms that occur in both the aircraft and tower structures. These failures result from large 

scale deformations of the materials as well as from exceeding the strength of bolted, welded, and riveted 

connections. At the connection locations, complex behaviors are influenced by variations in material 

cross sectional geometry and material properties that produce large stress and strain concentrations. 

The overall approach applied to modeling the impact damage and failure of the structures is to use 

engineering damage mechanics within the framework of the LS-DYNA finite element analyses. This 

analysis methodology is also referred to as local damage modeling and has been applied successfully in a 

wide variety of applications (e.g., Mudry 1985; Beremin 1983; Simons et al., 1999). Detailed fracture 

mechanics analysis of all the failures that occurred is beyond the scope of this project. The damage is 

calculated within each element in the impact simulations and damage development is based on local 

(element) quantities including plastic strain and stress state. When the specified failure criterion in an 

element is exceeded, the element is eroded (deleted) from the calculation. The erosion of elements allows 

for the propagation of failure through a structure. 

Calculation of the failure of structural components is further complicated by the scale of the global impact 

analyses. Local damage modeling is often applied to the analysis of smaller components and failure 

initiation. In these applications, a relatively fine mesh can be applied, and damage regions around a local 

fracture can be resolved on a fine scale. For the global impact of an aircraft into a WTC tower, element 

sizes will have to be on average a few in. to maintain a model size at approximately 2 million to 3 million 

elements. At this resolution, the gradients around a fracture cannot be accurately resolved, and the 

damage criteria need to be adjusted to obtain the appropriate strength and ductility of the structures. In 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–13

the following subsections, various local damage modeling techniques applied to estimate failure of the 

aircraft and tower structures are described. 

Mesh Refinement Effects 

One of the significant modeling issues for the analysis of aircraft impact damage and failure is the effect 

of mesh refinement. The global impact analyses of the aircraft impacts into the WTC towers are very 

large analyses of complex structures and will require the refinement of the model to be reduced 

significantly from the detailed component analyses. As the mesh refinement is reduced, it is important to 

ensure that the damage mechanisms and extent of impact damage are properly captured. 

A preliminary example of the effects of mesh refinement on the response was introduced earlier in the 

analysis of the material tests on the WTC tower steels. Figure C–1 showed both a fine mesh and a coarser 

mesh (referred to as medium mesh) version of the ASTM 370 rectangular tensile specimen for plate 

material. A comparison of the calculated necking behavior for two 75 ksi specimens immediately 

preceding failure is shown in Fig. C–13. The fine mesh is able to better resolve the strain gradients in the 

necking region and as a result has higher peak strain values at the same level of specimen displacement. 

The effects of further reductions in mesh refinement required for global impact analyses can be 

demonstrated with the ASTM 370 tensile test example used in this section. The extreme limit for a coarse 

model of the tensile specimen would use a single shell element to model the entire specimen gage section. 

An example of coarse shell element model of the tensile specimen is shown in Fig. C–14. This coarse 

tensile specimen model would have a single stress and a single strain value for the gage section and does 

not have sufficient independent degrees of freedom to model the localization that occurs during necking. 

As a result, the critical plastic strain criterion for this single shell element model would be approximately 

equal to the engineering failure strain of 0.20 (see Fig. C–6). 

The calculated engineering stress-strain behavior obtained with the three different mesh resolutions is 

shown in Fig. C–15. In these calculations, a maximum plastic strain criterion was used and the critical 

failure strain was shifted until each calculation failed at a value matching the average measured 

engineering failure strain. The corresponding critical plastic strains were 1.00 for the fine mesh 

resolution, 0.56 for the medium mesh resolution, and 0.18 for the coarse mesh resolution (shell element 

model). 

The medium mesh resolution used in the above analysis of the tensile test corresponds to the mesh 

resolution applied in the exterior column component impact analyses described in Section C.4. 

Therefore, the critical failure strain of 0.56 would be carried forward to the detailed component analyses. 

The coarser shell element subassembly and global impact models are typically developed with shell 

elements and a resolution that might correspond to the coarse model shown in Fig. C–14. Therefore, the 

measured engineering elongation of the specimens is used for the critical strain. Additional modifications 

for regions with stress concentrations, such as along welds, is described later in this section. 
Appendix C 

C–14

(a) Fine mesh necking behavior (red = 75 percent plastic strain) 

(b) Medium mesh necking behavior (red = 50 percent plastic strain) 

Figure C–13. Calculated necking response in the 75 ksi tensile specimen. 

Figure C–14. Coarse shell element mesh for the 75 ksi tensile specimen. 
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C–15

Figure C–15. Mesh refinement effects in the calculated 75 ksi tensile test. 

Stress Triaxiality Effects 

Using a local damage approach to model fracture with a damage model incorporating stress triaxiality 

effects has been successful in predicting the response and failure of welded steel structures to dynamic 

loads (Kirkpatrick, Giovanola, and Simons 1996). The strength of welded steel structures subjected to 

dynamic loads can be strongly influenced by the weld strengths and connection details. The following 

describes this approach to modeling fracture for impact damage to the WTC towers. 

The damage analyses use a local damage approach within a finite element code to simulate the 

development of damage as the structure deforms. The fracture model is a simple form of a ductile 

fracture criterion (Mudry 1985). It assumes that failure of a material location occurs when the damage 

within a surrounding microstructural characteristic volume, VMIC, exceeds a critical value. 

Mathematically, the damage function in the failure criterion can be written in the form: 

D = 

⌡



⌠ dε p

eq

εc(σmean/σeq)

 = 1 over VMIC ≈ (RMIC)3 (6)

where: 

D = normalized damage parameter 

dε p

eq = increment in plastic strain 

εc(σmean/σeq) = critical failure strain as a function of the stress triaxiality 

The stress triaxiality is defined as the ratio of the mean stress to the equivalent stress. 
Appendix C 

C–16

This critical strain function can be determined by a series of notched tensile tests with specimens of 

varying notch radii (e.g., Mackenzie, Hancock, and Brown 1977). VMIC is a characteristic volume of the 

material, which can be interpreted as the critical microstructural process zone. In turn, RMIC, the 

representative linear dimension of the volume VMIC, can be associated with microstructural dimensions 

such as grain size or spacing of the microvoid nucleating inclusions. RMIC is therefore a constant length 

dimension that can be used to introduce a scaling effect in the fracture simulations. In its present form, 

the model does not account for a possible strain rate sensitivity of the damage growth and has been 

calibrated using static data for the failure strain as a function of stress triaxiality. 

The damage function in Equation (6) provides the failure criterion based on the accumulation of damage 

over the history of the material. However, this damage law requires an appropriate definition of a critical 

failure strain function. In this type of model, the performance of the damage law is typically better if the 

form of the damage law has a physical basis from observations of the microstructural failure processes. 

Previous studies have shown that the rate of void growth in the ductile fracture process is an exponential 

function of the stress triaxiality (Rice and Tracey 1969) in the form: 

dR

Ro = 0.28 dεeq exp[1.5(σmean/σeq)] (7)

This type of void growth law can be used to develop a critical strain function of the form: 

εc(σmean/σeq) = α 















)]/ emean q (5.1exp[

0.1

σσ - εshift (8)

where α and εshift are constants. The critical strain function in Equation (8), combined with the damage 

law in Equation (6), form the basis of the local fracture model. 

The effect of stress triaxiality on damage development was measured by Project 3 for the core column 

steel materials using the specimen geometry shown in Fig. C–16. Only the core column materials were 

selected since the thickness of the components was sufficiently large to fabricate the notched round bar 

specimens. A series of 12 tests were performed using two different steel strengths and three different 

notch radii with a repeat test for each configuration. A summary of the notched round bar tests is given in 

Table C–2. 

To determine the triaxiality in the three different notched round bar specimen geometries, models were 

developed of the specimens and the tests were analyzed. The models for the three different specimen 

geometries are shown in Fig. C–17. The models had a fine mesh (average element size of approximately 

0.008 in.) capable of resolving the gradients in stress and strain across the specimen. It was found that the 

triaxiality and damage across the gage section is relatively uniform with a maximum along the axis of the 

specimen (failure is expected to initiate at the center of the specimen and propagate to the edges). 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–17

Figure C–16. Notched round bar specimen dimensions. 

Table C–2. Summary of notched round bar tensile tests. 

Specimen Number 

Gage Dia. 

(in.) 

Specimen 

Length (in.) 

Notch 

Radius (in.) 

Failure Dia. 

(in.) 

Failure 

Strain 

Stress 

Triaxiality 

B6152-1-F1-1-1-05 0.2519 3.005 0.245 0.18646 0.2598 0.7 

B6152-1-F1-1-1-06 0.2488 3.008 0.25 0.19223 0.2274 0.7 

B6152-1-F1-1-2-01 0.2526 3.007 0.065 0.21466 0.1502 1.2 

B6152-1-F1-1-2-02 0.2518 3.003 0.065 0.20518 0.1851 1.2 

B6152-1-F1-1-2-03 0.2513 3.005 0.148 0.18571 0.2610 0.9 

B6152-1-F1-1-2-04 0.2507 3.006 0.125 0.19837 0.2087 0.9 

C71-1-F1-07 0.2506 3.005 0.245 0.19835 0.2085 0.7 

C71-1-F1-08 0.2519 3.004 0.249 0.20325 0.1931 0.7 

C71-1-F1-09 0.2487 3.005 0.125 0.21888 0.1199 0.9 

C71-1-F1-10 0.2519 3.01 0.13 0.21818 0.1339 0.9 

C71-1-F1-11 0.2514 3.005 0.064 0.23301 0.0732 1.2 

C71-1-F1-12 0.2525 3.005 0.063 0.23919 0.0527 1.2 
Appendix C 

C–18

Figure C–17. Notched round bar tensile specimen models. 

The calculated triaxiality at the specimen center as a function of damage, parameter D in Equation (6), for 

the three different notched round bar specimens is shown in Fig. C–18. The calculated responses for all 

three specimens show an initial spike in triaxiality and then settle to a constant stress triaxiality over a 

wide range of specimen deformation. The average value of triaxiality from these calculations is 

summarized in the last column of Table C–2. 

Figure C–18. Calculated notched round bar stress triaxiality. 
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C–19

A summary of the WTC steel notched round bar tests is shown in Fig. C–19. In addition, the figure 

shows failure criteria fit to the notched round bar data using Equation (8). For the WTC steels of interest, 

the presence of sulfide inclusions can be used to justify that the nucleation of voids occurs nearly 

instantaneously and εshift can be set to zero. The figure compares the C71-1-F1 tests with the failure 

criterion using a coefficient value α=0.5. Similarly the tests with the B6152-1-F1-1 steel (36 ksi yield) 

are compared to the failure criterion using a coefficient value α=0.90. Although neither set of material 

test is in perfect agreement with the failure criteria, the overall trends of the aggregate data set match and 

suggest that the form of the failure criteria is appropriate for the steels. 

Figure C–19. Calculated critical plastic strain as a function of 

notched stress triaxiality. 

A user-defined constitutive and failure model was developed and incorporated into LS-DYNA in this 

project. The model is identical to the piecewise linear constitutive model applied previously for the 

analysis of the WTC tower steels with the addition of the above stress triaxiality damage function 

incorporated. The simulations of the B6152 steel tensile and notched round bar tests were repeated with 

the user-defined constitutive model. The load-displacement relationships were compared to the previous 

analyses to validate the stress-strain relationships in the user-defined constitutive model. The calculated 

failure of the specimens is compared to the corresponding failure criteria in Fig. C–20. The agreement of 

the failure strains is a validation of the damage algorithm within the user-defined constitutive model. 

In the following subsection, the user defined constitutive and damage model is applied to investigate 

failure of components in the WTC tower structure. A region of the WTC towers where the stress 

triaxiality is expected to be relatively high and play a role in the failure strength is the welded joints in the 

exterior column flanges and webs. This welded connection is analyzed below. 
Appendix C 

C–20

Figure C–20. Simulation of the notched and smooth round bar tests. 

Weld Zone Constitutive Modeling 

Photographs of the WTC towers immediately after impact, and inspection of the recovered exterior wall 

panels in the impact zone, have shown that failure along the weld zone was a characteristic feature of the 

impact damage. An example exterior column from the impact zone that has significant fractures along 

the weld zone for the outer web is shown in Fig. C–21. The amount of energy dissipated by these weld 

fractures is very small compared to the overall impact energy. As a result, the failure of the welds will 

have little effect on the subsequent damage to the interior structures and contents of the towers. However, 

to capture the damage mechanisms of the impacted exterior columns and to develop models and failure 

criteria for the global impact analysis, a failure model for the weld zone is required. 

Modeling of the constitutive behavior for the weld and heat affected zone (HAZ) material is a challenging 

task due to the lack of significant material testing in these regions. To date, the data obtained on the weld 

properties consist of a micrographic characterization of an exterior column weld with microhardness 

indentation tests performed at various locations across the weld geometry. The specific weld geometry 

and microhardness characterization locations are shown in Fig. C–22. The corresponding hardness 

measurements across the base, HAZ, and weld materials are shown in Fig. C–23. 
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C–21

Figure C–21. Photograph of an exterior column with weld zone fractures. 

Figure C–22. Micrograph of an exterior column weld geometry. 
Appendix C 

C–22

150

200

250

300

H

ard

n

e

s

s, H

V

HAZ Weld

500 g

200 g

Base

150

200

250

300

H

ard

n

e

s

s, H

V

HAZ Weld

500 g

200 g

Base

Figure C–23. Microhardness characterization of the weld and HAZ materials. 

The microhardness measurements shown in Fig. C–23 were used to develop approximate plasticity 

behaviors for the weld and HAZ material regions. The approach used was to use the base material 

constitutive behavior and shift the flow stress of the HAZ and weld materials by 12 percent and 

18 percent, respectively. These shifts correspond to the relative magnitude of the average measured 

hardness in each material region. 

Analyses were performed to investigate the weld failure behavior using a detailed model of a simplified 

two-dimensional (2-D) weldment geometry representative of an exterior column side flanges and welded 

outer web as shown in Fig. C–24. A short duration pressure load is applied across the front web to 

introduce a dynamic loadcondition on the welded connection. The boundary conditions were 

approximated by fixing the displacements at the top and bottom of the side plates, seen in Fig. C–24 (b). 

The entire model is constrained to a plane strain condition. The weldment model includes weld and HAZ 

regions as shown in Fig. C–24 (c). 

The base metal for the weldment is the 55 ksi steel. An approximate damage criterion was developed for 

the steel based on the results of the tensile tests on the material. The criterion is that of Equation (8) with 

a coefficient value α=0.92. This value was obtained by using an approximate value for the average 

triaxiality of the tensile specimen and the failure strain obtained from the previous analyses of the tensile 

tests described earlier in Section C.2.1. The estimate of the average triaxiality is approximate since the 

triaxiality is changing in the region of the necking prior to failure. The user-defined constitutive model 

with this estimated damage model was used to simulate the tensile test. The comparison to the data and 

previous analyses is shown in Fig. C–25. The approach underestimated the engineering failure strain by 

approximately 10 percent. 

0 0.2 0.4 0.6 

Distance (in)
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–23

Weld analysis model

(a) Cross section of an exterior column 

(b) Weld analysis model 

(c) Weld region detail 

Figure C–24. 2-D weldment model developed for analysis of failure behavior. 
Appendix C 

C–24

Figure C–25. Simulation of the 55 ksi steel tensile test. 

Two different mesh refinements were developed for the weldment model as shown in Fig. C–26. The 

model developed using the fine mesh has 10,240 solid brick elements, as shown in Fig. C–26 (a). The 

fine mesh model should have sufficient resolution to capture the gradients in stress and strain around the 

weld and HAZ. The model developed using the medium mesh has 702 solid brick elements, as shown in 

Fig. C–26 (b). This model was developed with a mesh refinement that could be applied to a threedimensional analysis of a column for component impact analyses. 

 

 (a) Fine mesh resolution (b) Medium mesh resolution 

Figure C–26. Mesh resolutions used in the 2-D weldment failure model. 
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C–25

An example fracture analysis of the weldment model using the fine mesh model is shown in Fig. C–27. 

The loading was provided by a rectangular pressure pulse with 0.10 ms duration and amplitude of 38 ksi 

(262 MPa). The failure initiates at the toe of the weld and propagates through the heat affected zone and 

outer web plate. This is the most common weld failure mode observed in the recovered WTC exterior 

columns. The pressure load results in a downward velocity of the front web. The motion at the ends of 

the web is restrained by the welds at the flanges and a moving plastic hinge behavior develops in the web. 

The deformations result in bending and tension at the weld location and eventually the local deformations 

initiate a fracture at the toe of the weld that propagates through the HAZ and web base metal. The details 

of the calculated fracture behavior are shown in Fig. C–28. 

(a) Initiation of fracture 

(b) Complete fracture through the web 

Figure C–27. Calculated weldment deformations and failure. 

 

 (a) Initiation of fracture (b) Complete fracture through the web 

Figure C–28. Detail of the calculated weldment failure behavior. 
Appendix C 

C–26

As described in previous analyses, the application of a coarser mesh resolution does not resolve the same 

gradients in stress and strain and the failure criteria need to be adjusted. The failure criteria parameters 

used in the two analyses are given in Table C–3. The comparison of the fracture behavior for the two 

mesh resolutions is shown in Fig. C–29. Similarly the kinetic and internal energies for the two 

calculations are compared in Fig. C–30. The comparisons show that both models have the same 

deformation and failure modes at very similar energy levels. 

Table C–3. 2-D weldment model comparison. 

Mesh Refinement Fine Medium 

Element class Brick Brick 

Base metal failure coefficient (α) 1.35 0.76 

HAZ metal failure coefficient (α) 0.81 0.45 

Weld metal failure coefficient (α) 1.10 0.62 

 

 (a) Fine mesh resolution (b) Medium mesh resolution 

Figure C–29. Mesh refinement effects on calculated weldment failure behavior. 

The remaining stage in developing the weldment failure model is to perform three-dimensional 

component impact analyses of the exterior column and develop a coarser shell element description of the 

weld region that can be applied in the global aircraft impact analyses. The problem analyzed is the drop 

test configuration shown in Fig. C–31. The drop test configuration shown has a 550 lb steel impactor 

with an impact velocity of 37.4 mph. The impactor is 12 in. wide and 5 in. across. The nose of the 

impactor has a reduced area that is 12 in. wide and 2 in. across with a one-half in. radius rounded edge 

around the impact face. The length of the column section represents a portion of an exterior column 

between spandrels. 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–27

Figure C–30. Calculated energy balance for the 2-D weldment models. 

Figure C–31. Drop test model for column weld fracture analysis. 

The two different column and weld models applied are shown in Fig. C–32. The medium mesh resolution 

was previously applied in the 2–D weldment fracture analyses. The half symmetry medium mesh column 

model contained 63,680 brick elements. The coarse shell element model has significantly fewer elements 

with approximately 4 in. elements to define the column and 1 in. wide elements in the weld zone. The 
Appendix C 

C–28

half symmetry coarse shell element column model contained 144 linear shell elements. Obviously, the 

coarse shell element model is not capable of capturing the stress and strain gradients to the same extent as 

the brick element model. As a result, the shell element model applies the simpler critical plastic strain 

criterion available in the default piecewise linear constitutive model. 

 (a) Medium mesh resolution (b) Coarse mesh resolution 

Figure C–32. Models developed for column weld fracture analysis. 

The drop test was simulated first using the medium resolution brick element model. The results of the 

calculated impact response and failure were then used to develop an appropriate failure strain for the shell 

element model weld zone. The strain profile calculated in elements along the upper surface of the web in 

the weld fracture region is plotted in Fig. C–33. The figure shows a strong gradient in the calculated 

strains with the peak strains of approximately 15 percent plastic strain near the toe of the weld. A 

corresponding critical strain of 8 percent was selected for the corresponding one-in.-wide single shell 

element weld zone. This strain is indicated by the red line in Fig. C–33. 

The comparison of the resulting impact behavior for the two models is shown in Fig. C–34. Both models 

have similar impact deformations and the lengths of the weld failures are in good agreement. 

These analyses have been used to develop improved engineering fracture criteria in the weld region. 

There is still significant uncertainty in the details of the weld geometry and damage development in the 

weld region with variations in material properties and stress distributions. However, reasonable bounds 

on the effects of these variations will be assessed later in the uncertainty analyses. 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–29

Figure C–33. Calculated strain profile for the weld zone. 

(a) Medium resolution brick element model 

(b) Coarse resolution shell element model 

Figure C–34. Calculated drop test fracture behavior. 
Appendix C 

C–30

Bolt Material Constitutive Modeling 

The primary bolts of interest for the impact analysis are those used at the connections between the 

exterior columns on the WTC towers. Within the impact zone, the connections are typically made using 

0.875 in. in diameter steel bolts grade A325. Initially, there was no test data available that could be 

applied to develop a bolt model. The modeling approach was to develop a brick element bolt model and 

use it to develop a corresponding beam element bolt model for the majority of the impact analyses. A 

description of these bolt analyses are given in Section C.4.2, along with the component analyses. 

Subsequently, a series of tests was performed by Project 3 on bolts recovered from the WTC towers. A 

summary of the bolt testing is given in Fig. C–35. The bolts were found to yield at a load of 

approximately 50 kip and have an ultimate failure load of approximately 68 kip. The measured 

elongation at failure is approximately 0.18 in. 

Figure C–35. Measured bolt load-displacement behavior. 

The beam element model for the bolt, described in Section C.4.2, was compared to the bolt test data. The 

comparison showed good agreement in the strength of the bolt, but it also indicated that the beam model 

overestimated the ductility. This may be a result of not capturing the details of the stress concentrations 

in the region of the threaded connection and nut. The bolt test data was used to correct the ductility of the 

beam element bolt model and the resulting comparison of the model and data is shown in Fig. C–36. The 

bolt model shows a bilinear elastic-plastic behavior that is stiffer in the elastic region and yields at a 

higher stress level than the data. The inability of the simplified model to capture stress gradients in the 

regions of the bolt head, threads, and nut may cause this type of response. However, the overall strength 

and ductility of the model and test data as well as the strain energy capacity agree reasonably well that 

further model development was not required. 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–31

Figure C–36. Comparison of the measured and calculated bolt behavior. 

C.2.3 Concrete Constitutive Models 

There are several concrete models in LS-DYNA. Each has different capabilities for modeling rate effects 

and nonlinearity associated with damage and failure behavior. Potential concrete models in LS-DYNA 

are Types 5 (soil and crushable/non-crushable foam model), 16 (pseudo-tensor concrete model), 

25 (kinematic hardening cap model), 78 (soil and concrete model), and 111 (Holmquist-Johnson-Cook 

concrete model). An important factor in determining the behavior of a concrete structure in compression 

or bending is its lateral confinement. The concrete floor slabs in the WTC towers were not highly 

confined so a material model suitable for this loading condition is needed. 

In this study, the ability to accurately model the low confinement damage and softening behavior is 

important. Damage caused by cracking in the concrete degrades the strength in the low confinement 

regime. Inclusion of this damage growth provides a more accurate representation of the stress-strain 

response. Based on this capability, the LS-DYNA material Type 16 (pseudo-tensor concrete model) is 

selected for modeling the concrete floor slabs. This model also accounts for the high strain-rate 

sensitivity of reinforced concrete. 

As implemented in LS-DYNA, the pseudo-tensor model can be operated in two major modes: (1) a 

simple tabular pressure-dependent yield surface, and (2) a model with two pressure-dependent yield 

functions and a damage-dependent function to migrate between curves. The first option is well suited for 

implementing standard geologic material behaviors such as a Mohr–Coulomb yield surface with a Tresca 

limit and has been used successfully for the analysis of ground shock and soil-structure interactions under 

high confinement. The second option, applied here, allows for implementation of tensile failure and 

damage scaling, which are more dominant material behaviors at low confinement. 
Appendix C 

C–32

The pseudo-tensor model, as applied to the analysis of the lightweight concrete in the WTC towers, has 

two pressure-dependent yield functions. By defining suitable yield functions for the undamaged and fully 

damaged concrete and an appropriate tabular interpolation between the curves, the behavior of the 

damage under low confinement can be properly captured. 

Material constitutive parameters for the pseudo-tensor model were developed for a 3 ksi compressive 

strength lightweight concrete. A simulation was performed of a standard unconfined concrete 

compression test to check the constitutive model behavior. The simulated behavior of the concrete 

specimen is shown in Fig. C–37. The calculated compressive stress-strain response is compared to 

measured compression data for 2.3 ksi and 3.8 ksi strength concretes in Fig. C–38 (Wischers 1978). 

Currently, the same material parameters are being used for the concrete in both the core (normal weight 

concrete) and truss floor (lightweight concrete) areas. 

Initial Configuration 2 % Compression 

Figure C–37. Finite element analysis of the unconfined compression test. 

C.2.4 Aircraft Constitutive Models 

No material testing was performed to characterize the structural materials in the aircraft or develop the 

constitutive model parameters for these materials. Therefore, the constitutive and failure properties for 

the aircraft materials were developed from data available in the open literature. The principal sources of 

data for the airframe materials are the Military Handbook (MIL-HDBK-5F), 1987 and Aerospace 

Structural Metals Handbook (Brown et al. 1991). Additional sources of data are used to verify and 

supplement the information obtained from these primary data sources. 

Complete engineering stress-strain curves were provided in the MIL-HDBK-5F for various 2024 and 

7075 aluminum alloys that are commonly used in the construction of the 767 airframe structures. These 
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Figure C–38. Comparison of the calculated unconfined 

compression behavior with concrete compression test data. 

curves were digitized for the various 2024 and 7075 alloys as shown in Fig. C–39 and Fig. C–40, 

respectively. Representative stress-strain curves were then converted into true stress and true strain as 

described earlier and used to develop tabular curves for constitutive models. The calculated true stressstrain curves and tabular constitutive model fits are shown in Fig. C–41 and Fig. C–42, respectively. 

C.3 DEVELOPMENT OF AIRCRAFT MODEL 

Development of the model of the Boeing 767-200ER aircraft is being accomplished through a three-step 

process. These are data collection, data interpretation and engineering analysis, and finally meshing of 

the structure. Data collection is nearing completion, and additional efforts will mainly focus on final 

details of non-structural contents and material properties for primary structural parts. Reviewing, 

organizing, and analyzing the data as well as meshing of the structure are also at the final stages. 

C.3.1 Aircraft Data Collection 

Data collection for the Boeing 767-200ER is nearing completion. Significant information on the aircraft 

structure and contents has been gathered from (1) documentary aircraft structural information, and 

(2) data from measurements on Boeing 767 aircraft. 

C.3.2 Description of the Aircraft Model 

The model for the Boeing 767-200ER is nearing completion. Certain components, such as the PW4000 

engine shown in Fig. C–43, have been completed. Construction of the wings is in the final stage with 

only the inboard flaps and ailerons left to be modeled. Work has begun on the fuselage as well. Some 

details of the airframe model are shown in Fig. C–46 later. The LS-DYNA model of the aircraft is 

generated and meshed using the TrueGrid software (TrueGrid Manual 2001). 
Appendix C 

C–34

Figure C–39. Digitized engineering stress-strain curves 

for various 2024 aluminum alloys. 

Figure C–40. Digitized engineering stress-strain curves 

for various 7075 aluminum alloys. 
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Figure C–41. True stress-strain curves developed for 

various aircraft aluminum alloys. 

Figure C–42. Tabular true stress-strain curves developed 

for various aircraft aluminum alloys. 
Appendix C 

C–36

Figure C–43. Pratt and Whitney PW4000 turbofan engine. 

Engine Model Development 

The Pratt and Whitney PW4000 turbofan engine has a very complex structure as shown in Fig. C–43. 

The engine is an important component of the aircraft with the potential to produce significant impact 

damage to the WTC tower structures (e.g., fail core columns). As a result, special care was given to the 

development of the engine model to include all the details of the engine construction. 

To develop a structural model of the engine, the primary structural components in the engine were 

identified and approximated with simplified geometry as illustrated in Fig. C–44. Known engine 

dimensions were used to determine the scale factor for the drawing. The simplified geometry of the 

engine structures could then be captured using a common digitization procedure. Once the engine’s 

internal geometry was captured, the digitized geometry was imported into TrueGrid and used to generate 

surface definitions and part geometries for the engine model. The engine model was developed primarily 

with shell elements. The objective was to develop a mesh with typical element dimensions between one 

and two in. However, smaller element dimensions were required at many locations to capture details of 

the engine geometry. Brick elements were used for some of the thicker hubs and the roots of the 

compressor blades. The various components of the resulting engine model are shown in Fig. C–45. A 

summary of the elements used in the engine model is given in Table C–4. 
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Figure C–44. PW4000 engine cross sectional geometry and simplification. 

Figure C–45. Pratt and Whitney PW4000 turbofan engine model. 
Appendix C 

C–38

Table C–4. Engine model parameters. 

PW4000 Engine Model 

Number of brick elements 9,560 

Number of shell elements 54,788 

Total nodes 101,822 

Preliminary engine model weight 7,873 lb (3,571 kg) 

Adjusted engine model weight 9,447 lb (4,285 kg) 

After the known structural components of the engine were included in the engine model, the weight of the 

model was calculated at 7,873 lb (3,571 kg). The dry weight of the PW4000 engine is listed at 9,400 lb. 

The difference in weight potentially results from the nonstructural components (tubing, pumps, seals, 

bearings, etc) that were not captured in the model. To account for the difference, the density of all of the 

material models used for engine components was increased by 20 percent. This effectively smears the 

missing mass in proportion to the original mass distribution in the model. The resulting adjusted engine 

model mass is 9,447 lb (4,285 kg). 

Information available from the Aviation Safety Network (http://aviation-safety.net/) indicates that 

American Airlines Flight 11 was powered by two General Electric CF6-80A2 engines, while United 

Airlines Flight 175 was powered by two Pratt and Whitney JT9D–7R4D engines. Review of these 

engines indicates that the PW4000 turbofan engine is very similar to the General Electric CF6-80A2 and 

the PW JT9D-7R4D engines. Comparisons of specific physical characteristics of the engines are given in 

Table C–5. In fact, the JT9D-7R4 and PW4000-94 are almost identical as they are in the same family of 

Pratt and Whitney aircraft engines. The PW4000 was labeled the “new technology JT9D” when it began 

replacing the latter engine on 767s built after 1987 where the PW4000-94 is 5.8 percent heavier than the 

JT9D-7R4 but produces up to 10 percent more thrust. Aside from an additional set of long stator blades 

and elongated exit nozzle, the CF6-80C2 is also of similar weight and dimensions to the PW4000. Due to 

these similarities, the PW4000 engine model will be used for all impact simulations. 

Table C–5. Boeing 767 engine comparison. 

Engine 

Pratt and Whitney 

PW4000-94 

Pratt and Whitney 

JT9D-7R4a, b

General Electric 

CF6-80C2c, d

Fan blade diameter 94 (in.) 94 (in.) 93 (in.)e

Length 153 (in.) 153 (in.) 161–168 (in.)f

Dry weight 9,400 (lb) 8,885 (lb) 9135–9860 (lb) 

a. Reference value of 106 in. also found––may include cowling. 

b. The “tail” of the CF6-80C2 is much longer than the PW4000. This potentially accounts for the additional 15 in. 

in length. 

c. The CF6-80C2 has an additional set of long stator blades for the excess fan air that is not present in the 

PW4000. 

d. The second stage compressor blades in the CF6-80C2 are closer to the central shaft than the PW4000 and do not 

appear to have counter weights. 

e. The JT9D-7R4 and PW4000-94 are almost identical: (1) They are in the same family of Pratt & Whitney 

aircraft engines, and (2) the PW4000 was labeled the “new technology JT9D” when it began replacing the latter 

engine on 767s built after 1987. 

f. The PW4000-94 is 5.8 percent heavier than the JT9D-7R4, but produces up to 10 percent more thrust. 
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Airframe Model Development 

All significant structural components are being included in the airframe model of the Boeing 767-200ER. 

Figure C–46 shows the status of the initial version of the complete aircraft model. Detailed models of the 

empennage and landing gears, not shown in this figure, are shown in Figs. C–47 and C–48, respectively. 

Ribs, spars, rudder, and elevator have all been modeled in detail in the empennage. Tires and hubs, the 

main strut and truck, and support bracing have all been included in the landing gear model. The 

underside of the airframe in the model is shown in Fig. C–49 illustrating the position of the retracted main 

landing gear in the wheel well. 

The models of the fuselage, empennage, and wing structures are developed using shell elements. The 

model is being developed in parameterized form where the mesh resolution is determined by a single 

element characteristic size parameter. This approach was selected early in the development to allow 

flexibility in the model size and resolution as the model development and impact analyses progressed. 

The objective was to develop a mesh with typical element dimensions between one and two in. for small 

components, such as spar or rib flanges, and element dimensions of 3 in. to 4 in. for large parts such as 

the wing or fuselage skin. A summary of the current model size is shown in Table C–6. 

Figure C–50 shows the entire wing structure modeled to-date, including the center wing which attaches 

the port and starboard outboard wings. The wing stringers were not explicitly modeled to help reduce the 

size of the model. The stringers have a z-section geometry with typical dimensions of approximately one 

in. flanges and a two in. web with a thickness of approximately 1/8 in. These stringers run spanwise over 

the top and bottom of the wing ribs. In the model, an ‘effective’ wing skin has been used to account for 

the weight and strength of the riveted skin/stringer construction. 

Wing Section Component Model Development 

Two wing segment models have been developed to perform the component level and subassembly level 

analyses. The smaller segment, for component analyses, is shown in Fig. C–51. The large wing segment 

for subassembly analyses is shown in Fig. C–52. The wing section components are modeled with shell 

elements. The main spars, wing ribs, leading edge ribs, nosebeams, leading edge slats, and outboard flaps 

have been included in the model. Nonstructural components, such as hydraulic lines, and mechanical 

components, such as slat actuators, are not included in the model geometry. The density of the spars, ribs 

and other structural components have been increased to account for this nonstructural mass. 
Appendix C 

C–40

Figure C–46. Finite element model of the Boeing 767-200ER (under construction). 
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(a) Top View 

(b) Side View (c) Oblique View 

Figure C–47. Complete empennage for the 767 aircraft model. 

Figure C–48. Retracted landing gear components for the 767 aircraft model. 
Appendix C 

C–42

Figure C–49. Underside of the 767 airframe model (skin removed) 

showing the position of retracted main landing gear. 

Table C–6. Summary of Boeing 767-200 aircraft model 

size (under construction). 

Number of Elements 

Number of brick elements 70,000 

Number of shell elements 562,000 

Total nodes 740,000 

(a) Complete wing model 

 

(b) Center wing structures 

Figure C–50. Complete wing structures for the 767 aircraft model. 
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 (a) Small Wing Section Model (b) Internal Structure (skin not shown) 

Figure C–51. Small wing section model for component-level analyses. 

(a) Large wing section model 

(b) Wing section internal structures (skin not shown). 

Figure C–52. Wing section model developed for the 

subassembly impact simulations. 
Appendix C 

C–44

C.3.3 Analysis of Fuel Distribution at Impact 

An important factor for determining impact damage and subsequent fire initiation is the distribution of the 

fuel in the aircraft at the time of impact. Both United Airlines and American Airlines have provided 

estimates for the quantity and distribution of fuel for UAL Flight 175 and AA Flight 11 at the time of 

impact.1,2 UAL estimates that Flight 175 contained approximately 62,000 lb or 9,118 gal of fuel at impact 

with the “fuel evenly distributed between both main tanks.”1

 American Airlines estimates that Flight 11 

contained 66,081 lb or 9,717 gal of fuel at impact and “the fuel was evenly distributed between left and 

right wing tanks of the aircraft.”2

Fuel tank locations and capacities for the Boeing 767 are shown in Fig. C–53. The Boeing 767 uses an 

integral fuel tank where the wing skin, ribs, and spars serve as the fuel tank. There are three classes of 

fuel tanks onboard the 767-200ER, a main tank, a surge tank, and auxiliary tanks. The auxiliary tanks 

consist of port, starboard, and center fuel tanks. All tanks are shown for the port wing in Fig. C–54 along 

with the associated internal structures. The main tank is from rib 3 to rib 31, the port and starboard 

auxiliary tanks are from the inboard closure rib to rib 3. The center auxiliary tank is between the port and 

starboard closure ribs and the surge tank is from rib 31 to rib 34. A dry bay is located above the engine at 

the forward part of the main tank between ribs 6 and 9. Baffle ribs are located at rib 5 and 18. 

Figure C–53. Flammable material locations in a Boeing 767 aircraft (www.boeing.com). 

 

1

 Communication between United Airlines and NIST, September 5, 2003, “NIST WTC Flammable Contents Request.” 2

 Communication between American Airlines and NIST, August 12, 2003, “In re September 11 Litigation C&F Ref.: 

DTB/MH28079.” 
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Figure C–54. Layout of fuel tanks in the Boeing 767 wing. 

The functionality of the wing fuel tanks is such that a typical wing rib allows some fuel to flow along the 

wing but acts as a two-way fuel baffle to minimize fuel slosh. However, there are special rib designs that 

alter the position and flow of fuel within the tank. The ribs in the dry bay region, between ribs 6 and 9, 

include a fuel barrier running parallel to the rear wing spar. In addition, baffle ribs (ribs 5 and 18) include 

a series of fuel dams that act as a one-way valve allowing fuel flow inboard toward the sump areas (low 

point of tank). According to the statements from both airlines regarding fuel distribution, it is most likely 

that the surge tanks and all auxiliary tanks were dry at the time of impact. 

Overall tank dimensions and geometry were estimated using the dimensions and approximated geometry 

shown in Fig. C–55. As shown in Fig. C–56, the front spar height is a good approximation for fuel depth 

in a full wing section. Using these approximate dimensions, the fuel tank capacity as a function of the 

distance along the wing buttock line (see Fig. C–57) was calculated. The maximum capacity of each 

main tank was calculated to be approximately 6,500 gal. The actual main tank capacity is 6,070 gal 

(Fig. C–53) so the calculated fuel capacity distribution was modified to match this maximum value, as 

shown in Fig. C–57. Notice that the main tank capacity inboard of baffle rib 18 is approximately the 

same volume as the fuel onboard each aircraft at the time of impact. 

The exact location and distribution of fuel at the time of impact is complicated by the flight conditions 

prior to and at the time of impact. The terrorist pilots likely performed extreme flight maneuvers prior to 

impact causing most of the fuel to flow inboard. Extreme banking maneuvers with inappropriate trim 

could cause the fuel to flow inboard quickly. High loads on the wings due to the extreme flight regime at 

the time of impact would also cause fuel to flow inboard by increasing the dihedral angle of the wing. 

Since the baffle ribs restrict fuel from flowing outboard, it is reasonable to assume that all fuel that could 

flow inboard was actually inboard at the time of impact. 
Appendix C 

C–46

Figure C–55. Approximate fuel tank dimensions (in.). 

Figure C–56. Wing cross sections at various rib locations. 
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Figure C–57. Fuel tank capacity. 

For simplicity, it was assumed that all fuel has moved inboard at the time of impact. Since the fuel tank 

capacity at the outboard baffle rib and the fuel onboard are approximately the same, a good first estimate 

is that the main tanks are full inboard of baffle rib 18 at the time of impact. A small amount of fuel is 

outboard of this rib for AA flight 11. This is shown graphically in Fig. C–58 for smooth and level flight 

with an undeformed wing shape. A full wing out to baffle rib 18 and dry outboard of this rib is taken as 

the nominal case in subsequent analyses. 

C.4 COMPONENT-LEVEL ANALYSES 

The primary objectives of component modeling are to (1) develop understanding of the interactive failure 

phenomenon of the aircraft and tower components and (2) develop the simulation techniques required for 

the global analysis of the aircraft impacts into the WTC towers. The approach taken for component 

modeling is to start with finely meshed, brick element models of key components of the tower structure 

and progress to relatively coarsely meshed beam and shell element representations that will be used for 

the subassembly and global models. This is done to develop reduced finite element models appropriate 

for high fidelity global impact analyses, as modeling each component with fine details would be too 

demanding from a computational standpoint and an inefficient use of resources. 
Appendix C 

C–48

Figure C–58. Approximate fuel locations for smooth and level flight. 

In addition to determining the optimal element size and type for global modeling, other key technical 

areas are addressed in the component modeling phase of the program. These issues include material 

constitutive modeling, treatment of connections, and modeling of aircraft fuel. The following component 

modeling scenarios were outlined at the start of the project: 

• An exterior column impacted by an aircraft engine 

• An interior column impacted by an aircraft engine 

• An exterior column impacted by an aircraft wing segment with and without fuel 

In addition to the above component impact analysis scenarios, a range of additional component analyses 

were identified that were considered important and helpful in developing the global impact models and 

analysis methods. These additional component analysis scenarios included: 

• Bolted column end-plate connections with approximated dynamic loading 

• Bolted spandrel connections with approximated dynamic loading 

• Floor system with concrete slab impacted by an engine 

The approach has deviated somewhat compared to the tasks above to maximize efficiency and to produce 

the most meaningful structural loading scenarios. Once preliminary calculations were performed it was 

found, for example, that the load generated by an impacting engine would totally overwhelm a single 

interior or exterior column. Simplified models based on this severe loading would match the detailed 

brick model but the subtle response from lesser loading might not be as accurate. To capture the more 

subtle response in the column components, wing sections with and without fuel were studied as impactors 

instead of engines. It was found that the empty wing section impact produced damage to the exterior 

columns that is near the failure threshold. Similarly, a fuel-filled wing section impacting both wide flange 
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and box type core column resulted in damage near the failure threshold. As a result, these impact 

scenarios were used primarily for the exterior and core column component impact analyses. 

Included in this appendix are the impact analyses for exterior columns, core columns, bolted column 

connections, bolted spandrel connections, and floor systems. 

C.4.1 Analysis Methodology 

The impact analyses were performed using the LS-DYNA finite element code (LS-DYNA 2003). 

LS-DYNA is a commercially available nonlinear explicit finite element code for the dynamic analysis of 

structures (LSTC 2003). The initial foundation of LS-DYNA was the public domain DYNA3D finite 

element code developed at the Lawrence Livermore National Laboratory. Since 1987, the code has been 

extensively developed and supported by the Livermore Software Technology Corporation (LSTC) and is 

used for a wide variety of crash, blast, and impact applications. 

LS-DYNA has several unique capabilities for this project such as Arbitrary-Lagrangian-Eulerian (ALE) 

and Smooth Particle Hydrodynamics (SPH) algorithms that can be applied to the analyses of fluidstructure interaction and large-scale fracture and fragmentation of structures. These capabilities are 

critical for the analyses of the fuel tank breakup and dispersion of fuel inside the towers upon impact. 

The fuel and debris dispersion is crucial for assessing the impact loads inside the tower structures and the 

corresponding damage to the mechanical systems. 

The impact analyses described in this report use a variety of capabilities and algorithms in LS-DYNA. A 

brief description of these capabilities is described in this section. A significantly greater detailed 

description of the analysis methods is provided in the LS-DYNA Theoretical Manual (1998). 

The fine mesh detailed component analyses typically use 8-node solid hexahedron (brick) elements with 

single point integration. This is the most commonly used solid element type in LS-DYNA due to its 

computational efficiency. The biggest disadvantage of the single point integration is the potential for 

hourglassing or zero energy modes. There are several methodologies for controlling hourglass modes in 

LS-DYNA. The typical approach used in these impact analyses is to apply a viscous hourglass control 

where a viscous damping is introduced that suppresses the formation of hourglass modes but does not 

significantly influence the global modes. 

The component impact analyses using solid elements typically have a fine mesh. As a result, damage and 

failure are included strictly through the constitutive algorithms. Damage criteria (such as maximum 

plastic strain) are tracked for each element within the constitutive model evaluation, and elements are 

eroded when the failure criteria are exceeded. This allows for a direct evaluation of damage and failure 

within the impact simulations. 

The eroded elements allow for the initiation and extension of fracture in the model. Eroded elements no 

longer support any stress, and the strains in the eroded elements are no longer calculated. The associated 

mass of the elements remains with the nodes in the calculation. If adjacent elements have not reached the 

failure surface, the nodes remain attached to the structure. If all of the elements connected to a specific 

node have failed, the node becomes a free particle. Free nodes can either be eliminated from the 

calculation or remain in the calculation with associated inertial properties and potential for impacts 

against other structural components (free nodes remain in contact algorithms). 
Appendix C 

C–50

As the mesh refinement and model size are reduced, the components are typically modeled using 

Belytschko-Lin-Tsay shell elements. This is a four node shell element with single point integration. The 

Belytschko-Lin-Tsay element is a computationally efficient alternative to the Hughes-Liu element in 

LS-DYNA and again is the most widely used shell element formulation within LS-DYNA for crash, 

impact, and metal forming applications. Results generated with the Belytschko-Lin-Tsay element 

typically agree with those using the Hughes-Liu element. As used in the solid elements, the most 

common approach to introducing damage and failure for the shell elements is through the constitutive 

models and element erosion. 

In specific applications, unique algorithms are required to introduce failure modes in the analysis. An 

example is the interface between the skin and internal frame structures of the aircraft. Rivets are used for 

the primary connection between the airframe and skin. The approach used to model this connection and 

failure during the impact event is the tied interface with failure. In this approach, interface segments 

(shell elements) are constrained to move together until a failure criterion is exceeded. The failure 

criterion is a quadratic combination of the normal and shear failure stresses at the interface. After failure, 

the segments are allowed to move independently but not allowed to penetrate each other (typical contact 

algorithm behavior). 

Overall contact in the impact analyses is modeled using the automatic single surface contact algorithm in 

LS-DYNA. Interacting components are defined by a material list, and contact segments are automatically 

generated by LS-DYNA. This greatly simplifies the specification of contact between various components 

in the aircraft and tower structures. The type 1 soft constraint option is used in the contact algorithm that 

determines the contact stiffness based on stability considerations, time step size, and nodal mass. This 

soft constraint option was found to be more robust than the default penalty formulation for modeling the 

complex contact behaviors in large impact and crash simulations. 

C.4.2 Exterior Column Impact Analyses 

Various exterior column component impact analyses were performed with different objectives. The 

preliminary exterior column component impact analyses were performed on a single column using a 

highly refined mesh of brick elements. These analyses were used to investigate the details of the column 

response and develop analysis techniques that can be applied to the subassembly and global impact 

analyses. These preliminary exterior column impact analyses used a simplified wing section impactor 

that was developed prior to gathering detailed structural information of the Boeing 767 wing design. 

Subsequent exterior column impact analyses were performed using less refined models of the exterior 

columns in full panel configurations impacted by detailed wing section component models with and 

without fuel. The primary objective of these component analyses was to study the impact response of the 

aircraft wing structures and investigate various modeling techniques for including aircraft fuel in the 

analyses. 

Figure C–59 shows the calculated response of a single exterior column component, impacted by an empty 

wing section. The figure compares the column damage calculated with two different models of very 

different resolutions. The column model on the left is a fine mesh made of brick elements and the column 

on the right is a coarser mesh of shell elements. The failure strains of the coarse model were estimated by 

calibrating the response of the coarse model against the refined model (see Section C.2.2). Contours of 
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resultant displacements are shown on the column components. It can be seen that the overall response, 

both in column and wing segment damage, is very similar. These analyses were used to develop the 

coarse column models used in subsequent analyses. 

 

(a) Fine brick element column (b) Coarse shell element column 

Figure C–59. Exterior column response comparison, showing contours of the 

displacement magnitude (in.). 

Empty Wing Section Impact Analysis 

Figure C–60 shows the impact of the small empty wing section into two exterior wall panels at a speed of 

500 mph (223 m/s). These analyses use a wing section model with significantly improved structural 

fidelity over the preliminary wing segment model used in the column component impact analyses 

described above. The development of the wing section component model used here was described 

previously in Section C.3.2. The wing section model shown in Fig. C–60 consists of approximately 

24,000 shell elements. 

The model for the two exterior panels is made primarily of shell elements with a medium mesh resolution. 

The exceptions are the butt plates, which were made of solid brick elements and the bolted butt plate 

connections using beam element bolts, as described later. The boundary conditions at the ends of the 

exterior columns are a bolted connection to an adjacent butt plate with constrained displacements at the 

edges. In addition, the displacements in the direction of the impact were constrained on the spandrel 

plates at the location of the floor slab. The resulting model for the exterior panel had 54,096 shell 

elements (columns and spandrels), 2,112 solid brick elements (24 butt plates), and 48 beam elements for 

the bolts. 
Appendix C 

C–52

 (a) Impact Configuration (b) Calculated Response 

Figure C–60. Calculated impact damage with the empty short wing 

section component model. 

The calculated impact response produces large scale damage and fragmentation of the empty wing section 

and significant damage to the exterior columns. The calculated impact damage to the exterior column 

panel is shown in Fig. C–61. The damage includes significant distortion of the columns, large plastic 

strains, and fracture of the plate connections within the columns. However, the columns are not 

completely severed and still maintain some load carrying capacity. 

Figure C–61. Damage produced by the empty wing section impact. 
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Wing Section with Fuel Impact Analysis 

A significant portion of the weight of a Boeing 767 wing is from the fuel in its integral fuel tanks. At the 

time of impact, it is estimated that each aircraft had approximately 10,000 gal of fuel on board. Upon 

impact, this fuel is responsible for large distributed loads on the exterior columns of the WTC towers and 

subsequently on interior structures, as it flows into the building. It therefore could have a significant 

effect on the damage done to the building structure. Accurate modeling of the fluid-structure interaction 

is needed to accurately predict the extent of this damage and the fuel dispersion within the building to 

help establish the initial conditions for the fire dynamics modeling. 

A number of approaches to solving Fluid-Structure Interaction (FSI) problems are available in 

LS-DYNA. One approach is the standard Lagrangian finite element analysis with erosion, where the fuel 

is modeled using a solid mechanics approach. This approach accounts for the inertial effects of the fuel, 

but does not simulate the fuel flow during impact. The ALE method has been developed as one good 

approach to solve fluid and solid material interaction. With this methodology, fluids are modeled with a 

fixed Eulerian mesh, which allows for materials to flow between mesh elements. Solid materials are 

modeled with a moving Lagrangian mesh. With ALE, both mesh types can interact. An alternative 

approach is to use mesh-free methods such as SPH. SPH modeling for fuel effects has the advantage of a 

smaller mesh size and potentially much faster run times than ALE analyses. Both methods will be 

applied to the analysis of fuel impact and dispersion. 

The small wing segment was used for performing component level analyses of the wing with fuel. The 

small wing segment is from rib 14 to rib 18, the outboard baffle rib. For this location, the segment was 

considered to be completely full of fuel. Figure C–62 shows the fuel-filled wing section model with an 

SPH mesh for the fuel, shown in blue. The wing section model is identical to that used in Fig. C–60, but 

with the addition of 4,400 SPH fuel particles. 

Figure C–62. SPH fuel in the small wing segment. 
Appendix C 

C–54

For the wing section with fuel component analyses, the impacted structures are the same two exterior 

panels as used in the empty wing section analysis above. As shown in Fig. C–63, the columns of the 

exterior panels are completely destroyed due to impact. Both the SPH and ALE methods simulate this 

extensive damage. This was not the case for the empty wing section model, as discussed in the previous 

section. 

(a) Prior to impact (b) After impact 

Figure C–63. Exterior panels impact behavior for a wing 

segment with fuel (SPH fuel model). 

Figures C–64 through C–67 show the fuel dispersion and wing break up predicted by the two fuel 

modeling methods (ALE and SPH). As most clearly shown in the side view, the current SPH modeling 

method predicts greater fuel dispersion and wing break up than from using ALE. Current efforts are 

underway to study the effect of mesh density on these results. Both analysis methods calculate an impact 

load sufficiently large to easily rupture the exterior columns. 

Run-times from these component analyses clearly indicate that the SPH method will be more practical for 

the global impact analyses. The current SPH model runs significantly faster than the ALE method as it 

requires a smaller mesh and does not need to rezone after each time step, as is done in the ALE method. 

In addition, the ALE method requires a mesh for both the fuel region and the void zone into which the 

fuel can flow. In the example shown, this required the addition of approximately 110,000 solid elements 

for the ALE analysis. These preliminary calculations indicate that the ALE analysis run-times are as 

much as 10 times longer than those for the SPH analyses. However, additional work is underway to 
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C–55

determine the mesh refinement in each approach required to calculate the fuel impact load effects and 

dispersion with sufficient fidelity. 

Figure C–64. SPH Analysis of structural damage and fuel dispersion (top view). 

Figure C–65. ALE analysis of structural damage and fuel dispersion (top view). 
Appendix C 

C–56

Figure C–66. SPH analysis of structural damage and fuel dispersion (side view). 

Figure C–67. ALE analysis of structural damage and fuel dispersion (side view). 
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Bolted Connection Modeling 

There are a wide variety of different connections required for the assembly of the towers and aircraft. 

Wherever possible, evidence gathered from WTC steel will be applied to determine the importance of 

including connection details in the model and failure modes of those connections. For example, 

photographic and structural debris evidence clearly demonstrated that the external column connections 

played a significant role in the mode of column failure and extent of the external damage. 

Two connections are of particular interest to the impact analysis of the aircraft into the WTC towers and 

are modeled in this section. These include the bolted connections at exterior column butt plates and the 

bolted spandrel connections. The objective of connection component analyses is to develop connection 

models for the global impact analyses that accurately capture the capacity and failure modes of the 

connection. Various connection component models developed include both fine models of these complex 

connection components (e.g., 3-D brick element models of bolts), to simple models such as beam element 

bolt models and tied constraints with failure. The constitutive behavior of the beam element bolt was 

described previously in Section C.2.1. A tied constraint with failure requires that two nodes or a node and 

a surface segment (shell element or solid element face) have tied degrees of freedom until a failure 

criterion is exceeded. 

The coarse component models utilize many of the simplified connection and element types available in 

LS-DYNA to approximate the behavior observed in the fine models. For example, in the exterior column 

bolted connections, the bolts were modeled with elastic-plastic beam elements calibrated to match the fine 

model. Bolted joints, such as in the spandrel connections, were approximated with tied node algorithm 

that constrains degrees of freedom of adjacent nodes and element faces. Various options are investigated 

in these component analyses and final selections of the modeling methodology are based on both the 

fidelity and efficiency of the modeling approaches. 

Component modeling of the exterior column butt plate connections has been completed. As shown in 

Fig. C–68 the detailed model includes individual bolts modeled with solid brick elements. The simplified 

model uses coarse brick butt plates joined by beam element representations of the bolts. A dynamic 

analysis was carried out to calibrate the beam element bolt model. The loading condition was a dynamic 

separation of the two butt plates. The velocity profile used to separate the butt plates was obtained from a 

preliminary engine impact analysis against the exterior wall similar to those described in the next 

subsection. The profile is a linearly increasing separation velocity between the butt plates with an initial 

velocity of zero and a velocity of 43 ft/s (13 m/s) at a time of 5.0 ms. 

Failure strain in the beam models was calibrated such that the beam bolts failed at the same time as the 

brick element bolts. Failure of the bolts occurs at a time of approximately 3.0 ms. These connection 

models were used in the corresponding brick and shell models of the exterior column component impact 

analyses shown previously in Fig. C–59. Connection failure at the column ends was quite similar in both 

cases. 

The spandrel connections consist of an overlapping splice plate across a spandrel joint with a row of bolts 

on either side of the joint. Typical failure of these spandrel joints in the impact zone resulted from bolt 

bearing shear failures, typically in the spandrel plate, as shown in the photographs in Fig. C–69. A 

common configuration in the impact zone would have six bolts on either side of the joint. The bolt 

bearing shear failure mechanism would be difficult to model using a beam element bolt as applied 
Appendix C 

C–58

previously to the exterior column bolted connections. In the exterior columns, the bolts fail primarily in 

tension and details of the contact between bolts and butt plates are not as important for capturing the 

failure behavior. As a result, an alternate modeling approach was required for the spandrel splice failures. 

(a) Brick element bolts (b) Beam element bolts 

 (butt plates shown as transparent) 

Figure C–68. Exterior column end connection modeling. 

 

Figure C–69. Typical bolt bearing shear failures of spandrel connections. 

Spandrel connections are modeled using a splice plate made up of shell elements, as shown in 

Fig. C–70. Connections corresponding to individual bolts are treated by tying single nodes on the splice 

plates (center of green squares in Fig. C–70) to the spandrels. Two material definitions are used to make 

up the splice plate to allow for contact between the splice plates and the spandrels as well as having tied 

contact. The first splice plated material definition (shown in green in Fig. C–70) is used to allow the 

center node tied constraint (representing the bolt connection) to be aligned with the center of the spandrel 

plate. Since this material includes the tied constraint, it is not included in the automatic contact definition. 

The second material definition in the splice plate (shown in red in Fig. C–70) has a standoff distance 

equal to one half of the combined thickness of the spandrel and splice plate and is included in the 

automatic contact definition. Both material definitions have the same constitutive properties; however, 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

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using a single material definition would sometimes result in a numerical instability due to conflicting 

constraint and interface algorithms on those segments. The application of the spandrel splice plate 

connection model is demonstrated in the engine impact component analyses below. 

 (a) Detail of spandrel connection (b) Multiple panel connections 

Figure C–70. Bolted spandrel connection modeling. 

Engine Impact Analysis 

An example of an engine impacting an assembly of exterior panels is shown in Fig. C–71. The analysis 

includes an engine impacting an exterior panel of WTC 1 and is centered on panel 124 at floor 96 (the 

impact is centered on the middle spandrel). The exterior wall model does not contain any boundary 

conditions or components to represent the truss floor behind the panels. The initial engine speed was 

500 mph (223 m/s). In this example, a medium mesh density shell element panel was used. Columns 

were connected with beam element models of individual bolts. Spandrels were merged together in this 

model—splice plates were not used. Fixed butt plates were bolted at the free column end and no 

boundary conditions were applied to represent the floors. Velocity time-histories for representative 

engine materials are shown in Fig. C–72. The plot shows an overall reduction in speed of about 

13 percent after impact with the exterior wall. 

A second engine impact analysis was performed with similar conditions except the impact location was 

moved downward by one half floor to create an impact centered between spandrels. A comparison of the 

two calculated impact behaviors is shown in Fig. C–73. In both analyses the engine breaks through the 

exterior wall with relatively little breakup of the engine core. The impact centered between spandrels 

results in a reduction in the velocity of the engine core of 56 mph (25 m/s). 
Appendix C 

C–60

(a) Initial configuration (b) Impact response at 40 ms 

(c) Impact response at 80 ms 

Figure C–71. Example engine impact analysis with exterior columns. 
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Figure C–72. Engine velocity history for the exterior wall impact. 

(a) Spandrel centered impact (b) Between-spandrel impact 

Figure C–73. Example engine impact analysis with different impact locations. 

A final revision of the above analysis included adding splice plates at the spandrel connections as 

explained in the previous subsection. Figure C–74 presents a comparison of both models. The figure 

shows exterior wall damage as seen from the outside without engine components. Contours of plastic 

strain are shown in the plot with blue being zero strain and red being at the failure strain threshold 

(20 percent in this case). Material exceeding the maximum failure strain is eroded and no longer shown 
Appendix C 

C–62

in Fig. C–74. The analyses differ most in spandrel failure modes. Spandrels fail at the column 

connection in the merged spandrel case, while the connection fails in the splice plate case. In the latter 

case, a realistic bearing stress type tear-out mode is seen in the splice plate. Engine core velocities for the 

three engine impact analyses are compared in Fig. C–75 for a single representative engine component. 

The splice plate model resulted in a 74 mph (33 m/s) velocity reduction of the engine core. The splice 

model results in the largest reduction in velocity in the engine. 

 

(a) Merged spandrel analysis (b) Spliced spandrel analysis 

Figure C–74. Example engine impact analysis with different spandrel 

connection treatments. 

Figure C–75. Engine velocity history for the exterior wall impact. 
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The above comparison suggests that the splice plate has a relatively small influence on the exterior wall 

strength. The addition of the splice plate has approximately a 10 percent effect on the change in engine 

velocity during impact and penetration of the exterior wall. However, the spandrel connection model 

does not introduce a large computational cost and results in a more appropriate failure mode for the 

spandrels in the impact zone. Therefore, the spandrel splice connections will be maintained in subsequent 

impact analyses. 

C.4.3 Core Column Impact Analyses 

Engine Impact Analysis 

Preliminary analyses of an engine impacting a single core column indicated that the impact load was 

sufficient to penetrate and overwhelm the column. Subsequent analyses of an engine impacting core 

columns have included multiple impacts and will be reported later along with the subassembly analysis 

and the uncertainty analyses. 

Wing Section with Fuel Impact Analysis 

Component analyses for core columns impacted by fuel-filled wing sections were conducted for both 

wide flange and box type columns (Figs. C–76 through C–79). Similar to exterior column analyses, the 

primary purpose was to progress from the initial finely meshed brick element models to coarser shell 

element models. Figure C–76 (wide flange core column) and Fig. C–78 (box section core column) 

compare the fine brick model and coarser shell model response under the same loading conditions. 

Figures C–77 and C–79 show corresponding displacement and kinetic energy comparisons for, 

respectively, the wide flange and box section core column models. The figures indicate that the response 

of the coarser shell models is very similar to that of the fine brick models. Therefore, the shell element 

formulation and mesh refinement of the coarse model are sufficient to capture the impact damage 

mechanisms in this component impact scenario. 

C.4.4 Combined Engine Impact Analyses 

An example of engine impact analysis is presented in this section to demonstrate the damage response to 

both exterior and interior columns. The impact configuration is an engine impacting at 560 mph 

(250 m/s) against a set of exterior columns in a single exterior panel, an interior box column, and an 

interior wide flange column. The columns are modeled using shell elements. The spacing between 

exterior and core columns was reduced to shorten the run time necessary for the complete impact 

scenario. The simulation included three external panels stacked vertically such that the impacted column 

was bolted to additional panels both above and below. The core columns models were several floors tall 

to reduce the influence of the clamped boundary conditions at the ends. 
Appendix C 

C–64

 

(a) Fine brick element column (b) Coarse shell element column 

Figure C–76. Wide flange core column response comparison, showing contours of the 

displacement magnitude (in.). 

Figure C–77. Displacement and kinetic energy comparison for wide flange core column 

wing impact analysis. 
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C–65

 

(a) Fine brick element column (b) Coarse shell element column 

Figure C–78. Box core column response comparison, showing contours of the 

displacement magnitude (in.). 

Figure C–79. Displacement and kinetic energy comparison for box core column wing 

impact analysis. 

The impact scenario is illustrated in Fig. C–80 (a). The calculated impact damage is shown at a time of 

90 ms in Fig. C–80 (b). The calculated engine impact response completely fails all of the columns with a 

residual velocity of the engine of approximately 224 mph (100 m/s). The deformations of the column 

include large lateral displacements which would be significantly reduced if the constraint effects of the 

concrete floor slab were added. The deceleration profile of the major engine debris fragment is given in 
Appendix C 

C–66

(a) Initial configuration 

(b) Impact response at 90 ms 

Figure C–80. Example engine impact analysis with interior and exterior columns. 
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Fig. C–81. The majority of the engine structure has been broken into fragments by the combination of the 

three impacts. The resulting size, strength, and velocity of the engine debris are not likely to produce 

severe impact damage or failure to a subsequent core column. 

Figure C–81. Example engine impact analysis with interior and exterior columns. 

This example illustrates the large level of damage produced by a massive aircraft component, such as an 

engine, at high impact velocity. The impact energy is sufficient to overwhelm a single core column and, 

therefore, makes it difficult to determine the effects of column component modeling parameters on the 

impact response using this impact scenario. 

C.4.5 Floor Assembly Component Analyses 

An additional component model analyzed is the truss floor assembly. The failure and penetration of the 

floor structures is important for assessing the extent of damage and the spread of fuel and debris through 

the structure. The integrity of the floor structures could also be significant in the analysis of the 

subsequent fire behavior in the towers. Loading of the floor structure was achieved by direct impact by 

the engine and a simplified impactor. 

Component models of a section of the composite floor assembly outside the core were generated and used 

in various impact analyses. Initial floor component models used the detailed combination of brick, shell, 

and beam elements. Subsequent floor assembly models were less refined with shell and beam elements 

only. A comparison of the two floor system models and analyses is given in Table C–7. The 

modifications have reduced the size of the model by an order of magnitude and the run times by more 

than 80 percent. 
Appendix C 

C–68

Table C–7. Truss floor assembly component analyses comparison. 

Model Type Fine Brick Model Coarse Shell Model 

Number of beam elements 6,928 3,440 

Number of brick elements 230,778 0 

Number of shell elements 148,256 39,000 

Total nodes 372,084 48,971 

CPU time 16,796 s (4.7 h) 2,482 s (0.7 h) 

Elapsed time 26,553 s (7.3 h) 4,454 s (1.2 h) 

The impactor used in the initial component modeling is a simplified plow type impactor which promotes 

repeatable damage, not complicated by all the debris and randomness associated with an engine-floor 

impact. The weight of the plow impactor is comparable to an engine and the impact velocity was 

500 mph. An example of a plow impactor analysis with the fine mesh floor model is shown in Fig. C–82. 

The calculated impact damage with a corresponding shell element floor system model is shown in 

Fig. C–83. This component impact configuration is useful for comparing the differences in response with 

changes in the modeling methods or refinement. Additional simulations with an engine impactor are 

being performed to validate the modeling approach and will be reported at a later date. 

C.5 SUMMARY AND PRELIMINARY FINDINGS 

The objectives of this project are to calculate the aircraft impact response of the WTC towers, including 

damage to WTC tower structural systems, acceleration environment, and fuel and debris dispersion. The 

impact analyses are conducted at various levels, including: (1) the component level, (2) the subassembly 

level, and (3) the global level to estimate the probable damage to the towers due to aircraft impact. The 

analyses also include simplified and approximate methods. Analysis of uncertainties using the 

component, subassembly, global, and simplified analyses will also be conducted to assess the effect of 

variability associated with various parameters on the damage estimates. 

Significant progress has been made to identify the proper constitutive relationships, including high strainrate effects and failure criteria for the various materials included in the analysis of aircraft impacts into 

the WTC towers. These materials include steels used in the exterior walls and core columns of the 

towers, weldment, bolts, reinforced concrete, and aircraft materials. 

The development of the Boeing 767 aircraft model for impact analysis is nearing completion. The engine 

and wing models have been completed and are being used in the component and subassembly analyses. 

Work is underway to finalize the model of the fuselage, nose, tail, and nonstructural components of the 

aircraft. 

The WTC towers and Boeing 767 aircraft are extremely complex structural systems, and a large database 

of detailed structural information has been collected on both the towers and the aircraft. In the model 

development process, the objective was to include all of the structural components and details of both the 

aircraft and towers that would influence the impact response and damage. This approach results in very 

large models for the tower and aircraft. The application of the models in the component and subassembly 

analyses were used to determine model simplifications that can reduce the overall model size while 

maintaining fidelity in determination of the impact damage. 
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(a) Initial configuration 

(b) Impact response at 0.10 s 

Figure C–82. Floor assembly impact analysis with brick element concrete slab. 
Appendix C 

C–70

(a) Initial configuration 

(b) Impact response at 0.10 s 

Figure C–83. Floor assembly impact analysis with shell element concrete slab. 

A series of component impact analyses were performed using models of tower exterior and core columns 

with wing section and engine component models as impactors. These models are used to develop the 

simulation techniques required for the global analysis of the aircraft impacts into the WTC towers. The 

following results were obtained from the component impact analyses: 

• A 500 mph (223 m/s) engine impact against an exterior wall panel results in a penetration of 

the exterior wall and failure of impacted exterior columns. If the engine does not impact a 

floor slab, the majority of the engine core will remain intact through the exterior wall 

penetration with a reduction in velocity between 10 percent and 20 percent. The residual 

velocity and mass of the engine after penetration of the exterior wall is sufficient to fail a core 
Interim Report on Analysis of Aircraft Impact into the WTC Towers 

C–71

column in a direct impact condition. Interaction with additional interior building contents 

prior to impact or a misaligned impact against the core column could change this result. 

• A normal impact of the exterior wall by an empty wing segment from the wing tip region will 

produce significant damage to the exterior columns but not necessarily complete failure. This 

is consistent with photographs showing the exterior damage to the towers due to impact. 

Specific details of the damage will depend on details of the impact orientation and locations 

of internal wing components such as control surface actuators and arms. 

• A fuel-filled wing section impact results in extensive damage to the external including 

complete failure of the exterior columns. This is also consistent with photographs of the 

exterior damage. The resulting debris propagating into the building maintains the majority of 

the initial momentum prior to impact. 

• Three different numerical techniques were investigated for modeling impact effects and 

dispersion of fuel: (1) standard Lagrangian finite element analysis with erosion, (2) Smoothed 

Particle Hydrodynamics (SPH) analysis, and (3) Arbitrary-Lagrangian-Eulerian (ALE) 

analysis. Of these approaches, SPH analyses appear to offer the greatest potential for 

modeling fuel in the global impact analysis due to the combination of both computational 

efficiency and modeling fidelity. 

C.6 REFERENCES 

ASTM Designation A 370 - 03a. 2003. “Standard Test Methods and Definitions for Mechanical Testing 

of Steel Products,” Approved June 10. 

Brown, W. F. Jr., H. Mindlin, and C. Y. Ho. eds. 1991. Aerospace Structural Metals Handbook. 

CINDAS/Purdue University Publishers, vol. 3 & 4. 

Kirkpatrick, S. W., J. H. Giovanola, and J. W. Simons. 1996. DYNA3D analysis of dynamic fracture of 

weldments, IA’96, Proceedings of International Seminar on Quasi-Impulsive Analysis. 

November 20–22, Osaka, Japan, pp. A6.1-15. 

LS-DYNA Keyword User’s Manual. 2003. Livermore Software Technology Corporation, version 970. 

April. 

Mackenzie, A. C., J. W. Hancock, and D. K. Brown. 1977. On the influence of state of stress on ductile 

failure initiation in high-strength steels, Eng. Fracture Mechanics, 9, 167–188. 

Military Handbook. 1987. Metallic Materials and Elements for Aerospace Vehicle Structures. 

U.S. Department of Defense, MIL-HDBK-5F. 

Mudry, F. 1985. Methodology and applications of local criteria for prediction of ductile tearing, ElasticPlastic Fracture Mechanics. L.H. Larson, ed. ECSC, EEC, EAEC, Brussels and Luxembourg, 

Belgium, pp. 263-283. 
Appendix C 

C–72

Rice, J.R., and D.M. Tracey. 1969. On the ductile enlargement of voids in triaxial stress fields, J. Mech. 

Phys. Solids, 17, 201–217. 

Wischers, G. 1978. Application of effects of compressive loads on concrete, Betontech, nos. 2 and 3. 

Berlin. 

XYZ Scientific Applications, Inc. 2001. TrueGrid Manual, version 2.1.0. September. 
D–i

TABLE OF CONTENTS - Appendix D

List of Figures.......................................................................................................................................... D–ii

List of Tables ..........................................................................................................................................D–iii

Appendix D

Interim Report on Preliminary Stability Analysis of the WTC Towers .............................. D–1

D.1 Introduction................................................................................................................................. D–1

D.2 Model Description and Assumptions.......................................................................................... D–1

D.2.1 Steel Properties................................................................................................................ D–2

D.2.2 Boundary Conditions: Spring Supports........................................................................... D–2

D.2.3 Floor Systems .................................................................................................................. D–3

D.2.4 Applied Loads ................................................................................................................. D–3

D.3 Analysis Procedures.................................................................................................................... D–5

D.3.1 Staged Construction......................................................................................................... D–6

D.3.2 Eigenvalue Buckling Analysis ........................................................................................ D–7

D.3.3 Redistribution of Forces within the Core Areas .............................................................. D–8

D.3.4 Nonlinear Analysis with Plastic Hinges ........................................................................ D–10

D.4 Results....................................................................................................................................... D–11

D.4.1 Results of Linear Stability Analysis .............................................................................. D–11

D.4.2 Results of Redistribution of Forces within the Core Analysis ...................................... D–12

D.4.3 Results of Nonlinear Analysis ....................................................................................... D–14

D.5 Summary and Preliminary Findings ......................................................................................... D–19
D–ii

LIST OF FIGURES

Figure D–1. Plan of typical truss-framed floor with loading areas indicated. ........................................ D–4

Figure D–2. Plan of typical core column layout. .................................................................................... D–9

Figure D–3. North and south elevations of WTC 1 indicating columns and spandrels removed due 

to aircraft impact and fire effects...................................................................................... D–10

Figure D–4. Load vs. deformation in column 111 at floor 98 (north face, west side of damage). ....... D–16

Figure D–5. Load vs. deformation in column 145 at floor 95 (north face, east side of damage). ........ D–17

Figure D–6. Load vs. deformation in column 332 at floor 97 (south face, west side of damage)........ D–17

Figure D–7. Displacements and locations of plastic hinges in the north and south exterior walls of 

WTC 1 after impact. ......................................................................................................... D–19

Figure D–8. Displacements and locations of plastic hinges in the north and south exterior walls of 

WTC 1 after impact and fire............................................................................................. D–20
D–iii

LIST OF TABLES

Table D–1. Steel strength used in the reduced tower model.................................................................. D–3

Table D–2. Superimposed dead load and service live load on typical floor.......................................... D–5

Table D–3. Dead, superimposed dead, and live loads on mechanical floors......................................... D–5

Table D–4. Core columns removed from WTC 1, assumed destroyed by aircraft impact. ................... D–9

Table D–5. Ultimate tensile capacities of core column splices............................................................ D–13

Table D–6. Tensile loads on columns above damaged area, and column load to ultimate capacity 

ratios for Core Damage Case 1. ........................................................................................ D–14

Table D–7. Tensile loads on columns above damaged area, and L/C ratios indicating likely 

progression of splice failures for Core Damage Case 2. ................................................... D–15

Table D–8. Distribution of loads on exterior walls and core columns at floor 99 for Core Damage 

Case 2................................................................................................................................ D–18

Table D–9. Hinge states of members where plastic hinges formed during nonlinear analysis............ D–18
Appendix D 

D–iv

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D–1

Appendix D

INTERIM REPORT ON PRELIMINARY STABILITY ANALYSIS OF THE 

WTC TOWERS

D.1 INTRODUCTION 

The objective of this appendix is to present preliminary system stability analyses of the World Trade 

Center (WTC) towers to: (1) examine the overall stability of the towers when floors are removed; 

(2) study possible redistribution mechanisms when core columns are destroyed by aircraft impact; and 

(3) study the response of the tower when columns and spandrels in the exterior walls and columns in the 

core are destroyed by aircraft impact, and columns in the exterior are damaged due to the subsequent 

fires, as observed in photographs and videos of WTC 1. The analyses use a reduced and modified 

version of the global reference structural model of WTC 1 and the model of a typical truss-framed floor 

(floor 96 of WTC 1), both developed within the framework of Project 2 of the investigation using 

SAP2000, version 8 (see Appendix B). Although analyses are conducted using models of WTC 1, some 

of the results and findings apply to WTC 2 as well. 

The analyses use a staged construction technique to account for the sequential construction of the towers, 

especially in the zone of the hat trusses. Linear buckling analysis and nonlinear analysis with plastic 

hinges are used in the reduced global model of WTC 1 to study the effects of removal of, respectively, 

floors and damaged exterior and core columns, representing the effects of aircraft impact and subsequent 

fire effects. In addition, a linear analysis of the typical floor model is used to study the load redistribution 

mechanisms after losing columns in the core of the tower. 

Section D.2 presents a description of the reduced global model of WTC 1 used in this study, including the 

modifications that were made to the reference model. Section D.3 outlines the analysis procedures, 

including the staged construction methodology, the eigenvalue buckling analysis, the nonlinear analysis 

with plastic hinges, and the linear analysis of the typical floor model. The results of these analyses are 

presented in Section D.4, and Section D.5 presents a summary of the analysis and results. 

D.2 MODEL DESCRIPTION AND ASSUMPTIONS 

The models considered for the preliminary system stability analyses of the WTC towers were based on 

the reference structural analysis global model of WTC 1 and the typical truss-framed floor model 

developed by the firm of Leslie E. Robertson Associates, R.L.L.P. (LERA) under contract from NIST 

within the framework of Project 2. These reference models, developed using Computers and Structures, 

Inc.’s SAP2000 Software, Version 8, were reviewed and approved by National Institute of Standards and 

Technology (NIST) (see Appendix B). The reference global models are linear elastic, three-dimensional 

structural analysis models and include the 110-story above grade structure and 6-story below grade 

structure for each of the two towers. The original models use frame elements to represent the exterior 

columns and spandrels, the core columns, and the hat trusses. Each element in the models is assigned 

cross-sectional properties and steel strength according to the original design documents, as well as later 

modifications made to the towers. 
Appendix D 

D–2

A reduced version of the original WTC 1 global model was used in this project to assess typical behavior 

of the intact structure, as well as the performance of the damaged structure, due to aircraft impact and fire 

effects. The intent of the reduced model was to minimize the computational effort without a major 

sacrifice in model performance. The reduced model included the global model of the structure above 

floor 84 of WTC 1. The structure below was removed and replaced with equivalent springs as 

summarized in Section D.2.2. The modifications and loads applied to the model are summarized below. 

The model of a typical truss-framed floor (floor 96 of WTC 1) was used to study the load redistribution 

mechanisms inside the core upon losing core columns due to aircraft impact. The floor model contains all 

primary structural members of the floor system, including primary and bridging trusses, beams in the 

core, strap anchors, viscoelastic dampers, exterior and core columns above and below floor level, spandrel 

beams, and concrete slabs. The gravity loads applied to the model, including dead loads, superimposed 

dead loads, and service live loads are presented in Section D.2.4. 

D.2.1 Steel Properties 

The values of the yield and ultimate strengths of the structural steel used in the WTC 1 reduced tower 

model were set to match the room-temperature properties, which were determined by Project 3 of the 

NIST investigation, by replacing the nominal strength included in the reference models with actual 

strength values. Project 3, “Mechanical and Metallurgical Analysis of Structural Steel,” provided 

estimates for typical steel properties based on test results from a limited number of steel specimens from 

the towers, construction documents that indicate the occasional substitution of higher strength steels for 

lower strength steels, and historical data from steels of that era. These estimates differentiate among 

steels with the same designation from different manufacturers and from different areas of the buildings; in 

particular, steel in the exterior columns, core columns, and floor trusses each had slightly different 

properties for steels with the same designation. In cases where steel with a particular designation had 

different properties in the exterior and core columns, the properties associated with the steel in the 

exterior columns were used in the reduced global model. The steel properties used for the work reported 

herein are listed in Table D–1. 

D.2.2 Boundary Conditions: Spring Supports 

The reduced model of WTC 1 was supported by vertical springs assigned to each joint (core and exterior) 

at floor 84. The spring stiffness coefficients were obtained from a separate model of the tower below 

floor 84. For that purpose, a concentrated gravity load was applied to each column node at floor 84 of the 

tower model below floor 84, and the spring stiffnesses were estimated by dividing the applied load by the 

measured vertical displacement of each column at floor 84. At the bottom of the reduced model, each 

joint with an assigned vertical spring was restrained from horizontal translation and rotation about all 

three axes. These boundary conditions provided results that most closely matched those obtained from 

analyzing the whole tower (i.e., all 116 floors). 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–3

Table D–1. Steel strength used in the reduced tower model. 

Model Yield Strength Model Ultimate Strength 

Design Yield Strength 

ksi (MPa) 

Fy 

ksi (MPa)

Fu 

ksi (MPa)

36 (248.2) 35.6 (245.5) 61.2 (422.0) 

42 (289.6) 53.1 (366.1) 74.9 (516.4) 

45 (310.3) 53.1 (366.1) 74.9 (516.4) 

46 (317.2) 53.1 (366.1) 74.9 (516.4) 

50 (344.7) 54.0 (372.3) 75.6 (521.2) 

55 (379.2) 60.8 (419.2) 82.6 (569.5) 

60 (413.7) 62.0 (427.5) 87.3 (601.9) 

65 (448.2) 69.6 (479.9) 90.4 (623.3) 

70 (482.6) 76.7 (528.8) 92.0 (634.3) 

75 (517.1) 82.5 (568.8) 96.8 (667.4) 

80 (551.6) 91.5 (630.9) 99.4 (685.3) 

85 (586.1) 104.8 (722.6)a 116.0 (799.8)a

90 (620.5) 104.8 (722.6)a 116.0 (799.8)a

100 (689.5) 104.8 (722.6) 116.0 (799.8) 

a. The steel fabricator used steel with a nominal strength of 100 ksi in place of steels with specified 

strengths of 85 ksi and 90 ksi. 

D.2.3 Floor Systems 

Floor systems distribute gravity loads to the core and exterior columns. Actual member properties of the 

floor elements have relatively little effect on the towers’ stability, but would significantly increase model 

complexity and decrease its efficiency. To capture their effect, each floor of the tower in the reduced 

global model was modeled with a rigid diaphragm, except for floor 107 to the roof, which were modeled 

using flexible diaphragms as described in Appendix B. Rigid diaphragms constrain all nodes at a 

particular floor to move as a single unit. Flexible diaphragms differ only in the level of constraint. Both 

types of diaphragms do not affect the relative vertical displacements of the nodes. Modeling floors with 

diaphragms (rigid or flexible) ignores the floor’s capability of redistributing loads from column to column 

in a damaged case (particularly within the core columns). More detailed models developed within the 

framework of Project 2 for the aircraft impact analysis and Project 6 for the thermal-structural and 

collapse initiation analysis will address this issue. 

D.2.4 Applied Loads 

Gravity loads from the floor systems were applied as joint loads on columns in the model. They are 

comprised of: dead load (DL), which includes the self-weight of all structural members and the floor 

systems; superimposed dead load (SDL), which includes additional static dead loads such as partitions, 

fireproofing, ceiling systems, and floor coverings; and service live load (LL) which includes occupants 

and furnishings. Each load was applied to the model as a separate load case. Identical loads were used in 

the reduced global model of WTC 1 on floors 85 through 106. These floors were considered to be typical 

floors as described below. Loads varied on floors 107 through 110, and on the roof. The antenna load 
Appendix D 

D–4

(724 kips) was distributed among eight points near the center of the roof. These loads should be 

considered preliminary; NIST is working with LERA to further refine them. 

The loads were developed based on a realistic assessment of service loads and their distribution 

throughout the towers. The service live loads were assumed to be 25 percent of the design live loads. 

This parametric value can easily be varied in a sensitivity analysis. Actual self-weights were used for the 

dead loads, and additional loads on the mechanical floors were accounted for explicitly. 

The detailed model of a typical truss-framed floor, floor 96 of WTC 1 (see Appendix B), was used to 

determine the actual loads on floors 85 through 106 of the reduced global model. The detailed floor 

model contains four distinct areas, each with its own load, as shown in Fig. D–1. The superimposed dead 

and live loads applied to each of these areas were determined from the design documents and are listed in 

Table D–2. In the core, the design live loads varied from 40 psf (1.92 kN/m2

) to 100 psf (4.79 kN/m2) 

and were scaled to service live loads from 10 psf (0.479 kN/m2

) to 25 psf (1.20 kN/m2), while the 

superimposed dead load varied from 29 psf (1.39 kN/m2

) to 49 psf (2.35 kN/m2). The loads due to the 

slab, trusses, beams, columns, and spandrels were all calculated based on the actual weights of the 

members. The reactions at each column due to the loads and self-weight of this typical floor were 

calculated separately for the dead load and superimposed dead load cases. The weights of the columns 

and spandrels were then subtracted from the DL results to determine the load on each column from the 

floor system. These loads were then applied as point loads to the reduced global tower model. The loads 

due to the self-weight of the columns and spandrels were calculated by the analysis software (SAP2000) 

for the reduced global tower model. 

Figure D–1. Plan of typical truss-framed floor with 

loading areas indicated. 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–5

Table D–2. Superimposed dead load and service live load on typical floor. 

 Core 

Long One-way 

Slabs 

Short One-way 

Slabs Two-way Slabs 

Area [ft2 (m2)] 9274 (861.6) 13186 (1,225.1) 5228 (485.7) 12233 (1,136.5) 

Superimposed Dead Loads [psf ( kN/m2)] 

Mechanical & electrical 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 

Ceiling 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 

Floor covering 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 

Fireproofing 2.0 (0.096) 2.0 (0.096) 4.0 (0.192) 

Total SDL Varies 8.0 (0.383) 8.0 (0.383) 10.0 (0.479) 

Live Loads [psf ( kN/m2)] 

Service LL Varies 17.5 (0.838) 21.3 (1.020) 13.8 (0.661) 

For the mechanical floors (floors 107 through 110) and roof, the design DL, design SDL, and service LL 

values were determined from the design documents and information provided by LERA. These loads, 

which are listed in Table D–3, were applied as uniform loads to the typical floor model to estimate the 

corresponding column reactions. The column reactions were then applied as point loads on the reduced 

tower model. 

Table D–3. Dead, superimposed dead, and live loads on mechanical floors. 

Floor 107 108 109 110 Roof 

Dead Loads [psf (kN/m2)] 

Concrete slab 100.0 (4.788) 69.0 (3.304) 69.0 (3.304) 104.0 (4.980) 48.1 (2.304) 

Reinforcing steel 2.0 (0.096) 3.0 (0.144) 3.0 (0.144) 3.0 (0.144) 2.0 (0.096) 

Steel deck 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 2.0 (0.096) 

Structural steel 13.0 (0.622) 20.0 (0.958) 20.0 (0.958) 20.0 (0.958) (a) 

Total DL 117.0 (5.602) 94.0 (4.501) 94.0 (4.501) 129.0 (6.177) 52.1 (2.496) 

Superimposed Dead Loads [psf (kN/m2)] 

Partitions 12.0 (0.575) - - - - 

Ceiling 2.0 (0.096) 10.0 (0.479) 10.0 (0.479) - - 

Mech. & elec. 2.0 (0.096) 3.0 (0.144) 3.0 (0.144) 50.0 (2.394) 50.0 (2.394) 

Fireproofing 2.0 (0.096) 5.0 (0.239) 5.0 (0.239) 5.0 (0.239) 5.0 (0.239) 

Flooring 57.0 (2.729) 31.0 (1.484) 31.0 (1.484) - 5.0 (0.239) 

Total SDL 75.0 (3.591) 49.0 (2.346) 49.0 (2.346) 55.0 (2.633) 60.0 (2.873) 

Live Loads [psf (kN/m2)] 

Service LL 25.0 (1.197) 18.8 (0.898) 37.5 (1.796) 18.8 (0.898) 37.5 (1.796) 

a. The roof structural steel is explicitly included in the tower model. 

D.3 ANALYSIS PROCEDURES 

Both linear and nonlinear analyses were performed on the reduced global model of WTC 1 to examine the 

tower stability and assess how the tower responded to the representative impact and fire damage. Due to 
Appendix D 

D–6

the difference in stiffness between the core and the exterior columns, and the presence of the hat trusses, it 

was necessary to use nonlinear, staged construction to analyze the intact structure. Subsequently, two 

different series of analyses were performed independently. 

An eigenvalue-based buckling analysis was performed using the reduced global model of WTC 1 to 

determine the reserve capacity of the columns to buckling, and to determine how much the unsupported 

column length would need to increase, through floor-constraint removal, before the columns lacked any 

reserve capacity. 

A nonlinear analysis of the tower with damage to exterior walls and core columns was performed on the 

reduced global model of WTC 1 to determine if the tower could withstand that level of structural damage, 

and to assess the response of the tower when columns are lost due to aircraft impact and fire effects. 

In addition, an analysis was conducted of the typical truss-framed floor model to study the mechanism by 

which the floor loads were redistributed when the core columns were severed by aircraft impact. In this 

analysis, the core columns that were assumed to be missing were replaced by equivalent vertical springs, 

representing the stiffness of the hat trusses and columns between the affected floors and hat trusses. The 

following describes the details of the various analyses. 

D.3.1 Staged Construction 

From a linear analysis of the response of the intact WTC towers to gravity loads, it was determined that a 

simple linear analysis does not produce realistic stress distributions in the core and exterior columns. All 

loads in a linear model are applied instantaneously, which is not unreasonable for most structural models. 

Tall buildings sustain loads gradually, as the structure is built from the ground up, and any differential 

deformation is accounted for during construction. In addition, the hat trusses atop the tower were applied 

stress-free to the existing structure subjected to dead loads, but prior to the application of live loads. The 

linear analysis (without staged construction) of the tower models resulted in unrealistic, large forces and 

stresses in some hat truss members, connecting spandrels, and core columns within the hat trusses, due to 

differential settlement between the core and exterior columns in the model. 

A staged construction analysis of the towers eliminates these nonexistent, large stresses. This method 

more closely approximates the way in which the towers were constructed and the loads applied. The 

staged construction analysis had three stages. First, the floors below the hat truss (up to floor 106) 

constituted the initial model. The dead and superimposed dead loads were applied to these elements, and 

the model was analyzed. Second, the upper, remaining stories including the hat truss were added to the 

model. These newly added components did not initially have internal forces or stresses, even though the 

components added in the first stage were loaded and stressed. The remaining portions of the dead and 

superimposed dead loads were then added to these top floors, and the model was again analyzed; this 

analysis continued from the stress and strain state at the end of the first stage. In the third construction 

stage, the live and antenna loads were added to the entire model, and the analysis continued from the end 

of the second stage. This analysis method produced reasonable stresses in the hat truss region of the 

undamaged towers. 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–7

D.3.2 Eigenvalue Buckling Analysis 

The objective of this analysis was to assess the overall stability of one of the towers, namely WTC 1, 

under service loads, without any aircraft impact damage and subject to a progression of floor removal. 

Stability was measured through column buckling strength, which was reduced in each column as floors 

were removed in the model and column unbraced lengths were increased. Floor removal was modeled by 

removing the rigid diaphragm constraint at all columns, discussed in Section D.2.3, for that particular 

floor. Each node within that floor was then free to translate (e.g., buckle) in either lateral direction. The 

four columns above and below each removed floor were subdivided into sixteen segments per floor to 

achieve sufficient resolution for estimation of buckling loads. If instability was identified using a linear 

stability (eigenvalue buckling) analysis, the analysis was rerun after buckled columns were removed and 

their loads were redistributed to neighboring columns. This process continued until either the structure 

was stable or the progression of local instabilities indicated overall system instability. 

The buckling analysis began with the “removal” of floor 96. The analysis calculated the load factor 

(eigenvalue), λ, for the first buckled column. If λ was greater than one, all columns were stable under the 

given loading condition, which signifies system stability. If λ was less than one, a column had buckled 

under the applied loads. This column was identified by visually examining the buckled mode shape of the 

structure at the end of the analysis. Only the first buckling mode was considered in the analysis. 

Buckling of a single column might not result in a collapse of the tower due to the load-redistribution 

capability of the structure. To investigate overall stability, the buckled column was removed from the 

model above and below the removed floor(s). Any joint loads applied to a removed column were 

distributed to neighboring nodes. This eliminated any load carrying capacity of the failed column without 

eliminating its applied load, but rather redistributing it. The analysis was rerun, and the next buckled 

column was identified until λ was greater than one or until the progression indicated that a global 

instability had likely been attained. If the structure attained stability, floor “removal” progressed 

sequentially to floors 95, 97, 94, 98, etc. 

The linear bucking analysis in SAP2000 only provided the load factor, λL, for the linear combination of 

DL, SDL, service LL, and antenna loads, but without staged construction. Since staged construction was 

employed to best represent the application of these loads, λ must be obtained from a relationship with λL. 

The buckling load in the linear case (staged construction not used) is equal to the axial force in the critical 

(i.e., buckled) column, FL, times λL. Since the buckling load is the same in either the linear case or the 

staged construction case, the load factor is defined as: 

SC

LL

F

λ F λ = (1)

where FSC is the axial load in the same column from the staged construction analysis. 

The procedure described above was also performed on the undamaged WTC 1 reduced global model with 

a reduced modulus of elasticity (E’) applied to all core and exterior columns directly above and below 

removed floors. A value for E’ equal to 21,460 ksi (E’ = 0.74E), corresponding to a uniform column 

temperature of 600 oC, was used in the analysis. 
Appendix D 

D–8

D.3.3 Redistribution of Forces within the Core Areas 

Analyses of the global models of the towers indicated that when columns are severed in the exterior walls, 

the walls can redistribute their load through the vierendeel action of the wall above the severed columns. 

However, when columns are severed in the core, the possible load redistribution mechanisms include: 

(1) load redistribution to neighboring core columns through the nonlinear, large deflection, tensile 

membrane action of the floor, (2) load redistribution to the hat truss through tensile loads on the columns 

between the affected floors and the hat truss, or (3) a combination of both. The objective of this analysis 

is to determine the actual mechanism that occurs for a given damage pattern in the core columns of 

WTC 1. 

The reduced global model of the tower lacks a complete floor system. As described in Section D.2.3, the 

floor systems were modeled as rigid or flexible diaphragms, which do not provide a path for vertical loads 

to be redistributed within the floors. Instead, when a core column is assumed to be damaged, all loads on 

that column from floors above the damage zone are redistributed through the hat truss in the model. This 

causes large tension forces in the damaged core columns. 

A two-step approach was used to examine how the loads might redistribute. First, the typical floor model 

was analyzed with assumed damage to core columns. The severed columns were replaced by equivalent 

vertical springs, representing the combined stiffness of the hat truss and the axial stiffness of the columns 

between the floor and hat truss. In the analysis of the floor system, damage to the exterior walls of the 

tower was ignored, since it is assumed that the walls are capable of redistributing their loads. This 

analysis estimated what portion of the load would be redistributed as forces in the springs that will be 

transmitted to the hat truss, and what portion would be redistributed to neighboring columns through the 

floor system. Second, the tensile capacities of the core column splices between the affected floors and the 

hat trusses were estimated to determine if the columns could carry the calculated tensile loads. 

To determine the equivalent stiffness of the hat truss, a separate model of the hat truss was first analyzed. 

For that purpose, a concentrated gravity load was applied at the node corresponding to the severed 

column, and the spring stiffness was estimated by dividing the applied load by the measured vertical 

displacement at that node. Then the axial stiffness of each of the columns above the damaged area was 

calculated. Finally, the model of the typical floor (Floor 96 of WTC 1) was modified to simulate the case 

of a floor above the damaged zone of the tower. The vertical support was removed from the base of the 

severed core columns, and spring restraints equivalent to the combined stiffness of the columns above and 

the hat truss were added to the tops of these columns. The floor model was then analyzed to determine 

how the loads would redistribute. 

Two damage patterns in the core of WTC 1 were considered for this analysis: the first assumes 

destruction of fifteen columns (Core Damage Case 1), and the second assumes that only eight columns 

were severed (Core Damage Case 2). Table D–4 contains a list of core columns that were assumed to be 

destroyed for both damage cases (see Fig. D–2 for column locations). 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–9

Table D–4. Core columns removed from WTC 1, assumed destroyed by 

aircraft impact. 

Core Core Damage Case 1 Core Damage Case 2 

Column Lowest Floor Highest Floor Lowest Floor Highest Floor 

503 96 

504 95 96 94 97 

505 95 96 94 96 

506 95 

603 96 

604 95 96 94 97 

605 95 96 94 95 

606 94 

703 96 94 95 

704 96 94 97 

705 96 

706 96 94 

803 96 

804 96 

805 96 

903 95 96 

Figure D–2. Plan of typical core column layout (courtesy FEMA). 
Appendix D 

D–10

D.3.4 Nonlinear Analysis with Plastic Hinges 

This analysis considered the nonlinear response of WTC 1 when an estimated pattern of damage had 

occurred. The damage scenario considered for this analysis included the following: 

• Representative aircraft impact damage: based on photographic evidence, members in the north 

exterior wall of WTC 1 that were visibly severed or missing were assumed to be incapable of 

carrying load and were removed from the model, while members that appeared to be mostly intact 

were assumed to be capable of still carrying full load. This damage case also includes an exterior 

panel in the south face of the tower (columns 329 through 331 between floors 94 and 96) that was 

destroyed by the aircraft impact. In addition, eight columns in the core were assumed severed 

(see Core Damage Case 2 in Table D–4). 

• Representative fire damage: 24 columns on the south face of WTC 1 between floors 96 and 98 

were assumed to have buckled and lost all load carrying capacity. This assumption is based on 

video evidence that indicates that columns in this area were visibly deformed inward a few 

minutes before the tower collapsed. 

The exterior members that were removed in this damage scenario are indicated in Fig. D–3. 

Figure D–3. North and south elevations of WTC 1 indicating columns and spandrels 

removed due to aircraft impact and fire effects. 

Aircraft impact damage 

Fire damage
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–11

To estimate how the damaged structure responded, the analysis considered geometric nonlinearities (large 

deflections and P-∆ effects) and material nonlinearities through a series of nonlinear, plastic hinges that 

were added to capture the post-yield behavior of structural members. The plastic hinges were placed in 

the reduced global model of WTC 1 based on a linear analysis of the damaged structures to determine the 

most stressed zones using a demand/capacity analysis. These hinges allow the members to act as 

nonlinear components, yielding once the stress on the member exceeds the material yield stress, 

continuing to accept some load at a reduced stiffness, and finally failing once an ultimate strain has been 

reached at an assumed ductility of 6. Hinges that considered both axial and bending forces (PMM hinges) 

were used in columns and hat truss members. Hinges that considered bending about the primary axis of 

the member, and shears in both the primary and secondary directions (MVV hinges) were used for most 

of the spandrels. Hinges that considered only bending about the primary axis (M3 hinges) were used for a 

small number of spandrels at the tower corners. 

This analysis does not account for local bucking of columns; neither does it consider the failure or the role 

of the floor system in redistributing the loads. More detailed models, currently being developed within 

the framework of Projects 2 and 6, will account for these factors. 

The damage due to aircraft impact analysis started from the end of the staged construction, described in 

Section D.3.1. At this stage, the set of damaged structural members that represent members destroyed by 

aircraft impact were removed from the reduced global model of WTC 1. This was followed by another 

stage, where the set of damaged structural members that represent members severely weakened by fire 

were removed from the model. For all analysis stages, room-temperature mechanical properties were 

used for all steels. 

D.4 RESULTS 

D.4.1 Results of Linear Stability Analysis 

An initial analysis of the reduced undamaged model of WTC 1 under service loads with the 96th floor 

removed (i.e., the diaphragm constraint removed for all nodes at floor 96) produced a load factor for 

staged construction, λ, of 1.91. This indicated that no columns buckled under the application of DL, 

SDL, service LL, and antenna loads, and that the structure was stable. The structure was still stable with 

the additional removal of floor 95 (λ = 1.03). The analysis with floors 95, 96, and 97 removed yielded 

λ = 0.65 and the buckled column (core column 705, see Fig. D–2) was identified through visual 

observation of the first buckled mode shape. 

Column 705 was removed from the model between floors 94 and 98, and the column’s joint loads at 

floors 95, 96, and 97 were evenly distributed to joints of columns 704, 706, and 804. The analysis 

produced λ = 0.78 and indicated that column 704 had buckled. Column 704 was then removed from the 

model in a similar fashion to column 705. The combined joint loads of columns 704 and 705 were then 

distributed to neighboring joints at columns 703, 706, 803, 804, and 805. The load redistribution was 

proportional to the distance of each joint from the point halfway between joints 704 and 705. This 

analysis produced λ = 1.38, which indicated a stable tower with three floors removed. 

The rigid diaphragm constraint at floor 94 was then removed from this latest model, i.e., floors 94 through 

97 were unconstrained, columns 704 and 705 were omitted between floors 93 and 98, and the joint loads 
Appendix D 

D–12

at each removed floor from these two columns were redistributed as above. The eigenvalue buckling 

analysis produced a load factor of 0.92 for this model and indicated that column 601 buckled. Column 

601 was removed between floors 93 and 98, and its joint load was distributed to columns 501, 502, 602, 

and 701. This analysis produced λ = 0.97 and indicated column 608, similarly located along the 

perimeter of the core columns like column 601, buckled. The model with column 608 removed and its 

load distributed to columns 508, 507, 607, and 708 yielded λ = 1.25. Thus, the tower was stable with four 

floors removed and with the redistribution of loads from removed columns 705, 704, 601, and 608. 

Subsequent models with floor 98 removed suggested that the structure did not attain λ greater than one, 

and that the structure had likely reached a failed state. 

When the modulus of elasticity was reduced from E = 29,000 ksi to E’ = 21,460 ksi in columns above and 

below removed floors, the conclusions changed slightly but the progression of column failures remained 

the same. The tower maintained its overall stability with floor 96 removed (λ = 1.38). With floors 96 and 

95 removed, the model of the intact structure indicated that column 705 buckled (λ = 0.83), but that 

stability was achieved through the removal of column 705 (λ = 1.02). With the removal of a third floor 

(97th), column 704 was also removed and its load redistributed in the model to maintain overall stability 

(λ = 1.11). The additional removal of a fourth floor (94th) produced a series of buckled columns 

(601, 608, 904, and 604) that indicated the structure would likely not achieve overall stability. 

To summarize, the eigenvalue analysis examined the stability of the undamaged tower under service loads 

through increased unbraced column lengths in the absence of material nonlinearities. For the case with 

columns at room temperature, the tower was stable when two floors were removed. Two core columns 

buckled when three floors were removed, but the tower maintained its overall stability. Similarly, the 

tower maintained its stability when four columns buckled with four floors removed. This analysis 

suggested that global instability of the tower occurred when five floors were removed from the model. 

The case with columns at the region of removed floors at temperature of 600 oC showed the tower 

maintained overall stability with one floor removed, with two floors removed and one buckled column, 

and with three floors removed and two buckled columns. This case produced tower instability with four 

floors removed from the model. 

D.4.2 Results of Redistribution of Forces within the Core Analysis 

The typical floor model was analyzed with 15 severed core columns (Core Damage Case 1) replaced with 

springs representative of the combined stiffness of the columns and hat truss. The analysis indicated that 

most of the load redistribution would take place initially through the hat truss, with the columns above the 

damaged zone carrying large tensile forces. A small portion of the load would be redistributed within the 

floor system. This is due to the greater stiffness of the hat truss-column assembly relative to the flexural 

stiffness of the floor system with fifteen severed columns. 

When eight columns were assumed severed in the core (Core Damage Case 2), the floor system had a 

larger stiffness than that with fifteen columns severed (Core Damage Case 1). Consequently, the 

contribution of the floor in redistributing the gravity loads was larger, and the tensile forces in the 

columns above the damaged zone were reduced relative to the tensile forces for the case of fifteen severed 

core columns. 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–13

Above the impact levels, the core columns were typically two or three stories high, wide flange segments 

that were connected together with bolted splices. In the upper floors of WTC 1, where the column tensile 

capacities were analyzed, the columns were spliced at floors 98, 101, 104, and 106. Only the splices for 

columns that were assumed to be damaged were analyzed. The splice connections on these columns 

typically consisted of two splice plates, one bolted to each flange of the column, connected to the columns 

by eight or twelve, 3/4 in. (19 mm) A325 bolts. At floor 106, where the columns connected to the hat 

truss, 7/8 in. (22.2 mm) bolts were used. The splice plates were made with A36 steel, which was assumed 

to have an ultimate tensile capacity of 61 ksi (422 MPa) (see Table D–1). Since the connections were 

bearing connections with the bolts in single shear, the ultimate shear capacity of each 3/4 in. (19 mm) bolt 

was estimated to be 31.8 kip (219 MPa). When the strengths of both the splice plates and bolts were 

estimated, the splice plates were found to be consistently stronger than the bolts, so the columns would 

fail in tension through shearing of the bolts. Table D–5 lists the ultimate capacities of the splices on each 

of the columns assumed to be damaged in this analysis. 

Table D–5. Ultimate tensile capacities of core column splices. 

Column Floor 98 Floor 101 Floor 104 Floor 106 

Number kip (kN) kip (kN) kip (kN) kip (kN) 

503 519.6 (2,311) 381.6 (1,697) 381.6 (1,697) 346.4 (1,541) 

504 381.6 (1,697) 381.6 (1,697) 254.5 (1,132) 346.4 (1,541) 

505 381.6 (1,697) 381.6 (1,697) 254.5 (1,132) 346.4 (1,541) 

506 519.6 (2,311) 381.6 (1,697) 381.6 (1,697) 346.4 (1,541) 

603 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

604 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

605 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

606 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

703 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

704 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

705 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

706 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

803 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

804 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

805 254.5 (1,132) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

903 381.6 (1,697) 254.5 (1,132) 254.5 (1,132) 346.4 (1,541) 

A comparison of the columns’ tensile forces with the capacities of column splice connections is presented 

in Table D–6 for Core Damage Case 1. The table indicates that, for the assumed service loads and 

damage pattern, the splice connections for interior core columns at the 700 and 800 lines are capable of 

resisting the tensile forces imposed on them. Splice connections on the 600 line are at the critical stage, 

but splices at the north perimeter core columns (500 line) are likely to fail. Failure of the column 

connections at the 500 line will require the floor to redistribute its loads to neighboring intact columns. 

This will result in overloading the columns at the 600 line, and consequently, the floor has to redistribute 

its loads through nonlinear, large deflection, tensile membrane action. 
Appendix D 

D–14

Table D–6. Tensile loads on columns above damaged area, and column load to 

ultimate capacity ratios for Core Damage Case 1. 

Floor 98 Floor 101 Floor 104 Floor 106 

Column Load Column Load Column Load Column Load

Column 

Number kip (kN) 

Load to 

Capacity

Ratio kip (kN) 

Load to 

Capacity

Ratio kip (kN) 

Load to 

Capacity

Ratio kip (kN) 

Load to 

Capacity

Ratio 

503 109.3 (486) 0.21 273.2 (1215) 0.72 437.1 (1944) 1.15 546.3 (2430) 1.58 

504 82.8 (368) 0.22 207.0 (921) 0.54 331.2 (1473) 1.30 414.0 (1841) 1.20 

505 92.7 (412) 0.24 231.8 (1031) 0.61 370.9 (1650) 1.46 463.6 (2062) 1.34 

506 214.7 (955) 0.41 375.7 (1671) 0.98 536.7 (2387) 1.41 644.0 (2865) 1.86 

603 64.6 (287) 0.25 161.5 (718) 0.63 258.4 (1149) 1.02 323.0 (1437) 0.93 

604 57.5 (256) 0.23 143.8 (640) 0.57 230.1 (1024) 0.90 287.7 (1280) 0.83 

605 72.3 (321) 0.28 180.7 (804) 0.71 289.1 (1286) 1.14 361.3 (1607) 1.04 

606 138.7 (617) 0.55 242.8 (1080) 0.95 346.9 (1543) 1.36 416.2 (1851) 1.20 

703 39.1 (174) 0.15 97.8 (435) 0.38 156.5 (696) 0.61 195.6 (870) 0.56 

704 20.1 (90) 0.08 50.4 (224) 0.20 80.6 (358) 0.32 100.7 (448) 0.29 

705 27.3 (121) 0.11 68.2 (303) 0.27 109.1 (485) 0.43 136.3 (606) 0.39 

706 27.2 (121) 0.11 68.0 (303) 0.27 108.9 (484) 0.43 136.1 (605) 0.39 

803 24.8 (110) 0.10 62.0 (276) 0.24 99.2 (441) 0.39 124.0 (552) 0.36 

804 36.2 (161) 0.14 90.4 (402) 0.36 144.7 (644) 0.57 180.9 (805) 0.52 

805 18.9 (84) 0.07 47.1 (210) 0.19 75.4 (335) 0.30 94.3 (419) 0.27 

For the case where eight columns were assumed severed in the core (Core Damage Case 2), the results are 

presented in Table D–7. The Table indicates that the columns splice connections are capable of resisting 

the tensile loads except for column 505, where the load to capacity ratio is approximately 1.25 at 

floor 104 and 1.15 at floor 106. Successive removal of columns that were assumed to lose their splice 

connections indicated that the failure of connections would propagate to the 500 and 600 column lines in 

the core. Splice connections at columns 704, 705, and 903 should remain intact. Table D–7 indicates, 

however, that the load to capacity ratio at the splices did not exceed 1.25 for all cases considered. These 

values might not be conclusive to determine connection failure or survival due to the uncertainties in the 

loads on the floors and the capacities of the splice connections. 

D.4.3 Results of Nonlinear Analysis 

Two cases were considered for this analysis based on the results presented in Section D.4.2. The first 

assumed that core column splices above the eight severed columns (Core Damage Case 2) had failed, and 

the load was being distributed through the floor system to neighboring columns (Case A). The second 

case assumed that splices were intact, and the load was being transmitted to the hat truss via tensile forces 

in the columns (Case B). For both cases, the results of the nonlinear analysis show that WTC 1 had 

significant reserve structural capacity after aircraft impact. Moreover, the loads and deformations in 

critical members varied little between the two cases. The results are described in detail for Case A only. 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–15

Table D–7. Tensile loads on columns above damaged area, and L/C ratios indicating 

likely progression of splice failures for Core Damage Case 2. 

Floor 98 Floor 101 Floor 104 Floor 106 

Column Column Load Column Load Column Load Column Load

Number kip (kN) 

L/C 

Ratio kip (kN) 

L/C 

Ratio kip (kN) 

L/C 

Ratio kip (kN) 

L/C 

Ratio 

Loads with all column splices intact 

504 34.7 (154) 0.09 138.9 (618) 0.36 243.1 (1081) 0.96 312.5 (1390) 0.90

505 79.5 (354) 0.21 198.8 (884) 0.52 318.1 (1415) 1.25 397.6 (1769) 1.15 

604 21.3 (95) 0.08 85.2 (379) 0.33 149.1 (663) 0.59 191.7 (853) 0.55

605 77.0 (343) 0.30 154.0 (685) 0.61 231.0 (1028) 0.91 282.4 (1256) 0.82 

703 45.0 (200) 0.18 90.0 (400) 0.35 134.9 (600) 0.53 164.9 (734) 0.48 

704 3.6 (16) 0.01 14.3 (64) 0.06 25.1 (112) 0.10 32.2 (143) 0.09 

706 21.0 (93) 0.08 36.7 (163) 0.14 52.4 (233) 0.21 62.9 (280) 0.18 

903 36.2 (161) 0.09 90.6 (403) 0.36 145.0 (645) 0.57 181.2 (806) 0.52 

Loads with splices at columns 504 and 505 failed 

504 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

505 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

604 29.0 (129) 0.11 115.9 (515) 0.46 202.8 (902) 0.80 260.7 (1160) 0.75 

605 103.6 (461) 0.41 207.3 (922) 0.81 310.9 (1383) 1.22 380.0 (1690) 1.10 

703 43.1 (192) 0.17 86.2 (383) 0.34 129.3 (575) 0.51 158.0 (703) 0.46 

704 3.0 (13) 0.01 12.0 (53) 0.05 20.9 (93) 0.08 26.9 (120) 0.08 

706 20.1 (89) 0.08 35.1 (156) 0.14 50.2 (223) 0.20 60.2 (268) 0.17 

903 36.2 (161) 0.09 90.5 (403) 0.36 144.8 (644) 0.57 181.0 (805) 0.52 

Loads with splices at columns 504, 505, and 605 failed 

504 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

505 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

604 42.8 (191) 0.17 171.4 (762) 0.67 299.9 (1334) 1.18 385.6 (1715) 1.11 

605 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

703 45.8 (204) 0.18 91.5 (407) 0.36 137.3 (611) 0.54 167.8 (746) 0.48 

704 3.5 (16) 0.01 14.0 (62) 0.05 24.5 (109) 0.10 31.5 (140) 0.09 

706 22.6 (101) 0.09 39.6 (176) 0.16 56.5 (251) 0.22 67.8 (302) 0.20 

903 36.2 (161) 0.09 90.5 (403) 0.36 144.8 (644) 0.57 181.0 (805) 0.52

Loads with splices at columns 504, 505, 604, and 605 failed 

504 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

505 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

604 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

605 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 0.0 (0) 0.00 

703 57.6 (256) 0.23 115.2 (513) 0.45 172.9 (769) 0.68 211.3 (940) 0.61 

704 5.1 (23) 0.02 20.6 (91) 0.08 36.0 (160) 0.14 46.3 (206) 0.13 

706 25.5 (113) 0.10 44.6 (198) 0.18 63.7 (284) 0.25 76.5 (340) 0.22 

903 36.1 (161) 0.09 90.3 (402) 0.35 144.4 (642) 0.57 180.5 (803) 0.52 
Appendix D 

D–16

The analyses show that the most stressed members were the columns next to the damaged area on the 

north wall of the tower. The analyses show that the tower also remained standing after losing columns in 

the south wall due to fire effects with some reserve capacity left. This indicates that additional loss or 

weakening of columns in the core, weakening of additional columns in the exterior, or additional loss of 

floors is needed to collapse the tower. 

Figures D–4 and D–5 show the load deformation curves for columns 111 and 145 (see Fig. D–3) on either 

side of the damage on the north face of the tower. Both of these columns yielded during the impact 

damage analysis, but had sufficient strength and ductility to resist the peak loads. Note that the loads on 

these two columns were slightly reduced during the fire damage analysis. This occurred because the 

south face of the tower lost stiffness when members were lost to the fire, which caused the upper portion 

of the tower to rotate slightly toward the south. This redistributed a small portion of the load on the north 

face through the hat trusses to the core. The columns at the edge of the damage on the south face 

experienced the opposite effect. As can be seen in Fig. D–6, column 332 on the west side of the damage 

on the south face (see Fig. D–3) did not have a significant change in its load during the impact damage 

stage, but received a large additional load during the fire damage analysis. This column remained within 

the linear response range throughout the analysis. 

0

200

400

600

800

1000

1200

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Deformation (inches)

Axial Force (kips)

Construction Stages

Impact Damage

Fire Damage

Figure D–4. Load vs. deformation in column 111 at floor 98 

 (north face, west side of damage). 

Table D–8 shows the distribution of axial loads in columns at floor 99, immediately above the damaged 

zone for the various loading stages. For the floors below the hat trusses (construction stage 1) about 

57 percent of the dead load was carried by the core columns, with the rest distributed among the four 

exterior walls. The mechanical floors and roof tended to have a large percentage of their loads carried by 

the exterior, so at the end of the final construction stage, the load was nearly evenly distributed between 

the core and exterior. Significant load redistribution occurred during the damage cases; however most of 

the redistribution was from the north and south walls to the east and west walls. Only 1.7 percent of the 

total load was redistributed from the exterior to the core. 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–17

0

200

400

600

800

1000

1200

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Deformation (inches)

Axial Force (kips)

Construction Stages

Impact Damage

Fire Damage

Figure D–5. Load vs. deformation in column 145 at floor 95 

 (north face, east side of damage). 

0

200

400

600

800

1000

1200

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Deformation (inches)

Axial Force (kips)

Construction Stages

Impact Damage

Fire Damage

Figure D–6. Load vs. deformation in column 332 at floor 97 

 (south face, west side of damage). 
Appendix D 

D–18

Table D–8. Distribution of loads on exterior walls and core columns at floor 99 for Core 

Damage Case 2. 

Const. Stage 1 Const. Stage 2 Const. Stage 3 

Impact 

Damage Fire Damage 

Case (kip) (%) (kip) (%) (kip) (%) (kip) (%) (kip) (%) 

Total axial force 27,719 45,694 54,639 54,641 54,641 

Force Distribution between Core and Exterior 

Core columns 15,828 57.1 24,466 53.5 27,397 50.1 27,791 50.9 28,318 51.8 

Exterior columns 11,891 42.9 21,228 46.5 27,242 49.9 26,850 49.1 26,323 48.2 

Force Distribution between Exterior Faces 

100 face 3,562 12.9 6,104 13.4 7,610 13.9 6,732 12.3 6,519 11.9 

200 face 2,389 8.6 4,551 10.0 6,084 11.1 6,539 12.0 6,765 12.4 

300 face 3,562 12.9 6,064 13.3 7,548 13.8 7,000 12.8 6,445 11.8 

400 face 2,378 8.6 4,509 9.9 6,000 11.0 6,579 12.0 6,594 12.1 

Even with the loss of 34 columns on the north face and 24 columns on the south face, relatively few 

structural members were overstressed. As listed in Table D–9, plastic hinges had formed in only nine 

members, and only three members had more than a small amount of plastic deformation. Of the members 

with plastic hinges, six were exterior columns, and three were exterior spandrels. None of the core 

columns had hinges form. Three beams in the hat truss at floor 107 experienced some yielding. These 

members were all light, short connecting members, and the yielding was most likely due to modeling 

idealization rather than overloading that would have occurred in the real structure. The hinges in the 

columns and spandrels all formed near the sides of the openings created by the aircraft impact and at the 

edges of the region that appeared to fail inward due to the fires. Figure D–7 shows the displacements and 

plastic hinges that occurred in the north and south faces of the tower during the impact damage analysis 

stage, while Fig. D–8 shows the displacements and plastic hinges from the fire damage analysis stage. 

Table D–9. Hinge states of members where plastic hinges formed during 

nonlinear analysis. 

Columns with Plastic Hinges 

Column Line Lower Floor Upper Floor Impact Damage Fire Damage 

111 98 99 Some strain hardening Some strain hardening 

111 99 99 mid floor Yielded Yielded 

111 99 mid floor 100 Some strain hardening Some strain hardening 

145 94 95 Yielded Yielded 

145 95 95 mid floor Yielded Yielded 

145 95 mid floor 96 Some strain hardening Some strain hardening 

Spandrels with Plastic Hinges 

Floor Level Start Column End Column Impact Damage Fire Damage 

99 110 111 Yielded Yielded 

96 144 145 Yielded Yielded 

98 331 332 - Yielded 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–19

(a) North Face (b) South Face 

Figure D–7. Displacements and locations of plastic hinges in the north and south 

exterior walls of WTC 1 after impact. 

D.5 SUMMARY AND PRELIMINARY FINDINGS 

Preliminary system stability analyses of the WTC towers have been performed to: (1) examine the 

overall stability of the undamaged tower upon removal of floors, (2) study possible load redistribution 

mechanisms upon losing columns in the core due to aircraft impact, and (3) study the response when 

columns in both the exterior walls and the core are assumed destroyed due to aircraft impact, and columns 

in the exterior are damaged due to the subsequent fires, as observed in photographs and videos of WTC 1. 

The analyses used the typical truss-framed floor model and a reduced version of the global reference 

model of WTC 1 with proper modifications. Modifications included adding vertical springs at the bottom 

of the reduced models to account for the removed lower portion of the towers, and using actual 

(vs specified) steel properties and service loads on the towers. The analyses used the staged construction 

technique to account for the sequential construction of the towers, especially in the zone of the hat trusses. 
Appendix D 

D–20

(a) North Face (b) South Face 

Figure D–8. Displacements and locations of plastic hinges in the north and south 

exterior walls of WTC 1 after impact and fire. 

Linear buckling analysis and nonlinear analysis with plastic hinges were used to study the effects of 

removal of floors and loss of exterior and core columns, respectively. In addition, analysis of the floor 

system, where severed core columns were replaced by equivalent springs representative of the combined 

stiffness of the hat trusses and columns between the floors and hat trusses, was conducted to study the 

mechanism by which the floor loads were redistributed when the core columns were destroyed by aircraft 

impact. 

The following presents some preliminary findings based on the analyses under service loading conditions: 

• Linear stability analysis was used to examine the stability of the undamaged WTC 1 under 

service loads through increased unbraced column lengths (floor removal). The tower was stable 

when two floors were removed. Two core columns buckled when three floors were removed, but 

the tower maintained its overall stability. The tower also maintained its stability when four 

columns buckled with four floors removed. The analysis suggested that global instability of the 

tower occurred when five floors were removed from the model. Assuming that all columns at the 
Interim Report on Preliminary Stability Analysis of the WTC Towers 

D–21

region of removed floors reached a temperature of 600 oC (reduced modulus of elasticity), the 

analysis indicated that removal of four floors would induce global instability. 

• Analysis of the typical truss-framed floor model with fifteen core columns assumed severed 

indicated that, under service loads, the floors first attempted to redistribute their loads to the hat 

trusses through tension in the columns above the damage. The load followed this path due to the 

relatively large stiffness of the hat trusses-column system compared to the flexural stiffness of the 

floors. This resulted, however, in the ultimate tensile capacity of some column splices below the 

hat trusses to be exceeded, and ultimately, the floors would have redistributed their loads directly 

to neighboring core columns. When only eight core columns were assumed severed, the analysis 

indicated that the tensile forces in the columns were smaller, due to the relatively larger stiffness 

of the floor. These forces may still have failed the columns at the splices. Since the load to 

capacity ratio at the splices did not exceed 1.25 when eight columns were severed, and due to the 

uncertainties in the loads on the floors and the capacities of the splice connections, the results are 

not conclusive as to whether splice failure would occur or not. 

• Nonlinear analysis that included geometric nonlinearities and material nonlinearities using plastic 

hinges was conducted on the reduced global model of WTC 1. The model assumed the following 

damage to the tower: (1) due to aircraft impact, loss of columns and spandrels in the north face, 

and an exterior panel in the south face of the tower (both based on photographic evidence), as 

well as eight columns in the core; and (2) due to fire, loss of columns in the south face, which 

were shown in videos to be bowing inward a few minutes prior to collapse. The analysis 

indicated that after aircraft impact, the tower maintained its stability, where the highest stressed 

elements were the exterior columns next to the damaged area on the north face of the tower. The 

tower also maintained its stability after losing columns in the south wall due to fire effects with 

some reserve capacity left, indicating that additional loss or weakening of columns in the core, 

weakening of additional columns in the exterior, or additional loss of floors is needed to collapse 

the tower. More detailed models will account for local bucking of columns, and the failure and 

role of the floor system in redistributing the loads, factors that are not considered in this analysis. 
Appendix D 

D–22

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E–i

TABLE OF CONTENTS - Appendix E

List of Figures...........................................................................................................................................E–ii

List of Tables .......................................................................................................................................... E–iii

Appendix E

Interim Report on Contemporaneous Structural Steel Specifications ..............................E–1

E.1 Scope of Report ...........................................................................................................................E–1

E.2 Units and Abbreviations ..............................................................................................................E–2

E.3 Sources of Information ................................................................................................................E–2

E.4 Tower Design – Structural Steel Documents...............................................................................E–3

E.4.1 Specification of Steel Grades (Minimum Yield Strength) ...............................................E–3

E.4.2 Structural Overview..........................................................................................................E–3

E.5 Contemporaneous Steel Specifications......................................................................................E–15

E.5.1 Standards Called for in the Steel Contracts....................................................................E–16

E.5.2 Steels Used in Construction............................................................................................E–20

E.5.3 Recommended Values for Mechanical Properties..........................................................E–29

E.5.4 Sources of Information...................................................................................................E–34

E.6 Contemporaneous Construction Specifications .........................................................................E–37

E.6.1 Fabrication of the Various Components.........................................................................E–37

E.7 Reference Lists ..........................................................................................................................E–41

E.7.1 References from Publicly Available Sources .................................................................E–41

E.7.2 Private Communications ................................................................................................E–44

E.7.3 References from Nonpublic Sources ..............................................................................E–45

Attachment 1

Steel Companies Involved in the World Trade Center ......................................................E–49 

Attachment 2

Notes on ASTM Standards for Structural Steel .................................................................E–53 
E–ii

LIST OF FIGURES

Figure E–1. Schematic diagram of the tower structure..........................................................................E–4

Figure E–2. WTC tower floor plan and column numbers......................................................................E–5

Figure E–3. Cross-section of perimeter columns; sections with and without spandrels........................E–6

Figure E–4. Characteristic perimeter column panel consisting of three full columns connected by 

three spandrels....................................................................................................................E–7

Figure E–5. Partial elevation of exterior bearing-wall frame showing perimeter column panel 

construction. Highlighted panel is three stories tall (36 ft) and spans four floors. 

Distance between panels has been exaggerated. ................................................................E–8

Figure E–6. Typical welded box members and rolled wide flange shapes used for core columns 

between floors 83 and 86 (to scale)....................................................................................E–9

Figure E–7. Core column layout in WTC towers. ...............................................................................E–10

Figure E–8. Schematic diagram of a floor truss...................................................................................E–11

Figure E–9. Hat truss in upper floors...................................................................................................E–13

Figure E–10. Schematic diagram of the stress-strain behavior of a structural steel. .............................E–21
E–iii

LIST OF TABLES

Table E–1. Metric equivalents of common yield strengths. ...................................................................E–2

Table E–2. Number of WTC 1 and WTC 2 perimeter columns damaged by aircraft impact. .............E–14

Table E–3. Number of core columns with a given minimum yield strength within the floors 

penetrated by the aircraft....................................................................................................E–14

Table E–4. Number of perimeter columns of specified grades in floors with significant fire. ............E–14

Table E–5. Number of core columns of specified grades in floors with significant fire......................E–15

Table E–6. Steels specified as acceptable by PONYA in its contract with steel fabricators................E–17

Table E–7. Summary of mechanical properties from relevant ASTM International structural steel 

standards from WTC era. ...................................................................................................E–18

Table E–8. Summary of chemistry data from relevant ASTM International structural steel 

standards from WTC era. ...................................................................................................E–19

Table E–9. Summary of Japan Industrial Standard structural steel standards from 1974. ...................E–20

Table E–10. Steel companies involved in WTC construction and their contracts..................................E–21

Table E–11. Properties of Laclede ASTM A 242 steels obtained from Laclede mill test reports..........E–22

Table E–12. Specified properties for Yawata contemporaneous steel grades........................................E–24

Table E–13. Reported properties for Yawata contemporaneous steel grades. .......................................E–25

Table E–14. Mechanical properties of U.S. Steel and Bethlehem V-series steels..................................E–27

Table E–15. Chemistry and mechanical property data for a Fuji Steel plate and a Colvilles plate 

used for core columns. .......................................................................................................E–28

Table E–16. Databases searched for WTC information. ........................................................................E–28

Table E–17. Estimated yield strengths (Fy) for grades of steel above Floor 9. ......................................E–33

Table E–18. Trade journals examined for WTC steel information. .......................................................E–35

Table E–19. Sources examined for mill test reports and other construction information, other than 

the (LERA) archives. .........................................................................................................E–36

Table E–20. Comparison of chemistry requirements for ASTM A 325 “Standard Specification for 

High-Strength Bolts for Structural Steel Joints, including Suitable Nuts and Plain 

Hardened Washers” between 1970 and 2002 standards.....................................................E–39
Appendix E 

E–iv

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E–1

Appendix E

INTERIM REPORT ON CONTEMPORANEOUS STRUCTURAL 

STEEL SPECIFICATIONS

The purpose of Project 3, Mechanical and Metallurgical Analysis of Structural Steel, of the National 

Institute of Standards and Technology (NIST) World Trade Center (WTC) Investigation is to analyze 

structural steel available from WTC 1, 2, and 7 for determining the metallurgical and mechanical 

properties and quality of the metal, weldments, and connections, and providing these data to other 

investigation projects. (For test plan details, see <http://wtc.nist.gov/media/WTCplan_new.htm#proj3>.) 

The properties determined under this project will be used in two ways: 

1. Properties will be correlated with the design requirements of the buildings to determine if the 

specified steel was in place in the towers. 

2. Properties will be supplied for other projects in the Investigation as input for models of 

building performance. 

E.1 SCOPE OF REPORT 

This appendix describes the WTC tower structure and critical structural elements to be characterized in 

Project 3. This includes the structural design and properties specified by the structural engineers for 

columns, floor systems, and connections. 

Contemporaneous (late 1960s era) specifications are described for various types and grades of steel 

designated by the ASTM International, the American Institute of Steel Construction (AISC), and other 

national and international organizations. It also includes information from numerous suppliers of the steel 

for the structure. The structural steel for the towers was supplied through at least a dozen contracts to 

suppliers and fabricators. Substantial understanding of the consistency, quality, and actual strength of the 

steel (as opposed to specified minimum values) can be gained if the production practices and quality 

control procedures used by the various steel suppliers are understood. Practices and data from the 

numerous WTC steel suppliers have been investigated and are reported for both structural steel and 

construction practices. In addition, this information has been used to estimate typical mechanical 

property values for the many of the grades of steel. These typical values can serve as a guide for the 

properties to be inserted into the finite element models of building performance and as a point of 

comparison for actual properties measured on the recovered steel. 

The appendix also includes a review of the standards and specifications used in welding the built-up 

columns, and those used in the erection of the towers. 
Appendix E 

E–2

E.2 UNITS AND ABBREVIATIONS 

Yield strengths of the steels and the dimensions of the building are expressed in English units with metric 

(SI) equivalents. The steels were specified to English unit-based ASTM standards, and the building was 

built to foot and inch dimensions. ASTM standards differentiate between English and metric units by 

denoting them with completely different designations and frequently by publishing them as separate 

documents. This appendix uses English units for values that were contractually specified during the 

construction (primarily component dimensions and steel strengths). Table E–1 shows the SI equivalents 

of the common yield strength grades of steel. 

Table E–1. Metric equivalents of common yield strengths. 

ksi MPa 

36 248 

42 290 

45 310 

46 317 

50 345 

55 379 

60 414 

65 448 

70 483 

75 517 

80 552 

85 586 

90 621 

100 689 

In reviewing some of the historical documents, NIST found ambiguities in the use of the measure “ton.” 

NIST has assumed that in any source originating in the United States, a “ton” refers to 2,000 lb (i.e., a 

short ton). For sources originating in Japan, NIST assumes that a “ton” refers to 1,000 kg (= 2,204.6 lb, 

i.e., a metric ton). For any source originating in Great Britain, NIST assumes that a “ton” is 2,240 lb (a 

“long” or U.K. ton) and that a “tonne” is 1,000 kg. In this appendix, all weights in tons are converted to 

short tons (= 2,000 lb). 

This appendix follows the American Iron and Steel Institute (AISI) convention and denotes yield strength 

with the symbol Fy. The ASTM International uses the symbols YS (or YP) and Sy. 

E.3 SOURCES OF INFORMATION 

This appendix is based on three different types of sources. Open literature sources like journal and trade 

magazine articles, books, historical standards, and publicly searchable databases comprise the first type. 

The second type comprises personal interviews by NIST investigators with individuals and company 

representatives, and information they provided voluntarily. Sources of information where NIST has 
Interim Report on Contemporaneous Structural Steel Specifications 

E–3

entered into material release agreements with organizations or individuals comprise the third type. 

Documents provided by Leslie E. Robertson Associates (LERA), which is the source of most of the 

contemporaneous information on the construction of the buildings, is an example of the third type. This 

archive has been useful in identifying the specific steels and standards used in the construction. Although 

it is voluminous, the LERA archive does not include every document generated during the construction of 

the towers. Section E.5.4 summarizes the search strategies for open literature information and provides 

details on the companies and individuals contacted and the information they provided 

This report identifies the type of source in the reference. For example, a reference to a book or other 

publicly available document appears as (Smith 1968). The symbol † denotes a personal communication 

to a NIST investigator, for example (Jones 2003 †). In the case of a source bound by a Material Transfer 

Agreement, the symbol § appears in the reference, for example (Monti 1969 §). The reference lists 

appear as Section E.7. 

E.4 TOWER DESIGN – STRUCTURAL STEEL DOCUMENTS 

E.4.1 Specification of Steel Grades (Minimum Yield Strength) 

Specifications (ASTM, AISI, etc.) typically place limits on chemical composition or mechanical 

properties or, most commonly, both. Various mechanical properties may be specified, such as tensile 

strength, minimum yield strength, ductility, and toughness. Other material properties may not appear in a 

specification, yet are critical in building design; the most important such property is perhaps the elastic 

modulus, or stiffness, which does not appear in specifications because there is little variability among the 

various steels. 

In practice, the material property of greatest importance for characterizing a particular steel is the yield 

strength (Fy). In the United States, steel is often referred to according to its yield strength; for example, a 

“50 ksi steel” is steel with a minimum yield strength of 50,000 lb/in2

. Skilling, Helle, Christiansen, & 

Robertson (SHCR), structural engineers for the WTC towers, followed this convention, and the structural 

engineering plans are marked with the minimum yield strength for each piece of structural steel. 

E.4.2 Structural Overview 

The WTC tower buildings had a frame-tube construction consisting of closely spaced perimeter columns 

coupled to a rectangular service core (Fig. E–1). The buildings had a square footprint, 207 ft 2 in. 
Appendix E 

E–4

Exterior Framed Tube

Central Core

Columns Floor Slab

59 columns per side

Trough Decking Concrete Slab

Open Web Bar Joist

Air-conditioning Duct

Electric Duct

Floor

Covering

Technical Services

Technical Services

Sky Lobby

Technical Services

Sky Lobby

Express Elevators

Technical Services

Plaza Level Local Elevators

Local Elevators

Figure E–1. Schematic diagram of the tower structure. 

(63.14 m) on a side with chamfered corners. From floor 9 to floor 107, the perimeter columns consisted 

of closely spaced built-up box columns. The service core at the building center was approximately 87 ft 

by 137 ft (26.5 m by 41.8 m) and connected to the perimeter columns by a floor panel system that 

provided an essentially column-free office space, see Fig. E–2. In addition to showing the location of 

perimeter and core columns, Fig. E–2 describes the column numbering scheme used to identify each 

column on a given floor. 

The WTC tower structural steel plans (SHCR 1967 §) point out the major structural elements of interest. 

The main features of structural interest are the perimeter columns, the core columns and associated 

framing, the trusses that supported the floors, and the connections between and within these elements. In 

addition, a hat truss located within floors 107 to 110 tied the core to the perimeter columns and provided a 

base for the television mast atop WTC 1 and support for a proposed mast atop WTC 2. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–5

Source: McAllister 2002. 

Figure E–2. WTC tower floor plan and column numbers. 

The structural engineering plans provide the location, cross-sections, and grade of steel (i.e., required 

minimum yield strength) for each of the thousands of structural elements in the buildings. In all, 14 

different grades of steel were specified, ranging in yield strength from 36 ksi to 100 ksi. In addition to 

yield strength requirements, Port of New York Authority (PONYA) documents provided by LERA 

specified allowable steels using ASTM or other standards (details in Section E.5 in this report). 

Requirements for bolts and welds are also given. 

Perimeter Columns and Spandrels 

Between floors 9 and 107, the perimeter structure consisted of closely spaced, built-up box columns. 

Each building face consisted of 59 columns spaced at 40 in. (1.02 m). The columns were fabricated by 
Appendix E 

E–6

welding plates of steel to form an approximately 14 in. (0.36 m) square section (Fig. E–3). Adjacent 

columns were interconnected at each floor level by deep spandrel plates, typically 52 in. (1.32 m) deep 

(Fig. E–4). 

15.75 in.

40.0 in.

14.0 in.

13.5 in.

11.0 in.

Inner Web (Plate 3)

Section at individual column

Section at spandrel

Spandrel Plate (Plate 4)

Splice Plates

Flange 

(Plate 1)

Outer Web (Plate 2)

Inside of building

Outside of building

Figure E–3. Cross-section of perimeter columns; sections with and without spandrels. 

The perimeter columns were prefabricated into panels, typically three stories tall and three columns wide 

(Fig. E–4). Heavy end, or “butt” plates with Fy = 50 ksi and 1.375 in. to 3 in. (3.5 mm to 7.6 mm) thick 

were welded to the top and bottom of each column. Fillet welds were used inside the columns along three 

edges, with a groove weld on the fourth, outside edge. During erection, abutting spandrels were bolted 

together, and columns were bolted to the adjacent columns, all using ASTM A 325 bolts except for the 

heaviest butt plates, which used ASTM A 490 bolts. Other than at the mechanical floors, panels were 

staggered (Fig. E–5) so that only one third of the units were spliced (i.e., connected) in any one story. At 

the mechanical floors (75 and 76 in the upper level of the buildings), however, every perimeter column 

was spliced at the same level, floors 74 and 77. These splices were both welded and bolted. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–7

Figure E–4. Characteristic perimeter column panel consisting of three full columns 

connected by three spandrels. 

Fourteen grades of steel were specified in the design documents for the perimeter columns, with 

minimum yield strengths of (36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100) ksi. Twelve 

grades of steel were specified for the spandrels, with the same strength levels as the columns but without 

the two highest strength steels. The structural engineering plans indicate that the flanges and webs of a 

given column section consist of a single grade (i.e., minimum yield strength) of steel, but each column 

and spandrel within a single prefabricated panel could be fabricated from different grades of steel. 

Columns in the upper stories were typically fabricated of lighter gage steel, as thin as 0.25 in. (6.35 mm), 

with the grade of steel dictated by the calculated gravity and wind loads. In this manner the gravity load 

on the lower stories was minimized. In the lower stories the perimeter column flanges were often more 

than 2 in. (51 mm) thick. 

The spandrels formed an integral part of the columns: there was no inner web plate at spandrel locations. 

(Fig. E–3). Spandrels were generally specified with a yield strength lower than that of the column webs 

and flanges, as well as a heavier gage than the adjacent inner webs. 
Appendix E 

E–8

Floor

Floor

Floor

Floor

Floor

Floor

Figure E–5. Partial elevation of exterior bearing-wall frame showing perimeter column 

panel construction. Highlighted panel is three stories tall (36 ft) and spans four floors. 

Distance between panels has been exaggerated. 

Core Columns 

Core columns were of two types: welded box columns and rolled wide flange (WF) shapes (Fig. E–6). 

The columns in the lower floors were primarily very large box columns as large as 12 in. by 52 in. 

(0.30 m by 1.32 m) composed of welded plates up to 7 in. (178 mm) thick. In the upper floors the 

columns shifted to the rolled WF shapes. The transition floors are indicated in Fig. E–7 for each of the 

core columns. Core columns were typically spliced at three-story intervals. The splices in the impact and 
Interim Report on Contemporaneous Structural Steel Specifications 

E–9

fire zones were at floors 75, 77, 80, 83, 86, 89, 92, 95, 98, and 101. Diagonal bracing was used at the 

mechanical floors and in the area of the hat truss. Core box columns were 36 ksi or 42 ksi. Core wide 

flange columns were specified to be one of four grades, but were primarily 36 ksi and 42 ksi steel; only 

about 1 percent of all the core columns were made of 45 ksi or 50 ksi steel. 

22.44 in.

22.0 in.

15.0 in.

22.0 in.

14.0 in.

15.87 in. 13.91 in.

4.91 in.

1.62 in.

1.50 in.

1.31 in.

0.64 in.

17.89 in. 15.825 in. 10.00 in.

14WF730

14WF219

14WF61

Columns 1001 1008 

501 508

Column 504

Column

701

Columns 607 906

Column 705

Figure E–6. Typical welded box members and rolled wide flange shapes used for core 

columns between floors 83 and 86 (to scale). 

The core area was framed conventionally with beams. There were numerous openings in the core area 

floor for elevators and stairwells. Since fewer elevators were needed at the upper floors, part of the core 

area was not needed for services. In Fig. E–7, the dashed line shows the perimeter of the core, and shaded 

areas indicate typical enclosed areas for elevators and other services. 
Appendix E 

E–10

83

501

83

502

83

701

95

801

7

802

95

803

9

804

7

805

7

806 89

807

901

89

1001

80

1002

77

1003

77

1008

80

1004

86

1005

86

1006

77

1007

77

902

48

903

80

904

77

905

86

906

48

908

48

907

48

7

702

75

703

<1

704

7

705

24

706

7

707

92

708

83

503

86

504

86

505

83

506

83

507

80

602

80

603

80

605

80

606

80

607

80

608

80

604

508

83

80

601

}

83

502 Column #

WF Column Shape of column at 84th floor

Box Column

Typical enclosed area for elevators and other services

Floor of transition from box to WF columns

Figure E–7. Core column layout in WTC towers. 

Flooring System 

In the great majority of floors, the floor area outside the central core was supported by a series of 29 in. 

(0.74 m) deep, composite open web bar joists (“floor trusses”) that spanned between the core and 

perimeter wall (see Fig. E–8). At the core, the floor trusses were bolted to seats generally attached to 

channels that ran continuously along the core columns. At the perimeter columns, the floor trusses were 

bolted and then welded to seats, mounted on spandrels at every other column. The floor trusses were 

approximately 60 ft (18.3 m) or 35 ft (10.7 m) long (depending upon the relative orientation of the 

building core), spaced at 6 ft 8 in. (2.0 m). There were of dozens of variants. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–11

Source: McAllister 2002. 

Figure E–8. Schematic diagram of a floor truss. 

The prefabricated floor modules were typically 20 ft (6.1 m) wide, containing two sets of doubled trusses 

in the interior and a single truss along each edge. Thus, each seat supported either a double truss within a 

floor panel, or two single trusses from adjacent floor panels. In addition, the bottom chord of each pair of 

trusses was attached to perimeter spandrels with visco-elastic dampers. Bridging trusses ran 

perpendicular to the main bar trusses and were spaced at 13 ft 4 in. (4.06 m). The floor panels were 

covered with a corrugated steel floor deck that rested on the bridging trusses. Flutes in the deck ran 

parallel to the main trusses. Once in place, 4 in. (100 mm) of lightweight concrete was poured for the 

floor. Figure E–4 shows an assembled floor panel before the concrete floor was poured. 
Appendix E 

E–12

The minimum yield strength of the steel for the floor trusses was specified to be 50 ksi “unless otherwise 

noted.” In practice, several of the designs specified 36 ksi steel as well as 50 ksi steel (see Section E.5.2 

for complete details). 

All seats were specified to be of 36 ksi minimum yield strength. There were over 30 varieties of 

perimeter seats, with various thicknesses from 3/8 in. to 7/8 in. in 1/8 in. increments (9.5 mm to 22.2 mm 

in 3.2 mm increments). Core seats were 7/16 in., 1/2 in., 5/8 in., or 3/4 in. thick (11.1 mm, 12.7 mm, 

15.9 mm, or 19 mm). 

The floor in the core area was typically framed with rolled structural steel shapes acting compositely with 

formed concrete slabs. Certain floors outside the core were also supported by rolled structural steel 

shapes rather than trusses. These included the mechanical floors and the floors just above the mechanical 

floors (e.g., floors 75, 76, and 77). Beam framing was typically W271

 beams in the long span region and 

W16 beams in the short direction with beams spaced at 40 in. The floor was 5.75 in. thick, normalweight concrete poured on a 1.5 in. fluted steel deck, acting compositely with the steel beams. The 

concrete on the beam-framed floors above the mechanical floors was 8 in. thick, normal-weight concrete 

in the core area and 7.75 in. thick normal-weight concrete outside the core. 

Floors 107 to 110 

At the top of each tower (floor 107 to the roof), a hat truss interconnected the core columns (Fig. E–9). 

Diagonals of the hat truss were typically W12 or W14 wide flange members. In addition, four diagonal 

braces (18 in. by 26 in. box beams spanning the 35 ft gap, and 18 in. by 30 in. box beams spanning the 

60 ft gap) and four horizontal floor beams connected the hat truss to each perimeter wall at floor 108 

spandrel. The hat truss was designed to provide a base for antennae atop each tower, although only the 

WTC 1 antenna was actually built. 

Perimeter columns for floors 107 to 110 also differed from the lower floors, and were alternating small 

tube columns or wide flange columns, with the wide flange columns supporting the floor system. 

Impact Zone 

The impact zones of the two towers are of particular interest, and special testing of the steels in this region 

will be conducted. High strain-rate mechanical tests and high-temperature mechanical property tests will 

focus on those steels most prominent in the impact zones, as indicated below. 

In WTC 1, the perimeter columns torn out or otherwise damaged by the airplane impact (as judged from 

photographs of the building) were predominantly specified as 55 ksi and 60 ksi steel. In WTC 2, most 

damaged columns were specified in the 55 ksi to 65 ksi range, though there was a wide range of steel 

 

1

 The “W” in W27 beam denotes the shape of the beam, which is like the letter “H” (see Figure E–6). The number following the 

“W” is the nominal depth of the beam in inches. The second number denotes the weight of the beam in pounds per foot. A 

W27 by 114 beam is 27.28 in. high and weighs 114 lb/ft. W shapes should not be confused with HP (sometimes called H) 

shapes. Like W shapes, HP shapes have flanges with parallel faces, but unlike W shapes, the webs and flanges of HP shapes 

have equal thickness. The common I-beam is denoted as an S shape, which differs from a W shape in that the flange faces are 

not parallel. Instead, the inside flange surface has a slope of 1/6. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–13

Source: McAllister 2002:Fig. 2-10; Leslie Robertson Associates. 

Figure E–9. Hat truss in upper floors. 
Appendix E 

E–14

grades involved. Table E–2 summarizes the steel grades in the perimeter columns damaged by the 

impact. In the table, the impact zone is defined as floors 94–98 in WTC 1 and floors 78–83 in WTC 2. 

Table E–2. Number of WTC 1 and WTC 2 perimeter columns damaged by aircraft impact. 

Column Design Minimum Yield Strength Fy (ksi) 

 50 55 60 65 70 75 80 85 90 100 

WTC 1 3 27 17 5 - - - - - - 

WTC 2 1 6 13 16 2 1 1 - 2 1 

Although the extremities of the airplanes extended onto surrounding floors, these are the floors over 

which the airplanes penetrated into the buildings. 

The number of core columns damaged by the impact is not known. In the WTC 1 impact zone, the core 

columns were almost entirely wide flange shapes. In the WTC 2 impact zone, the core columns were a 

mix of box and wide flange shapes. As is typical of all core columns, the steel was predominantly 

specified as 36 ksi and 42 ksi minimum yield strength. Table E–3 describes the distribution of core 

column types in the impact zones. 

Table E–3. Number of core columns with a given minimum yield strength within the 

floors penetrated by the aircraft. 

Yield Strength Fy (ksi) Column 

Type WTC 1 (floors 94 to 98) WTC 2 (floors 78 to 83) 

 36 42 45 50 36 42 45 50 

Box 0 3 - - 38 15 - - 

Wide flange 88 44 3 3 81 6 0 1 

Note: Core columns were three stories tall and were spliced at floors 77, 80, 83, 86, 89, 92, 95, and 98. The splice is 

several feet above the floor at the story indicated. Therefore, in the WTC 1 impact zone there were three sets comprising 

141 individual columns. 

Floors Involved in Post-Impact Fires 

Special attention will be given to characterizing the performance of the structural steel found in floors 

engaged in the post-impact fires. The steels most vulnerable to heat from the fires were located in the 

zone damaged by the impact since those members were already under additional loads. Table E–4 lists 

the perimeter column types and grades of steel within these floors, defined here as floors 92 to 100 for 

WTC 1, and floors 77 to 83 for WTC 2. Table E–5 lists this information for the core columns. 

Table E–4. Number of perimeter columns of specified grades in floors with 

significant fire. 

Perimeter Column Design Minimum Yield Strength Fy (ksi) 

Floors 45 46 50 55 60 65 70 75 80 85 90 100 

WTC 1 92 to 100 0 1 26 225 246 196 122 83 40 16 7 16 

WTC 2 77 to 83 1 3 34 217 255 88 29 25 26 40 91 105 
Interim Report on Contemporaneous Structural Steel Specifications 

E–15

Table E–5. Number of core columns of specified grades in floors with 

significant fire. 

Yield Strength Fy (ksi) Column 

Type WTC 1 (floors 92 to 100) WTC 2 (floors 77 to 83) 

36 42 45 50 36 42 45 50 

Box 0 7 - - 69 16 - - 

Wide flange 115 58 3 5 86 13 1 3 

E.5 CONTEMPORANEOUS STEEL SPECIFICATIONS 

This section integrates information from many sources on the steels used in the WTC and has three 

primary goals. First, contemporaneous (1960s era) American and Japanese steel specifications are 

summarized. Second, relevant information on steel properties from the construction documents and open 

literature sources is presented. Finally, estimated values for typical yield and tensile strengths and 

elongations for the numerous steels in the buildings are given (as opposed to the specified minimum 

values). 

The first and second goals are approached from several directions. As is common practice, the structural 

engineering plans (obtained from LERA) only specify the minimum yield strengths and dimensions of the 

beams and columns. The steel contracts that the Port Authority (PONYA 1967, Ch. 2 §) awarded for the 

fabrication provided the specifications for the allowable steels to meet those minimum yield strengths. 

Those contracts allowed the fabricators to use steels that conformed to certain ASTM Standard 

Specifications. In addition, the contracts also permitted the fabricators to use certain proprietary steels 

from U.S. steel mills. These were required to conform to specific, dated and published data sheets that 

the steel mills provided. Finally, the contracts also allowed other proprietary steels not listed in the 

contract, provided that the Port Authority chief engineer of the project reviewed and formally approved 

their specifications (PONYA 1967, Clause 1). In all cases, the steels required extensive documentation to 

be acceptable for use. 

Regarding the third goal, the best documentation of typical steel properties is contained in the mill test 

reports that detail the properties (Fy, tensile strength, elongation, chemistry, etc.) of the individual steel 

plates and shapes for the steels supplied. During late 2002 and early 2003, NIST investigators contacted 

the fabricating companies still in existence, their successors where possible, or in many cases their former 

employees, in a search for these mill test reports for steels used in the fire and impact zone, as well as 

other documents. None of the individuals or corporations retained these records. Section E.5.4 

summarizes these contacts. The sources for steel properties NIST has obtained to date sometimes supply 

inconsistent values for the properties, so this report is a best effort to supply the steel properties. 

This section focuses on the steels used in the area of the impact and fire: the floor panels, the perimeter 

columns, the welded core box columns, and the rolled core columns, fabricated by Laclede, Pacific Car & 

Foundry, Stanray Pacific, and Montague-Betts, respectively. It does not consider any of the sections of 

the buildings remote from the impact and fire sites, so fabricators of sections below the 9th floor (Mosher, 

Drier, Levinson, Pittsburgh-Des Moines, and Atlas) are not addressed, although Attachment 1 provides 

some background information on these companies. 
Appendix E 

E–16

In this document, “contemporaneous” refers to the standards in effect at the time of construction, in 

contrast to contemporary (or present-day) standards. ASTM standards are modified and renewed at 

regular intervals, so the current requirements of a standard may not have been in force during the WTC 

era. This distinction is also important because historical versions of standards can be difficult to locate. 

Attachment 2 summarizes the generally minor differences between the contemporaneous and 

contemporary versions of the relevant standards. 

E.5.1 Standards Called for in the Steel Contracts 

The Port Authority had a generic contract that listed allowable steel standard specifications, which went 

to all the fabricators. Generally, it specified that a given steel was acceptable for use if it conformed to 

one of a list of ASTM standards that were in force during September 1967. It also allowed several steels 

that were modifications of these ASTM standards. In addition, it allowed a number of proprietary steels 

made by U.S. steel mills. Finally, it allowed the use of other proprietary steels after formal approval by 

the Project Engineer, an employee of PONYA. It was by this last method that Pacific Car and Foundry 

(PC&F) received approval to use the Japanese steels in the perimeter columns. 

It is important to remember that an ASTM standard can admit a wide variety of steel compositions and 

strengths. A specific steel might be capable of meeting several distinct ASTM steel standards. For 

instance, in the WTC construction era, ASTM A 36 only specified a minimum 36 ksi yield strength, an 

upper and lower tensile strength and carbon, manganese, silicon, phosphorus, and sulfur contents. Many 

high-strength low-alloy steels designed to meet other ASTM structural steel standards (e.g., A 572, 

A 242) will also meet A 36. Simply identifying a specific steel as meeting a given ASTM standard will 

not uniquely identify its composition or mechanical properties. 

In terms of shapes and tolerances, all the steel was required to meet ASTM A 6, “General Requirements 

for Delivery of Rolled Steel Plates...” 

Steels 

Table E–6 summarizes the allowable steels listed in the contract (in “Chapter 2 (Materials)”) between the 

Port Authority and all the fabricators. Note that it does not list ASTM A 572, a common, current standard 

for niobium-vanadium structural steels, which was established only in 1966. The proprietary steels 

allowed by the contract do include U.S.S. EX-TEN and Bethlehem V-series, however. These steels 

would conform to ASTM A 572, which was under development in that era. Tables E–7 and E–8 

summarize the relevant structural steel specifications from the WTC construction era, including data on 

the various “modified” standards allowed in the Materials chapter of the fabricators’ contracts. 

Although Japanese steel mills supplied much of the steel, NIST has found no evidence that PONYA or 

the fabricators ever referred to any Japanese standards. Table E–9 summarizes the relevant Japanese 

Industrial Standard (JIS) from the era. They not as detailed as the corresponding ASTM steel standards, 

and mostly just specify minimum yield strength and maximum carbon content. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–17

Table E–6. Steels specified as acceptable by PONYA in its contract with steel fabricators. 

Standard Fy (ksi) Description of Standard 

Structural Steels 

A 36 36 Structural steel 

A 242 50 High-strength structural steel 

A 440 50 High-strength structural steel 

A 441 50 High-strength manganese vanadium steel 

A 441 modifieda 50 As A 441 with Cr and increased Cu 

A 514 100 Quenched and tempered alloy steel plate for welding 

A 514 Modified 100 As A 514, but TS requirements waived 

USS CON PAC Grades 70 and 80 

Bethlehem V series Grades 42, 45, 50, 55, 60, 65 

Lukens Grades 45, 50, 55, 60, 80 

USS EX-TEN Grades 42,45,50,55, 60, 65, 70 

USS COR-TEN “considered to conform to A 441 modified” 

Lukens COR-TEN “considered to conform to A 441 modified” 

Pressure Vessel Steels 

A 302 Manganese molybdenum steel for pressure vessels 

A 302 modified 

A 533 Mn-Mo and Mn-Mo-Ni steels for pressure vessels 

A 533 modified 

A 542 Cr-Mo steel for pressure vessels 

a. Apparently (Irving 1968) “A 441 modified” was a catch-all term for a group of steels that were codified in 1968 under ASTM 

A 588 “High-Strength Low-Alloy Structural Steel with 50,000 psi Minimum Yield Point to 4 in. Thick.” 

Key: Cr, chromium; Cu, copper; Mn, manganese; Mo, molybdenum; Ni, nickel; TS, tensile strength. 
Appendix E 

E–18

Table E–7. Summary of mechanical properties from relevant ASTM International 

structural steel standards from WTC era. 

Standard Title 

Fy

Min. 

(ksi) 

TS 

Min. 

(ksi) 

TS 

Max. 

(ksi) 

Elong 

Min. 

(%) Notes 

A 36-66a Structural steel 36 58 80 20 For shapes; plates have higher C, Mn, 

and Si requirements 

A 242-66a High-strength low-alloy structural 

steel 

50 70 18 Plates and bars t <= 0.75 in.; Group 

1&2 shapes 

A 440-67a High-strength structural steel 50 70 18 Plates and bars t <= 0.75 in.; Group 

1&2 shapes 

A 440-67a High-strength structural steel 46 67 19 Plates and bars 0.75 in. < t <=1.5 in.; 

Group 3 shapes; elongation reductions 

based for t > 0.75 in. 

A 440-67a High-strength structural steel 42 63 16 Plates and bars 1.5 in. < t <=4 in.; 

Group 4&5 shapes.; elongation 

reductions for t > 3.5 in. 

A 441-66a High-strength low-alloy structural 

manganese vanadium steel 

50 70 18 Plates and bars t <= 0.75 in.; Group 

1&2 shapes 

A 441- 

modifieda

As A 441, but modified by PONYA 50 70 19 Plates & bars 0.75 in.<= t<= 4 in.; 

Group 1,2,3 shapes 

A 441-66a High-strength low-alloy structural 

manganese vanadium steel 

46 67 19 Plates and bars 0.75 in. < t <=1.5 in.; 

Group 3 shapes.; elongation 

minimums relaxed for t > 0.75 in. 

A 441-66a High-strength low-alloy structural 

manganese vanadium steel 

42 63 16 Plates and bars 1.5 in. < t <= 4 in.; 

Group 4&5 shapes 

A 441-66a High-strength low-alloy structural 

manganese vanadium steel 

40 60 Plates and bars 4 in. < t <= 8 in.; 

elongations on 2 in. GL 

A 514-65a High-yield-strength, quenched and 

tempered alloy steel plate, suitable for 

welding 

100 115 135 18 t <= 0.75 in. 

A 514-65a High-yield-strength, quenched and 

tempered alloy steel plate, suitable for 

welding 

100 115 135 18 0.75 in. < t <= 2.5 in. 

A 514-65a High-yield-strength, quenched and 

tempered alloy steel plate, suitable for 

welding 

90 105 135 17 2.5 in. < t <= 4 in. 

A 514- 

modifieda

 100 x x See 

std. 

As A 514, but TS waived in PONYA 

steel contract 

A 529-64 Structural steel with 42 ksi minimum 

yield point 

42 60 85 19 

A 572-70 High strength low-alloy columbium 

vanadium steels of structural quality 

50 65 18 6 grades: Fy = (42 45 50 55 60 65) ksi; 

different C contents 

A 573-70 Structural carbon steel plates of 

improved toughness 

35 65 77 20 2 grades 65 ksi or 70 ksi TS 

A 588-70 High-strength low-alloy structural 

steel with 50 ksi minimum yield point 

to 4 in. thick 

50 70 18 9 chemistries 

a. Allowed by PONYA Steel contract, Chapter 2 “Materials.” 

Key: C, carbon; Elong, elongation to failure; Fy, specified minimum yield strength; Mn, manganese; Si, silicon; 

TS, tensile strength. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–19

Table E–8. Summary of chemistry data from relevant ASTM International structural steel 

standards from WTC era. 

Chemistry (%)

Standard 

C 

Max. 

Mn 

Max. 

Si 

Max. Ni Cr 

V 

Min. 

Cu 

Min. 

P 

Max. 

S 

Max. Other/Notes 

A 36-66 shapes 0.26 NR NR 0.2 0.04 0.05 Cu where specified

A36-66 plates with 

t≤0.75 in. 

0.25 NR NR 0.04 0.05 Cu where specified

A36-66 plates with 

0.75 in. <t≤1.5 in 

0.25 0.8-1.2 NR 0.04 0.05 Cu where specified

A36-66 plates with 

1.5 in. <t≤2.5 in 

0.26 0.8-1.2 0.15-0.3 0.04 0.05 Cu where 

specifieda

A 242-66 0.22 1.25 0.05 Type 1 

A 242-66 0.15 1.40 0.05 Type 2 

A 440-67 0.28 1.1-1.6 0.3 0.2 0.06 0.05 

A 441-66 0.22 0.85-1.25 0.3 0.02 0.2 0.04 0.05 

A 441-modified 0.19 0.85-1.25 0.15-0.3 0.4-0.65 0.02 0.25-0.4 0.04 0.05 

A 441-66 0.22 0.85-1.25 0.3 0.02 0.2 0.04 0.05 

A 514-65 8 individual 

chemistries, with 

Cr, Mo, B 

A 529-64 0.27 1.2 0.2 0.04 0.05 

A 572-70 0.22 1.35 0.3 0.04 0.05 4 variants with Nb 

or Va or Nb+Va, 

or V+N 

A 573-70 0.24 0.85-1.25 0.15-0.30 0.04 0.05 

A 588-70 9 individual 

chemistries, 

generally with Cr, 

Ni, V, Nb 

a. A 36 plates have different requirements for thicker sections that include higher carbon allowables and slightly different 

manganese requirements. 

Key: B, boron; C, carbon; Cr, chromium; Cu, copper; Mn, manganese; Mo, molybdenum; Nb, niobium; Ni, nickel; NR, no 

requirement; P, phosphorus; Si, silicon; S, sulfur; V, vanadium. 
Appendix E 

E–20

Table E–9. Summary of Japan Industrial Standard structural steel standards from 1974. 

Standard Grade 

Fy

Min. 

(ksi) 

TS 

Min. 

(ksi) 

TS 

Max. 

(ksi)

C 

Max. 

(%) 

Mn 

Max. 

(%) 

Si 

Max. 

(%) 

Cr 

(%) 

Cu 

Min. 

(%) 

P 

Max. 

(%) 

S 

Max. 

(%) Other 

SM50a 45 71 88 0.20 1.5 0.55 0.04 0.04 Add any element 

“if necessary” 

SM50b 

SM50c 

45 71 88 0.18 0.04 0.04 Add any element 

“if necessary” 

SM50Ya 

SM50Yb 

51 71 88 0.20 1.5 0.55 0.04 0.04 Add any element 

“if necessary” 

SM53b 

SM53c 

51 75 92 Add any element 

“if necessary” 

JISG3106-73 Rolled 

Steel for Welded 

Structure 

SM58 65 82 104 0.18 1.5 0.55 0.04 0.04 

SMA50a 

SMA50b 

SMA50c 

51 71 88 0.19 1.4 0.75 0.3–1.2 0.2–0.7 0.04 0.04 + Mo or Nb or Ni 

or Ti or V or Zr 

JIS G3114-73 Hot 

Rolled Atmospheric 

Corrosion Resistant 

Steel for Welded 

Structure SMA58b 65 82 104 0.19 1.4 0.75 0.3–1.2 0.2–0.7 0.04 0.04 +Mo, Ni, Nb, Ti, 

Va and or Zr 

SS55 57 78 0.30 1.6 0.04 0.04 Add any element 

"if necessary" 

JIS G3101-73 Rolled 

Steel for General 

Structure 

SS50 40 71 88 0.05 0.05 

Key: C, carbon; Cr, chromium; Cu, copper; Fy, yield strength; JIS, Japan Industrial Standard; Mo, molybdenum; Nb, niobium; 

Ni, nickel; P, phosphorus; Si, silicon; S, sulfur; Ti, titanium; TS, tensile strength; V, vanadium; Zr, zirconium.

Note: Compositions are given as mass fractions. Thickness range for all standards is 16 mm< t < 40 mm. 

Source: World Steel Standards, Handbook of Comparative (1974). 

Fasteners 

Section E.6 covers fastener standards in the section on connections (bolts and welds). 

E.5.2 Steels Used in Construction 

Information from the suppliers and fabricators was used to identify the specific steels supplied to meet 

those contractual requirements. Table E–10 and Attachment 1 provide background information on the 

various fabricators of WTC steel, including tons of steel reported in their contracts. The rest of this 

section summarizes information on the steels used in the impact and fire zones of the towers. 

Floor Trusses 

Laclede Steel manufactured the trusses for the composite floor panels for both WTC 1 and WTC 2 from 

steel they made and rolled at their mill in Alton, Illinois. The chords were fabricated from hot-rolled 

angles, while the web was fabricated from hot-rolled round bar, Fig. E–10. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–21

Table E–10. Steel companies involved in WTC construction and their contracts. 

Fabricator Current Status Component Tons 

Pacific Car & Foundry, Co. Sold in 1974 Exterior columns and spandrels 55,800 

Montague Betts, Co. Inc No longer a steel 

fabricator 

Rolled columns and beams above 9th 

floor 

25,900 

Pittsburgh-Des Moines Steel 

Co. 

Bifurcation columns (“trees”) 4th to 9th 

floor 

6,800 

Atlas Machine & Iron Works No longer in business Box columns below the bifurcation 

columns to 4th floor 

13,600 

Mosher Steel Co. Currently active Core box columns below the 9th floor 13,000 

Stanray Pacific Corp. Closed in 1971 Core box columns above the 9th floor 31,100 

Levinson Steel Co. Sold in 1997, parent 

company in bankruptcy 

Supports for slabs below grade 12,000 

Laclede Steel Co. Bankrupt in 2001, new 

owners of rolling mill 

Floor trusses Unknown 

Drier Structural Steel Co, Inc. Unknown Grillages Unknown 

 Total 141,170 

Source: Feld 1971. 

Figure E–10. Schematic diagram of the stress-strain behavior of a structural steel. 
Appendix E 

E–22

According to internal Laclede documents (Bay 1968 †), the top chord angles, as well as most round bars, 

were fabricated to meet ASTM A 242 (Fy = 50 ksi). Only 1.09 in. (27.7 mm) and 1 13/16 in. (46.0 mm) 

round bars and the bottom chord angles were specified as ASTM A 36. Conversations with Laclede 

metallurgists (Brown 2002 †) active during the WTC construction revealed that even for components 

specified as ASTM A 36, Laclede would have supplied a vanadium, micro-alloyed steel with a typical 

Fy = 50 ksi, similar to a contemporary A 572 steel. In all the Laclede documents NIST examined, there 

were only two different mill test reports on A 242 steel, both from mid-1969; see Table E–11. These mill 

test reports indicate that the A 242 steel supplied is a niobium-containing steel similar to modern ASTM 

A 572 steels with yield points that exceed the specified minimum by about 10 ksi. 

Table E–11. Properties of Laclede ASTM A 242 steels obtained from Laclede mill 

test reports. 

Element Composition (%) 

Component 

Fy 

(ksi) C Mn P S V Nb Source 

2 in. by 1.5 in. by 0.25 in. bulb 

angle heat 83033 

62.8 0.20 0.86 0.014 0.044 NR 0.020 (Kamper 1968 †) 

3 in. by 2 in. by 0.25 in. bulb 

angle heat 83162 

60.1 0.19 0.77 0.013 0.043 NR 0.015 (Kamper 1968 †) 

1.14 in. rod heat 76056 54 0.19 0.80 0.005 0.024 NR NR (White1969b †) 

2 tests 

Key: C, carbon; Mn, manganese; Nb, niobium; NR, not reported; P, phosphorous; S, sulfur; V, vanadium. 

Note: Compositions are reported % mass fractions. 

Perimeter Columns and Spandrels 

The perimeter wall columns, fabricated by PC&F, comprise three important sub assemblies: the columns, 

the spandrels, and the truss seats. The structural engineering plans called for the columns to be fabricated 

from 14 grades of steel with Fy = (36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100) ksi. Above 

floor 75, more than half of the columns have yield strengths greater than or equal to 55 ksi and less than 

or equal to 70 ksi. The spandrels were fabricated from 12 grades of steel with Fy = (36, 42, 45, 46, 50, 55, 

60, 65, 70, 75, 80, and 85) ksi. The truss seats were specified to be fabricated from steel with Fy = 36 ksi 

minimum. 

Yawata Iron and Steel Co. supplied most of the steel to PC&F for the perimeter columns and spandrels. 

In general, the exterior (or web) and side (or flange) plates of each column and the spandrels were 

fabricated from Japanese steel, and the inner web plate (plate 3, see Fig. E–3) was fabricated from 

domestic steel (Symes 1969a §; White 1969a §). Searches of archival material yielded no information on 

the steels for the truss seats beyond the fact that they were specified as Fy = 36 ksi. 

A contemporaneous Yawata document (Yawata 1969 †) indicates that Yawata shipped 46,000 metric tons 

of WEL-TEN 60, 60R, 62, 70, and 80 to PC&F. That document refers to WEL-TEN 80, rather than 

WEL-TEN 80C, which is a Yawata steel with a different chemistry, but identical yield strength. The 

document certainly must actually mean WEL-TEN 80C, because all other sources, including other 

Yawata sources, that mention WEL-TEN steels refer to WEL-TEN 80C. Most sources, for instance, Feld 

(1971), put the PC&F contract at 55,800 tons. Assuming the Yawata document (1969 †) refers to metric 

tons, that would still leave a minimum of 5,100 tons from other sources. The inner web plate (plate 3, 

Fig. E–3) represents about 12 percent of the total area of a perimeter column panel. The 5,100 tons 
Interim Report on Contemporaneous Structural Steel Specifications 

E–23

unaccounted for in the Yawata contract is not inconsistent with the assertion that the inner web was 

usually fabricated from domestic steel, while the remaining plates were fabricated from Yawata steel. 

Several sources (ENR 1967; Monti 1967a §; White 1967a §; Feld 1967a §) indicate that Kawasaki Steel 

also supplied PC&F, but apparently only 36 ksi grade (Feld 1967a §). Ronald Symes (2002 †), PC&F 

chief engineer, could not remember any other foreign steel suppliers other than Kawasaki. However, the 

fabricators only interacted with the Japanese import companies rather than with the steel mills directly. 

Mitsui (now Mitsui USA) imported the Japanese steel for PC&F. Because the flanges and spandrels are 

the primary structural components of the perimeter columns, and they were all fabricated from Yawata 

steel, the properties of the perimeter columns can be based on the mechanical properties of the Yawata 

steels. 

During the 1960s Yawata produced a number of named, proprietary grades (such as WEL-TEN and 

YAW-TEN series) of weldable steels with specified minimum properties. Several of these named grades 

supplied to PC&F (WEL-TEN 60, WEL-TEN 62, WEL-TEN 80C) are common in the contemporaneous 

literature, and open literature publications (Ito 1965a, Ito 1965b; Goda 1964) describe many of their 

physical and mechanical properties other than specified minimum strength quite extensively. For two of 

the named, proprietary grades that Yawata supplied to PC&F (WEL-TEN 60R and WEL-TEN 70), NIST 

has been unable to find corroborating specifications or mechanical property data, even in consultation 

with Nippon Steel. It is possible that these names were assigned simply for convenience for the WTC 

construction. Chemically, WEL-TEN 60, 60R and 62 are similar to contemporary ASTM A 588, with 

their Cr additions and high silicon contents, though none would meet that specification exactly. 

WEL-TEN 60, 62, and 70 are heat-treated steels, while WEL-TEN 60R is a hot-rolled steel. 

WEL-TEN 80C is a Cr-Mo steel that is very similar to contemporary A 514 steels, and possibly could 

have been manufactured to meet that contemporary specification. According to PC&F documents (Symes 

1967c §), Yawata intended to supply grades that would meet the “ASTM A 441-modified” specification 

(see Table E–7) of PONYA for the lower strength column plates. From the proposed specification, these 

“A 441-modified” compositions were similar to contemporary A 588 steels, with their added Cr and use 

of Nb for strengthening. Their chemistries do not correspond to any other named grade of Yawata steel, 

for example WEL-TEN 50, WEL-TEN 55, YES 36, YES 40 or YAW-TEN 50. For the intermediate 

strength plates (55 ksi, 60 ksi, and 65 ksi), Yawata intended to furnish heat-treated WEL-TEN grades for 

the thicker sections and the hot-rolled “A 441 modified” grades for the thinner sections. Tables E–12 and 

E–13 summarize these specifications and representative properties, obtained from a variety of documents. 

Note that not all the sources agree on yield strength or chemistries, probably because Yawata could tailor 

the steels for specific applications. The entries at the top of the table are for the steels that a PC&F memo 

(Symes 1967c §) mentions, while the bottom entries detail representative data culled from many literature 

sources for all grades of Yawata weldable steels. 

NIST has located a total of six mill test reports (tests performed at the Yawata rolling mill) describing 

135 plates (Symes 1969b §; Barkshire 1969a §; White 1969c §) of Yawata steels: two for Fy = 75 ksi, one 

for Fy = 70 ksi, two for Fy = 50 ksi, and one for Fy = 45 ksi. When the originals were microfilmed after 

the construction was completed, the technician did not rotate the landscape pages into portrait orientation, 

so the sheets only show the measured yield point, tensile strength and elongation, but not chemistry. For 

each steel, the measured yield strength of the plates increases with decreasing thickness. The thickest 

WEL-TEN 62 plates (t = 1.5 in.) plates typically have yield strengths 5 ksi higher than the specified yield 

strength. The thinnest plates (t = 0.375 in.) have yield strengths 15 ksi to 20 ksi higher than the stated 
Appendix E 

E–24

Table E–12. Specified properties for Yawata contemporaneous steel grades. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–25

Table E–13. Reported properties for Yawata contemporaneous steel grades. 
Appendix E 

E–26

yield strength. For the lower strength plates (Fy = 45 ksi and Fy = 50 ksi) the measured yield strength 

increases less rapidly with decreasing thickness: to a first approximation, their strength is independent of 

thickness. They average 7.4 ksi and 11.8 ksi higher than the specified yield strength, respectively. 

Contemporaneous documents indicate that PC&F also purchased V-series (White 1968a §; 2003 †) and 

modified V-series plate from Bethlehem Steel (Symes 1967a §), EX-TEN and modified EX-TEN from 

U.S. Steel (Symes 1967a §; White 2003 †; Barkshire 1968a §), and various Kaisaloy grades (Barkshire 

1968b §) from Kaiser steel, for use in the interior plates. The inner plate (plate 3, see Figure E–3) is 

usually half the thickness of the flange plates, and never exceeds 15/16 in. thick, and so represents at most 

5 percent of the mass of steel in the entire contract. Status reports from mid-1968 indicate that PC&F 

phased out U.S. Steel and Kaiser and replaced them with Bethlehem as the only domestic 

supplier (Barkshire 1968c §). Presumably most of the inner web plates (plate 3) in the columns (see 

Fig. E–3) near the impact floors were made from hot-rolled Bethlehem V-series steels. Table E–14 

summarizes the properties of the V-series (Alloy Digest 1970) and modified V-series 

steels (Symes 1967b §). 

In summary, NIST has extensive data from open literature sources for properties other than chemistry and 

yield strength for the 65 ksi WEL-TEN 60, the 70 ksi WEL-TEN 62, and the 100 ksi WEL-TEN 80C. 

Properties for the “A 441-modified” grades and for WEL-TEN 70 and WEL-TEN 60R must be estimated 

theoretically, from accepted literature values for plate steels, or experimentally from tests on recovered 

steels. 

Core (Welded Box Columns) 

Stanray Pacific Corp. fabricated the welded core columns in both buildings above floor 9. The plans 

called for two grades of steel with 36 ksi and 42 ksi minimum yield strengths. Contemporaneous Stanray 

Pacific documents (Morris 1967 §; Warner 1967 §) indicate that Stanray Pacific purchased nearly all the 

steel plate for the core columns from two sources. The 10,240 tons from Colvilles Ltd. (Dalzell Works, 

Motherwell, Scotland) and 21,760 tons from Fuji Iron and Steel (Hirohata Works). The total of 

32,000 tons is close to the 31,100 tons that Feld (1971) reported in his Civil Engineering article that 

summarizes the construction of the WTC towers. It is likely, then, that these two mills supplied nearly all 

the steel for the welded core columns. Telephone conversations with M. McKnight (2003 †), formerly 

with the British Steel Export Association, which imported the steel to the United States, confirmed 

Colvilles as a supplier to Stanray Pacific. 

The same document (Warner 1967 §) that details the major suppliers, indicates that Fuji Steel supplied all 

plates thinner than 1.75 in. Both mills supplied plates with t≥1.75 in., but even there, Fuji supplied about 

60 percent of the total mass of steel used. In the fire and impact floors of WTC 1 (floor 94 to floor 98), 

only three of the columns are welded, box columns, and all three are made from plate thinner than 1.75 in. 

In the fire and impact floors of WTC 2 (floor 78 to floor 84) only 9 of 52 welded box columns are made 

from plate 1.75 in. or thicker. In terms of steel properties for modeling, then, the columns can be modeled 

with the properties of the Fuji-supplied plates alone. NIST has located a mill test report for a single Fuji 

Steel A 36 plate (Morris 1969 §), and a third-party chemical analysis of a Colvilles A 36 

plate (Walton 1968 §) (Table E–15). Other than these, NIST has located no other mill records. See 

Table E–16 for the search details. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–27

Table E–14. Mechanical properties of U.S. Steel and Bethlehem V-series steels. 
Appendix E 

E–28

Table E–15. Chemistry and mechanical property data for a Fuji Steel plate and a Colvilles 

plate used for core columns. 

Description 

Fy

(ksi)

TS 

(ksi) 

Elong 

(%) 

C 

(%) 

Mn 

(%) 

Si 

(%) 

Ni 

(%) 

Cr 

(%) 

V 

(%) 

Cu 

(%) 

P 

(%) 

S 

(%) Other Source

12.6 ton A 36 plate 

3 in. by 65.5 in. by 

453.75 in. Rolled at 

Hirohata works, Fuji Steel; 

tested August 5, 1969. 

38.4 64.9 32 0.2 0.96 0.2 ND ND ND ND 0.013 0.008 (a) 

Chemical analysis of a 6 in. 

by 52 in. by 18 ft 0.75 in. 

Colvilles A 36 plate Heat 

H218 Slab 1804H by 

Materials Testing 

Laboratory, Los Angeles, 

CA, February 2, 1968. 

ND ND ND 0.2 0.99 0.3 0.2 <0.01 0.005 0.2 0.017 0.035 0.01 Mo, 

0.02 Co 

(b) 

a. Morris 1969. 

b. Walton 1969. 

Key: C, carbon; Cr, chromium; Cu, copper; Elong, elongation to failure; Fy, specified minimum yield strength; ND, not determined; 

Mn, manganese; Ni, nickel; P, phosphorus; S, sulfur; Si, silicon; TS, tensile strength; V, vanadium. 

Note: Compositions expressed as mass fractions. 

Table E–16. Databases searched for WTC information. 

Database Query Earliest year covered 

Cambridge Scientific Databases: 

Metadex, Weldasearch 

Yawata 

World Trade Center 

1966 (Metadex) 

1967 (Weldasearch) 

OCLC FirstSearch 

Database: WorldCat 

Search for steel periodicals—used to identify 

possible additional sources of information 

Search for library holdings of 

contemporaneous steel catalogs. Also 

searched on Alibris for used steel data sheets. 

19th century 

American Society of Civil 

Engineers Database 

www.pubs.asce.org 

World Trade Center, Yawata, Stanray, Pacific 

Car and Foundry: no useful information 

1973 

The LERA archives contain several examples of steel substitutions from other mills. Because the sum of 

the Colvilles and Fuji contracts, 32,000 tons (Warner 1967 §), is larger than the PONYA value of the 

contract, 31,100 tons (Feld 1971), these were probably isolated, uncommon occurrences. They are not 

relevant for estimating steel properties. The documents were probably retained because they documented 

substitutions that required authorization by PONYA. These records include mill test reports for a single 

A 7 plate purchased from Crest Steel and rolled by Nippon Kokan Steel (Fukuyama, Japan) 

(Tarkan 1969 §) and eight A 36 plates with 3.25 in.>t>7.25 purchased from Lukens Steel (Morris 

1969b §). A report (Yamada 1967 §) of the first shipment of plates from Japan lists the plates as being 

A 36 and A 572 grade 42. The appearance of A 572 is notable, because it is not listed in the PONYA 

steel contract list of steels that could be used without requesting formal approval. The document is not a 

mill test report, however, so it not completely certain that the 42 ksi plates were actually supplied to 

A 572. It is also possible that documents authorizing the use of steels meeting A 572 have not survived. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–29

Core (Rolled Wide flange Shapes) 

Montague-Betts Steel fabricated all the rolled WF shapes for the core columns as well as all the beams in 

both towers above floor 9. These rolled shapes represent a significant fraction of the total core columns 

in the fire and impact zone. Above floor 80 in WTC 2, more than half of the core columns were WF 

shapes, and above floor 94 in WTC 1, 43 of the 46 columns are WF shapes. The plans called for steels 

with 36 ksi, 42 ksi, 45 ksi, and 50 ksi minimum yield strengths, but very few of the rolled shapes used the 

45 ksi or the 50 ksi material. Various sources (Davis 2002 †; Yawata 1969 †) confirm that 

Montague-Betts purchased about 12,000 tons (of a total contract of 25,900 tons) of A 36 and A 441 wide 

flange shapes from Yawata Iron and Steel, Sakai Works. An additional 1,200 tons came from 

Dorman-Long, Lackenby Works, Middlesborough, England (Gallagher 1968 §; Goode 1967 §). Given 

the size of the Yawata contract, it is likely that it represents the majority, if not all, of the WF core 

columns. Because Yawata engineers felt that the “A 441-modified” composition was protected by a 

U.S. Steel patent (Clarkson 1967 §), they also obtained permission to supply high-strength steel to 

different “A 441-modified” composition (see Table E–3) with 0.2 percent to 0.4 percent mass fraction 

added Ni (White 1967b §). Whether this approval represents a complete substitution of a Yawata specific 

alternate “A 441 modified” for the original A 441 modified, or simply an alternate specification for use in 

limited instances, is unknown. Montague-Betts CEO William Davis (2002 †), who worked on the 

project, confirmed that Montague-Betts also purchased steel from Bethlehem and U.S. Steel, the only two 

domestic mills that produced 14WF rolled sections heavier than 87 lb/ft (AISI, 1973). To date, NIST has 

found no mill records for chemistry or mechanical properties for any of the column steels used in the 

Montague-Betts contract. See Table E–15 for the search details. 

E.5.3 Recommended Values for Mechanical Properties 

From the data NIST has recovered from various sources, it is possible to recommend values for estimated 

yield strength for the various steels for use in analyzing the performance of the buildings. These sources 

include mill test reports of WTC steels from corporate archives and contemporaneous studies of the 

properties of plates and shapes of structural steels. These estimates should be confirmed with results of 

mechanical tests on recovered steel, because they include assumptions about general steelmaking 

practices that may not have been employed for the specific steels in the WTC. 

Central to the estimation of properties is the data from the mill test report that accompanies every piece of 

steel sold to ASTM International structural steel specifications. In this report, the steel mill attests to 

certain measures of the quality and properties of the steel supplied. To understand the steel mechanical 

properties, it is important to recognize the limitations on the information contained in the mill test reports. 

Most of the characterization for structural steel for buildings is conducted on a per heat (or ladle) basis. A 

heat of steel weighs up to several hundred tons and represents the unit at which the steel mill modifies the 

chemistry for the intended application. Heats of molten steel are poured into ingots to solidify. After 

solidification and homogenization, the steel mill rolls the ingots into plates and structural shapes. A 

single heat may supply the steel for many ingots, and a single ingot can supply the steel for many plates 

or shapes. 

For structural steel intended for buildings, both in the WTC era and now, the mill test report contains the 

results of a single chemical analysis of the steel, taken before the ladle of molten steel is poured into the 
Appendix E 

E–30

ingot to solidify. It also contains the results of one or two tension tests, depending on the size of the heat, 

to evaluate the yield and tensile strengths of the steel. Yield strength is the stress at which the steel first 

begins to deform permanently, rather than elastically. Buildings are designed so that the stresses do not 

exceed the yield strength of the steel. Tensile strength represents the maximum stress the steel can carry. 

The test specimen does not necessarily come from the plate purchased, nor is it likely that it originates 

from a plate of identical thickness to the one purchased. Therefore, the properties of the plate may differ 

slightly from the properties that the mill test reports. In essence, the mill test report is a quality control 

tool. It represents a check that the properties of that production of plates or shapes are in the range that 

they should be. The mill test report is not the average of a collection of tests, nor is it a guarantee that the 

entire plate or shape has a yield strength or chemistry that would meet the specification. 

During the 1960s and 1970s, several studies (AISI 1973; Galambos 1976; Alpsten 1972) characterized the 

variability in properties of steels supplied to various standards. These studies asked the question, “If I 

buy a 36 ksi steel, what is the mean value of the yield strength of the plates that the steel mill supplies to 

me?” They answered this question by examining thousands of mill test reports, but not by doing 

independent product testing. Because the tension test to certify the mechanical properties is conducted 

near the end of the production process, scrapping a heat of steel because it did not meet the intended 

specification is undesirable. Thus, steel mills generally strive to make steels in which the strength 

exceeds the intended specification. Typically, the yield strengths in the mill test reports exceeded the 

specified minimum values. The exact value depended on the value of the yield strength specified in the 

standard, and whether the steel was supplied as plates or shapes. Of course, the yield strength in the mill 

test report will never be less than the standard calls for, because the steel could not have been sold as 

meeting the standard. The results of these studies are useful in estimating the properties of WTC steels 

when no other corroborating evidence is available. 

A second question that some studies attempted to answer was, “If I buy an A 36 plate (a steel with 

Fy = 36 ksi), what is the probability that a tensile test that I do on that plate will yield a value less than 

36 ksi?” Here, the question is about tests that the user conducts, and the studies attempted to characterize 

the distribution of strengths, rather than the mean value. The American Iron and Steel Institute 

commissioned the most complete and relevant of these studies in the early 1970s. It compared the results 

of subsequent tensile tests to the value listed on the mill test report. The most important conclusion from 

this study is that it is not uncommon for a product tension test to produce a yield strength that is less than 

the standard allows. The executive summary of the AISI report shows an example for an A 36 steel 

(specified minimum yield strength of 36 ksi) where measured yield strength on the mill test report is 

38 ksi (i.e., 6 percent over the minimum). Even in this plate, there is a 22 percent probability that a 

second test will produce a yield strength less than 36 ksi, i.e., below the specified minimum yield 

strength. Because the distribution of yield strengths is reasonably narrow, there is only a 0.1 percent 

probability that the test will have a yield strength less than 30 ksi, however. It is likely, therefore, that 

some tension tests done on recovered steel as part of the WTC investigation will produce yield strengths 

that are less than the relevant standard called for. The occasional appearance of a low yield strength in 

tests of recovered steel cannot be interpreted as meaning that the steel was defective, or even that it did 

not meet the standard to which it was supplied. 

Several further corrections must be made in estimating the deformation properties of structural steel for 

modeling. The three most important of these arise because the tensile test method specified in the 

standard does not perfectly match real deformation conditions. In the mill tension test, a test specimen is 
Interim Report on Contemporaneous Structural Steel Specifications 

E–31

cut from the plate and machined into the proper shape. It is then pulled in tension in a testing machine at 

a constant, prescribed elongation rate, while measuring the resulting load on the specimen. In contrast, 

the steel in a building supports loads that can be considered to be quasi-static. 

The elongation rates used in the mill tension tests are relatively high, to maximize throughput. 

ASTM A 370 allows a maximum strain rate of 0.001 s-1

, which causes yielding within 5 s for most 

structural steels. The yield strength of structural steel increases slightly with increasing testing rate. For 

modeling the static behavior of the building, the relevant strength is not the one measured in the mill test, 

but instead is the so-called “static yield strength.” This is the strength that would be measured at 

infinitesimally slow deformation rate, which is naturally the relevant rate for the gravity loads in a 

statically loaded building. Typically, the static yield strength is 1 ksi to 4 ksi less than the value on the 

mill test report, as established in extensive studies from the 1960s (Rao 1966; Johnson 1967; 

Galambos 1976), and methods exist to calculate the expected static yield strength from tests conducted 

dynamically. These corrections are necessary to estimate properties relevant to the airplane impact 

conditions. 

A second correction that must be made arises from the microstructural behavior of low alloy steels (such 

as steels specified to A 36 and A 440) near the yield strength. During tensile testing, these steels often 

exhibit what is known as a yield drop (Fig. E–10). The stress necessary to initiate the first bit of 

permanent deformation (yield) is larger than the stress necessary to continue the deformation. During the 

elongation in the test, the load rises linearly until permanent deformation initiates at a single location in 

the test specimen. Frequently the stress can drop 3 ksi to 5 ksi upon yielding. A localized band of 

deformation passes through the test specimen, and the load drops to a lower constant value. Because the 

specimen is tested at constant extension rate, rather than at constant load, the deformation band 

propagates through the test specimen until the entire specimen has begun to deform permanently. This 

behavior manifests itself as a region of constant stress deformation known as yield point elongation. For 

all structural steels with specified yield strengths less than 100 ksi, ASTM standards allow the mill test to 

report the maximum value of the stress reached before the load drop, called the yield point, rather than the 

lower, constant value, called the lower yield stress. The yield point phenomena occur only in tests that 

have uniform stress states. Beams loaded in bending, for example, will not show this sort of stress-strain 

behavior. For modeling purposes the lower yield stress is the relevant parameter for modeling yield 

behavior. 

For estimating the properties of rolled wide flange shapes, one must correct for variation in yield strength 

with location from which the test specimen is taken. During the WTC era, but not currently, ASTM 

standards specified that the test specimen for the mill test report be taken from the web section (in the 

“cross bar” of an “H” shaped specimen) of the rolled shape, rather than the flange. In typical rolled 

shapes, however, the flange is the thicker section, and accounts for most of the load-carrying capacity of 

the column. Because it is thicker, it cools more slowly from the rolling temperature, and generally has a 

lower yield strength than the flange. Many studies, summarized by Alpsten (1972), have characterized 

this difference as being 2 ksi to 4 ksi for nominally 36 ksi shapes. It was not uncommon for the yield 

strength of a flange to be 1 ksi to 2 ksi below the specified nominal value (from the web) for the standard. 

There is a similar problem for estimating the yield strength of plates. The steel community has 

recognized that for a given specified minimum yield strength, thinner plates often exhibit a higher yield 

strength than thicker plates. Thinner plates have had more hot working and cool faster than thicker plates 

rolled from the same heat (Alpsten 1973; Galambos 1978). Indeed, the high-strength plates that the 
Appendix E 

E–32

surviving Yawata mill test reports describe (Section 0) show this effect, but the low-strength plates do 

not. The effect is difficult to model, however, because mills can adjust the composition of heats intended 

for specific thicknesses, while still meeting the chemistry requirements of the standard specification, to 

keep the actual yield strength close to the specified minimum. In the absence of a well-defined method 

for estimating the thickness effect, it must be regarded as a source of uncorrectable uncertainty. 

Table E–17 lists estimated yield strengths for the relevant steels from the impact and fire zone. It 

contains two columns. The first, labeled “Estimated mill Fy” is an estimate of the average value of the 

yield strength that would have been reported on the mill test reports. It is based on surviving mill test 

reports where they are available, and on literature estimates where no mill test reports have survived. The 

values were estimated by multiplying the specified minimum value by a constant, k, where k = 1.12 for 

plates (Baker 1969) and k = 1.2 for shapes (Alpsten 1972). The second column corrects the estimated 

average mill test report Fy to the value for the static yield strength, using the correction factor of Rao 

(1966) of –3.6 ksi. The value for the rolled core shapes is further corrected to the expected yield strength 

for the flanges (since the mill test reports are for specimens taken from the webs) using the value from the 

AISI report (1973) of –2.4 ksi. 

Floor Trusses 

Based on the mill test reports summarized in Table E–11, NIST recommends using Fy = 58.4 ksi for 

angles specified as either A 242 or A 36, and 50.4 ksi for rounds specified as A 242. Based on the 

conversations with Laclede personnel (Brown 2002), A 36 rounds are estimated to have Fy = 43.4 ksi. 

Table E–17 summarizes these recommendations. 

Perimeter Columns and Spandrels 

Table E–17 provides the current best estimate of the properties of the Yawata grades for each indicated 

minimum yield strength. Most important in Table E–17 is the entry that shows that PC&F obtained 

permission (White 1968b) to substitute Fy = 100 ksi (WEL-TEN 80C) material for Fy = 90 ksi 

applications, but not to upgrade any other yield strengths by 10 ksi or larger anywhere else. Documents 

(Symes 1969a §) from early 1969 indicate that PC&F did not use any Fy = 85 ksi steel in the building, so 

any steel specfied as Fy = 85 ksi would have to have been supplied at Fy = 100 ksi as well. Nicholas 

Soldano, PC&F General Manager in 1969 (2002 †), confirmed that they had also been granted permission 

to substitute 45 ksi steel for 42 ksi. Ronald Symes (2002 †), project engineer for PC&F, confirmed that 

they followed the 5 ksi yield increments, so with the exception of the Fy = 85+ ksi and Fy = 42 ksi steels, 

there would be grades for each yield strength. Estimates of the yield stress use the average values of the 

plates in the mill test reports NIST has located (for “A 441-modified” with Fy = 45 ksi and Fy = 50 ksi and 

WEL-TEN 62 with Fy = 70 ksi and Fy = 75 ksi). Where no data from mill test reports exist, NIST 

recommends using the literature value as described above. Estimates for WEL-TEN 80C use the average 

values for the data found in a literature report (Ito 1965b). 
Interim Report on Contemporaneous Structural Steel Specifications 

E–33

Table E–17. Estimated yield strengths (Fy) for grades of steel above Floor 9. 

Grade 

Fy

(ksi) 

Estimated 

Mill test 

report Fy 

(ksi) (1) 

Estimated static 

Fy

(ksi) (2) Thickness Range Steel Source Notes 

Perimeter Column Plates 1, 2, 4 (flanges, exterior of building, and spandrels) –see Fig. E–3) 

36 40.3 35.6 Yawata A 36 (3) 

42 56.8 53.2 Yawata "A 441 modified" (4,5) 

45 56.8 53.2 Yawata "A 441 modified" (5) 

50 57.7 54.1 Yawata "A 441 modified" (5) 

55 61.6 58.0 For plates with t<=1.5 in. Yawata "A 441 modified" (6) (7) 

55 61.6 58.8 For plates with t> 1.5 in. Yawata WEL-TEN 60 (3) (7) 

60 67.2 63.6 For plates with t<=1.25 in. Yawata "A 441 modified" (6) (7) 

60 67.2 63.6 For plates with t>1.25 in. Yawata WEL-TEN 60 (3) (7) 

65 72.8 69.2 For plates with t>0.5 in. Yawata WEL-TEN 60 (3) (7) 

65 72.8 69.2 For plates with t<=0.5 in. Yawata WEL-TEN 60R (3) (7) 

70 78.4 74.8 Yawata WEL-TEN 62 (5) 

75 84.0 80.4 Yawata WEL-TEN 62 (5) 

80 89.6 86.0 Yawata WEL-TEN 70 (3) 

85 105.0 101.4 Yawata WEL-TEN 80C (8) (9) 

90 105.0 101.4 Yawata WEL-TEN 80C (8) (9) 

100 105.0 101.4 Yawata WEL-TEN 80C (9) 

Perimeter Column Plate 3 (faces interior of building –see Fig. E–3) 

42 47.0 43.4 Bethlehem V42 (3) 

45 50.4 46.8 Bethlehem V45 (3) 

50 56.0 52.4 Bethlehem V50 (3) 

55 61.6 58.0 Bethlehem V55 (3) 

60 67.2 63.6 Bethlehem V60 (3) 

60 67.2 63.6 0.75in.<t<=1.5 in. Bethlehem V60-modified (3) 

65 72.8 69.2 t<=0.375 in. Bethlehem V65 (3) 

65 72.8 69.2 0.375in.<t<=1.5 in. Bethlehem V65-modified (3) 

75 84.0 80.4 t<=1.0 in. Bethlehem V75-modified (3) 

Core Box Columns 

36 40.3 36.7 Fuji Steel, Colvilles (3) 

42 47.0 43.4 Fuji Steel, Colvilles (3) 

Core Rolled Columns 

36 43.2 37.3 Yawata + others (3) 

42 50.4 44.5 Yawata + others (3) 

45 54.0 48.1 Yawata + others (3) 

50 60.0 54.1 Yawata + others (3) 

Floor Trusses

50 62.0 58.4 A 242 and A 36 angles (6) 

36 41.6 38.1 d = 1.09 in. and 1 13/16 in. Laclede A 36 rounds (6) 

50 54.0 50.4 All other rounds Laclede A 242 rounds (6) (10) 
Appendix E 

E–34

Table E–17. Estimated yield strengths (Fy) for grades of steel above the Floor 9 

(continued). 

Grade 

Fy 

(ksi) 

Estimated 

Mill test 

report Fy 

(ksi) (1) 

Estimated actual 

Fy 

(ksi) (2) Thickness Range Steel Source Notes 

Notes:

1 This Fy is the estimated average Fy on that would have been reported on the mill test reports, had they been available. For WF shapes, it 

represents the value from a specimen taken from the web.

2 Estimated average mill test report Fy corrected for rate and location effects. For a WF shape, this represents the value appropriate for the 

flange. 

3 Based on reported literature properties for plates (Galambos 1978:Table 3; citing Baker 1969) and rolled shapes: (Alpsten 1975: Fig. 13; 

Galambos 1978). Estimated flange Fy reduced by 2.4 ksi (AISI 1973:Table 22). 

4 Fy = 42 ksi steel substituted with Fy = 45 ksi (Soldano 2002 †).

5 Based on averages from Yawata mill test reports (Symes 1969b §; Barkshire 1969a §; White 1969c §). 

6 Based on Laclede mill test reports (Table E–11) and conversations with Laclede metallurgists (Brown 2002). 

7 Use of A 441-modified vs. WEL-TEN based on White memo (White 1969a). 

8 Fy = 85 ksi and Fy = 90 ksi steel substituted with Fy = 100 ksi (Symes 1969a; White 1968).

9 Based on typical values from manufacturer reports. 

10 Assumed to be chemically identical to A242 angles. 

Core (Welded Box Columns) 

In the absence of any confirming mill test reports, the best estimate of yield strength for the core columns 

is 12 percent higher than specified value (Baker 1969), also listed in Table E–17. 

Core (Rolled Wide flange Shapes) 

Given the tonnages of wide flange shapes supplied, it is likely that Yawata supplied all the rolled core 

columns, but NIST has found no confirming evidence of this. Furthermore, NIST has found no open 

literature information on chemistry or typical mechanical properties of Yawata rolled shapes. In the 

absence of mill records or steel mill source identification, the best estimate of the yield strength for the 

expected average mill test report Fy for core rolled shapes is 20 percent higher than the specified 

minimum yield strength, as detailed by Alpsten (1972) and corrected for the difference between flanges 

and webs (AISC 1973), summarized in Table E–17. 

E.5.4 Sources of Information 

Preliminary searches used open literature sources of information, including trade journals to locate 

information on the various companies and steels involved in construction. Table E–18 lists the journals 

examined, and the strategy for locating WTC specific information. As mentioned, Table E–16 lists 

similar information for the databases and search strategies used to locate WTC information. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–35

Table E–18. Trade journals examined for WTC steel information. 

Journal Search Method 

Acier Stahl Steel 1966 to 1972 Tables of Contents. 

Civil Engineer-ASCE 1965 to 1973 Index on WTC. 

Engineering News Record 

Also, see compilation volume of all articles published (ENR 1972)

1967 to 1973 Index on WTC, New York 

City. 

The Iron Age 1966 to 1968 Index on Japan, WTC, 

structural steel, fabricator and steel company 

name. 

Iron and Steel Page-by-page for 1968 to 1971. 

Iron and Steel Engineer 1967 to June 1968 Table of Contents, 

Dateline column, Industry news column. 

Index is not topical. 

Japan’s Iron and Steel Industry 1967-1970 1967 to 1970 page-by-page. 

Metal Construction Page-by-page. 

Metal Progress Page-by-page. 

Modern Steel Construction Tables of Contents. 

Nihon Kinzoku Gakkaishi (J. Jap. Inst. Metals) Cursory, WTC era. 

Steel 1966 to 1969 Index on Japan, WTC, 

fabricator and steel company name. 

Transactions of the Iron and Steel Institute of Japan 1965 to 1969 Table of Contents and news 

pages. 

Stahlbau 1966 to 1973 Index under Hochbau. 

Steelways – 

Structural Engineer 1966 to 1972 cursory. 

Welding Design and Fabrication Cursory. 

West of Scotland Iron and Steel Institute Journal 1966 to 1969 Tables of Contents. 

After identifying the fabrication companies, NIST contacted Laclede Steel Corporation, Nippon (formerly 

Yawata) Steel, PACCAR (formerly Pacific Car and Foundry), Montague Betts, Dovell Engineering, and 

several former employees of Stanray Pacific and Pacific Car and Foundry. NIST did not attempt to 

contact fabricators that were only involved in the lower floors (Atlas Machine and Iron Works, Levinson, 

Mosher, and Drier). Table E–19 summarizes these contacts and information. Most of the information in 

this report came from the archives of LERA. 

Initially, NIST had hoped to find the mill test reports for the steel used, which would have provided 

complete yield (Fy) and tensile strength and chemistry information for all the steels. Each fabricating 

company, as part of the quality control program required by their contract with PONYA, supplied this 

information to Tishman, the general contractor, to SHCR, the structural engineers, and to PONYA. 

Unfortunately, Laclede, Montague-Betts (Davis 2003 †), PACCAR (Bangert 2002 †) (the new name of 

Pacific Car and Foundry), SHCR (Magnussen 2002 †), and Tishman (Christensen 2003 †) all confirm that 

they have no mill test reports from that era. 
Appendix E 

E–36

Table E–19. Sources examined for mill test reports and other construction information, 

other than the (LERA) archives. 

Contact Background Result 

Laclede Steel Corporation 

David McGee 

Larry Hutchison 

Laclede fabricated the trusses for the 

floor panels. 

During Nov. 2002 NIST personnel visited 

Laclede, which shared material from its 

archive, including two mill test reports. 

Ronald Symes Former Chief 

Engineer, PC&F 

PC&F fabricated the perimeter 

columns. 

Symes did not retain any WTC documents 

relating to steel properties, but he did have 

information on welding 

Nicholas Soldano Former 

general manager, PC&F 

– Soldano provided information on steel 

substitutions, but had no WTC documents. 

D. Bangert, VP for facilities 

PACCAR 

PACCAR owned Pacific Car & 

Foundry before selling it in 1974. 

PACCAR retained no records relating to 

any aspect of PC&F 

Nippon Steel USA 

Tomokatsu Kobayashi, VP 

Nippon Steel formed by the merger 

of Yawata and Fuji Steel, which 

together supplied most of the 

Japanese steel. 

Nippon located several 1960s era data 

sheets for Yawata WEL-TEN steels, but 

no mill test reports for steels used in the 

WTC. 

Mitsui USA, Janet Garland Mitsui imported the steel for PC&F Mitsui has no WTC records. 

Carl Lojic, former president, 

Joseph Tarkan, former Chief 

Engineer, Stanray Pacific 

Stanray Corp closed its fabricating 

business in 1969, and has apparently 

gone out of business. 

Neither Lojic nor Tarkan retained any 

documents from the project. 

Corus Construction & 

Industrial Homi Sethna 

Corus (formerly British Steel) owns 

the works that rolled the thicker 

plate for the welded core columns. 

Corus was unable to locate any records 

from the WTC era. 

Tony Wall, President, Dovell 

Engineering 

Dovell was the detailer for Stanray 

Pacific. 

The Northridge earthquake damaged their 

building. During clean-up they disposed 

of all WTC documents. 

William Betts, CEO 

Montague-Betts 

Montague-Betts fabricated all rolled 

shapes above the 9th floor. 

Six years after completion, 

Montague-Betts destroyed, as per 

company policy, all records relating to the 

WTC construction. 

Marubeni-Itochu Steel 

Tadashi Yaegashi Chief 

Administrative Officer

Marubeni-Itochu succeeded 

Marubeni-Iida, which imported the 

Yawata steel for Montague-Betts. 

“All sales transactions going back to the 

1960’s have been destroyed” 

SGS US Testing Company 

Rich Franconeri 

SGS succeeded US Testing and The 

Superintendence Co., both of which 

inspected the Japanese steel. 

SGS was unable to locate any documents 

from that era. 

Skilling, Ward, Magnussen, 

and Barkshire (SWMB); Jon 

Magnussen, partner 

SWMB is the successor to the 

structural engineering firm that 

designed the towers. 

SWMB retained no WTC records. They 

transferred everything to LERA. NIST has 

access to these records. 

Tishman Realty and 

Construction; Linda 

Christensen 

Tishman was the general contractor 

for the construction. 

“[O]ur archive facility has standing orders 

that any and all files over seven years in 

age are to be destroyed.” 

NIST also contacted several of the inspection companies (Franconeri 2003 †) and the steel mills 

(Sethna 2003 †) and steel importing companies (Garland 2004 †; Yaegashi 2003†), as well as Crest Steel, 

which some Stanray Pacific communications mention (Steinberg 2002 †). All confirmed that they 

retained no records relating to steel for the WTC. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–37

NIST investigators located six pages of mill test reports for PC&F in the LERA archives, and several 

individual mill test reports in the Laclede archives. 

E.6 CONTEMPORANEOUS CONSTRUCTION SPECIFICATIONS 

Section E.4, Contemporaneous Steel Specifications, traces the sources and grades of steel used to 

fabricate structural steel components for the WTC towers. This section supplements that by extending 

further into the construction process, specifically adding information on the fabrication (welding) of 

components and the erection of the buildings. 

E.6.1 Fabrication of the Various Components 

Floor Trusses 

Laclede Steel manufactured the trusses for the floor panels for both WTC 1 and WTC 2 from steel they 

made at their mill in Alton, Il. The chords of the trusses were fabricated from hot-rolled angles, while the 

web was from hot-rolled round bar. The web and the chord angles were joined by resistance welding 

(Laclede 1969). 

Little information is available on the standards used for fabrication of the floor trusses. However, floor 

joist standards existed since 1929. The AISC Steel Construction Manual (1972) Standard Specifications 

for Open Web Steel Joists specifies that 36 ksi and 50 ksi minimum yield strength steel are permitted in 

such bar joists, and that “Joint connections and splices shall be made by attaching the members to one 

another by arc or resistance welding or other approved methods.” A Technical Digest from the Steel Joist 

Institute (Somers 1980) also confirms the use of resistance welding. 

Exterior Wall Columns and Spandrels 

The perimeter column panels, fabricated by PC&F, comprise three important sub assemblies: the 

columns, the spandrels, and the seats. A Welding Design and Fabrication article (1970a) describes the 

fabrication sequence, which began with forming the inside wall of the panels (using a butt joint to link the 

spandrel plates to the inner column webs), followed by the addition of the flanges and outer web plate of 

the columns by six simultaneous submerged arc welds. PC&F constructed a 16-station automated 

production line to keep up with the schedule of 55,800 tons of perimeter column panels between 

November 1967 and August 1970, an average of 1,400 tons per month. 

The construction contract states that the submerged arc electrodes used in the WTC were purchased to the 

requirements of ASTM Standard A 558 “Specification for Bare Mild Steel Electrodes and Fluxes for 

Submerged Arc Welding.” This standard was withdrawn in 1969, and was replaced by an equivalent 

American Welding Society (AWS) Standard A 5.17 “Bare Mild Steel Electrodes and Fluxes for 

Submerged Arc Welding.” The period 1965 to 1969 was one of transition, during which AWS assumed 

the responsibility of maintaining the standards for welding filler materials. Because the contract was 

awarded in 1967, the fabrication was likely started with the requirements of the 1965 version of the 
Appendix E 

E–38

ASTM Standard (ASTM A 558-65T, jointly published by AWS as AWS A 5.17-65T), but later perimeter 

column panels may have included some minor changes associated with the conversion to the 1969 version 

of the AWS Standard (AWS A 5.17-69). Distorted columns were straightened in the conventional 

manner by heating just after column assembly, so any low-strength areas in the steel plates and changes in 

microstructure should not be interpreted solely in terms of the airplane impact and subsequent fires. 

The Welding Design and Fabrication article (1970b) further states that PC&F inspected the perimeter 

column panel welds using either ultrasonic, or visual and magnetic particle techniques. 

The inner wall assembly (the spandrels and inner plates of the perimeter column panels) was joined with 

complete joint penetration welds according to the requirements of AWS D 2.0 “Specifications for Welded 

Highway and Railway Bridges.” This probably refers to the 1966 version of AWS D 2.0. They may have 

chosen this standard over D 1.0 “Code for Welding in Building Construction” because, at the time, D 1.0 

was limited to steel strengths under 60 ksi (Fenton 1966). AWS D 2.0 specifies various dimension and 

strength requirements for the assemblies and their welds (e.g., paragraphs 302 and 403). This standard, 

like most standards, lags the steel technology of the time. Thus, it seems to be mostly designed around 

the application of fairly old steels, like A 7, A 36, and A 373. However, newer steels, such as the higher 

strength steels used in the WTC towers, could be used after formal approval. 

Once the inner wall was ready, the columns were assembled from outer web plates, butt plates, diaphragm 

plates, and flange plates (Welding Design 1970a). Once assembled and preheated, the plates were joined 

in the main fillet weld gantry, a station that made six, 0.75 in (19 mm) fillet welds simultaneously along 

the length of the perimeter column panel. Then the panel was jacked 90 degrees, and the other six fillet 

welds were made along the length of the panel. At full production, this gantry laid down 2,900 lb 

(1,300 kg) of weld metal a day. These large fillet welds started 6 in. (150 mm) from the ends of the 

columns, so manual welding was used to finish the welding of the ends and to make any repairs. 

Core (Welded Box Columns) 

Stanray Pacific Corp. fabricated the welded core columns in both buildings above floor 9. Like PC&F, 

they used large assembly fixtures and triple submerged arc welding stations to achieve high production 

rates. Review of some of the correspondence generated during the initial stages of the fabrication shows 

the level of attention to welding and inspection details needed to meet the requirements of PONYA and 

SHCR as described below. 

A September 1967 draft of the contract between PONYA and The United States Testing Laboratory (a 

third-party inspector) lists the documentation that would be required of the work at Stanray Pacific 

Corp (White 1967c §). This contract prescribes daily and weekly written reports of components that are 

accepted, those that are rejected, and a summary of any problems, with copies going both to the 

construction manager and to SHCR. In addition, a weekly report was sent with all the chemical and 

physical (mechanical) tests performed. The inspectors checked the various steps from plate delivery 

(checking heat number, specification conformance and condition), through fabrication (alignment, 

100 percent visual inspection of the welds, and selection of regions for non-destructive testing), to final 

inspection (perpendicularity of milled ends, overall length, cleaning, and marking). PONYA also had a 

procedure to inspect the steel from all sources. The procedure included double-checking the mill 

certificates by performing a tensile test and a check analysis on 1 out of 10 heats selected at 
Interim Report on Contemporaneous Structural Steel Specifications 

E–39

random (Monti 1967b §). The requirements were still higher for steel with strengths above 50 ksi or from 

foreign sources. The welding procedures, welders and welding operators were qualified in accordance 

with requirements of Appendix D of AWS Codes D1.1-66 and D 2.0-66. The welding electrodes for 

manual metal arc welding conformed to ASTM A 233-64T, E60 and E70 series (also AWS A 5.1-64T). 

Mild steel electrodes and fluxes for submerged arc welding conformed to ASTM A 588-65T (also 

AWS A 5.17-65T) and to Section 1.17.3 of the AISC Specification for Structural Steel Buildings. 

By October 1967, welders were being qualified, magnetic particle inspector qualification was being 

discussed (based on a minimum of 40 hours of training), and chemical analysis of the steel was underway 

(Chauner 1967a). The level of inspector oversight continued to increase until by November 10 “U.S. 

Testing inspectors are all over the place and recording a lot of information” (Chauner 1967b). The level 

of attention to detail increased even more after a surprise visit to Stanray by Hugh Gallagher, a PONYA 

inspector, on November 20, 1967 (Gallagher 1967). 

While reading the correspondence, one senses that toward the beginning of the contracts, the various 

fabricators faced major (and perhaps unexpected) challenges introduced by both the tight production 

schedule and PONYA and SHCR’s strict quality requirements. 

Connections (Bolts and Welds) 

The Port Authority contract allowed the use of ASTM A 307, A 325, and A 490 fasteners. The WTC 

Design Standards book (p. DS1-6) calls for the use of ASTM A 325 bolts with no indication of type. 

According to the standard, they would have therefore been supplied as Type 1. As in the contemporary 

version of ASTM A 325, Type 1 bolts in 1970 had Fy = 120 ksi for diameters up to and including 1 in, 

and Fy = 105 ksi for larger diameters. ASTM A 325-70 does differ significantly from ASTM A 325-02 in 

several ways. In particular, the specification for Type 2 bolts was withdrawn in 1991. ASTM A 325-02 

also admits three new chemistries for Type 1 bolts. In ASTM A 325-02, the specification for Type 1 

Carbon Steel bolts most closely approximates the Type 1 bolts of A 325-70. Table E–20 compares the 

chemistry requirements of the two standards. A 325-70 also admits a slightly wider range of acceptable 

hardness, which is currently in Table 3 of A 325-02. 

Table E–20. Comparison of chemistry requirements for ASTM A 325 

“Standard Specification for High-Strength Bolts for Structural Steel Joints, 

including Suitable Nuts and Plain Hardened Washers” 

between 1970 and 2002 standards 

Element 

ASTM A 325-70 

(% mass fraction) 

Maximum 

ASTM A 325-02 

(% mass fraction) 

Maximum 

C 0.27 0.28–0.55 

Mn 0.47 0.57 

P 0.048 0.048 

S 0.058 0.058 

Si – 0.13–0.32 

Key: C, carbon; Mn, manganese; P, phosphorus; S, sulfur; Si, silicon. 

Note: Data are for product, not heat, analysis. Mechanical property requirements are identical between 

versions. 
Appendix E 

E–40

Spandrels of adjacent perimeter column panels were attached together with high-strength bolted shear 

connections. Adjacent spandrels were butted to each other with splice plates on the inside and outside 

(Fig. E–3). For floors 9 to 107, each spandrel was connected to the splice plates with anywhere from 6 

to 32 bolts, depending on design load. Splice plates were all 36 ksi steel regardless of spandrel grade. 

Bolts for all connections between spandrels conformed to ASTM A 325. Minutes of a May 1967 

(Feld 1967a) meeting between PC&F, PONYA, and Koch, state that no A 490 bolts were to be used for 

the spandrel splice plates, and that only A 325 bolts were to be used there. “Bow-tie” spandrels in trees 

below the floor 9 were connected with heavy 42 ksi splice plates with A 325 or A 490 bolts in friction 

connections. 

Perimeter columns were bolted via the butt plates to those immediately above and below, with four bolts 

in the upper stories and six bolts in the lower stories. Other than at the mechanical floors, panels were 

staggered (Fig. E–4) so that only one third of the units were spliced in any one story. At the mechanical 

floors, every column contained a splice, and columns were welded together as well as bolted. 

Seats for the trusses that supported the floor were welded to spandrels in the perimeter column panels and 

to channels or core columns at the central core. The trusses were positioned on the seats and held in place 

with construction bolts until welded to the seats. The construction bolts generally remained in place after 

welding. 

Construction (On-site Assembly) 

During fabrication, Karl Koch Erecting Co. used a combination of bolting, shielded metal arc (SMA) 

welding (E7018), and gas metal arc welding (semiautomatic Fab Co 71 with CO2 shielding) to join the 

components (Welding Design 1970b). The E7018 low-hydrogen SMA electrode would likely have been 

produced to ASTM Standard A 233-64T (also published by AWS as A 5.1-64T), then AWS 

Standard A 5.1-69 for the later parts of the fabrication. The 3/32 in. (2.4 mm) diameter Fab Co 71 (sic, 

probably should be FabCO 71, a trademark of Hobart Brothers Company) was an E70T-1 flux cored arc 

(FCA) electrode and would likely have been produced according to ASTM A 559 (withdrawn in 1969), 

then AWS A5.20-69. Higher-strength SMA electrodes (ASTM A 316 until 1969, then AWS A 5.5-69) 

were also permitted by the contract. More than 48,000 lb (22,000 kg) of electrodes were used in each of 

the towers (Welding Design 1970b). Koch used a combination of visual and ultrasonic inspection on the 

joints. They estimated that rework would cost three times as much as the original weld, so they inspected 

early and often to minimize any rework. One reason that rework was so expensive is that some welds 

took as many as 200 passes, so they wanted to catch any problems before the later passes made access 

more difficult.

Perhaps the most common construction standard for buildings of the period was AWS D 1.0 “Welding in 

Building Construction” (Fenton 1966). This document was subject to frequent revisions by the 

responsible committee. Some versions that may have been specified for parts of the WTC towers were 

the versions published in 1966, 1967, and 1968. The 1967 and 1968 revisions addressed issues such as 

the details on the use of multiple-electrode submerged arc welding, more requirements on qualification of 

the welders (especially tack welders), and the addition of radiographic inspection. Many of these 

revisions may have been driven by the needs of the WTC design. Because the D2.0 code referenced in 

the discussion on fabrication of perimeter column panels above only covers the use of submerged arc and 
Interim Report on Contemporaneous Structural Steel Specifications 

E–41

shielded metal arc welds (unless through special application of Section 5), use of D1.0 (specifically 

through the use of Section 502) might have been the easiest way to cover the use of FabCO 71 electrode. 

Incidentally, the apparent misspelling of FabCO 71 in one of the references points out the problem of 

inconsistencies in some of the references. The likely explanations include both authors’ faulty memories 

of some details, but also changes that occurred after an article (perhaps based on the near-term 

construction plans) went to press. An apparent example of the later case involves the plan to use 

electroslag welding to fabricate the “trees,” the branching columns that formed the transition from the 

10 ft (3 m) spacing of columns in the lobby area to the 40 in. (1 m) spacing of columns for all the upper 

floors. Gillespie’s book (1999) describes the fabrication of these trees by electroslag welding. However, 

Koch’s book (2002) describes their inability to get the electroslag process operating under field conditions 

(in a location described as the “belly band,” halfway up between the front doors and the branching of the 

trees), so they welded all these large joints manually. 

Examination of the perimeter columns shipped to NIST revealed arc welds at the ends of the trusses, 

where they were attached to the columns during erection. These welds supplemented the bolt attachment 

at the seats, and were probably produced by gas metal arc or shielded metal arc electrodes. 

E.7 REFERENCE LISTS 

E.7.1 References from Publicly Available Sources 

This section lists references from the open literature, primarily magazine and journal articles and books. 

Alloy Digest. 1965. WEL-TEN 100N. Serial #826. 

Alloy Digest. 1967. WEL-TEN 50. Serial #966. 

Alloy Digest. 1968. WEL-TEN 60-LT. Serial #1003. 

Alloy Digest. 1969. WEL-TEN 80C-LT. Serial #1061. 

Alloy Digest. 1970. Bethlehem V Steel. Filing Code SA-257. 

Alpsten, G. A. 1975. Variations in mechanical and cross-sectional properties of steel. In Tall Building 

Criteria and Loading vol. 1b of Planning and Design of Tall Buildings. Edited by Thomas C. 

Kavanagh et al. New York: American Society of Civil Engineers. 755–805. 

American Institute of Steel Construction (AISC) 1974. The Variation of Product Analysis and Tensile 

Properties Carbon Steel Plates and Wide Flange Shapes. Contributions to the Metallurgy of Steel. 

1-120. 

American Institute of Steel Construction (AISC). 1973. Manual of Steel Construction. New York: 

American Institute of Steel Construction. 
Appendix E 

E–42

American Institute of Steel Construction (AISC). 1972. Standard Specifications and Load Tables for 

Open Web Steel Joists, Longspan Steel Joists, Deep Longspan Steel Joists. New York: American 

Institute of Steel Construction. 

American Welding Society (AWS). 1965. Tentative Specification for Bare Mild Steel Electrodes and 

Fluxes for Submerged-Arc Welding. New York: American Welding Society and American Society 

for Testing and Materials. 

American Welding Society (AWS). 1966. Specifications for Welded Highway and Railway Bridges. 

New York: American Welding Society. 

American Welding Society (AWS) Structural Welding Committee. 1967. Addenda to D1.0-66 Code for 

Welding in Building Construction on and SRI-65 Gas Metal-Arc Welding with Carbon Dioxide 

Shielding. New York: American Welding Society. 

American Welding Society (AWS) Structural Welding Committee. 1968. Supplement to AWS D1.0-66, 

Code for Welding in Building Construction, and AWS D2.0-66, Specifications for Welded Highway 

and Railway Bridges, on Requirements for Submerged Arc Welding with Multiple Electrodes. New 

York: American Welding Society. 

American Welding Society (AWS). Committee on Filler Metal. 1969. Specification for Bare Mild Steel 

Electrodes and Fluxes for Submerged-Arc Welding. New York: American Welding Society. 

Baker, M. J. 1969. Variations in the mechanical properties of structural steels. In Reports of the 

Working Commissions, Symposium on Concepts of Safety of Structures and Methods of Design. 

International Association for Bridge and Structural Engineering. London. 165–174. 

Brockenbrough, R. L., and B. G. Johnston. 1968. The structural steels and their mechanical properties. 

Chap. 1 in USS Steel Design Manual. United States Steel Corporation. Pittsburgh, PA. 

Civil Engineering-ASCE. 1970. World Trade Center shows on New York City skyline, 40:81. 

Engineering News Record (ENR). 1967. Steel Ahoy! November 11. 

Engineering News Record (ENR). 1972. The World Trade Center, its planning, finance, design and 

construction as reported through the weeks for more than a decade. New York:McGraw-Hill. 

Feld, L. S. 1971. Superstructure for the 1,350-ft World Trade Center. Civil Engineering-ASCE. 

41:66–70. 

Fenton, Edward A. 1966. Code for Welding in Building Construction. New York: American Welding 

Society. 

Galambos T. V. and M. K. Ravindra. 1978. Properties of steel for use in LRFD. Journal of the 

Structural Division-ASCE, 104(9):1459–1468. 

Gillespie, Angus Kress. 1999. Twin Towers: The Life of New York City's World Trade Center. New 

York: New American Library. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–43

Goda, S., T. Ito, H. Gondo, I. Kimura, and J. Okamoto. 1964. Present status of weldable high strength 

steel, Yawata Technical Report (Yawata Seitetsu Kenkyu). 248:5086–5163. 

Groner, Alex. 1981. PACCAR The Pursuit of Quality, Bellevue, Wa: Documentary Book Publishers. 

Handbook of Comparative World Steel Standards. 1974. Tokyo: International Technical Information 

Institute. 

Iron Age. 1966. Yawta advertisement, vol. 198. October 6. 

Irving, R.R. 1968. Construction, a materials arena. The Iron Age. November 1:59–66. 

Ito, T., K. Moriyama, and Y. Sogo. 1965a. Usability of WEL-TEN 80C welded joints of high-tension 

steel (in English). Yawata Technical Report (Yawata Seitetsu Kenkyu). 253:6307-6316. 

Ito, T., K. Moriyama, and Y. Sogo, 1965b. Usability of WEL-TEN 80C welded joints of high-tension 

steel (In Japanese). Yawata Technical Report (Yawata Seitetsu Kenkyu). 253 6232-6244 (1965). 

Japan’s Iron and Steel Industry. 1968. New products and processes. In Japan’s Iron and Steel Industry 

1968 ed., edited by Sukeyuki Kawata. Tokyo: Kawata Publicity, Inc. 

Johnson, R. F. 1967. The measurement of yield stress. In Strong Tough Structural Steel. England: The 

Iron and Steel Institute. 51-60. 

Koch, Karl, and Richard Firstman. 2002. Men of Steel: the Story of the Family that Built the World 

Trade Center. New York: Crown Publishers. 

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, 

Preliminary Observations, and Recommendations. FEMA 403. Federal Emergency Management 

Agency. Washington, DC, May. 

Otani, M. 1966. Fundamental properties of high tension steel (in Japanese). Yawata Technical Report 

(Yawata Seitetsu Kenkyu). 257:7066–7117. 

Rao, N. R. N., M. Lohrman, and L. Tall. 1966. Effect of strain rate on the yield stress of structural steels. 

ASTM Journal of Materials. 1(1):241–262. 

Rodengen, Jeffrey L. 2000. The Legend of Trinity. Fort Lauderdale, FL: Write Stuff Enterprises. 

Stanray Corp. 1970. 1969 Annual Report. 

Stanray Corp. 1971. 1970 Annual Report. 

Somers, Robert E., and Theodore V. Galambos. 1980. Welding of Open Web Steel Joists. Technical 

Digest No. 8. Steel Joist Institute Richmond, VA. Based on inclusion of statistics on U.S. joist 

production in 1979. 
Appendix E 

E–44

Welding Design and Fabrication. 1970a. Welding and abricating 55,000 tons of steel for the World Trade 

Center, July:50–52. 

Welding Design and Fabrication. 1970b. Welding and inspecting the World Trade Center-giant among 

giants, February:53–56. 

Woldman's Engineering Alloys. 1990. Materials Park, OH: ASM International. 

Zaizen, T. K. Moriyama, and M. Kawshima. 1968. On the Yawata’s structural steel plates for low 

temperature applications. Yawata Technical Report (Yawata Seitetsu Kenkyu). 262:8239–8250. 

E.7.2 Private Communications 

The text identifies references that are private communications or unpublished works that are not bound by 

any material transfer agreement with the symbol, †. All contemporaneous memoranda referring to 

Laclede are from the Laclede archives in Alton, Il. NIST obtained Yawata documents from Nippon Steel 

USA, New York office. 

Bangert, D. 2002. Telephone interview with William Luecke, NIST. Confirmation that PACCAR has no 

records of the WTC contract. Structural steel division was sold “years ago.” October 21. 

Bay, R. D. 1968. Memorandum No. 11, Laclede internal memorandum showing grades of steel for bar 

joists, February 28. 

Brown, Dale. 2002. Telephone interview with Richard Fields, NIST. Brown, the Laclede metallurgist 

during the WTC construction, confirmed that for ASTM A 36, Laclede would have specified a 

microalloyed steel similar to current ASTM A 572. November 20. 

Christensen, Linda (VP and General Counsel, Tishman Construction). 2003. Letter to William Luecke, 

NIST. Tishman has retained no records from the WTC construction, in response to letter sent on 

19 February. April 2. 

Davis, William. 2002. Telephone interview with William Luecke, NIST. Montague Betts furnished 

rolled beams for the core of both towers as well as the antenna base for one tower. He remembered 

about 60 percent of the steel was American, and the rest was Japanese or British. Confirmed that they 

did buy steel from Yawata, and that Yawata had better tolerances than the US steel mills did. All 

records of the job were destroyed after 6 years. Montague Betts closed its steel operations in 1992. 

November 5. 

Franconeri, Rich. 2003. Letter to William Luecke, NIST. SGS U.S. Testing Company was unable to 

locate any records relevant to the World Trade Center. April 25. 

Garland, Janet. 2004. Electronic mail to William Luecke, NIST. Mitsui and Co. USA, New York office. 

confirms that they have no records relevant to steel imported for the World Trade Center. January 20. 

Kamper, H. G. 1968. Internal Laclede memo to R. D. Bay, Laclede, shows chemistry and yield strength 

for A 242 steel. August 8. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–45

Laclede Steel. 1968. Standard Resistance and Arc Weld Connections used in Truss Manufacture. 

February 28. 

Magnusson, Jon D. 2002. Letter to William Luecke, NIST. Skilling Ward Magnussen Barkshire (the 

current name of SHCR) has “absolutely no documents relating to the WTC at our firm here in 

Seattle.” November 12. 

McKnight, M. E. 2003. Telephone interview with William Luecke, NIST. January 21. 

Steinberg, P. 2002. Electronic mail to William Luecke, NIST. Steinberg worked for Crest Steel in the 

1960s, which supplied steel to Stanray Pacific. They have no records from that era. December 26. 

Symes, R. C. 2002. Telephone interview with William Luecke, NIST. PC&F made a serious effort to 

follow the 5 ksi yield stress increments as noted in the plans. December 20. 

Tarkan, Y. N. 2002. Telephone interview with William Luecke, NIST. Tarkan was chief engineer for 

Stanray Pacific. He remembered that all the steel for Stanray's contract came from Japan. When 

questioned about the Crest Steel note in the LERA documents, he thought that Crest might have been 

the distributor for that Japanese mill (Nippon Kokan). December 17. 

Wall, Tony. 2002. Telephone interview with William Luecke, NIST. Former president of Dovell 

Engineering is in no position to provide details of the WTC construction. December 3. 

White, James. 2003. Telephone interview with William Luecke, NIST. PC&F used (in addition to 

Yawata) U.S.S. and Bethlehem Steel for plates (probably for Fy less than 60 ksi). February 11. 

White, James. 1969b. Memo from SHCR to R. D. Bay, Laclede, returning mill test reports for A 242 

steel. Laclede Steel Archives December 29. 

Yaegashi, Tadashi. 2003. Letter to William Luecke, NIST. Marubeni-Itochu Steel America confirms that 

they have no records relating to steel imported for the World Trade Center. February 7. 

Yawata Iron and Steel Co. Ltd. 1969. New York World Trade Center Building. Internal Yawata 

document. 

Yawata Iron and Steel Co. Ltd. 1969b. WEL-TEN 80. Data sheets for WEL-TEN 80 steel. 

E.7.3 References from Nonpublic Sources 

The text denotes with the symbol § references that are private communications or unpublished works 

bound by material transfer agreements. 

Barkshire, Art. 1968a. Internal SHCR report to J. White on fabrication at PC&F. Contains 5 page 

attachment showing instances of U.S.S. steel used in columns mostly in floors 20 to 30. Tower not 

specified. 7 pages. May 15. 
Appendix E 

E–46

Barkshire, Art. 1968b. Internal SHCR to J. White showing spandrel plate of Kaisaloy 50-SG in panel 

203-16-19A. 6 pages. December 4. 

Barkshire, Art. 1968c. Internal SHCR memo to J. White stating that U.S.S. and Kaiser are being phased 

out as suppliers with Bethlehem (Seattle) and Japanese mills furnishing all steel. 2 pages with 

5 pages attached. June 5. 

Barkshire, Art. 1969. Corner Panel Stiffener Plates Memo to R. Symes, PC&F, approving substitution of 

steel. Contains three Yawata mill test reports. 6 pages. January 23. 

Chauner, Richard. 1967a. Internal SHCR memo to James White. October 27. 

Chauner, Richard. 1967b. Internal SHCR memo to James White. November 10. 

Clarkson, William W. 1967. Memo from Montague-Betts Steel to R. M. Monti, PONYA, requesting 

permission to have Yawata supply steel similar to ASTM A 441-modified but with 0.2 percent to 

0.4 percent mass fraction Ni, to avoid the U.S. Steel patent on the A 441-modified composition. 

October 9. 

Equivalent Carbon Contents. 1967. Worksheet that calculates equivalent carbon content for various 

steels. It indicates that steel would be supplied in 5 ksi intervals. Chemistries correspond to “Yawata 

Proposition.” June 23. 

Feld, Lester S. 1967. Internal PONYA memo to H. Tessler summarizing meeting between PC&F, Koch, 

Tishman, SHCR, and PONYA to discuss engineering changes. Discusses spandrel plate splices, 

A 325 bolts, not using A 490 bolts for the spandrel splices, and a statement by N. Soldano, PC&F, 

that Yawata would furnish imported steel with Fy>36 ksi, and Kawasaki would furnish 36 ksi steel. 

5 pages. May 9. 

Gallagher, H. B. 1968. Internal PONYA memo. W. Borland detailing inspection trip to Great Britain to 

visit Colvilles mills at Motherwell and Mossend, and Dorman-Long. May 15. 

Gallagher, Hugh. 1967. Internal PONYA memo to D. Brown. December 11. 

Goode, Bob. 1967. Internal SHCR memo to Leslie Robertson on Worthington, Skilling, Helle, Jackson 

letterhead stating that Dorman-Long will produce 1,200 t of wide flange (WF) s for Montague-Betts. 

September 8. 

Monti, R. M. 1967a. Letter from PONYA to R. C. Symes, PC&F, mentioning discrepancies between 

purchase orders and inspection reports for Kawasaki steel plates. August 21. 

Monti, R. M. 1967b. Memo to R. Morris, Stanray Pacific. November 13. 

Morris, R. E. 1969. Letter to James White, SHCR, with attached mill test report for Fuji Steel Plate that 

appears in other documents. September 10. 

Morris, R. E. 1969b. Letter to James White, SHCR, with attached mill test report for Lukens A 36 

plates. August 29. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–47

Morris, R. E. 1967. Letter from Stanray Pacific to R. M. Monti, PONYA, showing Colvilles, British 

Steel Export Assn., and Fuji Steel as source of plate for contract. September 8. 

Port of New York Authority (PONYA). 1967. The World Trade Center Contract WTC-214.00 

Fabricated Steel Exterior Wall 9th Story Splice to Roof North and South Tower. This contract was 

between PONYA and PC&F, but the materials chapters of the Laclede (WTC 221, Laclede) contract 

are identical. February 25. 

Port of New York Authority (PONYA). 1967. The World Trade Center Contract WTC-214.00 

Fabricated Steel Exterior Wall 9th Story Splice to Roof North and South Tower. Clause 1 of the 

contract between PONYA and PC&F defines the term Engineer (who was responsible for approving 

proprietary steels) as follows. “‘Engineer’ shall mean the Chief of the Planning and Construction 

Division of the WTC of the World Trade Department of the Authority for the time being, or his 

successor in duties, acting personally or through his authorized representative, except where provided 

herein to be acting personally, who is at present the Construction Manager of the WTC.” 

February 25. 

Port of New York Authority (PONYA). 1967. Change slip DM-116 to Stanray Pacific. June 6. 

Skilling, Helle, Christiansen and Robertson (SHCR). 1967. WTC Structural Drawing Books. 

Symes, R. C. 1969a. Memo from PC&F to M. Gerstman, Tishman, requesting adjustment to payment 

because of steel changes. States that plates 1, 2, and 4 (flange, outside web, and spandrel) were made 

from imported steel (presumably Yawata) and plate 3 (inside web) was fabricated from domestic 

steel. Also contains a table showing tons of steel used by grade and thickness. 6 pages PCF#T-40. 

February 5. 

Symes, R. C. 1969b. Memo PC&F to R. M. Monti, PONYA, requesting approval for material 

substitution, contains Fy = 45 ksi and Fy = 50 ksiYawata mill test reports. February 24. 

Symes, R. C. 1967a. Memo from PC&F to R. Monti, PONYA, requesting approval of Bethlehem 

V-series steels outside of the published plate sizes. 2 pages. Denied without full information on 

September 8, 1967, requested again with further documentation on November 2, 1967. Provisionally 

approved November 18, 1967 (no PCF letter #). August 14. 

Symes, R. C. 1967b. Letter PC&F to R, Monti (PONYA) requesting approval to use modified 

Bethlehem V-series steels outside the published thickness range, with full specifications attached. 

(PCF #666-39.) 7 pages. Approved November 30, 1967. November 2. 

Symes, R. C. 1967c. Memo from PC&F to R. M. Monti, PONYA, including Yawata data sheets. 

June 6. 

Tarkan, Y. N. 1969. Memo from Stanray Pacific to James White, SHCR, requesting approval for use of 

a welded plate. Includes mill sheet showing that the plate originated from Nippon Kokan Steel 

Fukuyama Works and was supplied by Crest Steel. August 12. 
Appendix E 

E–48

Walton, W. E. 1968. Letter to Malcolm Levy, PONYA, with attached ultrasonic and metallurgical report 

(Magnaflux Corp) on plate of “British” (i.e., Colvilles) steel. Details chemical analysis, weld quality 

and (poor-quality) micrographs. February 8. 

Warner, H. L. 1967. Memo from Stanray Pacific to Malcolm Levy, PONYA, detailing distribution of 

plate thicknesses between British and Japanese steels. Total is 32,000 tons. July 7. 

White, James. 1969a. Memo from SHCR to R. Monti, PONYA, documenting use of heat-treated steel 

above (PC&F) and below (PDM) the 9th floor splice. Contains statement that plate 3 (inside web) 

was fabricated from domestic steel, while plates 1, 2, and 4 (flange, outside web, and spandrel) are 

imported steel. Also contains table that shows where ASTM 441-modified and WEL-TEN grades 

were used, by thickness and yield strength. 28 pages. July 28. 

White, James. 1969c. Memo approving the April 4, 1969, PC&F steel substitutions. Has WEL-TEN 62 

mill test report. May 2. 

White, James. 1968a. Memo from SHCR to R. M. Monti, Port Authority, approving PC&F substitution of 

Fy = 100 ksi steel for Fy = 90 ksi steel in exterior columns. February 15. 

White, James. 1968b. Memo from SHCR to R. M. Monti, Port Authority, approving use of Bethlehem 

V60, V65, and V75 steels as specified for PC&F. January 4. 

White, James. 1967a. Memo from SHCR to R. M. Monti, PONYA, asking for clarification on origin of 

Japanese 36 ksi steel (Yawata or Kawasaki). 2 pages. September 6. 

White, James. 1967b. Memo from SHCR to R. M. Monti, PONYA, approving Ni-containing 

A 441-modified steel. Has two-page specification for Yawata A 441-modified. October 18. 

White, James. 1967c. Memo to PONYA. September 1. 

Yamada, H. 1967. Memo from Marubeni-Iida, the firm that imported the Fuji steel for Stanray Pacific, to 

PONYA, listing 1,441 tons of A 36 and A 572 grade 42 steel plates up to 3 in. thick. Plates are 36 ft 

3/4 in. long and 36 in. to 94 5/8 in wide. July 18. 
E–49

Attachment 1 

STEEL COMPANIES INVOLVED IN THE WORLD TRADE CENTER 

Most of the fabrication firms that worked on the steel for the World Trade Center (WTC) are no longer in 

business. This section summarizes the contributions of each of the major steel firms involved, and their 

current status. 

1.1 ATLAS MACHINE AND IRON WORKS 

Contract WTC212 

Atlas fabricated the 27 in. by 32 in. perimeter box columns, spandrels, and X-bracing below the 4th 

floor (Feld 1971) (13,600 tons). This contract was the first major use of electroslag welding in the United 

States (Feld 1971). 

Most recent address: 

Atlas Machine and Iron Works 

13951 Lee Highway 

Gainesville, VA 22065 

Arthur X. Miles, President and Registered Agent 

The Virginia Corporation Commission indicates that Atlas went out of business in 1999. The address is 

at the intersection of US 29 and I-66 in Gainesville, Virginia. A drive past the site on November 24, 

2002, confirmed that it is inactive. 

1.2 DRIER STRUCTURAL STEEL 

Drier fabricated the foundation load distribution system (base plates and grillages) (Feld 1971). No 

information is available on its current status. 

1.3 DOVELL ENGINEERING 

Dovell was the detailer for Stanray Pacific. (The detailer makes the detailed fabrication drawings of the 

columns and beams.) 

Current Address 

9901 Paramount Blvd Suite 202 

Downey, CA 90241 

562-927-4770 

Dovell President Tony Wall (Wall 2002 †) indicated that the former owner, who was active in the WTC 

project, is not in a position to provide details of the WTC project. 
Appendix E 

E–50

1.4 GRANITE CITY STEEL 

Granite City fabricated the electrical/telephone ducts and the floor deck system (Feld 1971). 

1.5 HOBART BROTHERS CO./ITW 

Hobart provided the electrodes used for on-site erection by Karl Koch Erecting Company 

ITW purchased it several years ago, but it still maintains its headquarters in Ohio. 

Current Address 

400 Trade Square East 

Troy, OH 45373 

www.hobartbrothers.com 

1.6 KARL KOCH ERECTING COMPANY 

Koch erected the towers (McAllister 2002). 

Skanska, an international construction company, purchased Koch in 1982. Karl Koch III is still alive, and 

recently wrote a book “Men of Steel” that includes information about the project (Koch 2002). 

1.7 LACLEDE STEEL CO. 

Contract WTC226 

Laclede fabricated the trusses for the floor system (Feld 1971). It entered bankruptcy on November 30, 

1998, but re-emerged in January 2001 only to reenter bankruptcy again July 27, 2001. Currently a group 

of former employees has purchased the assets. 

Current address 

211 N Broadway 

St Louis, MO 63102 

314-425-1400 

1.8 LEVINSON STEEL 

Contract WTC230 

Levinson fabricated the below-grade area (12,000 tons of 14WF sections), the plaza, and the damping 

units (Feld 1971). Metals USA acquired Levinson in March 1998. The www.metalsusa.com Web site 

does not list any information on Levinson, however. Metals USA went bankrupt in August 2001, but was 

reported to be emerging from bankruptcy on October 31, 2002. 

1.9 MONTAGUE-BETTS 

Contract WTC226 

Montague-Betts fabricated all the rolled columns and beams in the core of both towers, 25,900 tons 

(Feld 1971). Their contract was for “all rolled columns and beams, including cover-plated sections 
Interim Report on Contemporaneous Structural Steel Specifications 

E–51

throughout both towers...including horizontal trusses on 2nd floor... and exterior wall steel above 107th 

floor and the weldments for supporting future T.V. masts,” (Feld 1971). 

Current address of former owners: 

1619 Wythe Rd 

PO Box 11929 

Lynchburg, VA, 24501 

William Davis, President 

434-522-3200 

William Davis (2002 †), son of the founder (now age 91), confirmed that they furnished all the rolled 

beams for the core of both towers as well as the antenna base. Montague-Betts closed its steel fabrication 

business in 1992, though the family still owns a majority interest in one steel fabrication business in 

Lynchburg. 

1.10 MOSHER STEEL 

Mosher fabricated the elevator core framing system to the 9th floor (Feld 1971) (13,000 tons). 

Trinity Industries acquired Mosher Steel in November 1973, which is still in business. Rodengen’s 

(2000, p. 58) book has only a partial chapter on Mosher, and only notes that it “shipped more than 

13,000 tons of steel for the lower portion...” 

1.11 PACIFIC CAR AND FOUNDRY 

Contract WTC214 

Pacific Car and Foundry fabricated the perimeter column panels from the 9th to 107th floors (Feld 1971), 

55,800 tons. It changed its name to PACCAR in 1972. As PACCAR, they manufacture Kenworth and 

Peterbilt trucks. 

Contact Info: 

PACCAR Inc. 

777 106th Avenue N.E. 

Bellevue, WA 98004 

Telephone 425-468-7400; Fax 425-468-8216 

Dick Bangert (2002 †) (VP for facilities) confirmed that PACCAR sold the structural steel division “years 

ago” and has no records from that business. Ron Symes (2002 †), chief engineer for PC&F during the 

WTC construction, confirmed that the division was sold in 1974. The PACCAR corporate 

history (Groner 1981) reports that the WTC contract was not profitable for the Structural Steel Division 

because it had estimated the job based on shipping the completed sections by barge to New York, but 

were unable to obtain insurance to do that. As a result, they had to ship by rail, which nearly doubled the 

shipping costs. These losses, plus concessions to settle strikes in 1969 and 1970 sent the division into a 

decline from which it never recovered. Nicholas Soldano (2002 †), former general manager, remembered 

that the metals recycler Schnitzer bought the Seattle property where the perimeter columns were 

fabricated. 
Appendix E 

E–52

1.12 PITTSBURGH-DES MOINES STEEL 

Pittsburgh-Des Moines (PDM) fabricated the perimeter bifurcation columns from the 4th to the 9th floors, 

6,800 tons (Feld 1971). The bifurcation columns are also referred to as the “tuning forks” or the trees. 

Civil Engineering (1970) reported that Lukens Steel “supplied seven basic grades of carbon and alloy 

plate steels for use in the welded ‘trees... steels meet yield strength requirements from 36,000 to 

65,000 min psi.” Reliance Steel and Aluminum (www.rsac.com) acquired PDM Steel Service Centers in 

July 2001. 

1.13 STANRAY PACIFIC CORP 

Stanray Pacific fabricated the welded core box columns and built-up beams above the 9th floor, 

31,100 tons (Feld 1971). 

The California business portal report indicates that the company is no longer in business 

(Record # C0388500). According to its annual reports, the parent corporation, Stanray (1969, 1970), 

decided to close the Stanray Pacific (based in Los Angeles, California) subsidiary during 1969. Joe 

Tarkan, Stanray Pacific chief engineer for the WTC contract, confirmed this (Tarkan 2002 †). 
E–53

Attachment 2 

NOTES ON ASTM STANDARDS FOR STRUCTURAL STEEL

This attachment summarizes the important aspects of the relevant standards that governed the structural 

steel supplied and compares contemporary (current) and contemporaneous (1960s) standards. In general, 

the differences between the contemporaneous and contemporary standards are minor, and are usually 

additions or deletions of individual steel chemistries or small changes in test protocol. However, because 

of these changes, it is possible that a steel that met a construction-era version of a standard might not meet 

that same standard today, because the chemistry or elongation requirements have changed. This 

statement should not be interpreted to mean that the steel in question as used was unsuitable, however. 

The ASTM International defines a standard as “a document that has been developed and established with 

the consensus principles of the Society and that meets the approval requirement of ASTM procedures and 

regulations.” A standard may be a document that specifies the properties of a material, as in the case of 

steel standard specifications such as A 36. Other standards are test methods that define the way in which 

the properties in a specification must be measured. An example of this is A 370, which defines the test 

methods for establishing the strength of steel. An important aspect of ASTM standards is that they are 

consensus documents, established by committees where membership is open to all individuals and 

organizations. Except for military construction, the U.S. Government does not establish structural steel 

standards for the industry. Instead, the ASTM committees that establish steel standards are required to 

have balanced membership among producers, users, and independent experts. The standards they 

produce allow the producers and consumers to efficiently specify materials, without requiring them to 

include all possible properties and methods in a contract. This report, to avoid confusion with other uses, 

will use the term “standard” to refer to all ASTM documents, regardless of their status as Specifications, 

Test Methods, Terminology Standards, or Practices. 

The ASTM issues its standards annually in a multi-volume “Annual Book of ASTM Standards,” but 

revises an individual standard only when the committee in charge sees a need. ASTM does require that 

standards be reauthorized every five years, even if they have not been revised. The designation of a 

standard, for example A 36-66, comprises two parts. The first (for example “A 36”) is a shorthand for the 

general chemistry and mechanical property requirements, in the case of structural steels. Following the 

designation is a two digit number denoting the most recent revision year of the standard (for example 

“-66,” which denotes a substantial revision in 1966). The steel fabrication contracts stipulated that the 

appropriate standards were those in effect in September 1966. In some cases the relevant standard was 

not revised in 1966, and so bears a prior year revision mark. 

An individual ASTM standard does not contain all the information to uniquely characterize the steel. 

Instead, there is a “chain of standards” that defines the properties of the steel. The WTC steel contracts 

allowed the use of steels that conformed to certain ASTM standards (e.g., A 36, A 242, A 441, A 514). 

These standards define the mechanical and chemical properties of the steels, but in turn reference other 

standards that define how those properties shall be measured. For instance, all the steel standards, then 

and now, require that the steel conform to ASTM A 6 (“Requirements for Delivery of Structural Steel”), 

which specifies, among many things, the dimensional tolerances of plates and rolled shapes. The rest of 
Appendix E 

E–54

this section describes the minor differences between the ASTM standards that governed structural steel 

used for construction of the WTC, and those that exist today. 

2.1 A 6-65 VERSUS A 6-02 

ASTM A 6-65 “Standard Specification for General Requirements for Delivery of Rolled Steel Plates, 

Shapes, Sheet Piling and Bars for Structural Use,” specifies the tolerances for structural steel. Both 

versions specify that mechanical properties shall be determined in accord with A 370. At some point 

ASTM editorially amended the title of the standard to its present version “Standard Specification for 

General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling.” A 6-02 is a 

much longer and more complex document than A 6-65. 

For determining mechanical properties, A 6-65 specifies the size and shape of test specimens, while 

A 6-02 references (similar) specimens in A 370. Table 2–1 summarizes the significant differences in 

determining mechanical properties between A 6-65 and A 6-02. Two differences are particularly 

significant. A 6-65 specifies that steels shall be tested in the rolling direction (longitudinally), but A 6-02 

requires most plates to be tested in the transverse direction. The location of specimens from shapes is 

also different: in A 6 they are always taken from the web, but in A 6-02 for the large shapes used for 

columns, the specimen is taken from the flange. Typically, because the flange is thicker than the web, the 

flange yield stress will be less than the web yield stress (Alpsten 1975, AISC 1974). In summary, to 

conform to A 6, most A 36 specimens for the WTC projects would have been tested full thickness. Core 

column steels over 1.5 in thick would have been permitted to use the round 0.5 in. (12.7 mm) diameter 

specimen because of their thickness. Thin perimeter column plates would have been tested full thickness. 

In terms of chemistry, A 6-65 does not require any special method be used to determine the chemistry of 

the steel. In contrast, A 6-02 specifies that chemistry is to be determined in accord with ASTM A 751 

(“Standard Test Methods, Practices, and Terminology for Chemical Analysis of steel products”). A 6-65 

requires the mill test report to state the percentages of carbon, manganese, phosphorus and sulfur, as well 

as any element required by the individual standard. To that list, A 6-02 adds silicon, nickel, chromium, 

molybdenum, copper, vanadium, and niobium (referred to as columbium in the U.S. steel industry). The 

chemistry requirements have also been moved between standards. A 6-65 specifies two types of chemical 

analysis. The so-called ladle analysis is conducted at the steel mill on the steel before rolling. “Check” or 

product analyses are conducted on representative samples taken from the finished structural product All 

of the contemporaneous steel standards (e.g., A 36-66, etc) specify compositions determined in both ladle 

and check analyses, where the check analyses are slightly relaxed from the ladle analyses. In 

contemporary standards, the check analysis values (now called product analysis) have been removed from 

the standards to a single table in A 6-02. A spot check of the some of these for A 36-01 and A 242-01 

indicates that the values listed in Table B of A 6-02 (“Permitted Variations in Product Analysis”) are 

identical to the values listed under check analysis in the contemporaneous steel standards of the 1960s. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–55

Table 2–1. Differences in specimen sampling requirements between A 6-65 and A 6-02e. 

Shape Specimen Location Orientation Specimen type and size 

A 6-65 

Beams, channels or 

zees 

Web (Sec. 6.4) Longitudinal 

(Sec. 6.3) 

Full-thickness 

(Sec. 6.5) 

Shapes or plates 

except alloy steel 

plates over 1.5 in. 

thick 

Generally specified as 

corner in product 

specifications, but no 

apparent restrictions on 

position within 

thickness for 

non-full-thickness 

specimens. 

Longitudinal 

(Sec. 6.3) 

Full-thickness 

(Sec. 6.5) 

18 in. long specimen with 8 in. gage length 

or straight-sided specimen. For t>1.5 in. 

can use 0.505 in. diameter round specimen 

with 2 in. gage length 

Alloy steel plates 

0.75<t<=1.5 in. 

“ Longitudinal May use a round specimen with 

d = 0.505 in. very similar to A 370 02 

Fig. 4 

Alloy steel plates 

>1.5 in. thick 

“ Longitudinal May use a round specimen with 

d = 0.505 in. very similar to A 370 02 

Fig. 4 

A 6-02e 

Shapes: t ≤ 0.75 in. If w>6 in. from the 

flange, otherwise from 

the web (Sec. 11.3.2) 

Full thickness 

(Sec. 11.5.1) 

Longitudinal 

(Sec. 11.2) 

8 in. or 2 in. gage length flat specimen 

A 370 Fig. 3 

Shapes: t > 0.75 in. “ “ 0.5 in. diameter round specimen (A 370 

Fig. 4) or full thickness flat specimen 

(A 370 Fig. 3) if desired 

Plates: t ≤ 0.75 in. Corner (Sec. 11.3.1) Full thickness 

(Sec. 11.5.1) 

Transverse if 

w>24 in. 

(Sec. 11.2) 

8 in. or 2 in. gage length specimen A 370 

Fig. 3 

Plates: t>0.75 in. “ “ 0.5 in. diameter round specimen (A 370 

Fig. 4) or full thickness flat specimen 

(A 370 Fig. 3) if desired 

Steel products have a natural variability in mechanical properties. Because the mill test for yield and 

tensile strength represents only one or two specimens, it is possible that tests conducted on the finished 

product may yield properties that differ from the mill test report. Sometimes these tests will yield values 

that are lower than the appropriate standard specification. Should a specimen taken as part of the 

investigation exhibit a yield point or strength less than the applicable standard, this does not imply that 

the steel as a whole did not meet the standard. A 6-02 makes this quite clear: 

X2.1 The tension testing requirements of Specification A 6/A 6M are 

intended only to characterize the tensile properties of a heat of steel for 

determination of conformance to the requirements of the material 
Appendix E 

E–56

specifications. These testing procedures are not intended to define the 

upper or lower limits of tensile properties at all possible test locations 

within a heat of steel. It is well known and documented that tensile 

properties will vary within a heat or individual piece of steel as a 

function of chemical composition, processing, testing procedure and 

other factors. It is, therefore, incumbent on designers and engineers to 

use sound engineering judgement (sic) when using tension test results 

shown on mill test reports. The testing procedures of Specification 

A 6/A 6M have been found to provide material adequate for normal 

structural design criteria. 

Thus, the results of contemporary tension tests on WTC steels can only be used to assert that the steel in 

question is of a quality that could reasonably be expected to meet a given ASTM standard. It may be that 

an individual tension test might result in a measured yield point less than that acceptable in the standard. 

As long as the measured yield point is close to the specified minimum, the steel in question probably met 

the requirements of the standard. 

2.2 A 370-67 VERSUS A 370-02 

ASTM A 370, “Standard Methods and Definitions for Mechanical Testing of Steel Products,” controls the 

methods used for mill acceptance testing of heats (or plates) of steel. Aside from minor revisions in 1966, 

to incorporate A 443 (“Method of Notch Toughness of Turbine and Generator Steel Forgings”) A 370-67 

is identical to A 370-66. 

By and large A 370-67 and A 370-02 are very similar. Although the section numbers are different, much 

of the text is unchanged over the past 35 years. Table 2–2 summarizes the important differences between 

the two documents as they relate to tensile testing. As long as the loading rates are specified as the 

maximum rate in A 370-02, the test results will also meet A 370-67. 

2.3 E 6-66 VERSUS E 6-99ε2

ASTM E 6, “Standard Terminology Relating to Methods of Mechanical Testing,” defines the technical 

terms used in the various mechanical testing standards. The definitions of elastic limit, elongation, gage 

length, Poisson’s ratio, proportional limit, reduction of area, and tensile strength are word-for-word 

identical in the two standards. The definitions of yield point and yield strength differ textually, but not in 

spirit. Table 2–3 summarizes the textual differences between the two versions. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–57

Table 2–2. Differences between A 370-67 and A 370-02. 

A 370-67 A 370-02 

Section 10d suggests that tests defined in terms of 

strain rate are acceptable, but not feasible with 

production grade equipment 

Section 7.4. specifically allows tests defined in terms 

of strain rate 

No such language. Note 2 specifically disallows tests in load control 

No restriction on minimum extension rate for tests Section 7.4.1 requires that minimum speed for testing 

shall not be less than one-tenth of the maximum rate 

for determining yield point or yield stress 

No such language Allows maximum testing rate to be less than 100,000 

psi, min 

Section 13 (Determination of yield point) has different 

language but is similar in spirit 

Section 12(b)(1) specifies a so-called “divider method” 

for measuring yield point. 

Absent from Section 13 

In section 13 (determination of yield strength) the order 

of the methods is reversed. 

In Section 13 the extension under load method may “be 

used only when the product specification permits.” 

No such recommendation 

Section 13 allows the yield point to be reported as the 

yield strength if the load drop occurs before the 

specified offset is reached. 

No such allowance 

A Class B1 extensometer is required for all offset 

method determinations of yield strength. 

Section 13.2.2 allows the use of a Class B2 

extensometer for determining yield strength if the 

offset is ≤0.2 % 

Table 2–3. Differences in the definitions of yield point and yield stress in ASTM E 6. 

E 6-66 E 6-99ε2

Yield Point 

“[FL-2] the first stress in a material, less than the 

maximum attainable stress, a which an increase in 

strain occurs without an increase in stress 

Note—It should be noted that only materials that 

exhibit the unique phenomenon of yielding have a 

‘yield point.’” 

“YP [FL-2], n – a term used, by E 8 and E 8M, for the 

property which is now referred to as upper yield 

strength.” 

”Upper yield strength UYS, [FL-2], n –in a uniaxial test, 

the first stress maximum (stress at first zero slope) 

associated with discontinuous yielding at or near the 

onset of plastic deformation.” 

Yield Strength 

“[FL-2] The stress at which a material exhibits a 

specified limiting deviation from the proportionality 

of stress to strain. the deviation is expressed in terms 

of strain.” 

Notes on the offset and total extension under load 

methods follow. 

“YS or Sy [FL-2], n–the engineering stress at which, by 

convention, it is considered that plastic elongation of the 

material has commenced. This stress may be specified 

in terms of (a) a specified deviation from a linear 

stress-strain relationship, (b) a specified total extension 

attained, or (c) maximum or minimum engineering 

stresses measured during discontinuous yielding.” 

Discussion of the offset and specified extension under 

load methods follows, as well as discussion of upper and 

lower yield strengths, differences between the results of 

the two methods and of rate effects. 
Appendix E 

E–58

2.4 A 36-66 VERSUS A 36-01 

All chemistry requirements of Table 2 are identical for carbon, manganese, sulfur, and phosphorus. 

A 36-01 requires that steel for plates and shapes other than group 1 be killed or semi-killed, while 

A 36-66 only requires “where improved notch toughness is important, the material may be specified to be 

silicon killed fin grain practice.” As a consequence, A36-01 limits silicon to 0.4 percent for all sizes of 

plates and shapes, while A 36-66 only specifies silicon for plates with t>1.5 in. A 36-66 requires the 

material to pass a bend test defined as “The bend test specimens shall stand being bent cold through 

180 deg without cracking on the outside of the bent portion, to an inside diameter which shall have a 

relation to the thickness of the specimens as prescribed in Table IV.” The bend test is absent from 

A 36-01. A 36-66 requires that the steel be made by “open-hearth, basic-oxygen, or electric-furnace” 

A 36-01 has no such requirements. The elongation requirements differ between the two standards. 

A 36-66 has relaxed elongation requirements for thicker plates that are missing from A 36-01, and does 

not differentiate between plates and shapes for elongation requirements. Other than these differences, the 

standards are identical. 

2.5 A 242-66 VERSUS A 242-01 

The yield and tensile requirements are unchanged in the two standards, but the chemistry requirements 

differ substantially. A 242-66 admits high and low carbon variants. A 242-01 admits only a low carbon, 

low manganese type. During the WTC construction era, A 242 was revised to include the Type 1 variant 

of A 242-01. Table 2–4 compares the chemistry requirements between the two standards. Another 

difference is that A 242-01 prescribes the method for determining the atmospheric corrosion resistance, 

while A 242-66 only states, “If the steel is specified for materially greater atmospheric corrosion 

resistance than structural carbon steel with copper, the purchaser should so indicate and consult with the 

manufacturer.” The elongation requirements are relaxed for thicker plates and shapes in A 242-66. The 

current standard also adds some required elongations when specimens with 2 in. gage length are tested. 

Finally, A 242-01 no longer mandates that steel pass a bend test. Requirements for bend testing are now 

included as a non-mandatory appendix in A 6-02. 

Table 2–4. Differences in chemistry requirements 

between A 242-66 and A 242-01. 

Element A 242-66 A 242-66 

A 242-01 

(Type 1) 

C (max.) 0.22 0.15 0.15 

Mn (max.) 1.25 1.4 1.00 

S (max.) 0.05 0.05 0.05 

P (max.) NR NR NR 

Cu (min.) NR NR 0.20 

Key: C, carbon; Cu, copper; Mn, manganese; NR, no requirement; 

P, phosphorus; S, sulfur. 

Note: Compositions expressed in % mass fraction. 

The A 242 steel that Laclede supplied for the floor trusses would have met the chemistry requirements of 

A 242-66, but would not meet the chemistry requirements of A 242-01, because of its elevated carbon 

content. In terms of its load-carrying capacity, these differences are irrelevant, however. 
Interim Report on Contemporaneous Structural Steel Specifications 

E–59

2.6 A 441-66 VERSUS A 572-01 

ASTM A 441 was withdrawn in 1989. A 441-66 and A 572-01 are similar in several ways. Both are 

standards for vanadium-containing steels with minimum yield points greater than those specified in A 36. 

To some degree it can be argued that A 572 replaced A 441. The carbon, manganese, and silicon levels in 

both standards are similar but not identical. However, in terms of chemistry, most steels that met 

A 441-66 would probably meet A 572-01. A 572-01 admits a wider range of minimum yield points in 

much thicker sections as well, see Table 2–5. 

Table 2–5. Differences between 

A 441-66 and A 572-01. 

A 441-66

YP (ksi) 

Thickness 

t (in.) 

A 572-01

YP (ksi) 

40 4 in. <t<=8 in. 

t <= 6 in. 42 

42 1.5 in.<t<=4 in. 

t<=4 in. 50 

t<=2 in. 55 

46 3/4 in.<t<=1.5 in. 

t<=1.25 in. 60 

t<=1.25 in. 65 

50 t<=3/4 in. 

2.7 A 514-65 VERSUS A 514-00A 

A 514-65 differs from A 514-00a at dozens of points. Table 2–6 summarizes the substantial ones. Unlike 

standards such as A 36, which have simple, non-proprietary chemistry requirements, each variant 

chemistry in A 514 represents a single mill’s 100 ksi steel. For instance, Brockenbrough and Johnson 

(1968) identify A 514 Grade F as USS T1, A 514 Grade B as USS T1 Type A, and A 514 Grade H as 

USS T1 Type B. 

2.8 YIELD POINT VERSUS YIELD STRENGTH 

Both E 8 and A 370 distinguish between yield point and yield strength. For steels of interest to the 

investigation, all standards for steels with yield strength under 80 ksi, whether contemporary or 

contemporaneous, specify yield point instead of yield strength. ASTM E 6-99ε2

 (Standard Terminology 

Relating to Methods of Mechanical Testing) defines them as follows: 

• yield point, YP [FL-2], n – a term used, by E 8 and E 8M, for the property which is now referred 

to as upper yield strength. 

• upper yield strength, UYS, [FL-2], n –in a uniaxial test, the first stress maximum (stress at first 

zero slope) associated with discontinuous yielding at or near the onset of plastic deformation. 
Appendix E 

E–60

Table 2–6. Differences in ASTM A 514-65 and A 514-00a. 

A 514-65 A 514-00a 

Sampling 

requirements 

One tension test from each of two plates from 

each lot (Sec. 10.2) 

Brinell hardness from all plates not 

tension-tested (Sec. 7.1) 

One tension test from every plate in each lot 

(Sec. 8.1) 

Brinell hardness may be substituted for plates 

3/8 in. and under, with tension test from at 

least two plates (Sec. 7.2) 

Test specimen 

orientation 

No special requirement Plates over 24 in. wide must be tested in the 

transverse direction (8.1) 

Strength t ≤ 3/4 in. 115 ksi ≤ TS ≤ 135 ksi 

3/4 in. < t ≤ 2.5 in. 115 ksi ≤ TS ≤ 135 ksi 

2.5 in. < t ≤ 4 in. 105 ksi ≤ TS ≤ 135 ksi 

(Table 2) 

 t ≤ 3/4 in. 110 ksi ≤ TS ≤ 130 ksi 

3/4 in. <t ≤ 2.5 in. 110 ksi ≤ TS ≤ 130 ksi 

2.5 in. <t ≤ 6 in. 105 ksi ≤ TS ≤ 130 ksi 

(Table 2) 

Elongation in 

2 in. (%) 

2.5 in.<t≤4 in.: 17 % 

special elongation reduction allowances for 

plates under 5/16 in. (Sec. 6.2) 

2.5 in. <t ≤ 6 in.: 16 % 

No such allowance 

Chemistry 

(compositions 

expressed in 

% mass 

fraction) 

Admits Types D, G 

Type D 0.13-0.2C 0.4-0.7Mn, 0.035P, 0.04S 

0.2-0.35Si, 0.85-1.2Cr, 0.15-0.25Mo 

0.04-0.1Ti, 0.2-0.4Cu, 0.0015-0.005 B 

Type G 0.15-0.21C, 0.8-1.1Mn, 0.035P, 

0.04S, 0.5-0.9Si, 0.5-0.9Cr, 0.4-0.6Mo 

0.05-0.15Zr, 0.0025 Max B 

Types D, G absent 

Admits new types J, K, M, P, Q, R, S, T. 

Chemistry Most S allowables are 0.04 % Most S allowables are 0.035 % 

• yield strength, YS or Sy [FL-2], n –the engineering stress at which, by convention, it is 

considered that plastic elongation of the material has commenced. This stress may be specified in 

terms of (a) a specified deviation from linear stress-strain relationship, (b) a specified total 

extension attained, or (c) maximum or minimum engineering stresses measured during 

discontinuous yielding. 

The definitions of yield point and yield strength differ textually, but not semantically, between ASTM 

E 6-99ε2 and E 6-66, and are contrasted in Sec. 0 and Table 2–7. 

In terms of mechanical properties, it matters little whether yield point or yield strength is specified. 

Almost certainly the yield point of plain carbon steels (like A 440 and A 36) will exceed the yield 

strength by only 1 KSI to 4 ksi, because they typically exhibit a yield drop after yielding. Of the relevant 

standards, only A 514 specifies steel in terms of yield strength. Both contemporary and contemporaneous 

version of A 36, A 242, A 441, and A 572 specify yield point rather than yield strength. The AISC 

Manual of Steel Construction (AISC 1973, p.1-3) treats them identically: 

As used in the AISC Specification, “yield stress” denotes either the 

specified minimum yield point (for those steels that have a yield point) or 

specified minimum yield strength (for those steels that do not have a 

yield point). 
Interim Report on Contemporaneous Structural Steel Specifications 

E–61

Table 2–7. Methods for determining Yield Point and Yield Strength in ASTM A 370. 

A 370-67 A 370-02 

Yield Point 

“Drop of the beam” method 

Section 12(a)(1) 

“Drop of the beam” method 

Section 13.1.1 

Position of the knee 

Section 12(a)(2) 

Position of the knee 

Section 13.1.2 

Total extension under load (at a suggested strain of 

ε = 0.005) 

Section 12(b)(2) 

Total extension under load (at a suggested strain of 

ε = 0.005) 

Section 13.1.3 

“Divider method” Section 12(b)(1) 

Yield Strength 

Offset method with no suggested value but with an 

example that uses ε = 0.002 

Section 13(b) 

Offset method with no suggested value but with an 

example that uses ε = 0.002 

Section 13.2.1 

Extension under load with no required or suggested 

strain value: “this approximate method be used only 

when the product specification permits” 

Section 13(a) 

Extension under load with no suggested strain, but with 

an example that uses of ε = 0.005 

Section 13.2.2 

A 370-02 permits three different methods for measuring yield point and two methods for yield strength, 

summarized in Table 2–2. The “drop of the beam” method applies to testing machines that prescribe the 

loading rate, rather than the extension rate. 

Interestingly, neither A 370-67 nor A 370-02 mandates a specific value of the total extension under load 

determining either yield point or yield stress when using the total extension under load method. It does 

suggest a value of ε = 0.005, but does so in a nonmandatory note. Furthermore, A 370-67 does not 

require the mill to report which method it used for measuring yield point. Neither A 6-65 nor A 370-67 

has any requirements as to the contents of a mill test report. A 6-02 does have a detailed section on Test 

Reports, however. 
Appendix E 

E–62

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F–i

TABLE OF CONTENTS - Appendix F

List of Figures...........................................................................................................................................F–ii

List of Tables ...........................................................................................................................................F–iii

Appendix F 

Interim Report on Inventory and Identification of Steels Recovered from the 

WTC Buildings ........................................................................................................................F–1

F.1 Purpose of Report ........................................................................................................................F–1

F.2 Scope of Report ...........................................................................................................................F–1

F.3 Background Information Related to Recovery of WTC Structural Steel ....................................F–2

F.4 Structural Elements Recovered from the WTC Buildings...........................................................F–3

F.4.1 Present Location and Labeling of Structural Steel Elements ...........................................F–3

F.4.2 Identification of WTC Structural Steel Elements.............................................................F–5

F.5 Structural Steel Elements of Special Importance.......................................................................F–35

F.5.1 Samples Located in or Around the Floors Impacted by the Airplane ............................F–35

F.5.2 Samples Representing the Various Types of Steel Specified in the Design Drawings ..F–37

F.6 Summary....................................................................................................................................F–38

F.7 References..................................................................................................................................F–40

F.7.1 References from Publicly Available Sources .................................................................F–40

F.7.2 References from Nonpublic Sources ..............................................................................F–40

Attachment 1 

Data on Recovered WTC Steel.............................................................................................F–41
F–ii

LIST OF FIGURES

Figure F–1. Characteristic “overall” view of the samples taken for each piece received......................F–4

Figure F–2. Location of the exterior panels recovered from the top third of WTC 1 and 2..................F–7

Figure F–3. Example of stampings on the interior base of the middle column for each panel..............F–9

Figure F–4. Example of stampings placed on one end of a core column. ...........................................F–10

Figure F–5. Example of stamping placed on flange indicating the column type (120), 

and schematic indicating the various plates corresponding to Table F–5........................F–11

Figure F–6. Characteristic stenciling found on the lower portions of the exterior column panels 

for sample C-14 and on an interior core column for sample B-6152...............................F–13

Figure F–7. Schematic showing the derrick divisions that hoisted the specific columns for 

WTC 1 and WTC 2. .........................................................................................................F–14

Figure F–8. Schematic showing the sample M-10 as two separate exterior column panels, M-10a 

and M-10b........................................................................................................................F–22

Figure F–9. Schematics displaying the various types of exterior column panels. ...............................F–23

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1. ................................................................F–24

Figure F–11. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 2. ................................................................F–31

Figure F–12. Interpreted column damage, from photographic evidence, to WTC 1, with overlay 

of samples in NIST’s possession......................................................................................F–36

Figure F–13. Interpreted column damage, from photographic evidence, to WTC 2, with overlay 

of samples in NIST’s possession......................................................................................F–37
F–iii

LIST OF TABLES

Table F–1. Identified exterior column panel pieces from WTC 1 and WTC 2....................................F–6

Table F–2. Partially identified exterior column panel from WTC 1 or WTC 2. ..................................F–7

Table F–3. Identified pieces of core column material from WTC 1 and WTC 2.................................F–8

Table F–4. Other built-up box columns and wide flange sections from WTC 1 and WTC 2 with 

ambiguous stampings and/or markings..............................................................................F–8

Table F–5. Examples of column types with corresponding plate gages.............................................F–12

Table F–6. Specified and observed minimum yield strengths for positively identified exterior 

column panels. .................................................................................................................F–16

Table F–7. Specified and observed column types for positively identified exterior 

column panels. .................................................................................................................F–17

Table F–8. Specified minimum yield strengths from WTC 1 and WTC 2, along with the observed 

stampings, used to positively identify some exterior column panels. ..............................F–18

Table F–9. Specified column types of exterior panels from WTC 1 and WTC 2, along with the 

observed stampings, used to positively identify some exterior column panels................F–19

Table F–10. Information used to determine the identification of exterior panel M-2..........................F–21

Table F–11. Listing of recovered exterior column panels with specified minimum yield strengths 

and thicknesses for columns and spandrels......................................................................F–39
Appendix F 

F–iv

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F–1

Appendix F

INTERIM REPORT ON INVENTORY AND IDENTIFICATION OF STEELS 

RECOVERED FROM THE WTC BUILDINGS

F.1 PURPOSE OF REPORT 

The purpose of Project 3 of the National Institute of Standards and Technology (NIST), World Trade 

Center (WTC) Investigation, Mechanical and Metallurgical Analysis of Structural Steel, is to analyze 

structural steel available from WTC 1, 2, and 7 to determine the metallurgical and mechanical properties 

and quality of the metal, weldments, and connections and to provide these data to other investigation 

projects. (For test plan details, see http://wtc.nist.gov/media/WTCplan_new.htm#proj3.) The properties 

determined under this project will be used in two ways: 

• Properties will be correlated with the design requirements of the buildings to determine if the 

specified steel was in place in the towers. 

• Properties will be supplied for other projects in the Investigation as input for models of 

building performance. 

This interim report is an output of Task 1 of Project 3. Task 1 is defined in the NIST plan as “Collect and 

catalog the physical evidence (structural steel components and connections) and other available data, such 

as specifications for the steel, the location of the steel pieces within the buildings, and the specified steel 

properties.”

F.2 SCOPE OF REPORT 

The Task 1 report comprises three parts: 

• Tower Design – Structural Steel Documents. 

• Contemporaneous Structural Steel and Construction Specifications. 

• Inventory and Identification of Steels Recovered from the WTC Buildings. This appendix 

covers part 3; Appendix E presents the structural design of the WTC towers and the 

specifications used for the steel and construction of the buildings. 

Part 1, which is covered in Appendix E of this Progress Report, describes the tower structure and critical 

structural elements to be characterized in Project 3. This includes the structural design and properties 

specified by the structural engineers for columns, floor systems, and connections. 

Part 2, also covered in Appendix E, describes the contemporaneous (late 1960s era) specifications for 

various types and grades of steel designated by the ASTM International, the American Institute of Steel 

Construction, and other national and international organizations. It also includes information from 

numerous suppliers of the steel for the towers. The structural steel for the towers was supplied through at 

least a dozen contracts to suppliers and fabricators. Substantial understanding of the consistency, quality, 
Appendix F 

F–2

and actual strength of the steel (as opposed to specified minimum values) can be gained if the production 

practices and quality control procedures used by the various steel suppliers are understood. Practices and 

data from the numerous WTC steel suppliers have been investigated and are reported for both structural 

steel and construction practices. In addition, this information has been used to estimate typical 

mechanical property values for many of the grades of steel. These typical values can serve as a guide for 

the properties to be inserted into the finite element models of building performance and as a point of 

comparison for actual properties measured on the recovered steel. 

Part 3, covered in this appendix, documents the steel recovered for the WTC Investigation. 

Approximately 236 pieces of WTC steel were available for study at NIST. These pieces represent a small 

fraction of the steel examined at the various recovery yards where the steel was sent as the WTC site was 

cleared. 

F.3 BACKGROUND INFORMATION RELATED TO RECOVERY OF WTC 

STRUCTURAL STEEL 

Beginning in October 2001, members of the Federal Emergency Management Agency (FEMA), 

American Society of Civil Engineers (ASCE), Building Performance Assessment Team (BPAT), 

members of the Structural Engineers Association of New York (SEAoNY), and Professor A. Astaneh-Asl 

of the University of California, Berkeley, California, with support from the National Science Foundation, 

began work to identify and collect WTC structural steel from the various recovery yards where debris, 

including the steel, was taken during the cleanup effort. Dr. J. Gross, a structural engineer at NIST and a 

member of the FEMA/ASCE BPAT, was involved in these early efforts. 

There were four major sites where debris from the WTC buildings was shipped during the clean-up effort 

in which the volunteers worked. These were: 

• Hugo Nue Schnitzer, Inc., Fresh Kills Landfill in Staten Island, New Jersey; 

• Hugo Nue Schnitzer East, Inc., Claremont Terminal in Jersey City, New Jersey; 

• Metal Management, Inc., in Newark, New Jersey; and 

• Blanford and Co. in Keasbey, New Jersey. 

The volunteers searched through unsorted piles of steel and other debris for pieces from the WTC 

buildings, specifically searching for (McAllister 2002): 

• Exterior column panels and interior core columns from WTC 1 and WTC 2 that were exposed 

to fire and/or impacted by the aircraft; 

• Exterior column panels and interior core columns from WTC 1 and WTC 2 directly above 

and below the impact zones; 

• Badly burned pieces from WTC 7; 

• Connections from WTC 1, WTC 2, and WTC 7 (e.g., seat connections, single-shear plates, 

and column splices); 
Interim Report on Inventory and Identification of Steels 

F–3

• Bolts in all conditions; 

• Floor trusses, including stiffeners, seats, and other components; and 

• Any pieces that in the engineers’ professional opinion might be useful. 

Once identified for recovery, the samples were marked as “SAVE” and given an alphanumeric code 

relative to the recovery yard from which they came and an accession number. Some pieces were not 

saved in their entirety, but instead, small portions were removed, hereafter called coupons. (Coupons 

were also removed in the field for WTC 5, held at Gilsanz Murray Steficek, LLP [GMS, LLP], and later 

brought to NIST.) 

Facing concern that the identified steel may not be properly preserved in the recovery yards, NIST 

arranged for the steel to be shipped to its campus in Gaithersburg, Maryland, starting in March 2002. 

Professor Astaneh-Asl also granted NIST permission to take custody of the steel that he had personally 

marked. Before the samples were shipped to the NIST campus, environmental testing for asbestos and 

analysis of the paint for lead was conducted. Volunteers from SEAoNY, with assistance from additional 

NIST personnel, continued their presence at the recovery yards and identified, catalogued, and shipped 

steel specimens to NIST through October 2002. The structural components recovered now constitute the 

material base from which samples are being removed for further evaluation and or testing relative to the 

fire and structural response of the WTC buildings as part of the WTC Investigation. 

Structural steel elements were also collected and held by the Port Authority of New York and New Jersey 

(PANYNJ) in Hanger 17 located at John F. Kennedy International Airport (JFK). The main goal of the 

Port Authority project was to decontaminate and preserve the steel, as well as other WTC artifacts, for 

future exhibits and memorials. A complete listing of the pieces held by PANYNJ can be found in the 

Preservation and Inventory Report prepared by Voorsanger and Associates Architects, PC 

(Voorsanger 2002). NIST personnel visited the hanger and identified 12 additional pieces that were 

considered important to the Investigation. Six of these samples were moved whole to the Gaithersburg 

campus. The remaining pieces had portions removed and sent to NIST, with the bulk of the structural 

element remaining at JFK. 

F.4 STRUCTURAL ELEMENTS RECOVERED FROM THE WTC BUILDINGS 

F.4.1 Present Location and Labeling of Structural Steel Elements 

At present, NIST possesses 236 labeled samples from the WTC buildings. While the majority of the 

NIST-held samples reside on the Gaithersburg campus, some samples were shipped to the Boulder 

campus for mechanical property testing following initial documentation. 

As samples were delivered, overall images of the pieces were taken for record-keeping purposes. An 

example is shown in Fig. F–1. Samples are identified by their original alphanumeric identification codes 

assigned by SEAoNY to be consistent with the FEMA report. However, there were cases in which two 

different codes were found on one piece. In these instances, if the pieces were already undergoing 
Appendix F 

F–4

Figure F–1. Characteristic “overall” view of the 

samples taken for each piece received. Sample shown 

here is C-14. 
Interim Report on Inventory and Identification of Steels 

F–5

documentation procedures, the first code noted was used. Samples that arrived lacking a code were 

labeled as part of the U series. Additionally, samples brought from Hanger 17 at JFK maintained their 

“B”-series labels provided in the Voorsanger report (Voorsanger 2002). 

Attachment 1 is a complete list of each sample received, in alphanumeric order, with its classification, a 

brief description of the component, and the location of the piece on the NIST campus. These samples 

range from full exterior column panels to pieces of bolts and bags of glass and other debris fragments. 

The pieces were classified into one of eight categories: 

Classification No. of Pieces Symbol 

Exterior column panel sections (flat wall or corner) 94 C, CC, or Cn 

Bowtie pieces 2 BT 

Rectangular built-up box column (not perimeter column) 11 RB 

Wide flange sections 44 W 

Floor trusses 23 J 

Channels 25 Ch 

Coupons from WTC 5 7 Cn5 

Miscellaneous (isolated bolts, floor hanger components, or other) 30 B,H,O 

Attachment 2 lists the pieces separated by type, and Attachment 3 displays characteristic photographs of 

the various pieces. 

F.4.2 Identification of WTC Structural Steel Elements 

Information from Leslie E. Roberts Associates indicates that all structural steel pieces in WTC 1 and 

WTC 2 were uniquely identified by stampings (recessed letters and numbers) and/or painted stencils 

(Faschan 2002). NIST has been successful in finding these identification markings on many of the 

perimeter panel sections, core columns, and other wide flange members. Of the 94 pieces of perimeter 

panel labeled in Attachment 1, 90 distinct panels were observed. (The other four pieces of perimeter 

column had become separated from the main panel during salvage and were subsequently labeled C-13a, 

C-16a, C-28b, and K-16a.) At this time, of the 90 panels, 41 distinct exterior column panels have been 

identified and 1 partially identified. Tables F–1 and F–2 list these samples, respectively, with Fig. F–2 

showing the relative locations of the identified exterior panels within the top third of the buildings. 

Significantly more pieces were recovered from WTC 1 than WTC 2. Table F–3 lists the 12 core columns 

in NIST’s possession that have been positively identified through their stampings. An additional sample, 

C-83, is also listed in this group. Though no markings were found on the piece, the shape and dimension 

of this sample are in conformance with the design drawings for core columns and it has a similar 

appearance to core column C-90. Additionally, there are 13 pieces of wide flange sections that have 

stampings and/or markings with different codes that are not presently understood (see Table F–4). NIST 

is still investigating the identification of these pieces. 

The positive identification of the structural elements was made possible by deciphering the stampings 

and/or stencils found on them. During the fabrication process, the exterior panel sections were stamped at 

the bottom of the center column on the inside face. These stampings indicated the building, center 

column line number, and floors spanned by the columns. The core columns had stampings placed at the 
Appendix F 

F–6

Table F–1. Identified exterior column panel pieces from WTC 1 and WTC 2. 

NIST 

Name Type Description Bldg. Column Floors 

Derrick

Division 

B-1024 C Full panel WTC 2 154 21 – 24 NA 

B-1043 C Full panel WTC 2 406 40 – 43 NA 

B-1044 C Full panel WTC 2 409 40 – 43 NA 

C-10 C Full panel WTC 1 451 85 – 88 5x 

C-13 

C-13a 

CC 

C 

Rectangular column with spandrel 

Partial of single column 

WTC 2 

WTC 2 

200 

159 

90 – 92 

90 – 92 

569 

569 

C-14 C 1 column, lower 1/3 WTC 2 300 85 – 87 570 

C-18 C 3 columns, bottom 2/3 WTC 2 230 93 – 96 NA 

C-22 C 3 columns, lower 1/2 WTC 1 157 93 – 96 69 

C-24 C 3 columns, upper 1/3 WTC 2 203 74 – 77 NA 

C-25 C 1 column, lower 1/2 WTC 1 206 89 – 92 69 

C-40 C 2 columns, lower 2/3 WTC 1 136 98 – 101 6x 

C-46 C Nearly full panel WTC 2 157 68 – 71 569 

C-48 C Nearly 2 full columns WTC 2 442 91 – 94 NA 

C-55 C 1 column, lower 1/3 WTC 1 209 94 – 97 NA 

C-89 C 2 full columns WTC 2 215 12 – 15 NA 

C-92 C 1 column, lower 1/3 WTC 2 130 93 – 96 NA 

C-93 C 1 column, lower 1/3 WTC 1 339 99 – 102 NA 

CC C 2 full columns WTC 1 124 70 – 73 NA 

K-1 C 3 columns, lower 1/3 WTC 1 209 97 – 100 NA 

K-2 C 1 column, lower 2/3 WTC 1 236 92 – 95 NA 

M-2 C Full panel WTC 1 130 96 – 99 63 

M-10a C 3 columns, middle section 1/3 WTC 2 209 82 – 85 NA 

M-10b C 3 columns, lower 1/2 WTC 2 206 83 – 86 569 

M-20 C 2 columns, lower 1/3 WTC 1 121 99 – 102 63 

M-26 C Full panel WTC 1 130 90 – 93 6x 

M-27 C 2 columns, lower 3/4 WTC 1 130 93 – 96 63 

M-28 C 3 columns, lower 1/4 WTC 2 345 98 – 101 NA 

M-30 C 2 columns, lower 1/3 WTC 1 133 94 – 97 65 

N-1 C 2 full columns WTC 1 218 82 – 85 NA 

N-7 C Full panel WTC 1 127 97 – 100 NA 

N-8 C Full panel WTC 1 142 97 – 100 67 

N-9 C Nearly full panel WTC 1 154 101 – 104 69 

N-10 C 2 columns, lower 2/3 WTC 1 115 89 – 92 6x 

N-12 C 2 full columns WTC 1 206 92 – 95 69 

N-13 C 3 columns, lower 1/3 WTC 1 130 99 – 102 63 

N-99 C Nearly full panel WTC 1 148 99 – 102 67 
Interim Report on Inventory and Identification of Steels 

F–7

Table F–1. Identified exterior column panel pieces from WTC 1 and WTC 2 (continued). 

NIST 

Name Type Description Bldg. Column Floors 

Derrick 

Division 

N-101 C Full panel WTC 1 133 100 – 103 65 

S-1 C 2 columns, lower 1/3 WTC 1 433 79 – 82 47 

S-9 C Full panel WTC 1 133 97 – 100 NA 

S-10 C 2 columns, lower 1/2 WTC 1 224 92 – 95 NA 

S-14 C Full panel WTC 2 218 91 – 94 557 

Key: NA, information not available. 

Note: “x” in Derrick Division: Unreadable. 

Table F–2. Partially identified exterior column panel from WTC 1 or WTC 2. 

NIST 

Name Type Description Bldg. Column Floors 

C-117 C 3 columns, lower 1/3 NA NA 100 – 104 

Figure F–2. Location of the exterior panels recovered from the top third of WTC 1 and 2. 

WTC 1 North face

WTC 2 West face

WTC 1 East face 

WTC 2 North face

WTC 1 South face

WTC 2 East face

WTC 1 West face 

WTC 2 South face 
Appendix F 

F–8

Table F–3. Identified pieces of core column material from WTC 1 and WTC 2. 

NIST Name Type Description Bldg. Column Floors 

Derrick 

Division 

FY 

(ksi) 

B-1011 RB Heavy rectangular column WTC 1 508 51 – 54 55 36 

B-6152-1 RB Heavy rectangular column WTC 1 803 15 – 18 52 36 

B-6152-2 RB Heavy rectangular column WTC 1 504 33 – 36 51 36 

C-83a RB Heavy rectangular column NA NA NA NA NA 

C-88a RB Heavy rectangular column WTC 2 801 80 – 83 550 42 

C-88b RB Heavy rectangular column WTC 2 801 77 – 80 550 42 

C-90 RB Heavy rectangular column WTC 2 701 12 – 15 549 36 

C-30 or S-12 W Wide flange section WTC 2 1008 104 – 106 NA 36 

C-65 or S-8 W Wide flange section WTC 1 904 86 – 89 52 36 

C-71 W Wide flange section WTC 1 904 77 – 80 NA 36 

C-80 W Wide flange section WTC 1 603 92 – 95 51 36 

C-155 W Wide flange section WTC 1 904 83 – 86 52 36 

HH or S-2 W Wide flange section WTC 1 605 98 – 101 53 42 

a. C-83 was not positively identified but due to similar size and shape was deemed a core column. 

Key: NA, information not available. 

Table F–4. Other built-up box columns and wide flange sections from WTC 1 

and WTC 2 with ambiguous stampings and/or markings. 

NIST Name Type Description Markings 

C-79 RB Thin rectangular column 101A 81 – 85 – 87 – 92 52 

C-101 RB Thin rectangular column 78A 10 27 50 

C-154 RB Thin rectangular column 825: 107 – 108 52 

C-26 W Three connected wide flange sections 604/605 107 64 50 

C-44 W Wide flange section 59 S 563 

C-45 W Wide flange section 16 S2 563 Fy 50 

C-60 W Wide flange section 193 S1 69 

C-61 W Wide flange section 150 S 69 

C-62 W Wide flange section 224 (S) <48> Fy 50 

M-17 W Wide flange section 163 (9) 62 Fy 36 

M-23 W Wide flange section F 2010 

M-37 W Wide flange section 130 (8x – 92) <50> 

M-38 W Wide flange section Fy 42 

Note: “x”, unreadable. 
Interim Report on Inventory and Identification of Steels 

F–9

Figure F–3. Example of stampings on the interior base of the 

middle column for each panel. 
Appendix F 

F–10

lower end of the component near the connector. The building was typically represented as “A” for 

WTC 1 and “B” for WTC 2. An example of a stamping found on an exterior column is shown in 

Fig. F–3, where the stamping indicates that the piece was from WTC 2, with center column line number 

206, spanning floors 83 through 86. Core column material was found to have similar markings 

(Fig. F–4). Other stampings have also been found on the flanges of the perimeter columns that indicated 

the column type (Fig. F–5 and Table F–5) as well as the specified minimum yield strength of the column. 

Additional stampings are located on the flanges, but are not yet understood. 

NIST is still investigating the significance of these codes. All of these stampings typically reside within 

1 meter from the bottom of the column. 

Figure F–4. Example of stampings placed on one end of a core column. 
Interim Report on Inventory and Identification of Steels 

F–11

Figure F–5. (a) Example of stamping placed on flange indicating the column type (120), 

and (b) schematic indicating the various plates corresponding to Table F–5. 

Plate 2 

Plate 1 

Plate 3 

a) 

b) 
Appendix F 

F–12

Table F–5. Examples of column types with corresponding 

plate gages. 

Column Type 

Plate 1 

(in.) 

Plate 2 

(in.) 

Plate 3 

(in.) 

120 1/4 1/4 1/4 

121 5/16 1/4 1/4 

122 3/8 1/4 1/4 

123 7/16 1/4 1/4 

124 1/2 1/4 1/4 

125 9/16 1/4 1/4 

126 5/8 1/4 1/4 

128 3/4 1/4 1/4 

129 13/16 5/16 5/16 

133 1-1/16 3/8 3/8 

149 2-1/16 11/16 11/16 

150 2-1/8 3/4 3/4 

152 2-1/4 3/4 3/4 

334 1-1/8 3/8 3/8 

335 1-3/16 7/16 7/16 

520 1/4 1/4 1/4 

522 3/8 1/4 1/4 

Each of the structural elements was additionally stenciled in white or yellow lettering with similar 

building information. For the exterior panel sections, the stenciling was located on or near the lower 

spandrel on the interior face. Figure F–6 (a) shows a typical stenciling found on a perimeter panel, 

indicating this piece was in WTC 2, with center column line number 300, spanning floors 85 through 87. 

For the core columns, both stenciling and handwritten codes have been observed on the recovered pieces. 

Figure F–6 (b) shows one of these stencilings from a core column located in WTC 1. 

Also seen in Fig. F–6 (a) are two other indicators, 3T and <570>, found on the exterior panel sections. 

These markings are the estimated piece tonnage (1 ton equals approximately 907 kg) and the erector’s 

derrick division number, respectively. This information was also stamped on some of the core column 

pieces (see Fig. F–4). The erector, Karl Koch Erecting Co., Inc., assigned derrick divisions 47 through 70 

for WTC 1 and derrick divisions 547 through 570 for WTC 2 (PONYA 1967). Each division was 

assigned to a specific area of the building and shared a crane with other nearby derrick divisions. 

Therefore, a single crane may have lifted pieces from derrick divisions 65, 67, and 69. Figure F–7 shows 

the derrick division numbers that hoisted the specific columns for both buildings, according to the derrick 

numbers found on structural elements with positive identification (also shown in Tables F–2 and F–3). 

Of the 41 positively identified exterior panels, 25 had specific markings giving all the information needed 

(building, column, floors) to locate the structural element within the buildings from one or both codes 

(i.e., stampings or stencils). The flange stampings, which indicated the specified yield strength and 

column type, were used to confirm the findings (Tables F–6 and F–7). The only deviation noted was that 

100 ksi steel was substituted for the 85 ksi and 90 ksi grades that were specified. This can be observed in 
Interim Report on Inventory and Identification of Steels 

F–13

Figure F–6. (a) Characteristic stenciling found on the lower portions of the exterior 

column panels for sample C-14. (b) Characteristic stenciling found on an interior core 

column for sample B-6152. 

a) 

b) 
Appendix F 

F–14

Source: McAllister 2002. 

Figure F–7. Schematic showing the derrick divisions 

that hoisted the specific columns for (a) WTC 1, and 

(b) WTC 2. 

63 6x

63

65

65

6x 67 67 69 69

69 

47 51

5x 

6x

63

a) 

69 

64

63

55

52

52

53 51

557 

569

570 

b) 569 

569 

550

549

563
Interim Report on Inventory and Identification of Steels 

F–15

Table F–6 for samples B-1043, B-1044, C-10, and M-10b. This substitution is consistent with (PANYNJ) 

documents of the construction period, indicating that 100 ksi steel was used for all steel specified as 

85 ksi or 90 ksi. (See Appendix C, Contemporaneous Structural Steel and Construction Specifications.) 

Sixteen other panels were positively identified using a combination of the stampings, including the 

specified minimum yield strength (Table F–8) and column type (Table F–9), the stenciled derrick division 

number (Table F–8), or association to another panel, as follows: 

• C-10: The stampings indicated that the center column line number was 451 and the panel 

spanned floors 85 through 88, but the building identification information was obscured by a 

weld bead. The building can be identified by a derrick division number in the 50 series, 

which corresponds to WTC 1 (Fig. F–7). (Note that the flange stampings indicated that the 

steel used is 100 ksi, while the building design drawings indicated that 85 ksi was specified. 

As mentioned above, substitution of the specified 85 ksi, as well as the 90 ksi grades, by 

100 ksi steel was approved.) 

• C-24: This piece was readily identifiable as a mechanical or service floor due to the nonuniform width of the columns. Unfortunately, only the upper portion of the panel was 

recovered, and thus no stampings were found. However, the end connections to these floors 

were welded in addition to the typical bolting. In doing so, the end plate and a small portion 

of the column from the panel above this piece remained after the collapse, and the stamping 

of “B 203 77-78” identifying the panel above this sample was clearly visible. 

• C-55: The stampings indicated that the center column line number was 209 and the panel 

spanned floors 94 through 97, however, no building information was observed. By reviewing 

the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

• C-92: Stenciling on the piece indicated that it was from WTC 2, floors 93 through 96. 

However, the center column line number was partially obscured, with 13x visible. By 

reviewing the flange stampings (Tables F–8 and F–9), the piece center column line number 

was determined to be 130. 

• C-93: The stampings indicated that the center column line number was 339 and the panel 

spanned floors 99 through 102; however, no building information was observed. By 

reviewing the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

• CC: The stampings indicated that the center column line number was 124 and the panel 

spanned floors 70 through 73; however, no building information was observed. By reviewing 

the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

• K-1: The stampings indicated that the center column line number was 209 and the panel 

spanned floors 97 through 100; however, no building information was observed. By 

reviewing the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 
Appendix F 

F–16

Table F–6. Specified and observed minimum yield strengths for positively identified 

exterior column panels.a

a. Columns 1, 2, and 3 are viewed left to right as viewed from the inside of the building. 

Key: NA, information not available. 
Interim Report on Inventory and Identification of Steels 

F–17

Table F–7. Specified and observed column types for positively identified exterior 

column panels.a

a. Columns 1, 2, and 3 are viewed left to right as viewed from the inside of the building. 

Key: NA, information not available. 
Appendix F 

F–18

Table F–8. Specified minimum yield strengths (ksi) from WTC 1 and WTC 2, along with the observed stampings, used to 

positively identify some exterior column panels.a

a. Columns 1, 2, and 3 are left to right viewed from inside the building. 
Interim Report on Inventory and Identification of Steels 

F–19

Table F–9. Specified column types of exterior panels from WTC 1 and WTC 2, along with the observed stampings, 

used to positively identify some exterior column panels.a

a. Columns 1, 2, and 3 are left to right viewed from inside the building. 
Appendix F 

F–20

• K-2: The stampings indicated that the center column line number was 236 and the panel 

spanned floors 92 through 95; however, no building information was observed. By reviewing 

the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

• M-2: No information was available from the stampings at the base of the middle column, 

and very little information was recovered from the stenciling on the spandrel. A derrick 

division number of <63> was observed, placing the element in WTC 1 (Table F–8). The only 

other information was – 9, indicating that some portion of the panel was located in the 

90s-floor-level range. The flange stampings from the recovered piece specified that all three 

columns were of the 122 type, with FY 55 ksi steel. In addition, columns 1 and 3 had floor 

truss seats, while column 2 had gusset plates for the diagonal bracing straps. Reviewing the 

building design drawings, it was found that five panels meet the 122 column type, with 55 ksi 

steel in the 90s range (Table F–10). Of these, only two panels had columns 1 and 3 with floor 

truss seats (130: 96 through 99 and 330: 96 through 99). As shown in Fig. F–7, the derrick 

division of <63> identifies the panel as 130: 96 through 99. 

• M-10a: The sample was identified solely by association to another panel (bolted spandrel 

connection). The sample M-10 retrieved by SEAoNY was actually composed of pieces from 

two different exterior column panels (Fig. F–8). Therefore, with the positive identification of 

M-10b via the stampings and stencils, M-10a’s connection to it allowed its identification as 

WTC 2,209: 82 through 85. 

• M-28: The stampings indicated that the center column line number was 345 and the panel 

was located in WTC 2. However, the markings of the floors spanned were partially obscured; 

9x – 1xx. By reviewing the building design drawings, the only panel that could fit spanned 

floors 98 through 101. 

• M-30: The stampings found were x33 94-97, where the “x” signifies missing information 

due to a weld bead running across this area. Thus, the building and exact center column line 

numbers were unknown. However, a derrick division number of <65> was visible on the 

interior spandrel. From this information, as well as the specified minimum yield strength 

(Table F–8) and column type (Table F–9), M-30 was determined to belong to WTC 1, with a 

center column line number of 133. 

• N-1: The stampings indicated that the columns spanned floors 82 through 85; however, no 

building information was observed, and a weld bead ran through the middle of the center 

column line number, yielding only 2x8. By reviewing the building plans, only column line 

218 spanned the floors specified, and the flange stampings (Tables F–8 and F–9) indicated 

that the piece belonged to WTC 1. 

• N-7: The stampings indicated that the center column line number was 127 and the panel 

spanned floors 97 through 100, however, no building information was observed. By 

reviewing the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

• N-12: The stampings found were x06 92-95 where the x signifies missing information due to 

a weld bead running across this area. Thus, the building and exact center column line 

numbers were unknown. However, a derrick division number of <69> was visible on the 
Interim Report on Inventory and Identification of Steels 

F–21

Table F–10. Information used to determine the identification of exterior panel M-2. 

1) 5 panels in WTC 1, floors 90-99 that meet the criterion of 3 columnsa with column type 122 and 55 ksi

COL 1 COL 2 COL 3 COL 1 COL 2 COL 3 COL 1 COL 2 COL 3

127 94 97 97 5110 1411 5210 96 5110 1411 5210 95 5110 1411 5210130 96 99 99 1411 5210 1411 98 1411 5210 1411 97 1411 5210 1411330 96 99 99 1411 5210 1411 98 1411 5210 1411 97 1411 5210 1411133 97 100 100 5210 1411 5110 99 5210 1411 5110 98 5210 1411 5110333 97 100 100 5210 1411 5110 99 5210 1411 5110 98 5210 1411 5110

2) Only two panels that meet the additional criterion of columns 1 and 3 having truss seat attachments and column 2 having gusset plate attachments 

 - Seat detail 5110 and 5120 are gusset plates for diagonal bracing straps - Seat detail 1411 are truss seat attachments

COL 1 COL 2 COL 3 COL 1 COL 2 COL 3 COL 1 COL 2 COL 3

130 96 99 99 1411 5210 1411 98 1411 5210 1411 97 1411 5210 1411330 96 99 99 1411 5210 1411 98 1411 5210 1411 97 1411 5210 1411

3) Derrick Division suggests that panel came from North face of WTC 1, i.e., panel in the 100-series

COL 1 COL 2 COL 3 COL 1 COL 2 COL 3 COL 1 COL 2 COL 3

130 96 99 99 1411 5210 1411 98 1411 5210 1411 97 1411 5210 1411

FLOOR

SEAT DETAIL

FLOOR

SEAT DETAIL

Center 

Col #

Story @ Splice

FLOOR

SEAT DETAIL

Lower Upper

PANEL NUMBER SPANDREL 1 SPANDREL 2 SPANDREL 3

FLOOR

SEAT DETAIL

FLOOR

SEAT DETAIL

Center 

Col #

Story @ Splice

FLOOR

SEAT DETAIL

Lower Upper

PANEL NUMBER SPANDREL 1 SPANDREL 2 SPANDREL 3

FLOOR

SEAT DETAIL

FLOOR

SEAT DETAIL

Center 

Col #

Story @ Splice

FLOOR

SEAT DETAIL

Lower Upper

PANEL NUMBER SPANDREL 1 SPANDREL 2 SPANDREL 3

a. Columns 1, 2, and 3 are left to right viewed from inside the building.
Appendix F 

F–22

Figure F–8. Schematic showing the sample M-10 as two separate exterior column 

panels, M-10a and M-10b. 

interior spandrel. From this information, as well as the specified minimum yield strength 

(Table F–8) and column type (Table F–9), it was determined that N-12 belonged to WTC 1, 

with a center column line number of 206. 

• S-10 or C-17: The stampings indicated that the center column line number was 224 and the 

panel spanned floors 92 through 95, however, no building information was observed. By 

reviewing the flange stampings (Table F–8), the piece was determined to belong to WTC 1. 

In addition to the overall images taken for record-keeping purposes, the exterior column panels were 

mapped to indicate how much of the panel was recovered after the collapse. Figure F–9 displays 

schematics of typical exterior panels recovered, and Figs. F–10 and F–11 show these maps, with the 

recovered portion indicated, for the identified samples from WTC 1 and WTC 2, respectively. Special 

note should be given to the fact that these diagrams are drawn as if viewed from the outside of the 

building. B-1043, B-1044, and C-24 were samples located at the mechanical floors of the building. C-13 

and C-13a (pieces of the same exterior panel) and C-14 were exterior wall panels located at the corner of 

the building. 

For the 12 samples identified as core column material (Table F–3), all but 2 were clearly marked. Sample 

C-30 had markings that clearly indicated the building and column; however, the floors were partially 

M-10a M-10b 

Bottom of columns 
Interim Report on Inventory and Identification of Steels 

F–23

Figure F–9. Schematics displaying the various types of exterior column panels. 

Outside of building looking in at panel Outside of building looking in at panel

Typical Mechanical Floor Panel 

Outside of building looking in at panel

Typical Corner Panel 

Typical Exterior Panel 

Column

Spandrel

Column 

Spandrel 

Column

Spandrel
Appendix F 

F–24

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1. 

Outside of building looking in at panel

C-10

A451: 85-88 

Un-recovered 

portion 

Outside of building looking in at panel

C-22

A157: 93-96 

Outside of building looking in at panel

C-25 

A206: 89-92 

Outside of building looking in at panel

C-40 

A136: 98-101 
Interim Report on Inventory and Identification of Steels 

F–25

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

CC 

A124: 70-73 

Outside of building looking in at panel

K-1 

A209: 97-100 

Also labeled as 

K-13

Outside of building looking in at panel

C-93 

A339: 99 - 102 

Outside of building looking in at panel

C-55

A209: 94-97 

Un-recovered 

portion 
Appendix F 

F–26

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

M-26 

A130: 90-93 

Outside of building looking in at panel

M-20

A121: 99-102 

Outside of building looking in at panel

M-2 

A130: 96-99 

Outside of building looking in at panel

K-2 

Also labeled as 

K-40

A206: 92-95 Un-recovered 

portion 
Interim Report on Inventory and Identification of Steels 

F–27

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

N-1 

A218: 82-85 

Outside of building looking in at panel

N-7 

 A127: 97-100 

Outside of building looking in at panel

M-30 

A133: 94-97 

Outside of building looking in at panel

M-27 

A130: 93-96 Un-recovered 

portion 
Appendix F 

F–28

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

N-12 

Also labeled 

as M-13 

A206: 92-95 

Outside of building looking in at panel

N-10 

A115: 89-92 

Outside of building looking in at panel

N-9 

Also labeled 

as M-8 

A154: 101-104 

Outside of building looking in at panel

N-8 

A142: 97-100 

Also 

labeled as 

M-7 

Un-recovered 

portion 
Interim Report on Inventory and Identification of Steels 

F–29

Figure F–10. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

N-101 

A133: 100-103 

Also labeled 

as M-21 

Outside of building looking in at panel

S-1 

A433: 79-82 

Also labeled 

as EE 

Outside of building looking in at panel

N-99 

A148: 99-102 

Outside of building looking in at panel

N-13 

A130: 99-102 Un-recovered 

portion 
Appendix F 

F–30

Figure F–10. Exterior column panel maps indicating the portion of the specific 

exterior column panel section recovered from WTC 1 (continued). 

Outside of building looking in at panel

S-10

A224: 92-95 

Outside of building looking in at panel

S-9 

A133: 97-100 Un-recovered 

portion 
Interim Report on Inventory and Identification of Steels 

F–31

Figure F–11. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 2. 

Outside of building looking in at panel

B-1024 

B154: 21-24 

Un-recovered 

portion 

Outside of building looking in at panel

B-1043 

B406: 40-43

Mechanical Floor 

Broken columns from 

upper panel 

3 bolts and 

welds intact

Outside of building looking in at panel

B-1044 

B409: 40-43 

Mechanical Floor 

Broken columns from 

upper panel 

3 bolts and 

welds intact 

Outside of building looking in at panel

C-13 and C-13a

(separated) 

Also labeled as 

S-19

B200: 90-92

Corner Panel 

C-13

C-13a

Also labeled as 

S-11
Appendix F 

F–32

Figure F–11. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 2 (continued). 

Outside of building looking in at panel

C-46 

B157: 68-71 

Outside of building looking in at panel

C-14 

Also labeled as 

S-18

B300: 85-87 

Corner Panel 

Outside of building looking in at panel

C-24 

B203: 74-77 

Mechanical Floor 

Outside of building looking in at panel

C-18 

B230: 93-96 

Un-recovered 

portion 
Interim Report on Inventory and Identification of Steels 

F–33

Figure F–11. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 2 (continued). 

Outside of building looking in at panel

C-89 

B215: 12-15 

Outside of building looking in at panel

C-92 

B130: 93-96 

Outside of building looking in at panel

C-48

B442: 91-94 

Outside of building looking in at panel

M-10a 

B209: 82-85 

Un-recovered 

portion 
Appendix F 

F–34

Figure F–11. Exterior column panel maps indicating the portion of the specific exterior 

column panel section recovered from WTC 2 (continued). 

Outside of building looking in at panel

S-14 

B218: 91-94 

Also labeled as 

C-20

Outside of building looking in at panel

M-28 

B345: 98-101 

Outside of building looking in at panel

M-10b 

B206: 83-86 Un-recovered 

portion 
Interim Report on Inventory and Identification of Steels 

F–35

obscured: “x04 – 10x”. As the 24 ft section has both connector ends, it spanned only two floors and fit 

with the floor levels of 104–106. The second sample was C-88b, which did not have any stampings or 

markings, but was welded to C-88a (identified by stampings). A final sample, C-83, was also found 

among this group. While no markings were found on the sample, it was recorded as a core column due to 

its shape, which was very similar to C-90. 

There were 13 other wide flange sections that had stampings and/or markings that did not correspond to 

the code as discussed above (Table F–4). Instead, there were typically three distinct grouping of numbers 

and/or letters. Two examples are: 

 Sample C-44: “59 S 563” 

 Sample M-17: “163 9 62” 

Given the position of the last grouping and the numbers typically found there, this is probably the derrick 

division. The first two most likely indicate the as-built locations of the pieces within the building. NIST 

is still investigating the identification of these samples. 

Floor trusses were also recovered; however, attempts to identify their specific as-built locations within the 

buildings were not successful. No stampings were found. Of the 23 pieces held by NIST, 8 are of 

significant size but are badly tangled and twisted as a result of the collapse and subsequent handling of the 

material. The remaining pieces consist of shorter sections of chord and rod material in addition to welded 

sections that connected the trusses to the floor seats. 

At present, there are seven samples from WTC 5, all in the GZ-series (see Attachment 1.2.9). These are 

coupons that were removed at the WTC site and held by GMS, LLP. They were subsequently sent to 

NIST once the Investigation officially began. 

No structural elements have been positively identified from WTC 7. However, the columns were 

fabricated from conventional 36 ksi, 42 ksi, and 50 ksi steel that complied with ASTM specifications. 

F.5 STRUCTURAL STEEL ELEMENTS OF SPECIAL IMPORTANCE 

Of the 41 exterior column panels and 12 core columns positively identified, many were considered 

especially important to this Investigation. Two major categories of steel are considered to be of special 

value: 

• Samples located in or around the floors impacted by the airplane 

• Samples that can represent 1 of 14 grades of steel specified for the exterior columns, 1 of 4 

grades of steel specified for the core columns, and 1 of the 2 grades of steel for the floor 

trusses 

F.5.1 Samples Located in or Around the Floors Impacted by the Airplane 

Interpretation of the photographic evidence revealed that damage to WTC 1 due to aircraft impact 

occurred from floor 94 to floor 99 and was bounded by columns 111 through 152. For WTC 2, the 

impact area was lower with damage found from floor 77 to floor 85. While the damage appears to be 

bordered by column lines 411 and 440, columns closer to the southeast corner of the building may also 

have been affected. However, few images were obtained where smoke is not obscuring this portion of the 
Appendix F 

F–36

south face of WTC 2 to complete the analysis. From this information, NIST was able to determine which 

perimeter panels and core columns could be used to comment on damage and possible failure 

mechanisms in this area. Figure F–12 shows the sample overlay of the exterior panels in NIST’s 

possession in and around the impact zone of WTC 1. Sample C-80, a core column, was also identified as 

residing near the impact zone. The recovered portion of each column is approximately represented in this 

image. Unfortunately, there were no similar corresponding exterior panels for WTC 2, but two core 

columns were recovered, (Fig. F–13). Later reports will describe the type of damage and failure 

mechanisms associated with each sample. 

Figure F–12. Interpreted column damage, from photographic evidence, to WTC 1, 

 with overlay of samples in NIST’s possession. 
Interim Report on Inventory and Identification of Steels 

F–37

Figure F–13. Interpreted column damage, from photographic evidence, to WTC 2, 

with overlay of samples in NIST’s possession. 

F.5.2 Samples Representing the Various Types of Steel Specified in the Design 

Drawings 

The other grouping of samples that was deemed important was that which belonged to one of the different 

grades of steel specified in the buildings’ construction. The following minimum yield strengths, in ksi 

(1 ksi equals 1,000 pounds per square inch), were specified for each structural element: 

• Columns of the exterior panels: 36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100 

• Core columns: 36, 42, 46, and 50 

• Floor truss material: 36 and 50 
Appendix F 

F–38

From the recovered steel, sufficient representative samples from each important class of steel groups are 

available for a full examination (i.e., chemical, metallurgical, and mechanical property analyses) to 

investigate why and how WTC 1 and WTC 2 collapsed following the initial impact of the aircraft. From 

Table F–11, it can be seen that 10 of the 14 types of steel specified for the columns are represented, and 

10 of the 12 grades of spandrel material have been identified. Additionally, sample ASCE-3 (as-built 

location in the building not identified) has a flange stamping of 45 for the minimum yield requirement, 

which would increase the total number of perimeter column material types to 11. One important note is 

that from the observed stampings of the recovered elements and other documents (see Appendix C), it 

appears that 100 ksi steel was substituted for the 85 ksi and 90 ksi grades in the construction of the 

exterior panels (Table F–6). Considering both column and spandrel material, samples of all grades 

specified for the perimeter panels are available. While only two of the four grades of steels were obtained 

(36 ksi and 42 ksi) for the core columns (Table F–3), 99 percent of the total number of core columns were 

fabricated from these two grades. For the floor truss material, the samples could not be identified as to 

their precise, as-built locations within the buildings. However, initial chemical and mechanical property 

analyses have shown that both minimum yield strength materials specified have been recovered. 

Characterization of these samples will be covered extensively in a later report. 

F.6 SUMMARY 

NIST has 236 samples from the WTC buildings, the majority belonging to WTC 1 and WTC 2. These 

samples represent roughly 0.25 percent to 0.5 percent of the 200,000 tons of structural steel used in the 

construction of the two towers. NIST believes the collection of steel from the WTC towers is sufficient 

for the Investigation. This assertion is drawn from the following two statements. First, recovery of 

material from locations in or near the impact and fire damaged regions of WTC 1 and WTC 2 was 

remarkably good, including four exterior panels directly hit by the airplane and three core columns 

located within these areas. Second, sufficient representative samples exist for all 14 grades of exterior 

panel material, 2 grades of the core column material (which represents 99 percent, by total number, of 

columns), and both grades for the floor truss material. 

This report identifies the structural steel elements recovered from the WTC towers. Later reports will 

determine the physical and mechanical properties of the steels and weld metal and the characteristics of 

the metal, weldments, and connections from WTC buildings. Additionally, a damage assessment/failures 

mode examination of the recovered structural steel elements will be performed. This information will be 

utilized in an effort to determine why and how WTC 1 and WTC 2 collapsed following the initial impact 

of the aircraft. 
Interim Report on Inventory and Identification of Steels 

F–39

 

Table F–11. Listing of recovered exterior column panels with specified minimum yield strengths and thicknesses for 

columnsa and spandrels. 
Appendix F 

F–40

F.7 REFERENCES 

F.7.1 References from Publicly Available Sources 

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, 

Preliminary Observations, and Recommendations. FEMA Report 403. Federal Emergency 

Management Agency. Washington, DC, May. 

F.7.2 References from Nonpublic Sources 

Faschan, W. 2002. Leslie E. Robertson Associates, New York, NY, personal communication to F. Gayle, 

Project Leader, World Trade Center Investigation, NIST, Gaithersburg, MD, May 21. 

PONYA (Port of New York Authority). 1967. to Steel Fabricators, Detailers, and Erectors on “Shop 

Drawing Procedures and Marking Systems,” May 1. 

Voorsanger and Associates Architects, PC. 2002. WTC Archives Interim Storage Facility, JFK 

International Airport: Preservation and Inventory Report, Draft 2. New York, NY, November. 
F–41

Attachment 1 

DATA ON RECOVERED WTC STEEL

1.1 DATABASE OF RECOVERED STEEL 

Table 1–1. List of all WTC steel elements recovered for NIST investigation. 
Appendix F 

F–42

Table 1–1. List of all WTC steel elements recovered for NIST investigation (continued). 
Interim Report on Inventory and Identification of Steels 

F–43

Table 1–1. List of all WTC steel elements recovered for NIST investigation (continued). 

Key: 202, Bldg 202, high bay; 205, Bldg 205, parking lot; PL, Bldg 202, parking lot; 236, Bldg 236, parking lot; B, 

bolt; BT, bowtie section of exterior wall; C, flat wall, exterior column panel section; CC, corner panel section of 

exterior wall; Ch, channel; Cn, coupon of exterior column; Cn5, coupon from WTC 5; H, hanger; J, floor truss; NSF, 

pieces contributed by A. Asteneh salvaged under NSF contract; O, other; RB, rectangular, built-up box column; W, 

wide flange section; Lab, Bldg 223, Rm B253; JFK, Hanger 17, JFK Airport; JFK/PL, Main piece at JFK, portion at 

NIST. 
Appendix F 

F–44

Table 1–2. List of identified exterior panel sections. 

Table 1–3. List of partially identified exterior panel sections. 
Interim Report on Inventory and Identification of Steels 

F–45

Table 1–4. List of unidentified exterior panel sections. 
Appendix F 

F–46

Table 1–5. List of identified core columns. 

Table 1–6. List of built-up box beams and wide flange sections with 

ambiguous stampings. 
Interim Report on Inventory and Identification of Steels 

F–47

Table 1–7. List of unidentified wide flange sections. 
Appendix F 

F–48

Table 1–8. List of recovered floor truss material. 

Table 1–9. List of recovered channel material. 
Interim Report on Inventory and Identification of Steels 

F–49

Table 1–10. List of material from WTC 5. 

Table 1–11. List of miscellaneous material. 
Appendix F 

F–50

Table 1–12. Strength/gage combination of columns recovered by NIST. 

45 1.75 1

50 0.5 2

50 0.5625 2

50 1.0625 2

50 1.8105 1

50 2.0625 1

50 2.125 1

50 2.25 1

50 2.5 1

50 2.625 1

55 0.25 12

55 0.3125 5

55 0.375 6

55 0.4375 3

55 0.5625 3

55 1.375 1

55 1.6875 1

60 0.25 5

60 0.3125 6

60 0.375 1

60 0.5 1

65 0.25 7

65 0.375 1

65 0.8125 1

70 0.25 7

70 0.4375 2

70 0.75 1

75 0.25 3

75 0.4375 2

80 0.25 3

80 0.625 1

80 1.1875 1

85 – 100 0.25 12

85 – 100 0.5625 3

85 – 100 1.125 2

85 – 100 1.1875 3

Flange FY (ksi)

Flange Gage 

(in.)

Number of Columns Recovered and 

Identified by NIST
Interim Report on Inventory and Identification of Steels 

F–51

Table 1–13. Strength/gage combinations of spandrels recovered by NIST. 

36 3/8 16

36 9/16 3

36 1 1/4 3

36 1 3/8 3

42 3/8 24

45 3/8 7

46 3/8 4

50 3/8 5

50 7/16 2

50 15/16 2

55 3/8 2

60 3/8 6

60 15/16 1

65 3/8 1

65 9/16 1

65 5/8 2

65 15/16 1

70 3/8 2

75 3/8 1

80 9/16 1

Spandrel FY 

(ksi) Spandrel Gage (in.)

Number of Spandrels Recovered by 

NIST
Appendix F 

F–52

1.2 REPRESENTATIVE PICTURES OF RECOVERED WTC STEEL 
Interim Report on Inventory and Identification of Steels 

F–53

Figure 1–1. Exterior column panel, sample C-46 shown. 
Appendix F 

F–54

Figure 1–2. Floor truss seats shown from sample N-8. 

Seat with 2 intact bolt holes for floor truss 

attachment. Intact bolt remains in far hole. 

Welded gusset plate 

Seat 

Stand off plates used to attach 

seat to column interior 
Interim Report on Inventory and Identification of Steels 

F–55

Figure 1–3. Damping Unit shown from sample N-8. 

Figure 1–4. Gusset plate shown from sample N-8. 

Welded gusset plate used in place of seat on 

alternate column/spandrel intersections. One 

method used to attach diagonal bracing strap to 

exterior wall 

Damping unit with missing viscoelastic 

material. Damping unit has been bent into an 

upright position. 

Gusset plate for viscoelastic damping unit 

attachment to column interior. 

Seat 
Appendix F 

F–56

Figure 1–5. (left) Diagonal bracing strap shown on sample C-25, (top), and 

single strap labeled M-29 (bottom). 

On Sample C-25 

Diagonal bracing strap attached 

directly to exterior column 

Sample M-29 
Interim Report on Inventory and Identification of Steels 

F–57

B-5004 at JFK 

B-5004 portion cut and moved to NIST campus 

Figure 1–6. Bowtie section of exterior wall. 
Appendix F 

F–58

Figure 1–7. Recovered rectangular built up box sections used as core columns. 

Samples C-83 and C-90 
Interim Report on Inventory and Identification of Steels 

F–59

Figure 1–8. Recovered wide flange sections used as core columns. 

Sample C-65 

Sample C-80 
Appendix F 

F–60

Figure 1–9. Other recovered wide flange sections, shown is sample C-42. 
Interim Report on Inventory and Identification of Steels 

F–61

Figure 1–10. Recovered floor truss material, shown are portions of sample C-53. 
Appendix F 

F–62

Figure 1–11. Recovered inner channel material used to connect floor trusses to core 

columns; shown is sample C-129. 

Figure 1–12. Coupons removed in the field from WTC 5; shown is sample GZ-1. 

Seat 
Interim Report on Inventory and Identification of Steels 

F–63

Figure 1–13. Examples of recovered bolts from various samples. 
Appendix F 

F–64

Figure 1–14. Examples of miscellaneous materials recovered. 

Square tubular piece 

Sample C-103 

Rectangular tubular piece 

Sample C-135 

Assorted pieces from within column 

Sample M-30 Associated 
G–i

TABLE OF CONTENTS - Appendix G

List of Tables ........................................................................................................................................... G–ii

Appendix G 

Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to September 11, 2001...... G–1

G.1 Background................................................................................................................................. G–1

G.2 FDNY Fire Reports and Fire Investigation Reports ................................................................... G–1

G.2.1 Fire Record Forms........................................................................................................... G–2

G.2.2 Overview of Fire Incidents 1970–2001 from FDNY Records ........................................ G–3

G.2.3 Fire Incidents Occurring in WTC 1................................................................................. G–5

G.2.4 Fire Incidents Occurring in WTC 2................................................................................. G–8

G.2.5 Additional Fires Involving the Deployment of Standpipe Lines in WTC 1 and 2 .......... G–9

G.2.6 Fire Incidents Occurring in WTC 7............................................................................... G–10

G.3 Summary................................................................................................................................... G–10

G.4 Attachments to This Fire History.............................................................................................. G–11

G.5 Conclusions............................................................................................................................... G–11

G.6 References................................................................................................................................. G–12

Attachments G–A.1 through G–A.3

Numeric Codes Used in the Fire Reports .......................................................................... G–13 

Attachments G–A.4 through G–A.7

Fire Reports Produced by the FDNY.................................................................................. G–29 
G–ii

LIST OF TABLES

Table G–1. Categorization of WTC 1, 2, and 7 fires from FDNY records............................................. G–3

Table G–2. Significant fires in WTC 1 extinguished by sprinklers and/or multiple standpipe lines...... G–5

Table G–3. Significant fires in WTC 2 extinguished by sprinklers and/or multiple standpipe lines...... G–8

Table G–4. Significant fires in WTC 7 extinguished by sprinklers and/or multiple standpipe lines.... G–10
G–1

Appendix G

INTERIM REPORT ON SIGNIFICANT FIRES IN WTC 1, 2, AND 7 PRIOR 

TO SEPTEMBER 11, 2001 

Fires occurred in World Trade Center (WTC) 1, 2, and 7 prior to September 11, 2001. This appendix 

documents the facts of significant fires in the building after first occupancy as they relate to the 

performance of the automatic sprinkler, manual suppression, fire detection, and smoke purge systems. 

The ultimate goal of this review was to identify from New York City Fire Department (FDNY) records 

significant but not well known fires for further study. 

G.1 BACKGROUND 

The fire protection engineering department of the Port Authority of New York and New Jersey 

(PANYNJ) maintained records of all significant fire events in the WTC buildings. These records were 

lost in the collapse of the towers. 

Two significant fire events involving WTC 1 are well known. On February 14, 1975, a fire started on the 

11th floor of WTC 1. Workers reported the fire to WTC police headquarters. When police reached the 

fire floor, they reported a serious fire and ordered the heating, ventilating, and air conditioning (HVAC) 

system be placed into the smoke purge mode. Fire spread through unprotected floor openings in utility 

closets. Fire damage occurred on floor 10 through floor 19. Approximately 800 m2

 (9,000 ft2) of the 

floor 11 contents were destroyed or damaged. At that time, sprinklers had not been installed in the office 

spaces. However, fire barriers divided the floor into quadrants. The fire on floor 11 was confined to the 

southeast quadrant. Fire damage on other floors was confined to the utility closets. The fire was 

extinguished by FDNY. More details about this fire incident can be found in Powers (1975), Lathrop 

(1975), and a report that is being prepared for the National Institute of Standards and Technology (NIST) 

by Hughes Associates. 

At 12:18 PM on February 26, 1993, a bomb exploded in an underground parking garage of the 

WTC complex. The explosion occurred on the B2 level in the area of the garage under WTC 3 and 

adjacent to WTC 1. The explosion resulted in a loss of normal electric power in WTC 1 and WTC 2. 

HVAC systems shut down. Smoke spread throughout WTC 1 and to a lesser extent in WTC 2. More 

details about this fire can be found in Isner and Klein (1993a, 1993b). The only historic record of smaller 

fire incidents in WTC 1, 2, and 7 known to this investigation are the fire reports and fire investigation 

reports prepared by the FDNY. These reports were provided to NIST by FDNY for use in this 

investigation. 

G.2 FDNY FIRE REPORTS AND FIRE INVESTIGATION REPORTS 

The FDNY released 397 Bureau of Operations Fire Reports and 112 Bureau of Fire Investigation Records 

(Fire Marshals’ Reports) which served as the basis for a summary of the fire history in the WTC 1, 2, and 

7. NIST obtained reports of fires for the period of 1970–2001 and fire investigation records between 

1977 and 2001 for WTC 1, 2, and 7, which in total, consisted of over 500 documents on which to report. 
Appendix G 

G–2

These records included all responses to fires in buildings 1, 2, and 7 by the FDNY. All of these records 

consist of standardized forms that may be supplemented with other materials. Many were for minor fire 

events, such as fires that were extinguished by occupants before FDNY arrival. These were not of 

interest for this investigation. The records of significant fires were identified. 

Significant fire incidents were those that exercised the fire suppression systems, specifically multiple 

sprinklers or multiple standpipes (with or without the activation of at least one sprinkler). These fires will 

be discussed individually, organized by the building in which they occurred. In addition to these fires, 

generalized facts relating to those fires involving the use of one standpipe line and one sprinkler and the 

use of one standpipe line will be provided throughout this report. As an aside, the majority of fire records 

for significant fires documented the performance of the detectors and sprinkler systems, but almost all 

reports lacked information about the performance of the smoke purge system. 

G.2.1 Fire Record Forms 

Depending upon the type and date of the incident, a specific fire report form was used by the FDNY to 

document the incident. For each type of emergency responded to by the FDNY, responders either 

completed a form that would describe a structural fire (BF–24) or a form that would describe any other 

type of emergency (BF–25), such as a nonstructural fire, transportation fire, and/or any other non-fire 

emergency. For this historical summary, only those events logged and organized under the structural fire 

form, 345 documents total, were of interest and used. A structural fire form is a one-page document 

(unless additional information is recorded on separate sheets) that gives valuable information about the 

fire event on various subjects, including: 

• Alarm–the date and time of the received alarm 

• Injuries and casualties–the numbers of each for the incident 

• Extinguishment–details of the sprinkler and standpipe performance 

• Ignition–information on the equipment involved in ignition, the form of the ignition source, 

the material type and form that was ignited, and the ignition factor (cause) 

• Structure–information on the class of construction, the use of the building, and its status 

(vacant, occupied, under construction, etc.) 

• Fire origin–the fire location and classification 

• Fire extension–the means of fire extension and number of buildings/vehicles involved 

• Damage–information on the damage done by flame, smoke, and water 

• Detectors–the type, power source and performance of the detectors in the fire area 

Each subject of the incident is given a set of codes or numbers that correspond to any incident, and in 

order to read the fire records successfully, an understanding of the codes is necessary (see 
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–3

Attachments G-A.1, G–A.2 and G–A.3). For the nonstructural B-25 record forms, the only fire related 

subjects included are the injury and casualty numbers, ignition, and structure information. 

Depending upon the date of the fire incident, certain information is lacking from the structural fire form. 

Before 1980, a different record form for structural fire incidents was used which left out the following 

subjects: fire extension, damage, detectors, and portions of the ignition data. Because of this, detection 

data are not available for the majority of the fires occurring before 1980. 

G.2.2 Overview of Fire Incidents 1970–2001 from FDNY Records 

Table G–1 contains the categorization of all structural fire incidents contained in the FDNY records for 

WTC buildings 1, 2, and 7 available to this investigation. The table contains information on the category 

of fire incident (whether or not the detection and/or sprinkler systems activated), the time period over 

which the fires occurred, the numbers of records in that category, and a descriptive statement about the 

category. 

 Table G–1. Categorization of WTC 1, 2, and 7 fires from FDNY records. 

WTC 1 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

1980–2001 66 Unattended food/appliances, overheated elevator 

equipment, discarded material, welding 

operations, electrical failure and suspicious fires 

No detection 

information and no 

sprinklers 

1970–1979 79 Trash can fires, discarded material, food on stove, 

electrical failure, overheated equipment 

Detection, no sprinklers 1980–2000 57 Unattended food/appliances, overheated elevator 

equipment, discarded material, welding 

operations, electrical failure 

[Detection] and 

sprinklers 

1977–1999 18 

Suspicious, electrical failure, discarded material 

WTC 2 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

1980–1999 37 Discarded material, welding too close, overheated 

equipment, suspicious, elevator motor 

 No detection 

information and no 

sprinklers 

1975–1979 40 Discarded material, fire in office furniture, trash 

can fires 

Detection, no sprinklers 1981–1999 40 Food on stove, small elevator fire, electrical 

failure, suspicious, overheated equipment 

[Detection] and 

sprinklers 

1977–2000 5 

Mechanical failure, suspicious 

WTC 7 

Category Dates Number Generalization of Incidents 

No detection, no 

sprinkler 

2000 1 Trash can fire/discarded material 

Detection, no sprinklers 1990 1 Electrical switch on floor – explosion 

[Detection] and 

sprinklers 

1988 1 Suspicious 
Appendix G 

G–4

All FDNY records provided to NIST, unless the records were not readable, contained relevant 

information about the type and performance of the suppression system. Because of this, reports of 

incidents in which the sprinkler system activated can range from 1970 to 2001. When the table lists 

“[detection]” in brackets, this is meant to symbolize that either detection was present or no information on 

detector performance was included on the form (as is the case with the older records). An attempt was 

made to compare all investigation records with the fire reports, especially those which activated the 

suppression system. Looking at the records in Table G–1, it is clear that only 24 fires activated the 

sprinkler system from 1970–2001 from all three buildings. Many of the other structural fires without 

sprinkler activation were labeled as suspicious, trash can fires, electrical failures, unattended 

food/appliances, or overheated equipment. 

In order to report on significant structural fires occurring in WTC 1, 2, and 7, the FDNY records had to be 

reviewed for those incidents that activated sprinklers, detectors, or were extinguished by hose line and 

those smaller fires that self-extinguished or could be extinguished using a fire extinguisher. The 

structural fire incidents without detection information (before 1980), had to be reviewed to locate any 

fires that activated the sprinkler system. 

The retrofit installation of sprinklers into WTC buildings 1 and 2 was accomplished in two phases. 

During the first phase in 1976, sprinkler risers/mains were installed throughout WTC 1 and WTC 2. 

Sprinklers were installed to protect corridors, storage rooms, lobbies, and certain tenant/PANYNJ spaces. 

In the second phase of the retrofit from 1983 to 2001, sprinklers were installed in all remaining places in 

the complex (PACO 2002; shown in Attachment G–B). Prior to the retrofit only the sub-grade areas and 

selected hazard areas were protected by automatic sprinklers. This retrofit proceeded throughout the 

buildings as much as practical when other renovations of the office spaces were underway, such as when 

change of tenants occurred. 

After the installation of the sprinkler risers in 1976, tenants had the option of providing sprinklers or 

compartmentation for fire protection in compliance with Local Law 5. It was therefore possible that 

during the period of time when retrofit installation of sprinklers was under way, a fire that occurred may 

or may not have been in an area protected by automatic sprinklers. 

The forms used by the FDNY after 1987 give a detailed description of the event and whether or not a 

system was present at the time of the fire; however, a fire recorded before 1987 will give data only on the 

number of sprinklers opened. Because of this, an effort was made to look through all reports, especially 

those that mentioned detection performance, in order to identify fires involving the use of standpipe lines 

by the FDNY as an alternate indication of a significant fire. 

The next section of the report will highlight significant fires occurring in WTC 1, 2, and 7. The 

significant fires will be described individually by WTC building, and organized by the date on which they 

occurred in the building. In addition to these significant fires, (1) the fires that activated one sprinkler 

head and involved the use of one standpipe and (2) the fires that involved the use of only one standpipe, 

due to the number of incidents, will be generalized as to the nature of the incidents and the procedures 

followed by the FDNY. 
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–5

G.2.3 Fire Incidents Occurring in WTC 1 

After reviewing all the FDNY records of fire incidents in WTC building 1 since 1970, the significant fires 

were selected. There were 12 significant fires found for WTC 1, and the fire reports are included in 

Attachment G–A.4. Table G–2 provides a summary of the fire incident information from FDNY records, 

which is followed by individual paragraphs about each incident. 

Table G–2. Significant fires in WTC 1 extinguished by sprinklers and/or multiple 

standpipe lines. 

Significant 

Fire 

Incident 

Date Fire Location 

# Sprinklers 

Activated 

# Standpipes 

Used Cause of Fire 

Material 

Ignited 

1 9/9/77 B-6 level storage 

room 

2 0 None listed Not listed 

2 9/23/77 Dumpster on B-4 

level 

2 0 Not 

classified 

Trash/waste 

3 10/16/81 19th floor office area - 2 Discarded 

material 

Furniture 

4 12/23/83 2 dumpsters on B-4 

level 

2 1 Suspicious Trash/waste 

5 1/27/85 Office space on 

mezzanine level 

(Floor 2) 

2 1 Incendiary Trash/waste 

6 9/10/85 Garbage dumpster in 

service elevator 

lobby on floor 43 

2 1 Suspicious Trash/waste 

7 11/1/85 Storage closet on B-4 

level 

3 1 Suspicious Supplies/ 

stock 

8 6/7/86 Dumpster fire on 

floor 106, compactor 

room on floor 107 

2 1 None listed Trash/waste 

9 9/30/91 Office on B-4 level ≥1 2 Discarded 

material 

Trash/waste 

10 11/19/91 Electrical closet on 

floor 93 

0 2 Short circuit Electrical 

wire or cable 

insulation 

11 7/23/92 Level B-5 at the 

power distribution 

panel 

0 2 Electrical 

failure 

Electrical 

wire or cable 

insulation 

12 11/10/99 Computer room on 

floor 104 

3 ≥1 None listed Plastics, 

electronic 

equip 

Key: ≥ symbol denotes that at least one of the units of the suppression system was used (and not specifically identified by the 

fire report); - indicates that the report acknowledges 0 sprinklers open; however, due to the date of the fire, the space may not 

have had a sprinkler system installed. 
Appendix G 

G–6

Significant Fire #1

On September 9, 1977, at 11:04 p.m., the FDNY received an alarm for a fire in the B-6 level storage room 

at the address of WTC 1. The fire activated two sprinklers, and was noted to be extinguished before the 

FDNY’s arrival. 

Significant Fire #2

Another fire occurred on September 23, 1977, at 11:48 p.m., in a dumpster on the B-4 level of WTC 1. 

This fire also activated two sprinklers, and the FDNY noted that the fire had been extinguished prior to 

their arrival. 

In both cases, no injuries or casualties resulted from these fires, and the damage was confined to the area 

of origin. 

Significant Fire #3

Six years later, on October 16, 1981, at 7:12 p.m., a fire occurred on floor 19 of WTC 1. The FDNY 

noted that they used two standpipe lines to extinguish the fire and that one person was evacuated from the 

scene. Again, the fire report notes that no sprinklers opened, but does not note whether or not sprinklers 

were present at the time of the fire. Given the date of the incident, sprinklers are not expected to be 

located on floor 19. The fire was caused by discarded material and involved furniture in an office area of 

the floor. 

Significant Fire #4

Six years later on December 23, 1983, at 2:50 a.m., the FDNY responded to an alarm of fire and heavy 

smoke conditions on the B-4 level of WTC 1. The FDNY found two dumpsters fully involved in separate 

locations on the same floor and noted that the two activated sprinklers extinguished a major portion of the 

fire. The FDNY extinguished the rest of the flames by stretching hose from the standpipe system. Again, 

no injuries or casualties resulted from this fire. The cause noted on the report was suspicious and the 

damage was confined to the origin of the fire. 

Significant Fire #5

On January 27, 1985, at 8:53 p.m., the FDNY was called for a fire located in an unoccupied office on the 

mezzanine level of WTC 1. Two sprinklers contained the incendiary (involving arson) fire consuming 

trash paper/waste. When the FDNY arrived, they extinguished the remaining fire with one standpipe line. 

Building and content damage was confined to less than 15 percent of the space. Also, no injuries or 

casualties were reported. 

Significant Fire #6

Eight months later on September 10, 1985, at 4:05 p.m., the Port Authority Police informed the FDNY on 

arrival of a sprinkler flow and smoke condition on floor 43. A medium smoke condition was report by 

the FDNY on floor 43, where a fire was extinguished by two sprinklers. The fire report notes the use of 

one standpipe line; however, this was used during the overhaul process. This fire originated suspiciously 
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–7

in a garbage dumpster in a service elevator lobby. There was no building or content damage as well as no 

injuries or casualties reported. 

Significant Fire #7

On November 1, 1985, at 4:05 a.m., the FDNY was called for another suspicious fire producing heavy 

smoke on the B-4 level under WTC 1 and WTC 2. This fire occurred in a storage closet of the men’s 

bathroom, and the FDNY noted that three sprinklers activated to keep the fire under control until their 

arrival. Upon arrival, the FDNY extinguished the remaining fire in the closet area with one standpipe 

line. Again, the damage was noted to be confined to the area of origin. 

Significant Fire #8

Less than a year later, on June 7, 1986, at 9:49 a.m., the FDNY received an alarm for a heavy smoke 

condition on floor 110. For this call, fires were burning in two separate places; a garbage dumpster on 

floor 106 and the compactor room on floor 107. Sprinklers were noted in operation in both locations and 

seemed to control the fires, until the FDNY could complete extinguishment with one standpipe line on 

floor 106. There was no report of injuries or casualties for the previous two fires. 

Significant Fire #9

An additional fire occurred in WTC 1 where multiple standpipe lines were used along with the activation 

of the sprinkler system. This fire occurred on September 30, 1991, at 6:32 p.m., in an office on the B4 

level. The fire report noted that the sprinkler system operated; however, there is no mention of how many 

sprinklers or even their activation in the Operations/Comments section of the report. Two 1 3/4 in. or 

larger hose lines were used by the FDNY to extinguish this fire. The cause of the fire was abandoned 

material (cigarette) igniting boxes/carton material in an office. The fire damage was confined to the area 

of origin and smoke damage was confined to the floor. There was one uniformed officer injured and no 

civilian injuries or casualties. 

Significant Fire #10

A fire occurred on November 19, 1991, at 6:27 pm., and two 2 1/2 in. standpipe hose lines were used by 

the FDNY. The FDNY responded to WTC 1 for this fire due to a report of fire and smoke condition in 

electrical closets on possibly four floors (floors 93–96) and an alarm transmitted from floors 93–98. 

According to the fire report, the sprinklers were in service, but did not operate for this fire. The noted 

cause of this fire was a short circuit and the material that was ignited was electrical wire or cable 

insulation. The fire and smoke damage was confined to its area of origin (electrical closet). Two 

occupants were removed from stalled elevators during this incident, and occupants were evacuated from 

the scene, although an exact number is not given. Also, two occupants were injured and required first aid. 

Significant Fire #11

The FDNY responded to WTC 1 on July 23, 1992, at 10:02 p.m., due to a transformer fire on the 5th sub 

basement level. Firefighters found a fire situation in a large power distribution panel, where a firefighter 

was knocked unconscious by a shock blast from the panel. Similar to the fire in November of 1991, two 

2-1/2 in. standpipe hose lines were used by the FDNY on this fire. The cause of the fire was an electrical 
Appendix G 

G–8

failure and the material ignited was electrical wire or cable insulation. No appreciable damage is noted. 

As mentioned earlier, one firefighter was injured as well as three civilians. 

Significant Fire #12

The final fire associated with WTC 1 was one that occurred on November 10, 1999, at 11:01 p.m., in a 

computer room on floor 104. The FDNY noted that the fire was “knocked down” by three sprinklers 

when they arrived and they completed extinguishment with a line extended from the standpipe. The 

flame damage was confined to the area of origin and computer equipment was involved in fueling the fire. 

There was one injury and no casualties reported in the FDNY record for this fire. 

Table G–2 presents the 12 significant fires in WTC 1. Five of the 12 fires occurred on the basement 

levels and two occurred on the upper levels (above floor 100). The causes of these significant fires 

include suspicious, discarded materials, and electrical failures. 

G.2.4 Fire Incidents Occurring in WTC 2

Table G–3 presents the significant fire occurring in WTC 2. There were three significant fires found for 

WTC 2, and the fire reports are included in Attachment G–A.5. Table G–3 provides a summary of the 

fire incident information from FDNY records, which is followed by individual paragraphs about each 

incident. 

Table G–3. Significant fires in WTC 2 extinguished by sprinklers and/or multiple 

standpipe lines. 

Significant 

Fire 

Incident 

Date Fire Location 

# Sprinklers 

Activated 

# Standpipes 

Used 

Cause of 

Fire 

Material 

Ignited 

1 5/19/75 Floor 32 - 3 Incendiary Trash/waste 

2 4/12/77 Duct work over 

grill in 

restaurant on 

floor 107 

2 0 None listed Duct work 

3 3/22/93 Fan motor room 

on floor 108 

2 0 Mechanical 

failure 

Not 

classified 

- Indicates that the report acknowledges 0 sprinklers open, however due the date of the fire, the space may not have had a 

sprinkler system installed. 

Significant Fire #1

A fire occurred on May 19, 1975, at 9:38 p.m., on floor 32 of WTC 2. The FDNY noted that they used 

three standpipe lines to extinguish the fire and that the Port Authority reported occupants trapped on 

floors 31 and 32. The fire report notes that no sprinklers opened, but does not note whether or not 

sprinklers were present at the time of the fire. Given the date of the incident, sprinklers are not expected 

to be located on floors 31 and 32. The fire was labeled as incendiary and involved trash/waste. The 

FDNY stated that the fire involved the core area of the floor and was confined to that area. Over 20 

people (civilians and uniformed personnel) were injured by this incident. 
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–9

Significant Fire #2

On April 4, 1977, at 1:15 p.m., the FDNY was called to WTC 2 for a fire in the duct work over the grills 

in a restaurant on floor 107. The FDNY record on this fire noted that the fire was extinguished prior to its 

arrival. The damage was confined to the area of origin, and the fire caused no injuries or casualties. 

Significant Fire #3

The second fire occurred on March 22, 1993, at 8:39 a.m., and caused a smoke condition on floor 108. 

The fire activated two sprinklers due to an overheated bearing in a fan motor room on floor 108. The 

damage to the area did not exceed 15 percent of the space, and there were no injuries or casualties 

reported. 

Table G–3 presents the three significant fires in WTC 2. No fires were discovered in WTC 2 where 

multiple sprinklers or standpipes were used with another suppression system. Two of the three fires 

occurred on the upper levels (above floor 100) and the other occurred on floor 32. The causes of these 

significant fires included incendiary and mechanical failures. 

G.2.5 Additional Fires Involving the Deployment of Standpipe Lines in WTC 1 and 2 

The fires described in this section (31 in total) involve the use of one standpipe, with and without the 

activation of one sprinkler for WTC 1 and WTC 2. Four of the 31 reports describe fires that were 

extinguished with one sprinkler and one standpipe line (see Attachment G–A.6.1). Three of these fires 

were located in WTC 1 between the years of 1986–1991 and the other in WTC 2 in 1981. Two of these 

fires occurred in basement levels, one occurred on floor 106 of WTC 1, and the last on floor 5 in WTC 1. 

In some of the fire reports, the FDNY noted that the sprinkler controlled the fire, and the standpipe was 

used to actually extinguish the remaining fire. Half of the fires were labeled as incendiary/suspicious, one 

was an electrical failure, and the last was unknown. 

In addition, 27 of the 31 fire reports describe fires that were extinguished using one standpipe line (see 

Attachment G–A.6.2). Twenty of these fires occurred in WTC 1 and the other seven occurred in WTC 2. 

A majority of these fires (19) are labeled as incendiary/suspicious or unknown, while the other causes of 

the fires are attributed to short circuits, abandoned material/cigarette, welding close to combustibles, and 

a mechanical failure. The dates of occurrence for these fires range from 1973–1999, with a majority (23) 

occurring between the years of 1973–1985. These fire incidents did not result in any casualties, but five 

civilians and one uniformed officer were injured. 

Two of the 27 fires involved a 300 person (April 19, 1980) and a 1,500 person (April 17, 1981) 

evacuation. These will be described in further detail. On April 19, 1980, at 2:06 p.m., the FDNY 

received reports of an activated smoke detector in the return air duct on floor 106 of WTC 1. The FDNY 

also received reports of heavy smoke on floor 106, light smoke on floor 109, and heavy odor of smoke in 

stairways A and B. The report notes that while only one standpipe was used, approximately 300 people 

were evacuated from the Windows on the World restaurant on floor 107 via stairway C (which was clear 

of smoke). The fire cause was labeled as abandoned or discarded material and involved plastic material. 

This fire did not cause any injuries or casualties. 
Appendix G 

G–10

On April 17, 1981, at 9:18 a.m., the FDNY was informed of a fire on floor 7 and a smoke condition on 

floors 7 through 11 of WTC 1. The FDNY hooked up one standpipe and extinguished the fire located in 

an air conditioning unit in the “MER” room on floor 7. The cause of this fire was labeled as a mechanical 

failure. The fire report notes that the Port Authority personnel reported an evacuation of approximately 

1,500 people from floors 9 through 23. However, no injuries or casualties were reported from this fire. 

G.2.6 Fire Incidents Occurring in WTC 7.

Table G–4 presents the significant fire occurring in WTC 7. There was one significant fire found for 

WTC 7, and the fire report is included in attachment G–A.7. Table 4 provides a summary of the fire 

incident information from FDNY records, which is followed by an individual paragraph on the incident. 

Table G–4. Significant fires in WTC 7 extinguished by sprinklers and/or multiple 

standpipe lines. 

Significant 

Fire 

Incident 

Date Fire Location 

# Sprinklers 

Activated 

# Standpipes 

Used Cause of Fire 

Material 

Ignited 

1 5/20/88 Construction shanties 

on floor 3 

Multiple, # not 

listed 

1 Suspicious Shanties 

Significant Fire #1

In WTC 7, a fire occurred on May 20, 1988, at 12:38 a.m., in the construction shanties on floor 3. 

Although the fire report does not specifically note the number of sprinklers that activated, the operations 

notes state that Ladder Truck 10 found the sprinklers (noting more than one) in operation and shut them 

down. The FDNY had to complete the extinguishment by stretching a line from the standpipe to the fire 

source. This fire is noted by the report as being suspicious in nature and the flame damage was confined 

to the area of origin. 

It is possible that the fire incidents that were not specifically highlighted, especially those in the areas 

without sprinklers, involved other methods of extinguishment before FDNY arrival, such as a WTC 

houseline (pre-connected standpipe hose), hand extinguisher, or bucket of water, as noted on some of the 

FDNY reports. All other fires, the majority, included in other categories were either self-extinguished, 

extinguished prior to FDNY arrival (by staff, etc.), or a hand extinguisher was used by the FDNY. 

G.3 SUMMARY 

In summary, 16 significant fires occurred in WTC 1, 2, and 7, with 12 occurring in WTC 1, three in WTC 

2, and one in WTC 7. In addition to these, 31 fires occurred in WTC 1 and WTC 2, which involved the 

use of one standpipe (with or without the activation of one sprinkler). Of these additional 31 fires, 23 

occurred in WTC 1 and eight occurred in WTC 2. The following paragraphs will summarize findings 

from the 16 significant fires that occurred in all three buildings. 

After reviewing the 16 significant fires, trends developed relating to the time of day that the fires 

occurred. Overall, 12 of the 16 fires occurred between the hours of 6 p.m. and 4 a.m. The fires that 

occurred during office hours (between 7 a.m. and 6 p.m.) included a dumpster fire in the floor 43 elevator 

lobby (WTC 1), a dumpster fire on floor 106 (WTC 1), a kitchen fire on floor 107 (WTC 2), and a bearing 

overheating in the fan motor room on floor 108 (WTC 2). Almost all of the incendiary (arson) and 
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–11

suspicious fires (5 out of 6 fires) and unclassified or unlisted fires (4 out of 5 fires) occurred after business 

hours (before 7 a.m. and after 6 p.m.). 

In addition to the time of day of the fire, trends in the cause of the fire and the materials involved in the 

fire can be highlighted. Of the 16 fires and their causes, five were labeled as unlisted or unclassified, six 

as suspicious or incendiary, two as discarded material, and three as an electrical failure or mechanical 

failure. For the material involved in the fire, eight reports noted trash, waste, and supplies; two reported 

not listed or not classified; one reported furniture; three reported electrical equipment; one reported duct 

work; and one reported shanties were the material involved in the fire. 

Lastly, the location of the fires throughout the buildings was of interest. Of the 16 fires, four fires were 

concentrated above floor 100 or and six fires were located in the basement. The others (6 fires) were 

spread throughout the rest of the building. 

G.4 ATTACHMENTS TO THIS FIRE HISTORY 

Attachments G–A.1 through G–A.7 are included as a supplement to this report. The first three sections, 

G–A.1 through G–A.3 are explanations of the numeric codes used in the fire reports by the FDNY. 

Attachment G–A.1 is included to explain the codes for the fire reports produced prior to and including 

1980, Attachment G–A.2 is included to explain the fire reports produced from 1981 to May 31, 1987, and 

Attachment G–A.3 is included to explain the fire reports produced from June 1, 1987, to the present. The 

report code explanations are divided into the same sections as the fire report and give short descriptions 

for the numbers used in the fire report under each section. For example, if the ignition factor for a fire 

occurring in 1990 was given a number code of 54, the reader can find that the cause of the fire was a 

“short circuit, ground fault.” 

Attachments G–A.4 through G–A.7 include the actual fire reports produced by the FDNY on the 

significant fires highlighted in the sections above. The reader can use Attachments G–A.1 through G–A.3 

(depending upon the date of the fire) to read the fire reports in more detail than what is provided in this 

fire history report. 

G.5 CONCLUSIONS 

From the information contained in FDNY fire reports and fire investigation records provided to NIST, 47 

fires occurred in WTC building 1, 2, and 7 that were of sufficient size and duration to activate multiple 

sprinklers or were estimated by NIST to be capable of doing so, over the time period the buildings were 

occupied. This total does not include the major 1975 office fire in WTC 1 or the 1993 bombing. 

The records indicate that in areas protected by automatic sprinklers, no fire activated more than 3 

sprinklers. Three sprinklers would provide coverage for a floor area of approximately 63 m2

 (675 ft2). 

This area is much smaller than the 800 m2

 (9,000 ft2) damaged by the 1975 fire in an office space 

unprotected with automatic sprinklers. 

Many of the fires that occurred were recorded as suspicious or unknown in cause, occurred during offpeak work hours, and involved materials such as trash or paper-based supplies. In cases where sprinklers 
Appendix G 

G–12

were activated, the FDNY records indicated that the sprinklers either extinguished the fire completely or 

aided in controlling the spread. 

G.6 REFERENCES 

Isner, M. S., and T. J. Klem. 1993a. Explosion and fire disrupt World Trade Center. NFPA Journal. 

National Fire Protection Association. 91-104. 

Isner, M. S., and T. J. Klem. 1993b. World Trade Center Explosion and Fire, New York, New York, 

February 26, 1993. Fire Investigation Report. National Fire Protection Association. Quincy, MA. 

PACO Group. 2002. World Trade Center General Description of All Building Systems and the Capital 

Program. August. 

Powers, W. R. 1975. One World Trade Center Fire New York, N.Y., February 13, 1975. National Fire 

Protection Association. Boston, MA. 1-15. 
G–13

Attachment G-A.1 

Explanation of Numeric Codes Used on Fire and Emergency Reports - 

Prior to 1980 
Appendix G 

G–14
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–15
G–16

This page intentionally left blank 
G–17

Attachment G-A.2 

Explanation of Numeric Codes Used on Fire and Emergency Reports - 

From 1981 to May 31, 1987 
Appendix G 

G–18
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–19
Appendix G 

G–20
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–21
G–22

This page intentionally left blank
G–23

Attachment G-A.3 

Explanation of Numeric Codes Used on Fire and Emergency Reports - 

From June 1, 1987 to present 
Appendix G 

G–24
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–25
Appendix G 

G–26
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–27
Appendix G 

G–28
G–29

Attachment G-A.4 

Significant fire incidents occurring in WTC 1 (12) 

Significant 

Fire 

Incident 

Date Fire Location 

# Sprinklers 

Activated 

# Standpipes 

Activated Cause of Fire 

Material 

Ignited 

1 9/9/77 B-6 level storage 

room 

2 None listed Not listed 

2 9/23/77 Dumpster on B-4 

level 

2 Not 

classified 

Trash/waste 

3 10/16/81 19th floor office area - 2 Discarded 

material 

Furniture 

4 12/23/83 2 dumpsters on B-4 

level 

2 1 Suspicious Trash/waste 

5 1/27/85 Office space on 

mezzanine level 

(Floor 2) 

2 Incendiary Trash/waste 

6 9/10/85 Garbage dumpster in 

service elevator 

lobby on floor 43 

2 1 Suspicious Trash/waste 

7 11/1/85 Storage closet on B-4 

level 

3 1 Suspicious Supplies/stock 

8 6/7/86 Dumpster fire on 

floor 106, compactor 

room on floor 107 

2 1 None listed Trash/waste 

9 9/30/91 Office on B-4 level ≥1 2 Discarded 

material 

Trash/waste 

10 11/19/91 Electrical closet on 

floor 93 

0 2 Short circuit Electrical wire 

or cable 

insulation 

11 7/23/92 Level B-5 at the 

power distribution 

panel 

0 2 Electrical 

failure 

Electrical wire 

or cable 

insulation 

12 11/10/99 Computer room on 

floor 104 

3 ≥1 None listed Plastics, 

electronic 

equip 
Appendix G 

G–30
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–31
Appendix G 

G–32
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–33
Appendix G 

G–34
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–35
Appendix G 

G–36
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–37
Appendix G 

G–38
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–39
Appendix G 

G–40
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–41
Appendix G 

G–42
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–43
Appendix G 

G–44
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–45
Appendix G 

G–46
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–47
Appendix G 

G–48
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–49
Appendix G 

G–50
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–51
Appendix G 

G–52
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–53
Appendix G 

G–54
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–55
Appendix G 

G–56
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–57
Appendix G 

G–58
G–59

Attachment G-A.5 

Significant fire incidents occurring in WTC 2 (3) 

Significant 

Fire 

Incident 

Date Fire Location 

# Sprinklers 

Activated 

# Standpipes 

Activated 

Cause of 

Fire 

Material 

Ignited 

1 5/19/75 Floor 32 - 3 Incendiary Trash/waste 

2 4/12/77 Duct work over 

grill in restaurant 

on floor 107 

2 None 

listed 

Duct work 

3 3/22/93 Fan motor room 

on floor 108 

2 Mechanic

al failure 

Not 

classified 
Appendix G 

G–60
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–61
Appendix G 

G–62
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–63
Appendix G 

G–64
G–65

Attachment G-A.6.1 

Additional fire incidents involving the deployment of standpipe lines in WTC 1 and WTC 2 – 

• Fires involving the use of one standpipe line and the activation of one sprinkler (4 in total) 
Appendix G 

G–66
Interim Report on Significant Fires in WTC 1, 2, and 7 Prior to 9/11/01 

G–67
Appendix G 

G–68
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G–69
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Attachment G-A.6.2 

Additional fire incidents involving the deployment of standpipe lines in WTC 1 and WTC 2 – 

• Fires involving the use of one standpipe line (27 in total) 
Appendix G 

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G–73
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G–75
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G–77
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G–83
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G–85
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G–87
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G–89
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G–91
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G–93
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G–95
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G–99
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G–103
Appendix G 

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G–105
Appendix G 

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G–107
Appendix G 

G–108
G–109

Attachment G-A.7 

Significant fires occurring in WTC 7 (1) 

Significant 

Fire 

Incident 

Date Fire Location 

# sprinklers 

activated 

# standpipes 

activated 

Cause of 

fire 

Material 

Ignited 

1 5/20/88 Construction 

shanties on floor 

3 

Multiple, # not 

listed 

1 Suspicious Shanties 
Appendix G 

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G–113

Attachment G-B 

PACO 2002 Report: World Trade Center General Description of All Building Systems and 

the Capital Program. Extracted page. 

Miscellaneous Life Safety Improvements and Sprinklerization Program 
Appendix G 

G–114
H–i

TABLE OF CONTENTS - Appendix H

List of Figures.........................................................................................................................................H–iii

List of Tables ........................................................................................................................................... H–v

Appendix H

Interim Report on Evolution of WTC Fires, Smoke, and Damage Based on 

Image Analysis....................................................................................................................... H–1

H.1 Collection and Analysis of Visual Material................................................................................ H–1

H.1.1 Sources ............................................................................................................................ H–2

H.1.2 Procedures ....................................................................................................................... H–2

H.1.3 Contents........................................................................................................................... H–3

H.2 Databasing and Cataloging ......................................................................................................... H–3

H.2.1 Digital Storage................................................................................................................. H–3

H.2.2 Digitizing Techniques ..................................................................................................... H–4

H.2.3 Searchable Database........................................................................................................ H–4

H.3 Timing of Photographs and Video Clips..................................................................................... H–8

H.3.1 Digital Timestamps ......................................................................................................... H–8

H.3.2 Reference Time ............................................................................................................. H–15

H.3.3 Timing Techniques........................................................................................................ H–15

H.3.4 Absolute Time Accuracy............................................................................................... H–16

H.4 Analysis of Visual Images ........................................................................................................ H–16

H.4.1 Window Numbering ...................................................................................................... H–16

H.4.2 Fire Properties ............................................................................................................... H–18

H.4.3 Window-by-Window Assessment ................................................................................. H–18

H.5 Initial Damage Patterns on WTC 1 and WTC 2 Due to the Plane Strikes................................ H–22

H.5.1 WTC 1 ........................................................................................................................... H–22

H.5.2 WTC 2 ........................................................................................................................... H–25

H.6 Fire Behaviors in the Two Towers............................................................................................ H–32

H.6.1 WTC 1 ........................................................................................................................... H–35

H.6.2 WTC 2 ........................................................................................................................... H–36

H.7 Evidence for Collapsed Floors in WTC 2................................................................................. H–37

H.7.1 Hanging Objects ............................................................................................................ H–37

H.7.2 Molten Material............................................................................................................. H–38
Appendix H 

H–ii

H.8 Progress on Collection of Images and Analysis for WTC 7 ..................................................... H–39

H.9 Summary with Key Findings .................................................................................................... H–40

H.10References................................................................................................................................. H–43
H–iii

LIST OF FIGURES

Figure H–1. An example of the VideoList data entry sheet for video assets. ....................................... H–5

Figure H–2. An example of the first page of the Cumulus data entry sheet for photographic assets.. H–12

Figure H–3. An example of a Cumulus asset screen display for the video database .......................... H–12

Figure H–4. An example of the PhotoTiming sheet for calculating times for photographs 

containing Exif meta data................................................................................................ H–13

Figure H–5. An example of the VideoList sheet for calculating clip times for video assets. ............. H–14

Figure H–6. Photograph taken at 9:26:20 a.m. on September 11, 2001, showing the east face of 

WTC 2............................................................................................................................. H–17

Figure H–7. This photograph is cropped from the image shown in Figure H–6. It was taken on 

September 11 and shows the east face of WTC 2 at the northeast corner from floor 77 

to floor 82........................................................................................................................ H–18

Figure H–8. The key used to describe observations with regard to fire, smoke, and window 

breakage in Excel data files for individual windows in the two towers.......................... H–19

Figure H–9. A portion of the Excel spreadsheet describing fires on the east face of WTC 1 around 

9:42 a.m........................................................................................................................... H–20

Figure H–10. A representation of fires for floors 91 to 100 on the east face of WTC 1 around 

9:42 a.m........................................................................................................................... H–21

Figure H–11. A single frame from a time-dependent visualization generated by Smokeview............. H–21

Figure H–12. A drawing of the damage to the steel façade of WTC 1 ................................................. H–23

Figure H–13. The condition of windows is shown for the four faces of WTC 1 around 8:47 a.m. 

shortly after it was struck by American Airlines Flight 11 ............................................. H–24

Figure H–14. Photograph showing a full panel section lying in Cedar Street near its intersection 

with West Street .............................................................................................................. H–26

Figure H–15. Series of sequential cropped frames taken from a video shot on September 11, 2001, 

showing the plane approaching WTC 2 .......................................................................... H–27

Figure H–16. Plots of pixel locations for the nose and tail of the plane that struck WTC 2 as a 

function of time............................................................................................................... H–28

Figure H–17. Frames from a composite video ...................................................................................... H–29

Figure H–18. The time when null points are observed in the difference video following the plane 

strike on WTC 2 are plotted versus the null point number ............................................. H–30

Figure H–19. A drawing of the damage to the steel facade of WTC 2 ................................................. H–31

Figure H–20. The condition of windows is shown for the four faces of WTC 2 around 9:03 a.m. 

shortly after it was struck by United Airlines Flight 175................................................ H–33

Figure H–21. An enhanced photograph of the north face of WTC 2 taken at 9:58 a.m. shows the 

east side of the north face................................................................................................ H–34
Appendix H 

H–iv

Figure H–22. Images of the east face of WTC 2 taken shortly after the plane struck and shortly 

before the tower collapse ................................................................................................ H–38
H–v

LIST OF TABLES

Table H–1. Attributes for photographic assets..................................................................................... H–6

Table H–2. Attributes for video assets................................................................................................. H–9

Table H–3. Times for major events of September 11, 2001. ............................................................. H–15
Appendix H 

H–vi

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H–1

Appendix H

INTERIM REPORT ON EVOLUTION OF WTC FIRES, SMOKE, AND 

DAMAGE BASED ON IMAGE ANALYSIS

H.1 COLLECTION AND ANALYSIS OF VISUAL MATERIAL 

Photographic and video images of damage and fires in the World Trade Center (WTC) towers and WTC 7 

are critical for guiding the investigation led by the National Institute of Standards and Technology 

(NIST). The conditions of the towers immediately following the plane strikes, the rates of fire 

development and spread through the buildings, and indications as to the floors on which the structural 

collapses may have begun and their causes are examples of issues that are being addressed using imagery. 

Observations discussed below demonstrate the importance of such visual evidence. 

This appendix is designed to provide an update on NIST efforts to collect and analyze visual material 

available for the WTC disaster. This effort is part of Project 5, Reconstruction of Thermal and Tenability 

Environment, and this is the focus of the material presented. It is important to recognize that the effort is 

coordinated with the other projects that form the NIST-led WTC Investigation, and the visual material is 

being used as the basis for additional analysis in these projects. 

The amount of visual material recorded on September 11, 2001, was extraordinary. The terrorist attacks 

occurred in an area that is the national home base of several news organizations, has several major 

newspapers, and is the center of the fashion industry. As a result, there were likely hundreds of 

professional photographers and videographers equipped with excellent equipment and the knowledge to 

use it in the immediate area. New York City is also a major tourist destination, and visitors often carry 

cameras to record their visits. 

The WTC towers (WTC 1 and WTC 2) were immense, and they dominated the New York City skyline. 

When WTC 1 was struck by American Airlines Flight 11 around 8:46 a.m., the approach of the plane was 

captured by at least two videographers who were coincidentally filming nearby. Other photographers and 

videographers in the vicinity began recording within a few seconds of the impact. As fires grew in the 

tower, smoke pouring from the building formed a plume that could be seen for miles in all directions in 

the clear air of September 11, 2001. People in Manhattan, Brooklyn, Queens, and New Jersey began to 

turn their cameras toward the WTC complex. The major news organizations began coverage almost 

immediately and began moving professionals into position to cover the event. Numerous other 

videographers and photographers, both professional and amateur, started moving toward the WTC in 

order to create their own visual records. 

At the time United Airlines Flight 175 struck WTC 2, around 9:03 a.m., the approach and collision of the 

aircraft were recorded by numerous cameras from a variety of directions. Many people continued to 

record images until WTC 2 collapsed, around 9:59 a.m. Following this collapse, the amount of visual 

material decreased markedly as people rushed to escape the area and the huge dust clouds generated by 

the collapse obscured the site. This situation was only exacerbated by the collapse of WTC 1, around 
Appendix H 

H–2

10:28 a.m. The visual record between the period following the collapse of WTC 1 and the collapse of 

WTC 7, around 5:21 p.m., is much less complete, but there is still a substantial amount of material. 

Even as the disaster unfolded, it was clear that a large amount of visual material was being recorded that 

was being used to inform the public, demonstrate the immensity of the disaster, and to chronicle the 

associated human suffering. It is now clear that the imagery of September 11, 2001, is the most extensive 

ever recorded of such a single tragic event. The resulting visual record offers an unparalleled opportunity 

to contribute to the technical understanding of the tragedy of September 11. Even though it was clear that 

an extensive visual record of the events of September 11 existed, approaches for obtaining access to 

photographs and videos and cataloging the material had to be developed. These critical aspects of the 

task have required a great deal of time and effort. 

H.1.1 Sources 

Potential sources of visual material have been identified in a number of ways. Recordings of newscasts 

from September 11, 2001, and afterwards, documentaries and other remembrances, provided information 

directly, but also pointed toward other potential sources of material. The major photo clearinghouses, 

such as AP, Reuters, and Corbis, have World Wide Web sites that were reviewed for material related to 

September 11. Several members of the media suggested sources. Several collections of visual material 

have been assembled for charitable or historical purposes. Collections from the Here is New York City 

exhibition and the September 11 Digital Archive were reviewed. Many photographs and videos began 

appearing on the Web as early as September 11. These could often be identified by Web searches, and in 

many cases contact information was provided. Public appeals for visual material were made during 

Investigation news conferences and updates. News accounts of these events led many to contact NIST 

using the toll-free number or the Investigation Web site. Frequently, a new source would provide 

information about other potential sources. 

NIST hired a visual media consultant, Mr. Valentine Junker, to act as its representative in the New York 

City area. In addition to interacting with a number of individuals, his efforts were particularly valuable in 

interfacing with the major television networks and local New York City stations as well as the major 

photographic news services. 

H.1.2 Procedures 

The identification of sources was only the first step in the collection process. It was then necessary to 

contact the source, request the material, and make arrangements for its transfer. Special considerations 

such as copyright and privacy issues often needed to be addressed. Once an agreement was reached, 

arrangements were made to review and transfer copies of the material to NIST. 

In the collection process, emphasis has been placed on obtaining material in a form that is as close as 

possible to the original in order to maintain as much spatial and timing information as possible. In the 

case of digital photographs and videos this implies a direct digital copy. For film or slide photographs, it 

would be a high-resolution digitized version of the original media, and for analog video, a direct copy 

from the original source. While it was not always possible to maintain this standard, the majority of 

material ultimately collected was handled in this manner. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–3

H.1.3 Contents 

Significant progress has been made in collecting visual material related to September 11, 2001. Thus far, 

in excess of 150 hours of video have been assembled. At the time of preparation of this update, video 

footage has been provided by NBC, CBS, ABC, CNN and local New York City stations WABC, WCBS, 

WNBC, WPIX, WNYW and New York City One. In many cases, the videos provided not only include 

material broadcast (known as air checks), but also material that was recorded but not broadcast (known as 

outtakes). Additionally, videotapes recorded by more than 20 individuals have been received. 

Photographs have been provided by a number of sources dominated by commercial photo services, the 

New York City Police Department (NYPD), the New York City Fire Department (FDNY), and 

individuals. Well in excess of 6,000 photographs, representing more than 185 photographers, have been 

received. Professional news organizations that have provided material include AP, Corbis, Reuters, the 

New York City Times, the Daily News, and the Star Ledger. As for the videos, many of these 

organizations have provided access to unpublished photographs. The majority of photographs have come 

from individual photographers, both professional and amateur. 

It is difficult to estimate the actual amount of relevant visual material recorded on September 11, 2001, 

and thus to estimate how complete the collection efforts have been. There is certainly material that has 

not been identified and collected. However, NIST believes that the extraordinarily large collection of 

video material that it possesses is sufficient for the Investigation. 

H.2 DATABASING AND CATALOGING 

It would be impossible to effectively use the vast amount of visual material collected for the Investigation 

without some means of organizing and cataloging the material. 

H.2.1 Digital Storage 

Very early in the task, the decision was made to save all material in digital format on large digital data 

storage devices. This approach has several advantages. Because the material is in digital form, it can be 

assessed quickly. It is not necessary to search for a particular photographic collection or videotape, and 

no special equipment is required to display it. Because most material is received in other forms, the 

digital storage is in effect a backup system for the original. Additional redundancy is provided by 

backing up the entire digital storage system at regular intervals. Because videos are saved digitally, they 

can be analyzed using a variety of commercially available editing software. 

Various storage solutions were considered. An approach was finally adopted in which a central server 

along with two 325 gigabyte and one 160 gigabyte external hard drives were connected with eight 

personal computers equipped with 70 gigabyte hard drives. The personal computers not only provide 

additional disk storage, but also serve as workstations for data entry and analysis. All of the systems are 

connected by high-speed ethernet to form a single network configured such that the entire system 

becomes, in effect, a single mass storage device. The total amount of storage available is roughly 

1.4 terabytes. 
Appendix H 

H–4

Due to security concerns related to the sensitive nature of some of the visual material and copyright 

issues, the computer network has been set up with its own dedicated connections and is isolated from the 

internet backbone of NIST. Policies have been adopted that require all viewing and analysis of the 

material to be done in secured rooms using secured networks. 

H.2.2 Digitizing Techniques 

When new visual material is received at NIST, it is stored digitally on the dedicated system. If the 

material is already in digital form this simply means copying and saving it on the system. Analog 

material must be first digitized in some manner. For instance, a photograph might be scanned and 

digitized, or a video might be converted to a digital video format (typically mini-DV) and then copied to a 

hard disk. 

Each arriving video is logged into VideoList, a Microsoft Access database written specifically for this 

application. There it is assigned a unique identification number. Pertinent information concerning the 

tape is recorded, including its duration, the network and broadcast date if applicable, its physical format 

(e.g., VHS, Hi-8, or mini-DV), where the tape is stored, whether the tape is an original or a copy, its 

source, whether it has been digitized, whether it contains embedded timecode, and general notes on its 

content. Figure H–1 shows an example of the entry sheet for the VideoList database. Videos to be stored 

digitally are copied onto mini-DV media, and each copy is also logged into the database. VideoList also 

contains a calculator for assisting in the calculation of clip timing that is described in H.3.1. Selected 

video material is then transferred to hard disk for storage. Video material is often found to have natural 

breaks, such as when the camera is turned off and on (e.g., by an individual videographer) or when 

multiple cameras are used (e.g., during a newscast). It is advantageous to treat each of these breaks as the 

end of an individual video. This is accomplished by a process known as “clipping.” By using Adobe 

Premiere software and a personal computer to control the video player, it is possible to identify and note 

such breaks in a “clip file.” The clip file can also contain notes related to the material. Once a clip file 

has been generated for an entire tape, the software goes through and automatically generates multiple data 

files containing the video material. The material is stored in “avi” format, which maintains all of the 

digital information. The maximum video file size that can be handled by this system is 1 gigabyte. This 

corresponds to slightly more than 4 1/2 min of avi video. Longer continuous video segments are broken 

into lengths having roughly this period. Breaking longer videos up in this manner also makes them easier 

to search and catalog. 

H.2.3 Searchable Database 

As noted earlier, a vast amount of visual material has been collected and saved digitally as part of the 

investigation. Without some organization, it would be impossible to use this material effectively. A 

commercial database program written specially for organizing visual material, Cumulus, was chosen for 

this purpose. This software is designed to collect individual “assets” in specified catalogs and to allow 

the assets to be characterized with a variety of attributes. It is possible to generate specific attributes and 

include these in specially designed forms for data entry. Once a catalog has been assembled, it is possible 

to search for assets having a specific attribute or combinations of attributes. Quite sophisticated searches 

can be created. It is also possible to order assets based on a particular attribute. As an example, when 

dates and times are assigned, the assets can be ordered in chronological order. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–5

Figure H–1. An example of the VideoList data entry sheet for video assets. 

Two separate catalogs, one for photographs and one for video clips, have been created for visual materials 

collected as part of the Investigation. Each catalog has a similar set of attributes that is used to 

characterize the assets that are included. These attributes were chosen based on the needs of this task 

dealing with fire conditions within WTC 1, 2, and 7 and by consultation with members of other project 

teams. Tables H–1 and Table H–2 list the attributes used for photographic and video catalogs, 

respectively. A description of each attribute is provided along with details on how information 

concerning the attribute is input into the worksheet. Figure H–2 shows an example of the first screen for 

the photographic data entry form. 

Cumulus allows thumbnails of entire catalogs or selected subsets to be displayed. This makes it possible 

to review large numbers of photographs and video clips quickly and to decide which are most likely to be 

useful for a particular purpose. A variety of asset characteristics can also be shown simultaneously. 

Typically, the asset name and the time the asset was recorded are displayed. Figure H–3 shows an 

example of thumbnails taken from the video database. 
Appendix H 

H–6

Table H–1. Attributes for photographic assets. 

Attribute Definition Entry Choice 

Asset Reference Location of photograph in file system Set by Cumulus 

Categories List of categories under which the photograph 

is listed, typically the photographer’s name or 

source 

Set by Cumulus 

Record Name File name of photograph Set by Cumulus 

Photographer Photographer’s name Text 

Received from Where photograph was obtained (“Other” may 

refer to a third party, for example) 

Photographer 

WWW 

Other 

Original Source How photograph was added to the collection Digital Copy of Original 

Digital Copy from Program 

Digitized Slide or Negative 

Digitized Photograph 

Uploaded from Web 

Use Limited Photographer has requested that use of the 

photograph be limited 

Checkbox 

Copyright A copyright exists Checkbox 

Copyright Agreement Usage agreement with NIST Text 

Shot From Location of photographer Text 

Date Recorded Date and time of shot Date and time 

Time Uncertainty (s) Number of seconds uncertainty in the time 

assigned 

Integer 

View Direction Location of photographer with respect to the 

WTC 

North 

Northeast 

East 

Southeast 

South 

Southwest 

West 

Northwest 

WTC Faces 

 WTC 1 North Face 

 WTC 1 East Face 

 WTC 1 South Face 

 WTC 1 West Face 

 WTC 2 North Face 

 WTC 2 East Face 

 WTC 2 South Face 

 WTC 2 West Face 

 WTC 7 North Face 

 WTC 7 East Face 

 WTC 7 South Face 

 WTC 7 West Face 

Building face(s) visible in the photograph Checkbox for each choice 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–7

Attribute Definition Entry Choice 

Distance 

 Near 

 Medium 

 Far 

Clarity of the photograph 

 Near = Can make out details in windows 

 Medium = Can count windows 

 Far = Unable to count windows 

Checkbox for each choice 

Building 

 WTC 1 

 WTC 2 

 WTC 7 

 Other Building 

Building(s) visible in photograph Checkbox for each choice 

1st Plane Strike Photograph shows the plane strike on WTC 1 Checkbox 

2nd Plane Strike Photograph shows the plane strike on WTC 2 Checkbox 

WTC 1 Collapse Photograph shows the collapse of WTC 1 Checkbox 

WTC 2 Collapse Photograph shows the collapse of WTC 2 Checkbox 

WTC 7 Collapse Photograph shows the collapse of WTC 7 Checkbox 

Street Street scene, or a street is visible in the 

photograph 

Checkbox 

Debris 

 Aircraft Debris 

 Collapse Debris 

 Debris Inside Building 

 Street Debris 

Debris is visible in the photograph 

Type of debris: 

 Aircraft = Can be identified as plane debris 

 (e.g., tires, engines) 

 Collapse = Resulting from collapse 

 Inside Building = Visible through windows 

 or openings 

 Street = On street 

Checkbox for each choice 

Fireball Initial fireball from plane strike is visible Checkbox 

Thermal The thermal is a tall region of the smoke plume 

that results from the lift caused by the hot gases 

of the initial fireball 

Checkbox 

Plume Smoke plume generated by the fires within the 

towers and blown downwind. This marker is 

checked if the smoke plume in the photograph 

extends farther than a single tower width. 

Checkbox 

Flames Visible Flames are visible in the photograph Checkbox 

People 

 Inside 

 Falling 

 Outside 

The photograph includes people 

 Inside = People inside the buildings, at the 

 windows or climbing down 

 Outside = People on the street 

Checkbox for each choice 

Falling building 

component 

The photograph shows a building component 

falling (e.g., aluminum cladding) 

Checkbox 

Streamers Falling The photograph shows a streamer, an object 

that emits smoke as it falls and leaves a trail 

Checkbox 

Dripping Molten material dripping from WTC 2 is 

visible 

Checkbox 

Hanging Floor A sagging or hanging object suggesting a floor 

is visible within the windows 

Checkbox 
Appendix H 

H–8

Attribute Definition Entry Choice 

Building Core Photograph shows the core of WTC 1 or 

WTC 2 –– both remained standing briefly 

during collapse before falling 

Checkbox 

FDNY 

 FDNY Apparatus 

 FDNY Personnel 

FDNY personnel or vehicles are visible, 

including EMTs, fire trucks, and ambulances 

Checkbox for each choice 

NYPD 

 NYPD Apparatus 

 NYPD Personnel 

NYPD personnel or vehicles are visible, also 

includes FBI and other police officials 

Checkbox for each choice 

Impact Aircraft Photograph shows aircraft approaching WTC 1 

or WTC 2 before or during the strike 

Checkbox 

Other Aircraft Aircraft other than the impact aircraft are 

included in the photograph, such as helicopters 

or fighter jets 

Checkbox 

Good for Analysis Mark photograph for possible window-bywindow analysis Checkbox 

Analyzed The photograph has been used for window-bywindow analysis Checkbox 

Notes Notes, including a description of how the 

photograph was timed 

Checkbox 

Not all collected visual material is incorporated into the two catalogs. Photographs and videos judged not 

to contain information directly relevant to the Investigation are not included. Even so, the number of 

photographs and clips included in the catalogs is huge. At the time of writing, the photographic catalog 

includes 6,759 assets and the video catalog includes 6,911 assets. 

H.3 TIMING OF PHOTOGRAPHS AND VIDEO CLIPS 

Since one of the major goals of this task is the development of time lines for fire growth and spread in 

WTC 1, 2, and 7, it is important to assign times of known accuracy to assets included in the two catalogs. 

This task is complicated by the absence of accurate times for the majority of visual materials collected. 

H.3.1 Digital Timestamps 

Modern photographic and video digital cameras often record camera clock times as part of their output. 

For photographs, this information is usually stored as an integral part of the image in a header known as 

an Exif file. Similarly, digital video cameras often embed a variety of information, including the camera 

clock time, as part of what is known as meta data. Software is available for reading these clock times 

from Exif and other meta data media file formats. While a great help, these times usually still require 

some adjustment because people do not generally set their camera clocks accurately. In some cases, 

camera clocks were off by days or even years. Even so, the relative times over the short time period of 

the events of September 11, 2001, are quite accurate. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–9

Table H–2. Attributes for video assets. 

Attribute Definition Entry Choice 

Asset Reference Location of video clip in the file system Set by Cumulus 

Categories List of categories under which the video clip is 

listed, typically the photographer’s name or 

source 

Set by Cumulus 

Record Name File name of video clip Set by Cumulus 

Photographer Photographer’s name Text 

Content Content of video clip 

 WTC 9/11 Footage = Events before collapse 

 of WTC 7 

 Street Scene (no timing) 

 Debris field = Ground Zero after WTC 7 

 collapse 

 Construction = Construction of WTC towers 

 from documentary 

 Normal Operation = Normal operation of 

 building, usually from documentary 

 Animation = Animation of 9/11 events from 

 documentary 

 Still(s) = Photographs contained within 

 documentary 

 Interview = Clip only shows interview 

WTC 9/11 Footage 

Street scene (no timing) 

Debris field 

Construction 

Normal operation 

Animation 

Still(s) 

Interview 

Use Limited Videographer has requested that use of the 

videotape be limited 

Checkbox 

Copyright A copyright exists Checkbox 

Copyright Agreement Usage agreement arrangements with NIST Text 

Shot From Location of videographer Text 

Date Recorded Date and time of beginning of video clip Date and time 

End Recording Date and time of end of video clip Date and time 

Duration Number of minutes:seconds contained in clip Real number 

Time Uncertainty (s) Number of seconds uncertainty in the time 

recorded / end recording 

Integer 

View Direction Location of videographer with respect to the 

WTC 

North 

Northeast 

East 

Southeast 

South 

Southwest 

West 

Northwest 
Appendix H 

H–10

Attribute Definition Entry Choice 

WTC Faces 

 WTC 1 North Face 

 WTC 1 East Face 

 WTC 1 South Face 

 WTC 1 West Face 

 WTC 2 North Face 

 WTC 2 East Face 

 WTC 2 South Face 

 WTC 2 West Face 

 WTC 7 North Face 

 WTC 7 East Face 

 WTC 7 South Face 

 WTC 7 West Face 

Building face(s) visible in the video clip Checkbox for each choice 

Distance 

 Near 

 Medium 

 Far 

Clarity of the video clip 

 Near = Can make out details in windows 

 Medium = Can count windows 

 Far = Unable to count windows 

Checkbox for each choice 

Building 

 WTC 1 

 WTC 2 

 WTC 7 

 Other Building 

Building(s) visible in video clip Checkbox for each choice 

1st Plane Strike Clip shows the plane strike on WTC 1 Checkbox 

2nd Plane Strike Clip shows the plane strike on WTC 2 Checkbox 

WTC 1 Collapse Clip shows the collapse of WTC 1 Checkbox 

WTC 2 Collapse Clip shows the collapse of WTC 2 Checkbox 

WTC 7 Collapse Clip shows the collapse of WTC 7 Checkbox 

Street Street scene, or a street is visible in the video 

clip 

Checkbox 

Debris 

 Aircraft Debris 

 Collapse Debris 

 Debris Inside Building 

 Street Debris 

Debris is visible in the video clip 

Type of debris: 

 Aircraft = Can be identified as plane debris 

 (e.g., tires, engines) 

 Collapse = Resulting from collapse 

 Inside Building = Visible through windows 

 Street = On street 

Checkbox for each choice 

Fireball Initial fireball from plane strike is visible Checkbox 

Thermal The thermal is a tall region of the smoke plume 

that results from the lift caused by the hot gases 

of the initial fireball 

Checkbox 

Plume Smoke plume generated by the fires within the 

towers and blown downwind. This marker is 

checked if the smoke plume in the video clip 

extends farther than a single tower width. 

Checkbox 

Flames Visible Flames are visible in the video clip Checkbox 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–11

Attribute Definition Entry Choice 

People 

 Inside 

 Falling 

 Outside 

The video clip includes people 

 Inside = People inside the buildings, at the 

 windows, or climbing down 

 Outside = People on the street 

Checkbox for each choice 

Falling building component The video clip shows a building component 

falling (e.g., aluminum cladding) 

Checkbox 

Streamers Falling The video clip shows a streamer, an object that 

emits smoke as it falls and leaves a trail 

Checkbox 

Dripping Molten material dripping from WTC 2 is 

visible 

Checkbox 

Hanging Floor A sagging object suggesting a floor is visible 

within the windows 

Checkbox 

Building Core Video clip shows the core of WTC 1 or 

WTC 2 – both remained standing briefly during 

collapse before falling 

Checkbox 

FDNY 

 FDNY Apparatus 

 FDNY Personnel 

FDNY personnel or vehicles are visible, 

including EMTs, fire trucks, and ambulances 

Checkbox for each choice 

NYPD 

 NYPD Apparatus 

 NYPD Personnel 

NYPD personnel or vehicles are visible, also 

includes FBI and other police officials 

Checkbox for each choice 

Aircraft 

 Impact Aircraft 

 Other Aircraft 

Aircraft are visible in the video clip 

 Impact: Shows aircraft approaching WTC 1 

 or WTC 2 before or during the strike 

 Other: Helicopters or fighter jets 

Checkbox for each choice 

Major Change 

 Major Fire Change 

 Major Smoke Change 

 Windows Opened 

One of the following events takes place in the 

video clip: 

 Major Fire Change: Fire flares up, dies 

 down, or spreads to a new region 

 Major Smoke Change: Smoke bursts, dies 

 down, or spreads to a new region 

 Windows Opened: Window breaks open, 

 either due to fire or to people 

Checkbox for each choice 

Good for Analysis Mark video clip for possible window-bywindow analysis Checkbox 

Analyzed The video clip has been used for window-bywindow analysis Checkbox 

Notes Notes, including a description of how the video 

clip was timed 

Text 
Appendix H 

H–12

Figure H–2. An example of the first page of the Cumulus data entry sheet 

for photographic assets. Thumbnail © 2001 Mark Stetler. 

Figure H–3. An example of a Cumulus asset screen display for the video 

database. Thumbnails are shown along with the time and dates when the 

recording started and ended. Thumbnails © 2001 Scott Meyers. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–13

Occasionally analog photo and video cameras imprint a time stamp on their outputs that can provide 

relative times similar to Exif or meta data, but generally there is no time information available, and such 

material must be timed in some other way. Some of the approaches used are described later in this 

section. 

Photograph Tools 

In order to make the best use of the information embedded in digital photographs, software was required 

to retrieve the Exif file information and software to adjust the recorded clock times. The commercial 

software package CatDV is able to retrieve meta data embedded in a variety of media formats, including 

digital photographs and mini-DVs. The Access database PhotoTiming was written for the purpose of 

determining the actual times for a set of photographs given the Exif time for each and an accurate time 

reference. For a set of photographs sharing a common clock from the same digital camera, an accurate 

time for a single photograph is sufficient to set the times for the entire set. Figure H–4 shows a 

PhotoTiming data sheet for a selected photographer. A file generated by CatDV containing the Exif data 

for each photograph, if available, is read into PhotoTiming. The equivalent Exif and actual times are 

entered into the appropriate fields at the upper right of the data sheet. Selection of the Calculate Photo 

Times button fills the Actual Time column with the appropriate value for each Exif time. In this example, 

the Exif time was found to be off by 62 s. 

Figure H–4. An example of the PhotoTiming sheet for calculating times for 

photographs containing Exif meta data. 
Appendix H 

H–14

Video Tools 

In addition to containing the video database described in Section H.2.3, VideoList also assists with timing 

the clips from a videotape. This function is similar to that in the PhotoTiming tool. For a broadcast video 

that was filmed in real time, the timing of every clip in the video, except for replays, can be set from 

knowing the time at a single point. An example is shown in Figure H–5. A clip file generated in Adobe 

Premiere for a specified video is read into VideoList. The mini-DV time of an event in the video whose 

timing is known, such as the moment of the second plane strike, is identified. Both times are entered into 

the fields at the upper right of the data sheet. Clips to be timed (excluding replays) are identified by a 

check mark, and the requested calculation results in the actual times in and out for each clip as shown in 

Figure H–5. This tool is also useful in calculating timings for continuous video segments broken into 

multiple clips. 

Figure H–5. An example of the VideoList sheet for calculating clip times for video assets. 

For each mini-DV video that contains meta data, CatDV is used to extract the clock times for the In and 

Out point for each clip. These values enable the timing of every clip in the video from a single reference 

time. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–15

H.3.2 Reference Time 

Faced with the timing considerations above, a timing scheme was developed in which all of the times in 

the databases are placed on a relative time scale tied to a single well-defined event. Due to the large 

number of different views available, the moment the second plane struck WTC 2 was chosen to be this 

time. This event was defined to have occurred at 9:02:54 a.m. based on times for major events included 

in the earlier Federal Emergency Management Agency (FEMA) report (McAllister 2002) describing the 

events of September 11, 2001. 

H.3.3 Timing Techniques 

Once the reference time was chosen, it was possible to place times on videos that showed the second 

plane strike. By matching other photographs and videos to these initially assigned videos, the 

assignments were extended to visual materials that did not include the primary event. By such a bootstrap 

process, it was possible to place photographs and videos extending over the entire period of the event on a 

single time line. Sets of photographs containing Exif times and video clips that either contained meta data 

or were continuous over relatively long periods were particularly useful for this purpose because a single 

time assignment would allow the entire series to be timed. Sets of photographs recorded on film or 

analog videos that were frequently turned on and off caused the most difficulty in timing, and individual 

matches were required for each photo or video clip. 

Matching visual images and assigning times has turned out to be a demanding task requiring unique 

approaches. A variety of characteristics have been employed to match times in different photographs and 

videos. These include distinct shadows cast on the buildings by the smoke plumes, the appearance and 

locations of smoke and fire plumes, the occurrence of well-defined events such as a falling object or the 

sudden appearance of smoke, and a variety of other unlikely clues such as a clock being recorded in an 

image. 

To assist in the timing process, relative times for the five major events of September 11, 2001: first plane 

strike, second plane strike, collapse of WTC 2, collapse of WTC 1, and collapse of WTC 7 have been 

determined with 1 second accuracy. These times are summarized in Table H–3. Note that the building 

collapse times are defined to be when the entire building is first observed to start to collapse. In the case 

of WTC 7, a penthouse on the roof sank into the building before the main collapse started. 

Table H–3. Times for major events of September 11, 2001. 

Event 

Relative Time from 

Visual Analysis 

Adjusted Time from 

Television Broadcasts 

Time Reported in the 

FEMA Study 

First plane strike 8:46:25 a.m. 8:46:30 a.m. 8:46:26 a.m. 

Second plane strike 9:02:54 a.m. 9:02:59 a.m. 9:02:54 a.m. 

Collapse of WTC 2 9:58:54 a.m. 9:58:59 a.m. 9:59:04 a.m. 

Collapse of WTC 1 10:28:20 a.m. 10:28:25 a.m. 10:28:31 a.m. 

Collapse of WTC 7 5:20:47 p.m. 5:20:52 p.m. 5:20:33 p.m. 

It is not only important to assign relative times for photographs and videos, but also to estimate how 

accurately they are known. For this reason, timing uncertainties are estimated for each determination and 

are included in the databases. 
Appendix H 

H–16

The bootstrap timing process was initially quite difficult. However, team members’ timing skills 

improved with practice at the same time as more visual material became available and the number of 

timed assets increased. At the present time, 3,032 of the 6,759 catalogued photographs and 2,673 of the 

6,911 video clips in the databases are timed with assigned relative accuracies of 3 seconds or better. 

H.3.4 Absolute Time Accuracy 

Many of the news broadcasts on September 11, 2001, included small clocks, known in the industry as 

“bugs,” imprinted on the screen. As such broadcasts were timed, it became apparent that there were small 

differences between times for the second plane strike based on these bugs and the time used as the basis 

for the database. Checks with several broadcasters indicated that the bugs should be quite close to the 

actual time because their clocks are regularly updated from highly accurate sources such as 

geopositioning satellites or the precise atomic-clock-based timing signals provided by NIST as a public 

service. Careful checks showed small time differences between different video recordings, but these were 

generally less than 1 second. These small discrepancies are likely due to variations in transmission times 

resulting from the different pathways that the video signals take to the sites where they are recorded. 

Based on four such video recordings, the time of the second plane impact is estimated as 9:02:59 a.m., or 

5 seconds later than the time assumed in developing the database. The estimated uncertainty is 1 second. 

Table H–3 compares times for the major events taken from the database, adjusted to television time, and 

reported in the FEMA report (McAllister 2002). Possible explanations for the observed differences are 

still under investigation. Because times based on the television broadcasts appear to be accurate (i.e., 

those in column 3 of Table H–3), 5 seconds will be added to times included in the databases when precise 

times are reported for the Investigation. 

H.4 ANALYSIS OF VISUAL IMAGES 

Once the two visual databases became available, it was possible to use the images to begin characterizing 

the events of September 11, 2001. Some of the images are quite close up and can be used to learn 

specific details concerning the towers. As an example, Fig. H–6 shows an image of the east face of 

WTC 2 recorded at 9:26:20 a.m., and Fig. H–7 shows an enlarged portion of the same photograph. The 

photograph has been enhanced using Adobe Photoshop, and lettering has been added to indicate the floors 

and the numbering system used to identify specific windows in the tower. The amount of detail available 

is evident. For instance, large piles of debris are present on the north side of the tower on floors 80 and 

81, and locations with fires visible or with windows missing are easily identified. 

H.4.1 Window Numbering 

The system used to describe window locations in the two towers and WTC 7 requires some elaboration. 

It is based on the outer-wall column numbering system used in plans for the buildings. First, consider the 

towers. In these structures individual windows were placed between two exterior columns. In order to 

refer to a particular window the designation for the column to the right as viewed from the outside is 

assigned to that window. These columns are numbered from 1 to 59 from right to left across a tower face, 

and windows are numbered from 1 to 58. Faces for the towers are also assigned numbers as follows; 

WTC 1—north: 1, east: 2, south: 3, and west: 4, and WTC 2—west: 1, north: 2, east: 3, and south: 4. By 

combining the floor number, the face number and a column number, a specific window on one of the 
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H–17

Figure H–6. Photograph taken at 9:26:20 a.m. on September 11, 2001, showing the east 

face of WTC 2. It has been enhanced, and lettering indicating floors and columns has 

been added. 

towers can be identified. As an example, for WTC 1, the number 94-214 refers to the fourteenth window 

from the right on the east face of floor 94. 

The window numbering system is somewhat different for WTC 7. It is also based on the outside column 

numbers, but in this building the numbering of columns was continuous around the structure and ranged 

from 1 to 57. Column 1 was located at the northwest corner of the building, and the numbering 

proceeded counter clockwise around the building faces with columns 15, 28, and 42 located at the 

southwest, southeast, and northeast corners, respectively. Note that the total number of perimeter 

columns is actually 58. An extra column, numbered 14A, was included on the west face between 

columns 14 and 15. Unlike the towers, the number of windows to the right of a given column varied from 

one to five depending on location. In some cases, the windows are located in front of the column. 

Individual windows to the right of a column are assigned letters increasing from left to right as seen from 

the outside. As an example, 12-45c refers to a window on the north face of WTC 7 that is the third 

window to the right of column 45 on floor 12. 
Appendix H 

H–18

Figure H–7. This photograph is cropped from the image shown in Figure H–6. It was 

taken on September 11 and shows the east face of WTC 2 at the northeast corner from 

floor 77 to floor 82. Note the large piles of debris evident on floor 80 and floor 81. 

H.4.2 Fire Properties 

Photographs and video images have been used to characterize a number of properties relevant to fire 

growth and spread in the towers as a function of time. Specific properties addressed include whether or 

not fire and smoke are present and whether windows are still in place. When smoke and/or fire are 

present, additional details concerning their appearances are documented. A numbered coding system is 

used to describe these characteristics. The key for this numbering system is shown in Fig. H–8. 

H.4.3 Window-by-Window Assessment 

The key in Fig. H–8 is used as the basis for a window-by-window assessment of the towers. The results 

are coded in three separate data sheets using Microsoft Excel. The floor and window locations are 

identified using the numbering system described in the last section. Separate files containing the three 

data sheets are generated for each face of a tower and time analyzed. Figure H–9 shows a portion of such 

a data sheet describing fires (i.e., sheet one) on the east face of WTC 1 around 9:42 a.m. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–19

Figure H–8. The key used to describe observations with regard to fire, smoke, and 

window breakage in Excel data files for individual windows in the two towers. 

KEY FOR ANALYSIS 

Sheet #1: Fire Visible 

 0 No fire 

 1 Spot fire 

 2 Fire visible inside 

 3 External flaming 

 9 Not visible 

Sheet #2: Smoke 

 0 No smoke evident 

 1 “Light smoke” 

 2 “Heavy smoke” 

 9 Not visible 

Sheet #3: Windows 

 0 Window open 

 1 Window in place 

 9 Not visible 
Appendix H 

H–20

Figure H–9. A portion of the Excel spreadsheet describing fires on the east face of 

WTC 1 around 9:42 a.m. is shown. The numbers at the left refer to floors, and those at 

the top are the window numbers. 

While the data sheets capture the desired behaviors, it is very difficult to use them to track changes 

without visualizing the results in some way. Two approaches have been developed for this purpose. The 

first employs a Web-based system that generates color-coded maps of the results contained in the data 

sheets. Figure H–10 shows such a map for the fire data included in the data sheet shown in Fig. H–9. 

The second approach uses the program Smokeview (Forney and McGrattan 2003; Forney, 

Madrzykowski, and McGrattan 2003) to generate a time-dependent visualization of the results. 

Smokeview was developed at NIST in order to display the results of fire dynamics calculations. In the 

current application, it is used to visualize the properties of interest on a three-dimensional representation 

of a tower façade as a function of time. Because Smokeview allows the point of view to be varied at will, 

this approach is a powerful means for investigating the temporal behavior of the fires on different faces of 

the tower. Figure H–11 shows a frame taken from a visualization in which results from the fire and 

windows data sheets for WTC 2 have been combined. 
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H–21

Figure H–10. A representation of fires for floors 91 to 100 on the east face of WTC 1 

around 9:42 a.m. is shown. Results are taken from the Excel spreadsheet shown in 

Figure H–13. The color coding is based on the key shown. The color assignments are: ▐

0-No fire,▐ 1-Spot fire,▐ 2-Fire visible inside,▐ 3-External Flaming, and▐ 9-Can’t see. 

Figure H–11. A single frame from a time-dependent visualization generated by 

Smokeview is reproduced here. The frame is a three-dimensional representation of the 

condition of windows and fires on WTC 2 from the time the second tower was struck at 

9:02:59 a.m. until it collapsed at 9:58:59 a.m. The color assignments are: ▐ – window in 

place, ▐ – missing window, ▐ – external flaming, ▐ – fire inside, and ▐ – spot fire. 
Appendix H 

H–22

H.5 INITIAL DAMAGE PATTERNS ON WTC 1 AND WTC 2 DUE TO THE 

PLANE STRIKES 

Close-up photographs and videos have been used to characterize the initial damage to the façades of the 

towers struck by the two planes along with precise determinations of the locations of the plane strikes. 

For WTC 2, analysis of videos has also been employed to estimate the speed of the airplane that struck 

the tower and to show that the tower swung back and forth for several minutes after it was struck. The 

period of the swinging has also been determined. 

H.5.1 WTC 1 

Damage Resulting from Plane Strike 

A detailed drawing of the damage to the steel façade of WTC 1 was included in the FEMA World Trade 

Center Building Performance Study (McAllister 2002). A careful inspection using photographs and 

videos in the database confirmed the accuracy of this analysis. Figure H–12 shows a drawing that 

represents this damage. It is similar to that included in the FEMA report, but it incorporates several minor 

changes that better reflect the geometry of the north face of WTC 1 in the vicinity of the plane strike. 

It was observed that the wing tips and the end of the vertical stabilizer at the plane’s tail section damaged 

the aluminum column covers on the steel façade without cutting through the steel below or completely 

removing the covers. By inspection it was possible to map out locations on columns where the wingtips 

and the vertical stabilizer struck the tower. These locations were then transferred to the representation of 

the damaged steel façade shown in Fig. H–12 and are represented by dashed lines, with wings to the right 

and left and the vertical stabilizer in the center. The good agreement between the damage pattern and the 

wing tip locations is evident. It is reported in the FEMA report (McAllister 2002) and widely in the 

media that American Airlines Flight 11 struck floors 94 to 98 of WTC 1. The dotted horizontal lines on 

the left side of Fig. H–12 indicate the locations of concrete floors. It can be seen that while the tip of the 

left wing of the aircraft struck very close to the base of floor 94, the wing end marked column 153 at the 

very top of floor 93. It is evident from the figure that the right wing actually struck well up on floor 99 on 

column 109. The impacted floors therefore range from floor 93 to floor 99. 

Fireballs and Missing Windows 

Additional insights into the initial damage inflicted on the towers by the plane strikes can be obtained by 

considering locations where fireballs are observed immediately following the plane strikes as well as 

locations where windows are missing. Videos and photographs recorded during and immediately 

following the plane strike on WTC 1 show that significant fireballs formed at the plane strike location on 

the north face, as well as near the center of the east face and on the western side of the south face. 

Figure H–13 compares window damage for the four sides of WTC 1 immediately after the plane strike. 

The floors shown extend from 91 to 100. The missing windows on the north face are consistent with the 

plane strike location and strike angle. The plane struck very close to the center of the face. Interestingly, 

the damage on the east and west faces appears to be asymmetric with a much higher number of windows 

missing on the east face than on the west. This observation is consistent with the formation of a fireball 

on the east side of the tower and not on the west side. Areas obscured by smoke are also much larger on 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–23

 

Figure H–12. A drawing of the damage to the steel façade of WTC 1. The dark dotted 

lines show locations where the airplane wings and vertical stabilizer marked the 

aluminum cladding on columns. 

the east side suggesting that more fire is present on this face as well. An asymmetry is also apparent on 

the south face where only a single window is missing on the east side while numerous windows are 

missing and significant smoke is present on the west side. Taken together, these observations suggest that 

debris and fuel from the airplane as well as any building materials and contents tended to pass straight 

across the building on the west side, while material on the east side was somehow reflected and more 

heavily damaged the east face. 
Appendix H 

H–24

Figure H–13. The condition of windows is shown for the four faces of WTC 1 around 8:47 a.m. shortly after it was struck by 

American Airlines Flight 11. The colors represent ▐ - window open, ▐ - window in place, and ▐ - not visible.
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–25

Panel Section in Street 

A photograph supplied by the NYPD provided additional details with regard to the initial damage 

suffered by WTC 1. Figure H–14 shows a full three-story three-column-wide steel panel section lying on 

the corner of Cedar Street near its intersection with West Street. This location is to the south of and 

roughly 210 m from the south face of WTC 1. The photograph was taken prior to the collapse of either 

tower. Closer inspection shows that there is an aircraft wheel embedded in one of the windows. The 

most likely source location for this panel section has been identified as being near the center of the south 

face of WTC 1 (i.e., columns 329 to 331) and extending from the middle of floor 93 to the middle of floor 

96. This conclusion remains tentative since, as indicated in Fig. H–13, the area is obscured by smoke in 

all of the close-up photographs of the area in NIST’s possession. If the location is identified correctly, the 

wheel is stuck in window 95-329. 

H.5.2 WTC 2 

Calculation of Plane Speed 

One of the videographers who provided material to the Investigation filmed from the top of his apartment 

building located to the east of the WTC complex. His camera was located on a tripod so that the images 

are very steady. One of the events he captured was United Airlines Flight 175 as it approached WTC 2. 

Figure H–15 is a series of cropped frames captured from this video that show the plane approaching the 

building. 

The images included in Fig. H–15 have been used to determine the speed of the plane as it approached the 

tower. This is done by identifying the locations of the nose and tail of the airplane relative to a fixed 

point defined to be the point on the frame where the plane passes out of sight behind the corner of the 

building. The plane is very nearly level relative to this point, so it is appropriate to simply count the 

number of picture elements, pixels, between this location and the two measurement points on the aircraft. 

This analysis, which presumes that the aircraft at this time travels in a straight path such that the nose and 

tail pass through the same point in space, has the advantage of being independent of the orientation of the 

flight path with respect to the line of sight of the observer. 

Figure H–16 shows the locations of the two points as a function of time. Using linear least squares curve 

fits, the exact relative times when the nose and tail pass the reference location are estimated. The 

difference between these two times is the period required for the entire length of the aircraft to pass the 

reference location. The result is 0.1939 s. Since the length of the plane is known to be 155.0 ft, the speed 

can be determined simply by dividing this length by the passage time to give 155.0 ft/0.1939 s = 799 ft/s 

= 545 mph. An uncertainty estimate based solely on the uncertainty in the determined time difference 

yields a value of ±18 mph with 95 percent confidence. 
Appendix H 

H–26

Figure H–14. Photograph showing a full panel section lying in Cedar Street near its 

intersection with West Street. An aircraft wheel can be seen imbedded in one of the 

windows. The building behind the panel is Saint Nicholas Greek Orthodox Church and 

the lower section of WTC 2 can be seen across Liberty Street. 

Note that the airplane speed and uncertainties are slightly different than listed in an earlier report 

(NIST 2003) due to a correction of the plane length to reflect the actual distance between the nose and the 

end of the body at the rear stabilizer and a math error in the uncertainty calculator. Uncertainties 

associated with aircraft motion that are not aligned with the aircraft body are judged to be less than the 

uncertainties in plane passage time. 

Observation of WTC 2 Sway Following the Plane Strike 

Close examination of the video revealed a perceptible movement of WTC 2 after it was struck by the 

aircraft. The building rocked back and forth much as a pendulum for at least 4 minutes. Image 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–27

Figure H–15. Series of sequential cropped frames taken from a video shot on 

September 11, 2001, showing the plane approaching WTC 2. The frames, ordered 

from left to right and top to bottom, are separated by 33.3 ms. 
Appendix H 

H–28

Figure H–16. Plots of pixel locations for the nose and tail of the plane that struck WTC 2 

as a function of time taken from the images shown in Fig. H–15. Straight lines are the 

results of linear least squares curve fits to the data. Extrapolation of the lines to pixel 0 

allows the time for the passage of the plane to be calculated. 

analysis was used to enhance this motion and estimate the period required for the building to sway 

through one complete cycle. This was accomplished by creating a new video in which a single frame just 

prior to the plane strike was subtracted from subsequent frames. In this way, small differences between 

images can be identified. If the image is unchanged from the initial frame, the result should be a black 

image, but any changes in location or color will appear in the difference video. When this approach was 

applied to the video, a region of windows was observed on the building that seemed to appear and 

disappear. Figure H–17 shows several frames of a composite video formed by overlaying half frames of 

the original video and the difference video. In the initial frame (time = 0.0 seconds), the plane has not yet 

appeared and the difference frame is black. In the next frame (time = 10.7 seconds), the plane is 

approaching the building. The plane is evident in the difference frame since it represents a change in the 

frame. WTC 2 is still dark except near the top where changes due to smoke movement are apparent. In 

the third frame (time = 11.3 seconds), the plane has struck the building and dramatic changes in the 

appearance of the building façade in the difference frame occur. Careful inspection shows what appear to 

be curved lines running across the face of WTC 2. These curves result from an interaction between the 

straight lines formed by the windows on the tower and the straight lines of picture elements that make up 

the detector in the digital video camera. This well-known behavior is called the moiré effect. The moiré 

effect also provides a sensitive approach for determining the displacement of the building. Such an 

analysis is in progress and will be reported at a later time. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–29

Figure H–17. Frames from a composite video are shown. The half frames on the left are 

taken from a video showing the plane strike on WTC 2, while the half frames on the right 

are generated by subtracting a frame recorded prior to the plane strike from all 

subsequent frames in the original video. Times refer to the period since the 

start of the difference video. 

Following the plane strike, areas of the tower face above the strike floors become hidden by smoke, and it 

is difficult to see the moiré patterns in the difference frames. However, the area of the tower below the 

strike floors to the left of the building in the foreground continues to show a distinct difference pattern 

because it is not obscured by smoke. This pattern is apparent in the fifth frame (time = 30.9 seconds). On 

the other hand, frames 4 and 6 (times = 28.0 seconds and 33.5 seconds, respectively) have been chosen 

because they are near null points, and the area appears dark in the difference frame because the location 

of the building is essentially unchanged from its position before the plane struck. When the video is 
Appendix H 

H–30

played, the moiré patterns in this area of the tower face alternately appear and disappear in the difference 

video. 

Because the absence of color is easiest to identify, it is straightforward to determine times when the null 

points occur in the difference image. Figure H–18 shows a plot of time versus null point number obtained 

from the difference video. The points fall on a straight line having a slope of 5.647 seconds 

± 0.008 seconds (95 percent confidence interval). Because the building passes through a null point twice 

during a single full oscillation, the period required for a single oscillation is 11.3 seconds. 

Figure H–18. The time when null points are observed in the difference video following 

the plane strike on WTC 2 are plotted versus the null point number. The points fall on a 

very good straight line having a slope of 5.647 seconds ± 0.008 seconds. 

The measured oscillation period is consistent with measurements that are available from WTC 1 that 

yielded periods of 10.9 seconds in the east-west direction (averaged over a 9-year period that ended in 

1993) and 11.6 seconds in the north-south direction (averaged over a 14-year period that also ended in 

1993). The cores of the two towers were oriented perpendicular to each other so the motion monitored 

here should be comparable to the east-west direction of WTC 1. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–31

Damage Resulting from the Plane Strike 

The results of an analysis of the damage to the steel façade of the south face of WTC 2 are provided in 

Fig. H–19. Much of the steel damage pattern is revealed, but it should be noted, as indicated, that a 

portion of this face on the east side of the plane strike location was constantly obscured by smoke, and the 

detailed pattern could not be discerned. The FEMA report (McAllister 2002) also includes a figure 

describing the damage to the steel façade inflicted by United Airlines Flight 175. The pattern in 

Fig. H–19 differs somewhat from that provided in this earlier study. Some inconsistencies in façade 

dimensions have also been corrected in the current version. 

Figure H–19. A drawing of the damage to the steel facade of WTC 2. The dark dotted 

lines show locations where the airplane wings and tail marked the aluminum cladding on 

the columns. The area shaded in gray was hidden by smoke and could not be observed. 
Appendix H 

H–32

As was true for WTC 1, in areas of the façade struck by the wing tips and the upper portion of the vertical 

stabilizer the aluminum covering was marked, but the aluminum covers were not removed and the steel 

was not cut through. Measurements for the location of the left wing tip were mapped out as shown in 

Fig. H–19. As already noted, the area at the end of the right wing was obscured by heavy smoke. 

However, there were brief periods when the location of the last column struck by the wing tip could be 

observed. This location is indicated on column 404 of floor 95 in Fig. H–19. The center of the plane 

strike is clearly located towards the east side of the face. The left wing mark extends to the bottom of the 

spandrel located below floor 78. The actual location of the concrete floor is well above this point, which 

means the lowest point struck lies on floor 77. Thus, the plane strike location on WTC 2 extends from 

floor 77 to floor 85. This can be contrasted to the FEMA study (McAllister 2002) and most media 

sources that report the floors struck extended from floor 78 to floor 84. 

Fireballs and Missing Windows 

Intense fireballs were observed on the south, east, and north faces of WTC 2 following the plane strike. 

Figure H–20 compares missing windows on floor 77 to floor 86 for the four faces of WTC 2 shortly after 

the plane struck at 9:02:59 a.m. 

The distribution of missing windows on the south face traces roughly the outline of the plane strike, with 

missing windows increasing in height from left to right. Recall that a portion of the east side of this face 

could not be observed due to smoke obscuration. The analysis indicates that a very large number of 

windows were removed on the east face by the collision and subsequent fireball. This is particularly true 

on floor 80 to floor 82. Photographs and videos show that extensive areas of the aluminum covering the 

façade and holding windows were removed, exposing the steel panels, as a result of the plane strike and 

fireballs. This damage is much more extensive than observed on the east face of WTC 1, consistent with 

the plane strike occurring closer to this face. In contrast to the extensive damage on the east face of the 

tower, no missing windows were found on the west face. 

A large number of windows are also missing on the north face of WTC 2. A substantial area of the 

aluminum façade was also removed during the plane strike and subsequent fireball. The missing 

windows on this face almost appear to be a mirror image of the south face with damage towards the 

center being on lower floors than on the eastern edge. This suggests that a great deal of debris passed 

through the entire length of the building. This hypothesis is supported by close up images that show large 

piles of debris on the east side of the north face on floor 80 and floor 81, and on floor 79 near the center 

of the face. Figure H–21 includes a photograph showing these debris piles. Recall that piles of debris 

were also evident on floor 80 and floor 81 on the north side of the east face (see Fig. H–7). 

H.6 FIRE BEHAVIORS IN THE TWO TOWERS 

Analysis of the fire spread in the towers is ongoing as this update is being prepared, but sufficient 

information is available to allow some of the fire characteristics to be described. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–33

Figure H–20. The condition of windows is shown for the four faces of WTC 2 around 9:03 a.m. shortly after it was struck by 

United Airlines Flight 175. The colors represent ▐ - window open, ▐ - window in place, and ▐ - not visible. 
Appendix H 

H–34

Figure H–21. An enhanced photograph of the north face of WTC 2 taken at 9:58 a.m. shows the east side of the north face. 

Piles of debris are evident on floor 80 and floor 81 on the northeast corner and on floor 79 to the right of column 230. The 

“cold” area extends over floor 80 to floor 82 from roughly column 229 to 240. There is a break in column 253 on floor 81, 

and the adjacent column, 254, is bent. Floors 80, 81, and 82 appear to be partially collapsed. It appears as if the collapsed 

floor 83 in the northeast corner has broken into sections. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–35

H.6.1 WTC 1 

It has already been mentioned that substantial fireballs formed on the north, east, and south faces 

immediately following the plane strike. A brief period of intense burning from openings on these faces 

was observed after the fireballs dissipated, but in a short period (on the order of 60 seconds) the fires 

seemed to “damp down” and very little flame and only light smoke was evident from the outside. This 

period of light burning lasted several minutes before fires began to reappear. 

Rapid early fire growth was observed on the east side of the north face on floor 96 and floor 97, the center 

of the east face on floor 94 and floor 97, and the western side of the south face on floor 96. Even though 

relatively little initial damage was sustained by the west face, heavy smoke followed shortly by flame 

appeared around window 97-437 at 8:55 a.m. After this time, a very rapid fire spread was observed 

across the west face on this floor. Within a couple of minutes, over half of the windows were emitting 

smoke, and flames were visible in many. Even though floor 92 was not directly struck by the airplane, 

fire appeared on the east side of the tower on this floor shortly after 9:00 a.m. 

Following the initial development of large fires, fire spread continued until WTC 1 collapsed around 

10:28 a.m. At times the fires displayed the systematic, relatively slow spread expected for fire growth in 

a typical building. For instance, after the initial rapid growth phase, fires on floors 92, 94, 96, and 97 on 

the east face began to move deliberately toward the south. As they spread, the fires would burn intensely 

at a given location for a while before dying down. As a result, these fires developed the appearance of a 

wave moving slowly across the building. 

There were also certain times and locations during which fire appeared to spread quite rapidly. Some of 

these episodes were clearly connected with rapid fire growth and likely flashover in rooms. During the 

first half hour, significant fires were observed toward the centers of floors 92, 94, 96, and 97 on the east 

face that were spreading towards the north. Each of these fires eventually reached a certain point where 

further fire spread was inhibited for many minutes. A review of building plans showed that walls of 

offices or meeting rooms were presented at the locations where fire spread was inhibited. 

Apparently, these walls served as effective fire breaks that protected against further fire spread. However, 

for each of these floors fire and smoke eventually appeared at one of the windows beyond the walls, and 

after one of these windows was broken fire growth was extremely rapid and robust across the remaining 

windows. These observations are consistent with the occurrence of flashover within an enclosed space. 

At other times, unusually rapid fire growth apparently occurred in areas that are believed to have been 

relatively open and not constrained by walls. One of these episodes occurred around 9:54 a.m. on the 

north face. Fire suddenly appeared on floor 96, a location to the west of the damage inflicted by the 

airplane. Within a very short period of time, fire could be seen in roughly 10 windows covering a 

distance of more than 30 ft. 

Another example of very rapid fire growth appeared to take place on floor 98. In the early period of the 

fire, this floor did not appear to be heavily involved, and this remained true for quite a while. However, 

after 9:30 a.m., fire began to appear on this floor and by 10:00 a.m., fires were observed over significant 

lengths on all four faces of the tower. 
Appendix H 

H–36

One of the more unusual fire spread episodes in WTC 1 occurred just after the collapse of WTC 2 around 

9:59 a.m. Within a couple of minutes, a large intense fire suddenly appeared on the south side of the west 

face on floor 104 in an area well above any other apparent fire. This unusual jump in fire location is 

difficult to explain, but is likely associated with vertical shafts located in the core of the tower. 

For most of the time following the plane strike, no fire was observed on any of the floors on the south 

face over lengths extending from the eastern edge of the tower to near the center of the face. Fires were 

not observed in this region of the building until around 10:00 a.m. By the time this tower collapsed 

roughly 25 minutes later, intense fires extending over significant lengths of the originally uninvolved area 

were burning on floor 94 to floor 98 in this area. 

A final example of rapid fire spread and growth in WTC 1 was described previously in the May 2003 

Progress Report for the Investigation (NIST 2003). In this case, a line of smoke appeared suddenly over 

a significant length of floor 92 on the north face of WTC 1 at 10:18:48 a.m., or roughly 9 minutes before 

the collapse of the tower. Puffs of smoke were observed simultaneously on the north face from floors 94, 

95, and 97. More isolated puffs were seen at the same time from floor 92 and floor 95 on the west face 

and from floor 92 on the south face. Very shortly (seconds) after the appearance of the smoke, a localized 

fire on floor 95 to the west of the plane strike location grew very rapidly and flames erupted from 

windows. Following the smoke release, a large fire began to spread rapidly across the western side of 

floor 92 on the north face. Previous to the appearance of the smoke, only small fires were evident on this 

floor. By the time the tower collapsed, this fire had spread across most of the floor and had reached the 

western wall. This fire was responsible for the large burst of flame from the north face observed when 

this tower collapsed. 

H.6.2 WTC 2 

The fire behavior observed in WTC 2 was qualitatively different than occurred in WTC 1. Intense 

fireballs were created by the released jet fuel on the south, east, and north faces immediately after the 

airplane struck the building. As observed for WTC 1, the fireballs were followed by a brief period (on the 

order of a minute) of intense flaming from windows over a large area of the building. Most of these 

flames then “damped down” as observed in WTC 1, but two regions of intense burning remained. One of 

these areas was located on floor 81 and floor 82 at the northeast corner of the tower. Flames were evident 

from windows on either side of the corner as well as the corner itself, which had become exposed by 

removal of the corner facing during the plane strike. This area is in the vicinity of large piles of debris 

formed during the plane strike. The second fire was located primarily on floor 79 just to the left of the 

center (roughly from windows 79-231 to 79-238) of the north face. This is in the area of the second 

debris pile described earlier. Both of these fires died slowly with time when compared to fires at other 

locations in WTC 1 and WTC 2. Both were still burning lightly when the tower fell 56 minutes after the 

plane strike. 

A curious aspect of the fire behavior is the existence of an area of the building façade between these two 

fire locations on the north face where very little fire and/or smoke was observed before the tower 

collapsed. This area is roughly rectangular in shape, covering floor 80 to floor 82 and extending across 

windows 249 to 239. Infrared images recorded shortly following the plane strike showed that this region 

was quite cool relative to other sections close to the fires. This area will be referred to as the “cold spot.” 

Spreading fires seemed to move around this cold spot. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–37

In general, the fires in WTC 2 appeared to be less active than those observed in WTC 1. The fires 

covered a smaller area of the façade and did not spread as quickly. This is true even when the shorter 

time between the plane strike and collapse for WTC 2 (1 hour 42 minutes for WTC 1 and 56 minutes for 

WTC 2) is taken into account. Nevertheless, there was significant fire spread, and instances of rapid fire 

growth similar to those seen in WTC 1 did take place. 

Around 9:29 a.m., large flames and heavy smoke erupted from an area on the north face just to the right 

of the cold spot (around window 83-236) on floor 83. Four minutes and forty-one seconds later, flames 

suddenly appeared at a separate location on the same floor further to the right near window 83-226. 

Another area of fire formed just to the right of the cold spot on floor 82 around 9:54 a.m. or 5 minutes 

before the collapse. The fires on floor 79 of the north face also spread towards the west, approaching the 

western edge of the tower just prior to the collapse. 

Initial fire growth on the east face was on floor 82. Around 9:12 a.m., flames could be seen in nearly half 

of the windows on this floor, and heavy smoke was pouring from additional windows. Only limited fire 

was evident on lower floors at this time. The fires on floor 82 grew smaller after this time, and most were 

no longer visible when the tower collapsed. Around 9:35 a.m., heavy flames and smoke appeared over 

large areas of floor 79 and floor 80. These fires abruptly died down 45 seconds later, before growing 

back slowly during the remainder of the time before the tower collapsed. 

In the early period following the plane strike, fire growth on the south face was seen primarily on floor 81 

with active fires present on both sides of the airplane strike location. Smaller isolated fires were present 

on other floors around the area damaged by the airplane. These fires were relatively quiet and stationary 

until just prior to the collapse. At 9:56 a.m., there was a sudden release of smoke along much of floor 80 

extending from the area of the plane strike to near the western edge. During the next 2 minutes, an 

intense fire developed covering approximately windows 81-441 to 81-454. 

No smoke or fire was observed near the floors struck by the airplane on the west face of WTC 2. Some 

smoke was apparent at windows higher on the face. This was most likely coming from windows broken 

by occupants located on these floors. 

H.7 EVIDENCE FOR COLLAPSED FLOORS IN WTC 2 

H.7.1 Hanging Objects 

In the May 2003 Investigation Progress Report (NIST 2003), a photograph was shown in which there 

appeared to be a floor draped across a number of windows extending roughly from 310 to 342 across the 

east face of floor 82 of WTC 2. Figure H–22 compares an image taken shortly after the plane strike at 

9:03 a.m. and one taken at 9:55 a.m. shortly before the tower collapsed. At the earlier time, the hanging 

object is already present, but is seen through the windows draped much higher on the floor 82. An 

interpretation consistent with these observations is that floor 83 along the east side of WTC 2 was 

partially collapsed over a significant fraction of its length by the passage of the plane through the 

building. At the later time the floor has sagged further. By reviewing a number of photographs and 

videos, it has been determined that the change in floor position occurred between 9:34 a.m. and 9:38 a.m. 
Appendix H 

H–38

Figure H–22. Images of the east face of WTC 2 taken shortly after the plane struck and 

shortly before the tower collapse are shown. An object that is most likely a floor can be 

seen through windows on floor 82 as indicated by the arrows. The object has dropped 

lower between the times the two photographs were taken. 

Very similar objects, albeit of shorter length, are seen hanging in windows in images taken from the 

north. These objects are apparent in Fig. H–21 hanging below floors 81, 82, and 83. As seen through 

windows on floor 82 (corresponding to the floor 83), the floor appears to have split into at least two 

sections. 

H.7.2 Molten Material 

It has been reported in the FEMA report (McAllister 2002) as well as in the media that what appeared to 

be molten metal was observed pouring from the north face near the northeast corner. This is the area 

where the sustained fires were seen. Video records and photographs indicate that the material first 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–39

appeared at 9:51:52 a.m. and continued to pour intermittently from the building until the time of collapse. 

Some of the material can be seen falling in Fig. H–21. Close-up video and photographs of the area where 

the material is pouring from have been examined and show that it is falling from near the top of 

window 80-256. The most likely explanation for this observation is that the material had originally 

pooled on the floor above, that is, floor 81, and that it was allowed to pour out of the building when this 

floor either pulled away from the outer spandrel or sank down to the point where the window was 

exposed. The fact that the material appears intermittently over a several minute period suggests that the 

floor was giving way bit by bit. 

The composition of the flowing material can only be the subject of speculation, but its behavior is 

consistent with it being molten aluminum. Visual evidence suggests that significant wreckage from the 

plane passed through the building and came to rest in the northeast corner of the tower in the vicinity of 

the location where the material is observed. Much of the structure of the Boeing 767 is formed from two 

aluminum alloys that have been identified as 2024 and 7075 and closely related alloys. These alloys do 

not melt at a single temperature, but melt over a temperature range from the lower end of the range to the 

upper as the fraction of liquid increases. The Aluminum Association handbook (Aluminum Association 

2003) lists the melting point ranges for the alloys as roughly 500 °C to 638 °C and 475 °C to 635 °C for 

alloys 2024 and 7075, respectively. These temperatures are well below those characteristic of fully 

developed fires (ca. 1,000 °C), and any aluminum present is likely to be at least partially melted by the 

intense fires in the area. 

H.8 PROGRESS ON COLLECTION OF IMAGES AND ANALYSIS FOR WTC 7 

Visual material is also required to characterize the initial damage to, fire spread in, and collapse behavior 

of WTC 7. Considerable useful material has been collected, but the visual record for times between the 

collapses of WTC 1 and WTC 7 is much less complete than those for the two towers. The reasons for this 

are easy to understand. Following the collapses of the towers, most people were focused on escape or 

rescue. A large dust cloud was formed by the collapses, and fires developed that generated large amounts 

of smoke. Both tended to obscure views of WTC 7, particularly from the south due to the northwesterly 

wind direction on September 11, 2001. 

Both photographs and videos have been included in the database that show fires and damage to the east, 

north, and west faces of WTC 7. Some of this material has been timed, but in general the record is 

insufficient to allow generation of a complete time line of fire behavior for the relevant period. Numerous 

images show the upper portion of WTC 7 from the south, but the actual face of the building is generally 

obscured by smoke. No clear images of the lower portion of the south face have been obtained despite a 

careful search and repeated appeals for the public’s help. This is particularly unfortunate since most of 

the damage caused by the collapses of the towers, and particularly WTC 1, should have occurred on this 

face. 

There is considerable interest in images showing the collapse of WTC 7. Currently, there are at least four 

videos in the database that include the collapse, primarily from northerly directions, as well as several 

photographs. While not ideal, these are providing adequate information for characterizing the collapse 

sequence, and some progress along these lines has been made. 
Appendix H 

H–40

An effort has begun to map out the same information concerning fires, smoke, and windows as in the 

towers using visual material in the database. This effort will continue with a goal of mapping out as much 

of the fire time line as possible based on the material in the database. 

H.9 SUMMARY WITH KEY FINDINGS 

This section provides a brief summary of progress on the collection and analysis of visual data along with 

key findings. 

The approaches used to identify and obtain visual material related to the WTC disaster are described 

along with the approaches employed by NIST to archive and catalog the material. Material is either saved 

in its original digital format or digitized and saved, and a commercial software package has been used to 

provide data entry, a searchable database, and ready access to assets for review. The large numbers of 

attributes used to characterize the photographs and videos are included. 

Separate databases are provided for photographic and video materials. A major effort has focused on 

assigning accurate times to the material, and the approaches used are summarized. In excess of 

6,700 photographs and 6,900 video clips have been included in the databases and 45 percent and 

39 percent, respectively, of these have assigned times accurate to 3 seconds or better. 

Major events timed to an accuracy of 1 second are: 

• First plane strike on WTC 1: 8:46:30 a.m. 

• Second plane strike on WTC 2: 9:02:59 a.m. 

• Collapse of WTC 2: 9:58:59 a.m. 

• Collapse of WTC 1: 10:28:25 a.m. 

• Collapse of WTC 7: 5:20:52 p.m. 

An approach has been developed to characterize the observed fire behaviors at the periphery of the 

buildings on a window-by-window basis by determining whether windows are open or closed and 

whether smoke and/or fire are observed. If smoke is present, it is characterized as “light” or “heavy”, and 

fires are characterized as “spot” (a small local fire), “fire inside,” and “external flaming.” The 

observations are coded in separate electronic spreadsheets for each building, façade, and time. 

Two approaches are used to visualize the fire-related parameters. The first is a Web-based application 

that displays single sides of the towers at a single time. The second is a time-dependent threedimensional representation based on Smokeview (Forney and McGrattan 2003; Forney, Madrzykowski, 

McGrattan, and Sheppard 2003). 

Photographs and videos have been used to characterize several aspects related to the plane strikes on the 

towers and the distribution of damage on the external faces. For WTC 1, locations where the ends of the 

wings and vertical stabilizer of the tail section struck the north face and the damage to the steel façade are 

mapped. The behavior of fireballs generated by the release of fuel as a result of the collision of the 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–41

aircraft with the tower and initial tower damage as reflected in broken windows is used to characterize the 

distribution of damage to the facades of the tower. In addition, it has been shown that an exterior panel 

section from the south face was dislodged and landed on the ground. It contained an aircraft wheel that 

passed through the tower. 

The following conclusions are reached concerning the immediate effects of the plane strike on WTC 1: 

• The airplane struck columns on the north face ranging from 109 to 152 and covering floor 93 

to floor 99. 

• Damage and initial fire growth were greater on the east face of the tower than on the west. 

Significant damage and early fire growth occurred on the west side of the south face, but not 

on the east side. 

• A three-story panel section was knocked from the south side of the tower and had an aircraft 

wheel lodged in window 95-329. 

Visual evidence related to the plane strike on the south face of WTC 2 is more extensive than for WTC 1. 

This has allowed additional analyses beyond the mapping of damage on the plane strike face and façade 

damage to the remaining faces. The following conclusions have been reached concerning the immediate 

effects of the plane strike on WTC 2: 

• The aircraft struck the tower with a measured speed of 545 miles per hour ± 18 miles per 

hour. 

• The collision of the aircraft caused a measurable sway of the tower that lasted more than 

4 minutes. The period of oscillation was 11.3 seconds. 

• The airplane struck columns on the south face ranging from 404 to 443 and covering floor 77 

to floor 85. 

• Large areas of the façade were removed and/or damaged along the east face of the tower and 

on the eastern side of the north face. No façade damage or window breakage was evident on 

the west face. 

• Debris piles are observed in the northeast corner of the tower primarily on floors 80 and 81. 

Debris is also evident towards the center of the north face on the floor 79. 

• Column 253 on the north face is broken on the floor 81 and the column 254 is severely 

distorted. 

Detailed maps for fire behavior are currently being made. This update characterizes general fire 

behaviors for the two towers and notes some particularly interesting observations. For WTC 1, the 

following observations are highlighted. 

• Extensive fires observed immediately following the plane strikes and which are most likely 

associated with released jet fuel damped down after roughly 60 seconds. 
Appendix H 

H–42

• In the period following the plane strikes fires tended to reappear over a period of many 

minutes. Initial fire growth was principally observed on floor 96 and floor 97 on the north 

face, floor 94 and floor 97 on the east face, floor 96 on the south face, and floor 97 on the 

west face. 

• Observed fire spread rates were quite variable. Examples of both relatively slow and very 

rapid apparent fire spread are described. 

• Interior walls at several locations were inferred to protect areas of the towers for a period of 

many minutes, though they were typically eventually breached by nearby fires. 

• Following the collapse of WTC 2, a large fire appeared and grew rapidly on the west face at 

floor 104. 

• There was an extensive area of the façade on the eastern side of the south face for which no 

fire was observed until at least 1 hour following the plane strike. When fires finally did 

appear in this area their growth was rapid over multiple floors. 

• A large amount of smoke was suddenly released from floor 92 on the north face at 

10:18:40 a.m. Smoke was expelled simultaneously from other floors and faces. Immediately 

after the smoke release rapid fire growth was observed at an isolated location on floor 95 and 

across much of the west side of the north face on floor 92. 

Observed fire behaviors for WTC 2 were somewhat different than for WTC 1. This is true even when the 

differences in times between plane strikes and collapses (1 hour and 42 minutes for WTC 1 and 56 

minutes for WTC 2) are considered. The following observations concerning fire behavior in WTC 2 are 

emphasized: 

• Extensive fires observed immediately following the plane strikes and most likely associated 

with released jet fuel damped down after roughly 60 seconds. 

• Two regions of intense fire remained following the initial fire period due to jet fuel burning. 

These fires were located on floor 81 and floor 82 in the northeast corner and towards the 

center of the north face on floor 79. These fires burned for longer periods than observed 

elsewhere in WTC 1 and WTC 2. They are located in regions of the tower where debris piles 

are observed. 

• No large fires were observed in a multi-floor region on the north face located between the 

two fire areas described in the last bullet. 

• Initial fire growth in areas away from the sustained fires was along the east face of floor 82. 

Large fires did not appear on lower floors of this face until later and were sporadic in space 

and time. 

• Prior to the tower collapse, fire spread primarily from east to west was observed on floors 79, 

82, and 83 of the north face. 
Interim Report on Evolution of WTC Fires Based on Image Analysis 

H–43

• A sudden release of smoke from windows on the west side of the south face on floor 80 

occurred at 9:56:37 a.m. This was followed very shortly by the appearance of heavy fire. 

A number of photographs and videos show what appears to be floor 83 hanging across window openings 

over a large fraction of floor 82 on the east face of WTC 2. This object is observed very shortly after the 

plane strike and is found to drop lower prior to the tower collapse. On the north face, shorter lengths of 

what appear to be floors 81, 82, and 83 are seen hanging through the windows below. 

Starting around 9:52 a.m., a molten material began to pour from the top of window 80-256 on the north 

face of WTC 2. The material appears intermittently until the tower collapses at 9:58:59 a.m. The 

observation of piles of debris in this area combined with the melting point behaviors of the primary 

aluminum alloys used in the Boeing 767 suggest that the material is molten aluminum derived from 

aircraft debris located on floor 81. 

The visual record for the period following the collapses of the two towers is much less complete than 

prior to this time. In addition to the general chaos caused by the collapses, significant dust and smoke 

from fires started by the collapses obscured the site. As a result, it has not been possible to identify clear 

visual images showing the damage to the south face of WTC 7 caused by the collapses of WTC 1 and 

WTC 2. The number of videos and photographs showing fires on the east, north, and west faces of 

WTC 7 is limited and sporadic. The images that are available are being used to generate an approximate 

time line for fire growth and spread. 

H.10 REFERENCES 

Forney, G. P., and K. B. McGrattan. 2003. User’s Guide for Smokeview Version 3.1: A Tool for 

Visualizing Fire Dynamics Simulation Data. NISTIR 6980. National Institute of Standards and 

Technology. Gaithersburg, MD. 

Forney, G. P., D. Madrzykowski, K. B. McGrattan, and L. Sheppard. 2003. Understanding fire and 

smoke flow through modeling and visualization, IEEE Computer Graphics and Applications, vol. 23, 

p. 6-13. 

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, Preliminary 

Observations, and Recommendations. FEMA 403. Federal Emergency Management Agency. 

Washington, DC, May. 

NIST (National Institute of Standards and Technology). 2003. May 2003 Progress Report on the Federal 

Building and Fire Safety Investigation of the World Trade Center Disaster. NIST Special Publication 

1000-3. U.S. Department of Commerce, Washington, DC, May. 

The Aluminum Association. 2003. Aluminum Standards and Data - 2003. The Aluminum Association, 

Inc., Washington, DC. 
Appendix H 

H–44

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I–i

TABLE OF CONTENTS - Appendix I

List of Figures........................................................................................................................................... I–ii

List of Tables ............................................................................................................................................I–iv

Appendix I 

Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers––

Methodology.............................................................................................................................I–1

I.1 Introduction...................................................................................................................................I–1

I.2 Sensitivity of Thermal Response to Fireproofing Geometery ......................................................I–1

I.3 In-Place Conditions of Fireproofing before Impact......................................................................I–7

I.3.1 History of WTC Fireproofing............................................................................................I–7

I.3.2 Specified Thickness of Fireproofing .................................................................................I–8

I.3.3 As-Applied Thickness and Variability ..............................................................................I–9

I.3.4 Equivalent Thickness.......................................................................................................I–20

I.3.5 Thickness of SFRM for Use in Analyses ........................................................................I–23

I.4 Thermal Properties......................................................................................................................I–25

I.4.1 Thermal Conductivity Measurements .............................................................................I–25

I.4.2 Specific Heat Measurements ...........................................................................................I–26

I.4.3 Density Measurements ....................................................................................................I–27

I.5 Response to Impact.....................................................................................................................I–27

I.5.1 Mechanical Properties of SFRM .....................................................................................I–28

I.6 Summary.....................................................................................................................................I–31
I–ii

LIST OF FIGURES

Figure I–1. Model used to study effects of fireproofing thickness and variability of thickness on 

steel temperature ...................................................................................................................I–3

Figure I–2. Temperature distribution after 1 h of exposure to gas temperature of 1,100 °C 

(1,373 K). ..............................................................................................................................I–3

Figure I–3. Variation of steel temperature (at a point 6 in. from end of plate) with time for 

different average thicknesses of fireproofing........................................................................I–4

Figure I–4. Example of “gap” in fireproofing on diagonal member of a bridging floor truss.................I–5

Figure I–5. Effects of gap in fireproofing................................................................................................I–6

Figure I–6. Example of plot matrix from sensitivity study of the effects of missing fireproofing 

and variability of fireproofing thickness on steel temperature..............................................I–7

Figure I–7. Dotplot of average thickness from floor trusses for floors 23 and 24, histogram of 

average thickness, normal probability plot of average thickness, histogram of 

natural logarithm of average thickness, and probability plot of natural logarithm of 

average thickness. ...............................................................................................................I–12

Figure I–8. Example of measurement procedure used to estimate fireproofing thickness from 

photographs.........................................................................................................................I–14

Figure I–9. Normal probability plot of estimated fireproofing thickness based on photographs, 

and normal probability plot of natural logarithms of thickness. .........................................I–16

Figure I–10. Dotplot of individual thickness measurements on floor trusses from Port Authority 

Construction Audit Reports, histogram of thickness measurements, normal 

probability plot of thickness measurements, histogram of natural logarithms of 

thickness measurements, and normal probability plot of natural logarithm of 

thickness measurements......................................................................................................I–18

Figure I–11. Fireproofing thickness on floor trusses in upgraded portions of WTC towers ...................I–19

Figure I–12. Individual and average thickness for core columns, normal probability plot of 

individual measurements on columns, individual and average thickness for core 

beams, and normal probability plot of individual measurements on beams. ......................I–20

Figure I–13. Randomly generated thickness profiles with average thickness of 0.75 in. and 

standard deviation of 0.3 in., cumulative element size, and deformation of 1 in. bar 

compared with deformation for uniform thickness of fireproofing. ...................................I–22

Figure I–14. Randomly generated thickness profiles with average thickness of 2.5 in. and 

standard deviation of 0.6 in., normal probability plots of thickness values, and 

deformation of 1 in. bar compared with deformation for uniform thickness of 

fireproofing. ........................................................................................................................I–24

Figure I–15. Preliminary test results: thermal conductivity as a function of temperature, and 

specific heat as a function of temperature...........................................................................I–26

Figure I–16. Pull-off test of SFRM applied to steel plate........................................................................I–29
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers––Methodology 

I–iii

Figure I–17. Derivation of acceleration to dislodge SFRM from planar substrate..................................I–29

Figure I–18. Derivation of acceleration to dislodge SFRM surrounding a round bar. ............................I–30 
I–iv

LIST OF TABLES

Table I–1. Specified passive fire protection. ..........................................................................................I–9

Table I–2. Average fireproofing thickness from six measurements taken in 1994 on each of 

16 random floor trusses on floors 23 and 24 of WTC 1......................................................I–11
I–1

Appendix I

INTERIM REPORT ON ASSESSMENT OF SPRAYED FIREPROOFING IN 

THE WTC TOWERS––METHODOLOGY

I.1 INTRODUCTION 

The structural steel in the World Trade Center (WTC) towers was “fireproofed” with sprayed fire 

resistive materials (SFRMs). These materials are packaged as dry ingredients, and water is added by a 

pressurized system as the materials are sprayed onto the steel. The water mixes with the cementitious 

materials and provides “stickiness” that allows the SFRM to adhere weakly to the steel. With time, the 

cementitious materials harden, and excess water evaporates. When dry, SFRMs provide an insulation 

barrier to reduce the vulnerability of the steel to excessive temperature rise during a fire. 

Analysis of the effects of the fires on the structural capacity of the damaged WTC towers as a function of 

time requires knowledge about the condition of fireproofing on the various structural components, 

namely, the exterior columns, the spandrel beams, the floor trusses, and the core columns. Because of the 

method of application, sprayed fireproofing will have variable thickness, especially when applied to long, 

thin elements such as the diagonals and chords of the floor trusses. In addition, fireproofing was 

dislodged during the impact, either from direct impact by debris or from vibrations of the members. The 

thermal properties of the fireproofing also need to be known as a function of temperature. 

The thermal-structural analysis of the WTC towers focused on two objectives: (1) analysis of the 

undamaged buildings exposed to postulated fires, and (2) analysis of damaged buildings exposed to the 

fires that occurred after impact. In order to reduce the uncertainties in the calculated thermal histories of 

various structural elements, the condition of the sprayed fireproofing as it existed on September 11, 2001, 

needs to be estimated as accurately as possible. In addition, reasonable estimates of the extent of 

fireproofing dislodged by the aircraft are needed. This appendix discusses the approach that will be used 

for this purpose. 

To gain an understanding of the effect of fireproofing thickness and its variability on the steel temperature 

during exposure to fire, a simple finite-element model was used for a sensitivity study. The information 

gained from that study is reviewed first. A brief summary of the construction history of the sprayed 

fireproofing in WTC 1 and WTC 2 is presented. This is followed by a quantitative assessment of in-place 

thickness and its variability based on available data. The rationale for the thickness of fireproofing to be 

used in the structural fire endurance analyses is presented. The tests conducted to determine the thermal 

properties of fireproofing materials similar to those used in the WTC towers are reviewed. The approach 

used to gain an understanding of the inherent fragility of sprayed fireproofing is discussed, and the 

scheme for estimating the extent of damage during impact is summarized. 

I.2 SENSITIVITY OF THERMAL RESPONSE TO FIREPROOFING GEOMETERY 

The fireproofing thickness has a great effect on the thermal response of the structural elements for a given 

fire condition. While others have considered the effect of thickness of fireproofing, the effect of the 
Appendix I 

I–2

variation of thickness along the length of a member is not well known. A sensitivity study using finite 

element modeling of heat transfer was conducted to investigate the sensitivity of steel temperature to the 

variability in fireproofing thickness. 

The simplified model that was used is shown in Fig. I–1. A 1 in. thick, 60 in. long steel plate (cyan color) 

was coated with fireproofing material (purple color) and subjected to the uniform radiative flux arising 

from a 1,100 °C fire. As shown in Fig. I–1 (b), the fireproofing is modeled with a layer of finite elements 

(0.125 in. thick and 0.6 in. long) having the thermal properties of fireproofing (purple). A parametric 

study was conducted with average thickness of fireproofing varying from 0 in. to 2 in. in increments of 

1/4 in. The effect of variability in thickness was modeled by imposing a normal probability distribution 

on the fireproofing thickness along the length of the steel plate. The assumed standard deviation varied 

from 0 in. (uniform thickness) to 1 in. A psuedo-random number generator was employed to determine 

the thickness at each cross section based on the assumed average thickness and standard deviation. The 

layer representing fireproofing was taken to be twice the average thickness, and the thickness of 

fireproofing at any cross section was modeled by assigning a low heat capacity and a high thermal 

conductivity to those elements that do not provide fireproofing. Figure I–1 (c) shows an example of 

variable thickness fireproofing; in this case, the average thickness is 1 in. and the standard deviation is 

3/8 in. 

When the model in Fig. I–1 is exposed to the thermal flux representing an 1,100 °C fire, the surface of the 

insulation heats up quickly to the gas temperature (1,100 + 273 = 1,373 K). Numerical simulation was 

performed over a 2-h period, and the steel temperature at five locations was recorded at 30 min, 60 min, 

90 min, and 120 min of exposure. The temperature recording locations are 6 in. from each end and at 

12 in. intervals, which are shown as numbers 1 to 5 in Fig. I–1 (a). The initial temperature of the model is 

300 K. 

Figure I–2 shows temperature contours (in K) through the fireproofing and steel at 60 min after initial 

exposure for the model shown in Fig. I–1 (a). The fireproofing surface temperature is close to the gas 

temperature of 1,373 K, while the steel temperature is 311 K. If the fireproofing were of uniform 

thickness, the isotherms would be a series of lines parallel to the plate. It is seen that, when the thickness 

of fireproofing is variable, the isotherms follow the shape of the fireproofing surface contour. Thus, the 

temperature history at any point in the steel depends on the loc al thickness of the fireproofing. 

Figure I–3 shows the steel temperature at the far sensor #1 (6 in. from the end) as a function of time for 

various insulation thicknesses ranging from 0 in. to 2 in. (the thickness is indicated by the numbers on the 

curves). For the case in Fig. I–3 (a), the fireproofing is of uniform thickness, and for the cases in 

Fig. I–3 (b), the thickness varies with a standard deviation of 1 in. The time to reach a temperature of 

600 °C is used as a measure of relative performance. It is seen that the presence of high variability in 

thickness has a detrimental effect of the protection provided by the fireproofing. For example, for a 

uniform thickness of 0.5 in., it takes about 60 min for the steel at point #1 to reach 600 °C; but when the 

standard deviation of the thickness is 1 in., the average thickness has to be 1.75 for the same level of 

thermal protection. 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–3

Figure I–1. Model used to study effects of fireproofing thickness and variability of 

thickness on steel temperature: (a) physical model used in analyses (points 1 to 5 are 

locations where temperatures are monitored), (b) finite element mesh used to represent 

physical model, and (c) finite element model to represent variable thicknessof 

fireproofing (purple) (the elements in red represent material of high thermal 

conductivity). 

Figure I–2. Temperature distribution after 1 h of exposure to gas temperature of 

1,100 °C (1,373 K). 

(a) 

2 in.

2 in.

60 in. 

1 in. thick steel plate Fireproofing 

1 2 3 4 5

(b) 

(c) 
Appendix I 

I–4

Figure I–3. Variation of steel temperature (at a point 6 in. from end of plate) with time for 

different average thicknesses of fireproofing (shown as numbers on the curves): 

(a) uniform thickness, and (b) variable thickness with a standard deviation of 1 in. 

(a) 

(b) 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–5

In addition to the effect of variation in thickness, it is important to understand the effect of missing 

fireproofing over a portion of a member. As an example, Fig. I–4 shows missing fireproofing from a 

diagonal of a bridging truss of the WTC towers floor system. Figure I–5 (a) shows an example of a 

numerical model with missing fireproofing. In this case, there is 12 in. of missing fireproofing on the 

steel plate, which is otherwise protected by 2 in. of uniform thickness fireproofing. Figure I–5 (b) shows 

the temperature contours (isotherms) at time 50 min. For comparison, Fig. I–5 (c) shows isotherms at the 

same time in a plate with no gap in the fireproofing. As expected, the bare steel at the missing 

fireproofing is at the gas temperature, but more importantly the “gap” in fireproofing leads to a “leakage” 

of heat into the interior steel. 

Figure I–4. Example of “gap” in fireproofing on diagonal member of a bridging floor 

truss. 

The combined effects of variation in thickness of the fireproofing and length of missing fireproofing were 

examined by a factorial study with the following factors: 

• Average thickness of fireproofing varying from 0 in. to 2.0 in. in 1/4 in. increments; 

• Standard deviation of fireproofing thickness of 0 in., 0.25 in., 0.5 in., 0.75 in. and 1.0 in.; and 

• Length of missing fireproofing varying from 0 in. to 30 in., in 6 in. increments. 

Missing 

Fireproofing 
Appendix I 

I–6

Figure I–5. Effects of gap in fireproofing: (a) model of plate with fireproofing having 2 in. 

uniform thickness and 12 in. gap, (b) isotherms (K) at time = 50 min with 12 in. gap, and 

(c) isotherms without gap. 

The results of the sensitivity study can be summarized in a series of plot matrices, which show the time 

histories of the steel temperature for different combinations of gap length and variability in fireproofing 

thickness. For example, Fig. I–6 shows the plot matrix for the temperature history at point #2 (18 in. 

from the end of the plate). Each plot contains a series of curves representing different average thickness 

of fireproofing, as in Fig. I–3. Each column of plots represents a constant value of thickness variability 

(standard deviation), and each row represents a constant gap length. The plot in the upper left corner 

represents the case of uniform thickness of fireproofing and no gap, which is the same plot as in 

Fig. I–3(a). (Note that for the case of uniform thickness and no gap, the steel temperature at any point in 

a cross section is the same along the length of the plate, as shown in Fig. I–5(c).) For gaps of 24 in. and 

30 in., the temperature at point #2 rises rapidly because there is no fireproofing on the plate at that 

location. This explains the shapes of the curves in the two lower rows. In going from left to right in one 

of the top four rows it is seen that as variability of thickness increases, the time histories shift upward, 

thereby reducing the time to reach 600°C. This is the same observation as shown in Fig. I–3. Moving 

from the top to the bottom in any column shows the effects of increasing gap length. The effect of gap 

length depends, of course, on where the steel temperature is measured. At a point within the portion of 

steel that is bare, the temperature rises quickly. At points within the steel that are surrounded with 

fireproofing, the gap provides a path for heat flow, as shown in Fig. I–5 (b). As a result, points in the 

steel within the vicinity of the missing fireproofing will experience higher temperatures, as indicated by 

the rising trend of the curves in going downward from the top of a column in Fig. I–6. The National 

(a) 

1 2 3 4 5 

(b) 

(c) 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–7

Institute of Standards and Technology (NIST) does not have sufficient information to determine the 

frequency of occurrence of these gaps or their typical locations. Therefore, gaps in fireproofing will not 

be considered in the thermal modeling. 

Figure I–6. Example of plot matrix from sensitivity study of the effects of missing 

fireproofing and variability of fireproofing thickness on steel temperature. Each graph is 

a temperature history of the steel at point #2 

(see Fig. I–5) for different thicknesses of fireproofing. 

I.3 IN-PLACE CONDITIONS OF FIREPROOFING BEFORE IMPACT 

I.3.1 History of WTC Fireproofing 

In Appendix 4 of the May 2003 Progress Report (NIST SP 1000-3), the history of the sprayed 

fireproofing, as reconstructed from available documentation, was reviewed. Basically, the following 

significant activities took place: 

• 1969: Decision made to use 1/2 in. of CAFCO BLAZE-SHIELD Type D (asbestos fibers) 

sprayed fireproofing. 
Appendix I 

I–8

• 1970: Use of CAFCO BLAZE-SHIELD Type D was discontinued at the 38th floor of 

WTC 1; remaining fireproofing to use CAFCO BLAZE-SHIELD Type DC/F (mineral wool 

fibers). 

• 1994: Thickness measurements of fireproofing on trusses of floors 23 and 24 of WTC 1. 

• 1995: Port Authority performed study to establish sprayed fireproofing thickness for tenant 

alterations. 

• 1999: Port Authority established guidelines for fireproofing repairs, replacement, and 

upgrades. 

• Late 1990s: Floor truss fire protection upgraded to 1 1/2 in. during tenant alterations using 

CAFCO BLAZE-SHIELD Type II. In-place measurements of thickness, density, and bond 

strength recorded. 

I.3.2 Specified Thickness of Fireproofing 

WTC project specifications for spray-applied fireproofing do not provide required material thickness or 

hourly ratings. However, a letter dated October 30, 1969, from Robert J. Linn (manager, Project 

Planning, WTC) to Mr. Louis DiBono (Mario & DiBono Plastering Co., Inc.) states, in part: 

...Tower “A” columns that are less than 14WF228 will require 2 3/16′′

thick of ‘Cafco [B]laze-Shield Type D’ spray-on Fireproofing. All 

Tower columns equal to or greater than 14WF228 will require 1 3/16′′ of 

fireproofing... 

All Tower beams, spandrels and bar joists requiring spray-on 

fireproofing are to have a 1/2′′ covering of “Cafco.” 

No reference is made in this letter to the required thickness of fireproofing of core box columns or 

exterior built-up columns. 

Alcoa was the supplier of the aluminum cladding on the exterior columns (Contract WTC 400.00), and 

the following “Note 11” was included among the “General Notes” of their drawings: 

11. Exterior column and spandrel fireproofing–CAFCO BLAZE 

SHIELD Type D Fireproofing. Interior column and spandrel 

fireproofing–Vermiculite plaster aggregate fireproofing with finished 

plaster coat on exposed areas of columns. (3 hr on spandrels, 4 hr on 

columns) 

Fireproofing Thickness 

Rating Cafco Vermiculite Aggregate 

4 hr (heavy column) 1 3/16′′ 7/8′′

3 hr (spandrels) 1/2′′ 1/2′′
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–9

In 1995, the Port Authority performed a study to establish the requirements for applying spray-on 

fireproofing to the floor trusses in the case of new construction (alterations conducted when tenants 

vacated the space) in the towers. The study estimated the fireproofing requirements for the floor trusses 

of the towers based on “the fireproofing requirements for Design No. G805 contained in the Fire 

Resistance Directory” of Underwriters’ Laboratories. The study concluded that 1 1/2 in. of spray-on 

mineral fiber fireproofing, “when applied directly to the chords and web members,” was sufficient to 

achieve the required 2 h rating for the floor trusses. In the years between 1995 and 2001, fireproofing 

was upgraded in a number of the floors affected by the fires on September 11, 2001. 

The specified fire protection is summarized in Table I–1. 

Table I–1. Specified passive fire protection. 

Structural Component Member Size Location Material Thickness (in.) 

Floor trusses All NA CAFCO DC/F 1/2 

Interior columns < 14WF228 NA CAFCO DC/F 2 3/16 a

≥ 14WF228 NA CAFCO DC/F 1 3/16 

“heavy” Exterior 

faces 

CAFCO DC/F 1 3/16 Exterior columns 

“heavy” Interior faces Vermiculite 

aggregate 

7/8 

Spandrel beams All Exterior face CAFCO DC/F 1/2 

All Interior face Vermiculite 

aggregate 

1/2 

a. No thicknesses specified for core beams and box columns. 

Key: NA, not applicable. 

In a letter dated July 25, 1966, from Emery Roth and Sons to the Port of New York Authority, it is stated 

“Since the deck is non-structural it will not be fire proofed.” Photographs show that in some areas the 

underside of the metal deck was indeed not fireproofed, while in other areas fireproofing appears to be 

present but of undetermined thickness and possibly resulting from overspray. Photographs reveal that the 

dampers and damper saddles were not fireproofed. Additionally, it is unclear whether the bridging trusses 

were required to be fireproofed in all areas. Subsequent to the design and construction of the WTC 

towers, some information has been found that further describes the elements of the structural systems that 

were indeed fireproofed. 

I.3.3 As-Applied Thickness and Variability 

The actual thickness of a spray-applied fire protection material generally exceeds the specified thickness 

by some amount. Since both towers collapsed on September 11, 2001, and most of the fireproofing was 

either dislodged or abraded (or scraped) off in the collapse, no examples remain of the “as installed” 

condition of the fireproofing. To make an estimate of the as-applied thickness and variability in 

thickness, several sources of information have been employed, including measurements taken by the Port 

Authority, condition surveys and anecdotal information, and photographs and video clips showing the 

condition of the fireproofing in selected areas. Each of the structural components or systems is 

considered here separately. 
Appendix I 

I–10

Steel Truss-Supported Floor System 

Qualitative information on the “as installed” fireproofing thickness for the floor system first appears in 

Sample Area Data Sheets from 1990, in which comments on the state of the in-place fireproofing were 

recorded. As an example, the data sheet for floor 29 of WTC 1 states the following for the South West 

quadrant of the floor: 

Fluffy spray-on fireproofing coating the support beams, joists, and deck 

above the ceiling. The thickness of the material on the beams and joists 

was consistently about 1/2′′ Regarding the deck it ranged from very 

sparce [sic] in areas to 1/4′′ other areas. 

Similar statements were recorded for the remaining quadrants of the floor. 

Information regarding quantitative inspection of existing fireproofing appears in documentation from 

1994. That year, the Port Authority performed a series of thickness measurements of the existing 

fireproofing on floors 23 and 24 of WTC 1. Six measurements were taken from “both flanges and web” 

of each of 16 randomly chosen trusses on each floor at those locations where the fireproofing was not 

damaged or absent. 

The averages of six measurements per joist that were recorded on the two floors are presented in 

Table I–2. Measured average thickness varied between 0.52 in. and 1.17 in. For the 32 measurements 

(16 on each floor), the overall average was 0.74 in. and the standard deviation of these averages was 

0.16 in. Four of the 32 floor trusses, had an average thicknesses between 0.52 in. and 0.56 in. These 

measurements suggest that the minimum average thickness exceeded 1/2 in. 

This same report stated that, on floor 23, 

... truss members located adjacent to the outside walls (within 3 ft) are 

devoid of fireproofing material. Visual inspection on floor 24 was not 

possible, as this area still has a lowered ceiling in place. 

The data in Table I–2 can be examined further to understand the variability of the fireproofing thickness 

in the non-upgraded locations. Figure I–7 (a) shows the average thicknesses measured on the floor 

trusses of floors 23 and 24. The values appear to be similar for the two locations in terms of overall 

average thicknesses and the variation in average thickness. A formal analysis of variance indeed 

indicated no statistically significant differences between the overall mean thicknesses for the two floors. 

Thus, the two groups of data can be combined into one. A question to be answered is whether the values 

of average thickness follow a normal distribution. To answer this question, histograms and normal 

probability plots are used. Figure I–7 (b) shows a histogram of the average thicknesses, and it appears to 

be non-symmetrical and skewed to the right, which is characteristic of a lognormal distribution.1

 

Figure I–7(c) is the normal probability plot of the average thicknesses for the combined data. If the points 

fall approximately on a straight line, it indicates that the data are normally distributed. It is seen that there 

are systematic deviations of the data from the best-fit line. To examine whether the data are represented 

better by a lognormal distribution, the average thicknesses, in Table I–2 were transformed by taking their 

 

1

 In a lognormal distribution, the natural logarithms of the values of a variate have a normal distribution. 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–11

natural logarithm. Figure I–7 (d) is a histogram of the natural logarithms of thickness, and Fig. I–7(e) is 

the corresponding normal probability plot. It is seen that the data are less dispersed about the straighter 

line, and the correlation coefficient has increased form 0.97 to 0.99. Thus, there is some indication that 

the distribution of fireproofing thickness is lognormal in the non-upgraded floor trusses. 

Table I–2. Average fireproofing thickness from 

six measurements taken in 1994 on each of 

16 random floor trusses on floors 23 and 24 

of WTC 1. 

Fireproofing Thickness (in.) 

Floor 23 Floor 24 

0.60 0.76 

0.53 0.60 

0.70 0.90 

0.76 0.72 

0.88 0.64 

0.89 0.80 

0.83 0.68 

1.17 0.65 

0.88 0.67 

0.71 0.77 

0.82 0.96 

0.52 0.66 

0.69 0.65 

0.52 1.11 

0.64 0.95 

0.52 0.56 

Source: Data provided by Port Authority of New York and New 

Jersey. 

A lognormal distribution for the average thickness of the fireproofing on the non-upgraded floor trusses is 

explained as follows. It is expected that the thickness of fireproofing will be highly variable due to the 

difficulty in spraying the material on the relatively thin members. If the overall thickness is low and the 

variability is high, a normal distribution would require a fraction of the surfaces to have negative values 

of fireproofing. If the thickness distribution is lognormal, the thickness cannot be zero, and there is a low 

likelihood of having thickness close to zero. If the underlying distribution of fireproofing thickness is 

lognormal, the average thickness overestimates the thickness expected to be exceeded with 50 percent 

probability, and the median is the appropriate statistic for the 50 percentile value. 
Appendix I 

I–12

Figure I–7. (a) Dotplot of average thickness from floor trusses for floors 23 and 24, 

(b) histogram of average thickness, (c) normal probability plot of average thickness, 

(d) histogram of natural logarithm of average thickness, and (e) probability plot of 

 natural logarithm of average thickness. 

0

0.2

0.4

0.6

0.8

1

1.2

22 23 24 25

Thickness, in.

Floor

(a)

0

2

4

6

8

10

12

0.3 0.5 0.7 0.9 1.1 1.3

Count

Average Thickness, in.

(b)

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

.01 .1 1 5 10 20 30 50 70 80 90 95 99 99.9 99.99

y = 0.74187 + 0.1607norm(x) R= 0.97209 

Thickness, in.

Percent

(c)

0

1

2

3

4

5

6

7

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4

Count

Ln (Thickness)

(d)

-0.8

-0.6

-0.4

-0.2

0

0.2

.01 .1 1 5 10 20 30 50 7080 90 95 99 99.9 99.99

y = -0.32122 + 0.21269norm(x) R= 0.98754 

Ln (Thickness)

Percent

(e)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–13

As stated, the standard deviation of the average thicknesses in Table I–2 is 0.16 in. Since each of the 

averages is based on six individual measurements, the variability in average thickness is less than the 

variability of the fireproofing thickness on a given element. If it is assumed that the true average 

thicknesses of fireproofing at the truss locations represented in Table I–2 are the same, it is possible to 

estimate the variability of individual measurements from the following well-known relationship: 

n

S S X = (I.1) 

where: 

X S = standard deviation of the average thicknesses 

S = standard deviation of the individual thickness measurements 

n = number of measurements to obtain the average thickness 

Thus, an estimate of the standard deviation of the individual measurements is ≈ 4.0616.0 in. Since it 

is unlikely that there is no difference in average fireproofing thickness at different cross sections, the 

standard deviation of 0.4 in. is an upper limit for the variability of fireproofing thicknesses in the nonupgraded floor trusses on the basis of the information provide in Table I–2. 

Analysis of Photographs 

Additional data regarding the thickness of fireproofing has been gathered by evaluating photographic 

evidence. Although photographic evidence of the state of the fireproofing is limited, two groups of 

photographs have been located and used for estimating fireproofing thickness. 

The first group of photographs was provided to NIST by Morse Zehnter Associates and includes images 

of floor trusses from WTC 1 (floors 12, 22, 23, and 27) and WTC 2 (floor 26). From this group, only 

photographs from floors 22, 23, and 27 of WTC 1 were analyzed. Photographs provided by Morse 

Zehnter Associates were taken in the mid-1990s and illustrate the fireproofing conditions prior to the 

upgrade carried out by the Port Authority. Thus, fireproofing thickness on the photographed trusses 

should be at least 1/2 in. as specified by the Port Authority on October 1969. 

The second group of photographs, taken in 1998, was provided by Gilsanz Murray Steficek (consulting 

engineers). This group illustrates the state of fireproofing after the upgrade program that was initiated in 

1995. The photographs were of trusses for floor 31 and below in WTC 1. 

Selection of which photographed trusses were used to estimate thickness of fireproofing was based on 

clarity of fireproofing edges and whether a feature of known dimensions was present. Thus, only 

photographs where reference measurements could be performed were used. The general approach to the 

analysis involved the estimation of distances based on the computed reference length per pixel. The 

procedure is summarized as follows: 

• A feature of known dimension (based on construction drawings) that could be used as 

reference was located in the photograph. For example, the dimension of the bare vertical leg 

of a damper saddle was a dimension that could be obtained from shop drawings. 
Appendix I 

I–14

• In the photograph, the length of the reference dimension was measured in pixels. 

• The scaling factor of length per pixel was computed by dividing the known dimension in 

inches by the number of pixels. For example, if the vertical leg of the damper saddle was 

measured as 48.2 pixels in the photograph, and it is known that the actual size of the leg was 

3.13 in., the scaling factor would be 3.13 in./48.2 pixels = 0.065 in./pixel. 

• Only truss webs or struts (diagonal bar at end of truss) located near and in the same plane as 

the reference object were selected for analysis. This selection was made to minimize error 

due to perspective. 

• It was assumed that the fireproofing on web bars was applied evenly around the perimeter of 

the bar. Based on this assumption, a “virtual” centerline along the length of the bar was 

drawn in the photograph. 

• Lines were drawn perpendicular to the “virtual” centerline. The number of pixels along the 

lines from the “virtual” centerline to the edge of the fireproofing was determined from the 

cursor positions indicated by the software. Measurements were made at regularly spaced 

intervals to avoid bias. Figure I–8 is an example of a series of measurements made on a strut. 

Figure I–8. Example of measurement procedure used to estimate 

fireproofing thickness from photographs. 

• Each measurement in pixels was multiplied by the scaling factor (in./pixel) to estimate the bar 

radius plus fireproofing thickness. 

Bar Radius + Fireproofing 

Thickness 

Center Line

Reference Length 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–15

• The radius of the bar was subtracted to provide the estimate of the fireproofing thickness. 

It was observed that the estimated thickness of fireproofing in the non-upgraded floors tended to be larger 

for the webs of the main trusses. Hence estimates of fireproofing thickness were divided into three 

groups: 

• Webs of main trusses, 

• Webs of bridging trusses, and 

• Diagonal strut at the exterior wall end of the truss. 

No estimates of fireproofing thickness on top and bottom chords were possible using photographs. For 

the upgraded floors in WTC 1 that were included in the second group of photographs, only estimates of 

the thickness on the web bars of the main trusses were made. Figure I–9 (a) shows normal probability 

plots of the fireproofing thickness estimated from the photographs. It is seen that the points for the 

“upgraded” main trusses follow a generally linear trend, which indicates that the estimated thicknesses for 

the upgraded main trusses are approximately normally distributed. The estimated thicknesses from the 

non-upgraded floors, however, do not follow linear trends on the normal probability plot. Figure I–9 (b) 

shows normal probability plots of the natural logarithms of the thicknesses. The transformed values for 

the non-upgraded fireproofing now follow generally linear trends, which means that a lognormal 

distribution is more appropriate for the non-upgraded floors. This reinforces the observation noted in the 

previous section. Thus there is strong evidence that the original fireproofing thickness on the floor trusses 

follows a log normal distribution. 

The average, standard deviation, and coefficient of variation were computed for the total number of 

measurements in each of these groups. The results are summarized as follows: 

• Main trusses before upgrade: Average thickness 0.6 in., standard deviation = 0.3 in., and 

coefficient of variation = 0.5. 

• Bridging trusses before upgrade: Average thickness 0.4 in., standard deviation = 0.25 in., and 

coefficient of variation = 0.6. 

• Diagonal struts before upgrade: Average thickness 0.4 in., standard deviation = 0.2 in., and 

coefficient of variation = 0.5. 

• Main trusses after upgrade: Average thickness 1.7 in., standard deviation = 0.4 in., and 

coefficient of variation = 0.2. 

Port Authority Data on Upgraded Fireproofing on Trusses 

As discussed in the May 2003 Progress Report (NIST SP 1000-3), the Port Authority provided 

information on fireproofing thickness from tenant alteration Construction Audit Reports prepared in 1997 

to 1999. Those reports included average thicknesses of fireproofing at the “bottom of truss.” In 2004, the 

Port Authority provided NIST reports of the individual measurements for many of the average thicknesses 

in the Construction Audit Reports. With the individual measurements, it is possible to investigate the 
Appendix I 

I–16

Figure I–9. (a) Normal probability plot of estimated fireproofing 

thickness based on photographs, and (b) normal probability plot 

of natural logarithms of thickness. 

variation of thickness at a cross section of a truss member and the variation in average thickness from 

truss to truss. To permit such analyses, only those data having the same number of individual 

measurements at each cross section were used. This resulted in 18 data sets for WTC 1 (including 

floors 93, 95, 98, 99, and 100) and 14 data sets for WTC 2 (including floors 77, 78, 88, 89, and 92). 

An analysis of the individual measurements was carried out to determine the underlying distribution for 

the measured thicknesses. Figure I–10 (a) is a dotplot of the individual measurements in WTC 1 

(144 measurements) and in WTC 2 (112 measurements). It is observed that the central values and ranges 

0

0.5

1

1.5

2

2.5

.01 .1 1 5 10 2030 50 7080 9095 99 99.999.99

Main

Bridging

Strut

Main-Upgrade

Thickness, in.

Percent

(a)

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

.01 .1 1 5 10 2030 50 7080 9095 99 99.999.99

Ln Main

Ln Bridging

Ln Strut

Ln Main-Upgrade

Ln Thickness

Percent

(b)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–17

are similar for the two towers, and the two groups of measurements were combined into one group. 

Figure I–10 (b) is the histogram of the individual measurements, and Fig. I–10 (c) is the corresponding 

normal probability plot. A straight line fit to the normal probability plot shows a tendency of the points to 

deviate from the line. Figure I–10 (d) is a histogram of the natural logarithms of the individual thickness 

values, and Fig. I–10 (e) is the corresponding lognormal probability plot. A comparison of the probability 

plots shows that natural logarithms fall closer to a straight line. Thus, it appears that the thickness of the 

upgraded fireproofing on the floor trusses is described by a lognormal distribution. This contradicts the 

observation based on analysis of photographs from lower floors discussed in the previous section. The 

overall average thickness of the 256 individual measurements is 2.5 in. with a standard deviation of 

0.6 in. Thus, the average thickness on the upgraded upper floors appears to be greater than that estimated 

from photographs taken on upgraded lower floors. 

The overall standard deviation of 0.6 in. includes two contributions: (1) the variation of thickness at the 

cross section (within-truss variability), and (2) the variation of average thickness between trusses 

(between-truss variability). Figure I–11 shows these two components of the thickness variability for the 

two towers. Figures I–11 (a) and (c) show the within-truss variability, and Figs. I–11 (b) and (d) show the 

variation of average thickness of each truss. From analysis of variance, it was found that the within-truss 

standard deviation is 0.4 in., and the between-truss standard deviation is also 0.4 in. The within-truss 

standard deviation of 0.4 in. is similar to the standard deviation of the estimated individual thickness 

obtained from analysis of the photographs of upgraded main trusses. 

Column Fireproofing Thickness 

NIST requested that the Port Authority provide available information on the thickness of fireproofing for 

the exterior and interior columns of the WTC towers. Specifically, the request included the following: 

• The fireproofing material used and the thickness on the various plates comprising the exterior 

columns and spandrels. 

• The fireproofing material used and the thickness on core columns. 

• Confirmation that the wide flange column sections were protected with CAFCO 

BLAZESHIELD Type DC/F with specified thickness of 2 3/16 in. for sections smaller than 

14WF228 and 1 3/16 in. for 14WF228 and larger. 

• Information on in-place fireproofing thickness. 

The Port Authority replied that, due to inaccessibility of exterior columns and core columns, there were 

no recent records of fireproofing thickness for these elements. The only available measurements of 

fireproofing thickness were for beams and columns accessible within elevator shafts. The most complete 

data set included measurements on beams and columns taken within shaft 14/15 in WTC 1. These 

measurements were taken in April 1999 and included measurements from floor 1 to floor 45. The 

thicknesses were recorded to the nearest 1/8 in., with a few thicknesses recorded to the nearest 1/16 in. 

The columns included 10 to 18 replicate measurements, and the beams included 11 to 16 replicate 

measurements. 
Appendix I 

I–18

Figure I–10. (a) Dotplot of individual thickness measurements on floor trusses from Port 

Authority Construction Audit Reports, (b) histogram of thickness measurements, 

(c) normal probability plot of thickness measurements, (d) histogram of natural 

logarithms of thickness measurements, and (e) normal probability plot of natural 

logarithm of thickness measurements. 

1

1.5

2

2.5

3

3.5

4

4.5

5

0123

Thickness, in.

Tower

(a)

0

5

10

15

20

25

30

35

40

0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2

Count

Thickness, in.

(b)

1

1.5

2

2.5

3

3.5

4

4.5

5

.01 .1 1 5 10 2030 50 7080 9095 99 99.999.99

y = 2.5449 + 0.57398norm(x) R= 0.98583 

Thickness, in.

Percent

(c)

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Count

Ln (Thickness)

(d)

0.4

0.6

0.8

1

1.2

1.4

1.6

.01 .1 1 5 10 2030 50 7080 9095 99 99.999.99

y = 0.90833 + 0.2266norm(x) R= 0.99261 

Ln (Thickness)

Percent

(e)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–19

Figure I–11. Fireproofing thickness on floor trusses in upgraded portions of WTC 

towers: (a) individual measurements in WTC 1, (b) average thickness in WTC 1, 

(c) individual measurements in WTC 2, and (d) average thickness in WTC 2. 

Figure I–12 (a) shows the individual and average fireproofing thickness on the core columns. Analysis of 

variance indicated no statistically significant differences among the average values and all data were 

pooled together. The average thickness for the columns is 0.82 in., the standard deviation is 0.20, and the 

coefficient of variation is 0.24. The information from the Port Authority indicated that the “minimum 

thickness required” for the columns was 0.5 in. Figure I–12 (b) is the normal probability plot of the 

individual thickness measurements. Because most of the thicknesses were reported to the nearest 1/8 in., 

the points are staggered instead of uniformly distributed. The plot, however, shows that the points follow 

a linear trend, and it appears that the thickness of the fireproofing on the core columns could be described 

by a normal distribution. Figures I–12 (c) and (d) shows the corresponding plots for the thickness of 

fireproofing on the beams. The average thickness for the beams is 0.97 in., the standard deviation is 

0.21 in. and the coefficient of variation is 0.21. The information from the Port Authority indicated that 

the “minimum thickness required” for the beams was 0.75 in. 

0

0.5

1

1.5

2

2.5

3

3.5

92 94 96 98 100 102

Average Thickness, in.

Floor

(b) WTC 1

0

1

2

3

4

5

92 94 96 98 100 102

Thickness, in.

Floor

WTC 1 (a)

0

1

2

3

4

5

75 80 85 90 95

Thickness, in.

WTC 2 (c)

Floor

0

0.5

1

1.5

2

2.5

3

3.5

75 80 85 90 95

Average Thickness, in.

Floor

(d) WTC 2
Appendix I 

I–20

Figure I–12. (a) Individual and average thickness for core columns, (b) normal probability 

plot of individual measurements on columns, (c) individual and average thickness for 

core beams, and (d) normal probability plot of individual measurements on beams. 

As might be expected, the variation in thickness of fireproofing for the beams and columns is lower than 

the variation observed in the floor trusses. The planar surfaces of the beams and columns result in more 

uniform application of the sprayed fireproofing than for the slender truss members. This results in 

reduced differences in the average thickness of fireproofing on different members and less variability 

within a member. 

I.3.4 Equivalent Thickness 

The sensitivity study summarized in Section I.2 indicated that variation in the thickness of fireproofing 

reduced the “effective thickness” of the fireproofing. It would be impractical to attempt to account for the 

variation in fireproofing thickness in the thermal modeling by introducing variable thickness fireproofing 

in the finite-element models. As an alternative, it was decided to attempt to determine the “equivalent 

uniform thickness” of fireproofing that would result in the same thermo-mechanical response of a 

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

.01 .1 1 5 10 20 30 50 70 80 90 95 99 99.9 99.99

y = 0.96562 + 0.1999norm(x) R= 0.97146 

Thickness, in.

Percent

(d)

0.0

0.5

1.0

1.5

2.0

0 10 20 30 40 50

Individual

Average

Thickness, in.

Floor

(a) Columns

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

.01 .1 1 5 10 20 30 50 70 80 90 95 99 99.9 99.99

y = 0.82308 + 0.18921norm(x) R= 0.96692 

Thickness, in.

Percent

(d)

0

0.5

1

1.5

2

0 10 20 30 40 50

Individual

Average

Thickness, in.

Floor

(c) Beams
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–21

member as variable thickness fireproofing. An approach similar to the methodology described in 

Section I.2 was used to model a 1 in. diameter by 60 in. long bar with fireproofing and subjected to the 

heat flux arising from a 1,100 °C fire. The bar was subdivided into 0.6 in. long elements, so that there 

were 100 elements along the length of the bar. The thermal history of the bar was calculated, and that 

history was used to calculate the length change of the unrestrained bar under a tensile stress of 12,500 psi. 

The bar was assumed to be similar to the steel used in the floor trusses, and the temperature dependence 

of the coefficient of thermal expansion and the modulus of elasticity were based on NIST measurements. 

The fireproofing thickness in the models was based on the measurements summarized in the previous 

section for the web bars of main trusses in the original condition and after the upgrade. Specifically, the 

following target values were investigated: 

• Original: average thickness = 0.75 in., standard deviation = 0.3 in., lognormal distribution. 

• Upgrade: average thickness = 2.5 in., standard deviation = 0.6 in., lognormal distribution. 

The variation of fireproofing thickness along the length of the bar was established by using a psuedo 

random number generator to select values from a lognormal distribution with central value and dispersion 

consistent with the above average values and standard deviation. Three sets of random data were 

generated for each condition. 

When the randomly selected thicknesses of each element were applied to the bar, it resulted in sudden 

changes in fireproofing thickness along the length of the bar. This resulted in a “rough” surface texture as 

shown by the dotted thickness profile in Fig. I–13 (a). It was felt that this rough texture (see also 

Fig. I–1 (c) might not be representative of actual conditions, so an alternative approach was to use 5-point 

averaging to reduce the roughness of the fireproofing profile. The solid line in Fig. I–13 (b) shows such a 

“smooth” profile. The two profiles in Fig. I–13 (a) have approximately the same average value and 

standard deviation and have similar cumulative distribution of fireproofing thickness as shown in 

Fig. I–13 (b). 

As stated, the calculated thermal histories of the bar elements were used to calculate the unrestrained 

length change of the bar due to thermal expansion and an applied stress of 12,500 psi. Work is currently 

underway to examine the performance of the bar under fully restrained conditions in which the induced 

stress history is computed. For comparison, the deformation of the bar with different but uniform 

thickness of fireproofing was calculated. The “equivalent thickness” was taken as the uniform thickness 

that resulted in similar deformation as under the variable thickness conditions. Figure I–13 (c) shows the 

results of these calculations for the original fireproofing. The three continuous curves are the 

deformation-time relationships for uniform thickness of 0.4 in., 0.5 in., and 0.6 in. The solid symbols 

represent the results for three cases with “rough” texture, and the open symbols are for the “smooth” 

texture. The following values summarize the six variable thickness profiles: 

• Rough 1: average = 0.79 in., standard deviation = 0.29 in. 

• Rough 2: average = 0.77 in., standard deviation = 0.27 in. 

• Rough 3: average = 0.79 in., standard deviation = 0.31 in. 
Appendix I 

I–22

Figure I–13. (a) Randomly generated thickness profiles with average thickness of 0.75 in. 

and standard deviation of 0.3 in., (b) cumulative element size, and (c) deformation of 1 in. 

bar compared with deformation for uniform thickness of fireproofing. 

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 10 20 30 40 50 60

Rough Texture

Smooth Texture

Thickness, in.

Bar Position, in.

(a)

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 20 40 60 80 100 120

Rough Texture

Smooth Texture

Thickness, in.

Cumulative Number of Elements

(b)

0.00

0.20

0.40

0.60

0.80

0 5 10 15 20 25 30

Rough 1

Rough 2

Rough 3

Smooth 1

Smooth 2

Smooth 3

0.4" Uniform FP

0.5" Uniform FP

0.6" Uniform FP

Deformation (in)

Time (min)

Load = 9800 lbs

(c)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–23

• Smooth 1: average = 0.79 in., standard deviation = 0.28 in. 

• Smooth 2: average = 0.78 in., standard deviation = 0.31 in. 

• Smooth 3: average = 0.78 in., standard deviation = 0.32 in. 

Figure I–13 (c) shows that the “rough” texture reduces the effectiveness of the fireproofing by a small 

amount compared with the “smooth” texture. As noted above, it is believed that the “smooth” texture is 

more representative of the actual conditions. On the basis of these analyses, it is concluded that 

fireproofing with an average thickness of 0.75 in. and a standard deviation of 0.3 in. provides equivalent 

protection to 0.6 in. of uniform thickness. 

The results for the upgraded fireproofing are shown in Fig. I–14. Only the “smooth” texture was used, 

and the values for the three cases are as follows: 

• Case 1: average = 2.50 in., standard deviation = 0.71 in. 

• Case 2: average = 2.43 in., standard deviation = 0.51 in. 

• Case 3: average = 2.55 in., standard deviation = 0.63 in. 

Figure I–14 (a) shows the three profiles, and Fig. I–14 (b) shows the normal probability plots of thickness 

values. Because the three randomly generated profiles do not have the same averages and dispersions, the 

responses show more scatter than in Fig. I–13 (c). On the basis of these analyses, it is concluded that an 

average thickness of fireproofing of 2.5 in. with a standard deviation of 0.6 in. is equivalent to 2.2 in. of 

uniform thickness. 

I.3.5 Thickness of SFRM for Use in Analyses 

Analyses of available data on fireproofing thickness and thermal modeling revealed the following: 

• From measurements of fireproofing thickness, the average values exceeded the specified 

thickness. 

• Fireproofing thickness was variable, and the distribution of thickness in the floor trusses 

appears to be described best by a lognormal distribution. 

• The standard deviation of fireproofing thickness on the trusses varied between about 0.3 in. to 

0.6 in. 

• The standard deviation of fireproofing on columns and beams from the core tended to be 

lower, with a value of 0.2 in. for the available data. 

• No information is available on the fireproofing thickness on the exterior columns and 

spandrel beams. 

• Variation in thickness reduces the effectiveness of fireproofing, and the equivalent uniform 

thickness is less than the average thickness. 
Appendix I 

I–24

Figure I–14. (a) Randomly generated thickness profiles with average thickness of 2.5 in. 

and standard deviation of 0.6 in., (b) normal probability plots of thickness values, and 

(c) deformation of 1 in. bar compared with deformation for uniform thickness 

of fireproofing. 

Based on the above findings, the following uniform thickness for the undamaged fireproofing will be 

used in calculating thermal response under various fire scenarios: 

• Original fireproofing on floor trusses: 0.6 in. 

• Upgraded fireproofing on floor trusses: 2.2 in. 

• Fireproofing on other elements: the specified thickness. 

The choice of specified thickness for those members lacking data is justified by offsetting factors as 

follows: (1) measured average thicknesses exceed specified values, and (2) variation in thickness reduces 

the effectiveness of fireproofing. 

0

1

2

3

4

5

0 10 20 30 40 50 60

Case 1

Case 2

Case 3

Thickness, in.

Bar Position, in.

(a)

1

1.5

2

2.5

3

3.5

4

4.5

5

.01 .1 1 5 10 20 30 50 7080 90 95 99 99.9 99.99

Case 1

Case 2

Case 3

Thickness, in.

Percent

(b)

0.00

0.20

0.40

0.60

0.80

1.00

0 20 40 60 80 100

Case 1

Case 2

Case 3

2.0 in. Uniform

2.2 in. Uniform

Deformation, in.

Time, min

(c)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–25

I.4 THERMAL PROPERTIES 

Based on the information provided by the manufacturers, three SFRMs have been identified in WTC 1, 2, 

and 7: (1) CAFCO BLAZE-SHIELD Type DC/F, (2) CAFCO BLAZE-SHIELD Type II, and 

(3) Monokote MK-5. Of the three SFRMs, only CAFCO BLAZE-SHIELD Type II is currently sold in 

the U.S., and CAFCO BLAZE-SHIELD Type DC/F is sold in Canada. 

CAFCO BLAZE-SHIELD Type DC/F is manufactured by Isolatek International (Stanhope, New Jersey) 

and was used in the interior columns, floor systems, and the exterior faces of the exterior columns of 

WTC 1 and WTC 2. CAFCO BLAZE-SHIELD Type II, also from Isolatek, was used in subsequent 

retrofit of WTC 1 floor systems. CAFCO BLAZE-SHIELD Type DC/F and Type II are portland cementbased products. Monokote MK-5 a gypsum-based SFRM, was manufactured by W.R. Grace and Co. 

(Cambridge, Massachusetts) and used in WTC 7. W.R. Grace stopped the production of Monokote MK-5 

in the 1980s. In addition to these three SFRMs, vermiculite plasters, manufactured by W.R. Grace until 

the 1970s, were used on the interior faces of the exterior columns of WTC 1 and WTC 2. 

To provide thermophysical property data for the modeling effort in fire-structure interaction, the thermal 

conductivity, specific heat and density of each SFRM were determined as a function of temperature up to 

1200 °C. Tests were performed by Anter Laboratories, Inc. in Pittsburgh, PA through an open-bid 

contract. Anter Laboratories is an ISO 9002 certified company. 

Samples of CAFCO BLAZE-SHIELD Type DC/F and Type II were prepared by Isolatek, Inc. in 

Stanhope, New Jersey, and sample of Monokote MK-5 were prepared by W.R. Grace and Co. in 

Cambridge, Massachusetts according to their respective application manuals. Since Monokote MK-5 is 

no longer on the market, it was specially manufactured by W.R. Grace according to the original MK-5 

formulation. The samples were made from the same batch of raw material, shipped to NIST for 

examination and documentation, and sent to Anter Laboratories for testing. The sample is 9 in. long, 

4.5 in. wide, and 3 in. thick. Three samples of each material were sent for testing. Two of them were 

used for the thermal conductivity measurements, and the third was used to prepare specimens for the 

other measurements involved. 

I.4.1 Thermal Conductivity Measurements 

The thermal conductivity measurements were performed according to ASTM C 1113 Test Method for 

Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique). This 

test method is based on heating two specimens with a platinum wire placed between them. The thin 

platinum wire serves not only as a heater, but also as a temperature sensor, since the variation of its 

electrical resistance during the test is converted into variation of temperature. Thermal conductivity is 

calculated based on the rate of temperature increase of the wire and power input. It was reported that 

substantial shrinkage during the measurements occurred for the three materials. The two MK-5 

specimens shrunk, exposing the platinum wire positioned between them. For this reason, no thermal 

conductivity measurement could be performed for this material at 1,200 °C. Figure I–15 (a) shows 

preliminary results for thermal conductivity as a function of temperature. The results show similar trends 

of increased thermal conductivity with increasing temperature; however, the Monokote MK-5 specimens 

had a different behavior than CAFCO BLAZE-SHIELD Type DC/F and Type II at temperatures above 

500 °C. 
Appendix I 

I–26

Figure I–15. Preliminary test results: (a) thermal conductivity 

as a function of temperature, and (b) specific heat as 

a function of temperature. 

I.4.2 Specific Heat Measurements 

For the specific heat capacity measurements, the same instrument (Unitherm Model QL-3141) was used 

with a slight modification. A thermocouple was added to the system and mounted on the specimen, 

parallel with the platinum wire at a known distance from the thermocouple. The test was performed in a 

similar manner as the thermal conductivity measurements, but from the thermocouple output the thermal 

diffusivity of the material was derived. Knowing the thermal conductivity, the thermal diffusivity, and 

the density calculated from the thermal expansion results and the thermogravimetric analysis, the specific 

heat of the material was calculated. Figure I–15 (b) shows preliminary results for specific heat as a 

function of temperature. It is seen that the materials had similar increasing trends with temperature, but 

the actual values differed. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800 1000 1200 1400

Monokote MK-5

BLAZE-SHIELD II

Thermal Conductivity (W/mK)

BLAZE-SHIELD DC/F

Temperature, oC

(a)

600

800

1000

1200

1400

1600

0 200 400 600 800 1000 1200 1400

Monokote MK-5

BLAZE-SHIELD II

BLAZE-SHIELD DC/F

Specific Heat, J/(kg K)

Temperature, oC

(b)
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–27

I.4.3 Density Measurements 

Densities of the samples were not measured directly (except at room temperature) but were calculated 

from TGA (thermal gravimetric analysis) and thermal expansion measurements. The TGA tests were 

performed according to ASTM Test Method E1131 using an Orton Model ST-736 TGA instrument. The 

thermal expansion tests were performed according to ASTM Test Method E228 using a Unitherm Model 

1161 instrument. Since the materials were not isotropic, separate tests were performed for the X and Z 

orientations. It was assumed that the X and Y directions had the same thermal expansion. The 

Z direction was defined as the direction perpendicular to the fibrous strands in the specimens. The 

specimens were tested from room temperature to 1,200 °C at a heating rate of 2 °C/min. All of the 

specimens shrank during the tests and, in all cases, lost contact with the pushrod before reaching the 

maximum test temperature. 

From the thermal expansion test results, the change in volume for each material was calculated at each 

temperature of interest. The density values were calculated from the results of the TGA and thermal 

expansion. 

I.5 RESPONSE TO IMPACT 

In order to estimate the extent of damage or loss of SFRM due to aircraft impact, the detailed finite 

element analysis of aircraft impact into the WTC towers, conducted within the framework of Project 2 of 

the investigation, will provide the following information: 

Debris Field––A database and graphics of the major fragments of the aircraft and destroyed structural 

components of the towers, including their mass, approximate size, speed, and trajectory will be developed 

in the global analysis of aircraft impact into WTC 1 and WTC 2. The trajectory of each fragment will 

consist of the initial point of entry, point of exit or resting place. This debris field database will be used to 

estimate which areas within the impacted floors would likely have lost their fireproofing due to direct 

impact by debris. 

Deformations and Accelerations––Estimates of accelerations and deformations, including localized 

effects, as a function of time on steel members in each of the two towers will be developed in the global 

analysis of the aircraft impact. Accelerations will be determined at representative locations on the floor 

truss systems and columns in the impact-affected zones of both towers (floors 93 to 98 of WTC 1 and 

floors 78 to 83 of WTC 2). These accelerations will be compared with the threshold values estimated 

from the adhesion and cohesion properties of SFRM developed in the experimental and analytical study 

presented below to estimate the likely extent of damage to the fireproofing on the columns and floor 

systems. 

Preliminary results from the subassembly impact analysis of an aircraft engine into a strip of the towers 

with a width and height of single exterior panel (three exterior columns width and three floor height) 

extending all the way through the core indicate that the accelerations on the lower chords of floor trusses 

will need further analysis to account for high frequency vibrations and the short-duration sharp peaks in 

the computed acceleration time-histories and their effects on damage to SFRM. One possible approach is 

to low-pass filter the acceleration records to remove these high-frequency vibrations. Another approach 

is to develop “shock spectra” for a number of steel members with fireproofing configurations using finite 
Appendix I 

I–28

element analysis to determine, for a given frequency, the acceleration amplitude that is needed to dislodge 

the fireproofing based on its adhesive and cohesive strength. The shock spectra will then be compared 

with spectra of the calculated acceleration time-histories to estimate the extent of damage to the 

fireproofing. 

I.5.1 Mechanical Properties of SFRM 

The purpose of these tests is to develop a rational basis for estimating the extent of loss of SFRM as a 

result of impact loads on protected members. Tests will (1) determine the mechanical properties of 

CAFCO BLAZE-SHIELD Type DC/F, and (2) verify models for estimating loss of fireproofing when a 

protected member is subjected to impact-induced vibration. The mechanical properties to be measured 

are: 

• SFRM cohesive strength, and 

• SFRM adhesive strength to steel substrates with and without primer. 

The adhesive and cohesive strengths will be measured for static loads, as described below for Phase I 

tests. The tests will be done on 1/4 in. thick steel plate specimens, with and without primer (Tnemec 

Series 10 red primer), and for nominal SFRM thickness of 3/4 in. and 1 1/2 in. Specimens are fabricated 

and testing will be done during June and July, 2004. 

From the measured strength properties, estimates will be made of the local accelerations required to 

damage or dislodge the SFRM, as described below. These estimates will be verified by impact tests of 

plates and bars covered with SFRM and instrumented with accelerometers, as described in the Phase II 

tests. 

Phase I—Tensile Pull-off Test to Measure Adhesive Bond Strength and Cohesive 

Strength 

Specimen––Steel plates (8 by 16 by 1/4 in.) with CAFCO BLAZE-SHIELD Type DC/F and nominal 

thickness of 3/4 in. and 1 1/2 in. 

Pull-Off Test Procedure (see Fig. I–16) 

• Using a fine-tooth saw, cut into SFRM applied to plate to obtain 2 3/4 in. square test 

specimens to ensure that the area resisting the applied load is well defined. 

• Affix aluminum plates with two-component adhesive. 

• Allow adhesive to cure. 

• Measure force required to pull off the plate. 

• Record load and note failure mode (cohesive, adhesive, mixed). 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–29

Figure I–16. Pull-off test of SFRM applied to steel plate. 

If all failures are adhesive, the cohesive strength will be determined by bonding the SFRM block to a steel 

plate with adhesive and repeating the test. 

Phase II—Verification of Models to Predict Dislodgement of SFRM 

Impact tests of plate and bar specimens will be done to determine the impact loads needed to produce 

different levels of accelerations. Plates and bars with SFRM will be subjected to different levels of 

impact until the SFRM is dislodged. Two simplified models will be used to estimate the relationships 

between material strengths and impact required to dislodge the SFRM. Model predictions will be 

compared with test results. 

CASE 1: Planar Element 

The simplified model considers the substrate and SFRM as rigid bodies. The SFRM would dislodge 

when the inertial force exceeds the smaller of the adhesive bond strength or cohesive strength. 

Figure I–17, shows the free body of the fireproofing being acted upon by its inertial force and the 

adhesive force. The acceleration to dislodge the SFRM is: 

t

f a b

ρ = (I.1) 

Figure I–17. Derivation of acceleration to dislodge SFRM from planar substrate. 

t

f a

atAAf

amF

b

b

ρ

ρ

=

=

=

t 

fb SFRM 

A = area of SFRM 

t = thickness of SFRM 

ρ = density of SFRM 

fb = bond or adhesive strength 

Inertial Force Equilibrium 

SFRM

Al Plate 

Adhesive 

SFRM 
Appendix I 

I–30

where: 

fb = bond or adhesive strength 

t = thickness of SFRM 

ρ = density of SFRM 

For example, for an SFRM with cohesive and adhesive strength of 150 psf, a density of 15 pcf, and an 

applied thickness t = 1 in., we would find that a = 119g, where g is the gravitational acceleration. This 

shows that acceleration on the order of 100g would be required to dislodge this SFRM from a planar 

surface. 

CASE 2: Encased Round Element 

Again, a rigid body model is used. In this case, the SFRM would mobilize its cohesive tensile strength, ft, 

and adhesive bond strength, fb. Figure I–18 shows the derivation for the relationship between material 

strengths and acceleration to dislodge the SFRM from a round bar. The required acceleration is as 

follows: 

πρ

α

)(

))1((4

22

0

0

i

t i

dd

df d

a −

+ − = (I.2) 

where: 

ft = cohesive tensile strength of SFRM 

d0 = outside diameter of SFRM 

Figure I–18. Derivation of acceleration to dislodge SFRM surrounding a round bar. 

SFRM ft

fb

di do

ρπ

α

α π ρ

α

π ρ

)(

))1((4

4

)( ))1((

Let

)(

4

)( Mass

22

22

22

io

ot i

io

ot i

tb

ibiot

io

dd

df d

a

a

dd dfF d

ff

dfddfF

dd

m

−

−+ =

− =−+=

=

+−=

− ==

Inertial Force Equilibrium 
Interim Report on Assessment of Sprayed Fireproofing in the WTC Towers—Methodology 

I–31

di = steel bar diameter 

α = Rrtio of bond strength to cohesive strength of SFRM 

ρ = Ddnsity of SFRM 

For example, if the steel bar has a diameter of di = 1 in., the SFRM has an outside diameter of d0 = 2 in., a 

density ρ = 15 pcf, a cohesive tensile strength of ft = 300 psf, and a bond strength to cohesive strength 

ratio of α = 0.5, we would find that an acceleration of a = 152g is required to dislodge the SFRM from the 

bar. 

I.6 SUMMARY 

This appendix has focused on conditions of the fireproofing (or SFRM) in the WTC towers before and 

after aircraft impact. Results of simplified finite-element simulations of heat transfer under fire 

conditions have shown that variability in thickness of fireproofing reduces the effectiveness of the 

fireproofing so that protection is less than implied by the average thickness of the fireproofing. As a 

result, the NIST-led investigation sought available information on the in-place condition of the SFRM 

used in the WTC towers. Limited information was provided by the Port Authority in the form of 

thickness measurements taken at various times during the 1990s. Additional information was obtained 

from photographs of floor trusses provided to NIST. Analysis of the data indicated that fireproofing 

thickness was variable, as would be expected for application to floor truss members with small cross 

sections. Based on analyses of the available data, the following values were taken to be representative of 

the SFRM thickness on the floor trusses: 

• Original SFRM: Average thickness of 0.75 in. with a standard deviation of 0.3 in. (coefficient 

of variation = 0.40) 

• Upgraded SFRM: Average thickness of 2.5 in. with a standard deviation of 0.6 in. 

(coefficient of variation = 0.24) 

Based on finite-element simulations of a 1 in. round bar covered with SFRM having lognormal 

distributions for thickness that are consistent with the above values, it is concluded that the original 

fireproofing on the floor trusses is equivalent to a uniform thickness of 0.6 in. and the upgraded 

fireproofing is equivalent to a uniform thickness of 2.2 in. 

No information is available on in-place conditions of the fireproofing on the exterior columns and 

spandrel beams, and little information is available on the conditions of fireproofing on core beams and 

columns. In subsequent thermal analyses, the fireproofing on these elements will be taken to have 

uniform thicknesses equal to the specified values. This assumption is believed to be justified by the 

offsetting factors of measured average thicknesses tending to be greater than specified thicknesses and the 

reduced effectiveness of a given average thickness of fireproofing due to thickness variability. 

Another objective of this appendix is to review the methodology that will be used to estimate how much 

of the SFRM may have dislodged as result of aircraft impact. Simple static models have been developed 

for an order of magnitude estimate of the acceleration that would be required to dislodge the SFRM. 

Based on these models and assumed, but representative, values of density and strength (adhesive and 
Appendix I 

I–32

cohesive), it is estimated that acceleration on the order of 100g to 150g (where g is the acceleration due to 

gravity) would be needed dislodge the fireproofing. Additional analytical studies will be conducted to 

account for dynamic effects, and tests will be performed to verify these predictions. 
J–i

TABLE OF CONTENTS - Appendix J

List of Figures........................................................................................................................................... J–ii

List of Tables ..............................................................................................................................................J–i

Appendix J

Interim Report on Experiments to Support Fire Dynamics and Thermal 

Response Modeling ................................................................................................................J–1

J.1 Introduction...................................................................................................................................J–1

J.2.1 Description of Experiments...............................................................................................J–2

J.2.2 Preliminary Results ...........................................................................................................J–7

J.3 Experiments for Guiding the FDS Fire Growth Predictions.......................................................J–12

J.3.1 Description of the Experiments .......................................................................................J–12

J.3.2 Preliminary Results .........................................................................................................J–18

J.4 Experiments for Fire Model Validation......................................................................................J–27

J.4.1 Preliminary Results .........................................................................................................J–32

J.5 References...................................................................................................................................J–34

Attachment 1

Simulating the Fires in the World Trade Center.................................................................J–35 
J–ii

LIST OF FIGURES

Figure J–1. Experimental enclosure during construction, viewed with access panels removed..............J–3

Figure J–2. Compartment content layout.................................................................................................J–4

Figure J–3. Tubular column. ....................................................................................................................J–5

Figure J–4. Bar joist.................................................................................................................................J–5

Figure J–5. Simple bar. ............................................................................................................................J–6

Figure J–6. Measured heat release rate as a function of time during tests 1−6........................................J–7

Figure J–7. Temperature-time history for truss A in test 5. .....................................................................J–8

Figure J–8. Temperature-time history for truss A in test 3. .....................................................................J–9

Figure J–9. Comparison of gas temperatures, test 3, exhaust side of compartment. .............................J–10

Figure J–10. Comparison of gas temperatures, test 5, exhaust side of compartment. .............................J–10

Figure J–11. Comparison of heat fluxes to column, test 3.......................................................................J–11

Figure J–12. Comparison of heat fluxes to column, test 5.......................................................................J–11

Figure J–13. Photograph of right side of generic workstation. ................................................................J–13

Figure J–14. Photograph of left side of generic workstation. ..................................................................J–13

Figure J–15. Photographs of high-end workstation. ................................................................................J–15

Figure J–16. Photograph of workstation soon after ignition in test 2. ....................................................J–18

Figure J–17. Photograph of workstation near the peak heat release rate in test 2....................................J–19

Figure J–18. Heat release rate versus time for single workstation test 1. ................................................J–19

Figure J–19. Heat release rate versus time for single workstation test 2. ................................................J–20

Figure J–20. Heat release rate versus time for single workstation test 3. ................................................J–20

Figure J–21. Heat release rate versus time for single workstation test 4. ................................................J–21

Figure J–22. Heat release rate versus time for single workstation test 5. ................................................J–21

Figure J–23. Heat release rate versus time for single workstation test 6. ................................................J–22

Figure J–24. Preliminary comparison of predicted and measured HRR values.......................................J–26

Figure J–25. Plan view of test configuration. ..........................................................................................J–28

Figure J–26. Elevation view of test configuration. ..................................................................................J–29

Figure J–27. Views of interior of test fixture...........................................................................................J–30

Figure J–28. View of fire compartment before the start of test 6. ...........................................................J–33

Figure J–29. View of fire compartment 2 minutes after the start of test 6...............................................J–33

Figure J–30. Fire Dynamics Simulator 3D rendition of experimental enclosure.....................................J–34

Figure J–31. Comparison of measured and predicted heat release rate for test 1. ...................................J–34 
J–i

LIST OF TABLES

Table J–1. Test matrix. ...........................................................................................................................J–6

Table J–2. Summary of insulation on steel components.........................................................................J–6

Table J–3. Peak heat release rates and times to peak............................................................................J–22

Table J–4. Test matrix. .........................................................................................................................J–31
Appendix J 

J–iv

This page intentionally left blank. 
J–1

Appendix J

INTERIM REPORT ON EXPERIMENTS TO SUPPORT FIRE DYNAMICS AND 

THERMAL RESPONSE MODELING

J.1 INTRODUCTION 

The reconstruction of the World Trade Center (WTC) fires involves two computation models: 

• Fire Dynamics Simulator (FDS): This is the first large-domain CFD fire model that predicts and 

visualizes the spread, growth and suppression of a fire based on the underlying scientific 

principles governing fluid motion. The model numerically solves the conservation equations of 

mass, momentum and energy that govern low-speed, thermally driven flows with an emphasis on 

smoke and heat transport from fires. The companion software package, called Smokeview, 

graphically presents the results of the FDS three-dimensional (3D) time-dependent simulation. 

Smokeview animates in three dimensions the smoke particulates, heat fluxes, temperatures and 

fluid velocities within a building. Users of the package can view the enclosure from any angle 

and from inside or outside. 

• Fire-Structure Interface (FSI): This code effects the transfer of radiant and convective heat from a 

CFD fire model, such as FDS, to a coupled, transient, three-dimensional finite element model for 

the thermal response of structural members, such as ANSYS. The members may be simple (e.g., 

bare steel) or complex (e.g., insulation-coated steel). 

For application to the Investigation, each of these needs experimental data (a) to guide 

adaptation/development of the models for this specific purpose and (b) with which to ascertain the 

accuracy of the model predictions. Ideally the uncertainty in the agreement with experiments will be 

much smaller than the effect of varying the unknowns in the actual conditions on September 11, 2001. 

The following text describes three sets of experiments designed to accomplish this. All three sets have 

been completed, and the analysis of the data is under way. Full reports are in preparation and will be 

completed in summer 2004. Attachment 1 is a short paper on the modeling and experiments that will be 

presented at Interflam in July 2004. 

J.2 EXPERIMENTS FOR ACCURACY ASSESSMENT OF THERMAL 

ENVIRONMENT MODELING 

The purposes of these experiments, conducted in the National Institute of Standards and Technology 

(NIST) Large-scale Fire Laboratory from March 10 through 26, 2003, were to: 

• Assess the accuracy with which FDS predicts the thermal environment in a burning compartment 

and; 
Appendix J 

J–2

• Establish a data set to validate the prediction of the temperature rise of structural steel elements 

using FSI. 

Within a large test compartment, assorted steel members were exposed to controlled fires of varying heat 

release rate (HRR) and radiative intensity. The steel members were bare or coated with spray-applied 

fireproofing of two thicknesses. The thermal profile of the fire was measured at multiple locations within 

the compartment. Temperatures were also recorded at multiple locations on the surfaces of the steel, the 

insulation, and the compartment. Prior to each test, a prediction of the thermal environment in the 

compartment was determined using FDS. Following the tests, the prediction and experimental results 

were compared. 

J.2.1 Description of Experiments 

The test compartment consisted of a steel stud frame lined with calcium silicate board. The internal 

dimensions of the compartment were 3 m high, 7 m deep, and 4 m wide. There were four openings in the 

west wall through which air entered the room; they totaled 1.75 m2

 (10.8 ft2) in area and were located 1 m 

(3.3 ft) above the floor. There were four openings in the east wall through which heat and combustion 

products were emitted; they also totaled 1.75 m2

 (10.8 ft2) in area and were located 2 m above the floor. 

A photograph is shown in Fig. J–1 and a schematic in Fig. J–2. 

In each of the six tests, the four test subjects were a bar, two trusses, and a thin-walled tubular column. 

These are depicted in Figs. J–3 through J–5. Depending on the test, these specimens were either left 

unprotected or were coated with spray-applied fire protective insulation material, BlazeShield DC/F. The 

fibrous insulation was applied by an experienced applicator who took considerable care to apply an even 

coating of the specified thickness. As such, the insulated test subjects represent a best case in terms of 

thickness and uniformity. 

The fires consisted of liquid hydrocarbon fuels sprayed by a two-nozzle spray burner onto a 1 m × 2 m 

(3.3 ft × 6.6 ft) pan. The fuels were (a) heptanes and (b) a mixture of nominally 60 percent (by mass) 

heptanes with 40 percent toluene. The latter fuel produced a significantly sootier flame. 

The instrumentation for the tests comprised up to 352 channels of data. 

• The combustion products were collected in a 6 m × 6 m (21.5 ft × 21.5 ft) hood. Instrumentation 

in the exhaust duct enabled calculation of the rate of heat release throughout a test. 

• Fourteen heat flux gauges were placed strategically around the compartment to measure the 

transport of radiant energy; in addition, there were four slug calorimeters measuring the total heat 

flux parallel to the trusses. 

Most of the channels were for thermocouples that measured the temperatures on the surface of the walls 

and ceiling, within the walls, on the surface of the steel components, and at the surface of the sprayapplied insulation. With the large number of measurements, it was possible to go beyond the traditional 

point-by-point comparison and discover why the model either under- or overpredicted a given 

measurement. A description of the test series appears in Table J–1. Table J–2 shows the dimensions and 
 Interim Report on Experiments to Support Fire Dynamics 

J–3

variability of the spray-applied insulation. The measurements were taken at numerous locations along the 

perimeter and length of each specimen using a pin-thickness gauge specifically designed for this type of 

insulation. 

Figure J–1. Experimental enclosure during construction, viewed with access 

panels removed. 

Steel Truss 

Supply Vents 

Baffle 

Fire Pan 

Exhaust Vents 
Appendix J 

J–4

1.2 SECTION A-A

7.04 m 

Fire Pan

Trusses 

Baffle 

Access panels

Column Suspended bars

PLAN VIEW

(i

Truss support 

3.82 m 

3.68 m 

Baffle

 Trusses

Inlet Vents 

Column 

Truss support

(

Fire Pan 

 3.68 m 

 Inlets 

 WEST 

Figure 2.4: Compartment Content Layout (not to scale). 

 Outlets 

Suspended 

bar 

Column 

support 

X 

Y 

(0,0) 

(0,0) 

NORTH 

NORTH 

Figure J–2. Compartment content layout (not to scale).
Appendix J 

J–5

Figure J–3. Tubular column. 

Figure J–4. Bar joist. 

0.8 m 

4.6 m 

25.6 mm 

76 mm

76 mm

6.5 mm 

6.4 mm 

64 mm

64 mm 

6.4 mm 

6.3 mm 

Web Bar Top Chord Angles Bottom Chord Angles

3.35 m 

0.26 m

 0.36 m 

6.31 mm

6.25 mm
Appendix J 

J–6

Figure J–5. Simple bar. 

Table J–1. Test matrix. 

Test 

Measured Heat 

Release Rate (MW) Fuel 

Planned Insulation 

Thickness (mm) 

Planned Test 

Duration (min) 

1 2.0 Heptanes None 15 

2 2.4 Heptanes/toluene None 15 

3 2.0 Heptanes/toluene None 15 

4 3.2 Heptanes Same as test 5 15 

5 3.0 Heptanes See Table J–2 50 

6 3.0 Heptanes See Table J–2 50 

Table J–2. Summary of insulation on steel components. 

Applied Thickness (mm) 

Test Item Specified Thickness (mm) Mean Std. Deviation 

5 Bar 19.1 23.0 5.5 

 Column 38.1 41.0 3.0 

 Truss A 19.1 26.9 7.3 

 Truss B 38.1 40.5 8.2 

6 Bar 19.1 25.3 4.6 

 Column 19.1 21.4 3.5 

 Truss A 19.1 26.0 6.9 

 Truss B 19.1 25.6 6.9 

The HRR are shown in Fig. J–6. The important features of the HRR curves are: 

• The expected (from the calorific value of the fuels) and measured heat release rates agreed 

within the relative expanded experimental uncertainty of ±11%. 

• The heat release rates are steady over the time interval in which the burner was turned on. 

25.5 mm

 3 m length 
 Interim Report on Experiments to Support Fire Dynamics 

J–7

Figure J–6. Measured heat release rate as a function of time during tests 1−6. 

In each test, the baseline signals from all the measurement devices were established; then the burner was 

ignited and continued burning at a steady rate until the temperature at any steel surface approached 

approximately 600 °C. (Above this temperature, there was concern that loss of strength might lead to 

collapse and accordant damage to the test facility.) At that point, the burner was turned off. In test 2, the 

steel reached the target temperature at about 6 min and was terminated at that time. During test 4, the 

Omega GG glass braid wire thermocouple extension cables failed leading to erroneous thermocouple 

readings. This was likely due to the opening-up of the Kaowool thermal insulation protective blanket 

around the thermocouple wires and subsequent shorting. Thus the test was terminated early, and the data 

have not been processed. To prevent reoccurrence of this problem during tests 5 and 6, the extensions 

were water cooled and double wrapped with Kaowool insulation. Each layer of Kaowool insulation was 

secured to the thermocouples using steel wire. Additionally, the thermocouple extensions were rerouted 

through the cooler west side of the compartment. After tests 5 and 6, the thermocouple extensions were 

visually inspected and were found to be undamaged.

J.2.2 Preliminary Results 

Figure J–7 shows typical temperature data obtained in the tests. These data are for truss A in test 5. The 

thermocouple location notation is as follows: TU: Truss Upper Chord, TM: Truss Middle (Web), TL: 

Truss Lower Chord; 1 to 4: locations across the length of the test specimen; S: on the steel surface, I: on 

the outer surface of the insulation; A: truss A. For the informative nature of this progress report, it is only 

important to note the following: 

• The curves that rise sharply from the beginning of the test are those for temperatures on the 

outside of the insulation. 

• The curves that rise more gently are those for temperatures at the interface between the steel and 

the insulation. 

0

500

1000

1500

2000

2500

3000

3500

0 1000 2000 3000 4000

Test 1 HRR

Test 2 HRR 

Test 3 HRR

Test 4 HRR

Test 5 HRR

Test 6 HRR

Time (s)
Appendix J 

J–8

From the figure, one can see that: 

• The 19.1 mm (0.75 in.) insulation delays the rise to a peak steel temperature by almost an hour at 

all locations. 

• The highest temperature reached at the steel surface is approximately 300 °C lower than the 

temperature at the outside face of the insulation material. 

The curve patterns for the other steel specimens in the tests with insulated steel are similar in shape. 

Test 5: Truss A

0

100

200

300

400

500

600

700

800

900

0 600 1200 1800 2400 3000 3600 4200 4800

Time (sec)

TU1SA

TU2SA

TU3SA

TU4SA

TM1SA

TM2SA

TM4SA

TL1SA

TL2SA

TL3SA

TL4SA

TU1IA

TU2IA

TU3IA

TU4IA

TM2IA

TM3IA

TM4IA

TL1IA

TL2IA

TL3IA

TL4IA

Figure J–7. Temperature-time history for truss A in test 5. 

By contrast, Fig. J–8 shows the same plot for truss A from test 3, in which the truss was not insulated and 

the fire was of shorter duration and lower intensity. Nonetheless, the outer surface of the steel reached the 

targeted maximum temperature (just short of 600 °C) in about one-third the time. As expected, this result 

is typical of the fire response of the uninsulated steel specimens in tests 1 through 3. 

Figures J–9 through J–12 show comparisons of the modeled and measured temperatures and heat fluxes 

for tests 3 (2.0 MW heptanes/toluene fire) and test 5 (3.0 MW heptanes fire). The agreement for the 

highest temperatures is excellent. Analysis of both the model and the thermocouple measurements is 

under way to determine the source of their differences at the lower temperatures, especially in test 5. The 

spikes in the heat flux plots are artifacts—the result of the periodic nitrogen blasts to reduce soot 

accumulation on the gage surface. 
 Interim Report on Experiments to Support Fire Dynamics 

J–9

0

100

200

300

400

500

600

0

109218327436545654763872981

10901199130814171526163517441853196220712180228923982507261627252834294330523161327033793488

Time (sec)

Temp (C)

TU4SA

TU3SA

TU2SA

TU1SA

TM4SA

TM3SA

TM2SA

TM1SA

TL1SA

TL4SA

TL3SA

TL2SA

Figure J–8. Temperature-time history for truss A in test 3. 

While the data analysis is not yet complete, the following are valuable preliminary observations: 

• The prediction of the upper layer temperatures was within experimental uncertainty. Since the 

heat flux to the walls and objects within the upper layer is highly dependent on the upper layer 

temperature, these predictions were also accurate.The prediction of the time for the steel surfaces 

to reach 600 °C was accurate. 

• The sootier burning fuel led to similar temperature rise in the ceiling and the steel above the fire 

plume. 

• The model predicted the asymmetric shape of the fire plume, caused by obstructions to uniform 

flow through the compartment. 

Further analysis will use Smokeview for visual comparison of the test results and the model predictions. 

This will determine how well FDS captures both the fire phenomena and the thermal patterns in the 

compartment. Quantitative analysis of the data will then determine the numerical accuracy of the 

predictions. Similar analysis will be performed to determine the accuracy of the finite element modeling 

of the thermal patterns within the bare and insulated steel components. 
Appendix J 

J–10

WTC Phase 1, Test 3, East Aspirated TCs

Time (s)

0 200 400 600 800 1000 1200

Temperature (C)

0

100

200

300

400

500

600

365 cm (Exp)

215 cm (Exp)

35 cm (Exp)

365 cm (FDS)

215 cm (FDS)

35 cm (FDS)

Figure J–9. Comparison of gas temperatures, test 3, 

exhaust side of compartment. 

WTC Phase 1, Test 5, East Aspirated TCs

Time (s)

0 1000 2000 3000

Temperature (C)

0

200

400

600

800

1000

365 cm (Exp)

215 cm (Exp)

35 cm (Exp)

365 cm (FDS)

215 cm (FDS)

35 cm (FDS)

Figure J–10. Comparison of gas temperatures, test 5, 

exhaust side of compartment. 
 Interim Report on Experiments to Support Fire Dynamics 

J–11

WTC Phase 1, Test 3, Column Heat Flux Gauges, East Facing

Time (s)

0 200 400 600 800 1000 1200

Heat Flux (kW/m2

)

0

10

20

30

40

High East (Exp)

Low East (Exp)

High East (FDS)

Low East (FDS)

Figure J–11. Comparison of heat fluxes to column, test 3. 

Figure J–12. Comparison of heat fluxes to column, test 5. 

WTC Phase 1, Test 5, Column Heat Flux Gauges, North Facing

Time (s)

0 1000 2000 3000

Heat Flux (kW/m2

)

0

20

40

60

80

100

High North (Exp)

Low North (Exp)

High North (FDS) 

Low North (FDS)
Appendix J 

J–12

J.3 EXPERIMENTS FOR GUIDING THE FDS FIRE GROWTH PREDICTIONS 

In the early stages of this Investigation, the FDS combustion module was enhanced to enable the 

inclusion of charring materials, such as those that comprise much of the office furniture. Thermophysical 

property data from the combustion of small (100 mm × 100 mm) samples of the furnishings were 

obtained using the Cone Calorimeter. These data became input to the fire simulation. A set of real-scale 

experiments was then designed and performed (July and August 2003) to identify any need for further 

enhancements to the fire model. In each of these experiments, a single workstation (i.e., an office cubical 

or module), similar to those in the WTC offices, was burned under a hood. A soffitted ceiling allowed for 

the collection of hot fire effluent and the accordant thermal radiation to the test specimen. Some of the 

tests examined the effects on the burning rate of jet fuel and/or noncombustible material occluding a 

fraction of the workstation surfaces. 

J.3.1 Description of the Experiments 

Materials 

Workstations come in a variety of styles and finishes. However, they tend toward similar size and mass. 

They are also fabricated of materials with similar burning behavior, e.g., the work surfaces are generally 

laminated particleboard or wood. Most of the workstations burned here were of a single generic type. 

For comparison, one high-end unit (identical to those in the aircraft impact floors in WTC 1) was also 

burned in a manner identical to one of the tests of the generic units. 

The generic workstation examined is shown photographically in Figs. J–13 and J–14. The layout, 

including the placement of the various nonstationary items, was suggested by personnel from a company 

that supplied office furnishings to the occupants of WTC 1. Information on the distribution of papers and 

other office items was provided by a frequent visitor to these offices. The workstation covered a footprint 

nominally 2.44 m × 2.44 m (8 ft × 8 ft) and was surrounded by privacy panels. 

• The panels were 1.22 m (4 ft) high, except that on one side the panel was 1.52 m (5 ft) tall and 

supported a bookcase. On the side opposite the bookcase, there was a 1.22 m (4 ft) wide open 

entrance opening. The panels were made of a steel and softwood frame, covered on both sides 

with layers of fiberglass padding and perforated steel and a thermoplastic cover fabric. A few 

sheets of copier paper were tacked to the cubicle walls on three sides. 

• The work surfaces were formed from four sections of laminated medium density fiberboard 

supported by steel brackets from the wall panels. Four document boxes contained a total of four 

reams of copier paper. Additional paper was stacked horizontally on the desk surface. 

• The seat and back of the office chair were a nonthermoplastic fabric over polyurethane foam 

supported by a one-piece thermoplastic shell; its five-legged base was thermoplastic with steel 

framing and support elements. 
 Interim Report on Experiments to Support Fire Dynamics 

J–13

Figure J–13. Photograph of right side of generic workstation. 

Figure J–14. Photograph of left side of generic workstation. 
Appendix J 

J–14

• The three file cabinets (0.91 m wide, 0.51 m deep, 0.68 m high [36 in. × 20 in. × 27 in.], with two 

horizontal drawers) were painted steel; they rested directly on the carpet. Two of the cabinets 

contained two reams each of copier paper as a rough means of assessing the extent to which paper 

in file drawers might contribute to a fire. 

• The bookcase (1.22 m [48 in.] long) had a steel shelf and top but these were supported only on 

their ends by combustible end panels; the steel front closure panel was fabric-covered steel and it 

was open (on top of the bookcase). Ten document boxes held about 13 reams of copier paper. 

• The carpet tiles were nylon fiber-faced over a dense foam rubber backing. A square area 

2.74 m × 2.74 m (9 ft × 9 ft) was covered with 36 carpet tiles. 

• The computer monitor was a nominally 17 in. CRT-based unit. Its front face was taped with 

fiberglass tape and it was pointed toward the wall panel opposite the cubicle opening for safety in 

the event of an implosion. The keyboard was placed in its normal location, parallel to the sloped 

segment of the work surface. The computer processor (tower-type container with plastic only on 

the front face of the container) was placed on the floor next to a waste paper basket (both on the 

side opposite the cubicle opening). 

• The wastebasket was thermoplastic and contained one ream of copier paper atop five balled-up 

paper ream wrappers. 

Thermophysical characterizations of six of the generic workstation materials (carpet, panels, work 

surface, chair seat, paper stack, and computer monitor shell) were obtained using the Cone Calorimeter. 

These data were to serve as input to FDS. Since the physical behavior of at least some of the work station 

materials (and the objects from which they were taken) was expected to be more complex than was 

revealed in the Cone tests, it was anticipated that these full-scale tests would provide clues as to necessary 

empirical adjustments in the FDS predictions. 

The high-end unit was similar to the generic workstation, with the principal differences being: 

• The wall panel construction was somewhat different having a 3 mm (0.125 in) layer of flameretarded polyester fiber beneath the outer fabric, a more open steel panel beneath this, a central 

fiberboard layer (3 mm thick) and an all steel peripheral frame (no wood). The fiberboard 

roughly doubled the amount of woody fuel within the wall panels of the enclosure and put it into 

a much higher surface area form in which it could be expected to burn appreciably faster. It 

should be noted, however, that this increase in woody fuel was only about 10 percent to 

15 percent of the total available in the desk surfaces. Also, its enclosure deep within the wall 

panels delays its burning. 

• The file cabinets (four, with a total face length of 2.67 m rather than three with a total face length 

of 2.44 m) had a flammable, charring plastic surface on the drawer fronts that added 15 percent to 

20 percent of flammable area. 

The chair was constructed somewhat differently (seat and back as separate pieces) and behaved as if its 

upholstered surfaces were flame-retarded. The high-end workstation is shown in Fig. J–15. 
 Interim Report on Experiments to Support Fire Dynamics 

J–15

Figure J–15. Photographs of high-end workstation. 

Test Configuration and Instrumentation 

The workstation to be tested was placed on top of a double layer of 13 mm (1/2 in) thick calcium silicate 

sheets. These in turn rested on a set of four weighing cells, one at each corner. Each workstation 

(including furnishings) weighed approximately 730 kg (1,600 lb) and contained approximately 300 kg 

(660 lb) of combustible material. The weighing cells are accurate to ±0.1 kg (0.2 lb). The entire 

assembly was placed beneath the hood of the NIST 10 MW calorimeter hood to allow continuous heat 

release rate measurement. 
Appendix J 

J–16

A fire typically forms a layer of hot, smoky combustion products near the ceiling of a room. Thermal 

radiation from this layer plays a significant role in fire spread. Thus the test fixture included partitions to 

hold the combustion products from these test fires. The ceiling was a 3.66 m × 3.66 m (12 ft × 12 ft) 

section of 13 mm thick calcium silicate board. It was supported on a water-cooled steel frame 2.74 m 

(9 ft) above the floor of the workstation. To keep the gases from flowing quickly across the ceiling and 

thus not forming an appropriate layer, the ceiling was surrounded on all four sides by a steel skirt that 

draped down 0.61 m (2 ft) from the ceiling. 

The test instrumentation included: 

• Instrumentation in the hood exhaust duct for measurement of rate of heat release. 

• Four video cameras placed to record the progression of flame spread over the objects in the 

cubicle. Their view of the combustibles became observed by the wall panels and the flames 

themselves as the fire grew in intensity. An observer narrated the fire growth to supplement what 

the cameras recorded directly. 

• In two tests, an upward-facing, water-cooled Schmidt-Boelter total heat flux gage 127 mm (5 in) 

above the cubicle floor, near the cubicle center. 

• An external total heat flux gage mounted so as to view the entire fire from one side. This 

provided a signal that was proportional to the instantaneous HRR of the fire and was useful for 

certain timing issues. 

• In two tests, six 24-gage chromel/alumel thermocouples to follow the progress of the fire on the 

underside of the desk surfaces. 

Ignition Scenario 

The initiation of the tests was in keeping with the workstation being one of a large array on a given floor 

of a large office space and a fire propagating through the array. Thus a large ignition source (a 2 MW, 

four-nozzle spray burner over a 2 m × 1 m pan) was placed immediately adjacent to the exterior of one 

wall panel of the test station, simulating the burning of the adjacent workstation. The size and placement 

(pan bottom 0.81 m above the floor) of this ignition source were guided by preliminary FDS predictions. 

The igniter fire was supported by a flow of commercial-grade liquid heptanes (mixed isomers) sprayed at 

a nominally steady rate from the four nozzles pointing downward toward the pan. The heptane mix was 

supplied from a reservoir by a variable-speed pump. The desired fuel flow was preset before the test and 

measured in triplicate by catching the flow from each nozzle in a volumetric cylinder for a typical period 

of 20 s. The nominally 2 MW fire supported by the heptane flow typically impinged almost continually 

on the ceiling above the igniter. There was thus an essentially continuous wall of flames radiating toward 

the workstation along the central three-fourths of the length of one panel. In addition, the workstation 

was subject to radiation from the hot ceiling and the hot smoke captured below the ceiling. 
 Interim Report on Experiments to Support Fire Dynamics 

J–17

Test Variables 

There are numerous variables that might influence the burning of a workstation such as that examined 

here, starting with the nature of the materials and their spatial arrangement, the ignition conditions, etc. 

There are also variables unique to the WTC fires: the amount of paper-based clutter in a workstation, the 

possible presence of jet fuel (and its amount), the possible presence and amount of inert rubble generated 

by the airplane impact (fallen ceiling tiles, inert dust from pulverized concrete and/or wallboard), varying 

degrees of impact-induced break-up and compaction of the work station itself, and presence of 

combustible solids from the airplane. 

The tests were designed keeping the purposes in mind: (a) identify the current capability of FDS to 

predict complex burning behavior using combustion data from small-scale specimens and (b) obtain clues 

to improving the combustion algorithm should the predictions be of insufficient accuracy. Thus the two 

selected variables were those that could affect the workstation fires in manners that test FDS and are 

important in the WTC context: the presence or absence of both jet fuel and of inert rubble. The inert 

rubble was taken to be representative of fallen ceiling tiles. 

The primary test set thus focused on two levels of two variables examined in a full factorial design. This 

calls for four tests: (1) no inert rubble and no jet fuel, (2) rubble but no jet fuel, (3) jet fuel but no rubble 

and (4) rubble and jet fuel. The levels of the two variables were estimated to produce differences in 

burning behavior that would be clearly observable. Thus the inert rubble was chosen to cover 

approximately 30 percent of the horizontal surfaces facing the hot ceiling. As performed, 24 of the 

40 ceiling tiles were on the horizontal desk surfaces; 14 were on the central, open floor area; and 2 were 

on the chair seat. The fraction of the plan view horizontal area covered by these tiles was approximately 

31 percent. A total of 4 L (approximately 1 gal) of Jet A was spread over these same horizontal surfaces.1

Two additional tests were conducted. The first test (in the entire series) utilized what was nominally onehalf of a generic workstation, though it included both a full chair and full computer. This was done to 

gage the burning behavior of what was an entirely unknown system. The fourth test was of the high-end 

workstation. Both of these were conducted with no jet fuel and no inert rubble present. 

Test Procedure 

The workstation was assembled a few hours before a test. Since the ambient humidity was high 

(approximately 70 percent) for most of the tests, the paper was covered with plastic sheeting if it was to 

be exposed for more than 2 hours, although this probably did not preclude significant moisture pick-up in 

the outer portions of the paper piles. That moisture would be expected to somewhat slow the ignition 

process relative to a more normal humidity of 50 percent. 

At the beginning of each test, all instruments were calibrated. [The heat release rate was also measured 

for a few minutes after the test ended in order to verify the calorimeter baseline.] The heptanes flow was 

measured in triplicate. The test was initiated by starting the heptanes flow and immediately igniting it 

with a torch. This defined time zero. The heptanes flow was left on until late in the test unless there was 

 

1

 Areas close to the impact could have been drenched with higher quantities. The workstations would also have had to be 

extensively fragmented, which is not a situation being examined here. Another pragmatic factor that kept the jet fuel loading 

down was the probability that higher levels would have pushed the calorimeter beyond its maximum allowable capacity. 
Appendix J 

J–18

an indication that the fire was going to significantly exceed the 10 MW calorimeter capacity; this 

happened only once (test 5). The full test was video taped by all four cameras and a narration was fed to 

one camera, describing the sequence of ignition events that spread the fire over the accessible flammable 

surfaces. In two cases (tests 3 and 4) the residual weight of the paper piles in the two file drawers was 

obtained as a measure of the participation of this paper in the overall fire. 

In the two tests in which jet fuel was placed on the horizontal surfaces prior to the start of a test, this was 

done just before ignition of the 2 MW spray burner. A sprinkling can was used in two separate 

operations, each involving 2 L of the liquid fuel. First, the liquid was sprinkled on the horizontal desk 

surfaces and the objects on them (i.e., the various paper stacks or document boxes, the computer monitor 

and the keyboard) using a timed movement that attempted to allot an equal amount of liquid to each onethird section of the total work surface. The desk surface had been leveled after installation so that the 

liquid would not run preferentially in one direction. Next, an equal amount of the liquid was sprinkled in 

a similarly timed manner on the central open section of the carpet (not under the desk surfaces). Since the 

chair occupied a portion of this space, the allotment for that portion went onto the chair seat and back 

surfaces. For the objects on or in contact with the desk surface, there was some tendency for the jet fuel 

to wick into them if they were porous. This was true of the paper, the inert tiles and the wall panel fabric 

just above the desk surface. 

J.3.2 Preliminary Results 

Figures J–16 and J–17 show the workstation early in a test and at a time just before the peak heat release 

was reached. Figures J–18 through J–23 show the heat release rate curves from the six fire tests. 

Figure J–16. Photograph of workstation soon after ignition 

in test 2. 
 Interim Report on Experiments to Support Fire Dynamics 

J–19

Figure J–17. Photograph of workstation near the peak heat 

release rate in test 2. 

Figure J–18. Heat release rate versus time for single workstation test 1. 
Appendix J 

J–20

Figure J–19. Heat release rate versus time for single workstation test 2. 

Figure J–20. Heat release rate versus time for single workstation test 3. 
 Interim Report on Experiments to Support Fire Dynamics 

J–21

Figure J–21. Heat release rate versus time for single workstation test 4. 

Figure J–22. Heat release rate versus time for single workstation test 5. 
Appendix J 

J–22

Figure J–23. Heat release rate versus time for single workstation test 6. 

Table J–3 lists the peak HRR values from each test plus the time to that peak, rounded to the nearest 10 s. 

The HRR peak is given in two ways: first, as the absolute highest single reading recorded (for a 1 s 

interval) and second, as the average of the value at this peak plus values up to 5 s to either side of this 

peak. The latter compensates for both noise in the calorimeter system and in the fire itself; it is doubtful 

that objects in a real room can respond in any meaningful way to small HRR fluctuations in a fire that is 

under 10 s in duration. The averaged values are about 3 percent lower than the absolute peak values, a 

result of the sharpness of the peaks in every case. 

Table J–3. Peak heat release rates and times to peak.

Test Test Specimen 

Jet Fuel 

(Yes/No) 

Inert Tiles 

(Yes/No) 

Peak HRR 

(kW) 

Time to Peak 

HRR (s) 

1 Half of generic workstation N N 5920/5770 490 

2 Generic work station N N 8700/8480 530 

3 Generic work station N Y 7560/7300 590 

5 Generic work station Y N 9120/8910 160 

4 High-end work station N N 9890/9660 510 

6 Generic work station Y Y 7960/7690 200 

The videotapes of the fires show that the peak HRR corresponds closely to simultaneous burning of all 

the “accessible” combustible surfaces in the workstation interior. This included the top of the desk 

surface, the objects on it, the exposed area of the materials in the bookcase, the full chair area (but see 

below), the exposed area of the carpet (i.e., not that under the three steel file cabinets), objects on the 
 Interim Report on Experiments to Support Fire Dynamics 

J–23

carpet (computer processor case and wastebasket) and the underside of the desk, except above the steel 

file cabinets where the air access was limited. The interior surfaces of the wall panels contributed 

negligibly because the thermoplastic fabric melted and rolled downward into a mass having much less 

surface area than the original fabric prior to igniting. This type of behavior is not captured in Cone 

Calorimeter tests. 

The following observations emerge from comparison of the results in Table J–3: 

• The peak fire intensity from the half workstation is about two-thirds that of the full workstation. 

This is probably primarily the result of two factors: 

− The same chair was present in both cases; this chair has an estimated HRR peak in the 

neighborhood of 1/2 MW by itself. 

− The inert steel file cabinets cover twice as much of the carpet in the second half of the 

workstation, precluding its participation in the HRR peak; this lowers the HRR 

contribution from the second half of the workstation. 

• The computer was also totally combusted within the first half of the experiments, though this was 

a substantially lesser heat source than was the chair. 

• The ceiling tiles reduced the peak HRR in proportion to their coverage of the burning surfaces; 

both just under 15 percent. While there was 30 percent coverage of the upward facing horizontal 

surfaces, there was no coverage of the underside of the desk, the carpeting below the desk or the 

underside and back of the chair. As noted above, all of these were burning at the peak. (The 

reduction might become more than linear if nearly all of the upward facing horizontal surfaces 

were covered since this could preclude the progressive flame spread that gets all accessible 

surfaces involved) 

• The principal effect of the presence of 4 L of Jet A on the horizontal surfaces was in shortening 

the time to involvement of all accessible combustible surfaces, and thus the time to the peak 

HRR. The peak itself was boosted upward only about 5 percent, presumably because the Jet A 

helped boost the overall fuel gasification rate somewhat while adding its high heat of combustion. 

When the inert tiles were also present, the Jet A was poured across their top surfaces temporarily 

rendering these surfaces flammable. Since the tiles were porous, the Jet A burning rate on them 

was reduced, however, and the tiles still managed to produce a 13 percent to 14 percent reduction 

in the peak HRR relative to the case with Jet A and no tiles. 

From examination of the videotapes and the commentary, NIST determined that, when there was no Jet A 

present, there was a fairly reproducible progression of ignition events leading up to the HRR peak: 

• The onset of the igniter fire bathed the entire workstation in radiant heat. 

• The igniter fire was not the pilot flame that ignited other objects, although FDS simulations 

suggest that the peak fluxes on surfaces facing the igniter were of the order of 30 kW/m2

 or more, 

well above the minimum flux for piloted ignition of the various exposed surfaces. Even the top 

of the computer monitor shell, which gasified extensively only 20 cm from the spray burner 
Appendix J 

J–24

flames, did not appear to ever have directly been ignited by those flames. Instead the progression 

of flaming was initiated by the top sheets on the paper stacks. Its presence in many places made 

it highly instrumental in spreading the flames to the computer monitor shell, the desktop and the 

top of the chair back. Flaming material from the chair fell onto the carpet, igniting it. 

Subsequent spread on the carpet was delayed, however, until the chair was fully involved, along 

with the remainder of the upward-facing surfaces of the desktop. Later, paper ignition brought 

flames to the two other sides of the desk area in the workstation. 

• Once the entire area at and above the top of the desk surface was burning, the “compartment” 

under the desk flashed over from the radiant heat from the upper compartment and especially the 

heat from the burning chair. With all flammable surfaces ignited, the HRR quickly peaked. The 

specific construction of the chair and its location were critical to this fire growth process: 

− One corner of the seat was deliberately placed to extend approximately 15 cm under the 

desk surface on which the computer keyboard rested. This assured that some heat would 

reach this space early on, even though the chair flames would contact the relatively 

ignition resistant underside of the desk. 

− When only the upholstered surfaces of its seat and back were burning, the chair retained 

its original shape, and little of its heat reached the lower compartment area. 

− When the thermoplastic support shell of the chair began to melt and flow to the floor, 

extensive heat flowed directly into the “compartment” under the desk. 

− The partial steel skeleton kept the chair from collapsing and maintained burning from 

desk level to floor level. The flame radiation to the cavity under the desk quickly ignited 

the materials located there, while the upper part of the chair flames played on the desk, 

assisting ignition of its underside. 

Since chairs of different designs (and thus different burning behavior) could be fabricated from the same 

materials, the detailed fire behavior of the chair cannot be inferred simply from the Cone Calorimeter data 

for the component materials. Thus, empirical treatment of its HRR process is necessary. 

As expected, the progression of ignition events in the presence of Jet A was different. Ignition of the 

materials in the cubicle occurred more quickly. However, the manner of the acceleration was not what 

might have been expected. 

• Since the flash point of Jet A is at least 46 °C (approximately 20 °C above the ambient 

temperature), initially there would be no flammable mixture of vapors above the liquid fuel 

surface. As the strong radiant flux from the spray burner bathes the workstation, the videos show 

an increasingly dense aerosol rising from the various wetted surfaces near the spray burner. 

These did not ignite, as evidently turbulent mixing of the vapor plume above the surfaces quickly 

diluted the fuel vapor below the lean flammable limit. Instead, in the first Jet A test, a random 

piece of flaming material rose from the burner area and drifted down on top of the desk, 

immediately igniting the Jet A. In the second test, a flaming piece of debris from the burner 

landed on top of the bookcase where there was no jet fuel and it had no effect. The paper stacks 
 Interim Report on Experiments to Support Fire Dynamics 

J–25

on the work surface next to the spray burner finally dried out, began to char, and then transitioned 

into flaming, igniting the jet fuel. 

• The Jet A flames then spread rapidly, but did not sweep continuously around the desktop. 

Presumably the initial evaporation period left some dry spots that stopped the spread. 

• The carpet was ignited by flaming matter dropping from the chair. Apparently the turbulent 

vapor plume dilution process mentioned above prevented the flames from jumping downward to 

the carpet from the flaming desk surface, even though the carpet was emitting an aerosol. At any 

rate, the subsequent flame spread on the carpet was rapid. 

• None of these steps is resolved in the HRR curves of Figs. J–21 and J–22 due to the rapidity with 

which the curves jumped rapidly to their peak values soon after the Jet A ignited. As in the “dry” 

tests, the peak corresponds to all accessible combustible surfaces burning simultaneously. 

The rapid decay in HRR after the peaks in all tests presumably reflects several factors that should be 

captured in the combustion algorithm in FDS: 

• The various paper piles developed a thick ash layer that would drive down their burning rate. 

• Char formation on the desk surfaces drove down its burning rate. 

• The carpet began to burn out. 

However, a number of geometric changes occurred that are beyond the capability of FDS to reproduce: 

• The chair fire rapidly collapsed to a pool fire on the floor whose reduced burning area meant a 

reduced HRR. 

• The front of the bookcase, resting on top of that unit, typically fell, changing the location of its 

13 reams of paper. 

• The desk surface bowed progressively as it charred through, and then it collapsed, with separate 

sections doing so at differing times. The initial desk collapse probably did not greatly affect its 

burning rate, but ultimately what was left was a complex rubble pile whose burning would not be 

predictable from any knowledge of the original configuration coupled with Cone Calorimeter 

data. 

• The wall panels collapsed at random times, both inward and outward, typically rather late in the 

fire. 

Thus, the further one goes out on the HRR curve, past the peak, the less it is predictable by an FDS 

calculation that retains the original geometry. Fortunately, the major effects appear to occur well after the 

desk surfaces collapse and the time when contiguous workstations would become ignited and dominate 

the heat release. 
Appendix J 

J–26

Numerical grids of 10, 20, and 40 cm were used to model the fires and ensure that the model was not 

sensitive to grid cell size. Figure J–24 shows a preliminary comparison of the FDS HRR prediction with 

the measured values for test 2 (no jet fuel, no inert tiles). The quality of fit is typical of the test series. 

Figure J–24. Preliminary comparison of predicted and measured HRR values. 

For these simulations, the thermal properties of the major materials making up the workstations were 

derived from Cone Calorimeter experiments. The carpet and privacy panel were modeled as 

thermoplastics, that is, the burning rate was assumed to be proportional to the heat flux from the 

surrounding gases. The desk was modeled as a charring solid, in which a pyrolysis front propagates 

through the material leaving a layer of char behind that insulates the material and reduces the burning 

rate. Details of the pyrolysis models can be found in the FDS Technical Reference Guide (McGrattan et 

al. 2002). Each feature of the experimental curve was related (using annotations made during the tests 

and from the video tapes) to specific aspects of the workstation combustion. 

There are similarities and discrepancies between the experimental data and this prediction. 

• The shape and magnitude of the two curves is encouragingly similar, as is the total heat release 

(area under the curves). 

• The peak HRR occurred sooner in the simulation. In the experiment, the time to peak HRR was 

strongly influenced by the melting of the chair plastic onto the carpet. As noted above, this level 

of detail is not captured in the numerical model. 
 Interim Report on Experiments to Support Fire Dynamics 

J–27

• At long times, the simulation drops to the residual HRR of the 2 MW burner somewhat more 

abruptly than does the experimental curve, indicating that it has run out of combustible mass 

sooner. The importance of this effect is modest, given the geometric changes during the test 

(listed above) and the similarity of the total heat released. 

A more complete analysis will be detailed in the forthcoming documentation report. 

J.4 EXPERIMENTS FOR FIRE MODEL VALIDATION 

Following the experiments described above and the accordant improvements in FDS, a series of largescale experiments was conducted in the NIST Large Fire Laboratory between November 4 and December 

10, 2003. The six experiments were designed to assess the accuracy with which FDS predicts the fire 

spread, heat release rate, and thermal environment in a compartment burning multiple workstations in a 

configuration characteristic of that found in the WTC buildings. In each of these experiments, sets of 

three workstations, identical to the generic ones tested in Section J.3, were burned in a large compartment 

(see Fig. J-25). The challenges to the model included varying the location of the ignition burner (and 

thus the fire ventilation), adding jet fuel and/or noncombustible material occluding a fraction of the 

workstations’ surfaces, and “rubblizing” the workstations. FDS simulation of each test was carried out 

before the test was conducted. 

The steel-frame experimental enclosure was 10.8 m long × 7.0 m wide × 3.4 m tall (35.5 ft × 23 ft × 11 ft) 

and was lined with three layers of 13 mm (0.5 in) calcium silicate board (see Fig. J-26). There was a 

subfloor (not included in the above dimensions) to house instrumentation. The enclosure had openings on 

the front mimicking window openings through which fresh air entered and heat and combustion products 

were emitted. The narrowed openings limited the amount of fresh air that entered the burning enclosure. 

Each of three workstations was placed on an isolated platform made of calcium silicate board. The top 

surface of each platform was flush with the floor of the compartment. Each platform was supported on 

water-cooled load cells, located in the subfloor, to monitor the mass of the workstation throughout the 

test. The load cells were the same as those described in Section J.3. 

Two of the workstations were contiguous, exemplifying a part of the type of cluster that exists in many 

large office spaces. The third workstation was separated from the other two by an aisle, representing a 

part of a second cluster. This array was to enable assessment of FDS's ability to replicate two different 

modes of cubicle-to-cubicle fire spread: direct flame impingement and radiative ignition from the hot 

ceiling layer. 
 Interim Report on Experiments to Support Fire Dynamics 

J–28

Figure J–25. Plan view of test configuration. 

FIRE PAN AND 

SPRAY BURNER 

LOCATION 1 

WEST 

CARPET 

COLUMN 

0.75 m 

OPENING 0.75 m 

2.6 m 0.9 m 2.6 m 

APPROXIMATE 

DIMENSIONS 

0.5 m 0.5 m 

CUBICLE 3 CUBICLE 2 

EAST 

NORTH 

CUBICLE 1 

Thermocouple Tree 4 

Thermocouple Tree 3

Thermocouple 

Tree 2

Thermocouple 

Tree 1
 Interim Report on Experiments to Support Fire Dynamics 

J–29

Figure J–26. Elevation view of test configuration. 

The west end of the enclosure was located under a 10 m × 12 m hood for collection of the effluent and 

measurement of the heat release rate. 

Other instrumentation included: 

• Four floor-to-ceiling trees of thermocouples to measure vertical profiles of temperature(see 

Fig. J-27); 

• Thermocouples on the desk surfaces to track flame spread; 

• Two downward-facing, water-cooled Schmidt-Boelter total heat flux gages in the ceiling to 

measure radiative heat flux; 

• Two water-cooled Schmidt-Boelter total heat flux gages mounted on the west wall; and 

• Four video cameras placed to record the progression of flame spread over the objects in the 

cubicle. 

CUBICLE 3 CUBICLE 1 

INSTRUMENTATION SUB-FLOOR (external to experimental enclosure) 

APPROXIMATE 

DIMENSIONS 

0.3 m 

COLUMNS 

0.5 m wide 

OPENINGS 

2.1 m tall 

0.45 m wide 

FIRE PAN AND 

SPRAY BURNER 

LOCATION 1 

0.9 m 
J–30

Interim Report on Experiments to Support Fire Dynamics 

Figure J–27. Views of interior of test fixture.

Thermocouple trees 

burner 
Interim Report on Experiments to Support Fire Dynamics 

J–31

The liquid spray burner, pan, and fuel (mixture of heptanes) were the same as used in the Section J.3 

experiments. Depending on the test, the burner was located abutting the top of a workstation partition at 

the east end of cubicle 1 or the west end of cubicle 2. The ignition fire intensity was a nominal 2 MW 

fire. The spray burner was operated for the first few minutes of the tests, for either 2 min or 10 min 

depending on the test scenario. 

Materials 

The workstations were of the same type as the generic units used in the experiments reported in 

Section J.3. The carpet tiles, which covered the floor of the cubicles and the aisle, were also the same 

type used in experiments reported in Section J.3. 

Test Variables 

The experimental matrix is shown in Table J–4. The experiments investigated the impact of several 

parameters on the fire behavior: 

Table J–4. Test matrix.

Test Ceiling Tiles Jet Fuel Burner Location Workstations Windows 

1 None None Front Intact No 

2 None None Front Intact No 

3 Present Present Front Intact No 

4 Present None Rear Intact No 

5 Present Present Rear “Rubble” No 

6 None Present Rear Intact Yes 

• Location of the burner. This was placed either abutting the west end of cubicle 1 or abutting the 

east end of cubicle 2. These two sites resulted in significantly different access to the air needed 

for combustion. In the former (“front”) location, much of the oxygen in the air initially entering 

the enclosure was consumed by the burner and the burning cubicle 1, with the result that limited 

oxygen was available for combustion in the middle and rear of the compartment. With the burner 

in the latter (“rear”) location, the fresh air passed directly to the rear of the compartment. 

• The application of 12 L of jet fuel evenly distributed about each workstation. The procedure was 

the same as used in the experiments reported in Section J.3.The presence of fallen ceiling tiles. 

Having seen the effect of coverage of 30 percent of the top surfaces in the previous test series, 

NIST covered approximately 70 percent of the top surfaces here. 

• Fractured furniture. In one experiment (test 5), NIST investigated the effect of different degrees 

of “rubblizing” the furniture. 

− In cubicle 1, the workstation pieces were placed unassembled on top of each other, 

occupying the same footprint as the assembled workstation. The same mass of 

combustibles was present as in the fully assembled cubicle tests. No steel filling cabinets 

were used. Ceiling tiles and broken up drywall were intermixed with the rubble. 
Appendix J 

J–32

− Cubicle 2 was the same as cubicle 1, except without the drywall. 

− For cubicle 3, the workstation was partially assembled. The same mass of ceiling tile and 

drywall as in cubicle 1 were intermixed with the cubicle components. 

− Window breakage. Test 6 had four glass windows mounted on the north end of the west 

wall. During the course of the fires in the WTC towers, a number of windows were 

broken, presumably by the heat from the fires. These result in a change in both the 

degree and pattern of ventilation. 

Test Procedure 

This was similar to that followed in the experiments reported in Section J.3, except that the ignition 

burner was turned off after approximately 2 min for tests 3, 5, and 6 (when jet fuel was present) and for 

10 min for Tests 1, 2, and 4. The tests continued until the HRR fell below 0.5 MW, which was typically 

60 min after ignition. 

J.4.1 Preliminary Results 

Figures J–28 and J–29 show the east view of the compartment before and during a test, respectively. A 

few observations about the tests were: 

• The peak HRR was approximately 11 MW for four of the tests. In test 5, the peak value was only 

approximately 6 MW. In test 6, the peak HRR reached almost 16 MW. 

• As in the single workstation tests, the peak value was reached earlier when jet fuel was present. 

Figure J–30 shows how the enclosure was represented in FDS. The computational grid size was 0.4 m 

(1.3 ft) on a side. Note that the chair, computer monitor, and paper have been collected together into 

“boxes” with comparable mass to the various items that were spread throughout the workstations. To the 

right are five windows that are similar in size to those of WTC 1 and WTC 2. 

The preliminary plot in Fig. J–31 depicts the degree to which the heat release rate measurements agreed 

with those predicted by FDS. 

The overall degree of agreement between the model and the experimental data is quite good, despite some 

modest local differences. In the analysis leading to candidate improvements in the modeling, it is 

important to maintain perspective of the effect of these differences on the accuracy needed in 

reconstructing the actual WTC fires. For fires that are sufficiently severe that they threaten the structural 

integrity of the building, many such workstations will be burning concurrently. These workstations will 

be at various stages of their combustion. Thus, for example, features occurring at long times in Fig. J–31 

may not merit closer replication, while those features occurring at short times (and thus have a bearing on 

the ease of fire spread among workstations) may merit attention. 
Interim Report on Experiments to Support Fire Dynamics 

J–33

Figure J–28. View of fire compartment before the start of test 6. 

Figure J–29. View of fire compartment 2 minutes after the start of test 6. 
Appendix J 

J–34

Figure J–30. Fire Dynamics Simulator 3D rendition of experimental enclosure. 

WTC Phase 2, Test 1 Heat Release Rate

Time (s)

0 500 1000 1500 2000 2500 3000

Heat Release Rate (kW)

0

2000

4000

6000

8000

10000

12000

14000

16000

Experiment

FDS

Figure J–31. Comparison of measured and predicted heat release rate for test 1. 

J.5 REFERENCES 

McGrattan, K.B., H.R. Baum, R.G. Rehm, G.P. Forney, J.E. Floyd, K. Prasad and S. Hostikka. 2002. Fire 

Dynamics Simulator (Version 3), Technical Reference Guide. NISTIR 6783. National Institute of 

Standards and Technology, Gaithersburg, MD. 
J–35

Attachment 1 

Simulating the Fires in the World Trade Center 

1.1 INTRODUCTION 

In the months following the attacks on the World Trade Center (WTC) and the Pentagon, there was an 

active debate in the fire protection engineering community about the fires that erupted following the 

impact of the aircraft on the buildings. Because fires of this magnitude in these types of buildings are 

rare, there is a wide spectrum of opinion about the fire temperatures and their effect on the structural steel. 

Much of the fire literature consists of empirical correlations derived from experiments ranging from 

bench scale to room scale. Extrapolating these well-known correlations to the WTC requires a 

reexamination of the underlying assumptions. Many of these correlations are appropriate for a narrow 

range of fire sizes and building geometries, and cannot be directly applied to the WTC fire scenarios. As 

a result, computer fire models that have been developed over the past decade are being applied to the 

analysis. 

As part of the investigation, the National Institute of Standards and Technology (NIST) has conducted 

simulations of the fires in each building using a computational fluid dynamics (CFD) model known as the 

Fire Dynamics Simulator (FDS). This attachment will describe the experiments conducted at NIST to 

calibrate and validate the FDS model for use in the WTC project, and it will describe the techniques 

developed to simulate the very extensive fires that spread over 6 to 12 floors in the different buildings. 

1.1.1 Experimental Program 

Two large-scale test series were conducted to provide validation for the FDS, plus various small-scale 

experiments were conducted to provide the model with input data for different materials. The large-scale 

test programs are referred to as Phase 1 and Phase 2. Both test series involved fires in compartments with 

the same ceiling height as a floor in WTC 1 or WTC 2. Phase 1 was a series of fire tests with a liquid fuel 

spray burner generating a fixed amount of energy in a compartment with various targets and obstructions, 

like columns, trusses and other steel objects. These tests were designed to test the accuracy of the model, 

and its sensitivity to changes in various input parameters. Phase 2 was a series of fire tests in which office 

workstations similar to those used in WTC 1, WTC 2, and WTC 7 were burned in a compartment with 

limited openings to simulate the under-ventilated conditions of the WTC fires. These tests were designed 

to test the model’s ability to characterize the burning behavior of real furnishings under conditions typical 

of the WTC fires. Only the Phase 2 work will be discussed in this attachment. 

1.1.2 NIST Fire Dynamics Simulator 

FDS is a CFD model of fire-driven fluid flow. It solves numerically a form of the Navier-Stokes 

equations appropriate for low-speed, thermally-driven flow with an emphasis on smoke and heat transport 

from fires (McGrattan et al. 2002). Version 1 was publicly released in February 2000. The core 

algorithm is an explicit predictor-corrector scheme, second order accurate in space and time. Turbulence 

is treated by means of the Smagorinsky form of Large Eddy Simulation (LES). For most applications, 
Appendix J 

J–36

FDS uses a mixture fraction combustion model. The mixture fraction is a conserved scalar quantity that is 

defined as the fraction of gas at a given point in the flow field that originated as fuel. The model assumes 

that combustion is mixing-controlled, and that the reaction of fuel and oxygen is infinitely fast. The mass 

fractions of all of the major reactants and products can be derived from the mixture fraction by means of 

“state relations,” empirical expressions arrived at by a combination of simplified analysis and 

measurement. 

Radiative heat transfer is included in the model via the solution of the radiation transport equation for a 

non-scattering gray gas, and in some limited cases using a wide band model. The equation is solved using 

a technique similar to finite volume methods for convective transport; thus the name given to it is the 

Finite Volume Method (FVM). Using approximately 100 discrete angles, the finite volume solver 

requires about 15 percent of the total CPU time of a calculation, a modest cost given the complexity of 

radiation heat transfer. FDS approximates the governing equations on a rectilinear grid. The user 

prescribes rectangular obstructions that are forced to conform with the underlying grid. 

All solid surfaces are assigned thermal boundary conditions, plus information about the burning behavior 

of the material. Usually, material properties are stored in a database and invoked by name by the user. An 

extensive effort was undertaken to characterize the thermal properties of common items found in an office 

setting, like privacy panels, stacks of paper, computer monitors, office chairs, pressboard tables, desks, 

and carpeting. These materials will be described next. 

1.2 CALIBRATION AND VALIDATION EXPERIMENTS 

The experimental program concentrated on the thermal properties of the office furnishings that 

constituted the bulk of the combustible fuel within the WTC buildings under study. Several types of 

office workstations typical of those used in WTC 1 and WTC 2 were purchased at area office supply 

stores. The thermal properties of the major materials making up the workstations were derived from cone 

calorimeter experiments. These properties were input into FDS, which was used to simulate the burning 

behavior of a single workstation burning under a 2.5 m ceiling with baffles to contain a hot layer of 

smoke above the burning workstation. Other than the baffled ceiling, no walls surrounded the workstation 

other than its own privacy panels. The thermal properties of the workstation components were adjusted 

slightly so that the FDS prediction of the heat release rate would match the experiment. Then the model 

was used to predict the heat release rate of 3 workstations burning within a large enclosure. The purpose 

of this exercise was to determine if FDS could simulate the dynamics of a fire in a setting similar to 

WTC 1, WTC 2, and WTC 7. 

1.2.1 Description of the Workstation Components 

Cone calorimeter experiments at three different heat fluxes were performed for the carpet, desk (wood), 

computer monitor, chair, privacy panel, and stacked paper. For the simulations of the WTC fires, only the 

carpet, desk and privacy panel data were used directly. The carpet and privacy panel were modeled as 

thermoplastics, that is, the burning rate is assumed to be proportional to the heat flux from the 

surrounding gases. The desk was modeled as a charring solid, in which a pyrolysis front propagates 

through the material leaving a layer of char behind that insulates the material and reduces the burning 

rate. Details of the pyrolysis models can be found in the FDS Technical Reference Guide (McGrattan et 

al. 2002). 
 Interim Report on Experiments to Support Fire Dynamics 

J–37

The desk was modeled as a charring solid. The thermal properties of the wood and its char were taken 

from both the calorimeter experiments and the work of Ritchie et al. (1997). It is 2.8 cm thick with 

density 450 kg/m3

, specific heat 1.2 kJ/kg/K at 20 °C and 1.6 kJ/kg/K at 900 °C, conductivity 

0.13 W/m/K at 20 °C and 0.16 W/m/K at 900 °C. The ignition temperature is 360 °C and the heat of 

combustion is 14,000 kJ/kg ± 800 kJ/kg. Its total available energy content is 210 MJ/m2

 ± 50 MJ/m2. 

The carpet was modeled as a thermoplastic with density 750 kg/m3

, specific heat 4.5 kJ/kg/K, 

conductivity 0.16 W/m/K, ignition temperature 290 °C, thickness 6 mm, heat of vaporization 2,000 kJ/kg, 

heat of combustion 22,300 kJ/kg ± 600 kJ/kg, and total available energy content 61 MJ/m2

 ± 2 MJ/m2. 

The privacy panel was modeled as a thermally-thin thermoplastic. The product of specific heat, thickness 

and density is 0.73 kJ/m2

/K. Its surface density is 0.25 kg/m2, ignition temperature 380 °C, heat of 

vaporization 6,000 kJ/kg, heat of combustion 30,000 kJ/kg ± 500 kJ/kg. Its total available energy content 

is 6.0 MJ/m2

 ± 1.3 MJ/ m2. 

The test compartment walls and ceiling were made of three layers of 1.27 cm (0.5 in) thick Marinite I, a 

product of BNZ Materials, Inc. (http://www.bnzmaterials.com).2

 The manufacturer provided the thermal 

properties of the material used in the calculation. The density is 737 kg/m3

, conductivity 0.12 W/m/K. 

The specific heat ranged from 1.2 kJ/kg/K at 93 °C to 1.4 kJ/kg/K at 425 °C. 

In the simulations of the fires within the WTC, the chair, computer, paper, and other miscellaneous items 

within the workstation were modeled as a single item by lumping the mass together into large “boxes” 

and distributing them throughout the workstation. It is common practice in fire protection engineering to 

use surrogate materials for fire experiments, and this practice has been extended to numerical modeling. 

Over the years, various items have been developed that are representative of various types of 

commodities. For example, wood cribs are often used to represent ordinary combustibles found in 

residential or light industrial settings. Paper cartons with various amounts of plastic within are also used 

as surrogates for a wide range of retail commodities. One in particular is called the FMRC (Factory 

Mutual Research Corp.) Standard Plastic Commodity, or more commonly, Group A Plastic. This test fuel 

is often used in sprinkler approval testing at Factory Mutual and Underwriters Laboratories in the US, and 

similar test fuels have been developed in Europe. In the late 1990s, FDS was used to simulate large scale 

rack storage fires to determine the effectiveness of the combined use of sprinklers, roof vents and draft 

curtains (curtain boards). As part of this effort, a considerable amount of work was done to characterize 

the thermal properties of Group A Plastic (Hamins and McGrattan 2003). Because Group A Plastic has 

been shown to be fairly representative of fires fueled by a mixture of paper (cellulosic materials) and 

plastic, and because it has been used in numerous FDS simulations, it was decided to model the contents 

of the office workstations with a fuel similar to Group A Plastic. Blind predictions of the single open 

workstation burns were made using the material properties obtained during the sprinkler/roof vent study, 

and then these properties were adjusted to match the results of the experiments. Thus, the single 

workstation burns served to calibrate the model. They were not intended to be validation experiments. 

The validation experiments consisted of burning 3 workstations at a time in an under-ventilated 

compartment. 

 

2

 Certain commercial equipment, instruments, or materials are identified in this document. Such identification does not imply 

recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products 

identified are necessarily the best available for the purpose. 
Appendix J 

J–38

The surrogate fuel is modeled as a homogenous solid whose density is 172 kg/m3. The paper carton is 

treated as a thermally-thin material whose density × specific heat × thickness is 1.0 kJ/m2

/K. Its ignition 

temperature is 370 °C and the heat of combustion is 30,000 kJ/kg. The heat release rate of the boxes 

ramps up to 450 kW/m2

 in about 1 min. Note that this fuel package is similar, but not the same, as Group 

A Plastic. The density has been increased to account for all the miscellaneous items within the 

workstation. Also note that unlike the desk, partition and carpet, the boxes are simply given a burning 

rate rather than a heat of vaporization, meaning that the boxes will burn at the given rate regardless of the 

heat flux upon them as long as the surface temperature remains above its ignition temperature. The 

reason for this is that it is not possible to predict the burning rate using the heat feedback approach 

because the geometry of the scattered fuel is too complex to directly predict the response of the material 

to the thermal insult. By collecting all the scattered items into boxes, the geometry of the combustibles is 

greatly simplified, and as a result the burning behavior must be simplified as well. 

1.2.2 Description of the Simulations 

The geometry of the compartment is relatively simple. The overall enclosure is rectangular, as are the 

vents and most of the obstructions. Numerical grids of 20 and 40 cm were used to model the fires. The 

purpose of the grid variation was to ensure that the model was not sensitive to the change in grid cell size. 

Typically, enclosures of this size are modeled using 10 cm grid cells. However, for the simulations of 

WTC 1, WTC 2, and WTC 7, a 40 cm grid was used. By simulating the experiments at 20 cm and 40 cm, 

NIST can test if the model produces significantly different results with grid cells of different sizes. 

Figure 1–1 is a snapshot of a simulation showing the fire and the major geometric features of the 

compartment for the simulations. Note that the surrogate fuel packages are placed roughly where the 

computer monitor, chair and paper were located. Six tests were performed, with various ignitor locations 

and fuel arrangements. A 2 MW burner was pl aced either near the windows of the compartment 

overlooking the workstation nearest the openings in Tests 1, 2, and 3. The burner was placed towards the 

rear of the compartment overlooking the workstation in the rear of the compartment in Tests 4, 5, and 6. 

In Tests 3, 5, and 6 Jet A fuel was poured over the workstations and surrounding carpet. To simulate this 

in the model, spray nozzles were positioned over the center of each workstation, 2 m above the floor. 

These nozzles are normally used to simulate water sprinklers, but in this case, the water was replaced by a 

liquid having similar properties to Jet A. The nozzles were activated for 2 s, in which time the equivalent 

amount of liquid as in the tests was ejected and spread over the furnishings. 

In Test 5, Workstations 1 and 2 were disassembled prior to the burn and the contents were piled on top of 

the respective load cells. To model this scenario, the burning rate of the collective fuel packages was 

reduced by one half to account for the decrease in burning area of the fuel pile. The choice of one half 

was somewhat ad hoc. No free burns of workstation parts had been performed. This was the only test in 

which the simulated fuel packages had to be modified from their free-burn values. In this regard, Test 5 

was used to calibrate, not validate, the model. 
 Interim Report on Experiments to Support Fire Dynamics 

J–39

Figure 1–1. Geometry of the Phase 2 simulations. 

From a modeling perspective, the objective of the simulations of the Phase 2 experiments was to 

demonstrate that a simplified model of an office workstation can be used to predict the burning behavior 

of a group of workstations in an enclosure with features similar to WTC 1, WTC 2, and WTC 7. Because 

of the magnitude of the simulations of the building fires, the model of the workstation had to be fairly 

crude. However, because of the many uncertainties in the initial conditions of the fire simulations, the 

lack of detail in the model is not considered to be a problem. The model fires had similar growth patterns, 

peak heat release rates, decay patterns, and compartment temperatures. 

The model also captured the major features of the individual tests. For example, Tests 1 and 4 were 

similar in design except for the burner location. In Test 1 the burner was near the windows; in Test 4 it 

was near the rear of the compartment. The peak heat release rate was reached in about 15 min in Test 1, 

whereas it was reached in about 10 min in Test 4. The model shows a similar trend. The faster growth of 

Test 4 is probably due to the fact that the compartment heated up more quickly with the fire deep inside 

rather than near the windows, leading to more rapid spread of the fire across the pre-heated furnishings. 

Even though ceiling tiles were distributed over the desk and carpet in Test 4, this did not seem to have a 

noticeable effect on the growth, or at the very least the burner position seemed to have a far greater role in 

explaining the difference between Tests 1 and 4. The comparison of HRR between model and experiment 

is shown for Test 1 in Figs. 1–2. The upper layer temperature in the rear of the compartment for this 

same test is shown in Figs.1–3. The results for the other tests are comparable. The peak HRR and 

temperature are predicted well, as well as the duration of the fires. Both the peak values and the duration 

of the burning are important for the WTC simulations because it is not only important to predict the 

temperatures that the structural steel was exposed to, but also the duration of the exposure. 
Appendix J 

J–40

WTC Phase 2, Test 1 Heat Release Rate

Time (s)

0 500 1000 1500 2000 2500 3000

Heat Release Rate (kW)

0

2000

4000

6000

8000

10000

12000

14000

16000

Experiment

FDS

Figure 1–2. Comparison of HRR for Phase 2, Test 1. 

Time (s)

0 1000 2000 3000

Temperature (C)

0

200

400

600

800

1000

1200

Experiment

FDS

Figure 1–3. Upper layer gas temperature in the rear of the compartment, 

Phase 2, Test 1. 
 Interim Report on Experiments to Support Fire Dynamics 

J–41

1.3 SIMULATIONS OF THE FIRES IN WTC 1 AND WTC 2 

This section describes how the physical geometry of the buildings was described in the numerical model. 

Information about the layout of the relevant floors was obtained from architectural drawings provided by 

the occupants. For floors where information was not available, the geometry of a nearby floor or a floor of 

similar use was substituted. Information about exterior damage and window breakage was obtained by 

studying thousands of photographs and videos. There was no attempt made to predict the window 

breakage in the simulations. This information was provided as a boundary condition. 

1.3.1 Numerical Grid

The windows in WTC 1 and WTC 2 were nominally spaced 1 m apart. In addition, the external columns 

plus their aluminum cladding were assumed to be 0.5 m wide. The slab-to-slab floor spacing was 

assumed to be 3.6 m. Because of these approximations, a uniform numerical mesh consisting of cells 

whose dimensions were 0.5 m × 0.5 m × 0.4 m was used. In the model, each tower face consisted of 

58 windows, 61 columns, and two 0.5 m spacers next to each corner column. In the real tower geometry, 

these spacers formed the bevel. Figure 1–4 shows a single floor of the WTC 1 as it is approximated by 

the numerical model. 

The numerical grid for each floor of WTC 1 and WTC 2 was of dimension 128 × 128 × 9 cells. The 

128 cells in the horizontal directions allow for several meters of simulation outside of the external walls. 

The calculations were run in parallel, thus each floor was assigned to a different processor. The floor 

slabs, core walls, and workstations were approximated as thin obstructions. As described in the previous 

section, the contents of each workstation were collected into boxes and distributed throughout. 

Penetrations in the floor slabs representing elevator shafts and HVAC ducts were created in the model by 

defining rectangular plates on top of the floor slab that were removed at the start of the calculation. This 

served to carve out holes in the floor. Window breakage was modeled by removing thin obstructions 

serving as windows at times obtained from the analysis of photographs and videos. Broken external 

columns were removed the same way. 

1.3.2 Parallel Processing 

Modeling the fires on multiple floors of WTC 1 and WTC 2 is computationally intensive, both in terms of 

CPU time and memory. Up to this point in its development, FDS has been limited to calculations small 

enough to run on a single CPU and fit into the memory of a desktop personal computer. The WTC study 

is an example of a large-scale fire modeling problem that is impossible to analyze without the use of 

parallel processing. In terms of parallelization, the exact details of FDS are not important. The approach 

taken to run the code on a cluster of machines can be applied to virtually any CFD code, in particular 

those that involve three spatial dimensions and time. In such cases, the computational demand is fairly 

well represented by the product of the number of computational grid cells and the number of time steps 

taken to advance the solution of the governing equations in time. For example, if the computational grid 

consists of 1 million cells and the simulation requires ten thousand time steps, the demand is 

1010 cell-cycles. The overall demand can be broken down into memory requirement and CPU time. The 

memory requirement is a function of the number of grid cells; the CPU time is a function of the number 

of time steps. 
Appendix J 

J–42

Figure 1–4. Plan view of a typical floor in WTC 1. 

Roughly speaking, state of the art 32 bit processors can complete roughly 100,000 FDS cell-cycles per 

second. Realistic simulations of fires such as those in the WTC require on the order of 500,000,000,000 

cell-cycles, or about two months of calculation on a single 2 GHz processor. Plus, the calculation would 

require 6 to 12 gigabytes of memory (RAM), well over the 4 gigabyte address space of 32 bit processors. 

Because of this, the WTC calculations are not only impractical on single processor systems, they are 

impossible on any 32 bit processor. A 64 bit processor system may theoretically handle the static memory 

requirements of a large simulation time, but the run times for large calculations remain prohibitive. 

Because of the computational and memory issues of large fire simulations, a parallel version of the fire 

model must meet the two fundamental requirements discussed above, as well as satisfy a number of 

practical implementation concerns. Both the computational and the memory requirements must be 

distributed across multiple processors. The simulation must be done so that each processor uses less than 
 Interim Report on Experiments to Support Fire Dynamics 

J–43

4 gigabytes of memory, while enough processors must be used to reduce the simulation to a practical 

length of time, of the order of one week. 

Because the computational load is distributed throughout much of the source code, NIST has chosen to 

break up the calculation into multiple spatial blocks, with each block essentially doing the same type of 

calculation. A feature common to most CFD codes is multi-block or multi-mesh structure in which more 

than one structured grid is used in the calculation. This feature is exploited by simply putting the data and 

computation for each block on a different processor. This has advantages and some limitations. The 

advantages are (1) a natural and scalable extension of the existing code, (2) the amount of data 

communication will be kept to a minimum, since only overlap information needs to be communicated, 

rather than the data for full blocks, (3) source code changes are localized in small communication 

routines, (4) development is fairly fast. The disadvantages to the multi-block approach are (1) equal 

distribution of work across processors (load balancing) depends on spatial symmetry in the simulation, 

such as the translationally symmetric geometry of the WTC floors, (2) the level of parallelism and the 

speed up of the calculation is limited to the number of spatial blocks that can be used in the calculation. 

These limitations are not severe in many cases, including the WTC. 

Because NIST is interested in a scalable, portable code, Message Passing Interface (MPI) is used. This is 

a standard, well-documented system of implementing parallel processing, that can work with shared 

memory, distributed memory, or combinations of those architectures (Gropp, 1999). Our goal in using 

MPI was to produce a code that, except for the requirement of the MPI library, would be as portable and 

standardized as the sequential version. The parallel code runs on most computer platforms, including 

networked Windows-based PCs. NIST opted for a cluster of commodity personal computers running 

Linux, connected by a gigabit ethernet network. The individual processors are in the range of 2.0 to 

2.8 GHz, and dual processor machines were chosen to save space and to allow the addition of OpenMP 

code as a future extension to the MPI-based code. For production work in the NIST laboratory, two 

clusters are used: a smaller, development cluster to develop and debug the code, and a larger cluster with 

128 processors. Using both clusters provides the capability to run six to eight large parallel processing 

jobs simultaneously. 

1.3.3 Sample Simulation 

Shown in Fig. 1–5 is a sequence of snapshots showing the predicted upper layer temperatures on a floor 

of WTC 1 at time increments of 15 min. The first image is a cut-away showing the damage to the north 

face of the tower and the layout of the walls and furnishings. The subsequent images are color coded by 

temperature, with the red (or dark) patches representing temperatures in the vicinity of 1,000 °C. Initially, 

these hot areas of active burning are near the impact zone at the north end (foreground of picture), but 

migrate towards the south as the combustible furnishings are exhausted. Driving the progress of the fires 

is the breaking of windows that provide air to the oxygen starved fire. The window breakage is not 

predicted by the model; it is an imposed boundary condition resulting from the analysis of thousands of 

photographs and videos recorded that day by eye witnesses. The uncertainty in the window break times is 

on the order of 5 min in areas not obscured by smoke. 
Appendix J 

J–44

Figure 1–5. Predicted upper layer temperatures of a floor of WTC 1. 

The burning behavior shown by the simulation is similar to that of the fires in experiments conducted by 

Ian Thomas and Ian Bennetts (1999). They looked at fire spread in long and wide enclosures with a 

single ventilation opening, where the fires were ignited at various points deep within the bench-scale 

compartments used. The fires would rapidly spread across the liquid or solid fuels covering the floor 

without consuming much of the fuel. The fires would then surround the compartment opening and burn 

back into the compartment as the fuel near the opening was exhausted. In the WTC simulations, fires are 

ignited over a wide area by simulated spray nozzles ejecting a liquid with properties of aircraft fuel. 

Much of the available oxygen is consumed rapidly, driving the fires to the openings created by the 

aircraft. The fires move away from the initial impact area as the nearby furnishings are exhausted, and as 

windows are broken out away in other parts of the building. 
 Interim Report on Experiments to Support Fire Dynamics 

J–45

1.4 SUMMARY 

The investigation into the cause of the collapse of WTC 1, WTC 2, and WTC 7 by NIST will not be 

completed until the fall of 2004. Work is on-going to simulate the weakening of the structural steel due to 

the aircraft impacts and the fires. Nevertheless, the fire experiments and simulations performed to date 

have improved our ability to analyze the response of any large building or structure to fire. In the years 

ahead, these techniques will become increasingly widespread due to faster computers and the ability to 

harness an entire set of off-the-shelf personal computers to perform very large calculations. Effective 

modeling is a combination of fast computers, efficient algorithms, and well-planned small and large scale 

experiments to provide both input to the model and a validation of results. Projects as complicated as the 

WTC study are rarely conducted using modeling alone. There is and will always be a need to coordinate 

computation and experiment to reconstruct the dynamics of large fires. 

1.5 REFERENCES 

Gropp, W., E. Lusk, and A. Skjellum. 1999. Using MPI, MIT Press, Cambridge, Massachusetts, USA. 

Hamins, A., and K.B. McGrattan. 2003. Reduced-Scale Experiments on the Water Suppression of a RackStorage Commodity Fire for Calibration of a CFD Fire Model. Fire Safety Science: Proceedings of 

the Seventh International Symposium. International Association for Fire Safety Science. 

McGrattan, K.B., H.R. Baum, R.G. Rehm, G.P. Forney, J.E. Floyd, K. Prasad and S. Hostikka. 2002. Fire 

Dynamics Simulator (Version 3), Technical Reference Guide. NISTIR 6783. National Institute of 

Standards and Technology, Gaithersburg, MD. 

Ritchie, S.J., K.D. Steckler, A. Hamins, T.G. Cleary, J.C. Yang, and T. Kashiwagi. 1997. The Effect of 

Sample Size on the Heat Release Rate of Charring Materials. Fire Safety Science: Proceedings of the 

Fifth International Symposium. International Association for Fire Safety Science. 

Thomas, I.R. and I.D. Bennetts. 1999. Fires in Enclosures with Single Ventilation Openings–Comparison 

of Long and Wide Enclosures. Fire Safety Science: Proceedings of the Sixth International 

Symposium. International Association for Fire Safety Science. 
Appendix J 

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K–i

TABLE OF CONTENTS - Appendix K

List of Figures.........................................................................................................................................K–iii

List of Tables .......................................................................................................................................... K–vi

Appendix K

Interim Report on Subsystem Structural Analysis of the WTC Towers............................ K–1

K.1 Purpose........................................................................................................................................ K–1

K.2 Scope of Work ............................................................................................................................ K–1

K.3 Description of Subsystem Structures .......................................................................................... K–2

K.3.1 Full Floor Subsystem Description ................................................................................... K–2

K.3.2 Exterior Wall Subsystem Description ............................................................................. K–3

K.4 Loads........................................................................................................................................... K–4

K.5 Materials ..................................................................................................................................... K–4

K.5.1 Concrete........................................................................................................................... K–5

K.5.2 Steel ................................................................................................................................. K–8

K.5.3 Welds............................................................................................................................. K–15

K.5.4 Bolts............................................................................................................................... K–15

K.5.5 Coefficient of Friction ................................................................................................... K–16

K.5.6 Symbols......................................................................................................................... K–16

K.6 Model Conversion From SAP to ANSYS ................................................................................ K–17

K.6.1 Translation Procedure.................................................................................................... K–18

K.6.2 Challenges ..................................................................................................................... K–19

K.6.3 Status ............................................................................................................................. K–19

K.7 Full Floor Subsystems............................................................................................................... K–24

K.7.1 Full Floor Model............................................................................................................ K–24

K.7.2 Knuckle Analysis........................................................................................................... K–26

K.7.3 Column Truss Seats....................................................................................................... K–31

K.7.4 Modeling Connection Failure by Break Elements ........................................................ K–40

K.7.5 Truss Model................................................................................................................... K–42

K.8 Exterior Wall Subsystem .......................................................................................................... K–57

K.8.1 Description of Exterior Wall Subsystem Model............................................................ K–61

K.8.2 Validation of the Exterior Wall Subsystem Model........................................................ K–61

K.8.3 Model of One-Story High Exterior Column.................................................................. K–61
Appendix K 

K–ii

K.8.4 Model of Nine-Story High Exterior Model ................................................................... K–61

K.8.5 Models of the Column Splice ........................................................................................ K–61

K.8.6 Prefabricated Panel Model ............................................................................................ K–61

K.8.7 Ongoing Work on the Exterior Wall Subsystem Model................................................ K–61

K.9 Floor Truss Dynamic Response Due to Impact of Dropping Floor.......................................... K–61

K.9.1 Impact of Dropping Floor.............................................................................................. K–61

K.9.2 Purpose and Scope......................................................................................................... K–61

K.9.3 Method of Analysis ....................................................................................................... K–61

K.9.4 Results ........................................................................................................................... K–61

K.9.5 Conclusions ................................................................................................................... K–61

K.10 References ............................................................................................................................. K–61
K–iii

LIST OF FIGURES

Figure K–1. Temperature–dependent concrete properties. ................................................................ K–6

Figure K–2. Concrete stress-strain curves.......................................................................................... K–7

Figure K–3. Temperature-dependent properties for all steels............................................................ K–9

Figure K–4. Stress-strain relationships for Material ID 1 steel........................................................ K–11

Figure K–5. Creep behavior at elevated temperatures for Material ID 1 steel................................. K–13

Figure K–6. Maximum plastic strain from the finite element analysis and limiting 

plastic strain. ................................................................................................................ K–14

Figure K–7. 7/8 in. A325 bolt load–elongation curves at elevated temperatures. ........................... K–15

Figure K–8. Converted ANSYS model for floor 96 of WTC 1: overall view. ................................ K–20

Figure K–9. Converted ANSYS model for floor 96 of WTC 1: partial view near corner 

of building.................................................................................................................... K–20

Figure K–10. Converted ANSYS model for floor 96 of WTC 1: close-up view at corner 

of building.................................................................................................................... K–21

Figure K–11. Converted ANSYS model for floor 96 of WTC 1: view of floor beams 

and columns. ................................................................................................................ K–21

Figure K–12. Deformed shape of gravity load case for SAP floor model. ........................................ K–22

Figure K–13. Deformed shape of gravity load case for ANSYS floor model with BEAM44 

(Euler beam) elements. ................................................................................................ K–22

Figure K–14. Dominant mode shape of floor structure for SAP floor model.................................... K–23

Figure K–15. Dominant mode shape of floor structure sase for ANSYS floor model. ..................... K–23

Figure K–16. Transverse shear test of a knuckle. .............................................................................. K–27

Figure K–17. Longitudinal shear test of a knuckle. ........................................................................... K–27

Figure K–18. Finite element models of knuckle shear tests............................................................... K–29

Figure K–19. Compressive stresses in longitudinal shear finite element model................................ K–29

Figure K–20. Compressive stresses in transverse shear finite element model................................... K–30

Figure K–21. Shear force versus displacement from finite element model for longitudinal 

shear of two knuckles. ................................................................................................. K–30

Figure K–22. Shear force versus displacement from finite element model for transverse 

shear of two knuckles. ................................................................................................. K–30

Figure K–23. Interior seat. ................................................................................................................. K–32

Figure K–24. Exterior seat. ................................................................................................................ K–32

Figure K–25. Truss seat detail location on northeast quadrant of floor 96 of WTC 1....................... K–33

Figure K–26. Finite element model of exterior seat........................................................................... K–35

Figure K–27. Failure sequence of the exterior seats against tensile force. ........................................ K–36
Appendix K 

K–iv

Figure K–28. Typical tensile force resistance from exterior seat....................................................... K–36

Figure K–29. Strength of combined vertical and horizontal force..................................................... K–38

Figure K–30. Basic mathematical model of connection failure......................................................... K–41

Figure K–31. Simplified model of interior seat. ................................................................................ K–43

Figure K–32. Results of simplified seat model capturing failure from truss walking off 

interior seat. ................................................................................................................. K–44

Figure K–33. Results of simplified seat model capturing failure from exceeding the interior 

seat vertical shear capacity........................................................................................... K–44

Figure K–34. Simplified model of exterior seat................................................................................. K–45

Figure K–35. Simplified model of knuckle........................................................................................ K–46

Figure K–36. Truss model.................................................................................................................. K–48

Figure K–37. Boundary conditions. ................................................................................................... K–50

Figure K–38. Break elements at the interior end of slab.................................................................... K–50

Figure K–39. Comparison of resistance weld strength and yield strength of web member at 

elevated temperatures. ................................................................................................. K–52

Figure K–40. Displacement versus temperature. ............................................................................... K–53

Figure K–41. Axial force in truss members versus temperature........................................................ K–54

Figure K–42. Axial stress contour in the truss members at 663 oC.................................................... K–55

Figure K–43. Force in the knuckles versus temperature.................................................................... K–55

Figure K–44. Reaction forces at seats................................................................................................ K–56

Figure K–45. Finite element analysis results from increasing gravity............................................... K–56

Figure K–46. Exterior wall subsystem structure................................................................................ K–58

Figure K–47. Column and floor number materials and splice types.................................................. K–59

Figure K–48. Spandrels and spandrel splices..................................................................................... K–60

Figure K–49. Exterior wall subsystem model, viewed from inside of WTC 1.................................. K–61

Figure K–50. Portion of exterior wall subsystem model showing number of elements used. ........... K–61

Figure K–51. Schematic representation of columns used in the exterior wall 

subsystem model.......................................................................................................... K–61

Figure K–52. One-story exterior column model. ............................................................................... K–61

Figure K–53. Load-deflection of column at room temperature and 700 °C. ..................................... K–61

Figure K–54. Local buckling of column at room temperature........................................................... K–61

Figure K–55. Plastic hinge in column at room temperature............................................................... K–61

Figure K–56. Deformed shape of column at maximum axial load at 700 °C. ................................... K–61

Figure K–57. Nine-story column model. ........................................................................................... K–61

Figure K–58. Variation of maximum temperature and corresponding yield stress with time, 

fire scenario G.............................................................................................................. K–61
 Appendix K 

K–v

Figure K–59. Maximum compressive and tensile axial stress and corresponding yield stress 

with time, fire scenario G............................................................................................. K–61

Figure K–60. Deformed shape of column at 400 s, 3,200 s, and 5,000 s (floors 95–97)................... K–61

Figure K–61. Plate model of column splice, floors 97-98. ................................................................ K–61

Figure K–62. Simplified model of column splice. ............................................................................. K–61

Figure K–63. Column splice details, plate model and simplified model. .......................................... K–61

Figure K–64. Variation of axial displacement with axial load........................................................... K–61

Figure K–65. Variation of lateral displacement with shear load........................................................ K–61

Figure K–66. Variation of rotation with moment, out of wall plane.................................................. K–61

Figure K–67. Variation of rotation with moment, in plane of wall.................................................... K–61

Figure K–68. Variation of twist angle with torque. ........................................................................... K–61

Figure K–69. SAP2000 model of prefabricated panel. ...................................................................... K–61

Figure K–70. ANSYS model of prefabricated panel showing geometry and number of 

elements used............................................................................................................... K–61

Figure K–71. ANSYS model of prefabricated panel showing loading and 

boundary conditions..................................................................................................... K–61

Figure K–72. Deflection of prefabricated panels under 100 kip lateral load. .................................... K–61

Figure K–73. Deflection of prefabricated panels under 100 kip transverse load............................... K–61

Figure K–74. Deflection of prefabricated panels under 10 kip vertical load. .................................... K–61

Figure K–75. Schematic of full truss or partial truss drop and diagonal crushing at impact. ............ K–61

Figure K–76. Target truss resistance against increasing acceleration................................................ K–61
K–vi

LIST OF TABLES

Table K–1. Steel types used in WTC 1 and WTC 2. ........................................................................ K–9

Table K–2. Parameters for k(T) and n(T)........................................................................................ K–11

Table K–3. Uniaxial plastic strain at fracture ................................................................................. K–14

Table K–4. Comparison of SAP and ANSYS results for gravity load case. .................................. K–19

Table K–5. Comparison of SAP and ANSYS Modal Analysis Results. ........................................ K–23

Table K–6. Material properties used for truss seat calculations. .................................................... K–34

Table K–7. Interior seat capacity against vertical force.................................................................. K–37

Table K–8. Interior seat capacity against tensile force. .................................................................. K–38

Table K–9. Exterior seat capacity against vertical force. ............................................................... K–39

Table K–10. Exterior seat capacity against horizontal tensile force................................................. K–39

Table K–11. Compression strength of gusset plate. ......................................................................... K–40

Table K–12. Material assignments in truss model............................................................................ K–51

Table K–13. Resistance weld strength.............................................................................................. K–51

Table K–14. Column sectional properties. ....................................................................................... K–60

Table K–15. Spandrel splice details. ................................................................................................ K–60

Table K–16. Column splice details................................................................................................... K–60

Table K–17. Validation results. ........................................................................................................ K–61

Table K–18. Demand-to-Capacity ratio of long-span truss for static gravity load........................... K–61

Table K–19. Demand-to-Capacity ratio of long-span truss for dynamic impact load from 

full truss drop............................................................................................................... K–61

Table K–20. Peak deflection response due to static gravity and dynamic impact............................ K–61
K–1

Appendix K

INTERIM REPORT ON SUBSYSTEM STRUCTURAL ANALYSIS OF THE 

WTC TOWERS

K.1 PURPOSE 

Project 6 addresses the first primary objective of the technical investigation led by the National Institute 

of Standards and Technology (NIST) of the World Trade Center (WTC) disaster: to determine why and 

how the WTC towers (WTC 1 and WTC 2) collapsed following the initial impacts of the aircraft. 

Specifically, the objectives of this project are to determine the response of the structural components and 

systems to the fire environment in WTC 1 and WTC 2 and to identify probable structural collapse 

mechanisms. This appendix documents the progress achieved to date on Project 6 in thermal/structural 

modeling of WTC 1 and WTC 2. 

Project 6 seeks to determine the response of structural components and systems to the fire environment in 

the WTC 1 and WTC 2 and to identify probable structural collapse mechanisms by (1) evaluating the 

response of floor and column systems under fire conditions, (2) evaluating the response of the WTC 

towers without and with aircraft impact damage under fire conditions, (3) conducting tests of structural 

components and systems under fire conditions, and (4) evaluating competing failure hypotheses for the 

WTC towers. 

K.2 SCOPE OF WORK 

The scope of the work consists of the following three tasks: 

• Task 1, Subsystem Structural Analysis. The objective of Task 1 includes structural 

analysis of components and two subsystems, a full-floor subsystem, and an exterior wall 

subsystem. Task 1 is intended to provide guidance for the development of the global finite 

element models (FEMs) with respect to element types and sizes, appropriate constitutive 

models, and failure criteria for any given structural component. The subsystem analyses also 

will help to validate the accuracy of the global analyses, and correlate the results of the fine 

mesh component analyses with the coarser mesh global analyses of Task 2 and Task 3. 

• Task 2, Global Analysis of the WTC Towers’ Response to Fire without Impact Damage. 

The objectives of this task are to determine the general vulnerability of the towers to 

fire-initiated collapse and the role of fire in the towers with respect to structural stability, 

sequential failures of components and subsystems, and collapse initiation for the towers 

without impact damage. 

• Task 3, Global Analysis of the WTC Towers’ Response to Fire with Impact Damage. 

The objectives of this Task are to determine the relative roles of the impact damage and fires 

in the towers with respect to structural stability and sequential failures of the components and 

subsystems and to determine probable structural collapse initiation sequences. 
Appendix K 

K–2

Work under Task 1 includes the following: 

• Develop and validate ANSYS models of the full floor and exterior wall subsystems. 

• Evaluate structural responses for the following loading conditions. 

− Service loads due to gravity (dead and live loads). 

− Elevated structural temperatures. 

• Identify the possible, likely and most likely failure modes and failure sequences, and the 

associated temperatures at failure and times-to-failure. 

• Identify the changes in mechanical properties or geometry at initiation of component and 

subsystems collapse. 

• Identify simplifications for the global structural models and/or analyses of subsystem models 

to use in Task 2 and Task 3. 

The scope of this report is to present the progress made in Task 1 work. 

K.3 DESCRIPTION OF SUBSYSTEM STRUCTURES 

The full floor subsystem modeled is floor 96 of WTC 1. The model is believed to be typical in the upper 

floors of both towers. The exterior wall subsystem is a nine-column (three-panel) wide by nine-story 

(three-panel) high section of the WTC 1 between floor 91 and floor 100 and column 150 and column 158. 

This area is typical of the exterior walls of the towers and connects to a part of the floor system near the 

corner with different types of trusses. 

K.3.1 Full Floor Subsystem Description 

Floor 96 of WTC 1 was identified as an office floor with typical floor construction and loading and, 

therefore, was selected as the basis for this model. Components of the floor subsystem are examined for 

performance under loads and elevated temperatures in different possible failure modes. Understanding of 

these component behaviors is used to define the floor models for global analyses of WTC 1 and WTC 2. 

The full floor subsystem of floor 96 of WTC 1 includes both office area and core area horizontal framing, 

as well as columns immediately above and below this floor. 

The floor support in the office area consisted of pairs of steel floor trusses (nominally 60 ft in north-south 

and 36 ft in east-west directions) that span between exterior walls and the central core at 6 ft 8 in. on 

center. Each of these primary trusses consisted of top and bottom chords fabricated from steel angles and 

diagonals fabricated from round bars that extended 3 in. above the top chord at the panel points into the 

concrete slab in the form of a knuckle. The top chords of the primary trusses were supported at the 

central core by truss seats connected to a steel channel that ran continuously between the core columns. 

Each pair of trusses was connected to this channel with a seat that included two 1 3/4 in. long slotted 

holes and two 5/8 in. bolts (one bolt in each truss) as shown in Fig. K–25. Note that the floor truss was 

not welded to the seat support. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–3

At the exterior wall, the truss pair was supported by a seat angle and fastened with two 5/8 in. diameter 

bolts in 2 in. long slotted holes. In addition, a gusset plate welded to the spandrel and to the truss top 

chord tied the supporting column to the truss, and a pair of straps welded to the top chord and to adjacent 

columns tied those columns into the primary trusses. Primary trusses were interconnected by a transverse 

bridging system consisting of bridging trusses and bridging angles. These bridging trusses were of 

similar construction to the primary trusses, although the knuckles for the diagonals did not project above 

the top chords. The top chord of the bridging trusses sat 1 1/2 in. below the top chord of the primary 

trusses and provided support for the 1 1/2 in., 22 gauge steel deck and the 4 in. thick lightweight concrete 

slab. At each corner of the building core, a 36 ft long transfer truss extended out from the corner core 

column to the exterior wall and supported the 60 ft long primary trusses. The core area floor consisted of 

a 5 in. thick normal-weight concrete slab on 1 1/2 in., 22 gauge steel deck, supported by wide flange 

girders and beams connected to the core columns. 

Task 1 analyses use the nominal dimensions and design details shown on the drawings, without 

modifications resulting from any construction deviations or tenant modifications. Those modifications 

are considered in subsequent Task 3 analyses, which are based on the reference model developed by 

Leslie E. Robertson Associates (LERA) under a contract for Project 2. Material properties are based on 

information provided by Project 3. 

K.3.2 Exterior Wall Subsystem Description 

Each side of the towers’ exterior wall consisted of fifty-nine 14 in. square box columns spaced at 3 ft 4 in. 

on center, with 52 in. deep spandrel plates at each floor level. The exterior wall was constructed from 

shop-welded prefabricated panels, each consisting, in general, of three columns and three spandrel beams, 

13 ft 4 in. wide by 36 ft high. Except at mechanical floors, the base and top of the structure, vertical 

splices in prefabricated panels were staggered such that within any story, every third prefabricated panel 

had a vertical splice. Exterior column splices at the upper stories typically consisted of four 7/8 in. 

diameter ASTM A325 bolts fastened through the welded butt plates at the tops and bottoms of adjoining 

columns. Special prefabricated panels existed for the mechanical floors where no stagger existed at floors 

7, 41, 75, and 108. At these mechanical floors, the column splice detail included supplemental field 

welding in addition to the bolted connection. Horizontal (spandrel-to-spandrel) connections between 

prefabricated panels were all field-bolted using splice plates. Corner panels that connected the orthogonal 

walls at corners were two-stories tall (24 ft) and consisted of two columns, two spandrel plates, and a 

third column midway between the two columns on alternate floors. 

Various grades of steel, having yield strengths ranging between 42 ksi and 100 ksi, were specified to 

fabricate the perimeter column and spandrel plates. However, fewer grades were actually used with 

somewhat coarser gradation in yield strength than specified. Plate thicknesses also varied, both vertically 

and around the building perimeter. Plate thicknesses in the exterior wall were as thin as 1/4 in. at the 

upper stories, and increased toward the base of the building. The specified plate thicknesses and material 

yield strengths differed between the two towers, among NS and EW directions and through the height of 

the tower. 

An exterior wall subsystem model, nine columns wide and nine floors high, was selected to study the 

structural behavior and failure modes of the exterior wall system. This subsystem model represents the 

exterior wall of WTC 1 between floors 91 and 100 and includes column lines 150 through 158. This area 
Appendix K 

K–4

is located near the corner of the tower (column 159 is at the corner of the north face of WTC 1. See 

Fig. K–27.) The wall subsystem allows evaluations of the interaction of the wall subsystem with thermal 

expansion of the floor near the corners. It also connects to various types of trusses with different 

behaviors. 

K.4 LOADS 

The subsystems and components are analyzed for Dead (D), Live (L), and thermal (Ta)loads. The dead 

load consists of structural weights and superimposed dead loads. The superimposed dead loads for floors 

outside the core consist of the weights of ceiling, mechanical and electrical, fireproofing, and floor finish, 

estimated at 8 psf. The superimposed dead load and design live load are defined in the World Trade 

Center Design Criteria (LERA 2001). Twenty five (25) percent of the design live load is selected as 

a reasonable approximation of the load that likely existed at the time of the collapse. (For example, 

25 percent of the design live load results in a load of 13.75 psf for the long-span trusses in the two way 

zone of floor 96 with 55 psf design live load.) The service dead and live loads are applied first, followed 

by the thermal loads. 

The dead and live loads are defined as weights, so that during the collapse process, the gravity loads 

remain acting on the structure. The weight of debris from the plane will be considered where provided by 

Project 2. 

The thermal loads, Ta, are temperature time histories for all structural members provided by Project 5 for 

the standard test fire ASTM E119 and between three and five representative building fire scenarios of 

different intensities and three fire protection conditions. 

For analysis of some of the components, discrete values of temperature or temperature distributions in the 

form of a ramp from 0 °C to 700 °C (or to a temperature below 700 °C that results in the failure of the 

component) over 0.5 h followed by a constant temperature of 700 °C for another 0.5 h are used. Failure 

modes of the components are evaluated at room temperature and at different elevated temperatures, as 

failure modes and failure loads may change with increasing temperature. 

Although wind may have had a minor role in the collapse of the towers, Task 1 analyses do not include 

wind load effects. 

K.5 MATERIALS 

The mechanical properties of both steel and concrete are affected significantly by temperature. In the 

following sections, the material properties used in this project are specified as a function of temperature. 

A material properties catalog is prepared and made accessible to all analysis models. For use in ANSYS, 

each material is identified with a number; steels are Material ID 1 through Material ID 29, and concretes 

are Material ID 51 through Material ID 83. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–5

K.5.1 Concrete 

Aggregate Types 

Two types of concrete were generally used for the flooring inside the towers; lightweight concrete was 

used in the office areas, and normal-weight concrete in the core area. Thermal properties of 

normal-weight concrete depend on the type of aggregate. Petrographic inspection by SGH of several 

samples of lightweight concrete taken from the debris at NIST showed siliceous sand in the lightweight 

concrete. Because source of coarse and fine aggregates is usually the same, the available data for 

normal-weight concrete with siliceous aggregates are used. 

Actual Compressive Strength 

Specified concrete strength for lightweight concrete is 3,000 psi and for normal-weight concrete either 

3,000 psi or 4,000 psi, as shown on Drawing Book 8, Sheet AB1–2.1 (SHCR 1973). The actual strength, 

af , of in-place concrete at room temperature is calculated from the specified strength, cf ′ , as follows: 

ca FFFff 321 = ′ ⋅ ⋅ ⋅ (1)

where the factor F1 is the ratio of the average strength of cylinders to specified strength, F2 is the ratio of 

in-situ 28-day strength to 28-day cylinder strength, and F3 accounts for the change in concrete strength 

with age. 

By using F1 = 1.25 and F2 = 0.95 (Bartlett and MacGregor 1996) and F3 = 1.16 based on the formula 

specified in Section 2.2.1 of American Concrete Institute (ACI) 209 for change of concrete strength with 

age of concrete, the mean of the ratio of actual strength of in-place concrete to the specified concrete 

strength ca / ff ′ = 1.38. Based on this mean value, the actual strength of in-place concretes are 

af = 5,500 psi for the specified 4,000 psi normal-weight concrete, 4,100 psi for the specified 3,000 psi 

normal-weight concrete, and 4,100 psi for the specified 3,000 psi lightweight concrete. 

Concrete Properties 

The unit weight of the lightweight concrete is 100 pcf according to the WTC Design Criteria 

(LERA 2003); however, 110 pcf is used based on the two concrete samples described above. The unit 

weight of the normal-weight concrete is 150 pcf, according to LERA. 

Poisson’s ratio, ν c , of 0.17 is used for both normal-weight and lightweight concrete at all temperatures. 

Temperature dependent properties of concrete are modulus of elasticity, instantaneous coefficient of 

thermal expansion, compressive strength, and tensile strength: 

Modulus of elasticity at room temperature is evaluated by the following formula: 

c ac RTE f 5.1 = 33)( γ (2)

The actual strength, af , is used as room temperature compressive strength, and a 5 f is used as room 

temperature tensile strength. Effects of elevated temperature on the listed properties are based on NIST 

research (Phan 1996, 2003), and plotted in Fig. K–1. 
Appendix K 

K–6

0 200 400 600

0

1 .106

2 .106

3 .106

4 .106

5 .106

Normal-weight (3000psi)

Normal-weight (4000psi)

Light-weight (3000psi)

Temperature (°C)

Modulus of Elasticity (psi)

0 200 400 600 800

0

1 .10 5

2 .10 5

3 .10 5

4 .10 5

5 .10 5

Normal-weight

Light-weight

Temperature (°C)

Coefficient of Thermal Expansion (1/°C)

 (a) Modulus of elasticity (b) Instantaneous coefficient of thermal expansion 

0 200 400 600

1000

2000

3000

4000

5000

6000

Normal-weight (3000psi)

Normal-weight (4000psi)

Light-weight (3000psi)

Temperature (°C)

Compressive Strength (psi)

0 200 400 600

0

100

200

300

400

Normal-weight (3000psi)

Normal-weight (4000psi)

Light-weight (3000psi)

Temperature (°C)

Tensile Strength (psi)

 (c) Compressive strength (d) Tensile strength 

Figure K–1. Temperature–dependent concrete properties. 

Concrete Stress-Strain Relationships 

The compressive stress-strain curve, based on the formula by Seanz (1964), is given by: 

3

1

2

1 1

1

1 















+ 















+















+

















=

c c c

c

cc

cba

fK

ε

ε

ε

ε

ε

ε

ε

ε

σ (3)
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–7

where: 

 , 21 , , 2 1

)1(

1

2 − −= −+= −

− = c

e

c

e

s Kcacb

K

K

K

K

c

c

c

c s e c E

f K = K = K = , 41.1 , c1 = K

85.0

1 , 2 ε

In tension, stress increases linearly up to the tensile strength. When strained in tension beyond its 

strength will soften and the stress will drop. However, the descending branch of stress-strain relationship 

causes significant numerical instability problems which can be avoided by assuming that concrete 

becomes plastic in tension. Figure K–2 shows a few examples of concrete stress-strain curves at room 

and elevated temperatures. 

0.015 0.01 0.005 0

4000

3000

2000

1000

0

T=RT

T=300°C

T=600°C

Strain (in/in)

Stress (psi)

0.015 0.01 0.005 0

4000

3000

2000

1000

0

T=RT

T=300°C

T=600°C

Strain (in/in)

Stress (psi)

 (a) Normal-weight concrete (3000psi) (b) Light-weight concrete 

Figure K–2. Concrete stress-strain curves. 

For the knuckle model in LS-DYNA, solid concrete elements are modeled with Pseudo Tensor material 

model, where the cap model is used. Since this material model is not temperature dependent, different 

material types are specified for the lightweight concrete at RT, 150 o

C, 300 oC, 450 oC, 600 oC, and 

750 o

C (Material IDs 51 through 56) with their different stress-strain relationships. 

The concrete slab in the truss is modeled with SHELL181 elements with a concrete material model that 

accounts for different behaviors in tension and compression. One such material model in ANSYS is the 

cast iron plasticity model which uses the Rankine maximum stress criterion in tension, and the expression 

for von Mises yield criterion in compression (ANSYS, Inc. 2004). Cast iron plasticity material models 

for specified 3,000 psi normal-weight concrete, specified 4,000 psi normal-weight concrete, and specified 

3,000 psi lightweight concrete are assigned to Material ID 81, 82, and 83, respectively. 
Appendix K 

K–8

K.5.2 Steel 

Steels used in WTC 1 and WTC 2 are listed in Table K–1 along with the yield and tensile strengths used 

in our analysis. 

Steel Properties 

Figure K–3 shows mechanical properties of steel at high temperatures: (a) modulus of elasticity; 

(b) Poisson’s ratio; (c) yield strength reduction factor; (d) tensile strength reduction factor; and 

(e) instantaneous coefficient of thermal expansion. All properties, except yield and tensile strength 

reduction factors for bolt steels, are the same for all steels shown in Table K–1. 

Stress–Strain Relationship 

Plasticity: Stress-strain relationships at room temperature were provided by Project 3. They were 

constructed from mill report data, actual test data, and literature information using the Voce hardening 

law. 

Stress–strain relationships at elevated temperatures, without consideration of creep, are obtained by the 

power law: 

)( )( Tn

CTS TKRR ep σ = ε (4)

where: 

0

21 5.0exp)04()(

1 2

k

tk

T

tk

T kkTK

k k

+































 



 



  + 

 



 −−= (5)

0

21 5.0exp)04()(

1 2

n

tn

T

tn

T nnTn

n n

+































 



 



  +  



 −= − (6)

The steel stress-strain relationships at different temperatures vary depending on the type of steel used in 

the construction of the towers. Values for RTS , RC , given in Table K–1, and parameters of TK )( and 

n(T) given in Table K–2, were provided by Project 3. The stress-strain curve is linear with Young’s 

modulus up to the “linearity limit”: At the linearity limit, the linear stress-strain curve intersects the power 

law stress strain curve. (Stress at the linearity limit is not necessarily equal to the yield stress. The 

linearity limit is required for ANSYS input.) 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–9

0 200 400 600 800 1000

1.5 .107

2 .107

2.5 .107

3 .107

Temperature (°C)

Modulus of Elasticity (psi)

0 200 400 600 800 1000

0.28

0.3

0.32

0.34

0.36

Temperature (°C)

Poisson's Ratio

(a) Modulus of elasticity (b) Poisson’s ratio 

0 200 400 600 800 1000

0

0.2

0.4

0.6

0.8

1

Construction Steel

Bolt Steel

Temperature (°C)

Yield Strength Reduction Factor

0 200 400 600 800 1000

0

0.2

0.4

0.6

0.8

1

Construction Steel

Bolt Steel

Temperature (°C)

Tensile Strength Reduction Factor

 (c) Yield strength reduction factor (d) Tensile strength reduction factor 

0 200 400 600 800 1000

1.2 .10 5

1.3 .10 5

1.4 .10 5

1.5 .10 5

1.6 .10 5

1.7 .10 5

Temperature (°C)

Coefficient of Thermal Expansion (1/°C)

(e) Instantaneous coefficient of thermal expansion 

Figure K–3. Temperature-dependent properties for all steels. 
Appendix K 

K–10

Table K–1. Steel types used in WTC 1 and WTC 2. 

Material ID Description 

σyRT

(psi) 

σuRT 

(psi) RTS RC 

1 All 36 ksi core box columns, plates, strapsa 36,720 64,470 1.086 0.857 

2 All 36 ksi core WF, channels, and tubes 36 ksi large area 

and large inertia “rigid” beams in SAP2000 modela

37,000 63,450 1.069 0.954 

3 All 42 ksi box columns (1<=0.75 in.) 51,400 79,200 1.070 0.884 

4 All 42 ksi box columns (0.75 in. < t <= 1.5 in.) 47,000 74,800 1.010 0.884 

5 All 42 ksi box columns (t > 1.5 in.) 42,600 70,400 0.951 0.880 

6 42 ksi or 45 ksi Group 3 WF core columns 53,800 74,400 1.005 0.977 

7 42 ksi or 45 ksi Group 3 WF core columns 49,000 71,040 0.960 0.954 

8 42 ksi Group 4&5 WF core columns 44,200 66,640 0.900 0.948 

9 45 ksi Group 4&5 WF core columns 47,800 71,074 0.960 0.939 

10 All 36 ksi Plates 1, 2, and 4 in perimeter columns 35,630 61,170 1.031 0.875 

11 All (42, 45, or 46) ksi Plates 1, 2, and 4 in. perimeter 

columns 

53,051 74,864 1.011 0.948 

12 All 50 ksi Plates 1, 2, and 4 in. perimeter columns. All 

50 ksi channels and platesa

53,991 75,618 1.021 0.978 

13 All 55 ksi Plates 1, 2, and 4 with t<=1.5 in. in perimeter 

columns 

60,817 82,558 1.115 0.903 

14 All 60 ksi Plates 1, 2, and 4 with t<=1.25 in. in perimeter 

columns 

62,027 87,250 1.178 0.894 

15 All 65 ksi Plates 1, 2, and 4 with t<=0.5 in. in perimeter 

columnsb

69,642 90,442 1.221 0.979 

16 All 70 ksi Plates 1, 2, and 4 in. perimeter columns 76,735 91,951 1.242 0.955 

17 All 75 ksi Plates 1, 2, and 4 in perimeter columns 82,469 96,821 1.308 0.936 

18 All 80 ksi perimeter columns steels, regardless of plate 91,517 99,442 1.343 0.987 

19 All (85, 90, 100) ksi perimeter column steels, regardless 

of plate 

104,783 115,983 1.566 0.976 

20 Laclede truss web bar rounds specified as A36 38,067 59,567 1.004 0.935 

21 Laclede truss chord angels (regardless of ASTM Spec) 

and all rounds specified as A242 

55,332 74,050 1.000 0.959 

22 A325 boltsc 104,783 115,983 1.566 0.976 

23 All 42 ksi Plate 3 in perimeter columns 42,600 67,216 0.900 0.912 

24 All 45 ksi Plate 3 in perimeter columns 45,900 69,831 0.940 0.921 

25 All 50 ksi Plate 3 in perimeter columns 51,400 74,188 1.000 0.935 

26 All 55 ksi Plate 3 in perimeter columns 56,900 78,546 1.070 0.906 

27 All 60 ksi Plate 3 in perimeter columns 62,400 83,903 1.130 0.949 

28 All 65 ksi Plate 3 in perimeter columns 67,900 87,261 1.190 0.975 

29 All 70 ksi and 75 ksi Plate 3 in perimeter columns 78,900 95,976 1.310 0.997 

a. Steels in the following members are assumed to have the properties shown in the table: 

 36 ksi plates and straps (Material 1). 

 36 ksi channels, tubes, and “rigid” beams (Material 2). 

 50 ksi channels and plates (Material 12). 

b. 65 ksi steels in perimeter columns with t>0.5 in. are assumed to have the same properties as those in Material 15. 

c. In the column model, stress-strain relationships of bolts are used. 

Note: Bolt properties are assumed to be the same as those in Material 19. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–11

Table K–2. Parameters for k(T) and n(T). 

σyRT = 36,000 psi σyRT > 36,000 psi 

tk1, °C 524.1812 511.8266 

tk2, °C 523.6799 511.8938 

k0, psi 29049.2 26472.1 

k1 9.4346 6.5764 

k2 9.3532 6.5971 

k4, psi 121605.6 122516.7 

tn1, °C 524.4304 519.634 

tn2, °C 521.241 499.6031 

n0, psi 0.1235 0.0342 

n1 19.0000 10.0000 

n2 19.0000 10.0000 

n4, psi 0.2168 0.1511 

Figure K–4 shows stress-strain curves of Material ID 1 (see Table K–1 for the material description) at 

room and elevated temperatures. Figure K–4 (a) is a close-up view of a low strain range, while 

Fig. K–4 (b) shows strain levels up to 0.3. 

The elastic-plastic behavior of steels is modeled with ANSYS material model “Multi–linear isotropic 

hardening von Mises plasticity.” 

0 0.005 0.01 0.015 0.02 0.025 0.03 0

1 .104

2 .104

3 .104

4 .104

5 .104

T=RT

T=300°C

T=500°C

T=700°C

Elastic + Plastic Strain (in/in)

Stress (psi)

0 0.05 0.1 0.15 0.2 0.25 0.3 0

2 .104

4 .104

6 .104

8 .104

T=RT

T=300°C

T=500°C

T=700°C

Elastic + Plastic Strain (in/in)

Stress (psi)

 (a) Strain < 0.03 (b) Strain < 0.30 

Figure K–4. Stress-strain relationships for Material ID 1 steel. 
Appendix K 

K–12

Creep: Steel creeps at elevated temperatures ( CT

ο ≥ 350 ), and the creep behavior for steels is based on 

the creep model by Fields and Fields (1991), expressed as: 

)( )(

5.35

60 )( 100

1 ),,(

Tc

yRT

Tb

cr

t Tt Ta 













 



 



 = σ

σ

σε (7)

where: 

Tc T CT

Tb T CT

CTC

CTC

CT

Ta

T

T

ο

ο

ο ο

ο ο

ο

725 for 0064.01.2)(

725 for 0035.01.1)(

10 500 for 725

10 350 for 500

0 350 for

)(

)00851.025.13(

)00573.01.6(

+= <

+−= <











<≤

<≤

<

=

−−

+−

This model is valid for the temperature range of CTCο ο 350 ≤≤ 725 . ANSYS uses the “time 

hardening creep” model, where creep strain rate is given by: 

)( )(

1 2 3 )( cr TC TCtTC

dt

d

σ

ε = (8)

and )(1 TC , )( 2 TC , and )( 3 TC are temperature-dependent parameters determined by Fields’ (1991) 

creep model given as: 

1)()(

)()(

5.35

60

1 )()( 1001 )(

3

2

)( )(

1

−=

=















 



 



 =

TbTC

TcTC

TbTaTC

Tc

yRT

Tb

σ

Figure K–5 illustrates creep behavior of steel at elevated temperatures for Material ID 1. Figure K–5 (a) 

shows creep strain rate at different stress levels and different temperatures, and Fig. K–5 (b) compares 

elastic, plastic, creep, elastic plus plastic, and total strains at CT

ο = 400 and after constant loading for 

1,800 s. 

Failure Criteria 

The failure criteria for steel are defined in terms of plastic strains. The multiaxial fracture strain criterion 

for different steels and temperatures (Fields 2004) is as follows: 





 



 = − σ

σ

αε m

f T

2

3 exp)( (9)

where stress and strain are true stress and true strain. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–13

0 2 .10 6 4 .10 6 6 .10 6 8 .10 6 1 .10 5 0

1 .104

2 .104

3 .104

4 .104

5 .104

T=400°C

T=500°C

T=600°C

Creep Strain Rate (in/in/sec)

Stress (psi)

0 5 .10 4 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

0

1 .104

2 .104

3 .104

4 .104

Elastic

Plastic

Creep

Elastic + Plastic

Total

Strain (in/in)

Stress (psi)

 (a) Creep strain rate (b) Comparison of different strains at 

 T=400 °C and t=1,800 s 

Figure K–5. Creep behavior at elevated temperatures for Material ID 1 steel. 

For the uniaxial stress condition, the plastic strain at fracture reduces to: 

)()5.0exp( ε _ unif = − α T (10)

Table K–3 shows the uniaxial plastic strain at fracture, _ unif ε , at different temperatures calculated by the 

equation above. This criterion is valid for the finite element analysis (FEA) with very fine mesh. For 

coarse mesh, the equivalent steel fracture criterion was determined numerically as follows. A standard 

tension test specimen was modeled in ANSYS. The gauge length, width, and thickness of the specimen 

were 8 in, 1.5 in, and 1 in., respectively, and Material ID 1 steel properties were used. Six different 

models (Model 0 to 5) were created, each having a different mesh size. Element sizes of Models 0 to 5 

were 0.025 in., 0.050 in., 0.0125 in., 0.250 in., 0.375 in., and 0.75 in. It was assumed that Model 0 was 

able to capture tensile fracture in a uniaxial tension. 

Model 0 was subjected to tension until the maximum plastic strain in the direction of applied 

displacement reached the uniaxial fracture strain determined by Eq. (10) for uniaxial stress condition, and 

the corresponding elongation of the specimen, ∆0, was obtained. Models 1 to 5 were then subjected to the 

same elongation, ∆0, and the maximum plastic strain in the direction of applied displacement was 

measured for each model. The maximum plastic strain due to the elongation of ∆0 is defined as the 

limiting plastic strain (equivalent fracture plastic strain) for the corresponding element size. 

From these six cases, a relationship between element size and equivalent uniaxial fracture plastic strain 

was established. This process was repeated for temperatures 20 °C, 100 °C, 300 °C, 500 °C, and 700 °C. 

Figure K–6 (a) shows the ratio of the maximum plastic strain in the direction of applied displacement due 

to displacement ∆0 to uniaxial plastic strain by Eq. (10) vs. element size at different temperatures. The 

FEA results were extrapolated up to the element size of 50 in. Plastic strain shown in Fig. K–6 (b) is used 

as fracture criterion for the corresponding element size in the FEA. 
Appendix K 

K–14

Table K–3. Uniaxial plastic strain at fracture by Eq. (10). 

Plastic Strain at Fracture in the Uniaxial Test 

Material ID 20 100 300 500 700 1000 

1 0.8411 0.6989 0.6610 1.0446 1.8100 3.5862 

2 0.8411 0.6989 0.6610 1.0446 1.8100 3.5862 

3 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

4 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

5 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

6 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

7 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

8 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

9 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

10 0.8891 0.7388 0.6987 1.1042 1.9142 3.7907 

11 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

12 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

13 0.2846 0.2364 0.2236 0.3534 0.6123 1.2132 

14 0.3774 0.3136 0.2965 0.4686 0.8120 1.6088 

15 0.5338 0.4436 0.4195 0.6629 1.1486 2.2758 

16 0.5623 0.4672 0.4418 0.6983 1.2099 2.3972 

17 0.7752 0.6442 0.6092 0.9628 1.6681 3.3051 

18 0.6545 0.5439 0.5143 0.8129 1.4084 2.7906 

19 0.4254 0.3535 0.3343 0.5283 0.9154 1.8137 

20 0.8411 0.6989 0.6610 1.0446 1.8100 3.5862 

21 0.4908 0.4078 0.3857 0.6095 1.0561 2.0924 

0.01

0.10

1.00

0.01 0.10 1.00 10.00 100.00

Element Size (in)

epmax / epmax of Model 0

T=RT

T=100°C

T=300°C

T=500°C

T=700°C

T=RT (power law)

T=100°C (power law)

T=300°C (power law)

T=500°C (power law)

T=700°C (power law)

0.0

0.1

0.2

0.3

0.4

0.5

0 100 200 300 400 500 600 700

Temperature (°C)

Plastic Strain at Fracture

EL size = 0.375 in EL size = 0.750 in EL size = 5.00 in

EL size = 10.0 in EL size = 50.0 in

 (a) FEA Results and Extrapolation (b) Recommended Limiting Plastic Strain 

Figure K–6. Maximum plastic strain from the finite element analysis 

 and limiting plastic strain. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–15

K.5.3 Welds 

The weld properties at all temperatures are assumed to be the same as those of the base metal of the same 

ultimate tensile strength. This assumption is validated by the following observations: the exterior column 

welds are strong enough to fail the base metal; the observed fractures in the exterior columns are mostly 

through the base metal; and the welds in trusses are resistance welds with no filler added. For the core 

columns, the area of the welds is significantly less than that of the base metal, and several fractures 

through the welds have been observed. Fractures in the truss seats and truss connections have also been 

observed. High temperature properties of the welding metals have not been found in the literature. 

Susceptibility of existing cracks in the welds to growth (fracture toughness) does not increase with 

temperature (Stevick 1994). 

K.5.4 Bolts 

A load-elongation relationship for 7/8 in. A325 bolt with 4 in. length at room temperature was provided 

by Project 3. Load-elongation relationships at elevated temperatures are constructed by scaling the loads 

by the yield and ultimate tensile strength reduction factors for bolt steels shown in Fig. K–3 (c) and (d). 

Figure K–7 shows the load-elongation relationships of a 7/8 in. bolt at different temperatures. Loadelongation relationships of A325 bolts of different size are scaled by proportioning the load by the ratio of 

the bolt thread area to the bolt body area for a 7/8 in. bolt. 

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

0.000 0.050 0.100 0.150 0.200 0.250

Elongation (in.)

Load (lbs)

t=RT

T=100°C

T=200°C

T=300°C

T=400°C

T=500°C

T=600°C

T=700°C

Figure K–7. 7/8 in. A325 bolt load-elongation curves at elevated temperatures. 

The load-elongation relationship for bolts with a different length than 4.0 in. is expected to be very 

similar to the load-elongation relationship of 4.0 in. length as deformations are localized. 

Based on the AISC formulas, C-J3–2 to C-J3–4, (AISC 2003), the shear strength for a single shear plane 

is calculated as 0.67 of the tensile strength given in Fig. K–7 when threads are excluded from the shear 

plane. When threads are not excluded from the shear plane, the nominal shear strength for a single shear 

plane is 0.53 of the tensile strength given in Fig. K–7. No shear ductility is assumed at failure. 
Appendix K 

K–16

K.5.5 Coefficient of Friction 

The coefficient of friction of 0.33 for calculation of shear in friction-type connections is the AISC LRFD 

(2003) friction coefficient for uncoated clean mill scale steel surfaces, or surfaces with Class A coatings 

on blast-cleaned steel surfaces. 

K.5.6 Symbols 

α T )( = temperature-dependent material property that defines fracture criterion 

T )( α c = instantaneous coefficient of thermal expansion of concrete 

T )( αs = instantaneous coefficient of thermal expansion of steel 

T )( β y = steel yield strength reduction factor due to elevated temperature 

T )( β u = steel ultimate strength reduction factor due to elevated temperature 

c γ = unit weight of concrete (110 pcf and 150 pcf for lightweight and normal-weight concrete, 

respectively) 

s γ = Unit weight (490 pcf = 0.284 pci for all steel types at any temperature) 

c1 ε = concrete strain at maximum compressive stress 

cr ε = creep strain of steel 

e ε = elastic strain 

ep ε = elastic plus plastic strain 

f ε = effective plastic strain at fracture 

_ unif ε = uniaxial plastic strain at fracture 

p ε = plastic strain 

t1 ε = concrete strain at maximum tensile strength 

tu ε = concrete strain at full crack formation (separation) in tension 

ν c = Poisson’s ratio of concrete 

ν s = Poisson’s ratio of steel 

σ = effective stress 

σ m = mean stress 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–17

σ yRT = room temperature yield strength of steel 

σ uRT = room temperature tensile strength of steel 

TE )(s = modulus of elasticity of steel 

TE )( c = modulus of elasticity of concrete 

F1 = mix design factor = ratio of the actual 28 day cylinder strength to cf ′

F2 = in-situ factor = ratio of in-situ 28 day strength to the 28 day cylinder strength 

F3 = aging factor = ratio of mature concrete strength to 28 day concrete strength 

af = actual strength of in-place concrete 

cf ′ = specified 28 day strength 

Tf )( c = compressive strength of concrete 

Tf )(t = tensile strength of concrete 

TK )( = sigmoidal function of temperature with six parameters 

Tn )( = sigmoidal function of temperature with six parameters 

RC = correction factor that has the following two functions: (1) to correct the strain rate effect 

introduced in the material testing and create the stress-strain curve for zero strain rate, 

and (2) to match the room temperature stress-strain curve at strain of 0.05 

RTS = ratio of the room temperature tensile strength of the steel of interest to the room 

temperature tensile strength of the steel used to develop the power law model 

K.6 MODEL CONVERSION FROM SAP TO ANSYS 

The SAP2000 (SAP) floor 96 Model of WTC 1 and the SAP Global Model of WTC 1 are converted into 

ANSYS. The goal of the conversion is to develop ANSYS models that match the SAP baseline models 

developed by Project 2 and can be used as a basis of the detailed thermal-structural evaluation. The 

converted ANSYS models will be modified to incorporate the nonlinear behaviors of the components and 

simplified for the thermal/structural evaluation of collapse initiation study. 
Appendix K 

K–18

K.6.1 Translation Procedure 

Automatic translation software was developed to partially convert the floor model and global model from 

SAP2000 to ANSYS 8.0: 

• The Joints, Frames, and Shells in the SAP model were translated into ANSYS Keypoints, 

Lines, and Areas. Using geometry definition instead of nodes and elements directly allows 

for ease in local mesh refinement. 

• Lines were meshed with both section and real constants so that a translation between 

BEAM44 and BEAM188 elements can be achieved by simply changing element types. 

Areas were meshed with SHELL63 elements in ANSYS to match the Shell elements in SAP. 

Eventually, Lines and Areas will be changed to nonlinear BEAM188 and SHELL181 

elements with a type change. 

• Material properties were assigned according to the Criteria Document based on the material 

definitions and Frame section properties in SAP. 

• Frame section properties in SAP were converted into Real Constants for BEAM 44 in 

ANSYS. Cross section properties in SAP were retained for future conversion into cross 

section data for BEAM188 elements. Shell thicknesses in SAP were converted into Real 

Constants for SHELL63 in ANSYS. 

• Joint restraints in SAP were translated into DOF constraints in ANSYS. 

• Frame distributed loads and area uniform loads were translated into surface loads on Lines 

and Areas in ANSYS. 

• The ANSYS BEAM44 elements support element moment releases, but the ANSYS nonlinear 

BEAM188 elements do not. Therefore, Frame releases in SAP were modeled by coincident 

nodes with coupled (CP) degrees of freedom in ANSYS. 

• The ANSYS BEAM44 elements allow beam end offsets in three directions, but the ANSYS 

nonlinear BEAM188 elements only allow beam end offsets perpendicular to the element axis 

through section offset (SECOFFSET) command. Frame insertion points in SAP were 

converted in two ways. For offsets along the element axis, additional nodes and rigid 

MPC184 elements with the proper lengths were used in ANSYS. For offsets perpendicular to 

the element axis, beam end offsets were defined using Real Constants for BEAM 44, and 

eventually will be defined using SECOFFSET command for nonlinear BEAM 188. 

• Frame offsets and rigid panel factor in SAP were modeled by adding additional nodes and 

rigid MPC184 elements with the proper lengths in ANSYS. 

Those parts of the model that were not converted by the translation software were converted manually. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–19

K.6.2 Challenges 

During the conversion of the SAP Floor Model, the following conditions were encountered and were 

resolved: 

• The SAP Floor Model allows automatic division of the frames at joints. This causes 

problems in the translation software because the frame connectivities in the Graphical User 

Interface do not show the actual internal element connectivities used in the SAP analysis 

engine. In order to resolve this problem, the translation software was modified to use the 

internal element connectivities. The table of internal connectivities was exported from the 

SAP model after the execution of the analysis. 

• Automatic offsets in the SAP model are not available in the ASCII SAP input file prior to the 

execution of the analysis. The table of element offsets was exported after the execution of the 

analysis. 

• There are both intentional and unintentional duplicate elements in the SAP Floor Model. 

Each leads to problems in the translator since ANSYS cannot have duplicate lines sharing the 

same key points. Some duplicate elements are used to model additional steel plates at the 

ends of the trusses. The duplicate elements were manually deleted and the section properties 

of the remaining elements were modified to account for the additional steel. Some duplicate 

elements are from frame elements which have different lengths and are overlapping each 

other. These were manually corrected. 

K.6.3 Status 

The automatic translation software developed to convert models from SAP2000 to ANSYS was applied 

to the floor model and will be applied to the global model shortly. Figures K–8 through K–11 show the 

converted floor model. 

The following analyses were performed to validate the converted ANSYS floor model against the original 

SAP model. 

• One static analysis with gravity loads as defined in SAP as Load Case “DEAD” which 

include self-weight plus 3.5 psf uniform load in the office area. 

• One modal analysis, using structural mass only. 

Table K–4 summarizes the comparison of the SAP and ANSYS results for the gravity load case. The total 

reactions for the SAP and ANSYS models are within 0.1 percent of each other. The maximum slab 

displacement predicted by the ANSYS model is 3.2 percent smaller than that obtained from the SAP 

model. This discrepancy is currently under study and is being resolved. The deformed shapes of the 

gravity load case for the SAP and ANSYS models are shown in Figs. K–12 and K–13. 

Table K–4. Comparison of SAP and ANSYS results for gravity load case. 

 SAP ANSYS (BEAM 188) 

Total reaction, kip 2,212.81 2,210.85 (-0.09 %) 

Maximum slab displacement, in. 0.718 0.695 (-3.2 %) 
Appendix K 

K–20

Figure K–8. Converted ANSYS model for floor 96 of WTC 1: overall view. 

Figure K–9. Converted ANSYS model for floor 96 of WTC 1: partial view 

 near corner of building. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–21

Figure K–10. Converted ANSYS model for floor 96 of WTC 1: close-up view 

 at corner of building. 

1

 

JUN 2 2004

16:30:28

ELEMENTS

SEC NUM

Figure K–11. Converted ANSYS model for floor 96 of WTC 1: view of 

 floor beams and columns. 
Appendix K 

K–22

Figure K–12. Deformed shape (x100) of gravity load case for SAP floor model. 

Figure K–13. Deformed shape (x100) of gravity load case for 

ANSYS floor model with BEAM44 (Euler beam) elements. 

Table K–5 summarizes the comparison of the SAP and ANSYS results for the modal analysis. The total 

masses of the SAP and ANSYS models are within 0.02 percent of each other. The dominant natural 

frequency of the floor predicted by the ANSYS model is 2.5 percent higher than that obtained from the 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–23

SAP model. This discrepancy is consistent with the discrepancy observed for gravity displacement, and 

is currently under study and is being resolved. The dominant mode shapes of the floor for SAP and 

ANSYS models are shown in Figs. K–14 and K–15. 

Table K–5. Comparison of SAP and ANSYS Modal Analysis Results. 

 SAP ANSYS (BEAM 188) 

Total mass, lb⋅sec2/in. 5448.7 5447.7 (-0.018 %) 

Dominant natural frequency of floor, Hz 4.32 4.43 (+2.5 %) 

Figure K–14. Dominant mode shape (frequency = 4.32 Hz) of floor structure 

for SAP floor model. 

Figure K–15. Dominant mode shape (frequency = 4.43 Hz) of floor structure 

for ANSYS floor model. 
Appendix K 

K–24

K.7 FULL FLOOR SUBSYSTEMS 

The full floor model is analyzed using the ANSYS general purpose finite element program Version 8.0. 

The objectives of the analysis are: 

• To identify the most likely failure modes, 

• To evaluate 

− Failure loads, 

− Temperatures at failure, 

− Time-to-failure, and 

− Changes in mechanical properties and geometry at failure. 

• To simplify the model and to reduce the computational efforts for incorporation into the 

global model. 

The failure modes and the failure loads of different components of the full floor subsystem are evaluated 

through analysis of detailed models of those components, using either hand calculations or FEAs. 

Simplified models that capture the failure loads and failure modes are then developed for each 

component. These simplified models of components are incorporated in the full floor subsystem model. 

In this chapter, after a general description of the full floor model, the analyses of important components 

are presented and discussed. 

K.7.1 Full Floor Model 

Model Description 

The floor model is developed using the converted SAP2000 model for floor 96, with the following 

modifications: 

1. Combine two adjacent trusses into a single truss. The elements in the truss model have 

double the areas of elements in each real truss. 

2. Change rigid beams at knuckle locations to user-defined elements with the properties of the 

knuckle determined by the component knuckle model. 

3. For compression diagonals, add user-defined elements to account for buckling of diagonals. 

4. For truss ends and connections, add user-defined elements to account for truss seat failure. 

5. Pin concrete slab for out-of-plane rotation at both its interior and exterior edges. 

6. Use user-defined elements along the edge nodes of the concrete slab to model the tensile 

strength of the concrete slab and the in-plane shear capacity at the connection to the spandrel 

plate. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–25

7. Remove the spandrels defined as beam sections in SAP2000 model and replace them with 

SHELL181 elements in ANSYS. (This modification eliminates the need for defining panel 

zone stiffness.) The new spandrels will wrap continuously around the floor. Each spandrel 

plate between columns will be represented by 16 elements, 4 in. height and 4 in. width. 

Material and geometry assignments are carried through to ANSYS. 

8. Change the elastic column elements as translated into ANSYS to user-defined sections with 

BEAM189 elements with plasticity and creep. 

9. Make new column sections within the limits of the spandrels with reduced Plate 3 thickness, 

say 0.005 in. in thickness, to insure correct modeling of torsional stiffness. Spandrel 

thicknesses should be reduced within the limits of the column by the same thickness. 

Connect the centerline of column to spandrel with rigid elements. 

Material Properties 

ANSYS’s multilinear isotropic hardening von Mises plasticity with time hardening for temperatures 

below 350 o

C is used for the beam elements representing the truss system, girders, beams, and columns in 

areas where plasticity is either anticipated or expected to occur by analysis. This material model with 

creep is used for temperatures above 350 °C. This material model is used for shell elements representing 

the spandrel plates, when appropriate. 

Loading 

The full floor model is analyzed for dead and live loads first, and then thermal loads are applied to model 

the path dependent nonlinear response. The thermal loads are provided by Project 5 and include 

temperatures and temperature gradient time-histories for all structural members in the full floor model for 

(1) standard fire, (2) representative building fire scenarios, and (3) different fire protection scenarios. 

Boundary Conditions 

The beam elements representing the columns are restrained vertically at floor 95. The outward and 

tangential displacements and all rotations of the column ends at floors 95 and 97 are fixed to restrain 

thermal expansion. Mass elements defined by the tributary dead and live loads are added to the top of the 

columns and at connections to floor 96. 

Failure Modes 

The possible failure modes of the floor subsystem are as follows: 

1. Sagging: Floor sagging along the axis of the main trusses may be caused by (1) loss of 

stiffness and softening of truss at high temperature, (2) catenary action of the truss due to 

plastic bending or buckling of critical members required for truss action, or (3) loss of 

composite action of floor-to-knuckle failure. These are discussed in some detail under truss 

failure modes. Floor sagging may result in component failure due to tension in the truss 

seats, tension in the floor subsystem, tension on the connections to the exterior walls, lateral 
Appendix K 

K–26

loads on columns, and increased demand on other components of the floor subsystem, for 

example, bridging trusses and transfer trusses and their connections. 

2. Edge Sagging: Edge sagging results from failure of truss seat connections at either the 

interior or exterior supports and is evidenced in videos. Edge sagging, similar to sagging, 

increases demand on other components of the floor subsystem, reduces buckling strength of 

columns, and can lead to failure of a floor. 

3. Loss of Support: Abrupt failure of the floor subsystem can result from loss of truss support 

for a large number of adjacent parallel trusses. Loss of a truss support can occur due to 

(1) vertical shear load due to debris and/or impact load of the dropping floor above, 

(2) vertical and horizontal shear loads resulting from slab expansion acting on column truss 

seats (3) tension acting on column truss seats, and (4) cooling of a truss shortened by plastic 

deformation and loss of composite action. Failure of truss support will increase the demand 

on the adjacent trusses and can result in sequence of truss seat failure, edge sagging, and 

ultimately failure of the floor subsystem. 

4. Expansion of Floor System: Expansion of floor results in deformation of columns and forces 

at corners of the exterior wall subsystem. Such corner forces can initiate a failure sequence 

of columns near the corners. Such a failure includes development of horizontal shear in the 

gusset plates and the exterior column truss seats, large forces in the straps, and large lateral x 

and y forces in columns, especially near the corners. 

K.7.2 Knuckle Analysis 

The “knuckle” is formed by the extension of the truss diagonals into the concrete slab and provides for 

composite action of the steel truss and concrete slab. The composite action is due to the shear transfer 

between the knuckle and the concrete slab both in the truss transverse and longitudinal directions. 

The objective of this analysis is to predict the knuckle capacity when the truss and concrete deck act as a 

composite member and to develop a simplified model of the knuckle behavior to be included in the full 

floor subsystem model. 

Knuckle Shear Tests 

Two sets of experiments were performed in 1967 at Laclede Steel Company in Saint Louis, Missouri, to 

determine the transverse and longitudinal shear capacities of the knuckle. 

The transverse shear test consists of double knuckles placed into two reinforced concrete blocks that were 

confined on the corners by angles as shown in Fig. K–16. The concrete density of 110 pcf corresponds to 

the lightweight concrete in the office areas. The concrete compressive strengths reported for 7 day and 

27 day cylinder tests were 1,330 psi, and 2,600 psi, respectively. The inner ends of the two knuckles were 

connected through channels to a #11 rebar and the rebar was loaded until the concrete failed. The tests 

were conducted at concrete age of 6 and 27 days. The primary failure mode observed was concrete shear 

failure. The pictures from the tests show formation of the shear crack in one of the concrete blocks and 

edging of the channel into the concrete. The transverse shear capacity of the knuckle as the average of the 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–27

Drawing provided by Laclede Steel. 

Figure K–16. Transverse shear test of a knuckle. 

two reported tests is 16.9 kip per knuckle. After adjusting it for the strength of in-place, mature, 

lightweight concrete in the slab of 4,100 psi relative to the average strength of the lightweight concrete 

used in the test of 1,965 psi, by multiplying by the ratio of 4,100 to 1,965 psi, the transverse shear 

capacity of the knuckle is approximately 35 kip per knuckle. 

The longitudinal shear test consists of double knuckles placed into two concrete blocks as shown in 

Fig. K–17. The test specification shows corner angles confining concrete blocks and no reinforcement for 

the concrete. However, the test pictures show reinforcement in both directions for each concrete block, 

with the corner angles dismantled. The test specification calls for concrete density of 152 pcf, which 

corresponds to a normal-weight concrete. The slab in office areas is of lightweight concrete. The average 

strength of two 28 day cylinders tested is 4,290 psi. A third sample, tested after 96 days, showed a 

strength of 2,850 psi. The test specification does not identify the weld size connecting the inner ends of 

the two knuckles to two channels. However, the primary failure mode observed for three tests is weld 

failure. Weld failure is not identified as the failure mode of the knuckle for the other two tests. The 

results of the shear tests of the knuckle in the longitudinal direction based only on these two tests are 

approximately 28.3 kip per knuckle. After adjusting for the strength of in-place, mature, lightweight 

concrete in the slab of 4,100 psi relative to the 28 day corrected average strength of the normal-weight 

concrete used in the test of 3,707 psi, by multiplying by the ratio of 4,100 psi to 3,707 psi, the 

longitudinal shear capacity of the knuckle is approximately 31 kip per knuckle. 

Drawing provided by Laclede Steel. 

Figure K–17. Longitudinal shear test of a knuckle. 
Appendix K 

K–28

The effect of temperature on the knuckle is as follows: 

• The steel knuckle conducts the temperature of the diagonal without much loss into the cool 

concrete. Concrete has a low coefficient of conductivity and does not respond rapidly to the 

rise of temperature. 

• Concrete in the intermediate proximity of the metal knuckle will heat to a temperature close 

to that of the steel. 

• Shear failure of the knuckle is initiated by the failure of concrete in close proximity to the 

knuckle. Final failure will engage not only the hot concrete in close proximity of knuckle, 

but the cooler concrete farther away. 

• It is reasonable to assume that for gas temperatures in the range of RT to 450 °C, 650 °C, 

850 o

C, and 1050 oC , the knuckle metal temperature is below 375 oC, 550 oC, 725 oC, and 

900 oC, and the average concrete temperature is below 300 oC, 450 oC, 600 o

C, and 750 oC, 

respectively. 

Neglecting the difference in thermal expansion of concrete and steel, for gas temperatures of RT, 450 o

C, 

650 o

C, 850 oC , and 1050 oC, the expected concrete strength is in the range of 4,100 psi, 3,300 psi, 

2,600 psi, and 2,000 psi, and the knuckle capacity in either direction is 30 kip, 24 kip, 19 kip, and 15 kip, 

respectively. 

Knuckle Test Finite Element Model 

Finite element models, shown in Fig. K–18, represent one quarter of the knuckle test specimens. The 

knuckle and channel members in the test set up are modeled by solid steel elements. Concrete Pseudo 

Tensor model and the LS-DYNA computer program were used for the analysis. An imposed ramped 

displacement was applied to the angle member. 

The concrete strength used in the finite element model for the longitudinal shear of the knuckle was 

4,100 psi and for the transverse shear of the knuckle was 2,500 psi. In addition 0.47 percent steel 

reinforcement representing welded wire fabric reinforcement of the slab was added in a distributed way to 

the concrete. Also, two different assumptions were made about the interface condition between the 

concrete and the steel: fully bonded and frictionless. 

The results of the analysis are shown in Figs. K–19 through K–22. They show significant dependence on 

the characteristic of the interface between the steel and concrete. The longitudinal shear test FEA results, 

shown in Fig. K–21, show that each knuckle has strength in the range of 15 kip to 35 kip, depending on 

interface. The test results show that the interface is closer to fully bonded case. For the transverse shear, 

the FEM results, Fig. K–22, show that transverse knuckle strength is about 24 kip for 2,500 psi concrete, 

corresponding to 39 kip for 4,100 psi concrete. Figure K–20 shows that for transverse shear concrete 

crushes in a small region next to the knuckle and extending in front of the shear load. Figure K–20 also 

shows large regions of crushing at the lower boundary of the model. These regions are the result of 

imposing the boundary condition UY=0. This boundary condition, and the crushing at the boundary, 

although realistic for the test, would not obtain in a pair of transversely loaded knuckles of the double 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–29

Figure K–18. Finite element models of knuckle shear tests. 

Figure K–19. Compressive stresses in longitudinal shear finite element model. 

UZ = 0 

(top edge of L -beam)

UXmax = -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket) UX = 0

UZ = 0 

(top edge of L -beam)

UXmax = -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(top edge of L -beam)

UXmax = -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(top edge of L -beam)

UXmax = -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(top edge of L -beam)

UXmax= -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket) UX = 0

UZ = 0 

(top edge of L -beam)

UXmax= -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(top edge of L -beam)

UXmax= -0.10 

(applied to all nodes 

on the end face of the 

L-beam)

UZ = 0 

(2.1 in. region along

base – support bracket)

UZ = 0 

(2.1 in. region along

base – support bracket)

UY = 0

UZ = 0

(top edge of L -beam)

UYmax = 0.10

UY = 0

UZ = 0

(top edge of L -beam)

UYmax = 0.10

Symmetry

UY = 0

UZ = 0

UYmax = 0.10

Symmetry

UY = 0

UZ = 0

UYmax = 0.10

UXmax = -0.10

Symmetry

UX = 0

UZ = 0 

UZ = 0 

(2.1 in. region)

UZ = 0 

(2.1 in. region)

UXmax = -0.10

Symmetry

UX = 0

UZ = 0 

UZ = 0 

(2.1 in. region)

UZ = 0 

(2.1 in. region)
Appendix K 

K–30

Figure K–20. Compressive stresses in 

transverse shear finite element model. 

Figure K–21. Shear force versus displacement from finite element 

model for longitudinal shear of two knuckles. 

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Displacement (in.)

Force (lbs.)

0 Friction Contact

Fully Bonded

Figure K–22. Shear force versus displacement from finite 

 element model for transverse shear of two knuckles. 

0

10000

20000

30000

40000

50000

60000

70000

80000

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Displacement (in.)

Force (lbs.)

0 Friction Contact

Fully Bonded
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–31

truss. The small crushing regions at the knuckle indicate that a pair of knuckles in a double truss can be 

expected to behave nearly independently of each other, and, therefore, have nearly double the capacity of 

a single knuckle. Unfortunately, test results are not available that would confirm this conclusion. 

Although the analysis shows the sensitivity of the results to the interface assumptions, it justifies the shear 

capacities computed from the test results. 

Knuckle Model 

The purpose of the detailed finite element analysis of the knuckle is to provide a basis for deriving a 

simple model for use in analyses of the full floor. The knuckle model includes segments of concrete floor 

and truss diagonal that protrudes into the 4 in. thick concrete. The dimensions of the concrete included in 

the model are one half of the double truss spacing of 40 in. The diameter of the truss diagonal included in 

the model is 1.09 in., and the center line of the knuckle is 3 9/16 in. from the center line of the double 

truss. The concrete slab wire mesh reinforcement is modeled by distributed reinforcement properties. 

The model is bounded by four planes. Two of these planes are parallel to the chord of the truss, and the 

other two planes are perpendicular to the chord. Symmetry conditions are applied to these planes 

consistent with the loads. For the tension loading, in addition to the symmetry conditions, the model is 

supported vertically at both symmetry planes that are perpendicular to the truss chord. For the shear load 

parallel to the chord, the model is supported ahead of the shear load in the direction parallel to the chord. 

The knuckle has the properties of ASTM A36 (Material ID 20) round bar steels and the concrete has 

lightweight concrete properties specified for LS-DYNA with concrete-cap model. 

K.7.3 Column Truss Seats 

In this section, likely failure modes of truss seats are identified, and the corresponding failure loads are 

determined. The following loading conditions were considered: vertical force, horizontal tensile force, 

horizontal compressive force, and combined vertical and horizontal force. 

Description of Column Truss Seats 

The floor truss is supported at the exterior wall and at the core by seats. The truss seat at the exterior wall 

and at the core will be referred to as exterior seat and interior seat, respectively. 

The interior seat consists of a horizontal plate with two vertical plate stiffeners as shown in Fig. K–23. 

These plates are fillet welded together and fillet welded to the core channel beam. Two 5/8 A325 bolts 

(one bolt in each truss) connect the truss to the seat. The bolt connection is a friction type connection 

with 1 3/4 in. long slotted holes in the seat and 7/8 in. oversize holes in the bearing angles. 
Appendix K 

K–32

Side View Front View Top View

Figure K–23. Interior seat. 

The exterior seat consists of a seat angle attached to the spandrel with two vertical plates (stand-off 

plates), and a gusset plate as shown in Fig. K–24. Fillet welds connect the seat angle to the stand-off, the 

stand-off to the column/spandrel, and the gusset plate to truss top chord. A complete-joint-penetration 

groove weld connects the gusset plate to the column/spandrel. Similar to the interior seat, each pair of 

trusses is attached to the exterior seat by two 5/8 in. A325 bolts. The bolt connection is a friction type 

connection with 2 in. long slotted holes in the seat angle and 7/8 in. oversize holes in the truss-bearing 

angle. 

Side View Front View Top View

Figure K–24. Exterior seat. 

In floor 96 of WTC 1, there are seven types of interior seats and eight types of exterior seats. The 

different types of interior seats are identified with Detail Numbers 15, 17, 20, 21, 22, 23, and 226A; and 

the exterior seats with Detail Numbers 1013, 1111, 1212, 1311, 1313, 1411, 1511, and 1611, as shown in

Fig. K–25. 

Stand-off

Seat angle

5/8 in. diameter bolt

5/8 in. diameter bolt

Bearing angle

Truss top chord

Strut

Truss top chord

Gusset plate

Strut

Column/spandrel

Bearing angle

Stand-off

Seat angle

Gusset plate

Column/spandrel

Horizontal plate

5/8 in. dia. bolt

Bearing angle

Truss top chord

Strut

Channel beam

Vertical plate stiffener

Truss top chord

Strut

Bearing angle

5/8 in. dia. bolt

Channel beam Vertical plate stiffener
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–33

Original drawing provided with permission from PANYNJ. 

Figure K–25. Truss seat detail location on northeast quadrant of floor 96 of WTC 1. 

All types of interior seats are similar in their design, but are all unique because of the variation in the size 

of the vertical and horizontal plates, the location of the bolt holes, and the size of the fillet welds. The 

thickness of the plates ranges from 0.375 in. to 0.75 in.; the distance between bolt holes ranges from 

8.5 in. to 10.5 in.; and the size of the fillet welds ranges from 0.25 in. to 0.375 in. All types of exterior 

seats are also similar in their design, but are all unique because of the variation in the size of the stand-off, 

and size of the seat angle, the size and shape of the gusset plate, the location of the bolt holes, and the size 

of fillet welds. The vertical height of the stand-off ranges from 8 in. to 11 in. The smallest seat angle size 

is L4 × 4 × 1/2, and the largest is L6 × 4 × 3/4. The shapes of the gusset plate are rectangular and 

trapezoid, and the plate ranges in width from 4.5 in. to 6 in. The distance between bolt holes ranges from 

3.25 in. to 10.5 in., where it is 3.25 in. when the seat is supporting a single truss. The size of the fillet 

welds ranges from 0.2125 in. to 0.375 in. 

Truss Seat Material Properties 

The material properties used in the calculations were selected from Table K–1 to best match the material 

properties indicated in the design drawings. Figure K–3 was used to determine the material mechanical 1411 1411 1411 1411 1411 1411 1411 1411 1411 1511 1411 1411 1311 1311 1313 

1013 

1212 

1212 

1212 

1611 

1111 

1111 

1111 

1111 

1111 

1111 

1111

23 23 23 23 22 22 21 21 20 20 

226A, 

17 

17 

17 

17 

17 

15 

15
Appendix K 

K–34

properties at high temperature. The material properties used for truss seat calculations are summarized in 

Table K– 6. 

Table K–6. Material properties used for truss seat calculations. 

Description Selected Material ID 

A325 bolts Material 22 

Fillet welds Material 7 

Exterior and interior seat 

Truss bearing angles Material 21 

Seat angle Material 1 

Gusset plate Material 12 

Stand-off Material 23 

Truss top chord angles Material 21 

Exterior seat 

Cover plate for bridging truss top chord Material 1 

Interior seat Vertical plate stiffener Material 12 

Horizontal plate Material 12 

Truss Seat Failure Modes and Sequence 

The failure modes of different truss seats are identified for vertical force, horizontal tensile force, 

horizontal compressive force, and combined vertical and horizontal force. 

Failure Modes of Interior Seat against Vertical Force: The location of the vertical load on the truss seat 

is eccentric to the plane of fillet weld connection between the truss seat and the channel beam. Hand 

calculations have shown that the fillet welds at this connection, which must resist shear and bending, 

control the truss seat capacity. The failure mode is fracture of the fillet welds at this connection, which 

results in loss of the truss vertical support. 

Failure Modes of Interior Seat against Horizontal Tensile Force: The failure modes considered are 

(1) bolt shearing, (2) bolt bearing, (3) bolt tear-out, and (4) block shear failure. Hand calculations have 

shown that the bolt shear strength controls the truss seat capacity. Bolt shear by itself, however, does not 

cause the truss to lose its vertical support, but it is the prerequisite to the truss walking off the seat. The 

travel distance required for the truss to walk off of the seat is 4 in. 

Failure Modes of Interior Seat against Horizontal Compressive Force: The concrete slab above the 

truss seat connection provides the compressive force resistance. If the concrete slab fails, the truss seat 

has resistance against compressive force from bolt friction and surface friction between the seat and 

bearing angles. Additional resistance is developed when the truss comes into contact with the channel 

beam. Travel distance for the truss to come into contact with the channel beam is 1/2 in. Under 

compressive force, the truss will not lose its vertical support. 

Failure Modes of Interior Seat against Combined Vertical and Horizontal Forces: Under combined 

vertical and horizontal forces, the failure modes are a combination of the individual failure modes for 

vertical and horizontal forces. 

Failure Modes of Exterior Seat against Vertical Force: The location of the vertical load on the seat is 

eccentric to the plane of connection between the seat and the spandrel. Because of this eccentricity, the 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–35

truss seat must resist both shear and bending. Finite element analysis of the truss seat was used to 

determine load paths and evaluate the behavior of the seat connection. 

Figure K–26 shows the finite element model of the seat connection, where half of the seat was modeled 

and symmetry boundary conditions were applied. The results of the finite element analysis show that 

shear force is carried primarily by the stand-off plates shown in Fig. K–24, while the bending moment is 

resisted by tensile force in the gusset plate and compressive force in the stand-off plate. The seat restrains 

the moment until horizontal force in the connection causes slippage between the seat angle and bearing 

angle. Fillet welds at the stand-off to spandrel connection, which must resist shear, bending, and 

compression, control the seat capacity. The failure mode is fracture of the fillet welds as this connection, 

which results in loss of truss vertical support. 

Figure K–26. Finite element model of exterior seat. 

Failure Modes of Exterior Seat against Horizontal Tensile Force: The failure modes considered are: 

(1) failure of the groove weld between gusset plate and spandrel, (2) failure of the fillet weld between the 

gusset plate and the truss top chord, (3) tensile failure of the gusset plate, (4) bolt shearing off, (5) bolt 

bearing, (6) bolt tear-out, and (7) block shear failure. For calculation purposes, the bolts are assumed to 

be centered in the slotted holes. The typical failure sequence of the truss seat is as follows: first the gusset 

plate yields, then it fractures, followed by truss deformation and bolt bearing against the slotted hole, the 

bolt shears off, and then finally the truss walks off the seat. The travel distance for the truss to walk off of 

the seat is 4 5/8 in. This failure sequence is illustrated in Fig. K–27 as path (A) and shown in Fig. K–28, 

where the relationship between the tensile force resistance from the seat connection and the truss travel 

distance is plotted. In this plot, frictional resistance between the seat angle and bearing angle was not 

included. 

Apply load and 

boundary conditions 

at diagonal strut 

Strut 

Apply load and 

boundary conditions 

at truss top chord 

Truss top chord

Symmetry constraints at the 

gusset plate and seat angle

Gusset plate 

Fixed boundary conditions 

at the gusset plate to 

spandrel connection 

5/8 in. diameter bolt 

Stand-off 

Bearing angle 

Seat angle 
Appendix K 

K–36

(A) Seat details 1111, 1311, 1411, 1511, and 1611 at all temperatures. 

(B) Seat detail 1013 at temperatures below 100 oC. 

(C) Seat details 1212 and 1313 at all temperatures, and detail 1013 at temperatures more than or equal to 100 oC.

Figure K–27. Failure sequence of the exterior seats against tensile force. 

0

20

40

60

80

100

120

140

160

0 0.2 0.4 0.6 0.8

20oC - 200oC

300oC

600oC

400oC

500oC

700oC

Truss travel distance (in.)

Tensile force resistance (kip)

600oC

500oC

400oC

300oC

20oC

50o

C

200oC

100oC

700oC

Gusset plate yields Gusset plate fractures

Bolt shears off

Slip resistance from 

bolt connection

At travel distance 4-5/8 in. 

truss walks off support 

Figure K–28. Typical tensile force resistance from exterior seat (Detail 1411). 

Seat details 1212 and 1313 have a wider gusset plate and follow path (C) which differs from the typical 

sequence where the bolts will bear against the slotted hole then shear off before the gusset plate 

connection fails. The failure sequence of seat detail 1013 is temperature-dependent. At temperatures 

below 100 o

C, the fillet weld connection between the gusset plate and the truss top chord fractures before 

bolts shear off. At temperatures greater than or equal to 100 oC the failure sequence is the same as for 

Details 1212 and 1313. 

Failure Modes of Exterior Seat against Horizontal Compressive Force: The concrete slab above the 

truss seat connection provides the compressive force resistance. If the concrete slab fails, the truss seat 

Truss 

walk-off seat

(C) 

(B) 

(A) 

Gusset plate 

yields

Gusset plate 

fractures

Fillet weld 

fractures

Bolt 

shears off

Bolt 

shears off

Fillet weld 

fractures
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–37

has resistance against compressive force provided by the gusset plate until it buckles, and from bolt 

friction and bolt shear until the bolt bears against the slotted hole and then shears off. Surface friction 

between the seat angle and bearing angles will also provide some resistance. Additional resistance is 

developed when the truss comes into contact with the spandrel. Travel distance for the truss to come into 

contact with the column spandrel is 1 1/2 in. Under compressive force, the truss will not lose its vertical 

support. 

Failure Modes of Exterior Seat against Combined Vertical and Horizontal Force: Under combined 

vertical and horizontal forces, the failure modes are a combination of the individual failure modes for 

vertical and horizontal forces. 

Truss Seat Capacity Calculations 

In this section, truss seat capacities corresponding to the failure modes described in the previous section 

are given. The capacity is computed for the different types of the truss seat at different temperatures. 

Calculation of the connection capacity was performed using the methods in the Manual of Steel 

Construction: Load and Resistance Factor Design (AISC 2001) with the resistance factor, φ, assumed to 

be equal to one. 

Capacity of Interior Seat against Vertical Force: Failure mode of the truss seat against vertical force is 

fracture of the fillet welds at the seat-to-channel beam connection. Strengths of the fillet welds at this 

connection are summarized in Table K–7. The symbol # in this table refers to seat detail number. 

Table K–7. Interior seat capacity against vertical force. 

Temp. Connection Capacity Against Vertical Force (kip) 

(°C) #15 #17 #20 #21 #22 #23 #226A 

20 226 226 265 221 187 187 385 

50 226 226 265 221 187 187 385 

100 226 226 265 221 187 187 384 

200 225 225 264 220 187 187 383 

300 220 220 258 215 182 182 374 

400 201 201 236 197 167 167 343 

500 160 160 188 156 132 132 272 

600 98 98 116 96 82 82 167 

700 45 45 53 44 37 37 76 

Capacity of Interior Seat against Horizontal Tensile Force: Failure loads were computed for the failure 

modes described above. Table K–8 summarizes the results for Seat Detail 22. This table shows that the 

shear strength of the two bolts controls the horizontal tensile strength of the truss seat connection. As can 

be seen from this table at temperature 500 °C, bolt shear capacity is reduced by half, and at 600 °C it is 

reduced to less than a quarter of the original capacity at room temperature. Other seat details also have 

the same failure mode, and, therefore, the same failure load. 
Appendix K 

K–38

Table K–8. Interior seat capacity against tensile force. 

Resistance against Tensile Force (kip) 

Temp. Bolt Bearing Bolt Tear-out Block Shear 

(°C) 

Bolt Slip 

Critical 

Bolt 

Shearing Off On Seat On Truss From Seat From Truss Of Seat Of Truss

20 6 44 124 69 87 101 60 59 

50 6 44 124 69 87 101 60 59 

100 6 44 124 69 87 101 60 59 

200 6 44 124 69 87 100 59 59 

300 6 42 121 68 85 98 58 57 

400 6 34 111 62 77 90 53 52 

500 6 21 88 49 61 71 42 42 

600 6 9 54 30 38 44 24 24 

700 6 4 25 14 17 20 10 10 

Capacity of Interior Seat against Horizontal Compressive Force: Under compressive force, the truss 

will come into contact with the channel beam before the bolt bears against the slotted hole. The truss seat 

connection does not fail under compressive force. 

Capacity of Interior Seat against Combined Vertical and Horizontal Force: A typical interaction 

relationship for combined vertical and horizontal tensile force is shown in Fig. K–29. As can be seen 

from this figure, the vertical shear strength of the seat reduces because of the additional horizontal tensile 

force that the fillet weld connection between the truss seat and the channel beam must resist. 

0

50

100

150

200

0 10 20 30 40 50

20oC 

50o

C 

100 oC 

200 oC

300oC

400oC

500oC

600oC

700oC

Horizontal tensile strength (kip) 

Vertical shear strength (kip) 

Figure K–29. Strength of combined vertical and horizontal force (Detail 22). 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–39

Capacity of Exterior Seat against Vertical Force: The failure mode of the truss seat against vertical force 

is fracture of the fillet welds at the stand-off-to-spandrel connection. Strength of the fillet welds at this 

connection is summarized in Table K–9. 

Table K–9. Exterior seat capacity against vertical force. 

Temp. Connection Capacity against Vertical Force (kip) 

(°C) #1013 #1111 #1212 #1311 #1313 #1411 #1511 #1611 

20 94 94 111 94 94 140 193 207 

50 94 94 111 94 94 140 193 207 

100 94 94 111 94 94 140 193 207 

200 93 93 110 93 93 139 192 206 

300 91 91 108 91 91 136 187 201 

400 84 84 100 84 84 126 172 184 

500 69 69 81 69 69 102 136 146 

600 45 58 53 60 45 78 84 90 

700 29 26 34 27 29 35 38 41 

Capacity of Exterior Seat against Horizontal Tensile Force: The connection capacity of truss seats that 

follow failure sequence (A), as shown in Fig. K–27, equals the failure load for mode (3) defined 

previously. The connection capacity of truss seats that follow failure sequence (B) equals the failure load 

for mode (2). The connection capacity of truss seats that follow failure sequence (C) equals the failure 

load for mode (4) plus the developed resistance from the gusset plate. The results of the exterior seat 

capacity calculations are summarized in Table K–10. Note that the strength of the truss seat #1013 

increases by about 38 percent at a temperature of about 100 °C. For temperatures less than 100 °C, the 

capacity is controlled by the gusset seat fillet weld strength, and for temperatures in excess of 100 °C, the 

bolt reaches the end of its travel in the elongated bolt hole and increases the capacity of the connection. 

Table K–10. Exterior seat capacity against 

 horizontal tensile force. 

Temp. Connection Capacity against Tensile Force (kip) 

(°C) #1013 #1111 #1212 #1311 #1313 #1411 #1511 #1611 

20 100 104 182 134 182 134 134 134 

50 100 104 182 134 182 134 134 134 

100 138 104 181 134 181 134 134 134 

200 135 103 180 133 180 133 133 133 

300 130 101 174 130 174 130 130 130 

400 115 93 156 120 156 120 120 120 

500 84 75 117 96 117 96 96 96 

600 42 49 67 62 67 62 62 62 

700 20 25 32 31 32 31 31 31 

Capacity of Exterior Seat against Horizontal Compressive Force: Under compressive force, the gusset 

plate will buckle before the bolts shear off. Compression strength of the gusset plate governs the truss 

seat capacity. The compressive strength of the gusset plate is summarized in Table K–11. 
Appendix K 

K–40

Table K–11. Compression strength of gusset plate. 

Temp. Compression Strength of Gusset Plate (kip) 

(C) #1013 #1111 #1212 #1311 #1313 #1411 #1511 #1611 

20 77 69 99 90 99 90 90 90 

50 76 68 98 89 98 89 89 89 

100 74 66 96 87 96 87 87 87 

200 71 63 91 83 91 83 83 83 

300 67 60 87 79 87 79 79 79 

400 62 55 80 72 80 72 72 72 

500 48 42 61 55 61 55 55 55 

600 20 17 25 22 25 22 22 22 

700 6 5 8 7 8 7 7 7 

Capacity against Combined Vertical and Horizontal Force: Interaction relationships for combined 

vertical and horizontal forces are under development. 

K.7.4 Modeling Connection Failure by Break Elements 

In this section, simplified finite element models of the exterior and interior seat, knuckle, stud on strap 

anchor, and stud on spandrel are described. These connection models were developed for incorporation 

in the floor truss analysis to capture the connection failure modes and determine the sequence of the 

failure modes that may lead to the failure of the floor truss. 

The developed simplified model of these connections simulates the loss of connection resistance after 

failure either by exceeding the connection force capacity or by exceeding the allowable deformation (truss 

walking off the seat). The connection capacity can also be temperature-dependent. The finite element 

modeling assumptions are as follows: 

Break element, a unidirectional linear spring element with the capability of turning on and off during an 

analysis, is used for modeling connection failure. The element is a part of the structure that connects two 

“active” nodes in the “on” mode and disconnects them in the “off” mode, depending on the relative 

displacement of two “control” nodes. The break element is defined as follows: 

Bm[iI,j,dofij);(k,l,dofkl);(K,∆0)] (11)

where m is the break element number, i and j are the active nodes, dofij is the degree of freedom for the 

active nodes, k and l are the control nodes, dofkl is the degree of freedom for the control nodes, K is the 

elastic stiffness of the break element, and ∆0 is the differential displacement limit of the control nodes. 

A beam element with temperature-dependent thermal expansion material properties is used to make the 

connection capacity temperature-dependent. This is done by using the deformation of the beam element 

from thermal expansion to control the status (on/off) of the break element. Figure K–30 illustrates the 

basic mathematical model of the connection. The connection capacity is made temperature-dependent by 

defining the thermal expansion of the beam element to be temperature dependent. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–41

Figure K–30. Basic mathematical model of connection failure. 

Multiple connection failure modes require use of different break elements that are connected together in a 

logical manner. For example, to model independent failure modes, that is, one failure mode that does not 

cause other failures, break elements are connected in parallel. If one break element turns off, the other 

break elements remain. For dependent failure modes, break elements are connected in series. If one 

break element turns off, then all elements turn off. 

Simplified Model of the Interior and Exterior Seat 

The failure modes of the interior seat include (1) the truss walking off the support, (2) exceeding the 

vertical temperature-dependent shear capacity of the truss seat, and (3) exceeding bolt 

temperature-dependent shear capacity when bolt bears against slotted hole. These failure modes are 

Basic mathematical model: 

– Let node l and i have same displacement. 

– In this example, control node k and l relative 

displacement has the form: 

– Control element status is by definition 

– Define break element to turn off when capacity, Pu, 

has been reached 

– Element capacity at room temperature is given as 

– Hence for a given Pu and ∆T relationship, the 

required thermal expansion to turn element off can 

be found by

TKL

PP urmu

∆

− = . α −UU lk

P

K

1

status: on status: off 

KP α ∆TL

P .rmu

∆0

TL

K

P UU lk α ∆+=−

off :status ,

on :status ,

0

0

∆>−

∆≤−

lk

lk

UU

UU

.rmu KP ∆= 0

TL

K

Pu

0 α ∆+=∆

i j k

P

Control element

- Stiffness, K 

Beam element

- Length, L 

l
Appendix K 

K–42

captured by using four break elements and two beam elements as shown in Fig. K–31. Results of the 

simplified seat model capturing failure from the truss walking off support and failure from exceeding seat 

vertical shear capacity are shown in Fig. K–32 and Fig. K–33, respectively, which depict the relationship 

between the horizontal and vertical seat forces and the horizontal truss travel distance. 

When truss reaction force on the seat is large in horizontal tension and small in vertical shear, the failure 

mode is bolt shearing off followed by truss walking off the support as shown in Fig. K–32. Bolt shear is 

controlling the seat horizontal resistance capacity. Bolt shear by itself however does not cause the truss to 

lose its vertical support, but it is the prerequisite of truss walking off the seat. The travel distance for a 

truss to walk off an interior seat is 4 in. When truss reaction force on the seat is large in vertical shear and 

small in horizontal tension, the failure mode is exceeding the seat vertical shear capacity as shown in 

Fig. K–33. This failure mode will cause the truss to lose both its vertical and horizontal support from the 

seat. 

The simplified model of the exterior seat is the same as the simplified model of the interior seat, except 

for an additional beam element and a break element to model failure of the gusset plate shown in 

Fig. K–34. 

Simplified Model of the Knuckle 

Knuckle failure modes that must be captured by the simplified model are the horizontal shear and vertical 

tensile failure, which are both temperature-dependent. Finite element modeling assumptions for the 

knuckle are: (1) the knuckle has resistance in all translational DOF, (2) the knuckle does not have a 

vertical compression capacity limit, (3) capacities in the horizontal shear and vertical tension are 

dependent, and (4) vertical compression resistance is independent of the capacities in the other directions. 

Knuckle failure is captured by using 15 control elements and 5 beam elements as shown in Fig. K–35. 

Simplified Model of the Stud on Strap Anchor and Stud on Spandrel 

Simplified models of stud on strap anchor and stud on spandrel were developed using the same technique 

as described for the knuckle model. 

K.7.5 Truss Model 

Objectives 

The objectives of the truss model study are to: 

• Capture the potential failure modes and failure sequence of the truss under combined gravity 

load and thermal load; 

• Develop an understanding of the relative importance of different structural features and 

failure modes; and 

• Develop a simplified model that replicates the expected failure and the limit loads of the truss 

to be used for analysis of the full floor subsystem model. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–43

Figure K–31. Simplified model of interior seat. 

Rigid beam 

Beam element 

Beam element 

Beam element 

(Out-of-plan) 

n2

3 n

6 n

5 n

n4

Rigid beam 

n1

Control element No. 1: Capture 

walk-off support 

B1[(2,3,UZ);(2,1,UY);(K,∆0)] 

n2 n3 n1

3 n

n2

Pz

Py

Control element No. 2:

Capture seat vertical shear capacity 

B2[(1,3,UZ);(4,2,UZ);(K,∆0)] 

Control element No. 4: 

Capture bolt shear capacity 

B4[(1,3,UY);(2,5,UY);(K,∆0)] 

Control element No. 3: 

Capture loss of horizontal 

resistance if seat fails vertically 

B3[(2,3,UY);(4,2,UZ);(K,∆0)] 

6 n 5 n

n4 Beam element No. 1: 

Make seat vertical shear 

capacity 

temperature-dependent

Beam element No. 2: 

Make bolt shear capacity 

temperature-dependent 

Y

Z

Coordinate system 

Rigid beam Constraint equations

Coupling displacement 

DOF of node 1 and 6 

Beam element 

(Out-of-plan) 

Fix rotational DOF 
Appendix K 

K–44

Figure K–32. Results of simplified seat model capturing failure 

from truss walking off interior seat. 

Figure K–33. Results of simplified seat model capturing failure 

from exceeding the interior seat vertical shear capacity. 

Loss of horizontal resistance 

because of seat vertical shear 

failure

Seat vertical 

shear failure 

Vertical resistance 

Horizontal resistance 

Truss walks off 

the support 

Bolt shears off 

Vertical resistance

Horizontal resistance 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–45

Figure K–34. Simplified model of exterior seat. 

Exterior seat model same as the 

interior seat 

n3

2 n

1 n

4 n

2 n 1 n

Y

Z

Coordinate system

4 n n3

Beam element: 

Make gusset plate tensile strength 

temperature-dependent

Exterior column 

centerline 

Fix rotational DOF 

Constraint equations

Coupling displacement DOF of 

node 1 and 3

Break element: 

Capture failure of gusset plate under tensile 

force 

B1[(1,2,UY);(4,2,UY);(K,∆0)] 
Appendix K 

K–46

Figure K–35. Simplified model of knuckle. 

Pz

Px

Py

1 n

2 n - Node on slab 

- Node on truss 

3 n

n3

5 n

3 n4 n

n4

n5 n4

n5

n1

Py Px

Pz

n2

6 n

n6

n2 6 n

n2

7 n

n8

11 n

12 n

9 n

10 n

Break element No. 5: 

Capture knuckle tensile failure 

B5[(3,1,UZ);(12,2,UZ);(K,∆0)] 

The purpose of break 

elements No. 11-15 is 

similar to break elements 

No. 6-10 

Constraint equations

Coupling displacement 

DOF of node 1 and 7

Point-to-point contact 

element to transfer 

vertical compressive force 

between node 1 and 2

Break element No. 8-9: 

Capture loss of horizontal 

resistance in the Y direction if 

knuckle fails horizontally in the X 

direction 

B8[(5,4,UY);(8,2,UX);(K,∆0)] 

B9[(6,5,UY);(2,9,UX);(K,∆0)] 

Break element No. 10: 

Capture loss of horizontal 

resistance in the Y direction if 

knuckle fails vertically 

B10[(2,6,UY);(12,2,UZ);(K,∆0)] 

Break element No. 1-4: 

Capture loss of vertical resistance if 

knuckle fails horizontally 

B1[(2,6,UZ);(8,2,UX);(K,∆0)] 

B2[(6,5,UZ);(2,9,UX);(K,∆0)] 

B3[(5,4,UZ);(10,2,UY);(K,∆0)] 

B4[(4,3,UZ);(2,11,UY);(K,∆0)] 

Break element No. 7: 

Capture knuckle horizontal tensile 

failure in the Y direction 

B7[(4,3,UY);(2,11,UY);(K,∆0)] 

Break element No. 6: 

Capture knuckle horizontal 

compression failure in the Y direction 

B6[(3,1,UY);(10,2,UY);(K,∆0)] 

Five beam elements are 

used to make the knuckle 

capacity 

temperature-dependent
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–47

Failure Modes 

The model can capture the following failure modes: 

Softening and Sagging of Truss—The top and bottom chords and diagonals of the truss are exposed to 

the hot gas layer below the floor slab. As described in Section K.5, the steel in the truss exhibits stiffness 

degradation, yield strength reduction, plastic softening, and creep at high temperatures. A truss with 

softened chords sags. The heat may also reduce the stiffness and strength of the concrete slab, especially 

its bottom layer where temperature is the highest, and around the knuckle where concrete temperature 

rises by conduction through the steel. 

In addition to direct thermal effects, sagging and weakening of the truss can be caused by the following 

failure modes: 

• Buckling or failure of web diagonal members, which reduces the truss action and causes the 

truss to act as a catenary; 

• Buckling or failure of the top and bottom chord members; 

• Knuckle failure and loss of composite action of the concrete slab and the steel truss; or 

• Weld failure between the diagonal and the chord. 

Loss of Support of Truss—The truss can fail by loss of support due to seat failure. Loss of support at 

either the exterior or interior seat can be caused by the extreme sagging and catenary action of the truss 

due to plastic deformation and buckling of truss members. 

As discussed under Boundary Conditions later in this section, the bottom chord of the truss is restrained 

in the lateral direction at the bridging truss locations. Although the out-of-plane deformation of the 

bottom chord due to thermal expansion of bridging trusses will result in a reduction in the vertical load 

capacity of a primary truss, the truss model studied here cannot capture this phenomenon. The interaction 

between the bridging trusses and the primary trusses is intended to be captured in the full floor model. 

Model Description 

Figure K–36 shows the truss model. A typical long-span truss designated C32T1 (SHCR 1973:WTC 

Drawing Book 7, Sheet AB1–2) is modeled to study its response to failure when subjected to dead and 

live loads and thermal loads. The model includes the following: 

• One truss of the pair of trusses at column line 143 of floor 96 in WTC 1; 

• Two exterior columns (columns 143 and 144) with half the area and bending properties, and a 

length of 24 ft (12 ft above and below the floor level); 

• The portion of the spandrel between the two exterior columns; 
Appendix K 

K–48

(a) Entire model 

 

(b) Cross section (c) Top and bottom chords and web diagonals 

Figure K–36. Truss model. 

• The portion of the slab (40 in. wide) between the two exterior columns; 

• One strap anchor that is attached to the truss top chord, concrete slab and the adjacent exterior 

column (column 144); and 

• Exterior and interior seats, and the gusset plate at the exterior end. 

A typical slab section consists of 4 in. thick lightweight concrete on 22 gauge metal deck with flutes 

6.8 in. on centers. Two layers of welded wire fabric were provided in the slab. The reinforcement ratios 

are 0.21 and 0.735 in the directions along and transverse to the truss, respectively. A flute is 2 in. wide at 

the top, 1.25 in. wide at the bottom, and 1.47 in. high. An equivalent thickness of 4.35 in. is used as the 

slab thickness in the truss model. By using the equivalent thickness, the bending stiffness in the direction 

transverse to the truss is about 15 percent higher than the actual stiffness. However, since the bending in 

the transverse direction in this truss model is insignificant, the slab is modeled as an isotropic plate. The 

metal deck and the welded wire fabric are not included in the truss model. 

The top and bottom chords and the diagonals of the truss are modeled by 3–D quadratic finite strain beam 

(BEAM189) elements with temperature-dependent elastic, plastic, and creep material properties. The top 

X Y

40 in. 

4.35 in. 

29 in. 

slab 

top chord 

bearing angle 

bottom chord 

web 

bottom chord 

web diagonal 

knuckle top chord 

288 in. 

span=713 in. 

y 

z 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–49

chord consists of double angles of 1 1/2 × 2 × 0.25 (long legs horizontal), while the bottom chord consists 

of double angles of 3 × 2 × 0.37 (long legs horizontal). Web members are round bars of either 1.09 in. or 

1.14 in. diameter. A typical diagonal member has a 1.09 in. diameter. Top and bottom chords are divided 

into four elements between panel points, and a diagonal is also divided into four elements between top 

and bottom chords. The concrete slab is modeled with 4-node finite strain shell (SHELL181) elements. 

The nodes of the concrete slab are located at the neutral plane of the concrete slab with an offset relative 

to the nodes of the top chords. The cast iron model (Hjelm model) can be used with the SHELL181 

elements that allow different “yield” in tension and compression. A low “yield stress in tension” is used 

to simulate cracking. 

At knuckle locations, the top chord elements and the elements representing the concrete slab are 

connected by control elements (COMBIN37) with capacities determined from the detailed knuckle 

analysis. By including point-to-point contact (CONTA178) elements, compression can always be 

transferred even after knuckles fail. Studs on the strap between the top chord and column 144 are also 

modeled by COMBIN37 elements that connect the strap to the slab and have temperature-dependent 

capacities. The slab and the strap are tied by the COMBIN37 elements horizontally while their vertical 

displacements are coupled. The exterior and core seats are modeled by COMBIN37 elements that have 

temperature-dependent capacities determined from the seat analysis. A stud on the spandrel is also 

modeled by a COMBIN37 element, which ties the spandrel with the slab and has temperature-dependent 

capacities. Because only one 5/8 in. stud was provided over 80 in. between the slab and the spandrel, a 

half stud capacity is considered for the stud in the model. In addition, the first stud on the strap is located 

near this stud on the spandrel. Therefore, COMBIN37 elements between the slab and the spandrel have a 

capacity of a combination of these studs, including a group effect. Damping unit connecting the truss 

bottom chord to the spandrel plate is assumed to have little effect on the behavior of the floor truss under 

sustained loading; therefore, it was not included in the model. 

Three–D elastic beam (BEAM44) elements model the exterior columns. SHELL63 elastic shell elements 

model the spandrel. 

Boundary Conditions 

Boundary conditions on the truss model are shown in Fig. K–37. 

The entire top chord of the truss is supported in the x direction. The bottom chord is supported in the x 

direction at four bridging truss locations. Two edges of the concrete slab are restrained against rotations 

about the y and z axes, but can move in the x direction. 

The interior truss seat is fixed in all directions. The exterior seat is fixed to the spandrel. The truss is 

pinned at both exterior and interior truss seats. 

The exterior end of the slab is tied to the spandrel by only COMBIN37 elements representing studs. The 

interior end of the slab is fixed in the z direction and in rotation about the z direction. In the y direction at 

the interior end of slab, break elements that have temperature-dependent tensile capacities are 

implemented as show in Fig. K–38. Therefore, the interior slab end is fixed in the y direction until the 

tensile force exceeds the capacity that is calculated based on the amount of steel reinforcement (#3@10′′

top and #4@12′′ bottom). 
Appendix K 

K–50

Figure K–37. Boundary conditions. 

Figure K–38. Break elements at the interior end of slab. 

In the analysis with increasing gravity load, a model different from the current model is used, where 

boundary conditions of the slab are slightly different from the current model described above. In this 

model, the exterior end of the slab is tied to the spandrel, and the core end of the slab is fixed in the y and 

z directions and in rotations about the x and z directions. Also, COMBIN37 elements for seats are not 

included in this model. 

Loading 

The loading on the truss model consists of gravity dead and live loads and temperature time-histories for 

all steel members, including the truss seats. The gravity loads include weight of the structure, 8 psf 

superimposed dead load (including nonstructural dead loads due to architectural items and fixed service 

equipment), and 13.75 psf of live load equal to 25 percent of design live load of 55 psf. The thermal load 

is a linear temperature gradient through the slab from 300 °C at the top surface of the concrete slab to 

X

Y

Z

x

y

z

Fixed UZ, UY, ROTX, ROTY, and ROTZ

Fixed UY, ROTX, 

ROTY, and ROTZ

Fixed ROTY and ROTZ, and 

UX is coupled, at two edges 

of slab

Fixed UX for the entire 

length of top chord

Fixed UX at four 

bridging truss locations

Fixed all DOFs

of Rigid Bar 

connected to the 

interior core

Fixed UZ, UY, 

and ROTY at 

the end of slab

X

Y

Z

x

y

z

Fixed UZ, UY, ROTX, ROTY, and ROTZ

Fixed UY, ROTX, 

ROTY, and ROTZ

Fixed ROTY and ROTZ, and 

UX is coupled, at two edges 

of slab

Fixed UX for the entire 

length of top chord

Fixed UX at four 

bridging truss locations

Fixed all DOFs

of Rigid Bar 

connected to the 

interior core

Fixed UZ, UY, 

and ROTY at 

the end of slab

Break Elements
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–51

700 °C at the bottom surface of the slab. The temperature is ramped from 20 °C to 700 °C in steel 

members; from 20 °C to 700 °C at the bottom surface of the slab and from 20 °C to 300 °C at the top 

surface of the slab at 1,800 s; thereafter, the temperatures do not change for another 1,800 s. Temperature 

is not applied to the columns. 

In order to determine the effect of debris load on the truss behavior, a parametric study will be performed. 

Material Properties 

Table K–12 shows material assignments for different structural components in the truss model. 

Table K–12. Material assignments in truss model. 

Structural Component 

Specified Yield 

Strength Material ID 

Top chord 50 ksi 21 

Bottom chord 50 ksi 21 

1.09 in. diameter web 36 ksi 20 

1.14 in. diameter web 50 ksi 21 

Strap 36 ksi 1 

Column 143 65 ksi 15 

Column 144 65 ksi 15 

Spandrel 42 ksi 11 

Lightweight concrete slab 3,000 psi (f’c) 83 

Columns 143 and 144 and the spandrel, use only elastic properties. In the current model, the concrete 

slab also remains elastic. 

Resistance Welds 

Table K–13 shows the resistance weld strength between a chord (double angles) and a diagonal based on 

the test data found at Laclede Steel. Weld strength shown in Table K–13 is the sum of the capacities of 

two resistance welds. Figure K–39 compares resistance weld strength between top or bottom chord and a 

diagonal with yield strength of a diagonal at elevated temperatures. As can be seen in Fig. K–39 (a), a 

typical diagonal (1.09 in. diameter) will yield before the resistance weld fails. For 1.14 in. diameter 

diagonal, the resistance weld strength cannot yield the bar at temperatures below 550 °C, as can be seen in 

Fig. K–39 (b). However, shop drawings show additional arc welds between the chord and 1.14 in. 

diameter bar at most locations. 

Table K–13. Resistance weld strength. 

Chord Diagonal Size (in.) Average Strength (kip) 

Top chord 1.09 36.9 

Top chord 1.14 37.7 

Bottom chord 1.09 41.0 

Bottom chord 1.14 40.5 
Appendix K 

K–52

0 200 400 600 800 0

10

20

30

40

50

Top Chord

Bottom Chord

Yield Strength

Temperature (°C)

Strength (kip)

0 200 400 600 800 0

10

20

30

40

50

60

Top Chord

Bottom Chord

Yield Strength

Temperature (°C)

Strength (kip)

 (a) 1.09 in. diameter bar (b) 1.14 in. diameter bar 

Figure K–39. Comparison of resistance weld strength and yield strength of web member 

at elevated temperatures. 

Current Status 

The truss model can capture the following: 

• Temperature-dependent elastic material properties for both steel and concrete; 

• Temperature-dependent steel plasticity; 

• Buckling of truss members; 

• Failure of knuckle—loss of composite action; 

• Failure of studs on the strap; 

• Failure of stud between the spandrel and the concrete slab; and 

• Failure of the exterior and interior truss seats. 

The following features are being added to the truss model: 

• Crushing and cracking of concrete; 

• Creep strain in steel at elevated temperatures; and 

• Failure of welds (Calculations show section yielding can occur prior to weld failure in nearly 

all cases.). 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–53

Model Verification 

The maximum vertical displacement is checked against the single truss model extracted from the ANSYS 

full floor model that was converted from the SAP full floor model. The difference in the vertical 

displacement is only 3.5 percent. 

FEA Results 

Gravity Loading—The maximum calculated vertical deflection is 1.1 in. downward. The maximum 

calculated horizontal column deflection is 0.022 in. inward. The maximum forces in top chord, bottom 

chord, and diagonal are 13,357 lb, 39,514 lb, and 7,647 lb, respectively. 

Gravity Plus Thermal Loading—The analysis is carried out dynamically with 5 percent Rayleigh 

damping. To shorten the run time, the total time period is set to 1.0 s for the temperature ramp. The 

analysis proceeded to a temperature of T=663 oC. Figure K–40 shows horizontal and vertical 

displacement results. A positive horizontal displacement indicates that the exterior columns are pushed 

out, and a negative vertical displacement indicates that the truss is deflected downward. At 340 o

C, the 

horizontal displacement at the exterior column starts to decrease. At 560 o

C, the exterior columns are 

pulled in, and the truss becomes catenary from that point on. 

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 200 400 600 800

Temperature (°C)

Displacement (in)

Node 3908

diagonal bucklingknuckle failurebolt shear failure

seat walk-off

-35

-30

-25

-20

-15

-10

-5

0

0 200 400 600 800

Temperature (°C)

Displacement (in)

Node 540

diagonal bucklingknuckle failurebolt shear failure

seat walk-off

(a) Horizontal displacement at column 143 (b) Vertical displacement at midspan 

Figure K–40. Displacement versus temperature. 
Appendix K 

K–54

Figure K–41 shows axial forces in the truss members. In the figure, Py is the axial force at yield and 

equals the product of the net area of the member and the yield strength which varies with temperature. Pc 

is the compressive strength per AISC formula (AISC 2003) for the top chord with fixed ends in 

Fig. K–41 (a) and for 1.09 in. diameter diagonal bar with pinned ends in Fig. K–41 (c). 

-100000

-80000

-60000

-40000

-20000

0

20000

0 100 200 300 400 500 600 700

Temperature (°C)

Axial Force (lb)

EL#8 EL#13 EL#61

EL#86 EL#92 EL#100

Py

diagonal bucklingknuckle failurebolt shear failureseat walk-off

-20000

0

20000

40000

60000

80000

100000

0 100 200 300 400 500 600 700

Temperature (°C)

Axial Force (lb)

EL#168 (mid)

EL#120 (near ext)

EL#199 (near int)

Py

diagonal bucklingknuckle failurebolt shear failureseat walk-off

(a) Top chord (b) Bottom chord 

-25000

-20000

-15000

-10000

-5000

0

5000

0 100 200 300 400 500 600 700

Temperature (°C)

Axial Force (lb)

EL#222 EL#236 EL#357 EL#371 EL#385 EL#399 EL#413 (1.14in) Py Pcr for L=32 in diagonal buckling knuckle failure

bolt shear failureseat walk-off

(c) Compression diagonal 

Figure K–41. Axial force in truss members versus temperature. 

Figure K–42 (a) shows the top chords yielding beyond 300 oC. This is due to a significant difference of 

coefficients of thermal expansion (CTE) between concrete and steel. At 500 oC, the CTE of steel is twice 

that of lightweight concrete. Bottom chords are still in the elastic range at the end of analysis. Some 

diagonals are bent significantly in the plane of the truss by high axial force and end moments (see 

Fig. K–42 for the deformed shape at the interior end). This diagonal buckling starts at approximately 

340 o

C. 

X 8 13 

222 

120 

236 

61 

168

86 92 100

199 

371 357 385 413 399 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–55

Figure K–42. Axial stress contour in the truss members at 663 o

C 

(displacement magnification factor = 1.0). 

Figure K–43 shows knuckle forces in the y direction (longitudinal truss direction) and the z direction 

(vertical direction). The capacity of a knuckle in the y direction is assumed to be 30,000 lb, and in the 

z direction 15,000 lb in tension. Knuckles 14 and 15 fail due to horizontal shear around 400 °C. 

Knuckle 1 also fails due to the horizontal shear around 650 °C. 

Figure K–43. Force in the knuckles versus temperature. 

Figure K–44 (a) and (b) show horizontal and vertical reaction forces at seats, respectively. At 510 oC, the 

interior seat bolt shears off. At 650 oC, the truss walks off the interior seat. At 660 oC, the gusset plate at 

the exterior end fails in tension. 

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 

-25000

-20000

-15000

-10000

-5000

0

5000

10000

15000

0 200 400 600 800

Temperature (°C)

Vertical Force (lb)

Knuckle 1

Knuckle 2

Knuckle 3

Knuckle 13

Knuckle 14

Knuckle 15

diagonal bucklingknuckle failurebolt shear failureseat walk-off

-30000

-20000

-10000

0

10000

20000

30000

0 200 400 600 800

Temperature (°C)

Horizontal Force (lb)

Knuckle 1

Knuckle 2

Knuckle 3

Knuckle 13

Knuckle 14

Knuckle 15

diagonal bucklingknuckle failurebolt shear failureseat walk-off

Knuckles 14 and 15 failed

Knuckle 1 failed

(a) Horizontal Force (b) Vertical Force 

MX

-23549

-18398

-13246

-8095

-2944

2207

7359

12510

17661

22812
Appendix K 

K–56

RFY at Interior and Exterior Seats (CTRUSS-08c)

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

25000

30000

0 100 200 300 400 500 600 700

Temperature (°C)

Horizontal Reaction (lb)

Interior Seat Exterior Seat

Gusset Plate Interior Seat Capacity

Exterior Seat Capacity Gusset Plate Capacity

bolt-shear

fracture

diagonal bucklingknuckle failureseat walk-off

0

2000

4000

6000

8000

10000

12000

14000

0 100 200 300 400 500 600 700

Temperature (°C)

Vertical Reaction (lb)

Interior Seat

Exterior Seat

walk-off

diagonal buckling

knuckle failure

bolt shear failure

(a) Horizontal reaction force (b) Vertical reaction force 

Figure K–44. Reaction forces at seats. 

Additional Debris Load 

The capacity of the truss model against additional debris load is determined by increasing the gravity 

loading at room temperature. The analysis is performed with the previous model, where boundary 

conditions of the slab are as described in the section “Bounding Conditions.” Let us define load factor as 

the ratio of the gravity load plus debris weight to the gravity load, where gravity load includes self weight, 

superimposed dead load, and 25 percent of the reduced live load. The analysis was terminated at a load 

factor of 3.4. Figure K–45 (a) shows midspan vertical displacement versus load factor. At 2.4 times the 

gravity loading, 11 knuckles from the core end fail in the truss direction. At 2.8 times the gravity loading, 

the fourth knuckle from the exterior end fails. Figure K–45 (b) shows the sum of horizontal reaction 

forces measured at the exterior columns. Note that seat capacities are not modeled in this analysis. 

Figure K–45. Finite element analysis results from increasing gravity. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10,000 20,000 30,000 40,000 50,000 60,000

Horizontal Reaction Force (lb)

Load Factor

(a) Load factor versus midspan vertical 

 displacement 

(b) Load factor versus horizontal reaction 

 force 

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12

Displacement (in)

Load Factor

11 knuckles from the core end 

failed in the truss direction (only 4 

knuckles are intact on the

side of the exterior end)

The 4 th knuckle from the exterior 

end failed (only 3 knuckles are left)

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–57

Summary and Discussion 

The truss behavior under the gravity plus thermal loading, where the temperature is ramped up to 663 oC 

can be summarized as follows: 

• Top chords yield above 300 oC due to the difference in CTEs of steel and lightweight 

concrete. 

• Compression diagonals start to buckle in the plane of the truss due to a high axial force and 

end moments at 340 oC. 

• At 400 oC, knuckles start to fail. 

• The interior seat bolt shears off at 510 oC. 

• The truss walks off the interior seat at 650 oC, followed by fracture of the gusset plate at the 

exterior end at 663 oC. 

The results for the additional debris weight show that the knuckles start failing when the load factor is 2.4. 

Most knuckles fail before load factor reaches 3.0. After the knuckle failure, the truss loses composite 

action between the truss and the concrete slab, and the vertical displacement increases significantly. As a 

result, horizontal reaction force increases. 

Models of the truss including knuckles with temperature-dependent capacities, diagonal weld failure, and 

concrete cracking and crushing are under study. 

Simplified Model 

To be used in the full floor subsystem model, the truss model will be simplified based on the results from 

the truss model analysis. Characteristics of the simplified truss model are listed in the following: 

• The geometry of the truss will be preserved. 

• Pin-ended Link elements will be used for truss members. 

• User-defined elements will be used to model failure modes of knuckles, seats, and diagonal 

members. They will be implemented at the ends of link elements. 

• Slab softening or cracking will be incorporated into the model. 

K.8 EXTERIOR WALL SUBSYSTEM 

The exterior wall subsystem represents the impact zone and includes nine prefabricated wall panels, three 

panels high by three panels wide. 

The exterior wall subsystem model includes nine columns, extending vertically from the column splice 

located below floor 91 to the column splice above floor 99, and nine spandrels, extending horizontally 
Appendix K 

K–58

from the spandrel splice located at mid-span between columns 149 and 150 to the spandrel splice at 

mid-span between columns 158 and 159, of the WTC 1 exterior wall. 

Figure K–46 shows the subsystem pictorially. Tables K–14 through K–16 give the properties of the 

column component plates, the spandrels, and the column and spandrel splices. Figure K–47 shows 

pictorially the spandrel plate thickness, nominal yield strengths, and spandrel splice types. Figure K–48 

shows the column plates notation used. 

The odd numbered columns support floor trusses. Pairs of strap anchors extend diagonally from the top 

chord of truss pairs to the even numbered columns. The trusses and the straps partially brace the columns 

both in-plane and out-of-plane of the exterior wall. 

Figure K–46. Exterior wall subsystem structure. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–59

a) Columns and Column Splices 

b) Spandrels and Spandrel Splices 

Figure K–47. Column and floor number materials and splice types. 
Appendix K 

K–60

Plate 1 Plate 1

Plate 2

Plate 3

Spandrel

14"

ECRL 13.5"

4.5"

6.5"

Figure K–48. Spandrels and spandrel splices. 

Table K–14. Column sectional properties. 

Column 

Type 

Plate 1 

l × t 

Plate 2 

l × t 

Plate 3 

l × t 

Col. Type 

ID 

120 13.5 × 0.25 13.5 × 0.25 15.75 × 0.25 0 

121 13.5 × 0.3125 13.375 × 0.25 15.75 × 0.25 1 

122 13.5 × 0.375 13.25 × 0.25 15.75 × 0.25 2 

123 13.5 × 0.4375 13.125 × 0.25 15.75 × 0.25 3 

124 13.5 × 0.5 13 × 0.25 15.75 × 0.25 4 

125 13.5 × 0.5625 12.875 × 0.25 15.75 × 0.25 5 

Note: All spandrels in wall model are 52 in. deep × 3/8 in. thick. 

Table K–15. Spandrel splice details.

Spandrel 

Splice 

Type 

Number of 

Bolts/Row 

Total 

Number of 

Rows 

Bolt 

Spacing Gage

Overall Splice 

Plate 

Dimensions 

Bolt to 

Centerline of 

Splice 

Gap B/W 

Spandrels 

Spandrel 

Splice ID 

101 6 2 5@9 49 × 6.75 ×.25 1.875 0.75 101 

102 8 2 3,6,3@

9,6,3 

 49 × 6.75 ×.25 1.875 0.75 102 

111 6 4 5@9 3 49 × 12.75 ×.25 1.875 0.75 111 

112 8 4 3,6,3@

9,6,3 

3 49 × 12.75 ×.25 1.875 0.75 112 

a. All spandrel splices use 7/8 in. A325 bolts; spandrel plate yield strength is 36 ksi. 

b. Holes in spandrel are 1/4 in. larger than bolts; holes in plates are bolt + 1/16 in. or option to match spandrel holes. 

Table K–16. Column splice details. 

Column 

Splice Type 

Butt Plate 

Thickness 

Number 

of Bolts 

Bolt 

Diameter Gage 

Bolt 

Spacing 

Column 

Splice ID 

411 1.375 4 0.875 3.5 6 411 

421 1.625 4 0.875 3.5 6 421 

431 1.875 4 1 3.5 6 431 

a. Butt plates have specified yield strength of 50 ksi. 

b. Bolts are A325. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–61

K.8.1 Description of Exterior Wall Subsystem Model 

Figure K–49 shows the model in elevation. BEAM189 elements model the columns. SHELL181 plate 

elements model the spandrels. Figure K–50 shows the number of elements used to model columns and 

spandrels. MPC184 rigid elements connect the center of gravity of a column to the mid-plane of a 

spandrel at each shell element. Figure K–51 shows this use of the MPC184. MPC184 rigid elements also 

model the spandrel connections. A simplified model, consisting of two BEAM189 elements for each of 

the four bolts, four pairs of CONTA178 contact elements at the faying (contact) surfaces, and MPC184 

rigid elements connecting the tops of the bolts to the CONTA178 contact elements, model the column 

splice. COMBIN37 elements model the fracture of the column splice bolts. 

Figure K–49. Exterior wall subsystem model, viewed from inside of WTC 1. 
Appendix K 

K–62

Figure K–50. Portion of exterior wall subsystem model showing 

number of elements used. 

 

Figure K–51. Schematic representation of columns used in the exterior wall 

subsystem model. 

The capabilities of the BEAM189 and SHELL181 elements include large deflections, plastic deformation, 

and creep at elevated temperatures. Materials are assigned as described in Section K.5. 

The loads on the model include the following: 

• Self weight; 

• Dead load of floor trusses; 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–63

• 25 percent of floor live load; 

• Column splice bolt preload; and 

• Temperatures of fire scenarios. 

A concentrated vertical load and an out-of-the-wall-plane moment due to the dead and live load of the 

structure above floor 99 load the top of each column. A concentrated vertical load and an 

out-of-the-wall-plane couple due to the dead and live load of the floor truss load each odd numbered 

column at the truss seats. Mean temperature at the center of gravity of the column and a linear gradient in 

each of two directions through the section of the column strain the BEAM189 elements at each node. 

Temperatures at the nodes strain the SHELL181 elements. Loads and/or deflections at the truss seats 

model the outward motion or the caternary action of the floor truss due to fire scenarios. The 

7/8 in. diameter column splice bolts are preloaded with 36.05 kip (AISC 1964).

Simple supports out of the plane of the wall restrain the tops and the bottoms of all columns in the model. 

In addition, supports horizontally in the plane of the wall restrain the top and the bottom of central 

column 154. Simple supports in the vertical direction restrain the bottoms of all columns in the model. 

Symmetry conditions are imposed on the spandrels at the extremities of the model, except that the 

spandrels are free to expand in the plane of the wall. In the plane and out of the plane of the wall 

restraints brace the column at floor truss seats and diagonal straps. 

The model captures the following failure modes: 

• Column collapse due to large lateral deformations; 

• Column buckling due to loss of bracing at floor truss seats and diagonal straps; 

• Failure of column splice bolts; and 

• Failure of spandrel splice bolts. 

The model does not capture the local buckling of the column plates and the formation of plastic hinges 

due to the interaction of local plate buckling and high stresses in the column from axial load and bending 

moments. Section K.8.4 below includes the justification for excluding this structural behavior from the 

wall subsystem model. 

K.8.2 Validation of the Exterior Wall Subsystem Model 

The behavior of models of the following components of the exterior wall subsystem validate the exterior 

wall subsystem model: 

• Model of a one-story-high exterior column. 

• Model of a nine-story-high exterior column. 
Appendix K 

K–64

• Detailed and simplified models of the column splice. 

• SAP2000 and ANSYS models of a prefabricated wall panel. 

K.8.3 Model of One-Story High Exterior Column 

Figure K–52 shows the model of a one-story-high exterior column. The model includes a one-story-high 

portion of column 151 extending from floor 95 to floor 96 and portions of spandrels at floor 95 and 

floor 96. The model also represents column 151 from floor 96 to floor 97 since the dimensions, plate 

thicknesses and material properties are identical to those of column 151 from floor 95 to floor 96. 

SHELL181 plate elements model the plates of the column and the spandrels. CERIG rigid elements 

connect the center of gravity of the column to its component plates and the spandrel at both the top and 

bottom of the model. The column is simply supported in three directions at the bottom and simply 

supported in the horizontal direction at the top. Increments of axial displacement applied at the top load 

the model. 

Figure K–52. One-story exterior column model. 

Figure K–53 shows the variation of axial load with enforced axial displacement and resulting lateral 

deflection at room temperature and 700 °C. This figure also shows the hand calculated column load 

levels at room temperature and 700 °C for: 

• Local buckling of Plate 2 and Plate 3; 

• Uniform yielding of the column; and 

• Axial load due to dead and live load at floor 96 in the exterior wall subsystem model. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–65

0

175

350

525

700

875

1050

1225

012345678

Displacement (in)

Axial Load (kip)

Vertical Displacement T=700-Load applied at

CG of column section away from spandrel

Vertical Displaceemnt T=700-Load applied at

CG of column at spandrel section

Horizontal Displacement T=700-Load applied

at CG of column at spandrel section

Horizontal Displacement-RT

Vertical Displacement -RT

Axial yield load of column at T(700) = 444 kips

Elastic local buckling load at T(700)= 682 kips

Elastic local buckling load at 

RT=1004kips

Axial yield load of column at RT=1177kips

Vertical 

displacement

Scenario G 

Horizontal 

displacement 

Scenario G 

Gravity load demand

Figure K–53. Load-deflection of column at room temperature and 700 °C. 

Figure K–54 shows the local bucking deformation of Plate 2 and Plate 3 at the maximum load level. 

Figure K–55 shows a plastic hinge at mid-height of the column for an axial displacement of 2 in. 

Figure K–56 shows the presence of local buckles in Plate 2 and Plate 3 at the maximum load. 

Figure K–53 shows that at room temperature Plate 2 and Plate 3 buckle locally at a load that is less than 

the maximum column load, but that at 700 °C the column yields before it buckles locally. This figure 

also shows that the expected column demand load of 175 kip is substantially lower than the local buckling 

load at room temperature and the column yield load at 700 °C. For these results, the axial displacement 

was applied along the center of gravity of the column cross section away from the spandrel. If axial 

displacement is applied at center of gravity of the column cross section at the spandrel, there will be 

additional bending moment in the column section away from the spandrel. The presence of moments 

reduces the axial load capacity of the column. The resulting load-deflection diagram is also shown in 

Fig. K–53. 

K.8.4 Model of Nine-Story High Exterior Model 

Figure K–57 shows the nine-story-high exterior column model. The model includes column 151 

extending from near mid-height between floor 91 and floor 92 to mid-height between floor 100 and 

floor 101, spandrels at floors 92 through 100, and column splices located at the mid-height between 

floors 94 and 95 and floors 97 and 98. SHELL181 plate elements model the plates of the column, the 

spandrels, the butt plates at the column splice, and the stiffeners. BEAM189 elements model the column 

splice bolts. CONTA174 and TARGE170 elements model the faying surfaces of the column splice. 

MPC184 rigid elements connect the tops of the bolts to the butt plates. At the bottom the column is 

restrained from displacement and rotation in all three directions. At the top the column is restrained from 

translating in the horizontal directions and from twisting. 
Appendix K 

K–66

Figure K–54. Local buckling of column at room temperature. 

Figure K–55. Plastic hinge in column at room temperature. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–67

Figure K–56. Deformed shape of column at maximum axial load at 700 °C. 

Figure K–57. Nine-story column model. 
Appendix K 

K–68

The capabilities of the BEAM189 and SHELL181 elements include large deflections and plastic 

deformation. For these elements, gives the material property identification numbers, which in turn are 

described in Chapter 4 above. 

The loads on the model include the following: 

• Self weight; 

• Dead load of floor trusses; 

• 25 percent of floor live load; 

• Column splice bolt preload; and 

• Temperature of Fire Scenario G. 

In Fire Scenario G, the fire starts on floors 95, 96, and 97 and spreads to floors 93 through 98. Gas 

temperature reaches 1,100 °C. Convection cools the outside face of the column. Radiation heats the 

other three faces. The inside face of the column is not fireproofed. Temperatures are provided at 200 s 

intervals up to 5,000 s. Figure K–58 shows the variation of the maximum temperature anywhere in the 

column with time and the yield stress at the point of maximum temperature. The temperature reaches a 

maximum of 706 °C at 5,000 s. 

0

10000

20000

30000

40000

50000

60000

70000

80000

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200

Time (sec)

Temperature ( C)

0

100

200

300

400

500

600

700

800

Yield stresses (psi)

Temperature

Yield stress

Figure K–58. Variation of maximum temperature and 

corresponding yield stress with time, fire scenario G. 

To account for the dead and live load of the structure above floor 100 and of the floors that connect to the 

column, concentrated vertical loads and bending moments about a horizontal axis in the plane of the wall 

are applied to the top of the column and at all truss seats. Furthermore, the 7/8 in. diameter column splice 

bolts are preloaded to 36.05 kip (AISC 1964). 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–69

Figure K–59 shows the variation of maximum tensile and maximum compressive stresses with time and 

the corresponding yield stress. Figure K–60 shows the deformed shape of the column at 400 s when the 

compression stress in the column is a maximum. Figure K–60 also shows the deformed shape of the 

column at 3,200 s when the tensile stress in the column is a maximum. Figure K–60 also shows the 

deformed shape of the column at 5,000 s when the temperature in the column is a maximum. 

-90000

-80000

-70000

-60000

-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200

Time (Sec)

Stresses (psi) Tension

Compression

Yield Stress

Yield Stress

Figure K–59. Maximum compressive and tensile axial stress and 

corresponding yield stress with time, fire scenario G. 

Figure K–59 shows that the tensile and compressive stresses exceed the yield stress for most times during 

the duration of the fire. 

Figure K–60 shows that for Fire Scenario G, an extreme scenario that assumes no fireproofing on the 

inside face of the column, plastic hinges do not form in the column. This justifies the exclusion of local 

buckling of the column plates from the wall subsystem model. 

K.8.5 Models of the Column Splice 

The plate model of the column splice, shown in Fig. K–61 includes a 92 in. tall section of the nine-story 

column model centered on the column splice located below floor 98. 

Figure K–62 shows a simplified model of the column splice. The simplified model consists of two 

BEAM 189 elements for each of the four bolts, four pairs of CONTA178 contact elements at the faying 

surfaces, and MPC184 rigid elements connecting the ends of the bolts to the CONTA178 contact 

elements. A BEAM189 element extends from each side of the splice to match the length of the plate 

model. Figure K–63 shows the details that model the faying surfaces of the splice. 
Appendix K 

K–70

Figure K–60. Deformed shape of column at 400 s, 3,200 s, 

and 5,000 s (floors 95–97). 

Figure K–61. Plate model of column splice, floors 97-98. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–71

Figure K–62. Simplified model of column splice. 

Figure K–63. Column splice details, plate model and simplified model. 

Both models are subjected to the following loads: 

• Axial tension; 

• Shear transverse to the plane of the wall; 

• Moment out of plane of the wall; 

• Moment in plane of the wall; and 

• Torsion. 
Appendix K 

K–72

Figure K–64 shows the variation of axial displacement with axial force load. Figure K–65 shows the 

variation of transverse displacement with transverse shear fore. These figures show excellent agreement 

of the simplified model with the plate model. 

0

10000

20000

30000

40000

50000

60000

70000

80000

0.005 -0.015 -0.035 -0.055 -0.075 -0.095 -0.115 -0.135 -0.155 -0.175

Vertical Displacement (in)

Axial Tension (lbf)

Plate

Simplified

Figure K–64. Variation of axial displacement with axial load. 

0

25000

50000

75000

100000

125000

150000

175000

200000

225000

250000

275000

300000

0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0

Lateral Displacement (in)

 Shear Load (lbf)

Plate

Simplified

Figure K–65. Variation of lateral displacement with shear load. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–73

Figure K–66 shows the rotation variation with out-of-plane of the wall moment. Figure K–67 shows the 

rotation variation with in-plane-of-the-wall moment. Figure K–68 shows the twist variation with torque. 

Figures K–66, K–67, and Fig. K–68 show large differences between the results of the simplified and plate 

models of the column splice. These differences are due to the fixed locations of pivot points in the 

simplified model, provided by pairs of CONTA178 point-to-point contact elements, about which the 

faying surfaces rotate. The CONTA174 and TARGE170 surface contact elements for the faying surfaces 

in the plate model permit the location of the pivot point to adjust to the demand of the applied moment. 

Adjusting the location of the point-to-point contact elements can minimize these differences, but they 

cannot be eliminated. In the exterior wall subsystem model, the locations of the single point contact 

elements in the column splices will be adjusted and the sensitivity of the response of the model results to 

these locations computed. 

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Rotation (radians) 

Out of Plane Moment (lbin)

Plate

Simplified

Figure K–66. Variation of rotation with moment, out of wall plane. 

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

2000000

0.00 0.01 0.01 0.02 0.02 0.03 0.03

Rotation (radians)

In Plane Moment (lbf.in) )

Simplified

Plate

Figure K–67. Variation of rotation with moment, in plane of wall. 
Appendix K 

K–74

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

2000000

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Twist Angle (radians)

Torque (lbf.in)

Plate

Simplified

Figure K–68. Variation of twist angle with torque. 

K.8.6 Prefabricated Panel Model 

Description of Model 

Figure K–69 shows the SAP2000 model of a typical prefabricated panel at floors 79 to 82. The model is 

modified as follows: 

• Eliminated self-weight from loading conditions. 

• Provided stiff members at the tops of the columns and replaced the four concentrated loads 

with a single concentrated load. 

• Added out-of-plane of the wall supports (UY) at top of columns for out-of-plane loading. 

Figure K–70 shows the ANSYS model for matching the behavior of the SAP2000 exterior wall 

subsystem model. In the ANSYS version of the panel model BEAM189 elements model the columns and 

MPC184 rigid elements attach the spandrels to the columns. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–75

Figure K–69. SAP2000 model of prefabricated panel. 

Figure K–70. ANSYS model of prefabricated panel showing 

geometry and number of elements used. 
Appendix K 

K–76

Both models are subjected to the following loadings at room temperature: 

• A concentrated vertical load (FZ) at the top of one of the outside columns. 

• A concentrated horizontal load in the plane of the wall (FX) at the top of one of the outside 

columns. The stiff members described above distribute this shear load evenly to the tops of 

all three columns. 

• A concentrated transverse load (FY) on the middle column at floor 81. 

The above loadings do not include self-weight. Figure K–71 shows the various loadings applied to the 

ANSYS model. 

Simple supports in the plane and out of the plane of the wall (UX,UY) restrain the tops of the columns. 

Simple supports in all three directions restrain the bottoms of the columns. The spandrels at the 

extremities of the model are free. See Figure K–71. 

Figure K–71. ANSYS model of prefabricated panel showing loading 

and boundary conditions. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–77

Validation Results 

Figures K–72 through K–74 show deflected shapes and indicate the displacement at the points of applied 

load for the SAP and ANSYS models. Table K–17 summarizes the differences in reactions and 

displacements between the SAP and ANSYS models. 

Figure K–72. Deflection of prefabricated panels under 100 kip lateral load. 

Figure K–73. Deflection of prefabricated panels under 100 kip transverse load. 
Appendix K 

K–78

Figure K–74. Deflection of prefabricated panels under 10 kip vertical load. 

Table K–17. Validation results. 

Loading Condition 

SAP2000/ANSYS Displacementsa 

Difference Range 

Lateral FX RX: -2 % to +1 % UX: 7 % 

Transverse FY RY: -6 % to +7 % UY: -13 % 

Vertical FZ RZ: -1 % to +2 % UZ: -4 % 

a. Displacements considered at tops of columns for FX and FZ, and at points of 

load application for FY. 

K.8.7 Ongoing Work on the Exterior Wall Subsystem Model 

The ongoing work includes the following: 

• Stability of a two-story-high exterior column unbraced at the middle floor. 

• Stability of a three-story-high exterior column braced at the top and bottom floor levels only. 

• Stability of nine-story-high exterior column (floor 92 to 100) unbraced at floors 96 and 97 

and subjected to fire scenarios. 

• Response of exterior wall model to fire scenarios 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–79

K.9 FLOOR TRUSS DYNAMIC RESPONSE DUE TO IMPACT OF DROPPING 

FLOOR 

K.9.1 Impact of Dropping Floor 

The failure of dropping floor may occur due to thermal response and/or additional debris weight on the 

truss, and/or as a result of the aircraft impact. A floor truss or a group of floor trusses could lose support 

at both the exterior and interior supporting ends and drop onto the floor below. This failure mode, which 

is shown in Fig. K–75, will be referred to as “full truss drop.” Alternatively, a floor truss or a group of 

floor trusses could lose support on one side and drop down to impact the floor below. This failure mode, 

which is also shown in Fig. K–75, will be referred to as “partial truss drop.” 

Before impact

After impact

L/4

Before impact

After impact

Schematic of full truss drop Schematic of partial truss drop

Before impact

After impact

L/4

Before impact

After impact

Schematic of full truss drop Schematic of partial truss drop

Figure K–75. Schematic of full truss or partial truss drop 

and diagonal crushing at impact. 

K.9.2 Purpose and Scope 

The purpose of this study is to determine the dynamic response of the target truss from the impact of full 

and partial truss drop, to determine whether the target truss seats can resist such an impact load and to 

determine whether the target truss will lose its composite action, become a catenary, and thus fail to 

restrain the exterior column to which it is connected against instability. 

K.9.3 Method of Analysis 

The simulation of a floor drop is idealized with a truss drop. This has the inherent assumption that all 

seats for the floor fail simultaneously to cause a full or partial drop. The dynamic response of the target 

truss from the impact of a dropping truss is calculated using conservation of energy principle. The 

potential energy of the truss just before drop, which is a function of drop height, converts to the kinetic 

energy of the truss just before impact. As the dropping truss starts to impact the target truss, the diagonal 

members of the dropping truss are assumed to deform plastically to absorb some of the kinetic energy. 
Appendix K 

K–80

The energy absorption due to crushing of the furniture and partitions are neglected in this study. The 

energy absorption due to diagonal member crushing reduces the kinetic energy available at impact to 

deform the target truss. All the diagonal members are assumed to deform plastically for the full truss 

drop case, while only one quarter of the diagonal member are assumed to deform plastically for partial 

truss drop, representing one quarter of the length of the truss that may come in contact at impact with 

floor below. The kinetic energy loss at the time of impact of the dropping truss and the target truss is 

calculated based on conservation of momentum. The two trusses are assumed to travel together after the 

impact, at one-half of the velocity of the dropping floor before impact. 

The dynamic load due to the impact of the dropping truss onto the target truss will result in the target 

truss to deform plastically beyond the static load due to the weight of the two trusses. The maximum 

dynamic deformation of the trusses is calculated by conservation of energy principle assuming that the 

resistance of the truss is a bilinear function of displacement. This assumption is based on fitting the FEA 

calculated acceleration-deflection relationship of target truss as shown in Fig. K–76. 

K.9.4 Results 

The ratios of demand-to-seat capacity for the gravity loads of the dropped and impacted trusses moving 

together for temperatures of 20 o

C, 400 oC, 600 oC, and 700 oC; and the gravity plus dynamic impact loads 

for temperatures of 20 o

C and 400 oC, are calculated. The demand-to-capacity ratio of less than one 

shows that the truss seat has sufficient capacity to resist the load, and the demand-to-capacity ratio of 

larger than one, implies that the seat could fail. The range of the demand-to-capacity ratios are due to the 

different assumptions for the amount of energy loss due to crushing of the diagonal members of the 

dropped truss. 

The demand-to-capacity ratio of the long-span truss for gravity loads is shown in Table K–18 and for 

gravity plus impact load is shown in Table K–19. The result for gravity load alone shows that both the 

exterior and interior truss seats have sufficient capacity to support the weight of two floors for all 

temperatures considered. The result for gravity plus impact load shows that at temperatures below 400 o

C 

neither the exterior nor interior truss seat is expected to fail. Peak deflection response due to gravity and 

the dynamic impact of the dropping truss is given in Table K–20. The results show that at room 

temperature, and more so at 400 o

C, the impacted truss will deflect to an extent that it loses composite 

action, and become a catenary. At 400 o

C the truss walks off the interior seat. Obviously, a catenary truss 

is not able to restrain the exterior column against transverse movement and cannot restrain it from 

instability. Although a truss response to increasing acceleration at 700 o

C has not yet been developed, the 

strength reduction of the truss seats clearly indicates that the failure of truss seats will occur. The results 

for long-span truss, for partial truss drop, and for the short-span truss are in progress. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–81

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60 80

Finite element results

Assumed resistance

TRUSS MID-SPAN DEFECTION (IN.)

ACCELERATION (G) 

T = 20oC 

T = 400o

C 

Knuckle No. 4 fail 

Increase T 

from 20 oC to 400o

C 

Diagonals start to buckle and 2 knuckles 

from the interior end fail 

Bolts at the interior seat shears off 

Knuckle No. 1 to 6 and No. 13 fail, and 

truss walks off the interior seat 

All knuckle fail 

11 knuckles from the 

interior end fail 

Figure K–76. Target truss resistance against increasing acceleration. 

Table K–18. Demand-to-Capacity ratio of long-span truss for static gravity load. 

Temp. Capacity (kip) Demand/Capacity 

(°C) 

Demand 

(kip) Int. Seat Ext. Seat Int. Seat Ext. Seat 

20 26.4 187.3 140.0 0.14 0.19 

400 26.4 166.9 125.7 0.16 0.21 

600 26.4 81.6 77.8 0.32 0.34 

700 26.4 37.2 35.5 0.71 0.74 

Table K–19. Demand-to-Capacity ratio of long-span truss 

for dynamic impact load from full truss drop. 

Temp. Capacity (kip) Demand / Capacity 

(°C) Demand (kip) Int. Seat Ext. Seat Int. Seat Ext. Seat 

20 38.6 - 65.3 187.3 140.0 0.21 - 0.35 0.28 - 0.47 

400 39.1 - 45.2 166.9 125.7 0.23 - 0.27 0.31 - 0.36 
Appendix K 

K–82

Table K–20. Peak deflection response due to 

static gravity and dynamic impact. 

Temp. (°C) 

Static 

Deflection (in.) 

Dynamic 

Deflection (in.) 

20 2.3 7.6 - 25.4 

400 24.2 66.4 - 89.6 

K.9.5 Conclusions 

At room temperature, the impact of a dropping truss will not cause failure of truss seats, but will cause the 

impacted truss to deform into a catenary. At 400 o

C, the impacted truss will walk off the interior seat. In 

either case, the impacted floor will not restrain the exterior column against transverse movement and 

instability. The impact of a dropping truss at 700 o

C will cause failure of truss seats. 

K.10 REFERENCES 

ACI (American Concrete Institute) Committee 209. 1992. “Prediction of Creep, Shrinkage, and 

Temperature Effects in Concrete Structures”, ACI 209R–92. 

AISC (American Institute for Steel Construction). 2001. Manual of Steel Construction: Load and 

Resistance Factor Design, 3rd ed. 

AISC (American Institute for Steel Construction). 2003. Manual of Steel Construction, Load and 

Resistance Factor Design. 

ANSYS, Inc. 2004. Theory Reference, ANSYS, Inc. 

ASTM (ASTM International). 2001. “Standard Test Methods for Fire Tests of Building Construction 

and Materials,” ASTM Standard Test Method E119. 

Bartlett, F.M. and MacGregor, J.G. 1996. “Statistical Analysis of the Compressive Strength of Concrete 

in Structures,” ACI Materials Journal, March–April, pp. 158–168. 

Fields, B.A. and Fields, R.J. January 1991. The Prediction of Elevated Temperature Deformation of 

Structural Steel Under Anisothermal Conditions, NIST. 

Fields, R. March 22, 2004. Recommended Multiaxial Fracture Criterion, NIST. 

LERA (Leslie E. Robertson Associates). 2001. The World Trade Center Design Criteria. Worthington, 

Skilling, Helle, and Jackson, original document dated 8/20/65. 

LERA (Leslie E. Robertson Associates). August 2003. Development of Structural Databases and 

Baseline Models for the World Trade Center (WTC) Towers–Progress Report #4: Structural Analysis 

Computer Models. 

Phan, Long T. December 1996. Fire Performance of High-Strength Concrete: A Report of the 

State-of-the-Art, NIST. 
Interim Report on Subsystem Structural Analysis of the WTC Towers 

K–83

Phan, Long T. December 2003. Properties of Concrete at Elevated Temperature, NIST. 

Seanz, L.P. 1964. “Discussion of Equation of the Stress-Strain Curve of Concrete by Desayi and 

Krishman,” Journal of the American Concrete Institute, Col. 61, No. 9, pp. 1229–1235. 

SHCR (Skilling, Helle, Christiansen, Robertson). 1973a. World Trade Center Drawing Book 8, Concrete 

Reinforcing, Chapters 1-4. 

SHCR (Skilling, Helle, Christiansen, Robertson). 1973b. World Trade Center Drawing Book 7, Floor 

Panels and Truss Details, Chapters 1-5. 

Stevick, G.R. March 1994. “Failure of Welds at Elevated Temperatures,” Welded Research Council 

Bulletin 390, Welding Research Council, pp. 1–39. 
Appendix K 

K–84

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L–i

TABLE OF CONTENTS - Appendix L

List of Figures...........................................................................................................................................L–ii

List of Tables ...........................................................................................................................................L–iv

Appendix L

Interim Report on WTC 7........................................................................................................L–1

L.1 Building Description....................................................................................................................L–1

L.1.1 Purpose .............................................................................................................................L–1

L.1.2 Scope of Work..................................................................................................................L–1

L.1.3 Introduction ......................................................................................................................L–1

L.1.4 Foundations ......................................................................................................................L–3

L.1.5 Con Edison Substation .....................................................................................................L–4

L.1.6 Floor Systems ...................................................................................................................L–5

L.1.7 Columns..........................................................................................................................L–13

L.1.8 Column Transfer Trusses and Girders............................................................................L–14

L.1.9 Lateral System................................................................................................................L–15

L.2 Observations of Structural Collapse ..........................................................................................L–17

L.2.1 Damage from WTC 1 and WTC 2 Collapses .................................................................L–17

L.2.2 Observed Fire Locations.................................................................................................L–22

L.2.3 WTC 7 Collapse Observations .......................................................................................L–26

L.2.4 Interpretation of Collapse Initiation Observations in Elevation .....................................L–29

L.2.5 Interpretation of Collapse Progression Observations .....................................................L–30

L.2.6 Debris Field ....................................................................................................................L–33

L.2.7 Summary ........................................................................................................................L–33

L.3 Collapse Hypothesis...................................................................................................................L–34

L.3.1 Introduction ....................................................................................................................L–34

L.3.2 Collapse Initiation Scenarios..........................................................................................L–36

L.3.3 Vertical Progression Scenarios.......................................................................................L–41

L.3.4 Horizontal Progression Scenarios...................................................................................L–46

L.3.5 Summary of Working Collapse Hypothesis ...................................................................L–50

L.3.6 Technical Approach for Analysis of the Working Collapse Hypothesis........................L–52

L.4 References..................................................................................................................................L–52
L–ii

LIST OF FIGURES

Figure L–1. WTC complex.................................................................................................................L–2

Figure L–2. Size comparison of WTC 7.............................................................................................L–2

Figure L–3. WTC 7 to foundations.....................................................................................................L–4

Figure L–4. Con Ed substation location relative to the WTC 7 building. ..........................................L–5

Figure L–5. Floors 8 to 45 plan. .........................................................................................................L–6

Figure L–6. Floor 1 plan.....................................................................................................................L–8

Figure L–7. Floor 2 plan.....................................................................................................................L–8

Figure L–8. Floor 3 plan.....................................................................................................................L–9

Figure L–9. Floor 4 plan.....................................................................................................................L–9

Figure L–10. Floor 5 plan...................................................................................................................L–10

Figure L–11. Floor 5 diaphragm plan.................................................................................................L–10

Figure L–12. Floor 6 plan...................................................................................................................L–11

Figure L–13. Floor 7 plan...................................................................................................................L–11

Figure L–14. Floor 46 plan.................................................................................................................L–12

Figure L–15. Floor 47 plan.................................................................................................................L–12

Figure L–16. Roof layout. ..................................................................................................................L–13

Figure L–17. Typical built-up column details. ...................................................................................L–14

Figure L–18. 3D schematic view of transfer trusses and girders between Floors 5 and 7. ................L–15

Figure L–19. Perimeter lateral system elevations...............................................................................L–16

Figure L–20. Core lateral system........................................................................................................L–16

Figure L–21. Photograph of roof after WTC 1 collapse.....................................................................L–19

Figure L–22a. Debris damage around Floor 18 of the southwest corner. ............................................L–20

Figure L–23a. Pedestrian bridge and debris on Vesey Street after WTC 1 collapsed..........................L–22

Figure L–24a. Fires on Floors 11 and 12 on the east face....................................................................L–25

Figure L–25. East penthouse kink. .....................................................................................................L–27

Figure L–26. East penthouse sinks (2.2 s)..........................................................................................L–28

Figure L–27. Center screenwall and west penthouse sink (7.9 s).......................................................L–28

Figure L–28. Global collapse. ............................................................................................................L–29

Figure L–29. Plan view of regions for collapse initiation. .................................................................L–31

Figure L–30. Likely region of initiating component failures based on videographic and 

photographic records of fire and collapse.....................................................................L–31

Figure L–31. Plan View of Collapse Progression...............................................................................L–32
Interim Report on WTC 7 

L–iii

Figure L–32. Aerial view of WTC 7 after collapse. ...........................................................................L–34

Figure L–33. Collapse initiation and vertical progression on the east side of WTC 7. ......................L–35

Figure L–34. Horizontal progression to the west side of WTC 7.......................................................L–35

Figure L–35. Phases of WTC 7 building collapse. .............................................................................L–36

Figure L–36. Collapse initiation scenarios. ........................................................................................L–37

Figure L–37. Column 79 capacity versus temperature and unbraced length K..................................L–39

Figure L–38. Effects of temperature gradient on interior column 79.................................................L–40

Figure L–39. Steel strength versus temperature. ................................................................................L–41

Figure L–40. Vertical collapse progression scenarios. .......................................................................L–42

Figure L–41. Geometry changes for removal of column 73...............................................................L–43

Figure L–42. Geometry changes for removal of columns 70 and 67. ................................................L–43

Figure L–43. Geometry changes for removal of columns 64 and 61. ................................................L–44

Figure L–44. Geometry changes for removal of column 76...............................................................L–45

Figure L–45. Geometry change for removal of column 79. ...............................................................L–46

Figure L–46. Horizontal collapse progression scenarios....................................................................L–47

Figure L–47. Transfer components between Floors 5 and 7. .............................................................L–49

Figure L–48. Horizontal progression mechanism for truss #1 failure. ...............................................L–49

Figure L–49. Horizontal progression mechanism for truss #2 failure. ...............................................L–50
L–iv

LIST OF TABLES

Table L–1. Timeline of WTC 7 collapse as observed from the northwest. ...........................................L–27
L–1

Appendix L

INTERIM REPORT ON WTC 7 

L.1 BUILDING DESCRIPTION 

L.1.1 Purpose 

Project 6 addresses the first primary objective of the technical investigation led by the National Institute 

of Standards and Technology (NIST) of the 47-story World Trade Center (WTC) disaster: to determine 

why and how WTC 7 collapsed. Specifically, the objective of this Project is to determine the response of 

structural components and systems to the impact damage and fire environment in WTC 7, and to identify 

probable structural collapse mechanisms. 

L.1.2 Scope of Work 

The structural response of WTC 7 to damage from debris and fires is being evaluated to identify possible 

collapse sequences and critical components that are consistent with the videographic and photographic 

records, interview accounts by individuals that were in or around WTC 7, and other available data. This 

work is being conducted in two tasks: 

• Task 1, Structural response analysis to identify critical components 

• Task 2, Structural analysis of possible collapse initiation hypotheses 

The analytical work is being conducted with the assistance of Gilsanz Murray Steficek LLP. 

The scope of work under Task 1 includes (a) develop a nonlinear global structural model of WTC 7 and 

evaluate its performance under design gravity loads, (b) identify credible failure sequences for the 

structural model with service loads and initial structural damage by analyzing the effect of component 

failures (that may have occurred directly or indirectly from fires) on the structural system stability, 

(c) identify dominant failure modes for critical components and subsystems determined in (b) for service 

loads and elevated structural temperatures, (d) conduct parametric studies of critical subsystems to 

identify influential parameters, and (e) develop approaches to simplify structural analyses for global 

modeling and analyses. 

Selected technical results and finding for progress on Task 1 (a), (b), and (c) are presented in the 

following sections: a description of the WTC 7 structural design, observations of damage, fires, and the 

structural collapse, and the working collapse hypothesis developed to date. 

L.1.3 Introduction 

WTC 7 was a 47 story commercial office building, completed in 1987. Its location relative to the WTC 

Plaza is shown in Fig. L–1. It contained approximately 2 million ft2

 of floor area. The overall 

dimensions of WTC 7 were approximately 330 ft long, 140 ft wide, and 610 ft tall. The typical floor was 
Appendix L 

L–2

similar in size to a football or soccer field (see Fig. L–2). The gross floor area was about 75 percent of 

that contained in the Empire State Building. The building was constructed over a pre-existing electrical 

substation owned by Con Edison. The original plans for the Con Ed Substation included supporting a 

high-rise building, and the foundation was sized for the planned structure. However, the final design for 

WTC 7 had a larger footprint than originally planned. Section L.1.4 describes the WTC 7 foundation. 

Figure L–1. WTC complex. 

A. Comparison to Football Field B. Comparison to Soccer Field 

Figure L–2. Size comparison of WTC 7. 

N 

WTC 7
Interim Report on WTC 7 

L–3

WTC 7 was located immediately to the north of the main WTC Complex, approximately 350 ft from the 

north side of WTC 1. It occupied the block bounded by Vesey Street on the south, Barclay Street on the 

north, Washington Street on the west, and West Broadway on the east. It was connected to the WTC 

complex with a 120 ft wide elevated plaza at the Floor 3, and a 22 ft wide pedestrian bridge, also at 

Floor 3. 

Above Floor 7, the building had typical steel framing for high-rise construction. The floor systems had 

composite construction with steel beams supporting concrete slabs on metal deck, with a floor thickness 

of 5.5 in. The core and perimeter columns supported the floor system and carried their loads to the 

foundation. The perimeter moment frame also resisted wind forces. Columns above Floor 7 did not align 

with the foundation columns, so braced frames, transfer trusses, and transfer girders were used to transfer 

loads between these column systems, primarily between Floors 5 and 7. Floors 5 and 7 were heavily 

reinforced concrete slabs on metal decks, with thicknesses of 14 in. and 8 in., respectively. The following 

sections describe the components and subsystems of WTC 7. 

L.1.4 Foundations 

WTC 7 and the electrical substation were supported on caisson foundations. When the substation was 

constructed in 1967, provision was made for a future office tower by including capacity to carry both the 

substation and the weight of a future building. Caissons were also installed in the property adjacent to the 

substation, for the proposed future building. When WTC 7 was constructed approximately 20 years later, 

it was significantly larger than the originally proposed building, and required additional caissons to be 

installed, as shown in Fig. L–3. 

The typical caisson consisted of several components: a 30-in., 36-in., or 42-in. diameter steel casing, a 

heavy rolled or built-up steel core shape, vertical reinforcing bars, spiral rebar, and concrete fill. At the 

base of the caisson core, a pattern of shear studs was placed to help transfer the load from the steel 

caisson core into the encompassing concrete, from which it passed into the rock. The caissons extended 

through the soil, and were socketed (seated) in the bedrock, approximately 60 ft below the surface. There 

were vertical caissons as well as battered (or sloped) caissons to carry the lateral load. Above the 

caissons were heavy grillages composed of built up steel girders. Grillages transferred loads between the 

building columns and the caissons. 

The distance between the caisson grillages and the first floor varied between 8 ft and 30 ft. This region 

was braced by reinforced concrete walls with thicknesses varying from 1 ft to 2.5 ft. Many of the WTC 7 

steel columns were embedded in these walls, and supporting steel braces were made composite by the 

addition of shear studs along the height of embedment. 

Areas between the concrete walls were backfilled with compacted gravel fill and then covered with a 

concrete slab on grade or framed slab to form closed cells and bring the structure up to the required 

elevation. In some cases, the area was left unfilled and used to house fuel tanks. 
Appendix L 

L–4

Source: McAllister 2002. 

Figure L–3. WTC 7 to foundations. 

L.1.5 Con Edison Substation 

The Con Ed Substation was constructed in 1967 and consisted of a steel framed structure with cast-inplace concrete floors and walls. It was placed on the northerly portion of the site and extended 

approximately 40 ft north of the north facade of WTC 7, as shown in Fig. L–4. Its southerly boundary 

was irregular, but extended approximately one-third to two-thirds of the width of WTC 7. The Con Ed 

Substation was three stories in height. 

The substation’s lateral system consisted of a moment frame along the northern row of interior columns. 

Along the south edge of the substation there was a braced frame. This braced frame was coincident with 

the north side of the WTC 7 core, at columns 64, 67, 70, and 73. Lateral loads from WTC 7 were passed 

directly from the core above to the Con Ed braced frame below. There were also two moment frames 

within the substation oriented in the north-south direction, one on each end of the WTC 7 core. 

The WTC 7 columns, which were within the perimeter of the substation, were supported by substation 

columns. During the construction of WTC 7, heavy plates were welded to the tops of the existing 

substation columns which then received the new building columns. 

Indicates original caisson 

Indicates additional WTC 7 caisson 

Con Edison 

Substation 

WTC 7 

Footprint 

Face of WTC 7 (above) 

Ramp 
Interim Report on WTC 7 

L–5

N

Access Ramp to WTC 

Con-Edison Substation 

64 67 70 73

WTC 7 3rd Floor 

Figure L–4. Con Ed substation location relative to the WTC 7 building. 

L.1.6 Floor Systems 

Typical Floor Systems Above Floor 7 

The typical floor framing system, shown in Fig. L–5, was composed of rolled steel wide-flange beams 

with composite metal decking and concrete slabs. Floors 8 through 45 had essentially the same framing 

plan, but the core layout varied over the height of the building. 

Floors 8 through 45 had floor slabs that were composed of 3 in., 20 gage metal deck with 2.5 in., 

3,500 psi normal weight concrete, for a total thickness of 5.5 in. There was one layer of 6x6 W1.4xW1.4 

welded wire mesh within the concrete. The drawings show a second layer of mesh placed over girders at 

the slab edges. The fastening requirements for the metal deck are not shown on the drawings, but standard 

practice provides puddle welds 12 in. on-center at the beams and side lap welds, screws, or buttonpunching at 36 in. on-center between adjacent panels of deck. The drawings contain a note calling for 

1.5 in., 20 gage deck with 4 in. concrete topping (5.5 in. total) in the elevator lobbies, where there was a 

3 in. floor finish specified by the architect. 

Typical floor framing for Floors 8 through 20 and Floors 24 through 45 consisted of 50 ksi wide-flange 

beams and girders. Between the core columns was a grid of beams and girders. Core girders ranged in 

size from W16x31 to W36x135, depending on the span and load. (W16x31 describes a steel wide-flange 

beam, sometimes referred to as ‘I’ beams; the nomenclature indicates the cross-section is nominally 16 in. 

deep and weighs 31 lb per lineal foot.) Beams spanned directly between the core and the exterior of the 

building, at approximately 9 ft on-center spacing. On the north and east sides, the typical beam was a 

W24x55 with 28 shear studs, spanning 53 ft. On the south side, the typical beam was a W16x26 with 

24 shear studs spanning 36 ft. Between the exterior columns were moment connected girders that formed 

part of the lateral system of the building. On Floors 10, 19, and 20, a portion of the floor framing was 
Appendix L 

L–6

Figure L–5. Floors 8 to 45 plan. 

Figure L–5. Floors 8 to 45 plan. 

reinforced with plates attached to the bottom flange. Certain connections at these floors were also 

reinforced. 

Floors 21 to 23 had slightly heavier steel framing than the typical floors. Core girders were generally one 

size class larger than the typical floor; the beams between the core and the south facade were W16x31 

instead of W16x26. There were additional studs on the W24x55 beams on the north and west sides. 

Most of the beams and girders were made composite with the slabs through the use of shear studs. 

Typically, the shear studs were 0.75 in. in diameter by 5 in. long, spaced 1 ft to 2 ft on center. Studs were 

Elevator Banks Stairs Stairs

Core 

Area Composite 

Floor System 

Column 

Number

Floor 

Beams 

Girders 
Interim Report on WTC 7 

L–7

not indicated on the design drawings for many of the core girders. The design drawings specified design 

forces for connections and suggested a typical detail, but did not show specific connection designs; this is 

standard practice on the U.S. east coast. The erection drawings indicate that design shear forces for the 

typical beam and girder connections were to be taken from the American Institute of Steel Construction 

(AISC) beam design tables for beams without shear studs, using 1.5 times those forces for beams with 

shear studs. 

According to a paper by Salvarinas (1986), who was the project manager for Frankel Steel, which 

fabricated the steel for WTC 7, the typical floor beam to girder and girder to core column connection was 

a single shear plate, although end plate and double angle connections were also used. The typical beam to 

exterior column connection was a seated connection. The typical bolt size for the simple shear 

connections is cited as 0.875 in. in diameter ASTM A325, where A325 is a standard specification for a 

structural bolt specified by ASTM International. The bolt size used for heavier brace and moment 

connections was 1 in. in diameter ASTM A490. Information on the specific connection details used is 

unavailable at this time. 

Other Floors 

The remaining floors, Floors 1 to 7 and Floors 46 to 47, were atypical and are described below and in 

Figs. L–6 through L–15. 

Floor 1 was built adjacent to the substation and included the truck ramp for the WTC complex. The first 

floor is shown in Fig. L–6. The floor was framed with steel beams that were encased in a formed 

concrete slab. The floor slab was 14 in. thick, with typical #5 reinforcement bars (5/8 in. rebar) at a 10 in. 

to 12 in. spacing and #6 rebar at 9 in. spacing for the bottom reinforcement; #5 rebar at 12 in. spacing 

was used for temperature reinforcement. The southeast portion of the floor above the WTC truck ramp 

had a 6 in. formed concrete slab with #4 rebar at 12 in. spacing for top and bottom reinforcement; 

#4 rebar at 18 in. spacing was used for temperature reinforcement. 

The floor slab for Floors 2, 3, 4, and 6 had a 3 in., 20 gage metal deck with 3 in. 3,500 psi normal weight 

concrete, for a total thickness of 6 in. Floors 2 and 3 were also partial floors adjacent to the substation. In 

addition, they had a floor opening on the south side to form the atrium above the ground level lobby (see 

Figs. L–7 and L–8). Floor 4 was above the substation and had a large opening over most of the south side 

of the building, to form a double-height space above the 3rd floor lobby (see Fig. L–9). Floor 6 had two 

openings on the floor to form a double-height mechanical space, one at the east side and the other one at 

the southeast corner (see Fig. L–12). Truss #2 and column 80 were located in this double-height 

mechanical space. 

The 5th floor slab was 11 in. of 3,500 psi normal weight concrete on top of a 3 in., 18 gage composite 

metal deck for a total slab thickness of 14 in. The slab was heavily reinforced, with #7 rebar at 12 in. 

spacing for top reinforcement in both directions and #9 rebar at 12 in. spacing for bottom reinforcement 

that acted as additional diaphragm chord reinforcement in many areas. This floor also had 36 ksi steel 

WT sections (W, or wide-flange, sections cut in half to look like a ‘T’ section) embedded in the 11 in. 

concrete slab above the deck. The WT sections were designed to act as a horizontal truss within the plane 

of the floor between the perimeter and core columns (see Figs. L–10 and L–11). 
Appendix L 

L–8

Figure L–6. Floor 1 plan. 

Figure L–7. Floor 2 plan. 

Ramp

Con Ed Substation 

WTC 7

Ground Lobby 

Atrium over 

Lobby 

Atrium over 

Lobby 

WTC 7

Con Ed Substation
Interim Report on WTC 7 

L–9

Figure L–8. Floor 3 plan. 

Figure L–9. Floor 4 plan. 

Open Floor over Third Floor Lobby 

Atrium over 

Lobby 

Atrium over 

Lobby 

WTC 7

Con Ed Substation
Appendix L 

L–10

Figure L–10. Floor 5 plan. 

Figure L–11. Floor 5 diaphragm plan. 

14 in. Reinforced Slab & Metal Deck

WT Sections 
Interim Report on WTC 7 

L–11

Figure L–12. Floor 6 plan. 

Figure L–13. Floor 7 plan. 

The 7th Floor slab consisted of 5 in. of 3,500 psi normal weight concrete on top of 3 in., 18 gage 

composite metal deck, for a total thickness of 8 in. The slab was reinforced with #5 rebars at 6 in. oncenter in both directions. Regions of the slab on the south side of the building had 8 in. of formed 

concrete without any metal deck. In these regions two layers of steel reinforcing were provided (see 

Fig. L–13). 

 8 in. Slab & Metal Deck

8 in. Formed Slab

Two-Story 

Mechanical 

Space 

Truss #2

Truss #1

Two-Story 

Mechanical 

Space 

Plenum 

Plenum 

80 
Appendix L 

L–12

Figure L–14. Floor 46 plan. 

Figure L–15. Floor 47 plan. 

Floors 41 and 43 had the east half removed to provide double height spaces. Columns in these areas and 

areas of Floors 40 and 42 had been reinforced to provide adequate capacity for the additional height and 

change in use by tenants. By 2001, Floors 41 and 43 had been reconstructed to provide full floor space. 

Specifics of this reconstruction are not available at this time. 

The 46th Floor had heavier framing to support the cooling towers and dunnage on the north side, 

(alternating W36x150 with W36x260 under the posts) and the setback roof on the south side (alternating 

W21x44 with W36x150 under the posts). There was a 6 in. reinforced concrete slab in a portion of the 

core and under the cooling towers (see Fig. L–14). 

Heavier Framing for 

Cooling Towers 

Roof Setback 

Two-Story Space for 

Cooling Towers 
Interim Report on WTC 7 

L–13

Floor 47 had a double height space extending from the 46th Floor to the underside of the roof for the 

cooling towers on the north side. There was also a setback roof on the south side at Floor 46 

(see Fig. L–15). 

Roof and Penthouses 

The roof had a concrete slab on metal deck, the top of which sloped 3 in., from an 8.5 in. thickness to a 

5.5 in. thickness, to provide drainage. The wire mesh in this slab was 6x6 W2.4xW2.4, which was 

70 percent heavier mesh than at the typical floor. There were slab openings for the cooling towers on the 

north and the setback roof on the south. The area above the cooling towers was framed in steel, with 

areas of grating spanning between the beams. A series of diagonal WT 6x9 members under the grating 

provided diaphragm action in this area. The east side of the floor was reinforced to carry the east 

penthouse and its contents. Specifics of this reinforcement are not available at this time. 

The west penthouse roof was framed in steel with the floor slab increased to a 6 in. thickness. The 

framing and roof reinforcement for the east penthouse and the mechanical equipment screenwall are not 

available at this time. Layout of these areas has been determined from photographs, as shown in 

Fig. L–16. 

Figure L–16. Roof layout. 

L.1.7 Columns 

Core columns were primarily rolled wide-flange shapes of grade 36 or 50 steel. As the loads increased 

towards the base of the building, many of these column sizes were increased through the use of built-up 

shapes. These built-up columns had a W14x730 core with cover plates welded to the flanges (to form a 

box) or web plates welded between the flanges as shown in Fig. L–17. The reinforcing plate welds were 

57 56 55 45 53 52 51 50 49 48 47 46 45 44 43 

58 61 64 67 70 73

59 

62 65 68 71 74

76

79 

77 80 

60 63 66 69 72 75 78 81 

EAST 

PENTHOUSE WEST SCREEN WALL PENTHOUSE 

COOLING TOWERS BELOW 

N 

Roof Setback 

Cooling Towers 

Below 

East West 

Screenwall Penthouse 

Penthouse
Appendix L 

L–14

specified to be continuous 0.5 in. fillet welds at the cover plates and 0.313 in. minimum at the web plates. 

Plate thickness ranged from 1.5 in to 8 in. Reinforcing plates were specified as follows: 

Plate thickness t (in.): 

2 < t < 4 ASTM A588 Grade 50 

4 < t < 6 ASTM A572 Grade 42 

t > 6 ASTM A588 Grade 42 

Figure L–17. Typical built-up column details. 

Typical core column splices were shown on available erection drawings. The adjoining surfaces of 

columns were specified to be milled. The splice plates were welded or bolted to the outsides of the 

column web and flanges. Built-up columns were also milled at their bearing ends but the splice plates 

were fillet welded to the cover plates. 

Perimeter columns were nominally 14 in. wide-flange shapes (W14) of ASTM A 36 steel. Perimeter 

column splices were similar to the core column splices. 

L.1.8 Column Transfer Trusses and Girders 

The layout of the substructure and Con Edison columns did not align with the column layout in the upper 

portion of WTC 7. Therefore a series of column transfers were constructed. These transfers occurred 

primarily between Floors 5 and 7. See Fig. L–18 for a schematic rendering of the transfers. 

Columns 47 through 54, at the north facade, were transferred at Floor 7 by cantilever girders to bring 

them in line with the substation columns, offset 6 ft to 9 ft to the south. The back-span of these 

cantilevers was supported by the north side core columns. The eastern most cantilever girder was 

connected to truss #1, and the western most cantilever girder was connected to truss #3 (see Fig. L–18). 

W14 × 730

Cover plate 

0.5 in. Welds 

Web plate

W14 × 730 

0.313 in. 

Welds 
Interim Report on WTC 7 

L–15

Figure L–18. 3D schematic view of transfer trusses and girders between Floors 5 and 7. 

Column 76 was supported at Floor 7 by truss #1. The west side of truss #1 is supported by column 73, 

while the east side is supported by a transfer girder running north-south which is, in turn, supported by 

columns E3 and E4 at Floor 5. 

Columns 58, 59, and 78 were transferred by simply supported girders at Floor 7. Column 78 was 

supported at Floor 7 by a transfer girder that was supported at its north end by truss #2. Column 77 was 

also supported by truss #2. Truss #2 was supported by column 74 at its west end and by column 80 at its 

east end. 

Column 61 was supported by truss #3. Truss #3 runs north-south and was supported by columns 62 and 

61A. Truss #3 has a 10 ft cantilever span between column 61 and column 61A and an 18 ft back span to 

column 62. 

L.1.9 Lateral System 

Above Floor 7, WTC 7 had a perimeter moment frame. Exterior columns were typically rolled W14 

shapes of ASTM A36 grade steel. Column trees were fabricated for the east and west facades with field 

splices occurring every other story in the columns and at the spandrel beam midspan between columns, 

where the tree stubs were spliced with a bolted connection. On the north and south facades, the moment 

frames were constructed with spandrel connections at the face of the columns. Some column splices were 

shown on the erection drawings to be partial penetration groove welds between the column flanges. 
Appendix L 

L–16

At Floors 5 to 7 and Floors 22 to 24, there was a perimeter belt truss, shown in Fig. L–19. Below Floor 7, 

a combination of moment and braced frames around the perimeter and a series of braced frames in the 

core, is shown in Fig. L–20. The strong diaphragms of Floors 5 and 7 transferred load from the perimeter 

to the core. Above the loading dock at the south facade, two of the columns were hung from the belt truss 

at Floors 5 through 7. Above the Con Edison vault at the north facade, eight columns were also hanging 

from the belt truss between Floors 5 and 7. 

Figure L–19. Perimeter lateral system elevations. 

Figure L–20. Core lateral system. 

64 73 70 67 75 72 69 66

N 

W W
Interim Report on WTC 7 

L–17

L.2 OBSERVATIONS OF STRUCTURAL COLLAPSE 

This section presents observed data and events from available drawings, photographic and videographic 

records, interviews, and other data sources for WTC 7 to identify damage and fire locations. Damage to 

WTC 7 from debris impact from WTC 1 and WTC 2 is summarized in Section L.2.2, followed by known 

fire growth and progression in Section L.2.3. The observed exterior sequence of collapse events from 

photographic and videographic records are described in Sections L.2.4 and L.2.5, where collapse 

observations are considered from the plan and elevation views of the structure, respectively. These 

observations have been used for developing possible collapse initiation locations and progression 

mechanisms, which are presented in Section L.3. 

L.2.1 Damage from WTC 1 and WTC 2 Collapses 

To place the events leading to the global collapse of WTC 7 into context, it is helpful to summarize the 

events of September 11, 2001: 

8:46 a.m. WTC 1 was struck by an aircraft 

9:03 a.m. WTC 2 was struck by an aircraft 

9:59 a.m. WTC 2 collapsed 

10:28 a.m. WTC 1 collapsed 

5:21 p.m. WTC 7 collapsed 

After WTC 1 collapsed, the south face of WTC 7 was obscured by smoke, making direct observation of 

damage from photographs or videos difficult or impossible. The source of the smoke is uncertain, as 

large fires were burning in WTC 5 and WTC 6, as well as those noted below in WTC 7. The light but 

prevalent winds from the northwest caused the smoke to rise on the leeward, or south, side of the 

building. The following information about damage seen in WTC 7 was obtained from interviews of 

people in or near the building: 

After WTC 2 collapsed: 

• Some south face glass panes were broken at lower lobby floors 

• Dust covered the lobby areas at Floors 1 and 3 

• Power was on in the building and phones were working 

• No fires were observed 

Reported close to time of WTC 1 collapse: 

• East stair experienced an air pressure burst, filled with dust/smoke, lost lights 

• West stair filled with dust/smoke, lost lights, swayed at Floors 29 through 30, and a crack was 

felt (in the dark) on the stairwell wall between Floors 27 through 28 and Floors 29 through 30 

• Floors 7 and 8 had no power, air was breathable but not clear 
Appendix L 

L–18

• Phone lights on Floor 7 were on but could not call out 

After WTC 1 collapsed: 

• Heavy debris (exterior panels from WTC 1) was seen on Vesey Street and the WTC 7 

promenade structure at the third floor level 

• Southwest corner damage extended over Floors 8 to 18 

• Damage was observed on the south face that starts at the roof level and severed the spandrels 

between exterior columns near the southwest corner for at least 5 to 10 floors. However, the 

extent and details of this damage have not yet been discerned, as smoke is present. 

• Damage to the south face was described by a number of individuals. While the accounts are 

mostly consistent, there are some conflicting descriptions: 

− middle one-fourth to one-third width of the south face was gouged out from Floor 10 to 

the ground 

− large debris hole near center of the south face around Floor 14 

− debris damage across one-fourth width of the south face, starting several floors above the 

atrium (extended from the ground to 5th floor), noted that the atrium glass was still intact 

− from inside the building at the 8th or 9th Floor elevator lobby, where two elevator cars 

were ejected from their shafts and landed in the hallway north of the elevator shaft, the 

visible portion of the south wall was gone with more light visible from the west side 

possibly indicating damage extending to the west 

At 12:10 to 12:15 p.m.: 

• Firefighters found individuals on Floors 7 and 8 and led them out of the building 

• No fires, heavy dust or smoke were reported as they left Floor 8 

• Cubicle fire was seen along west wall on Floor 7 just before leaving 

• No heavy debris was observed in the lobby area as the building was exited, primarily white 

dust coating and black wires hanging from ceiling areas were observed 

Photographs support some of these reports and show additional damage at the upper portions of the 

building. Figure L–21 is an aerial view of WTC 7 after the collapse of WTC 1. There is no visible debris 

on the roof; some minor damage is seen on the south side at the parapet wall. Figures L–22a and L–22b 

show the reported damage between Floors 8 to 18 at the southwest corner. Much of the damage above 

Floor 18 appears to be nonstructural. The black areas on the facade indicate areas of burned out fires. 

Note the heavy smoke obstructing any observations along the south face. Study of this photograph 

indicates that at least two exterior columns were severed. Figures L–23a and L–23b show the debris on 

Vesey Street in front of WTC 7 after the collapse of WTC 1. The pedestrian bridge (L–23a) and the 
Interim Report on WTC 7 

L–19

promenade (L–23b) appear to be standing, although damaged. Exterior panels from WTC 1 can be seen 

on Vesey Street and on the promenade. The approximate extent of possible damage due to debris from 

WTC 1 is shown in Fig. L–23c. 

Figure L–21. Photograph of roof after WTC 1 collapse. 

©2001. NYPD Aviation 
Appendix L 

L–20

Figure L–22a. Debris damage around Floor 18 of the southwest corner. 

SW Corner 

Damage 

Starting at 

Floor 18 
Interim Report on WTC 7 

L–21

Figure L–22b. Debris damage around Floor 8 of the southwest corner. 

SW Corner Damage 

down to Floor 8 
Appendix L 

L–22

Figure L–23a. Pedestrian bridge and debris on Vesey Street after WTC 1 collapsed. 

L.2.2 Observed Fire Locations 

Photographs and videos were used to determine fire locations and movement within WTC 7. Most of the 

available information is for the north and east faces of WTC 7. Information about fires in other areas of 

the building was obtained from interviews, and is summarized as follows: 

From 11:30 a.m. to 2:30 p.m.: 

• No diesel smells reported from the exterior, stairwells, or lobby areas 

• No signs of fire or smoke were reported below the 6th Floor from the exterior, stairwells or 

lobby areas 

• In the east stairwell, smoke was observed around Floors 19 or 20, and a signs of a fully 

involved fire on the south side of Floor 23 were heard/seen/smelled from Floor 22. 

• Interviews place a fire on Floor 7 at the west wall, toward the south side, at approximately 

12:15 p.m. 
Interim Report on WTC 7 

L–23

Figure L–23b. WTC 7 Promenade and debris on Vesey Street after WTC 1 collapsed. 

Figure L–23c. Possible extent of debris damage in plan. 

©2001. William 

Pedestrian 

Bridge Promenade

78A

60

59 63 66 69 72 75 78 81

Southwest Corner 

Debris Damage 

Possible Roof and 

Upper Level Damage

Possible Region of Impact 

Damage by WTC1 Debris
Appendix L 

L–24

• From West and Vesey Streets near the Verizon Building, fires were observed in floors 

estimated to be numbered in the 20s and 30s. 

Looking from the southwest corner at the south face: 

• Fire was seen in the southwest corner near Floor 10 or 11 

• Fire was seen on Floors 6, 7, 8, 21, and 30 

• Heavy black smoke came out of a large, multi-story gash in the south face 

Looking from the southeast corner of the south face: 

• Fire seen on Floor 14 (reported floor number) on south face; the face above the fire was 

covered with smoke 

• Fire on Floor 14 moved towards the east face 

Looking at the east face: 

• Fire on Floor 14 (reported floor) moved along east face toward the north side 

Photographs and videos were used with these interview accounts to document fire progression in the 

building. The fires seen in photographs and videos are summarized: 

Before 2:00 p.m. 

• Figures L–22a shows fires that had burned out by early afternoon on Floors 19, 21, 22, 29, 

and 30 along the west face near the southwest corner. 

2:00 to 2:30 p.m. 

• Figure L–24a shows fires on east face Floors 11 and 12 at the southeast corner. Several 

photos during this time show fires progressing north. 

3:00 to 5:00 p.m. 

• Around 3 p.m., fires were observed on Floors 7 and 12 along the north face. The fire on 

Floor 12 appeared to bypass the northeast corner and was first observed at a point 

approximately one third of the width from the northeast corner, and then spread both east and 

west across the north face. 

• Some time later, fires were observed on Floors 8 and 13, with the fire on Floor 8 moving 

from west to east and the fire on Floor 13 moving from east to west. Figure L–24b shows 

fires on Floors 7 and 12. 

• At this time, the fire on Floor 7 appeared to have stopped progressing near the middle of the 

north face. 
Interim Report on WTC 7 

L–25

Figure L–24a. Fires on Floors 11 and 12 on the east face. 
Appendix L 

L–26

Figure L–24b. Fires on Floors 7 and 12 on the north face. 

• The fire on Floor 8 continued to move east on the north face, eventually reaching the 

northeast corner and moving to the east face. 

• Around 4:45 p.m., a photograph showed fires Floors 7, 8, 9, and 11 near the middle of the 

north face; Floor 12 was burned out by this time. 

L.2.3 WTC 7 Collapse Observations 

The collapse of WTC 7 was recorded on several videos from locations northeast and northwest of the 

building. Study of these videos led to the development of the timeline in Table L–1, which lists the 

visible external sequence of events. Figures L–25 to L–28 are images from a CBS News Archives video 

that show key points observed during the collapse. 

The deformed shape of the east penthouse roof shows that the middle fell before the sides (see Fig. L–25), 

as the whole penthouse drops into the main building (see Fig. L–26). This may imply that support 

initially remained on the east and west edges of the east penthouse. Therefore, the perimeter columns on 

©2001. William 
Interim Report on WTC 7 

L–27

the east side of the building which have not already been considered least likely, may be considered less 

likely locations for collapse initiation. 

Table L–1. Timeline of WTC 7 collapse as observed from the northwest. 

Time 

Interval 

(s) 

Total 

Time (s) Observation from CBS Video 

0.0 0.0 - First movement of east penthouse roofline downwards 

0.9 0.9 - East penthouse kink between columns 44 and 45 (Fig. L–25) 

- First 2 windows at Floor 40 fail between columns 44–45 (windows 9 and 

11 from east end) 

0.3 1.2 - 4 more windows fail at Floor 40 

- East penthouse submerged from view (now inside building) 

0.5 1.7 - 3 windows break at Floor 41, Floor 43, Floor 44 

0.5 2.2 - East penthouse completely submerged (Fig. L–26)

1.8 4.0 - Windows break along column 46 at Floors 37 and 40 

3.0 7.0 - West penthouse and screenwall begin to move downward into building 

- Movement of entire north face of WTC7 (visible above Floor 21) 

0.2 7.2 - West end of roof starts to move 

0.5 7.7 - East end of roof starts to move 

- Kink formed in north facade along column 46-47 

0.4 8.2 - West penthouse and screenwall submerged 

- Windows fail between Floors 33–39 around column 55 

- Global collapse initiates (Fig. L–27 and L–28)

Figure L–25. East penthouse kink. 

Penthouse 

Kink 

North-East 

corner 

North-West 

corner 

East 

Penthouse 

Screenwall 

West 

Penthouse 

Louvers in 

front of 

cooling 

tower 
Appendix L 

L–28

Figure L–26. East penthouse sinks (2.2 s). 

Figure L–27. Center screenwall and west penthouse sink (7.9 s). 

East Penthouse 

Sinks into 

Building 

West Penthouse 

Drops 
Interim Report on WTC 7 

L–29

Figure L–28. Global collapse. 

Possible Locations of Collapse Initiation 

Columns 76, 77, 78, 79, 80, and 81 appear to have direct influence on the collapse initiation of the east 

penthouse. A failure of any of these columns, truss #1 or #2, or the east transfer girder, or some 

combination of these components, with possible contribution of adjacent framing and floor systems, could 

be considered possible locations of the initiating events that led to the observed collapse of the east 

penthouse. 

L.2.4 Interpretation of Collapse Initiation Observations in Elevation 

In addition to determining some possible locations of the collapse initiation locations within the plan of 

the structure, it is also helpful to use the available collapse documentation to identify possible locations in 

the building elevation for the initial failure. 

[SOURCE] [SOURCE] 

North

Facade Kink

N

Approximate View Angle 

Key 

Plan: 
Appendix L 

L–30

Least Likely Locations of Collapse Initiation–Penthouse Failure Mechanism 

Because the first visible failure is in the east penthouse, one possible collapse initiation mechanism 

involves a local failure of the penthouse framing, which then progressed down the structure with floors 

sequentially impacting upon those below. There are two reasons that this scenario may be considered 

unlikely. 

First, there was no visible abnormal loading locally applied to cause a local failure at the East Penthouse. 

The photograph in Fig. L–21 shows the east penthouse sustained no damage due to the collapse of the 

WTC towers. The videographic records do not show any visible fire in or near the penthouse prior to 

collapse. 

Second, Fig. L–25 shows a snapshot as the east penthouse starts to collapse. When the roof of the 

penthouse starts to fall, a line of windows (roughly in line with columns 79 to 81) has broken over the 

entire height of the visible region. In free fall, it would take 3 to 4 seconds for an object to fall from the 

roof elevation to the height of the bottom visible broken window, around Floor 33. Since the bottom 

window is broken nearly simultaneously when the kink is seen at the east penthouse, the initial failure 

may be assumed to have propagated upward from the lowest window breakage rather than propagated 

downward from the top of the building. Therefore, initial failure within the penthouse may be considered 

unlikely. 

Less Likely Locations of Collapse Initiation–High Elevation Column Failure Mechanism 

Another possible collapse initiation mechanism may be the failure of a column in the upper elevations of 

the building. The collapse could have progressed vertically upward by pulling down the floors above the 

failed column as debris landed on and sequentially crushed the floors below. 

The timing required for this mechanism, in accordance with gravitational acceleration, requires that any 

column locations significantly above the 13th floor (the lowest visible floor in photographic and 

videographic records) may be considered unlikely failure initiation locations. The lack of observed fires 

in the floors above Floor 13 also reduces the likelihood of failure initiating in this region of the building. 

Possible Locations of Collapse Initiation Mechanism 

Based on review of the photographic and videographic records, a failure of any column within the plan 

area shown in Fig. L–29, and below Floor 13, likely contributed to the collapse initiation. This includes 

columns 76, 77, 78, 79, 80, and 81, truss #1, truss #2, column 78A, the east transfer girder and adjacent 

framing and floor systems within this region (see Fig. L–30). 

L.2.5 Interpretation of Collapse Progression Observations 

Interior columns 79, 80, and 81, were located directly below the east penthouse on the roof and supported 

large tributary areas. It appears that some sequence of component failures in the region identified in 

Figs. L–29 and L–30 led to the failure of one or more of these columns, as discussed above. The failure 

progressed vertically upward within the failed bay to the roof level, based upon observations of window 

breakage relative to failure of rooftop structures, and was first visible from the exterior when the east 

penthouse lost support (see Fig. L–26). 
Interim Report on WTC 7 

L–31

Figure L–29. Plan view of regions for collapse initiation. 

 

 Ground 

7th

13th

North

West Face East Face 

Figure L–30. Likely region of initiating component failures based on videographic 

and photographic records of fire and collapse. 

58 61 64 67 70 73

59 62 65 68 71 74

76 79

80

60 N 63 66 69 72 75 81

Possible Location of Initial Failure

Less Likely Location of Initial Failure

Least Likely

Truss #1

Truss #3

78

77 Truss #2

78A

West 

Transfer 

Girder

East Transfer 

Girder 

Cooling Towers

Screenwall West 

Penthousee

East 

Penthouse

Setback Roof 

 

Approximate Location of 

Window Breakage Below

57 56 55 54 53 52 51 50 49 48 47 46 45 44 43

# Columns Designation

Approximate Location

 of First kink in East 

 Penthouse

Southwest Corner 

Debris Damage 

Approximate Region of Impact 

Damage by Large WTC1 Debris 

Roof and Upper 

Level Debris 

Damage 
Appendix L 

L–32

The 5 s to 6 s delay between the failure of the east penthouse and the failure of the screenwall and west 

penthouse (shown in Fig. L–27) approximates the time it would take for the debris pile from the vertical 

failure progression on the east side of the building to reach Floors 5 to 7 and damage the transfer trusses 

and girders in this area. 

A kink developed in the north facade approximately where column 76 projects to the north face. The 

kink may have formed in the plane of the north facade or it may represent a displacement in the structure 

along this line towards the south. The area of this kink correlates to the easternmost cantilever transfer at 

Floor 7. All of the Floor 7 cantilever transfer girders had back spans supported along the line of the north 

core columns, of which the easternmost one was supported by truss # 1. This north facade kink also 

coincides with the girders at the eastern edge of the cooling tower area at Floor 46. 

When the screenwall and the west penthouse sank into the building, a line of windows broke from 

Floor 44 down to the bottom of the visible range, which is approximately at Floor 33 on the west side of 

the structure (see Fig. L–27). This area aligns with column 61, which is supported by the cantilevered 

end of transfer truss #3 between Floors 5 and 7, as shown in Fig. L–31. This suggests that the observed 

window breakage may be related to the failure of column 61 or truss #3. 

Figure L–31. Plan View of Collapse Progression. 

Approximate Region of Impact 

Damage by Large WTC1 Debris 

TRUSS #1

TRUSS #2

58 61 64 67 70 73

59

62 65 68 71 74

76

79

77 80

60 63 66 69 72 75 78 81

Region of Initial Collapse

Region of Secondary Collapse 

Unknown Time of Collapse

Final Collapse 

Approximate Location of 

North Façade Kink

N

TRUSS #3 

WEST 

TRANSFER 

GIRDERS 

EAST 

TRANSFER 

GIRDER 

78A 

Approximate Location of 

Kink in East Penthouse 

West 

Penthouse 

Approximate Location 

of Vertical line of 

window breakage 

Cooling 

Towers 

Screenwall 

East 

Penthouse 

Southwest Corner 

Debris Damage 

Roof and Upper Level 

Debris Damage 
Interim Report on WTC 7 

L–33

The simultaneous failure of screenwall and west penthouse structures, window breakage on the west side 

of the north facade, and initiation of global collapse (see Fig. L–28) indicates that the building loads could 

no longer be supported. Horizontal progression of the collapse appears to have occurred after the vertical 

collapse on the east side of the building. The greater strength of Floors 5 and 7 relative to the other floors 

and the transfer trusses between these floors suggests that this region of the building played a key role in 

destabilizing the remaining core columns, and the global collapse occurred with few external signs prior 

to the system failure. 

All of the photographic and videographic records show the north facade collapsing from below the visible 

area; the facade appears to sink into the ground without any sign of the other floors in the visible portion 

of the building collapsing. This may indicate that the collapse of the facade starts below the area visible 

in the photographic and videographic records. 

L.2.6 Debris Field 

The debris of WTC 7 was mostly contained within the original footprint of the building. From aerial 

photos, the debris visible on top of the pile is mostly façade structure. This failure sequence suggests that 

the interior of the building collapsed before the exterior. See Fig. L–32. 

L.2.7 Summary 

The possible region of collapse initiation and progression has been refined and can be limited based upon 

available data as follows: 

• Based upon the observed fire locations, it appears that the initiating collapse event may have 

occurred on Floors 5 through 13. 

• Due to the pattern of window breakage, it appears that the initiating collapse event may have 

occurred below Floor 13 and then progressed vertically upward to the east penthouse. 

• Since the middle of the east penthouse roofline appears to fall first, it is possible that the 

initiating collapse event occurred at columns or transfer components with direct influence on 

the footprint of the east penthouse. 

• The north facade kink and the window breakage on the west side of the north facade as the 

screenwall and west penthouse began to fall into the building core suggest that a horizontal 

collapse mechanism occurred between Floors 5 and 7, as there are vertical discontinuities in 

line with each of these elements between Floors 5 and 7. 

• The relatively small debris field, with the exterior moment frame visible on top of the 

building debris, an internal collapse mechanism is likely. 
Appendix L 

L–34

Figure L–32. Aerial view of WTC 7 after collapse. 

The working collapse hypothesis can be summarized in Figs. L–33 and L–34, which illustrate the 

components of the observed collapse event: collapse initiation and vertical progression, horizontal 

progression, and global collapse. 

L.3 COLLAPSE HYPOTHESIS 

L.3.1 Introduction 

WTC 7 suffered a global collapse. The initiating cause or causes of this collapse, and its sequence of 

events, are still being investigated though fire appears to have played a key role and there may have been 

some physical damage on the south side of the building. 

To develop a working hypothesis for the collapse sequence, it is useful to subdivide the problem into 

several phases. Many factors and structural components may have contributed to the start of the collapse, 

but there must have been an initiating event. After the collapse initiated, it progressed to other parts of 

the building, leading to their failure as well. From the observations of the collapse (see Section L.2), it 

appears that first there was a vertical failure progression, from some point in the lower eastern portion of 

the building up to the east penthouse. After a time lag of approximately five seconds, the screenwall and 

west penthouse were observed to begin sinking into the core area. This suggests that there was a 

horizontal progression of the collapse towards the west. Since the screenwall and west penthouse fell 

almost simultaneously, it is reasonable to assume that the horizontal progression captured all the columns 

that support these building parts. 

WTC 7

WTC 5

WTC

6

30 West 

Broadway

Verizon

US Post 

Office
Interim Report on WTC 7 

L–35

Figure L–33. Collapse initiation and vertical progression on the east side of WTC 7. 

Figure L–34. Horizontal progression to the west side of WTC 7. 

Substation Columns

Horizontal 

Collapse 

Progression

Debris Impacts Strong Floor /

Truss Below

East Penthouse 

Sinks into Building

Substation

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Sky Visible

Horizontal 

Progression 

Initiates 

Failure in 

 

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Global 

Collapse

Window 

Breakage

Remainder of 

Building Sinks

East 

Penthouse

West 

Penthouse 

Substation

48th Fl 

40th Fl 

30th Fl 

13th Fl 

Collapse 

Initiation 

5th Fl 

7th Fl 

Column 

above 

Initiation 

Penthouse 

Kink

Screenwall East Penthouse Screenwall West 

Penthouse 

Window 

Breakage 

 around 

Fl 44

Substation

48th Fl 

40th Fl 

30th Fl

13th Fl 

Vertical Collapse 

Progression 

5th Fl 

7th Fl 

Failed 

Column 
Appendix L 

L–36

Within each phase of the collapse shown in Fig. L–35, the initiating event, vertical progression, horizontal 

progression, and global collapse, exist many possible scenarios. Scenarios have been developed from 

available observations of the collapse and are explored with event trees. Preliminary analyses, combined 

with the observations of collapse, can be used to prune the list of postulated scenarios to a relatively small 

number of possible collapse hypotheses. These possible hypotheses will then be analyzed in successive 

levels of detail to try to determine one or more probable sequence of events leading to the building 

collapse. 

Figure L–35. Phases of WTC 7 building collapse. 

L.3.2 Collapse Initiation Scenarios 

For the collapse to have started, there must have been a component or group of components that failed 

first, referred to here as the initiating event, as shown in Fig. L–36. The initiating event may have 

included structural components severed or damaged by falling debris (I1.1) and/or structural components 

affected by fires (I1.2). 

I1.1 Initiating Components Fail Due to Debris Damage From WTC 1 of WTC 2: The initiating 

components may have included perimeter or interior columns that were severed or damaged by falling 

debris from WTC 1 or WTC 2. 

• I2.1 Debris Damage to South Facade Columns: Perimeter columns on the south face and 

the southwest corner were reported or observed in photographic and videographic records to 

have been severed or damaged after WTC 1 collapsed. If the initiating event was due to 

damage to the perimeter moment frame, then it would have started along the south or 

southwest facade. Photographic and videographic records show that columns on the north 

and east facades were undamaged by debris impact. 

− I3.1 Perimeter Moment Frame Arrests Failure Progression: Analysis of the global 

structure indicates that the structure redistributed loads around the severed and damaged 

areas. A progression of column failure to adjacent columns would have been arrested by 

the vierendeel action of the perimeter moment frame, which could span across a sizeable 

opening due to the strength and stiffness of the frame. 

− I2.2 Debris Damage to Interior Columns: Interior columns may have been severed or 

damaged by impacting debris. 

− I3.2 Interior Columns Fail Immediately: If interior columns had been severed or 

severely deformed, they may have failed immediately. 

Initiating Event Vertical Collapse 

Progression 

Horizontal Collapse Global Collapse 

Progression 
Interim Report on WTC 7 

L–37

Figure L–36. Collapse initiation scenarios. 

I1.1 Initiating 

event components 

damaged by 

impacting debris 

I2.1 South facade 

damaged 

I2.2 Interior 

columns and 

adjacent framing 

damaged 

I3.1 Perimeter 

moment frame 

redistributes loads 

around damage 

I4.1 Local failure 

only 

I2.3 Components 

on floors with fires 

that burned out 

before midafternoon 

I2.4 Components 

on floors 5-8 and 

11-13 with fires in 

afternoon 

I3.5 Fire fails floor 

systems, impair 

lateral column 

support 

I3.6 Fire fails 

interior columns, 

east transfer 

girder, and/or 

transfer truss 

components 

I3.4 Components 

cool after fires, 

portions of floors 

fail and leave 

columns unbraced

I4.3 Interior columns 

stable for multi-story 

lengths. Local floor 

failure only. 

I4.2 Interior columns 

stable for multi-story 

lengths. Local floor 

failure only. 

I4.4 Columns fail by 

lateral forces from 

thermal expansion 

of floor system 

section. Go to 

Vertical Progression 

in Figure L-41 

I4.6 Critical elevated 

temperature of steel 

components reached, 

components fail to 

support loads. Go to 

Vertical Progression 

in Figure L-41. 

I4.5 Temperature 

gradient across 

column section fails 

cover plate welds or 

column splice. Go to 

Vertical Progression 

in Figure L-41 

I3.2 Interior 

columns fail 

immediately 

I3.3 Weakened 

components more 

susceptible to fire 

effects. Go to I1.2.

I1.2 Initiating 

event components 

fail due to fire 

effects 
Appendix L 

L–38

o I4.1 Localized Failure at Interior Columns: If the interior columns failed just 

after impact, this likely resulted in a local failure only, since the building continued to 

stand for almost 7 hours after WTC 1 collapsed. This failure could have progressed 

vertically upward to the roof level within the bays immediately adjacent to the failed 

columns, yet from the northern vantage point of the photographic and videographic 

observations, would not have been visible. 

− I3.3 Interior Columns Remain Standing But Damaged: If interior columns were 

weakened by damage from debris, but retained sufficient capacity to carry their loads, 

then additional loading and/or fire effects would have been required to cause their failure. 

Debris impact may have damaged the structural steel fireproofing without significantly 

deforming the structural component. 

I1.2 Initiating Components Fail Due to Fire Effects: Fires had been burning in WTC 7 for many 

hours, as observed in the photographic and videographic records (see section L.2). The initiating event 

may have been caused by fire effects on structural components. 

• I2.3 Components on Floors With Burned Out Fires: If the initiating components failed 

from fire effects, then locations where fires had burned out by mid afternoon could possibly 

been affected by the cooling which occurs after a fire. No fire was observed or reported in 

the afternoon on Floors 1–5, 10, or above Floor 13. 

− I3.4 Floor Systems Fail: The cooling that may have occurred as the fires burned out in 

an area may have generated thermal contraction forces, which may have induced tensile 

forces at floor-to-column connections. 

o I4.2 Unbraced Columns: If floor systems failed, one or more columns may have 

lost lateral bracing. At a floor where fires were noted, interior columns were 

comprised of W14x730 cores and reinforcing plates, and could support several 

stories unbraced without failure. As an example, the column capacity curve of 

column 79 between Floors 5 to 9 is shown in Fig. L–37. Column load-carrying 

capacities shown in this figure are based on the AISC column capacity formulas 

(AISC 2001). The column is not very sensitive to the number of stories of unbraced 

column length, K. This column, which had a service load stress of approximately 

21 ksi, would be approaching its load carrying capacity for an unsupported length of 

four stories if it was also subject to a uniform temperature of 500 °C. 

• I2.4 Components on Floors With Fire: If the initiating components failed because of fire 

effects, then locations with uncontrolled fires would be more likely for the initiating event. 

From available data of fire locations in WTC 7, likely locations would include Floors 5, 6, 7, 

8, 9, 11, 12, and 13. No fires were observed on Floor 5, but the lack of windows and the 

presence of fuel systems on the south, west, and north floor areas indicate that fire should be 

considered as a possibility on this floor. 

− I3.5 Floor System Failure: The fires could have caused the failure of portions of one or 

more floor system and its framing connections. 
Interim Report on WTC 7 

L–39

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0 0.5 1 1.5 2 2.5 3

λc

Stress (ksi)

T = 0

T = 100

T = 300

T = 500

T = 700

Existing

K=1

K=2

K=4

Figure L–37. Column 79 capacity versus temperature and unbraced length K. 

o I4.3 Unbraced Columns: If floor systems failed, one or more columns may have 

lost lateral bracing. See I4.2 for discussion. 

− I3.6 Columns, Transfer Girders or Transfer Trusses Fail: The fires could have 

failed interior columns, transfer girders, transfer trusses, or their framing connections. 

o I4.4 Lateral Displacements: Fire effects may have caused column instability 

failure by lateral displacements from asymmetric thermal expansion of the floor 

system. Such thermally-induced displacements must overcome the restraining effect 

of the remaining floor system against further lateral deflection of the column. 

o I4.5 Temperature Gradients: Fire effects may have caused the failure of columns 

and other components through the forces induced by temperature gradients through 

their cross section. Bending and shear forces may be induced that are sufficient to 

yield either the column splice or reinforcing plate welds. Analysis of a one-story 

segment of interior column 79 indicates that the cover plate weld would begin to 

yield at a mean temperature of 490 °C with a 200 °C gradient across the section, as 

shown in Fig. L–38. Other mean temperature and gradient combinations may also 

cause this type of failure. 
Appendix L 

L–40

Figure L–38. Effects of temperature gradient on interior column 79. 

o I4.6 Uniform High Temperatures: If initiating event components were sufficiently 

exposed to fire effects to be uniformly heated to elevated temperatures, the steel 

strength would be reduced below that required to support the load. Figure L–39 

shows that for interior columns subject to service loads (shown as approximately 

20 ksi of compressive stress), uniform steel temperatures of approximately 570 oC 

would result in column failure. 

0 510

20

16

12

8

4

0

4

8

12

16

20

Weld Capacity

Shear Flow

Weld Capacity

Shear Flow

Location (ft)

Shear Flow (kip/in)

0 200 400 600

2

1

0

1

Expansion 

Creep

Total

Col. Lateral Deflection vs. Temperature

Temperature (deg C)

Lateral Displacement @ Mid-Height (in)

.

Maximum temperature (deg C) Axial 

thermal expansion 

Creep 

Total Displacement 

Welds reach yield 

strength at 600 deg C 

Temperature Distribution 

across Column Section 

400oC 600oC 

26 in. x 3 in. Cover 

Plate 

0.5 in. Welds 

Cross Section
Interim Report on WTC 7 

L–41

 

Figure L–39. Steel strength versus temperature. 

L.3.3 Vertical Progression Scenarios 

After the initiating component or components failed, there must have been a progression of the failure 

from the initiating event to other locations. To reflect the observed failure of the east penthouse, the 

failure likely progressed vertically upwards. Figure L–40 shows possible vertical progression scenarios. 

The initiating component could have failed by any of the failure sequences listed under the collapse 

initiation scenarios in Fig. L–36. This component could have been one of the columns under the east 

penthouse. It could also have been one of transfer trusses #1 or #2 under the east penthouse. 

A collapse mechanism model was created to capture possible collapse initiation at the roof and the east 

penthouse. The model seeks to simulate only the kinematics of the collapse mechanism when columns 

are removed. Several columns were tested for removal. The resulting geometry change was then 

compared to the observed collapse of WTC 7. 

V1.1 Perimeter Columns Fail: Had the initiating component been any perimeter column, most likely it 

would have been at floor levels with debris impact damage (possible range extends from the ground level 

up to floors 15 to 20) or the floors possibly experiencing fire (Floors 5, 6, 7, 8, 9, 11, 12, or 13). 

• V2.1 Collapse Does Not Progress: If a group of perimeter columns failed, the perimeter 

framing above this area would have redistributed its loads, due to the redundancy of the 

moment frame. 

V1.2 Core Columns Not Directly under East Penthouse Fail: Had the initiating component been a 

core column that was not under the east penthouse, most likely it would have been at floor levels with 

debris impact damage (possible range extends from the ground level up to Floors 15 to 20) or the floors 

100 200 300 400 500 600 700

0

10

20

30

40

50

Fy

Example of stress in steel

Yield Strength vs. Temperature

Temperature (deg C)

Yield Strength (ksi)

50

0 ksi ⋅

r F.y( ) 50 T ⋅

20

25 T 725
Appendix L 

L–42

Figure L–40. Vertical collapse progression scenarios. 

possibly experiencing fire (Floors 5, 6, 7, 8, 9, 11, 12, or 13). However, a core column may have failed 

following the failure of adjacent columns or framing members. 

• V2.2 Collapse Does not Progress: If core columns failed, the loads above the failed 

columns may have been redistributed to adjacent columns through the core floor system. If 

the loads could not be redistributed, then additional failures in one or more components 

would have been necessary to progress the collapse. 

• V2.3 Collapse Progresses: From this initial failure, the portion of the column above the 

failure could have fallen, progressing the failure vertically upwards. 

V1.1 Any perimeter 

column fails 

V1.2 Core columns 

not directly under east 

penthouse fail at floors 

5-8 or 11-13 Initiating 

event 

scenarios 

from I4.4 

to I4.6 

V2.1 Collapse does 

not progress vertically 

V2.4 Collapse does 

not progress vertically, 

need additional 

columns to fail – go to 

V1.4 

V2.5 Collapse 

progresses vertically at 

columns 76, 77, 78 

V2.2 Collapse does 

not progress vertically 

V2.3 Collapse 

progresses vertically 

V1.3 Truss #1 or #2, 

east transfer girder, or 

columns 78 or 78A fail 

V1.4 Interior columns 

79, 80, and/or 81 fail at 

floors 5-13 

V2.7 Collapse 

progresses vertically 

V2.6 Collapse does 

not progress vertically, 

need additional 

columns to fail – go to 

V1.3 

V3.5 East penthouse 

collapses as seen in 

video records. Go to 

Horizontal Progression 

in Figure L-41 

V3.1 Screenwall or 

west penthouse 

collapse first 

V3.2 Screenwall or 

west penthouse 

collapse first 

V3.3 East penthouse 

collapses as seen in 

video records if 

columns 79, 80, or 81 

also fail. Go to 

Horizontal Progression 

in Figure L-41 

V3.4 Screenwall or 

west penthouse 

collapse first 
Interim Report on WTC 7 

L–43

• V3.1 Something Else Besides East Penthouse Observed to Collapse First: Had the failure 

of core columns progressed upwards, then the first exterior sign of the internal failure likely 

would have been seen in the screenwall or west penthouse, which are located above the core 

columns. A collapse mechanism analysis performed for the removal of columns 61, 64, 67, 

70, and 73 produced geometry changes that differed from the observed collapse. For the 

scenario where each of these columns fails and the failure progresses upwards to the roof line 

as the adjacent floors cannot redistribute the loads, the screenwall or the west penthouse 

collapses, and no kink develops in the east penthouse (see Figs. L–41, L–42, and L–43). 

. 

Figure L–41. Geometry changes for removal of column 73. 

Figure L–42. Geometry changes for removal of columns 70 and 67. 

Column 70 Removed Column 67 Removed 

Image of Penthouse Kink Column 73 Removed [CBS Video] 
Appendix L 

L–44

Figure L–43. Geometry changes for removal of columns 64 and 61. 

V1.3 Truss #1 or Truss #2 East Transfer Girder, or Columns 78 or 78A Fail: Had the initiating 

component been truss #1 or truss #2, most likely there would have been debris impact damage or 

possibly fires at Floors 5 or 6. However, truss #2 failure could have followed the failure of the east 

transfer girder or columns 78 or 78A. 

• V2.4 Collapse Does Not Progress: If truss #1 or #2 failed, the floor framing, including the 

Floor 7 diaphragm, may have redistributed the loads to adjacent columns. Had this occurred, 

additional failures in one or more components would have been necessary to progress the 

collapse. For instance, the columns 76, 77, 78, 79, 80, or 81 may also have failed, and the 

combined effect of both component failures could have been sufficient to overcome the 

supporting strength of the floor systems. 

• V2.5 Collapse Progresses: If truss #1 failed, column 76 would lose its support at Floor 7, 

and the failure could have progressed vertically upwards if the floors could not redistribute 

column 76 loads. If truss #2 failed, columns 77 and 78 would lose their support at Floor 7, 

and the failure could have progressed vertically upwards if the floors could not redistribute 

the loads from columns 77 and 78. 

o V3.2 East Penthouse Collapses Differently Than Observed: If truss #1 or truss #2 

failed and the failure progressed vertically upward to the roof level, the exterior 

deformations observed in the roof structures would be different from what was 

actually observed. Column 76 supported the west side of the east penthouse and the 

east end of the screenwall. A collapse mechanism analysis performed for the 

removal of column 76 produced a geometry change that shows the west side of the 

east penthouse and the east end of the screenwall deflecting downward 

(see Fig. L–44). 

Column 64 Removed Column 61 Removed
Interim Report on WTC 7 

L–45

Figure L–44. Geometry changes for removal of column 76. 

o V3.3 East Penthouse Collapses as Observed: Had the failure of columns 76 or 77 

and 78 been followed by the failure of columns 79, 80, or 81, such that the failure of 

column 79, 80, or 81 progressed upwards, while the vertical progression of failure 

above columns 76, 77, and 78 was arrested, then the first exterior sign of the internal 

failures could have been observed at the center of the east penthouse roof. 

V1.4 Interior Columns 79, 80 or 81: Had the initiating component been column 79, 80 or 81, most 

likely the failure would have occurred at the floors possibly experiencing fire (Floors 5, 6, 7, 8, 9, 11, 12, 

or 13). 

• V2.6 Collapse Does Not Progress: If only one of columns 79, 80, or 81 failed, the floor 

systems above the failure area may have redistributed the column loads to adjacent columns. 

Had this occurred, additional failures in one or more components would have been necessary 

to progress the collapse vertically upwards. For instance, both columns 79 and 80 may have 

failed, and the loads of both columns could have been sufficient to overcome the supporting 

strength of the floor systems. 

• V2.7 Collapse Progresses: If only one of columns 79, 80, or 81 failed, the floor systems 

above the failure area may have not been able to redistribute the column loads to adjacent 

columns. The floor system above Floor 7 had beams and girders, concrete slabs on metal 

deck, wire mesh in tenant floor areas, and rebar in the core area slabs. These floor systems do 

not appear to have sufficient bending or catenary action to redistribute loads for failure of 

column 79, 80, or 81. A calculation of the catenary action that might be developed by the 

beams and girders framing into column 79, assuming the floors try to redistribute the loads 

above the area of column 79 failure, found that the girder connections reach their capacity at 

approximately 10 percent of the estimated service loads and the connections probably fail at 

approximately 25 percent of the service loads. If the floor-to-column connections had not 

Column 76 Removed
Appendix L 

L–46

failed, the beams would have started to yield axially at approximately 40 percent of the 

service load present. 

o V3.4 East Penthouse Collapses Differently than Observed: The collapse could 

have progressed upwards, but the failure caused in the east penthouse could be 

different than what was actually observed. 

o V3.5 East Penthouse Collapses as Observed: Had the failure of the column 

progressed upwards, then it could have been reflected in the observed collapse of the 

east penthouse, which sits directly above columns 79, 80, and 81. Also, the kink 

observed in the roof of the east penthouse was in line with these columns. A collapse 

mechanism analysis performed for the removal of column 79 produced a deformed 

shape with a kink in the roof of the east penthouse (see Fig. L–45). This is a possible 

collapse scenario. 

Figure L–45. Geometry change for removal of column 79. 

L.3.4 Horizontal Progression Scenarios 

After the east penthouse was observed to sink into the building core, approximately five seconds lapsed 

before the screenwall and west penthouse were observed to also sink into the building core. The 

screenwall and west penthouse movements occurred almost simultaneously with the global collapse of the 

structure. From these external observations, it appears that after the vertical progression failure on the 

east side of the building, the failure progressed horizontally across the core. The horizontal progression 

of the collapse could have started due to any of the likely vertical progression scenarios, which are shown 

in Fig. L–46. 

Column 79 Removed 
Interim Report on WTC 7 

L–47

Figure L–46. Horizontal collapse progression scenarios. 

The likely region in which the horizontal progression occurred is in the lower portion of the building, 

around Floors 5 and 7. Floor 5 had a 14 in. reinforced concrete slab on metal deck. The slab was heavily 

reinforced, and contained steel WT sections embedded in the slab. The WT sections were arranged in a 

diagonal pattern, like a horizontal truss, within the plane of the floor between the perimeter and core 

columns. Floor 7 had an 8 in. reinforced concrete slab on metal deck with rebar in each direction. The 

beams between interior columns at Floors 5 and 7 were much larger than at other floors, and the beam-tocolumn connections were able to transfer more of the beam axial and bending load capacity. These strong 

lateral ties between the interior columns may have been able to impose lateral displacements on adjacent 

columns. Transfer trusses and girders between Floors 5 and 7 transferred loads from the columns above 

Floor 7 to the foundation columns below Floor 5. 

Assuming that a vertical collapse of one or more bays occurred over the height of the building, a large 

pile of debris would have fallen on Floor 7 and below. Such a large amount of debris is likely to have 

Debris from vertical 

collapse impacts and 

damages or severs 

adjacent 

components 

H1.3 Truss #1 fails, 

column 76 fails, floors 5/7 

exert lateral forces on 

adjacent columns 

H2.4 Collapse does not 

progress horizontally 

H2.5 Collapse progresses 

horizontally at columns 73, 70, 

67, 64, 61; leads to sudden 

onset of global instability 

H2.1 Horizontal progression 

unlikely 

H2.2 Structure can resist 

horizontal loads and 

progression stops, need 

additional failures, go to H1.3 

H2.3 Lateral forces cause 

many core columns to fail; 

leads to sudden onset of 

global instability 

H2.6 Collapse does not 

progress horizontally 

H2.7 Collapse progresses 

horizontally at columns 74, 71, 

68, 65, 62; leads to sudden 

onset of global instability 

H1.1 Any typical floor 

system above floor 7 fails

H1.2 Floors 5 and/or 7 fail 

and exert lateral forces 

on adjacent columns 

H1.4 Truss #2 and/or 

east transfer girder fail, 

columns 77 and 78 fail, 

floors 5/7 exert lateral 

forces on adjacent 

columns 
Appendix L 

L–48

severed the Floor 7 slab and damaged or severed any transfer truss or girders in the vicinity. For a 

vertical collapse on the east side of the building, transfer trusses #1 and #2 and the east transfer girder 

may have been damaged, particularly the east diagonals of the trusses. The scenarios below describe 

possible responses of Floors 5 and 7 following a vertical collapse of one or more bays. 

H1.1 Floor Systems above Floor 7: Typical tenant floors above Floor 7 were constructed with concrete 

slabs metal deck with wire mesh reinforcement. The steel framing connections were designed for shear 

loads only, though they could likely resist some degree of tensile catenary forces. 

• H2.1 Collapse Does Not Progress: For any interior column failure above Floor 7, the 

tenant and core floor systems are not able to develop sufficient axial tensile loads for 

imposing lateral deflections on adjacent columns. It is likely that the floor system within a 

bay will fail before a column failure is propagated horizontally to adjacent columns. 

H1.2 Floors 5 and 7: Floors 5 and 7 were thicker and more heavily reinforced than the typical floor 

systems, and may have been subjected to a large debris load from a vertical collapse within one or more 

bays.

• H2.2 Collapse Does Not Progress: Floors 5 and 7 may fail at connections to adjacent 

columns before developing any tensile forces large enough to cause other column failures 

through lateral displacements, halting the horizontal progression. 

• H2.3 Collapse Progresses: Floors 5 and 7 may impose large tensile forces at the adjacent 

columns to cause lateral displacements that fail the columns. The failure mechanism could 

occur at the column splice, located just above Floor 5 and Floor 7, rather than through the 

column section. The simultaneous occurrence of column instability in many core columns 

would cause a sudden and large change in the structural system capacity. 

H1.3 Truss #1: If one of the diagonals of truss # 1 (see Fig. L–47) was damaged or severed by collapse 

debris from the vertical progression, there would be a horizontal force developed in the Floor 7 slab as 

column 76 became unstable. The floor beam between column 76 and column 73 would try to restrain 

column 76 movement through tensile forces to column 73. 

• H2.4 Collapse Does Not Progress: The horizontal tensile force would tend to pull the line 

of columns 73, 70, 67, 64, and 61 towards the east. The continuity of the Floor 7 slab and the 

presence of braced frames around the north core column line makes the simultaneous lateral 

displacement of the core columns less likely, as such displacements within a rigid slab may 

similarly displace other columns, including perimeter columns. 

• H2.5 Collapse Progresses: The failure of column 76 may create its own vertical collapse, 

due to the inability of the floor systems above to redistribute the loads and fail at the column 

splices near Floors 5 and 7 as shown in Fig. L–48. If column 76 cannot be restrained and 

there is a vertical collapse of the surrounding bay, it would cause a debris pile at the lower 

floors which may then destabilize adjacent columns. 
Interim Report on WTC 7 

L–49

Figure L–47. Transfer components between Floors 5 and 7. 

79 76 73 70 67 64

5th

7th

9th 

Column Fails 

in Bending

E

Truss #1

Splices Fail 

in Bending

Figure L–48. Horizontal progression mechanism for truss #1 failure. 

N

65 68 71 74 77

80 Truss #2

78A

Truss #1

78

79

Truss 

#3

Braced 

Frames

Cantilever 

Transfers

81
Appendix L 

L–50

H1.4 Truss #2 and/or East Transfer Girder: If one of the diagonals of truss # 2 and/or the east 

transfer girder was damaged or severed by collapse debris from the vertical progression, there would be a 

horizontal force developed in the Floor 7 slab as columns 77 and 78 became unstable. 

• H2.6 Collapse Does Not Progress: The Floor 7 slab may fail at adjacent columns prior to 

imposing lateral displacements sufficient to fail the columns or their splices. 

• H2.7 Collapse Progresses: The horizontal tensile force would tend to pull the line of 

columns 74, 71, 68, 65, and 62 towards the east. The general absence of the Floor 7 slab and 

braced frames around the center core column line, due to the presence of elevators shafts, 

creates a more likely scenario for the simultaneous lateral displacement of the center core 

columns without similarly displacing other core columns. The possible result is a failure of 

all the columns at their splices, as shown in Fig. L–49. 

Figure L–49. Horizontal progression mechanism for truss #2 failure. 

L.3.5 Summary of Working Collapse Hypothesis 

The working collapse hypothesis has been developed around four phases of the collapse that were 

observed in photographic and videographic records: the initiating event, a vertical progression at the east 

side of the building, and a horizontal progression from the east to west side of the building, leading to 

global collapse. 

From an analysis of the observed collapse sequence, the following general sequence of events appears 

possible: 

1. Debris damaged the south face of the perimeter moment frame and some interior core 

framing on the south side. The debris impact severed approximately a quarter to a third of 

80 77 74 71 68 65

5th

7th

9th 

Truss #2 

Splices Fail in 

Bending 

Column Fails in 

Bending E
Interim Report on WTC 7 

L–51

the south face perimeter columns. The damaged floors are less certain, but reports indicate 

they occurred between the ground and up to Floors 15 or 20. The extent of damage, both 

structural and to fireproofing, of core framing is not known, but damage to elevator cars and 

shafts was reported to have occurred around columns 69 to 78 at Floors 8 or 9. 

2. Fires were observed after the collapse of WTC 1. Fires were observed after 2 pm on 

Floors 7, 8, 9, 11, 12, and 13. Fires were not observed on Floor 5, but this may be due to the 

lack of windows. The presence of a fuel distribution system and the possibility of damage at 

the south face from WTC 1 debris impact, indicates that fires may have been present on Floor 

5. 

3. The initiating event may have included a number of structural components, though the 

relative role of impact damage and fire need further investigation. Possible components that 

may have led to the failure of columns 79, 80, and/or 81 include interior columns 69, 72, 75, 

78, and 78A, the east transfer girder (which supports column 78A and frames into transfer 

truss #2), and adjacent framing and floor systems. 

4. A vertical collapse appears to have occurred after interior columns 79, 80, and/or 81 failed. 

This failure mechanism would progress vertically upward within the failed bay to the roof 

level, as analysis indicates that the floors would not be able to redistribute their loads. 

5. The debris from a 40-story vertical collapse on the east side of the building would fall down 

onto the strong diaphragms at Floors 5 and 7 and possibly onto transfer trusses #1 and #2, 

and/or the east transfer girder. Damage and loading on these floors and transfer components 

would generate lateral forces which would cause the failure of the remaining core columns. 

The horizontal progression requires further analysis and investigation, but observations 

indicate that the remaining core columns appeared to fail almost simultaneously, 

approximately 5 second after the east penthouse failed. 

6. The core columns failed and redistributed loads until the building loads could no longer be 

supported. Once the core columns failed, the cantilever girders which supported the north 

facade also failed. The remaining perimeter columns at the east, south, and west facades 

were either left unsupported or were pulled down with the interior collapse. The global 

collapse occurred with few external signs prior to the system failure. 

The working hypothesis, for the collapse of the 47-story WTC 7, if it holds up upon further analysis, 

would suggest that it was a classic progressive collapse that included: 

• An initial local failure due to fire and/or debris induced structural damage of a critical 

column, which supported a large span floor area of about 2,000 ft2

, at the lower floors 

(below Floor 14) of the building, 

• Vertical progression of the initial local failure up to the east penthouse bringing down the 

interior structure under the east penthouse, and 

• Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7 

that were much thicker and more heavily reinforced than the rest of the floors), triggered by 
Appendix L 

L–52

damage due to the vertical failure, resulting in disproportionate collapse of the entire 

structure. 

The working hypothesis will be revised and updated as results of ongoing, more comprehensive analyses 

become available. 

L.3.6 Technical Approach for Analysis of the Working Collapse Hypothesis 

There are many possible collapse scenarios that have been postulated in the preceding section. Many of 

the scenarios will not produce the observed sequence of global collapse events and can be classified as 

unlikely. Likely collapse scenarios will be identified through analyses that test the postulated phases of 

collapse against observations. It is equally important to test scenarios that are not predicted to match the 

observed data. The testing of the postulated collapse scenarios will be conducted through hand 

calculations, simplified nonlinear thermal-structural analysis, and full nonlinear thermal analysis. 

L.4 REFERENCES 

American Institute of Steel Construction Inc., Manual of Steel Construction, Load and Resistance Factor 

Design, Third Edition, 2001. Chapter 16-E. 

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, Preliminary 

Observations, and Recommendations. FEMA 403. Federal Emergency Management Agency. 

Washington, DC, May. 

Salvarinas, John J. 1986. Seven World Trade Center, New York, Fabrication and Construction Aspects, 

Proceedings of the 1986 Canadian Structural Engineering Conference, Vancouver, Canadian Steel 

Construction Council, Ontario. February 24-25. 
M–i

TABLE OF CONTENTS - Appendix M

List of Figures......................................................................................................................................... M–ii

Appendix M 

Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire..........M–1

M.1 Summary.....................................................................................................................................M–1

M.2 Introduction and Review.............................................................................................................M–1

M.3 Structural Model .........................................................................................................................M–1

M.4 Material Properties......................................................................................................................M–2

M.5 Loading .......................................................................................................................................M–4

M.6 Results.........................................................................................................................................M–6

M.7 Conclusions...............................................................................................................................M–11

M.8 Note .......................................................................................................................................M–12

M.9 References.................................................................................................................................M–12
M–ii

LIST OF FIGURES

Figure M–1. Yield strength of steels used in model. .............................................................................M–3

Figure M–2. Modulus of elasticity of steels used in model. ..................................................................M–3

Figure M–3. Mechanical properties of concrete slab as modeled..........................................................M–4

Figure M–4. Temperature distribution of “real fire” scenario 2-04.......................................................M–5

Figure M–5. Deformed shape at 1,273 K...............................................................................................M–7

Figure M–6. Horizontal force between column and floors versus temperature.....................................M–7

Figure M–7. Vertical deflections of floors versus temperature .............................................................M–8

Figure M–8. Overall deflected shape for fire scenario 2-04 ..................................................................M–8

Figure M–9a. Maximum deflection of floors 95–98 for fire scenario 2-04.............................................M–9

Figure M–9b. Maximum deflection of floors 95–98 for fire scenario 2-04: details of 

Fig. M–9a at high temperatures. .......................................................................................M–9

Figure M–10. Perimeter column lateral deflection for fire scenario 2-04..............................................M–10

Figure M–11. Horizontal force at connection between floor and internal column for 

fire scenario 2-04. ...........................................................................................................M–10

Figure M–12. Horizontal force at connection between floor and external column for 

fire scenario 2-04. ...........................................................................................................M–11

Figure M–13. Overall deflection of 12-floor model (floors 91–102) subjected to gravity loads 

and with floors 95 and 96 under conventional fire..........................................................M–13

Figure M–14. Details of floors 93–98 for the 12-floor model shown in Fig. M–13..............................M–14

Figure M–15. Details of temperature distribution of floors 92–95 for 12-floor model .........................M–14
M–1

Appendix M

INTERIM REPORT ON 2-D ANALYSIS OF THE WTC TOWERS UNDER 

GRAVITY LOAD AND FIRE

M.1 SUMMARY 

A two-dimensional (2-D) finite element model is developed to provide insight and evaluate some aspects 

of a possible collapse sequence for the World Trade Center (WTC) towers. For a prescribed temperature 

distribution that corresponds to a two-story, quarter-span fire, and for a three-story fire derived from fire 

dynamics simulation, diagonals of the heated trusses buckle inelastically, causing considerable sag in the 

fire floors. This behavior puts a high-tension demand on the truss connections to the perimeter column, 

which remains at moderate temperatures in this model and does not experience buckling. Because neither 

the prescribed nor the derived temperature distributions are necessarily representative of the actual fire, 

and the material properties are approximate, further work is needed to evaluate the collapse sequence and 

develop findings regarding the actual event. 

M.2 INTRODUCTION AND REVIEW 

Within days of the collapse of the WTC towers on September 11, 2001, publications postulating the 

mechanism of the collapse began to circulate. A substantial effort was launched by the Federal 

Emergency Management Agency (FEMA) and the American Society of Civil Engineers (ASCE), 

culminating in a preliminary building performance study (McAllister 2002). Quintiere et al. calculated 

the elastic buckling strength of a single diagonal of a floor truss, assuming pinned end conditions, and 

suggested that the buckling of such thin members exposed to fire might have initiated the collapse 

(Quintiere et al. 2002). More recently, Usmani et al. performed a series of 2-D, nonlinear finite element 

analyses of a 12-story vertical frame that comprises a perimeter column and, at each floor, a truss and 

floor slab supported by the column and the tower core (Usmani et al. 2003). The temperature distribution 

in the steel and concrete members was characterized by an assumed time-dependent profile. Usmani et al. 

concluded that column instability caused by the loss of bracing normally provided by floors led to overall 

structural collapse (Usmani et al. 2003). 

The objective of this report is to present a simplified analysis approach to evaluate some aspects of the 

collapse sequence of the WTC towers. The analysis is based on a 2-D model that is simple and can be 

easily used to evaluate a wide variety of conditions. The structural system was modeled independently of 

connection details. At this stage, connections are the object of a separate analysis that can draw on the 

results presented here concerning demand upon connections at various stages of fire development. 

M.3 STRUCTURAL MODEL 

The vertical plane considered in the model includes perimeter column 109 on the North face of WTC 1, 

and five longitudinal floor trusses and slabs (floors 94 to 98). The center of the airplane impact was at 
Appendix M 

M–2

floor 96, and column 109 was the intact column closest to the edge of the initial damage zone 

(McAllister 2002). The column extends 22 m (72 ft) to a height of six floors, and both its upper and 

lower ends are pinned, with the upper end free to translate vertically. The upper chords of the floor 

trusses are simply supported at the internal end, and connected to the perimeter column by hinges. In the 

actual structure, a double floor truss carries a tributary floor slab 2 m (80 in.) wide and is supported by 

two perimeter columns, whereas in the present model a single truss supported by a single perimeter 

column carries a 1 m (40 in.) wide slab. The floor spans 18.3 m (60 ft) from the perimeter column to the 

core. The model is similar to that of Usmani et al. (2003), except it has fewer floors. 

The principal reason for including only five floors in the analysis is to have the simplest model that will 

still capture salient features of the collapse of the towers. The fire applied to the model only heats two 

floors, and the remaining floors remain cool and provide lateral restraint to the perimeter column under 

study. Since the ends of the perimeter column in this simple model are hinged, the model ignores the 

rotational restraint supplied by the continuous column if additional floors are considered. Thus the short 

model is less stiff than a taller model (such as the 12-floor model developed by Usmani [2003] and would 

buckle sooner (or at a lower mechanical or thermal load), if global buckling should occur at all. As far as 

translational restraint is concerned, only a very small amount of lateral bracing can have a tremendous 

effect on the buckling strength (Winter 1958), and the cool floors one or two stories away from the fire 

can be replaced by a support that does not allow horizontal translation. One additional reason for 

including only five floors is to provide a guide for and allow comparisons with the results of a threedimensional study, where several full floors are included. The size of the three-dimension model is a 

concern. 

The trusses, slabs and the column that supports them are simulated by three-node beam finite elements, 

capable of modeling a wide variety of cross sections, with a mesh density and number of integration 

points specified by the user. One particularly attractive feature of these elements is the capability of 

supporting linear temperature gradients across their section and along their length. 

M.4 MATERIAL PROPERTIES 

The various steels range in nominal yield strength from 250 MPa (36 ksi) in the floor trusses to 450 MPa 

(65 ksi) in the column (McAllister 2002). They are all modeled by bilinear stress-strain curves, with a 

tangent modulus about 0.5 percent of the elastic modulus. Figures M–1 and M–2 show the steel 

properties for the temperature range used in the analysis. Usmani et al. (2003) used similar steel 

properties. 

The lightweight concrete slab is also modeled as a bilinear, ductile material (Fig. M–3), with compressive 

strength of 20 MPa or 3,000 psi (McAllister 2002). The top chord of the floor truss is assumed to act in a 

perfectly composite way with the slab and allow the tensile strength at the bottom of the slab to be equal 

in magnitude to the compressive strength at the top. This choice of a simple, bilinear material 

overestimates the tensile capacity of the slab. As well, the simplification inherent in transforming the 

steel top chord into an equivalent concrete section disregards the differential thermal expansion between 

steel and concrete. A more accurate concrete model (currently being developed) may show slab failure or 

a smaller horizontal tension at the connection between floor and column than the present results. 
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–3

0

50

100

150

200

250

300

350

400

450

500

273 373 473 573 673 773 873 973 1073 1173 1273

Temperature (K)

Yield strength (MPa)

65 ksi

60 ksi

50 ksi

36 ksi

Figure M–1. Yield strength of steels used in model. 

60

85

110

135

160

185

210

273 373 473 573 673 773 873 973 1073 1173 1273

Temperature (K)

E (GPa)

Figure M–2. Modulus of elasticity of steels used in model. 
Appendix M 

M–4

0

5

10

15

20

25

273 373 473 573 673 773 873 973 1073 1173 1273

Temperature (K)

Compressive strength, MPa

Modulus of elasticity, GPa

Figure M–3. Mechanical properties of concrete slab as modeled. 

M.5 LOADING 

The floor slabs are acted upon by a dead load of 3.3 kPa (70 psf) and a live load of 720 Pa (15 psf). The 

column load, determined by a linear, static finite element analysis of the global, damaged structure, 

includes the weight of the floors above and a surcharge due to load transfer from the columns damaged or 

missing after the airplane impact (Appendix D, Section D.2.4 of this report). The top of the column is 

loaded by a 1,100 kN (250 kip) axial compressive force and a 2,000 N⋅m (18 kip⋅in.) clockwise moment 

(compared to 540 kN or 120 kip, and 1,800 N⋅m or 16 kip⋅in. counter clockwise moment before damage). 

In addition, the column self-weight is applied along its length. For comparison, Usmani et al. (2003) used 

loading consistent with the FEMA report (McAllister 2002) and applied 40 percent of the gravity loads of 

the tributary floor strips above the model to the top of the perimeter column. 

The behavior of the structure and its eventual collapse are greatly influenced by thermal loads. This 

report first performs an analysis based on a conventional fire, which provides a useful first approximation 

to the behavior of the building in fire. For comparison with the work of Usmani et al. (2003), a single 

temperature distribution T represented by an exponential function of time t, with a reference temperature 

T0 = 300 K, is used. The time rate of change of the temperature, represented by coefficient a = 0.005, 

depends, among other factors, on the location and intensity of the fire, and the quality of the insulation. 

)1()()( 0 max 0

at eTTTtT − −−+= (1)
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–5

A two-floor fire, with maximum temperature Tmax = 1,273 K, heats the structure on floors 95 and 96, over 

the quarter-span closest to the perimeter column. Over that span, the slab of floor 95 is uniformly heated, 

whereas the slabs of floors 94 and 96 have linear temperature gradients across their thickness, with the 

bottom of slab 94 and the top of slab 96 remaining at 300 K at all times. In the three-quarters of the span 

not directly under fire, the temperature decreases linearly from the maximum at quarter-span to room 

temperature at the core. Between floors 94 and 96, the column temperature is also described by Eq. (1), 

with Tmax = 400 K, whereas the rest of the column remains at 300 K at all times. 

The finite element model was used further by applying to it a second temperature distribution (Fig. M–4) 

that corresponds to a more realistic, physics-based fire, generated by NIST’s Fire Dynamics Simulator in 

a manner consistent with initial conditions appropriate for the WTC towers following the aircraft impacts. 

The gas temperatures associated with the fire were used to calculate structural member temperatures and 

temperature gradients, assuming an insulation thickness of 19 mm (3/4 in.) for truss members and 36 mm 

(1.4 in.) for the perimeter column. The fire considered in this application is more widely spread than the 

conventional fire, and covers four floors, with the entire floor span heated. Floor 94 (lowest) remains 

unheated, and the column is only moderately heated. Linear temperature gradients are modeled across the 

column section and the slab thickness of heated floors. The peak temperature of 1,230 K is obtained at 

the end of 25 temperature load steps, each 200 s apart. 

Figure M–4 Temperature distribution (K) of “real fire” scenario 2-04. 
Appendix M 

M–6

M.6 RESULTS 

Nonlinear, static, large deformation analysis accounting for the magnification of flexural deflections due 

to axial load (P-delta effect) was performed. Member stiffness matrices are updated during the analysis to 

account for the P-delta effect, and when a member stiffness gets close to zero, excessive lateral deflection 

occurs and the member is considered to have buckled. The first analysis proceeded in eight load steps, 

the first corresponding to gravity loads at the start of the fire (normal room temperature). Subsequent 

steps occurred at 200 s intervals, with the maximum temperature attained, to within 1 K, at 1,400 s. As 

required in the computation, the load steps are further divided into substeps (up to several hundreds). 

Results for the conventional fire are shown in Figs. M–5, M–6, and M–7. 

At room temperature, even under the severe load redistribution due to the damage caused by the airplane 

impact, the structure still behaves linearly. The maximum floor sag is 35 mm (1.4 in.), causing the 

horizontal span to decrease and the column to pull in slightly. Approaching 200 s and a temperature of 

915 K (the temperatures referred to in these results are the hottest temperatures in the structure at any 

given time), the heated truss begins to show distress, especially in the compressed diagonal and vertical 

web members, which buckle inelastically. This means these heated steel members do not buckle 

elastically, but rather reach yielding in compression. Buckling is then governed by the tangent modulus 

of steel, which is about 0.5 percent of the elastic modulus, and the members immediately buckle after 

yielding, in the inelastic range. At 200 s, the maximum floor sag increases to 335 mm (13.2 in.), and the 

column is pushed out (peak of 38 mm or 1.5 in.) by the thermal expansion of floor 95. At that time, the 

connection of floor 95 to the perimeter column experiences its maximum compression of 125 kN (28 kip). 

Because slab 96 has a thermal gradient with its top surface at room temperature, its lateral expansion is 

much smaller than for slab 95, and its sag is larger. The connection between slab 96 and the column is 

always in tension (Fig. M–6). As expected, slab 94, heated at the top and cool at the bottom, bows 

upward. As the temperature continues to rise, more of truss 95 web members buckle inelastically, and the 

increasing sag begins to pull the column in. The horizontal deflection of the column becomes positive 

(inward), and the connection force between column and floor 95 turns to tension. This inward movement 

of the column relieves the tension in the connection between the column and floor 96. Further 

temperature rise causes further weakening in truss 96, which eventually becomes active in pulling the 

column in. At the peak temperature of 1,273 K, the maximum lateral deflection in the column (183 mm 

or 3.3 in.) occurs at floor 96, inward, and the connection between the column and floor 96 experiences a 

tension of 185 kN (42 kip). 

Under the second fire scenario, the structure exhibits similar behavior. Figures M–8, M–9, and M–10 

show the resulting deflections. Inelastic buckling of the diagonals causes considerable vertical deflection 

of the heated floors beyond a maximum temperature of 900 K. At 1,220 K, the sag of floors 96 and 97 

overcomes the outward push on the perimeter column due to thermal expansion of floors, and pulls the 

column inward. This transition causes the column to temporarily straighten up, causing the overall floor 

deflection to be less. Figures M–11 and M–12 show severe horizontal tension greater than 120 kN 

(27 kip) at the connection of floors with the internal column (floor 95) and external column (floor 96). 
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–7

Figure M–5. Deformed shape at 1,273 K (not to scale). 

Figure M–6. Horizontal force (kN) between column and floors versus temperature (K). 
Appendix M 

M–8

Figure M–7. Vertical deflections (mm) of floors versus temperature (K). 

Figure M–8. Overall deflected shape for fire scenario 2-04 (not to scale). 
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–9

Figure M–9a. Maximum deflection of floors 95–98 for fire scenario 2-04. 

Figure M–9b. Maximum deflection of floors 95–98 for fire scenario 2-04: details of 

Fig. M–9a at high temperatures. 
Appendix M 

M–10

Figure M–10. Perimeter column lateral deflection for fire scenario 2-04. 

Figure M–11. Horizontal force at connection between floor and 

internal column for fire scenario 2-04. 
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–11

Figure M–12. Horizontal force at connection between floor and 

external column for fire scenario 2-04. 

NIST analysis of connections is ongoing and will indicate whether this or other connections can supply 

the calculated demand, and if not, at what temperatures connection failures will occur. In this regard, the 

loss of composite behavior of the concrete slab with the steel truss may occur at a temperature and strain 

level yet to be determined. 

For comparison with Quintiere et al. ( 2002), the present results show that the truss diagonals buckle 

inelastically, and there is considerable reserve strength after the first diagonal buckles. At the highest 

temperatures analyzed, seven diagonal and the vertical web members had buckled in each of the floors 

heated by the conventional fire. This conclusion assumes that the various structural connections maintain 

their integrity throughout the fire. 

M.7 CONCLUSIONS 

A model has been developed to provide insight and evaluate some aspects of a possible collapse sequence 

of the WTC towers. Its results are subject to the following qualifications: (1) the approximate nature of 

the material properties used, especially the concrete slab; (2) connection failures are not considered, 

although information is provided on demand experienced by the connections; and (3) the model is twodimensional. In one of the two cases covered by this report, the temperature distribution of the members 

is selected from among those assumed by Usmani et al. (2003). In the second case, the temperature 

distribution is physically based, and was obtained by using the NIST Fire Dynamics Simulator with 

reasonable initial conditions associated with a damaged tower. In both cases, the diagonals buckle 

inelastically, causing considerable sag in the fire floors. This behavior puts a high-tension demand on the 
Appendix M 

M–12

column, which remains at moderate temperatures in this model (same temperature as in Usmani et al. 

[2003]), and does not experience buckling. This is the major difference between these results and Usmani 

et al., even though the heated trusses in the present model are exposed to a much higher temperature and 

the column to a more severe load that reflects load redistribution in the damaged structure. One possible 

explanation for the difference is that failure modes may be sensitive to material properties. 

M.8 NOTE 

For confirmation, a 12-floor model (from floors 91 to 102) was developed and loaded with the same floor 

load and conventional temperature distribution described by Eq. 1 (Usmani et al. 2003) and mentioned 

above. Compared to case 1 reported earlier, the hinged column end conditions, the heated floors (95 and 

96) and the bending moment applied on top of the perimeter column are the same, but the axial 

compression is reduced (950 kN or 210 kip) because of the fewer floors above the model. Results of the 

12-floor model, shown in Figs. M–13, M–14, and M–15, are very similar to those of the 5-floor model, 

thus confirming the discussion in Section M.3. 

M.9 REFERENCES 

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection, 

Preliminary Observations, and Recommendations. FEMA 403. Federal Emergency Management 

Agency. Washington, DC, May. 

Quintiere, J. G., M. DiMarzo, and R. Becker. 2002. A suggested cause of the fire-induced collapse of the 

World Trade Towers, Fire Safety Journal, vol. 37, no. 7. October, pp. 707-716. 

Usmani, A. S., Y. C. Chung, and J. L. Torero. 2003. How did the WTC towers collapse: a new theory, 

Fire Safety Journal, vol. 38, no. 6. October, pp. 501-533. 

Winter, G. 1958. Lateral bracing of columns and beams, ASCE Journal of Structural Division, 

Proc. 1561. March. Also in ASCE Transactions 3044. 
Interim Report on 2-D Analysis of the WTC Towers Under Gravity Load and Fire 

M–13

Figure M–13. Overall deflection (not to scale) of 12-floor model (floors 91–102) 

subjected to gravity loads and with floors 95 and 96 under conventional fire: at 

the maximum temperature of 1,272 K, the maximum deflection is 1.08 m. 
Appendix M 

M–14

Figure M–14. Details of floors 93–98 for the 12-floor model shown 

in Fig. M–13 (compare with Fig. M–5). 

Figure M–15. Details of temperature distribution of floors 92–95 for 

12-floor model: note temperature gradients across slab thickness of 

 floors 93 and 95, and cool column. 
N–i

TABLE OF CONTENTS - Appendix N

List of Figures.........................................................................................................................................N–iii

List of Tables .......................................................................................................................................... N–iv

Appendix N 

Interim Report on Analysis of First-Person Accounts from Survivors of the WTC 

Evacuation on September 11, 2001...................................................................................... N–1

N.1 Introduction................................................................................................................................. N–1

N.2 Background Literature ................................................................................................................ N–2

N.3 Study Objectives ......................................................................................................................... N–3

N.4 Methodology............................................................................................................................... N–4

N.4.1 Content Analysis ............................................................................................................. N–5

N.4.2 Variables Considered....................................................................................................... N–6

N.4.3 Procedure......................................................................................................................... N–8

N.5 Study Results .............................................................................................................................. N–9

N.5.1 Profile: Gender and Age.................................................................................................. N–9

N.5.2 Location at the Beginning of the Event........................................................................... N–9

N.5.3 Means of Egress Used ................................................................................................... N–10

N.5.4 First Cue Reported......................................................................................................... N–11

N.5.5 Time to Start Evacuation ............................................................................................... N–11

N.5.6 Conditions on Floors and in Stairwells.......................................................................... N–12

N.5.7 Obstructions During Evacuation ................................................................................... N–13

N.5.8 Announcement............................................................................................................... N–14

N.5.9 Location When WTC 2 Was Hit ................................................................................... N–14

N.5.10Location When WTC 2 Collapsed................................................................................. N–15

N.5.11Location When WTC 1 Collapsed................................................................................. N–15

N.5.12Location When They Saw Firefighters.......................................................................... N–15

N.5.13Time of Exit................................................................................................................... N–15

N.5.14Help Received and Help Given ..................................................................................... N–16

N.5.15Occupants with Disabilities or Injuries ......................................................................... N–16

N.5.16Phone Calls.................................................................................................................... N–17

N.5.17Knowledge of Situation................................................................................................. N–17

N.5.18 Influence of Others........................................................................................................ N–17
Appendix N 

N–ii

N.5.19Perception of Others...................................................................................................... N–18

N.5.20Technology to Gain Information ................................................................................... N–20

N.5.21 Impact of the 1993 Evacuation...................................................................................... N–20

N.6 Summary Results ...................................................................................................................... N–20

N.7 Future Work.............................................................................................................................. N–22

N.8 Acknowledgments .................................................................................................................... N–23

N.9 References................................................................................................................................. N–23

Attachment 1

WTC First-Person Accounts Code Book ........................................................................... N–27
N–iii

LIST OF FIGURES

Figure N–1. Distribution of publication dates of accounts...................................................................... N–9

Figure N–2. Gender and age distribution. ............................................................................................. N–10

Figure N–3. Distribution of time to start evacuation............................................................................. N–12

Figure N–4. Obstructions encountered during evacuation in both towers. ........................................... N–14

Figure N–5. Knowledge of situation in the towers................................................................................ N–17

Figure N–6. Distribution of perception of others between the towers. ................................................. N–19

Figure N–7. Distribution of gender and perception of others................................................................ N–19
N–iv

LIST OF TABLES

Table N–1. Questions on manifest information. .................................................................................... N–6

Table N–2. Questions on latent information.......................................................................................... N–6

Table N–3. Means of egress used within the towers............................................................................ N–10

Table N–4. First cues of event within the towers................................................................................. N–11

Table N–5. Adverse conditions on floor at impact. ............................................................................. N–13

Table N–6. Adverse conditions reported in the stairs during evacuation. ........................................... N–13

Table N–7. Time out of towers. ........................................................................................................... N–16

Table N–8. Gender and influence of others. ........................................................................................ N–18

Table N–9. Distribution of age and perception of others..................................................................... N–20
N–1

Appendix N

INTERIM REPORT ON ANALYSIS OF FIRST-PERSON ACCOUNTS FROM 

SURVIVORS OF THE WTC EVACUATION ON SEPTEMBER 11, 2001 

N.1 INTRODUCTION 

In the days following the September 11, 2001, attacks on the World Trade Center (WTC), the National 

Fire Protection Association (NFPA) in collaboration with the National Research Council of Canada 

decided to collect survivors’ stories to document the event and to use this background material to develop 

future studies on occupant behavior during the evacuation of the World Trade Center. First-person 

accounts were collected from newspapers, radio and television programs, e-mail exchanges, and a variety 

of websites. Additional accounts were received at a later date from the National Institute of Standards 

and Technology (NIST). This large quantity of material was coded and analyzed to obtain a better 

understanding of the personal evacuation experiences of different survivors located on the different floors 

of the two towers. This report documents that analysis. The analysis was solely based on written 

accounts and does not include first-person interviews conducted as part of the NIST investigation. 

Rather, it provided background for the development of the telephone and face-to-face interviews 

conducted for the NIST investigation. 

At 8:46 a.m. on Tuesday, September 11, 2001, American Airlines Flight 11, a hijacked Boeing 767, hit 

WTC 1 of the WTC. This impact caused extensive damage on five floors, from 93 to 99 of the 110-story 

high tower, trapping those above. Sixteen and a half minutes later, at 9:03 a.m., a second hijacked 

Boeing 767, United Airlines Flight 175, struck WTC 2 of the WTC, damaging nine floors, from 77 to 85. 

Despite the massive localized damage caused by the impact, each structure remained standing. However, 

as each aircraft impacted the building, jet fuel on board ignited. Part of this fuel immediately burned off 

in large fireballs that erupted at the impact floors. Remaining fuel flowed across the floors and down 

elevator and utility shafts, igniting intense fires throughout upper portions of the buildings. 

At 9:59 a.m., WTC 2, the second building to be hit, collapsed after burning intensely for 56 minutes. 

WTC 1 withstood its injury longer than the South tower, collapsing to the ground at 10:28 a.m. after 

burning for 102 minutes (FEMA BPAT 2002). It is estimated that approximately 2,800 people were 

killed and 800 others injured by the attacks and eventual collapse of the towers on September 11, 2001, 

including building occupants and first responders (Cauchon 2001). 

Although the events of September 11, 2001, involved the WTC, the Pentagon and the hijacked airliners, 

the evacuation of the two towers is the focus of this research. The attacks precipitated the evacuation of 

the entire WTC complex. The evacuation of WTC 1 and WTC 2 was largely initiated by the occupants 

themselves. 

The WTC was a complex of seven buildings, including the two 110-story office towers. Approximately 

50,000 people worked in each tower (100,000 total), with an estimated 70,000 visitors to the complex 

during the course of a normal business day (Yamasaki 2002). However, the occupancy of the towers on 
Appendix N 

N–2

the morning of September 11, 2001, was not at its maximum capacity. According to USA TODAY, 

5,000 to 7,000 people were in each tower at 8:46 a.m. that morning, the time of first impact 

(Cauchon 2001). It has been suggested that the towers were not at their maximum capacity for several 

reasons. That morning marked New York City’s mayoral primary, and it is assumed that many people 

stopped to cast their ballots before heading in to work. The New York Stock Exchange does not open 

until 9:30 a.m., therefore many people from trading firms had not come into work yet. Tuesday, 

September 11, 2001, was the first day of school in several primary school districts, and many parents 

accompany their children to school on this day. Visitor hours had not started yet, as the viewing platform 

in the South Tower did not open to the public until 9:30 a.m. Perhaps the biggest factor of all was the 

early hour––many simply had not arrived at work by 8:46 a.m. In addition, dozens of investment firms in 

the WTC had closed their offices or cut employment sharply. Some offices were leased but empty or 

under renovation (Cauchon 2001). 

By certain measures, the evacuation of the WTC on September 11, 2001, could be termed a success 

(Cauchon 2001). Under the impacted floors, nearly every occupant who could physically get out did get 

out. According to USA TODAY, in each tower, 99 percent of the civilian occupants below the crash sites 

survived. Their analysis shows that two-thirds of WTC 2 occupants started their evacuation of the upper 

floors during the 16.5 minutes between the attacks, and survived. Among the occupants under the 

impacted floors in WTC 1, 72 people died, whereas under the impacted floors in WTC 2, 4 people died. 

It should be noted that some fraction of the deaths below the impacted floors in WTC 1 occurred in the

elevators, which were carrying people at the time of impact. 

N.2 BACKGROUND LITERATURE 

Understanding the basic concepts of human behavior in fire is necessary to envision occupants’ likely 

response during an emergency. Human behavior in fire is a scientific field that identifies facts, concepts 

and relationships established through systematic observation and experimentation. What is known about 

human behavior in fire is that the three dimensions of the emergency, namely the occupant, building and 

fire characteristics, interact to explain or predict occupant response to fires (Proulx 2001). 

During a fire, the nature of the information obtained, the limited time to react and the assessment of 

danger will create a feeling of stress. This stress is not an abnormal reaction; on the contrary, stress is 

regarded as a necessary state to motivate reaction and action. During the course of the event, the intensity 

of stress experienced will vary as a function of the information newly-perceived and the assessment of the 

decision taken (Proulx 1993). Key factors which increase stress include: the perception of threat to 

oneself or others, the suddenness of the threat and the available time to respond or prepare, the amount of 

sensory input needed to be processed, and the degree of physical effort (aerobic and anaerobic output) 

that is engaged during the incident. If the individual is physically wounded or injured, the effect will be 

even greater (Grossman 2002). Taking all of these factors into account, it can be said that most evacuees 

of the WTC were experiencing extremely high levels of stress. 

Most people assume that individuals cease to act in a predictable, orderly fashion in the face of disaster, 

and that norms which govern our behavior collapse into Durkheim’s anomie, a state of normlessness 

(Fisher 1998). This mindset, known as disaster mythology, has been greatly nourished by the mass media 

and movie industry to capitalize on strong emotional images (Proulx 2002). Today, it is largely known 

that in the face of the extreme stress of a disaster, there is an absence of widespread, irrational, antisocial 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–3 

and dysfunctional behavior that has often been described as ‘panic’ (Quarantelli 1998). Thus, the false 

but common belief that people will panic in disaster situations is a myth (Sime 1980, and Keating, 1982). 

In human behavior fire research, it is found that panic behavior is extremely rare (Proulx 2002). Decision 

making during an emergency is, however, different from day to day decision making for three main 

reasons (Janis and Mann 1977). First, there is much more at stake in emergency decisions––often the 

survival of the person and of the people he or she values the most is at risk. Second, the amount of time 

available to make a decision before crucial options are lost is limited. Third, the information on which to 

base a decision is ambiguous, incomplete and unusual. Further it is usually impossible to look for more 

appropriate information due to the lack of both time and the means to get information (Proulx 1993). 

Turning to the literature of the evacuation of the WTC following the 1993 terrorist bombing, it was 

concluded that there was a lack of panic flight during the evacuation, even though the occupants had to 

descend the crowded and smoky stairwells in total darkness. No official evacuation orders were issued by 

recognized emergency and building officials after the explosion, and no official information was provided 

about safe or proposed exit routes (Isner and Klem 1993). However, it was found that there was an 

overall mood of orderly evacuation during the 1993 evacuation (Wenger et al. 1994; Fahy and Proulx 

1995). This lack of panic during the 1993 evacuation may be explained by the fact that although the 

explosion created immediate danger, it was not perceived by participants as particularly severe. 

Secondly, people were not alone; they were with coworkers, friends and associates. This web of social 

integration among participants works against the adoption of norms that would support individual, 

competitive flight behavior to favor the emergence of cooperative, altruistic, helping and orderly behavior 

(Wenger et al. 1994). 

In contrast to the panic scenario of a competitive scramble towards an exit, Dr. Jonathan Sime argues that 

the most common behavior during a fire is movement towards familiar persons and places (Sime 1985). 

This is known as the affiliation model which suggests that detached groups will attempt to reunite before 

evacuating, and evacuation movement is most likely to be through a familiar way in and out of the 

building (Sime 1985). The grouping of people during an emergency is sometimes known as the milling 

process: the communication process that takes place among participants in a crisis setting as they attempt 

to define the situation, propose and adopt new appropriate norms for behavior and seek coordinated, 

collective action (Wenger et al. 1994). High levels of social interaction were reported during the 1993 

evacuation as the tenants engaged in milling behavior regarding the definition of the situation, the attempt 

to give meaning to the crisis (i.e. to determine “What is happening?”), the appropriate response to it or 

proposed cues for action, and the attempt to give direction to the behavior of the participants by offering 

new, emergent norms (i.e., “What should we do? What is appropriate?”) (Wenger et al. 1994). 

Identification of the information available to occupants in defining the situation is essential in attempting 

to understand the decision-making process during an emergency. The social context of the occupants and 

the opportunity to observe and interact with others are also fundamental considerations when attempting 

to understand occupant response and the overall outcome of evacuations. 

N.3 STUDY OBJECTIVES 

This exploratory research project was conducted in order to gain an overall understanding of the 

circumstances surrounding the evacuation of the WTC towers on September 11, 2001. More specifically, 

this research project endeavors to gain insight into the variability of human behavior and response time 
Appendix N 

N–4

displayed during the evacuation, with the findings to be used as a guide for future research. This study 

can also provide insight for the NIST investigation into the WTC event. Human behavior data gathered 

from this project will help to create a better understanding of the individual experiences of occupants in 

specific locations by documenting, to the extent possible, the information available to occupants, such as 

conditions on their floor and along their evacuation route, perceived behavior of others, and escape 

conditions and timing. 

N.4 METHODOLOGY 

In the moments following the attack of the WTC towers on September 11, 2001, journalists started 

interviewing survivors to obtain the story of their evacuation. These first-person accounts were presented 

on television or radio and published in newspapers, magazines, or websites and later reported in books 

and special media programs. During the three months following the events, over 280 first-person 

accounts were collected. These accounts included media reports (newspapers, magazines, television and 

radio), as well as material from books, training videos, personal websites and emails. The information 

provided in some of these accounts was so detailed that it provided sufficient material for a study. 

Additional accounts were gathered over the next year for a total of 745 first-person accounts from 465 

individuals, as some survivors provided multiple accounts through different sources. The 435 accounts 

retained for analysis are from evacuees of WTC 1 and WTC 2 only. Although numerous accounts were 

found from occupants of the surrounding WTC complex, only those civilians who had evacuated the 

actual towers were considered. For those survivors for whom numerous accounts were found, the 

information across the accounts was collapsed into one highly detailed account, containing the combined 

information from all of the given accounts. For instance, 16 survivors provided five to 12 different 

accounts to the media. These survivors had dramatic accounts and, therefore, were of particular media 

interest. The study involved no first-person interviews. 

It is recognized that the use of first-person accounts published in the media as main sources of 

information for a study has many limitations. The questions asked by reporters are unknown and can be 

different for each journalist and with each interview. It is also noted that the date an account was 

published does not necessarily represent the date that the specific interview took place; the account could 

have been held at some point and then published at a later date. It is suspected that the most dramatic 

experiences are reported and that some information may be emphasized or left unreported for the purpose 

of the article. As stated by Dr. H.W. Fischer, the mass media has greatly reduced the level of flamboyant 

exaggeration in what they report as typical behavioral and organizational response to disasters over the 

last 50 years; however, since a larger portion of the news is now devoted to reporting disasters, a less than 

accurate image is still commonly portrayed both in the print and broadcast media (Fischer 1998). It also 

must be stressed that the findings in this study are representative only of the individual experiences 

captured in first-person accounts and cannot be generalized to the population of the two towers. 

Recall of details of an event depends on many factors, including the intensity of the event, time since the 

event, and influence from other information sources. Recollection of extreme events such as the WTC 

attacks may be far better than ordinary daily events. Conversely, traumatic situations may result in 

memory impairment or “critical incident amnesia.” Finally, with the intense media attention that the 

events of September 11, 2001, received, it is highly likely that this coverage influenced 

survivors’ recollection of events. This phenomenon, referred to as “contamination,” occurs when 

information outside of the actual experience is integrated into the reconstruction of memory 
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(Grossman 2002). Since different occupants of the WTC had a range of experiences on and after 

September 11, it is unclear to what extent memory issues impacted the reports included in this study. 

Despite the drawbacks of using media sources for the basis of research, however, some of the accounts 

contained such a high level of detail, particularly the ones written by survivors themselves, they provided 

justification for the analysis of this information. It should also be stressed that these media accounts are 

the only documented descriptions of the WTC evacuation and immediate reactions of the survivors, as no 

research has been conducted or published 2 years after the events, regarding human behavior surrounding 

the events of September 11, 2001. Since documenting human behavior is time sensitive and considerable 

time has passed since the event, it may be said that these initial media accounts may hold significant 

detailed and accurate information that may only be available in these accounts. 

N.4.1 Content Analysis 

The most appropriate social research method for analyzing media communications is content analysis. To 

extract the important content from the accounts, a “questionnaire” was developed to “interview” each 

account. This procedure was used by Johnson (1987) to analyze police file statements related to the 

“Who Concert Stampede;”1

 it is also explained in some detail in Gamso’s book “The Strategy of Social 

Protest” (1975). The approach relies on a series of identical questions used to “interview” each 

document. Once the information is gathered in a qualitative or descriptive database, codes are developed 

to reduce the variety of answers to each question to a manageable number. To ensure reliability of the 

coding, at least two researchers independently review each account and compare their coding. Any 

disagreement is discussed and resolved. 

Questions to “interview” each account were designed to obtain manifest and latent information from the 

745 first-person accounts. A majority of the questions, 30 of them, rely on manifest information or 

elements specifically reported in the account, such as the person's location at certain key moments. They 

are listed in Table N–1. The remaining three questions called for latent information, such as words 

describing emotions. They are listed in Table N–2. Data was retrieved from the accounts and entered 

into a qualitative database. Nominal and ordinal categories were conceptualized, which can be found in 

the coding scheme presented in Attachment 1. It is important to note that not all questions were answered 

for each account gathered, as a number of the accounts were incomplete. The fact that an individual’s 

account is silent on some point does not mean that this factor was not important or relevant in that 

individual’s evacuation. It simply means that it was not included in the published account or that it was 

never mentioned by the individual. For those questions not addressed in the account, the category was 

awarded the code “9” or “99,” accounting for the lack of information regarding that specific question. 

This lack of information for some items is the equivalent in a questionnaire survey to a respondent who 

did not answer some of the questions. The information gathered in the qualitative database was coded 

and transformed into a quantitative matrix from which descriptive statistics were calculated. 

 

1

 On December 3, 1979, 11 people were crushed to death as fans rushed the entrance of a stadium in Cincinnati, Ohio, to see a 

sold-out concert. 
Appendix N 

N–6

Table N–1. Questions on manifest information. 

What is the date of published account? Heard fire alarm? 

Gender? Location at WTC 2 impact? 

Age? Location at WTC 2 collapse? 

In which building was the person at the time of 

first cue? 

Location at WTC 1 collapse? 

On what floor was the person at the time of first 

cue? 

Location when met firefighters? 

What was the first cue of event? At what time person exited the building? 

How long did the person take to start evacuation? Who helped person during evacuation? 

Did the person delay start time? Was the person disabled? 

What mode of egress was used? Was the person injured? 

What was the condition on floor? Location when person placed phone call? 

What was the condition on the stairs? Who was the phone call recipient? 

Were obstructions encountered during evacuation? Was there social influence on decision making? 

Heard announcement? Use other (non-phone) communication technology?

Location when WTC 2 announcement heard? Was person at the WTC during 1993 bombing? 

Action after hearing WTC 2 announcement? Did the person rest during evacuation? 

Table N–2. Questions on latent information. 

What was the person's knowledge of the situation in the initial moment? 

How serious did the person judge the situation to be? 

What was the person's perception of others? 

N.4.2 Variables Considered 

Conceptualization and operationalization involve precisely defining how the variables were measured and 

ensuring the attributes within those variables are mutually exclusive and exhaustive. There were 33 

questions providing data ranging from demograhics and physical location, to response time and social 

interaction during the evacuation. Coded data included the evacuees' gender, age and which building and 

floor they were located in, as well as the date the account was published. The floors of the towers were 

categorized as lower (basement to floor 42), mid (floor 43 to floor 76), and upper (floor 77 to floor 110) 

based on the common areas referred to as “skylobbies” on the 44th and 78th floors, which separated the 

towers into three strata. The skylobbies on floors 44 and 78 served the occupants of floors 43 and 77, 

respectively. Mode of egress was recorded as stairs, elevators or a combination of both. 

The first cue of the event was categorized according to whether the cues were “audio,” such as hearing an 

explosion, crash or rumbling; “visual,” such as seeing the plane approach the towers, or seeing fire, 

smoke or debris. Another category of first cue was “building movement” and was represented by feeling 

the building shake, sway or tremble, whereas moving office furniture, falling ceilings, jolting in the 

elevator and flickering lights were attributes of the variable category “contents movement.” The 

remaining three categories were “warned by others” (either verbally or through their behavior), 

“physically impacted” (e.g., burned, fell from chair, thrown against a wall), and “smelled smoke or fumes 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–7 

or felt heat.” These cues were coded as check-off items so that multiple initial cues could be captured. 

Whether or not evacuees heard a building alarm during their evacuation was recorded in a separate field, 

if mentioned. 

Time to start evacuation was recorded as immediately, shortly after impact (where the occupant took less 

than 5 minutes to retreive belongings before evacuating), delayed (representing those who took more than 

5 minutes to start evacuating, taking time to search floors or gather company documents, etc.), stayed or 

stuck. 

Conditions of floors immediately after the building was hit were recorded in two ways. One field 

summarized the conditions as follows: “devastated,” meaning combinations of conditions such as 

scattered debris, fire, darkness and fallen ceilings and walls were reported; “abnormal,” in that there was 

some smoke, heat or the smell of jet fuel; and “normal,” represented by accounts describing usual 

conditions on their floors. A series of check-off columns then recorded whether a person’s account 

reported the presence of specific conditions: door jammed, debris (e.g., from wall, floor or ceiling 

collapses), smoke, dust, no power or darkness, smell of fumes, water, fire, crowds or injured people, 

entrapment, or normal conditions. If the individual was not on an office floor when the building was 

struck, that was recorded in an additional check-off column. This allowed the recording of multiple 

conditions for each individual. 

Similar check-off columns were used to record observations of conditions in stairwells during evacuation: 

normal, door locked or jammed, crowded and/or hot, no power, water, cracked wall, debris, smoky or 

smell of fumes. 

If and where the evacuees heard the announcement stating that WTC 2 was secure were also noted, as 

were their actions after hearing the announcment (i.e., continued evacuating, continued but saw others 

return to offices, or returned to or remained in office). The survivors’ location at the time of impact, 

collapse of the towers and meeting of firefighters were also coded, as well as who helped them during the 

evacuation. Those who provided help were categorized as firefighters, Port Authority employees, 

external officials such as NYPD, FBI, and coworkers. Individuals’ disabilities were coded as “visually 

impaired,” “hearing impaired,” “physically challenged” (e.g., obese, pregnant, or with asthma or heart 

conditions), “wheelchair users,” or ‘injured.” People who mentioned that they had aided a disabled or 

injured person during the evacuation were also noted in this variable category, as were those who reported 

seeing injured or disabled people during their evacuation. 

Whether or not a person was present at the WTC during the 1993 bombing was recorded, as was whether 

or not each person delayed his or her evacuation on September 11, 2001. Where the evacuee placed a 

phone call and the recipient of it were coded, along with whether or not they rested and where they rested. 

A series of check-off columns recorded if a person experienced obstructions, such as door jams, debris, 

smoke, no power, smell of fuel, water, fire, crowds, injured and disabled people or became trapped during 

the evacuation. Multiple entries were possible for each individual. 

Other variables included the survivors’ knowledge of the situation, recorded as “high” for those who 

knew a plane had struck the towers or that there had been a terrorist attack; “moderate” for those who 

thought there was a fire, bombing or judged the situation as a serious emergency; and “low” for those 

who were not aware of the reasons behind the evacuation. The evacuees’ indication of the level of 

seriousness was recorded as “very serious,” “somewhat serious” and “not serious” based on the perceived 
Appendix N 

N–8

tone of the account. The variable “perception of others” included the categories of “calm,” in that 

evacuees felt others to be orderly and composed; “upset,” which represented those who judged others as 

nervous, anxious or visibly bothered. For survivors who described others as hysterical or pushing and 

shoving, this field recorded their perception of others as “momentarily panicked.” When accounts 

reported that those around them lent assistance to others, this field was coded as “helpful.” 

Social influence on decision making was categorized according to who influenced the evacuee: authority 

figures, such as bosses or managers; groups of coworkers; or both authority figures and groups of 

coworkers. If a person indicated that he or she took on a leadership role, that was also captured. Use of 

new communication technology including utilizing text messaging over pagers or wireless e-mail devices, 

TV or radio to gain information was noted. (See Attachment 1 for further variable category definitions.) 

The time that people reached the outside was recorded. It must be stressed that most accounts did not 

report specific times at which people took different actions. However, several occupants mentioned their 

location at key moments such as where they were when WTC 2 was hit or when WTC 1 or WTC 2 

collapsed. For example, one survivor of WTC 1 reports, “When we got to the twentieth (floor) I 

remember hearing a rumble. One of the fellows looked at me and we knew it didn’t sound good. It must 

have been WTC 2 coming down” (Fink and Mathias 2002). Thus, it was deduced that this survivor was 

on the 20th floor of WTC 1 at 9:59 a.m., when WTC 2 collapsed. Similarly, for many people, the time 

they reached the outside could be estimated from their description of events (e.g., WTC 2 being struck, 

WTC 2 collapsing) as they reached the outside. 

N.4.3 Procedure 

Various media avenues were utilized in gathering first-person accounts including television, radio, 

newspapers, magazines, websites, books and special media programs. Personal websites and e-mails 

written by survivors themselves were also used and are of particular interest, as they have not been altered 

by media editors in any way, but appear in their full, original format. During the three months following 

the events, over 280 first-person accounts were collected. Eventually, a total of 745 accounts were 

gathered from 465 individuals, as numerous survivors gave multiple stories to different journalists. 

The accounts, which were gathered over a period of 18 months, were published up to 14 months after 

September 11, 2001. The distribution of published accounts over time is shown in Fig. N–1. Among the 

dated accounts studied, 51 percent were published in the first two weeks after September 11, 2001, with 

another influx of accounts surfacing around the one-year anniversary, 10 months to 12 months after the 

disaster. 

Content analysis was performed on the 745 accounts using 33 questions for which the data was entered 

into a qualitative spreadsheet. Duplicate accounts were merged, resulting in a final study size of 435 

individuals who were present either in WTC 1 or WTC 2. The data was then coded and transferred into a 

matrix for analysis. 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–9 

0

2 5

5 0

7 5

100

125

150

175

200

225

250

275

9/11/01

10/2

10/2311/13

12/4

12/25

1/15

2/5

2/263/19

4/9

4/305/216/11

7/2

7/238/13

9/3

9/24

10/15

11/5

W e e k o f P u b lic a tio n

Number of Accounts

Figure N–1. Distribution of publication dates of accounts. 

N.5 STUDY RESULTS 

The raw data for each account was entered into an Excel spreadsheet and then coded. The coded data was 

transferred into SPSS 11.0 for statistical analysis. The statistical analysis conducted was essentially 

descriptive statistics to organize and summarize the information. Inferential statistical tests were not 

conducted since the data obtained is not a representative sample of the population. Results presented in 

this report should not be generalized to all occupants of the two towers on September 11, 2001. Although 

they are reported using terms such as “the occupants” and “the survivors,” the results refer only to the 

accounts analyzed. 

N.5.1 Profile: Gender and Age 

The study contained accounts from 435 survivors, ranging in age from 20 to 89 years old (mean = 39.5, 

standard deviation = 11.8). Included were accounts from 118 women (27 percent) and 314 men 

(72 percent); three accounts did not mention their gender (1 percent). It is speculated that the substantially 

higher number of men involved in these accounts occurred because there were more men working in the 

two towers than women or that men may be more likely to talk to the media than women. The breakdown 

by gender and age is shown in Fig. N–2. 

N.5.2 Location at the Beginning of the Event 

There were 251 individuals who were located in WTC 1, comprising 58 percent, with the remaining 

42 percent or 184 people from WTC 2. In WTC 1, 90 people (36 percent) were from upper floors 

(77 to 110), 79 people (31 percent) were from mid levels (43 to 76) and 58 people (23 percent) were from 

the lower floors of WTC 1. Another 22 people (9 percent) were in elevators and two people did not 

specify a location. In WTC 2, 94 people (51 percent) were from upper floors, 57 people (31 percent) 

were from mid floors, 28 people (15 percent) were from the lower levels of WTC 2 and five people did 

not specify a location. Although the distribution of accounts in the two buildings was not identical, 
Appendix N 

N–10

reports were obtained from the three strata in both buildings. It is likely that the higher fraction of 

individuals in WTC 1 and in higher floors reflects the more dramatic stories of those closest to the 

airplane impact locations in WTC 1 and WTC 2. 

6 8

6 4

2 6

3

3 1

2 7

1 1

2

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

20-35 36-50 51-65 66+

Age

Frequency

M a le

F e m a le

Figure N–2. Gender and age distribution. 

N.5.3 Means of Egress Used 

On September 11, 2001, almost all individuals from WTC 1 (198 people or 98 percent) reported using the 

stairs to evacuate while three used both stairs and elevator and one used the elevator only. The person 

who used the elevator for evacuation reported that he was in an elevator when the building was struck, 

and the elevator stopped on one of the floors. He was able to use the elevator to move people from that 

floor to the lobby. Two of the three who used both stairs and elevators were initially trapped in an 

elevator behind a 50th floor restroom. After freeing themselves, they were directed by firefighters to an 

elevator to the 44th floor, from which point they walked down. The third person who used both stairs and 

elevators rode with a person he was assisting from the 52nd floor to the 44th floor. Unable to find a 

working elevator on the 44th floor, he walked down the rest of the way. In WTC 2, 114 (72 percent of 

the total for that building) used the stairs while 18 people (11 percent) used elevators and 26 (16 percent) 

used a combination of elevators and stairs. These results are shown in Table N–3. Of the 44 people who 

used the elevator to evacuate WTC 2, 37 were from floors served by the 78th sky lobby and 7 were from 

floors between the 44th and 78th sky lobbies. From these accounts, it seems that the higher up people 

were in WTC 2, the more likely they were to use the elevator as a means of egress. 

Table N–3. Means of egress used within the towers. 

WTC 1, N=202 WTC 2, N=158 

Stairs 198 people (98.0 %) 114 people (72 %) 

Elevator 1 person (0.5 %) 18 people (11 %) 

Stairs and elevator 3 people (1.5 %) 26 people (16 %) 
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N.5.4 First Cue Reported 

The first cues of the event that were mentioned in the accounts were found to differ depending on which 

tower the person was located. For WTC 1, the first building hit, the most common first cue of the event 

reported by 146 people (69 percent of people in that tower) was “building movement,” such as feeling the 

building sway and tremble––many thought the building was going to tip over. WTC 2 occupants most 

commonly reported first becoming aware of the event from visual cues (96 people) such as fire, debris 

and smoke, most likely coming from WTC 1. Several people reported more than one first cue, so they 

may appear more than once in Table N–4 and percentages total more than 100 percent. 

Table N–4. First cues of event within the towers. 

First Cues WTC 1, N=212 WTC 2, N=145 

Audio cues: heard explosion, crash, rumble 107 (50 %) 69 (48 %) 

Visual cues: saw fire, incoming plane, debris, smoke 87 (41 %) 96 (66 %) 

Building movement: felt building sway, tremble, jolt 146 (69 %) 30 (21 %) 

Contents movement: furniture movement, ceiling falling 66 (31 %) 11 (8 %) 

Warning from others 14 (7 %) 34 (23 %) 

Impact 29 (14 %) 1 (1 %) 

Smelled fumes or felt heat 12 (6 %) 16 (11 %) 

Interestingly, only 25 people made any mention of building alarms in their evacuation accounts. Of 

those, eight in WTC 1 and one in WTC 2 reported hearing alarms but did not specify where. Two in 

WTC 1 and one in WTC 2 heard alarms while on their floors and one person in each tower heard alarms 

while in the stairs. Eight people in WTC 1 stated that they did not hear alarms. Three people in WTC 2 

said they never heard alarms, but two of them were outside the building when it was hit. 

N.5.5 Time to Start Evacuation 

After perceiving these first cues, 101 people from WTC 1 (47 percent) immediately started evacuating, 

while 84 people (52 percent) immediately started their evacuation of WTC 2. As can be seen in Fig. N–3, 

similar numbers of people from both towers started evacuating shortly after the first cue of the event 

(28 in WTC 1 versus 27 in WTC 2). Another 46 people in WTC 1 and 40 people in WTC 2 delayed their 

evacuation. Some 23 people in WTC 1 (11 percent) reported they initially stayed, while 10 people from 

WTC 2 (6 percent) also said they initially remained on their floors. Of the 16 people who reported being 

stuck and therefore temporarily unable to start their evacuation, all but one were from WTC 1. 

Among occupants who initially decided to stay, it is noteworthy to mention a group in WTC 1. Two 

survivors reported that a group of about 16 employees gathered in a conference room on Floor 64 of 

WTC 1. The group stayed in the room discussing the situation for approximately 1 hour before deciding 

to evacuate the building. 

Most of those who were not stuck but who took more than 5 minutes to begin evacuation delayed because 

they took the time to complete activities such as searching the floor, securing documents, making calls, or 

giving instructions, or because they felt it was the right thing to do. Twenty-one of 63 people in WTC 1 

(33 percent) and 13 of 45 people in WTC 2 (29 percent) delayed starting their evacuation because 
Appendix N 

N–12

101

2 8

4 6

2 3

1 5

8 4

2 7

4 0

1 0

1

0

2 0

4 0

6 0

8 0

100

120

Im m e d S ho rtly D e la y e d S ta y e d S tuc k

S ta rt T im e

Frequency

W TC 1

W TC 2

Figure N–3. Distribution of time to start evacuation. 

they were completing activities such as those described above. Of those in WTC 1 who did not begin 

their evacuation within 5 minutes, 12 people simply decided to stay (19 percent), compared to 20 people 

in WTC 2 (44 percent). In WTC 1, 17 of those who did not begin their evacuation within 5 minutes 

(27 percent) were helping others or required assistance themselves, compared to only four people 

(9 percent) in WTC 2. 

N.5.6 Conditions on Floors and in Stairwells 

It was possible to code multiple reported conditions on floors and in stairwells for each individual. Six 

people in WTC 1 and seven people in WTC 2 indicated that conditions on their floor were normal after 

their building was struck. For the 191 evacuees who commented on adverse conditions on their floors 

after the plane hit their tower, similar results emerged between the towers, in terms of the large 

proportions reporting smoke or debris and collapse damage on their floor. Specifically, the most 

frequently reported adverse conditions in WTC 1 were smoke (55 percent or 74 people), debris or 

collapse of wall, ceiling or floor (72 people or 54 percent), fire (41 people or 31 percent), darkness or loss 

of power (20 people or 15 percent) and smell of fuel (13 people or 10 percent). In WTC 2, the most 

frequently reported adverse conditions were debris or collapse of wall, ceiling or floor (38 people or 

67 percent), smoke (25 people or 44 percent), darkness or loss of power (18 people or 32 percent), dust 

(10 people or 18 percent), smell of fuel (7 people or 12 percent) and injured people (7 people or 

12 percent). Seven people in WTC 1 who mentioned jammed doors were in the upper strata of the 

building. Two people in WTC 2 who reported jammed doors had moved to middle floors of their 

building after the first impact. The complete details on conditions are presented in Table N–5. 

A large number of evacuees (106 people) mentioned that the stairwells were crowded and hot during their 

evacuation (71 people in WTC 1 and 35 in WTC 2). A total of 27 indicated that conditions in the 

stairwells were otherwise normal. For the 155 evacuees who commented on adverse conditions in the 

stairwells during their evacuation (other than crowdedness), the majority in both towers reported smoke 

and the smell of fuel in the stairs (79 people or 72 percent in WTC 1 and 29 people or 63 percent in 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–13 

WTC 2). For other types of conditions in stairwells, responses between the two towers were quite 

different, as shown in Table N–6. 

Table N–5. Adverse conditions on floor at impact. 

WTC 1, N=134 WTC 2, N=57 

Debris (collapse) 72 (54 %) 38 (67 %) 

Smoke 74 (55 %) 25 (44 %) 

Fire 41 (31 %) 20 (35 %) 

No power, dark 20 (15 %) 18 (32 %) 

Smell of fumes 13 (10 %) 7 (12 %) 

Dust 9 (7 %) 10 (18 %) 

Water 7 (5 %) 3 (5 %) 

Door jammed 7 (5 %) 2 (4 %) 

Crowds, people injured 2 (1 %) 7 (12 %) 

Trapped 5 (4 %) 2 (4 %) 

Table N–6. Adverse conditions reported in the stairs during evacuation. 

WTC 1, N=109 WTC 2, N=46 

Smoke, smell of fuel 79 (72 %) 29 (63 %) 

Water 49 (45 %) 4 (9 %) 

Dark, no power 14 (13 %) 9 (20 %) 

Debris (damage or belongings) 9 (8 %) 14 (30 %) 

Cracked walls 5 (5 %) 14 (30 %) 

Doors locked, jammed 12 (11 %) 2 (4 %) 

N.5.7 Obstructions during Evacuation 

Tables N–5 and N–6 display details on the adverse conditions that resulted at the time of impact. These 

were things that were observed but that might not have presented an obstacle. (For example, a person 

might have reported seeing smoke or debris, without being impeded by that debris.) Obstructions are 

things that limited or otherwise affected a person's ability to evacuate. Many of the same items were cited 

as both adverse conditions and obstructions. More than one obstruction during evacuation could be 

recorded for each person. A total of 153 people in WTC 1 and 59 people in WTC 2 indicated 

encountering obstructions during their evacuation. Almost half of the evacuees in each tower reported 

encountering crowds and injured people in the stairways, and indicated that they interfered to some 

degree in their evacuation (46 percent in each tower). The next most frequently reported obstructions 

were smoke and debris. The details are shown in Fig. N–4. 

Of the 22 evacuees who reported encountering jammed or locked doors, 20 were in WTC 1, and all but 

three were located on upper floors. One of the WTC 2 evacuees reported that an elevator door was 

jammed by debris, and the other reported a locked door on reaching the bottom of the stairs. Of the 25 

evacuees who reported being trapped, nine were in elevators, eight were trapped by debris or smoke when 
Appendix N 

N–14

their building was hit, five were trapped in the collapse of WTC 2, and three were trapped when WTC 1 

collapsed. 

2 0

3 6

7 1

4 9

2 0 1 9

1 1

2

2 1

2 7

2 2

8 6 8

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

D o o r ja m D e bris C ro w ds S m o ke N o po w e r T ra ppe d F ire

O b s tru c tio n

Percent

W TC 1

W TC 2

Figure N–4. Obstructions encountered during evacuation in both towers. 

N.5.8 Announcement 

It is estimated that the WTC 2 announcement came over the public address system at approximately 9 

a.m. The majority of survivors said they heard it just minutes before WTC 2 was struck, which occurred 

at 9:03 a.m. As one survivor from the 103rd floor of WTC 2 describes it, “When we reached the 70th 

floor we heard the announcement. The building was secure; no one needed to evacuate. We had 

descended down 3 more floors to the 67th when the second plane hit our tower” (csmonitor.com 2001). 

Of the 184 WTC 2 occupants, 96 people (52 percent) mentioned hearing this announcement in their 

accounts. The majority of them, 69 survivors, decided to disregard the instructions of the message and 

continue their evacuation; however, 16 people (17 percent) said they remained in their offices or decided 

to return back up to their offices after hearing the message. Those returning did not have time to travel 

very far before the second plane hit; at that point they all resumed their evacuation. 

N.5.9 Location When WTC 2 Was Hit 

Of the 273 survivors who mentioned their location at the time WTC 2 was hit, 36 people reported being 

somewhere inside the stairwells of WTC 1, while 14 people reported being on various floors of WTC 1. 

Fifty-six did not give a specific location, and 15 had already reached the outside. Of the survivors from 

WTC 2, 65 people reported they were in the stairs and 52 occupants reported they were on various floors 

within WTC 2. Four did not give a specific location, and 31 had already left the building. Of the people 

who were on the floors within WTC 2, 19 were on the upper floors (77th and above) at impact and 

survived. One of these occupants, who survived the plane impact on the 78th floor of WTC 2, describes 

the stairwell: “a tornado of hot air and smoke and ceiling tiles and bits of drywall came flying up the 

stairwell. In front of me, the drywall split from the bottom up” (csmonitor.com 2001). 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–15 

N.5.10 Location When WTC 2 Collapsed 

WTC 2 was the first of the towers to collapse at 9:59 a.m. Of the 296 survivors who mentioned their 

location at the time of WTC 2’s collapse, 230 people (78 percent) were outside of the buildings, on the 

streets and surrounding areas. Some 47 people (16 percent) were still inside WTC 1 on lower levels from 

the basement to the 42nd floor, and three people (1 percent) were on mid levels (43 to 76) in WTC 1 

when WTC 2 fell. Thirteen did not give exact locations, and one was in an elevator. Three individuals 

were on the lower levels of WTC 2 (concourse) when it collapsed, and they survived. 

N.5.11 Location When WTC 1 Collapsed 

WTC 1, the second tower to collapse, fell at 10:28 a.m. As approximately 1 hour and 42 minutes had 

passed since the initial WTC 1 impact, almost everyone who reported their location at the time WTC 1 

collapsed was outside (263 people or 98 percent). Four people were on the lower levels of WTC 1, and 

two were in the concourse when it collapsed, and they survived. 

N.5.12 Location When They Saw Firefighters 

For the evacuees who mentioned seeing firefighters during their evacuation, the location where they met 

them was recorded to gain an understanding of the dispersion of emergency workers throughout the 

towers. For the 169 people who reported meeting firefighters, 143 people saw them in WTC 1, with only 

26 people in WTC 2 mentioning their presence. In terms of floor location within WTC 1, it was found 

that a majority of the people (76 people) saw firefighters in WTC 1 on the lower levels (basement to 

43rd); 74 of them saw firefighters in the stairwells, and two on a floor. Another 21 people saw 

firefighters on the mid floors (43rd to 76th)––17 of them were in the stairs while the other four people 

were on floors. Another three people saw firefighters on the upper floors (77th to 110th) in office areas. 

All three were trapped on the 83rd floor. One survivor stated: “We saw two flashlights belonging to two 

New York City firemen. They told us to leave all of our possessions and to quickly follow them.” 

(Manning 2001). At the mezzanine, lobby or concourse level, 11 people reported seeing firefighters. 

The remaining 31 occupants who saw firefighters inside WTC 1 did not give a location. 

Among the 26 people who mentioned seeing firefighters in WTC 2, eight saw them on the lower floors 

(basement to 42nd), and two saw firefighters in the mid floors of the building (43rd to 76th). Seven 

people saw firefighters at the mezzanine, lobby or concourse levels, while six people in WTC 2 

mentioned seeing firefighters but did not indicate their locations. Three people indicated that they met 

firefighters outside WTC 2. 

N.5.13 Time of Exit 

For evacuees from both towers who indicated at what time they exited, it was found that as more time 

passed, a progressively greater number of people exited the building, as shown in Table N–7. Of the 183 

WTC 2 occupants who indicated what time it was when they left the building, 77 exited between 9:31 and 

9:58 a.m. WTC 2 collapsed at 9:59 a.m. Of the 211 WTC 1 occupants who indicated the time they left 

their building, 70 exited between 9:59 and 10:27 a.m. WTC 1 fell at 10:28 a.m. The six people who 

exited the towers after 10:28 a.m. were rescued from the rubble by firefighters up to several hours after 

the collapse.
Appendix N 

N–16

Table N–7. Time out of towers. 

 WTC 1 

(impact - 8:46 a.m.) 

(collapse - 10:28 a.m.) 

N=211 

WTC 2 

(impact - 9:03 a.m.) 

(collapse - 9:59 a.m.) 

N=183 

8:48 – 9:02 a.m. 

(before WTC 2 impact) 

19 37 

9:03 – 9:30 a.m. 45 68 

9:31 – 9:58 a.m. 

(before WTC 2 collapse) 

72 77 

9:59 – 10:27 a.m. 

(after WTC 2 collapse) 

70 0 

10:28 a.m. 

(after WTC 1 collapse) 

5 1 

N.5.14 Help Received and Help Given 

Among the 435 accounts, 203 survivors described receiving help from others during their evacuation, 

with some mentioning more than one source of help. Some 84 people (41 percent) were helped by Port 

Authority personnel. Firefighters provided direct help to 65 people (32 percent). Another 65 people 

(32 percent) were helped by other first responders such as NYPD or other rescuers. Help from coworkers 

was received by 34 people (17 percent). 

Overall, 166 people mentioned being comforted and reassured by passing firefighters. Several occupants 

of the two towers helped others during the evacuation. Among the first-person accounts, 20 people said 

they helped people with disabilities and 14 said they helped people who were injured during the event. 

N.5.15 Occupants with Disabilities or Injuries 

Among the 27 persons reporting a disability in their account, two were visually impaired, three were 

hearing impaired, three used wheelchairs and 19 others were physically challenged such as suffering from 

a heart condition, asthma, obesity, etc. Twenty-two people mentioned seeing people with disabilities. 

Another 47 people who provided first-person accounts were injured that morning. Some accounts from 

people who suffered injuries reported exiting the buildings later in the evacuation process. However, in 

numerous accounts occupants mention moving aside in the stairwells to let badly injured and burned 

people pass, thus it is assumed that those with extreme injuries who were mobile exited the building faster 

than the majority of others. For instance, one survivor from floor 88 of WTC 1 who suffered burns to 

over 77 percent of her body reported that crowds parted in the stairwell to let her through (Kugler 2002). 

These victims were all accompanied by coworkers or emergency workers. Twenty-five people mentioned 

seeing injured people coming down in the stairwells. 

Twenty-three individuals with disabilities and 43 with injuries mentioned a time to start. Of these 

66 people, 50 percent (13 people with disabilities and 20 injured) started evacuating immediately, 

5 percent (two disabled and one injured) left shortly after, 29 percent (7 disabled and 12 injured) delayed 

evacuating, 14 percent (one wheelchair user and eight injured) initially decided to stay, and 3 percent (two 

injured people) were initially stuck.
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–17 

N.5.16 Phone Calls 

An overwhelming 87 percent of those who placed phone calls (151 people) were trying to contact their 

families and friends to let them know their whereabouts and gather information from them. Only 

12 people (7 percent) tried contacting authorities, such as building security or calling 911, and 20 people 

(12 percent) placed calls to their boss or colleagues. Eleven people (6 percent) did not say who they 

called. 

The majority of people who placed phone calls that morning did so once they were outside (93 people or 

54 percent); however, many did not get through. Forty-four people (25 percent) mentioned that they 

placed calls from their offices before evacuating, 13 people (8 percent) called from other floors and 

10 people (6 percent) attempted to make phone calls while in the stairwells. 

N.5.17 Knowledge of Situation 

In judging the evacuees’ knowledge of the situation, categories were created. A “high level” of 

knowledge indicated knowing that planes had hit the towers or that there had been an explosion within the 

towers. Those who speculated about a bombing saw fire and debris or had reason to believe an 

emergency was occurring were said to have a “moderate level” of knowledge. Survivors who were not 

aware of the reasons behind the evacuation were classified as having a “low level” of knowledge. Level 

of knowledge was coded for 330 people. As shown in Fig. N–5, survivors with “high levels” of 

knowledge totaled 69; 214 people were judged to have a “moderate level” of knowledge and 47 survivors 

had a “low level” of knowledge regarding the events of that morning. 

3 7

120

3 2 3 2

9 4

1 5

0

2 0

4 0

6 0

8 0

100

120

High M oderate Low

L e v e l o f K n o w le d g e o f S itu a tio n

Frequency

W TC 1

W TC 2

Figure N–5. Knowledge of situation in the towers. 

N.5.18 Influence of Others 

One hundred and ninety-one survivors reported that their decisions during the evacuation were influenced 

by others. It appeared that 28 people were influenced by authority figures, such as their boss or manager, 
Appendix N 

N–18

and complied with their instructions. Another 97 survivors seemed to be influenced by groups of people 

and coworkers. One person appeared to have been influenced by both authority figure(s) and the group. 

Many individuals indicated that they took on leadership roles that morning. Sixty-six people reported 

they directed people to the stairs, searched for others, gave orders or somehow took part in organizing the 

evacuation. 

Males were more likely to perceive themselves as taking on leadership roles that morning than females 

(see Table N–8). Thirty-eight women (59 percent of the females for whom influence could be inferred) 

were influenced by groups of coworkers, whereas only 58 men (46 percent) were apparently influenced 

by the group. Concerning leadership roles, 52 men (41 percent) reported adopting this behavior, 

compared to the 14 women who mentioned taking a leadership role (22 percent of the women). 

Table N–8. Gender and influence of others. 

 Males, N=127 Females, N=64 

Authority figures (boss, manager) 17 (13 %) 11 (17 %) 

Groups/coworkers 58 (46 %) 38 (59 %) 

Both authority and groups 0 (0 %) 1 (2 %) 

Took a leadership role 52 (41 %) 14 (22 %) 

N.5.19 Perception of Others 

How survivors perceived others during the evacuation was recorded for 268 people––others could have 

been perceived as “calm,” “momentarily panicked,” “upset,” or “helpful.” Multiple responses could be 

coded for each person. The results show that the majority (154 people or 57 percent) described people 

around them as calm and orderly. Some 84 people (31 percent) judged others as “upset,” which included 

crying, shouting, nervous or anxious, but rational. There were 78 people (29 percent) who described 

others as “momentarily panicked,” in that they were pushing, shoving or generally displaying behavior 

associated with chaos, while 59 people (22 percent) found others to be “helpful.” More details are 

presented in Fig. N–6. 

It was found that of 155 people in WTC 1, 93 survivors judged others to be “calm,” compared to 61 of 

113 people in WTC 2. Only 33 people in WTC 1 described others as “momentarily panicked,” compared 

to 45 people in WTC 2. For the people in WTC 2, the perception of “panic” occurred before WTC 2 was 

hit for at least three occupants, while another 29 survivors described others around them as “panicky” 

after WTC 2 was hit. For two others, the “panicky” behavior was reported at the point in time when each 

tower collapsed. It was not clear from the other 11 accounts from WTC 2 when the people around them 

were “panicky.” 

This variance in perception of others between the towers is illustrated by contrasting the following two 

accounts. One survivor from the 65th floor of WTC 1 said that those in the stairwells “maintained their 

calm really well” and went on to say that, a couple of people started crying a little, but we said, ‘We’re 

going to get out of here, we just have to take it one step at a time.’ It wasn’t quiet, people were talking–in 

fact someone was laughing, it was pretty normal (Anderson 2001). It is proposed that the occupants of 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–19 

WTC 2 observed others “momentarily panicking” mainly once their tower had been hit. One survivor 

from the 70th floor of WTC 2 said she and her coworkers walked down to the 59th floor and took an 

elevator to the 44th floor, when at that point, another plane hit their tower and then there was a mad 

scramble down the stairs with people pushing, shoving and yelling (Black 2001). 

9 3

3 3

4 6

4 2

6 1

4 5

4 1

1 7

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

C a lm M o m e nta ry pa nic U ps e t H e lpful

P e rc e p tio n o f O th e rs

Frequency

W TC 1

W TC 2

Figure N–6. Distribution of perception of others between the towers. 

Perception of others and gender are compared in Fig. N–7. 

115

5 5

6 5

3 8 3 7

2 2 2 2 2 2

0

2 0

4 0

6 0

8 0

100

120

C a lm M o m e nta ry P a nic U ps e t H e lpful

P e rc e p tio n o f O th e rs

Frequency

M a le

F e m a le

Figure N–7. Distribution of gender and perception of others. 

The distribution of perception of others by age group is shown in Table N–9. It is interesting to note that 

some of the most dramatic language (“chaos,” “total chaos,” “mayhem”) was used by the youngest males. 
Appendix N 

N–20

Table N–9. Distribution of age and perception of others. 

21-35 yrs old (N=74) 36-50 yrs old (N=58) 51-65 yrs old (N=21) 

Calm 39 (53 %) 31 (53 %) 9 (43 %) 

Panicked 25 (34 %) 14 (24 %) 6 (29 %) 

Upset 31 (42 %) 22 (38 %) 4 (19 %) 

Helpful 16 (22 %) 17 (29 %) 8 (38 %) 

N.5.20 Technology to Gain Information 

In addition to the people mentioned earlier who made cell phone calls from the stairwells, 10 people used 

technology such as wireless e-mail devices and text pagers as a means of gathering information about the 

situation. Thirteen listened to the radio or watched television, among them three evacuees who stopped to 

watch TV on the mid floors (43 to 76) of WTC 1 and saw live media coverage of the events. 

N.5.21 Impact of the 1993 Evacuation 

Only nine percent, 41 people, reported being present during the 1993 bombing and evacuation of the 

WTC. Of them, three people explained that their experience in 1993 helped them decide to start their 

evacuation immediately on September 11, 2001. Five people who were present in 1993 mentioned being 

better prepared this time with evacuation kits. These emergency escape kits were described as being 

equipped with flashlights, masks, glow sticks, whistles and water (Murphy and Levy 2001). Another 18 

people specifically mentioned that 1993 was on their mind during their evacuation, although they were 

not present during the events of 1993.

Four survivors reported seeing photoluminescent stripes on the stairs, railings and stairwell doors––an 

improvement the Port Authority made following the 1993 bombing. As one survivor stated, “All you had 

to do was follow those yellow-green stripes. They were wonderful.” The stripes were especially valuable 

when the emergency stairs stopped and people had to travel horizontally through mechanical equipment 

spaces that had many doors (Masetti 2001). 

A paraplegic survivor from WTC 1 who was also present for the 1993 evacuation of the WTC 

commented on the successful use of an evacuation chair on September 11, 2001. The evacuation chairs 

were part of the improvements made to the WTC evacuation process after the 1993 bombing, and this 

survivor credits the chair with saving his life. In 1993, he was bounced down the stairs in his electric 

wheelchair from floor 69 to floor 43, where he was then transferred to a stretcher and carried down the 

rest of the way. It took him 6 hours to evacuate from floor 69 in 1993. On September 11, 2001, using the 

evacuation chair enabled him to escape floor 69 of WTC 1 and get to street level in 1 hour and 

30 minutes. He went on to say, “If it weren’t for the evacuation chair and the 10 people that brought me 

down, I would not have made it, that's for sure. That evacuation chair made the difference” (Fink and 

Mathias 2002). 

N.6 SUMMARY RESULTS 

Although it is recognized that content analysis of first-person accounts has limitations, and the results 

cannot be generalized to all occupants of the towers, this methodology was found to be particularly useful 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–21 

in this case. With the large number of accounts that were gathered from a variety of sources (print 

media, television, radio, internet, emails, etc.), the similar themes and experiences within these texts 

became more than merely anecdotal stories. Considering that a great majority of the accounts became 

public within three weeks following the events and that recollection of human behavior is delicately time 

sensitive, it was important to analyze this information. This methodology could prove useful in future 

projects dealing with first-person accounts, although events of the magnitude of September 11, 2001, 

which produced such a large number of first-person accounts, are extremely rare. 

For the accounts gathered from media sources, it is recognized that they may represent the most dramatic 

stories of the evacuation. At the same time, those survivors who have dramatic stories of escape may be 

more inclined to share them compared to other survivors who may judge their evacuation as less eventful. 

However, the accounts analyzed were from survivors located in several areas in each tower, providing a 

distribution of floors from the upper, middle and lower strata of the two towers. In total, 745 accounts 

were analyzed, representing 435 survivors from WTC 1 and WTC 2. 

An interesting and important observation involves the emergence of new first-person accounts from 

survivors who had not previously shared their stories, around the first anniversary of the event. In trying 

to explain this phenomenon, it is speculated that survivors who had not previously shared their stories 

were now prepared to do so after having time to cope and deal with their experience. Many of the 

evacuees mentioned that telling their stories proved to be a therapeutic exercise. Media sources may have 

also held accounts gathered from an earlier date or searched for new, untold stories and published them as 

part of the anniversary coverage. 

An important observation stemming from the accounts analysis encompasses the issue of evacuation 

strategies. It was found that 44 people, about 24 percent of WTC 2 occupants in this study, used the 

elevators at some point during their evacuation. It has long been accepted among fire safety experts that 

people know they should not use elevators as a means of egress during an emergency, but those in WTC 2 

who chose to use the elevators may have thought it was the quickest or safest route of escape and may 

have believed that because they were not in immediate danger, they were justified in their decision to use 

the elevators to evacuate. The same theme is echoed when examining the reactions of the 96 WTC 2 

occupants who heard the public address announcement, which told them their building was secure and to 

return to their offices. Only 16 people took heed of this message and stopped their evacuation, making 

their way back to, or remaining in, their offices. Through all accounts studied (with the possible 

exception of one) there was no doubt that people understood the message, as there were no audibility or 

intelligibility issues; the content of the message was clear. However, the majority of 69 occupants made 

their decision based on the information that they had at that point in time and decided to disregard the 

order and continue evacuating. As one survivor stated, “I was thinking that there is a real difference of 

opinion here about what my eyes are seeing and what the announcement was saying” (Murphy and 

Levy 2001). The decision to carry on with the evacuation may also reflect the concept of commitment: as 

these occupants had already made the decision to leave, they pursued this task. 

It is also interesting to note that the official procedure for emergencies in the WTC was to meet in the 

lobby area on each floor and wait for instruction. Nevertheless, the majority of occupants of both towers 

decided to evacuate on their own after WTC 1 was hit, without waiting for an official building 

announcement. Thus, this is further evidence that people will make decisions based on what they judge 

the proper action to take despite official procedures. 
Appendix N 

N–22

Those who had experienced the 1993 terrorist bombing of the WTC left promptly. Although their past 

experience could have suggested that the evacuation was going to be long and difficult and that people 

who stayed behind would be evacuated by rescuers later on, very few used this rationale. Instead, most 

occupants with experience from 1993 felt an urgency to leave immediately. 

The results show that 18 people who were identified as having “high levels” of knowledge delayed 

evacuating. Those who delayed their evacuation reported that they rushed to gather their belongings or 

went to backup important company files, for they suspected they would not be returning to the building 

for an extended period of time. These are rational actions; therefore, it is concluded that those with “high 

levels” of knowledge who delayed evacuating had to have been in areas where the perceived threat to 

personal safety was not high. 

The overall impression of the emotional atmosphere during the evacuation, after reading all 745 accounts, 

was that of calm and order. Although some reported crying and being anxious or nervous, the majority 

viewed themselves and others as composed. A stark contrast in perceived behavior was found to exist 

between the two towers, with the majority of WTC 1 occupants reporting others as “calm” (93 of 

155 people), where as a large proportion of WTC 2 occupants perceived others to be “panicked” (45 of 

113 people). The perception of “panic” occurred before WTC 2 was hit for at least three occupants, while 

another 29 survivors perceived others as “panicked” after WTC 2 was hit. After their building had been 

struck, WTC 2 occupants may have realized they were under attack, which could possibly explain the 

heightened level of anxiety in the tower. (It is important to note, however, that the colloquial use of the 

word panic more often describes a state of mind––high anxiety, for example––rather than the irrational 

actions that more correctly define “panic.”) 

Emergency crews disrupted evacuation in the stairwells while going against traffic, but many evacuees 

who mentioned seeing firefighters felt reassured and safe due to their presence. It is assumed that this 

counter flow did not prevent occupants from evacuating, as the last people to exit reported being alone in 

the stairs while they were descending rapidly seconds before the collapse. Evacuees used technology 

such as cell phones, wireless e-mail devices, and text messaging over pagers during their descent as a 

means of gathering information about the situation unfolding around them. 

N.7 FUTURE WORK 

Future research is needed to fully understand the evacuation behavior of the occupants who were in the 

two towers of the WTC on September 11, 2001. A variety of approaches should be used to gather this 

information such as interviews and questionnaires. Unfortunately, the extended amount of time that has 

elapsed since the events is an important factor to mitigate, since occupants’ recollection may be 

incomplete and contaminated by what has been seen, read, or heard since September 11, 2001. 

Technology clearly played a role in providing occupants with information about the event during their 

evacuation. This phenomenon raises important issues regarding the information age and how new 

technologies can be taken advantage of to aid in emergency situations. If technology can help to 

disseminate timely information to the public in times of crises, strategies should be developed to enable 

authorities to fully utilize such technology. 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–23 

This major event, which was repeatedly broadcast on television around the world, may also influence fire 

safety in high-rise buildings in general. It is essential to study how the perception of risk in high-rise 

buildings has changed since September 11, 2001. Do people who live, work or visit high-rise structures 

feel more at risk of a potential fire or fear that the building might collapse if there is a fire? If the 

occupants feel more at risk, what is their likely behavior and response in future emergencies? Studies 

should be conducted to explore the impact of high-rise risk perception on intended behavior in future 

emergencies. Are occupants prepared to follow procedures and instructions? Would they comply with a 

protect-in-place approach or to move to a refuge floor? If all occupants want to evacuate to the ground 

floor or exit during an emergency, requirements for stair design and building height might need to be 

revisited. Drills should be conducted to observe unannounced emergency evacuations in high-rise 

buildings, varying evacuation strategies and information provided to occupants to assess actual response. 

Longitudinal studies should also be conducted to assess the impact of September 11 over time on highrise building occupants. 

N.8 ACKNOWLEDGMENTS 

The authors would like to express their appreciation to Amber Walker, formerly of the National Research 

Council of Canada, for the work she did developing the original matrix format and undertaking the early 

analyses that form the basis of this study. They would also like to thank Joseph Molis and 

Norma Candeloro of NFPA for their contributions to this work.

N.9 REFERENCES 

Anderson, P., 2001. We the living: One company that made it. CNN, September 21. 

Black, B., 2001. How the telephone linked an anxious family on September 11. Concordia’s Thursday 

Report, November 22. 

Cauchon, D., 2001. For many on September 11, survival was no accident,” USA TODAY, December 19. 

csmonitor.com/2001/0917/pls1-usgn.html, The Christian Science Monitor, accessed September, 2002. 

Federal Emergency Management Agency (FEMA), 2002. World Trade Center Building Performance 

Study: Data Collection, Preliminary Observations, and Recommendations. Federal Insurance and 

Mitigation Administration, Washington, D.C. 

Fahy, R., G. Proulx. 1995. Collective Common Sense: A study of the human behavior during the World 

Trade Center evacuation, NFPA Journal, Vol. 89, No. 2, pp. 59-67. 

Fink, M., L. Mathias. 2002. Never Forget. An Oral History of September 11, 2001. Harper Collins 

Publishers, New York, NY, pp. 124-127. 

Fischer III, H.W., 1998. Response to Disaster. Fact versus Fiction and Its Perpetuation. University Press 

of America, Maryland, pp. 13-45. (With foreword written by Quarantelli) 

Gamson, A.W., 1975. The Strategy of Social Protest, The Dorsey Press, Homewood, Illinois. 
Appendix N 

N–24

Grossman, D., B. Siddle. 2002. Critical Incident Amnesia: The psychological basis and the implications 

of memory loss during extreme survival stress situations, The Firearms Instructor, Issue 31. 

Hays, W., 1994. Statistics, 5th ed. Harcourt Brace College Publishers, Orlando, Florida. p. 869. 

Isner, M.S., T.J. Klem. 1993. Fire Investigation Report: World Trade Center Explosion and Fire, New 

York, New York, February 26. National Fire Protection Association, Quincy, MA, p. 32. 

Janis, L., L. Mann, 1977. Decision Making. Free Press, Macmillan Publishing Co., New York, NY. 

Johnson, R.N. 1987. Panic at The Who Concert Stampede: An Empirical Assessment, Social Problems, 

Vol. 34, No. 4, pp. 362-373. 

Keating, P.J., 1982. The Myth of Panic, Fire Journal, Vol 76, No.3, National Fire Protection 

Association, Quincy, MA, pp. 57-61. 

Kugler, S., 2002. WTC victim leaves hospital, Boston Globe, January 30. 

Manning, C., 2001. Goffstown man tells of his escape from 83rd floor, The Union Leader, September 15. 

Section A, Page 1. 

Masetti, A., 2001. Personal e-mail correspondence. December 10. 

Murphy, D., Levy, C., 2001. The evacuation that kept a horrible toll from climbing higher, The New York 

Times, September 21. 

Proulx, G., 1993. A stress model for people facing a fire, Journal of Environmental Psychology, Vol.13, 

pp. 137-147. 

Proulx, G., 2001. As of the year 2000, what do we know about occupant behavior in fire?, The Technical 

Basis for Performance Based Fire Regulations, United Engineering Foundation Conference, San 

Diego, pp. 127-129. 

Proulx, G., 2002. Understanding human behavior in stressful situations. Workshop to Identify Innovative 

Research Needs to Foster Improved Fire Safety in the United States, National Academy of Sciences, 

Delegate Binder Section 7, Washington, D.C., pp. 1-5. 

Sime. J.D., 1980. The Concept of Panic, Fires and Human Behaviour, D. Canter, Editor, John Wiley and 

Sons, Ltd., pp. 63-81. 

Sime, J.D., 1985. The outcome of escape behavior in the Summerland fire: panic or affiliation?, 

International Conference on Building Use and Safety Technology, Los Angeles, CA. 

Wegner, D., B. Aguirre, G. Vigo, 1994. Evacuation behavior among tenants of the World Trade Center 

following the bombing of February 26. Hazard Reduction Recovery Center Publications, Texas 

A&M University. 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–25 

Yamasaki, M., 2002. World Trade Center Fact Sheet, Prepared by Minoru Yamasaki Associates, 

Architects. www.m-yamasaki.com/projs/wtcfs.htm. Accessed on December 9. 
Appendix N 

N–26

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N–27

Attachment 1 

WTC FIRST-PERSON ACCOUNTS CODE BOOK

1. BLDG - ‘Building Location at the Time of Awareness / Perception of First Cue’ 

1 = Tower 1, North Tower 

2 = Tower 2, South Tower 

3 = Plaza/Outside 

4 = Concourse 

5 = Mall 

6 = PATH Train 

7 = Bldg 7 or Bldg 3 

99 = n/a 

2. FLR - ‘Floor Location at Perception of First Cue’ 

SPLIT COLUMN EXACT FLOOR AND CATEGORY 

1 = T1 Lower (basement-42) in stairs 

2 = T1 Lower (basement-42) on a floor 

3 = T1 Mid (43-76) in stairs 

4 = T1 Mid (43-76) on a floor 

5 = T1 Upper (77-110) in stairs 

6 = T1 Upper (77-110) on a floor 

7 = T1 in stairs, level not specified 

8 = T1 location not specified 

9 = T1 mezzanine, lobby, concourse 

10 = T2 Lower (basement-42) in stairs 

11 = T2 Lower (basement-42) on a floor 

12 = T2 Mid (43-76) in stairs 

13 = T2 Mid (43-76) on a floor 

14 = T2 Upper (77-110) in stairs 

15 = T2 Upper (77-110) on a floor 

16 = T2 in stairs, level not specified 

17 = T2 location not specified 

18 = T2 mezzanine, lobby, concourse 
Appendix N 

N–28

19 = Outside 

22 - T1 elevator - lower floors 

23 - T1 elevator - mid floors 

24 - T1 elevator - upper floors 

25 - T1 elevator, level not specified 

99 = n/a 

3. SEX - ‘Sex of Evacuee’ 

1 = male 

2 = female 

99 = n/a 

4. AGE_CODE - ‘Age of Evacuee’ 

SPLIT COLUMN EXACT AGE AND CATEGORY 

1 = 21-35 

2 = 36-50 

3 = 51-65 

4 = 66 + 

99 = n/a 

5. DATE - ‘Date of Record’ 

SPLIT COLUMN EXACT DATE MENTIONED 

1 = Week of (09/11/2001-09/15/2001) 

2 = 2 weeks after (09/16/2001-09/30/2001) 

3 = 1-3 months after (10/01/2001-12/31/2001) 

4 = 4-6 months after (1/01/2002-3/31/2002) 

5 = 7-9 months after (4/01/2002-6/30/2002) 

6 = 10-12 months after (7/01/2002-9/30/2002) 

99 = n/a 

6. EGRESS - ‘Evacuation Method’ 

1 = Stairs 

2 = Changed stairwells 

3 = Elevator 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–29

4 = Combo of stairs and elevator 

99 = n/a 

7. FSTCUE - ‘First Cue of Event’ 

COLUMN CHECKED OFF FOR EACH INITIAL CUE MENTIONED 

1 = Audio (boom, crash, explosion, thunder, blast, roar, rumbling) 

2 = Visual (smoke, fire, bodies, plane approaching, panicked people, debris falling) 

3 = Building Movement (impact, sway, shake, earthquake, rocking, jolt) 

4 = Content Movement (chairs moving, ceiling falling, bounce in elevator, debris in 

halls/offices, lights flickering, change in air pressure, burned by fire) 

5 = Warn by others (directly told or behavior of others) 

6 = Physically impacted (burned, fell or thrown out of chair) 

7 = Smelled fumes or Felt heat 

99 = n/a 

8. ALRM – Heard Alarm 

1 = Yes, heard alarm 

2 = Heard alarm on floor 

3 = Heard alarm in stairs 

4 = ‘I did not hear an alarm’ 

99 = n/a 

9. STTIME - ‘Time to Start Evacuation’ 

1 = Immediately (ran, right away, rapidly): 1 minute 

2 = Shortly after (short delay, picked up belongings, warn others): up to 5 minutes after 

3 = Delayed (gathered belongings, look out window, make phone calls, watch TV, kept 

working, checked security, planned with coworkers, shut equip off, Post T2 Impact) 

4 = Stayed (to help: headcount, direct people, assisted coworkers, waited to be 

rescued/given instructions; went up) 

5 = Stuck (behind debris, walls, in elevator) 

99 = n/a 

10. CNDFL - ‘Condition on Floor When Building was Hit’ 

1 = Devastated (combo of debris, fire, walls collapsed, ceiling/lights down, 

darkness, water/sprinklers, smoke, jet fuel, glass, bodies) 
Appendix N 

N–30

2 = Abnormal (some smoke, heat, smell fuel, power out, dusty, debris past 

windows, some reason for alarm/evacuation) 

3 = Normal (usual working conditions) 

99 = n/a (incl. not on floor when building was hit) 

11. CNDFL – ‘Condition on Floor’ 

COLUNM CHECKED OFF FOR EACH CONDITION MENTIONED. 

1 = Normal 

2 = Door Jammed 

3 = Debris – Wall, ceiling collapsed 

4 = Smoke 

5 = Dust 

6 = No power – dark 

7 = Smell 

8 = Water 

9 = Fire 

10 = Crowd, injuries 

11 = Trapped 

12 = Not on a floor 

99 = n/a 

12. STRS - ‘Condition in Stairwell During Evacuation’ 

COLUMN CHECKED OFF FOR EACH CONDITION MENTIONED. 

1 = Normal 

2 = Door locked, jammed 

3 = Crowd, hot 

4 = No power 

5 = Water 

6 = Cracked wall 

7 = Debris 

8 = Smoky, smelly 

99 = n/a 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–31

13. ANCHRD - ‘Heard Announcement’ 

1 = T1 Yes 

2 = T1 No (mentioned specifically not hearing message) 

3 = T2 Yes 

4 = T2 No (mentioned specifically not hearing message) 

99 = n/a 

14. ANCACT - ‘Action After Hearing T2 Announcement’ 

1 = Continued evacuating 

2 = Continued evacuating saw some returned 

3 = Returned to office/Stay on location 

99 = n/a 

15. ANCFLR - ‘Location when T2 Announcement Heard’ 

10 = T2 Lower (basement-42) in stairs 

11 = T2 Lower (basement-42) on a floor 

12 = T2 Mid (43-76) in stairs 

13 = T2 Mid (43-76) on a floor 

14 = T2 Upper (77-110) in stairs 

15 = T2 Upper (77-110) on a floor 

16 = T2 in Stairs not specified 

17 = T2 Location not specified 

18 = T2 mezzanine, lobby, concourse 

19 = Outside 

20 = T2 in Elevator 

99 = n/a 

16. LT2IMP - ‘Location at T2 Impact’ 

1 = T1 Lower (basement-42) in stairs 

2 = T1 Lower (basement-42) on a floor 

3 = T1 Mid (43-76) in stairs 

4 = T1 Mid (43-76) on a floor 

5 = T1 Upper (77-110) in stairs 

6 = T1 Upper (77-110) on a floor 

7 = T1 in stairs, level not specified 
Appendix N 

N–32

8 = T1 location not specified (incl. Inside elevator) 

9 = T1 mezzanine, lobby, concourse 

10 = T2 Lower (basement-42) in stairs 

11 = T2 Lower (basement-42) on a floor 

12 = T2 Mid (43-76) in stairs 

13 = T2 Mid (43-76) on a floor 

14 = T2 Upper (77-110) in stairs 

15 = T2 Upper (77-110) on a floor 

16 = T2 in stairs, level not specified 

17 = T2 location not specified (incl. Inside elevator) 

18 = T2 mezzanine, lobby, concourse 

19 = Outside 

99 = n/a 

17. LT2COL - ‘Location at T2 Collapse’ 

1 = T1 Lower (basement-42) in stairs 

2 = T1 Lower (basement-42) on a floor 

3 = T1 Mid (43-76) in stairs 

4 = T1 Mid (43-76) on a floor 

5 = T1 Upper (77-110) in stairs 

6 = T1 Upper (77-110) on a floor 

7 = T1 in Stairs not specified 

8 = T1 in Elevator 

9 = T1 mezzanine, lobby, concourse 

10 = T2 mezzanine, lobby, concourse 

11 = T2 Lower (basement-42) in stairs 

12 = Outside 

13 = Other WTC building 

14 = Subway 

99 = n/a 

18. LT1COL - ‘Location at T1 Collapse’ 

1 = Lower T1 (basement-43) stairs 

2 = T1 mezzanine, lobby, concourse 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–33

3 = Outside 

99 = n/a 

19. LFFS - ‘Location When Met Firefighters’ 

1 = T1 Lower (basement-42) in stairs 

2 = T1 Lower (basement-42) on a floor 

3 = T1 Mid (43-76) in stairs 

4 = T1 Mid (43-76) on a floor 

5 = T1 Upper (77-110) in stairs 

6 = T1 Upper (77-110) on a floor 

7 = T1 in stairs, level not specified 

8 = T1 location not specified 

9 = T1 mezzanine, lobby, concourse 

10 = T2 Lower (basement-42) in stairs 

11 = T2 Lower (basement-42) on a floor 

12 = T2 Mid (43-76) in stairs 

13 = T2 Mid (43-76) on a floor 

14 = T2 Upper (77-110) in stairs 

15 = T2 Upper (77-110) on a floor 

16 = T2 in stairs, level not specified 

17 = T2 location not specified 

18 = T2 mezzanine, lobby, concourse 

19 = Outside 

99 = n/a 

20. HELP - ‘Who Helped Evacuee during Evacuation’ 

COLUMN CHECKED OFF FOR EACH HELPER MENTIONED 

1 = Firefighter 

2 = Port Authority (building staff/security) 

3 = External Official (police, FBI, EMT, rescue workers) 

4 = Coworkers 

5 = Passed Firefighters in Stairs 

99 = n/a 
Appendix N 

N–34

21. DSBLD - ‘Evacuee Disability and Injury’ 

1 = Visual impairment 

2 = Hearing impairment 

3 = Physically challenged (obese, asthma, heart condition) 

4 = Wheelchair user 

5 = Injured during event (burned, sprained ankle, broken bones, emotional trauma) 

6 = Helped disabled (during the evacuation) 

7 = Saw disabled (during the evacuation) 

8 = Helped injured 

9 = Saw injured 

99 = n/a 

22. B1993 - ‘1993 WTC Bombing Presence’ 

1 = Yes 

2 = Yes, prepared since (evacuation packs) 

3 = Yes, reason evacuated early 

4 = Yes, reason stayed 

5 = No 

6 = 1993 bombing in the back of their mind but were probably not there at the time 

99 = n/a 

23. DELAY - ‘Reason for Delay in Evacuation’ 

1 = Decide to stay 

2 = Activity to complete before leaving (search floor, secure document, made calls, 

instruct others) 

3 = Went Up/Return 

4 = Stuck or trapped 

5 = Help others, disabled or injured/Being helped 

6 = Told to stay 

99 = n/a 

24. LPHONE - ‘Location when Evacuee Made Phone Call’ 

1 = Office 

2 = Other floor 

3 = Stairs 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–35

4 = Outside 

5 = Multiple locations 

99 = n/a 

25. WPHONE - ‘Recipient of Evacuee Phone Call’ 

COLUMN CHECKED OFF FOR EACH GROUP MENTIONED 

1 = Family and friends (spouse, parents, home) 

2 = Colleague or boss 

3 = Authorities (building security, 9-1-1) 

99 = n/a 

26. REST - ‘Rest during Evacuation’ 

1 = T1 Lower (basement-42) in stairs 

2 = T1 Lower (basement-42) on a floor 

3 = T1 Mid (43-76) in stairs 

4 = T1 Mid (43-76) on a floor 

5 = T1 Upper (77-110) in stairs 

6 = T1 Upper (77-110) on a floor 

7 = T1 in stairs, level not specified 

8 = T1 location not specified 

9 = T1 mezzanine, lobby, concourse 

10 = T2 Lower (basement-42) in stairs 

11 = T2 Lower (basement-42) on a floor 

12 = T2 Mid (43-76) in stairs 

13 = T2 Mid (43-76) on a floor 

14 = T2 Upper (77-110) in stairs 

15 = T2 Upper (77-110) on a floor 

16 = T2 in stairs, level not specified 

17 = T2 location not specified 

18 = T2 mezzanine, lobby, concourse 

19 = Outside 

99 = n/a 
Appendix N 

N–36

27. OBSTCN - ‘Obstructions Encountered During Evacuation’ 

COLUMN CHECKED OFF FOR EACH OBSTRUCTION MENTIONED 

1 = Door Jam (locked or jammed) 

2 = Debris (wall falling, floor collapse, material damaged) 

3 = Smoke 

4 = No power 

5 = Smell (of fuel) 

6 = Water 

7 = Fire 

8 = Crowd, disabled, injured 

9 = Trapped by building rubble 

99 = n/a 

28. TMOUT - ‘Time Evacuee Exited Building’ 

1 = T1: 8:48-9:02 

2 = T1: 9:03-9:30 

3 = T1: 9:31-9:58 

4 = T1: 9:59-10:27 

5 = T1/T2: 10:28+ 

6 = T2: 8:48-9:02 

7 = T2: 9:03-9:30 

8 = T2: 9:31-9:58 

99 = n/a 

29. KNWSIT - ‘Evacuee’s Knowledge of the Situation in the Initial Moment’ 

1 = High (terrorism/plane attack/ T2 collapsed/saw plane approaching/hitting building) 

2 = Moderate (fire/bomb/earth quake/serious emergency/speculated plane/rumors) 

3 = Low (reason for evacuation unknown or limited) 

99 = n/a 

30. SRSNSS - ‘Level of Seriousness to Themselves in the Initial Moment’ 

1 = Very serious (fear, scared, want to get out ASAP) 

2 = Somewhat serious (worried, did not know what was happening) 

3 = Not serious (not concerned) 

99 = n/a 
Interim Report on Analysis of First-Person Accounts from Survivors of WTC 

N–37

31. SOINFL - ‘Social Influence on Evacuee’s Decisions’ 

1 = Authority figure (boss, supervisor, manager) 

2 = Coworkers/Group influence 

3 = Survivor took leadership role 

4 = Boss and group influence 

99 = n/a 

32. TCINFL - ‘Technological Influence on Knowledge during Evacuation’ 

1 = Cell phone 

2 = Blackberry, Text pager (deaf) 

3 = TV, radio 

4 = Walkie Talkie 

99 = n/a 

33. PERCEP - ‘Perception of Others During Evacuation’ 

COLUMN CHECKED OFF FOR EACH PERCEPTION MENTIONED 

1 = Calm/Orderly (civil, supportive, chatty, composed) 

2 = Momentarily Panicked (running, pushing, shoving) 

3 = Upset (crying, shouting, fearful, anxious) 

4 = Helpful (assisting others) 

99 = n/a 
Appendix N 

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O–i

TABLE OF CONTENTS - Appendix O

List of Figures.........................................................................................................................................O–iii

List of Tables .......................................................................................................................................... O–iv

Appendix O 

Interim Report on Telephone Interviews ............................................................................. O–1

O.1 Scope and Objective ................................................................................................................... O–1

O.2 Methodology............................................................................................................................... O–1

O.2.1 Sampling Frame............................................................................................................... O–2

O.2.2 Tracking and Screening the Sample ................................................................................ O–2

O.2.3 Design Parameters........................................................................................................... O–2

O.2.4 Expected Dispositions ..................................................................................................... O–3

O.2.5 Reserve Sample ............................................................................................................... O–3

O.2.6 Disproportionate Allocation ............................................................................................ O–3

O.2.7 Final Sample Disposition Analysis.................................................................................. O–3

O.3 Telephone Interview Schedule.................................................................................................... O–5

O.3.1 Preparedness and Training .............................................................................................. O–6

O.3.2 Initial Experience on September 11, 2001....................................................................... O–6

O.3.3 Interim Experience on September 11, 2001 .................................................................... O–7

O.3.4 Evacuation Experience on September 11, 2001 .............................................................. O–7

O.3.5 Respondent Background.................................................................................................. O–8

O.4 Preliminary Results..................................................................................................................... O–8

O.4.1 Response Rate Analysis .................................................................................................. O–8

O.4.2 Initial Building Populations............................................................................................. O–9

O.4.3 Occupant Characteristics ............................................................................................... O–11

O.4.4 Previous Experience ...................................................................................................... O–13

O.4.5 Preparedness and Training ............................................................................................ O–13

O.5 Summary................................................................................................................................... O–14 

Attachment 1

CATI Advance Letter to Occupants.................................................................................... O–19 

Attachment 2

Oral Consent Statement ...................................................................................................... O–21 
Appendix O 

O–ii

Attachment 3

Telephone Interview Instrument......................................................................................... O–23 
O–iii

LIST OF FIGURES

Figure O–1. Employment start date at WTC......................................................................................... O–12
O–iv

LIST OF TABLES

Table O–1. Disposition of the CATI sample and the total sample by tower. ........................................ O–4

Table O–2. Summary disposition rates by tower................................................................................... O–5

Table O–3. Response rate analysis for WTC 1...................................................................................... O–9

Table O–4. Response rate analysis for WTC 2...................................................................................... O–9

Table O–5. Reports of WTC decedents. .............................................................................................. O–10

Table O–6. Occupancy estimates on September 11, 2001, by tower................................................... O–11

Table O–7. Age for WTC 1 respondents. ............................................................................................ O–11

Table O–8. Age for WTC 2 respondents. ............................................................................................ O–12

Table O–9. WTC fire drills in 12 months prior to September, 11, 2001. ........................................... O–13
O–1

Appendix O

INTERIM REPORT ON TELEPHONE INTERVIEWS

O.1 SCOPE AND OBJECTIVE 

Interviews with survivors of the World Trade Center (WTC) attacks were conducted using three methods: 

telephone interviews, face-to-face interviews, and focus groups. This appendix will review four aspects 

of the telephone interviews: methodology, sample disposition, telephone questionnaire, and preliminary 

results of the telephone interviews for the pre-September 11, 2001, data. Significant additional analysis 

will be completed over the next several months to develop as clear an understanding as possible of the 

evacuation of WTC 1 and WTC 2 on September 11, 2001. These findings will be enhanced and 

compared with findings from many other sources, including face-to-face interviews, focus groups, 

published accounts (see Appendix N of this report for a discussion of published accounts analysis), 9-1-1 

records, and other materials. 

The multimethod approach was selected for several reasons. First, multiple methods increase confidence 

in the conclusions and findings when more than one method arrives at the same conclusions. Second, the 

multiple objectives of the investigation mandated complementary approaches to accomplish all the goals. 

In other words, it is difficult to establish a scientific foundation for general findings while also broadly 

investigating and establishing new facts and discovering unique events using only one method. Finally, 

concerns associated with the time latency since September 11, 2001, suggest the use of different 

approaches and techniques in order to increase memory recall and accuracy. 

The telephone interview questions and protocols met all Federal requirements regarding the Common 

Rule for the Protection of Human Subjects, including Institutional Review Board (IRB) and National 

Institute of Standards and Technology (NIST) approvals. Further, the telephone interview questions met 

the requirements of the Paperwork Reduction Act, subject to Office of Management and Budget (OMB) 

approval number 0693-0044. 

O.2 METHODOLOGY 

The survey objectives of the telephone interviews called for collecting 800 computer assisted telephone 

interviews (CATI) of persons occupying either of the two WTC towers (WTC 1 and WTC 2) at the time 

of the terrorist attacks on September 11, 2001. The sample size of 800 and allocation of n=400 to each 

tower were determined to simultaneously maximize the statistical precision within each tower. Primary 

statistical analyses are in the form of tabulations and linear statistics (e.g., reporting of percentages and 

average/means). Estimates of percentages from tower-specific survey data (at n=400) exhibit sampling 

errors no greater than 2.5 percentage points, and 95 percent confidence intervals of percentages are no 
Appendix O 

O–2

greater than +/- 5 percentage points. This level of precision is more than adequate for examining 

characteristics of occupants and egress attributes.1

Attempts were made to equally divide the respondents among WTC 1 and WTC 2 occupants (i.e., 

n=400 occupant interviews from each tower). Within each of the WTC buildings, independent 

proportionate stratified samples of survivors were drawn. In other words, each occupant of a particular 

tower had an equal probability of being selected. 

O.2.1 Sampling Frame 

The sampling frame (i.e., the list from which the sample was drawn) consisted of the names of occupants 

from badge lists of WTC 1 and WTC 2. All occupants of the WTC were required to provide personal 

data in support of issuing badges to clear through the security station at the entrance of each tower. The 

badge lists were provided to NIST by the Port Authority of New York and New Jersey. The lists provide 

name, floor of occupancy, employer, and social security number, the only available means of uniquely 

identifying individuals. 

O.2.2 Tracking and Screening the Sample 

The badge lists contained September 11, 2001, occupants, occupants who were absent on the day of the 

attacks, decedents, former occupants, and nonperson listings (false names used in sample testing). This 

means that a screening effort was needed to identify “eligible” badge list members––namely, those who 

were inside WTC 1 or WTC 2 during the attacks. Moreover, the absence of telephone numbers for the 

badge holders on the list necessitated a tracking/locating effort. The primary tracking mechanism was to 

search public databases using commercially available batch matching and Web-based search utilities. 

This necessitated a large sample to generate the 800 completed interviews. 

O.2.3 Design Parameters 

The number of occupant selections drawn into the sample was contingent on four key design parameters: 

• The percentage of individuals from badge listings for whom a working telephone number 

could be found (initial estimate: 80 percent tracking success) 

• The percentage of badge listings that corresponded to a surviving WTC 1 or WTC 2 occupant 

on September 11, 2001 (initial estimate: 14 percent) 

• The cooperation rate for screening the occupants (initial estimate: 65 percent) 

 

1

 Multivariate modeling such as correlation analyses, multiple linear regressions, and path analyses, are also a prominent part of 

the survey analyses. Like the tabulations, these analyses are being conducted independently by tower. A sample size of n=400 

per tower provides more than ample statistical power for the F tests used to determine the significance of the regression models 

(i.e., testing the null hypothesis that the ratio of explained variance to error/residual variance is equal to zero). For instance, in 

a multiple regression analysis featuring 20 independent variables, the sample size of 400, and 0.05 level of significance (Type I 

error), the power of the F test to detect an r2 statistic (i.e., proportion of explained variance) of 0.06 is just over 81 percent. See 

also Chapter 9 of Cohen, J., 1988, Statistical Power Analysis for the Behavioral Science, Lawrence Erlbaum Associates, Inc., 

Hillsdale, N.J. Multivariate modeling results will be presented at a later date. 
 Telephone Interviews 

O–3

• The interview response rate among September 11, 2001 survivors (initial estimate: 

50 percent) 

O.2.4 Expected Dispositions 

In planning the CATI survey, a number of design parameters needed to be quantified in order to 

determine the number of persons to draw from the badge list. The expected disposition of the sample was 

developed using the parameters defined in the aforementioned paragraph. A total sample of 

22,735 persons from the badge list was needed to generate the desired 800 completed interviews. The 

expected disposition by tracking efforts, screening, and interviewing are discussed later. 

O.2.5 Reserve Sample 

A reserve sample of about 14 percent (or about n=3,265) was added in the event additional sample size 

was needed due to unanticipated circumstances (e.g., the eligibility rate is lower than anticipated). This 

brought the total sample size to 26,000. The reserve was held “in reserve” while the main sample was 

worked. Working the main sample allowed preliminary estimates of all design parameters to be 

monitored so that an informed decision could be made on the necessity of releasing none, some, or all of 

the reserve. 

O.2.6 Disproportionate Allocation 

The badge list contained different counts of persons from each tower, yet our sample design called for 

equal samples to be drawn from the collections of badge holders in WTC 1 and WTC 2. Thus, a 

disproportionate design (across tower strata) was employed. But within a tower, independent 

proportionate samples were drawn using stratification by floor (within tower), employer (within floor) 

and last name (within employer). This served to increase the statistical precision of the tower-specific 

samples. 

Thus, equal-sized samples of 13,000 selections were drawn from each of WTC 1 and WTC 2 badge lists. 

Each tower-specific sample was partitioned into 20 random replicates (comprising 5 percent of the total), 

and the reserve sample was determined by the last several random replicates for each tower. It is 

important to note that all badge holders from WTC 1 floors 92 and above were omitted from sampling 

because there were no survivors from those floors. 

O.2.7 Final Sample Disposition Analysis 

A total sample of 26,000 was drawn, comprising 13,000 names for each tower. Table O–1 summarizes 

the final disposition of the CATI sample and the total (locating) sample. The table is comprised of two 

sets of rows. The top set pertains to the CATI sample and represents those sample persons for whom an 

initial telephone number was identified prior to commencing the CATI survey operations. The bottom set 

of rows with the heading “Total Sample Disposition” represents the results of our locating/tracking effort 

used to identify usable telephone numbers associated with the sample subjects. (Recall that only name, 

SSN and employer were available; no other contact information was readily available). 
Appendix O 

O–4

Table O–1. Disposition of the CATI sample and the total sample by tower. 

CATI Disposition WTC 1a WTC 2a Total % Distn 

 Interview 427 376 803 4.0 % 

 Partial interview 47 37 84 0.4 % 

 9/11 decedent 20 40 60 0.3 % 

 Other decedent 49 39 88 0.4 % 

 Not eligible 3,712 3,752 7,464 37.5 % 

 Language barrier 135 129 264 1.3 % 

 Eligible refused to interview 138 139 277 1.4 % 

 Other refusal 224 181 405 2.0 % 

 Respondent not interviewed 247 168 415 2.1 % 

 Can’t contact/locate respondent 4,987 5,076 10,063 50.5 % 

CATI total 9,986 9,937 19,923 100.0 % 

 Total sample disposition: WTC 1 WTC 2 Total % Distn 

 Found initial telephone number 9,986 9,937 19,923 76.6 % 

 Unable to find a telephone number 3,014 3,063 6,077 23.4 % 

Sample total 13,000 13,000 26,000 100.0 % 

a. Table data are unweighted. Tower location as indicated in the badge list and may differ from 

reported tower location. 

The bottom set of rows shows that telephone numbers were identified for just over three quarters 

(76.6 percent) of the sampled subjects. Moreover, this rate was fairly uniform across towers. The 

19,923 individuals with an initial telephone number were then loaded into the CATI sample management 

system for calling. Ultimately, all reserve samples were used in the telephone survey. In the initial 

design parameters, it was assumed that 82 percent of the subjects would be locatable. While 76.7 percent 

is close, many of the numbers were obsolete (e.g., disconnect, wrong number) and necessitated additional 

tracking during CATI operations. Ultimately, by the end of data collection, only half the sample 

(49.5 percent) represented confirmed contacts with subjects. 

The top set of rows in Table O–1 presents the final disposition of the sample by tower as well as for the 

overall sample. Several statistics in the percentage distribution (rightmost) column are notable. First, we 

were unable to contact subjects for half the sample (50.5 percent), due either to failures to answer the 

phone, answering machines, unusable numbers (e.g., wrong number, disconnected, business), etc. Most 

of these telephone numbers represent “unlocatable” subjects––subjects for whom the initial telephone 

number was incorrect. It bears reiterating that substantial additional research during CATI operations was 

conducted using powerful subscription-based Web-based search engines. Unfortunately, little 

information was available for these individuals. 

A second result of interest is the prevalence of ineligible subjects––those not in the building on the 

morning of September 11, 2001. An assessment of eligibility rates appears later in this appendix. A third 

result is the existence of decedents––some from the September 11 attack and others from causes not 

necessarily related to September 11, 2001 (e.g., cause unknown, natural causes). Most of the September 

11, 2001, decedents were encountered due to a difference in the full (formal) name of the subject and the 

name that appeared on the badge list (e.g., the badge list sometimes contained maiden names, middle 
 Telephone Interviews 

O–5

names, nicknames, misspelled first or last names, out-of-sequence names, titles, and so on). This 

impeded the ability to remove known decedents prior to calling. 

The outcome of CATI operations on the final outcome rates is presented by tower in Table O–2. The 

table shows screening rates, interview rates, and rates of eligible occupants (among those who responded 

to the screening questions). The first row shows that screening response rates were relatively uniform 

across towers at about 46 percent. A screening response rate of 65 percent had been planned. Similarly, 

interview response rates (among screened eligible subjects) were relatively stable across towers at about 

49 percent. This is consistent with the planned interview response rate of 50 percent. 

Table O–2. Summary disposition rates by tower. 

Disposition Rate WTC 1 WTC 2 Total 

 Screen 46.5 % 45.8 % 46.1 % 

 Interview 48.6 % 49.5 % 49.0 % 

 Eligibility 18.9 % 16.7 % 17.8 % 

 Overall 22.6 % 22.7 % 22.6 % 

Note: Definitions for “Rates” consistent with American Association of 

Public Opinion Research (AAPOR) Standards, which may be found at 

http://www.aapor.org/pdfs/standarddefs2004.pdf. 

The eligibility rates were higher than expected – about 18 percent overall compared to the 14 percent 

expected. The eligibility rate among WTC 1 subjects was slightly higher than those of WTC 2. 

However, the overall response rates are essentially uniform across towers, at 22.6 percent. 

O.3 TELEPHONE INTERVIEW SCHEDULE 

The telephone interview was conducted by a trained interviewer using a computer program which 

provides questions and answer categories for the interviewer. Prior to calling, subjects received a letter 

that outlined the scope and purpose of the investigation, the purpose of the interview, and the telephone 

call that came several days later. A full informed consent statement appeared in the letter, as well. A 

copy of the letter can be found in Attachment 1 of this appendix. 

When interviewers reached the subjects by telephone, the respondents were provided a description of the 

survey, the confidentiality of responses, the length of the interview, and the voluntary nature of 

participation. They were then asked if they wished to participate, thereby obtaining oral informed 

consent. The full text of the informed consent statement appears after the advance letter as Attachment 2. 

The telephone interview instrument, Attachment 3 at the end of this appendix, includes the questions, 

variable names, response options, and skip patterns directly from the computer program used by the 

interviewers. Variable names are used as shorthand for subsequent data analysis. Questions had a variety 

of response option categories: multiple choice, interval, Likert scale, or open-ended. Open-ended 

responses were minimized where possible due to the analysis burden and the fact that face–to–face 

interviews are also being conducted. Skip patterns reduce burden on the respondent by skipping 

questions that would not apply to a particular respondent. For example, a respondent would not be further 

questioned about fire drills if they did not receive fire drill training. Subsequent discussion of the 

questions indicates whether a respondent was read a list of choices or was expected to give a free 

response. 
Appendix O 

O–6

The interview was designed with five primary groups of questions, covering emergency training and 

preparedness, three stages of evacuation experience, and background information about the respondent. 

O.3.1 Preparedness and Training 

The first group of questions served to measure the extent to which an occupant had any special level of 

knowledge about the building, other than what would be obtained by performing their job. The most 

prevalent special knowledge would be formal evacuation training, or fire drills. If a respondent indicated 

that they participated in evacuation training during the 12 months prior to September 11, 2001, further 

questions were asked about the content of the training. The occupant’s understanding of the emergency 

procedures, or the way it was ‘supposed to go,’ was also measured. Next, a Likert Scale2

 measured the 

usefulness of the evacuation training in the context of their egress experience on September 11, ranging 

from very helpful to very unhelpful. Finally, the respondent was asked whether he or she knew that there 

was a floor warden for their floor. 

O.3.2 Initial Experience on September 11, 2001 

The second group of questions covered the first moments of September 11, 2001, as experienced by the 

respondent, also known as the initial awareness period. How a person first became aware that something 

was not normal, whether in their building or the neighboring building, may have influenced subsequent 

decisions. Examples of awareness channels may include sensory perception, such as feeling, hearing, or 

seeing the building shake, seeing or smelling fire or smoke, or may include a conversation with a person 

inside or outside the WTC complex. Next, the respondent was asked to provide context to the initial 

moment of awareness. Context was first created by identifying what activity the respondent was 

performing. Activities may include, but are not limited to, working, conversing with coworker(s), eating, 

or participating in a meeting. The respondent was then asked to recall the number of other people they 

were with at the first moment of awareness. People in groups often defer to group decisions rather than 

making their own evacuation decisions. Next, a list of observations was read aloud and the respondent is 

asked to indicate whether they noticed the event during the period of initial awareness. These events 

included smoke, fire, fireballs, collapsed walls, jet fuel, severely or fatally injured people, sprinklers 

going on, fire alarm sounding, power outage or flickering lights, fallen ceiling tiles, and extreme heat. 

The event proximity was probed for every affirmative response to determine whether the observed event 

was in the immediate area or outside the building. If no affirmative responses were indicated, the 

respondent was asked whether they observed any disaster related events not previously mentioned. 

Finally, the extent of any injuries to the respondent or those in the immediate area was ascertained, as 

well as whether the respondent felt that their life or the lives of other people were in danger. 

 

2

 A Likert Scale measures the degree to which the respondent agrees or disagrees with a statement. In this case, the scale 

measured helpfulness, including very helpful, helpful, unhelpful, and very unhelpful. A neutral response was not included. 
 Telephone Interviews 

O–7

O.3.3 Interim Experience on September 11, 2001 

The format of the interim experience group of questions mirrored the format of the initial awareness 

questions. The interim time period was defined as the time after initial awareness, but before the person 

entered a stairwell or elevator to leave the building. This time period may range from moments to tens of 

minutes. The objective of the interim period questions was to determine what motivated/forced people to 

either immediately evacuate or delay their evacuation by some period of time. 

Information about the nature of the event often forms the basis for decision-making during the interim 

period. Many people may have found the environmental cues from the initial awareness period sufficient 

to initiate an immediate evacuation. Others may have required additional information in order to feel 

comfortable leaving the workplace. Occupants could have obtained information in two ways: passively 

and actively. Passive information is information received without seeking it out. In other words, the 

information was received regardless of whether the person felt it was needed. Active information is 

information which the respondent actively seeks and considers important with respect to their decision to 

evacuate. The respondent was first asked whether they received any additional information about the 

event during the interim period. If so, the source (who), the nature (what), and the channel (how) of the 

information was probed. Next, additional information sought out by the respondent was probed, 

including the source, nature, channel, and whether the process was successful in gathering additional 

information. 

The perception of risk to the respondent’s life, as well as the lives of others was asked in the same way as 

during the initial period, in order to determine whether the sense of risk was increasing or decreasing over 

time. The interviewer probed about the activities of other people in the proximity of the respondent, 

which may influence the respondent’s subsequent choices. Whether people began evacuating prior to the 

respondent was specifically asked. Next, the respondent was asked about the activities they undertook 

during the interim period, as well as activities that they wanted to do but could not. These activities 

included work-related actions, such as saving files or shutting machines down; personal actions, such as 

gathering belongings or calling people; or emergency-related actions, such as fighting fires/smoke, and 

searching for or helping others. If a respondent was unable to accomplish an action, the action and the 

reason for being prevented from accomplishing the action was gathered. 

As with the initial period, any observations of building damage or distress were collected. If the 

respondent received help in any way before initiating evacuation, the nature and source of the assistance 

was determined. The respondent was asked what the primary cue was which initiated their evacuation on 

September 11 and how many minutes passed before they started evacuating. Finally, the respondent was 

asked whether anything prevented them from evacuating sooner than they reported. 

O.3.4 Evacuation Experience on September 11, 2001 

The next group of respondents completed the questions about the September 11, 2001, evacuation 

experience and focused on time spent in the stairwell and/or elevator(s). The respondent was first asked 

whether they began their evacuation alone or with other people. Which stairwell (or elevator) the 

respondent entered was collected as either the stair identification letter (A, B, or C) or the geographic 

location, if known. Knowing where the stairwell emptied out at the bottom may also narrow down which 

stairwell was used, which was collected near the end of this group of questions, [Stairs A/C (44 in. wide) 
Appendix O 

O–8

emptied out to the upper, Mezzanine level, while Stair B (54 in. wide) went to the lower, Concourse 

level]. Next, the respondent’s rationale for using a particular stairwell was probed. The respondent was 

then asked whether they left the stairwell or turned back for any reason during the evacuation and, if so, 

why? 

Some events and features of the stairwells aided the progress of the evacuation, while other features 

constrained the progress of the evacuation. The following features or events were identified to the 

respondents, who were asked to indicate whether it acted as an aid to their egress: instructions or 

assistance from their floor warden, a police office, or fire fighter, support/encouragement from others, exit 

signage, and photoluminscent paint. The following items were identified to determine whether they 

served to constrain the evacuation: crowded stairwells, counterflow (people moving up the stairs, against 

the flow of occupants), disabled or injured people being taken down the stairwell, locked doors, poor 

lighting, confusing or missing signage, and lack of clear instructions. 

As with the initial and interim time periods, environmental cues related to fire smoke, jet fuel, and other 

disaster-related observations were probed, as well as whether the observation was in the immediate area 

or outside the tower. The final question about the respondent’s own evacuation estimated the elapsed 

time from entering the stairwell until they left the building. A concluding evacuation question determined 

whether they knew why someone on their floor did not survive the WTC attack, if applicable. 

O.3.5 Respondent Background 

The final group of questions explored the background of the respondent relevant to evacuation. The first 

question identified any preexisting disabilities or injuries which made evacuation more difficult. The 

respondent’s age, gender, and primary language were collected. If the respondent was working in the 

building prior to 1993, they were asked whether they were present during the February 26, 1993 bombing. 

If so, respondents were asked questions about their evacuation experience. 

The interview concluded with an open-ended opportunity for the respondent to say anything additional 

about their evacuation experience on September 11, 2001. Respondents who indicated that they had a 

disability, were near the floors of impact, observed fire, smoke, or fireballs in their immediate area, or had 

a role of building responsibility on September 11, 2001, were asked if they would be willing to participate 

in a follow-up face-to-face interview. 

O.4 PRELIMINARY RESULTS 

The following section is a preliminary analysis of the telephone interview data. For this interim report, 

only pre-September 11, 2001 questions, or occupant background, preparedness, and training data, are 

analyzed and presented. Data related to September 11, 2001, evacuation experiences are currently being 

analyzed in the context of other data, such as face-to-face interviews and 9-1-1 tapes. 

O.4.1 Response Rate Analysis 

The response rate analysis of the telephone interview sample indicated an inverse relationship between 

floor height and the rate of response in WTC 1, as shown in the last column of Table O–3. The 

nonresponse weight adjustment is the inverse of the overall response rate. For example, the inverse of 
 Telephone Interviews 

O–9

25.3 percent is 3.95. In general, the weight adjustment for WTC 1 indicates that representative results 

should reflect that a single interview with a respondent high in the building is representative of more 

occupants than a single interview with a person lower in the building. 

Table O–3. Response rate analysis for WTC 1. 

Floor Stratum 

Number 

of 

Selections 

Number 

of 

Interviews Screen Eligibility Interview Overall 

Non-response 

Weight 

Adjustment 

1 to 42 4,464 256 46.2 % 22.6 % 54.8 % 25.3 % 3.95 

43 to 75 3,714 137 48.6 % 16.6 % 45.8 % 22.3 % 4.49 

76 to 92 1,802 34 42.7 % 14.7 % 30.1 % 12.9 % 7.78 

Floor missing 6 0 50.0 % 0.0 % N/A N/A 

Total 9,986 427 46.5 % 18.9 % 48.6 % 22.6 % 

While a similar analysis of telephone interview response rates for WTC 2 (shown below in Table O–4) 

does not indicate a significant need to weight the results, it is a conservative assumption to be consistent 

with WTC 1 analysis and the results will be weighted. 

Table O–4. Response rate analysis for WTC 2. 

Floor 

Stratum 

Number 

of 

Selections 

Number 

of 

Interviews Screen Eligibility Interview Overall 

Non-response 

Weight 

Adjustment 

1 to 42 4,339 143 44.8 % 14.8 % 49.7 % 22.3 % 4.49 

43 to 75 3,187 134 45.0 % 17.7 % 52.8 % 23.8 % 4.21 

76 to 110 2,203 94 48.3 % 19.5 % 45.2 % 21.8 % 4.58 

Floor missing 208 5 50.5 % 9.5 % 50.0 % 25.2 % 3.96 

Total 9,937 376 45.8 % 16.7 % 49.5 % 22.7 % 

All subsequent telephone interview data analysis will thus reflect weighting of the results in order to more 

accurately generalize the results. By convention, when a sample number is indicated (n = ), the sample 

number will be the actual number of responses. Where percentages are indicated, however, the 

percentages were weighted to allow for generalization, unless otherwise indicated. 

O.4.2 Initial Building Populations 

The total building population is the sum of survivors and decedents. At the time of this report, the City of 

New York has officially determined 2,749 people to be killed at the WTC on September 11, 2001; no 

official breakdown of where people were killed presently exists. While an analysis of this issue by 

Dennis Cauchon,3

 a reporter for USA Today, in the months immediately following September 11, 2001, 

was remarkably complete, differences between his projections and the official numbers from the City of 

New York and other official sources exist. These differences are shown in Table O–5. For example, the 

number of first responders depends upon the definition of first responder. The City of New York 

published an occupational analysis of WTC decedents based upon a Census of Fatal Occupational Injuries 

 

3

 Cauchon, Dennis. ‘For many on September 11, survival was no accident.’ USA Today, December 20, 2001. 
Appendix O 

O–10

(U.S. Department of Labor, Bureau of Labor Statistics, in cooperation with the New York City 

Department of Health and Mental Hygiene and State and Federal agencies). Four hundred and thirtythree decedent’s occupations were listed as firefighting, police, or security. This number exceeds by 30 

the number of FDNY, NYPD, and PAPD reported killed. This may be attributable to private security 

forces present inside the towers on September 11, 2001, and/or first responders not employed by New 

York City or the Port Authority. NIST is attempting to resolve these differences in order to fully 

understand the initial building population. 

Table O–5. Reports of WTC decedents. 

Decedent 

Official 

Numbers 

USA 

Todaya

WTC 1 occupants 1,434 

At or above impact 1,360 

Below impact 72 

WTC 2 occupants 599 

At or above impact 595 

Below impact 4 

First responders (total) 433 b,c 479 

FDNY 343e 

NYPD 23f 

PAPD 

 403d

 37g 

UA 175 and AA 11 157d 157 

Uncertain location in towers 147 

Bystanders 10 

Total number of decedents 2,749b,h 2,826 

a. Cauchon, Dennis. ‘For many on Sept.11, survival was no accident.’ 

USA Today, December 20, 2001. 

b. Summary of Vital Statistics 2002: The City of New York. 

Bureau of Vital Statistics, New York City Department of Health and 

Mental Hygiene. December 2003. 

c. Table WTC 8: Occupation of Decedents. All decedents classified as 

‘protective service’ occupations, which includes firefighting, police, 

and guards. 

d. World Trade Center Building Performance Study. FEMA 403. 

May 2002. 

e. Increasing FDNY’s Preparedness (McKinsey Report). Available at: 

http://www.ci.nyc.ny.us/html/fdny/html/mck_report/index.shtml 

f. Available at http://www.ci.nyc.ny.us/html/nypd/html/memorial_01.html 

g. Available at: http://www.panynj.gov/AboutthePortAuthority 

/PortAuthorityPolice/InMemorium/ 

h. Does not include 10 airplane hijackers for whom the City has not 

issued death certificates. 

Using the known eligibility rates allows for a projection of the survivors of WTC 1 and WTC 2 present in 

the building at 8:46 a.m. on September 11, 2001. The analysisindicates that WTC 1 had approximately 

7,470 ± 750 surviving occupants, while WTC 2 had approximately 7,940 ± 920 occupants. Thus, the total 

population of survivors from both towers was 15,410 ± 1,180. Table O–6 summarizes the projection of 

population of WTC 1 and 2 on September 11, 2001. Pending resolution of decedent locations, the total 
 Telephone Interviews 

O–11

building population at the time of the first airplane impact was 17,440 ± 1,180, calculated using the 

building decedent locations reported by Cauchon. 

Table O–6. Occupancy estimates on September 11, 2001, by tower. 

WTC 1 WTC 2 Total 

Number in sampling frame 39,454a 47,608 87,062 

Survivor occupancy rate 18.9 % 16.7 % 17.7 % 

Estimated total population of survivors 7,470 7,940 15,410 

Statistical Precision Calculations 

Sample n 427 376 803 

Standard error (p)~ 1.90 % 1.92 % 1.36 % 

Standard error (total) 750 920 1,180 

Confidence limits at 5 % ±1,470 ±1,790 ±2,320 

Number of Occupant/Decedents 

Decedents 1,434b 599b 2,033 - 2,192c

Total Building Population 

 8,900 8,540 17,440 

a. Includes only occupants below floor 92. 

b.Calculated from Cauchon as 1,434 + 599. 

c. Calculated as 2,749 – 403 first responders – 157 airplane passengers.

O.4.3 Occupant Characteristics 

The results of the background analysis of the average WTC occupant are identical to the precision 

presented whether the data was weighted or unweighted. Occupants of the WTC towers were twice as 

likely to be male as female (65 percent male [n=284]) for WTC 1 and 69 percent [n=250] for WTC 2). 

As shown in Table O–7 and Table O–8 below, the average age of the occupants was mid-forties, with a 

range of people from their early twenties to mid-seventies. The vast majority of respondents (92 percent 

(n=739)) spoke English as their primary language, although no attempt was made to account for the fact 

that some telephone contacts ended with a language barrier and no interviews were conducted in any 

language other than English. 

Table O–7. Age for WTC 1 

respondents.a

N Valid 439 

Refuse 1 

Mean 45 

Median 46 

Minimum 22 

Maximum 73 

a. Mean and Median values are weighted. N, 

Min, and Max are unweighted.
Appendix O 

O–12

Table O–8. Age for WTC 2 

respondents.a

N Valid 361 

Refuse 2 

Mean 45 

Median 44 

Minimum 21 

Maximum 74 

a. Mean and Median values are weighted. N, 

Min, and Max are unweighted. 

Tenant and employee turnover at the WTC was not uncommon. Figure O–1 shows the reported start 

dates for respondents in WTC 1 and WTC 2. In WTC 1, 4 percent (n=18) of the occupants had worked in 

the building since 1975. Further, 25 percent (n=110) had been working in the building prior to the 1993 

bombing, although only 15 percent (n=64) of the WTC 1 respondents were present on February 26, 1993. 

For WTC 1, 67 percent (n=287) of the occupants had started working in the building in the last four years 

(1998–2001). The mean residence time in WTC 1 was over 5.6 years, while the median was 2 years. 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR STARTED AT WTC 1

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

Percent

197519761978197919801981198219831984198519861987198819891990199119921993199419951996199719981999

20002001

YEAR STARTED AT WTC 2

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

Percent

Percentages are weighted. 

Figure O–1. Employment start date at WTC. 

Occupant tenure in WTC 2 demonstrated a similar trend. While only one respondent had worked in the 

building since 1975, 25 percent (n=91) of the respondents had been working in the building prior to the 

1993 bombing (with 16 percent (n=59) present on the day of the bombing). Another 51 percent (n=185) 

started working in the building in the previous 4 years (1998–2001). The mean residence time in WTC 2 

(n=360) was 5.9 years, while the median was 3 years. 

Overall, 7 percent (n=56) had a formal responsibility or special knowledge about the building. These 

respondents were fire safety staff, floor wardens, searchers, building maintenance, or security staff. 

Approximately 13 percent (n=105) of the respondents were employed by the Port Authority, which may 

not imply a special knowledge of the building as some Port Authority employees had job duties related to 

functions outside the WTC. 
 Telephone Interviews 

O–13

Some 6 percent (n=52) reported having a limitation which impacted their ability to evacuate. These 

limitations included obesity, heart condition, needing assistance to walk, pregnancy, asthma, elderly, 

chronic condition, recent surgery or injury, and other. 

O.4.4 Previous Experience 

Whether an occupant had a previous evacuation experience may have affected the decisions an individual 

made during the September 11, 2001, evacuation. Further analysis will develop this hypothesis. Of the 

WTC 1 occupants present on September 11, 2001, 16 percent (n=64) were also present during the 

1993 Bombing. In WTC 1, 60 percent (n=38) of evacuees in 1993 reported that they evacuated 

immediately, 30 percent (n=20) reported that they waited to evacuate, and 9 percent (n=6) did not recall. 

Most (95 percent [n=53]) who were able to recall their 1993 evacuation decision felt that they made the 

right decision, while 5 percent (n=3) did not believe they made the right decision. 

Similarly, 16 percent (n=59) of WTC 2 evacuees on September 11, 2001, also evacuated in 1993. In 

WTC 2, however, only 75 percent (n=42) felt that they made the right decision in 1993, possibly due to 

the fact that many more waited to evacuate in 1993 in WTC 2 (69 percent (n=39)) than did so in WTC 1. 

Only 31 percent (n=17) who reported their decision evacuated immediately from WTC 2 in 1993, keeping 

in mind that the bomb had a more significant impact upon WTC 1 in 1993. 

O.4.5 Preparedness and Training 

Long a cornerstone of public policy on the emergency preparedness of office workers around the country, 

the Port Authority required tenants to conduct regular fire drills and appoint employee floor wardens and 

searchers. Overall, 66 percent (n=529) of WTC 1 and WTC 2 occupants reported participation in at least 

one fire drill in the 12 months immediately prior to September 11, 2001. Another 17 percent (n=139) 

reported that they did not participate in any fire drills in the 12 months prior to September 11, 2001, and 

17 percent (n=135) did not know. Fire drill participation rates were similar between the two towers, as 

shown in Table O–9. 

Table O–9. WTC fire drills in 12 months 

prior to September, 11, 2001. 

Number of Drills WTC 1a WTC 2a

None 18 % (n=78) 17 % (n=61) 

1 13 % (n=57) 8 % (n=29) 

2 21 % (n=90) 24 % (n=88) 

3 11 % (n=47) 15 % (n=53) 

4 10 % (n=44) 9 % (n=32) 

5 – 11 7 % (n=31) 9 % (n=32) 

12 or more 3 % (n=13) 4 % (n=13) 

Don’t know 18 % (n=80) 15 % (n=55) 

a. Percentages are weighted, n values unweighted. 

One of the goals of fire drill training is to make occupants aware of the location of the emergency exits. 

Of respondents who reported participation in a fire drill, 93 percent (n=490) were instructed about the 
Appendix O 

O–14

location of the nearest stairwell. However, of the respondents who reported being shown a stairwell, 

82 percent (n=432) did not enter or use the stairwell. Some 17 percent (n=92) reported that they did use 

the stairs during a drill, while approximately 1 percent (n=5) reported not knowing. Overall, more than 

half (51 percent (n=415)) of the occupants had never used a stairwell in WTC 1 or WTC 2 prior to 

September 11, while 48 percent (n=386) had used a stairwell. Two persons reported not knowing 

whether they had used the stairs previously. 

Another goal of the fire drills was to introduce the floor warden system and evacuation procedures. Most 

occupants (82 percent (n=528)) with fire drill training were aware that there was a floor warden for their 

floor. Approximately 70 percent (n=557) of all occupants reported that they were aware of the evacuation 

procedures. When asked what those evacuation procedures comprised, however, answers varied 

significantly, including: wait in hallway for further instructions; do not use elevators, use stairs; meet at a 

designated site outside the building for a head count; or proceed down (varied number of) flights of stairs 

and wait. Further analysis of the understanding and implementation of the emergency procedures is under 

way. 

O.5 SUMMARY 

Eight hundred and three occupants of WTC 1 and WTC 2 were interviewed by telephone. Sample 

disposition analysis indicated differential nonresponse, particularly for WTC 1. In other words, the closer 

the occupant was to the impact area in WTC 1, the more likely it was that they would choose not to 

complete the telephone interview. Telephone interview percentages were then weighted to adjust for this 

effect. 

On the morning of September 11, 2001, 17,440 people (± 1,180) were present at WTC 1 and WTC 2. 

This does not include first responders. The initial population of both towers was similar: 8,900 (± 750) in 

WTC 1 and 8,540 (± 920) in WTC 2. 

The average age of an occupant of the WTC towers was mid-forties. Two-thirds of WTC 1 occupants 

had started working in the building during the previous 4 years (1998–2001), while half of WTC 2 

occupants had begun working there during the same time period. Overall, 7 percent of occupants reported 

having special knowledge about the building, and 6 percent reported a preexisting limitation to their 

mobility. 

Of those present on September 11, 2001, 16 percent were also present during the 1993 bombing. Twothirds of occupants reported having participated in a fire drill in the 12 months immediately prior to 

September 11, while 17 percent reported that they received no training during that same period. Ninetythree percent of those participating in fire drills were instructed about the location of the nearest stairwell. 

Slightly over half of the occupants, however, had never used a stairwell at the WTC prior to 

September 11. 

Significant additional analysis is presently under way. It is particularly important that results of questions 

related to the events, observations, and activities within the towers on September 11, 2001, be analyzed 

within the context of the findings coming from face-to-face interviews, focus groups, and other data 

collection activities.
O–15

Attachment 1 

CATI ADVANCE LETTER TO OCCUPANTS

Dear [Name]: 

You are being asked to voluntarily participate in the federal investigation of the collapse of World Trade 

Center structures on September 11, 2001. The National Institute of Standards and Technology (NIST) is 

investigating the cause of the collapse of the World Trade Center towers on September 11 in order to 

improve the way that building professionals, emergency responders, and regulatory authorities prepare for 

and respond to future emergency events. 

Because you were an occupant of the WTC buildings, you have been identified as a person who can 

provide NIST with information critical to its investigation. Your cooperation with the investigation 

involves participating in a 20 minute telephone interview with a representative of our survey research 

contractor, Datasource. The purpose of the interview is to gather information about where you were in 

the WTC buildings at the time of the September 11 events, what you observed and experienced, and how 

you evacuated the building. 

You may also be asked to participate in a voluntary face-to-face interview. Participating in the telephone 

survey does not obligate you to participate in the face-to-face interview. 

NIST and its contractors NuStats and Datasource will keep the identity of all participating individuals as 

confidential as possible. To the extent permitted by law, no one other than NIST, authorized Federal 

officials, NIST contractors NuStats and Datasource, and Essex Institutional Review Board will have 

access to your identity. Access to identifying information will only be provided to staff members on an 

as-needed basis. Data will be reported in summary form. 

NIST is a non-regulatory agency within the U.S. Department of Commerce and is conducting this 

investigation under the authority of the National Construction Safety Team Act (P.L. 107-231). The 

investigation involves strict fact-finding. No part of the NIST Investigation report can be used in any suit 

or action for damages. For more information, see http://wtc.nist.gov. 

A representative of Datasource will phone you in the next week or two. Please be aware that he / she will 

want to conduct the interview at your convenience. If you agree to do the survey, you may choose not to 

answer any question. If you wish, you can choose to withdraw your responses at any time during the 

interview or at the end of the interview. 

If you have any questions or comments regarding your participation in the NIST investigation, please feel 

free to contact Dr. Johanna Zmud, NuStats project director, at 800-447-8287, ext. 2225 or Jason Averill, 

NIST project director, at 301-975-2585. If you have any questions about your rights as a participant or if 

you have any concerns, you may contact the Essex Institutional Review Board, Inc. (IRB), 121 Main 

Street, Lebanon, NJ; Phone: 908-236-7735. The IRB is a committee that has reviewed this research 

investigational plan to help ensure that your rights and welfare are protected and that the investigation is 

carried out in an ethical manner. 

Sincerely, 

NIST OFFICIAL 
Appendix O 

O–16

This page intentionally left blank. 
O–17

Attachment 2 

ORAL CONSENT STATEMENT

THE TELEPHONE INTERVIEWER USES THE FOLLOWING SEQUENCE OF STATEMENTS AND 

QUESTIONS TO EFFECT ORAL CONSENT PRIOR TO BEGINNING THE INTERVIEW: 

SUBJECT NAME: _______________________ 

Hi, may I please speak with <SUBJECT NAME>? 

YES, CONTINUE 1 

NO 2 SET CALLBACK 

Hi, my name is ______ and I am calling on behalf of the National Institute of Standards and Technology 

(NIST). NIST is conducting the federal investigation of the World Trade Center disaster. Information 

about the investigation is available at the website “wtc.nist.gov” or we can provide you a toll-free 

number to call. 

We are interviewing people about their experiences on September 11. We sent you a letter about the 

study informing you of our call. Did you receive the letter? 

YES 1 Æ ASK IF THERE ARE ANY QUESTIONS 

NO 2 Æ ASK IF THERE ARE ANY QUESTIONS 

SCREENER: 

First, I need to ask you a few questions because we want to speak to people who had certain types of 

experiences that may be especially helpful to NIST. For this study, we are conducting interviews with 

people who were in WTC 1 or WTC 2 during the September 11, 2001 attacks. 

(SCREEN) At the time of the attack, were you in WTC 1 or WTC 2 at the World Trade Center? 

YES 01 

NO 02 [THANK AND TERMINATE] 

RF 99 [THANK AND TERMINATE] 

(SCREEN) Which tower were you in? 

WTC1 01 

WTC2 02 

OTHER, specify 97 [THANK AND TERMINATE] 

RF 99 [THANK AND TERMINATE] 

(SCREEN) What floor were you on? 

<Enter floor number> 
Appendix O 

O–18

BASEMENT 990 

CONCOURSE/LOBBY 991 

PLAZA 992 

OTHER, SPECIFY 997 

DK 998 

RF 999 

PROGRAMMER NOTE: NEED CODED FLOOR NUMBER CATEGORIES FOR SAMPLE 

TRACKING: 

LOWER FLOORS (T1: BASEMENT – 42) 01 

LOWER FLOORS (T2: BASEMENT – 42) 02 

MIDDLE FLOORS (T1: 43 – 76) 03 

MIDDLE FLOORS (T2: 43 – 76) 04 

UPPER FLOORS (T1: 77 – 91) 05 

UPPER FLOORS (T2: 77 – 110) 06 

We would like you to participate in our study. Before we start, I’d like to read a statement to you about 

this study to help you decide if you wish to participate: 

In this study, we want to ask about when and how you left the tower you were in during the attack on 

9/11. The information you provide will help engineers and emergency planners to improve the safety and 

evacuation procedures for high rise buildings. The interview length is about 20 minutes and your 

participation is voluntary. Because this interview involves recalling a traumatic event, you may 

experience emotional discomfort. You are free to skip over any question you do not wish to answer. You 

may take a short break or stop the questions at any time. We can also provide you counseling referrals if 

you like. Your identity will be kept as confidential as possible. To the extent permitted by law, no one 

other than NIST, authorized Federal officials, NIST contractors NuStats and Datasource, and Essex 

Institutional Review Board will have access to your identity. There are no direct benefits to participants. 

If you have any questions or comments regarding your participation in the NIST investigation, you may 

contact Dr. Johanna Zmud, NuStats project director, at 800-447-8287, extension 2225. If you have any 

questions or concerns about your rights as a participant, you may contact the Essex Institutional Review 

Board at 908-236-7735. 

3a. Are you willing to participate? 

 YES, NOW 1 

 YES, LATER 2 [SET CALLBACK APPOINTMENT] 

 NO 99 THANK AND TERMINATE
O–19

Attachment 3 

TELEPHONE INTERVIEW INSTRUMENT

I would like to start by getting some background information. What year did you first start working at the 

World Trade Center? RANGE: 1975 – 2001 

$E 1975 2001 

DK 9998 

RF 9999 

«YRWRK» 

On September 11, 2001, were you in any of the following positions with the World Trade Center? 

PORT AUTHORITY STAFF 1 

FIRE SAFETY STAFF 2 

FLOOR WARDEN OR SEARCHER 3 

MAINTENANCE OR SECURITY STAFF 4 

NONE OF THESE 0 X 

DK 8 

RF 9 

«ROLES_01» 

«ROLES_02» 

«ROLES_03» 

«ROLES_04» 

During the year from September 11, 2000 to September 11, 2001, how many fire drills did you take part 

in at the World Trade Center? 

$E 0 99 

NONE 00 => SWLOC 

DK 98 => SWLOC 

RF 99 => SWLOC 

«FIRED» 

During these drills, were you ever instructed about the location of the emergency stairwell nearest to your 

office? 

YES 1 

NO 2 => SWLOC 

DK 8 => SWLOC 

RF => SWLOC 

«DEXIT» 

How many emergency stairwells were you shown? 

ONE 1 

TWO 2 
Appendix O 

O–20

THREE => LVFSW 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«HMEXT» 

«O_HMEXT» 

Before September 11, had you learned in other ways about the locations of the three emergency 

stairwells? 

YES 1 

NO 2 

DK 8 

RF 9 

«SWLOC» 

SKIP IF NO FIRE DRILLS 

=> USESW 

Else => +1 

if FIRED=00,98-99 

«SOUT1» 

During any of the fire drills, did you leave your floor using one of the stairwells? 

YES 1 

NO 2 => USESW 

DK 8 => USESW 

RF 9 => USESW 

«LVFSW» 

Which stairwells did you use? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSW1_01» 

«WHSW1_02» 

«WHSW1_03» 

«WHSW1_04» 

«O_WHSW1» 
Interim Report on Telephone Interviews 

O–21

Which side of the building was the stairwell located on? 

=> +1 

if NOT WHSW1=8 

NORTH 1 

SOUTH 2 

EAST 3 

WEST 4 4 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSL1» 

«O_WHSL1» 

Had you ever used any of the emergency stairwells prior to September 11? 

=> DHELP 

if LVFSW=1 

YES 1 

NO 2 => DHELP 

DK 8 => DHELP 

RF 9 => DHELP 

«USESW» 

SKIP FOR NO DRILLS AND NO USE OF STAIRWELLS 

=> AEVOF 

Else => +1 

if FIRED=00,98,99 AND USESW>1 

«SOUT2» 

Which stairwell did you use? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSW2_01» 

«WHSW2_02» 

«WHSW2_03» 

«WHSW2_04» 

«O_WHSW2» 

SKIP IF NO FIRE DRILLS 
Appendix O 

O–22

=> AEVOF 

Else => +1 

if FIRED=00,98-99 

«SOUT3» 

When you were evacuating on September 11, how helpful was your experience during these drills? 

=> +1 

if FIRED=00 

VERY HELPFUL 1 

SOMEWHAT HELPFUL 2 

SOMEWHAT UNHELPFUL 3 

VERY UNHELPFUL 4 

DK 8 

RF 9 

«DHELP» 

Prior to September 11, were you aware of the evacuation procedures for your floor? 

YES 1 

NO 2 => FLWAR 

DK 8 => FLWAR 

RF 9 => FLWAR 

«AEVOF» 

Prior to September 11, what was the evacuation procedure you were told to follow? 

LEAVE BUILDING IMMEDIATELY 1 

GO TO ELEVATOR LOBBY 2 

GO TO FLOORS UP OR DOWN 3 

GO TO ROOF 4 

STAY WHERE YOU ARE 5 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«EVACP» 

«O_EVACP» 

Did you know that there was a Floor Warden for your floor? 

=> +1 

if ROLES=1-4 

YES 1 

NO 2 

DK 8 
Interim Report on Telephone Interviews 

O–23

RF 9 

«FLWAR» 

The next questions ask about 3 different time periods. The first series of questions asks about when you 

first became aware that something had happened at the World Trade Center. This is a period of just a few 

seconds. The next series of questions asks about the time from when you first became aware that 

something had happened, to the time you first entered a stairwell or elevator to exit the building. The 

third series of questions asks about what happened during your evacuation, meaning the time from when 

you first entered a stairwell or elevator until you exited the tower. At the end of the interview, I will ask 

you if there is anything else about your experience on September 11 that you would like to contribute. 

CONTINUE 1 D 

«IFAWA» 

Now thinking back to the morning of September 11, how did you first become aware that something had 

happened at the World Trade Center? 

$E 1 9 

HEARD SOMETHING (BOOM, CRASH, EXPLOSION, 

BLAST, ROAR, RUMBLING, ALARM) 01 

SAW SMOKE OR FLAMES 02 

SAW DEAD BODIES 03 

SAW A PLANE 04 

SAW DEBRIS 05 

FELT SOMETHING (BUILDING MOVING, IMPACT, SHAKING, 

SWAYING, ROCKING, JOLT, EARTHQUAKE) 06 

FELL DOWN/FELL OFF CHAIR 07 

WARNED BY SOMEONE AROUND ME 08 

CONTACTED VIA PHONE 09 

CONTACTED VIA EMAIL 10 

PUBLIC ADDRESS SYSTEM 11 

NEWS MEDIA (TELEVISION, RADIO) 12 

OFFICE FURNITURE OR FIXTURES FALLING 13 

FURNITURE OR OTHER ITEMS FALLING OVER/DOWN 14 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«FAWAR» 

«O_FAWAR» 

What were you doing when you first became aware that something had happened to the World Trade 

Center? PROBE: Anything else? 

$E 1 9 

WORKING INDEPENDENTLY 01 

IN MEETING 02 
Appendix O 

O–24

ON PHONE 03 

CHECKING/WRITING EMAIL 04 

WAITING FOR ELEVATOR 05 

RIDING IN ELEVATOR 06 

CHATTING WITH COWORKERS 07 

EATING/HAVING COFFEE 08 

ENTERING BUILDING 09 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«ACTV1_01» 

«ACTV1_02» 

«ACTV1_03» 

«ACTV1_04» 

«ACTV1_05» 

«ACTV1_06» 

«ACTV1_07» 

«ACTV1_08» 

«ACTV1_09» 

«ACTV1_10» 

«O_ACTV1» 

At the moment when you first became aware that something had happened at the World Trade Center, did you 

notice any of the following? FOLLOW UP: Was that in your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the Tower

Smoke    

Fire or Flames    

Fireballs    

Collapsed walls    

Jet Fuel    

Severely or fatally injured people    

Sprinklers going on    

A fire alarm sounding    

Power outage or flickering lights    

Fallen ceiling tiles    

Extreme heat    

«NOT01_01» 

«NOT01_02» 

TIME PERIOD: 1 

Were there any disaster related events going on around you at this time? 

=> WHTW2 

if OR[NOT01-NOT11]=2-3 

YES 1 

NO 2 => WHTW2 

DK 8 => WHTW2 

RF 9 => WHTW2 

«OEVEN» 
Interim Report on Telephone Interviews 

O–25

TIME PERIOD: 1 

What was going on? 

ENTER RESPONSE 1 O 

DK 8 

RF 9 

«GOING» 

«O_GOING» 

TIME PERIOD: 1 

Were you still in<WHTOW>at this time? IF YES, SELECT APPROPRIATE CHOICE IF NO, ASK 

WHICH TOWER THEY WERE IN 

WTC 1 1 

WTC 2 2 

DK 8 

RF 9 

«WHTW2» 

TIME PERIOD: 1 

And were you still on the<WHFLO>floor at this time? RANGE: 1st - 110th FLOOR IF YES, 

SELECT/ENTER FLOOR IF NO, ASK WHICH FLOOR THEY WERE ON AND SELECT/ENTER IT 

$E 1 110 

BASEMENT 990 

CONCOURSE/LOBBY 991 

PLAZA 992 

IN ELEVATOR 993 

OTHER, SPECIFY 997 O 

DK 998 

RF 999 

«WHFL2» 

«O_WHFL2» 

TIME PERIOD: 1 

At the moment when you first became aware that something had happened to the World Trade Center, 

approximately how many people were with you? RANGE: 0 - 999 PEOPLE WE WANT THE 

NUMBER OF PEOPLE THAT WERE IN THE SAME LOCATION AS THE RESPONDENT. (IN 

THEIR LINE OF SIGHT) 

$E 0 999 

NONE 00 => YOUIN 

DK 98 => YOUIN 

RF 99 => YOUIN 

«PEOP1» 
Appendix O 

O–26

TIME PERIOD: 1 

Were any of these people injured at that time as a result of the event? 

YES 1 

NO 2 

DK 8 

RF 9 

«PEOIN» 

TIME PERIOD: 1 

Were you injured at that time, as a result of the event? 

YES 1 

NO 2 => ORISK 

DK 8 => ORISK 

RF 9 => ORISK 

«YOUIN» 

TIME PERIOD: 1 

Would you say your injury was a ... 

AN INJURY THAT DID NOT IMPACT YOUR ABILITY TO EVACUATE, 1 

AN INJURY THAT DID IMPACT YOUR ABILITY TO EVACUATE BUT 

WAS NOT LIFE THREATENING, OR 2 

A LIFE THREATENING INJURY 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«NATIN» 

«O_NATIN» 

TIME PERIOD: 1 

Still thinking about the moment when you first became aware that something had happened at the World 

Trade Center, did you believe that other people were in danger of being killed? 

YES 1 

NO 2 

DK 8 

RF 9 

«ORISK» 

TIME PERIOD: 1 

Did you believe you were in danger of being killed? 

YES 1 
Interim Report on Telephone Interviews 

O–27

NO 2 

DK 8 

RF 9 

«YRISK» 

TIME PERIOD: 2 

Now please think about the time period between when you first became aware that something had 

happened and when you first entered a stairwell or elevator to leave the tower. During this entire time 

period, were you given any additional information about what was going on? AFTER BECOMING 

AWARE OF THE EVENT, BUT BEFORE EVACUATION. 

YES 1 

NO 2 => SEEKI 

DK 8 => SEEKI 

RF 9 => SEEKI 

«GETIN» 

TIME PERIOD: 2 

Who gave you this information? PROBE: Anyone else? 

MANAGER/SUPERVISOR 1 

COWORKER INSIDE BUILDING 2 

FAMILY/FRIEND OUTSIDE BUILDING 3 

POLICE/FIREFIGHTER 4 

FLOOR WARDEN 5 

MEDIA PERSON (TV/RADIO) 6 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHINF_01» 

«WHINF_02» 

«WHINF_03» 

«WHINF_04» 

«WHINF_05» 

«WHINF_06» 

«WHINF_07» 

«O_WHINF» 

TIME PERIOD: 2 

What information did you get? PROBE: Any other information? 

INFORMATION ABOUT WHAT HAD HAPPENED 1 

INSTRUCTIONS TO LEAVE 2 

INSTRUCTIONS TO STAY 3 

OTHER, SPECIFY 7 O 

DK 8 X 
Appendix O 

O–28

RF 9 X 

«WHATI_01» 

«WHATI_02» 

«WHATI_03» 

«WHATI_04» 

«O_WHATI» 

TIME PERIOD: 2 

How did you get this information? PROBE: Any other way? 

FACE TO FACE 1 

TELEPHONE 2 

EMAIL/BLACKBERRY 3 

PA ANNOUNCMENT 4 

TV/RADIO 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«HOWGT_01» 

«HOWGT_02» 

«HOWGT_03» 

«HOWGT_04» 

«HOWGT_05» 

«HOWGT_06» 

«O_HOWGT» 

TIME PERIOD: 2 

And during this same time period, did you try to get additional information about what was going on? 

AFTER BECOMING AWARE OF THE EVENT, BUT BEFORE EVACUATION 

YES 1 

NO 2 => ORIS2 

TRIED, BUT WAS UNABLE TO GET INFORMATION 3 => ORIS2 

DK 8 => ORIS2 

RF 9 => ORIS2 

«SEEKI» 

TIME PERIOD: 2 

Who did you go to for this information? PROBE: Anyone else? 

MANAGER/SUPERVISOR 1 

COWORKER INSIDE BUILDING 2 

FAMILY/FRIEND OUTSIDE BUILDING 3 

POLICE/FIREFIGHTER 4 

FLOOR WARDEN 5 

MEDIA PERSON (TV/RADIO) 6 
Interim Report on Telephone Interviews 

O–29

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«GOINF_01» 

«GOINF_02» 

«GOINF_03» 

«GOINF_04» 

«GOINF_05» 

«GOINF_06» 

«GOINF_07» 

«O_GOINF» 

TIME PERIOD: 2 

What type of information did you try to find? PROBE: Anything else? 

INFORMATION ABOUT WHAT HAD HAPPENED 1 

INSTRUCTIONS TO LEAVE 2 

INSTRUCTIONS TO STAY 3 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHAI2_01» 

«WHAI2_02» 

«WHAI2_03» 

«WHAI2_04» 

«O_WHAI2» 

TIME PERIOD: 2 

How did you get this information? PROBE: Any other way? 

FACE TO FACE 1 

TELEPHONE 2 

EMAIL/BLACKBERRY 3 

PA ANNOUNCMENT 4 

TV/RADIO 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«HOWG2_01» 

«HOWG2_02» 

«HOWG2_03» 

«HOWG2_04» 

«HOWG2_05» 

«HOWG2_06» 

«O_HOWG2» 

TIME PERIOD: 2 
Appendix O 

O–30

And during the time between when you first became aware that something had happened at the World 

Trade Center and when you first entered the stairwell or elevator to leave the tower, did you believe that 

other people were in danger of being killed? AFTER BECOMING AWARE OF THE EVENT, BUT 

BEFORE EVACUATION 

=> YRIS2 

if ORISK=1 

YES 1 

NO 2 

DK 8 

RF 9 

«ORIS2» 

TIME PERIOD: 2 

During that time period, did you believe you were in danger of being killed? 

=> PEODO 

if YRISK=1 

YES 1 

NO 2 

DK 8 

RF 9 

«YRIS2» 

TIME PERIOD: 2 

During this time period, what were the people around you doing? PROBE: Were they doing anything 

else? AFTER BECOMING AWARE OF THE EVENT, BUT BEFORE EVACUATION 

$E 0 10 

NOONE AROUND/WAS ALONE 00 X 

TALKING TO OTHERS 01 

GATHERING PERSONAL/WORK ITEMS 02 

SEARCHING FOR OTHERS 03 

CALLING OTHERS 04 

FIGHTING FIRE/SMOKE 05 

LOCKING UP 06 

WORKING 07 

EVACUATING THE TOWER 08 

CRYING, RUNNING AROUND, IN SHOCK 09 

HELPING OTHERS 10 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«PEODO_01» 
Interim Report on Telephone Interviews 

O–31

«PEODO_02» 

«PEODO_03» 

«PEODO_04» 

«PEODO_05» 

«PEODO_06» 

«PEODO_07» 

«PEODO_08» 

«PEODO_09» 

«PEODO_10» 

«O_PEODO» 

TIME PERIOD: 2 

Did the people around you start evacuating before you did? 

=> DOBEF 

if PEODO=08 

YES 1 

NO 2 

DK 8 

RF 9 

«EVACB» 

TIME PERIOD: 2 

Did you do any of the following before starting your evacuation? 

$E 1 9 

TALK TO ANOTHER PERSON FACE TO FACE 01 

GATHER PERSONAL ITEMS 02 

TELEPHONE OTHER PEOPLE 03 

CONTINUE WORKING 04 

SAVE OR TRANSFER COMPUTER FILES 05 

SEARCH FOR OTHERS 06 

FIGHT FIRE OR SMOKE 07 

MOVE TO ANOTHER FLOOR 08 

HELP OTHERS 09 

LOGGING OFF/SHUTTING DOWN COMPUTER 10 

NONE OF THESE 11 X 

«DOBEF_01» 

«DOBEF_02» 

«DOBEF_03» 

«DOBEF_04» 

«DOBEF_05» 

«DOBEF_06» 

«DOBEF_07» 

«DOBEF_08» 

TIME PERIOD: 2 
Appendix O 

O–32

Did you do anything else during this time? 

ENTER RESPONSE 1 O 

NO OTHER ACTIVITIES 0 

DK 8 

RF 9 

«OACTI» «O_OACTI» 

TIME PERIOD: 2 

Before you began your evacuation, was there anything you wanted to do, but couldn't? 

YES 1 

NO 2 => SEE01 

DK 8 => SEE01 

RF 9 => SEE01 

«WANTD» 

TIME PERIOD: 2 

What was that? PROBE: Anything else? 

$E 1 7 

GATHER WORK ITEMS 01 

GATHER PERSONAL BELONGINGS 02 

CALL FRIEND/FAMILY MEMBER 03 

FIND FRIEND/COWORKER 04 

HELP FRIEND/COWORKER 05 

LOCK UP 06 

EVACUATE IMMEDIATELY 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WANAC_01» 

«WANAC_02» 

«WANAC_03» 

«WANAC_04» 

«WANAC_05» 

«WANAC_06» 

«WANAC_07» 

«WANAC_08» 

«O_WANAC» 

TIME PERIOD: 2 

Why couldn't you do that/those things? 

$E 1 9 
Interim Report on Telephone Interviews 

O–33

AFRAID 01 

LOCKED DOORS 02 

PHONE LINES DEAD 03 

INJURED 04 

EXIT BLOCKED 05 

TOO CROWDED 06 

TOLD TO STAY IN BUILDING 07 

TOLD TO LEAVE 08 

FATIGUE 09 

DISABLED 10 

SMOKE 11 

DAMAGE TO FLOOR 12 

WAS HELPING OTHERS 13 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«WHYNO_01» 

«WHYNO_02» 

«WHYNO_03» 

«WHYNO_04» 

«WHYNO_05» 

«WHYNO_06» 

«WHYNO_07» 

«WHYNO_08» 

«WHYNO_09» 

«WHYNO_10» 

«WHYNO_11» 

«WHYNO_12» 

«WHYNO_13» 

«WHYNO_14» 

«O_WHYNO» 

Still thinking about the time between when you first became aware that something had happened at the World Trade 

Center and when you entered the stairwell or elevator to leave the tower, did you notice any of the following? 

FOLLOW UP: Was that in your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the Tower

Smoke    

Fire or Flames    

Fireballs    

Collapsed walls    

Jet Fuel    

Severely or fatally injured people    

Sprinklers going on    

A fire alarm sounding    

Power outage or flickering lights    

Fallen ceiling tiles    

Extreme heat    
Appendix O 

O–34

«SEE01_01» 

«SEE01_02» 

TIME PERIOD: 2 

Were there any disaster related events going on around you at this time? 

=> EVACF 

if OR[SEE01-SEE11]=2-3 

YES 1 

NO 2 => HELPY 

DK 8 => HELPY 

RF 9 => HELPY 

«ODISE» 

TIME PERIOD: 2 

What was going on? 

ENTER RESPONSE 1 O 

DK 8 

RF 9 

«GOIN2» 

«O_GOIN2» 

TIME PERIOD: 2 

Were you still on the<WHFL2>floor at this time? RANGE: 1st - 110th FLOOR IF YES, 

SELECT/ENTER FLOOR IF NO, ASK WHICH FLOOR THEY WERE ON AND SELECT/ENTER IT 

$E 1 110 

=> +1 

if (AND[SEE01-SEE11]=1) AND PEODO>0 AND PEODO<98 

BASEMENT 990 

CONCOURSE/LOBBY 991 

PLAZA 992 

ELEVATOR 993 

OTHER, SPECIFY 997 

DK 998 

RF 999 

«EVACF» 

«O_EVACF» 

TIME PERIOD: 2 

Did anyone help you in any way before you started your evacuation? 
Interim Report on Telephone Interviews 

O–35

YES 1 

NO 2 => DECID 

DK 8 => DECID 

RF 9 => DECID 

«HELPY» 

TIME PERIOD: 2 

Who helped you? PROBE: Anyone else? WE WANT THEIR ROLE NOT THE NAME OF THE 

PERSON 

POLICE OFFICER/FIREFIGHTER 1 

COWORKER 2 

STRANGER 3 

FLOOR WARDEN 4 

MANAGER/SUPERVISOR 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHOHE_01» 

«WHOHE_02» 

«WHOHE_03» 

«WHOHE_04» 

«WHOHE_05» 

«WHOHE_06» 

«O_WHOHE» 

TIME PERIOD: 2 

What did they help you with? PROBE: Anything else? 

$E 1 7 

LOCATING OTHERS 01 

HELPING OTHERS 02 

FINDING EXITS 03 

TREATING YOUR INJURIES 04 

PROVIDED INFORMATION/INSTRUCTIONS 05 

GATHER BELONGINGS 06 

CALM DOWN/EMOTIONAL ASSISTANCE 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHATD_01» 

«WHATD_02» 

«WHATD_03» 

«WHATD_04» 

«WHATD_05» 

«WHATD_06» 
Appendix O 

O–36

«WHATD_07» 

«WHATD_08» 

«O_WHATD» 

TIME PERIOD: 2 

What was the one thing that made you decide to evacuate? 

WAS TOLD TO EVACUATE 1 

FRIENDS CO-WORKERS EVACUATED 2 

AFRAID/FELT IN DANGER 3 

FIRE ALARM WAS GOING OFF 4 

SAW SMOKE 5 

SAW FIRE 6 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«DECID» 

«O_DECID» 

How many minutes had passed before you started to evacuate? IF NEEDED: How much time passed 

between when you first became aware that something had happened to the World Trade Center and when 

you entered the stairwell or elevator to leave the tower. THIS IS NOT TIME TO EVACUATE. 

PLEASE CLARIFY WITH RESPONDENT IF TIME APPEARS TOO LONG. RESPONDENT WAS 

IN<WHTW2> RANGE FOR WTC 1: 1 - 103 MINUTES RANGE FOR WTC 2: 1 - 75 MINUTES 

$E 1 103 

DK 998 

RF 999 

«TIMEP» 

SKIP FOR TOWERS 

=> EVAC2 

Else => +1 

if WHTW2=2 

«SKIP1» 

Did you begin your evacuation... WE ARE INTERESTED IN WHAT THEY KNOW NOW. THEY 

MAY NOT HAVE KNOWN WHEN THEY WERE EVACUATING, BUT NOW THEY CAN TELL US 

WHEN IT WAS. 

BEFORE THE PLANE HIT WTC 2 1 

AFTER THE PLANE HIT WTC 2, BUT BEFORE THE WTC 2 

COLLAPSE 2 

AFTER THE WTC 2 COLLAPSE 3 

DK 8 

RF 9 
Interim Report on Telephone Interviews 

O–37

«EVAC1» 

SELECT1 

$S NS=2 CO=1 IN=EVAC1<=1 ;CO=2 IN=EVAC1<=2 ; 

BEFORE THE PLANE HIT WTC 2 1 

AFTER THE PLANE HIT WTC 2, BUT BEFORE THE WTC 2 COLLAPSE 2 

AFTER THE WTC 2 COLLAPSE 3 

DK 8 

RF 9 

«SEL1» 

SELECT2 

BEFORE THE PLANE HIT WTC 2 1 

AFTER THE PLANE HIT WTC 2, BUT BEFORE THE WTC 2 COLLAPSE 2 

AFTER THE WTC 2 COLLAPSE 3 

DK 8 

RF 9 

«SEL2» 

Did you begin your evacuation... 

=> EVCSO 

if EVAC1>0 

BEFORE THE PLANE HIT WTC 2 1 

AFTER THE PLANE HIT WTC 2 2 

DK 8 

RF 9 

«EVAC2» 

SELECT4 

$S CO=1 IN=EVAC2<=1 ; 

BEFORE THE PLANE HIT WTC 2 1 

AFTER THE PLANE HIT WTC 2 2 

DK 8 

RF 9 

«SEL3» 

Was there anything that kept you from evacuating sooner? 

YES, RECORD RESPONSE 1 O 

NO 2 

DK 8 

RF 9 
Appendix O 

O–38

«EVCSO» 

«O_EVCSO» 

TIME PERIOD: 3 

When you began your evacuation, were you alone or with other people? PEOPLE THAT THEY KNOW, 

PEOPLE THAT THEY WERE TALKING WITH 

ALONE 1 

WITH OTHER PEOPLE 2 

DK 8 

RF 9 

«ALONE» 

TIME PERIOD: 3 

Which stairwell did you use for your evacuation? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

USED ELEVATOR 4 => FOLA1 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«STAIR_01» «STAIR_02» 

«STAIR_03» 

«STAIR_04» 

«STAIR_05» 

«O_STAIR» 

TIME PERIOD: 3 

Which side of the building was the stairwell located on? 

=> /WHYST 

if NOT STAIR=8,7 

NORTH 1 

SOUTH 2 

EAST 3 

WEST 4 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHISI» 

«O_WHISI» 

TIME PERIOD: 3 
Interim Report on Telephone Interviews 

O–39

Why did you choose that/those stairwell(s) for your evacuation? PROBE: Any other reason? 

CLOSEST ONE 1 

FOLLOWED OTHER PEOPLE TO IT 2 

OTHER EXITS WERE BLOCKED 3 

SAME AS I USED IN PREVIOUS EMERGENCY 4 

I WAS TOLD TO USE THIS STAIRWELL 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHYST_01» 

«WHYST_02» 

«WHYST_03» 

«WHYST_04» 

«WHYST_05» 

«WHYST_06» 

«O_WHYST» 

TIME PERIOD: 3 

At any time during your evacuation, did you leave that/those stairwell(s)? DO NOT INCLUDE PEOPLE 

WHO FOLLOWED THE PASSAGE WHERE THE STAIRWELLS START AND END. 

YES 1 

NO 2 => FOLA1 

DK 8 => FOLA1 

RF 9 => FOLA1 

«LEVST» 

TIME PERIOD: 3 

Which floor were you on when you left the stairwell? IF RESPONDENT UNSURE, SELECT 997 AND 

RECORD RANGE OF FLOORS EXAMPLE: 34-40 

$R 1 110 

UNSURE, RECORD RESPONSE 997 O 

«FLLST» 

«O_FLLST» 

TIME PERIOD: 3 

Why did you leave the stairwell? PROBE: Any other reason? 

$E 1 9 

I GOT LOST 01 

WAS TOLD TO LEAVE STAIRWELL 02 

TO HELP SOMEONE 03 

TO GO BACK AND GET SOMETHING 04 

TOO CROWDED 05 
Appendix O 

O–40

SMOKE IN STAIRWELL 06 

PATH OBSTRUCTED 07 

A LOCKED DOOR 08 

STAIRWELL LED TO A FLOOR 09 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«WHYLS_01» 

«WHYLS_02» 

«WHYLS_03» 

«WHYLS_04» 

«WHYLS_05» 

«WHYLS_06» 

«WHYLS_07» 

«WHYLS_08» 

«WHYLS_09» 

«WHYLS_10» 

«O_WHYLS» 

Screen [Template 3] -> FLOA5 

=> +1 

if FLWAR>1 

Did any of the following help you evacuate while you were in the building?

 Yes No DK RF

Instructions or assistance from your floor warden     

Instructions or assistance from Police or Firefighters     

Support and encouragement from others     

Exit signs     

Photo luminescent paint in stairwells     

«FOLA1» 

Screen [Template 3] -> EVCM7 

=> +1 

if NOT STAIR<4 
Interim Report on Telephone Interviews 

O–41

Did any of the following make your evacuation more difficult while you were in the building?

 Yes No DK RF 

Crowded stairwells     

Firefighters or Police moving up stairwell     

Disabled or injured people being taken down stairwell     

Locked doors     

Poor lighting     

Confusing or missing signs     

Lack of clear instructions     

«EVCM1» 

 

Screen [Template 3] -> EXP11 

Please tell me if you noticed any of the following at any time during your evacuation. FOLLOW UP: Was that in 

your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the Tower

Smoke    

Fire or Flames    

Fireballs    

Collapsed walls    

Jet Fuel    

Severely or fatally injured people    

Sprinklers going on    

A fire alarm sounding    

Power outage or flickering lights    

Fallen ceiling tiles    

Extreme heat    

«EXP01_01» 

«EXP01_02» 

TIME PERIOD: 3 

During your evacuation, did you turn back at any time? “TURN BACK” MEANS “GO BACK UP". 

YES 1 

NO 2 => EXITS 

DK 8 => EXITS 

RF 9 => EXITS 

«TURNB» 
Appendix O 

O–42

TIME PERIOD: 3 

Why did you turn back? PROBE: Any other reason? 

$E 1 7 

I GOT LOST 01 

I WAS TOLD TO TURN BACK 02 

TO HELP SOMEONE 03 

TO GET SOMETHING 04 

IT WAS TOO CROWDED 05 

SMOKE IN THE STAIRWELL 06 

MY PATH WAS OBSTRUCTED 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHYTB_01» 

«WHYTB_02» 

«WHYTB_03» 

«WHYTB_04» 

«WHYTB_05» 

«WHYTB_06» 

«WHYTB_07» 

«WHYTB_08» 

«O_WHYTB» 

TIME PERIOD: 3 

Did you exit the stairwell or elevator to the mezzanine or to the concourse? 

MEZZANINE 1 

CONCOURSE 2 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«EXITS» 

«O_EXITS» 

TIME PERIOD: 3 

How much time passed between the moment you first began your evacuation to when you exited the 

Tower? PLEASE CLARIFY WITH RESPONDENT IF TIME APPEARS TOO LONG. RESPONDENT 

WAS IN<WHTW2> RANGE FOR WTC 1: 1 - 103 MINUTES RANGE FOR WTC 2: 1 - 75 MINUTES 

$E 1 103 

DK 998 

RF 999 

«TIMP2» 

SKIP FOR TOWERS 
Interim Report on Telephone Interviews 

O–43

=> +2 

Else => +1 

if WHTW2=2 

«SKIP2» 

TIME PERIOD: 3 

Did you exit the tower... 

ELIMINATE -> 2 

ACCORDING TO NOT SEL1-SEL 2 

BEFORE THE PLANE HIT WTC 2 => GETOU 

AFTER THE PLANE HIT WTC 2 BUT BEFORE THE 

WTC 2 COLLAPSE, OR 2 => GETOU 

AFTER THE WTC 2 COLLAPSE 3 => GETOU 

DK 8 => GETOU 

RF 9 => GETOU 

«EXIT1» 

TIME PERIOD: 3 

Did you exit the tower... 

Eliminate -> 1 

According to NOT SEL 3 

Before the plane hit WTC 2, or 1 

After the plane hit WTC 2 2 

DK 8 

RF 9 

«EXIT2» 

Please remember that this study is intended as a fact finding mission and not a fault finding mission. It is 

crucial that we determine why some people were successful in their evacuation while others were not. 

Was there anyone on your floor that was not successful in their evacuation? 

YES 1 

NO 2 => PHYSI 

DK 8 => PHYSI 

RF 9 => PHYSI 

«GETOU» 

Why didn't they make it out? PROBE: Any other reason? 

$E 1 8 

WAS INJURED 01 

WAS DISABLED 02 

REFUSED TO LEAVE 03 

DID NOT THINK IT WAS SERIOUS 04 
Appendix O 

O–44

STAYED BACK TO HELP SOMEONE 05 

WAS TOLD TO STAY 06 

STRUCTURAL DAMAGE 07 

SMOKE OR FIRE 08 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHYNG_01» 

«WHYNG_02» 

«WHYNG_03» 

«WHYNG_04» 

«WHYNG_05» 

«WHYNG_06» 

«WHYNG_07» 

«WHYNG_08» 

«WHYNG_09» 

«O_WHYNG» 

On September 11, 2001, did you have any physical problems that made it more difficult for you to leave 

the tower? Please do not include injuries caused by the incident or evacuation. 

YES 1 

NO 2 => AGE 

DK 8 => AGE 

RF 9 => AGE 

«PHYSI» 

What type of physical problem? PROBE: Anything else? 

$E 1 9 

BLIND/PARTIALLY BLIND 01 

DEAF 02 

IN WHEELCHAIR 03 

NEED WALKING ASSISTANCE 04 

OBESITY 05 

HEART CONDITION 06 

PREGNANT 07 

ASTHMA 08 

ELDERLY 09 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99X 

«LIMIT_01» 

«LIMIT_02» 

«LIMIT_03» 

«LIMIT_04» 

«LIMIT_05» 

«LIMIT_06» 
Interim Report on Telephone Interviews 

O–45

«LIMIT_07» 

«LIMIT_08» 

«LIMIT_09» 

«LIMIT_10» 

«O_LIMIT» 

What is your age? RANGE: 1 - 98 YEARS 

$E 1 99 

RF 99 

«AGE» 

READ ONLY IF YOU CAN'T TELL. What is your gender? 

MALE 1 

FEMALE 2 

RF 9 

«GEND» 

What language do you speak best? 

ENGLISH 1 

SPANISH 2 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«PLANG» 

«O_PLANG» 

Were you working in WTC 1 or WTC 2 during the 1993 bombing? 

=> SAY11 

if YRWRK>1993 

YES 1 

NO 2 => CONCR 

DK 8 => CONCR 

RF 9 => CONCR 

«WBOMB» 

During the 1993 bombing, did you evacuate immediately or wait to evacuate? 

EVACUATE IMMEDIATELY 1 

WAIT TO EVACUATE 2 

DK 8 => +2 

RF 9 => +2 

«EVBOM» 
Appendix O 

O–46

At the time of the 1993 bombing, did you feel you that your decision to<EVBOM>was the right decision? 

YES 1 

NO 2 

DK 8 

RF 9 

«DEC93» 

 

After the 1993 bombing how concerned were you that terrorists would attack the World Trade Center? 

Were you... 

EXTREMELY CONCERNED 1 

VERY CONCERNED 2 

MODERATELY CONCERNED 3 

SLIGHTLY CONCERNED 4 

NOT AT ALL CONCERNED 5 

DK 8 

RF 9 

«CONCR» 

 

Is there anything else you would like to say about your experience on September 11? 

YES, RECORD RESPONSE 1 O 

NO 2 

DK 8 

RF 9 

«SAY11» 

«O_SAY11» 

 

IMPACT FLOOR FLAG 

=> * 

if IF(((WHTW2=1 AND WHFL2>91 AND WHFL2<99) OR (WHTW2=2 AND WHFL2>77 AND 

WHFL2<111)),1,0) 

IMPACT FLOOR FLAG 1 

«FFLAG» 

 

163: LFLAG 

 Single 

min = 1 max = 1 l = 1 

2003/09/18 15:21 

LOCATION FLAG 

=> * 

if IF((WHFL2>990 AND WHFL2<994),1,0) 

LOCATION FLAG 1 
Interim Report on Telephone Interviews 

O–47

«LFLAG» 

 

EVENT FLAG 

=> * 

if IF(((AND[NOT02-NOT06]=2-3) OR (AND[SEE02-SEE06]=2-3) OR (AND[EXP02-EXP06]=2-

3)),1,0) 

EVENT FLAG 1 

«EFLAG» 

DISABILITY FLAG 

=> * 

if IF((PHYSI=1),1,0) 

DISABILITY FLAG 1 

«DFLAG» 

 

ROLE FLAG 

=> * 

if IF((ROLES=1-4),1,0) 

ROLE FLAG 1 

«RFLAG» 

 

We may be interested in learning more about your experience on September 11. Would it be okay if we 

follow up with you sometime in the future to get more detailed information on your evacuation 

experience? 

=> +1 

if FFLAG+LFLAG+EFLAG+DFLAG+RFLAG==0 

YES 1 

NO 2 

«FOLUP» 

 

PRESS ENTER TO CONTINUE 

Those are all the questions we have. The valuable information you provided will help designers and 

engineers improve building safety, and help emergency planners improve building evacuation procedures. 

Thank you so much for taking the time to talk with me, and have a good day/evening. Good-bye. 

END OF SURVEY 1 D => /INT99 

«THANK» 
Appendix O 

O–48

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P–i

TABLE OF CONTENTS - Appendix P

List of Figures...........................................................................................................................................P–ii

List of Tables ...........................................................................................................................................P–iii

Appendix P 

Interim Report on Emergency Communications..................................................................P–1

P.1 Introduction..................................................................................................................................P–1

P.2 Report Objectives ........................................................................................................................P–1

P.3 Radio and Telephone Communications .......................................................................................P–2

P.3.1 Telephone Communications Recordings..........................................................................P–3

P.3.2 First-Person Accounts of Telephone Communications ....................................................P–3

P.4 Communications Files Processing and Preliminary Evaluation ..................................................P–3

P.4.1 Audio Data Files and Processing......................................................................................P–3

P.4.2 Audio Data Computer Software .......................................................................................P–4

P.5 Analysis of Audio Communications Files ...................................................................................P–5

P.5.1 Communications Transcription ........................................................................................P–5

P.5.2 Transcription Methods......................................................................................................P–6

P.5.3 Assessment of Radio Communications Quality ...............................................................P–6

P.5.4 Training of Transcribers ...................................................................................................P–7

P.6 Radio Communications Concepts................................................................................................P–7

P.7 Communications Data Analysis...................................................................................................P–7

P.7.1 PAPD Radio Communications .........................................................................................P–8

P.7.2 FDNY Radio Communications ........................................................................................P–8

P.7.3 NYPD Radio Communications ......................................................................................P–11

P.7.4 Radio Communications Readability Analysis................................................................P–12

P.7.5 Observations on Radio Communications .......................................................................P–14

P.7.6 Preliminary Chronology of Emergency Communications .............................................P–15

P.8 Findings .....................................................................................................................................P–24

P.9 References..................................................................................................................................P–25

Attachment 1

Communications of the Port Authority of New York and New Jersey.............................P–27
P–ii

LIST OF FIGURES

Figure P–1. PAPD police desk Channel 26/W plot of percent transmission versus time.....................P–10

Figure P–2. FDNY City-wide high-rise Channel 7 (PAPD Channel 30) plot of percent 

transmission versus time. ...................................................................................................P–10

Figure P–3. NYPD Division 1 channel plot of percent transmission versus time.................................P–11

Figure P–4. NYPD Special Operations Division channel plot of percent transmission 

versus time. ........................................................................................................................P–11

Figure P–5. PAPD police desk Channel 26/W radio communications readability................................P–12

Figure P–6. FDNY City-wide high-rise repeater Channel 7 at the WTC site (PAPD Channel 30), 

radio communications readability......................................................................................P–13

Figure P–7. NYPD Special Operations Division channel radio communications readability...............P–13

Figure P–8. NYPD Division 1 channel radio communications readability between FDNY 

personnel in the lobby of WTC 2 and FDNY personnel some 40 or more floors up 

inside the same building.....................................................................................................P–14
P–iii

LIST OF TABLES

Table P–1. Comparison of radio transmissions before and after the first aircraft impact. ....................P–9

Table P–2. Comparison of average and maximum radio transmission times before and after first 

aircraft impact. .....................................................................................................................P–9
Appendix P 

P–iv

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P–1

Appendix P

INTERIM REPORT ON EMERGENCY COMMUNICATIONS

P.1 INTRODUCTION 

On September 11, 2001, radio and telephone communications played a significant role in the operations 

of emergency responders at the World Trade Center (WTC). Radio and telephone communications were 

a primary means of communicating information to emergency responders concerning the incident. These 

forms of communication were also used by emergency responders to communicate with people trapped in 

the WTC buildings and people attempting to evacuate from the buildings. They were used to 

communicate between members of the same emergency responder departments for planning and 

operations at the incident, and they were used to communicate between different departments or 

responding organizations at the incident. 

Each of the governmental departments that had emergency responders at the WTC—New York City Fire 

Department (FDNY), New York City Police Department (NYPD), and the Port Authority Police 

Department (PAPD)—depended on their ability to communicate to accomplish their mission and to 

obtain information related to operations safety at the incident. Personnel from each of the departments 

used radios, cellular phones, and wired or landline telephones for communications during the incident. In 

addition, the emergency responders relied on the most basic form of communication, direct face-to-face 

communications. 

As a normal practice during a typical emergency response many radio and telephone communications are 

recorded by the respective departments that respond to an incident. These recordings are normally made 

by the departments to provide an accurate record of operations during an incident. These records are 

often used by departments during review of incident operations. They are also used for investigative 

purposes and are sometimes used as evidence in legal cases. During the attack on the WTC many of the 

emergency responder communications were recorded and preserved. This study is based on these 

recordings. In addition, information gathered by personnel from the National Institute of Standards and 

Technology (NIST) during first-person interviews with more than 100 emergency responders has 

contributed to the report. 

P.2 REPORT OBJECTIVES 

The objective of this study is to develop a better understanding of the role that emergency 

communications played during the attack on the WTC, and to quantify information related to 

communications effectiveness. Although there have been numerous reports of radio equipment failures 

during the emergency response at the WTC, the only radio system examined in this study is that of the 

FDNY WTC site high-rise repeater that was installed by the Port Authority. This report does not address 

issues related to the technical capabilities of any other radio equipment. The analysis of handie-talkies 

and other radio equipment is in progress and will be addressed in a following report. 
Appendix P 

P–2

Many factors are associated with the ability of emergency communications to be successful. The 

following objectives were set in this report: 

• To document radio and telephone communications operations 

• To document radio communications readability or understandability 

• To quantifying radio communications traffic volume 

• To understand the impact of traffic volume on communications readability and the transfer of 

information 

• To identify communications associated with dispatch and arrival of responders 

• To identify communications related to evacuation and emergency response operations 

• To identity communications related to building conditions at the WTC and the impact of this 

information on the emergency response 

P.3 RADIO AND TELEPHONE COMMUNICATIONS 

Both the Port Authority of New York and New Jersey (PANYNJ) and the NYPD supplied copies of audio 

recordings from the emergency response operations at the WTC. The PANYNJ provided digital copies of 

the audio communications tapes recorded by them during the incident. These recordings included 

communications from emergency response personnel, maintenance personnel, PAPD personnel, and a 

recording of the FDNY’s Channel 30 radio repeater that was located at the WTC. Channel 30 was a Citywide channel designated by FDNY for use in high-rise building operations. The Port Authority had 

installed this radio repeater system at the WTC for use by FDNY after the 1993 bombing. 

The NYPD submitted their communications to NIST in the form of audio tapes that were copies of the 

original tapes recorded on September 11, 2001. These tapes included radio communications from NYPD 

internal department operations. 

FDNY communications recordings were not available from the incident location that day because the 

primary Field Communications truck was in the shop for repairs and a backup Field Communications van 

was used in its place. The backup Field Communications van did not have the capability to record the onscene incident command or tactical communications; also, the backup van was destroyed when the towers 

collapsed. Therefore, the best record of radio communications available to NIST on FDNY operations 

came from the FDNY/PAPD Channel 30 tape and first-person accounts provided by FDNY personnel 

during their interviews. The Channel 30 tape provides a limited amount of information on FDNY 

communications and operations at the incident, but it does provide insight into FDNY operations inside 

WTC 2. 

Each audio communications file was received from the source with the starting and ending times marked 

on the media jacket or the surface of the media. A list of all communications recordings acquired from 

the various departments is found in Attachment 1 at the end of the report. 
Interim Report on Emergency Communications 

P–3

P.3.1 Telephone Communications Recordings 

Because telephone communication (both landline and cellular phone) was a contributing part of the 

emergency communications process during the incident, NIST received copies of telephone emergency 

response communications from the PANYNJ. Identification information for these recordings is also 

listed in Attachment 1. The City of New York provided NIST with opportunities to review their 

telephone recordings for 9-1-1 Emergency Operators and FDNY fire dispatchers in their New York City 

offices. At this time, the telephone recordings have been reviewed and documented, and the analysis 

work is still in progress. A detailed analysis of emergency telephone communications will be covered in 

a following report. 

P.3.2 First-Person Accounts of Telephone Communications 

As mentioned earlier, more than 100 first-person interviews were conducted with emergency responders 

that reported to the WTC incident. The following information was drawn from these interviews: 

• Before the attack occurred on the WTC both the landline and cellular systems appeared to be 

working normally. 

• Only moments after the first aircraft impacted WTC 1, the landline and cellular telephone 

systems were stressed by increased caller volume that made it difficult to get messages 

through. This condition continued for many hours following the attack. 

• Telephone calls from the WTC to the 9-1-1 emergency operators and statements from various 

individuals being interviewed shows that even though WTC 1 and WTC 2 were severely 

damaged by the aircraft impact and fires, many of the landline telephones in the buildings 

continued to work up until the collapse of WTC 2. 

• After the collapse of WTC 2, a number of cellular phone systems were not functional in the 

area of lower Manhattan. 

• After the collapse of WTC 2, there were still some landline telephones working within the 

city block areas adjacent to the WTC site. 

P.4 COMMUNICATIONS FILES PROCESSING AND PRELIMINARY 

EVALUATION 

P.4.1 Audio Data Files and Processing 

An evaluation of methods for listening to the recorded communications files was carried out. 

Comparisons were made between the functionality of using tape recorders versus that of using digital 

computer-based software for listening to the various emergency response communications files. It 

became apparent that the computer based listening system had advantages over the use of tape recorders. 

Some of the advantages of the computer based system are the ease of operation, ability to use the 

computer monitor for visually observing the beginning and end of communications periods, and the 

ability to easily and accurately reverse through a recording to a selected location so that a selected section 
Appendix P 

P–4

of a communication could be listened to multiple times. As a result, it was decided to conduct the audio 

communications study using the computer based audio software system. This decision had a direct 

impact on the type of data format and media that would be needed for conducting the audio 

communications study. Therefore, NIST requested that audio communications be provided in a digital 

format on CD-ROM disks. 

The communications recordings provided by the PANYNJ were digital files that were copied onto 

CD-ROMs, and they were in a format that could be played by computers while using audio player 

computer software. The audio recordings on each of the NYPD cassettes had to be converted to a digital 

format, and each file was then recorded onto a CD-ROM disk. In addition, some of the recordings that 

were received were recorded at very low amplitude that made it difficult to hear the communications. 

NIST used professional-quality computer audio software to increase the low audio volume recordings to a 

usable audio level. 

P.4.2 Audio Data Computer Software 

Three different types of software were used while conducting communications analysis on the audio 

recordings. Each of these software packages incorporated a clock for timing the audio recording and 

important communications during the incident. 

The first, Sound Forge 6.0, a product of Sonic Foundry, Inc., of Madison, Wisconsin, is a professional 

digital audio editor (Sonic 2002). It possesses tools that can assist with increasing audio quality and 

volume of digital audio recordings. It has a graphic output to the computer monitor that allows for rapid 

evaluation of large audio files. It is also capable of operating as a tool for spectrum analysis. As a 

spectrum analyzer, it can be used to analyze waveforms by frequency, and it helps to identify noise 

problems in communications data. In addition, the audio waveforms can be expanded on various scales 

for detailed analysis. This software was used throughout the study for analysis of the recordings that 

required audio adjustments to improve quality. 

The other two software packages were used as general purpose audio players. They both possess the 

same basic capabilities and were applied in this audio analysis process based on user preference. 

Windows Media Player, a digital media player software package, is a product of Microsoft, Inc. 

(Microsoft 2003). This media player can be downloaded from the Internet. The player allows for 

viewing of audio wave forms from the digital audio files that are being listened to. It allows for easily 

changing a computer’s audio volume, and it may be used effectively for locating specific points on an 

audio recording. The software also allows for movement through an audio file in a reverse direction so 

that selections of an audio file can be listened to multiple times. 

WinAmp3 is a media playback software package for Windows that can be downloaded from the Internet, 

and it is a product of Nullsoft, Inc. (Nullsoft 2002). This player allows for viewing of audio wave forms 

from the digital audio files that are being listened to. It allows for easily changing a computers audio 

volume and it may be used effectively for locating specific points on an audio recording. The software 

also allows for movement through an audio file in a reverse direction so that selections of an audio file 

can be listened to multiple times. 
Interim Report on Emergency Communications 

P–5

P.5 ANALYSIS OF AUDIO COMMUNICATIONS FILES 

Analysis of the communications recordings was a multistep process that began with sorting and 

cataloging the files. The initial sort separated radio communications files from telephone 

communications files. The files were also cataloged as it related to emergency response operations: 

PANYNJ, PAPD, FDNY, and NYPD. The respective files were then checked for content and primary 

emergency response channel files were selected for analysis first. Primary emergency response channels 

were channels specifically used by PAPD, NYPD, and FDNY for conducting emergency response 

operations at the WTC. The secondary channels relate to maintenance channels and other emergency 

responder channels that were not directly associated with operations at the WTC. 

Analysis was carried out using the computer based software media players described above. The 

professional quality digital audio software, Sound Forge 6.0, was used for listening to and enhancing 

audio files that were difficult to hear. The two other media player software packages, Windows Media 

Player and WinAmp3, were used to listen to the majority of audio recordings. 

P.5.1 Communications Transcription 

Two different processes were used to transcribe the emergency communications. Data for the primary 

emergency communications files were put into a spreadsheet format so that a detailed analysis of results 

could be made. The overall analysis work is continuing; however, some of the data put into the 

spreadsheets was used to assist in quantifying communications quality and the radio traffic volume as 

related to time, as will be discussed in Section P.7. The second and simplest form of communications 

transcription was the verbatim transcription of the communications into a word processor data file, which 

was used to record the secondary communications files. 

The primary communications audio files were selected for complete transcription to generate information 

concerning the quality of communications. The files selected were the FDNY Channel 7/PAPD 

Channel 30 radio repeater, the PAPD police desk radio channel, and the NYPD Special Operations 

Division channel and Division 1. These files included the following: 

• Time of the radio transmission (radio transmission time was taken from the media player 

clock and was adjusted for the start time supplied with the communications file.) 

• Type of radio transmission (voice or tone only for primary emergency response 

communications channels) 

• Readability signal quality (done only for the primary emergency response communications 

radio channels) 

• Content of the communication 

As the communications transcripts were being prepared, the names of individuals identified during the 

communications were deleted to protect the identity of individuals and to adhere to the confidentiality 

agreements with the various organizations that supplied the communications data files. 
Appendix P 

P–6

P.5.2 Transcription Methods 

As mentioned earlier, the communications transcripts were generated using three different computer 

based media players. The media players were installed on computer systems that were stand alone and 

isolated from the internet. The process for preparing a communications transcript was the following: 

• The communications data file was loaded onto the computer. 

• The media player was opened and the data file was selected. 

• The spreadsheet on the computer was opened and prepared for data input. 

• The transcriber would queue the communications recording to the beginning and check to be 

sure that the media player clock time was zeroed. 

• The data file starting time was put into the spreadsheet. 

• The communications recording was started, and the output was written into the spreadsheet. 

• To improve accuracy of the transcripts, a second transcriber checked sections of the transcript 

against the audio recording. 

For audio passages that were difficult to understand on the first pass, multiple passes of the section were 

used to improve comprehension. 

P.5.3 Assessment of Radio Communications Quality 

The Readability, Signal Strength, and Tone system for rating the quality of radio communications is used 

widely throughout the field of radio communications and is described in The ARRL Handbook for Radio 

Communications (ARRL 2003). This system is broken into three distinctive groups that can be rated: 

Readability, Signal Strength, and Tone. The rating for tone is only used to identify the quality of radio 

communications for “Continuous Wave” transmissions, and it does not apply to this analysis as “Tone” 

does not relate to voice communications. For voice radio communications, only “Readability” and 

“Signal Strength” are used. Signal Strength” is usually read from a signal strength meter at the time of 

the actual radio communication and is not available on the audio recordings. 

Thus, in this study “Readability” only was used for rating the primary emergency responder radio 

communications channel recordings. It is recognized that this form of analysis is subjective, and it relates 

to the ability of an individual to hear and understand the radio communications. In an attempt to 

minimize the influence of the subjective rating system, individuals with extensive experience using radio 

communications and project staff trained by the experienced personnel were used to conduct the analysis. 

In addition, communication periods from the various recorded data sets were reviewed by more than one 
Interim Report on Emergency Communications 

P–7

person where radio communications readability was difficult. The rating table for communications 

readability is listed below (ARRL 2003): 

 Readability (the term “readable” means “understandable”): 

 1 – Unreadable2 – Barely readable, occasional words distinguishable 

 3 – Readable with considerable difficulty 

 4 – Readable with practically no difficulty 

 5 – Perfectly readable 

P.5.4 Training of Transcribers 

Four NIST personnel were used to transcribe the emergency responder communications files. This 

included the Project Leader and three other staff personnel. The transcription protocol listed above was 

planned and tested by the two senior NIST personnel, including the project leader. When the protocol 

was found to be acceptable, the two other NIST personnel were trained by the senior members of the 

group. After the basic transcription training was completed, each of the new transcribers was given a 

communications file to transcribe. This communications file had previously been transcribed by the two 

senior personnel. After the file was transcribed by the new transcriber, their results were compared to that 

transcribed by the senior personnel. When it was demonstrated that the new transcriber had a full 

understanding of the transcription process, they were then assigned communications files to transcribe. 

P.6 RADIO COMMUNICATIONS CONCEPTS 

Currently, for most emergency responder radio systems the only way to produce a totally clear 

communication that can be received and understood is for only one communications signal to be 

transmitted at a time on a given radio frequency. This means that only one person can transmit a radio 

message at a time without creating communications interference on that radio frequency. With these 

systems, if two or more radio transmissions are made on the same radio frequency at the same time, signal 

mixing may occur and the communications may not be understandable. This difficulty with radio 

communications is often referred to as doubling. Under these conditions, usually the radio with the 

highest transmitting power will override transmissions from the lower-power radios and only the highestpower radio signal will be heard. This is often the case where an emergency response radio system uses a 

higher-power base station for dispatch communications or where a repeater is used to amplify a radio 

system’s signal output. Where multiple radio communications are received by a radio repeater, signal 

mixing is likely to occur and the communications will not be understandable (ARRL 2003). 

Over the last several years radio communications technology has undergone some significant 

advancements, particularly with cellular phones. These new systems can increase the effective use of the 

radio frequency/time factors related to radio communications (ARRL 2003), and are now beginning to be 

applied to emergency responder communications equipment. 

P.7 COMMUNICATIONS DATA ANALYSIS 

This analysis of communications addresses five major factors: (1) radio traffic volume, 

(2) communications duty cycle, (3) readability of communications, (4) operation of the FDNY site high-
Appendix P 

P–8

rise repeater at the WTC, and (5) the development of a chronology of radio communications from the 

incident. 

The first two factors, radio traffic volume and communications duty cycle, are directly related, and each 

has an impact on readability, the ability to understand and also deal with the information being 

communicated. Generally, as radio traffic rate increases, the operations duty cycle approaches overloaded 

conditions. With very high traffic volumes it becomes more difficult for personnel at central 

communications facilities and personnel in the field to respond to the volume of traffic. Human operators 

of communications equipment become overloaded with work because not only do the operators have to 

verbally communicate with personnel over the radio, but they must often transfer the information gained 

to other locations. The transfer of information may also be done verbally using other communications 

systems or it may be done by hand through keyboard inputs or by both methods. Analysis of the radio 

traffic for each of the departments shows periods where radio traffic rates during the surge conditions 

potentially resulted in situations where base station radio operators were unable to relay important 

information. 

P.7.1 PAPD Radio Communications 

All radio communications evaluated for this report experienced traffic volume surge conditions as a result 

of the attack. The traffic volume surge greatly exceeded the traffic volume experienced under normal 

operating conditions. 

PAPD Channel 26/W is used to demonstrate typical radio communications and operations conditions that 

occurred with the PAPD before and during the incident. This radio channel is used by PAPD police 

officers, NYPD supervisors and FDNY Engine 10 and Ladder 10 for communications at the WTC site. 

Tables P–1 and P–2 compare the number of transmissions and their length of time before and after the 

first aircraft impacted WTC 1. The percent of radio transmissions versus time are also shown on 

Fig. P–1. This percent of radio transmission, as well as others discussed in this report, was calculated 

based on the sum of transmission time and no transmission time over a given period of time. These data 

show that there was a 13 percent rate of radio transmissions on the PAPD police desk channel just prior to 

the aircraft impact. After the first aircraft impact on WTC 1 the radio communications were occurring 

87 percent of the time. This surge in communications significantly impacted the functional capability of 

the radio system. After approximately 10 minutes, communications dropped to a steady operating level 

of 48 percent capacity. 

P.7.2 FDNY Radio Communications 

The communications for this FDNY, City-wide, high-rise building, radio Channel 7 was recorded by 

PAPD on their Channel 30. The Port Authority installed this high-rise repeater at the WTC for FDNY 

following the 1993 bombing. This FDNY channel was used primarily by FDNY personnel during 

operations in WTC 2. Personnel using this channel were FDNY Chief Officers, Company Officers, 

Aides, and firefighters. 
Interim Report on Emergency Communications 

P–9

Table P–1. Comparison of radio transmissions before and 

after the first aircraft impact. 

Department 

Number of transmissions 

before first aircraft impact 

(20 min period) 

Number of transmissions 

after first aircraft impact 

(20 min period) 

PAPD 42 176 

FDNY 39 134 

NYPD 

Division 1 

7a 225 

NYPD Special 

Operations 

Division 

No data 192 

a. Data only available for 2 min prior to first aircraft impact 

Table P–2. Comparison of average and maximum radio transmission 

times before and after first aircraft impact. 

Department 

Average time per transmission before 

first aircraft impact and maximum (s) 

Average time per transmission after 

first aircraft impact and maximum (s) 

PAPD 3.8 (maximum 21.8) 3.3 (maximum 19.7) 

FDNY 3.8 (maximum 50.9) 3.1 (maximum 19.5) 

NYPD 

Division 1 

1.9 (maximum 5.9) 3.4 (maximum 12.6) 

NYPD 

Special 

Operations 

Division 

No data 5.7 (maximum 31.5) 

Note: All minimum transmission times were typically less than 1 s and were often related to the keying of a 

microphone. 

While looking at these data it is important to keep in mind that several FDNY personnel at the incident 

did not think that the WTC site, high-rise channel, radio repeater was working. This is based on radio 

communications tests that were conducted by two Chief Officers working inside WTC 1 when the first 

Command Post was being set up in that lobby. A record of this radio communications test was recorded 

on the PAPD Channel 30 tape. Following this radio test, a Chief Officer involved in the test chose to use 

different channels for command and tactical communications during the incident. However, as FDNY 

operations increased in WTC 2, it was determined by members of the FDNY that the high-rise channel 

was functioning and use of the channel developed. 

Preliminary analysis by NIST indicates that the repeater was operating at the WTC; however, there also 

appears to be some type of malfunction with the communications equipment. This malfunction was 

detected by the FDNY officers during the initial communications test, but it was not identified. As a 

result, this radio frequency was not primarily being used by many emergency responders. Two 

hypotheses are currently being studied related to the malfunction: (1) damage to the repeater antenna 

system and (2) failure of the radio hand set located at the Fire Command Desk in the lobby of WTC 1. 

Two failure modes are being considered, (1) the radio handset was broken, and (2) the volume on the 

handset was turned down. The evaluation of the repeater and its operation is still under way, and final 
Appendix P 

P–10

conclusions have not yet been drawn concerning the repeater’s performance. Additional information will 

be covered in the WTC Investigation final report. 

Traffic load for this FDNY channel is summarized in Tables P–1 and P–2 and in Fig. P–2. Figures P–1 

and P–2 shows that there was a significant peak in radio traffic that approached an 80 percent level which 

then dropped to a near steady high level of operations several minutes following the aircraft impact. The 

communications traffic level following the aircraft impact was four times greater than the level prior to 

impact. 

0

10

20

30

40

50

60

70

80

90

100

0 20 40

Time (minutes)

Percent Transmission (%)

First Aircraft Impact

Second Aircraft Impact

Figure P–1. PAPD police desk Channel 26/W plot of percent 

 transmission versus time. 

0

10

20

30

40

50

60

70

80

90

100

0 20 40

Time (Minutes)

Percent Transmission (%)

First Aircraft Impact

Second Aircraft Impact

Figure P–2. FDNY City-wide high-rise Channel 7 (PAPD Channel 30) 

plot of percent transmission versus time. 
Interim Report on Emergency Communications 

P–11

P.7.3 NYPD Radio Communications 

The third example illustrates radio communications for the NYPD Division 1 channel and the NYPD 

Special Operations Division channel. The Division 1 channel was used by police officers and supervisory 

police officers. The Special Operations Division channel was used by senior level NPYD management, 

supervisory police officers, Emergency Service Unit personnel, and aviation unit personnel. 

The communications recordings provided by NYPD did not typically contain communications that 

preceded the attack. However, the Division 1 radio channel recording did start approximately 2 minutes 

before the first aircraft impacted WTC 1. This 2 minute period provides a limited sample of the level of 

radio communications prior to the attack. The volume of NYPD communications is shown in Tables P–1 

and P–2. These data demonstrate that NYPD had a similar surge in radio traffic immediately following 

the attack. Figure P–3 shows that the level of transmissions before the attack was at approximately 

15 percent. Following the attack the level of transmissions jumped to over 90 percent and then settled 

down to a level of 63 percent. Radio traffic on the Special Operations Division channel was even higher, 

as shown in Fig. P–4, peaking near the 95 percent level and staying in the 80 percent range over the 

remainder of the sample period. 

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20 22

Time (minutes)

Percent Transmission (%)

First Aircraft Impact

Second Aircraft Impact

Figure P–3. NYPD Division 1 channel plot of percent transmission versus time. 

0

10

20

30

40

50

60

70

80

90

100

0 10 20

Time (M inutes)

First Aircraft Impact at Zero Time

Second Aircraft Impact

Figure P–4. NYPD Special Operations Division channel plot of 

percent transmission versus time. 
Appendix P 

P–12

P.7.4 Radio Communications Readability Analysis 

As each of the communications files was transcribed a readability value was assigned for each attempt to 

communicate. Results of this analysis are shown in Figs. P–5 through P–8. Analysis of these data 

showed that the ability to transmit a complete message was difficult during the communications surge. 

Data showed that approximately one-third to one-half of the radio communications for each of the three 

departments did not exceed a readability level of 2. These emergency communications were not complete 

and may have not been fully understood. The largest fraction of readability for all radio communications 

analyzed was recorded at a readability level of 3. This means that this fraction of communications was 

readable, but audio and radio transmission problems were being experienced. Some conditions that will 

cause poor communications quality are: 

• Background noise either at the transmission point or receiving point or both, 

• Operating health of transmitting and receiving radios and antenna systems, 

• Doubling or crossing of radio signals caused by multiple transmissions at the same time on 

the same radio frequency, and 

• Radio transmissions that may be affected by alternating materials or electromagnetic 

interference. 

2

24%

4

24%

5

0% 1

9%

3

43%

Note: Readability scale: 

 1 – Unreadable 4. – Readable with practically no difficulty 

 2 – Barely readable, occasional words distinguishable 5. – Perfectly readable 

 3 – Readable with considerable difficulty 

Figure P–5. PAPD police desk Channel 26/W 

 radio communications readability. 
Interim Report on Emergency Communications 

P–13

3

42%

2

26%

1

10%

5

4%

4

18%

Note: Readability scale: 

 1 – Unreadable 4. – Readable with practically no difficulty 

 2 – Barely readable, occasional words distinguishable 5. – Perfectly readable 

 3 – Readable with considerable difficulty 

Figure P–6. FDNY City-wide high-rise repeater Channel 7 

 at the WTC site (PAPD Channel 30), radio communications readability. 

3

32%

2

35%

4

19%

5

4%

1

10%

Note: Readability scale: 

 1 – Unreadable 4. – Readable with practically no difficulty 

 2 – Barely readable, occasional words distinguishable 5. – Perfectly readable 

 3 – Readable with considerable difficulty 

Figure P–7. NYPD Special Operations Division 

 channel radio communications readability. 
Appendix P 

P–14

5

8%

1

11%

2

26%

3

32%

4

23%

Note: Readability scale: 

 1 – Unreadable 4 – Readable with practically no difficulty 

 2 – Barely readable, occasional words distinguishable 5 – Perfectly readable 

 3 – Readable with considerable difficulty 

Figure P–8. NYPD Division 1 channel radio communications readability 

between FDNY personnel in the lobby of WTC 2 and FDNY personnel 

 some 40 or more floors up inside the same building. 

In addition, approximately 25 percent of the radio communications had readability levels 4 or above. 

Typically, the higher readability levels were produced by the various department base stations that 

operate at a higher radio transmission output power than the hand held radios. However, there is one 

exception: several of the radio communications on the FDNY City-wide high-rise radio channel, the 

PAPD recording of Channel 30. It appears that the repeater was operating at the WTC site. Several of the 

radio communications on this channel were assigned readability values of four and five as the FDNY 

began its operations in WTC 2. In addition, some of these 4 and 5 readability value radio 

communications were occurring between FDNY personnel in the lobby of WTC 2 and FDNY personnel 

some 40 or more floors up inside the same building. 

P.7.5 Observations on Radio Communications 

All of the radio systems analyzed appeared to work well during the period of normal operations before the 

attack on the WTC. It was noted that Channel W of the PAPD was experiencing some difficulty with a 

handie-talkie radio transmitting a carrier wave as a result of an open or keyed microphone, which 

disrupted communications on that channel. PAPD personnel recognized the problem and were busy 

trying to correct it just before the first plane struck WTC 1. The keyed microphone problem continued 

after the attack occurred. NYPD also had a problem with an open or keyed microphone after the incident 

began, occurring on the Special Operations Division channel. The problem was recognized and efforts 

were also made by the NYPD desk operator to get the problem cleared up. Initial attempts to correct the 

open microphone problem appeared to be successful. 
Interim Report on Emergency Communications 

P–15

Also, the data above for the various departments demonstrate the significant changes that occurred in 

radio communications traffic during the incident. It is evident that PAPD, FDNY, and NYPD all 

experienced similar surges in radio traffic volume following the first aircraft impact into WTC 1. It is 

also interesting to note that when the second aircraft struck WTC 2, 17 minutes later, there was no major 

surge in radio communications. This may be attributed to the fact that the initial emergency response 

assignments had already been made and that operations had already begun at the WTC, and an additional 

surge in radio communications was not needed. In addition, it is observed from the communications 

recordings and from first-person interviews that the emergency responders were trying to limit their use 

of the radios to reduce interference on their operating frequencies. 

P.7.6 Preliminary Chronology of Emergency Communications 

The following are lists of selected chronological communications messages that provide information 

concerning (1) dispatch and arrival of emergency response units, (2) evacuation, (3) emergency response 

operations, (4) emergency response communications, and (5) observations of building conditions. 

Note: These chronologies are based on the best possible data provided to NIST for the analysis. The 

times are given to represent the exact event sequence. Based on the variations of recorded clock times for 

the data and times assigned for each recording provided by the departments, it is estimated that the error 

for time with these chronologies is likely to be on the order of ±2 minutes. 

Dispatch and Arrival of Emergency Responders Chronology 

This chronology clearly demonstrates that the emergency response to the World Trade Center was 

immediate. Within the first 3 minutes of the aircraft impact on WTC 1, PAPD was responding by 

providing information on the incident to the police desk, FDNY had dispatched 26 units to the incident, 

and NYPD had called of a department mobilization that included dispatching aviation units to the incident 

for visual assessment. In less than 10 minutes, PAPD had called a chemical mobilization; NYPD had 

dispatched five ESU teams and had two aviations units at the scene providing observations. In less than 

30 minutes, 121 FDNY units had been dispatched to the scene and 30 units had signaled their arrival at 

the scene. This response combined with the activities undertaken demonstrates a high level of 

professionalism by the various departments. 

8:46 a.m. FDNY Chief makes report that an airplane has struck the upper floors of a WTC building and 

transmits a first and second alarm. 

 PAPD officer reports to the police desk an explosion at the WTC. 

8:48 a.m. 26 FDNY units dispatched. NYPD calls for a department mobilization. 

8:49 a.m. NYPD requests for aviation to get in the air and make a visual assessment. 

8:50 a.m. PAPD officer calls for a chemical mobilization. 

8:52 a.m. 5 NYPD Emergency Service Units dispatched. 

 NYPD aviation requests landing zone in the vicinity of the WTC. 

 NYPD aviation unit arrives at the WTC and examines possibilities of roof rescue. 

8:54 a.m. NYPD aviation advises they have two units in the air to do aerial survey. 

8:59 a.m. FDNY Chief calls for all but one Rescue Squad to the WTC. 

9 a.m. 66 FDNY units have been dispatched at this time. 

9:03 a.m. FDNY Marine unit reports that a second plane struck WTC 2. 

9:15 a.m. 121 FDNY units dispatched and 30 FDNY units signal*

 their arrival. 

9:29 a.m. FDNY dispatcher relays that a department-wide recall has been instituted. 
Appendix P 

P–16

9:59 a.m. 171 FDNY units dispatched and 74 FDNY units signal their arrival. 

10:29 a.m. 214 FDNY units dispatched and 103 FDNY units signal their arrival. 

*

Note: Arrival times are determined from 10–84 signals transmitted by units as they arrive at their 

assigned location. A 10-84 signal is sent by a firefighter from a fire department vehicle by pressing a 

button on the communications console. 

Evacuation Chronology 

The evacuation chronology exhibits a mix of responses to the incident. It provides insight into the 

successes and shortcomings of the evacuation from the WTC buildings and site. The first noteworthy 

event is that multiple orders were given by a senior PAPD police officer to evacuate the WTC buildings 

and the entire complex. There is no evidence that these orders were transmitted to appropriate personnel 

at the site to initiate the full evacuation of the complex. Data from these communications also show that 

the evacuation process was not always orderly and controlled. This is demonstrated by the fact that the 

first people jumped from WTC 1 at 8:52 a.m., only 6 minutes after the first aircraft struck WTC 1. In 

addition, it was reported that people were running from the PATH trains, and a report came in to the 

PAPD police desk from a police officer in WTC 5 stating that “I have people going crazy.” However, it 

is a fact that most of the evacuation process from the WTC complex was orderly. This chronology also 

provides a view of the professionalism of the PA, PAPD, and building security personnel that held their 

posts in the face of life threatening conditions to assist people in the evacuation. In addition, these 

communications provide some basic information related to the status of people trapped in the buildings 

and the fact that the buildings elevators were not functioning or dangerous to use. Finally, several cases 

are listed where injured, elderly, or physically impaired people are not able to walk down the stairs in the 

building and need assistance to evacuate. 

8:47 a.m. PAPD police desk receives a message to evacuate the building (WTC 1) and send people out 

towards WTC 5 

 PAPD police desk receives a message from a PAPD officer instructing employees to avoid 

the Concourse. 

8:48 a.m. PAPD police desk receives two orders from a senior police officer calling for the evacuation 

of the building. 

8:52 a.m. PAPD police desk receives report from police officer that people are jumping out of the 

windows from WTC 1. 

8:53 a.m. PAPD police desk report indicates that people are running from the PATH trains. 

8:55 a.m. PAPD police desk handles message calling for the evacuation of the Plaza 

 FDNY dispatcher relays information that people are trapped on floor 106 of WTC 1. 

8:56 a.m. PATH trains are still bringing people into the WTC site. 

 PAPD police desk message attempts to assemble personnel at WTC 1 exits to the plaza to 

show people how to get out. One Port Authority person responds to the message that he 

cannot get over to the building exits because glass is falling all over the place. 

 PAPD police desk receives a radio message that they need assistance in WTC 4 because 

people are attempting to exit the building. 

8:57 a.m. PAPD police desk: a message is sent stating, “Don’t let anyone in the building evacuate to the 

Plaza at this time.” 

8:58 a.m. PAPD police desk instruction to security guards: hold your post and don’t allow people into 

the Plaza or out onto the Courtyard. 

 PAPD police desk reports that people are trapped on floor 79 WTC 1. 

8:59 a.m. PAPD police desk: a senior PAPD officer calls for the evacuation of WTC 1 and WTC 2. 
Interim Report on Emergency Communications 

P–17

 PAPD police desk: a senior PAPD officer calls for the evacuation of the entire WTC 

complex, all buildings. 

9 a.m. PAPD police desk: a police officer asks if building five should be evacuated and he was told 

to stand by. 

 PAPD police desk receives a report that there are people trapped inside suite 4711 of WTC 1 

and can’t get out. 

 PAPD police desk: orders were given to evacuate WTC 1, B4 level. 

 PAPD police desk: a Port Authority employee calls in that he is on floor 27 in the C staircase 

and has a man in a wheel chair and needs assistance. 

9:01 a.m. PAPD police desk: a senior PAPD officer calls for the evacuation of all buildings in the WTC 

complex. 

9:02 a.m. PAPD police desk: Port Authority person calls in reporting that he is stuck in an elevator on 

floor 78 of WTC 1 in car number 81A. 

9:03 a.m. A second aircraft strikes WTC 2. 

9:04 a.m. PAPD police desk: a call is made to evacuate everybody from the building now. Note: 

Building not identified. 

9:05 a.m. PAPD police desk: a police officer indicates that WTC 4 is being evacuated. He is then 

going to WTC 5. He also reports that, “I have people going crazy.” 

 PAPD police desk: a report comes in that Port Authority employees heard people stuck inside 

of some elevators and also report that they are getting them out. Note: Building and location 

not identified. 

 PAPD police desk: a call comes in to get everybody off the complex. 

9:07 a.m. PAPD police desk: a report comes in that somebody is stuck in an elevator on floor 76. 

Note: Building not identified. 

9:08 a.m. PAPD police desk report from an officer that debris is falling from WTC 2 by WTC 4 and 

Liberty Street, and to let the people out of WTC 4. 

9:09 a.m. PAPD police desk report is received indicating that FDNY is entering elevator bank 11, 12. 

Note: Building not identified. Elevators 11 and 12 are shuttle elevators. Elevator 11 goes 

from the lobby to the 44th floor. Elevator 12 goes from the lobby to the 78th floor. 

9:10 a.m. PAPD police desk receives a report that the express elevators could be in jeopardy of falling. 

Note: Building not identified. 

9:12 a.m. PAPD police desk receives a radio report from the Command Desk in the lobby of WTC 2 

that they cannot pick up the Warden phones and that they are making announcements telling 

people not to stay at the Warden phones. Note: This communication indicates that the 

Warden phones in WTC 2 were not working. Warden phones are located on each floor of the 

building for the use of floor wardens. They are wired for communications with the fire 

command desk in the building lobby. 

9:14 a.m. PAPD police desk receives confirmation that no elevators are working. Note building not 

identified. 

9:16 a.m. FDNY radio dispatcher advises a chief that there are people trapped in WTC 1 at the 

following locations: floor 82 east side; floor 83, room 8311; floor 103, room 103 near the 

corner, floor 104; and floor 106. WTC 2 at the following locations: floor 82 west side, 

floor 88, and floor 89. 

9:17 a.m. PAPD police desk reports that four callers have made contact and need assistance on 

floor 106 of WTC 1. 

9:20 a.m. PAPD police desk receives a message from an officer that no one is down on the B4 level of 

WTC 1. 

9:23 a.m. FDNY radio dispatcher advises FDNY Field Communications Unit that 100 people are 

overcome in WTC 1 on the northwest and southwest corner of floor 103. The dispatcher also 

reports that Ladder 3 reports numerous injuries in the stairwell of floor 35 on up. 
Appendix P 

P–18

9:24 a.m. PAPD police desk receives a report from an officer that people from floor 64 are now coming 

down onto the courtyard level of WTC 1. 

9:28 a.m. PAPD police desk receives a radio report of an injured person with burn injuries caused by a 

falling elevator. Note: Location of injured person was provided as A20. This may mean 

WTC 1 on floor 20 on the A stairway. 

9:29 a.m. PAPD police desk reports that there is a medical emergency in the B stairway; there is a 

person that cannot walk down. The people are coming down from floor 51, and the person 

needing assistance has asthma. Note: Building not identified. 

9:30 a.m. PAPD police desk receives a report that two elderly people on floor 51, 

 B stairway, WTC 1, cannot walk down and need medical assistance. 

9:37 a.m. PAPD police desk recorded the following message: “All World Trade Center units to the 

Command Post. All World Trade Center units escort everybody over the land bridge on West 

Street to the Financial Center. Do not, repeat, do not send people out into the Concourse on 

to south side.” 

9:45 a.m. PAPD police desk receives a report that officers are sending people down, evacuating on the 

A stairway in WTC 1. 

 PAPD police desk copies a request for crowd control on Broadway. Answer to the request is 

that the City police should be responding. 

9:56 a.m. PAPD police desk receives a report that WTC 1 is not completely evacuated and that people 

are still coming out of each stairway. 

Emergency Response Operations Chronology 

This section provides a view of the emergency response operations carried out by FDNY, NYPD, and 

PAPD at the WTC. The chronology highlights several communications that identify cases where 

emergency responders are assisting injured people, call for EMS assistance, and search for functioning 

elevators to help evacuate injured people. Some fires in the buildings are identified and some fire fighting 

operations in WTC 2 are identified. Communications from PAPD provide information on the locations of 

many people that were trapped in the buildings and in elevators. Several communications provide insight 

into FDNY operations in WTC 2 and show that some fire fighters actually reached the 78th floor in 

WTC 2. This assent to the 78th floor was assisted by the use of an elevator that operated up until just 

before the building collapsed. The elevator became stuck in the elevator shaft and the firefighter 

operating the elevator was chopping his way out when the building collapsed. Several radio 

communications provide insight into the difficulty that emergency responders had trying to climb the 

stairs of the WTC. Cases are noted where FDNY personnel had to stop and rest. Radio communications 

for the FDNY channel 7 repeater also point out the difficulty that some firefighters had with the 

identification of the two buildings. The exchange of communications by FDNY personnel at 9:29 a.m. 

clearly shows this difficulty. Several communications from NYPD aviation units show how the aircrews 

repeatedly accessed the possibility of landing on the roof of WTC 1 and reported that conditions were not 

safe for landing. However, at 9:38 and 9:40 a.m. an aviation unit calls in for permission to land on the 

roof of WTC 1. No evidence has been found that indicates that people were seen on the tower roof or that 

conditions had improved when these radio requests were made. Interviews with aviation personnel 

indicate that many of them were highly troubled by the number of occupants trapped in the buildings and 

the number of people jumping from the buildings, and they were distressed that they were unable to help 

them. At 9:43 a.m. the order came from a senior police department official that no one from the aviation 

units is to rappel on the building’s roofs. Communications in this section also provides information that 

many people were coming to the WTC to volunteer their assistance. This assistance was turned away as 

the emergency responders felt that they needed to get everybody away from the WTC complex. 
Interim Report on Emergency Communications 

P–19

8:46 a.m. An aircraft strikes WTC 1. 

8:49 a.m. PAPD police desk a message is received that Emergency Medical Service (EMS) is needed 

because there is an injured security guard. The message was not complete; the location was 

not understandable. 

8:50 a.m. FDNY establishes a command post in the lobby of WTC 1. 

 PAPD police desk message from an officer on the B2 level of WTC 1 that there are two 

workers injured on that level and that EMS is needed ASAP. 

 PAPD police desk receives a radio call from FDNY Ladder 10 requesting information from 

PAPD about which building was struck and the location of the fire, WTC 1 or WTC 2. 

FDNY uses a Port Authority Radio for the communication. 

8:52 a.m. NYPD aviation unit arrives at the WTC to examine possibilities for roof rescue. 

8:56 a.m. PAPD police desk recording: an officer calls for an ambulance at WTC 4 for an injured 

person. 

8:58 a.m. NYPD aviation unit advises that they are unable to land on the roof due to heavy smoke 

conditions. 

9 a.m. PAPD police desk receives a message that there is an injured person between floors 14 and 

15 of WTC 2. 

9:01 a.m. PAPD police desk receives a report of a fire in a parking lot. 

9:02 a.m. PAPD police desk receives a report of a gas leak. (Incomplete message, location of leak not 

identified.) 

9:03 a.m. An aircraft strikes WTC 2. 

9:03 a.m. PAPD – by this time a PAPD senior officer has called three times for the evacuation of the 

World Trade Center, WTC 1 and WTC 2, and then “all buildings in the complex.” 

9:03 a.m. PAPD police desk reports that another aircraft has stuck WTC 2. 

9:05 a.m. PAPD police desk, an officer calls in and requests that every ambulance that can be spared be 

sent to the WTC. 

9:10 a.m. FDNY dispatcher receives message that people are trapped on floor 86 of WTC 2. 

9:11 a.m. FDNY reports that Engine 10 requests that all responding units stop short of the WTC 

buildings, either north or south of Liberty and West Street because of the large number of 

parked ambulances and debris falling from the buildings. 

9:17 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30) 

indicates that they are using an elevator for operations in WTC 2. 

9:18 a.m. FDNY radio communication from WTC 2 indicates they have one elevator working to 

floor 40, and it is staffed by a firefighter from Ladder 15. 

 PAPD police desk receives a report that FDNY is abandoning its command post and going 

across the street. 

9:22 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30) 

states that a Battalion Chief is on floor 43 or WTC 2 in the B stairway. 

 FDNY Battalion Chief now located on floor 43 of WTC 2 receives a message from a FDNY 

member in the lobby that NYPD Emergency Service police officers (Emergency Service 

Unit) want to provide support for him. The Battalion Chief gives the Emergency Service 

Unit police officers direction to his location on floor 43 in the B stairway. 

9:26 a.m. NYPD aviation unit advises that it is impossible to land on the roof at this time. 

9:29 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

Battalion Chief is communicating that he is located inside “Tower 2, the South Tower.” A 

firefighter follows the communication attempting to correct the Chief by saying that he was 

actually in the “North Tower, Tower 2.” This communication confused the actual location of 

the Battalion Chief, who later came back on the radio reporting that he was in the South 

Tower. Interviews with FDNY personnel conclusively show that the Battalion Chief was 

actually inside WTC 2, the South Tower. 
Appendix P 

P–20

9:30 a.m. PAPD police desk receives a report that EMS is setting up a triage station in the lobby of 

WTC 2. 

9:32 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): 

firefighters have been able to get to floor 55 inside WTC 2. 

9:38 a.m. NYPD aviation unit calls in to request a landing on the roof of the North Tower as soon as 

possible. 

9:39 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): 

FDNY officer inside WTC 2 indicates that he is sending 10 to 15 injured people down to 

floor 40 and that the firefighter at that location should take the injured to the building’s lobby 

in the elevator. The officer also requests that the firefighter operating the elevator bring an 

EMS crew back up with him. 

9:40 a.m. NYPD officer advises that they need the aviation units on the roof as soon as possible. 

9:41 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): 

Hazmat 1 reports that they are on floor 48 of WTC 2 in the B stairway. 

9:42 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30) a 

firefighter informs the Battalion Chief that he cannot find any elevator banks that are 

operating above floor 40. The Chief advises the firefighter that he should climb the 

B stairway from his location. 

 PAPD police desk receives a report that people have arrived and want to volunteer to help 

and where should they be sent. Answer: “Right now just send everybody away from the 

World Trade. We are not letting anybody come close to it.” 

 PAPD police desk receives a radio report that a triage center has been set up at WTC 4 at 

Victoria’s Secret. 

9:43 a.m. NYPD officer advises that no one is to rappel onto the top of the buildings. 

 Note: The term “rappel” in this case refers to the process of emergency responders using 

ropes suspended from a helicopter to descend onto the roof of a building. 

9:44 a.m. PAPD police desk receives a communication that “They haven’t evacuated the Fire 

Command over here in building 2 or 1.” 

9:45 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

firefighter calls the Battalion Chief and reports that they had to take their coats off. 

9:49 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): 

Battalion Chief instructs firefighter that it is imperative that he get down to the lobby 

command post to get some people up to floor 40. Injured people are being sent down from 

floor 70. The firefighter is inside an operating elevator and is reporting that it is not operating 

properly and expresses concerns about the elevator becoming stuck in the shaft. 

9:50 a.m. PAPD police desk receives a message that FDNY needs a resuscitator on floor 19, B corridor 

of WTC 1. 

9:54 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

Battalion Chief calls for a ladder company in the A stairway to extinguish two fires. They are 

attempting to stretch building hose lines on about floor 78. 

 FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

firefighter calls to the Battalion Chief that he is on floor 55 and must stop to rest. 

 PAPD police desk message indicates that an officer is located on floor 22, fire command 

center and that there is heavy traffic in the B stairway. The person indicates that they cannot 

release any emergency locked doors due to fire and the loss of electrical power. 

Note: Communication appears to originate from WTC 1. 

 PAPD police desk receives a report that there are 18 passengers stuck in an elevator on 

floor 78 sky lobby of WTC 2 and that firefighters are working to get them out. They request 

EMS at the location on the double. 
Interim Report on Emergency Communications 

P–21

9:56 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): 

inside WTC 2, a firefighter states they are in the B stairway and that they will have to put 

some fire out in order to get to the A stairway. 

9:57 a.m. PAPD police desk receives reports by radio on Channel X and by phone at 435-2131 from 

floor 78 of WTC 2 that people are coming out of the elevator banks. 

 At and below floor 79 of WTC 2, FDNY, NYPD, and PAPD personnel are evacuating 

occupants, assisting the injured, and fighting fires. 

 FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

firefighter in WTC 2 reports that he is trapped in an elevator in the elevator shaft and that 

they are chopping their way out. 

9:59 a.m. FDNY Marine unit reports the collapse of WTC 2. 

10:28 a.m. FDNY Marine unit advises that the second WTC tower collapsed. 

Emergency Response Communications Chronology 

This chronology provides information on communications difficulties experienced by NYPD and PAPD 

following the attack on the WTC. Much has been published concerning the communications difficulties 

experienced by FDNY during the incident, and first person interviews with FDNY personnel confirms 

some of these difficulties. However as shown by this chronology, FDNY was not the only emergency 

responder department that experienced radio equipment and communications difficulties. There are 

reports of radios not working well and communications showing that some personnel were not being 

heard or responded to. Also, some of the radio transmissions demonstrate the failure to communicate as a 

result of radio traffic surge conditions. 

The chronology provides examples of numerous cases where radio transmissions were not understood 

because of “crossing or doubling” of radio signals when too many people are trying to talk at one time. 

8:45 a.m. PAPD police desk Channel W: requests a radio check to locate an open microphone on a 

handie-talkie radio. 

8:49 a.m. PAPD police desk requests, as a result of the surge in radio traffic volume, that police officers 

stay off the air. 

 PAPD police desk Channel W: extended period with an open microphone, lots of background 

noise and people talking. 

8:50 a.m. PAPD police desk receives a message that the officer did not copy the previous transmission 

and asks what is going on. 

8:51 a.m. NYPD Special Operations Division channel: a dispatcher advises a police lieutenant that his 

message was crossed and to repeat it. A message came through that he can’t get ahold of 

someone on the cell phone. 

8:53 a.m. NYPD Special Operations Division channel: a dispatcher advises a police department truck 

that their radio message is cutting off and all that the dispatcher got was something about the 

upper floors. 

8:54 a.m. PAPD police desk is reporting that it is having trouble reading incoming radio transmissions. 

 PAPD police desk receives a message that an officer is having trouble reading radio messages 

because of so much commotion on the floor. 

8:59 a.m. NYPD Special Operations Division channel: a dispatcher advises a police truck that their 

radio message is breaking up, and the dispatcher asks what units he wants to respond. 

9 a.m. NYPD Special Operations Division channel: dispatcher advises that various units are crossing 

each other and that the dispatcher cannot understand them. 

9:01 a.m. PAPD police desk Channel Y: a microphone is stuck open, interfering with communications. 
Appendix P 

P–22

9:02 a.m. NYPD Special Operations Division channel: a police officer asks the dispatcher if the last 

transmission was heard. The police office asks twice. There is no answer. 

9:03 a.m. PAPD police desk receives a report that someone has found a supervisor’s radio that has been 

lost. 

 NYPD Special Operations Division channel: an officer in an NYPD car requests that units 

give their messages slowly. 

9:05 a.m. FDNY chief officers conduct tests of the City-wide, high-rise repeater located at the WTC. 

9:07 a.m. NYPD Special Operations Division channel: a police officer requests that the air be cleared 

for emergency vehicles and personnel unimpeded. 

9:08 a.m. NYPD Special Operations Division channel: a dispatcher advises officers directing traffic 

that they are coming over the air. Approximately 30 s later the dispatcher advises a second 

time that the officers directing traffic are coming over the air and requests that they stop. 

9:09 a.m. NYPD Special Operations Division channel: a Special Operations Division officer requests 

that the dispatcher designate two channels for this emergency, one for units on the scene and 

one for units that are responding. 

9:11 a.m. NYPD: a backup transmitter for City-wide communications is put into service in anticipation 

of potential problems with the primary transmitter. 

9:12 a.m. NYPD City-wide channel: a dispatcher advises that “We need to keep this frequency clear 

unless it is in regards to the level four mobilization.” 

 NYPD Special Operations Division channel: dispatcher state “Only emergency transmissions 

are to be made on this frequency.” 

 FDNY Chief begins using the FDNY channel 7 repeater while working inside WTC 2. 

 PAPD police desk receives a radio report from the Fire Command Desk in the lobby of 

WTC 2 that they cannot pick up the Warden phones and that they are making announcements 

telling people not to stay at the Warden phones. Note: This communication indicates that the 

Warden phones in WTC 2 were not working. 

9:15 a.m. PAPD police desk Channel W: a radio microphone is stuck open. 

9:19 a.m. NYPD Special Operations Division channel: a dispatcher advises a police officer that his 

message was being cut off and that only part of the message was copied. 

9:20 a.m. NYPD Special Operations Division channel: a dispatcher advises that there is an open carrier 

and the units should check their radios. 

9:22 a.m. NYPD Special Operations Division channel: a dispatcher advises a police truck that his radio 

message was unreadable. 

 NYPD Special Operations Division channel: the dispatcher advises a second time that there is 

an open carrier and that messages are not being understood. 

9:23 a.m. NYPD Special Operations Division channel: the dispatcher advises that the two frequencies 

are the Manhattan IO (Interoperability Channel) and the City-wide. The dispatcher also 

advises that the various units are crossing. 

9:25 a.m. PAPD police desk Channel W: a radio microphone is stuck open and interfering with 

communications. Radio signals are garbled and broken, and there is a high level of 

background noise. 

9:30 a.m. PAPD police desk Channel X has a communication indicating that everybody should turn 

their phone off. 

9:31 a.m. PAPD police desk Channel X: a Port Authority officer is questioned as to whether they have 

brought any red bags with radios for the fire department. The answer is no, and is it safe to 

go into the building. 

9:32 a.m. NYPD City-wide channel: a unit advises that he cannot communicate, his radio is going in 

and out and the cell phone is not working. 

9:36 a.m. NYPD Special Operations Division channel: a police officer reports that the telephones at his 

location are not working. Note: Location not identified. 
Interim Report on Emergency Communications 

P–23

9:43 a.m. NYPD Special Operations Division channel: a police officer advises that he heard over an 

AM radio that a plane had crashed into the Pentagon. 

9:49 a.m. PAPD police desk instructs officer that he was speaking too fast and that he must slow down 

so that he could be understood. 

9:53 a.m. NYPD Special Operations Division channel: the dispatcher advises all units to check their 

portable radios for an open carrier. 

9:54 a.m. NYPD City-wide channel: a dispatcher requests “Keep the air clear. We have problems in 

the City. Keep the air clear right now.” 

9:55 a.m. PAPD police desk Channel Z: police officers are having trouble reading communications 

over the radio and indicate that they will try to call on the telephone. 

9:57 a.m. PAPD police desk Channel X: a report is received that an officer is responding to WTC 1 Fire 

Command and that he had been trying to contact the Command Center on floor 22, but they 

didn’t know how to operate the other set of communications equipment. 

9:59 a.m. NYPD Special Operations Division channel: an Emergency Service Unit police officer calls 

several times for the dispatcher. The dispatcher answers each time and apparently was not 

heard by the calling unit. 

10:03 a.m. NYPD Special Operations Division channel: a dispatcher requests that some units standby 

while the needs of other units are addressed. 

10:05 a.m. NYPD Special Operations Division channel: the dispatcher advises that all units need to talk 

one by one. The dispatcher further advises that units are cutting each other off. 

10:09 a.m. NYPD Special Operations Division channel: an Emergency Service Unit advises the 

dispatcher that he can hear the dispatcher but is not sure if the dispatcher is hearing him. 

10:10 a.m. NYPD Special Operations Division channel: the dispatcher advises that there are three units 

trying to talk at the same time and requests, “One at a time.” 

Condition of the WTC Towers Chronology 

Information provided by this chronology partially describes the variable conditions found in WTC 1 and 

WTC 2 towers. It is shown that the impact of the first aircraft into WTC 1 produced an explosive 

condition all the way down to the building’s basement. The impact of the aircraft into what appears to be 

WTC 2 produced jet fuel fires in the building on the 51st floor. Other communications indicate that there 

was no smoke or fire on the 68th, 73rd, or 74th floors, the walls in stairway B had been breached. A 

telephone call to a New York City Radio 9-1-1 telephone operator at 9:36 a.m. indicates that a floor in the 

90’s level of WTC 2 had collapsed. Information from this call concerning the floor collapse appears to be 

misstated by the NYPD Division 1 radio operator in the message transmitted at 9:41 a.m. and again at 

9:51 a.m. Communications from the NYPD aviations units describe a steady deterioration of the two 

WTC towers before they collapsed. 

8:47 a.m. PAPD police desk reports that there is a fire on floor 22 of WTC 1. 

 PAPD police desk receives a report that there is a lot of debris on floor 22 of WTC 1. 

8:49 a.m. PAPD police desk reports that there is damage and a lot of debris on floor 22 of WTC 1. 

8:51 a.m. PAPD police desk receives a call that an explosion was observed in the basement of the 

B1 level of WTC 1. The police desk informs the officer on the B1 level that what he saw 

resulted from an explosion on the upper floors of the building. 

8:57 a.m. PAPD police desk receives report that water pipes are broken on the B4 level of WTC 1. 

9:02 a.m. PAPD police desk receives message from a person trapped in an elevator on floor 78 of 

WTC 1 that the area has smoke, and water and debris are coming down from above. 

9:10 a.m. PAPD police desk receives a report that there is burning jet fuel on floor 51 of one of the 

towers. Note: Communications suggest this is WTC 2. 

9:13 a.m. PAPD police desk receives a report that WTC 1 is flooding. 
Appendix P 

P–24

9:32 a.m. PAPD police desk receives a message from an officer that the WTC Concourse is flooding. 

9:36 a.m. New York City 9-1-1 telephone operator receives a message from an occupant of WTC 2 that 

a floor had collapsed below them in the 90’s level. 

9:41 a.m. NYPD dispatcher advises units that floor 106 in WTC 2 is collapsing and that the message 

comes from someone on that floor. 

9:47 a.m. FDNY radio communications on the City-wide, high-rise Channel 7 (PAPD Channel 30): a 

firefighter inside WTC 2 reports that he is standing in the B stairway on floor 74 and there is 

no smoke or fire problem. He reports that the stairway walls have been breached on floors 73 

and 74. Another FDNY unit in the same stairway reports that the walls were also breached 

on floor 68. 

9:49 a.m. NYPD aviation unit gives a radio report stating that “large pieces” are falling from WTC 2. 

9:51 a.m. NYPD dispatcher advises that at WTC 2, floor 106 is crumbling per communications with 

victims trapped on the floor. 

9:58 a.m. NYPD aviation unit advises that the south tower is coming down. 

10:06 a.m. NYPD officer advises that it isn’t going to take much longer before the north tower comes 

down and to pull emergency vehicles back from the building. 

10:20 a.m. NYPD aviation unit reports that the top of the tower might be leaning. 

10:21 a.m. NYPD aviation unit reports that the north tower is buckling on the southwest corner and 

leaning to the south. 

 NYPD officer advises that all personnel close to the building pull back three blocks in every 

direction. 

10:27 a.m. NYPD aviation unit reports that the roof is going to come down very shortly. 

10:28 a.m. NYPD officer reports that the tower is collapsing. 

P.8 FINDINGS 

The following is a list of preliminary findings based on the current status of emergency responder 

communications analysis: 

1. After the first aircraft struck the WTC, there was a peak increase in emergency responder 

radio communications by approximately a factor of 5, followed by an approximate factor of 

3 steady level of radio communications. 

2. A surge in communications traffic volume made it more difficult to handle the flow of 

communications and delivery of information. 

3. Analysis of the radio communications records received by NIST indicates that roughly onethird to one-half of the radio messages transmitted during these radio traffic surge conditions 

were not complete messages nor understandable. 

4. Preliminary analysis of the FDNY City-wide, high-rise Channel 7 (PAPD Channel 30) 

recording indicates that the WTC site repeater was operating. 

5. Communications records and interviews indicate that smoke and heat conditions on the top of 

the two WTC buildings prevented the NYPD helicopters from conducting safe roof 

evacuation operations. 

6. NYPD aviation unit personnel reported critical information about the impending collapse of 

the WTC towers several minutes prior to their collapse. No evidence has been found to 
Interim Report on Emergency Communications 

P–25

suggest that the information was further communicated to all emergency responders at the 

scene. 

P.9 REFERENCES 

ARRL. (American Radio Relay League). 2003. The ARRL Handbook for Radio Communications. 

Newington, CT. 

Microsoft (Microsoft, Inc.). 2003. Windows Media Player. http://www.microsoft.com. 

Nullsoft (Nullsoft, Inc.). 2002. WinAmp 3. http://www.winamp.com. 

Sonic (Sonic Foundry, Inc.). 2002. Sound Forge 6.0. Madison, WI. 
Appendix P 

P–26

This page intentionally left blank.
P–27

Attachment 1 

COMMUNICATIONS OF THE PORT AUTHORITY OF

NEW YORK AND NEW JERSEY

The National Institute of Standards and Technology received duplicates of many radio and telephone 

channels. The tapes listed below cover a wide range of times. All recordings cover at least from 0845 to 

0958. The remainder of the communications can be placed in two categories as follows: 0705 to 0958 

and 0705 to 1800. 

Central Police Desk (CPD): Police Command Channel 2 to 39 – Each one of these recordings is 

198 minutes long. 

CPD Ch. 002 – CPD.wav – Not assigned 

CPD Ch. 003 – CPD.wav – Not assigned 

CPD Ch. 004 – CPD.wav – Not assigned 

CPD Ch. 005 – CPD.wav – Not assigned 

CPD Ch. 006 – CPD.wav – Not assigned 

CPD Ch. 007 – CPD.wav – Not assigned 

CPD Ch. 008 – CPD.wav – Not assigned 

CPD Ch. 009 – CPD.wav – Not assigned 

CPD Ch. 010 – CPD.wav – Not assigned 

CPD Ch. 011 – CPD.wav – Not assigned 

CPD Ch. 012 – CPD.wav – Not assigned 

CPD Ch. 013 – CPD.wav – Not assigned 

CPD Ch. 014 – CPD.wav – Not assigned 

CPD Ch. 015 – SPEN 1 State Police Emergency Network.wma 

CPD Ch. 016 – Radio SPEN 2 State Police Emergency Network.wma 

CPD Ch. 017 – Radio (Ch. A) PA Area Wide.wma 

CPD Ch. 018 – Radio (Ch. W) LT Police.wma 

CPD Ch. 019 – CPD.wav – Not assigned 

CPD Ch. 020 – CPD.wav – Not assigned 

CPD Ch. 021 – Phone 9-1-1 Emergency.wma (Note: Recording was blank.) 

CPD Ch. 022 – Phone 9-1-1 Emergency.wma (Note: Recording was blank.) 

CPD Ch. 023 – Phone SGT's Desk - 201-216-6800.wma 

CPD Ch. 024 – Phone Clerk - 201-216-6800.wma 

CPD Ch. 025 – Phone TTY NY- 201-216-6800.wma 

CPD Ch. 026 – Phone Clerk Extra - 201-216-6800.wma 

CPD Ch. 027 – Phone TTY NJ - 201-216-6800.wma 

CPD Ch. 028 – Phone Absence Control Line 1 - 201-216-6988.wma 

CPD Ch. 029 – Phone Absence Control Line 2 - 201-133-6988.wma 

CPD Ch. 030 – Phone 800 number SGT’s Desk - 201-216-6858.wma 

CPD Ch. 031 – Desk TTY number 3.wma 

CPD Ch. 032 – CPD.wav – Not assigned 

CPD Ch. 033 – CPD.wav – Not assigned 

CPD Ch. 034 – CPD.wav – Not assigned 

CPD Ch. 035 – Phone 201-963-7247 Assignment Line 800-776-8580.wma 
Appendix P 

P–28

CPD Ch. 036 – Phone 201-963-7248 Assignment Line 800-776-8580.wma 

CPD Ch. 037 – Phone 201-963-7249 Assignment Line 800-776-8580.wma 

CPD Ch. 038 – Phone 201-659-3028 Toll Rob 800-TOLL-ROB.wma 

CPD Ch. 039 – Phone 201-216-6794 Drug Tip 800-828-PAPD.wma 

PATH Police Command: Ch. 02 to 31 – Recordings vary in length from 106 minutes to 193 minutes. 

PATH Ch. 02 – Phone Desk Right.wma 

PATH Ch. 03 

PATH Ch. 04 

PATH Ch. 05 

PATH Ch. 06 – SGT. desk.wma 

PATH Ch. 07 – Tour Commander.wma 

PATH Ch. 08 – Report Room.wma 

PATH Ch. 09 – Juvenile Room.wma 

PATH Ch. 10 – Reserve Room 216-6078.wma 

PATH Ch. 11 – Phone Desk Left.wma 

PATH Ch. 12 – Jersey City Fire Department.wma 

PATH Ch. 13 – Jersey City Medical Center.wma 

PATH Ch. 14 – Jersey City Police.wma 

PATH Ch. 15 – NYPD.wma 

PATH Ch. 16 

PATH Ch. 17 

PATH Ch. 18 

PATH Ch. 19 – Conference Room 1.wma 

PATH Ch. 20 – Conference Room 2.wma 

PATH Ch. 21 – Radio (R2) Train Master.wma 

PATH Ch. 22 – PD Wall (Desk Area).wma 

PATH Ch. 23 – Court Office 1.wma 

PATH Ch. 24 – Court Office 2.wma 

PATH Ch. 25 – Court Sgt.wma 

PATH Ch. 26 – Radio (R1) Train Master.wma 

PATH Ch. 27 – Radio (R30) Communications.wma 

PATH Ch. 28 

PATH Ch. 29 

PATH Ch. 30 

PATH Ch. 31 

WTC Police Desk 1: Ch. 002 to 039 – Each one of these recordings is 171 min. 

Ch. 002 WTC.wav 

Ch. 003 WTC.wav 

Ch. 004 WTC.wav 

Ch. 005 WTC phone 435-8456 clerk.wav 

Ch. 006 WTC phone 435-8462 clerk.wav 

Ch. 007 WTC phone 435-2135 TC.wav 

Ch. 008 WTC phone 435-3541 desk left.wav 

Ch. 009 WTC phone 435-3541 desk center.wav 

Ch. 010 WTC phone 435-3541 desk right.wav 

Ch. 011WTC phone 435-8460 conf. room.wav 

Ch. 012WTC .wav 
Interim Report on Emergency Communications 

P–29

Ch. 013 WTC phone 435-3519 office.wav 

Ch. 014 WTC direct line FDNY.wav 

Ch. 015 WTC direct line NYC EMS.wav 

Ch. 016 WTC phone 435-7666 floor warden.wav 

Ch. 017 WTC direct line fire command WTC 1.wav 

Ch. 018 WTC direct line fire command WTC 2.wav 

Ch. 019 WTC.wav 

Ch. 020 WTC.wav 

Ch. 021 WTC phone 435-2133 police reserve rm.wav 

Ch. 022 WTC phone 435-2131 SHO desk.wav 

Ch. 023 WTC phone 435-2948 desk.wav 

Ch. 024 WTC radio Ch. A.wav 

Ch. 025 WTC radio Ch. B.wav 

Ch. 026 WTC radio Ch. W.wav 

Ch. 027 WTC radio Ch. X.wav 

Ch. 028 WTC radio Ch. Y.wav 

Ch. 029 WTC radio Ch. Z.wav 

Ch. 030 WTC FDNY radio.wav 

Ch. 031 WTC.wav 

Ch. 032 WTC.wav 

Ch. 033 WTC.wav 

Ch. 034 WTC.wav 

Ch. 035 WTC.wav 

Ch. 036 WTC.wav 

Ch. 037 WTC.wav 

Ch. 038 WTC.wav 

Ch. 039 WTC.wav 

Newark International Airport: Police Command – Ch. 02 to 39 

EWR Ch. 002 

EWR Ch. 003 

EWR Ch. 004 

EWR Ch. 005 

EWR Ch. 006 

EWR Ch. 007 

EWR Ch. 008 – Phone 733-7525 – Newark PD.wma 

EWR Ch. 009 – Phone PL234846 – Eliz. PD.wma 

EWR Ch. 010 – Phone PL92866- Newark FD.wma 

EWR Ch. 011 – PL234881 – Eliz. FD.wma 

EWR Ch. 012 – Phone PL230333 – AFA.wma 

EWR Ch. 013 

EWR Ch. 014 – Phone PL234979 – REMCS.wma 

EWR Ch. 015 – FAA Tower Crash Alarm.wma 

EWR Ch. 016 

EWR Ch. 017 

EWR Ch. 018 – PNPD PVL - OSNA660-650.wma 

EWR Ch. 019 – Phone 589-6321 - PNPD.wma 

EWR Ch. 020 – Phone 589-0292 - PNPD.wma 

EWR Ch. 021 – Phone 961-6666 - Line 3.wma 

EWR Ch. 022 – Phone 961-6666 - Line 4.wma 
Appendix P 

P–30

EWR Ch. 023 – Radio – EWR Command – 800Mhz.wma 

EWR Ch. 024 – Radio – EWR ARFF – 800Mhz.wma 

EWR Ch. 025 – Radio – EWR TAC 1 - 800Mhz.wma 

EWR Ch. 026 – Radio – Central police desk - 800Mhz.wma 

EWR Ch. 027 – Radio – EWR Detectives.wma 

EWR Ch. 028 – Police desk left phone – 961-6230.wma 

EWR Ch. 029 – Police desk phone center – 961-6230.wma 

EWR Ch. 030 – Police CAD desk phone – 961-6230.wma 

EWR Ch. 031 – Police desk right phone – 961-6230.wma 

EWR Ch. 032 – Phone 961-6666 – Line 2.wma 

EWR Ch. 033 – Phone 961-6666 – Line 1.wma 

EWR Ch. 034 

EWR Ch. 035 

EWR Ch. 036 – Radio Ch. Z – Operations & Terminals.wma 

EWR Ch. 037 

EWR Ch. 038 – Radio – Ch. X – Facility maintenance.wma 

EWR Ch. 039 – Radio – Ch. B – Maintenance.wma 

NYPD WTC Communications: 

 NYPD Special Operations Division 

 Tape 1, 08:46 – 09:33 

 Tape 2, 09:32 – 10:18 

 Tape 3, 10:18 – 11:04 

 NYPD City-wide 1 radio, Tape 4, 08:40 – 09:27 

 Tape 4b, 09:27 – 10:12 

 Tape 5c, 10:12 – 11.59 

 Tape 5d, 10:59 – 11:46 

 NYPD Division 1, Tape 6, 08:45 – 9:30 

 Tape 7, 09:29 – 10:15 

 Tape 8, 10:14 – 11:00 
Q–i

TABLE OF CONTENTS - Appendix Q

Appendix Q 

NIST’s Working Hypothesis for Collapse of the WTC Towers........................................... Q–1

Q.1 Objectives ................................................................................................................................... Q–1

Q.2 Approach..................................................................................................................................... Q–1

Q.3 Factors to Evaluate the Working Hypothesis.............................................................................. Q–2

Q.4 The Working Hypothesis ............................................................................................................ Q–2

Q.5 Issues Still Being Investigated .................................................................................................... Q–6
Appendix Q 

Q–ii

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Q–1

Appendix Q

NIST’S WORKING HYPOTHESIS FOR COLLAPSE OF THE 

WTC TOWERS 

In response to the terrorist attacks of 

September 11, 2001, and the resulting collapse of 

the World Trade Center (WTC) buildings, the 

National Institute of Standards and Technology 

(NIST) has been investigating possible collapse 

scenarios. Establishing the sequence of events 

that led to the towers collapsing is important in 

determining which factors allowed the buildings 

to hold up for as long as they did without 

collapsing, and which factors, if any, could have 

delayed or prevented the collapse of the WTC 

towers. Understanding these factors will provide 

valuable information on which to base 

recommendations for improvements to buildings 

practices, standards, and codes that may be 

warranted. 

Q.1 OBJECTIVES 

The objectives of the NIST analysis are: 

• Establish how and why the WTC 

towers collapsed after the aircraft 

impact, i.e., the ‘triggering event’ 

• Determine the most probable collapse 

sequence 

• Identify the factors that have the 

strongest influence on the most 

probable sequence 

Q.2 APPROACH 

To identify the most probable of the technically possible collapse sequences, NIST is adopting an 

approach that combines mathematical modeling, well-established statistical and probability based analysis 

methods, laboratory experiments, and analysis of photographic and video evidence. The approach 

accounts for variations in models, input parameters, analyses, and observed events. It allows for 

Background

NIST has estimated that 17,400 occupants (± 1,200) 

were present in the WTC towers on the morning of 

September 11, 2001, about equally divided between 

the two buildings (8,900 in WTC 1 and 8,540 in 

WTC 2). 2,159 building occupants and an additional 

433 first responders, including security guards, were 

reported to have lost their lives that day. This does not 

include aircraft passengers and crew or bystanders. 

Approximately 87percent of the WTC tower occupants 

were able to evacuate successfully. More than 

99 percent of occupants below the crash impact areas 

had sufficient time prior to collapse of the buildings to 

safely evacuate. WTC 1 stood for one hour and 

43 min after impact; WTC 2 collapsed 56 min after it 

was struck. 

Preliminary estimates indicate that about 20 percent or 

more of those in the WTC towers who lost their lives 

may have been alive in the buildings just prior to their 

collapse. This includes nearly all of the first 

responders and 76 building occupants below the floors 

of impact. There were 72 fatalities reported in WTC 1 

and 4 fatalities reported in WTC 2, not including first 

responders, below the floors of impact. 

Buildings are not designed to withstand the impact of 

fuel-laden commercial airliners. However, Port 

Authority documents indicate that the impact of a 

Boeing 707 flying at 600 mph and possibly crashing 

into the 80th floor had been analyzed during the 

design of the WTC towers in February/March 1964. 

While NIST has not found evidence of the analysis, the 

documents state that such a collision would result in 

localized damage only, and that it would not cause 

collapse or substantial damage to the WTC towers. 

The effect of fires due to jet fuel dispersion and ignition 

of building contents was not considered in the 

1964 analysis. Loss of life in the immediate area of 

aircraft impact was anticipated, but loss of life from fire 

and smoke was not considered. 
Appendix Q 

Q–2

evaluation and comparison of possible collapse hypotheses based on different damage states, fire paths, 

and structural responses to determine the following: 

1. The most probable sequence of events from the moment of aircraft impact until the initiation 

of global building collapse; 

2. How and why WTC 1 stood nearly twice as long as WTC 2 before collapsing (103 min 

versus 56 min), though they were hit by virtually identical aircraft; 

3. What factors, if any, could have delayed or prevented the collapse of the WTC towers. 

Q.3 FACTORS TO EVALUATE THE WORKING HYPOTHESIS 

In further evaluating the working hypothesis for the collapse of the WTC towers, NIST is considering the 

following factors: 

• The relative contributions of aircraft impact damage and subsequent fires 

• How safe each building was immediately after aircraft impact but before fire weakened the 

structures, i.e., to what extent the capacity of the buildings to carry design loads1

 was reduced 

• Relative roles of the perimeter and core columns2

 and the composite floor system,3 including 

connections 

• The role played by fireproofing, especially the extent to which fireproofing may have been 

damaged due to aircraft impact 

• Whether the undamaged towers would have remained standing in a “maximum credible fire”4

• The role compartmentation (i.e., areas divided by fire-rated walls) may have played, i.e., what 

would have happened if the floors had been separated into 7,500 or 10,000 ft2

 compartments 

with 1 h fire-rated partition walls or separations 

Q.4 THE WORKING HYPOTHESIS 

NIST has developed a working hypothesis to explain the sequence of events from aircraft impact until the 

initiation of global structural collapse. This hypothesis will be further refined based on the results of 

 

1

 The design of the WTC towers was governed by gravity and lateral wind loads. 

2

 The perimeter columns were designed to carry both gravity and wind forces and acted together as a framed-tube system. The 

core columns were designed to carry only gravity loads and not required to provide frame action. 3

 The composite floor truss system, which included long-span open-web bar joist elements, was designed to carry floor loads to 

the supporting core and perimeter columns. It also acted as a diaphragm that distributed wind forces to the perimeter columns 

of the framed-tube system and provided lateral stability to the perimeter columns. 4

 A maximum credible fire for the WTC towers is assumed to have the following characteristics: the sprinkler system is 

compromised, overwhelmed, or not present; there is no active firefighting; combustible building contents averaging 10 psf (in 

the range of about 5 to 20 psf for conventional office buildings); floor-to-floor fire spread to next upper floor at 30 or 60 min; 

and ventilation from windows broken by fire and a total of 50 ft2

 of air leakage between floors. 
NIST’s Working Hypothesis for Collapse of the WTC Towers 

Q–3

Role of the Hat Truss System 

The purpose of the hat truss was to support 

gravity and wind loads on the antenna. It 

was not designed to resist lateral forces on 

the towers, and, in an undamaged state, it 

did not have a significant role in carrying 

gravity loads. Lateral loads due to wind 

were distributed to the framed-tube system 

via diaphragm action of the floor system. 

The hat truss was connected to each 

perimeter face at only four points, all at the 

same level (at the 108th floor just below the 

concrete floor slab). The 47 core columns 

were connected to diagonal elements, 

heavier transfer beams, or smaller beam 

elements in the hat truss. Most of the core 

columns extended to the roof level, but four 

core columns, which were only minimally 

connected to the hat truss, terminated at 

floor 110. The hat truss provided minimal 

redistribution of loads (less than 

10 percent) from perimeter columns to core 

columns. Most of the load redistributed 

due to aircraft impact damage occurred on 

the external face through vierendeel action.

NIST’s continuing comprehensive analyses to identify specific load redistribution paths and damage 

scenarios that are possible for each building, from which the most probable collapse sequence will be 

identified. NIST welcomes comments from technical experts and the public on the working hypothesis. 

NIST’s working hypothesis is based on analysis of the available evidence and data, consideration of a 

range of hypotheses (including those postulated publicly by experts), and a newly enhanced 

understanding of structural and fire behavior. It is consistent with all current evidence held by NIST, 

including photographs and videos, eyewitness accounts, and emergency communication records. NIST’s 

analysis allows for different sequences of events and different possible event paths for each building. 

To accommodate the aircraft impact and subsequent fire damage, the structure redistributed loads from 

structural element to structural element via redundant load paths and maintained overall structural 

stability. Structural collapse began when the structure was not able to redistribute loads any further. The 

working hypothesis addresses the following chronological sequence of major events; specific load 

redistribution paths and damage scenarios are currently under analysis: 

1. Aircraft impact damage to perimeter columns 

with redistribution of column loads to adjacent 

perimeter columns and to the core columns via 

the hat truss; 

2. After breaching the building’s exterior, the 

aircraft continued to penetrate into the 

buildings, damaging core columns with 

redistribution of column loads to other intact 

core and perimeter columns via the hat truss 

and floor systems; 

3. The subsequent fires, influenced by 

post-impact condition of the fireproofing, 

further weakened columns and floor systems 

(including those that had been damaged by 

aircraft impact), triggering additional local 

failures that ultimately led to column 

instability; 

4. Initiation and horizontal progression of 

column instability ensued when redistributing 

loads could not be accommodated any further. The collapses then ensued. 

Aircraft Impact Damage to Perimeter Columns 

Initially, the WTC towers withstood the impact virtually identical aircraft. Based on video that NIST has 

obtained, it is known that WTC 2, which collapsed first and in about half the time as WTC 1, vibrated for 

over 4 min at an oscillation rate nearly equal to that measured for the undamaged building after it was 

struck, indicating that the buildings were continuing to respond normally. The lightly damped (about 

1.2 percent of critical damping) oscillation had a maximum amplitude of approximately 20 in. at the roof 
Appendix Q 

Q–4

level, where sway was about 3 ft to 4 ft under design wind conditions. Based on this information, 

structural damage to perimeter columns as a result of aircraft impact of the framed-tube system appears to 

have played a minimal role in initiating the collapse. Perimeter column bowing prior to collapse occurred 

on other faces (i.e., fire floors on the south face of WTC 1 and east face of WTC 2) that were not severed 

by the aircraft. 

Aircraft Impact Damage to Core Columns 

The core columns were designed to carry only gravity loads and not required to provide frame action. 

The aircraft trajectory at impact suggests damage to the core columns occurred as follows: 

WTC 1—The aircraft was traveling about 450 mph and hit the tower near the center of the north face 

damaging floors 93 to 99. The aircraft fully entered the core area and severed or damaged central core 

columns in the north-south direction. Aircraft and building debris accumulated in the remaining core area 

and south-side floor areas as contents were displaced from the point of impact. 

WTC 2—The aircraft was traveling about 550 mph and hit the tower near the southeast corner of the 

building damaging floors 77 to 85. Core columns to the south and east were severed or damaged. 

Aircraft and building debris accumulated in the core area and floor areas to the east and north. 

Severed core columns redistributed their loads in three ways, depending on how many and which core 

columns were severed. 

1. Isolated core columns were severed. Severed column and tributary floor loads at and above 

the point of impact were redistributed locally at each floor to adjacent intact core columns via 

core floor framing. This was limited by shear/bending capacity of floor-framing connections 

to adjacent columns. 

2. Critical (e.g., corner) core columns and/or several other core columns were severed. The 

severed column and tributary floor loads, at and above impact floors, redistributed to intact 

core columns via the hat truss. Significant hat truss deflections may have occurred if there 

was adequate connection capacity since the severed core columns and the associated floors 

were hanging from the hat truss which was not designed to carry such loads. This was 

limited by the tensile capacity of bolted splices in the severed core columns, 

tensile/compression capacity of hat truss members, and tensile capacity of column 

connections to the hat truss. 

3. Extent of core column failures precluded redistribution through the hat truss and/or exceeded 

redistribution capacity of the hat truss: severed column and associated floor loads, at and 

above floors of impact, redistributed to intact core and perimeter columns via the core and 

composite truss floor system. Floors were subjected to combined bending and diaphragm 

action (e.g., consider the scenario of no core columns in the floor span direction to visualize 

this action). The overall capacity of the floors was limited by shear capacity of 

floor-to-column connections (including perimeter columns) and tensile/bending capacity of 

composite truss floor connections to core or perimeter columns. Significant sagging of the 

hat truss system may have occurred if its capacity was exceeded. 
NIST’s Working Hypothesis for Collapse of the WTC Towers 

Q–5

Role of Fireproofing 

The post-impact condition of the fireproofing 

played a key role in the structural response 

to fires. The post-impact condition of the 

fireproofing depends on the condition of the 

fireproofing prior to aircraft impact and the 

extent to which fireproofing was damaged 

due to aircraft impact. The fire-affected 

floors in WTC 1 had, in general, upgraded or 

thicker fireproofing (1.5 in. specified) while, 

in general, those in WTC 2 did not have 

upgraded fireproofing (0.5 in. specified].

Relative Roles of Fires and Aircraft Impact 

Fires played a major role in collapse initiation. The tower structures withstood the initial aircraft impacts 

and remained stable. While aircraft impact damage did not, by itself, initiate building collapse, it had the 

following harmful effects that then contributed greatly to the subsequent fires: 

• Compromised the sprinkler and water supply systems, 

• Dispersed jet fuel and ignited building contents over large areas, 

• Created large accumulations of combustible matter containing aircraft and building contents, 

• Increased air supply into the buildings (through broken windows and holes in the sides of the 

buildings, and between floors due to damaged floors, vertical shafts, and columns) permitted 

significantly higher energy release rates than would normally be seen in ventilation limited 

building fires, allowing the fires to spread rapidly within and between floors, and 

• Damaged ceilings enabling “unabated” heat transport over the floor-to-ceiling partition walls 

and to the floor trusses, spandrels, and tops of columns. 

The jet fuel, which ignited the fires, was mostly consumed within the first few minutes after impact. The 

fires that burned for almost the entire time that the buildings remained standing were due mainly to 

burning building contents and, to a lesser extent, aircraft contents, not jet fuel. 

Thermal Effects on Columns and Floors 

Some floors in WTC 2 experienced partial collapse due to 

aircraft impact. For example, partially collapsed floor slabs 

were visible on the east and north faces. This included 

failures at the edges with perimeter columns causing floor 

edge sagging. There is no visible evidence of hanging floors 

in WTC 1. 

• Fires may have had the following thermal 

effects: core columns and core floors may have 

been further weakened, with reduced ability to 

carry and/or redistribute load, causing such 

loads to be redistributed to other core and 

perimeter columns consistent with the residual 

reserve capacities of these columns and the transfer mechanisms (i.e., hat truss and floor 

system). 

• The floor system may have been further weakened, either along the span of the floor system 

or localized at connections with columns. The weakening floor system may have pulled the 

perimeter columns inward (observed on the south face of WTC 1 and the east face of WTC 2 

minutes prior to building collapse) and then initiated connection failures at perimeter or core 

columns. 
Appendix Q 

Q–6

• Perimeter columns may have been further weakened, with reduced ability to carry loads. 

Thermal effects could also cause inward bowing of perimeter columns due to differential 

temperatures between the inner and outer faces of the columns. The loads that could no 

longer be carried by the weakened columns would have been redistributed to adjacent 

perimeter columns. 

Column Instability and Collapse Initiation 

The perimeter columns were designed as part of a framed-tube system to carry both gravity and wind 

forces. Instability of perimeter columns resulted from a combination of (1) redistributed loads from the 

core columns via the floor system and possibly the hat truss, (2) inward bowing due to thermally 

weakened and sagging floors, (3) increased unsupported length due to failed floors, and (4) thermal 

effects directly on the perimeter columns. 

The instability of a few perimeter columns spread instability across the entire face and around the corners 

just before or during collapse initiation. The initiation or spread of perimeter column instability also may 

have been facilitated by the hoop stress demand on the framed-tube system exceeding the capacity of the 

spandrels (horizontal steel plates) that tied the perimeter columns together (e.g., at the northeast corner of 

WTC 2). 

The initiation of global collapse for both towers was first observed by the tilting of the sections above the 

impact regions of both WTC towers. The top section of WTC 1 rotated to the south (observed via 

antenna tilting in a video recording) and the top of WTC 2 rotated to the east and south and twisted in a 

counterclockwise motion. The primary direction of tilt of each tower was around the weaker axis of the 

core (north-south for WTC 1 and east-west for WTC 2). The rigid body rotation associated with the 

tilting and the propagation of column instability are synchronous processes that initiated global collapse. 

The rigid body rotation may have caused forces such as shear and torsion to spread the column instability 

laterally. 

Q.5 ISSUES STILL BEING INVESTIGATED 

Over the next few months, NIST will continue to investigate the following technical issues and modify its 

working hypothesis as needed. Findings on these issues will be included in the final report. 

• Aircraft impact damage to structural components, fireproofing, and hat truss connections. 

• Distribution of aircraft/building contents. 

• Thermal effects on core columns and core floors, especially extent of fires and growth 

history. 

• Thermal effects on welded perimeter columns, especially temperature gradients on columns. 

• Extent of load redistribution to intact core columns and their reserve capacity to 

accommodate thermal loads. 
NIST’s Working Hypothesis for Collapse of the WTC Towers 

Q–7

• Capacity of hat truss connections to perimeter columns, especially to meet the demands of 

aircraft impact and any torsional effects. 

• Capacity of hat truss to accommodate the load redistribution from severed columns. 

• Capacity of bolted splices in the severed core columns to carry loads to the hat truss. 

• Relative magnitude of the load redistribution provided by the local core floor, hat truss, and 

the core-truss floor system for each tower. 

• Axial/shear/bending capacity of floor connections to core and perimeter columns. 

• Effect of localized fires on floor truss connections. 

• Mechanisms to propagate instability laterally in the perimeter columns (e.g., shear and torsion 

forces induced by a rigid body movement) 

• Capacity of spandrels, including splices, to carry shear transfer in the framed-tube system, 

especially at the corners. 

• Role of bolted splices on instability of perimeter columns. 

• Outward bowing of perimeter columns due to thermal expansion of floors. 

• Effect of uneven floor thermal expansion on perimeter column instability due to potential 

biaxial bending. 

• Comparison and reconciliation of working hypothesis with observed facts (photographs and 

videos, eyewitness accounts, emergency communication records). 

• Examination of other possible or probable hypotheses. 
Appendix Q 

Q–8

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