NIST NCSTAR 1-3E

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Physical Properties of Structural

Steels

Stephen W. Banovic

Christopher N. McCowan

William E. Luecke

National Institute of Standards and Teclinology • Technology Administration • U.S. Deportmenl of Commerce

NIST NCSTAR1-3E

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Physical Properties of Structural

Steels

Stephen W. Banovic

Christopher N. McCowan

William E. Luecke

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

September 2005

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

Michelle O'Neill, Acting Under Secretary for Technology

National Institute of Standards and Technology

William Jeffrey, Director
Disclaimer No. 1

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 No. 2 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.

Disclaimer No. 3

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is "not directly related to the building failure being investigated" and that "disclosure of that information would inhibit the

voluntary provision of that type of information" (15 DSC 7306c).

In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been

provided to NIST under nondisclosure agreements.

Disclaimer No. 4 NIST takes no position as to whether the design or construction of a WTC building was compliant with any code

since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the

properties and condition of the materials used, or changes to the original construction made over the life of the

buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining

compliance when implementing building codes. Where an Investigation report states whether a system was

designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating

whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the requirement was met.

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 National Construction Safety Team Act Report 1-3E

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-3E, 162 pages (September 2005)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2005

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
Abstract

This report describes the physical properties of the structural steel recovered from the World Trade Center

(WTC) towers. Analytical techniques were used to determine and evaluate the chemistry, microstructurc,

and thermal properties of the steels. While not a physical property, hardness of the steels was also

measured and discussed in relation to strengthening mechanisms of the material. The primary focus was

on structural components with known as-built locations from WTC 1 and WTC 2. Evaluation of samples

without known as-built locations was conducted in order to fully characterize all of the structural elements. The physical property information was found useful in helping to identify specific grades and

producers of steel used for the various components. In addition, the thennal properties were developed

for the use in the models of the building response to fire. Although no recovered structural elements were from WTC 7, physical property data of steels from this building were estimated based upon values found

in the literature. In addition to the structural steel, chemistry information was measured for a piece of the aluminum fa9ade used on the WTC towers and the sprayed fire-resistive material applied to the structural elements of WTC 1 and WTC 2.

Keywords: Chemistry, hardness, microstructurc, physical property, steel, thermal properties. World Trade

Center.

NISTNCSTAR 1-3E, WTC Investigation iii
Abstract

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IV NISTNCSTAR 1-3E, WTC Investigation
Table of Contents

Abstract iii

List of Figures ix

List of Tables xiii

List of Acronyms and Abbreviations xv

Metric Conversion Table xvii

Preface xxi

Acknowledgments xxxi

Executive Summary xxxiii

Chapter 1 Introduction 1

1.1 Scope of Report 1

Chapter 2 Background of Structural Steels 3

2. 1 Brief Review of Structural Steels Specified in the Construction of the WTC Towers 3

2.1.1 Specified and Contemporaneously Available Steels for Construction of the

WTC Towers 4

Chapter 3 Experimental Procedures 5

3.1 Sample Documentation and Removal 5

3.2 Chemical Analysis 5

3 . 3 Metall ography 6

3.3.1 Sample Preparation 6

3.3.2 Ferrite Grain Size Measurements 6

3.3.3 Pearlite Volume Fraction Measurements 6

3.4 Hardness Testing 7

3.5 Furnace Exposure 7

Chapter 4

Chemical Analysis Results 1

1

4. 1 Exterior Panel Material 1

1

4.1.1 Perimeter Columns 11

NISTNCSTAR1-3E, WTC Investigation v
Table of Contents

4.1.2 Spandrel Plates 14

4. 1 .3 Spandrel Splice Plates 14

4. 1 .4 Floor Truss Connectors 15

4.2 Core Material 16

4.2.1 Wide Flange Core Columns 16

4.2.2 Built-Up Box Core Columns 16

4.2.3 Channel 16

4.2.4 Core Truss Seat 17

4.3 Floor Truss Material 17

4.3.1 Rod 17

4.3.2 Chord/Angle 17

4.4 Summary 17

Chapters

Metallographic Analysis Results 31

5 . 1 Exterior Panel Material 31

5.1.1 Perimeter Columns 31

5.1.2 Spandrel Plates 34

5.1 .3 Spandrel Splice Plates 36

5. 1 .4 Floor Truss Connectors 36

5.2 Core Material 38

5.2. 1 Wide Flange Core Columns 38

5.2.2 Built-Up Box Core Columns 38

5.2.3 Channel 38

5.2.4 Core Truss Seat 39

5.3 Floor Truss Material 39

5.3.1 Rod 39

5.3.2 Chord/Angle 39

5 .4 Furnace Exposure of WTC steel 40

5.4. 1 Hot-Rolled Flange and Spandrel Plates 40

5.4.2 Quenched-and-Tempered Flange Plate 41

5.4.3 Exterior Panel Floor Truss Seat 41

5.5 Summary 41

VI NISTNCSTAR 1-3E, WTC Investigation
Table of Contents

Chapter 6 Hardness Analysis Results 93

6. 1 Perimeter Panel Material 93

6.2 Core Column Material 93

6.3 Floor Truss Material 93

6.4 Panel Splice Conneetors and Truss Connectors 93

6.5 Hardness Traverses Through Welds on Perimeter Columns 94

6.6 Hardness of Furnace Exposed Flange Plates 94

6.6. 1 Hot-Rolled Flange and Spandrel Plates 94

6.6.2 Quenched-and-Tempered Flange Plate 95

6.6.3 Exterior Panel Floor Truss Seat 95

6.7 Summary 95

Chapter 7 Thermal Properties of WTC Structural Steel 103

7.1 Introduction 103

7.2 Specific Heat 103

7.2.1 Recommended Value 104

7.2.2 Uncertainties 104

7.3 Coefficient of Thermal Expansion 104

7.3.1 Recommended Value 106

7.3.2 Uncertainties 106

7.4 Thermal Conductivity and Diffusivity 106

7.4.1 Recommended Values 109

7.4.2 Uncertainties 109

7.5 Notes on Data Sources 109

7.5.1 Thermal Expansion Coefficient 109

Chapter 8 Analysis of Aluminum Fa9ade Used on the WTC Towers 117

Chapter 9

Analysis of Sprayed Fire-Resistive Material 119

Chapter 10

Findings and Conclusions 123

NISTNCSTAR 1-3E, WTC Investigation vii
Table of Contents

Chapter 11 References 125

1

vm NISTNCSTAR 1-3E, WTC Investigation
List of Figures

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

disaster xxiii

Figure 3-1. a) Photograph prior to sample removal from structural element, b) photograph

subsequent to sample removal from structural element, and c) photograph displaying

samples to be cut for further analysis from the specimen 8

Figure 3-2. Schematic indicating the viewing orientations for metallographic analysis 9

Figure 4-1. Plots of mass fraction element as a function of specified minimum yield strength for the flange plates from perimeter columns 18

Figure 4-2. Plots of mass fraction element as a ftinction of specified minimum yield strength for the outer web plates from perimeter columns 19

Figure 4-3. Plots of mass fraction element as a function of specified minimum yield strength for the inner web plates from perimeter columns 20

Figure 4-4. Plots of mass fraction element as a function of specified minimum yield strength for the spandrel plates from the exterior panels 21

Figure 5-1. Representative microstructures of hot-rolled perimeter column flange plates as a function of strength level 42

Figure 5-2. a) ASTM ferrite grain size number as a function of specified minimum yield strength

of the plate, and b) volume fraction of pearlite as a function of specified minimum

yield strength of the plate 45

Figure 5-3. Representative micrographs of ferrite morphologies observed from plates with the

specified minimum yield strength less than 70 ksi 46

Figure 5-4. Banding of microstructural constituents observed in a 60 ksi plate 47

Figure 5-5. Distribution of microstructural constituents observed in a 60 ksi plate 48

Figure 5-6. Morphologies of pearlite observed 49

Figure 5-7. Possible bainite or degenerate pearlite in lower strength plates 50

Figure 5-8. Non-metallic inclusions of MnS observed in the rolled plates 51

Figure 5-9. Representative microstructures of quenched-and-tempered perimeter column flange

plates as a ftinction of strength level 52

Figure 5-10. Microstructures from hot-rolled steels used for all inner web plates 54

Figure 5-11. Polished and etched cross-section of weld between inner web and flange 55

Figure 5-12. Representative micrographs from perimeter column welds of a hot-rolled steel with

Fy = 55ksi 56

NISTNCSTAR1-3E,'tAn'C Investigation ix
List of Figures

Figure 5-13. Representative microstructure of HAZ near fusion line from perimeter column weld of

a quenched-and-tempered steel with F, = 100 ksi 59

Figure 5-14. Microstructure from a column butt plate 60

Figure 5-15. Representative microstructures of hot-rolled perimeter column spandrel plates as a function of strength level 61

Figure 5-16. Representative microstructures of quenched-and-tempered perimeter column spandrel

plates as a function of strength level 65

Figure 5-17. Representative microstructure of spandrel splice plate 67

Figure 5-18. Partially decarburized zone found near the surface of a perimeter floor truss seat 68

Figure 5-19. Microstructure from an ASTM A 325 construction bolt 69

Figure 5-20. Microstructures observed for standoff plates cormecting floor truss seats to spandrel

plates 70

Figure 5-21. Etched cross-section of weld between spandrel plate 71

Figure 5-22. Etched cross-section of intact weld between a standoff plate and truss seat 72

Figure 5-23. Microstructure of a hot-rolled gusset plate welded to the top chord of the floor trusses 73

Figure 5-24. Microstructure of a hot-rolled damper plate 74

Figure 5-25. Microstructure of a hot-rolled gusset plate used to attach the damper units and the

diagonal bracing straps to the perimeter columns 75

Figure 5-26. Microstructure of a hot-rolled diagonal bracing strap 76

Figure 5-27. Microstrucmre of a hot-rolled 36 ksi rolled wide flange 77

Figure 5-28. Microstructure of a hot-rolled 42 ksi rolled wide flange 78

Figure 5-29. Microstructure of a hot-rolled 36 ksi plate from a built-up box core coluirai 79

Figure 5-30. Microstructure of a hot-rolled 42 ksi plate from a built-up box core column 80

Figure 5-3 1 . Microstructures from channel material located in the core 81

Figure 5-32. Microstructure of a hot-rolled core floor truss seat 82

Figure 5-33. Microstructure from floor truss rods 83

Figure 5-34. Microstructures from floor truss angles 85

Figure 5-35. Change in microstmcture of a 60 ksi flange plate that was heat treated in a laboratory

furnace at 625 °C for various times 86

Figure 5-36. Change in microstructure of a 42 ksi spandrel plate that was heat treated in a laboratory furnace at 625 °C for various times 87

Figure 5-37. Change in microstructure of a 100 ksi flange plate that was heat treated in a laboratory

furnace at 625 °C for various times 88

Figure 5-38. Change in microstructure of a truss seat that was heat treated in a laboratory furnace at 625 °C for various times 89

X NISTNCSTAR 1-3E, WTC Investigation
List of Figures

Figure 6-1 . Knoop hardness traverses through welded joints 96

Figure 6-2. Knoop hardness traverses through welded joints 97

Figure 6-3. Hardness as a function of time and temperature for ftimace exposure of 60 ksi flange

plate 98

Figure 6-4. Hardness as a function of time and temperature for furnace exposure of 42 ksi spandrel plate 98

Figure 7-1 . Heat capacity as a function of temperature for several low-alloy steels and pure iron 110

Figure 7-2. Thermal expansion of pure iron showing the discontinuity in thermal expansion

coefficient at the phase boundary 1 1

1

Figure 7-3. Instantaneous thermal expansion coefficient for several low-alloy steels 112

Figure 7-4. Thermal conductivity as a function of temperature for 12 low-alloy steels 113

Figure 7-5. Fractional error in estimating steel thermal conductivity from chemistry using

Eq. 7-15 114

Figure 7-6. Thermal conductivity as a function of temperature, calculated from Eq. 7-15 for four

grades of steel 114

Figure 8-1. Differential thermal analysis scan for aluminum fafade used on the WTC towers 117

Figure 9-1. Location of sprayed fire-resistive material that was scraped from a perimeter column

of panel S-1 120

NIST NCSTAR 1-3E, WTC Investigation xi
List of Figures

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xii NISTNCSTAR 1-3E, WTC Investigation
List of Tables

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

Table P-2. Public meetings and briefings of the WTC Investigation xxv

Table 4-1 . Comparison of chemistry results from two outside contractors and NIST 22

Table 4-2. Chemistry results of flange plates from perimeter columns 23

Table 4-3. Chemistry results of outer web plate from perimeter columns 24

Table 4-4. Chemistry results of inner web plate from perimeter columns 24

Table 4-5. Chemistry results from samples that had significantly outlying values from average

values of specified plate 25

Table 4-6. Chemistry results of panel spUce connectors and floor truss connectors 26

Table 4-7. Chemistry results of spandrel material from perimeter columns (in mass fraction x

100). Shown are the averages with standard deviations given directly below 27

Table 4-8. Chemistry results of core coluinn material (in mass fraction x 100) 28

Table 4-9. Chemistry results of floor truss material (in mass fraction x 100) 29

Table 5-1. ASTM grain size number and volume fraction pearlite for plates from the exterior

panels 90

Table 5-2. ASTM grain size number, volume fraction pearlite, and hardness results for panel

splice connectors and floor truss connectors 91

Table 5-3. ASTM grain size number and volume fraction pearlite for core column material 92

Table 5^. ASTM grain size number, volume fraction pearlite, and hardness results for floor

truss material 92

Table 6-1. Rockwell hardness data for exterior panel material 99

Table 6-2. Vickers hardness data for exterior panel material 100

Table 6-3. Rockwell and Vickers hardness data for core columns 101

Table 6—4. Hardness values for various furnace exposed WTC steel 102

Table 7-1. Mean thermal expansion coefficient 115

Table 8-1 . Chemistry results for the aluminum facade used on the WTC towers 118

Table 9-1 . Chemistry analysis of sprayed fire-resistive material 1 2

1

NIST NCSTAR1-3E, WTC Investigation xiii
List of Tables

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XIV NISTNCSTAR 1-3E, WTC Investigation
List of Acronyms and Abbreviations

MCrOliy illo

AISC American Institute of Steel Construction

ASCE American Society of Civil Engineers

ASTM ASTM International

CE carbon equivalent

CTE coefficient of thermal expansion

DTAP dissemination and technical assistance program

EDS energy dispersive spectroscopy

FCAW flux cored arc welding

FEMA Federal Emergency Management Agency

HAZ heat-affected zone HRB Rockwell Hardness B-Scale

HSLA high-strength, low alloy

NIST National Institute of Standards and Teclinology

R&D research and development

SFRM sprayed fire-resistive material

bMAW shielded metal arc welding

use United States Code

WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

WTC 2 World Trade Center 2 (South Tower)

WTC 7 World Trade Center 7

Abbreviations

°C degrees Celsius

op degrees Fahrenheit

|j.m micrometer

|am" square micrometer

Al aluminum

NISTNCSTAR 1-3E, WTC Investigation XV
List of Acronyms and Abbreviations

B boron

C carbon

Cr chromium

Cu copper

ft foot

Fy yield strength (AISC usage) -

.

g gram

gal gallon

h hour

in. inch

K kelvin ^ , kg kilogram

ksi 1 ,000 pounds per square inch

m" square meter

min minute

mL milliliter mm millimeter

mm' square millimeter

Mn manganese

Mo molybdenum

N nitrogen

Nb niobium

Ni nickel

P phosphorus

s second

S sulfur

Si silicon

Ti titanium

V vanadium

Zr zirconium

XVI NISTNCSTAR 1-3E,WTC Investigation
Metric Conversion Table

To convert from to Multiply by

AREA AND SECOND MOMENT OF AREA

square foot (ft")

square inch (in.")

square inch (in.")

square yard (yd")

square meter (m")

square meter (m")

square centimeter (cm")

square meter (m")

9.290 304 E-02

6.451 6 E-04

6.451 6 E+00

8.361 274 E-01

ENERGY (includes WORK)

kilowatt hour (kW h)

quad (1015 BtuIT)

therm (U.S.)

ton of TNT (energy equivalent)

watt hour (W h)

watt second (W • s)

joule (J)

joule (J)

joule (J)

joule (J)

joule (J)

joule (J)

3.6 E+06

1.055 056 E+18

1.054 804 E+08

4.184 E+09

3.6 E+03

1.0 E+00

LENGTH

foot (ft)

inch (in)

inch (in.)

micron (m)

yard (yd)

meter (m)

meter (m)

centimeter (cm)

meter (m)

meter (m)

3.048 E-01

2.54 E-02

2.54 E+00

l.OE-06

9.144 E-Ol

MASS DIVIDED BY AREA

pound per square foot (lb/ft")

pound per square inch

{not pound force) (lb/in.")

kilogram per square meter (kg/m") 4.882 428 E+00

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

MASS DIVIDED BY LENGTH

pound per foot (lb/ft)

pound per inch (lb/in.)

pound per yard (lb/yd)

kilogram per meter (kg/m)

kilogram per meter (kg/m)

kilogram per meter (kg/m)

1.488 164 E+00

1.785 797 E+01

4.960 546 E-01

NISTNCSTAR 1-3E, WTC Investigation xvn
Metric Conversion Table

To convert from to Multiply by

PRESSURE or STRESS (FORCE DIVIDED BY AREA)

kilogram-force per square centimeter (kgf/cm") pascal (Pa)

kilogram-force per square meter (kgf/m") pascal (Pa)

kilogram-force per square millimeter (kgf/mm") pascal (Pa)

kip per square inch (ksi) (kip/in.") pascal (Pa)

kip per square inch (ksi) (kip/in.") kilopascal (kPa)

pound-force per square foot (Ibf/ft") pascal (Pa)

pound-force per square inch (psi) (Ibf/in.") pascal (Pa)

pound-force per square inch (psi) (Ibf/in.") kilopascal (kPa)

psi (pound-force per square inch) (Ibf/in.") pascal (Pa)

psi (pound-force per square inch) (Ibf/in.") kilopascal (kPa)

9.806 65 E+04

9.806 65 E+00

9.806 65 E+06

6.894 757 E+06

6.894 757 E+03

4.788 026 E+01

6.894 757 E+03

6.894 757 E+00

6.894 757 E+03

6.894 757 E+00

TEMPERATURE

degree Celsius (°C)

degree centigrade

degree Fahrenheit (°F)

degree Fahrenheit (°F)

kelvin (K)

kelvin (K)

degree Celsius (°C)

degree Celsius (°C)

kelvin (K)

degree Celsius (°C)

T/K = t/°C + 273.15

t/°C = t /deg. cent.

t/°C = (t/°F- 32)/1.8 T/K = (t/°F + 459.67)/l.

t/°C = T/K 2 273.15

TEMPERATURE INTERVAL

degree Celsius (°C)

degree centigrade

degree Fahrenheit (°F)

degree Fahrenheit (°F)

degree Rankine (°R)

kelvin (K)

degree Celsius (°C)

degree Celsius (°C)

kelvin (K)

kelvin (K)

1.0 E+00

1 .0 E+00

5.555 556 E-01

5.555 556 E-01

5.555 556 E-01

VELOCITY (includes SPEED)

foot per second (ft/s)

inch per second (in./s)

kilometer per hour (km/h)

mile per hour (mi/h)

mile per minute (mi/min)

meter per second (m/s)

meter per second (m/s)

meter per second (m/s)

kilometer per hour (km/h)

meter per second (m/s)

3.048 E-01

2.54 E-02

2.777 778 E-01

1.609 344 E+00

2.682 24 E+01

XVlll NISTNCSTAR 1-3E, WTC Investigation
To convert from to Multiply by

VOLUME (includes CAPACITY)

cubic foot (ft')

cubic inch (in/ )

cubic yard (yd')

gallon (U.S.) (gal)

gallon (U.S.) (gal)

liter (L)

ounce (U.S. fluid) (fl oz)

ounce (U.S. fluid) (fl oz)

cubic meter (m')

cubic meter (m')

cubic meter (m')

cubic meter (m')

liter (L)

cubic meter (m')

cubic meter (m')

milliliter (mL)

2.831 685 E-02

1.638 706E-05

7.645 549 E-01

3.785 412 E-03

3.785 412 E+00

1.0 E-03

2.957 353 E-05

2.957 353 E+01

NISTNCSTAR 1-3E, WTC Investigation
Metric Conversion Table

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XX NISTNCSTAR 1-3E, WTC Investigation
Preface

Genesis of This Investigation

Immediately following the terrorist attack on the World Trade Center (WTC) on September 1 1 , 2001 , the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began

planning a building perfomiance study of the disaster. The week of October 7, as soon as the rescue and

search efforts ceased, the Building Performance Study Team went to the site and began its assessment. This was to be a brief effort, as the study team consisted of experts who largely volunteered their time

away from their other professional commitments. The Building Performance Study Team issued its report in May 2002, fulfilling its goal "to determine probable failure mechanisms and to identify areas of

future investigation that could lead to practical measures for improving the damage resistance of buildings

against such unforeseen events."

On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of

Standards and Technology (NIST) announced its building and fire safety investigation of the WTC

disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-23 1), was

signed into law. The NIST WTC Invesfigation was conducted under the authority of the National

Construction Safety Team Act.

The goals of the investigation of the WTC disaster were:

• To investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster.

• 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; and - Improved public safety.

The specific objectives were:

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 injuries and fatalities were so high or low depending on location,

including all 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; and

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

practices that warrant revision.

NISTNCSTAR 1-3E, WTC Investigation XXI
Preface

NIST is a nonregulatory agency of the U.S. Department of Commerce's Technology Administration. The

purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United

States, and the focus is on fact finding. NIST investigative teams are authorized to assess building

performance and emergency response and evacuation procedures in the wake of any building failure that has resulted in substantial loss of life or that posed significant potential of substantial loss of life. NIST

does not have the statutory authority to make findings of fault nor negligence by individuals or

organizations. Further, no part of any report resulting from a NIST investigation into a building 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 Public

Law 107-231).

Organization of the Investigation

The National Construction Safety Team for this Investigation, appointed by the then NIST Director, Dr. Arden L. Bement, Jr., was led by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as Associate Lead Investigator, Mr. Stephen A. Cauffman served as Program Manager for Administration,

and Mr. Harold E. Nelson served on the team as a private sector expert. The Investigation included eight

interdependent projects whose leaders comprised the remainder of the team. 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 Fig. P-1.

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

Technical Area and Project Leader Project Purpose

Analysis of Building and Fire Codes and

Practices; Project Leaders: Dr. H. S. Lew

and Mr. Richard W. Bukowski

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 WTC 1, 2, and 7. Baseline Structural Perfonnance and

Aircraft Impact Damage Analysis; Project

Leader: Dr. Fahim H. Sadek

Analyze the baseline perfonnance of WTC 1 and WTC 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; Project Leader: Dr. Frank

W. Gayle

Detennine 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; Project Leader: Dr. David

D. Evans; Dr. William Grosshandler

Investigate the perfonnance 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 Thennal and Tenability

Environment; Project Leader: Dr. Richard

G. Gann

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; Project Leaders: Dr. John

L. Gross and Dr. Theresa P. McAllister

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

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

of composite steel-trussed floor systems, and detennine the most

probable structural collapse sequence for WTC 1, 2, and 7. Occupant Behavior, Egress, and Emergency

Communications; Project Leader: Mr. Jason

D. Averill

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.

Emergency Response Technologies and

Guidelines; Project Leader: Mr. J. Randall

Lawson

Document the activities of the emergency responders from the time

of the terrorist attacks on WTC 1 and WTC 2 until the collapse of

WTC 7, including practices followed and technologies used.

xxu NISTNCSTAR 1-3E, WTC Investigation
Preface

NIST WTC Investigation Projects

WTC Buildi Performanc

Recommendatto

Government,

Industry,

Professional,

Academic Inputs

Figure P-1. The eight projects in the federal building and fire safety

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction

Safety Team Act. The initial members of the committee were appointed following a public solicitation. These were:

• Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety

Team Advisory Committee Chair

• John Barsom, President, Barsom Consulting, Ltd.

• John Bryan, Professor Emeritus, University of Maryland

• David Collins, President, The Preview Group, Inc.

• Glenn Corbett, Professor, John Jay College of Criminal Justice

• Philip DiNenno, President, Hughes Associates, Inc.

NISTNCSTAR 1-3E, WTC Investigation xxin
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

• Charles Thornton, Co-Chairman and Managing Principal, The Thomton-Tomasetti Group,

Inc.

• Kathleen Tiemey, Director, Natural Hazards Research and Applications Information Center,

University of Colorado at Boulder

• Fonnan Williams, Director, Center for Energy Research, University of California at San

Diego

This National Construction Safety Team Advisory Committee provided technical advice during the

Investigation and commentary on drafts of the Investigation reports prior to their public release. NIST

has benefited from the work of many people in the preparation of these reports, including the National

Construction Safety Team Advisory Committee. The content of the reports and recommendations,

however, are solely the responsibility of NIST.

Public Outreach

During the course of this Investigation, NIST held public briefings and meetings (listed in Table P-2) to

solicit input from the public, present preliminary findings, and obtain comments on the direction and

progress of the Investigation from the public and the Advisory Committee.

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site contained extensive infonnation on the background and progress of the Investigation.

NIST's WTC Public-Private Response Plan

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed,

constructed, maintained, and used, especially with regard to major events such as fires, natural disasters,

and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support

from Congress and the Administration, has put in place a program, the goal of which is to develop and

implement the 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:

• 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

building, and the associated evacuation and emergency response experience.

• A research and development (R&D) program to (a) facilitate the implementation of

recommendations resulting from the WTC Investigation, and (b) provide the technical basis

for cost-effective improvements to national building and fire codes, standards, and practices

that enhance the safety of buildings, their occupants, and emergency responders.

XXIV NISTNCSTAR 1-3E, WTC Investigation
Preface

Table P-2. Public meetings and briefings of the WTC Investigation.

Date Location Principal Af^enda

June 24, 2002 New York City, NY Public meeting: Public comments on the Draft Plan for the pending WTC Investigation.

August 21, 2002 Gaithersburg, MD Media briefing announcing the formal start of the Investigation.

December 9, 2002 Washington, DC Media briefing on release of the Public Update and NIST request

for photographs and videos. April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person interviews. April 29-30, 2003 Gaithersburg, MD NCST Advisory Committee meeting on plan for and progress on WTC Investigation with a public comment session. May 7, 2003 New York City, NY Media briefing on release of May 2003 Progress Report.

August 26-27, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status of the WTC

investigation with a public comment session. September 17, 2003 New York City, NY Media and public briefing on inifiation of first-person data

collection projects.

December 2-3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results and release of the Public Update with a public comment session. February 12, 2004 New York City, NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in formulating final recommendations.

June 18,2004 New York City. NY Media/public briefing on release of June 2004 Progress Report. June 22-23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and

preliminary findings from the WTC Investigation with a public

comment session. August 24, 2004 Northbrook, IL Public viewing of standard fire resistance test of WTC floor system at Underwriters Laboratories, Inc. October 19-20, 2004 Gaithersburg, MD NCST Advisory Committee meeting on status and near complete

set of preliminary findings with a public comment session.

(iVJal?t 1 1lllCi riprcoUUl ni tro" 1VJ_L/ \A INV^ljI /ALIViJiUiy V, Ulllllll I ICC LiiJ>C LloolUll KJll Lllail dlLLlUal 1 C UtJl I IVJ Congress, a public comment session, and a closed session to discuss pre-draft recommendations for WTC Investigation.

April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse

sequence for the WTC towers and draft reports for the projects on

codes and practices, evacuation, and emergency response. June 23, 2005 New York City, NY Media and public briefing on release of all draft reports for the WTC towers and draft recommendations for public comment.

September 12-13,

2005

Gaithersburg, MD NCST Advisory Committee meeting on disposition of public

comments and update to draft reports for the WTC towers. September 13-15,

2005

Gaithersburg, MD WTC Technical Conference for stakeholders and technical community for dissemination of findings and recommendations

and opportunity for public to make technical comments.

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

construction and building community in ensuring timely adoption and widespread use of

proposed changes to practices, standards, and codes resulting from the WTC Investigation

and the R&D program, and (b) 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 buildings, occupants, and first responders safer in future disaster events.

NISTNCSTAR 1-3E, WTC Investigation XXV
Preface

National Construction Safety Team Reports on the WTC Investigation

A final report on the collapse of the WTC towers is being issued as NIST NCSTAR 1. A companion

report on the collapse of WTC 7 is being issued as NIST NCSTAR 1 A. The present report is one of a set that provides more detailed documentation of the Investigation findings and the means by which these

technical results were achieved. As such, it is part of the archival record of this Investigation. The titles of the full set of Investigation pubhcations are:

NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse ofthe World Trade

Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September.

NIST (National Institute of Standards and Technology). 2006. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. NIST NCSTAR 1 A. Gaithersburg, MD.

Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Design, Construction, and Maintenance ofStructural and Life Safety

Systems. NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Design and Construction ofStructural Systems.

NIST NCSTAR 1-1 A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Ghosh, S. K., and X. Liang. 2005. Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Comparison ofBuilding Code Structural Requirements. NIST

NCSTAR 1-lB. National Institute of Standards and Technology. Gaithersburg, MD, Septem.ber.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Maintenance and Modifications to Structural

Systems. NIST NCSTAR 1-lC. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and

Construction of World Trade Center ], 2, and 7 and Post-Construction Provisions Applied after Occupancy. NIST NCSTAR 1-lD. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Razza, J. C, and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time ofthe

Design and Construction of World Trade Center 1, 2, and 7. NIST NCSTAR 1-lE. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safet}>

Investigation ofthe World Trade Center Disaster: Comparison ofthe 1968 and Current (2003) New

xxvi NIST NCSTAR 1-3E, WTC Investigation
Preface

York City Building Code Provisions. NISTNCSTAR 1-lF. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safely Investigation ofthe World

Trade Center Disaster: Amendments to the Fire Protection and Life Safely Provisions of the New

York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in Use. NIST NCSTAR 1-lG. Nationalhistitute of Standards and Technology. Gaithersburg, MD,

September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safely Systems

of World Trade Center 1 and 2. NIST NCSTAR 1-lH. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life

Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR l-ll. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Design, Installation, and Operation ofFuel System for Emergency Power in World Trade Center 7. NIST NCSTAR 1-1 J. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Sadek, F. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Baseline Structural Performance and Aircraft Impact Damage Analysis ofthe World Trade Center

Towers. NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Faschan, W. J., and R. B. Garlock. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of

the World Trade Center Towers. NIST NCSTAR 1-2A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson, R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Analysis ofAircraft Impacts into the World Trade Center Towers, NIST

NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Gayle, F. W., R. J. Fields, W. E. Luecke, S. W. Banovic, T. Foecke, C. N. McCowan, T. A. Siewert. and

J. D. McColskey. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Mechanical and Metallurgical Analysis ofStructural Steel. NIST NCSTAR 1-3. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Contemporaneous Structural Steel

Specifications. NIST Special Publication 1-3 A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

NISTNCSTAR 1-3E, WTC Investigation xxvn
Preface

Banovic, S. W. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Steel Invenloiy and Identification. NIST NCSTAR 1-3B. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Damage and Failure Modes ofStructural Steel Components. NIST

NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., J. D. McColskey, C. N. McCowan, S. W. Banovic, R. J. Fields, T. Foecke,

T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Mechanical Properties ofStructural Steels. NIST NCSTAR 1-3D.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Banovic, S. W., C. N. McCowan, and W. E. Luecke. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Physical Properties ofStructural Steels. NIST

NCSTAR 1-3E. National Institute of Standards and Technology. Gaithersburg, MD, September.

Evans, D. D., R. D. Peacock, E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Active Fire Protection

Systems. NIST NCSTAR 1-4. National Institute of Standards and Teclinology. Gaithersburg, MD,

September.

Kuligowski, E. D., D. D. Evans, and R. D. Peacock. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Post-Construction Fires Prior to September 11,

2001. NIST NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Hopkins, M., J. Schoenrock, and E. Budnick. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Fire Suppression Systems. NIST NCSTAR 1-4B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Keough, R. J., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation ofthe

World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR I-4D. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Gann, R. G., A. Hamins, K. B. McGrattan, G. W. Mulholland, H. E. Nelson, T. J. Ohlemiller,

W. M. Pitts, and K. R. Prasad. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Reconstruction ofthe Fires in the World Trade Center Towers. NIST NCSTAR 1-5. National Institute of Standards and Technology. Gaithersburg, MD, September.

Pitts, W. M., K. M. Butler, and V. Junker. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis.

NIST NCSTAR 1-5A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

xxvni NIST NCSTAR 1-3E, WTC Investigation
Preface

Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemillcr, M. Donnelly,

J. Yang, G. Mulholland, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal

Building and Fire Safety Invesligation ofthe World Trade Center Disaster: Experiments and

Modeling ofStructural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Ohlemiller, T. J., G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Fire Tests ofSingle

Office Workstations. NIST NCSTAR 1-5C. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhorn, and P. A. Hough. 2005.

Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Reaction of

Ceiling Tile Systems to Shocks. NIST NCSTAR 1-5D. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Hamins, A., A. Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofMultiple Workstations Burning in a Compartment. NIST NCSTAR I-5E. National

Institute of Standards and Technology. Gaithersburg, MD, September.

McGrattan, K. B., C. Bouldin, and G. Forney. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Computer Simulation ofthe Fires in the World

Trade Center Towers. NIST NCSTAR 1-5F. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Prasad, K. R., and H. R. Baum. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Structure Interface and Thermal Response ofthe World Trade Center

Towers. NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Gross, J. L., and T. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Fire Response and Probable Collapse Sequence ofthe World Trade Center

Towers. NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Carino, N. J., M. A. Stames, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski.

2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Passive

Fire Protection. NIST NCSTAR 1-6A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005. Federal Building and

Fire Safety Investigation ofthe World Trade Center Disaster: Fire Resistance Tests ofFloor Truss

Systems. NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Zarghamee, M. S., S. Bolourchi, D. W. Eggers, O. O. Erbay, F. W. Kan, Y. Kitane, A. A. Liepins,

M. Mudlock, W. I. Naguib, R. P. Ojdrovic, A. T. Sarawit, P. R Barrett, J. L. Gross, and

NIST NCSTAR 1-3E, WTC Investigation XXIX
Preface

T. P. McAllister. 2005. Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis. NISTNCSTAR 1-6C.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Zarghamee, M. S., Y. Kitane, O. O. Erbay, T. P. McAllister, and J. L. Gross. 2005. Federal

Building and Fire Safety^ Investigation ofthe World Trade Center Disaster: Global Structural

Analysis of the Response ofthe World Trade Center Towers to Impact Damage and Fire. NIST

NCSTAR 1-6D. National Institute of Standards and Technology. Gaithersburg, MD, September.

McAllister, T., R. W. Bukowski, R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, L. Phan,

W. M. Pitts, K. R. Prasad, F. Sadek. 2006. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade

Center 7. (Provisional). NIST NCSTAR 1-6E. National Institute of Standards and Technology.

Gaithersburg, MD.

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva,

J. Oh, K. Rubenacker. 2006. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Analysis ofthe Response of World Trade Center 7 to Debris Damage

and Fire. (Provisional). NIST NCSTAR 1-6F. National Institute of Standards and Technology.

Gaithersburg, MD.

Kim, W. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Analysis ofSeptember II, 2001, Seismograni Data. (Provisional). NIST NCSTAR 1-6G.

National Institute of Standards and Technology. Gaithersburg, MD.

Nelson, K. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: The Con Ed Substation in World Trade Center 7. (Provisional). NIST NCSTAR 1-6H.

National Institute of Standards and Technology. Gaithersburg, MD.

Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, P. A. Reneke, and

H. E. Nelson. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Occupant Behavior, Egress, and Emergency Communication. NIST NCSTAR 1-7. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Fahy, R., and G. Proulx. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Analysis ofPublished Accounts ofthe World Trade Center Evacuation. NIST

NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Zmud, J. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Technical Documentationfor Sur\'ey Administration. NISTNCSTAR 1-7B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Lawson, J. R., and R. L. Vettori. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of

Standards and Technology. Gaithersburg, MD, September.

XXX NISTNCSTAR 1-3E, WTC Investigation
Acknowledgments

The National Institute of Standards and Technology (NIST) thanks the volunteers of the Structural

Engineers Association of New York for their efforts in the recovery of the steel components. Countless

hours were unselfishly spent in the recovery yards searching for these invaluable pieces that are an

integral component of this investigation. Without their efforts, this report would not have been possible.

The Federal Emergency Management Agency/American Society of Civil Engineers Building

Performance Study Team; Professor A. Astaneh-Asl of the University of California, Berkeley, California;

and the National Science Foundation are also acknowledged for their help in the recovery effort.

Mr. David R. Kelley of NIST is recognized for all of his efforts in sample removal, metallographic

preparation, and hardness evaluation of the World Trade Center (WTC) steels.

Dr. Kil-Won Moon of NIST is recognized for the differential thermal analysis of the aluminum fa9ade

used on the WTC towers.

Ms. Maureen E. Williams of NIST is recognized for characterizing the sprayed fire-resistive material

removed from a perimeter column from the WTC towers.

Mr. Rodney D. Jiggetts of NIST is recognized for filming sample removal during the initial stages of the

investigation.

Ms. Sandra Claggett of NIST is recognized for conducting the initial volume fraction pearlite and grain

size measurements.

Dr. Donald E. Kramer and Dr. Michael F. Savage, both formerly of NIST, are recognized for their help in sample removal in the early stages of the investigation.

Dr. Gregory C. Turk, Dr. John R. Sieber, and Dr. Michael R. Winchester of NIST are recognized for their contributions to the chemical analysis of the structural steels.

Mr. Arlan O. Benscoter and Dr. Arnold R. Marder of the Department of Materials Science and

Engineering, Lehigh University, are thanked for the numerous conversations concerning the

metallography of carbon steels and review of this report.

NISTNCSTAR 1-3E, WTC Investigation xxxi
Acknowledgments

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xxxn NISTNCSTAR 1-3E, WTC Investigation
Executive Summary

This report describes the physical properties of the structural steel recovered from the World Trade Center

(WTC) towers, including the chemical composition, microstructure, and thermal properties of the steels. While not a physical property, hardness of the steels also was measured and discussed in relation to strengthening mechanisms of the material. The primary focus was on structural components with known

as-built locations from WTC 1 and WTC 2. hi order to fully characterize all of the structural elements, some samples were evaluated that did not have known as-built locations. The physical property

infonnation was found useful in helping to identify specific grades and producers of steel used for the

various components located within the towers. In addition, the thermal properties were developed for use

in the models of the building response to fire. Ahhough no recovered structural elements were from

WTC 7, physical property data of steels from this building were estimated based upon values found in the

literature.

In addition to the structural steel, chemistry information was obtained for a piece of the aluminum fa9ade

used on the WTC towers and the sprayed fire-resistive material (SFRM) found on the structural elements

from WTC 1 and WTC 2.

The major findings of this report are:

• Two different microstructural types were observed for the recovered WTC steels: hot-rolled

for low strength components and quenched-and-tempered for high strength components.

• In general, the steel used for the construction of the WTC towers was found to be of good

quality. There did not appear to be atypical defects in the microstructure as a result of

processing of the individual plates. Further, the weldments used to build-up the box columns

and attach the floor truss connectors to the columns were sound with no visible flaws.

• For a given yield strength (F,.), the flange, outer web, and spandrel steel in the perimeter

panels were found to be nearly identical, with indistinguishable chemistries and similar

microstructures and hardness values. The inner web was found to be distinguishable from the

other plates, for F,. less than 80 ksi, in terms of the physical properties evaluated. Irmer webs

with F, equal to 80 ksi and 100 ksi were the same steel as used for the flanges. These results support the contemporaneous documents stating that the flange, outer web, and spandrel

plates were produced by a single company (Yawata Iron and Steel Co.) and that the inner web

plates were primarily supplied domestically.

• The recovered core columns were found to vary in composition for a given F, and shape

(buih-up box, rolled wide-flange). This resuh is consistent with the contemporaneous

documents stating that numerous suppliers produced the steel to be used for these structural components.

• Butt plates of the perimeter panel met chemistry specifications for several Yawata steels.

NISTNCSTAR 1-3E, WTC Investigation xxxni
Executive Summary

• Construction bolts were either A 325 or A 490 bolts. This was primarily determined by the

stampings on the head. One bolt designated as A 325 was analyzed and met the chemistry

specifications for A 325 Type 1.

• Chords and rods that composed the floor trusses, supplied by Laclede Steel Co., were either

ASTM International (ASTM) A 36 or ASTM A 242.

• Other structural elements (floor truss connectors, core channels, spandrel splice plates, etc.) met specifications for ASTM A 36. Compositions varied significantly, which may indicate

that numerous suppliers completed the steel orders for these components.

• Discernible changes to the microstructure of furnace exposed WTC steel were observed when

exposed to a temperature of 625 °C for as little as 0.25 h. At or below 500 °C, no indication

of microstructural change was apparent using light optical microscopy. The hardness of these

samples varied for a given temperature and material type, but correlated well with

microstructural observations: '

- Hot-rolled steels below 500 °C experienced a slight increase in hardness, which may be

due to strain-age hardening effects;

- Hot-rolled steels at 500 °C and 625 °C experienced a decrease in hardness. Depending

upon the prior thermomechanical history, an increase in hardness was observed if the extent of spheroidization was such that large carbide particles fonned; and

- Quenched-and-tempered steels at 625 °C had a continual decrease in hardness up to the

maximum exposure time of 2 h.

These results were used in the report "Damage and Failure Modes of Structural Steel

Components" (NIST NCSTAR 1-3C)' to assess the steel for fire-related damage.

• Microhardness traces across weld cross-sections from perimeter panel columns displayed an

increase in hardness near the fusion line in the heat affected zone of the base plate. This was

related to the alteration of microstructure due to the heat input from the welding process.

• Thermal property values as a function of temperature were provided for modeling the

structural steel response to fire. These properties include specific heat {Cp), coefficient of

thermal expansion (a), and thermal conductivity (k).

• A piece of the aluminum facade used on the WTC towers was analyzed and found to be

consistent with the non-heat treatable aluminum-manganese alloy AA 3003.

• A sample of SFRM scraped from a recovered perimeter column of WTC 1 was chemically

analyzed. While the chemical composition of the material was proprietary, the results were

consistent, on a qualitative basis, with the information presently found on the manufacturer's

web site.

This reference is to one of the companion documents from this Investigation. A list of these documents appears in the Preface

to this report.

xxxiv NIST NCSTAR 1-3E, WTC Investigation
Chapter 1 Introduction

The purpose of the mechanical and metallurgical analysis of structural steel was to analyze structural steel available from World Trade Center (WTC) 1, 2, and 7 for detennining the metallurgical and mechanical

properties and quality of the metal, weldments, and connections, and providing these data to other

investigation projects. The properties determined were used in two ways: (1) properties were correlated

with the design requirements of the buildings to determine whether the specified steel met the design

requirements, and (2) properties were supplied as input for models of building performance.

This report is an output of a specific task of the WTC Investigation defined as:

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 coluirms, welds,

spandrels, trusses, truss seats, and fasteners. The identification will

include composition, microstructure, mechanical, and impact properties.

This project 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 and propagation of structural collapse

in the project on Structural Fire Response and Collapse Analysis.

1.1 SCOPE OF REPORT

This report describes in detail the physical properties of the structural steel used in the construction of the WTC towers, including the chemical composition, microstructure, and thermal properties of the steels. While not a physical property, hardness of the steels was also measured and discussed in relation to strengthening mechanisms of the material. Physical property information was useful in helping to identify specific grades and producers of steel used for the various components located within the towers. These analyses were made only for steel from WTC 1 and WTC 2 as no steel was recovered from

WTC 7. Physical property data of steels from this building were estimated based upon values found in the literature. In addition, thermal properties were developed for use in models of the building response

to fire.

In addition to the structural steel, chemistry information was obtained for the aluminum fafade used on

the WTC towers and the sprayed fire-resistive material applied to the structural elements of WTC 1 and

WTC 2.

NISTNCSTAR 1-3E, WTC Investigation 1
Chapter 1

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2 NISTNCSTAR 1-3E, WTC Investigation
Chapter 2

Background of Structural Steels

2.1 BRIEF REVIEW OF STRUCTURAL STEELS SPECIFIED IN THE

CONSTRUCTION OF THE WTC TOWERS

The report "Contemporaneous Structural Steel Specifications" (NIST NCSTAR 1-3A)' presents a detailed discussion of the types of structural steel specified in the design drawings for the World Trade

Center (WTC) towers, and thus, these steels will be only briefly reviewed here. The major structural steel components used in the WTC towers were primarily low carbon alloy steels with carbon contents less than 0.2 percent. (The reader is referred to other sources for an in-depth review of the microstructures of

the Fe-C steels [Irvine 1967; Krauss 1980; Leslie 1981; Samuels 1980; Gladman 1996; Bramfitt and

Benscoter 2002].) Both low-alloy hot-rolled and quench-and-tempered high-strength, low alloy grades were used in the WTC towers; the latter for applications where a higher strength to weight ratio was

beneficial, such as for the exterior columns on upper floors of the building. Unfortunately no direct

references discussing specific production conditions for WTC steels were noted in the open literature.

It was found that Skilling, Helle, Christiansen, & Robertson, structural engineers for the WTC towers,

specified the type of steel to be used for each structural piece by the minimum specified yield strength.

For example, a "50 ksi steel" is a steel with a minimum yield strength of 50,000 pounds per square inch.

Thus, this report will continue with this designation for the steel The structural plans called for the

following:

• Perimeter columns were fabricated from 12 grades of steel with F, = (36, 42, 45, 46, 50, 55,

60, 65, 70, 75, 80, and 100) ksi. (Two additional grades, 85 ksi and 90 ksi, were specified,

but later replaced with 100 ksi steel).

• Spandrels were to be fabricated from 12 grades of steel with F,. = (36, 42, 45, 46, 50, 55, 60,

65, 70, 75, 80, and 85) ksi.

• Built-up core box columns were to be fabricated from two grades of steel with 36 ksi and

42 ksi minimum yield strengths.

• Rolled wide-flange shapes were to be fabricated from steels with F, = (36, 42, 45, and

50) ksi, but there were very few 45 ksi or 50 ksi core columns.

• For the floor trusses, the top chord angles, as well as most round bars, were to be fabricated to meet ASTM International ( ASTM) A 242 (F, = 50 ksi); some of the round bars and the

bottom chord angles were specified as ASTM A 36.

• Floor truss seats were specified to be fabricated from steel with F,. = 36 ksi minimum.

This reference is to one of the companion documents from this Investigation. A Hst of these documents appears in the Preface

to this report.

NIST NCSTAR 1-3E, WTC Investigation 3
Chapter 2

For the floor truss components, specific ASTM materials were specified for fabrication. However, while

specifications (ASTM, American Iron and Steel Institute, etc.) typically place limits on chemical

composition or mechanical properties or, most commonly, both, it is important to remember that an ASTM standard can permit a wide variety of steel compositions and strengths. Therefore, a specific steel might be capable of meeting several distinct ASTM steel standards. For instance, ASTM A 36 only

specifies 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. Thus, many of the steels used in the construction of the WTC towers may not be

unambiguously identified as to their exact specifications.

2.1.1 Specified and Contemporaneously Available Steels for Construction of the

WTC Towers

There were many domestic and Japanese steels allowed in the contracts between the Port Authority of

New York and New Jersey and all the fabricators. A collection of relevant ASTM structural steel standards from 1970, as well as data relating to the physical and mechanical properties of the Japanese

steels, can be found in NIST NCSTAR 1-3 A.

4 NIST NCSTAR 1-3E, WTC Investigation
Chapter 3

Experimental Procedures

This chapter describes the practices and procedures used to analyze and evaluate the physical properties

of the recovered World Trade Center (WTC) steel.

All samples for analysis underwent similar documentation and removal procedures from the recovered

structural members. Once a specific sample was identified to be removed, the sample was given an alphanumeric identification code, marked on the component with rolling direction of the plate specified,

and a photograph taken of the sample to indicate location and orientation with respect to the entire

element (Fig. 3-1 a). The sample was then removed through one of two methods depending upon

physical access to the sample, the size, and/or thickness of the plate being section. The removal methods

used were either mechanical sawing (reciprocating saw, circular saw with carbide tipped blade) or flame

cutting with an oxyacetylene torch. Samples were typically cut larger then necessary in order to preserve

a central portion of the sample that was unaffected (in terms of the original structure and properties) by

the removal method. Once the sample was free of the structural element, a second photograph was taken

with the sample positioned near its original location (Fig. 3-lb). Additional documentation was made in the laboratory after the sample had been marked for further sectioning for the analyses to be conducted

(Fig. 3-1 c). This section was done either by an abrasive cutoff saw or high speed saw with diamond or alumina blade. For all samples, two metallographic samples ("//" was a sample where the rolling plane was viewed and was a longitudinal section perpendicular to the rolled surface), a sample for chemical

analysis ("C"), and a sample for Rockwell Hardness testing ("H") were sectioned from the main piece.

Portions of all samples were sent to an outside contractor (Chicago Spectro Service Laboratory, Inc., Chicago, Illinois) for optical emissions spectroscopy. The following elements were analyzed: C, Mn, P,

S, Si, Ni, Cr, Mo, Cu, V, Nb, Ti, Zr, Al, B, and N. Iron was assumed to be the balance. Tests were

conducted in accordance to ASTM International (ASTM) E 415 for all elements except C; ASTM E 1019 was used for this element. The company has an annually renewed accreditation with the American

Association of Analytical Laboratories.

During the initial stages of the investigation, a second contractor (Laboratory Testing, Inc., Hatfield,

Pennsylvania) was also used to check for accuracy of results. Nineteen samples were analyzed with

measurements made according to ASTM standards E 227, E 404, E 415, E 607, and E 1086 using optical

emission spectroscopy for all elements except carbon, nitrogen, and sulfur. The concentrations of carbon,

nitrogen, and sulfur were determined using LECO gas analysis according to ASTM E 1019.

Two samples were tested internally at the National Institute of Standards and Technology (NIST). The

two methods used were X-ray fluorescence and Glow Discharge Optical Emission Spectroscopy. Both

3.1 SAMPLE DOCUMENTATION AND REMOVAL

3.2 CHEMICAL ANALYSIS

NISTNCSTAR 1-3E, WTC Investigation 5
Chapter 3

measurements were made using instrumentation calibrated with NIST Steel Standard Reference

Materials.

3.3 METALLOGRAPHY

3.3.1 Sample Preparation

Standard metallographic procedures were used to prepare the metallographic samples. Typical

metallography of a sample consisted of evaluation on both the rolling and longitudinal planes, as defined

in Fig. 3-2. Once sectioned from the main piece, burrs were ground off by hand using 120 grit Si02

papers with water as a lubricant. The samples were then flushed with water followed by flushing with

ethyl alcohol and drying using forced air. Sample labels were affixed to the samples and then encased in cold setting epoxy. Most samples were prepared using an automated grinding/polishing machine.

Grinding steps were conducted with SiO^ papers to 800 grit followed by polishing with diamond pastes of

6 micron and 0.25 micron. The final pohshing step was conducted by hand using a colloidal mixture of

SiOi and AI2O3 with sizes of approximately 0.05 ^m. Some samples that were too large to fit into the

grinder/polisher sample holder (e.g., weld cross-sections) were completely prepared by hand using similar

procedures listed above.

The underlying steel microstructural constituents were revealed using various etchants. Both hot-rolled

and quenched-and-tempered steel microstructures were revealed using a combination of 4 percent picral

(4 g

picric acid, 96 mL ethyl alcohol) followed by 2 percent nital (2 iriL concentration nitric acid, 98 mL

ethyl alcohol). Further, prior austenite grain boundaries were attempted to be revealed using Marshall's

reagent and Vilella's reagent (Bramfitt and Benscoter 2002). Following completion of etching, samples were ultrasonically cleaned in ethyl alcohol and then forced air dried.

3.3.2 Ferrite Grain Size Measurements

The ferrite grain size of the hot-rolled steels was determined according to ASTM E 1382-97, a standard

test method for detemiining the average grain size using semiautomatic and automatic image analysis.

The samples were etched prior to evaluation. The grain size was calculated using the Jeffries planimetric

method. The pearlite regions were removed from the analysis, so that only the area of the ferrite grains was used in the detemiination of the grain size. Grain size measurements were made on cross longitudinal planes. The number of fields measured for a specific sample depended on the thickness of

the steel. The first and last field of the grain boundary counts were taken from positions adjacent to the

top and bottom surfaces of the plates. Then measurements at 10 or more positions between these two

boundary fields were measured. Results were given in terms of an ASTM grain size number with a

• measure of relative accuracy also provided (defined in the standard).

3.3.3 Pearlite Volume Fraction Measurements

The volume fraction of pearlite in the hot-rolled steels was determined via an area detected measurement

using automatic image analysis techniques. The samples were etched prior to evaluation. These samples were evaluated on longitudinal planes. The qualitative terms used to characterize the pearlite

morphologies are taken from the ASTM E 1268 Standard Practice for Assessing the Degree of Banding or

Orientation of Microstructure.

6 NISTNCSTAR 1-3E, WTC Investigation
Experimental Procedures

3.4 HARDNESS TESTING

Tliree types of hardness tests were conducted on the WTC steels. Rockwell Hardness testing was

conducted on un-mounted samples using the B-scale or C-scale in accordance with ASTM E 18. Two

sides that were parallel to the rolling plane of the plate were ground parallel to 1 20 grit. Three

measurements were made on each sample after an initial indent was dropped to "seat" the sample. Most

hot-rolled samples were tested using the B-scale while the quenched-and-tempered samples were

primarily tested on the C-scale.

Vickers microhardness testing was conducted on polished metallographic samples (parallel to the rolling

plane) in accordance with ASTM E 384. Five indents were made on each sample with a load of 1 kg.

Measurements were conducted using 200 times magnification. Additionally, microhardness profiles were made through weld cross-sections using a Knoop indenter with 1 kg load and measured at 200 times

magnification in accordance with ASTM E 384.

Various plate materials were exposed to elevated temperatures through use of a laboratory furnace. All

samples from a give material were taken from within 1 ft of each other on the same plate. The samples were ultrasonically cleaned in ethyl alcohol and air dried. A standard laboratory furnace was set at a given temperature and allowed to stabilize for a minimum of 1 h; a thermocouple was placed in the

furnace at roughly the same location as the samples to monitor the temperature. Samples were exposed

for 15 min and 30 min at temperatures of 200 °C, 300 °C, 400 °C, and 500 °C. Additionally, exposure at 625 °C was conducted for times of 15 min, 30 min, 60 min. and 120 min. Samples were removed from

the furnace and allowed to cool in a medium of standard silica sand. The two surfaces of the plate

(parallel to the rolling plane) were ground parallel to 600 grit and tested using the Rockwell B scale in accordance with ASTM E 18.

3.5 FURNACE EXPOSURE

NISTNCSTAR 1-3E, WTC Investigation 7
Chapter 3

Source: NIST. Figure 3-1. a) Photograph prior to sample removal from structural element,

b) photograph subsequent to sample removal from structural element, and c) photograph

displaying samples to be cut for further analysis from the specimen. Note that the rolling

direction of the plate is indicated by the white arrow. Sample shown is from a 60 ksi

inner web plate from column 136 in the vicinity of the 99th floor of WTC 1 (panel C-40).

8 NISTNCSTAR 1-3E, WTC Investigation
Experimental Procedures

Rolling plane

RD

Longitudinal

Transverse

Figure 3-2. Schematic indicating the viewing

orientations for metallographic analysis.

Microstructural evaluation was conducted by viewing

the rolling and longitudinal planes.

NISTNCSTAR 1-3E, WTC Investigation 9
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NISTNCSTAR 1-3E, WTC Investigation
Chapter 4

Chemical Analysis Results

Chemical analysis of the recovered World Trade Center (WTC) structural steel was conducted to help

identify the steels used in the towers' construction. A portion of nearly every sample removed from the

structural elements was chemically analyzed (385 chemical analyses). As discussed in Sec. 3.2, in the

initial stages of the investigation, two independent laboratories were sent samples from the same plate at a distance of no further than 1 ft apart. This was done for an independent check on accuracy. Comparison

of two of the nineteen samples that were duplicated can be seen in Table 4-1. While not exact, the results were similar. (The choice of one laboratory over the other to continue on with the bulk of the work is not meant to imply that one laboratory out performed the other.) Further, these two samples were also tested

at the National Institute of Standards and Technology (NIST). Two different methods were used to obtain

the results: X-ray fluorescence and Glow Discharge Optical Emission Spectroscopy. Both measurements

were made using instrumentation calibrated with NIST standard reference materials for steel. Again

similar results were observed.

The following sections discuss the chemical analysis results in relation to available structural steel at the

time of the WTC towers' construction. When referring to individual chemistry specifications for contemporaneous materials, see NIST NCSTAR 1-3 A. "Again, it should be noted that only 12 grades of

steel were used in the construction of the towers as 85 ksi and 90 ksi steels were replaced with 100 ksi

steel."

4.1 EXTERIOR PANEL MATERIAL

4.1.1 Perimeter Columns

Flange Plates

Table 4-2 displays the chemical analysis results for the flange plates of the perimeter columns. The

samples were separated first by specified minimum yield strength (F, ) and then by thickness of the plate.

The number of individual plates analyzed that contributed to these results for the given F, and thickness was also shown. Each element is separately listed with the average compositional value, in percent, and

standard deviation shown. Further, Fig. 4-1 shows the average value for various elements plotted as a function of F,. for plates with a thickness of 0.25 in.

The steels used for the flange plates of the perimeter panels were low carbon steels with carbon contents

typically between 0.1 percent and 0.2 percent. These would be classified as high-strength, low alloy

(HSLA) steels.

The carbon equivalent (CE) for these materials was also calculated and shown in Table 4-2 for the

0.25 in. plates. The CE is used to account for those elements that may affect weldability of the steel. The

formula

NIST NCSTAR 1-3E, WTC Investigation 11
Chapter 4

%Mn VoCr + %Mo + VoV VoSi + %Ni + VoCu CE = VoC + + + (4-1)

6 5 15

reduces the significant composition variables into a single number (ASM 1983). As a rule of thumb, low

carbon steels with CE equal to or less than 0.40 typically do not require preheat or post-weld heat treating.

For CE more than 0.40 but less than 0.60, some preheating should be provided before welding. For CE

more than 0.60, both preheat and post-weld heat treatments should be applied. For the flange materials, it appears that preheating of the plates may have been necessary.

Based upon the chemical analysis of the flange plates, the following can be concluded regarding the

possible identification of the tested plates (see NIST NCSTAR 1-3A for descriptions):

Fy = 45 ksi : Met the Yawata "A441 -modified" chemistry specification.

Fy = 50 ksi: Both plate sizes met the Yawata "A441 -modified" chemistry specification, the

0.5625 in. plate also met "WEL-TEN 60", ''WEL-TEN 60R", ''WEL-TEN 62," and "WEL-TEN

70" chemistry specifications.

Fy = 55 ksi: The 0.5625 in. plate met the Yawata "A441 -modified", all other plates met this specification in addition to "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN 62," and "WELTEN 70" chemistry specifications.

Fy = 60 ksi: Both plate sizes met the Yawata "A441 -modified" chemistry specification, only the

0.3125 in. plate met the "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN 62," and "WEL-TEN

70" chemistry specifications.

Fy - 65 ksi : Met the Yawata "A441 -modified", "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN

62," and "WEL-TEN 70" chemistry specifications.

Fy = 70 ksi : All plates met the Yawata "A441 -modified", "WEL-TEN 60", "WEL-TEN 60R",

"WEL-TEN 62," and "WEL-TEN 70" chemistry specifications.

Fy ^ 75 ksi : Met the Yawata "A441 -modified", "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN

62," and "WEL-TEN 70" chemistry specifications.

F,- 80 ksi : The 0.25 in. plate met the "WEL-TEN 60" and "WEL-TEN 70," the 0.625 in. plate

met the "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN 62," and "WEL-TEN 70" chemistry

specifications.

Fy= 100 ksi : Both gauges met the "WEL-TEN 70" and "WEL-TEN 80C" chemistry

specifications.

Outer Web Plates

Table 4—3 displays the chemical analysis results for the outer web plates of the perimeter columns. The

samples were separated by specified minimum yield strength (F,.). The number of individual plates

analyzed was also shown. Each element is separately listed with the average compositional value, in

12 NIST NCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

percent, and standard deviation shown. Further, Fig. 4-2 shows the average value for various elements

plotted as a function of F, for plates with a thickness of 0.25 in.

In comparing Fig. 4-2 to Fig. 4-1, it can be seen that nearly identical results were observed for the outer web plates as found for the flange material. This lends credence to the assumption that the same supplier

(Yawata Iron and Steel Co.) was used for both the flange and outer web plates (NIST NCSTAR 1 -3A).

For example, the 60 ksi flange material with different thicknesses had chemistries that were similar but not exact, particularly for the C and V or Nb additions. However, the 0.25 in. 60 ksi plate for the flange

and 0.25 in. outer web have nearly identical compositions. One further note was the similarity between

50 ksi and 55 ksi web plates. The 50 ksi web contained Nb additions similar to that of the 55 ksi plate,

but these two materials were distinct in that there was a statistically significant difference in the Si contents.

Due to the similarity between these plates and the flanges, no further analysis of the outer web plates was

conducted. For more information regarding possible identification of material, please refer to the

discussion for the flange plates.

Inner Web Plates

Table 4-4 displays the chemical analysis results for the inner web plates of the perimeter columns. The

samples were separated by specified minimum yield strength (F,.) with only the F,. 50 ksi material having more than one gauge. Figure 4-3 shows the average value for various elements from Table 4—4 plotted as a function of F,. for plates with a thickness of 0.25 in. In addition to the number of individual plates

analyzed, an "a" was placed next to the number signifying that additional plates of this thickness and Fy

were not included in the average values as significant outlying chemistry values were observed. Samples

with significantly outlying chemistry values, compared to those that were statistically similar, were

individually shown in Table 4-5 and did not contribute to the average values found in Table 4-4.

In comparing the inner web chemistry results with those obtained for the flanges or the outer webs, there was a clear disparity for minimum yield strengths between 55 ksi and 75 ksi. The 80 ksi and 100 ksi

inner web plates had very similar chemistries to the flanges and outer web and were consistent with the

Yawata grades; the 50 ksi inner web plate was somewhat similar, but not exact. Contemporaneous

construction documents (NIST NCSTAR 1-3 A) indicate that the flanges, outer webs and spandrels of the

perimeter panels were generally fabricated from Japanese steel (primarily Yawata) and the inner web

from domestic steel. Other contemporaneous construction documents contain evidence that Bethlehem

steel supplied most of the domestically produced steel for the perimeter panel contract. The actual

fraction was unknown, and the same construction documents also show that the company that fabricated

the perimeter panels purchased, or requested approval to purchase, plate from nearly every domestic steel company. Because of the lack of information in the construction record, it was impossible to unambiguously identify the source of the steel used for the inner web.

Butt Plate

The butt plates for the individual perimeter columns were specified as having Fy = 50 ksi. No other

specifications were given. The chemistry results for these plates can be found in Table 4-6. Due to the

high silicon content, the steel did not meet the chemistry specifications for relevant ASTM structural

NIST NCSTAR 1-3E, WTC Investigation 13
Chapter 4

steels from that era. However, it did meet the chemistry specifications for the following: Yawata

"A441 -modified," "WEL-TEN 50," "WEL-TEN 55," all "WEL-TEN 60" designations, and

"WEL-TEN62."

4.1.2 Spandrel Plates ^

Table 4-7 displays the chemical analysis results for the spandrel plates of the exterior panels. The

samples were separated by specified minimum yield strength (F,) with only the F, 50 ksi and 65 ksi

materials having more than one thickness. Each element is separately listed with the average

compositional value, in percent, and standard deviation shown. Further, Fig. 4^ shows the average value

for various elements plotted as a function of F, for plates with a thickness of 0.375 in. In addition to the

number of individual plates analyzed, an "a" was placed next to the number signifying that additional

plates of this thickness and F, were not included in the average values as significant outlying chemistry

values were observed. Samples with significantly outlying chemistry values, compared to those that were

statistically similar, were individually shown in Table 4-5 and did not contribute to the average values

found in Table 4-7.

In comparing the results between the spandrel plates (Fig. 4^) and those from the flange (Fig. 4-1), it can be seen that very similar results were observed for material between 50 ksi and 75 ksi. There were minor differences associated with the Mn content for plates with F, between 60 ksi and 75 ksi where the

spandrel plates had slightly elevated levels. Additionally, the 60 ksi plate for the spandrel was

strengthened with Nb as opposed to V seen in the flanges. However, if comparing similar thickness plate,

it can be seen that the Nb levels for 0.3125 in. flange plate are nearly identical to the 0.375 in. spandrel

plate. This lends credence to the assumption that the same supplier (Yawata) produced the flange, outer web, and spandrel plates (NIST NCSTAR 1-3 A).

As similar results were observed between the spandrel and flange plates, only those materials not previously discussed in Sec. 4.1.1 will be commented on for possible identification of the plates with

regard to the chemistry data:

F,. = 36 ksi: Met the "A-36" chemistry specification.

F, - 42 ksi : Met the Yawata "A441 -modified", "WEL-TEN 60", "WEL-TEN 60R", "WEL-TEN

62," and "WEL-TEN 70" chemistry specifications.

F, = 46 ksi : Met the Yawata "A441 -modified", "WEL-TEN 60," "WEL-TEN 60R", "WEL-TEN

62," and "WEL-TEN 70" chemistry specifications.

4.1.3 Spandrel Splice Plates

The splice plates for the individual exterior panels were specified as having F,. = 36 ksi. No other

specifications were given. The chemistry results for these plates can be found in Table 4-6 and indicates

that chemistry specifications were met for ASTM A 36 and ASTM A 242.

14 NIST NCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

4.1.4 Floor Truss Connectors

Truss Seat

The structural engineering plans specified all truss seats were to have a minimum yield strength

Fy = 36 ksi, however, the plans do not require that the steel conform to a specific ASTM standard.

Table 4-6 shows the chemistry results and indicates that chemistry specifications were met for ASTM A 36 and ASTM A 242.

Construction Bolts

Two types of bolts were specified for construction of the towers, either A 325 or A 490 bolts. Stampings on the bolt heads clearly indicated the bolt type. Only one A 325 bolt was tested for chemistry. The

results shown in Table 4-6 indicated that it met the A 325 Type 1 chemistry specifications.

Standoff Plate

The structural engineering plans specified that all standoff plates were to have a minimum yield strength

Fy = 42 ksi. No other specifications were found. The chemistry results for these plates can be found in Table 4-6. Many of the specifications for contemporaneous ASTM structural steels were met, but none

could be specifically identified. However, the additions of V would suggest that care was taken to meet

the strength levels necessary.

Gusset Plate Welded to Top Chord of Floor Truss

No information was found specifying the grade of steel to be used for the gusset plates that were welded

to the top chord of the floor trusses. However, structural engineering plans stated that ''All steel shall be

ASTMA36 for locations where a specific strength requirement is not stated in the drawings."" Given

these facts, these gusset plates were assumed to be ASTM A 36. Table 4-6 shows the chemistry results and indicates that chemistry specifications were met for ASTM A 36 (bars).

Damper Plates

No information was found specifying the grade of steel to be used for the damper plates. However,

structural engineering plans stated that ''All steel shall be ASTMA36for locations where a specific

strength requirement is not stated in the drawings^ Given these facts, the damper plates were assumed to be ASTM A 36. Table 4-6 shows the chemistry results and indicates that chemistry specifications were met for ASTM A 36 (bars).

Gusset Plate for Damper Unit and Diagonal Bracing Strap Attachment

No information was found specifying the grade of steel to be used for the gusset plates that attached the damper units and diagonal bracing straps to the perimeter columns. However, structural engineering

plans stated that "All steel shall be ASTMA36for locations where a specific strength requirement is not stated in the drawings."" Given these facts, the gusset plates were assumed to be ASTM A 36. Table 4-6

shows the chemistry results and indicates that chemistry specifications were met for ASTM A 36.

NISTNCSTAR 1-3E, WTC Investigation 15
Chapter 4

Diagonal Bracing Strap

No information was found specifying the grade of steel to be used for the diagonal bracing straps.

However, structural engineering plans stated that ''All steel shall be ASTMA36for locations where a

specific strength requirement is not stated in the drawings."" Given these facts, the diagonal bracing

straps were assumed to be ASTM A 36. Table 4-6 shows the chemistry results and indicates that chemistry specifications were met for ASTM A 36.

4.2 CORE MATERIAL

Table 4-8 displays the individual chemical analysis results for each of the samples where the as-built

location was known. The columns were separated by type (built-up box column or wide flange section)

and then by specified minimum F, . The plate thickness was also indicated.

4.2.1 Wide Flange Core Columns

The chemistry of the six wide flange sections can be found in Table 4-8. Five of the samples were

specified as having a minimum yield strength of 36 ksi. While all of these core columns met ASTM A 36

chemistry specifications, sample C-30 (1008B: 104-106) was unique in that it had V additions. The other

wide flange section was specified as having a minimum yield strength of 42 ksi. Sample HH (605A:

98-101) met chemistry specifications for ASTM A 440, ASTM A 441, and ASTM A 572 (both type 2 and

type 4). As contemporaneous documents state that numerous suppliers were used to fill the steel orders

(NIST NCSTAR 1-3 A), attempts were not made to distinguish between the suppUers.

4.2.2 Built-Up Box Core Columns

Five built-up box columns were identified as to their as-built location. Three of these were specified as having a minimum yield strength of 36 ksi. Table 4-8 shows these chemistries with the results indicating

that they met chemistry specifications for ASTM A 36. The fourth and fifth columns were specified as 42 ksi material. (The core columns are labeled as C-88a (801B: 80-83) and C-88b (801B: 77-80). The

piece labeled "C-88c" was part of a flange that broke away from C-88a during transport to NIST.) While

chemically similar, the materials are not identical. Table 4-8. The plates do not conform to any of the

chemistry specifications mentioned in the Port of New York Authority steel contracts. However, there

are several steels listed in the contracts, most notably proprietary Lukens grades (NIST NCSTAR 1-3 A).

NIST was not able to locate typical or specified compositions, so possibly the steel could have been

shipped to one of those specifications.

4.2.3 Channel

No information was found specifying the grade of steel to be used for the channels. However, structural engineering plans stated that "'All steel shall be ASTMA36 for locations where a specific strength

requirement is not stated in the drawings'". Given these facts, the channels were assumed to be

ASTM A 36. Table 4-6 shows the chemistry results from two individual channels and indicates that chemistry specifications were met for ASTM A 36.

16 NIST NCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

4.2.4 Core Truss Seat

No information was found specifying the grade of steel to be used for the core truss seats. However,

structural engineering plans stated that ''All steel shall be ASTMA36for locations where a specific

strength requirement is not stated in the drawings''. Given these facts, the core truss seats were assumed

to be ASTM A 36. Table 4-6 shows the chemistry results from two individual seats and indicates that chemistry specifications were met for ASTM A 36. The two core chemistries were also compared to the

average values for the perimeter panel truss seats. One was found to be similar, but not identical, while

the other was found to be dissimilar, suggesting that multiple steel suppliers may have been used to fill the contracts.

4.3 FLOOR TRUSS MATERIAL

4.3.1 Rod

Table 4-9 displays the individual chemistry results for the truss rounds that were analyzed. The diameter

of the rod was also given. Chemically, all of the truss rods met chemistry specifications for ASTM A 36

and ASTM A 242 specifications. Contemporaneous construction documents indicated that Laclede Steel Co. supplied steel in the floor trusses to meet this specification as well as those for ASTM A 36 (NIST

NCSTAR 1-3 A).

4.3.2 Chord/Angle

Table 4-9 displays the individual chemistry results for the truss angles that were analyzed. Chemically,

all of the truss angles met chemistry specifications for ASTM A 242. One bottom angle (C53-BA-1) and

one top angle (C53-TA-1), with their lower Mn content, also met chemistry specifications for ASTM A 36. Contemporaneous construction documents indicated that Laclede Steel Co. supplied steel

in the floor trusses to meet these two specifications (NIST NCSTAR 1-3 A).

4.4 SUMMARY

Chemical analysis of the recovered WTC steels showed a division in the types of steels used. For the

lower strength components, Mn and Si were added for strength. Higher strength components contained

Cr and Mo additions. Nb and V additions were used through out both types of steel. These alloying

practices were typical for steels of this era. For the exterior panel material, the flange, outer web, and

spandrels plates were all chemically similar, indicating fabrication by the same supplier (Yawata Iron and

Steel Co.). Inner web plates with F, equal to 80 ksi and 100 ksi were also found to conform to these

chemistries. The chemistries of the inner web plates below 80 ksi were dissimilar to the flange plates for similar F, indicating that a different supplier(s) was most likely used to fabricate the components. These

results were consistent with contemporaneous documents. For the core columns, all chemistry

specifications were met for a given F, ; however, due to the number of possible supphers of steel, definitive identification of the steel was not possible. The other structural components (floor truss connectors, core chaimels, spandrel splice plates) analyzed met specifications for either ASTM A 36 or ASTM A 242 grade steels.

NIST NCSTAR 1-3E. V/TC Investigation 17
Chapter 4

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NISTNCSTAR 1-3E, WTC Investigation 21
Chapter 4

Table 4-1. Comparison of chemistry results from two outside contractors and NIST.

Element

Sample 1: 60 ksi flange material from

colmmi 141, floor 100 (panel N-S)

Sample 2: 55 ksi flange plate from

colmmi 155, floor 101 (panel N-9)

Lab #1 Lab #2

NIST metl-iods

Lab #1 Lab #2

MST methods

XRF GDOES GDOES

C 0.15 0.16 _NA ^ 0.166 0.18 0.18

Mn 1.35 1.42 1.45 1.44 1.37 1.42 "l'42'

"

P 0.013 0.019 0.021 0.0218 0.017 0.018 0,019 0.0188

S 0.011 0.014 0.026 0.0143 0.015 0.013 _0JI16 0.0128

Si 0.38 0.49 0.493 0.495 0.47 0.51 0.522^'"

Ni 0.01 0.011 0.016 0.0167 0.01 0.011 0.015

" T0T59

Cr 0.02 0.02 0.017 0.0173 0.02 0.019 0.016 0.0158

Mo <0.01 0.072 0.0036 0.0034 <0.01 0.072 0.0036 0.0033

Cu 0.05 0.048 0,047 0.05 0.05 0.046 0.048 0.0459

V <0.005 0.005 0,0038 0.0037 <0.005 0.004 0.0023 0.002

Nb 0.042 0.052 0,057 0.056 0,056 0.058 _0.063_ 0.0597

"

Ti <0.005 0,004 0,0035 0.0042 <0,005 0.003 ""a

0019"'"' To029"

Zr <0.005 0,001 0,0004 NA <0,005 0.001 NA NA

Al 0.031 0.034 0.039 0.0336 0.033 0.032 0.036 0.0318^

B <0.0005 0.0003 JNfA 0.0001_ <0.0005 0.0003 NA__na"

'

N 0.004 0.004 NA 0.0025 0.009 0,0047 NA NA

As NA NA 0.015 0.0143 NA NA 0.017

T0Ci56""'"

0.0176

Co NA NA 0,005 NA NA NA NA

' Sb""" NA NA 0,0029 NA NA NA 0.0033 NA

Sn NA NA 0J06 NA^_ iSTA NA 0.0062 NA

Ta NA NA J.0052^_ _NA_ NA NA 0.0059 NA

NA NA 0.0013 NA NA NA NA NA

22 NISTNCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

Table 4-2. Chemistry results of flange plates from perimeter columns

(in mass fraction x 100). Shown are the averages with standard deviations given directly

below. Also calculated was the CE.

Fv

(ksi)

Thickness

(in)

# of plates analyzed c Mn P s Si Ni Cr Mo Cu V .Nb Ti Zr Al B N CE

45ksi 0.4375 2 0.20 1.23 0.01 0.02 0.26 0 01 0 03 < 0 01 0 05 <0.005 0 043 <0 005 <0,005 <0.005 <0 0005 0 009 0,43

0.01 0.03 0 01 0.00 0 01 0 00 0 00 a^a 0 01 ri/a 0.004 n/a ri/a n/a 0 003

0 43

50 ksi

0.5 2 0.20 1.14 0 01 0.03 0 05 0 10 0.07 0.03 0.30 0,055 <0.005 <0 005 <0.005 <0,005 <0.0005 0 007

0,01 0.04 0.00 0.01 0.01 0.01 0.00 [1 0.01 0 n/a n/a 0 004

0 41 0.5625 4

0 16 1.35 0 02 0 02 0 28 0 01 0 02 < 0 01 0 04 <0 005 0 045 <0 005 <0 005 0 017 <0.0005 0 008

0 00 0.02 0 00 0 00 0 01 0 01 0 00 a^a 0.00 ri/a 0 002 n/a n/a 0 002 n/a 0 007

0.25 18

0, 17 1.30 0.02 0.02 0.42 0 01 0.02 < 0.01 0.05 <0,005 0.047 <0.005 <0 005 0,022 <0 0005 0 007 0 43

0 01 0 04 0 00 0 00 0 03 0 00 0 00 0 01 0 005 n/a n/a 0 005 n/a 0 004

0.3125 10

0. 17 1.30 0.02 0.02 0 42 0.01 0.03 < 0.01 0.04 <0 005 0,046 <0.005 <0.005 0,028 <0 0005 0,005 0,42

55 ksi

0.01 0.06 0.00 0.01 0.03 0 00 0 01 0 01 0 006 n/a 0,010 n/a 0,004

0 42

0.375 Q

0. 17 1.28 0.01 0.01 0 43 0 01 0 02 < 0 01 0.04 <0,005 0,046 <0.005 <0.005 0 031 <D 0005 0 005

0 01 0 03 0 00 0 on 0 02 0 00 0 00 a'a 0 01 a'a 0 004 n/a n/a 0 004 n/a 0 002

0.4375 2

0 17 1.28 0 02 0.02 0 41 0 01 0 02 < 0 01 0 04 <0 005 0 046 <0 005 <0 005 0 027 <0 0005 0 011 0.42

0 00 0 08 0 00 0 00 0 04 0 00 0 00 0 00 0 008 0 006 n/a 0 010

0 46

0.5625 6

0. 19 1.38 0 02 0.02 0 45 0.01 0.03 < c.Ol 0.04 <0.005 0 053 <0 005 <0.005 0,043 <0 0005 0,004

0 01 0 04 0 00 0 00 0 02 0 00 0 00 a'

a

0 00 ri/a 0 003 n/a n/a 0.005 n/a 0 002

0.25 3 0.20 1.39 0.02 0 02 0 43 0 02 0 04 < [1 01 0 05 0 012 <0,D05 <0 005 <0 005 0,031 U , U U Lt 0 002 0 47

60 ksi

0 00 0 02 0 00 0 01 0 02 0 00 0 01 0 00 0 002 n/a a'a 0 003 n/a 0 001

0,3125 8

0 16 1 ''i6 0 02 0 01 0 41 0 01 0 02 < 0 01 0 05 <0 005 0 047 <0 005 <0 005 0 033 <n nnri5 0 005 0.42

0.01 0.06 0.00 0.00 0.02 0.00 0 00 a'

a

0.00 a'

a

0.007 a^a a'a 0 005 Til's 0 002

65 ksi 0 25 6

0 16 1.27 0 02 0.02 0 39 0 01 0. 19 < 0 01 0 05 0 054 <0,005 <0,005 <0 005 0 033 <rj 0005 0 012 0.44

0.00 0.07 0.00 0.00 0.03 0 00 0.01 a'a 0.00 0 005 a^a a'a a'a 0 006 n/a 0 001

0.25 4

0. 14 1.09 0.01 0.01 0 40 0 01 0.21 < 0 01 0.05 0 022 <0.005 <0,005 <0 005 0. 113 <0.0005 0 004 0 39

70 ksi

0 01 0.04 0 00 0.00 0 01 0 00 0.01 0.00 0 002 0.005 n/a 0 002

0.39 0.4375 2

0 14 1 09 0 01 0.02 0 41 0 01 0.22 < 0 01 0.04 0 020 <0 005 <0.005 <0.005 0. 105 <0.0005 0 006

0 01 0 01 0 00 0 00 0 00 0 00 0 01 a'a 0 00 0 00! a'a a'a 0 007 0 001

0.75

0 12 1 10 0 01 0 02 0 40 0 04 0.21 < 0 01 0.05 0 024 < 0.005 < 0,005 < 0.005 0 097 < 0.0005 0 007 0.38

1 ni'ci n^ci rii'a n''a ii''a a'a a'

a

a'

a

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a

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75 ksi 0.25 4

0 15 1 13 0 02 0 01 0 41 0 01 0.23 < 0 01 0.05 0 044 <0.005 <0.005 <0 005 0 1 13 <0.0005 0 004 0 42

0 01 0.04 0 00 0 00 0 07 0 00 0.01 a^a 0.01 0 002 n/a n/a n/a 0 013 n/a 0 002

0.25 5

0 13 0.90 0.02 0.02 0.33 0 02 0.32 0.21 0 20 <--0.005 0 026 <0 005 <0 005 0 104 0.001 0 004 0.42

80 ksi

0 00 0.08 0 00 0 00 0 01 0 01 0 01 0 01 Oil a'a 0 004 a'a a'a 0 005 0.000 0 002

0.625 2

0 13 1.08 0.01 0.01 0.37 043 0.27 0.21 0.20 0.028 <0.005 <0.005 <0.005 0.041 0.002 0 009 0.43

0.01 0.00 0.00 0 00 0.00 0 00 0.00 0.00 0.00 0.000 a'a a'a a'a 0 001 0 001 0 006

0.25 11

0 15 0 86 0 01 0 02 0 25 0 01 0 90 0 43 0 26 <0.005 <0.005 <0,005 <0 005 0 083 0.001 0 005 0.59

100 ksi 0.02 0.03 0.00 0.00 0.02 0 01 0 03 0.03 0.03 a'a a/a a/a n/a 0.009 0.001 0.002

1.125

0 15 0.80 0.01 0.01 0 23 0 01 0.88 0.33 0.31 <0.005 <0.005 <0.005 <0.005 0.053 <0.0005 0.008 0.50

1 n'a a'a a'

a

n'a ai'a a/a a/a a^a n'a a'a n/a a'a n/a n/a a'a

NISTNCSTAR 1-3E, U/TC Investigation 23
Chapter 4

Table 4-3. Chemistry results of outer web plate from perimeter columns (in mass

fraction x 100). Shown are the averages with standard deviations given direct y below.

(ksi)

Thickness

(in)

# of plates

analyzed c Mn P s Si Ni Cr Mo Cu V Nb Ti Zr Al B N

0.25 3 0.16 1 35 0,02 0.02 0.28 0.01 0,02 <0.01 0.05 <0.005 0.048 <0.005 <0.005 0 029 <0,0005 0.009

50

0.00 0 01 0,01 0.00 0.01 0.00 0.01 a^a 0.01 ru'a 0.003 a^a a'a 0 005 a'a 0 009

0.5625 1 0.14 1.42 O.OI 0.02 0.42 0.04 0.01 <0.01 0.04 <0.005 0.054 <0.005 <0.005 0 026 <0.0005 0.007

rL''a n'a ru'a n'''a rt'a n^a nfa n/a n^a n/a n/a a'

a

a'a a'a n/a a'a

r c 0 25 14

0 17 1,30 0,02 0.02 0 43 0.01 0.02 <0 01 0 05 <0.005 0 049 <0 005 <0M5 0 029 <0.0005 0.005

J J 0.01 0 07 0,00 0.00 0.03 0.00 0.00 n^a 0 01 n,'a 0 006 a'a a'a 0 006 n/a 0 002

60 0.25 4

0.20 1,36 0.02 0.01 0.43 0.02 0.04 <0.01 0.05 0.011 <0.005 <0.005 <0.005 0 031 <0.0005 0.007

0 01 0 03 0.01 0 00 0.02 0.00 0.00 n^a 0 01 0,002 a^a a'a a'a 0 002 a'a 0.002

65 0 25 J 0 16 1 26 0 02 0.02 0.38 0.01 0.19 <0 01 0 05 0 054 <0 005 <0.005 <0,005 0 028 <0.0005 0.012

0.01 0 09 0 00 0.01 0 04 0.00 0.01 n/a 0 00 0,005 a.'a a'a a'a 0 005 a'a 0 000

70 0.25 3

0 13 1,12 0.01 0.01 0.38 0.01 0.23 <0.01 0.05 0.023 =-'0.005 <0.005 <0,005 0 104 <0 0005 0.004

0.01 0 02 0.00 0.00 0.03 0.00 0.01 n'a 0.01 0.003 a'a a'a a'a 0 015 n/a 0.001

75 0 25 2 0.16 1,17 0 02 0.01 0.42 0,01 0.24 <0.01 0 05 0.044 <0.005 <0.005 <0 005 0 115 <0.0005 0 006

0.01 0 01 0,00 0 01 0 02 0,00 0.00 n/a 0 01 0.001 a'a a'a a'a 0 007 a'a 0.002

80 0.25

0 15 0 99 0 02 0.02 0 36 0 02 0.33 0.21 0.26 <0.005 0 028 <U.005 <0,005 0 116 <0.0005 0 006

0 01 0 01 0,00 0.00 0 03 0,01 0.01 0.00 0.03 n/a 0 002 n/a n/a 0 022 n/a 0 002

100 0.25 3 0.14 0 88 0,02 0,01 0.26 0,02 0.93 0.45 0.29 <0.005 <0.005 <0.005 <0,005 0,094 <0.0005 0.007

0,02 0 04 0.01 0.00 0.01 0,01 0.04 0.03 0.03 n/a a''

a

a'a a'a 0 009 n/a 0 003

Table 4-4. Chemistry results of inner web plate from perimeter columns (in mass

fraction x 100). Shown are the averages with standard deviations given direct y below.

Fy

(ksi)

Thickness

(in)

# of plates analyzed

C Mn P s Si Ni Cr Mo Cu V Nb Ti Zr Al B N

50 ksi

025 0 18 1.34 0.02 0 02 0,43 0.01 0 02 <0 01 0,04 < 0.005 0.046 < 0 005 < 0.005 0 031 < 0.0005 0 002

0 01 0 05 0.00 0 00 0 00 0.00 0 00 n/a 0 00 n/a 0 002 n/a n/a 0 001 n/a 0 001

0.375 1 0 18 096 0.01 0,02 0,04 0.06 007 < 0 01 0 25 0 054 < 0 .005 < 0 005 0 0.08 < 0.005 < 0.0005 0.007

n.'a n.'a n/a n/a n/a n./a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

025 12" 021 1 17 001 0 02 004 0 07 0,06 <0 01 0 21 0.055 <0 005 <0 005 <0 005 <0.005 <0.0005 0 008

0.02 0 07 0.00 000 0.01 0.02 0 00 n/a 001 0 004 n/a n/a n/a n/a n/a 0.007

60 0 25 6

0.16 1.13 0.01 002 0.02 0.07 0 09 <0,01 020 0.053 < 0 005 < 0.005 < 0.005 0.024 < 0.0005 0.005

0 01 0 07 0 01 0 01 0 01 0 03 0 05 n/a 0 07 0 005 n/a n/a n/a 0 008 n/a 0 003

65 0.25 023 1.10 0,01 0,03 0 15 0.08 0,08 <0 01 0 21 0.055 < 0.005 < 0 005 < 0.005 < 0.005 < 0.0005 0.004

0 00 0 03 0,00 0 00 0 01 0 01 0 01 n/a 0 02 0 003 n/a n/a n/a n/a n/a 0 002

70 0 25

0 20 1 30 0,01 0 02 0 17 0,07 008 <001 0 22 0 071 < 0.005 < 0 .005 < 0.005 0.014 < 0.0005 0.005

0 02 0 04 0 00 0 00 0 01 0,00 0 01 0 00 0 01 0 006 n/a n/a n/a 0 000 n/a 0 000

75 0 25 1 0 23 1 39 0,01 002 0.19 0,08 0,07 <0,01 0,24 0.078 < 0.005 <q.005 <0.005 0.008 < 0.0005 0 009

n/a n.'a n/a n/a n/a n/a n/a n/a n/a n./a n/a n/a n/a n/a n/a n/a

80 025 2

0 15 0 96 0,01 0 01 0 34 001 0 32 0 20 0 26 < 0 005 0 027 < 0 005 < 0.005 0 120 0 001 0 004

0 01 0 05 0 00 0 00 0 01 0 00 0 00 0 01 0 00 n/a 0 000 n/a n/a 0 000 0 000 0 001

100 0.25 4= 0.16 0 91 0,01 0 02 0.27 0,03 0.97 0.46 0.28 < 0.005 < 0.005 < 0.005 < 0.005 0.091 < 0.0005 0.009 001 0 10 0.01 0.01 0.02 0,01 0 10 0 04 0 04 n/a n/a n/a n/a 0.013 n/a 0.005

a. Signifies that additional plates. Table 4-5.not included in these averages because of significant outlying values, are reported in

24 NISTNCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

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Chapter 4

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26 NISTNCSTAR 1-3E, WTC Investigation
Chemical Analysis Results

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NISTNCSTAR 1-3E. WTC Investigation 29
Chapter 4

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30 NIST NCSTAR 1-3E, WTC Investigation
Chapter 5

Metallographic Analysis Results

In addition to the chemical analysis, metallography of the recovered World Trade Center (WTC)

structural steels was conducted to help identify the steels used in the towers' construction. The

microstructure of every sample removed from the structural elements was evaluated (862 metallographic

samples). This number included base plate material (viewed in two different orientations) and welded

connections. Standard metallographic procedures for grinding, polishing, and etching were used to prepare the samples for examination with light optical microscopy.

5.1 EXTERIOR PANEL MATERIAL

5.1.1 Perimeter Columns

Flange and Outer Web Plates

Contemporaneous documents indicate that the flange and outer web plates of the perimeter panels were

produced by a Japanese steel mill (Yawata) and then shipped to the West Coast for assembly (NIST

NCSTAR 1-3 A). The chemistry analysis further confirmed that these plates were similar. It is likely that similar processing paths were used in fabricating the two types of plates, and thus, the microstructures of

the flange and outer web are discussed together below.

Two types of steels were observed for the WTC steels from the towers. Hot-rolled steels, with ferrite- pearlite microstructures, were used for the lower strength materials. The higher strength plates had

bainitic, martensitic, or a combination of the two structures as a result of quenching and tempering of the

plates ("quenched-and-tempered" steels). As the structures of the steels are different, they will be

discussed separately.

Hot-Rolled Steels: Ferrite-Pearlite

Plates with specified minimum yield strengths less than 70 ksi were observed to be hot-rolled steels (Fig. 5-1). Documents from Yawata state that this was the practice at the time (NIST NCSTAR 1-3A).

Plates with yield strength (F, ) less than 70 ksi were typically hot-rolled unless the following sizes were

exceeded for a given F,.: 0.5 in. for 65 ksi, 1.25 in. for 60 ksi, and 1.5 in. for 55 ksi. Above these plate

thicknesses for the given F,., the steel was produced as quenched-and-tempered. The main microstructural

constituents observed were ferrite and pearlite. The average ASTM International (ASTM) grain size

number for the ferrite and the volume fraction pearlite, as a function of specified minimum yield strength, for these microstructures are shown in Fig. 5-2. The following observations were made concerning the

microstructural features:

• Ferrite morphology. Relatively large polygonal and irregular ferrite moiphologies were

observed for plates with F,. equal to or less than 55 ksi (Fig. 5-3a). Similar ferrite morphologies were observed for plates with F, of 60 ksi and 65 ksi, except the ferrite grain

size was, on average, slightly smaller (Fig. 5-2a and Table 5-1). (A smaller grain size is

NIST NCSTAR 1-3E, WTC Investigation 31
Chapter 5

designated by a larger ASTM grain size number.) Additionally, the 0.25 in. plates with

minimum yield strength of 60 ksi contained a faint Widmanstatten structure (Fig. 5-3b).

Some of the plates, regardless of F,., also had ferrite grains that were slightly elongated in the

' direction of rolling, particularly near the centerline of the plate, while ferrite grains near the

surfaces were more equiaxed. The finer ferrite grain sizes observed in the higher strength

plates would most likely be attributed to the particular controlled rolling practice used. A

relatively faster cooling rate was suggested for the plates that developed the Widmanstatten

structure; this factor, as well as higher austenitizing temperatures, promotes a needle-like

morphology in 2-D (in 3-D, the shape is plate or disk-like). Further, the rolled plates were

continuously cooled as the Widmanstatten ferrite will not form as a result of isothermal

transformations.

• Pearlite distribution. Both banded (Fig. 5-4) and non-banded (Fig. 5-5) structures were

observed. The single 45 ksi plate example had moderate banding, while banding was nearly

absent for all plates with F, equal to 50 ksi or 55 ksi. Minimum yield strengths equal to or

greater than 60 ksi exhibited both types of constituent distribution. For those plates with

aligned microstructural constituents, the bands of ferrite and pearlite were elongated in the

rolling plane, resulting in a laminated-like structure in cross-section. The ferrite grains

appear coarser in the bands containing no pearlite than in the ones with pearlite. Further, some samples possessed a gradient of banding through the thickness of the plate (Fig. 5-5)

with the banding more prevalent near the centerline of the plate than near the surfaces,

regardless of plate thickness. The occurrence of constituent banding in low carbon steels is common in hot-rolled steel plate.

• Pearlite morphology. Two types of pearlite morphology were found. For plates that had low

banding features, dense, but distinguishable, lamellar plates (Fig. 5-6a) were observed in the

pearhte colonies. The smaller colonies of the heavily banded structures (typically higher F,.) had a "mottled" appearance (Fig. 5-6b) in addition to possible degenerate pearlite. The

lamellar spacing could not be resolved light optically, and the morphology of the pearlite

appeared almost granular at times. Bainite may also be present in these structures in places

where the ferrite was associated with small cementite particles or as large grains in the lower

strength plates (Fig. 5-7). Difference in pearlite appearance may be related to the chemistry

and deoxidation practices (Samuels 1980).

• Volume fraction pearlite. The volume fraction pearlite of the flange and outer web plates was

not observed to deviate significantly with strength of the plates (Fig. 5-2b and Table 5-1).

• Non-metallic inclusion morphology. The non-metallic inclusions observed in all plates were

typical for hot-rolled steel. The MnS inclusions had a stringer appearance when viewed in longitudinal cross-section (Fig. 5-8). Typically, for plates with F,, equal to or less than

55 ksi, the stringers were heavier and had a higher volume fraction (qualitatively). Plates

with Fy equal to 60 ksi and 65 ksi were also observed to have these morphologies, though not

to the extent that the lower strength plates were observed to have them.

"Quenched-and- Tempered" Steels: Bainitic/Martensitic

According to contemporaneous Yawata documents, steel plates with minimum yield strengths equal to or

greater than 70 ksi were produced using quenching and tempering steps (NIST NCSTAR 1-3 A). Further,

32 NIST NCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

plates exceeding the following sizes for a given F,, were also specified to be "qucnchcd-and-tcmpcred":

0.5 in. for 65 ksi, 1.25 in. for 60 ksi, and 1.5 in. for 55 ksi. The different processing variables (cooling

rates, tempering temperature, and tempering time) used to fabricate the plates can yield microslructures

(bainite, tempered bainite, martensite, tempered martensite, or a combination of these) that appear similar

using light optical microscopy techniques. In depth transmission electron microscopy (TEM) would be

needed to determine the true structure of these steels. Additionally, it is unknown if these plates were

truly "quenched" and then given a post-quench "temper" treatment or if the structures developed as a resuU of a single cooling path (accelerated cooling) without any post production steps as the actual

processing parameters used to produce these plates were proprietary. As a result of the lack of processing

history information, this section does not attempt to definitively identify the "quenched-and-tempered"

microstructures, but suggests which phases may be present (bainite, tempered bainite, martensite,

tempered martensite, or a combination of these).

Figure 5-9 displays representative structures of the quenched-and-tempered steels with varying minimum

specified yield strengths. The images were taken near the centerline of the plate. The following

microstructural features were observed:

• For the 70 ksi plate (Fig. 5-9a), broad needles and leaf-like ferrite grains were surrounded by

coarse cementite precipitates. Prior austenite grain boundaries were not observed. Due to the

morphology of the ferrite constituent, as well as the distribution of the cementite phase, it was

not believed that this was a tempered martensitic structure, but more likely a coarse Widmanstatten or bainitic structure developed during cooling of the plate.

• The structure for plates with Fy equal to 75 ksi, 80 ksi, and 100 ksi had similar looking

microstructures. Prior austenite grain boundaries were clearly visible. As the strength of the

plate increases, the remnants of ferrite lath boundaries become more distinct. Cementite

carbides were found within the prior austenite grain boundaries and what appears to be the

lath boundaries. As the strength of the plate decreases, the carbides become more discernible

and less widespread. Based upon these microstructural features, it was believed that these

structures were tempered martensite with the amount of tempering increasing with decreasing

plate strength.

Inner Web Plates

Documentation from the construction era indicates that the inner plates of the perimeter columns were

produced domestically (NIST NCSTAR 1-3 A). Consistent with these documents was the chemical

analyses indicating significant differences when comparing the inner web plates to the flange and outer web plates for Fy less than 80 ksi. Further, differences in the microstructures of plates with similar Fy were noted:

• Inner web plates were hot-rolled up to and including plates with F, = 75 ksi. Flange and

outer web plates were hot-rolled for F, less than or equal to 65 ksi. As any plate over 70 ksi from Yawata, regardless of thickness, was specified to be quenched-and-tempered, these

results were consistent with the contemporaneous documents that stated different suppliers

may have produced these plates (NIST NCSTAR 1-3 A).

NIST NCSTAR 1-3E, WTC Investigation 33
Chapter 5

• Plates contained primarily polygonal or irregular ferrite. A low percentage of Widmanstatten

ferrite was observed in all plates with the exception of the 50 ksi and 65 ksi plates; these two

plates were heavily banded compared to the other hot-rolled inner web plates.

• While ferrite grain sizes were statistically equivalent (Fig. 5-2a) for a given Fy, the volume

fraction of pearlite was higher in the inner web for Fy greater than 60 ksi (Fig. 5-2b).

• All plates had a combination of lamellar and mottled pearhte. Many times these two different

morphologies were within close proximity to each other (Fig. 5-1 Oa).

• The non-metallic inclusions were heavier and more prevalent throughout the inner web

structure of all strength levels than in the flange plates (Fig. 5- 10b).

Only plates with Fy of 80 ksi and 100 ksi were observed to have quenched-and-tempered microstructures;

these structures were similar to those observed for flange plates of similar strength and were believed to be Yawata steel. .

Welds Associated with Column Material

Welds between flange and web plates were cross-sectioned and metallographically examined. An

example of a weld between hot-rolled plates (55 ksi flange and inner web) is shown in Fig. 5-11. The

weld appears to have been made in one pass, and from the depth of the heat affected zone (HAZ) in both

plates, the heat input of the weld was high. The weld was sound with no visible flaws. A portion of the

overall microstructure can be seen in Fig. 5-12a. The weld material was primarily acicular ferrite with

coarse carbides dispersed throughout (Fig. 5-1 2b). The microstructure of the HAZ near the fusion line was found to have both blocky and acicular proeutectoid ferrite on the grain boundaries, with a Widmanstatten structure within the prior austenite grain boundaries (Fig. 5- 12c). Bainite may also be

present in the structure. Compared to the original plate microstructure of Fig. 5-1 c, significant grain

growth has occurred. Tempering of the original pearlite was also observed (Fig. 5-1 2d).

A weld cross-section was also prepared between plates with Fy = 100 ksi. Again, the weld was primarily

acicular ferrite with dispersed carbides. However, due to the relatively fast cooling rates with respect to the composition of the plates, a "quenched" structure was observed in the HAZ near the fusion line (Fig. 5-13).

Butt Plate

A butt plate was removed from the unidentified panel C-68. Microstructural evidence suggested that the

plate was either completely bainite (broad ferrite needles) with coarse carbides or a combination of the

bainite/carbide structure and tempered or un-tempered martensite (thinner needles with well dispersed

carbides), as shown in Fig. 5-14.

5.1.2 Spandrel Plates

Spandrel plates were also produced by the same Yawata steel mill that fabricated the flange and outer web plates (NIST NCSTAR 1-3 A). The chemistry analysis further suggested that the plates were similar.

34 NIST NCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Wliile similarities would be expected in the microstructures between these plates, there may be

differences noted as the thinnest plate for the spandrels was 0.375 in. while flange plates with 0.25 in. thicknesses were characterized above. Only one 0.375 in. flange plate was recovered (55 ksi), and this will be specifically discussed below in comparison to 0.375 in. 55 ksi spandrel.

Hot-Rolled Steels

Similar to the flange and outer web plates, spandrel plates with minimum yield strengths less than 70 ksi were observed to be hot-rolled with the exception of the 65 ksi plates with thicknesses over 0.5 in. Quenched-and-tempered microstructures were observed for these materials. Again, given the plate

thickness and specified minimum yield strength, these microstructures were expected (NIST

NCSTAR 1-3 A). Figure 5-15 shows the microstructures as a function of F,.. The following features were

observed in the microstructure:

• Ferrite Morphology. Similar to the flange plates, both polygonal and irregular ferrite morphologies were observed. However, the average ASTM grain size number for the

spandrel plates was typically smaller (Fig. 5-2a). A faint Widmanstatten morphology was

observed in the 36 ksi plate. Comparison between the two 55 ksi plates with a thickness of

0.375 in. showed similar ferrite morphologies.

• Pearlite distribution. To some degree, banding of the constituents was observed in all plates;

the 36 ksi and 65 ksi plates had the least. All microstructures were relatively uniform across

the plate thickness with the exception of the 45 ksi plate which had a gradient in banding

from the edge to the centerline of the plate, with the latter being more heavily banded. There was a similar degree of banding between the two 0.375 in. plates (flange and spandrel) with

Fy = 55 ksi.

• Pearlite morphology. For the 65 ksi material with plate thickness less than or equal to 0.5 in. and those with minimum yield strengths below 50 ksi, the lamellar spacing in the pearlite,

while being fine, was discernible. It also appeared that degenerate pearlite or bainite may

exist in the 65 ksi plate. A mottled appearance was observed for the 50 ksi to 60 ksi plates.

This was also observed for the 55 ksi flange plate with a thickness of 0.375 in.

• Volume fraction pearlite. The volume fraction pearlite was similar between the Fy values of

50 ksi to 65 ksi (Fig. 5-2b). The level was lower for the 45 ksi plate comparison, with the

42 ksi plate also having a similar volume fraction as the 45 ksi plate.

• Non-metallic inclusion morphology. The morphology of the non-metallic inclusions was

similar to that described above for the flange plates.

Quenched-and-Tempered Steels

For the plates with Fy values between 70 ksi and 80 ksi, as well as the 65 ksi plate with plate thickness

greater than 0.5 in., the steels were quenched-and-tempered. The highest specified minimum yield

strength for the spandrels, according to the engineering plans, was 85 ksi. An example of this plate was

not recovered. Figure 5-16 displays the microstructures. For all plates, the microstructures had very

NIST NCSTAR 1-3E, WTC Investigation 35
Chapter 5

similar features to that of the flange plates of similar Fy. The 65 ksi plate may consist of a coarse Widmanstatten ferrite structure or bainite.

5.1.3 Spandrel Splice Plates

The spandrel splice plates were hot-rolled steel with a relatively uniform distribution of the constituents.

Surrounding the fine pearlite was primarily polygonal ferrite (Fig. 5-17). Table 5-2 displays both the

ferrite grain size number and the volume fraction pearlite for the plates.

5.1.4 Floor Truss Connectors

Truss Seat

Numerous truss seats were removed from the exterior columns and analyzed. The steels were hot-rolled

with a relatively uniform distribution of the constituents (no banding of the pearlite). Some seats were

found to contain a partially decarburized zone near the surface with a depth ranging from 400 ^im to 500 |im (Fig. 5-18). There was no pattern (seat detail or location within the towers) to indicate which

seats may have had this microstructural feature. The ferrite was primarily polygonal, and the pearlite had

a mottled appearance. Table 5-2 displays both the ASTM grain size number for ferrite and the volume

fraction pearlite for the seats. Additionally, some samples had thin, non-uniform oxide islands located at the interface between the steel and the paint. It was believed that these oxide phases had formed prior to application of the paint as the paint conformed to the oxide without cracking.

Construction Bolts

The A 325 construction bolt removed from panel M-2 was found to be a quenched-and-tempered steel

(Fig. 5-19) with possible bainite and tempered martensite microstructures observed.

Standoff Plate

Two standoff plates were microstructurally analyzed. Both were observed to be hot-rolled steels with

relatively uniform distribution of the pearlite. However, the volume fraction and distribution within the

structures was very different. Figure 5-20 shows the structures from the two samples. Table 5-2

displays both the ASTM grain size number for ferrite and the volume fraction pearlite for the plates. The

sample from panel M-2 contained mostly polygonal ferrite, whereas the standoff plate from panel N-8

consisted of both polygonal and Widmanstatten morphologies. The chemistries for these samples varied

as the plate from M-2 contained Mo and V additions. It may be these alloying additions, in combination

with processing conditions (particularly the cooling path of the plates), that led to the different

morphologies between the samples. These differences would be expected as no specifications were found

for the type of material to be used for these structural components.

Welded Joint Between Spandrel and StandoffPlate

Two standoff plates were fillet welded directly to the spandrel plate with multiple pass beads observed

only on the outer side of the plates. The specific arc welding process was not found within the design

drawings, but due to the short welding length, shielded metal arc welding (SMAW) or flux cored arc

36 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

welding (FCAW) was most likely used. Typically, these joints were ovcr-wcldcd, meaning that a 3/8 in. or V2 in. weld may have been deposited where only a 5/16 in. bead was specified. Figure 5-21 shows one of these welds in cross-section. The standoff plate was a 42 ksi hot-rolled steel, and the spandrel shown

in the images was a 65 ksi plate. The heat affected zone in the standoff plate can be seen extending

through the entire thickness, while a shallower penetration was observed in the spandrel plate. This

difference would be related to welding procedures and angle of the heat source with respect to the two

plates. The microstructures of the weld and HAZ were very similar to that observed for welds between

the perimeter column plates.

Welded Joint Between StandoffPlate and Floor Truss Seat

Removal of the seat from the spandrel revealed that a double fillet weld was used to join the seat and the standoff plates. This clearly indicated that the standoff plate/seat assemblies were first prefabricated and

then welded to the spandrel. Again, SMAW or FCAW was assumed to be the process used. Figure 5-22

shows one of these welds in cross-section. The heat affected zone from both fillet welds spans the entire standoff plate. Again, a shallower penetration was observed in the seat angle. The microstructures of the weld and HAZ were very similar to that observed for welds between the perimeter column plates

(Sec. 5.1.1).

Gusset Plate Welded to Top Chord of Floor Truss

Three gusset plates from various locations within the tower were removed from columns and

microstructurally analyzed. The samples were found to be hot-rolled steel with banding of the structure

(Fig. 5-23). Polygonal and irregular ferrite grains were observed with those at the centerline of the plate

heavily elongated. There was no loss of structure at the surfaces of the plates. The average ASTM grain

size number of the ferrite can be found in Table 5-2. The pearlite content was also very high, above

0.40 volume fraction of the structure (Table 5-2).

Damper

One damper was removed from the perimeter panels and microstructurally analyzed. The sample was a

hot-rolled steel composed of a banded ferrite and pearlite structure (Fig. 5-24). There was no loss of

structure at the surfaces of the plates. The average ASTM grain size number of the ferrite and volume

fraction pearlite can be found in Table 5-2.

Gusset Plate for Damper Unit and Diagonal Bracing Strap Attachment

Three gusset plates used for attaching the damper units and diagonal bracing straps to the perimeter

columns were removed and analyzed metallographically. The plates were found to be hot-rolled steel with a relafively uniform distribution across the sections observed. A representative micrograph is shown

in Fig. 5-25. The majority of the ferrite is polygonal with some irregular grains. The average ASTM

grain size number of the ferrite and the volume fraction pearlite can be seen in Table 5-2.

NISTNCSTAR 1-3E, WTC Investigation 37
Chapter 5

Diagonal Bracing Strap

Two samples of diagonal bracing straps were removed and examined. Both were hot-rolled steels that consisted of ferrite and pearlite structures that were slightly banded (Fig. 5-26). The shapes of the ferrite grains are slightly elongated in the rolling direction, particularly in the center regions of the plates. The

average ASTM grain size number of the ferrite and volume fraction pearlite can be found in Table 5-2.

5.2 CORE MATERIAL

5.2.1 Wide Flange Core Columns

Five 36 ksi wide flange core columns with known as-built locations were recovered and available for analysis. The sections were found to be hot-rolled steels with a relatively unifonn distribution of the

constituents (ferrite and pearlite) across the sections observed. Representative micrographs are shown in Fig. 5-27. The majority of the ferrite was polygonal or blocky with some Widmanstatten. At low

magnification, very localized banding of the structure can be observed. The average ASTM grain size number of the ferrite was 10.5 (Table 5-3). The pearlite constituent, at a volume fraction of 0.36 for this sample (Table 5-3), was dense with no loss of structure near the edges of the section.

There was only one 42 ksi wide flange column identified from the recovered WTC steel. The section was

also a hot-rolled steel with a relatively uniform distribution of the constituents across the sections

observed. Representative micrographs are shown in Fig. 5-28. The majority of the ferrite is polygonal

with an average ASTM grain size number of 9.4 (Table 5-3). The pearlite constituent was dense with no

loss of structure near the edges of the section.

5.2.2 Built-Up Box Core Columns

Three 36 ksi built-up box columns were identified from the recovered WTC steel. The sections were

found to be hot-rolled steels with some banding of the constituents across the sections observed.

Representative micrographs are shown in Fig. 5-29. The majority of the ferrite was polygonal or blocky

with some irregular morphologies. At low magnification, banding of the structure can be observed. The

average ASTM grain size number of the ferrite was 9.6 (Table 5-3). The pearlite constituent, at a volume

fraction of 0.22 for this sample (Table 5-3), was dense with no loss of structure near the edges of the

plate.

Two 42 ksi built-up box columns were recovered and available for analysis. Five of the eight plates

available for inspection were examined. The plates were found to be hot-rolled steels with some banding

of the constituents across the sections observed. Representative micrographs are shown in Fig. 5-30.

The majority of the ferrite was polygonal or blocky with some Widmanstatten morphologies observed.

The average ASTM grain size number of the ferrite was 1 1.0 (Table 5-3). The pearlite constituent, at a volume fraction of 0.29 for this sample (Table 5-3), was dense with no loss of structure near the edges of

the plate.

5.2.3 Channel

Two samples were removed from the channels and microstructurally analyzed. Both were found to have

hot-rolled structures but with vastly different distributions of the constituents. The sample from piece

38 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

C-107 had a heavily banded structure (Fig. 5-3 la), while a uniform distribution of the pcarlitc was found

in the sample from M-24 (Fig. 5-3 lb). Their chemistries were quite different as well, Table 4-6, with the sample from C-107 having higher contents for both C and Mn. These differences would be expected as no specifications were found for the type of material to be used for these structural components.

5.2.4 Core Truss Seat

Two core seats were removed from the channels and microstructurally analyzed. The sections were found

to be hot-rolled steels with a relatively unifomi distribution of the constituents across the sections

observed. Representative micrographs are shown in Fig. 5-32. The majority of the ferrite was polygonal.

The average ASTM grain size number of the ferrite was 10.3 (Table 5-2). The pearlite constituent, at a volume fraction of 28.8 percent for this sample (Table 5-2), was dense with no loss of structure near the edges of the section.

Compared to the average for the perimeter panel floor truss seats, the grain size was statistically similar,

but the volume fraction of pearlite was significantly higher.

5.3 FLOOR TRUSS MATERIAL

5.3.1 Rod

Six different diameter rods were recovered from the floor trusses ranging in size from 0.68 in. to 1.13 in. All rods were observed to have hot-rolled microstructures with a relatively uniform distribution of the

pearhte constituent. Polygonal, irregular, and Widmanstatten morphologies were observed for the ferrite; though the volume fraction and distribution within the structures was very different. Figure 5-33 shows

the structures from three rods with three different diameters. The sample labeled Tl-SR-1 contained

mostly polygonal and irregular ferrite, whereas Tl-LR-1 consisted of all three types. These rods were removed from the same floor truss and had chemistries that were very similar (Table 4-9). The sample

M32-LR-1 also contained all three morphologies; however, it had a completely different distribution. The

chemistry for this sample varied from the first two in that it had small Mo additions and lacked V. It may

be these alloying additions, in combination with processing conditions (particularly the cooling path of

the rod), that attributed to the different morphologies between the samples. (Laclede produced all rods in house.) Table 5^ displays both the ASTM grain size number for fen^ite and the volume fraction pearlite for the individual samples.

5.3.2 Chord/Angle

Ten pieces of truss angle were evaluated from the floor truss material recovered. Again, all were

observed to have hot-rolled microstructures with a relatively good distribution of fine pearlite. The

ASTM grain size number for the ferrite and the volume fraction pearlite for some of the individual

samples can be found in Table 5^. Unlike the truss rod microstructures, angle samples with V additions

had a higher volume fraction of the Widmanstatten morphology for the ferrite constituent (Fig. 5-34).

Samples lacking this element contained primarily polygonal ferrite. This information suggests that the

processing conditions had a strong effect on the microstructure as the correlation between chemistry and

ferrite morphology is different between the rods and the angles. (Laclede produced both of these shapes in house.)

NISTNCSTAR 1-3E, WTC Investigation 39
Chapter 5

5.4 FURNACE EXPOSURE OF WTC STEEL

The effect of high temperature exposure due to pre-collapse fires on the structural steels is of great concern as the strength of the steel (and structural integrity of the component) decreases significantly at elevated temperature. Evidence of this exposure may be observed through changes in the steel microstructure. Therefore, four "as-fabricated" steel structures were chosen for furnace studies in order to simulate the exposure of steel to high temperature excursions. These samples were: a 60 ksi hot-rolled

flange plate, a 100 ksi quenched-and-tempered flange plate, a 42 ksi hot-rolled spandrel plate, and a hotrolled floor truss seat. The samples were chosen based upon their specified minimum yield strength and

location within the building (no pre-collapse fire exposure). Unless otherwise indicated, furnace

exposures consisted of placing small coupons of material into a furnace pre-heated to temperatures in the

range of 200 °C to 625 °C for times up to 2 h. The samples were then removed from the furnace and

allowed to cool in a medium of standard silica sand. Metallographic preparation of the samples was

similar to that used above.

5.4.1 Hot-Rolled Flange and Spandrel Plates

A 60 ksi flange plate was chosen from panel N-7 (WTC 1, column 126, 97th floor) and a 42 ksi spandrel

plate from panel N-9 (WTC 1, 102nd floor spandrel, near column 154). According to the time-fire

exposure maps of NIST NCSTAR 1-3C, pre-collapse fires were not observed near either location.

Further, there was no evidence, such as loss of paint or sooting on the column, that suggested postcollapse fires may have affected the material. For temperatures below 625 °C, only times of 0.25 h and

0.5 h were investigated.

For temperatures below 625 °C, there was no apparent change in structure using light optical microscopy.

However, when samples were exposed to 625 °C, alteration of the microstructure was evident after only

15 min of exposure. Figure 5-35 displays representative microstructures of the exposed 60 ksi flange

plate and Fig. 5-36 of the 42 ksi spandrel plate. Both figures have an "as-fabricated" structure (t = 0 h)

shown for comparison. For the 60 ksi flange plate, the banding of the structure remained through the 1 h

exposure with it being eliminated in the 2 h time frame. With time, the change in the cementite phase was

clearly visible. Initially, the pearlite was mottled, however, after the first 15 min of exposure, the

cementite lamellae within the pearlite began to spheroidize, with the precipitates becoming more

discernible at the longer times. After the 1 h exposure, cementite was found to form larger carbide

particles in the ferrite grain boundaries. The transformation of cementite to this morphology was nearly

complete after 2 h. For the 42 ksi spandrel plate, the transformation with time was also noticeable,

though not to the extent seen for the flange plate. After the 2 h exposure, remnants of the initial pearlite

colonies were still visible even though spheroidization was well in progress.

The difference in the kinetics of spheroidization for these two steels was most likely a result of the prior

thermomechanical history. The higher strength 60 ksi flange plate experienced a significantly different

control rolled schedule than the 42 ksi spandrel plate. This was clearly evident in the starting

microstructures (e.g., significant banding and smaller constituent size of the 60 ksi plate). As a result, the

microstructures responded differently with exposure to elevated temperatures with the 60 ksi plate having

faster spheroidization kinetics than the 42 ksi spandrel plate.

40 NIST NCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

5.4.2 Quenched-and-Tempered Flange Plate

A 100 ksi flange plate was chosen from panel C-10 (WTC 1, column 451, 88th floor). According to the

time-fire exposure maps of NIST NCSTAR 1-3C, pre-collapse fires were not observed near this location.

Further, there was no evidence that suggested post-collapse fires may have affected the material.

Exposure was conducted only at 625 °C for up to 2 h.

Figure 5-37 displays representative microstructures of the exposed samples, as well as an "as-fabricated"

structure. It can be seen that as the exposure time increased, the lath boundaries of the martensitic

structure become less distinguishable, while the cementite precipitates becoming more discernable.

5.4.3 Exterior Panel Floor Truss Seat

A floor truss seat was chosen from C-18 (WTC 2, column 231, 95th floor). According to the time-fire

exposure maps of NIST NCSTAR 1-3C, pre-collapse fires were not observed near this location. Further,

there was no evidence, such as loss of paint or sooting on the column, that suggested post-collapse fires may have affected the material.

For temperatures below 625 °C, there was no apparent change in structure using light optical microscopy.

However, when samples were exposed to 625 °C, alteration of the microstructure was evident after only

15 min of exposure. Figure 5-38 displays representative microstructures of the exposed floor truss seat. The as-fabricated structure (t = 0 h) was shown for comparison. At 0.25 h, some spheroidization can be

observed, however, it was very clear in the 0.5 h time frame. Continued exposure resulted in the

remnants of the initial pearlite colonies still being visible even though spheroidization was well in progress. Similar to that of the 42 ksi spandrel plate, the kinetics of spheroidization were much slower for

this component than for the 60 ksi flange plate.

5.5 SUMMARY

Two types of steels were observed from the recovered WTC steels: hot-rolled for the lower strength

materials and quenched-and-tempered for the higher strength plates/components. The hot-rolled steels had ferrite-pearlite microstructures while the quenched-and-tempered structures exhibited possible

bainitic/martensitic structures. For each of type of steel, the types of structures observed were expected

for steels of that era. Further, while microstructural features may have varied from plate to plate (for a given Fy) or across an individual plate thickness or component, there was no indication that the practices

followed to produce the plates were improper or that the mechanical properties of the component may

have been negatively affected.

In addition to the "as-fabricated" structures, it was shown that an elevated temperature exposure (625 °C)

for a minimum of 1 5 min was found to produce significant changes to the microstructure for both hotrolled and quenched-and-tempered steels. Temperatures at 500 °C and below showed no change using

light optical microscopy. Further, the prior thermomechanical treatment of the component appeared to

dictate the kinetic rate of spheroidization.

NIST NCSTAR 1-3E, WTC Investigation 41
Chapter 5

Source: NIST. Figure 5-1. Representative microstructures of hot-rolled perimeter column flange plates as a function of strength level. The images were taken near the centerline of the plate,

a) Fy = 45 ksi (panel ASCE-3, unidentified perimeter column with "45" flange stamping);

b) Fy = 50 ksi (panel M-26, WTC 1, 129, 90th floor). Both with 2 percent nital and 4 percent

picral etch.

42 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Mail' ^ — - "t • »»- <J»» .^^-^ ^

Source: NIST. Figure 5-1. Representative microstructures of hot-rolled perimeter column flange plates as a function of strength level. The images were taken near the centerline of the plate,

c) Fy = 55 ksi (panel M-20, WTC 1, column 121, 100th floor); d) Fy = 60 ksi (panel N-7,

WTC 1, column 126, 97th floor). Both with 2 percent nital and 4 percent picral etch

(continued).

NISTNCSTAR 1-3E, WTC Investigation 43
Chapter 5

Source: NIST. Figure 5-1. Representative microstructures of hot-rolled perimeter column flange plates as a function of strength level. The images were taken near the centerline of the plate,

e) Fy = 65 ksi (panel N-99, WTC 1, column 149, 99th floor). 2 percent nital and 4 percent

picral etch (continued).

44 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

a) 16

14

N

12

'«O

-4-1

10

8

6 • Flange

Inner web

Spandrel

40 50 60

Fy (ksi)

70 80

b) 50

45 ^ ^ 40

1 35

a

c 30 ol25

20

15

10

5

0

I I i

• Flange

A Inner web

Spandrel

40 50 60

Fy (ksi)

70 80

Figure 5-2. a) ASTM ferrite grain size number as a function of

specified minimum yield strength of the plate, and b) volume fraction

of pearlite as a function of specified minimum yield strength of the

plate. Various perimeter panel plates are shown in both graphs.

NISTNCSTAR 1-3E, WTC Investigation 45
Chapter 5

Source: NIST. Figure 5-3. Representative micrographs of ferrite morphologies observed from plates

with the specified minimum yield strength less than 70 ksi. a) Polygonal and irregular

ferrite grains (panel M-26, WTC 1, column 129, 90th floor), b) 0.25 in. thick plate with

Fy= 60 ksi, in addition to polygonal and Irregular ferrite grains, a faint Widmanstatten

structure (ferrite needles) was developed (panel K-1, WTC 1, column 210, 97th floor).

Both with 2 percent nital and 4 percent picral etch.

46 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-4. Banding of microstructural constituents observed in a 60 ksi plate,

a) and b) are opposite surfaces of the plate showing banding of the constituents;

c) was taken near the centerline of the plate showing heavy banding (panel N-7,

WTC 1, column 126, 97th floor). 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 47
Chapter 5

Source: NIST

Figure 5-5. Distribution of microstructural constituents observed in a 60 ksi plate,

a) and b) are opposite surfaces of the plate showing nearly uniform distribution of

constituents; c) was taken near the centerline of the plate showing some banding effects

(panel N-8, WTC 1, column 142, 97th floor). 2 percent nital and 4 percent picral etch.

48 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-6. Morphologies of pearlite observed, a) Pearlite with observable lamellar

spacing (panel ASCE-3, unidentified perimeter column with "45" stamping on flange

plate) and b) mottled appearance where the lamellar spacing were not visible and may

have a granular morphology (panel C-40, WTC 1, column 136, 98th floor). Both with

2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 49
Chapter 5

Source: NIST. Figure 5-7. Possible bainite or degenerate pearlite in lower strength plates, (panel M-27,

WTC 1, column 131, 93rd floor). 2 percent nital and 4 percent picral etch.

50 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

a)

•

— —

20 |im

b)

20 |Lim

Source: NIST. Figure 5-8. Non-metallic inclusions of MnS observed In the rolled plates, a) Thin

stringers aligned with the rolling direction of the plate (panel S-14, WTC 2, column 217,

92nd floor and b) thin stringers with other secondary phases (panel N-10, WTC 1,

column 114, 91st floor). Samples in the as-polished condition.

NISTNCSTAR 1-3E, WTC Investigation 51
Chapter 5

a): . --y_ .— * »..< - , p -i i .'•{ . ,..

^ ^ y

- , ' , -"- - - .. / ^a'^ - f ...-.>

' ' >.-^

' : ' . ,

. -J . , -J.

' '

'

' ''-^

- / ' •' '

.

• . j , ^ * * i 1/ . .- J - ' • .

, . / . ^ V

' '

^' ^ '

'

- ' ' . • , '•

.V ^ - " -,

'

'

. . y^g—.

.

^

. -. ' . : '-^ ,,

" ««ap^

- ; > •

- ^ /'^

: —"^or-

>b)-

20 |im

Source: NIST. Figure 5-9. Representative microstructures of quenched-and-tempered

perimeter column flange plates as a function of strength level. The images

were taken near the centerline of the plate, a) Fy = 70 ksi (panel S-10, WTC 1,

column 224, 94th floor); b) Fy = 75 ksi (panel C-22, WTC 1, 157, 95th floor).

Both with 2 percent nital and 4 percent picral etch.

52 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

20 lim

. • . ; " ""'1

- J" , ,\ • >

'

, ,.1., -

'

20 jim

Source: NIST. Figure 5-9. Representative microstructures of quenched-and-tempered perimeter

column flange plates as a function of strength level. The images were taken near the

centerline of the plate, c) Fy = 80 ksi (panel C-25, WTC 1, column 207, 89th floor),

d) Fy = 100 ksi (panel C-14, WTC 2, 259, 87th floor). Both with 2 percent nital and

4 percent picral etch (continued).

NISTNCSTAR 1-3E, WTC Investigation 53
Chapter 5

20 |im

Source: NIST

Figure 5-10. a) Microstructures from hot-rolled steels used for all inner web

plates, pearlite was observed to be mottled and lamellar; sample shown from

panel C-40 (WTC 1, column 135, 100th floor). 2 percent nital and 4 percent

picral etch, b) Heavy second phase observed in inner web, from panel M-20

(WTC 1, column 121, 100th floor). Sample in the as-polished condition.

54 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

NISTNCSTAR 1-3E, WTC Investigation 55
Chapter 5

a) Weld

material

1 Ci-

, {Ji '* t"" ^ -p. ,."v':

~ 20 unit'

Source: NIST.

Fusion

line

HAZ

t

Figure 5-12. Representative micrographs from perimeter column weids of a hotrolled steel with Fy = 55 ksi. a) Low magnification showing weld material and

portion of HAZ. Sample shown was from panel M-2 (WTC 1, column 129,

97th floor). 2 percent nital and 4 percent picral etch.

56 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

, tii/: -^-^rr -til

. _____—_—_ ———-— ; :

' ;

-

,\'

Source: NIST. Figure 5-12. Representative micrographs from perimeter column welds of a hot-rolled

steel with Fy = 55 ksi. b) Weld material that was primarily acicular ferrite with carbide

dispersion and c) HAZ near the fusion line showing grain growth and formation of

Widmanstatten and acicular ferrite Bainite may also be present in areas where ferrite

contains small cementite particles. Sample shown was from panel M-2 (WTC 1,

column 129, 97th floor). Both with 2 percent nital and 4 percent picral etch (continued).

NISTNCSTAR 1-3E, WTC Investigation 57
Chapter 5

1 ± ^ Source: NIST,

Figure 5-12. Representative micrographs from perimeter column welds of a hot-rolled

steel with Fy = 55 ksi, d) Tempering of the original pearlite in the structure. Sample

shown was from panel M-2 (WTC 1, column 129, 97th floor). 2 percent nital and 4 percent

picral etch (continued).

58 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

-

•

»

f

. a

*

•

* o

•

*

0

•

# 10 |im

Source: NIST. Figure 5-13. Representative micrestructure of HAZ near fusion line from

perimeter column weld of a quenched-and-tempered steel with Fy = 100 ksi.

Sample shown was from panel C-10 (WTC 1, column 452, 86th floor). 2 percent

nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 59
Chapter 5

20 |Lim

Source: NIST

Figure 5-14. Microstructure from a column butt plate. Sample shown was from

unidentified panel C-68. 2 percent nital and 4 percent picral etch.

60 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-15. Representative microstructures of hot-rolled perimeter column spandrel

plates as a function of strength level. The images were taken near the centerline of the

plate, a) Fy = 36 ksi (panel N-9, WTC 1, column 153, 102nd floor); b) Fy = 2 ksi

(panel C-40, WTC 1, 136, 99th floor). Both with 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 61
Chapter 5

|iin

Source: NIST. Figure 5-15. Representative microstructures of hot-rolled perimeter column spandrel

plates as a function of strength level. The images were taken near the centerline of the

plate, c) Fy = 45 ksi (panel N-99, WTC 1, column 147, 100th floor); d) Fy = 46 ksi

(panel S-14, WTC 2, 218, 92nd floor). Both with 2 percent nital and 4 percent picral etch

(continued).

62 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

I

* «C 4^- --4. II — , ^ *^~# — 4^

g-— ' ' -J.'JJ'. ' ^ y..--.,.., ...... ' J' I I,.. . .H

I M , ^ . ^ H W^FTIffg

Source: NIST. Figure 5-15. Representative microstructures of hot-rolled perimeter column spandrel

plates as a function of strength level. The images were taken near the centerline of the

plate, e) Fy = 50 ksi (panel N-12, WTC 1, column 206, 93rd floor); f) Fy = 55 ksi

(panel C-25, WTC 1, 206, 91st floor). Both with 2 percent nital and 4 percent picral etch

(continued).

NISTNCSTAR 1-3E, WTC Investigation 63
Chapter 5

Source: NIST. Figure 5-15. Representative microstructures of hot-rolled perimeter column spandrel

piates as a function of strength level. The images were taken near the centerline of the

plate, g) Fy = 60 ksi (panel C-22, WTC 1, column 156, 95th floor), h) Fy = 65 ksi with plate

thickness equal to or less than 0.5 in. (panel C-22, WTC 1, 157, 94th floor). Both with

2 percent nital and 4 percent picral etch (continued).

64 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

4Z> 20 |im

-b)<:r.

' .-5! * •'-« ^

' jT

' ''1^' v.-" - ' - ^ .V .

20 |Lim

Source: NIST. Figure 5-16. Representative microstructures of quenched-and-tempered perimeter

column spandrel plates as a function of strength level. The images were taken near the

centerline of the plate, a) Fy = 65 ksi with plate thickness greater than 0.5 in. (panel C-46,

WTC 2, 58, 69th floor b) Fy = 70 ksi (panel C-13a, WTC 2, column 159, 92nd floor). Both

with 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 65
Chapter 5

c)

•—" 20 j^m

20 \im

Source: NIST. Figure 5-16. Representative microstructures of quenched-and-tempered perimeter

column spandrel plates as a function of strength level. The images were taken near the

centerline of the plate, c) Fy = 75 ksi (panel C-14, WTC 2, 259, 86th floor) and d) Fy = 80 ksi

(panel C-24, WTC 2, column 202, 77th floor). Both with 2 percent nital and 4 percent

picral etch (continued).

66 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-17. Representative microstructure of spandrel splice plate. Sample was from

panel N-101 (WTC 1, column 132, 101st floor). 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 67
Chapter 5

HI

r

1

1. V

/•I

r . 50 urn

Source: NIST. Figure 5-18. Partially decarburized zone found near the surface of a perimeter floor truss

seat. Sample shown taken from panel M-2 (WTC 1, column 131, 99th floor). 2 percent

nital and 4 percent picral etch.

68 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

y

%^ '

- S

' J

- ~j

..J

.

- u't— , - .

^

, 20 |im|

Source: NIST. Figure 5-19. Microstructure from an ASTM A 325 construction bolt. Sample shown was

taken from a spandrel splice located at the 97th floor !evel next to column 131 of WTC 1 (panel M-2). 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 69
Chapter 5

Source: NIST. Figure 5-20. Microstructures observed for standoff plates connecting floor truss seats to

spandrel plates. Microstructures had relatively good distribution of the pearlite, however,

the volume fraction and distribution within the structures was very different, a) sample

with polygonal and irregular ferrite grains (panel M-2, WTC 1, column 129, 97th floor) and

b) sample with both polygonal, irregular, and Widmanstatten morphologies (panel N-8,

WTC 1, column 143, 98th floor). 2 percent nital and 4 percent picral etch.

70 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results
Chapter 5
Metallographic Analysis Results

Figure 5-23. Microstructure of a hot-rolled gusset plate welded to the top chord of the

floor trusses. High volume fraction of pearlite and banding of the structure was noted.

Sample shown was from panel N-8 (WTC 1, column 143, 98th floor). 2 percent nital and

4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 73
Chapter 5

Source: NIST. Figure 5-24. Microstructure of a hot-rolled damper plate. Banding of the structure was

noted. Sample shown from panel N-8 (WTC 1, column 143, 98th floor). 2 percent nital

and 4 percent picral etch.

74 NIST NCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST

Figure 5-25. Microstructure of a hot-rolled gusset plate used to attach the damper units

and the diagonal bracing straps to the perimeter columns. Ferrite was primarily

polygonal. Sample shown was from panel N-8 (WTC 1, column 142, 98th floor). 2 percent

nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 75
Chapter 5

%^ 1 00 urn

Figure 5-26. Microstructure of a hot-rolled diagonal bracing strap. Ferrite was primarily

polygonal and elongated in the RD. Sample shown was from panel N-8 (WTC 1,

column 142, 98th floor). 2 percent nital and 4 percent picral etch.

76 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-27. Microstructure of a hot-rolled 36 ksi rolled wide flange, a) The majority of

the ferrlte is polygonal or blocky with some Widmanstatten and b) fine pearlite with some

bainite possible where cementite precipitates are associated with ferrite. Sample is from

C-80 (WTC 1, core column 603, 92nd floor). Both with 2 percent nital and 4 percent

picral etch.

NISTNCSTAR 1-3E, WTC Investigation 77
Chapter 5

Source: NIST. Figure 5-28. Micrestructure of a hot-rolled 42 ksi rolled wide flange. The majority of the

ferrite is polygonal or blocky. Sample is from HH (WTC 1, core column 605, 98th floor).

Both with 2 percent nital and 4 percent picral etch.

78 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-29. Microstructure of a hot-rolled 36 ksi plate from a built-up box core column.

The majority of the ferrite is polygonal. Sample is from B-6152-1 (WTC 1, core column 803, 15th floor). Both with 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 79
Chapter 5

Source: NIST. Figure 5-30. Microstructure of a hot-rolled 42 ksi plate from a built-up box core column.

a) The majority of the ferrite is polygonal with some Widmanstatten ferrite, moderate

banding was observed, and b) Widmanstatten ferrite. Sample is from C-88b (WTC 2, core column 801, 80th floor). Both with 2 percent nital and 4 percent picral etch.

80 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

m^'**~mC/'-" . i ¥!sr< "Visual -I

„

"i:1

» . ^"i, .» . • 'ja,..—^T's, . -

'— —TT yr-

'

. - \—: —

Source: NIST. Figure 5-31. Microstructures from channel material located in the core. Both samples

were hot-rolled with different volume fraction and distribution of pearlite within the

structures, a) Banded microstructure of sample C-107 and b) uniform distribution of

sample M-24. Both with 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 81
Chapter 5

Source: NIST. Figure 5-32. Microstructure of a hot-rolled core floor truss seat. Majority of the ferrite is polygonal, but there are some irregular morphologies. Sample is from C-118. 2 percent

nital and 4 percent picral etch.

82 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Source: NIST. Figure 5-33. Microstructure from floor truss rods, a) Uniform distribution of

constituents of rod T1-SR-1. 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 83
Chapter 5 4

84 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

1

Jjf?^^^-^ 100 |im

Source: NIST. Figure 5-34. Microstructures from floor truss angles, a) Angle samples with V additions

had a higher volume fraction of the Widmanstatten morphology for the ferrite constituent

(sample from C-137a) and b) samples lacking V contained primarily polygonal ferrite

(sample from C-53a). Both with 2 percent nital and 4 percent picral etch.

NISTNCSTAR 1-3E, WTC Investigation 85
Chapter 5

86 NISTNCSTAR 1-3E, WTC Investigation
NISTNCSTAR 1-3E, mc Investigation 87
Chapter 5

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88 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

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NISTNCSTAR 1-3E, 1/VTC Investigation 89
Chapter 5

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90 NISTNCSTAR 1-3E, WTC Investigation
Metallographic Analysis Results

Table 5-2. ASTM grain size number, volume fraction pearlite, and hardness

results for panel splice connectors and floor truss connectors.

Element Component

(ksi)

Ave ASTM Pearlite

/U HRB HVN

Truss seat 36

9.7 17.9 26 6 183

0.8 0.9 1 Z. 1 1 J

Standoff plates 42

11.9 17.3 78.6 161

1 n

1. U 1 T Z. 1 Z. 0

''J fi

Gusset plate ^velded to the top

36

13.1 A3.

6

89.9 221

rVirifn nt flnriT' tnicic n 5 U.o ^ 1 0. 1 Perimeter panel

Bolt 120 n/a n,i''a 26.7 340

U. J

Damper plate 36

13.3 30.9 <;;7.9 187

0.7 3.5 n oU. Z J Gusset plate for attaching damper

36

10.3 13.4 81.3 169

unit and diagonal bracing sti"aps U. 0 U. / 0. / Zo

Diagonal bracing straps 36

10.5 19.9 86.7 190

U. / Z.u 3.4

Butt plate 50 n/a ru'a 90.8 203

Perimeter panel 0.3 11 J

bpjlLC

Splice plate 36

12.7

1 o o 18.8 on80.6 164

0.7 1.2 9.5 25

Channel 36

9.8 24.5 80.7 178

Core coliomn 1.1 2.5 3.2 12

truss connector Truss seat 36

10.3 28.8 85.3 212

0.6 0.7 4.3 17

NISTNCSTAR 1-3E, WTC Investigation 91
Chapter 5

Table 5-3. ASTM grain size number and volume fraction pearlite for core column material.

Element Component

Fy

(ksi)

Tliickness

(in)

Ferrite gi^i size Peai'lite %

Ave ASTM

grain size #

Relative

accuracy

Ave Stdev

Core Coluirm Box- flange 36 2.00 9.6 0.9 21.9 1.0 Core Colurnn E>ox- flange 42 1.55 11.0 1.1 28.9 3.0 Core Colmrm Wide- flange 36 1.38 11.1 1.1 36.6 2.7 Core Coluirm Wide- flange 36 1.50 28.2 1.9 Core Column Wide- flange 36 1.55 10.0 1.2 31.1 2.3 Core Column V'^ide- fl:arige 42 0.88 9.6 0.8

Table 5-4. ASTM grain size number, volume fraction pearlite, and hardness results for

floor truss material.

Component Sample ID Diameter ASTM grain size # Pearlite % HRB HVN

(in) Ave Relative accuracy Ave Stdev Ave Stdev Ave Stdev

Bottom C53-BA-3 NA 83,9 0 2 angle/chord Tl-BA NA 189 3 Tl-TA NA 10 0 5 22,6 2 0 87.8 0,1 197 4 C132-TA-3 NA 86.6 0,4 Top C137a-TA-3 NA 11.3 0 6 37 1 1 8 89.2 0.2 183 1 angle/chord C53-TA-2 NA 89.6 0.2 203 7 C137f-TA-5 NA 86,5 0.4 172 3 C53-TA-1 NA 11,7 1 24 4 2.2 88.3 0.8 209 3 C106-SR-1 0 68 10.2 1.1 25.6 3,8 83.5 0.8 Tl-SR-1 0.76 89,5 1.0 160 4

Truss rod C53-MR-1 0.93 12.9 0.8 34.2 3,6 92,9 04 203 4 Tl-LR-1 1.00 12.5 0 9 29 2 2,3 86,5 04 163 7 C53-LR-1 1.10 82,6 1.5 159 6 M32-LR-1 1 13 95,6 0.7 189 5

92 NISTNCSTAR 1-3E, WTC Investigation
Chapter 6

Hardness Analysis Results

Hardness is a mechanical property typically defined as a measure of the material's resistance to plastic

deformation. These values correlate with the ultimate tensile strength obtained from uniaxial tensile

testing. In the following section, both Rockwell hardness, with a sampling area of 1 mm" to 2 mm", and

microhardness, with a sampling area of 100 |^m" to 200 ^m", were used in the study for all base

plates/material of a component. Further, microhardness traces across weld cross-sections from perimeter

panel columns were conducted to determine if a change in the microstructure of the heat affected zone

occurred.

In addition to the "as-fabricated" plate hardness measurements, the furnace exposed samples described in Sec. 5.4 were tested. This testing was conducted to help detect changes in the mechanical properties as a

result of microstructure alteration from the elevated temperature exposure.

6.1 PERIMETER PANEL MATERIAL

Tables 6-1 and 6-2 show Rockwell and Vickers data sets, respectively, for the perimeter column

material. For Rockwell testing, most of the data for the high strength plates were reported in Rockwell C,

as they were too hard for the B-scale. Similar results were observed for both sets of data; as the specified

minimum yield strength increased, the hardness was also found to increase.

6.2 CORE COLUMN MATERIAL

Table 6-3 displays the individual hardness results for each of the 12 identified columns. In grouping

them by their specified minimum yield strength, the 36 ksi and 42 ksi materials all had similar hardness

values. However, the 36 ksi materials tested from the core columns had lower hardness values than were

found for the 36 ksi plate materials from the perimeter columns.

6.3 FLOOR TRUSS MATERIAL

Table 5^ displays the individual results from the floor truss material (rods and angles). As all the materials met chemistry specifications for ASTM International (ASTM) A 36 and ASTM A 242,

deviations in the results were minimal. Two exceptions were observed for the 0.93 in. and 1.13 in. diameter rods that had Rockwell hardness values above 90 Rockwell Hardness B-Scale (HUB).

6.4 PANEL SPLICE CONNECTORS AND TRUSS CONNECTORS

Table 5-2 displays hardness results for connectors for both the panels and the floor trusses. As most of

the materials were believed to have a minimum yield strength of 36 ksi, their average hardness values were very similar. Note that, with the exception of the bolt and the butt plate (both with only one sample

measured), the standard deviations are all large. This may suggest that these components were supplied

NISTNCSTAR 1-3E, WTC Investigation 93
Chapter 6

by multiple suppliers or that the specification was very relaxed, as is the case for A 36. The only

difference among this group was the A 325 bolt, which had a higher hardness due to the quenched-andtempered microstructure.

6.5 HARDNESS TRAVERSES THROUGH WELDS ON PERIMETER COLUMNS

Knoop hardness traverses were conducted across welded joints found on perimeter columns. Joints

between an inner web and flange were analyzed for base plates with specified minimum yield strengths of

55 ksi and 100 ksi. Figures 6-1 and 6-2 show the samples used for profiles through the joint for the

55 ksi and 100 ksi materials, respectively, with a line inserted schematically showing the approximate

location of the hardness trace. Multiple indents at various locations along the weld were also dropped

near the fusion line to yield more data points at various distances near this microstructural feature. To

accomplish this task, samples were etched in order that the fusion line of the weld could be marked.

Samples were re-polished before testing. Subsequent to Knoop indentation, the samples were again

etched to reveal the weld structure and allow for exact determination of the indents from the fusion line.

On both samples, a spike in hardness was noted near the fusion line. In Sec. 5.1.1, it was observed that the welding of these steels produced areas near the weld in the un-melted base material that experienced

diffusional transformations. As seen in the weld microstructures of Fig. 5-1 2c, a Widmanstatten

morphology developed in the heat-affected zones (HAZs) near the fusion line for the hot-rolled steels (Fig. 5- 12c) and the grain size was larger in this area compared to the as-fabricated plate. For the

quenched-and-tempered steels, the formation of what appears to be martensite was observed (Fig. 5-13),

thus increasing the hardness in this area.

The hardness data for exposed hot-rolled plates corresponds well with the observed microstructural

changes at the higher temperatures. For the 60 ksi plate exposed at 625 °C, an initial decrease was

observed with time for the first 30 min of exposure and then increased after that to a level greater than the

"as-fabricated" hardness (Fig. 6-3). The initial hardness decrease was due to the spheroidization of the

cementite lamellae found in the pearlite (Fig. 5-35). The strengthening mechanism for the increase in hardness with continued exposure was undetennined. Similar to the 60 ksi flange plate, the 42 ksi spandrel plate also had a decrease in hardness (Fig. 6-4). However, unlike the 60 ksi steels, there was no

hardness increase with long exposure times.

For exposure at 500 °C, both materials experienced a slight decrease in hardness. No changes were

observed in the microstructure at this temperature using light optical microscopy.

Samples exposed in the 200 °C to 400 °C range showed a slight increase in hardness. The mechanism for the increase in hardness was not determined.

6.6 HARDNESS OF FURNACE EXPOSED FLANGE PLATES

6.6.1 Hot-Rolled Flange and Spandrel Plates

94 NISTNCSTAR 1-3E, WTC Investigation
Hardness Analysis Results

6.6.2 Quenched-and-Tempered Flange Plate

For the quenched-and-tempered plate, the hardness was observed to continually drop over the 2 h period

at 625 °C (Table 6-4). This was most likely due to the diffusion of carbon out of the lattice to fonn

carbides in the lath boundaries; these precipitates were observed to be more discernible after the 2 h

period (Fig. 5-37).

6.6.3 Exterior Panel Floor Truss Seat

The truss seat exposed to 625 °C was observed to decrease in hardness with an increase in time

(Table 6-4). Similar to the 42 ksi spandrel plate, no increase in hardness was observed after the 2 h

exposure.

6.7 SUMMARY

Hardness values were observed to increase with increasing specified minimum yield strength of the

plate/component.

Microhardness traces across weld cross-sections from perimeter panel columns displayed an increase in hardness near the fusion line in the HAZ of the base plate. This was related to the aheration of

microstructure due to the heat input from the welding process, and subsequent rapid removal, and is common when welding low carbon steels.

Hardness of furnace exposed samples varied for a given temperature, time, and microstructure.

NISTNCSTAR 1-3E, WTC Investigation 95
Chapter 6

inner web

Source: NIST. b) 300

en oo

o

?50

f 200

o

150 -

X 100

50 -

flange

fusion liiie

HAZ

Traverse acnDss sample M2-C3M-Weld-1

Weld between inner web and flange (55 ksi material)

4 8 10

Distance (mm)

Figure 6-1. Knoop hardness traverses through welded joints. Joints shown were

between an inner web and flange for hot-rolled plates, a) photomicrograph indicating

trace through 55 ksi plates. 2 percent nital and 4 percent picral etch, b) Knoop hardness

trace through 55 ksi material.

96 NISTNCSTAR 1-3E, WTC Investigation
Hardness Analysis Results

Source: NIST. b)

«soo

o

500

450

400

350

300

250

200

150

100

50

0

flaiige

HAZfusion linfi

Traverse across sample ClO-CiM-n/^/ELD-1

Weldbetvi^een inner web and flange (100 ksi material)

-Trio

Wstance (mm)

Figure 6-2. Knoop hardness traverses through welded joints. Joints shown were

between an inner web and flange for quenched-and-tempered plates,

a) photomicrograph indicating trace through 100 ksi plates. 2 percent nital and 4 percent

picral etch, b) Knoop hardness trace through 100 ksi material.

NISTNCSTAR 1-3E, WTC Investigation 97
Chapter 6

98

96

94

90

86

60 ksi flange material 0

200

A 300

X400

X 500 • 625

0.0 0.5 1.0 1.5 2.0 2.5

Time (h)

Figure 6-3. Hardness as a function of time and temperature for furnace exposure of

60 ksi flange plate.

89

87

85

COg 83

X

81

79

77

42 ksi spandrel material 0

200

4 300

> 400

X 500 • 625

0.0 0.5 1.0 1.5 Time (h)

2.0 2.5

Figure 6-4. Hardness as a function of time and temperature for furnace exposure of

42 ksi spandrel plate.

98 NISTNCSTAR 1-3E, WTC Investigation
Hardness Analysis Results

fable 6-1. Rockwell hardness data for exterior panel materia

Fy Tliifkness Outer Inner Spandrel

Fy Thickness

Flanges Outer Inner Spandrel

(ksi) (in) web web (ksi) (in) web web

JU 0.375

84.7

0.25 96 1 94 9 94 0

5 8 11 2 6 2 2

42 0.375 86 9 0.375 97 3

Oj

0 2

0.375

86 8

0 5625

98 0 24 0 2

0.4375

81 7

0.625

96 6

0.8 C 2:

46 0.375

90.7

0.25

94.3 98 5 21.6' 13 3.2 0 6 0 5

0.25 91.9 95.3

0 375

95 2

0.8 2.0

/U

0 3

0.375

89.5 93 5 0.4375 97 4

0 8 1.4 1.8

jU 0.4375

92.2

0.75

94.9

2 5 04

0.5 90.3iJ 0 25

96.2 97.8 20.8'

2.9 3 0 0 7 0 3

0.5625

91.0 S5 3

0 375

96 7

1.7 0.0 0 2

0 25

95.3 93 3 92 9 0.25

21.2* 20.4' 23.2'

2.7 3.1 2 5

<?n 1.2 0.5 1.9

0.3125 934

6U

0 5625

20 r 99.8

3.0 0 3 0 4

<^jj 0.375

95.0 92.3 0.625

98 5

2.6 0

Q 0 8

0.4375

96.1 0.25

25.6" 25.2' 25.8"

1.0 1.4 1.3 15

0.5625

90.7

100 0.5625

23 4'

2.9 0.8

0 25

94.5 94.7 90.3

1.125 102.0

0.8 1.8 3.7 0.3

60 0.3125

96.1 a. Indicates that values are HRC.

1.0 Note: Unless indicated, all values are HRB. 1

0.375

95.3

1 i 1 • 1.4

NISTNCSTAR 1-3E, WTC Investigation 99
Chapter 6

Table (3-2. Vickers hardness data 'or exterior panel material.

Fv Thickness

Flanges Outer Inner Spandrel Fv Tliickness Flanges Outer InnerSpandrel

(ksi) (in) web web (ksi) (in) web web

36 0,375 176

0,25 228 222 195

21 9 21 19

42 0 375

183

0,375 238

16 65 4

0,375

178 0.5625 252

45

15

" 7

0.4375

150

" -

~

0 625

229

8 8

0,375 202

0.25 226 243 254 46 27 30 13 7

0.25 204 70 0 375

226

4 16 4

0.375 187 215

0.4375

J.S2, 4 21 8

50 0.4375 205

75 0.25 236 231 241

15 12 8 10

0 5 199

0 375

246 5

18 3

0,5625 205 168

0.25 257 242 277

11 6 8010 5 14

0,25 229 212 206

0.5625

261

18 19 16 6

0,3125 216

0.625 254

24 10

55 0,375 226 216

0.25 275 27? 281

19 18 13 12 11

0,4375 226100 0.5625 272

17 6

0,5625 200

1.125 275

18

0,25 213 220 195

8 15 19

60 0 3125

226

20

0 375 220

6

1

100 NISTNCSTAR 1-3E, WTC Investigation
Hardness Analysis Results

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t<z

f

r-j u

c>o r-- (-- t-- r- Cn] -=3- c-- CO CO

0.1 0.1 OJ OJ 0.1 0.1 0.1 U

O &

f5

CO Q oo CD CD cn c-o

1 CO

cn

f-O OO oo OO OCi oo On

i:^ on

OO

f-| oo 00

ON 0) (-- t-- CO C^O Qn CO CO £1

ro <; < CQ CQ OD CQ r-i < < CQ < 1-

-Ho

en o CO

CD oo oo OCi CO OO oo NCi On as

a

H

C-l oo oo oo OO OCi CD CD' •o ON oo oo oo OCi oo OO p: o n rji 6 6 6 U O rj 6

NISTNCSTAR 1-3E, WTC Investigation 101
Chapter 6

Table 6-4. Hardness values for various furnace

exposed WTC steel.

Panel C-10 C-18

Bldg YfTC 1 V^TC 2 Component Q&T flange plate Floor truss seat Colunm # 451 231

Floor 88tl-i 95th

Fy 100 ksi 36 ksi Tempeimg time HRB ave St Dev HRB ave St Dev

0 h 25.9* 0.0 71.2 0.9 0.25 h 98.6 0.2 69.6 0.2 0.5 h 96.5 0.2 67.7 0.3 Ih 94.0 0.1 67.5 0.4 2 h 92.1 0.3 67.2 0.4

a. Indicates value reported in HRC.

Note: Unless indicated, all values are HRB.

102 NISTNCSTAR 1-3E, WTC Investigation
Chapter 7

Thermal Properties of WTC Structural Steel

7.1 INTRODUCTION

This chapter provides thermal property values as a function of temperature, which are needed for modeling the structural steel response to fire. These properties include:

• Specific heat (Cp),

• Coefficient of thermal expansion (a), and

• Thermal conductivity (k).

In general, properties are provided from room temperature up to 600 °C or 700 °C. On heating to approximately 727 °C, iron carbon alloys, such as structural steel, transfomi from a mixture of a-phase

(ferrite) and cementite (FcsC) to a mixture of a and y (austenite). Most thermostructural properties are discontinuous at this transition, and the heat capacity becomes infinite. Furthermore the literature contains

much less information on properties at temperatures higher than the phase transformation. Generally,

though, the strength of steel above the phase transformation is reduced to 10 percent or less of the roomtemperature value, so other properties become of minor importance.

Of the three thermal properties described here, only the thermal conductivity varies by more than a few

percent among the different steels used in the building. The specific heat, Cp, and coefficient of thermal

expansion, k, as a function of temperature can be represented by individual equations for all World Trade

Center (WTC) steels. The thermal conductivity, k, depends strongly on chemical composition. In

principle, it is possible to estimate the thermal conductivity from the steel chemistry. In the trusses, it is possible to estimate the thennal conductivity of all the components because a single mill supplied all the

steel for the trusses. Similarly, for the perimeter columns enough is known about the chemistry of the

steels to estimate the thennal conductivity of an individual plate in a given column. For the wide flange

and welded core columns, the steel composition used in individual columns can not be uniquely

identified. (Two mills supplied steel for the welded core columns. For the rolled, wide-flange core columns and beams, three or more mills supplied identical ASTM International (ASTM) grades of steel with different chemistries (NIST NCSTAR 1-3 A). Those steel chemistries were relatively different, so it

is impossible to assign a specific thermal conductivity to a given column or beam.

7.2 SPECIFIC HEAT

The specific heat at constant pressure is defined as

(7-1)

NIST NCSTAR 1-3E, WTC Investigation 103
Chapter 7

where q is the heat added per unit mass. To a first approximation, the molar specific heat of elements and

mixtures at room temperature and above is a constant at approximately 3i?=24.94 J/(mol K), which is the

so-called law of Dulong and Petit. At the a-y phase transformation (above about 720 °C in carbon steels)

the heat capacity is effectively infinity, since all added energy goes into transforming the phase rather

than raising the temperature.

Structural steels generally contain ver}' low fractions of alloying elements. To a good approximation,

then, it is possible to use the heat capacity as a function of temperature for pure iron without introducing an error of more than 3 percent. Figure 7-1 shows some experimental detenninations of the heat capacity

of various low-alloy steels and pure iron. In principle, it is possible to correct for the presence of alloying

elements, but these corrections will be, at most, several percent.

7.2.1 Recommended Value

Of the many experimental determinations of the heat capacity of iron, the National Institute of Standards

and Technology (NIST) has selected that of Wallace, Sidles, and Danielson (1960), Usted in Touloukian

and Buyco (1970), because it spans the temperature range of interest, is relatively recent, and has an

uncertainty statement. To represent the data, a third-order polynomial was fitted to the data in the range

0 °C<T<650 °C:

C^{t) = c,+c^T + cJ' +c,T' - where co= 51.11±33.39 (7_2^

c, = 2.019±0.185

C2 = (-3.0135±0.320)xl0"^

C3= (1.829±0.175)xl0"^

where Cp is expressed in J/(kg K) and T in kelvins.

7.2.2 Uncertainties

Wallace, Sidles, and Danielson (1960) reports an uncertainty of 2 percent in the measured value of Q?.

The maximum deviation of Cp from other measured structural steels is at most several percent.

7.3 COEFFICIENT OF THERMAL EXPANSION

The coefficient of thermal expansion (CTE), a, is used to predict the length of a member, L, as a function

of temperature, T. There are two common definitions. The mean CTE, is simply the two-point slope

of the length of the specimen at two temperatures. As such, it ignores any changes in slope between the

two fixed points.

L-, Z/j

104 NISTNCSTAR 1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

The instantaneous CTE, a, which pemiits integration along the temperature path to find the length of a member at any temperature is defined as

a

L dT

(7-4)

For pure iron, Touloukian et al. (1977) provides a recommended function for the fractional length change

as a function of temperature, as well as tabulated values for the instantaneous coefficient of thermal

expansion, a. Although it is possible to derive an expression for a from the function for L(Tj using

Eq. 7-4, the resulting expression is mathematically more complicated than necessary. It is more

convenient to calculate the points of the expression for a (T) and then fit a cubic polynomial to the data. The expression for a (T) derived this way differs from the derived version by less than 1 part in 1 0,000.

-a^ +a^T + ajT' + a^T'

where

ao= 7.3633 x

10"^

a, = 1.8723 x

10-*' a2 = -9.8382 X

10"'" 1.6718x10"'^

(7-5)

and 7" is measured in kelvins, for 300 K < r< 900 K.

The American Institute of Steel Construction (AISC) Manual of Steel Construction (AISC 2001)

recommends

-(r-273.15) + 32

.5where

(9/5)6.1 X 10'^

(9/5) 1.9 X 10"^

(7-6)

and ris expressed in kelvins, relative to a base temperature of 31 1 K (100 °F). It makes no distinction

between grades of structural steel. The AISC Manual is not completely clear that this expression is for the mean coefficient of thermal expansion, rather than the instantaneous value. Converting to the

instantaneous value of coefficient of thermal expansion yields

a = b^ + b^T

where (7-7)

Z,, =7.4954 X 10-^ 62= 1.231 X 10-^

when ris expressed in kelvins for 31 1 K < r< 922 K (100 °F<r<1200 °F).

There are several difficulties in estimating the thermal expansion coefficient of structural steel from first

principles. One serious problem is that the thermal expansion is discontinuous at the a-y phase boundar>'

NISTNCSTAR 1-3E, WTC Investigation 105
Chapter 7

(910 °C in pure iron, and beginning as low as 723 °C in Fe-C alloys). Figure 7-2 illustrates the fraction

length change AL/Lo of iron as a function of temperature up to 1,650 °C, and shows the discontinuity at 910 °C at the a-y phase boundary. Of course, because the phase transfonnation is not instantaneous, the

details of the thennal expansion near the transformation will depend on the heating rate. Interpreting

experimental thermal expansion curves is also complicated by the time-dependence of the phase

transformation. Curves obtained on heating and cooling frequently differ because the transformation occurs at different rates, leading to a hysteresis. First principles calculation of the thermal expansion of

multi-phase materials, such as the Fe-FcsC in structural steel, is also difficult.

There is a great amount of literature on thermal expansion coefficient of steels; Touloukian et al. (1977)

has tabulated many of the values. Figure 7-3 plots the instantaneous coefficient of thermal expansion

(a in Eq. 7^) for several low alloy steels, as well as for pure iron and the AISC recommended value

(Eq. 7-7). The difference between these curves is of the same magnitude as the difference between the

different detenninations for nominally pure iron. All of the data sets are truncated before they approach

the a-y phase boundar)' near 700 °C. The behavior of steels 6 and 7, which have anomalously low values

of a near room temperature, can not be explained. Note also the 2001 AISC recommended value severely over predicts all the other low-alloy steels for T > 550 °C. The origin of the use of this equation for

structural steels has not been found.

7.3.1 Recommended Value

Given the relative insensitivity to composition and microstructure, it is recommended that Eq. 7^ be

used, the value for pure iron, for all structural steels, in the temperature range 20 °C < T <700 °C. If an

expression for CTE for T > 700 "C is necessary, it would be necessary to measure it on the steel in question.

Table 7-1 collects the values of the mean thermal expansion coefficient relative to both 273 K and

293 K calculated from Eq. 7-5.

7.3.2 Uncertainties

Touloukian et al. (1977) reports that the uncertainty in the expression for the change in length of iron with

temperature (i.e., the integral of the CTE) is ±3 percent for 300 K < T < 900 K ±5 percent for 900 K < T <

1,185 K.

7.4 THERMAL CONDUCTIVITY AND DIFFUSIVITY

For one-dimensional heat flow, the thermal conductivity, k, relates the quantity of heat per unit time, Q,

transported per unit area. A, to the temperature gradient dT I 8x :

A

(7-8)

106 NISTNCSTAR 1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

and has units of W/(m»K). For one-dimensional situations where the temperature depends on time, the

thermal diffusivity, a, is appropriate —dT d^T ^a— (7-9)

dt dx^

The thermal diffusivity, a,

k

(7-10)

is the ratio of the thennal conductivity, k, to the density, p, and specific heat Cp. It has units of a diffiision coefficient: m"/s.

Unlike the other themial properties (CTE, specific heat), the microstructure of the steel affects the thermal

conductivity. Identical chemistries processed differently will yield steels with different thermal

conductivities. Because the various alloying elements and processing steps modify the microstructure,

there is no generic relation between grade of steel and thermal conductivity. For instance. Fig. 1-^ plots

the thermal conductivity as a function of temperature for twelve low-alloy steels. Even near room

temperature, the data span 40 percent of the mean value.

In metals above room temperature, electronic conduction dominates the thermal conductivity. Conduction

by lattice vibrations (phonons) is not usually significant. Because the thermal conduction in steel (and

most metals) is primarily by electrons, the thennal (A') and electrical (CT=l/p) conductivity are related through the Wiedemann-Franz law:

k K=— (7-11)

where Lo is called the Lorenz constant. In principle, then, one can predict the thennal conductivity from

the electrical conductivity.

Data exist from a statistically planned experiment (Ludwigson 1971) to develop a relation between

chemical composition (for extremely low alloy steels) and electrical conductivity at room temperature.

To estimate the thermal conductivity of a given steel, NIST assumes that the effects of chemical

composition are independent of temperature, and that the temperature dependence of the resistivity

follows that of pure iron. For the former effect, NIST uses the electrical resistivity (p)-composition data of

Ludwigson (1971) (at 26.75 °C):

NISTNCSTAR 1-3E, WTC Investigation 107
Chapter 7

- p= 0.01

'

. (+10.1169 + 6.1995 [Mn]

':':>. : ' +14.3269 [P]

+10.3715 [S] (7-12)

^ +11.7499 [Si]

: ' +3.8815 [Cu]

. " ~ + 2.9775 [Ni] :

- + 5.5696 [Cr]) - .

' , ,

where the resistivity, p, is measured in )iQ'm and the [ ] denotes % in percent. It was also assumed that the temperature dependence of the electrical conductivity is that of pure iron (ppe), using the data reported

by Schwerer (1986) Table 3 in the range 300 K<T<1 173 K), but with the resistivity at 300 K equal to that of the constant term in Eq. 7-12:

P,.(T)= 100

(+(10.1169-9.7947)

-1.506

+0.02276 T

+4.1966 X 10"^ T- +2.5879 X

10"*^

)

where ppe is measured in iiH-m and Tin K. The total resistivity of the steel alloy as a function of

temperature and composition, evaluated from the combination of the two equations is

p-(T)= 0.01

(+(10.1169-9.7947)

+6.1995 [Mn] + 14.3269 [P]

-10.3715 [S] + 11.7499 [Si]

+3.8815 [Cu] (7-14)

+2.9775 [Ni]

+5.5696 [Cr]

-1.506

+0.02276 T

+4.1966 X 10"^ T' +2.5879 X 10"*^ T-'),

where p, is measured in p.f2"m and Tin K. The constant term accounts for the additional resistivity due to alloying elements not enumerated.

Equation 7-11 relates the thermal and electrical conductivity through the Lorenz constant, Lq.

Schwerer (1986) recommends a value of 6.8x10'^ cal ^Q. /K's (= 2.84648 x 10 2 J iiQ m /K^s). The final expression for thermal conductivity as a function of composition for temperatures 0 °C < T < 700 °C is

(7-13)

108 NISTNCSTAR 1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

k(T)^ 2.84648 X 10 ' 77

[0.01 ( +(10.1169-9.7947)

+6.1995 [Mn] + 14.3269 [P]

-10.3715 [S] + 11.7499 [Si] +3.8815 [Cu]

+2.9775 [Ni]

+5.5696 [Cr]

-1.506

+0.02276 T

+4.1966 X

10"-' T- +2.5879 X lO"*' r')]

where k is measured in J/m s K, 7 in K, and [ ] denotes % in percent. The expression is not valid above

725 °C due to the phase change from a+FcsC to a + y iron.

7.4.1 Recommended Values

Thermal conductivities can be calculated from Eq. 7-14.

7.4.2 Uncertainties

Figure 7-5 shows the fractional error ( k^^^ ) for several low alloy steels evaluated using

Eq. 7-14. In all cases the thermal conductivity was reported as a function of temperature, and then also

calculated using Eq. 7-14. In some cases the error can be greater than 20 percent, but it is usually less than 10 percent.

Figure 7-6 demonstrates that steels conforming to ASTM A 36 can have significant variation in thermal

conductivity due to the wide range of allowable compositions. There is no single value for thermal

conductivity of A 36. Because NIST cannot identify the mill that supplied the plate or wide flange shape

in the core, it is not possible to state to better than 20 percent uncertainty the thermal conductivity of any

core element.

7.5 NOTES ON DATA SOURCES

7.5.1 Thermal Expansion Coefficient

Several American Society for Metals publications tabulate thermal expansion coefficients as a function of

temperature for low-alloy steels. High-Temperature Property Data: Ferrous Alloys (Rothman 1988)

repeats data already published in the 9th edition of the Metals Handbook (ASM 1978). It differentiates

between different American Iron and Steel Institute 1000-grade steels, but has no sources for any of the information, so it is impossible to make any judgment on the reliability of this information.

NISTNCSTAR 1-3E, WTC Investigation 109
Chapter 7

J I I I I 1 1 1 I ! I i 1 I 1 I i ! 1 1 '

I i ! ' I I 'I I i I 0 100 200 300 400 500 600

T(C)

Sources: (1 ) (2) and (3) are Steels 4, 19, and 20 of the NPL report

(1946), (4) Curve 1 of Specification Table 182 of Touloukian and Buyco

(1970) (5) Curve 3 of Specification Table 182 of Touloukian and

Buyco (1970), (6) from Wallace, Sidles, and Danielson (1960),

Figure 7-1. Heat capacity as a function of temperature for several low-alloy steels and

pure iron.

110 NISTNCSTAR1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

T(°C)

200 400 600 800 1000 1200

T(K)

Source: Touloukian et al. 1977. Figure 7-2. Thermal expansion of pure iron showing the discontinuity in thermal

expansion coefficient at the phase boundary.

NISTNCSTAR 1-3E, WTC Investigation
Chapter 7

20

18

16

200 400 600

<0O

X ^4

LU

(-

o

12

10

- (1) Pure Fe - {2}0.10C0.42Mn

- (3)0.17C042yn

~ {4)0,20C 1.10yn ?.??S!O.50CrO.50Ni - (5)0.230 1.15Mn0.12Si0.06Cr0.04Ni - (6; 0 25C 0.0?JMn - {?) 0.4.2G Q3BUn O.ESS!

(8) Also Handbook

0.025

0.020

- 0.015

400 600 800 1000

T(K)

Sources: (1) Pure Fe (Touloukian et al. 1977), (2) Lucks, as reported in Touloukian

et al. (1977), (3) Mochel, as reported in Touloukian et al. (1977), (4,6,7) Souder (1922) as

reported in Touloukian et al. (1977), (5) NPL 1946, (8) AISC Manual of Steel Construction (AISC 2001).

Figure 7-3. Instantaneous thermal expansion coefficient (Eq. 7-4) for several

low-alloy steels.

112 NISTNCSTAR 1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

0 200 400 600

400 600 800

T(K)

Sources: Curves 1-20 correspond to steels 1-20 in a report from the National Physical

Laboratory (NPL 1946). Curves A-C correspond to curves 22, 24, and 79 of Specification Table 334 in Touloukian et al. (1970). Curve D appears in Specification

Table 335 in Touloukian et al. (1970); the original data are from (Powell 1956). Curve W

comes from a data sheet for WEL-TEN 80 (Yawata 1969).

Figure 7-4. Thermal conductivity as a function of temperature for 12 low-alloy steels.

NISTNCSTAR 1-3E, WTC Investigation
Chapter 7

0.15

900

Alloy 5 —m— Touloukian cur\^ 24 —^— Touloukian curve 75

Alloy 20

X Powell

Steel 19

Sources: Alloys 5, 19, and 20 correspond to steels 5, 19, and 20 in a report from the National Physical

Laboratory (NPL 1946). Curves 24, and 79 come from of Specification Table 334 in Touloukian et al. (1970).

Curve labeled Powell is from Pov\/ell (1956).

Figure 7-5. Fractional error in estimating steel thermal conductivity from chemistry

using Eq. 7-15.

60

50

2"

0) 45

E

3 40

35

30

25

200

Specification

• Hannathy

Holt

• Laclede

WTC Wideflange

400 600

T(K)

800 1000

Sources: Harmathy 1970; Holt 1964. . Figure 7-6. Thermal conductivity as a function of temperature, calculated from Eq. 7-15

for four grades of steel, all identified as conforming to ASTM A 36, Laclede and WTC

wide-flange from column line 1008B, floors 104-106 (this report), and the allowable

composition of A 36.

114 NISTNCSTAR 1-3E, WTC Investigation
Thermal Properties of WTC Structural Steel

Table 7-1. Mean (Eg. 7-3) thermal expansion coefficient, a^.

T T

1/°C 1 AT 1/°C

relative to 0 relative to ?0 '^f relative tn 20 '''C

25 1 60E-06 400 1 u u 1X . OfiE-05 U U X^ U ^ l,08E-05

50 7 S3E-0fi 8u. OIE-Ofi u X xj* uu 425 1X . 08F-05 U Ui-/ u^ 1 fi9F-05

75 8 DfiE-06 80 . 24F-0fi X_/ u u 450 " w'U 11 . 09F-05 U X_/ u ^ 100 8U . 28F-0fi ^ (J i—f u u U . ~ ^ i—<" u u 475 1X . 1X 1E-05 X J_/ u ^ 1. 1jE-05

1 ?5 8U . 49F-nfi ~ ^ l-i- L.' U 8U . fi7F-0fi U / X-j- u u 500U u 1X . 13E-05 X J X-/ u^ 1. 14E-05

150 8 70E-D6 8U . 88E-Qfi U U i—i u u 525 1X . 14E-05 X 1 j_/ u y 1.16E-05

175 8 QIE-Dfi J' . U ^ X-/' u u 550 1X . lfiE-Q5 1 U Lu U 1.17E-05

Q 1 ?F-nfi y . Cij' ilj- uu 575 1 17F-05 1 19F-Q5

795 Q 37F-nf^ y .^y 11/ U

U

uuu 1 19F-05 1X . 20E-05 ^ U i—/ U 75n P . J L LI/ UU 9^ . f*i8F-0f*i u 0 X_/ u u fi25 u 1 20F-05 1 21E-05

775 Q 71 F.rifi q^ . S7F-0fi u / 1-/ uu 650 U w'U 1X . 21E-05 ^ X X-/ u y 1.23E-05

300 9.89E-0(5 l.OlE-05 675' 1.23E-05 1.24E-05

325 l.OlE-05 1.02E-05 700 1.24E-05 1.25E-05

1.03E-05 1.04E-05 725 1.25E-05 1.26E-05 ^

iT4E-05 1.06E-05 750 1.26E-05 "T28E-O5

NISTNCSTAR 1-3E, WTC Investigation
Chapter 7

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116 NISTNCSTAR 1-3E, WTC Investigation
Chapter 8

Analysis of Aluminum Faqade Used on the WTC Towers

A piece of the aluminum facade from the World Trade Center (WTC) towers was recovered and available

for testing. These pieces fit around the outside face of the column and were one story in length. It was unknown which tower this piece was recovered from. Chemical analysis was conducted to determine the

alloy type. This testing was done in accordance with ASTM International E227, and the results can be

seen in Table 8-1. It appears that the chemistry would place it in the 3xxx-series alloys (Al-Mn alloy).

Differential thennal analysis of the aluminum was also conducted in a Ti gettered UHP Ar atmosphere.

The heating rate was 10 K/min. The second melting scan for the sample is shown in Fig. 8-1, where a

solidus temperature of 625 °C was observed. This was consistent with the non-heat treatable Al-Mn

aluminum alloy AA 3003.

550 575 600 625 650 675 700 725

Sample Temperature (°C)

Figure 8-1. Differential thermal analysis scan for aluminum facade used on the WTC

towers. A solidus temperature of 625 °C was observed upon cooling.

NISTNCSTAR 1-3E, WTC Investigation 117
Table 8-1. Chemistry results for the aluminum

fa9ade used on the WTC towers (in mass

fraction x 100).

Element Alumit:Lum facade

Si 0.19

Cu 0.13

Mg <0.05

Fe 0.69

Ti <0.G5

Zn <0.05

Mn 1.11 Pb <0.05

Sn <05

Ni <0.05

Cr <0.05

Others each <0.05

Others total <0.15

97.88

NISTNCSTAR 1-3E, WTC Investigation
Chapter 9

Analysis of Sprayed Fire-Resistive Material

A sample of sprayed fire-resistive material (SFRM) was scraped from a perimeter column of panel S-1

(A433: 79-82; west face of World Trade Center [WTC] 1). The exact location of removal was on column

433 in the vicinity of the 80th floor (Fig. 9-1 a). The material was mounted in cold setting cpoxy and

polished using standard metallographic procedures. The sample was then examined using a scanning

electron microscopy with x-ray energy dispersive spectroscopy (EDS). Figure 9- lb displays a secondary

electron image of the material. Chemical analysis of 10 different areas was conducted with standardless

EDS. The following conditions were used: accelerating voltage = 15 kV; probe current = 1 nA; working

distance = 39 mm. Table 9-1 shows the average composition in weight percent.

Information was sought from the manufacturer concerning the chemical composition of the SFRM; this information was found to be proprietary. From the company's (Isolatek) web site, the SFRM was

described as "a commercial density, compositely reinforced portland cement based SFRM. The formula

for this product was listed as "slag wool and inorganic binders" with the "chemical family" of "silicates and calcium sulfites." Qualitatively, the results of this investigation were found to be consistent with the

information presently on the manufacture's Web site.

NISTNCSTAR 1-3E, IV7C Investigation 119
Chapter 9

Source: NIST,

Figure 9-1. a) Location of sprayed fire-resistive material that was scraped from a perimeter column of panel S-1 (A433: 79-82). The exact location of removal was on

column 433 in the vicinity of the 80th floor, b) secondary electron image of the sprayed

fire-resistive material.

120 NISTNCSTAR 1-3E, WTC Investigation
Analysis of Sprayed Fire-Resistive Material

Table 9-1. Chemistry analysis of sprayed

KIjBV Ipnipn IIICll t1 r\ V CI " liv St Dpv

Ca 45.4 0.4

Si 33.9 0.6 Al 9.9 0.1 Mg 9.0 0.2

Si 1.1 0.1 K 0.7 0.1

NISTNCSTAR 1-3E, WTC Investigation 12]
Chapter 9

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122 NISTNCSTAR 1-3E, WTC Investigation
Chapter 10

Findings AND Conclusions

The physical properties assessment of the recovered World Trade Center (WTC) steel resulted in ihc

following findings and conclusions:

• Two different microstructural types were observed for the recovered WTC steels: hot-rolled

for low strength components and quenched-and-tempered for high strength components.

• In general, the steel used for the construction of the WTC towers was found to be of good

quality. There did not appear to be atypical defects in the microstructure as a result of

processing of the individual plates. Further, the weldments used to build-up the box columns

and attach the floor truss connectors to the columns were sound with no visible flaws.

• For a given yield strength (F^), the flange, outer web, and spandrel steel in the perimeter

panels were found to be nearly identical, with indistinguishable chemistries and similar

microstructures and hardness values. The inner web was found to be distinguishable from the

other plates for F, less than 80 ksi, in terms of the physical properties evaluated. Inner webs

with Fy equal to 80 ksi and 100 ksi were the same steel as used for the flanges. These results support the contemporaneous documents stating that the flange, outer web, and spandrel

plates were produced by a single company (Yawata Iron and Steel Co.) and that the inner web

plates were primarily supplied domestically.

• The recovered core columns were found to vary in composition for a given F, and shape

(buih-up box, rolled wide-flange). This result is consistent with the contemporaneous

documents stating that numerous suppliers produced the steel to be used for these structural components.

• Butt plates of the perimeter panel colunms met chemistry specifications for several Yawata

steels.

• Construction bolts were either A 325 or A 490 bolts. This was primarily determined by the

stampings on the head. One bolt designated as A 325 was analyzed and met the chemistry

specifications for A 325 Type 1.

• Chords and rods that composed the floor trusses, supplied by Laclede Steel Co., were either

ASTM International (ASTM) A 36 or ASTM A 242.

• Other structural elements (floor truss connectors, core channels, spandrel splice plates, etc.) met composition specifications for ASTM A 36. Compositions varied significantly, which

may indicate that numerous suppliers completed the steel orders for these components.

• Discernible changes to the microstructure of furnace exposed WTC steel were obsers ed \\ hen

exposed to a temperature of 625 °C for as little as 0.25 h. At or below 500 °C, no indication

NISTNCSTAR 1-3E, WTC Investigation 123
of microstructural change was apparent using light optical microscopy. The hardness of these

samples varied for a given temperature and material type, but correlated well with

microstructural observations:

- Hot-rolled steels below 500 °C experienced a slight increase in hardness, which may be

due to strain-age hardening effects;

- Hot-rolled steels at 500 °C and 625 °C experienced a decrease in hardness. Depending

upon the prior thermomechanical history, an increase in hardness was observed if the extent of spheroidization was such that large carbide particles formed; and

- Quenched-and-tempered steels at 625 °C had a continual decrease in hardness up to the

maximum exposure time of 2 h.

These results were used in the report "Damage and Failure Modes of Structural Steel

Components" (NIST NCSTAR 1-3C) to assess the steel for fire-related damage.

Microhardness traces across weld cross-sections from perimeter panel columns displayed an

increase in hardness near the fusion line in the heat affected zone of the base plate. This was

related to the alteration of microstructure due to the heat input from the welding process.

Thermal property values as a function of temperature were derived from the literature and

provided for modeling the structural steel response to fire. These properties include specific

heat (Cp), coefficient of thermal expansion (a), and thermal conductivity {kj.

A piece of the aluminum facade used on the WTC towers was analyzed and found to be

consistent with the non-heat-treatable aluminum-manganese alloy AA 3003.

A sample of sprayed fire-resistive material scraped from a recovered perimeter column of

WTC 1 was chemically analyzed. While the chemical composition of the material was

proprietary, the results were consistent, on a qualitative basis, with the information presently

found on the manufacturer's web site.

NIST NCSTAR 1-3E, WTC Investigation
Chapter 11

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126 NISTNCSTAR 1-3E, WTC Investigation