NIST NCSTAR 1-7 (Draft)

Federal Building and Fire Safety Investigation of the 

World Trade Center Disaster 

Occupant Behavior, Egress, and 

Emergency Communications (Draft) 

Jason D. Averill 

Dennis S. Mileti 

Richard D. Peacock 

Erica D. Kuligowski 

Norman Groner 

Guylene Proulx 

Paul A. Reneke 

Harold E. Nelson 

For Public Comment

NIST NCSTAR 1-7 (Draft) 

For Public Comment

Federal Building and Fire Safety Investigation of the 

World Trade Center Disaster 

Occupant Behavior, Egress, and 

Emergency Communications (Draft) 

Jason D. Averill 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Dennis S. Mileti 

University of Colorado – Boulder

Richard D. Peacock 

Erica D. Kuligowski 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Norman Groner 

John Jay College 

Guylene Proulx 

National Research Council Canada 

Paul A. Reneke 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Harold E. Nelson 

Consultant

September 2005 

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

Technology Administration 

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology 

Hratch G. Semerjian, Acting 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 USC 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 1-7 (Draft) 

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-7 (Draft), 298 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
NIST NCSTAR 1-7, WTC Investigation iii

ACKNOWLEDGMENTS

The authors wish to acknowledge the contributions of a number of people, without whom, this project 

would not have been successful. 

Robyn Gershon, Erin Hogan, and Stephen Morse, Columbia University; Nora Marshall and others from 

the National Transportation Safety Board; Henry Quarantelli, and Benigno Aguirre from the Disaster 

Research Center, University of Delaware; Linda Bourque – University of California – Los Angeles; 

Richard Mendelson, Occupational Safety and Health Administration; Jake Pauls; and Edwina Juillet all 

provided professional insight into issues, procedures, and pitfalls of conducting research involving human 

behavior in disasters. 

Rita Fahy, National Fire Protection Association analyzed media accounts of the September 11, 2001, 

attacks and provided the EXIT89 model and support in its use. 

Jay Cohen and George Duke, employees of the Corporation Counsel of the City of New York, provided 

access to emergency communications and 9-1-1 records that provided details of the experiences and 

conditions above the floors of impact in the WTC towers. 

Frank Lombardi, Saroj Bhol, and Nancy Seliga, PANYNJ provided access to numerous Port Authority 

records on the World Trade Center egress system and building procedures and facilitated access to Port 

Authority personnel with extensive knowledge of the building operation and the events of 

September 11, 2001. 

Patti Adler, Julie Gailus, Janet Jacobs, and Lori Peek – University of Colorado - Boulder; Martha van 

Haitsma and Virginia Bartot – University of Chicago provided guidance on the analysis of qualitative 

interview data. 

Peter Thompson, Integrated Environmental Solutions, Inc.; Ed Galea and Steven Gwynne, University of 

Greenwich – UK; and Jeremy Fraser-Mitchell, BRE – FRS, UK provided support and suggestions on 

appropriate techniques for modeling evacuation in high-rise structures. 

Finally, National Institute of Standards and Technology would like to gratefully acknowledge the more 

than 1,000 survivors and families of victims of the September 11, 2001, attacks that generously agreed to 

be interviewed about their difficult and often tragic experiences on September 11, 2001. Their detailed 

accounts provide a continuing memory of the attacks and the victims of September 11, 2001. 
Acknowledgments Draft for Public Comment 

iv NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation v

ABSTRACT

This report describes the occupant evacuation of World Trade Center (WTC) 1 and WTC 2 on 

September 11, 2001. Multiple sources of information were collected and analyzed: over 1,000 new 

interviews with survivors (including 803 telephone interviews, 225 face-to-face interviews, and 5 focus 

groups); over 700 published interviews; 9-1-1 emergency calls; transcripts of emergency 

communications, historical building design drawings, memoranda, and calculations; formal complaints 

filed with the Occupational Safety and Health Administration; and other relevant materials. 

The egress system, including stairwells and elevators, was described and compared to requirement of both 

contemporary and current code requirements. This report documents the emergency procedures, both as 

they were designed to be implemented, as well as how they were actually implemented on 

September 11, 2001. 

The population in WTC 1 and WTC 2 on September 11, 2001, at 8:46:30 a.m. was enumerated and 

described, where the characteristics of the population were relevant to the subsequent evacuation, 

including training, experience, mobility status, among others. The progress of the evacuation of both 

towers was described in a quasi-chronological manner from 8:46:30 a.m. when WTC 1 was attacked, until 

10:28:22 a.m., when WTC 1 collapsed. 

Causal models were built to explore the sources of evacuation initiation delay (why people did not 

immediately start to leave the building) as well as normalized stairwell evacuation time (how long the 

average occupant spent in the stairwells per floor). Issues identified as contributing to either speeding or 

aiding the evacuation process were explored. Egress simulations provided context for estimating how 

long WTC 1 and WTC 2 would have taken to evacuate with different populations, using different models, 

and subject to different damage to the building. 

Keywords: Building fires, egress, egress modeling, emergency communication, evacuation, human 

behavior, interviews, World Trade Center. 
Abstract Draft for Public Comment 

vi NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation vii

TABLE OF CONTENTS

Acknowledgments........................................................................................................................................iii

Abstract......................................................................................................................................................... v

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

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

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

Metric Conversion Table ..........................................................................................................................xvii

Preface ....................................................................................................................................................... xxi

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

Chapter 1 

Background and Introduction....................................................................................................1

1.1 Historical Incidents in Which Egress Was Significant.................................................................. 1

1.2 Previous Fires and Evacuation Incidents in the World Trade Center............................................ 4

1.3 Scope of Occupant Behavior, Egress, and Emergency Communications Project for the 

NIST World Trade Center Investigation ....................................................................................... 6

1.4 Report Organization ...................................................................................................................... 7

Chapter 2

Design of the World Trade Center Egress System ..................................................................9

2.1 Overall Building Description......................................................................................................... 9

2.1.1 Description of the Towers ............................................................................................. 10

2.2 Building Systems......................................................................................................................... 18

2.2.1 Egress Calculations........................................................................................................ 18

2.2.2 Stairwells ....................................................................................................................... 27

2.2.3 Elevators ........................................................................................................................ 32

2.2.4 Emergency Communication System.............................................................................. 35

2.3 The Human Element.................................................................................................................... 37

2.3.1 Responsibilities of the Fire Safety Director................................................................... 38

2.3.2 Responsibilities of the Deputy Fire Safety Director...................................................... 38

2.3.3 Responsibilities of the Operations Control Center Supervisor...................................... 39

2.3.4 Responsibilities of the Operations and Maintenance Management............................... 39

2.3.5 Floor Warden System .................................................................................................... 40

2.3.6 Occupant and Tenant Training ...................................................................................... 41
Table of Contents Draft for Public Comment 

viii NIST NCSTAR 1-7, WTC Investigation

2.4 Changes to the Egress System after the 1993 Bombing .............................................................. 42

2.5 Building Code Analysis............................................................................................................... 43

2.5.1 Egress in the Building Codes......................................................................................... 44

2.5.2 New York City Building Code (1968)........................................................................... 45

2.5.3 New York City Building Code (October 2003)............................................................. 46

2.5.4 International Building Code (2000)............................................................................... 46

2.5.5 NFPA 5000 & NFPA 101 – Life Safety Code (2003)................................................... 47

2.5.6 Comparison of Current Code Requirements.................................................................. 47

Chapter 3

First-Person Data Collection and Analysis Methods.............................................................49

3.1 Purpose and Scope of Collection of First-Person Data ............................................................... 49

3.2 Methods ....................................................................................................................................... 49

3.2.1 Published First-Person Accounts................................................................................... 49

3.2.2 Telephone Surveys ........................................................................................................ 50

3.2.3 Face-to-Face Interviews ................................................................................................ 59

3.2.4 Focus Groups................................................................................................................. 63

3.2.5 Audio, Video, Photographic, and Records Collection................................................... 64

3.3 Analysis of First-Person Data...................................................................................................... 64

Chapter 4

September 11, 2001, Before the Attacks.................................................................................67

4.1 Building Population on September 11, 2001............................................................................... 67

4.2 Occupant Characteristics ............................................................................................................. 68

4.3 Previous Experience .................................................................................................................... 70

4.4 Occupant Preparedness................................................................................................................ 70

Chapter 5

September 11, 2001, 8:46:30 a.m., Flight 11 Crashes into WTC 1 ........................................73

5.1 Introduction ................................................................................................................................. 73

5.2 Initial Observations In WTC 1 .................................................................................................... 75

Chapter 6

September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1......79

6.1 Conditions Worsen in WTC 1 ..................................................................................................... 79

6.1.1 Activities and Information............................................................................................. 81

6.1.2 Emergency Response at the Fire Command Station, Lobby, WTC 1 ........................... 85
Table of Contents Draft for Public Comment, 6/15/05 

NIST NCSTAR 1-7, WTC Investigation ix

6.1.3 Survivors Begin Their Evacuation................................................................................. 86

6.2 Initial Observations and Reactions from WTC 2 ........................................................................ 89

6.2.1 Activities and Information – WTC 2............................................................................. 91

6.2.2 Survivors Begin Their Evacuation................................................................................. 95

6.2.3 Elevator Use in WTC 2 ................................................................................................. 96

6.2.4 Announcements in WTC 2 ............................................................................................ 98

Chapter 7

September 11, 2001, 9:02:59 a.m., Flight 175 Crashes into WTC 2 ......................................99

Chapter 8

September 11, 2001, 9:02:59 a.m. – 9:58:59 a.m., 56 Critical Minutes................................105

8.1 Survivors From At or Above Floor 78 in WTC 2, After Impact ............................................... 106

8.2 Progress of Evacuation Below the Impact Region .................................................................... 107

8.3 Evacuation Route Through Concourse...................................................................................... 109

8.4 More Announcements in WTC 2............................................................................................... 110

8.5 Conditions Continue to Deteriorate in Both Towers ................................................................. 111

Chapter 9

September 11, 2001, 9:58:59 a.m. – 10:28:22 a.m., Collapse ..............................................115

9.1 Evacuating WTC 1 .................................................................................................................... 115

9.2 Location of Victims and Survivors Within the Building........................................................... 119

9.2.1 Fatalities in Elevators .................................................................................................. 122

Chapter 10

Discussion and Analysis........................................................................................................125

10.1 Overview of Total Evacuation Time ......................................................................................... 125

10.1.1 Evacuation Initiation Delay Time in WTC 1 and WTC 2 ........................................... 127

10.1.2 Time and Average Travel Speed in the Stairwells, WTC 1......................................... 129

10.2 Factors Influencing Total Evacuation Time .............................................................................. 130

10.2.1 Predicting Evacuation Delay In World Trade Center.................................................. 131

10.2.2 Predicting Normalized Stairwell Evacuation Time In WTC 1 On 

September 11, 2001 ..................................................................................................... 135

10.3 Issues That Impacted Occupant Evacuation.............................................................................. 138

10.3.1 Environmental Cues and Information.......................................................................... 138

10.3.2 Building Alarm Systems.............................................................................................. 141

10.3.3 Public Address Announcements.................................................................................. 142

10.3.4 9-1-1 System................................................................................................................ 145
Table of Contents Draft for Public Comment 

x NIST NCSTAR 1-7, WTC Investigation

10.3.5 Previous Evacuation Experience ................................................................................. 146

10.3.6 Occupant Activities ..................................................................................................... 147

10.3.7 Aids and Constraints to Evacuation............................................................................. 151

10.3.8 Emergency Responders and Building Authorities....................................................... 153

10.3.9 Occupant Experience................................................................................................... 158

10.3.10 Mobility-Challenged Occupants.................................................................................. 158

10.4 Evacuation Simulations ............................................................................................................. 160

10.4.1 Egress Simulation Results ........................................................................................... 160

10.4.2 Egress Modeling Conclusions ..................................................................................... 164

Chapter 11

Findings...................................................................................................................................165

11.1 Building Population and Demographics.................................................................................... 165

11.2 Evacuation ................................................................................................................................. 165

11.3 Delays in Evacuation................................................................................................................. 167

11.4 Emergency Management and Planning ..................................................................................... 168

11.5 Occupants with Mobility Challenges ........................................................................................ 169

Chapter 12

References...............................................................................................................................171

Appendix A 

Telephone Interview Script ....................................................................................................175

Appendix B 

Qualitative Analysis Coding ..................................................................................................211

Appendix C 

Causal Modeling .....................................................................................................................217

Appendix D 

Egress Modeling .....................................................................................................................237
NIST NCSTAR 1-7, WTC Investigation xi

LIST OF FIGURES

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

disaster. .............................................................................................................................xxiii

Figure 2–1. WTC site plan. ..................................................................................................................... 10

Figure 2–2. Typical WTC tower architectural floor plan........................................................................ 11

Figure 2–3. Shopping mall layout underneath WTC plaza. .................................................................... 13

Figure 2–4. WTC 1 lobby (concourse) level. .......................................................................................... 14

Figure 2–5. WTC 2 lobby (concourse) level. .......................................................................................... 15

Figure 2–6. Photographs of typical WTC tenant space........................................................................... 17

Figure 2–7. Photograph of a WTC trading floor ..................................................................................... 17

Figure 2–8. 1993 Windows on the World egress calculation, floor 106. ................................................ 25

Figure 2–9. 1993 Windows on the World egress calculation, floor 107. ................................................ 26

Figure 2–10. 44 in. stairwell in WTC 1 taken on September 11, 2001. .................................................... 27

Figure 2–11. Stairwells in the WTC towers. ............................................................................................. 29

Figure 2–12. Horizontal transfer floors in the WTC towers...................................................................... 30

Figure 2–13. Stairwell door signage as seen from inside stairwell A. ...................................................... 30

Figure 2–14. Elevator riser diagram for WTC 1 and WTC 2.................................................................... 33

Figure 2–15. Fire command station in lobby of WTC on September 11, 2001, as seen from 

mezzanine escalator, looking west....................................................................................... 37

Figure 2–16. Elevator communication panel in the fire command station of WTC 1, as operated on 

September 11, 2001. ............................................................................................................ 40

Figure 4–1. Employment start date at WTC............................................................................................ 69

Figure 5–1. WTC 1 impact, 8:46 a.m...................................................................................................... 73

Figure 5–2. Computer simulated impact damage to WTC 1 on floor 95 at 0.7 s after impact with 

stairwells superimposed. ...................................................................................................... 74

Figure 5–3. Calculated damage to floors 93 through 98 in WTC 1. ....................................................... 75

Figure 6–1. Observations of building damage after initial awareness but before beginning 

evacuation in WTC 1. .......................................................................................................... 81

Figure 6–2. Fire Command Station in WTC 1 on September 11, 2001. ................................................. 86
List of Figures Draft for Public Comment 

xii NIST NCSTAR 1-7, WTC Investigation

Figure 7–1. WTC 2 damage (computer simulated) at 0.62 s after impact with stairwells 

superimposed. ...................................................................................................................... 99

Figure 7–2. East-looking view through WTC 1 lobby as debris from WTC 2 impact travels past 

lobby windows onto plaza.................................................................................................. 100

Figure 7–3. Calculated damage to floors 78 through 83 in WTC 2 (see NIST NCSTAR 1-6)............. 101

Figure 7–4. Observations of damage from tenant spaces in WTC 2..................................................... 103

Figure 8–1. View from mezzanine level in WTC 1 looking east across WTC plaza, covered with 

debris. ‘Sphere,’ artwork by Fritz Koening, can be seen on the plaza............................... 109

Figure 8–2. Escalator from mezzanine to concourse level in WTC 1 on September 11, 2001............. 110

Figure 8–3. Evacuees leaving complex near WTC 5. .......................................................................... 110

Figure 8–4. Occupants seeking fresh air on floors 103 – 105 on north face of 

WTC 1 at 9:58:12 a.m........................................................................................................ 112

Figure 9–1. Collapse of WTC 2, viewed from north............................................................................. 116

Figure 9–2. Occupants leaving a previously stuck elevator on September 11, 2001 in the lobby 

of WTC 1. .......................................................................................................................... 123

Figure 10–1. Reported evacuation time for survivors of WTC 1 and WTC 2. ....................................... 126

Figure 10–2. Percentage of survivors remaining in the building for WTC 1 and WTC 2. ..................... 126

Figure 10–3. Percentage of occupants remaining in the building for WTC 1 and WTC 2. .................... 127

Figure 10–4. Stairwell travel speed in WTC 1 for all stairwells. ............................................................ 129

Figure 10–5. Model of evacuation delay................................................................................................. 132

Figure 10–6. Causal model for predicting normalized stairwell evacuation time................................... 136

Figure 10–7. Firefighter and occupants using a 44 in. stairwell in WTC 1 on September 11, 2001....... 154

Figure 10–8. Occupant and firefighter in bunker gear passing in a generic 44 in. stairwell................... 155

Figure 10–9. Simulation of full building evacuation of a WTC tower with different occupant loads.... 163
NIST NCSTAR 1-7, WTC Investigation xiii

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 2–1. Use of floors in the WTC towers ......................................................................................... 12

Table 2–2. Plan view of stairwells in WTC 1 and WTC 2..................................................................... 20

Table 2–3. Minimum stairwell design for 42,850 ft2 office plan........................................................... 47

Table 3–1. Comparison of media interviewees and badge list............................................................... 51

Table 3–2. Comparison of CNN victim list and badge list. ................................................................... 52

Table 3–3. Disposition of the CATI sample and the total sample by tower. ......................................... 54

Table 3–4. Summary disposition rates by tower.................................................................................... 55

Table 3–5. Response rate analysis for WTC 1....................................................................................... 59

Table 3–6. Response rate analysis for WTC 2....................................................................................... 59

Table 3–7. Example tabular face-to-face interview data entry. ............................................................. 61

Table 4–1. Occupancy estimates on September 11, 2001, by tower...................................................... 67

Table 4–2. Occupancy estimate uncertainty. ......................................................................................... 68

Table 4–3. Age for telephone survey respondents in WTC 1 and WTC 2............................................. 68

Table 4–4. WTC fire drills in 12 months prior to September 11, 2001. ................................................ 70

Table 5–1. How survivors in WTC 1 became aware something was wrong on 

September 11, 2001. ............................................................................................................ 76

Table 5–2. Observations of building damage in WTC 1 when occupants first became aware 

something was wrong on September 11, 2001..................................................................... 78

Table 6–1. Observations of conditions in WTC 1 before beginning evacuation. .................................. 80

Table 6–2. Activities prior to evacuation reported in telephone survey by survivors from WTC 1...... 82

Table 6–3. Information received and sought prior to beginning evacuation in WTC 1......................... 83

Table 6–4. Survivors perception of risk to self and others after airplane impact and prior to 

entering stairwell in WTC 1................................................................................................. 83

Table 6–5. Sources of help used by occupants prior to beginning their evacuation in WTC 1. ............ 84

Table 6–6. Single reason given by survivors to begin their evacuation in WTC 1................................ 85
List of Tables Draft for Public Comment 

xiv NIST NCSTAR 1-7, WTC Investigation

Table 6–7. Elapsed time to initiate evacuation for survivors from WTC 1. .......................................... 88

Table 6–8. Stairwell chosen for evacuation in WTC 1. ......................................................................... 89

Table 6–9. How survivors in WTC 2 became aware that something was wrong on 

September 11, 2001. ............................................................................................................ 89

Table 6–10. Activities prior to evacuation reported in telephone surveys by survivors from 

WTC 2.................................................................................................................................. 92

Table 6–11. Information received and sought prior to beginning evacuation in WTC 2......................... 93

Table 6–12. Survivors’ perception of risk upon airplane impact and prior to beginning their 

evacuation in WTC 2. .......................................................................................................... 93

Table 6–13. How survivors received help prior to beginning their evacuation from WTC 2.................. 94

Table 6–14. Single reason given by survivors to begin their evacuation from WTC 2. .......................... 94

Table 6–15. Elapsed time to initiate evacuation for survivors from WTC 2. .......................................... 95

Table 6–16. Stairwell chosen for evacuation in WTC 2. ......................................................................... 96

Table 8–1. Estimated times for survivors leaving the building in WTC 1 and WTC 2. ...................... 105

Table 8–2. Location of WTC 2 survivors at or above floors of impact at 9:03 a.m. ........................... 106

Table 8–3. Observations of conditions during evacuation of WTC 1 and WTC 2. ............................. 113

Table 8–4. Total evacuation time for survivors from WTC 2.............................................................. 114

Table 9–1. Total evacuation time for survivors from WTC 1.............................................................. 119

Table 9–2. World Trade Center decedent location. ............................................................................. 120

Table 10–1. Constraints to evacuation................................................................................................... 151

Table 10–2. Egress simulation matrix.................................................................................................... 161

Table 10–3. Results for phased evacuation simulations. ....................................................................... 161

Table 10–4. Total building evacuation time (simulated) for various occupant loads. ........................... 162

Table 10–5. Simulated evacuation results for fully-occupied WTC 1 and WTC 2 on 

September 11, 2001. .......................................................................................................... 164
NIST NCSTAR 1-7, WTC Investigation xv

LIST OF ACRONYMS AND ABBREVIATIONS

Acronyms 

ASTM ASTM International 

BPS Building Performance Study 

CATI Computer Assisted Telephone Interview 

ESU Emergency Services Unit 

FEMA Federal Emergency Management Agency 

FDNY New York City Fire Department 

IBC International Building Code 

NFPA National Fire Protection Association 

NIST National Institute of Standards and Technology 

NYC New York City 

NYPD New York City Police Department 

OSHA Occupational Safety and Health Administration 

PANYNJ Port Authority of New York and New Jersey 

PAPD Port Authority Police Department 

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 

°F degrees Fahrenheit 

ft foot 

ft2

 square foot 

in. inch 

L liter 

m meter 

μm micrometer 

min minute 

s second 
List of Acronyms and Abbreviations Draft for Public Comment 

xvi NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation xvii

METRIC CONVERSION TABLE

To convert from to Multiply by 

AREA AND SECOND MOMENT OF AREA 

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

square inch (in.2

) square meter (m2) 6.4516 E-04 

square inch (in.2

) square centimeter (cm2) 6.4516 E+00 

square yard (yd2

) square meter (m2) 8.361 274 E-01 

ENERGY (includes WORK) 

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

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

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

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

watt hour (W ⋅ h) joule (J) 3.6 E+03 

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

FORCE 

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

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

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

kip (1 kip=1,000 lbf) newton (N) 4.448 222 E+03 

kip (1 kip=1,000 lbf) kilonewton (kN) 4.448 222 E+00 

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

FORCE DIVIDED BY LENGTH 

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

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

HEAT FLOW RATE 

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

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

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

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

xviii NIST NCSTAR 1-7, WTC Investigation

To convert from to Multiply by 

LENGTH 

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

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

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

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

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

MASS and MOMENT OF INERTIA 

kilogram-force second squared 

per meter (kgf ⋅ s2

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

pound foot squared (lb ⋅ ft2

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

pound inch squared (lb ⋅ in.2

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

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

ton, short (2,000 lb) kilogram (kg) 9.071 847 E+02 

MASS DIVIDED BY AREA 

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

pound per square inch 

(not pound force) (lb/in.2

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

MASS DIVIDED BY LENGTH 

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

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

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

PRESSURE or STRESS (FORCE DIVIDED BY AREA) 

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

kilogram-force per square meter (kgf/m2

) pascal (Pa) 9.806 65 E+00 

kilogram-force per square millimeter (kgf/mm2

) pascal (Pa) 9.806 65 E+06 

kip per square inch (ksi) (kip/in.2

) pascal (Pa) 6.894 757 E+06 

kip per square inch (ksi) (kip/in.2) kilopascal (kPa) 6.894 757 E+03 

pound-force per square foot (lbf/ft2

) pascal (Pa) 4.788 026 E+01 

pound-force per square inch (psi) (lbf/in.2

) pascal (Pa) 6.894 757 E+03 

pound-force per square inch (psi) (lbf/in.2

) kilopascal (kPa) 6.894 757 E+00 

psi (pound-force per square inch) (lbf/in.2

) pascal (Pa) 6.894 757 E+03 

psi (pound-force per square inch) (lbf/in.2

) kilopascal (kPa) 6.894 757 E+00 
Draft for Public Comment Metric Conversion Table 

NIST NCSTAR 1-7, WTC Investigation xix

To convert from to Multiply by 

TEMPERATURE 

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

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

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

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

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

TEMPERATURE INTERVAL 

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

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

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

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

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

VELOCITY (includes SPEED) 

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

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

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

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

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

VOLUME (includes CAPACITY) 

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

cubic inch (in.3

 ) cubic meter (m3) 1.638 706 E-05 

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

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

) 3.785 412 E-03 

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

liter (L) cubic meter (m3

) 1.0 E-03 

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

) 2.957 353 E-05 

ounce (U.S. fluid) (fl oz) milliliter (mL) 2.957 353 E+01 
Metric Conversion Table Draft for Public Comment 

xx NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation xxi

PREFACE

Genesis of This Investigation 

Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the 

Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began 

planning a building performance 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 their 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 their 

report in May 2002, fulfilling their 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 107-231), was 

signed into law. (A copy of the Public Law is included in Appendix A.) The NIST WTC Investigation 

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. 
Preface Draft for Public Comment 

xxii NIST NCSTAR 1-7, WTC Investigation

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

purposes of NIST investigations under the National Construction Safety Team Act are to improve the 

safety and structural integrity of buildings in the United States, and the focus is on fact finding. NIST 

investigative teams are required 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 or 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 NIST Director, 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 Figure 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 Performance and 

Aircraft Impact Damage Analysis; Project 

Leader: Dr. Fahim H. Sadek 

Analyze the baseline performance 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 

Determine and analyze the mechanical and metallurgical properties 

and quality of steel, weldments, and connections from steel 

recovered from WTC 1, 2, and 7. 

Investigation of Active Fire Protection 

Systems; Project Leader: Dr. David 

D. Evans 

Investigate the performance of the active fire protection systems in 

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

and fate of occupants and responders. 

Reconstruction of Thermal and Tenability 

Environment; 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. Therese 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 determine 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 attacks on WTC 1 and WTC 2 until the collapse of WTC 7, 

including practices followed and technologies used. 
Draft for Public Comment Preface 

NIST NCSTAR 1-7, WTC Investigation xxiii

NIST WTC Investigation Projects NIST WTC Investigation Projects

Analysis of 

Steel

Structural 

Collapse

Evacuation

Baseline 

Performance

& Impact 

Damage

Analysis of 

Codes and 

Practices

Emergency 

Response

Active Fire 

Protection

Thermal and 

Tenability 

Environment

Video/

Photographic 

Records

Oral History Data

Emergency 

Response 

Records

Recovered 

Structural Steel

WTC Building 

Performance Study

Recommendations

Government, 

Industry, 

Professional, 

Academic Inputs

Public 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. 
Preface Draft for Public Comment 

xxiv NIST NCSTAR 1-7, WTC Investigation

• Robert Hanson, Professor Emeritus, University of Michigan 

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

Inc. 

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

University of Colorado at Boulder 

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

Diego 

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

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

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 information 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. 
Draft for Public Comment Preface 

NIST NCSTAR 1-7, WTC Investigation xxv

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

Date Location Principal Agenda 

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

pending WTC 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 National Construction Safety Team (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 the May 2003 Progress Report

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

investigation with a public comment session. 

September 17, 2003 New York City, NY Media briefing and public briefing on initiation of first-person 

data collection projects. 

December 2-3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results 

and the release of the Public Update with a public comment 

session. 

February 12, 2004 New York City, NY Public meeting: Briefing 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 briefing and public briefing on release of the 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. 

November 22, 2004 Gaithersburg, MD NCST Advisory Committee discussion on draft annual report to 

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 briefing 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. 

• 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. 
Preface Draft for Public Comment 

xxvi NIST NCSTAR 1-7, WTC Investigation

National Construction Safety Team Reports on the WTC Investigation 

A draft of the final report on the collapses 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 1A. 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 publications are: 

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

Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team 

on the Collapses of the World Trade Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September. 

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

Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team 

on the Collapse of World Trade Center 7. NIST NCSTAR 1A. Gaithersburg, MD, December. 

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 of Structural 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 of Structural Systems. 

NIST NCSTAR 1-1A. 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 of Building Code Structural Requirements. NIST 

NCSTAR 1-1B. 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: Maintenance and Modifications to Structural 

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

MD, September. 

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

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

Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after 

Occupancy. NIST NCSTAR 1-1D. National Institute of Standards and Technology. Gaithersburg, 

MD, September. 

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

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

Design and Construction of World Trade Center 1, 2, and 7. NIST NCSTAR 1-1E. 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 of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New 
Draft for Public Comment Preface 

NIST NCSTAR 1-7, WTC Investigation xxvii

York City Building Code Provisions. NIST NCSTAR 1-1F. National Institute of Standards and 

Technology. Gaithersburg, MD, September. 

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

Trade Center Disaster: Amendments to the Fire Protection and Life Safety 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 1-1G. National Institute of Standards and Technology. Gaithersburg, MD, 

September. 

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

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

of World Trade Center 1 and 2. NIST NCSTAR 1-1H. 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 

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

Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR 1-1I. National Institute of 

Standards and Technology. Gaithersburg, MD, September. 

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

Trade Center Disaster: Design, Installation, and Operation of Fuel System for Emergency Power in 

World Trade Center 7. NIST NCSTAR 1-1J. National Institute of Standards and Technology. 

Gaithersburg, MD, September.

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

Baseline Structural Performance and Aircraft Impact Damage Analysis of the 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 of the 

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 of the World Trade 

Center Disaster: Analysis of Aircraft 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. McCowan, T. A. Siewert, and 

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

Disaster: Mechanical and Metallurgical Analysis of Structural 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 of the World Trade Center Disaster: Contemporaneous Structural Steel 

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

Gaithersburg, MD, September. 
Preface Draft for Public Comment 

xxviii NIST NCSTAR 1-7, WTC Investigation

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

Disaster: Steel Inventory 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 of Structural Steel Components. NIST 

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

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

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

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

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

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

Investigation of the World Trade Center Disaster: Physical Properties of Structural Steels. NIST 

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

Evans, D. D., E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal Building and Fire 

Safety Investigation of the World Trade Center Disaster: Active Fire Protection Systems. NIST 

NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Kuligowski, E. D., and D. D. Evans. 2005. Federal Building and Fire Safety Investigation of the 

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 

of the 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 of the 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 of the 

World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National 

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

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

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

Center Disaster: Reconstruction of the Fires in the World Trade Center Towers. NIST NCSTAR 1-5. 

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

Pitts, W. M., and K. M. Butler. 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. 

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

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

Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and 
Draft for Public Comment Preface 

NIST NCSTAR 1-7, WTC Investigation xxix

Modeling of Structural 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 of the World Trade Center Disaster: Fire Tests of Single 

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 of the 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 of the World Trade Center Disaster: Experiments and 

Modeling of Multiple Workstations Burning in a Compartment. NIST NCSTAR 1-5E. National 

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

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

Investigation of the World Trade Center Disaster: Computer Simulation of the 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 of the World 

Trade Center Disaster: Fire Structure Interface and Thermal Response of the 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 of the World Trade 

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

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

September. 

Carino, N. J., D. P. Bentz, R. W. Bukowski, J. L. Gross, S. Kukuck, K. R. Prasad, and 

M. A. Starnes. 2005. Federal Building and Fire Safety Investigation of the 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 of the World Trade Center Disaster: Fire Resistance Tests of Floor Truss 

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

MD, September. 

Zarghamee, M. S., A. A. Liepins, F. W. Kan, M. Mudlock, O. O. Erbay, Y. Kitane, W. I. Naguib, 

A. T. Sarawit. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 

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

National Institute of Standards and Technology. Gaithersburg, MD, September. 
Preface Draft for Public Comment 

xxx NIST NCSTAR 1-7, WTC Investigation

Zarghamee, M. S., O. O. Erbay, Y. Kitane. 2005. Federal Building and Fire Safety Investigation of 

the World Trade Center Disaster: Global Structural Analysis of the Response of the 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. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, W. M. Pitts, K. R. Prasad. 2005. 

Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Structural Fire 

Response and Probable Collapse Sequence of World Trade Center 7. 2005. NIST NCSTAR 1-6E. 

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

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

J. Oh, K. Rubenacker. 2005. Federal Building and Fire Safety Investigation of the World Trade 

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

and Fire. NIST NCSTAR 1-6F. National Institute of Standards and Technology. Gaithersburg, 

MD, December. 

Kim, W. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 

Disaster: Analysis of September 11, 2001, Seismogram Data, NIST NCSTAR 1-6G. National 

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

Nelson, K. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 

Disaster: The ConEd Substation in World Trade Center 7, NIST NCSTAR 1-6H. National 

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

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

2005. Federal Building and Fire Safety Investigation of the 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 of the World Trade 

Center Disaster: Analysis of Published Accounts of the 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 of the World Trade Center 

Disaster: Technical Documentation for Survey Administration. NIST NCSTAR 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 of the World 

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

Standards and Technology. Gaithersburg, MD, September. 
NIST NCSTAR 1-7, WTC Investigation xxxi

EXECUTIVE SUMMARY

E.1 OVERVIEW 

While most attention has properly focused on the nearly three thousand people who lost their lives at the 

World Trade Center (WTC) site on September 11, 2001, five times that many people successfully 

evacuated from the WTC towers due to heroic efforts of occupants, as well as emergency responders. 

Understanding why many, yet not all, survived the WTC attacks was one of the four objectives of the 

Federal building and fire safety investigation of the WTC disaster led by the National Institute of 

Standards and Technology (NIST).

Success in evacuating a building in an emergency can be characterized by two quantities: the time people 

needed to evacuate and the time available for them to do so. To the extent the first time exceeded the 

second, it follows that there will be casualties. When the second time exceeds the first, perhaps by some 

suitable margin, nearly all should be able to evacuate the building. 

For the WTC towers, the times available for escape were cataclysmically established by the collapses of 

the buildings. Those times were not known in advance by the building occupants or the responders. The 

times were also considerably shorter, by a factor of three or four, than the time needed to clear the tenant 

spaces of WTC 1 following the 1993 bombing and an additional factor of two shorter than the time 

needed to clear the last person from the elevators in the building. Further, some occupants would have 

been unable to evacuate the buildings given any amount of time due to injuries, entrapment, and/or toxic 

exposure. 

NIST examined the design of the building, the behavior of the people, and the evacuation process in detail 

to ascertain the factors that factored prominently in the time needed for evacuation. 

In order to accomplish this objective, numerous sources of data were collected and analyzed, including: 

over 1,000 new interviews with survivors; a collection of over 700 published interviews with 

WTC survivors; 9-1-1 emergency calls; transcripts of emergency communication among building 

personnel and emergency responders; historical building design drawings, memoranda, and calculations; 

building modifications and upgrades; formal complaints filed with Occupational Safety and Health 

Administration; and other relevant material. 

There were three forms of interviews with survivors: 803 telephone interviews, over 225 face-to-face 

interviews, and 6 focus groups. The telephone interviewees were randomly selected using independent 

proportionate stratification from a list of occupants who had badges to enter WTC 1 or WTC 2 on 

September 11, 2001. In other words, each occupant of a particular tower had an equal probability of 

being selected. Roughly 400 occupants in each tower were interviewed in order to achieve a high level of 

statistical precision within each tower. Reported percentages from tower-specific survey data (n=400) 

exhibited sampling errors no greater than 2.5 percentage points, and 95 percent confidence intervals of 

percentages are no greater than ± 5 percentage points. This level of precision was more than adequate for 

examining characteristics of occupants and egress attributes. With telephone interview results, primary 

statistical analyses were in the form of tabulations and linear statistics (e.g., reporting of percentages and 
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xxxii NIST NCSTAR 1-7, WTC Investigation

average/means). The telephone interview results enabled a scientific projection of the population and 

distribution of occupants in WTC 1 and WTC 2, as well as causal modeling and multivariate regression 

analysis to explore fundamental egress issues such as sources of evacuation delay. 

The objective of the face-to-face interviews was to gather first-hand accounts and observations of the 

activities and events inside the buildings on the morning of September 11. This approach identified 

unknown information, aided in the evaluation of technical hypotheses, and explored motivations for 

occupant behaviors, while allowing for comparisons to the telephone interview data. There was no 

recording of the face-to-face interviews, other than random selections, with consent of respondents, for 

quality control purposes. A typical face-to-face interview averaged approximately two hours. The 

methodology for the face-to-face interviews was a synthesis of two established methodologies, designed 

to assist survivors in providing comprehensive and accurate accounts of their evacuation, given the 

latency between experience and interview. Some groups of occupants were specifically sought in order to 

explore targeted unknowns. These included occupants near the floors of impact, witnesses to fireballs, 

mobility-challenged occupants, floor wardens, building personnel with emergency response 

responsibilities, family members who spoke to an occupant after 8:46:30 a.m., and occupants from 

regions of the building not addressed by other groups in order to ensure adequate interview coverage for 

all areas of both towers. 

Six focus groups were conducted in order to elicit accurate group representations of specific events or 

themes and complement the findings of the telephone and face-to-face interviews. The focus groups and 

the corresponding objectives were: 

1. Occupants located near the floors of impact: to explore the extent of the building damage and 

how the damage influenced the evacuation process. 

2. Floor wardens: to explore the implementation of the floor warden procedures and the effect 

those actions had on the evacuation of the occupants on a floor and the evacuation of the floor 

warden. 

3. Mobility-challenged occupants: to explore the effect of a disability on the evacuation of the 

occupant and any other individuals who may have assisted or otherwise been affected by the 

evacuee. 

4. Persons with building responsibilities: to capture the unique perspective of non-traditional 

occupants, including custodians, security, maintenance, or other building staff. 

5. Randomly selected evacuees in WTC 1: to further explore the variables from the causal 

modeling which best explained evacuation delay and normalized stairwell evacuation time, 

including environmental cues, floor, and activities. 

6. Randomly selected evacuees in WTC 2: to further explore variables used in the causal 

modeling that best explained evacuation delay, including environmental cues, floor, risk 

perception, and use of elevators. 

NIST documented the WTC egress system, including the location of the three primary stairwells, exit 

doors, core hallways, transfer corridors, wall construction, location and layout of the 100+ elevators in 
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NIST NCSTAR 1-7, WTC Investigation xxxiii

each tower, and emergency communication devices. The design of the egress system was compared to 

building code requirements of the New York City Building Code, National Fire Protection 

Association 101 (Life Safety Code), and International Building Code. 

NIST documented the emergency procedures, both as they were planned to be carried out, as well as how 

they were actually implemented on September 11, 2001. The procedures included responsibilities for 

tenant safety through the floor warden system; pre-planned content of public address system 

announcements (which varied from public address system announcements made on September 11, 2001); 

responsibilities of the fire safety director, deputy fire safety director, building security, and supervisors of 

various contractors (including mechanical, vertical transportation, and electrical). Additionally, 

interaction among responding agencies such as the Port Authority of New York and New Jersey, the Port 

Authority Police Department, the New York City Police Department (NYPD), the New York City Fire 

Department, and contract security were documented. 

NIST estimates that there were 8,900 ± 750 people in WTC 1 at 8:46:30 a.m. on September 11, 2001. 

Similarly, NIST estimates that there were 8,540 ± 920 people inside WTC 2 at 8:46:30 a.m. New York 

City officially announced 2,749 fatalities at the WTC complex, including emergency responders, airplane 

passengers and crew (but not hijackers), and bystanders. NIST estimated that of the 17,400 ± 1,180 

occupants inside WTC 1 and WTC 2 at 8:46:30 a.m., 2,163 to 2,180 perished. No information could be 

found for 17 persons. More than twice as many occupants were killed in WTC 1 as WTC 2, largely due 

to the fact that occupants in WTC 2 used the 16 minutes between the attacks on WTC 1 and WTC 2 to 

begin evacuating, including the use of elevators by some occupants in WTC 2. 

The demographic characteristics of the evacuees was explored where the characteristics were relevant to 

the evacuation on September 11, 2001. Few differences in the characteristics of WTC 1 or WTC 2 were 

observed. Men outnumbered women roughly two to one. The average age was mid-forties. The mean 

length of employment at the WTC site was almost 6 years, while the median was 2 and 3 years for 

WTC 1 and 2, respectively. Sixteen percent of 2001 WTC evacuees were also present during the 1993 

bombing, although many other occupants were also knowledgeable about the 1993 evacuation. Twothirds of the occupants had participated in at least one fire drill during the 12 months immediately prior to 

September 11, 2001. Eighteen percent did not recall whether they had participated in a fire drill during 

that time period and 18 percent reported that they did not participate in a fire drill during that time period. 

In WTC 1, all three stairwells and the elevators were destroyed in the impact region, extending as low as 

floor 92. No occupant evacuated from above the 91st floor, although some survived until the building 

collapsed after 102 minutes. Helicopter rescue from the roof was considered by an NYPD aviation unit, 

but deemed not possible due to the heat and smoke from the building fire. Occupants of both towers 

delayed initiating their evacuation after WTC 1 was hit. In WTC 1, the median time to initiate evacuation 

was 3 minutes for occupants from the ground floor to floor 76, and 5 minutes for occupants near the 

impact region (floors 77 to 91). Occupants observed various types of impact indicators throughout the 

building, including wall, partition, and ceiling damage and fire and smoke conditions. The most severe 

damage was observed near the impact region, fatally trapping some occupants. Announcements in 

WTC 1 were not heard by the occupants, despite repeated attempts from the lobby fire command station 

to order an evacuation. Damage to critical communications hardware prevented announcement 

transmission. Evacuation rates reached a peak, steady-state in approximately 5 minutes, and remained 

roughly constant until the collapse of WTC 2, when the rate in WTC 1 slowed to about one-fifth of the 
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xxxiv NIST NCSTAR 1-7, WTC Investigation

peak, steady-state. WTC 1 collapsed at 10:28:22 a.m., resulting in approximately 1,500 occupant deaths, 

107 of which were estimated to be below the 92nd floor. 

The evacuation of WTC 2 was markedly different from the evacuation of WTC 1. There was a 16 minute 

period after WTC 1 was attacked, but before WTC 2 was attacked. During this time period, occupants 

were forced to decide whether to remain inside WTC 2, and if they decided to leave, they had to choose 

between using one of the three stairwells or using an elevator. Further complicating this decision process 

were multiple, conflicting announcements around 9:00 a.m., first instructing occupants to return to their 

offices, and then within one minute of impact, instructing them to begin an evacuation if conditions on 

their floor warranted that decision. Over 90 percent of WTC 2 survivors started to evacuate the building 

prior to its being attacked. Sixteen percent of the survivors used elevators to evacuate. Approximately 

75 percent of the occupants who were above the 78th floor (the lowest floor of impact) descended to at 

least below the impact region prior to the attack on WTC 2. Over 40 percent of the survivors had left 

WTC 2 prior to 9:02:59 a.m. After WTC 2 was attacked, at least 18 individuals used Stairwell A, located 

in the northwest corner and furthest from the impact damage, to descend below the 78th floor to evacuate 

the building. Additional public address announcements were made after the airplane strike on WTC 2, 

although occupants who survived generally did not hear those announcements. After the initial peak in 

evacuation rate, the rate reached a steady-state similar to the rate observed in WTC 1 until approximately 

20 minutes prior to collapse of WTC 2. The evacuation rate during the final 20 minutes dropped 

significantly, likely due to a decreased number of occupants remaining in the egress system below the 

78th floor. NIST analysis indicated only 11 occupants initially below the 78th floor were killed when 

WTC 2 collapsed at 9:58:59 a.m. Overall, NIST estimated that 630 occupants of WTC 2 perished. 

Using the statistical power of the telephone interview results, causal models were constructed to explain 

both evacuation initiation delay and average stairwell travel time per floor. The factors that best predicted 

evacuation initiation delay in WTC 1 were (1) which floor the respondent was on when WTC 1 was 

attacked, (2) whether occupants encountered environmental cues, and (3) seeking additional information 

(or milling) about the nature of the event. In WTC 2, the same process occurred as in WTC 1, except that 

perceived risk (sense of immediate danger) was a predictor of seeking additional information (along with 

floor and environmental cues). Analyses explored factors that affected time spent in the stairwells in 

WTC 1 exiting the building. The floor an occupant was on when WTC 1 was attacked (distance to safety) 

increased the probability of encountering an environmental cue (smoke, damage, fire, etc). Additionally, 

being on a higher floor predicted greater evacuation initiation delay times and encountering 

environmental cues, which predicted higher normalized stairwell travel time. Independently, interrupting 

evacuation for any reason increased the normalized stairwell travel time. 

Constraints or aids to the evacuation progress were documented. Building announcements were cited by 

many in WTC 2 as a constraint to their evacuation, principally due to the 9:00 a.m. announcement 

instructing occupants to return to their work spaces. Crowdedness in the stairwells, firefighter 

counterflow, lack of instructions and information, as well as injured or disabled evacuees in the stairwells 

were the most frequently reported obstacles to evacuation. The most commonly mentioned forms of aid 

were assistance from coworkers and emergency responders and the photoluminescent markings in 

stairwells. Six percent of survivors in WTC 1 and WTC 2 reported a mobility challenge which slowed 

their evacuation. Sometimes the evacuation speed of others in the immediate area slowed down occupant 

evacuation speed. Recent pre-existing injuries, medications, or medical treatments were the most 

commonly reported mobility challenges, while a small number used wheelchairs, were pregnant, or were 
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NIST NCSTAR 1-7, WTC Investigation xxxv

elderly. A rest station for mobility-challenged occupants was established in WTC 1 somewhere between 

floors 12 and 20. Less than 10 minutes prior to the collapse of WTC 1, the occupants and helpers on the 

floor were ordered to evacuate, although it remains unclear whether all rest station residents survived. 

Minutes prior to the collapse of WTC 2, an NYPD Emergency Services Unit (ESU) officer radioed from a 

floor in the 20s to the outside that he was having trouble ascending the stairwell due to the large number 

of occupants descending (Interview 24 NYPD [NIST 2004]). As only seven occupants who started 

evacuating below the impact region were known not to have survived, among several possibilities, a large 

group of occupants from above the impact floors may have identified the passable stairwell (Stairwell A) 

and may have been making their way out of the building as it collapsed. 

Multiple evacuation models were used to simulate different WTC tower evacuations, subject to a number 

of assumptions. The goal of the modeling was to frame an understanding of actual evacuation findings on 

September 11, 2001. Simulations demonstrated that a phased evacuation (also known as defend-in-place, 

whereupon occupants on the fire floor and the immediately surrounding floors descend to three floors 

below the fire floor) would have taken between 4 minutes to complete (without delays in evacuation 

initiation) and 11 minutes to complete (with evacuation initiation delays between 0 and 10 minutes). 

Total evacuation of a tower assuming a full occupant load without visitors (19,800) would have required 

as few as 92 minutes to 112 minutes. With visitors (total population 25,500 people) total evacuation 

would have required as little as 114 minutes to 142 minutes. The ranges reflect two different model 

outputs, each assuming two different delay times (no delay and a 10 minute distribution of delay times). 

An evacuation simulation for 8,800 people (approximately the number present in each tower on 

September 11, 2001) in the absence of any damage to the building, would have required at least 

52 minutes to 71 minutes, depending on the model or the delay times. Finally, the EXODUS model was 

‘calibrated’ to approximate the gross evacuation rates observed in WTC 1 and WTC 2 on 

September 11, 2001. Once the model input necessary to approximate the observables was determined, 

additional occupants were added in order to estimate how many occupants might have been unable to 

evacuate on September 11, 2001 (given the damage to the building and observed delay times) if the 

buildings had had larger occupant loads. NIST estimated that approximately 14,000 occupants would 

have been unable to evacuate from WTC 1 and WTC 2 on September 11, 2001, had the starting building 

population been 19,800 in each building. 

E.2 REFERENCE 

NIST (National Institute of Standards and Technology). 2004. NIST WTC Emergency Responder 

Interview Data Set. Gaithersburg, MD. 
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NIST NCSTAR 1-7, WTC Investigation 1

Chapter 1

BACKGROUND AND INTRODUCTION

On the morning of September 11, 2001, the World Trade Center (WTC) in New York City was attacked 

by hijacked commercial airplanes. The collision with each tower (WTC 1 at 8:46:30 a.m. and WTC 2 at 

9:02:59 a.m.) produced significant structural damage. The impact generated a large, luminous external 

fireball that consumed a portion of the jet fuel, with the remaining fuel acting as an ignition source for the 

combustible material within each tower. At 9:58:59 a.m., 56 minutes after it was struck, WTC 2 

collapsed due to a combination of the aircraft impact damage and subsequent fire. WTC 1 stood until 

10:28:22 a.m. 

This report provides an analysis of the overall evacuation of WTC 1 and WTC 2. The two towers were 

nearly identical buildings in height, geometry, and architectural features. The evacuation processes in 

these two buildings displayed both distinct similarities and differences. This report also focuses on the 

behaviors of the occupants, actions of the building personnel and emergency responders (covered more 

completely in NIST NCSTAR 1-81

), and the interactions among all three. This report documents the 

performance of the emergency egress system. 

This chapter begins with a discussion of significant egress events. It then reviews the design of the 

WTC egress system and emergency procedures, and outlines the National Institute of Standards and 

Technology (NIST) data collection methodology. The events of September 11, 2001, are detailed as they 

relate to the evacuation, including the moments prior to 8:46:30 a.m. Finally, an analysis of the events 

establishes key evacuation findings and conclusions. Note that individuals shown in photographs in this 

report may have been blurred to protect their identities. 

1.1 HISTORICAL INCIDENTS IN WHICH EGRESS WAS SIGNIFICANT 

Although the World Trade Center building collapses are arguably the most significant building events 

where building egress played a critical role, concern about the ability of occupants to escape from large 

buildings is hardly new. Indeed, many earlier lessons were based on analyses of high-rise fires in New 

York City. In 1911, the Triangle Shirtwaist Fire spread through the top three floors of a 10-story fireresistant building in New York (Fire Engineering 1977). The fire started in a corner of the eighth floor of 

the building and quickly spread over the entire floor as well as the floors above by the windows, stairs, 

and elevator shafts. There were 145 fatalities in the fire, all but one from the ninth floor of the building. 

While many of the fatalities were located on the ninth floor, approximately 40 jumped from the building 

to the street below to escape the flames, and another 10 perished when an exterior fire escape collapsed. 

The fire was extinguished with hose lines from two standpipe risers in the stairwells and was under 

control within 18 min. The upper three floors were a complete loss. Significant issues identified from the 

fire investigation included the fact that there was limited access to the stairwells due to partially-blocked, 

non-fireproof doors that opened inward, as well as exterior cast-iron fire escapes which loosened from the 

 

1

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

to this report. 
Chapter 1 Draft for Public Comment 

2 NIST NCSTAR 1-7, WTC Investigation

wall due to heat from the fire. Subsequent recommendations promoted fireproof egress stairways and 

automatic sprinklers for buildings taller than 18 m (60 ft). 

The Equitable Building fire in New York City in January 1912 is an early example of building collapse 

following fire (NFPA 1912). The Equitable Building was a group of five buildings, the tallest being 10 

stories tall. Erected beginning in 1869, the buildings were constructed of so-called fireproof construction 

with wood floors on brick or tile arches supported by wrought-iron and steel I-beams resting on columns 

made mostly of cast iron. The fire originated in the basement of one of the buildings from a discarded 

match and spread quickly to a tile-enclosed shaft containing two elevators and eleven small dumbwaiters 

enclosed in wood. Within 45 minutes, the fire had spread throughout the upper floors of the buildings 

and downward through numerous unprotected floor openings. Except for a few areas, the building was 

completely gutted by the fire. Three separate sections of the building collapsed, with the largest collapse 

involving all of the floors down to the basement on one side of the building. Since the fire occurred before 

business hours, loss of life was limited to three employees on the upper floors and three additional deaths 

attributed to collapse of cast-iron columns. Firefighter loss was limited to a single fatality, as all 

personnel were ordered out of the building prior to the first collapse. Egress and firefighter access was 

through a single continuous stairway from the basement level to the top floor, deemed inadequate for 

escape in the subsequent investigative report. Recommendations included the need for protection of floor 

openings, corridor partitions, and structural metal work, and inclusion of sprinklers in all portions of 

office buildings where fire is most likely to occur. Two remote stairways enclosed in fireproof shafts with 

fire doors at each floor were deemed necessary. Additional stairways were recommended such that travel 

distance to a stairway was limited to 27 m (90 ft). 

In 1945, a U.S. Army Air Force B-25 crashed into the Empire State Building in New York City resulting 

in a significant fire on parts of the 78th and 79th floors from an estimated 3 m3

 (800 gal) of gasoline 

sprayed from the plane crash (Hayne 1945). The crash and resulting fire caused 14 deaths and 

approximately 25 injuries. The crash occurred on a Saturday morning when few building occupants were 

present, and much of the office space surrounding the crash site was unoccupied. Several occupants of the 

79th floor took refuge in a metal and glass partitioned office and were later rescued by the fire 

department. According to the investigation report, the stairwells remained tenable throughout the incident 

and provided fire department access and a safe means of egress for occupants of the upper floors not 

involved with the initial gasoline fire. Fire department access was accomplished via elevator to the 

65th floor and by stairwell the remaining 13 to 14 floors. The fire was extinguished approximately 

35 minutes after the first fire department notification. Building design, timing of the fire on a Saturday 

morning, and fire department response were credited with limiting the resulting damage and loss of life. 

Important issues related to building egress identified in the investigation report include (1) limiting use of 

elevators as a means of egress from upper floors, since the crash of one of the elevators to the subbasement might create apprehension of the dependability of the remaining elevators; (2) an understanding 

that damage to stair shafts may be sufficient to prevent their use as a means of egress from the crash floor 

and floors above; and (3) a realization that fire resistive building construction does not preclude damage 

by fire involving building contents. 

In August 1970, a fire at the 50-story One New York Plaza building extensively damaged the 33rd and 

34th floors and spread significant smoke throughout the building (Powers 1970). The fire was first 

detected in the concealed ceiling space of the 33rd floor and spread to exposed polystyrene insulation in 

the south and west walls of the 33rd floor. The building was only partially occupied at the time of the 
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NIST NCSTAR 1-7, WTC Investigation 3

fire, with some of the floors above the fire unoccupied. Occupants evacuated either by elevators or down 

the stairwells. Heavy smoke conditions were noted on many floors of the building. Two security guards 

and two firefighters died from the fire, and 30 injuries resulted. Fire department access was accomplished 

via elevators to the 30th floor and by stairs to the fire. The fire was controlled within 5 h. Reducing the 

fire load of building contents, the need for automatic sprinkler systems, and the protection of steel 

members by materials that cannot be readily removed or damaged were important issues identified from 

the investigation. New York City Local Law No. 5, Fire Safety in High-Rise Buildings, resulted in large 

part from a reaction to this and several other high rise-fires in New York at the time. Among other 

provisions, Local Law 5 requires building compartmentation, with an exception for sprinklered spaces 

(New York City 1973). 

In February 1972 and February 1974, major high-rise fires occurred in Brazil, causing more than 

200 casualties. In February 1972, a fire in the 31-story Andraus Building in São Paulo resulted in 16 

fatalities and more than 375 injuries (Willey 1972). The fire developed on four floors of a department 

store and then spread up the exterior facade of the building, involving 28 floors of the building within 

25 minutes. The fire gutted most areas of the building and damaged structural supports. The department 

store occupied the lower seven floors above grade and was served by four open stairways and two 

elevators. The remainder of the building was of office occupancy with a single 1 m (39 in.) wide 

enclosed masonry spiral stairwell and five elevators. Door construction in the office stairwell was of 

hollow-core, wood, or metal construction. Combustible interior finish and exterior façade were credited 

for the rapid fire spread throughout all but the upper four floors of the building. It was reported that some 

people used elevators to egress the building, while others used the single stairwell. Once a stairwell door 

on the fifth floor failed, leaving the lower floors of the stairwell untenable, occupants fled toward the roof 

of the building. Approximately 300 people reached the roof level heliport, while another 200 became 

trapped in the stairwell. Rescue operations for those trapped in the stairwell included ladders from nearby 

buildings on the fifteenth and sixteenth floor. The use of areas of refuge by nearly 500 occupants was 

aided by stairway ventilation and wind velocity. 

An unfortunately similar fire two years later, which started on the 12th floor of the 25 story Joelma 

Building in São Paulo, and resulted in 179 deaths, 300 injuries, and total destruction of the building 

contents (Sharry 1974). Inability of helicopters to rescue occupants trapped on the roof of the building, 

inadequate means of egress from the building (a single 1.2 m (47 in.) unenclosed stairwell), lack of fire 

protection, and presence of combustible contents within the building were noted as significant in the fire. 

The majority of survivors of the fire made their escape through the use of the building’s four elevators. 

While this method was not recommended due to the possibility that occupants may become trapped, the 

success of the evacuation was attributed to two factors: the use of elevator operators allowed the 

elevators to be operated in an express mode (stopping only at desired floors), and the elevator power 

supply was unaffected early in the fire. 

In June 1989, a fire occurred in a 10-story office building in Atlanta, Georgia (Isner 1990). The Peachtree 

25th building was an H-shaped building with two connected 10-story towers and a population of 

approximately 1,500 people. Each tower measured approximately 76 m by 20 m, with the connection 

measuring 21 m by 24 m. The fire began on the 6th floor of the south tower at approximately 10:30 a.m. 

The ignition of the fire was attributed to an electrician working on an electrical switchbox. While the 

worker was attempting to return power to a section of the floor by replacing a 200-ampere fuse, severe 

arcing occurred. The arcing had sufficient energy to melt metal and ignite the interior-finish materials in 
Chapter 1 Draft for Public Comment 

4 NIST NCSTAR 1-7, WTC Investigation

the hallway. The electrician was severely injured and later died, although not as a direct result of the arc, 

which was estimated as having lasted 60 seconds or more. The fire growth rate was extremely high, and 

the fire spread was rapid. Multiple layers of wall covering promoted extraordinary fire spread rates, 

which was not an unfamiliar fire hazard to fire investigators (Bouchard 1982; Demers 1980). The wall 

coverings had completely burned out when the fire department arrived on the floor, only seven minutes 

after notification. The intense black smoke quickly trapped about 40 occupants on the floor of origin. 

Most occupants found a room and closed the door behind them, breaking out windows to vent incoming 

smoke and waited to be rescued. At some point, one woman jumped from a 6th floor window and 

sustained severe injuries. The fire department was not notified until an occupant of the building from a 

remote floor activated a manual pull-station at approximately 10:30 a.m. Most occupants of the building 

were leaning out of the windows in order to breathe when the fire department arrived on the scene at 

approximately 10:34 a.m. Fourteen occupants were rescued via ladder truck, and 14 people were rescued 

using the stairwells. In all, five people died because of this fire, the first multiple fatality high-rise office 

building fire in the United States in 17 years (Isner 1990). 

Several failure modes contributed significantly to the severity of the fire. There were no automatic 

sprinklers, which allowed the fire to spread. The electrician did not follow proper procedure when 

changing the fuse, resulting in the arc that ignited the wall linings and electrical equipment. The ignition 

source was so severe that a fire in the electrical room was inevitable, however. Multiple-layer, 

combustible interior-finishes also contributed to the rapid spread of the fire. 

In many instances, these significant egress events resulting from fires in buildings have shaped building 

codes requirements related to the egress system. Requirements for stairwell design, placement, and 

capacity all evolved as a result of significant past fire incidents. 

1.2 PREVIOUS FIRES AND EVACUATION INCIDENTS IN THE WORLD 

TRADE CENTER 

In February 1975, a fire in WTC 1 began on the 11th floor and ultimately spread from the 10th to the 

19th floor extending through telephone closets on each floor (Powers 1975). Although not important 

from an egress perspective, the fire provides an appropriate background for what occurred in the later 

terrorist attacks in 1993 and 2001. The fire was initially reported by manual alarm at 11:35 p.m. 

Automatic alarms from smoke detectors on the 11th floor through the 19th floor responded at about 1 min 

intervals after the manual alarm. It was believed that the fire originated in an executive office on the 

11th floor and spread to upper and lower floors through 0.30 m by 0.45 m (12 in. by 18 in.) openings in 

the floors of utility closets on each floor. Four steel floor trusses were distorted slightly. Approximately 

800 m2

 (9,000 ft2) of the 11th floor was damaged, destroying about half of the contents and damaging the 

remaining contents in this area. Virtually all combustibles, including fire retardant-treated wood paneling 

on the telephone closet walls of the 10th and 12th floors, were destroyed. Limited quantities of 

combustible furnishings on the 12th and 13th floors limited the spread of fires from the telephone closets 

on these floors. Recommendations resulting from the fire included (1) provisions for automatic sprinklers 

in areas where highly combustible material or large accumulations of combustibles are present, (2) the 

installation of detectors in return air shafts on each floor to purge the return air and stop the supply of 

fresh air to the fire area, and (3) fire stopping of all openings in floors or walls as well as in any wiring 

installations. It was noted that sprayed-on fireproofing may not adhere properly to surfaces or may be 

dislodged as other building services are installed. 
Draft for Public Comment Background and Introduction 

NIST NCSTAR 1-7, WTC Investigation 5

On August 3, 1977, two Fuerzas Armadas de Liberacion Nacional (F.A.L.N.) terrorist bombs exploded in 

midtown Manhattan, killing one person and injuring seven others. When a specific threat against the 

World Trade Center was phoned into a local TV news station at 9:45 a.m. (Breasted 1977), both 

WTC towers were evacuated, although not until after 12 noon. An employee of Windows on the World at 

the time, described the situation: 

“We were all scared. I started to shake. The ride down seemed to take 

two hours. I’m part of a team that was trained for fire drills, but I have 

no idea of what do if there was a bomb. This was more frightening than 

a fire because we are all equipped for a fire.” (Ivins 1977) 

An estimated 35,000 people were evacuated from WTC 1 and WTC 2, both of which reopened the same 

day, shortly after 3:00 p.m. (Ivins 1977). Overall, more than 100,000 people evacuated buildings in 

Manhattan that day (Breasted 1977). Many people, however, were reluctant to leave after having been 

docked wages after previous incidents for evacuating the building (Ivins 1977). 

At 12:18 p.m. on February 26, 1993, a terrorist attack resulted in an explosion in a sublevel parking 

garage in the World Trade Center complex, immediately killing six people (Isner and Klem 1993a; Isner 

and Klem 1993b) and causing an estimated $300 million damage. The explosion of at least 450 kg 

(1,000 lb) of explosive material caused extensive damage to several sublevels of the building and an 

intense fire that spread varying amounts of smoke in four of the seven buildings in the complex. Most of 

the complex’s estimated 150,000 occupants evacuated the buildings as a result of the incident, including 

approximately 50,000 from the affected towers. 2 According to the NFPA Investigation, 1,042 people 

were injured in the incident, including 15 who received blast-related injuries. At the peak of the incident, 

the fire reached 16 alarms and involved more than 700 firefighters (approximately 45 percent of the New 

York City Fire Department’s on-duty personnel) (Isner and Klem 1993a). As a comparison, on 

September 11, 2001, 22 alarms were called prior to the collapse of WTC 2, in addition to a 10-60 alarm 

(unique to special operations for large incidents) and a three alarm which staged additional units nearby. 

This resulted in the involvement of more than 1,000 firefighters being at the World Trade Center. 

The explosion significantly damaged floors, walls, and doorways in subgrade levels and forced large 

amounts of smoke well away from the immediate area. In one report, visibility was reduced to 0.3 m 

(1 ft) within about 1 min at the 44th floor of WTC 1, largely through the spread of smoke in elevator and 

stairwell shafts (Isner and Klem 1993b). Before beginning evacuation, many occupants experienced 

smoke on occupied floors and encountered even heavier smoke as they descended the buildings in the 

stairwells. Since the explosion disabled the emergency communication systems in the buildings, 

occupants responded to the event without the planned central guidance. Even without guidance, many 

occupants began evacuation early in the event. Egress was further complicated by a total loss of electrical 

power to emergency stairwell lighting within about 1 hour and 15 min. It was estimated that it took 

occupants from 11⁄2 hours to 3 hours to exit the building from the upper floors of the towers. Fortunately, 

the scarcity of combustibles in the subgrade levels and dilution of the fire gases limited the toxic potency 

of the resulting smoke. Although most of the injuries were smoke related, no fatalities due to smoke 

inhalation were noted even with prolonged exposure to dense smoke. 

 

2

 Text of undated presentation by Ted Stam, General Property Manager, World Trade Center. WTCI-619-P. 
Chapter 1 Draft for Public Comment 

6 NIST NCSTAR 1-7, WTC Investigation

Fire crews were assigned responsibility for searching five floor subsectors. Since the elevators were not 

operational, firefighters climbed the stairwells. It took more than two hours for crews to climb to the 

100th floor. By 4:00 p.m., approximately 4 hours after the blast, all occupants had evacuated tenant 

floors. Some elevator cars, however, had stopped in elevator shafts, portions of which had no doors 

leading from that section of the shaft (such as express elevators). Locating and evacuating the trapped 

elevator occupants was such a high priority that, rather than wait for the stairwells to empty to send the 

elevator technicians up into the building by stairwells, the technicians were delivered to the roof by 

helicopter. Even with that measure, it took approximately 5 hours to locate and free a group of 

kindergarten students and several adults trapped in an elevator in WTC 2 (Isner and Klem 1993b). By 

8:00 p.m., approximately 8 hours after the explosion, the last occupants trapped in elevators were 

evacuated. 

1.3 SCOPE OF OCCUPANT BEHAVIOR, EGRESS, AND EMERGENCY 

COMMUNICATIONS PROJECT FOR THE NIST WORLD TRADE CENTER 

INVESTIGATION 

The purpose of this project was to determine the behavior and fate of occupants and responders - both 

those who survived and those who did not - by collecting and analyzing information on occupant 

behavior, human factors, egress, and emergency communications in WTC 1, WTC 2, and WTC 7, and 

evaluating the performance of the evacuation system on September 11, 2001. 

This project was divided into six tasks as follows: 

Task 1—Gathered baseline information on the evacuation of the WTC buildings on September 11, 2001, 

through a comprehensive, systems-oriented, and interdisciplinary data collection effort focused on 

occupant behavior, human factors, egress, and emergency communications (including instructions given, 

interpretation of instructions, and response to instructions). This involved the collection of new data from 

people affected by the WTC attacks (e.g. building occupants, building operators, and emergency 

responders via direct accounts from survivors and families of victims), especially those who had to 

evacuate the buildings. Experts in human behavior and statistical sampling were retained to assist in 

developing a data acquisition strategy that considered various data collection methods, such as interviews 

and questionnaires. Inputs and suggestions were obtained from individuals with an interest in the data 

collection effort. Additionally, written accounts, transcripts of (emergency) communications, published 

accounts, and other sources of egress related information were obtained, in coordination with other data 

collection efforts for the overall investigation. 

Task 2—Collected archival records from prior WTC evacuation incidents (e.g., 1975 fire, 1977 blackout, 

1980 bomb scare, 1990 power outage, and 1993 bombing) and practice evacuations, including oral history 

data from floor wardens and fire safety directors. These records were compared and contrasted with the 

September 11, 2001, evacuation. Changes made to the evacuation procedures following the earlier 

incidents and in recent years were evaluated in the context of the experience on September 11, 2001. 

Task 3—Documented pre-event data for WTC Buildings 1, 2, and 7. This information included physical 

aspects of building egress components, such as stairs (width, number, location, vertical continuity), 

evacuation lighting, back-up power, elevators (number, operational before and after impact, role in 

evacuation), and active fire protection systems (sprinklers, manual suppression, fire alarms, smoke 
Draft for Public Comment Background and Introduction 

NIST NCSTAR 1-7, WTC Investigation 7

control). Building plans, emergency plans, type and frequency of evacuation drills, occupancy level and 

distribution on the morning of September 11th, and communications also constituted pre-event data. This 

information provided a baseline for evaluating the performance of the egress system. 

Task 4—Stored the information collected in task 1 in a database. Additionally, information from thirdparty sources, such as published media accounts, were assembled and analyzed in the database. 

Task 5—Analyzed the data to study the movement of people during the evacuations, decision-making and 

situational awareness, and issues concerning persons with disabilities. A timeline of the evacuation was 

developed using the results of these analyses together with other data sources. This timeline was 

compared with the timeline of the structural response, data on the development of the interior conditions 

(fire and smoke), as well as information on the activation of the active fire protection systems. The 

observed evacuation data was compared with results obtained using alternate egress models to better 

understand occupant behavior and identify needed improvements to existing egress models. In addition, 

the evacuation experience was compared with previous evacuation incidents in these buildings. The 

results were reviewed in the context of occupant protection practices for tall buildings, including the 

consideration of total evacuation and phased evacuation strategies. 

Task 6—Report preparation. The results of this project were synthesized into this report to describe the 

occupant behavior, egress, and emergency communications in WTC 1 and WTC 2, and the performance 

of the evacuation system. 

1.4 REPORT ORGANIZATION 

This report investigates the occupant behavior, egress, and emergency communications at the World 

Trade Center on September 11, 2001. 

This chapter explores historical fire incidents where egress played a significant role, as well as previous 

significant fire or evacuation incidents at the World Trade Center complex. It also describes the scope of 

the overall project. 

Chapter 2 describes the design of the World Trade Center egress system, including the stairwells, 

elevators and emergency communication systems. Emergency procedures, including the roles of building 

managers is described. Finally, changes to the egress system as a result of the 1993 bombing are detailed. 

Chapter 3 documents the overall technical approach of the project, including discussion of the collection 

and analysis of first-person accounts (face-to-face, telephone, and focus group interviews), collection and 

analysis of published media accounts, and collection and analysis of other relevant data, including audio, 

video, photographic, and design records. 

Chapters 4 through 9 chronologically detail the overall progression of the evacuation of WTC 1 and 

WTC 2, including occupant activities, observations, and reactions. 

Chapter 4 enumerates the occupants of WTC 1 and WTC 2, describes their basic characteristics as it 

relates to evacuation, and discusses the emergency preparedness of the occupants prior to the attacks. 
Chapter 1 Draft for Public Comment 

8 NIST NCSTAR 1-7, WTC Investigation

Chapter 5 documents the occupants’ awareness of and reaction to the impact of the first airplane with 

WTC 1, as well as observations of local damage and phenomena. 

Chapter 6 describes the period of time from immediately after WTC 1 was attacked until just prior to the 

attack on WTC 2. The overall evacuation rate, actions of the building managers, and occupant activities 

and behaviors are discussed. 

Chapter 7, paralleling Chapter 5, documents the impact of the second airplane with WTC 2, including 

awareness and reaction on the part of the occupants. 

Chapter 8 tracks the progress of the evacuation and overall emergency response in both towers until 

immediately prior to the collapse of WTC 2. 

Chapter 9 examines the collapse of WTC 1 and WTC 2, including an analysis of where the occupants 

likely were as each building collapsed. 

Chapter 10 discusses the important egress issues raised by the events of September 11, 2001, at the World 

Trade Center. Included are causal models, summary statistics on the overall building evacuation rates, 

egress modeling, and in-depth analysis of specific issues that affected the evacuation, including the role of 

alarms, announcements, mobility challenges, emergency responders, authorities, information flow, 

activities, evacuation experience, and constraints/aids to evacuation. 

Chapter 11 summarizes the key findings of this report and highlights the most important findings to 

consider in response to the evacuation of WTC 1 and WTC 2. 
NIST NCSTAR 1-7, WTC Investigation 9

Chapter 2

DESIGN OF THE WORLD TRADE CENTER EGRESS SYSTEM

The provision of access to and egress from buildings under emergency conditions relies on four primary 

components: stairwells, elevators, communication systems, and emergency responders (broadly defined to 

include Fire Department of New York (FDNY), New York Police Department (NYPD), Port Authority 

Police Department (PAPD), Port Authority personnel, building security, fire safety directors, floor 

wardens, and other individuals with formal response responsibilities). These are subsequently grouped 

into building systems and the human component. 

2.1 OVERALL BUILDING DESCRIPTION 

By 2001, the World Trade Center (WTC) complex had become an integral part of Manhattan. It was 

composed of seven buildings (here referred to as WTC 1 through WTC 7) on a 16 acre site, located near 

the southwest tip of the island, shown in Figure 2–1. Whether viewed from close up, from the Statue of 

Liberty across the Upper Bay or from an airplane descending to LaGuardia Airport, the WTC towers were 

a sight to behold. WTC 1 (often referred to as the North Tower) and WTC 2 (often referred to as the 

South Tower), were each 110 stories high, dwarfing the other skyscrapers in lower Manhattan and 

seemingly extending to all Manhattan the definition of “tall” set by midtown's Empire State Building. 

Groundbreaking for the towers was in 1966, while construction began in 1968. WTC 1 was first occupied 

in 1970; WTC 2 in 1972. 

Additionally, there was a six-story subterranean structure, largely below the WTC Plaza with connections 

to WTC 1, 2, 3, 4, 5, and 6, which included a shopping mall and the WTC PATH station. This was 

surrounded by a 3 ft (0.9 m) thick concrete wall that extended from ground level down 70 ft (21 m) to 

bedrock. Holding back the waters of the Hudson River, this wall had enabled rapid excavation for the 

foundation and served to keep the groundwater from flooding the underground levels. Commuter trains 

brought tens of thousands of workers and visitors to Manhattan from Brooklyn and New Jersey into the 

WTC station. A series of escalators and elevators took the WTC employees directly to an underground 

shopping mall and to the Concourse Level of both towers. 

WTC 3 (Marriott Hotel) was 22 stories. WTC 4 (South Plaza Building) and WTC 5 (North Plaza 

Building) were both 9-story office buildings. WTC 6 (U.S. Customs House) was an 8-story office 

building. These six buildings were built around a 5 acre plaza, named for Austin J. Tobin, and the 

centerpiece of which was a large globe art object. WTC 7, located north of the other six WTC buildings 

and separated by Vesey Street, was a 47-story office building. WTC 7 was completed in 1987 and was 

operated by Silverstein Properties, Inc., as an air rights building. 
Chapter 2 Draft for Public Comment 

10 NIST NCSTAR 1-7, WTC Investigation

Figure 2–1. WTC site plan. 

2.1.1 Description of the Towers 

WTC 1 and WTC 2 each consisted of 110 stories above the Concourse Level (or 109 stories above the 

plaza / Mezzanine Level) structure. There were also six basement levels below the Concourse Level. 

Although the towers were similar, they were not identical. The height of WTC1 at the roof level was 

1,368 ft (418 m) above the Concourse Level (6 ft taller than WTC 2), and WTC 1 additionally supported 

a 360 ft (110 m) tall antenna on the roof for television and radio transmission. Each tower had a square 

plan with the side dimension of 207 ft 2 in. (63.2 m). The corners of the tower were chamfered 6 ft 11 in. 

(2.1 m). Each tower had a core service area of approximately 135 ft x 87 ft (41 m x 27 m), although the 

core space changed on tenant spaces throughout the towers. A typical architectural floor plan in the tower 

is shown in Figure 2–2. As can be seen in this figure, placing all service systems within the core provided 

column-free floor space of roughly 31,000 sq ft (2,900 m2

) per floor outside the core. The long axis of the 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 11

core in WTC 1 was oriented in the east-west direction while the long axis of the core in WTC 2 was 

oriented in the north-south direct. 

Figure 2–2. Typical WTC tower architectural floor plan. 

The superb vistas from the top of such buildings virtually demanded public space from which to view 

them, and the Port Authority responded. The 107th floor of WTC 1 housed a gourmet restaurant and bar 

with views of the Hudson River and New Jersey to the west, the skyscrapers of midtown Manhattan to the 

north, the East River and Queens to the east, the Statue of Liberty to the southwest, and the Atlantic 

Ocean to the South. Similar views could be seen from observations decks on the 107th floor and the roof 

of WTC 2. 
Chapter 2 Draft for Public Comment 

12 NIST NCSTAR 1-7, WTC Investigation

Table 2–1 shows the use of the floors, which was similar but not identical in the two towers: 

Table 2–1. Use of floors in the WTC towers 

Floor(s) WTC 1 WTC 2 

Roof Antenna space and window washing 

equipment 

Outdoor observation deck and window 

washing equipment 

110 Television studios Mechanical equipment 

108, 109 Mechanical equipment Mechanical equipment 

107 Windows on the World Indoor observation deck 

106 Catering Tenant space 

79 through 105 Tenant space Tenant space 

78 Skylobby, tenant space Skylobby, tenant space 

77 Tenant space Tenant space 

75, 76 Mechanical equipment Mechanical equipment 

45 through 74 Tenant space Tenant space 

44 Skylobby, cafeteria, tenant space Skylobby, tenant space 

43 Port Authority space Tenant space 

41, 42 Mechanical equipment Mechanical equipment 

9 through 40 Tenant space Tenant space 

7, 8 Mechanical floors Mechanical floors 

Concourse through 6 6-story lobby 6-story lobby 

The Port Authority had managed the operation of the two towers since their opening three decades earlier. 

Silverstein Properties acquired a 99 year lease on the towers in July 2001. 

At the beginning of the workday, many of the roughly 40,000 people who worked in the towers and 

visited to tour or to conduct business, emerged from PATH trains in the massive subterranean station. 

They would take escalators and elevators to a large shopping concourse. Walking a few hundred feet led 

occupants to the spacious, 6-story-high lobby on the Concourse Level where they would cross paths with 

those who arrived on foot or by bus and cab. Figure 2–3 shows the layout of the shopping mall, located 

underneath the WTC plaza. Figure 2–4 shows the lobby configuration for WTC 1. Figure 2–5 shows the 

layout of the WTC 2 lobby. The WTC 1 and WTC 2 lobbies were at the same level as the underground 

shopping mall, often collectively referred to as the Concourse Level. The WTC outdoor plaza and the 

WTC 1 and WTC 2 Mezzanine were one story higher than the Concourse Level, often referred to as 

either the Mezzanine or plaza level.
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 13

Figure 2–3. Shopping mall layout underneath WTC plaza. 
Chapter 2 Draft for Public Comment 

14 NIST NCSTAR 1-7, WTC Investigation

Figure 2–4. WTC 1 lobby (concourse) level. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 15

Figure 2–5. WTC 2 lobby (concourse) level. 
Chapter 2 Draft for Public Comment 

16 NIST NCSTAR 1-7, WTC Investigation

Getting tens of thousands of people from the Concourse to their offices was no small task. This was 

accomplished by a then-novel array of 106 express and local elevators located within the building core (as 

shown later in Figure 2–14). Section 2.2.3 discusses the elevators system in WTC 1 and WTC 2. 

Also within the core were three sets of stairs that extended the full height of the tower. Section 2.2.2 

discusses the stairwells in each tower. However, upon entering a stairwell at an upper floor, one did not 

find a continuously descending staircase leading to the lobby. Principally at the mechanical floors, there 

were enclosed horizontal corridors that led around the massive elevator hardware. These corridors ranged 

in length from about 10 ft to about 100 ft. After traversing each of these, the pedestrians would resume 

their descent. 

Upon exiting the elevators (or stairs, for those who chose the more strenuous route), one was faced with a 

view typical of high-rise buildings. Surrounding the rectangular core corridor was a mixture of blank 

walls, door entries to firms, and glass-front reception areas. Above was a standard drop ceiling. 

Many of the floors had but a single tenant, and some of these tenants occupied multiple floors. By 2001, 

most of these companies, which had moved in since the installation of automatic sprinklers, had taken 

advantage of Yamasaki's design concept of a vast space that was virtually obstruction-free. The open 

landscaping included as many as 200 or more individual workstations, often clustered in groups of six or 

eight (Figure 2–6). Trading floors had arrays of long tables with multiple computer screens (Figure 2–7). 

Some of these floors had a few executive offices in the corners and along the perimeter. Many also had 

walled conference rooms. It was common for the multiple-floor tenants to have installed convenience 

stairs internal to their space. 

Other floors were subdivided to accommodate as many as 20 firms. Some of the smaller firms occupied 

space in the core area, reclaimed as elevator shaft space from local elevators was phased out throughout 

the towers. 

With so many workers and visitors in the buildings, there needed to be food available. The Port 

Authority maintained a cafeteria on the 43rd floor of WTC 1. A number of the companies maintained 

kitchen areas where catered food was brought in daily, making it unnecessary for their staff even to leave 

their floor for lunch. The underground Concourse Level mall also provided may options for eating. In 

addition, there were hundreds of restrooms, in both the tenant and the core spaces. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 17

Figure 2–6. Photographs of typical 

WTC tenant space. 

Source: Photos courtesy of The Port Authority of 

New York and New Jersey. 

Figure 2–7. Photograph of a 

WTC trading floor 

Source: Photo courtesy of The Port Authority of 

New York and New Jersey. 
Chapter 2 Draft for Public Comment 

18 NIST NCSTAR 1-7, WTC Investigation

2.2 BUILDING SYSTEMS 

2.2.1 Egress Calculations 

Determining the Number and Width of Stairwells 

In 1965, architects and engineers designing the World Trade Center towers were faced with an impending 

change to the NYC Building Code. The draft building code language had a significant impact on the 

design of emergency egress systems. In 1965, the Port Authority directed its designers to adopt the draft 

version of the new code for their final designs. Some of the advantages of the new draft code were noted 

to be the following (Levy 1965): 

• Fire towers3

 could be eliminated; 

• Provisions for exit stairs were more “lenient;” and 

• Criteria for partition weights were more “realistic.” 

It was not certain whether all the changes being proposed to the 1938 code would be incorporated into the 

final version of the new code. Thus, in 1966, the Chief Engineer of the Port Authority suggested that the 

“architect/engineers prepare a listing of the elements of the design which do not conform to old code 

requirements, but are acceptable under the new. With this list in hand, we could initiate discussions, at 

top level in the Building Department, to see if we can secure agreement to go along with our design” 

(Kyle 1966). 

A one-page document, dated “2/15/67”, with the initials “CKP” listed the following items:4

• Fire tower corridors [sic] eliminated. 

• Number of stairs reduced from 6 to 3. (Old plans had 5 stairs at 3’-8” and 1 stair at 4’-8” for a 

total population of 390.5

 New plans have 2 stairs at 3’-8” and 1 stair at 4’-8” allowing a 

population of 390.) 

• The size of doors leading to the stairs are [sic] changed from 3’-8” to 3’-0”. 

• All stairs exit through a lobby. Old plans had fire tower stair exiting through a fire enclosed 

corridor. 

• Shaft walls are changed from a 3 h rating to a 2 h rating. 

• Corridors are limited to a 100 ft dead end and with a 2 h rating. 

 

3

 A fire tower is an exterior stairwell of incombustible construction terminating at grade level designed to ensure that smoke 

conditions from an interior fire do not contaminate the fire tower. The fire tower was provided for firefighter ingress and did 

not count as a required stairwell for occupant egress. (NYC Building Code 1938) 4

 See appendix of NCSTAR 1-1 for a reproduction of this memo. 

5

 The 1938 NYC Building Code allowed 30 person per unit of exit width, while the 1968 NYC Building Code allowed 

60 persons per unit of exit width, effectively halving the egress capacity of new construction. Population calculations are 

per floor. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 19

• Additional (word(s) missing) changed from 20 pounds per ft2

 to 6 pounds per ft2 (based on 

partition weight of 50 pounds to 100 pounds per linear foot). 

Apparently, this list represented elements of the WTC design that would not have satisfied the 1938 code, 

but did satisfy the then-current draft version of the new code. Ultimately, WTC 1 and WTC 2 were 

designed with three stairwells, two 3’-8” (44 in.) wide and one 4’-8” (56 in.) wide, as discussed below. 

A unit of exit width in the 1968 NYC Building Code was (and continues to be) 22 in. (0.56 m). The NYC 

Building Code table specifying exit and access requirements (Table 6–1) required that for a business 

occupancy, the stairs would accommodate 60 persons per unit of exit width. As the WTC 1 and WTC 2 

tenant floor design occupancy load was 365 persons per floor6

 (Solomon 1968), this required 6.5 units of 

exit width. Twelve in. (0.3 m) was the minimum half-width acceptable in the code, therefore, three 

stairwells (two with two units of exit width (44 in. [1.1 m]) and one with 2.5 units (56 in. [1.4 m])) 

satisfied the minimum requirements of the 1968 NYC Building Code. Table 2–2 shows the location of 

the stairwells, core perimeter, and transfer hallways for occupied floors in WTC 1 and WTC 2. 

Egress Provisions from Windows on the World 

The 106th and 107th floors of WTC 1 (North Tower) contained the Windows on the World complex, 

consisting of the Windows on the World restaurant, the Greatest Bar on Earth, numerous banquet and 

function rooms, kitchens and support areas, and management offices for the dining complex. While the 

configuration of the space may have changed over the life of the building, these functions were all present 

from the time Windows on the World first opened in April 1976.7

 

Restaurants, bars, and function rooms are classified in building codes as assembly use, which carries a 

significant increase in occupant load and consequent provisions for egress. The design occupant load for 

such assembly space is 15 ft2

 per occupant as opposed to the 100 ft2 per occupant for the office use space 

in most of the rest of the buildings. Thus, while the design number of occupants on an office floor was 

365 – 390 (depending on the calculation method), the design number of occupants for these floors was 

over 1,000 each (the exact number depends on the area of kitchens, dishwashing, and office space on the 

floor, all of which is at 100 ft2

 per occupant). 

Locating assembly space high in a building poses particular challenges to egress design because the 

capacity of an egress component is not permitted to be decreased in the direction of travel. Thus, where 

more or wider stairs are provided to meet capacity requirements these must be continued all the way 

down through the building which affects space utilization for the entire structure. 

 

6

 A January 25, 1968 memo from J. Solomon (Emory Roth and Sons) to M. Levy (PANYA) subsequent to a NYC Building 

Department plan review, documents that the “largest floor area is about 36,500 ft2

 on the 106th floor. At one person per 

100 ft2

 there will be 365 persons per floor, well within the permissible maximum” of 390 persons based upon stairwell 

capacity. WTCI-477-P Note that this calculation did not account for the use of the 106th floor as an assembly space. 7

 PANYNJ response to formal NIST question. March 25, 2005. 
Chapter 2 Draft for Public Comment 

20 NIST NCSTAR 1-7, WTC Investigation

Table 2–2. Plan view of stairwells in WTC 1 and WTC 2. 

Lobby (Concourse) Level Layout 

Only Stairwell B Serviced the Concourse Level

Mezzanine (Plaza) Level Layout 

Only Stairwell B Serviced the Concourse Level

Floors 9 – 19 

Stairwells A was East in WTC 1, North in WTC 2.

Floors 20 – 26 

Core space previously used by the elevators in the northeast 

quadrant became leasable tenant space.

Floors 27 - 34 Floors 35 - 41 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 21

Floor 42 

Stairs A and C transferred outside the core. 

Floors 43, 45 – 47 

There was an escalator connecting floors 44-45 (skylobby). 

Floor 44 (Skylobby) Floor 48 

Stairs A and C transferred back inside the core. 

Floors 49 – 54 Floors 55 - 56 
Chapter 2 Draft for Public Comment 

22 NIST NCSTAR 1-7, WTC Investigation

Floors 57 – 75 

There was a slight change in Stairs A, C between floors 66-68. 

Floor 76 

Stairs A, B, and C transferred, with Stairwells A and C moved 

outside the core. 

Floors 77, 80, 81 

There was an escalator connecting floors 77-78 (skylobby). 

Floor 78 

Floor 82 

Stairs A and C transferred back inside the core. 

Floors 83 - 95 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 23

Floors 96 - 102 Floors 103 – 104 

Floors 105 - 106 Floor 107 

The document record contains a letter dated January 27, 1995 from Eugene Fasullo (PANYNJ) to 

Richard Visconti (Deputy Commissioner, NYC Department of Buildings [DOB]) confirming the results 

of a meeting on December 6, 1994 at which they reached agreement on a plan to address egress 

requirements from the 106th and 107th floors (Fasullo 1995). The details of the agreed solution are 

summarized below. The Deputy Commissioner, DOB, signed the letter to show concurrence with the 

agreed solution.8

 

It remains unclear what conditions existed from the date Windows on the World first opened to the time 

the agreed solution was implemented in 1995. The dates suggest that the issue was identified as a result 

of the Memoranda of Understanding (MOU) between PANYNJ and the NYC DOB and FDNY executed 

in 1993, in response to the bombing. A Windows on the World refurbishment after the 1993 bombing 

included these egress system changes.9

The basis for the agreed solution was to divide each floor into three areas of refuge (consistent with 

Section 27-372 [NYC Building Code]) to provide additional capacity to the existing stairs in accordance 

with Section 27-367 (NYC Building Code). These identical provisions existed in the version of the 

 

8

 WTCI-159-P. 

9

 PANYNJ response to formal NIST question. March 25, 2005. 
Chapter 2 Draft for Public Comment 

24 NIST NCSTAR 1-7, WTC Investigation

1968 NYC Building Code in effect when the buildings were built as sections C26-604.5 and C26-603.3, 

respectively (the NYC Building Code was renumbered) (NYCBC 1968). 

These code provisions allow for a doubling of allowed stair capacity when one area of refuge is provided 

on a floor and tripling the stair capacity for two or more areas of refuge on a floor. These areas of refuge 

must be separated by 2 h construction, be large enough for the expected occupant load at 3 ft2

 per 

occupant, each contain at least one stair, and have access to at least one elevator (above the 11th floor). 

Since three, distinct areas of refuge were provided on each floor, tripling of the capacity of each of the 

three stairs resulted in a maximum permitted occupant load of 1,170 people per floor (6.5 units of egress x 

60 persons per unit x 3). 

Attached to (and referenced in) the letter were two plans entitled “106th Floor Egress Plan” and “107th 

Floor Egress Plan” (shown in Figure 2–8 and Figure 2–9, respectively) that detailed the arrangement,. 

The 2 h separation walls snaked across the floors and were not aligned on the two floors. Some areas that 

needed to remain open to free passage were protected with Won doors (accordion doors that are fire rated 

and are closed automatically on activation of the fire alarm system). Details of the egress system design 

calculations and corresponding NYC Building Code requirements were included on the plans to 

demonstrate they met code requirements. 

By comparison, current model building codes, including the International Building Code (IBC) and 

National Fire Protection Association (NFPA) 5000, both permit a doubling (but not tripling except in IBC 

Type I-2 and I-3 institutional uses) of the stair capacity for the provision of a horizontal exit on a floor. 

The horizontal exit must consist of a 2 h fire rated separation, contain at least one stair on each side, and 

have sufficient space for the expected occupant load at 3 ft2

 per person. A horizontal exit must be 

continuous down through the building to grade10 (NFPA 11.2.4.3.1 and IBC 1021.2), unless the floor 

assemblies are at least 2 h with no unprotected openings. 

The solution to the egress problem was to provide a protected space in which occupants could wait to 

enter stairs that did not have adequate capacity for the numbers of people. Since the attacks took place in 

the morning (a non-peak time), NIST estimated that there were 188 occupants trapped in the Windows on 

the World floors. If the attacks had occurred when the facility was loaded near its capacity, as many as 

2,000 occupants could have lost their lives on those two floors alone, since there were no survivors above 

the impact floors of WTC 1. 

 

10 In other words, the stairway may not contain unprotected openings (such as opening out to a floor) until the occupant exits the 

building. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 25Figure 2–8. 1993 Windows on the World egress calculation, floor 106. Source: Reproduced with permission of The Port Authority of New York and New Jersey. Enhancement by NIST. 
Chapter 2 Draft for Public Comment 

26 NIST NCSTAR 1-7, WTC InvestigationFigure 2–9. 1993 Windows on the World egress calculation, floor 107. Source: Reproduced with permission of The Port Authority of New York and New Jersey. Enhancement by NIST. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 27

A similar condition existed on the 107th floor of WTC 2, commonly referred to as the Observation Deck. 

A tenant alteration application submitted by Ogden Entertainment (the tenant) to PANYNJ in late 1995 

and early 1996 utilized the areas of refuge provisions referred to previously with respect to the Windows 

on the World space.11 Taking advantage of a NYC Building Code provision which permits a lower basis 

for occupant load, the PANYNJ permitted a maximum occupant load of 1,170 persons on the floor 

(Indoor Observation Deck and Outdoor Observation Deck, combined), which was enforced by the lessee 

with periodic oversight by PANYNJ.12

2.2.2 Stairwells 

WTC 1 and WTC 2 each had three 

primary stairwells designed for 

emergency egress, designated as A, B, 

and C. There were additional 

stairwells located in the basement 

levels (B1 – B5), convenience stairs 

for tenants leasing multiple floors, 

and mechanical room stairs. These 

secondary stairs are not considered 

part of the emergency egress system 

and are not described here. Stairwells 

A and C were 1.1 m (44 in.) wide and 

extended from floor 2 (plaza or 

Mezzanine Level) to floor 110 (lower 

mechanical space). Figure 2–10 

shows a 44 in. (1.1 m) stairwell in 

WTC 1 taken on September 11, 2001, 

by John Labriola during his evacuation. 

Note the photoluminescent paint on the 

stair edge and landing. Stairwell B was 56 in. (1.4 m) wide and ran from the subgrade 6 levels below 

ground to floor 107 including the Concourse (main lobby); there was no exit from Stairwell B onto the 

2nd floor (plaza / Mezzanine Level). 

The 1968 NYC Building Code has requirements for the number and capacity of stairs and for the assumed 

occupant load that are similar to requirements in the other contemporaneous codes (see NIST 

NCSTAR 1-1, Appendix A). Codes of that time required that multiple stairs be located “as remote from 

each other as practicable.” NYC permitted scissor stairs13, and the code required the exit doors to be at 

least 4.6 m (15 ft) apart. Local Law 16 (1984) first imposed a remoteness requirement of 30 ft or onethird the maximum travel distance of the floor (whichever is greater). This requirement was not 

retroactive, so it did not apply to WTC 1 and WTC 2. However, this requirement did apply to WTC 7. 

 

11 Ogden Entertainment. 1996. Port Authority work number W96-2103-01. WTCI-180-P. 12 PANYNJ response to formal NIST question. “Re: Question for PA.” March 25, 2005. S. Bohl to S. Sunder. 13 Scissor stairs are two separate stairwells with two separate stairwell access doors, which share a common shaft space, often 

winding around each other. This results in an efficient use of space, but places the stairwells in direct contact (in other words, 

there is not a barrier separating the stairwells), thus allowing smoke or other threats to affect two stairwells simultaneously. 

Figure 2–10. 44 in. stairwell in WTC 1 taken on 

September 11, 2001. 
Chapter 2 Draft for Public Comment 

28 NIST NCSTAR 1-7, WTC Investigation

The 1968 NYC Building Code also states that, “ ...vertical exits should extend in a continuous enclosure 

to discharge directly to an exterior space or at a yard, court, exit passageway or street floor lobby ...” 

(C26-602.4). The 1965 BOCA Basic Building Code and 1966 NFPA 101 contained similar language, but 

not the 1964 New York State Building Construction Code or the 1966 Municipal Code of Chicago. 

Current model code language (2003 IBC, section 1003.6) defines continuous as: not “ ... interrupted by 

any building element other than a means of egress component.” 

The exit discharge language was the subject of discussion in that the stairs in WTC 1 and WTC 2 

discharged onto the Mezzanine Level, which was not at street level but rather at the Plaza level. The Port 

Authority took the position that the Plaza was like an underground street, and the arrangement met the 

intent of the Code, as demonstrated by a February 18, 1975 letter from Joseph Solomon (Emory Roth and 

Sons) to Malcolm Levy (PANYNJ), which covered six points. “We [Emory Roth and Sons] were 

instructed by the Port Authority to deviate from the code [1968 NYC Building Code].” The fourth point 

listed the “treatment of concourse level as ‘Underground Street’ noted by letter to the Port Authority on 

April 6, 1971, January 11, 1972, and May 7, 1973” (Solomon 1975). 

Transfer Hallways 

The WTC 1 and WTC 2 stairwells were occasionally routed horizontally around equipment on 

mechanical floors, through what were called transfer hallways, as shown in Figure 2–11. Table 2–2 

shows the overall layout of the stairwells in WTC 1 and WTC 2, including the basic core perimeter.14 

Stairwell B required a horizontal transfer at floor 76. For all other floors, stairwell B maintained vertical 

alignment through the building. Stairwells A and C required horizontal transfers (some longer than 

others) at floors 42, 48, 66, 68, 76, and 82. Horizontal transfer distances ranged from several feet to over 

100ft (33 m), including multiple right angles turns in the transfer on floors 42, 48, 76, and 82 for 

Stairwells A and C. Note that the mechanical floors were located on floors 41-42, 75-76, and 108-109. 

One problem with the horizontal transfers was that they extended the total evacuation time, when 

compared to a similar design without horizontal transfers. The World Trade Center Review Committee, 

formed by the New York City Building and Fire Commissioners in response to the 1993 WTC Bombing, 

found that “the occupants of the towers encountered changes in the path of egress that were unfamiliar, 

[contributing] to the general confusion during the evacuation process (New York City 1995).” 

Figure 2–12 shows a photograph of a horizontal transfer hallway in WTC 1 or WTC 2 taken after the 

1993 bombing, including photoluminescent markings. 

 

14 Core is defined in this report as the boundary of non-leasable common space, including egress hallways, stairwells, elevator 

shafts and lobbies, HVAC, plumbing and other mechanical spaces. This definition of core may differ from a structural 

definition of core, defined by the location of core columns, which did not change location in WTC 1 or WTC 2 on different 

floors. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 29

Figure 2–11. Stairwells in the WTC towers. 
Chapter 2 Draft for Public Comment 

30 NIST NCSTAR 1-7, WTC Investigation

Each stairwell had signage on both 

sides of the stairwell access doors 

indicating the letter designation of the 

particular stairwell. A sign on the 

inside of the stairwell indicated the 

floor number, the stairwell 

designation, and whether the floor was 

a “re-entry” or “non-re-entry” floor. 

Figure 2–13 shows a photograph of 

this signage taken after the 1993 

bombing. A non-re-entry floor was a 

landing in the stairwell where the door 

to the floor was locked from the 

stairwell side. If the particular floor 

was not a re-entry floor, the sign 

indicated the location of the nearest reentry location, every fourth floor (in the case of 

Figure 2–13, floors 74 and 78). The stairwell doors 

were required to be always open every fourth floor by 

the NYC Building Code and the door locks were 

controlled electronically at the Security Command 

Center (SCC) on floor 22. The NYC Building Code 

also required that, in the event of a power outage, the 

re-entry locking mechanism would default to the open 

position. 

Compartmentation 

The design of WTC 1 and WTC 2 featured large, open 

office spaces devoid of columns due to the innovative 

structural design. Tenants could (and often did) utilize 

open plan office layouts that permitted impressive 

views of the Manhattan skyline out the perimeter 

windows. 

The NYC Building Code and PANYNJ practice 

required partitions to separate tenant spaces from one 

another and from common spaces such as the corridors 

that served the elevators, stairs, and other common 

spaces in the building core. Fire rated partitions are 

intended to limit fire spread on a floor and to prevent 

spread of fire in one tenant space to that of another. 

Partitions separating tenant space from exit access 

corridors were permitted to be 1 h, although PANYNJ 

specified them to be 2 h (Kyle 1966). This allowed dead ends to extend to 100 ft (rather than 50 ft with 

1 h partitions), which permitted more flexibility in tenant layouts. Partitions separating tenant spaces (soFigure 2–12. Horizontal transfer floors in the 

WTC towers. 

Figure 2–13. Stairwell door signage as 

seen from inside stairwell A. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 31

called demising walls) were required to be 1 h. Enclosures for vertical shafts, including stairways and 

transfer corridors, elevator hoistways, and mechanical or utility shafts were required to be of 2 h fire rated 

construction. Protection of vertical shafts was intended to limit the spread of fire and smoke from floor to 

floor. 

Another influence on compartmentation of the buildings was the adoption of Local Law 5 

(New York 1973) (LL 5) amending the NYC Building Code. While it did not (legally) apply to the 

WTC buildings, PANYNJ policy was to follow the requirements voluntarily. LL 5 required 

compartmentation of unsprinklered spaces in existing office buildings over 100 ft in height “having airconditioning and/or mechanical ventilation systems that serve more than the floor on which the equipment 

is located” to be subdivided by 1 h fire separations into spaces or compartments not to exceed 7,500 ft2

. 

Floor areas could be increased up to 15,000 ft2

 if protected by 2 h fire resistive construction and smoke 

detectors. Regardless of the floor area, compartmentation is not required when complete sprinkler 

protection is provided (LL 5, Section 6). 

Shortly after the adoption of LL 5, PANYNJ began to add the required compartmentation as a part of new 

tenant layouts as evidenced by several subsequent tenant alteration contracts from this time. Following 

the 1975 fire a fire safety consultant report recommended to PANYNJ that the buildings be retrofitted 

with sprinklers to address possible smoke problems, which would also obviate the need for 

compartmentation and permit the unobstructed views for which the buildings were known. The decision 

left the interior WTC floor arrangements with only partitions separating tenant spaces from one other and 

from exit access corridors or common spaces in the core, and with shaft enclosures.15

Construction of Partitions and Shaft Enclosures 

Vertical shafts surrounding stairs, mechanical shafts (carrying supply and return air), elevator hoistways, 

and utility shafts were all contained within the building core of the WTC towers, and were enclosed by 

gypsum planking similar to fire separations commonly used today in single-family attached housing. 

These gypsum planks were 2 in. thick and 2 ft wide, reportedly with metal tongue and groove channels 

attached to the long sides. These were likely two 1 in. panels held together by the metal channels. Their 

length in WTC 1 and WTC 2 is unknown, but similar panels today are available in 8 to 14 foot lengths. 

The planks were placed into metal H-channels at the top and bottom and secured by drywall screws. 

The 1978 edition of the Gypsum Association (GA) Fire Resistance Design Manual lists several similar 

shaft wall constructions utilizing 2 in. gypsum planks consisting of two 1 in. gypsum core board panels 

with “metal channels on long edges.” The GA Manual lists shaft walls of a single 2 in. metal edged plank 

(WP7015) having a 1 h fire rating, a single 2 in. metal edged plank with one layer of Type X gypsum 

board on the unexposed side (WP7112) having a 2 h fire rating, and a single 2 in. metal edged plank with 

two layers of Type X gypsum board on the unexposed side (WP 7575) having a 3 h fire rating. 

Partitions separating tenant spaces on the same floor were constructed of two layers of 5/8 in. Type X 

gypsum board on steel studs and ran slab to slab. This construction is commonly recognized as a 2 h fire 

separation. Above the ceiling, penetrations for ducts or to allow for return air flow were fitted with rated 

fire dampers to preserve the fire rating. 

 

15 PONYA 1976 – Complete report reproduced in NIST NCSTAR 1-1H. 
Chapter 2 Draft for Public Comment 

32 NIST NCSTAR 1-7, WTC Investigation

Interior partitions not separating spaces occupied by different tenants were constructed of single or double 

layers of 5/8 in. Type X gypsum board on steel studs, and ran from the slab to the suspended ceiling but 

not above. Double layers of gypsum board were used when the tenant desired additional sound 

attenuation. These partitions were not required to be fire rated and fire rated doors were not used. 

However, a single layer of 5/8 in. Type X gypsum board on steel studs (16 in. on center) is generally 

considered to have a 1 h fire rating and two layers of 5/8 in. Type X gypsum on steel studs (16 in. on 

center) is considered to have a 2 h fire rating. For a ceiling-high partition to be considered as having a 

fire rating, the ceiling itself would have to be rated as well. The ceiling system used throughout these 

buildings was not fire rated. 

2.2.3 Elevators 

Getting thousands of people from the ground level to the offices, observation levels, and restaurants, some 

as high as a quarter-mile, was no small task. Thus, elevators were the primary mode of movement 

between floors of the World Trade Center. The World Trade Center complex contained more than 

240 elevators, with 99 elevators serving the above-ground levels in each of the two main towers and an 

additional 7 elevators serving primarily the sub-grade basement levels. In the towers, the elevators were 

arranged to serve the buildings in three sections divided by skylobbies, which served to distribute 

passengers among express and local elevators. Figure 2–14 shows an elevator riser diagram for WTC 1 

and WTC 2 for passenger elevators. 

• People traveling to floors 9 through 40 entered a bank of 24 local elevators at the Concourse 

Level. These were divided into four groups, with each stopping at a different set of eight or 

nine floors (9 through 16, 17 through 24, 25 through 31, and 32 through 40). 

• Those going to floors 44 through 74 took one of eight express elevators to the 44th floor 

skylobby before transferring to one of 24 local elevators. These 24 were stacked on top of 

the lower bank of 24, providing additional transport without increasing the occupied floor 

space. 

• Those going to floors 78 through 107 took one of 10 express elevators from the Concourse 

Level to the 78th floor before transferring to one of 24 local elevators. These were also 

stacked on the lower banks of 24. 

• Dedicated express elevators served the restaurant, bars, and meeting rooms on floors 106 and 

107 of WTC 1, as well as the observation deck in WTC 2. 

An occupant traveling to the 91st floor, for example, would have taken an express elevator from the lobby 

to the 78th floor and then would have had to transfer to another elevator to arrive at the 91st floor. The 

elevator trip would have taken several minutes travel time, depending upon the wait at the elevators. 

While providing an acceptable rate of people movement, this three-tier system also used less of the 

building footprint than the usual systems in which all elevators run from the entrance to the top of the 

building. Further, leasable floor space was reclaimed near the top of a given zone. At the top of each 

elevator bank, the machinery to lift the cabs occupied the next higher floor. From the next higher floor up 

to the bottom of the next elevator bank, there was no need for an elevator shaft. The concrete floor was 

extended into this space, providing additional rentable floor area for offices, conference rooms, storage, 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 33

etc. Figure 2–14, for example, shows that the space taken by Elevator Bank A (Elevators 24 – 29) in 

order to serve floor 9 to floor 16, was reclaimed for tenant use on floors 19 to 42. This resulted in a 

reclamation of approximately 750 ft2

 per floor. A calculation for reclaiming unused floor space above 

elevator banks A, B, and C for all three zones, reveals that roughly 100,000 ft2

 of potentially leasable 

office space could be recovered. Assuming $55 per ft2

 per year as a rental rate for a downtown 

Manhattan office building over 600,000 ft2

 (BOMA 2001), the reclamation could theoretically yield 

nearly $6 million per year of rental income. At the time WTC was built, the concept of skylobbies, 

served by express elevators and serving only one zone of the building, was innovative. Other tall 

buildings now use this concept. 

Figure 2–14. Elevator riser diagram for WTC 1 and WTC 2. 
Chapter 2 Draft for Public Comment 

34 NIST NCSTAR 1-7, WTC Investigation

In addition to the passenger elevators, there were seven freight elevators in each tower; most served a 

particular zone, while Car 50 served every floor. 

• Car #5: B1-5, 7, 9-40, 44 

• Car #6: B1-5, 44, 75, 77-107 

• Car #17: B1-1, 41, 43-78 

• Car #48: B1-7, 9-40 

• Car #49: B1-5, 41-74 

• Car #50: B6-108 

• Car #99: 107-11016 

There were two express elevators to Windows on the World (and related conference rooms and banquet 

facilities) in WTC 1 and two to the observation deck in WTC 2. There were five local elevators in each 

building: three that brought people from the subterranean levels to the lobby, one that ran between floors 

106 and 110, and one that ran between floors 43 and 44, serving the cafeteria from the skylobby. All 

elevators had been upgraded to incorporate firefighter emergency operation requirements. 

Local Law 5 (New York 1973) requires that elevators be provided with an emergency recall system. This 

requirement was incorporated subsequently into the American Society of Mechanical Engineers (ASME) 

A17.1, Safety Code for Elevators and Escalators, which governs elevator design and operation in all 

present U.S. building codes. The ASME Code required that: 

• All passenger elevators be marked with signs stating that they cannot be used during a fire; 

• Fire detectors installed in every elevator lobby and machine room be arranged to initiate a 

recall of the elevators to the ground floor where the doors open and the elevator is taken out 

of service; and 

• Fire service personnel can use a special key to operate any individual car in a manual mode as 

long as they feel it is safe to do so. 

• At least one elevator serving every floor be connected to emergency power. 

Currently, there are no U.S. building codes that permit elevators to be used as a means of occupant egress 

in emergencies, and ASME A17.1 (ASME 2000) requires signs at all elevators warning that they should 

not be used in fires. There are some recent exceptions to this requirement, but these are limited to special 

cases. For example, NFPA 5000 permits protected elevators as a secondary means of egress for air traffic 

control towers, and the City of Las Vegas accepted elevators as a primary means of occupant egress from 

Stratosphere Tower based on a performance-based design (Bukowski 2003). 

 

16 PANYNJ WTC Fire Safety Plan (Towers One and Two), 1995, WTCI-13-NYC. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 35

U.S. building codes (including NYC Building Code) require accessible elevators as part of a means of 

egress that may be used by the fire service to evacuate people with disabilities. These elevators must 

comply with the emergency operation requirements of ASME A17.1 (Phase II emergency operation by 

the fire service), be provided with emergency power, be accessible from an area of refuge or a horizontal 

exit (unless the building is fully sprinklered), and operate in a smoke protected hoistway. Phase II 

operation involves the use of an elevator by a firefighter for fire service access or for rescue of people 

with disabilities performed under manual control (with the use of a special key). 

In the event of a fire in WTC 1 or WTC 2, or other emergency requiring evacuation where the stairwells 

are unusable or cut off by fire and/or smoke, consideration of using elevators for occupant egress may be 

given in accordance with the following PANYNJ guidelines:

• Elevators may not be used if they also service the fire floor, except under specific instructions 

from the fire safety director or Fire Department; 

• If the elevators do not service the fire floor and their shafts have no opening to the fire floor, 

they may be used at the direction of the fire safety director or fire department; 

• Elevators under the direction of the fire department or trained building personnel may be 

used.17 

Every elevator lobby contained a sign reading, “IN CASE OF FIRE USE STAIRS UNLESS 

OTHERWISE INSTRUCTED.” The sign also included a diagram indicating the location of the sign and 

the location and letter designation of each stairwell serving the particular floor.18 

2.2.4 Emergency Communication System 

WTC emergency procedures specified that all building-wide announcements were to be broadcast from 

the fire command station of each WTC tower, in coordination with the fire safety director or life safety 

and security supervisor. The deputy fire safety director was likely to make all announcements. 

Appendix J of the World Trade Center Emergency Guidelines19 provided prepared text for a variety of 

emergency scenarios, including power failures, fires, and service interruptions. Prior to all emergency 

announcements, the following pre-announcement was made: 

“Your attention please, your attention please. An important public 

address announcement will be made in the main corridor of your floor in 

a few moments.” 

Evacuation for any reason, including fire or smoke, would have generated the following announcement, 

enabling a phased evacuation: 

“Your attention please. We are experiencing a smoke condition in the 

vicinity of your floor. Building personnel have been dispatched to the 

 

17 World Trade Center Emergency Procedures Manual 2001 – Confidential. Port Authority of New York and New Jersey. 18 The Port Authority of NY & NJ World Trade Center Fire Safety Plan (Towers One and Two). 1995. WTCI-13-NYC. 19 World Trade Center Emergency Procedures Manual 2001 – Confidential. Port Authority of New York and New Jersey. 
Chapter 2 Draft for Public Comment 

36 NIST NCSTAR 1-7, WTC Investigation

scene and the situation is being addressed. However, for precautionary 

reasons, we are conducting an orderly evacuation of floors _____. 

Please wait until we announce your floor number over the public address 

system. Then follow the instructions of your fire safety team. We will 

continue to keep you advised. We apologize for the inconvenience and 

we thank you for your cooperation.” 

The standard evacuation announcement for a particular floor was: 

“Your attention please. It is now time for your floor to be evacuated. In 

accordance with the directions from your fire safety team, please take the 

exit stairs nearest to your location. We remind you that communications, 

emergency lighting and other essential services are in service. We will 

continue to keep you advised. We apologize for the inconvenience and 

we thank you for your cooperation.” 

According to the Guidelines, however, the information and instructions broadcast to the building 

occupants could be modified to suit the nature of the emergency, at the discretion of the fire safety 

director.20 NIST NCSTAR 1-4 addresses the fire alarm systems in WTC 1 and WTC 2. 

Fire Command Station 

The fire command station, located in the lobbies of both WTC 1 and WTC 2, provided a command post 

for building personnel to orchestrate the response. The NYC Building Code requires that the computer 

screen in the fire command station monitor and display information regarding: 

• Manual fire alarms 

• Smoke detection 

• Sprinkler water flow 

• Elevator lobby smoke detectors 

• Fire signal activation 

• Central office notification 

• Fan system status 

• Fail safe locked door status 

 

20 World Trade Center Emergency Procedures Manual 2001 – Confidential. Port Authority of New York and New Jersey. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 37

• Fire system trouble 

• Fire signal trouble 

• Tamper switch alarm 

• Power source 

• Test/normal mode 

• Other information as desired, 

including the status of 

elevators. 

The primary value of the fire 

command station was its role as a 

convening point for key building 

personnel responding to a building 

incident. The roles of many of the 

key personnel are described in the 

following section. Figure 2–15 shows 

the fire command station in the lobby of WTC 1 on September 11, 2001, seen from the east end of the 

Mezzanine Level. The fire command station appears in the back right corner of the picture. 

2.3 THE HUMAN ELEMENT 

PANYNJ produced and regularly updated an emergency procedures manual for building personnel to 

follow in the event of a building incident,21 at least until Silverstein Properties formally had become 

leaseholder several months prior to September 11, 2001. While Silverstein Properties was formally 

managing WTC 1 and WTC 2, PANYNJ staff continued to be significantly involved in property 

management during the transition. The latest update to the manual was completed earlier in 2001. Note 

that PANYNJ was not responsible for responding to fires or alarms in WTC 3 (Marriott Hotel), WTC 6 

(US Customs House), or WTC 7. 

The fourteen chapters in the 2001 manual addressed such possibilities as bomb threats, fires, floods, gas 

leaks, elevator emergencies, power failures, medical emergencies, chemical and fuel releases, structural 

integrity, and political demonstrations, among other potential problems. Aircraft impact was not 

specifically addressed. Individual responsibilities for key personnel were enumerated, including 

interactions with non-PANYNJ personnel (including, as appropriate, FDNY, NYPD, and others). The 

following subsections of Chapter 2.3, are derived primarily from The 2001 WTC Emergency Procedures 

Manual, Chapter 7, entitled “Fire Emergencies.” 

 

21 World Trade Center Emergency Procedures Manual 2001 – Confidential. Port Authority of New York and New Jersey. 

Figure 2–15. Fire command station in lobby of WTC on 

September 11, 2001, as seen from mezzanine 

escalator, looking west. 
Chapter 2 Draft for Public Comment 

38 NIST NCSTAR 1-7, WTC Investigation

2.3.1 Responsibilities of the Fire Safety Director 

The fire safety director was a position required by Local Law 5 (New York 1973). Local Law 5 required 

all buildings (new or existing) occupied by more than 100 persons above or below the street level, or 

more than 500 people in the entire building to have a fire safety director, deputy fire safety director, and 

building evacuation supervisor. Each such building is required to have one employee designated as fire 

safety director and one or more employees designated as deputy fire safety director, who possess 

certificates of fitness from the commissioner qualifying the individual to conduct fire drills, evacuations, 

and related training. A certified individual is required to be on duty during normal working hours. 

Consistent with Local Law 5, the primary responsibility of a fire safety director at the WTC (according to 

the formal emergency procedures manual) was overall emergency management for a building incident 

(PANYNJ 2001b). The fire safety director reports to the Fire Command Station, or scene, and assumes 

the following duties: 

• Verify that FDNY has been notified and coordinate activities of FDNY and other emergency 

response personnel; 

• Confer with floor wardens of affected floor(s) to determine conditions on the floor and 

identify areas to be evacuated, route of evacuation, stairwells available, and potential refuge 

floors; 

• Initiate evacuation procedures; 

• Direct public address announcement(s), as necessary; 

• Deploy security officers to restrict access to affected and secure areas; 

• Dispatch “key runs”;22

• Ensure appropriate notifications are initiated; 

• Maintain a chronological record of the event; 

• Direct the Operations Control Center (OCC) to arrange for emergency elevator service; 

• Investigate cause of fire (in coordination with the FDNY Bureau of Fire Investigation, 

prepares appropriate reports). 

2.3.2 Responsibilities of the Deputy Fire Safety Director 

The role of the deputy fire safety director assigned to the Fire Command Station in the lobby was to 

execute and direct the fire safety plan, including: 

• Notify fire safety director, operations control center, and police desk of incident; 

 

22 A “key run” is a security officer charged with distributing and retrieving master key rings and Fire Department portable radios. 

These keys provided access to secure areas for emergency responders or building personnel during an incident. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 39

• Maintain communications with floor wardens and other members of the fire safety team; 

• Assist with crowd control and evacuation, as necessary; 

• Request that the elevator starter or OCC contact the elevator(s) to respond to the Lobby and 

await Fire Department personnel; 

• Complete necessary fire alarm notification forms. 

The deputy fire safety directors staffing the satellite (or skylobby) command stations were assigned a 

variety of responsibilities: 

• Maintain contact with the Fire Command Station in the lobby; 

• Contact the vertical security patrol via radio in order to assist with crowd control or 

evacuation; 

• Distribute information to floor wardens, fire safety teams, and tenants via telephone, when 

contacted; 

• Assist building security with securing elevators which service the affected areas; 

• Direct occupants to stairwells and elevators (if authorized) and assist with crowd control. 

2.3.3 Responsibilities of the Operations Control Center Supervisor 

Upon notification of a fire event, the supervisor on duty at the Operations Control Center was to first 

ensure that the fire command station and the fire safety director were notified. Next, the supervisor was 

to issue a general broadcast of information over all WTC radio channels, monitor all channels and ensure 

that radio silence is observed unless directly related to the ongoing incident, arrange for elevator service, 

update units with relevant information as necessary, and notify managers of Windows on the World 

(WTC 1) and Top of the World (WTC 2) of incident in order to “reduce anxiety to tenants, visitors, 

guests, etc. when numerous emergency vehicles respond.” The Operations Control Center was located in 

the B1 Level of WTC 1 and was a backup Fire Command Center. 

2.3.4 Responsibilities of the Operations and Maintenance Management 

Building operators and maintenance personnel were mobilized in order to provide emergency response 

assistance should the need arise. The duty supervisor established contact with the fire safety director, fire 

safety coordinator, or life safety and security supervisor and responded to the fire command station to 

assist as required. The operations group supervisor, who may have required self contained breathing 

apparatus, was assigned to respond to one floor below the scene of the incident, established 

communication with the fire command station using the floor warden telephone, assisted with the 

evacuation, and kept in contact with the fire command station. 
Chapter 2 Draft for Public Comment 

40 NIST NCSTAR 1-7, WTC Investigation

The supervisor of the mechanical contractors was to dispatch staff to the fire pumps in order to “stand by” 

for further instructions, dispatched staff to operate the smoke purge system as requested by the fire safety 

director or Fire Department, and dispatched staff to secure sprinkler water shutoff valves. 

The supervisor of electrical contractors was assigned to dispatch one contract electrician to one floor 

below the affected floor in order to assist should the incident involve electrical closets or fixtures, two 

electricians to the nearest sub-station below the affected floor, and a supervisor to the fire command 

station. Further, the electrical supervisor was to ensure that staff was standing-by in order to secure 

electrical power, if necessary, and that portable electrical power was available, as needed, and played a 

significant role in post-incident restoration of smoke detectors and/or alarm panels. In the event of a 

major disaster, all staff 

electricians were to report to 

the electrical shop/office. 

The elevator maintenance 

contract supervisor was to 

report to the fire command 

station in order to assist, as 

needed, as well as dispatch 

elevator mechanics to their 

appropriate posts to assist, 

as needed. Figure 2–16 

shows a WTC official 

(denoted by the vest 

identifying WTC Officials) 

attempting to communicate 

with elevator occupants in 

WTC 1 on September 11, 2001, from the fire command station in the lobby. 

2.3.5 Floor Warden System 

The WTC Emergency Procedures (PANYNJ 2001b) requires each floor of a high-rise building to 

designate a floor warden to coordinate the evacuation of the floor, consistent with NYC Building Code. 

Assisting the floor warden were deputy floor wardens and searchers, which constitute a tenant fire safety 

team. Once the order to evacuate a floor was given, those with building authority had specific 

responsibilities to insure an orderly evacuation: 

• In the event of an emergency, the floor warden was responsible for ensuring that an alarm 

was transmitted by either telephoning the police desk or activating a manual pull station. The 

floor warden reported the incident in detail to the Fire Command Station, and relayed 

instructions to building occupants. 

• The floor warden was responsible for notifying occupants of the floor that there was a fire 

and ensure that the occupants executed the fire safety plan (PANYNJ 1995). In an 

emergency, searchers would round up employees, and the deputy fire warden would move 

Figure 2–16. Elevator communication panel in the fire 

command station of WTC 1, as operated on 

September 11, 2001. 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 41

them into the corridors and make sure all occupants were accounted for. In the event 

occupants were reluctant to evacuate, searchers were not required to force evacuation. 

• In coordination with the Fire Safety Director, floor wardens selected the safest stairwell to 

use on the basis of the location of the fire, including checking the environment in the stair, 

and notifying the fire command station which stairwell was utilized. 

2.3.6 Occupant and Tenant Training 

WTC policy was to conduct fire drills every 6 months, consistent with NYC Local Law 5,23 or shortly 

after move-in for all new tenants in WTC 1 and WTC 2. Written procedures specified a three day 

advance notice prior to the drill for tenants, through the floor warden and deputy floor warden. The floor 

warden then notified all occupants of the floor. 

Immediately prior to the fire drill, the public address system would be used to announce that the drill was 

about to occur. Occupant attendance at drills was mandatory, with a small “skeleton staff” permitted for 

business continuity. An occupant who missed a fire drill as “skeleton staff” was required to attend the 

next fire drill. The occupants were required to assemble outside a designated stairwell. 

During the fire drill training, the fire alarm was sounded. The floor warden, deputy floor warden, and 

searchers ensured that occupants gathered in the central hallway, near a stairwell. The fire safety team 

then instructed the occupants not to attempt to fight fires, not to use the elevators, to obey all instructions 

from the deputy fire safety director, and what phone number to call if there was a problem. The location 

of the nearest stairwell was identified and the procedures for phased-evacuation (move three floors below 

the fire floor, as instructed by the floor warden and/or deputy fire safety director) (PANYNJ 1996). 

The standard instruction to the occupants was to evacuate downward (to three floors below the incident 

floor). The training did not explicitly instruct occupants not to evacuate upward or attempt to access the 

roof. Stairwells A and C went to the 110th floors, but only to serve as egress points to descend from the 

110th floor or the roof. The 110th floor was not a re-entry floor, and thus, occupants without an 

authorized badge or a key would have been unable to reach the door that led to the roof. Had the 110th 

floor been accessible, actually reaching the roof would have been prevented by two additional doors, in 

accordance with Federal Communication Commission regulations.24 The first door to access the stairwell 

to the roof was protected by an access card reader. Upon opening the first door, the individual would 

enter a vestibule where, upon showing ID to a closed-circuit television monitored at the Operations 

Control Center (OCC), the door would be electronically unlocked from the OCC. Access to the roof was, 

thus, limited to a small number of people certified to enter through a radio frequency hazard awareness 

class.24

Floor wardens, deputy floor wardens, and searchers were required as part of their training, to watch a 

video, prepared by PANYNJ. The video entitled “WTC Fire Safety” and provided to NIST by PANYNJ, 

 

23 Local Law 5-73. §C19-161.2.a.4. 24 The roof housed critical communications equipment, including broadcast facilities for major television stations, paging 

transmitters, FDNY transmitters, numerous mobile transmitters. [Information derived from Port Authority response to formal 

NIST question, April 2004] 
Chapter 2 Draft for Public Comment 

42 NIST NCSTAR 1-7, WTC Investigation

reviewed the emergency procedures, building fire safety systems, and the responsibilities of the members 

of the fire safety team (PANYNJ 1996). 

2.4 CHANGES TO THE EGRESS SYSTEM AFTER THE 1993 BOMBING 

The February 26, 1993, World Trade Center bombing precipitated a $100 million repair and life safety 

upgrade to the complex, including (PANYNJ undated): 

• Radio repeaters on the roof of WTC 5 for Fire Department communications. 

• Circulation improvements. 

• New North (to Vesey St.) and South (to Liberty St.) corridors for faster evacuation from the 

Concourse (mall). 

• Two escalators from the Concourse (mall): one to the plaza at WTC 5 and one up to WTC 4 

and onto Church St. 

• Photoluminescent paint on handrails, stair treads, and stair centerline. 

• Multiple power sources for stairwell lighting: 2 normal feeds, back-up generator, and a backup from the PATH system; battery backup for every other stairwell fixture (up to 

90 minutes). 

• LED exit signs for extra brightness and visibility through smoke conditions. 

• Fluorescent signs inside stairwells at all stair reentry doors along with raised porcelain type 

Braille. 

• Fire Command Stations in main lobbies. 

• Two sealed beam (with battery back-up) elevator lights and bells, in addition to normal 

lighting. 

• Upgraded elevator intercom system, monitored at Fire Command Station. 

• New decentralized Fire Alarm System (Style 7), with three separate data risers to 

transponders located every three floors; redundant control panels and electronics; multiple 

control station announcement capability. 

• Fire alarm system powered by normal emergency power, battery back-up, and tertiary power 

to equipment. 

• New modernized Operations Control Center with the capability to monitor all HVAC systems 

and elevators. 

• Elevators modernized to current code, including replacing relay system with microprocessor 

based system (only 50 percent complete on September 11, 2001). 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 43

• Sprinkler installation accelerated to completion, including Concourse. 

• Fire wardens equipped with flashlights, whistles, hats and special training. 

• Fire drills conducted in conjunction with the Fire Department. 

Further discussion of the changes to WTC 1 and WTC 2 subsequent to the 1993 bombing can be found in 

NIST NCSTAR 1-1. 

2.5 BUILDING CODE ANALYSIS 

NIST NCSTAR 1-1 addresses the building codes relevant to WTC, including provisions for egress system 

design. For most buildings constructed in the United States, building codes adopted by local jurisdictions 

establish minimum requirements for design and construction. However, because the PANYNJ is an 

interstate agency, which was established in 1921 under a clause in the U.S. Constitution, its construction 

projects are not required to comply with any local or national model building code. For the design of the 

WTC towers, which began in 1962, the Port Authority in May 1963 instructed the architect and engineers 

to prepare their designs of WTC 1 and WTC 2 to comply with the NYC Building Code.25 While not 

specifically stated in the 1963 letter to the architect, the 1938 edition of the Code was in effect at that 

time. In areas where the Code was not explicit or where technological advances made portions of the 

1938 Code obsolete, the Port Authority also directed the architect and engineers to propose designs 

“based on acceptable engineering practice.” When such situations occurred, the Port Authority required 

the architect and engineers to inform the Planning Division of the WTC. The Port Authority established a 

special WTC office that reviewed and approved plans and specifications, issued variances, and conducted 

inspections during construction instead of the city agencies that would normally perform these duties. 

In September 1965, the Port Authority instructed the architect and engineers to revise their designs for 

WTC 1 and WTC 2 to comply with the second and third drafts of the NYC Building Code that was under 

development and to undertake any design modifications necessary to comply with the new code 

provisions.26 Prior to issuance of this instruction, the Port Authority recognized that the draft version of 

the new New York City Building Code had incorporated advanced techniques and that the Port Authority 

favored the use of advanced techniques in the design of the WTC towers.27 By adopting the draft versions 

of the new NYC Building Code, WTC 1 and WTC 2 were classified as Type 1-B Construction instead of 

Type 1-A Construction (see Sect. 9.1.3 for definition and fire protection requirements of Construction 

Type), and several architectural features related to egress were modified in the final design (see Sect. 10.1 

of NIST NCSTAR 1-1). This relaxation of code requirements allowed the Port Authority to gain 

economic advantage.28 The new NYC Building Code (NYCBC 1968) was enacted by the City Council 

 

25 Letter dated May 15, 1963 from Malcolm P. Levy (Chief, Planning Division, World Trade Department, PANYNJ) to Minoru 

Yamasaki (architect, Minoru Yamasaki & Associates) (See Appendix A of NCTAR 1-1). 26 Letter dated September 29, 1965 from Malcolm P. Levy (Chief, Planning Division, World Trade Department, PANYNJ) to 

Minoru Yamasaki (architect, Minoru Yamasaki & Associates) (See Appendix A of NCSTAR 1-1). 27 Memorandum dated June 22, 1965 from John M. Kyle (Chief Engineer, PANYNJ) to Malcolm P. Levy (Chief, Planning 

Division, World Trade Department, PANYNJ) (See Appendix A of NCSTAR 1-1) 28 Memorandum dated January 15, 1987 from Lester S. Feld (Chief Structural Engineer, World Trade Department) to Robert J. 

Linn (Deputy Director, Physical Facilities, World Trade Department), (See Appendix A of NCSTAR 1-1) 
Chapter 2 Draft for Public Comment 

44 NIST NCSTAR 1-7, WTC Investigation

on October 22, 1968, approved by the Mayor on November 6, 1968, and became effective on 

December 6, 1968. 

2.5.1 Egress in the Building Codes 

The ability to evacuate thousands of occupants from buildings as massive as WTC 1 and WTC 2, was a 

function of three primary variables: how many stairs, how wide the stairs were, and where the stairs were 

located. Each of those three factors, in the context of building code requirements, are evaluated below. 

Building codes largely relate required egress capacity to the size of the area served and the nature of the 

use of the area served. WTC 1 and WTC 2 were square buildings (roughly 207 ft by 207 ft, measured 

internally), with a gross square footage for each tenant floor approximately 42,850 ft2

 (3,990 m2). The 

floor areas in the towers were typically one of two use categories: business or assembly. The distinction 

is important for calculating egress requirements as the number of people allowed in a given space would 

be significantly fewer if the space is used for office (business) activities, than if the same space were used 

for assembly activities (such as a restaurant or meeting space). The width and number of stairwells are 

then specified to equal or exceed the number of occupants on a floor. 

The size of the ‘core’ varied significantly throughout WTC 1 and WTC 2. Note that the size of the 

structural core (as defined by the location of interior load-bearing columns) did not change significantly 

from floor to floor. As discussed earlier in Chapter 2.2.3, however, on any given floor, the core space 

used for local elevators was reclaimed for leasable office space on successively higher floors within a 

zone. For example, while floors 42 to 48 had a core area of approximately 12,000 ft2

 (1,100 m2), 

floor 105 had a core area of 6,800 ft2

 (630 m2), or 57 percent of the core area of floors 42 through 48. 

The size of the core was important because some building codes (including the NYC Building Code) 

calculate occupant load on a net basis rather than a gross basis. A net basis reduces the square footage of 

a floor by an amount equal to the unoccupied space on a floor, such as elevator or machinery shafts, and 

common areas such as hallways. The logic of using net as a calculation basis rather than gross is that 

there does not need to be egress capacity provided for floor area where no occupants would be located. 

As an example, while floors in WTC 1 and WTC 2 had a gross square footage of approximately 

42,850 ft2

 (3,990 m2), floor 105 had a net square footage of approximately 36,500 ft2 (3,400 m2), which 

yielded an occupant load of 365 persons. Floors 106 and 107 in WTC 1 were designated an assembly 

space for Windows on the World and were discussed previously in Chapter 2.2.1. By comparison, the 

occupant load on floors 42 – 48 would have been approximately 313 when calculated using 100 persons 

per ft2

 net. Generally, however, the floor with the largest occupant load dictates the overall design of an 

egress system. 

In addition to local changes in the size of the core space, the stairwells in the WTC towers changed floor 

location throughout the building, as well. This meant that the remoteness (or the distance the stairwells 

are located apart from one another) of stairwells varied, as well. The greatest separation distance between 

any two of the three stairwells, as measured by a walking path measurement (assuming that the building is 

fully sprinklered, which WTC 1 and WTC 2 were) determines the stairwell remoteness. At the two 

extremes of remoteness found in WTC 1 or WTC 2, floors 83 and higher had Stairwell A and Stairwell B 

located about 70 ft (21 m) apart, while on floors 77 – 82, Stairwell A and Stairwell C were located 

approximately 175 ft – 200 ft (54 m – 63 m) apart (depending upon the walking path on a particular 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 45

floor). Coincidentally, WTC 1 was most heavily damaged on floors in the 90s (where the stairwells were 

the closest together) and all three stairwells were destroyed, while WTC 2 was attacked in a region where 

the stairwells were the most remote (floors 78 through 82) and one stairwell remained passable. The 

angle of the airplane impact, the length-wise orientation of the core, and the presence of elevator 

machinery near the passable stairwell may also have been contributing factors to the stairwell 

survivability, however. For context, most current codes require that two exits be located a distance apart 

no less than one-third of the diagonal distance of the area served (if the floor has full sprinkler protection) 

or no less than one-half of the diagonal distance of the area served (if the floor is not fully covered by 

sprinkler protection). One-half of the diagonal distance of the area served was 147 ft (45 m) and onethird of the diagonal distance was 98 ft (30 m). Thus, in separate areas within the same building, stairwell 

remoteness distances would have been less than that required for sprinklered buildings, as well as greater 

than that required for unsprinklered buildings. 

As described earlier in Chapter 2.2, WTC 1 and WTC 2 had three stairwells, two 44 in. wide and one 

56 in. wide. The 44 in. stairwells were served by doors on each floor measuring 34 in. (1 m), while the 

56 in. stairwell was served on each floor by a door measuring 44 in. The NYC Building Code was 

selected due to the PANYNJ instruction to architects and engineers to adhere to the NYC Building Code. 

IBC and NFPA 5000 and NFPA 101 were selected because they are national model codes. See NIST 

NCSTAR 1-1 for further discussion of building codes. 

2.5.2 New York City Building Code (1968) 

Table 6-2 in §C26-601 required 100 ft2

 per occupant (net) for business occupancies, yielding a nominal 

occupant floor load of 365 persons per floor, based upon the largest net square footage (36,500 ft2

). In 

order to provide sufficient capacity for 365 persons, six and one-half units of exit width (at 60 people per 

22 in. unit) would have been required, yielding an allowable floor load of 390 people for business 

occupancies. A minimum of two stairwells would have been required for an occupant load less than 

500 people [§C26-602 Exits from Floors], each equally sized, as no more than 50 percent of the 

occupants can be served by a single exit. Two equally sized stairwells would have been 78 in. wide each. 

Three stairwells, two 44 in. wide and one 56 in. wide, would also provide the minimum egress capacity 

for business occupancy floors. 

Floors 106 and 107 in WTC 1 and floor 107 in WTC 2, having had occupant loads of over 1,000 persons 

each, would have required four stairwells to serve each floor. Thus, the 1968 NYC Building Code would 

have required theses spaces be served by a minimum of four stairwells (as the occupant load was greater 

than 1,000 persons).29 The number of stairwells was not allowed to decrease in the direction egress 

travel, therefore, the entirety of WTC 1 and WTC 2 was required to have four stairwells. In 1995, once 

these spaces were considered formally between PANYNJ and NYC DOB, three stairwells were shown to 

provide adequate total capacity of 1,170 using the NYC Building Code exit reduction clause in §C27-367 

(Fasullo 1995). There was no mention, however, of the requirement for a fourth stairwell in either tower 

(which existed and continues to exist as a requirement in §C27-366), nor whether the floor and ceiling 

system satisfied the area of refuge requirement for a 2 h fire rating. 

 

29 Egress calculation performed in 1995 (contained as figures in Chapter 2.2.1) show a calculated egress capactity of 

1,170 persons for both floors 106 and 107 in WTC 1 and a calculated occupant load of 1,013 and 1,030 persons for floor 106 

and 107, respectively. Note that there was no mention in this memo of a fourth stairwell. 
Chapter 2 Draft for Public Comment 

46 NIST NCSTAR 1-7, WTC Investigation

Each stairwell would be required to have a door at least 0.9 m (36 in.) wide. The sum of two risers and 

one tread depth was required to be not less than 0.61 m (24 in.) nor greater than 0.65 m (25.5 in.).30 

(Thus, the ‘standard’ 7 in. riser and 11 in. tread depth would satisfy this formula, at 25 in. [7 in. + 7 in. + 

11 in.]). According to Table 6-4 in the NYC Building Code, however, the stair rise, may not exceed 

0.2 m (7.75 in.) and the tread depth must be greater than 0.24 m (9.5 in.). In 1968, the location of a floor 

exit was required to be as remote from the others as practicable. 

2.5.3 New York City Building Code (October 2003) 

As it pertains to the narrow scope of this egress analysis, the requirements related to the egress system in 

2003 would be identical to the requirements of 1968, with one significant exception: stairwell remoteness. 

In 1968, the location of a floor exit was required to be as remote from the others as practicable. New 

York City Local Law (LL) 16 (1984) imposed a remoteness requirement (not retroactive to an exiting 

building such as WTC 1 or WTC 2) of 9 m (30 ft) or one-third the maximum travel distance of the floor 

(55 m [180 ft]), whichever is greater, which for WTC 1 and WTC 2 was 55 m (180 ft). Thus, all floors of 

WTC 1 and WTC 2 had stairwell separations that exceeded the minimum separation distance requirement 

of New York City LL 16 (1984). 

2.5.4 International Building Code (2000) 

Chapter 10 of the 2000 International Building Code (IBC) require 100 ft2

 per occupant (gross), yielding a 

nominal occupant floor load of 429 persons per floor. A minimum of two stairwells would have been 

required (for occupant load less than 500 persons [Table 1005.2.1]), each equally sized. As WTC 1 and 

WTC 2 were fully sprinklered, Table 1003.2.3 requires a minimum of 0.005 m per occupant (0.2 in. per 

occupant) totaling 2.2 m (87 in.) of total stairwell width, or two 1.1 m (44 in.) stairwells. 

Floors 106 and 107 in WTC 1 and floor 107 in WTC 2, having occupant loads of over 1,000 persons 

each, would require four stairwells to serve each floor. The four stairwells would be required to be 

maintained to grade, as the number of stairwells shall not decrease in the direction of egress travel. 

Additionally, the floor system would be required to have at least a 2 h fire rating. If two areas of refuge 

were built on floors 106 and 107 (each area holding at least one stairwell), the IBC would permit four 

44 in. stairwells. 

Section 1003.3.1 requires that each stairwell have a door at least 0.8 m (32 in.) wide. Section 1004.2.2.1 

requires that for fully sprinklered buildings, the stairwell doors be located a distance of no less that onethird the length of the maximum overall diagonal dimension of the building or area to be served (30 m 

[98 ft] for WTC 1 and WTC 2). This requirement was met on some floors, but not all floors, as discussed 

previously. 

 

30 From the NFPA Life Safety Code Handbook (2003 Edition), this note was made about the formula formerly used by NFPA 

and currently used by NYC: “This requirement was deleted because it was based on a 300-year-old French formula in which 

an inch was a slightly larger unit of measure than it is today. Moreover, people’s feet and stride length – the basis of the 

formula – were somewhat smaller at that time. Also, the requirement was originally intended only for stairs of moderate 

steepness or pitch.” 
Draft for Public Comment Design of the World Trade Center Egress System 

NIST NCSTAR 1-7, WTC Investigation 47

2.5.5 NFPA 5000 & NFPA 101 – Life Safety Code (2003) 

The gross square footage for each WTC tower was 42,850 ft2

 (3,990 m2). Table 7.3.1.2 requires 100 ft2

per occupant (gross) for a business occupancy, yielding a nominal occupant floor load of 429 persons per 

floor. A minimum of two stairwells would be required (for occupant load less than 500 persons 

[Section 7.4]), each equally sized. Table 7.3.3.1 required a minimum of 0.0076 m per occupant (0. 3 in. 

per occupant) totaling 3.3 m (129 in.) of total stairwell width, which may be satisfied by two 1.7 m 

(65 in.) stairwells, or three stairwells, sized at 1.1 m (44 in.) each. Section 7.2.1.2 requires that each 

stairwell have a door at least 0.8 m (32 in.) wide. 

Floors 106 and 107 in WTC 1 and floor 107 in WTC 2, having occupant loads of over 1,000 persons 

each, would require four stairwells to serve each floor (Section 7.4.1.2). Four stairwells would be 

maintained to grade, as the number of stairwells shall not decrease in the direction of egress travel. 

Section 7.5.1.3 addresses the remoteness of stairwells, with the purpose “to minimize the possibility that 

more than one has the potential to be blocked by any one fire or other emergency condition.” 

Additionally, Section 7.5.1.3.3 requires that for fully sprinklered buildings, the stairwell doors be located 

a distance of no less that one-third the length of the maximum overall diagonal dimension of the building 

or area to be served (30 m (98 ft) for WTC 1 and WTC 2). This requirement was met on some floors, but 

not all floors, as discussed previously. 

2.5.6 Comparison of Current Code Requirements 

Differences in Stairwell Occupant Capacity 

The IBC allows a reduction in egress capacity for fully-sprinklered buildings, to 0.005 m (0.2 in.) per 

person. Thus, while IBC would require two 44 in. stairwells for 429 occupants, NFPA Life Safety Code 

would require two 65 in. stairwells from tenant floors in of dimension similar to WTC 1 or WTC 2. 

While NYC Building Code also uses 0. 3 in. per person of required exit width, the calculation is net 

square feet, rather than gross square feet, effectively reducing the requirements. However, minimum halfunits of exit width (12 in.) used in the NYC Building Code often force the designer to ‘round up’ the 

calculated egress capacity (from 365 to 390, e.g.). Two stairwells, each 78 in. in width, would be a 

minimum allowed by the NYC Building Code, as an alternative to the three stairwells (two 44 in., one 

56 in.). Table 2–3 summarizes the results of the calculation of the minimum number of stairwells for an 

office occupancy with a service area of 42,850 ft2

 gross (36,500 net) using each of the three building 

codes described above. 

Table 2–3. Minimum stairwell design for 42,850 ft2

 office plan. 

Building Code 

International Building 

Code (2003) 

NFPA Life Safety Code 

(2003) 

New York City Building 

Code (2003) 

Number and Width of 

Stairwells for a 

42,850 ft2

 Office Plan 

2 Stairwells 

44 in. each 

2 Stairwells 

65 in. each 

2 Stairwells 

78 in. each 
Chapter 2 Draft for Public Comment 

48 NIST NCSTAR 1-7, WTC Investigation

Net vs. Gross Occupant Load Calculations 

The IBC and NFPA model codes both calculate the number of occupants per floor (business occupancy) 

based on the gross square footage of the floor divided by 100. NYC Building Code, on the other hand, 

calculates the maximum occupant load by subtracting from the gross square footage, shafts, storage 

rooms, and stairs. Thus, the WTC had a design occupant load of 390 persons per floor, whereas IBC and 

NFPA 101 would have required egress capacity for 428 persons per floor. On the other hand, the egress 

system would have to reflect the higher occupant load, as well. WTC 1 and WTC 2 would have required 

7.5 (rather than 6.5) units of exit width if the occupant load calculation was on a gross basis rather than a 

net basis. Thus, three stairwells would need to have been 1.4 m (56 in.), rather than only Stairwell B, to 

accommodate the higher occupant load. 

Areas of Refuge and Egress Capacity 

When two stairwells are each located in separated areas of refuge, the capacity of each stairwell may be 

doubled. NYC Building Code, IBC, and NFPA 101 each permit doubling of a stairwell’s capacity using 

this method. As discussed in Section 2.2.1, however, NYC Building Code also allows the capacity to be 

tripled when three stairwells are each separated from the other two by fire-rated partitions complying with 

requirements for areas of refuge. IBC and NFPA 101 do not allow tripling of stairwell capacity. 

Stairwell Remoteness 

NYC Building Code calculates the minimum separation distance for stairwells based upon a fraction of 

the longest travel distance on a particular floor (one-third, if fully sprinklered and one-half if not fully 

sprinklered). IBC and NFPA 101 calculate the minimum separation distance for stairwells based upon a 

fraction of the maximum diagonal of the floor or area served (one-third, if fully sprinklered and one-half 

if not fully sprinklered). 
NIST NCSTAR 1-7, WTC Investigation 49

Chapter 3

FIRST-PERSON DATA COLLECTION AND ANALYSIS METHODS

3.1 PURPOSE AND SCOPE OF COLLECTION OF FIRST-PERSON DATA 

The purpose of first-person data collection was to capture the full range of occupant experiences from 

World Trade Center (WTC) 1, 2, and 7 from 8:46:30 a.m., when WTC 1 was attacked, until all survivors 

had successfully evacuated. The goal was to capture both common (frequent) evacuation experiences and 

unique observations or actions that may have contributed to a greater understanding of the events of 

September 11, 2001. Potential respondents included all occupants inside WTC 1 or WTC 2 between 

8:46:30 a.m. and 10:28:22 a.m. (when WTC 1 collapsed), building personnel, emergency responders, and 

family members who spoke to occupants inside WTC 1 or WTC 2 during the attack. 

3.2 METHODS 

To best capture both the generic evacuation experience and the unique observations and experiences, 

multiple interview methods were selected: face-to-face interviews, telephone interviews of a statistically 

representative sample of people, and focus group interviews. Each method contributed a unique strength 

to the overall objectives, complemented and contributed to understanding the data collected through the 

other methods, and established multiple measures of a variety of phenomena. In addition to interviews, 

published media accounts, video, and photographs were collected and analyzed. Each method is 

discussed below. 

3.2.1 Published First-Person Accounts 

National Institute of Standards and Technology (NIST) contracted with the National Fire Protection 

Association (NFPA) to collect first-person accounts from newspapers, radio and television programs, 

e-mail exchanges, and a variety of websites and to distill them into a searchable database (Fahy and 

Proulx 2003). Over a period of 18 months, a total of 745 first-person accounts were collected. These 

accounts had been published up to 14 months after the event. Although media accounts do not provide 

the rigor of a proper scientific study, they do present important insights into events. The objective of the 

analysis of the first-person accounts was to gain insight into the variability of human behavior and 

response time displayed during the evacuation, and to use the findings as a guide for additional 

investigation. For the NIST investigation, the accounts provided background for development of the 

telephone survey instrument and aided in identification of individuals with particularly compelling stories 

that were of interest for face-to-face interviews conducted as part of the investigation. 

A coding tool was developed for content analysis of the first-person accounts. Data were then entered 

into the database. The coding tool had 33 questions such as: “On what floor was the person?,” “What was 

the first cue of the event?,” “Was the person injured?,” and “What were the conditions in the stairs?” Not 

every account provided answers for all 33 questions, since some accounts lacked certain details, but this is 

not unlike the situation of a respondent who did not answer some questions in a survey. Once the 

745 first-person accounts were summarized, multiple accounts from the same person were merged into 
Chapter 3 Draft for Public Comment 

50 NIST NCSTAR 1-7, WTC Investigation

one, which provided accounts for 465 distinct individuals. (Some survivors provided multiple accounts 

through different sources.) Before any analysis began, the database was further limited to the 

435 building occupants who were actually in WTC 1 or WTC 2 on September 11, 2001. The accounts 

analyzed were from 435 individuals - 251 occupants of WTC 1 and 184 occupants of WTC 2 - 

representing occupants from low, middle, and high regions of both WTC 1 and WTC 2. 

The content analysis of first-person accounts has significant limitations. First, the actual questions asked 

by the journalists reporting the accounts are not usually contained in the accounts. Second, some details 

may have been left unreported; and third, more dramatic stories may have been over-represented. 

Consequently, while the results of the published accounts analysis cannot be generalized to the overall 

population of the towers of the World Trade Center, they provided valuable input to the NIST 

Investigation. 

3.2.2 Telephone Surveys 

The survey objective of the telephone interview phase of this study called for collecting 800 computer 

assisted telephone interviews (CATI) of persons occupying either of the WTC towers (WTC 1 and 

WTC 2) at the time of the terrorist attacks on September 11, 2001. The sample size of 800 and allocation 

of n=400 to each tower were chosen to maximize the statistical precision of estimates and projections 

within each tower.31 Primary statistical analyses were in the form of tabulations and linear statistics 

(e.g., reporting of percentages and average/means). Estimates of percentages from tower-specific survey 

data (at n=400) exhibit sampling errors no greater than 2.5 percentage points, and 95 percent confidence 

intervals of percentages are no greater than ± 5 percentage points. This level of precision was more than 

adequate for examining issues of interest in this investigation. Within WTC 1 and WTC 2, independent 

proportionate stratified samples of survivors were drawn. In other words, each occupant of a particular 

tower had an equal probability of being selected. 

Population and Sampling Frame 

The total population of people eligible to participate in a telephone interview consisted of individuals who 

were inside WTC 1 or WTC 2 between 8:46:30 a.m. and 10:28:22 a.m. on September 11, 2001, with the 

exception of emergency responders (FDNY, NYPD, OEM, FBI. Secret Service, ATF, and others). The 

sampling frame (i.e., the list from which the sample was drawn) consisted of the names of occupants from 

badge lists for persons authorized to be present in WTC 1 and WTC 2 and was assumed to represent the 

entire population of individuals eligible to participate in telephone interviews. All occupants who worked 

or regularly visited the World Trade Center were required to provide personal data to PANYNJ in order 

to be issued a badge to clear through the security station at the entrance of each tower. The badge lists 

were provided to NIST by the Port Authority of New York and New Jersey. The lists provide name, floor 

 

31 Multivariate modeling such as correlation analyses, multiple linear regressions, and path analyses, are also a prominent part of 

the survey analyses. Like the tabulations, these analyses were conducted independently by tower. A sample size of n=400 per 

tower provides more than ample statistical power for the F tests used to determine the significance of the regression models 

(i.e., testing the null hypothesis that the ratio of explained variance to error/residual variance is equal to zero). For instance, in 

a multiple regression analysis featuring 20 independent variables, the sample size of 400, and 0.05 level of significance (Type I 

error), the power of the F test to detect an r2

 statistic (i.e., proportion of explained variance) of 0.06 is just over 81%. See also 

Chapter 9 of Cohen, J., 1988, Statistical Power Analysis for the Behavioral Science, Lawrence Erlbaum Associates, Inc., 

Hillsdale, N.J. 
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NIST NCSTAR 1-7, WTC Investigation 51

of occupancy, employer, and social security number (the only available means of uniquely identifying 

individuals). 

Representativeness of Badge List 

Confidence in the assumption that the badge list accurately represented the WTC population was 

increased by comparing independent ‘lists’ of occupants to the badge list: survivors who were interviewed 

by the media and lists of decedents. The three sources of data that were compared were (1) the list of 

decedents from CNN web site, (2) a media list of survivors, and (3) the badge list of occupants. 

One limitation is that the independent list of media interviewees may not have sufficient information to 

indicate whether the listed person should have had a badge (and thus been listed on the badge list). 

After comparing the media list with the badge list, it was determined that 134 (93 percent) individuals of 

144 selected from the media list were authorized to be at WTC 1 or WTC 2 the day of the tragedy. 

Approximately 2 percent of all individuals were definitely not supposed to be at WTC 1 or WTC 2 the 

day of the tragedy and insufficient information existed to determine positively if the remaining 5% were 

supposed to be at WTC 1 or WTC 2 on September 11, 2001. 

Based on Table 3–1, a conservative estimate of coverage was obtained by taking the number of persons 

from the media list who were definitely authorized to be at WTC (134), and comparing them to the total 

possible number of authorized individuals from that list – (134 + 7) = 141. The resulting conservative 

coverage rate of the badge list, estimated based upon media interviews with survivors, was (134/141), or 

95 percent. 

Table 3–1. Comparison of media interviewees and badge list. 

Status Frequency Percent 

Definitely authorized to be at WTC 1 or WTC 2 134 93% 

Definitely not authorized to be at WTC 1 or WTC 2 3 2% 

Not enough information to determine 7 5% 

Total 144 100% 

A similar analysis was conducted using the victim list published on the web site of CNN. As Table 3–2 

shows, a conservative estimate of coverage was obtained by taking the number of persons from the 

decedent list who were definitely authorized to be at WTC (2,141), and comparing that number to the 

total possible number of authorized individuals from that list (2,141 + 79 = 2,220). The resulting 

conservative coverage rate of the badge list was, as estimated from the CNN victim list, was 

(2,141 / 2,220), or 96.4 percent. 

Thus, the assumption that the badge list was a complete universe of possible WTC survivors from which 

to select a representative sample was determined to be valid. 
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52 NIST NCSTAR 1-7, WTC Investigation

Table 3–2. Comparison of CNN victim list and badge list. 

Badge List Status 

WTC Status Appears in Badge List 

Does not Appear 

in the List Total 

Person authorized to be at WTC 2,141 0 2,141 

Not authorized to be at WTC N/A 408 408 

Insufficient information to 

determine WTC authorization 

N/A 79 79 

Total 2,141 487 2,628 

Telephone Interview Sample Selection 

The badge list contained September 11, 2001, occupants, occupants who were absent on the day of the 

attacks, decedents, former occupants, and non-person listings (false names used in sample testing input by 

PANYNJ prior to delivery to NIST but not removed). This meant that a screening effort was needed to 

identify “eligible” badge list members – namely, those who were inside WTC 1 or WTC 2 during the 

attacks and survived. Moreover, the absence of telephone numbers for the badge holders on the list 

necessitated a tracking/locating effort. The primary tracking mechanism was to search public databases 

using commercially available batch matching and web-based search utilities. Consequently, a large 

sample was needed to generate the 800 completed interviews. 

The number of occupant selections drawn into the sample was contingent on four key design parameters: 

• The percentage of individuals from badge listings for whom a working telephone number 

could be found (initial estimate: 80 percent tracking success) 

• The percentage of badge listings that corresponded to a surviving WTC 1 or WTC 2 occupant 

on September 11, 2001 (initial estimate: 14 percent) 

• The cooperation rate for screening the occupants (initial estimate: 65 percent) 

• The interview response rate among September 11, 2001 survivors (initial estimate: 

50 percent). 

In planning the CATI survey, a number of design parameters needed to be quantified in order to 

determine the number of persons to draw from the badge list. The expected disposition of the sample was 

developed using the parameters defined in the preceding paragraph. A total sample of 22,735 persons 

from the badge list was needed to generate the desired 800 completed interviews. The expected 

disposition by tracking efforts, screening and interviewing are discussed later. 

A reserve sample of about 14 percent (or about N=3,265) was added in the event additional respondents 

were needed due to unanticipated circumstance (if the eligibility rate was actually lower than anticipated). 

This brought the total sample size to 26,000. The reserve was initially held “in reserve” while the main 

sample was worked. Working the main sample allowed preliminary estimates of all design parameters to 

be monitored so that an informed decision could be made on the necessity of releasing none, some, or all 

of the reserve. 
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NIST NCSTAR 1-7, WTC Investigation 53

The badge list contained different counts of persons from each tower, yet the sample design called for 

equal samples to be drawn from the collections of badge holders in WTC 1 and WTC 2. Thus, a 

disproportionate design (across tower strata) was employed. Within each tower, independent 

proportionate samples were drawn using stratification by floor (within tower), employer (within floor) 

and last name (within employer). This served to increase the statistical precision of the tower-specific 

samples. 

Thus, equal-sized samples of 13,000 selections were drawn from each of WTC 1 and WTC 2 badge lists. 

Each tower-specific sample was partitioned into 20 random replicates (comprising 5 percent of the total), 

and the reserve sample was determined by the last several random replicates for each tower. It is 

important to note that all badge holders from WTC 1 floors 92 and above were omitted from sampling 

because there were no survivors from those floors. 

Table 3–3 summarizes the final disposition of the CATI sample and the total (locating) sample. The table 

is comprised of two sets of rows. The top set pertains to the CATI sample and represents those sample 

persons for whom an initial telephone number was identified prior to commencing the CATI survey 

operations. The bottom set of rows with the heading “Total Sample Disposition” represents the results of 

the locating/tracking effort used to identify usable telephone numbers associated with the sample subjects. 

(Recall that only name, SSN, and employer were available; no other contact information was readily 

available.) 

The bottom set of rows shows that telephone numbers were identified for just over three quarters 

(76.6 percent) of the sampled subjects. Moreover, this rate was fairly uniform across towers. The 

19,923 individuals with an initial telephone number were then loaded into the CATI sample management 

system for calling. Ultimately, all reserve respondents were used in the telephone survey. In the initial 

design parameters, it was assumed that 82 percent of the subjects would be locatable. While 76.7 percent 

is close, many of the numbers were obsolete (e.g., disconnect, wrong number) and necessitated additional 

tracking during CATI operations. Ultimately, by the end of data collection, only half the sample 

(49.5 percent) represented confirmed contacts with respondents. 

The top set of rows in Table 3–3 presents the final disposition of the sample by tower as well as for the 

overall sample. Several statistics in the percentage distribution (rightmost) column are notable. First, 

NuStats (under contract to NIST) was unable to contact subjects for half the sample (50.5 percent), due to 

failures to answer the phone, answering machines, unusable numbers (e.g., wrong number, disconnected, 

business), etc. Most of these unusable telephone numbers represent “unlocatable” subjects – subjects for 

whom the initial telephone number was incorrect. It bears reiterating that substantial additional attempts 

to locate individuals during CATI operations were conducted using powerful subscription-based webbased search engines. Unfortunately, little information was available for these individuals. 
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54 NIST NCSTAR 1-7, WTC Investigation

Table 3–3. Disposition of the CATI sample and the total sample by tower. 

CATI Disposition: WTC 1a WTC 2a Total % Distn 

 Interview 427 376 803 4.0 % 

 Partial Interview 47 37 84 0.4 % 

 9/11 decedent 20 40 60 0.3 % 

 Other decedent 49 39 88 0.4 % 

 Not Eligible 3,712 3,752 7,464 37.5 % 

 Language Barrier 135 129 264 1.3 % 

 Eligible Refused to Interview 138 139 277 1.4 % 

 Other Refusal 224 181 405 2.0 % 

 Respondent not Interviewed 247 168 415 2.1 % 

 Can't contact/locate Respondent 4,987 5,076 10,063 50.5 % 

CATI TOTAL 9,986 9,937 19,923 100.0 % 

 Total Sample Disposition: WTC 1 WTC 2 Total % Distn 

 Found initial telephone # 9,986 9,937 19,923 76.6 % 

 Unable to find a telephone # 3,014 3,063 6,077 23.4 % 

SAMPLE TOTAL 13,000 13,000 26,000 100 % 

a. Table data are unweighted. Tower location as indicated in the badge list and may differ from reported tower location. 

Second, the badge list contained a number of ineligible subjects (37.5 percent) – individuals on the badge 

list but not in the building on the morning of September 11, 2001. An assessment of eligibility rates 

appears later. Third, the badge list included decedent names (0.4 percent) – some from the 

September 11, 2001, attack (0.3 percent) and others from causes not necessarily related to September 11, 

2001 (e.g., cause unknown, natural causes, 0.1 percent). Most of the September 11, 2001, decedent 

names were encountered due to a difference between the full (formal) name of the subject and the name 

that appeared on the badge list (e.g., the badge list sometimes contained maiden names, middle names, 

nicknames, misspelled first or last names, out of sequence names, titles, and so on). This impeded the 

ability to remove known decedent names prior to calling. 

The final outcome rates of the CATI operations are presented by tower in Table 3–4. The table shows 

screening rates, interview rates, and rates of eligible occupants (among those who responded to the 

screening questions). The first row shows that screening response rates were relatively uniform across 

the towers, at about 46 percent. In other words, approximately 46 percent of successful telephone 

contacts resulted in determining whether the potential respondent was present at WTC 1 or WTC 2 on 

September 11, 2001. Similarly, interview response rates (among screened eligible subjects) were 

relatively stable across towers, at about 49 percent. 

The eligibility rates were higher than expected – about 18 percent overall compared to the 14 percent 

expected. The eligibility rate among WTC 1 subjects was slightly higher than those of WTC 2. 

However, the overall response rates are essentially uniform across towers, at 22.6 percent. 
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NIST NCSTAR 1-7, WTC Investigation 55

Table 3–4. Summary disposition rates by tower. 

Disposition Ratea WTC 1 WTC 2 Total 

Screen 46.5 % 45.8 % 46.1 % 

Interview 48.6 % 49.5 % 49.0 % 

Eligibility 18.9 % 16.7 % 17.8 % 

Overall 22.6 % 22.7 % 22.6 % 

a. Definitions for “Rates” consistent with American Association of Public Opinion Research (AAPOR) Standards, 

which may be found at http://www.aapor.org/pdfs/standarddefs_3.1.pdf . 

The telephone interview protocol resulted in 803 interviews with individuals who evacuated WTC 1 or 

WTC 2 on September 11, 2001, after 8:46:30 a.m. These 803 individuals were interviewed in roughly 

equal proportion (N = 440 for WTC 1 and N = 363 for WTC 2) between the two buildings. The interview 

results can be generalized to the entire population of survivors in both buildings with a high degree of 

statistical confidence. 

Telephone Questionnaire 

The telephone interview was conducted by trained interviewers using a computer program that provides 

questions and answer categories for the interviewer. Prior to being contacted by telephone, subjects 

received a letter that outlined the scope and purpose of the investigation and the purpose of the interview, 

and indicated that a telephone call would come several days later. A full informed consent statement also 

appeared in the letter, as well as in the script for the calls. 

When interviewers reached the subjects by telephone, they described the survey, the confidentiality of 

responses, the length of the interview, and the voluntary nature of participation. Subjects were then asked 

if they wished to participate, which served as the means of obtaining oral informed consent. 

The telephone interview instrument (see Appendix A in this report for the complete instrument) included 

the questions, variable names, response options, and skip patterns taken directly from the computer 

program used by the interviewers. Variable names are used as shorthand for subsequent data analysis. 

Questions had a variety of response option categories: multiple choice, interval, Likert scale, or openended. Open-ended responses were minimized where possible due to the analysis burden and the fact that 

face-to-face interviews also were being conducted. Skip patterns reduced the burden on the respondent 

by skipping questions that would not apply to a particular respondent. For example, a respondent would 

not be further questioned about fire drills if he or she had not received fire drill training. Subsequent 

discussions of the questions will indicate whether a respondent was read a list of choices or was expected 

to give a free response. 

The interview, which typically lasted approximately 20 minutes, was designed around five primary 

groups of questions, covering emergency training and preparedness, three stages of evacuation 

experience, and background information about each respondent. 
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56 NIST NCSTAR 1-7, WTC Investigation

Emergency Training and Preparedness 

The first group of questions measured the extent to which an occupant had any special level of knowledge 

about the building, other than what would be obtained by performing his or her job. The most prevalent 

special knowledge would be formal evacuation training, or fire drills. If respondents indicated that they 

participated in evacuation training during the 12 months prior to September 11, 2001, further questions 

were asked about the content of the training. The occupant’s understanding of the emergency procedures, 

or the way it was ‘supposed to go,’ was also measured. Next, a Likert scale32 measured the perceived 

usefulness of the evacuation training in the context of egress experiences on September 11, 2001, ranging 

from very helpful to very unhelpful. Finally, the respondent was asked whether he or she knew that there 

was a floor warden for his or her floor. 

Initial Experience on September 11, 2001 

The second group of questions covered the first moments of the September 11, 2001 attack on the World 

Trade Center as experienced by the respondent, also known as the initial awareness period. The manner 

in which a person first became aware that something was not normal (whether in the building or the 

neighboring building) may have influenced subsequent decisions. Examples of awareness channels may 

include sensory perception (such as feeling, hearing, or seeing the building shake; seeing or smelling fire 

or smoke) or may include a conversation with a person inside or outside the WTC complex. Next, the 

respondent was asked to provide context to the initial moment of awareness. Context was first created by 

identifying what activity the respondent was performing. Activities included, but were not limited to, 

working, conversing with coworker(s), eating, or participating in a meeting. The respondent was then 

asked to recall the number of other people he or she was with at the first moment of awareness. People in 

groups often defer to group decisions rather than making their own evacuation decisions. Next, a list of 

observations was read aloud, and the respondent was asked to indicate whether he or she noticed that 

event during the period of initial awareness. These events included smoke, fire, fireballs, collapsed walls, 

jet fuel, severely or fatally injured people, sprinklers going on, fire alarm sounding, power outage or 

flickering lights, fallen ceiling tiles, and extreme heat. The event proximity was probed for every 

affirmative response to determine whether the observed event was in the immediate area or outside the 

building. If no affirmative responses were indicated, the respondents were asked whether they observed 

any disaster related events not previously mentioned. Finally, the extent of any injuries to the respondent 

or those in the immediate area was ascertained, as well as whether the respondent felt that his or her life 

or the lives of other people were in danger. 

Interim Experience on September 11, 2001 

The format of the interim experience group of questions mirrored the format of the initial awareness 

questions. The interim time period was defined as the time after initial awareness, but before the person 

entered a stairwell or elevator to leave the building. This time period may have ranged from moments to 

tens of minutes. The objective of the interim period questions was to determine what motivated/forced 

people to either immediately evacuate or delay their evacuation by some period of time. 

 

32 A Likert scale measures the degree to which the respondent agrees or disagrees with a statement. In this case, the scale 

measured helpfulness, including very helpful, helpful, unhelpful, and very unhelpful. A neutral response was not included. 
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NIST NCSTAR 1-7, WTC Investigation 57

Information about the nature of the event obtained during the initial period often forms the basis for 

decision-making during the interim period. For example, many people may have found the environmental 

cues from the initial awareness period sufficient to initiate an immediate evacuation. Others may have 

required additional information in order to feel comfortable leaving the workplace. Occupants could have 

obtained information in two ways: passively and actively. Passive information is information received 

without seeking it. In other words, the information is received regardless of whether the person feels it is 

needed. Active information is information which the respondent actively seeks and considers important 

with respect to his or her decision to evacuate. In the interview, respondents were first asked whether 

they received any additional information about the event during the interim period. If so, the source 

(who), the nature (what), and the channel (how) of the information was probed. Next, additional 

information sought by the respondent was probed, including the source, nature, channel, and whether the 

process was successful in gathering additional information. 

The perception of risk to the respondent’s life, as well as the lives of others, was asked in the same way as 

during the initial period, to determine whether the sense of risk was increasing or decreasing over time. 

The interviewer probed about the activities of other people in the proximity of the respondent, which may 

have influenced the respondent’s subsequent choices. Whether other people began evacuating prior to the 

respondent was specifically asked. Next, respondents were asked about the activities undertaken during 

the interim period, as well as activities that they wanted to carry out but could not. These activities 

included work-related actions, such as saving files or shutting machines down; personal actions, such as 

gathering belongings or calling people; and emergency-related actions, such as fighting fires/smoke, and 

searching for or helping others. If a respondent was unable to accomplish an action, the action and the 

reason for being prevented from doing so was captured. 

As with the initial period, any observations of building damage were collected. If the respondent received 

help in any way before initiating evacuation, the nature and source of the assistance was determined. The 

respondent was asked for the primary cue was that initiated his or her evacuation on September 11, 2001 

and how many minutes passed before initiating evacuation. Finally, respondents were asked whether 

anything prevented them from evacuating sooner than they reported. 

Evacuation Experience on September 11, 2001 

The next group of questions which followed the evacuation sequence to its completion, focused on time 

spent in the stairwell and/or elevator(s). Respondents were first asked whether they began their 

evacuation alone or with other people. Which stairwell (or elevator) the respondent entered was collected 

as either the stair identification letter (A, B, or C) or the geographic location, if known. Knowing where 

the stairwell emptied at the bottom could also narrow down which stairwell was used, which was 

collected near the end of this group of questions (Stairs A/C [44 in. wide] emptied out to the upper, 

Mezzanine Level, while Stair B [56 in. wide] went to the lower, Concourse Level). Next, the 

respondent’s rationale for using a particular stairwell was probed. The respondent was then asked 

whether he or she left the stairwell or turned back for any reason during the evacuation and, if so, why. 

Some events and features of the stairwells aided the progress of the evacuation, while other features 

constrained the progress of the evacuation. The following features or events were identified to the 

respondents, who were asked to indicate whether it was an aid to their egress: instructions or assistance 

from their floor warden, a police officer, or firefighter; support/encouragement from others; exit signage; 
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58 NIST NCSTAR 1-7, WTC Investigation

and photoluminscent paint. The following items were identified to determine whether they constrained 

the evacuation: crowded stairwells, counterflow (people moving up the stairs, against the flow of 

occupants), disabled or injured people being taken down the stairwell, locked doors, poor lighting, 

confusing or missing signage, and lack of clear instructions. 

As with the initial and interim time periods, environmental cues related to fire, smoke, jet fuel and other 

disaster-related observations were probed, as well as whether the observation was in the immediate area 

or outside the tower. The final question about the respondent’s own evacuation estimated the elapsed 

time from entering the stairwell until they left the building. A concluding evacuation question determined 

whether they knew why someone on their floor did not survive the WTC attack, if applicable. 

Respondent Background 

The final group of questions explored the background of the respondent relevant to evacuation. The first 

question identified any pre-existing disabilities or injuries which made evacuation more difficult. The 

respondent’s age, gender, and primary language were collected. If the respondent was working in the 

building prior to 1993, they were asked whether they were present during the February 26, 1993 bombing. 

If so, respondents were asked questions about their evacuation experience on that day. 

The interview concluded with an open-ended opportunity for the respondent to say anything additional 

about their evacuation experience on September 11, 2001. Respondents who indicated that they had a 

disability, were near the floors of impact, observed fire, smoke, or fireballs in their immediate area, or had 

a role of building responsibility on September 11, 2001, were asked if they would be willing to participate 

in a follow-up face-to-face interview. 

Telephone Interview Response Rate Analysis 

The response rate analysis of the telephone interview sample indicated an inverse relationship between 

floor height and the rate of response in WTC 1, as shown in the last column of Table 3–5. In other words, 

an individual was somewhat less likely to complete a telephone interview if they were high in WTC 1 

than if they were lower in WTC 1. The non-response weight adjustment is the inverse of the overall 

response rate. For example, the inverse of 25.3 percent is 3.95.33 In general, the weight adjustment for 

WTC 1 indicates that representative results should reflect that a single interview with a respondent high in 

the building is representative of more occupants than a single interview with a person lower in the 

building. 

 

33 1 / 0.253 = 3.95. A lower overall percentage would, therefore, yield a higher weight adjustment. 
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NIST NCSTAR 1-7, WTC Investigation 59

Table 3–5. Response rate analysis for WTC 1. 

Floor 

Stratum 

Number of 

Selections 

Number of 

Interviews Screen Eligibility Interview Overall 

Non-Response 

Weight 

Adjustment 

1 to 42 4464 256 46.2 % 22.6 % 54.8 % 25.3 % 3.95 

43 to 75 3714 137 48.6 % 16.6 % 45.8 % 22.3 % 4.49 

76 to 91 1802 34 42.7 % 14.7 % 30.1 % 12.9 % 7.78 

Floor missing 6 0 50.0 % 0.0 % NA NA 

Total 9986 427 46.5 % 18.9 % 48.6 % 22.6 % 

Key: NA, not applicable. 

A similar analysis of telephone interview response rates for WTC 2 (shown below in Table 3–6) did not 

indicate a significant need to weight the results; however, to be consistent with WTC 1 analysis, the 

results were weighted. 

Table 3–6. Response rate analysis for WTC 2. 

Floor 

Stratum 

Number of 

Selections 

Number of 

Interviews Screen Eligibility Interview Overall 

Non-Response 

Weight 

Adjustment 

1 to 42 4339 143 44.8 % 14.8 % 49.7 % 22.3 % 4.49 

43 to 75 3187 134 45.0 % 17.7 % 52.8 % 23.8 % 4.21 

76 to 110 2203 94 48.3 % 19.5 % 45.2 % 21.8 % 4.58 

floor missing 208 5 50.5 % 9.5 % 50.0 % 25.2 % 3.96 

Total 9937 376 45.8 % 16.7 % 49.5 % 22.7 % 

All subsequent telephone interview data analysis in this report reflects weighting of the results in order to 

more accurately generalize the results. By convention, when a sample number is indicated (n= ), the 

sample number will be the actual number of responses. Where percentages are indicated, however, the 

percentages were weighted to allow for generalization, unless otherwise indicated. 

The source of differential non-response for floors 76 to 91 in WTC 1 when compared to floors 1 to 75 

was not specifically identified. 

3.2.3 Face-to-Face Interviews 

The objective of the face-to-face interview segment was to gather first-hand accounts and observations of 

the activities and events inside the buildings on the morning of September 11, 2001. Using this approach, 

NIST identified previously unknown information, evaluated technical hypotheses, and explored conscious 

motivations for occupant behaviors, while allowing for comparisons to the telephone interview data. 

There was no recording of the face-to-face interviews, other than random selections for quality control 

purposes. It is estimated that the average face-to-face interview lasted approximately two hours. 

The methodology for the face-to-face interviews is a synthesis of the Behavioral Sequence Interview 

Technique (BSIT), originally developed by Keating and Loftus (Keating and Loftus 1984), and the 
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60 NIST NCSTAR 1-7, WTC Investigation

Cognitive Interviewing Method (CIM), originally developed by Fisher (Fisher et al. 2000) and Geiselman 

(Geiselman 1986). These two interviewing methodologies were developed with the purpose of assisting 

persons in retrieving more comprehensive and accurate memories of incidents and sharing important 

details. Both approaches begin by allowing the informant to retell an unimpeded account without 

interruption by the interviewer, and both initially employ a chronological retelling of information. 

However, BSIT was designed to yield a database of qualitative information that could be subjected to 

systematic analysis and consolidation, while CIM was designed to facilitate investigative interviews. 

Since the Investigation is pursuing both goals (i.e., creation of a database of evacuation-related behaviors 

and an investigatory capture of information relevant to outcomes), the methodology combines these two 

approaches. 

Cognitive interviewing has been the subject of many empirical investigations. Fisher et al. (Fisher, 

Brennan, and McCauley 2002) summarized these findings, demonstrating that the methodology 

significantly increases the amount of information recalled without affecting rate of errors. Interviewing a 

large number of informants will allow corroboration of information, thereby compensating for the likely 

increase in the absolute number of errors. Accordingly, it is likely that this approach will be productive in 

achieving a holistic view of the building evacuations. 

The face-to-face interview methodology involved occupants who may have observed (knowingly or 

unknowingly) events important to completion of the objectives of the investigation. 

Enumerating the population: The population included the entire occupant and building management 

population of World Trade Center WTC 1, 2, and 7. 

Selecting the sample: The face-to-face interview sample was developed by identifying: (a) individuals 

who identified themselves as being willing to share information relevant to the objectives of the NIST 

investigation, (b) individuals identified from the telephone interview sample as having experiences or 

observation requiring further exploration, and (c) the snowball quota sample approach. A snowball quota 

sample approach asks individuals for the names of other people who may meet the selection criteria for 

the study. The people identified are subsequently contacted and asked the same question. The process 

continues until the quota has been reached. 

Data Collection: The face-to-face interviews followed a four step technique, including unimpeded, openended narrative, a structured narrative, technical probes, and closed-ended questions. Each step is 

described more fully below. 

Step 1: Unimpeded open-ended narrative account: Both BSIT and CIM begin the process by asking 

the participant to chronologically recount his or her “story.” For the NIST investigation, the starting point 

was when it became apparent that something unusual had occurred on the morning of 

September 11, 2001. The ending point was when the participant felt that he or she reached a location 

where he or she felt safe (or, alternatively, when he or she successfully reached the exterior of the 

building). Researchers and practitioners involved with cognitive interviewing believe that starting the 

face-to-face interviews in this manner both improves recall and helps build rapport between the 

participant and the interviewer. Fisher et al. also noted that asking questions may interfere with recall 

because a participant must divide his or her mental resources between recall and listening to the 

interviewer’s questions (Fisher, Falkner, Trevisan, and McCauley 2000). During the open-ended 
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NIST NCSTAR 1-7, WTC Investigation 61

narrative account, the interviewer often records notable information that was used for the probing phase 

conducted later. 

Step 2: Structured narrative account: After participants completed their stories, interviewers prompted 

the respondents to go through the story again, but this time working cooperatively with the interviewer to 

record entries into a table. This approach was employed by BSIT for three primary reasons: (1) to yield a 

structured account that can be entered into a database without further processing; (2) to avoid the biasing 

effects of having interviewers ask specific questions; and, (3) to enhance the effort at recall put forward 

by participants by encouraging their active collaborative participation, an advantage to open-ended 

formats as noted by Fisher et al. (Fisher, Falkner, Trevisan, and McCauley 2000). 

Each row of the table represented a single action. The approach was used based on the hypothesis that 

people encode narrative memory in a manner consistent with this format, thus facilitating both recall and 

data entry. Each column of the table represented three essential components of actions: a cue, an action, 

and the reason for taking that action. Cues can be either external (e.g., signs of a fire, someone saying 

something) or internal (e.g., remembering another means of escape). Actions are expressed using specific 

action verbs (i.e., “ran” instead of “went”) and may include objects (e.g., a fire extinguisher) used by the 

informant. Reasons are the intentional, goal-directed base for the action. The interviewer used the 

participant’s own words to the greatest extent possible. Participants were asked to review the data for 

accuracy. 

Table 3–7 is an example of actions recorded in this manner. 

Table 3–7. Example tabular face-to-face interview data entry. 

Cue Action Reason 

I heard but couldn’t see 

someone yell “I’ve found a 

clear path” 

So I stumbled in the dark towards where 

I thought the voice came 

So that I could find a way to 

escape 

My path was blocked by debris So I called out to whoever yelled, “I’m 

near the reception area. Where are 

you?” 

To try to get a better idea about 

where the person was 

Experimental findings in psychological research on memory (Nillson 2000) suggest that when people 

perform actions, their abilities to verbally recall those actions are significantly improved. Script theory 

(Schank and Abelson 1977) suggests that people naturally organize their knowledge of actions using 

narrative sequences of actions structured around their pursuit of goals. However, in the case of 

WTC interviews, gaps in the narrative are anticipated, especially given the long period of time that has 

elapsed between the event and the interview. For the NIST investigation, however, interviewers 

encouraged participants to report only those memories about which they were confident really occurred to 

them. 

Step 3: Probing for specific information: After completing the structured narrative account, 

interviewers asked specific open-ended questions (probes) intended to elicit specific information of 

particular value to the investigation. While some of this information was likely to have been part of the 

structured narrative account, participants could also recall other valuable information as well in response 

to probes. 
Chapter 3 Draft for Public Comment 

62 NIST NCSTAR 1-7, WTC Investigation

Interviewers may use “context reinstatement” from CIM to improve recall of important information, 

because laboratory experiments have demonstrated that contextual cues enhance recall of related 

information. Fisher et al. explain that context reinstatement may enhance recall because people use 

multisensory coding of events. Using this mnemonic method, interviewers ask participants to “mentally 

recreate the external environment, and their affective, physiological, cognitive, and emotional states that 

existed at the time of original event” (Fisher, Falkner, Trevisan, and McCauley 2000). 

Depending on the population, probes were used to try to elicit information including, but not limited to: 

• Location of the informant at the time of certain marker events (e.g., location in WTC 1 when 

WTC 2 collapsed); 

• Fire conditions (e.g., fire and smoke); 

• Other cues of interest (e.g., the smell of jet fuel); 

• Presence and activities of persons with disabilities; 

• Use of elevators by self or others; and 

• Knowledge of any obstacles to their progress while using the stairs. 

Because information about many of these areas of concern required precise responses, questions for openended probes were developed collaboratively between the contractor and NIST. Responses to probes 

were recorded using standardized formats where feasible. For example, all participants who observed 

smoke were asked to estimate the smoke density qualitatively. 

Quality Control for Face-to-Face Interviews: With the respondent’s consent, some of the face-to-face 

interviews were audiotaped. The audio tapes were used for quality control of both data collection and 

interview quality. The audiotapes were periodically reviewed to ensure that interviewers precisely 

followed the protocol and conformed to administrative requirements. 

Interview Responses 

Over 200 face-to-face interviews were conducted with survivors from WTC 1 and 2; 131 interviews were 

performed with survivors of WTC 1, and 73 interviews with survivors from WTC 2, with the remainder 

from WTC 7 or with building personnel who moved between both WTC 1 and WTC 2. An attempt was 

made to interview occupants from each zone: low, middle, and high (at least 20 occupants were 

interviewed from each zone) of WTC 1 and WTC 2), as well as to interview occupants with unique 

experiences: occupants who witnessed fireballs, occupants with mobility impairments, occupants trapped 

in elevators, and occupants near or above the floors of impact. Occupants with special roles in the 

building, such as floor wardens and PANYNJ employees or contractors, were interviewed. Finally, six 

family members who spoke with occupants inside WTC 1 or WTC 2 after 8:46:30 a.m. on 

September 11, 2001 were interviewed. 
Draft for Public Comment First-Person Data Collection and Analysis Methods 

NIST NCSTAR 1-7, WTC Investigation 63

3.2.4 Focus Groups 

Williams reported that in a group setting, people provide cues that evoke memories in others and that 

social pressures mediate against reporting misrepresentations of what they recall (Williams 1990). The 

overall goal of the WTC focus group interviews was to elicit accurate group representations of specific 

events or themes. The six focus groups and the corresponding objectives were: 

• Occupants located near the floors of impact. The objective of conducting a focus group with 

people near the floors of impact was to obtain information on the extent of the building 

damage and how the damage influenced the evacuation process. 

• Floor wardens. The objective of the floor warden focus group was to explore the 

implementation of the floor warden procedures and the effect those actions had on the 

evacuation of the occupants on a floor and the evacuation of the floor warden themselves. 

• Mobility-challenged occupants. The objective of this focus group was to explore the effect of 

a disability on the evacuation of the occupant and any other individuals who may have 

assisted or otherwise been affected by the evacuee. 

• Persons with Building Responsibilities: The objective of the focus group with persons with 

building responsibilities was to capture the unique perspective of non-traditional occupants, 

including custodians, security, maintenance, or other building staff. 

• Random evacuees in WTC 1. The objective of the focus group with random evacuees in 

WTC 1 was to further explore the variables used in the causal modeling (i.e., those obtained 

from the hone interviews) that best explained evacuation delay and normalized stairwell 

evacuation time, including environmental cues, floor, and activities. 

• Random evacuees in WTC 2. The objective of the focus group with random evacuees in 

WTC 2 was to further explore the variables used in the causal modeling (i.e., those obtained 

from the hone interviews) that best explained evacuation delay, including environmental 

cues, floor, risk perception, and use of elevators. 

Sample selection: The people selected for inclusion in this study were selected using non-probability 

sampling procedures, i.e. snowball quota sampling (Blalock 1972; Cochrane 1977). Respondents 

contacted for face-to-face interviews or for other reasons were asked to provide the names and contact 

information for people they knew in each of the categories described above. Every effort was made to 

include approximately five people in each of these categories in the focus group study. 

Data collection: The data collected in the focus group study produced qualitative and detailed narrative 

accounts of the experiences of several groups of people, including those near the floors of impact and 

those having mobility challenges. The focus group discussion was moderated by a trained and 

experienced facilitator. Notes were taken in duplicate and compiled into a single summary of the focus 

group. 
Chapter 3 Draft for Public Comment 

64 NIST NCSTAR 1-7, WTC Investigation

3.2.5 Audio, Video, Photographic, and Records Collection 

Numerous emergency communications were recorded between 8:46:30 a.m. and 10:28:22 a.m., a period 

during which building occupants, WTC personnel, emergency responders, and people outside the 

WTC complex used radios and telephones to cope with the unfolding disaster. Emergency 9-1-1 call 

records, made available by the City of New York, were analyzed in both audio and transcript format. 

Communications at the WTC complex, transcripts of which were made publicly available by PANYNJ, 

were analyzed in both audio and transcript format. These communications included radio channels 

internal to the WTC complex, such as maintenance, vertical transportation, security, and PAPD. They 

also included communications external to the WTC complex, such as NYPD, EMS, PATH, and Newark 

airport. 

In stark contrast to the number of photographs and video taken of the outside of WTC 1, WTC 2, and 

WTC 7, very few video or photographic records from inside WTC 1, WTC 2, or WTC 7 survived. A 

significant number of records related to the design and maintenance of the egress and communications 

systems, however, were collected and analyzed. In particular, records regarding the identities of the 

occupants authorized to enter the WTC complex (referred to as the badge list), architectural drawings, 

tenant alteration applications, building upgrades, emergency plans and procedures, and training materials 

contributed to a better understanding of the egress system performance on September 11. 

Finally, complaints filed with the Occupational Safety and Health Administration (OSHA) were 

reviewed. The issues raised by complainants, a combination of surviving occupants and families of 

victims, guided the development of interview instruments and identified additional avenues of 

investigatory pursuit for emergency preparedness and evacuation system performance. 

3.3 ANALYSIS OF FIRST-PERSON DATA 

A systematic, comprehensive approach was required in order to consolidate the enormous volume of data 

collected or made available. Two primary techniques were utilized in order to capture the full range of 

the data collection: quantitative and qualitative analysis. Neither technique would have sufficed by itself, 

as the two techniques were highly complementary. In general, the quantitative data analysis was used 

with the telephone interview data, while the qualitative data analysis was used with face-to-face 

interviews, focus groups, and emergency communications. 

Quantitative Data Analysis 

The quantitative data analysis, based on to a telephone interview data set collected according to rigorous 

statistical standards, provided the ability to generalize findings and conclusions to the entire population of 

WTC 1 and WTC 2 survivors. The results of the 803 telephone interviews were archived and analyzed 

using SPSS 12.0.1,34 a statistical analysis software package. This package provided the ability to apply 

weights to the data, compute mean, median, mode, skewness, and other relevant statistical measures, 

recode data using expert judgment, and automatically produce reports of subsets. 

 

34 See Federal Government product disclaimer contained at the beginning of this report. 
Draft for Public Comment First-Person Data Collection and Analysis Methods 

NIST NCSTAR 1-7, WTC Investigation 65

Qualitative Data Analysis 

The purpose of the face-to-face interviews and focus groups was to explore the events of 

September 11, 2001 at WTC 1, WTC 2, and WTC 7 in an investigatory manner, allowing the individuals 

to communicate their experiences, observations, and thoughts outside the constraints of a closed-ended 

interview format. While a framework of data collection format and a time frame (the morning of 

September 11, 2001) was imposed, respondents had complete freedom to express their experiences in 

their own words. The primary value of the qualitative data analysis was two-fold: first, to collect 

information on the entire range of observations and experiences, and second, to enhance and provide a 

deeper understanding of topics explored and generalized in the telephone interviews. ATLAS.ti 4.134 was 

used to conduct qualitative data analysis. Each face-to-face interview and focus group was coded for over 

130 different types of information, resulting in several thousand individual codings. The codings, along 

with a brief explanation of each coding, are included in Appendix B at the end of this report. The 

resulting dataset was queried for targeted exploration of various factors of interest. 

Protection of Human Subjects 

To ensure the protection of the respondents and to comply with the Common Rule for the Protection of 

Human Subjects, the protocols and informed consent forms for the conduct of the telephone interviews, 

face-to-face interviews, and focus groups were reviewed and approved by an institutional review board 

and by NIST. NIST NCSTAR 1-7B summarizes the procedures used in first-person interviews. 

Confidentiality of Respondents 

NIST noted the importance of maintaining the confidentiality of the respondents. Contractors and 

retained experts were bound by contractual obligation to protect the confidentiality of all interview 

respondents, whether interviewed by telephone, face-to-face, or focus groups. No identifying information 

(name, gender, floor number, job title, etc.) has been included in this report. Individuals may have 

previously spoken to the media or other individuals about their experiences and observations, however, 

which could diminish NIST’s ability to protect an identity. Interviews conducted as part of Project 7 were 

distinct from interviews with emergency responders (NIST NCSTAR 1-8 2005), which were conducted in 

accordance with a separate agreement between the City of New York and NIST and between the 

PANYNJ and NIST. 
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66 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 67

Chapter 4

SEPTEMBER 11, 2001, BEFORE THE ATTACKS

4.1 BUILDING POPULATION ON SEPTEMBER 11, 2001 

The total number of people inside WTC 1 and WTC 2 on September 11, 2001, is not known precisely, but 

it is necessary to make a good estimate of that number in order to provide context for understanding the 

evacuation of the buildings. On a typical Tuesday at 8:46:30 a.m., some businesses within the 

WTC complex would be largely staffed and operational, with others mostly empty, owing to a later 

corporate start time. In addition, September 11, 2001, was both the first day of the new school year for 

many children, as well as the date of primary election in New York City. Finally, tourists were not yet 

traveling to the observation deck in WTC 2 as it had not yet opened. These factors, among others, may 

have acted to limit the number of people who were at the WTC complex on that morning. 

The total number of building occupants is equal to the sum of survivors and decedents. Section 9.2 

contains an analysis of likely decedent locations at the time or aircraft impact. The response rate analysis 

for interviews presented in Section 3.2.2 leads directly to a projection of the number of people present in 

WTC 1 (8,900 ± 750) and WTC 2 (8,540 ± 920) on September 11, 2001, at the time of the first airplane 

impact. Table 4–1 indicates that the populations of WTC 1 and WTC 2 were similar (within statistical 

uncertainty intervals) and that 17,580 ± 1,180 individuals were inside the towers at 8:46:30 a.m. These 

numbers do not include any airplane passengers or crew, emergency responders, or bystanders. The total 

population was rounded to reflect uncertainty in the projection and decedent analysis. 

Table 4–1. Occupancy estimates on September 11, 2001, by tower. 

 WTC 1 WTC 2 Total 

Estimated Total Population of 

Survivors 7,470 7,940 15,410 

Estimated Number of Occupant 

Decedentsa 1,462 – 1,533 630 – 701 2,146 – 2,163 

Estimated Total Building 

Population 8,960 8,600 17,560 

a. See Section 9.2 of this report for analysis of occupant decedent locations in WTC 1 and WTC 2. 

While Table 4–1 shows the estimated total population for WTC 1 and WTC 2, Table 4–2 shows the 

uncertainty calculations at different levels of statistical confidence.35 The uncertainty is directly related to 

the number of interviews: more interviews completed results in less uncertainty in a projection (i.e., the 

number of occupants). Thus, the projection for WTC 1 has less uncertainty than the projection for 

WTC 2 (427 interviews and 376 interviews, respectively). 

 

35 The standard error expressed in Table 4–2 assumes that the probability distribution is approximately normal (Gaussian). The 

standard error then defines the interval over which the actual population of each tower (as opposed to the estimated population) 

existed with an approximate level of confidence of 68 percent. The range of population expands as the uncertainty decreases, 

as shown by the 95 percent confidence (confidence limit at 5 percent) row in Table 4–2. 
Chapter 4 Draft for Public Comment 

68 NIST NCSTAR 1-7, WTC Investigation

Table 4–2. Occupancy estimate uncertainty. 

 WTC 1 WTC 2 Total 

Number of Telephone Interviews 427 376 803 

Standard Error (p) 1.90% 1.92% 1.36% 

Standard Error (Total) 750 920 1,180 

Confidence Limits at 5% ±1,470 ±1,790 ±2,320 

4.2 OCCUPANT CHARACTERISTICS 

The survey data indicate that occupants of the WTC towers were twice as likely to be male as female 

(65 percent male (n=284) for WTC 1 and 69 percent (n=250) for WTC 2). As shown in Table 4–3 below, 

the average age of the occupants was mid-forties, with ages ranging from early twenties to mid-seventies, 

although one interviewee indicated that she attended the 90th birthday celebration for a fellow 

WTC evacuee (not included in the NIST sample) in 2003. The vast majority of respondents (92 percent 

(n=739)) spoke English as their primary language. It should be noted that some telephone contacts ended 

with a language barrier and that no interviews were conducted in any language other than English. 

Table 4–3. Age for telephone survey respondents in WTC 1 and WTC 2.a

 WTC 1 WTC 2 

N Valid 439 361 

Refuse 1 2 

Mean 45 45 

Median 46 44 

Minimum 22 21 

Maximum 73 74 

a. Mean and Median values are weighted. N, Minimum, and Maximum are unweighted. 

Building occupants become familiar over time with a building, including the location of the emergency 

egress components and emergency procedures and protocols. The median residence time of the overall 

occupant population could thus be a predictor of how likely it was that an individual received training if 

the training was conducted every six months. 

Tenant and employee turnover at the WTC was not uncommon. Figure 4–1 shows the reported start dates 

for respondents in WTC 1 and WTC 2. In WTC 1, 4 percent (n=18) of the occupants had worked in the 

building since 1975. Further, 25 percent (n=110) had been working in the building prior to the 1993 

bombing, although only 16 percent (n=64) of the WTC 1 respondents were present on February 26, 1993. 

For WTC 1, 67 percent (n=287) of the occupants had started working in the building in the last four years 

(1998 – 2001). The mean residence time in WTC 1 was over 5.6 years, while the median was 2 years 

(half the respondents had been there two years or less, while half the respondents had been there longer 

than two years). 

Occupant tenure in WTC 2 showed a similar trend. While only one respondent had worked in the 

building since 1975, 25 percent (n=91) of the respondents had been working in the building prior to the 
Draft for Public Comment September 11, 2001, Before the Attacks 

NIST NCSTAR 1-7, WTC Investigation 69

1993 bombing (with 16 percent (n=59) present on the day of the bombing). Another 51 percent (n=185) 

started working in the building at some point in the four years prior to the 2001 terrorist attack. The mean 

residence time in WTC 2 (n=360) was 5.9 years, while the median was 3 years. 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR STARTED AT WTC 1

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

Percent

197519761978197919801981198219831984198519861987198819891990199119921993199419951996199719981999

20002001

YEAR STARTED AT WTC 2

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

Percent

Note: Percentages are weighted. 

Figure 4–1. Employment start date at WTC. 

Overall, 7 percent (n=56) had a formal responsibility or special knowledge about the building. These 

respondents included fire safety staff, floor wardens, searchers, building maintenance, and security staff. 

Approximately 13 percent (n=105) of the respondents were employed by the PANYNJ, which may not 

necessarily imply a special knowledge of the building, as some PANYNJ employees had job duties 

related to functions outside the WTC. 

Some 6 percent (n=52) reported having a limitation that constrained their ability to evacuate. Both 

WTC 1 and WTC 2 had roughly the same fraction of the population who reported a mobility challenge 

(n=26 in each building). Six percent of the population of WTC 1 and WTC 2 corresponds to roughly 

1,000 people. The reported limitations included chronic illness, recent surgery or injury, obesity, elderly, 

heart condition, pregnancy, asthma, and other. Of these conditions, the most prevalent (n=20 of 52) was 

recent injury (in particular, severe knee and ankle injuries), followed closely (n=16 of 52) by reports of a 

chronic illness (such as cancer, leukemia, arthritis), or use of medications which hindered full mobility or 

cognitive ability. Four telephone interview respondents (of 52) reported being pregnant or having asthma, 

while three (of 52) reported having asthma-like conditions or indicated that age played a role in their 

ability to navigate the egress system. One person reported having a heart condition, while no telephone 

interviewee reported being blind, deaf, or requiring the use of a wheelchair. While the last three mobility 

challenges were not captured in the 803 telephone interviews, face-to-face interviews and published 

media accounts did provide information on people with these disabilities. A small number of respondents 

reported more than one mobility challenge. 
Chapter 4 Draft for Public Comment 

70 NIST NCSTAR 1-7, WTC Investigation

4.3 PREVIOUS EXPERIENCE 

Of the WTC 1 occupants present on September 11, 2001, 16 percent (n=64) were also present during the 

1993 bombing. In WTC 1, 60 percent (n=38) of evacuees who evacuated both in February 1993 and in 

September 2001 reported that they evacuated immediately in 1993, while 30 percent (n=20) reported that 

they waited to evacuate in 1993, and 9 percent (n=6) did not recall. Most (95 percent (n=53)) who were 

able to recall their 1993 evacuation decision felt that they made the right decision, while 5 percent (n=3) 

did not believe they made the right decision. 

Similarly, 16 percent (n=59) of WTC 2 survivors on September 11, 2001, also evacuated in 1993. In 

WTC 2, however, only 75 percent (n=42) felt that they made the right decision in 1993 (compared to 

95 percent in WTC 1), possibly due to the fact that many more waited to evacuate in 1993 in WTC 2 

(69 percent (n=39)) than did so in WTC 1. Only 31 percent (n=17) of those who reported their decision 

evacuated immediately from WTC 2 in 1993. It should be noted that the bomb had a more significant 

impact upon WTC 1 in 1993. 

4.4 OCCUPANT PREPAREDNESS 

Consistent with the NYC Building Code (Local Law 5 §C19-162.2.a.4) and Occupational Safety and 

Health Administration (OSHA) regulations (29 CFR 1910.38, “Employee Emergency Plans and Fire 

Protection Plans”),36 the tenants of the World Trade Center were required by the Port Authority to 

conduct regular fire drills. Further, they were required to appoint employee floor wardens and searchers. 

Overall, 66 percent (n=529) of WTC 1 and WTC 2 occupants reported participation in at least one fire 

drill in the twelve months immediately prior to September 11, 2001, and a significant proportion of 

occupants had taken part in two or more drills during that time. However, 17 percent (n=139) reported 

that they had not participated in any fire drills in the 12 months prior to September 11, 2001, and 

17 percent (n=135) did not know. Fire drill participation rates were similar between the two towers, as 

shown in Table 4–4. 

Table 4–4. WTC fire drills in 12 months prior to September 11, 2001.a 

Number of Drills WTC 1 WTC 2 

None 18 % (n=78) 17 % (n=61) 

1 13 % (n=57) 8 % (n=29) 

2 21 % (n=90) 24 % (n=88) 

3 11 % (n=47) 15 % (n=53) 

4 10 % (n=44) 9 % (n=32) 

5 – 11 7 % (n=31) 9 % (n=32) 

12 or More 3 % (n=13) 4 % (n=13) 

Don’t Know 18 % (n=80) 15 % (n=55) 

a. Percentages are weighted, n values unweighted.

 

36 While the NYC Building Code required fire drills every six months, OSHA regulations require fire drills at least annually. The 

PANYNJ required fire drills for all tenants every six months. 
Draft for Public Comment September 11, 2001, Before the Attacks 

NIST NCSTAR 1-7, WTC Investigation 71

One of the goals of fire drill training was to make occupants aware of the location of the emergency exits. 

Of respondents who reported participation in a fire drill, 93 percent (n=490) indicated that they had been 

instructed about the location of the nearest stairwell. However, of the respondents who reported being 

shown a stairwell, 82 percent (n=432) did not enter or use the stairwell during the fire drill. Some 

17 percent (n=92) reported that they did use the stairs during a drill, while approximately 1 percent (n=5) 

reported not knowing whether they had used the stairs. Overall, more than half (51 percent (n=415)) of 

the occupants had never used a stairwell in WTC 1 or WTC 2 prior to September 11, while 48 percent 

(n=386) had used a stairwell. Two persons reported not knowing whether they had used the stairs 

previously. It should be noted, however, that Local Law 5, adopted in 1973, prohibits occupants from 

being required by building management or employers to enter or use a stairwell during a fire drill (1973). 

This prohibition may reflect social concerns regarding liability of required stairwell use by occupants and 

the economic costs of decreased employee productivity. In contrast, the City of Chicago, Illinois, 

(reacting in 2002 to the collapse of WTC) now requires twice-yearly fire drills, which “may conclude 

when all participating occupants have fully entered and have begun using designated stairwells.” The 

City of Los Angeles, CA (Sixth Edition, 2002) in §57.33.19.C, entitled “Emergency Planning and 

Evacuation Requirements for High-rise Buildings,” requires that a “minimum of one fire drill annually on 

individual floors is mandatory. Total building evacuation is not required.” 

Another goal of the fire drills was to introduce the floor warden system and evacuation procedures. Most 

occupants (82 percent (n=528)) with fire drill training were aware that there was a floor warden for their 

floor. Approximately 70 percent (n=557) of all occupants reported that they were aware of the evacuation 

procedures. When asked what those evacuation procedures comprised, however, answers varied 

significantly, including: wait in hallway for further instructions; do not use elevators, use stairs; meet at a 

designated site outside the building for a head count; or proceed down (varied number of) flights of stairs 

and wait. 

In general, most (66 percent (n=212) in WTC 1 and 60 percent (n=167) in WTC 2) survivors who 

received fire drill training, reported that they found the training to be somewhat or very helpful to their 

evacuation experience on the morning of September 11, 2001. 
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Chapter 5

SEPTEMBER 11, 2001, 8:46:30 A.M. 

FLIGHT 11 CRASHES INTO WTC 1 

“I heard the roar of the plane, looked out of the window [and] saw the 

plane halfway in the building. I jumped up and ran out into the hallway 

and screamed 'Everybody get out!'“ Interview 1000749 (NIST 2004) 

5.1 INTRODUCTION 

Figure 5–1. WTC 1 impact, 8:46 a.m. 

At 8:46:30 a.m. on Tuesday, September 11, 2001, American Airlines Flight 11, a hijacked Boeing 767, hit 

the north face of WTC 1, as shown in Figure 5–1. This impact resulted in a direct hit on seven floors, 

from 93 – 99, with additional damage extending several floors above and below the direct impact area. 

As Flight 11 crashed into the North Tower, all access to safety for those at or above the impact was 

destroyed, including both elevators and stairwells. The fate of over 1,300 occupants located above the 

91st floor of WTC 1 was sealed at that instant. This chapter focuses on occupant reaction during the 

initial moments after aircraft impact. 

As the aircraft struck the building, jet fuel on board ignited. Part of this fuel immediately burned off in 

large fireballs that erupted at the impact floors. Remaining fuel flowed across the floors and down 

elevator and utility shafts, igniting intense fires throughout portions of the buildings. A fireball killed or 

injured several occupants in the Concourse Level lobby. Despite the massive localized damage caused by 

the impact, as shown in the computer simulation in Figure 5–2 and Figure 5–3, the structure initially 

remained standing. 
Chapter 5 Draft for Public Comment 

74 NIST NCSTAR 1-7, WTC Investigation

Figure 5–2. Computer simulated impact damage to WTC 1 on floor 95 at 0.7 s after 

impact with stairwells superimposed. 
Draft for Public Comment September 11, 2001, 8:46:30 a.m., Flight 11 Crashes Into WTC 1 

NIST NCSTAR 1-7, WTC Investigation 75

Figure 5–3. Calculated damage to floors 93 through 98 in WTC 1. 

5.2 INITIAL OBSERVATIONS IN WTC 1 

The New York City 9-1-1 call center was quickly besieged with calls, primarily from citizens outside 

WTC 1, reporting that the World Trade Center was on fire. Some callers reported observing a 

commercial airliner intentionally hitting the building, and a few quickly concluded it was an act of 

terrorism. Other callers reported missiles and bombs, while some called 9-1-1 seeking to find out what 

was going on, possibly preventing or delaying people in danger from accessing the 9-1-1 system. For 

example, occupants from an upper floor of WTC 1, finding they were unable to connect to 9-1-1, called a 

business colleague outside the building, who in turn, was able to contact 9-1-1 and report their plight.37

The plane strike was immediately obvious to occupants of WTC 1 throughout the building. Even below 

the floors of direct airplane impact, building occupants knew that a significant event had occurred, and 

many witnessed significant fire, smoke, or building damage. The majority of survivors in WTC 1 felt the 

building move from the airplane impact (63 percent, n=277). Table 5–1 shows a summary of how the 

survivors became aware that something was wrong. The reported percentages were roughly constant 

throughout the building. For example, 60 percent (n=157) in the upper third of the building (floors 78 

and above) felt the building move, 62 percent (n=86) in the middle third (floors 44 to 77) felt the building 

move, while 64 percent (n=34) of the respondents in the lower third (up to floor 43) felt the building 

 

37 New York City 9-1-1 Emergency Call Recordings, 2001. 

Floor 94 Floor 95

Floor 96 Floor 97 Floor 98

Floor 93 Floors 93 to 98 

Cumulative 

Damage 

Severe Floor Damage Fireproofing 

(SFRM) 

and partitions 

Floor system 

structural damage 

Floor system 

removed 
Chapter 5 Draft for Public Comment 

76 NIST NCSTAR 1-7, WTC Investigation

move. For some, this first cue was extreme; for others, less so. Still, it was clearly a significant event for 

all. For example: 

A survivor from a floor in the 70s felt tremendous movement in the 

building: “There were large vibrations, the building shuddering, the floor 

shaking violently. The initial explosion was a large cracking sound, and 

then boom. The building swayed heavily to the South the first time, and 

then a couple other times with decreasing severity.” Interview 1000103 

(NIST 2004). 

Forty floors below the impact, the effect was immediate and significant 

as a survivor from a floor in the 50s notes: “There were creaking noises 

in the closet. I walked out into the hall and stood there. There was no one 

else in the hallway. I heard whooshing noises in the closet. The door 

blew open from the closet door, causing my chair to hit the desk” 

Interview 1000054 (NIST 2004). 

Thoughts of terrorism sprang quickly to the minds of many, particularly 

those who had survived the 1993 Bombing: “I felt the building sway. I 

knew it was really bad. My co-worker said, ‘They did it again.’” Focus 

Group #4 (NIST 2004) 

Even occupants low in the building knew something major had occurred. 

A survivor from a floor in the 20s reported: “We felt the impact. The 

building swayed about seven times. Debris was falling down on the 

street. We gathered our belongings, I shut off the computer and headed 

towards the stairwell” Interview 1000559 (NIST 2004). 

Table 5–1. How survivors in WTC 1 became aware something was wrong on 

September 11, 2001. 

 Percent (n=440) 

Felt something (building moving, impact, shaking, swaying) 63 % 

Heard something (boom, crash, explosion, blast, roar, rumbling) 30 % 

Other, including saw a plane, smelled jet fuel, fell down/fell off chair, 

warned by someone 

7 % 

Source: NIST WTC Telephone Survey Data. 

Even at ground level, awareness of the incident was immediate. One survivor who was in the Concourse 

of the building recounts this experience: 

“I was walking through the mall toward Tower One to get to the elevator. 

The lights flickered. I stopped in my tracks and looked around. I saw a 

brown cloud coming down the center corridor in the lobby, and I feared 

for my safety. The brown cloud had a heavy density and reached from 

floor to ceiling. It looked to me like it was both smoke and debris. It first 

came from the center corridor, but by the time it reached the revolving 

doors (a split second later) it seemed to come from every direction. At 

this point, the revolving doors exploded. They seemed to vaporize.” 

Interview 1000046 (NIST 2004) 
Draft for Public Comment September 11, 2001, 8:46:30 a.m., Flight 11 Crashes Into WTC 1 

NIST NCSTAR 1-7, WTC Investigation 77

Like the Concourse Level, elevator lobbies throughout the building were particularly affected,38 likely by 

excess jet fuel ignited by the crash pouring down the elevator shafts.39 While only 3 percent (n=11) of the 

survivors reported seeing fireballs in their immediate area at the time of the airplane impact, the 

observations from the face-to-face interviews show the extreme nature of these events: 

A survivor from a floor in the 80s: “The entire corridor became an 

inferno outside our front door. Smoke began to enter our office. There 

was also debris falling. ... The fire on the corridor was at least 10 ft high, 

and it ran the ... good length of the corridor. Then I saw a fireball come 

down the elevator shaft and blew the elevator doors. The fireball came 

right at me; it was a really bright color.” Interview 1000055 (NIST 2004) 

A survivor from a floor in the 40s: “I saw the elevator in front me had 

flames coming out from it. The elevator was closed but the flames came 

from the front where the doors meet and on the sides. They reached 

about a foot and a half, with the flames standing from the floor to the 

ceiling. I saw a chandelier shaking; it was really moving. The corridor 

was dim. I also heard people screaming from the [nearby] floor. I felt the 

heat on my face and I thought that my eyebrows were going to get 

burned. Black smoke starting filling the corridor, it got really dense 

really fast.” Interview 1000109 (NIST 2004) 

A survivor in the basement: “I saw a big bright orange color coming 

through the basement with the smoke ... A fire ball came shooting out of 

the basement door.” Interview 100760 (NIST 2004) 

The elevator lobbies were not the only areas of the building damaged at the time of the airplane impact. 

Survivors noticed a range of damage and conditions throughout the building, from lost power to fire and 

smoke, to missing walls and floors. Table 5–2 shows observations at the time of first awareness. While 

some of the observations involve less severe phenomena (fallen ceiling tiles or flickering lights), others 

are more extreme, including collapsed walls, fire, and smoke. 

Observation of building damage during this initial awareness period were not as consistent over the height 

of the tower as the indications of the airplane impact. Table 5–2 presents a summary of observations 

reported during the telephone survey. While damage was more severe near the floors of impact, some 

damage was also evident at different locations lower in the building. Survivors provide a range of 

observations: 

A survivor from a floor in the 90s of WTC 1, just below the impact, 

recounts the severe damage on the floor: “In the hallway (from the 

bathroom to the elevator), there were no walls left (the wall board was 

blown off) and the bathroom seemed to be missing (the walls and the 

floor). There was a hole in the wall near the elevator (in the hall) and fire 

was coming up onto our floor through that hole.” Interview 1000052 

(NIST 2004) 

 

38 The majority of face-to-face interview respondents who observed fire inside the building, observed flames at or near the 

elevator shafts (NIST Interviews 2004). 39 See NCSTAR 1-5 for further information about the consumption of jet fuel in WTC 1 and WTC 2. 
Chapter 5 Draft for Public Comment 

78 NIST NCSTAR 1-7, WTC Investigation

A survivor from a floor in the 70s in WTC 1: “To me everything seemed 

normal, all the ceilings were fine, the electricity was fine, and the air 

conditioning was also working.” Interview 1000118 (NIST 2004) 

A survivor from a floor in the 20s in WTC 1: “I was close to the 

windows. The windows were broken and I saw things from the office 

were going out the window.” Interview 1000064 (NIST 2004) 

Table 5–2. Observations of building damage in WTC 1 when occupants first became 

aware something was wrong on September 11, 2001. 

 Percent (n=440) 

Fallen ceiling tiles 17 % 

Power outage / flickering lights 17 % 

Smoke 10 % 

Jet fuel 8 % 

Fire alarms 8 % 

Collapsed walls 6 % 

Other events, including fire, fireballs, 

injured people, fire sprinklers going off, 

extreme heat, debris 

45 % 

Note: Total does not add up to 100 percent because respondents may have observed more than one event 

indicative of damage. 

Source: NIST WTC Telephone Survey Data. 

While a significant event, not all occupants felt their lives were in danger initially. Of the survivors in 

WTC 1, 41 percent felt their life was at risk, and 48 percent felt others lives were at risk, at first 

awareness. Only 4 percent of the survivors reported being injured by the attack initially, while only 

6 percent reported others being injured. 

Most of the survivors were with other occupants when the event occurred. One of every eleven survivors 

reported being alone at the time. Sixty-one percent were in a group of 10 persons or less, although two 

respondents reported being in a group as large as 400. The average reported group size was 23 persons, 

while the median group size was 7 persons. This suggests that a few reports of very large groups 

(33 respondents [8 percent] reported being a group of greater than 100 people) skewed the average. 

During the initial moments after WTC 1 was attacked, occupants above the 91st floor were trapped. A 

few occupants below floor 92 but near the impact region were alive, although trapped as well, some in 

their offices, others in elevators. Elevators were rendered inoperable. Occupants observed smoke, 

fireballs, damaged walls, fallen ceiling tiles, and injured colleagues on many floors throughout WTC 1. 
NIST NCSTAR 1-7, WTC Investigation 79

Chapter 6

SEPTEMBER 11, 2001, 8:47 A.M. – 9:02 A.M.

OCCUPANTS REACT TO THE ATTACK ON WTC 1 

“We gathered the group together to figure out what we should do. One 

exit was filled with smoke and it was dark; we tried to shut the door to 

keep out the smoke. The other exit was of no use, [so] we regrouped and 

went toward the main exit (towards the elevator). When the smoke was 

building up inside the office, I was more inclined to possibly break open 

the window and get some fresh air and wait for help; one or two of the 

other people insisted that we start evacuating [recognizing] that we were 

in very serious trouble.” Interview 1000137 (NIST 2004) 

Between 8:47 a.m. and 9:02 a.m., the time period after World Trade Center (WTC) 1 was attacked but 

before WTC 2 was attacked, one of the nation’s largest building evacuations and emergency responses 

began to unfold. Occupants of WTC 1, aware that something significant was happening in their building, 

were assessing their situation, performing necessary duties, and actively seeking a way out of the 

building. With no operational elevators available to the occupants, the three stairwells began to fill, not 

only with occupants exiting the building, but also with emergency responders entering the building. Near 

and above the floors of aircraft impact, the fire and smoke continued to spread, threatening the lives of the 

trapped occupants. Some occupants fell or chose to jump from the building. Building fire safety staff 

coordinated the response with incoming personnel from the Fire Department of New York, New York 

Police Department, Port Authority Police Department (PAPD), emergency managers from the City of 

New York, and Federal agents. At 8:47 a.m., an evacuation order for WTC 1 was broadcast over the 

vertical transportation channel (Z). The first radio communication regarding evacuation of WTC 1 and 

WTC 2 was issued by a senior PAPD officer at 8:59 a.m. Two minutes later, at 9:01 a.m., an instruction 

was issued to evacuate all WTC complex building (PANYNJ 2001a). NIST NCSTAR 1-8 contains a 

detailed chronology of all radio communications with respect to evacuation. 

Analysis of face-to-face interviews showed that many occupants of WTC 2, if afforded a view to the 

north or west, often chose to see for themselves what was happening to WTC 1. Once the significance of 

the event was verified, they were forced to decide, first, whether to evacuate or stay in place, and second, 

whether to evacuate using the stairs or the elevators. Building fire safety staff were deciding whether to 

order an evacuation of the occupants of WTC 2, taking into account the safety of WTC 2 occupants and 

what effect an evacuation of WTC 2 may have had on the evacuees from WTC 1 and on the incoming 

emergency responders (NIST 2004b). 

6.1 CONDITIONS WORSEN IN WTC 1 

Within minutes of the aircraft impact, occupants above the 91st floor began to assemble in groups of 

various sizes, often taking refuge in offices with access to windows. They also began to reach out for 

assistance, calling 9-1-1, family and friends, or colleagues. Electricity and phone service in the region 
Chapter 6 Draft for Public Comment 

80 NIST NCSTAR 1-7, WTC Investigation

directly impacted by the airplane was apparently disrupted.40 Electricity and phone service to floors 

above the impact floors was maintained, however, as evidenced by a number of 9-1-1 calls from the 100th 

to 106th floors during this time period. 

Fire and smoke raced upward. As early as 8:50 a.m., occupants on the 106th floors reported worsening 

smoke conditions for about 100 people on that floor, some of whom took refuge in a back office.41 

Somewhat later, at 9:00 a.m., WTC Ch. 9 (PAPD Police Desk) received a call from a Windows on the 

World manager, who reported that floor 107 was “way too smoky” and most people had retreated to 

floor 106 (PANYNJ 2001a). 9-1-1 received reports of hazardous smoke conditions on 103, 104, 105, 

and 106 within ten minutes of aircraft impact. Each of those four floors reported having more than 

100 people trapped on the floor. For some, the conditions remained at least partially tenable during this 

time period. Others jumped or fell out of the building within minutes of the aircraft impact. Along with 

falling building debris, this created a hazardous situation for emergency responders, evacuees, and 

bystanders. 

Conditions for occupants below the 91st floor were deteriorating as well. More than one-third of the 

survivors reported seeing smoke after the initial airplane impact but before they left their initial floor to 

begin their evacuation, up from only 10 percent at first awareness. Eighteen percent of the survivors 

encountered the smell of jet fuel, up from 8 percent at first awareness. Table 6–1 shows a summary of 

these observations compared to those at first awareness. Observations of nearly all conditions increased as 

time progressed. 

Table 6–1. Observations of conditions in WTC 1 before beginning evacuation. 

 At Awareness During Interim Period 

Smoke 10 % 35 % 

Jet fuel 8 % 18 % 

Fallen ceiling tiles 17 % 21 % 

Power outage / flickering lights 17 % 17 % 

Fire Alarms 8 % 14 % 

Collapsed walls 6 % 10 % 

Fire 3 % 5 % 

Other events 45 % 48 % 

Note: Total does not add up to 100 percent because respondents may have observed more than one event indicative of 

damage. 

Source: NIST WTC Telephone Survey Data. 

In WTC 1, a number of people below the impact zone were trapped on their floors, unable to either leave 

their offices or reach the stairwells. Prior to 9:02:59 a.m., trapped occupants requested assistance by 

calling 9-1-1 from an elevator, from scattered floors in the 10s, 20s, 30s, 40s, 60s, and from most floors in 

the 80s.42 Among those requesting assistance, heavy smoke, wall damage, and occupant injuries were 

 

40 New York City 911 Emergency Call Recordings, 2001. 

41 New York City 911 Emergency Call Recordings, 2001. 

42 911 Emergency Call Records, City of New York, 2001. 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 81

common reports. Figure 6–1 shows graphically the distribution of observed conditions in WTC 1 after 

initial awareness, but before beginning evacuation. The information to develop Figure 6–1 was compiled 

from every source available to the NIST investigation, including interviews, published accounts, 

transcripts of emergency communication channels, and emergency 9-1-1 calls. Note that the “?” denotes 

a floor where there was no information found to record the absence or presence of observations. Further, 

the absence of an observation on any floor does not positively exclude the presence of that condition as it 

may simply not have been reported. 

Smoke

Sprinklers / water

Fatally injured people

Power outage

Jet fuelFallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heat

Fire

Fireballs

–– 110

???????????

*

???????????

–– 100

???????????

???????????

???????????

*

*

–– 90

–– 80

Smoke

Sprinklers / water

Fatally injured people

Power outage

Jet fuelFallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heat

Fire

Fireballs

–– 70

–– 60

–– 50

SmokeSprinklers / water

Fatally injured people

Power outageJet fuel

Fallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heat

FireFireballs

–– 40

–– 30

–– 20

???????????

–– 10

???????????

???????????

???????????

???????????

???????????

SmokeSprinklers / water

Fatally injured people

Power outageJet fuel

Fallen ceiling tiles

Fire alarms

Collapsed wallsExtreme heat

FireFireballs

–– 110

???????????

*

???????????

–– 100

???????????

???????????

???????????

*

*

–– 90

–– 80

SmokeSprinklers / water

Fatally injured peoplePower outage

Jet fuel

Fallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heatFireFireballs

–– 70

–– 60

–– 50

SmokeSprinklers / water

Fatally injured people

Power outageJet fuel

Fallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heat

FireFireballs

–– 40

–– 30

–– 20

???????????

–– 10

???????????

???????????

???????????

???????????

???????????

 

Figure 6–1. Observations of building damage after initial awareness but before 

beginning evacuation in WTC 1. 

6.1.1 Activities and Information 

Occupants of WTC 1 engaged in a variety of activities prior to leaving their floor and beginning their 

evacuation, including exchange of information, gathering personal items, helping or searching for others, 

and fire fighting. Table 6–2 summarizes activities reported in the telephone survey. 

Many respondents used the time prior to beginning their evacuation to gather information about the event 

or to call family members. Others helped injured coworkers: 

A survivor from a floor in the 90s: “I heard a sound that sounded like a 

giant aluminum can being crushed and I felt the building tilt. I tried 

calling my company’s home office but the line for long distance calls 

was not in service. I called home to test the phones and to let my family 

know that I was okay. I checked to see if our server was still up. I saw a 

man bleeding. I got a first aid kit and succeeded in halting the man’s 

bleeding. We saw debris and smoke and decided it was time to get out. I 
Chapter 6 Draft for Public Comment 

82 NIST NCSTAR 1-7, WTC Investigation

got my briefcase, a fire extinguisher, and four diet sodas, exited into the 

hallway and went towards stairwell C.” Interview 1000052 (NIST 2004) 

A survivor from a floor in the 30s: “I used a telephone in the trading 

room to call my wife. I wanted to see if she had seen anything on the 

news and could tell me what was wrong. I called my wife within 

4 minutes of the impact of our building--and I got through okay. She 

wasn’t there and I left a message. Because we saw the place of impact 

and fire coming out of the windows above us in our building, a 

co worker and I got our personal belongings and headed calmly to the 

stairwell.” Interview 1000042 (NIST 2004) 

Table 6–2. Activities prior to evacuation reported in telephone survey 

by survivors from WTC 1. 

Activities Before Evacuation Percent Reporting the Activity (n=440) 

Talked to others 70 % 

Gathered personal items 46 % 

Helped others 30 % 

Searched for others 23 % 

Talked on telephone 16 % 

Moved between floors 8 % 

Fought fire or smoke 6 % 

Shut down computers 6 % 

Continued working 3 % 

Other activities 25 % 

Note: Total does not add up to 100 percent because respondents may have taken multiple actions. 

Source: NIST WTC Telephone Survey Data. 

Some occupants, on the other hand, started their evacuation almost immediately: 

An occupant from a floor in the 60s in WTC 1: “It felt like the building 

was going to fall over. I grabbed my bag to leave the office floor. I was 

not waiting for anyone to tell me what to do.” Interview 1000122 

(NIST 2004) 

An occupant from a floor in the 20s in WTC 1: “I waited for building to 

stop shifting. I began to run straight out the nearest exit out of my office 

towards Stairway B. It was the nearest exit from my office and 

co-workers were just saying let’s go this way.” Interview 1000064 

(NIST 2004) 

While only 11 percent of the telephone survey respondents reported being given additional information 

about the event during this interim period without consciously seeking the information, 28 percent 

reported seeking such information. Table 6–3 shows the types of information received and sought by 

occupants. The majority of occupants received or sought information about the event, with a smaller 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 83

number looking for information on whether to evacuate or remain in the office. For those who sought 

additional information, 43 percent were unable to find the information they were seeking. 

Table 6–3. Information received and sought prior to beginning evacuation in WTC 1. 

 Information Given (n=50) Information Sought (n=122) 

Information about what had happened 57 % 81 % 

Instructions to leave 28 % 17 % 

Instructions to stay 17 % 12 % 

Other, including information about what to 

do and to receive assistance in evacuation, 

don’t know 

13 % 13 % 

Note: Total does not add up to 100 percent because respondents may have observed more than one event indicative of damage. 

Source: NIST WTC telephone survey data. 

Occupants tried to obtain information through a variety of means –face-to-face conversation; telephone, 

television, or radio; e-mail or handheld devices; and from building announcements: 

A survivor from a floor in the 70s: “I walked to my desk and spoke on 

the phone to find out what happened. I went on the Internet and I was 

informed of what happened; also through telephone conversations. I 

thought it was necessary to look around. I walked around the floor with 

the fire warden; I also stopped, looked, and took some pictures. I was 

there in 93, and I wanted to wait for some directions from someone, 

through the speaker system, fire alarms, etc.” Interview 1000576 

(NIST 2004) 

A survivor from a floor in the 60s: “During the time in which I was 

circulating in the northeast side of the floor, I heard a secretary say, 

‘Aren't we supposed to wait for an announcement?’ I saw other workers 

who were standing there talking and trying to assess what to do next. I 

went into the hall located between the stairwells A & C. People within 

the group helped each other make the correct decisions for evacuation.” 

Interview 1000639 (NIST 2004) 

Twenty-seven percent of survivors felt they began their evacuation before the people around them. Not 

surprisingly, survivors’ perceptions of risk to themselves and to others increased as conditions in the 

buildings worsened. Table 6–4 shows a comparison of survivors’ perception of risk at the point of 

airplane impact and in the interim period before they left their floor to begin their evacuation. 

Table 6–4. Survivors perception of risk to self and others after airplane impact and 

prior to entering stairwell in WTC 1. 

 Others Self 

At awareness 48 % 41 % 

Interim 63 % 52 % 

Source: NIST WTC Telephone Survey Data. 
Chapter 6 Draft for Public Comment 

84 NIST NCSTAR 1-7, WTC Investigation

Although occupants felt at a heightened risk during this time, many occupants helped others before 

beginning their own evacuation; 20 percent of the survivors reported being helped by someone; while 

30 percent reported helping others. Table 6–5 summarizes the responses of the survivors who received 

assistance. 

Table 6–5. Sources of help used by occupants prior to beginning their evacuation 

in WTC 1. 

 Percent (n=87) 

Co-worker 48 % 

Police officer / firefighter 16 % 

Floor warden 12 % 

Manager / supervisor 13 % 

Other / don’t know 13 % 

Stranger 8 % 

Note: Total does not add up to 100 percent because respondents may have received help from more than one person. 

Source: NIST WTC Telephone Survey Data. 

Requests for guidance or assistance were not uncommon. At 8:48 a.m., a mere two minutes after the 

plane hit WTC 1, an occupant from the 78th floor called the Port Authority Police desk requesting 

guidance: 

PAPD: Port Authority Police... 

Male: Yes, uh, we're on the 78th floor, at Hyundai Securities. Do we need to evacuate or 

not? 

PAPD: Right away. 

Male: Right now? 

PAPD: Right away. 

Male: Okay, which stairs do we take? 

PAPD: Uh, whichever is the easiest one nearest without too much smoke and everything. 

Try to get the best way down. 

Male: Well, there's...the hallways are full...full of smoke. 

PAPD: Okay. If you could find your way down one... 

Male: Okay, get out right now, right? 

PAPD: Right. Right, exactly. 

Male: Okay, bye. (PANYNJ 2001a) 

The telephone interviews revealed a variety of reasons which caused occupants decide to evacuate their 

floor. Table 6–6 summarizes the single predominant reason given by occupants for beginning evacuation 

in WTC 1. 

Some left due to observations of building damage or movement, others felt in danger, and still others left 

because co-workers left or told them to leave. Less than 1 percent said they left because they heard a fire 

alarm: 

A survivor from a floor in the 80s: “My boss told me that a plane came 

into the building. I was at a cubical with no window view. I was 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 85

screaming and crying, my boss came over to my location. We only had 

one door to enter or exit the office. The door was blocked with debris. 

We saw the ceiling caving in, but I don't recall any smoke or fire at this 

point. We began to dig out pieces of ceiling debris to open the door to 

exit.” Interview 1000722 (NIST 2004) 

A survivor from a floor in the 60s: “The big boss, the treasurer, he stated 

“Get out now”. I grabbed my pocketbook and started walking towards 

the stairwell.” Interview 1000834 (NIST 2004) 

A survivor from a floor in the 20s: “I saw the floors in the hallway were 

twisted. The burning stuff outside the window was getting heavier and I 

decided that maybe I should look for a stairwell.” Interview 1001667 

(NIST 2004) 

Table 6–6. Single reason given by survivors to begin their evacuation in WTC 1. 

 Percent (n=440) 

Building movement 20 % 

Afraid / felt in danger 20 % 

Was told to evacuate 14 % 

Friends / co-workers evacuated 9 % 

Saw debris 6 % 

Saw smoke 5 % 

Other, including saw fire, 1993 experience, saw / heard plane, people 

panicking, fire alarm going off 

26 % 

Source: NIST WTC Telephone Survey Data. 

6.1.2 Emergency Response at the Fire Command Station, Lobby, WTC 1 

Within minutes of the initial impact, personnel from the Port Authority, building security, FDNY, NYPD, 

FBI, Secret Service, and representatives of Silverstein Properties (principal leaseholder) were assembling 

at the Fire Command Station in the lobby of WTC 1, as shown in Figure 6–2. The deputy fire safety 

director (a contractor from O’Conner Security) after assisting a woman injured by glass in the lobby, 

quickly began to receive and log calls from floor wardens on floors above the mid-rise area, including 

floors above the impact area. As the first calls came in, announcements were made to the affected floors, 

indicating, in general terms, ‘We have received an alarm downstairs and the alarm is being investigated. 

Please stand by.’ Any information from the floor wardens about the condition of the floor or injuries was 

passed to the Fire Department personnel nearby. As multiple floors were reporting incidents, the deputy 

FSD took down the floor numbers on a pad and paper and awaited the arrival of a supervisor. Within ten 

minutes, it was determined that the attack was a multiple-floor event. Therefore, consistent with 

emergency procedures, building-wide public address system announcements were made informing 

occupants to evacuate the building using the stairs and not the elevators. Initially, the evacuation script 
Chapter 6 Draft for Public Comment 

86 NIST NCSTAR 1-7, WTC Investigation

was used, but later the 

deputy FSD simply 

told people over the 

public address system 

not to use the 

elevators because they 

were crashing and to 

use the stairs to leave 

the building (Other 

Interview 03 

[NIST 2004b]). 

Unfortunately, the 

individual was 

unaware of the 

condition of the 22nd 

floor, where critical 

communications 

hardware in the hidden security command center lay in ruins, likely preventing any building-wide public 

address announcements from reaching the occupants (PANYNJ 2001a; NIST 2004b). After the fact, a 

person familiar with the operation of the building suggested that the fire alarm closet on floor 22 

destroyed the riser. NIST NCSTAR 1-4 has a more complete analysis of the fire alarm and public address 

system in WTC 1 and WTC 2. 

The damage on floor 22 was also reported by several emergency responders (NIST 2004) and was noted 

several times in the NIST analysis of the published accounts (NIST NCSTAR 1-7A). NIST 

NCSTAR 1-8 (Chapter 4.4) contains additional information regarding the status of the 22nd floor 

command desk, as well as information flow between occupants, 9-1-1, and emergency responders. 

6.1.3 Survivors Begin Their Evacuation 

Most, but not all building occupants began their evacuation of the WTC 1 before the WTC 2 was hit. 

Ninety-one percent of the survivors in WTC 1 reported beginning their evacuation before Flight 175 

struck WTC 2. At this point, nearly all observations of types of building damage had become more 

widespread than those at first awareness. Survivors recalled a variety of conditions on the floors as they 

left for the stairwells, ranging from significant damage to damage insufficient to deter the occupant from 

completing a routine task: 

From a floor in the 70s in WTC 1: “As I was leaving it didn't seem as 

bad as I thought in the office and I decided that I would just walk all the 

way down and reassess the situation and go back to the office if things 

were ok.” Interview 1000129 (NIST 2004) 

From a floor in the 60s in WTC 1: “There was smoke and smell of jet 

fuel coming from the stairwell. I covered my nose/mouth with tissue. 

This smoke wasn't a lot; not to the extent that [you] could choke. The 

smoke was coming from the vents/corners of the stairwell.” Interview 

1000036 (NIST 2004) 

Figure 6–2. Fire Command Station in WTC 1 on September 11, 2001. 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 87

From a floor in the 60s in WTC 1: “The first time I faxed my documents, 

they did not go through. I felt safe because I watched previous 

documentaries and I was informed that I was safe in my building. I went 

back to the fax machine.” Interview 1000733 (NIST 2004) 

From a floor in the 50s in WTC 1: “I heard a chunk of ceiling fall and a 

woman screamed. We all stood and looked at each other and we tried to 

figure out what happened. We heard the cable snap in the freight elevator 

while we were talking, and the woman yelled “follow my voice”. I 

followed the woman's voice to find where the stairwell was at to get out.” 

Interview 1000054 (NIST 2004) 

From a floor in the 30s in WTC 1: “It [the stairwell] was the closest one 

to our office. I opened the doorway to the staircase... There was a lot of 

smoke and there was no one in it. I quickly closed the door.” The 

occupant went to another stairwell down the hall to leave the floor. 

1000009 (NIST 2004) 

For consistency in evacuation measurement, time to begin evacuation was defined as the interval from 

first awareness to the time the respondent left his or her floor to begin evacuation.43 On average, 

survivors in WTC 1 began their evacuation within 6 min. However, it is important to note that the 

statistical distribution of time to initiate evacuation was skewed in the direction of longer delays. In other 

words, while the most frequent response for survivors in all three zones in WTC 1 was one minute or less 

(referred to as the mode in statistics), and 50 percent of occupants had left their floor within 3 – 5 minutes 

(depending on zone), a few individuals took significantly longer (sometimes longer than 30 minutes) to 

start evacuating, thus disproportionately affecting the mean time to start evacuation. 

Table 6–7 summarizes the quartile, mode, and average times to start evacuation for survivors in WTC 1. 

Note that Table 6–7 separates the occupants into lower, middle, and upper floors based upon the location 

of the mechanical floors, which roughly divided the building into thirds. The reported times from the 

lower floors were not different than the reported times in the middle floors (tail probability from a logtransformed t-test comparing the two zones was 0.81). The upper floor evacuation initiation delay times, 

however, were statistically significant different when compared to both the middle and lower zones 

(significant at approximately the 99 percent confidence level). Thus, occupants nearer the impact area in 

WTC 1 delayed their evacuation for a longer period of time than occupants of the other two zones. This 

could have been due to the increased frequency of fire, smoke, building damage, and injured occupants on 

the upper floors, although that is only one explanation. A further discussion of evacuation initiation delay 

time and comparisons across regions of WTC 1 and WTC 2 is contained in Section 10.1. 

 

43 The time to begin evacuation was defined as the time while on the floor of origin due to the fact that, while many people 

decided to leave quickly, they often chose to perform several activities prior to actually entering the stairwell. Thus, time prior 

to entering a stairwell (or elevator) was a better measure of evacuation delay than a moment when the occupant ‘decided’ to 

evacuate, which may have been significantly prior to actually starting evacuation. 
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88 NIST NCSTAR 1-7, WTC Investigation

Table 6–7. Elapsed time (min) to initiate evacuation for survivors from WTC 1. 

Time for Survivors to 

Initiate Evacuationa

25% 

Initiation 

50% 

Initiation 

75% 

Initiation 

Mode of 

Responses 

Average Time 

(min) 

Lower floors 

(Basement – 42) 

1 3 5 1 5.7 

Middle floors (43-76) 1 3 5 1 4.8 

Upper floors (77-91) 2 5 10 1 7.4 

a. Time to begin evacuation is the time interval from first awareness to the time the respondent left their floor to begin evacuation. 

Source: NIST WTC Telephone Survey Data. 

As shown in Table 6–8, survivors below the 92nd floor typically reported choosing the stairwell closest to 

them at the time. All three stairwells below the impact region were in use throughout the evacuation. 

Some found an appropriate stairwell quickly: 

From a floor in the 80s in WTC 1: “The hallway was free of debris and 

well-lit. We (my boss, and co-workers, about ten to fifteen) went back to 

staircase C and proceeded to evacuate. The door on the staircase was not 

damaged. We entered staircase C. The staircase was well lit and fairly 

empty.” Interview 1000108 (NIST 2004) 

From a floor in the 60s in WTC 1: “The building shook and I thought 

something tremendous had struck the building. I looked out the building 

to see what had happened. The fire alarm went off. I went to the stairwell 

(the exit) to evacuate the floor.” Interview 1000025 (NIST 2004) 

From a floor in the 50s in WTC 1: “It [the stairwell] was closest, and we 

had been trained in emergencies to only use the stairwells, never the 

elevators. We (four of us from my immediate office) exited down 

stairwell B.” Interview 1000106 (NIST 2004) 

For others, finding an appropriate stairwell for evacuation was not always a straightforward process, as a 

survivor from a floor in the 30s recounts: 

“I opened the doorway to the staircase. There was a lot of smoke and 

there was no one in it. I quickly closed the door to not bring smoke into 

the floor. The group of people that was with me (about 10 people) started 

running back to the office. I began running after my coworkers and 

yelling at them to come back to find a different staircase. I was trying to 

do the right thing, and they were doing the wrong thing based on the fire 

drill training we had. The coworkers weren’t listening so I let them go 

their own way and I went by myself back out to the hallway to find a 

different staircase. I walked down the north-south hallway back past the 

original stairwell (the one with the smoke in it) and made a right down 

the other hallway because I wanted to go with the crowd. There was a lot 

of traffic, so it took a little longer. After a couple of minutes, I went into 

the stairwell.” Interview 1000009 (NIST 2004) 
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NIST NCSTAR 1-7, WTC Investigation 89

Table 6–8 shows the percentage of occupants that chose each stairwell for evacuation, as well as their 

primary reason for selecting a stairwell. 

Table 6–8. Stairwell chosen for evacuation in WTC 1. 

Stairwell Used for Evacuation Percent 

Stairwell A 17 % 

Stairwell B 25 % 

Stairwell C 19 % 

Stairwell A or C 10 % 

Don’t know 17 % 

Other, not applicable, used elevator 12 % 

Reason for Choosing Stairwell 

Closest one 66 % 

Followed others 17 % 

Was told to use 12 % 

Other exits blocked 6 % 

Other, including don’t know, used before, best 

conditions, not applicable 

18 % 

Note: Total does not add up to 100 percent because respondents may have given more than one reason for 

choosing their stairwell. 

Source: NIST WTC Telephone Survey Data. 

6.2 INITIAL OBSERVATIONS AND REACTIONS FROM WTC 2 

Many of the occupants of WTC 2 quickly became aware that something significant had happened in 

WTC 1. Table 6–9 shows how survivors in WTC 2 became aware that something was wrong on 

September 11, 2001. Most occupants in WTC 2 heard, saw, or felt the event in WTC 1 (81 percent of the 

363 respondents). Others were made aware after a short time by coworkers, telephone, or the news media 

coverage. 

Table 6–9. How survivors in WTC 2 became aware that something was wrong 

on September 11, 2001. 

 Percent (n=363) 

Heard something (boom, crash, explosion, blast, roar, rumbling) 51 % 

Saw something (smoke or flames, plane, debris) 19 % 

Warned by someone around me 13 % 

Felt something (building moving, impact, shaking, swaying) 11 % 

Other, including contacted via phone, lights flickered, news media 7 % 

Source: NIST WTC Telephone Survey Data. 
Chapter 6 Draft for Public Comment 

90 NIST NCSTAR 1-7, WTC Investigation

For occupants near windows which faced north and west throughout WTC 2, what they saw made it 

instantly clear that the damage to WTC 1 was severe: 

A survivor from a floor in the 90s in WTC 2: “I heard a large noise that 

sounded like muffled dynamite and looked out the window which faces 

the East - Brooklyn Bridge or into Queens to see if the noise was 

connected to anything outside. I saw glittering paper which made no 

sense and thought it was part of a promotional event, as if some one was 

throwing confetti out of a plane. I headed in the direction of the noise 

and saw a gigantic red fire ball at the cubical diagonally from my desk 

and smelled gasoline, which I later learned was jet fuel. I went back to 

my office to call my [spouse] to inform [him/her] that I was OK, and that 

I was leaving the building.” Interview 1000001 (NIST 2004) 

A survivor from a floor in the 90s in WTC 2: “I felt a wave of heat; very 

high temperature. I went to the window on my floor to find out what was 

happening. I saw the flames/fire outside through the window (in Building 

One, just about the same floor as mine) and I saw everybody going to the 

manager. The manager instructed that, ‘people without special 

responsibilities should evacuate the building.’” Interview 1000632 

(NIST 2004) 

A survivor from a floor in the teens of WTC 2: “I heard a loud roaring 

sound. I thought it was a window washer falling off its tracks. I walked 

into my office and looked out of the window. I saw people running in the 

plaza, away from building one. I saw paper and dust floating down from 

the sky. I saw the fireball coming from Building One. I thought it was an 

explosion of Windows on the World. However, I realized that there were 

still several floors above the explosion. Seeing the explosion and the 

panic in the plaza triggered my evacuation decision.” Interview 1000922 

(NIST 2004) 

For some in WTC 2, however, the event was not as obvious. 

A survivor from the 100s in WTC 2: “A co-worker came to my office 

and said “There's a fire in the first building, we recommend that you 

leave.” I grabbed my bag and packed up my belongings in order to leave 

the office. I grabbed my belongings for the reason in which I thought I 

was going to go to the gym & then would return to the office.” Interview 

1000767 (NIST 2004) 

Although aware of the event, some occupants of WTC 2 were unsure of appropriate action to take since 

the event at this time was limited to WTC 1. 

A survivor from a floor in the 50s in WTC 2: “I heard a loud, horrendous 

explosion. I turned and faced the source of the noise. I saw debris flying 

through the air outside the window. I saw large objects flying by, then 

reams of paper - some that were burning - like confetti. I looked up at 

One World Trade Center and saw a gaping hole and smoke. I called my 

wife and parents to reassure them, to tell them I was okay. Co-workers 

and I discussed what our course of action should be. We stood kind of 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 91

paralyzed, undecided as to what we should do. The Bloomberg headline 

said a plane had hit WTC 1. We didn't know if we should close the desk 

and leave; if we should assume the day was over. We were looking for 

guidance.” Interview 1000557 (NIST 2004) 

An occupant from WTC 2 at 8:49 a.m. also sought guidance from the Port Authority Police: 

PAPD: Port Authority Police... 

[Caller]: Yeah, this is [Caller], Securities Department from Morgan Stanley. 

PAPD: Uh-huh? 

[Caller]: Uh, what's the status right now as far as (overlap) 

PAPD: We're still checking. Everybody just get out of the building right now. 

[Caller]: All right. Have you guys announced an evacuation of Two? 

PAPD: We are trying to do that right now. 

[Caller]: All right, thank you. 

PAPD: All right? We are just advising everybody to get out of the building. 

[Caller]: All right, thank you, bye-bye. (PANYNJ 2001a) 

At 8:53 a.m., an occupant from WTC 2 called the Port Authority Police Department seeking advice. 

Male: Hi, um, I'm on the 95th floor of Two World Trade Center. 

PAPD: Yeah, just come on down anyway, sir. 

Male: Does that mean walk down the stairs? 

PAPD: You’d, be advised, right now at this time. 

Male: Should we evacuate all of our people? 

PAPD: Yes. Yes. Yes. Everybody. 

Male: Okay, thank you. (PANYNJ 2001a) 

6.2.1 Activities and Information – WTC 2 

Occupants of WTC 2 engaged in a variety of activities prior to leaving their floors and beginning their 

evacuation, including exchange of information, gathering personal items, helping or searching for others, 

and continuing work activity. Table 6–10 summarizes activities reported in the telephone survey. 

A survivor from a floor in the 60s of WTC 2 engaged in a variety of activities before beginning her 

evacuation. 

“I observed that the secretary was very upset. I went with [the secretary] 

and my manager to the lobby area to help care for [the secretary]. People 

were leaving the building and I didn’t know when I’d be returning to the 

building, so I gathered my stuff and I went to the [rest] room. I saw 

flying things in the air and everyone was in a commotion to see what was 

going on, so I went back to the windows in the northern part of the 

building to find out what was going on. I heard the announcement . . . 

and I went back to my desk. [There] I made two phone calls to my 

mother and my [spouse] to reassure them that I was not affected and that 

I was safe. Two of my co-workers came into my office area and stated 

that they saw people jumping from Building 1 and that we should leave. 
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92 NIST NCSTAR 1-7, WTC Investigation

I gathered my stuff again and prepared to leave.” Interview 1000877 

(NIST 2004) 

Table 6–10. Activities prior to evacuation reported in telephone 

surveys by survivors from WTC 2. 

Activities Before Evacuation Percent Reporting the Activity (n=363) 

Talked to others 75 % 

Gathered personal items 57 % 

Helped others 34 % 

Searched for others 32 % 

Talked on telephone 16 % 

Moved between floors 8 % 

Shut down computers 7 % 

Continued working 6 % 

Fought fire or smoke 1 % 

Other activities 20 % 

Note: Total does not add up to 100 percent because respondents may have observed more than one 

event indicative of damage. 

Source: NIST WTC Telephone Survey Data. 

Another occupant from a floor in the 100s in WTC 2, who had only worked in the WTC for a couple of 

months, took relatively few actions before beginning evacuation. 

“I heard a female co-worker who had a window cubicle shouting ‘Get 

out!’ I turned around from my inner office, grabbed my purse, and 

walked out my office door. I ran to another co-worker and asked her 

what was going on. [The co-worker] didn’t know and continued 

walking. I decided to follow the co-worker to [figure out] where to go 

and to find out what was going on. I followed [the co-worker] to a 

stairwell and began to go down the stairs.” Interview 1000897 

(NIST 2004) 

While 21 percent of the WTC 2 telephone survey respondents reported being given additional information 

about the event during this interim period without actively seeking the information, 18 percent reported 

actively seeking such information. Table 6–11 shows the types of information received and sought by 

WTC 2 occupants. The majority of occupants received or sought information about the event, with 

smaller number looking for information on whether to evacuate or remain in the office. For those who 

sought additional information, 39 percent were unable to find the information they were seeking. 

Before WTC 2 was hit, information about the event affected occupants of WTC 2 in different ways. Even 

if an occupant heard that something happened to WTC 1, he or she may have still felt safe in the building. 

For instance, an occupant beginning evacuation from a floor in the 50s in WTC 2 took to the stairs early, 

only to return to his/her desk. 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 93

“[After a few floors], I thought to myself, ‘Why am I running?’ I 

remembered that I was the fire warden for my floor.” The occupant then 

took an elevator back to his/her original floor “to see if anyone else was 

there. I thought about the training and what I was supposed to do.” 

After following the training procedure of calling security, the occupant 

“picked up the cell phone and called our [out-of-town] office to let them 

know that we were evacuating the building. I got ahold of them and they 

told me that a plane hit the building and to get out of there.” Interview 

1001666 (NIST 2004) 

Table 6–11. Information received and sought prior to beginning evacuation in WTC 2. 

 Information Given (n=77) Information Sought (n=64) 

Information about what had happened 65 % 92 % 

Instructions to leave 26 % 17 % 

Instructions to stay 35 % 13 % 

Other, including information to remain 

calm or a choice to evacuate or stay. 

10 % 5 % 

Note: Total does not add up to 100 percent because respondents may have received or sought information from more than one 

source. 

Source: NIST WTC Telephone Survey Data. 

The occupant then proceeded to look around the floor for other occupants as well as make a phone call 

home to a family member. The occupant took an additional phone call when WTC 2 was hit, which 

prompted an immediate evacuation. Interview 1001666 (NIST 2004) 

However, another fire warden from a floor in the 40s of WTC 2 used observations from and media 

information about WTC 1 to begin evacuation immediately. 

After seeing paper flying outside the window and smelling gas, “I ran 

across the hall and came across the boss and immediately informed [the 

boss] that we should leave. I saw the TV and saw what had happened on 

CNN and was informed that a plane hit the building, Tower 1. I went to 

the stairs.” Interview 1000867 (NIST 2004) 

Twenty-four percent of survivors felt they began their evacuation before the people around them. Not 

surprisingly, survivors perception of risk to themselves and to others increased as conditions in the 

buildings worsened. Table 6–12 shows a comparison of survivors’ perception of risk at the point of 

airplane impact and in the interim period before they left their floor to begin their evacuation. 

Table 6–12. Survivors’ perception of risk upon airplane impact and 

prior to beginning their evacuation in WTC 2. 

 Others Self 

At awareness 52 % 29 % 

Interim 67 % 42 % 

Source: NIST WTC Telephone Survey Data. 
Chapter 6 Draft for Public Comment 

94 NIST NCSTAR 1-7, WTC Investigation

Although occupants felt a heightened risk during this time, many occupants helped others before 

beginning their own evacuation. Seventeen percent of the WTC 2 survivors reported being helped by 

someone and 34 percent reported helping others. Table 6–13 summarizes the responses of the WTC 2 

survivors. 

Table 6–13. How survivors received help prior to 

beginning their evacuation from WTC 2. 

 Percent (n=60) 

Co-worker 56 % 

Manager / supervisor 15 % 

Floor warden 12 % 

Other / don’t know 11 % 

Police officer / firefighter 7 % 

Stranger 5 % 

Source: NIST WTC Telephone Survey Data. 

Even though the environmental cues were more obvious in WTC 1 than they were in WTC 2, 86 percent 

of the survivors in WTC 2 began their evacuation before their building was hit. Occupants gave a variety 

of reasons for beginning their evacuation, which are summarized in Table 6–14. 

Table 6–14. Single reason given by survivors to begin their 

evacuation from WTC 2. 

 Percent (n=363) 

WTC 1 observations 26 % 

Was told to evacuate 21 % 

Afraid / felt in danger 17 % 

Friends / co-workers evacuated 11 % 

1993 Experience (thought was a bomb) 6 % 

WTC 2 building hit 6 % 

Other, including jet fuel, information seek, 

evacuating the building just felt like the right 

thing to do 

7 % 

Source: NIST WTC Telephone Survey Data. 

Eighty-six percent to 91 percent of the occupants in WTC 2 began their evacuation before WTC 2 was 

hit.44 Approximately one in four WTC 2 occupants began evacuation because of their observations of 

WTC 1 from their office windows, while a similar percentage (21 percent) were told to evacuate. 

 

44 86 percent was calculated by summing the number of people who reported starting their evacuation in less than 16 minutes, 

whereas 91 percent of people reported starting their evacuation prior to WTC 2 being attacked. These two questions were 

asked independently of one another during the telephone interviews. Note that while the discrepancy may be partially 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 95

An occupant from a floor in the 90s in WTC 2 left because of 

observations of WTC 1: “[I] saw a gigantic red fire ball at the cubicle 

diagonal from my desk and smelled gasoline which I later learned was jet 

fuel. I went back to my office to call my [spouse] to tell [the spouse] that 

I was OK and that I was leaving the building.” Interview 1000001 

(NIST 2004) 

Another occupant from a floor in the 30s in WTC 2 was told by company 

management that the occupant “should begin a self-evacuation, instead 

of waiting for the building management to tell us to evacuate. I went 

back to my desk to prepare to leave. I decided to follow the group that 

was heading for the stairwell.” Interview 1000049 (NIST 2004) 

An occupant from a floor in the 90s in WTC 2 “heard a noise that 

prompted me to get up and look out the window. I saw people out of the 

corner of my eye, grabbing their bags and leaving. I turned and got my 

bag, with my pocketbook and things to leave my cubicle and follow the 

people [to the staircase]. I didn’t have to investigate; I just left [because] 

I saw other people leaving.” Interview 1000070 (NIST 2004) 

6.2.2 Survivors Begin Their Evacuation 

On average, survivors in WTC 2 began their evacuation within 6 min. However, as with WTC 1, the 

statistical distribution of time to initiate evacuation was skewed toward longer preparation periods. 

Overall, the most frequent response for survivors WTC 2 was one minute or less (although the middle 

floors had a mode of 5 min), and 50 percent of occupants had left their floor within 3 – 5 minutes 

(depending on zone), a few individuals took significantly longer (more than 30 minutes) to start 

evacuating, thus disproportionately affecting the average time to start evacuation. Table 6–15 

summarizes the quartile, mode, and average times to start evacuation for survivors in WTC 2. Note that 

Table 6–15 separates the occupants into lower, middle, and upper floors, based upon the location of the 

mechanical floors which roughly divided the building into thirds. The evacuation delay results may be 

biased for the upper floors in WTC 2, however, as only occupants who acted quickly to move below the 

78th floor before 9:02:59 a.m. could be interviewed. In other words, those who delayed for whatever 

reason, with few exceptions, did not survive. The impact of this potential bias was not quantified, but 

should be noted. A further discussion of evacuation initiation delay time and comparisons across regions 

of WTC 1 and WTC 2 is contained in Section 10.1. 

Table 6–15. Elapsed time (min) to initiate evacuation for survivors from WTC 2. 

Time for Survivors to 

Enter Stairwella

25% 

Initiation 

50% 

Initiation 

75% 

Initiation 

Mode of 

Responses 

Average Time 

(min) 

Lower floors 

(Basement – 42) 

1 4 5 1 6.3 

Middle floors (43-76) 2 5 10 5 7.1 

Upper floors (77-110) 1b 3 b 5 b 1 b 4.2 b

a. Time to begin evacuation is the time interval from first awareness to the time the respondent left their floor to begin evacuation. 

b. The evacuation delay results may contain significant bias for the upper floors in WTC 2, as discussed in the text. 

Source: NIST WTC Telephone Survey Data. 

 

attributable to the uncertainty in time recollection by the occupants, the uncertainty intervals for the two percentages overlap at 

the 95 percent confidence level (plus or minus 5 percentage points). 
Chapter 6 Draft for Public Comment 

96 NIST NCSTAR 1-7, WTC Investigation

Occupants typically chose the stairwell closest to them at the time. Below the impact region, all three 

stairwells were in use throughout the evacuation. Table 6–16 shows the percentage of occupants each 

stairwell for evacuation and why that stairwell was chosen. 

Table 6–16. Stairwell chosen for evacuation in WTC 2. 

Stairwell Used for Evacuation Percent 

Stairwell A 18 % 

Stairwell B 18 % 

Stairwell C 14 % 

Stairwell A or C 10 % 

Don’t know 14 % 

Other, including closest to the office, not applicable, 

used elevator 

27 % 

Reason for Choosing Stairwell 

Closest one 63 % 

Followed others 20 % 

Was told to use 10 % 

Other exits blocked 4 % 

Other, including don’t know, used before, best 

conditions, not applicable 

13 % 

Note: Total does not add up to 100 percent because respondents may have given more than one reason for 

choosing their stairwell. 

Source: NIST WTC Telephone Survey Data. 

An occupant in the 80s of WTC 2 notes that colleagues “ran down the stairs in the South part of the office 

to evacuate the building. The staircase that I chose was the only one that I was aware of.” Interview 

1000003 (NIST 2004) 

Another occupant in the 60s moved to the stairwell that was already being used. 

“I saw a person opening the door to a stairwell to the left of the reception 

area, near the men’s room. We walked really fast to enter the stairwell to 

make our way down.” Interview 1000526 (NIST 2004) 

An occupant in the 100s of WTC 2 was aware that he/she was assigned to use Stairwell B. 

“Due to the fire drill training, we were always told to use Stairwell B. I 

began going down Stairwell B in order to avoid elevators and to get out 

of the building safely.” Interview 1000906 (NIST 2004) 

6.2.3 Elevator Use in WTC 2 

Elevator usage by occupants played a significant role in reducing the total loss of life in WTC 2 on 

September 11, 2001. Sixteen percent of the occupants of WTC 2 used elevators for at least part of their 
Draft for Public Comment September 11, 2001, 8:47 a.m. – 9:02 a.m., Occupants React to the Attack on WTC 1 

NIST NCSTAR 1-7, WTC Investigation 97

egress from the building; another 2 percent used elevators in the basement levels. Occupants higher in the 

building were somewhat more likely to use elevators than occupants lower in the building: subdividing 

the 16 percent who used elevators, 4 percent were from floors below the 44th floor skylobby, 5 percent 

from floors 46 to 73, and 7 percent from floors above the upper skylobby on the 78th floor. Expressed 

another way, 12 percent of survivors in the lower floors reported that they used an elevator, while 

15 percent of survivors from the middle floors reported using an elevator, and 33 percent of survivors 

from the upper floors reported that they used an elevator to evacuate WTC 2. Interpretation of the latter 

percentage may be biased, however, as occupants who used stairwells or delayed evacuation may have 

been disproportionately trapped above the impact region after 9:02:59 a.m. when compared to occupants 

who initially chose to evacuate using elevators over stairwells. 

For some occupants, the physical challenge of navigating the stairwells was daunting enough to 

precipitate rapid use of the elevator system. 

An occupant from the 90s: “I think perhaps a survival instinct prompted 

my decision. I grabbed my briefcase and walked fast toward the 

elevator. I saw someone who needed to use crutches just as the elevator 

door was closing on the floor. We opened the door for him manually; 

the ‘open door’ button never functioned properly. I was taking a new 

medication and knew I should not walk down the stairs, so I took the 

elevator to the 78th floor and then took another elevator to the lobby.” 

Interview 1000553 (NIST 2004) 

For others, physical challenges presented themselves while in the stairwells. Even before WTC 2 was 

attacked, Port Authority communications (Channel 27) indicate that an occupant required assistance in the 

stairwells at 9:00 a.m. (PANYNJ 2001a) Others were able to switch egress modes from the stairwells to 

the elevators, although not without some difficulty, as one occupant from the 40s demonstrated: 

“My leg was hurting (it was a pre-existing condition) and I had 

bronchitis. The security guard wouldn't let me go down the express 

elevator, [so] this man agreed to come to my aid. [We] started our 

descent on the steps and continued to the 40th floor where he knew of 

another elevator bank. The man who was helping me announced to the 

people around us that he was helping somebody (me) and that we would 

stop at each landing and get out of their way so that they could pass us 

easily. We exited the staircase on the 40th floor and found and entered an 

elevator. Two women joined us because they thought we knew what we 

were doing. We rode the elevator from the 40th floor to the lobby. It 

made two stops before expressing to the lobby.” Interview 1000048 

(NIST 2004) 

Some occupants were successful in using an elevator to get to the lobby, but were turned around by 

building security, presumably in an effort to minimize the impact WTC 2 occupants would have posed on 

the evacuation of WTC 1 occupants: 

“I was in an elevator going up to my floor when Building One was hit. 

When I got to my floor, we took an elevator back down to lobby, but the 

guard sent the elevator full of people back up to the floor.” Focus 

Group #3 (NIST 2004) 
Chapter 6 Draft for Public Comment 

98 NIST NCSTAR 1-7, WTC Investigation

6.2.4 Announcements in WTC 2 

At 8:58 a.m., WTC Channel 15 (NYC EMS Direct Line) recorded a statement from an unknown source, 

stating, “I want to start a building evacuation, Building One and Building Two.” (PANYNJ 2001a). The 

timing was proximate with announcements in WTC 2, made from the lobby fire command station, first 

contradicting the Channel 15 statement, then supporting it. 

At 9:00 a.m., approximately three minutes before the second attack, the first building-wide public address 

system announcement was made to the occupants of WTC 2. Synthesizing the content of the 

announcement as reported by many interview respondents indicates that the general announcement went 

as follows: 

There is a fire condition in WTC 1. WTC 2 is secure. Please return to 

your offices. 

While some occupants interpreted the announcement as less of an instruction and more of a suggestion 

that it was safe to return to their offices, others interpreted the announcement as an instruction to remain 

where they were or to return to their offices. No respondent reported that the announcement told them to 

evacuate. 

Approximately two minutes later, at 9:02 a.m., one minute before WTC 2 was attacked, a follow-up 

announcement was made, contradicting the previous announcement. Recorded in the background of an 

answering machine recording an occupant calling home from a floor in the 90s and subsequently provided 

to the interviewer during the interview, the announcement indicated that it was now permissible to initiate 

an evacuation: 

“May I have your attention please. The situation is in Building 1. 

However, if conditions on your floor warrant, you may wish to start an 

orderly evacuation.” Interview 3000001 (NIST 2004) 

The 9:02 a.m. announcement was also noted by an occupant catalogued in the collection of published 

accounts (Fahy and Proulx 2003). A survivor in the 70s of WTC 2 was still on his or her floor when the 

9:02 a.m. announcement was given over the PA system. 

“I stayed on the floor [to] wait until everyone was cleared out. Most of 

the people on my floor evacuated using the elevators, which they took to 

the skylobby. It seemed to me [that] I would be in more danger by going 

out. I heard the phone ringing off the hook and my boss and I were 

going from phone to phone answering them to reassure the families that 

everyone was ok.” Interview 1000524 (NIST 2004). 

It was then that United Airlines Flight 175 crashed into WTC 2. 
NIST NCSTAR 1-7, WTC Investigation 99

Chapter 7

SEPTEMBER 11, 2001, 9:02:59 A.M. 

FLIGHT 175 CRASHES INTO WTC 2 

“On the phone, [the occupant from a floor in the 90s in WTC 2] kept 

describing the people jumping out of WTC 1. I could not hear any noise 

in the background; it was quiet with some slight conversations going on. 

When the second plane hit WTC 2, I then heard people screaming, “No, 

no. Oh my God, no!” Screaming and then silence. People's sounds 

disappeared. I could hear ‘whopping noises.’ The sounds were similar 

to wind blowing and I could hear the fire alarms going off. Then, 

nothing else. Just silence.” Interview 300003 (NIST 2004)

At 9:02:59 a.m., a second hijacked Boeing 767-200ER, United Airlines Flight 175, struck WTC 2, 

damaging nine floors, from 78 to 84, as shown in Figure 7–1. By the time WTC 2 was hit, 21 percent of 

the eventual survivors had already exited WTC 1, 

and 41 percent of the survivors had already exited 

WTC 2. With the second attack, occupants in 

WTC 2, like those from WTC 1, began a fullscale building evacuation. 

Alarm tones and public address announcements 

were overheard in 9-1-1 audio records from 

various floors from floors 82 to 105 after the 

impact, indicating that some power was available 

to most of the building. 

As when WTC 1 was struck, a significant surge 

in 9-1-1 calls occurred when WTC 2 was struck, 

as bystanders and people watching television 

called to notify authorities that they witnessed the 

event. Many witnesses immediately reported that 

a deliberate terrorist attack was underway. Once 

again, the WTC 2 attack was described to 

operators in a variety of ways, from a large 

military aircraft to a bomb, to a commercial 

aircraft.45 

 

45 New York City 911 Emergency Call Recordings, 2001. 

Figure 7–1. WTC 2 damage (computer 

simulated) at 0.62 s after impact with 

stairwells superimposed. 
Chapter 7 Draft for Public Comment 

100 NIST NCSTAR 1-7, WTC Investigation

In WTC 1, many occupants only had passing knowledge of the second plane strike from cell phone or 

electronic communications received by other occupants within the stairwells during their egress: 

A survivor from the 50s in WTC 1: “Very soon after entering the 

stairwell, someone with a Blackberry communication device received 

word that a corporate jet had hit the building. Later the info was clarified 

as to what kind of plane it was. Later a report of the second plane hitting. 

We could feel the building shake a little, not that strong though.” 

Interview 1000015 (NIST 2004) 

Other occupants sensed that something happened, but had no idea what the source of the disturbance was. 

A survivor in the 60s in WTC 1: “I felt the air clear up due to a big gush 

of wind that cleared the smoke (this happened when Building 2 was hit 

by the plane)” Interview 1000100 (NIST 2004) 

Occupants of WTC 1 with a view of the plaza observed large pieces of debris, some flaming, crashing 

down onto the central plaza. Evacuees from Stairwells A and C used escalators on the east side of the 

Mezzanine to proceed down to the lobby. The escalator access point, which often had a group of people 

waiting their turn to use the escalators, had a direct view of the debris, as shown in Figure 7–2. 

Figure 7–2. East-looking view through WTC 1 lobby as debris from WTC 2 impact travels 

past lobby windows onto plaza. 
Draft for Public Comment September 11, 2001, 9:02:59 a.m., Flight 175 Crashes into WTC 2 

NIST NCSTAR 1-7, WTC Investigation 101

As occupants exited the building onto the street or plaza level, many were unknowingly walking into 

danger, as debris rained down from the impact region in WTC 2. An occupant of WTC 2 from the 100s, 

having used an elevator to get to the lobby, exited WTC 2 at 9:03 a.m.: 

“I was guided by what I think were police officers who directed the 

crowd. I imagine they were guiding [us] away from where the debris 

was falling off Building 1. I walked out straight ahead towards the 

Liberty St. exit. Outside of Building 2, debris was falling down when the 

second plane hit. A piece of metal went into my arm. I went into shock 

due to severe blood loss [and] was assisted by NYPD, which [took] me 

to the hospital.” Interview 1000563 (NIST 2004) 

Due to the angle of the aircraft impact, large regions of survivability remained on some of the floors 

directly damaged by Flight 175. Figure 7–3 shows the individual and aggregated damage areas for floors 

78 – 83 in WTC 2. See NIST NCSTAR 1-2 and NIST NCSTAR 1-6 for additional building damage 

information. From the time of impact and for the next 20 minutes, occupants called 9-1-1 from the 

impact region and above to request assistance.46 

Figure 7–3. Calculated damage to floors 78 through 83 in WTC 2 (see NIST NCSTAR 1-6). 

 

46 New York City 911 Emergency Call Recordings, 2001. 

Floor 79 Floor 80 

Floor 81 Floor 82 Floor 83 

Floor 78 Floors 78-83 

Cumulative 

Damage 

Severe Floor Damage Fireproofing 

(SFRM) 

and partitions 

Floor system 

structural damage 

Floor system 

removed 
Chapter 7 Draft for Public Comment 

102 NIST NCSTAR 1-7, WTC Investigation

Standing on a floor in the 70s in WTC 2, a survivor described the immediate aftermath of the aircraft 

impact: 

“I heard this deep thud and the ceilings and walls started to crumble. I 

grabbed my laptop, ran out of my office. I ran towards Stairwell A 

[because] this was the closest stairwell to my office. One of my 

colleagues opened the door to Stairwell A, but we didn't go in and turned 

around and went south. Stairwell A (the NW stairwell) was pitch black, 

filled with heavy black smoke. There was falling debris from the ceiling 

making the other two stairwells inaccessible [so we] ran back to 

Stairwell A [because] that was only stairwell we had access to. There 

was a lot of soot and dust in the air on the way back to Stairwell A. I had 

to cover my mouth and nose with my shirt to be able to breathe.” 

Interview 1000625 (NIST 2004) 

A survivor from a floor in the 100s in WTC 2 made it to the 78th floor skylobby just prior to the impact 

of flight 175: 

I was walking down the stairs, and got off at 78 (stairwell C). I 

encountered a lobby full of people (found some people from my group). 

The plane hit the building. I went flying and I landed on my right arm. 

When that happened my body turned over onto its left side and was 

sliding towards the elevators that were being repaired. I thought I would 

go down into the elevator shaft (since the door was slightly ajar and you 

could see fire coming up and I thought that it would be the end). I 

stopped and I called out to my friends and they called back with their 

location. I walked over to them climbing over dead bodies. We made an 

assessment of what had happened (my boss was dead, other man had 

broken legs and my coworker was missing). I got up and was walking in 

the direction of the plane. As I was walking there I came across people 

who were upset and I was climbing over bodies and I came to realize 

there was no communication desk.” Interview 1000562 (NIST 2004) 

The majority of WTC 2 occupants were not on their usual work floors when WTC 2 was attacked; most 

people had initiated or completed evacuation. Therefore, relatively few observations of damage to the 

floors in WTC 2 were reported. Figure 7–4 shows a summary of the reported damage to floors in WTC 2. 

The information to develop Figure 7–4 was compiled from every source available to the NIST 

investigation, including interviews, published accounts, transcripts of emergency communication 

channels, and emergency 9-1-1 calls. Note that the “?” denotes a floor where there was no information 

found to record the absence or presence of observations. Further, the absence of an observation on any 

floor does not positively exclude the presence of that condition as it may simply not have been reported. 
Draft for Public Comment September 11, 2001, 9:02:59 a.m., Flight 175 Crashes into WTC 2 

NIST NCSTAR 1-7, WTC Investigation 103

Smoke

Sprinklers / water

Fatally injured people

Power outage

Jet fuelFallen ceiling tiles

Fire alarms

Collapsed walls

Extreme heat

FireFireballs

??????????? –– 110

???????????

???????????

???????????

???????????

–– 100

???????????

???????????

???????????

???????????

* *

???????????

???????????

??????????? –– 90

???????????

???????????

???????????

–– 80

???????????

SmokeSprinklers / waterFatally injured peoplePower outageJet fuel

Fallen ceiling tilesFire alarmsCollapsed wallsExtreme heatFireFireballs

–– 70

???????????

–– 60

???????????

???????????

???????????

???????????

??????????? –– 50

???????????

???????????

???????????

???????????

???????????

Smoke

Sprinklers / water

Fatally injured peoplePower outage

Jet fuel

Fallen ceiling tiles

Fire alarmsCollapsed walls

Extreme heat

Fire

Fireballs

–– 40

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 30

???????????

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 20

???????????

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 10

???????????

???????????

???????????

???????????

???????????

Smoke

Sprinklers / water

Fatally injured peoplePower outage

Jet fuelFallen ceiling tiles

Fire alarmsCollapsed walls

Extreme heat

Fire

Fireballs

??????????? –– 110

???????????

???????????

???????????

???????????

–– 100

???????????

???????????

???????????

???????????

* *

???????????

???????????

??????????? –– 90

???????????

???????????

???????????

–– 80

???????????

SmokeSprinklers / waterFatally injured peoplePower outageJet fuel

Fallen ceiling tilesFire alarmsCollapsed wallsExtreme heatFireFireballs

–– 70

???????????

–– 60

???????????

???????????

???????????

???????????

??????????? –– 50

???????????

???????????

???????????

???????????

???????????

Smoke

Sprinklers / water

Fatally injured peoplePower outage

Jet fuel

Fallen ceiling tiles

Fire alarmsCollapsed walls

Extreme heat

Fire

Fireballs

–– 40

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 30

???????????

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 20

???????????

???????????

???????????

???????????

???????????

???????????

???????????

??????????? –– 10

???????????

???????????

???????????

???????????

???????????

Figure 7–4. Observations of damage from tenant spaces in WTC 2. 
Chapter 7 Draft for Public Comment 

104 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 105

Chapter 8

SEPTEMBER 11, 2001, 9:02:59 A.M. – 9:58:59 A.M. 

56 CRITICAL MINUTES

With the attack on World Trade Center (WTC) 2, the events in both WTC 1 and WTC 2 unfolded 

similarly, with survivors in both buildings trying to or continuing to evacuate the buildings. For those 

trapped above the plane strike in WTC 1, there was no way out. For those above the impact in WTC 2, a 

single intact stairwell provided a critical lifeline for at least 18 occupants fortunate enough to both find 

and use it.47 

In both buildings, stairwells became not only a means of egress for occupants, but also the only 

significant route into the building for emergency responders. Interview analysis found that, prior to the 

collapse of WTC 2, the six stairwells in the two towers were relatively passable below the impact regions. 

While many occupants indicated that the conditions in the stairwells were better than the conditions 

experienced during the 1993 bombing, water, crowding, and the smell of jet fuel were cited as stairwell 

obstacles. All the while, building officials and emergency responders tried to cope with an incident of 

enormous scope and provide for as orderly an evacuation as possible. 

By the time WTC 2 was hit, 91 percent of the survivors in WTC 1 and 87 percent in WTC 2 had begun 

their evacuation. Over 20 percent of the survivors in WTC 1 and more than 40 percent in WTC 2 had 

already left the buildings at the time of the second airplane strike. Table 8–1 summarizes when survivors 

began their evacuation and when they left their respective building. 

Table 8–1. Estimated times for survivors leaving the building in WTC 1 and WTC 2. 

WTC 1 (n=440) WTC 2 (n=363) 

Begin 

Evacuation Leave Building 

Begin 

Evacuation Leave Building 

Before WTC 2 hit 91 % 21 % 87 % 41 % 

After WTC 2 hit, but before 

WTC 2 collapse 

7 % 67 % 13 % 58 % 

After WTC 2 collapse 0 % 9 % n.a. n.a. 

Key: n.a., not applicable; question not included in telephone survey. 

Note: Numbers may not total to 100 percent. Some survivors were unsure when they began their evacuation and/or when 

they left the building. 

Source: NIST WTC Telephone Survey Data. 

 

47 See Section 8.1 for further information. 
Chapter 8 Draft for Public Comment 

106 NIST NCSTAR 1-7, WTC Investigation

8.1 SURVIVORS FROM AT OR ABOVE FLOOR 78 IN WTC 2, AFTER IMPACT 

Above the floors directly damaged by 

Flight 175 in WTC 2, smoke traveled rapidly 

upwards. Within twelve minutes of WTC 2 

impact, a smoke layer was descending on 

floor 105,48 where multiple callers reported 

that between twelve and one hundred people 

were trapped.49 Through face-to-face 

interviews and analysis of published accounts, 

however, NIST identified 18 individuals at or 

above the floors of impact in WTC 2 after 

9:02:59 a.m. who escaped. The majority of 

these survivors were on the 78th floor, often 

referred to as the skylobby, waiting for an 

express elevator to the lobby or choosing a 

course of action just prior to the impact. 

Table 8–2 shows the location of the 18 

individuals at 8:46:30 a.m., as well as 

9:02:59 a.m. For the hundreds of occupants 

who remained above the skylobby when 

WTC 2 was hit, two of the buildings three 

stairwells had been destroyed by the impact. 

Only the staircase furthest away from the 

plane’s impact was passable (Stairwell A) and 

even this stairwell was severely damaged in 

places, as shown in Figure 7–1. According to 

eyewitness accounts, there was drywall and 

other debris scattered over the stairs, water 

was running down the stairs, presumably from 

the sprinkler system (Murphy 2002), and 

smoke filled the stairwell (Adler 2002). In the 

region near the airplane impact, the stairwell 

walls had collapsed, and occupants had to 

crawl over or under the debris in order to 

descend below the 78th floor (Adler 2002). 

According to NIST interviews and several 

published accounts, the conditions in the stairwells then improved significantly below the 78th floor. 

Published accounts indicate at least two individuals were below the 78th floor in WTC 2 at the time of 

impact, but ascended in order to help injured or trapped colleagues (Fahy and Proulx 2003; 

Murphy 2002). Both individuals were building occupants, and one individual was reported to have had 

 

48 New York City 911 Emergency Call Recordings, 2001. 

49 New York City 911 Emergency Call Recordings, 2001. 

Table 8–2. Location of WTC 2 survivors at or 

above floors of impact at 9:03 a.m. 

Location at 8:46 a.m. 

(WTC 1 Impact) 

Location at 9:03 a.m. 

(WTC 2 Impact) 

1 Floor 103 Floor 78 

2 Floor 103 Floor 78 

3 Floor 102 Floor 78 

4 Floor 101 Floor 78 

5 Floor 100 Floor 82 (Stairs) 

6 Floor 100 Floor 78 (Elevator) 

7 Floor 100 Floor 78 

8 Floor 97 Floor 78 

9 Floor 97 Floor 78a

10 Floor 86 Floor 78 

11 Floor 86 Floor 78 

12 Floor 84 Floor 84 

13 Floor 84 Floor 84b

14 Floor 84 Floor 84 (Elevator) 

15 Floor 81 Floor 81 

16 Floor 81 Floor 81 

17 Floor 79 Floor 78 

18 Floor 78 Floor 78 

a. While this individual did evacuate the building, the occupant died 

several days later as a result of injuries sustained on September 11, 

2001. 

b. After impact, this occupant ascended to floor 91 for a period of time 

before finally making it below the impact zone. 
Draft for Public Comment September 11, 2001, 9:02:59 a.m. – 9:58:59 a.m., 56 Critical Minutes 

NIST NCSTAR 1-7, WTC Investigation 107

an emergency response background. Neither individual successfully evacuated, although several 

survivors credited one of the individuals with saving their lives. 

8.2 PROGRESS OF EVACUATION BELOW THE IMPACT REGION 

For nearly all occupants below the floors of impact, the stairwells became the only means of egress from 

both towers. For some the egress went smoothly; for others it was far more difficult. Deteriorating 

conditions, coping with crowded stairwells, and the sheer physical effort required to descend dozens of 

flights of stairs presented a challenge for many occupants. 

A survivor who was on the 78th floor skylobby in WTC 2, then used the 

stairs down from there: “People were having general conversations, 

seemed calm, and walked at a steady pace, no sense of panic.” Interview 

1000825 (NIST 2004) 

A survivor who began on a floor in the 30s of WTC 1: “On about the 

29th or 30th floor, as I was leaning against the wall letting an injured 

person pass, I felt the wall heave (which apparently was due to the 

second plane hitting the other tower). This was about 9:10. On the stairs, 

near the 28th floor, there was a pile of shoes that accumulated from 

people kicking them off. Some of the people around me were tripping on 

them and warning others to watch out for them. Our speed of descent 

was very slow. There were many firemen clogging the stairs and spilling 

out onto the 28th floor. They were resting and taking care of one who 

was sick--was having a heart attack. They were also carrying much 

equipment. All of this was happening at about 9:20 am.” Interview 

1000042 (NIST 2004) 

Occupants also sought information from emergency responders by telephone. Advice from 9-1-1 

operators was often to remain in place and await rescue.50

At 9:20 a.m., also in WTC 1, occupants frustrated by a lack of information reached out to authorities, 

including the Port Authority Police Desk. 

Male: Oh, hi. We're on the 39th floor here. We're not getting any messages. Can 

somebody tell us what's up? 

PAPD: Come on down. Just come on down, everybody just come on down. Get down 

the staircase. Don't take the elevator. 

Male: Is there smoke in the stair? 

PAPD: Check the stairway. We're not sure yet. 

Male: We've got... 

Male: We're in one, 39. We've got, uh... 

PAPD: You should be low enough to get down, because it happened up high. 

Male: All right. 

Male: Thank you. (PANYNJ 2001a) 

 

50 New York City 911 Emergency Call Recordings, 2001. 
Chapter 8 Draft for Public Comment 

108 NIST NCSTAR 1-7, WTC Investigation

As the two communication records exemplify, calls to PAPD from occupants, in general, resulted in 

clearer, more specific information, such as where the emergency was. Further, some PAPD operators 

quickly recommended to occupants that they should leave the building, including WTC 2 prior to 9:02:59 

a.m., although that was not universally true. FDNY dispatch and 9-1-1 operators generally advised 

occupants to shelter in place and await rescue, consistent with existing protocols, but not the preferable 

course of action on September 11, 2001. 

These quotes and communication records point out several issues that occupants had to deal with during 

their evacuation. The environment in both buildings continued to worsen with time. In addition to the 

physical exertion required to descend the stairways, occupants were also faced with the need to allow 

injured persons to pass and firefighters and other emergency responders to travel up the stairs to respond 

to the fires. In addition, information from co-workers and managers at times conflicted with local and 

building-wide announcements about the event. These issues are discussed in more detail in Chapter 10. 

Occupants of both buildings faced a number of challenges in their attempts to evacuate once the planes hit 

both buildings. As building authorities and emergency responders worked to organize the evacuation 

under rapidly changing conditions, many occupants were directed out of the stairwells during their 

evacuation. Thirty-four percent of respondents to the telephone survey reported leaving a stairwell at 

least once during their evacuation. While this occurred from floors 3 through 78 in WTC 1, the largest 

percentages were found in the skylobbies on floor 78 (7 percent of telephone interview respondents) and 

floor 44 (9 percent), as well as on floor 13 (9 percent). Similarly, in WTC 2, the skylobbies on 78 

(13 percent of respondents) and 44 (25 percent) were mentioned most often. Some occupants left 

stairwells at the skylobbies or other locations to seek additional information or to find a better route out of 

the building. A survivor from a floor in the 90s in WTC 1 used multiple stairs during egress: 

“The staircase became crowded at about the 68th floor and I hoped to 

find a less crowded stairwell. I descended the stairs from 78 to about the 

50s, before exiting the stairs again. The stairs at 68 became more 

crowded because they were evacuating some injured or handicapped 

people. The speed slowed to a halt--so we exited to find another 

staircase. I entered staircase B (in the 50s), and descended to the 40s 

before exiting the stairs again to find better conditions as I exited the 

building.” Interview 1000052 (NIST 2004) 

Occupants were also directed to leave the stairwells. 

A survivor who began on a floor in the 80s of WTC 1: At the skylobby, 

we were re-directed by Port Authority personnel. We went to the 

stairwell on the far west side of the building. I saw many people trying to 

get into the little door of the stairwell. I went back to a different stairwell 

that was empty. The center stairwell was the one we ended up in that 

took us to the ground floor lobby. Interview 1000535 (NIST 2004) 

In WTC 1, occupants with mobility difficulties presented a particular challenge. A specific floor was 

designated by emergency responders to hold mobility-challenged occupants. The idea was to allow fully 

ambulatory occupants to egress, and then move the mobility-challenged occupants out once the stairwells 

had cleared somewhat. Numerous interview respondents indicated that a relatively small number of slowmoving occupants hindered the flow of faster moving occupants behind them in the stairwells. Reports of 
Draft for Public Comment September 11, 2001, 9:02:59 a.m. – 9:58:59 a.m., 56 Critical Minutes 

NIST NCSTAR 1-7, WTC Investigation 109

the specific floor number in WTC 1 varied (12, 18, or 20), but several survivors recalled the existence of a 

holding floor. 

An occupant from a floor in the 60s, confined to a wheelchair, was being 

assisted by four previously unknown occupants down the stairwells in 

WTC 1: “We saw a Port Authority person. He told us to go to the 18th 

floor [because] there was an evacuation station for people who needed 

special assistance. I was carried mostly by the two [occupants] who 

wandered onto my floor off the stairs [in the 60s], but then there were 

two more that also helped. They switched off. At about the 30th floor, 

we encountered firemen for the first time. We moved to the right. Focus 

Group #3 (NIST 2004) 

NIST found no evidence that a similar holding floor for mobility-challenged occupants existed in WTC 2. 

8.3 EVACUATION ROUTE THROUGH CONCOURSE 

The first firefighter killed at the WTC was reportedly killed by a falling person (FDNY CD 12/15 2001); 

many evacuees and bystanders were injured by debris. Figure 8–1 shows the WTC plaza from the 

WTC 1 Mezzanine Level, perhaps viewed by thousands of occupants as they prepared to walk down the 

escalators to the Concourse 

Level. Throughout the 

evacuation, the lobbies and 

particularly the Concourse 

area served as the primary 

route to safety for survivors 

of the WTC attacks. Within 

approximately 10 minutes of 

the attack on WTC 1, 

building officials and police 

in WTC 1 and WTC 2 had 

established a procedure to 

route evacuees from both 

buildings through the 

Concourse (mall) rather than 

the plaza in order to prevent 

occupants from being 

injured by falling debris or 

people or being upset by 

shocking sights. As 

presented in Section 2.2.2, 

Stairwells A and C in both 

WTC 1 and WTC 2 discharged to the Mezzanine Level, while Stairwell B discharged to the Concourse 

Level, without the choice of exiting at the Mezzanine Level. Occupants who used Stairwell A or 

Stairwell C were directed to use an escalator (see Figure 8–2) in order to descend to the Concourse Level. 

Figure 8–1. View from mezzanine level in WTC 1 looking east 

across WTC plaza, covered with debris. ‘Sphere,’ artwork by 

Fritz Koening, can be seen on the plaza. 
Chapter 8 Draft for Public Comment 

110 NIST NCSTAR 1-7, WTC Investigation

Many face-to-face interviewees who used 

Stairwell A or C reported that the escalators 

were turned off in order to allow occupants 

to descend easily using both escalators.51

Once through the underground shopping 

mall, an occupant originating from a floor 

in the 30s in WTC 2, recalled being 

directed out of the Concourse at 

approximately 9:03: “The security guards 

were like a human chain telling us which 

direction to go. We followed the security 

guards direction, half walking, half running 

towards Borders [retail book store] to now 

go up from the Concourse to get out.” 

Interview 1000842 (NIST 2004) Ascending 

the escalator from the shopping complex 

was captured in Figure 8–3. 

8.4 MORE ANNOUNCEMENTS 

IN WTC 2 

At 9:12 a.m., the fire command station in 

WTC 2 (310-B) used WTC Ch. 22 to 

broadcast that the fire (floor) warden 

phones in WTC 2 were not working.52 

Further, WTC personnel indicated that 

they were making announcements that 

wardens should not wait for further 

instructions over the floor warden phones, 

as per protocol. 

310-B to any units: Be advised that Building Two (inaudible) only 

(inaudible) warden phones. We can't pick up warden phones. We are just 

making straight announcements telling the people not to stay at the 

warden phones, because we can't pick them up. (PANYNJ 2001a) 

 

51 Analysis of Face-to-face Interviews (NIST 2004). 52 For an explanation of the emergency communication channel assignments, see NIST NCSTAR 1-8. 

Source: John Labriola 2001, edited by NIST. 

Figure 8–2. Escalator from mezzanine to 

concourse level in WTC 1 on September 11, 2001. 

Figure 8–3. Evacuees leaving complex 

near WTC 5. 
Draft for Public Comment September 11, 2001, 9:02:59 a.m. – 9:58:59 a.m., 56 Critical Minutes 

NIST NCSTAR 1-7, WTC Investigation 111

NIST found no other evidence to confirm or refute that the fire command station issued such an 

announcement or that any occupant heard an announcement around that time. 

At 9:20:10 a.m., approximately 17 minutes after the second tower was hit, the fire command station made 

another announcement over the public address system updating the occupants. The announcement was 

recorded in the background of multiple 9-1-1 calls.53 To the best of NIST’s ability to understand, the 

announcement went as follows: 

“This is the fire command station in building number two. We are in the 

lobby. We are holding down here. The condition seems to have 

subsided somewhat. There are people in the building. There are some 

people have left. If you wish to leave, you can now use the Concourse. 

You may walk to the Concourse on this side, avoiding building number 

one. The condition seems to have subsided in the lobby of building two. 

Some people have left the building. We are here monitoring the 

situation. The fire department are concentrating their efforts on building 

number one. People are leaving the building... 

(unintelligible)...Number 2...(unintelligible)...” 

The announcement lasted between 60 and 70 seconds, with the unintelligible portion comprising 

approximately the final 10 seconds. The content of the 9:20 a.m. WTC 2 public address system 

announcement varied significantly from the pre-planned building evacuation announcement covered in 

Section 2.2.4. However, no survivors from whom NIST collected first-person accounts reported hearing 

the 9:20 a.m. announcement in WTC 2. This may be attributable to the fact that approximately 75 percent 

of all WTC 2 survivors had left WTC 2 by 9:20. Two occupants, trapped above the floors of impact, 

however, did respond to the 9:20 a.m. announcement, according to 9-1-1 records.54

8.5 CONDITIONS CONTINUE TO DETERIORATE IN BOTH TOWERS 

As the events unfolded, conditions within both towers continued to worsen. At or above the floors where 

the airplanes hit the buildings, occupants had to cope with fire, smoke, and severe building damage. In the 

time period after 9:15 a.m., only four calls to 9-1-1 came directly from inside WTC 1. As Figure 8–4 

shows however, many occupants above the impact region in WTC 1 remained alive at 9:58:59 a.m., 

shortly before WTC 2 collapsed. 

Below the impact floors, occupants worked to evacuate the building while emergency responders made 

efforts to organize the evacuation and gain access to damaged floors to rescue injured or trapped 

occupants. At 9:35 a.m., a WTC Official in the security command center (SCC) reported to S2 trouble 

unlocking the stairwell doors: 

Male B: S2 to SCC. 

Female A: Go, S2. 

Male B: How are you doing up there? 

Female A: We’ve got a lot of smoke. 

 

53 New York City 911 Emergency Call Recordings, 2001. 

54 New York City 911 Emergency Call Recordings, 2001. 
Chapter 8 Draft for Public Comment 

112 NIST NCSTAR 1-7, WTC Investigation

Male B: We’re on 16 now. 

Female A: That’s a copy. We can’t use software right now to try to release the doors. 

(PANYNJ 2001a) 

Figure 8–4. Occupants seeking fresh air on floors 103 – 105 on north face of 

WTC 1 at 9:58:12 a.m. 

The reported difficulty unlocking doors did not appear to directly prevent any occupants from 

successfully evacuating, however, as every fourth door was always open and the doors were only locked 

from the stairwell side. The communication does reflect the condition of the SCC and the difficulty the 
Draft for Public Comment September 11, 2001, 9:02:59 a.m. – 9:58:59 a.m., 56 Critical Minutes 

NIST NCSTAR 1-7, WTC Investigation 113

conditions posed for operations originating from that office. Numerous other reports of fire, smoke, and 

damage were noted in transcripts of PANYNJ radio calls regarding the 22nd floor command center 

(PANYNJ 2001a). 

In WTC 1, more than half of the survivors noticed smoke in their immediate area during their evacuation 

compared to only 10 percent who reported noticing smoke at first awareness. At least 25 percent reported 

collapsed walls during their evacuation compared to only 6 percent who noticed collapsed walls at first 

awareness. 

Table 8–3 shows a summary of observations by the survivors during their evacuation from WTC 1 and 

WTC 2. All percentages increased from earlier observations on their floors prior to beginning evacuation. 

Table 8–3. Observations of conditions during evacuation of WTC 1 and WTC 2. 

Observation WTC 1 (n=440) WTC 2 (n=363) 

Smoke 57 % 21 % 

Sprinklers / water 55 % 3 % 

Fatally injured people 41 % 8 % 

Power outage / flickering lights 37 % 29 % 

Jet fuel 31 % 21 % 

Fallen ceiling tiles 29 % 12 % 

Fire Alarms 26 % 19 % 

Collapsed walls 25 % 11 % 

Extreme heat 10 % 7 % 

Fire 9 % 4 % 

Fireballs 3 % 2 % 

Conditions in the stairwell were challenging, as well. 

A survivor from a floor in the 20s in WTC 1: “The stairwell was lit the 

entire way down. There was a grayish color smoke which smelled like 

fuel. The more we reached the lower floors the stronger the smell 

became. On the 6th floor, the sprinklers were on, which slowed us down 

because we wanted to be cautious and not slip or fall.” Interview 

1000044 (NIST 2004) 

An occupant who survived the horror of aircraft impact on the 78th floor 

in WTC 2:”... not much debris, but there was a pipe that we had to go 

under, hanging wire, and water in the stairs; no counter flow after we 

were hit, stairs were well lit, but 78 had no lighting. On 40-well lit, a lot 

of doors were locked, stranger smell that I can't identify. As we were 

walking down, we saw fireman coming up and told us to get to 40 and 

that someone would take us in an elevator down to the lobby.” Interview 

1000562 (NIST 2004) Firefighters and security personnel would escort 
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114 NIST NCSTAR 1-7, WTC Investigation

the occupant down in the elevator, out through the Concourse area to a 

waiting ambulance. 

At 9:36 a.m., an occupant’s telephone call to a New York City 9-1-1 telephone operator indicated that a 

floor in the 90s of WTC 2 had collapsed. According to NYPD records, information from this call 

concerning the floor collapse appears to have been conveyed inaccurately by the 9-1-1 call taker and the 

NYPD radio dispatcher.55 The NYPD dispatcher transmitted the message at 9:41 a.m. and 9:51 a.m. 

Independent of the exact floor number, the call content demonstrated deteriorating conditions above the 

impact region in WTC 2. 

Table 8–4 shows the total evacuation time for survivors in WTC 2. Total evacuation time was measured 

from first awareness to exiting the building and included time spent on the starting floor. While the total 

evacuation time increased from the lower floors to the middle floors, as would be expected when the 

proportion of occupants who used elevators was roughly constant, the total evacuation time for survivors 

from the upper floors decreased due to the increased proportion (approximately twice as many, see 

Section 6.2.3) of survivors who used elevators to evacuate. 

Table 8–4. Total evacuation time (min) for survivors from WTC 2. 

Total Evacuation Timea (min) 

25% 

Evacuated 

50% 

Evacuated 

75% 

Evacuated 

Average Time 

(min) 

Lower floors (Basement – 42) 10 15 21 17.6 

Middle floors (43-76) 23 35 45 34.9 

Upper floors (77-110) 11 23 36 25.5 

All Survivors in WTC 2 13 21 35 25.0 

a. Total evacuation time was measured from first awareness to exiting the building. 

Note: Italics indicate the presence of a bias in the underlying data. Interpretation and comparison of data should consider 

this bias. 

Source: NIST WTC telephone survey data. 

Minutes or seconds prior to collapse of WTC 2, an NYPD ESU officer reported from the 20s that his 

team was having a difficult time ascending a stairwell in WTC 2 due to the volume of people in the 

stairwell (Interview NYPD 24 (NIST 2004)). As NIST identified only seven occupants who died below 

the floors of impact in WTC 2 (see Section 9.2), this strongly suggests that additional occupants from 

above the 77th floor had identified the surviving stairwell in the impact region and were making their way 

out of the building. 9-1-1 call records and face-to-face interviews documented that some occupants were 

alive above the floors of impact until 9:58:59 a.m., thereby establishing tenability above the impact 

region. Further, at least two individuals above the floor of impact were aware that Stairwell A was 

passable, although they may not have been the source of identification of a viable route through the 

impact region. These facts suggest that among several possibilities, a group of occupants from above the 

impact floors had identified a way through the impact region and were making their way down the 

stairwell some minutes before building collapse. 

WTC 2 collapsed at 9:58:59 a.m., leaving over 600 occupants and over 100 emergency responders dead. 

 

55 NYPD, McKinsey and Company, NYPD call-routing and message dispatch, “106th floor is collapsing.” Draft summary report 

July 23, 2002. 
NIST NCSTAR 1-7, WTC Investigation 115

Chapter 9

SEPTEMBER 11, 2001, 9:58:59 A.M. – 10:28:22 A.M. 

COLLAPSE

“The lights were off and it was dark. You could barely see your hand in 

front of your face and there was a lot of dust. We just stood there and 

waited for the dust to settle. The emergency lights went on and we tried 

resuming going back down those stairs for half a flight. People [were 

coming back] up the stairs and said, 'the egress is gone.'“ Interview 

1000594 (NIST 2004)

At 9:58:59 a.m., many of the 624 to 680 occupants and building personnel who had fought for survival in 

WTC 2 for the 56 minutes after the building was attacked were killed by the building collapse. Over 100 

emergency responders, unaware of impending collapse, were killed as well (NIST NCSTAR 1-8 2005). 

The effect of the collapse of WTC 2 was felt not only at the remaining WTC buildings, but all over Lower 

Manhattan, as shown in Figure 9–1. 

9.1 EVACUATING WTC 1 

Although occupants in WTC 1 may or may not have comprehended the magnitude of the collapse at the 

time, it still had a significant impact on the evacuation of remaining survivors in WTC 1. As described 

below, survivors related to NIST, during face-to-face interviews, the nature of the cloud of debris and 

smoke which filled the Concourse and lower portions of the stairwells in WTC 1, making evacuation that 

much more difficult for those still left in the building. 

A survivor in the shopping Concourse who began on a floor in the 50s of 

WTC 1: “We heard (what we later learned) the South Tower/WTC 2 

collapsing. We stopped near the joining of the Gap store and the PATH 

escalators. The sound, at first, was muted and rumbling. It was a 

scraping sound that grew louder over the course of 15-30 seconds. It 

seemed like it was going to envelop us. The sound became very loud 

(like steel on steel). Very quickly after WTC 2 collapsed, the lights went 

out in the Concourse. Immediately, people screamed and panicked, and 

started to run in the dark. This was the one time I thought I really was 

going to die. I saw what I thought then was a wall of water, like a 

tsunami, coming at me (I thought I was going to drown). What it turned 

out to be was the cloud of debris and smoke. It irritated my eyes and 

filled my mouth, and made me cough and spit. I called to the others (my 

staff/ co-workers) who were with me. I felt the need to be in touch with 

them, since we had come all this way together.” Interview 1000106 

(NIST 2004) 
Chapter 9 Draft for Public Comment 

116 NIST NCSTAR 1-7, WTC Investigation

Figure 9–1. Collapse of WTC 2, viewed from north. 

A survivor still in a stairwell who began on a floor in the 30s of WTC 1: 

“We descended on this stair until we got to the landing between floors 7 

and 8. The lights now went out in the stairwell. I entered the 8th floor 

and went down a corridor to a different staircase (I'm not sure which 

stair, but I believe it was the original one I started out on). People on the 

stairs below me (who apparently got a big rush of air) said the floors 

below us were collapsing. They turned around and started running back 

up and they were covered with a gray, powdery substance, such as ash or 

smoke -- so we believed them. Then when the fireman opened the door 

on the 8th floor and said to come onto the 8th floor, I readily followed 
Draft for Public Comment September 11, 2001, 9:58:59 a.m. – 10:28:22 a.m., Collapse 

NIST NCSTAR 1-7, WTC Investigation 117

his instructions. Some didn't follow those instructions and continued 

back upstairs. I descended the stairs from the 8th floor to what I believe 

was the 5th floor. It was still dark in the stairwell. I changed stairwells 

for the last time and descended to the Mezzanine Level. I exited the stairs 

onto the Mezzanine, headed toward the west wall, then the north wall, 

and finally exited through the emergency exit door.” Interview 1000042 

(NIST 2004) 

A survivor from a floor in the 50s in WTC 1: “We thought it was a bomb 

at this point, however, it was Building Two collapsing. Vibrations 

stopped; the noise was incredible; the lights went out and the sprinklers 

came on. We walked a lot more quickly. We were getting out of the 

building. It was hard to walk down the stairs because of the water 

coming from above from the sprinkler system. There was a large amount 

of smoke and I couldn't see the person in front of me. I guess we smelled 

jet fuel. I could barely see the emergency strips on the stairs, but you 

could see them [because] the lights had gone out.” Interview 1000054 

(NIST 2004) 

A survivor in the stairwell who began in the teens of WTC 1: “We heard 

a gigantic explosion. We reached the second floor landing and we 

stopped. It was pitch black; black and tremendous suffocating smoke and 

dust. We stopped there and choked and froze. The lights went back on 

after it seemed like forever. We were told by the firemen and rescue 

workers that we had to go up to the fourth floor, that the exit was 

blocked.” Interview 1000093 (NIST 2004) 

Whereas occupants were primarily using the cover of the underground Concourse Level to escape the 

WTC site prior to the collapse of WTC 2, the debris created by the collapse forced the remaining 

occupants to proceed outdoors after they left the stairwells. For occupants leaving Stairwells A and C on 

the Mezzanine Level, the exit on the North wall, West corner provided access to the Tobin Plaza, from 

which many proceeded North between WTC 5 and WTC 6; others used the bridge to cross West St to the 

World Financial Center. Occupants leaving Stairwell B on the Concourse Level moved West to the West 

Street VIP entrance, where many continued to the World Financial Center, while others traveled North 

along West St. 

The evacuation rate slowed dramatically in WTC 1 after WTC 2 collapsed. Clearly, the obstacles 

presented by the dust, debris, and loss of power from the WTC 2 collapse contributed to slowing the 

occupants travel speed. By 9:58:59 a.m., 88 percent of survivors (over 6,500 people) in WTC 1 had 

exited the building. Approximately 900 additional occupants and many emergency responders would exit 

WTC 1 over the remaining 29 minutes before the building collapsed. 

Many mobility-challenged occupants remained in a holding pattern on a lower floor in WTC 1 during this 

time. An FDNY Battalion Chief wrote “Last Man Down,” in which he described walking onto the 12th 

floor of WTC 1: 

“The office was filled with people. Forty, fifty, sixty of them. I didn’t 

stop to count, but there they were, in all shapes and sizes, of all ages. All 

of them were sitting quietly, patiently, apparently waiting for instruction 
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118 NIST NCSTAR 1-7, WTC Investigation

or assistance...There had to have been a couple dozen nonambulatory 

office workers in that room, easy, and each one of them had one or two 

or sometimes three friends helping them. Friends, coworkers, people 

they’d just met in the stairs, for all I knew.” (Picciotto 2002) 

Some occupants, however, had resisted the idea of delaying their evacuation by stopping on the 

designated holding floor. A mobility-challenged occupant who started in the 60s had been instructed 

while in the stairwell by a Port Authority employee to report to the 18th floor, when the time came to 

choose between resting or continuing to evacuate: 

“At about the 22nd floor, Building Two collapsed. The building swayed. 

There was smoke and dust from the collapse of building two. I had my 

mask on, but it was hard to breathe and see. We picked up speed. 

Firemen started coming back down (from above us). We had to leave the 

staircase because we couldn’t see or breathe. We went into a tenant’s 

space for 5 or 10 minutes. Then, we couldn’t find another staircase so 

we went back to the one we had just left. When we got to the 18th floor, 

we just kept going. The [helpers] asked me if I wanted to stay, I said 

“No.” I had been with them this far. After that we were moving more 

quickly.” [NIST Focus Group #3, 2004] 

Battalion Chief Picciotto described ordering the evacuation of the holding floor in “Last Man Down,” 

largely steering them down Stairwell B, at approximately 10:21 a.m. The fate of those mobilitychallenged occupants remains unclear. The conditions inside the stairwells, after WTC 2 collapsed, 

became more difficult. 

A survivor who began on a floor in the 60s of WTC 1: “The staircase 

was a mess; it was filled with concrete, pieces of metal sticking out of 

the stairs. There were also holes on the walls. There was also lots of 

water on the floor.” Interview 1000543 (NIST 2004) 

While building personnel attempted to provide building-wide announcements in WTC 1, only two 

survivors remembered hearing announcements in WTC 1, both near the time of the collapse of WTC 2. 

Specific details of the announcements were not recalled beyond “some sort of an emergency” or “to 

evacuate the building” Interviews 1000594 and 1000068 (NIST 2004), respectively. 

After WTC 2 collapsed, there was a sharp decrease in 9-1-1 call volume from inside WTC 1. Above the 

floors of impact in WTC 1, NIST found only one call to 9-1-1 from a person inside WTC 1.56 

Table 9–1 shows the total evacuation time for survivors in WTC 1 overall, as well as by zone (low, 

middle, and high). Comparisons of upper zone data with data from lower and middle zones should be 

carefully considered, however, as the upper zone represents a smaller region (fewer number of floors and 

less vertical height represented) than the lower and middle zones. This was due to the fact that no 

individuals survived from above the 91st floor. 

 

56 Several calls were received from persons relaying information on behalf of occupants in WTC 1, but the time delay in relaying 

the information from the occupant to the 9-1-1 dispatcher was unknown. 
Draft for Public Comment September 11, 2001, 9:58:59 a.m. – 10:28:22 a.m., Collapse 

NIST NCSTAR 1-7, WTC Investigation 119

The quartile and mean times for survivors to exit the building shown in Table 9–1 increased as the 

survivors were higher in the building. The first quartile of surviving evacuees from the middle zone were 

exiting WTC 1 at approximately the same time as the last quartile of surviving evacuees from the lower 

zone (approximately 40 minutes). Similarly, the first quartile of surviving evacuees from the upper zone 

were exiting WTC 1 at approximately the same time as the last quartile of surviving evacuees from the 

middle zone (approximately 60 minutes). Note, however, that the number of floors (and vertical range) of 

the upper zone was significantly less that the lower and middle zones. Therefore, comparisons of the 

upper zone times to the lower and middle zone times should factor the zone differential into the analysis. 

Table 9–1. Total evacuation time (min) for survivors from WTC 1. 

Total Evacuation Timea

(min) 

25% 

Evacuated 

50% 

Evacuated 

75% 

Evacuated 

Average Time 

(min) 

Lower floors (Basement – 42) 16 27 42 29.0 

Middle floors (43-76) 41 51 65 54.7 

Upper floors (77-91) 60 71 77 70.3 

All Survivors in WTC 1 25 41 58 41.9 

a. Total evacuation time was measured from first awareness to exiting the building. 

Note: Italics indicate the presence of a bias in the underlying data. Interpretation and comparison of data should consider 

this bias. 

Source: NIST WTC Telephone Survey Data. 

9.2 LOCATION OF VICTIMS AND SURVIVORS WITHIN THE BUILDING 

Once the two towers collapsed, the fate of 2,749 building occupants and emergency responders was 

sealed. Table 9–2 shows the likely location at time of impact of all decedents identified by the City of 

New York as decedents. It also categorizes types of emergency responders who perished in the disaster. 

To identify locations, NIST relied on more than 300 face-to-face interviews and 800 telephone surveys, 

various Web sites maintained by survivors or victims’ families and colleagues, several media outlets’ 

reports, and a badge list maintained by The Port Authority of New York and New Jersey. Table 9–2 

below shows the results of that analysis. 

The following sources were used to identify the likely location of decedents at time of impact: 

• September11Victims.com: This site is dedicated to the victims of September 11, 2001 

tragedy. 

• Portraits: 9/11/01: Published by the New York Times in 2003, this book includes short 

interviews with family members of many decedents. 

• CNN.com In-Depth Special (http://www.cnn.com/SPECIALS/2001/memorial/index.html): 

Tribute site for people to write remembrances of decedents. 

• Badge List maintained by Port Authority of New York and New Jersey: Includes name, 

employer, building, and floor for all occupants with badge-access to WTC 1 or WTC 2. 
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120 NIST NCSTAR 1-7, WTC Investigation

• Numerous memorial sites maintained by companies which lost employees: Includes names 

and remembrances of decedents. Examples include the Port Authority, Fire and Police 

Departments, Marsh & McLennan Companies, EuroBrokers, Fiduciary Trust, and others. 

• Newsday.com: Includes short stories written about specific decedents. 

Table 9–2. World Trade Center decedent location. 

Likely Location at Time of Impacta

World Trade Center 1 Occupants 1,462 

At or Above Impact 1,355 

Below Impact 107 

World Trade Center 2 Occupants 630 

At or Above Impact 619 

Below Impact 11 

Confirmed Below Impact in WTC 1 or WTC 2 30 

Unknown Location Inside WTC 1 or WTC 2 24 

Emergency responders (Total) 421 

FDNY 343 

NYPD 23 

PAPD 37 

Hospital/Paramedic 7 

Federal 2 

Volunteer Responders 9 

Bystander / Nearby Building Occupant 18 

American Flight 11 87 

United Flight 175 60 

No Information 17 

Total 2,749 

a. Where possible, NIST has used eyewitness accounts to place individuals. Where no specific 

accounts existed, NIST used employer and floor information to place individuals. 

As used in Table 9–2, location refers to the likely area where an individual was when his or her building 

was hit. The employer and floor number were generally used to designate location. An employee of 

Cantor Fitzgerald, for example, would be assumed to have died above the floors of impact in WTC 1, 

unless evidence to the contrary was discovered. Often, information about the activities or location of an 

individual was available from on of the sources described above, and that information was used to 

categorize individuals. Additional information was necessary to place the 18 individuals documented as 

bystanders or occupants of nearby buildings. There were 30 individuals who were known to have been 

below the floors of impact, but whose locations could not be determined. Typically, these individuals 

were security guards and fire safety staff who were observed performing activities below the floors of 
Draft for Public Comment September 11, 2001, 9:58:59 a.m. – 10:28:22 a.m., Collapse 

NIST NCSTAR 1-7, WTC Investigation 121

impact after the airplanes struck. There were 24 individuals who were likely in the building, but for 

whom no clarifying information could be uncovered indicating whether they were above or below floors 

of impact. Their occupations largely included maintenance, janitor, delivery, safety, or security functions. 

Emergency responders were defined to be people who arrived at the site from another location; thus, 

security staff and Port Authority staff (different from PA Police Officers) were not defined as emergency 

responders. 

No information could be determined for 17 individuals. 

Estimated Number of Decedents Below the Impact Regions 

The total number of people who perished below the floors of impact was: 

• 421 emergency responders; 

• 107 individuals in WTC 1; 

• 11 individuals in WTC 2; 

• 30 individuals below the impact region in either WTC 1 or WTC 2; 

• Up to 24 individuals were somewhere in the building, but may have been above or below 

impact regions in either building; 

• Up to 17 individuals for whom no information could be located; 

Thus, there were between 569 – 610 individuals who were below the impact floors but did not 

successfully evacuate. Assuming that all of these individuals were alive at the time that his or her 

respective building collapsed, approximately 20 percent of those lost on September 11, may have been 

alive just prior to collapse of the WTC towers. 

Estimated Number of Decedents At or Above the Impact Region 

The total number of individuals who perished who were at from above the floors of impact was: 

• 1,355 individuals above the 91st floor in WTC 1 at 8:46:30 a.m.; 

• 619 individuals above the 77th floor in WTC 2 at 9:02:59 a.m.; 

• Up to 24 individuals were somewhere in the building, but may have been above or below 

impact regions in either building; 

• Up to 17 individuals for whom no information could be located. 

Therefore, between 1,974 and 2,005 individuals were at or above the area of impact in WTC 1 and 

WTC 2 who did not successfully evacuate. 
Chapter 9 Draft for Public Comment 

122 NIST NCSTAR 1-7, WTC Investigation

Estimated Number of Aircraft Passengers and Crew 

The total number of airplane passengers and crew was 147, not including the 10 hijackers.57 American 

Flight 11 had 87 passengers and crew, while United Flight 175 had 60 passengers and crew. 

Additionally there were 18 people (non-WTC occupants) killed outside the building by debris and jet fuel. 

9.2.1 Fatalities in Elevators 

USA Today (Cauchon 2001) reported that as many as 200 occupants died in elevators in WTC 1 and 

WTC 2. While the NIST Investigation did not produce an estimate of the number of occupants who died 

in elevators, NIST did collect information related to elevators and elevator use that may be of benefit in 

identifying ways of reducing building occupant risk in general. 

The elevators were the normal means of vertical travel in WTC 1 and WTC 2, and it was likely that many, 

if not most, of the elevators were in use at the time of aircraft impact. There were two ways in which 

lives of building occupants may have been lost on September 11, 2001 while using the elevators. First, if 

the cables above the elevator car were severed, the car would have dropped to the bottom of the shaft 

(which may not be the basement, depending upon the specific elevator shaft). The safety brakes on the 

elevator car would have been activated by the governor cable when the governor wheel exceeded the 

design speed. Since the governor was mounted at the top of the hoistway, should the governor cable have 

been cut along with the hoist cables, the safety brake would not have been activated, and the car would 

have fallen. If the car dropped far enough, occupants of the car may not have survived the impact. For 

shorter falls, the occupants may have survived but remained trapped in the elevator car at the bottom of 

the shaft. 

Each of the towers had 99 elevators but most were locals that ran to only a few floors within one of the 

three elevator zones. For an elevator’s cables to be cut and result in dropping the car to the bottom of the 

shaft, the cables would need to have been in the aircraft impact debris path, floors 93 through 98 in 

WTC 1 or floors 78 through 83 in WTC 2. Inspection of the elevator riser diagram and architectural floor 

plans for WTC 1 shows that the following elevators met these criteria: cars 81 through 86 (Bank B) and 

87 through 92 (Bank C), local cars in Zone III; car 50, the freight elevator, and car 6, the Zone III shuttle. 

Inspection of the elevator riser diagram and architectural floor plans for WTC 2 shows that the following 

elevators met these criteria: cars 75 through 80 (Bank A), local cars in Zone III; car 50, the freight 

elevator, and car 6, the Zone III shuttle. Bank A local cars could have fallen a maximum of 6 floors, 

Bank B cars 16 floors, and Bank C cars 23 floors. Cars 6 and 50 could have fallen all the way to the pit in 

the sub-basement level, and car 50 in WTC 1 was reported to have done so.58 Elevator 81 (Bank B) in 

WTC 1 was apparently not destroyed, as WTC Radio Channel 25 (maintenance and electric) recorded a 

request for assistance in getting the elevator doors opened and noted smoke and water infiltration into the 

elevator cab (PANYNJ 2004). 

In WTC 1, several occupants seeking to use elevators reported that the elevators were not working 

immediately after impact, despite signage and a training program specifically prohibiting elevator usage 

 

57 New York City has not issued death certificates for any hijackers. 

58 NIST Interview, 2004. 
Draft for Public Comment September 11, 2001, 9:58:59 a.m. – 10:28:22 a.m., Collapse 

NIST NCSTAR 1-7, WTC Investigation 123

in the event of an emergency (see Chapter 4). In WTC 2, analysis of telephone interviews determined 

that approximately 18 percent of the survivors evacuated primarily using an elevator on 

September 11, 2001. As with WTC 1, however, it remains unknown how many occupants were using the 

elevators at the time of impact. In addition to the occupants who may have been killed while using 

elevators, several occupants reported numerous fatalities in the 78th floor skylobby, where occupants 

were either waiting for elevators or deciding whether to return to their offices when the second aircraft 

struck WTC 2. Interview 1000562 (NIST 2004) and (NIST NCSTAR 1-7A) 

Not all occupants in an elevator when their building was hit were killed. Some occupants were able to 

force doors open, a few ‘chopped’ their way through walls (one group famously used a window washing 

squeegee (Dimarco 2004)), while others were able to climb out of elevators which had fallen to the 

bottom of their respective shafts (Cauchon 2001). Figure 9–2 shows occupants leaving an elevator in the 

lobby of WTC 1 shortly prior to the collapse of WTC 2 on September 11, 2001. The reason the 

occupants were able to escape after an extended entrapment remains unclear. 

Figure 9–2. Occupants leaving a previously stuck elevator on September 11, 2001 in 

the lobby of WTC 1. 

Second, a number of safety systems and sensors in the WTC 1 and WTC 2 elevator systems were 

arranged to stop the elevator in place if they detected an abnormal condition (PA Interview 5 

[NIST 2004]). Stopping an elevator car generally represents the safest course of action. The intercom in 

every car would then be used to notify authorities of an entrapment. Given the size of WTC 1 and WTC 2 

and the number of elevators, however, occupant extraction could have taken hours, as evidenced by the 

five or more hour waits for some occupants during the 1993 bombing (Isner and Klem 2003). 

In fires and earthquakes, however, entrapment may result in additional risks to the occupants before they 

can be removed. In an earthquake, for example, one of the greatest safety risks comes from the 

possibility that the counterweight jumps its track and is struck by the car. Seismic switches on elevators 
Chapter 9 Draft for Public Comment 

124 NIST NCSTAR 1-7, WTC Investigation

in seismic zones are designed to detect the earthquake and cause the system to stop, move the car in the 

direction away from the counterweight to the next floor, stop and open the landing doors (ASME 17.1, 

Safety Code for Elevators and Escalators). 

Another issue regarding entrapment was the presence of “door restrictors.” A door restrictor is a safety 

device required on all new elevators (ASME 17.1, Safety Code for Elevators and Escalators) and retrofit 

on existing elevators (ASME 17.3, Safety Code for Existing Elevators) upon any elevator equipment 

upgrade. Door restrictors mechanically prevent the elevator car doors from being forced open unless the 

car floor is within 36 in. of a landing floor. They were installed in some, but not all, elevators in WTC 1 

and WTC 2 (Interview PA 5 2005). Door restrictors address the significant safety hazard that occurs 

when occupants force the car doors open and fall through the gap down the shaft or get crushed between 

the car and shaft wall. Door restrictors have all but eliminated these accidents, but have made self-escape 

by entrapped occupants nearly impossible. Elimination of door restrictors alone would lead to increased 

deaths from accidents. A better solution would address both issues. 

Occupants could have become entrapped at any level if the aircraft impact caused the elevators to stop. If 

the car floor were not within 36 in. (plus or minus 18 in.) of a landing floor, the door restrictor would 

have prevented the car doors from being forced open from the inside, and the roof hatch could only be 

opened from the top of the car, making for self-escape extremely difficult. If the car was within 36 in. of 

a landing, occupants could have forced the car doors open and then would have had to force open the 

landing doors or break through the elevator shaft wall, which was 3 in. to 3.5 in. of gypsum material. 
NIST NCSTAR 1-7, WTC Investigation 125

Chapter 10

DISCUSSION AND ANALYSIS

The attacks on the World Trade Center (WTC) towers were unprecedented events, extending well beyond 

the typical engineering design scenarios of buildings on many levels. Occupants were forced to deal with 

a full evacuation of severely damaged buildings, although training and procedures called only for partial 

evacuation near affected floors. Emergency responders and building management were forced to deal 

with multiple extreme disasters in the buildings while facilitating the safe egress of some occupants and 

attempting the rescue of others. 

This chapter provides a discussion of the factors that affected the overall safety of occupants on 

September 11, 2001. It is divided into several topics areas: a summary of total evacuation time for 

WTC 1 and WTC 2 from the telephone survey data, an analysis of the important factors that affected total 

evacuation time and its components of delay prior to evacuation, and evacuation time in stairwells or 

elevators. The face-to-face interview data also highlight additional factors that were not included in the 

telephone survey, but were prevalent in sufficient number in the face-to-face data to warrant discussion. 

10.1 OVERVIEW OF TOTAL EVACUATION TIME 

Figure 10–1 shows the percentage of survivors who reported evacuating over five minute intervals, 

measured from the moment that WTC 1 was attacked. There was a large peak in evacuation in WTC 2 

during the time period immediately surrounding the WTC 2 impact. Recall that WTC 2 was attacked at 

9:02:59 a.m., approximately 16 minutes after WTC 1 was attacked. Nearly one in six WTC 2 survivors 

reported having left the building in a single five minute period between 9:01 a.m. and 9:06 a.m. Over 

40 percent of the surviving occupants left WTC 2 prior to 9:02:59 a.m. That so many evacuated was 

largely due to occupant use of the elevators in WTC 2 to evacuate prior to impact. Approximately one in 

six surviving occupants (18 percent) initially used an elevator to evacuate WTC 2. After WTC 2, was hit, 

only one elevator was operating, under the control of the Fire Department of New York. 

Integrating the rates shown in Figure 10–1 yields Figure 10–2, which represents the fraction of survivors 

remaining in the building at different points in time. Within the bounds of statistical uncertainty, the 

same number of occupants successfully evacuated WTC 1 as WTC 2. Therefore, comparing the slopes of 

the curves is appropriate. For a period of approximately 30 minutes in WTC 2 (when only stairwells were 

available) the slope (or evacuation rate) roughly matches the slope of the first 70+ minutes of evacuation 

in WTC 1. This result would be expected given the similarity of the egress systems in the two buildings. 

The evacuation rate declines significantly for the final twenty minutes before each building collapses, due 

to the fact that over 90 percent of the occupants who were physically able to affect their own escape had 

done so by that time. Further, the collapse of WTC 2 slowed the evacuation rate in WTC 1 due to the 

significant quantity of dust and debris complicating the path from the bottom few stair landings out and 

away from WTC 1. 
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126 NIST NCSTAR 1-7, WTC Investigation

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105

0

2

4

6

8

10

12

14

16

18

20

Percentage of Survivors Exiting

Time Since WTC 1 Attack (min)

WTC 1

WTC 2

Figure 10–1. Reported evacuation time for survivors of WTC 1 and WTC 2. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100

Time from WTC 1 Attack (min)

Remaining Survivors in Building (%)

WTC 1

Attack

WTC 1

Collapse

WTC 2

Attack

WTC 2

Collapse

WTC 1

WTC 2

Figure 10–2. Percentage of survivors remaining in the building for WTC 1 and WTC 2. 
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NIST NCSTAR 1-7, WTC Investigation 127

While Figure 10–2 provides a comparison of overall rate of evacuation in WTC 1 and WTC 2, analyses 

should also take into account that many occupants did not survive the attacks. Recasting Figure 10–2 to 

include those who did not survive results in Figure 10–3, which shows the percentage of the total building 

population remaining in the building over time. Note that the slopes of the two curves in Figure 10–3 are 

not directly comparable since twice as many occupants perished in WTC 1 as did in WTC 2. When 

occupants who were unable to evacuate are included, more than 80 percent of the occupants of WTC 1 

survived and more than 90 percent of the occupants in WTC 2 survived. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60 70 80 90 100

Minutes From WTC 1 Attack

Percentage of People In Building

WTC 1

Attack

WTC 1

Collapse

WTC 2

Attack

WTC 2

Collapse

WTC 1

WTC 2

Figure 10–3. Percentage of occupants remaining in the building for WTC 1 and WTC 2. 

Evacuation time, however, has two primary components which contribute to the total time: the delay time 

prior to initiating evacuation and the time spent evacuating. Each of these components is discussed in 

detail below, incorporating multiple regression analysis (causal modeling) of each. 

10.1.1 Evacuation Initiation Delay Time in WTC 1 and WTC 2 

Evacuation initiation delay time for occupants of WTC 1 and WTC 2 was discussed briefly in Chapter 6. 

The analysis presented here explores whether statistically significant differences existed in the overall 

evacuation initiation times for occupants in WTC 1 and WTC 2 and among occupants located in the high 

(floor 77 and higher), middle (floors 43 to 76), and lower (basement to floor 42) regions of each tower 

(for a total of six zones). Histograms of reported delay time based upon the telephone interview results 

showed that each zone had a peak evacuation initiation around minute one or minute two, generally 

decreasing over time (or skewed right). The general distribution for all six zones was a non-normal 

distribution, approximated by a gamma or log-normal distribution. 
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128 NIST NCSTAR 1-7, WTC Investigation

Note that due to the probable bias in the data from the high region in WTC 2 discussed previously 

(occupants who delayed a length of time sufficient to prevent them from descending below the 78th floor 

prior to 9:02:59 a.m. were disproportionately removed from the telephone interview sample), no further 

consideration was given to comparing the results from the high zone in WTC 2 for the variable of 

evacuation initiation delay time. 

In order to calculate whether differences between the remaining five zones were statistically significant, 

the reported data were transformed to better approximate a normal distribution and, thus, satisfy statistical 

testing assumptions. A Box-Cox Power analysis revealed that all five zones had peak normal probability 

plot correlation coefficients (of approximately 0.95 or better) when the Box-Cox power was 

approximately zero, which indicated a log transformation. After the reported evacuation initiation delay 

times were log-transformed, an analysis of variance (ANOVA) F-statistic was calculated for WTC 1 and 

WTC 2. 

Meaningful differences in the evacuation initiation delay time in WTC 1 among the three zones (high, 

middle, low) existed, significant at the 1 percent level. Exploring the interzonal differences in WTC 1, 

there was not a statistically significant difference in the reported evacuation initiation delay times between 

the low zone and the middle zone. The high zone, however, exhibited significantly longer reported delay 

times, when compared to both the low zone and the middle zone. The increased delay time in the high 

zone compared to the low zone was significant at the 2 percent level, while the increased delay time in the 

high zone compared to the middle zone was significant at the 1 percent level. 

For WTC 2, evacuation initiation delay time was higher in the middle zone, compared to the lower zone; 

the difference was significant at the 5 percent level. Again, comparing the high zone to either the middle 

or lower zone in WTC 2 was not considered due to the bias in the upper zone delay time results. 

In conclusion, the general trend in WTC 1 and WTC 2 was for evacuation initiation delay to increase with 

building height, although there may be other explanations for this phenomenon. In WTC 1, occupants 

reported being trapped by debris and building damage, observing fire and smoke, and assisting injured 

colleagues at a higher rate near the impact region (high) than was reported in the lower or middle zones. 

The causal modeling in Section 10.2.1 explores this further through the independent variables ‘floor’ and 

‘environmental cues.’ 

Additionally, comparison of evacuation initiation delay times across buildings should be made with 

considerable caution. In WTC 1, the cues which led occupants to decide to evacuate were substantively 

different from those that likely influenced the decision process in WTC 2. In WTC 1, the building had 

been attacked, stairwells were the only available route out of the building, and there were generally no 

instructions delivered over the public address system. WTC 2 had not yet been attacked for over 

90 percent of the occupants by the time they initiated evacuation. Thus, for less than 10 percent of the 

WTC 2 survivors, their building had been attacked by the time they chose to initiate evacuation. In 

addition, in WTC 2 stairwells and elevators were available for most people to choose to use, and multiple 

(though conflicting) public address system announcements were made. Note, however, that many 

occupants in WTC 2 did not hear any announcements, because they had evacuated the building prior to 

the time the announcements were delivered. 
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NIST NCSTAR 1-7, WTC Investigation 129

0.00

0.03

0.06

0.09

0.12

0.15

0 0.5 1 1.5 2 2.5 3

Floors per Minute

Probability Density Function

10.1.2 Time and Average Travel Speed in the Stairwells, WTC 1 

The reported time spent in the stairwells for occupants of floor 10 to floor 91 in WTC 1, based on the 

NIST telephone interviews (n=368), yielded a mean normalized travel speed of 1.3 floors per minute. 

Normalized travel speed is defined as the total time from entering the stairwell until leaving the building, 

divided by the number of floor that had to be descended. According to Figure 10–4, 25 percent of the 

occupants traveled faster than 1.5 floors per minute, while 25 percent of the occupants traveled more 

slowly than 0.9 floors per minute. The median normalized travel speed was 1.2 floors per minute. The 

data include all interruptions to the evacuation process, including crowding, transfer floors, smoke, water, 

switching stairwells, and resting. Assuming an average floor height of 3.7 m (12 ft), the distance along 

the stair slope (including landings) would have been approximately 10 m (33 ft), yielding a movement 

speed of approximately 0.2 m/s (0.65 ft/s) for the median occupant in WTC 1 while in the stairwells, 

which is on the slow end of published scientific literature values for stairwell travel speeds. This is 

understandable given the frequency of crowding and the significant number of obstacles to evacuation 

reported by many occupants. 

Figure 10–4. Stairwell travel speed in WTC 1 for all stairwells. 

A “rule-of-thumb” for calculating evacuation speed is to assume that a standard size door at the bottom of 

the stairwell is capable of discharging approximately one person per second. (Fruin 1987)59 By that logic, 

with three stairwells, the WTC system would have been capable of moving approximately three people 

 

59 Fruin reported values of 40 – 60 persons per minute for free-swinging doors. The “rule-of-thumb” captures the high end of the

reported range. 
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130 NIST NCSTAR 1-7, WTC Investigation

per second from the occupied floors to the Mezzanine or Concourse. In WTC 1, where elevators were not 

operational for the duration of the evacuation, approximately 7,500 occupants used the stairwells over a 

100 minute period, yielding an exit rate of 1.3 people per second from the occupied floors. Even 

discounting the final 27 minutes (after the collapse of WTC 2 when the evacuation rate dropped 

significantly), the discharge rate was 1.5 people per second (or 0.5 people per second per door), or about 

one-half the “rule-of-thumb” discharge rate. This is consistent with the previous observation that the 

stairwell movement speed was slower than the published literature values would have predicted and that 

the slower rate was to be expected given the number of obstacles to egress encountered by the evacuees 

and the total travel distance required. 

Impact of Wider Stairwell on Stairwell Evacuation Times 

An analysis of the hypothesis that wider stairwells correspond to faster overall evacuation times is most 

reliable when all other variables are held constant. In WTC 1 on September 11, 2001, however, many 

other variables were not constant, complicating the comparison: (a) Stairwell B (56 in. wide) exited into 

the Concourse, while Stairwells A and C (44 in.) exited to the Mezzanine where occupants typically 

traversed to the escalator in order to descend to the Concourse; (b) Stairwell B only required one 

(relatively short) horizontal transfer section, while Stairwells A and C required multiple, sometimes 

lengthy (over 100 ft) horizontal transfers; (c) emergency response personnel preferentially used Stairwell 

B to climb to higher floors; and (d) an occupant may have switched stairwells during egress, introducing a 

significant uncertainty. Therefore, these four factors confound any conclusions regarding stairwell width 

which may be drawn from the evacuation of the WTC towers. Respondents reported three pieces of 

information critical to this analysis: number of floors they had to climb down, the total time spent in the 

stairwells, and which stairwell they used. Each reported time was normalized (divided by) with the 

number of floors descended in order to compare results independently of starting location. The 

normalized times were then averaged over all occupants who reported using that stairwell. 

In Stairwell B in WTC 1, the average occupant spent approximately 61 ± 38 s per floor, while in 

Stairwells A and C (the narrower stairwells), the average occupant spent 53 ± 34 s per floor. The 

uncertainty was calculated using a standard deviation. Unfortunately, the large uncertainty, relative to the 

average, in the data collected from WTC 1, combined with the confounding conditions referred to 

previously, do not allow differentiation of stairwell movement speeds for the wider stairwell compared to 

the narrower stairwells. 

10.2 FACTORS INFLUENCING TOTAL EVACUATION TIME 

The telephone interview results permit rigorous statistical analysis of factors that played a role in the 

evacuation process. In order to understand what factors influenced the overall evacuation time of the 

average occupant in WTC 1 or WTC 2, two primary dependent variables were predicted: how long an 

individual delayed initiating his or her evacuation, where initiation is defined as entering a stairwell or 

elevator with the intention of exiting the building; and how long an individual spent traversing the 

stairwells. Elevator travel time was not considered in this analysis. The sum of these two elements of the 

evacuation process is the total evacuation time. Multivariate regression modeling was utilized to establish 

factors which contributed to overall evacuation time. A more detailed discussion of both causal models, 

including methods, equations, and significance testing can be found in Appendix C: Causal Modeling. 

This chapter will only present the basic model structure and findings. 
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NIST NCSTAR 1-7, WTC Investigation 131

10.2.1 Predicting Evacuation Delay In World Trade Center 

The analysis of evacuation behavior consisted of two stages. The first stage, which focused on how long 

it took for people to begin evacuating, determined the factors (variables) and social processes (the major 

paths of causal links between variables) that influenced people delaying the initiation of their evacuation 

out of WTC 1 and WTC 2 on September 11, 2001. Evacuation delay was defined as the number of 

minutes that passed from when a person first became aware that something was wrong until he or she 

began evacuating. 

Model Description 

The model used to predict important factors in evacuation delay in the towers used variables that 

preliminary analyses suggested were salient and closely followed general evacuation theory from the 

social and fire sciences The model is diagrammatically illustrated in Figure 10–5. It can be described as 

follows: (1) delay in evacuation initiation (X7, the dependent variable in this analysis) was a direct 

consequence of environmental cues (X1), the floor on which occupants were located (X2), obtaining 

information without seeking it (X3), perceived risk (X4), seeking additional information (X5), and taking 

pre-evacuation actions (X6); (2) taking pre-evacuation actions was a direct consequence of environmental 

cues, floor, obtaining information without seeking it, perceived risk, and seeking additional information; 

(3) seeking additional information was a direct consequence of environmental cues, floor, obtaining 

information that was not sought, and perceived risk; and, finally, (4) perceived risk was a direct 

consequence of environmental cues, floor, and obtaining information without seeking it. This model was 

parsimonious (in other words, the model was constructed of no more or fewer components than were 

necessary), and consistent with theory that stems from research on existing evacuation and risk 

communication research, and the model accurately reflected findings from preliminary analyses on the 

many variables that could have impacted evacuation delay. With this conceptual model, a set of equations 

for evacuation delay, pre-evacuation actions, seeking additional information, and perceived risk that were 

assumed to depend linearly on the factors above were solved simultaneously to obtain the relative 

importance of each factor in predicting the four primary variables. 

Results 

The estimated parameters of the models for WTC 1 and WTC 2 revealed that the model had a very high 

degree of success in explaining evacuation initiation delay, pre-evacuation actions, seeking information, 

and perceived risk in both towers. The adjusted explained variance (R2

) for perceived risk was 55 and 

60 percent in WTC 1 and WTC 2, respectively. The adjusted explained variance for sought information 

was 25 percent in both towers. Respectively, the adjusted R2

 for pre-evacuation actions was 68 and 

69 percent for WTC 1 and WTC 2. Finally, the adjusted R2

 for delay in evacuation for WTC 1 and 

WTC 2 was, respectively, 49 and 56 percent. These were extraordinarily high levels of adjusted explained 

variance to observe in a study of human evacuation. The combined amount of variance in the dependent 

variable, evacuation thereby, established the strong predictive power of the models for both towers. All of 

the equations in the model for WTC 1 and WTC 2 were statistically significant at the 0.001 level or 

higher, indicating a high degree of confidence in the explanatory power of the model. 
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132 NIST NCSTAR 1-7, WTC Investigation

Figure 10–5. Model of evacuation delay. 

Direct Effects 

With a few exceptions, similar findings regarding evacuation delay emerged in both WTC 1 and WTC 2. 

Repeatability of findings lends validity to the conclusions that can be drawn from the analyses. The 

findings are presented below. First, each of the four equations in both models for both towers was 

considered; next, the models were interpreted as a whole so that the most significant paths of influence for 

each tower could be distinguished. 

Predicting Perceived Risk 

The findings that emerged regarding predicting the risk that people perceived were virtually identical 

across the two towers. ‘Perceived’ risk was considered more important than objective or actual risk 

because identical observations may be interpreted differently by individuals, and because people act on 

the basis of perceptions, or what they believe to be true. The R2

 for perceived risk was 55 percent in 

WTC 1 and 60 percent in WTC 2. In WTC 1, both environmental cues and floor had strong and similar 

impacts on predicting perceived risk, while obtained information had a weaker but statistically significant 
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NIST NCSTAR 1-7, WTC Investigation 133

impact. In WTC 2, once again, both environmental cues and floor had strong impacts on predicting 

perceived risk, and, once again, obtained information had a weaker but statistically significant impact. 

These findings suggest that the risk that people perceived before they began their evacuation increased 

largely as a function of their starting floor (distance to safety) and being exposed to environmental cues. 

Clearly, information had a lesser impact on risk perception than did the two more salient variables of 

experiencing environmental cues and floor height. The only difference in findings between the towers 

was that, in WTC 2, floor height was by far the strongest predictor of perceived risk, while in WTC 1, 

both floor and environmental cues were equal predictors. In other words, those who were on higher 

floors felt more at risk and moved more rapidly to evacuate.60

Predicting Seeking Information 

The variable ‘sought information’ may include verification of the nature of the attack, instructions or 

directions, or other information deemed relevant prior to evacuation initiation. Once again, the findings 

that emerged for predicting seeking information were almost identical across the towers. Explained 

variance (R2

) for seeking information was 25 percent in both WTC 1 and WTC 2. In WTC 1, 

environmental cues and floor both had the strongest and identical impacts on seeking information; 

obtained information had no statistically significant impact on seeking information; and perceived risk 

had a slight impact on seeking information. In WTC 2, environmental cues had the strongest impact on 

seeking information; the impacts of floor and obtained information were not significant; and perceived 

risk had a slight impact on seeking information. 

Seeking information in times of rapid onset emergencies is a typical human response, since people need 

to interpret and make sense out of an event before they act on it. The finding that the variable of 

environmental cues was the strongest predictor of seeking additional information is consistent with this 

theoretical finding about “milling” from past research. Obtained information had no impact on seeking 

information in either tower. This was likely because information to make sense out of the event had 

already been obtained. Perceived risk had a similar effect on seeking information—albeit lesser of an 

effect than environmental cues—in both towers. Interestingly, floor height (or distance from the exit) had 

a significant effect on seeking information in WTC 1, but not in WTC 2. Evacuation theory would 

predict that this effect would be present; it was present in the tower that was struck first, and it was not 

present in the tower struck second. 

Predicting Pre-Evacuation Actions 

Pre-evacuation actions included activities such as making telephone calls, gathering belongings, or 

performing any other tasks deemed necessary prior to evacuation initiation. The explained variance (R2

) 

for taking pre-evacuation actions in WTC 1 was 68 percent, and it was 69 percent in WTC 2. In both 

towers, the strongest predictor of taking pre-evacuation actions was floor. Environmental cues was also 

predictive of pre-evacuation actions. Once again, observing cues indicating that one is at risk and being 

high in the building with a longer path to safety emerged as strong predictors. Obtaining information had 

virtually no impact on pre-evacuation actions in either tower. Seeking information impacted preevacuation action likely because the information obtained supported the need to evacuate and, hence, 

 

60 Recall, however, that individuals who were not motivated to initiate evacuation on higher floors in WTC 2 were more likely to 

be trapped above the aircraft impact region when WTC 2 was attacked. 
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134 NIST NCSTAR 1-7, WTC Investigation

related to getting ready to leave. Finally, the impact of perceived risk on taking pre-evacuation actions 

was higher in WTC 2 than in WTC 1. 

Predicting Delay in Evacuation 

Explained variance (R2

) in evacuation delay was 49 percent in WTC 1 and 56 percent in WTC 2. The 

greatest predictor of evacuation delay in both towers was taking pre-evacuation actions. Obviously, doing 

anything before initiating evacuation—including preparation to leave—delayed departure. Setting this 

factor aside, some clear differences emerged between the two towers in terms of the relative impacts of 

the remaining variables in the model. Perceived risk had no direct effect on evacuation initiation delay. 

This finding is consistent with general evacuation theory where perceived risk’s impact on actual 

behavior is indirect through other factors. The three factors with the strongest direct effects on evacuation 

delay were the same in both towers. These were environmental cues, floor, and obtained information. In 

both towers, floor’s effect was negative, that is, the more floors one was from the exit, the quicker people 

were to initiate their evacuation. Environmental cues and information that was received passively both 

increased delay in the initiation of evacuation. Finally, seeking additional information had a minimal 

impact on evacuation delay. 

Paths of Greatest Influence and Conclusions 

Although each of the aforementioned findings are interesting in and of themselves, perhaps the most 

important findings that we can offer are those that emerge when all of the individual findings offered 

above are brought together and viewed at the same time in the context of the entire model. 

Bias 

As discussed previously, any conclusions about evacuation initiation delay time in WTC 2 should 

consider the impact of disproportional decedent location, particularly as a source of the disproportionality 

may be highly correlated to the variable of interest, evacuation delay. In other words, those who 

exhibited long delay times in one region of the building were unable to be interviewed, thus artificially 

shortening the average delay time for one-third of the building. In the causal modeling, this would affect 

the relationship between ‘floor’ and ‘delay initiating evacuation,’ likely tending towards zero a slightly 

negative estimate (-0.19) of the beta value between the two variables. As floor was not a primary path 

which directly predicted evacuation initiation delay in WTC 2, the impact of this bias was considered 

secondary. The effect of this bias as it worked through other variables was not considered. 

WTC 1 

Although there were other factors that had some lesser impacts on influencing what people did, the paths 

of causal influence that defined the main process that led to delay in the evacuation of WTC 1 on 

September 11, 2001 follows. 

Environmental cues (information from the physical environment that something was terribly wrong) and 

floor (increased distance to safety) caused people to set out to find additional information, most likely 

information about what was going on and what they should do. Next, the act of seeking additional 

information, that is, “milling” about to make sense out of the situation, led people to take actions to 
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NIST NCSTAR 1-7, WTC Investigation 135

prepare to evacuate. Finally, taking those actions to prepare to evacuate delayed the initiation of actually 

evacuating. 

In addition to this four step causal process, environmental cues and floor also had indirect impacts on 

evacuation delay as follows. Both factors increased the odds of seeking information and both factors 

increased the chances that people would take pre-evacuation actions prior to evacuating. Both factors also 

had direct impacts on actual evacuation delay. Environmental cues increased delay while floor decreased 

delay. 

WTC 2 

Although there were other factors that had lesser influence on what people did, the paths of causal 

influence that defined the main process that led people to delay in the evacuation of WTC 2 on 

September 11, 2001 were identical to WTC 1 with one decided difference. 

Environmental cues (information from the physical environment that something was terribly wrong) and 

floor (increased distance to safety) predicted perceived risk. Environmental cues, floor, and perceived 

risk caused people to set out to find additional information. Next, the act of seeking additional 

information, that is “milling” about to make sense out of the situation, and perceived risk both led people 

to take actions to prepare to evacuate. Finally, taking those actions to prepare to evacuate delayed the 

initiation of actually evacuating. 

In addition to this five step causal process, environmental cues and floor also had indirect impacts on 

evacuation delay. Both factors increased the odds of seeking information and both factors increased the 

chances that people would take pre-evacuation actions prior to evacuating. Both factors also had direct 

impacts on actual evacuation delay. Environmental cues increased delay, while floor decreased delay. 

10.2.2 Predicting Normalized Stairwell Evacuation Time In WTC 1 On 

September 11, 2001 

The second component of total evacuation time was the time spent in the stairwells. This analysis 

determined the factors and social processes that influenced the normalized stairwell evacuation time per 

flight of stairs for the people who evacuated out of WTC 1 on September 11, 2001. WTC 2 was excluded 

from this analysis because evacuees used stairs, elevators, and/or a combination of both for their 

evacuation and could not be separated for the analysis. Evacuation time was defined as the average 

number of seconds per flight of stairs that it took people from the time they entered a stairwell until they 

completed their evacuation out of the building. The model used to predict important factors in stairwell 

evacuation time again used variables that preliminary analyses and general evacuation theory suggested 

as salient, and is illustrated in Figure 10–6. It can be described as (1) normalized stairwell evacuation time 

is a direct consequence of floor, evacuation decision delay, environmental cues, emergency responders, 

crowding, and evacuation interruption; (2) evacuation interruption is a direct consequence of floor, 

evacuation decision delay, environmental cues, emergency responders, and crowding; (3) crowding is a 

direct consequence of floor, evacuation decision delay, environmental cues, and emergency responders; 
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136 NIST NCSTAR 1-7, WTC Investigation

Figure 10–6. Causal model for predicting normalized stairwell evacuation time. 

(4) emergency responders is a direct consequence of floor, evacuation decision delay, and environmental 

cues; (5) environmental cues is a direct consequence of floor and evacuation decision delay; and, finally, 

(6) evacuation decision delay is a direct consequence of floor. This model is parsimonious, consistent 

with the high rise fire evacuations theory, and it well-represented the positive findings of our preliminary 

analyses of the many variables that could have impacted normalized stairwell evacuation time. 

Results 

The estimated parameters of the model for WTC 1 revealed that the model had a very high degree of 

success in explaining normalized stairwell evacuation time. The adjusted explained variance (R2

) for 

normalized stairwell evacuation time was 44 percent, 11 percent for evacuation interruption, 72 percent 

for crowding, 57 percent for emergency responders, 79 percent for environmental cues, and 34 percent for 

evacuation initiation delay. With the exception of evacuation interruption, these are extraordinarily high 

levels of adjusted explained variance to observe in a study of human evacuation; and, these R2

’s, thereby, 

establish the strong predictive power of the model. All of the equations in the model were statistically 

significant at the 0.001 level or better. 

Direct Effects in the Model 

We first consider each of the factors individually, and then the model is interpreted as a whole so that the 

most significant paths of influence can be distinguished. 
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NIST NCSTAR 1-7, WTC Investigation 137

Predicting Evacuation Initiation Delay 

Evacuation initiation delay is the total time from first awareness that something was wrong on 

September 11, 2001, until entering a stairwell to evacuate. The findings that emerged regarding 

predicting delay in the initiation of evacuation from floor were that the R2

 was 34 percent. This 

relationship is already discussed in detail in the previous section. 

Predicting Environmental Cues 

Environmental cues were the visual, auditory, or other sensory perceptions that indicated danger on 

September 11, 2001. Explained variance (R2

) for observing environmental cues was 79 percent, and the 

equation was statistically significant at the 0.001 level. Floor had a very strong direct impact on observing 

environmental cues, and it was significant at the .001 level. The effect of delay in the initiation of 

evacuation was smaller, but still statistically significant at the 0.001 level. It would appear that the longer 

a person took to begin their evacuation, the more the physical impacts of the event grew and the more 

likely people were to experience them. 

Predicting Emergency Responders 

Emergency responder was denoted as either observed or not observed. The explained variance for 

predicting encountering emergency responders was 57 percent, and the equation was statistically 

significant at the .001 level. Experiencing environmental cues and floor both predicted encountering 

emergency responders. This makes sense when one considers that emergency responders would be most 

likely to go to areas experiencing the impacts that would also yield environmental cues, and the higher 

one was in the tower, the more stairwells one had to traverse, the longer the traversal time overall, and, 

thus, the greater the odds of encountering emergency responders. 

Predicting Crowding 

Stairwell crowding was denoted when a respondent indicated that the stairwell was crowded to the extent 

that it impacted progress down the stairs. The explained variance for perceived crowding on the 

evacuation stairwells was 72 percent. Perceived crowding largely increased as a result of environmental 

cues and encountering emergency responders. 

Predicting Evacuation Interruption 

Evacuation interruption was denoted by respondents who indicated that they left the stairwell or stopped 

in the stairwell by choice. The explained variance for interrupting evacuation was only 11 percent. Even 

though this equation was statically significant at the .001 level, none of the five predictor variables in the 

equation had a statistically significant impact on evacuation interruption. At best, these findings can be 

interpreted to mean that there was a slight but not statically significant tendency for people to interrupt 

their evacuation if they had more rather than fewer floors to traverse to safety, and if they encountered 

environmental cues (perhaps obstacles) in the process of evacuation. 
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138 NIST NCSTAR 1-7, WTC Investigation

Predicting Normalized Stairwell Evacuation Time 

Explained variance in predicting normalized stairwell evacuation time was 44 percent, and the equation 

was statistically significant at the .001 level. Only two factors in the equation were statistically 

significant, both at the .001 level. These were environmental cues and evacuation interruption. Clearly, 

the single factor that had the biggest impact on increasing the amount of time people spent, on average, 

per stairwell was environmental cues. The only other factor that had a significant impact was interrupting 

evacuation, obviously, because stopping egress would increase the amount of time needed to complete 

evacuation. 

Paths of Greatest Influence and Conclusions 

Although each of the aforementioned findings are interesting in and of their own right, the most important 

findings that we can offer are those that emerge when all of the above findings are brought together and 

viewed at the same time in the context of the model as a whole. Thus, the main process that led to 

increased normalized stairwell evacuation time in the evacuation of WTC 1 on September 11th was 

straightforward and clear. Floor (increased distance to safety) substantially increased the odds that people 

would encounter environmental cues. Floor also increased delay in starting evacuation (this relationship is 

elaborated upon in much detail previously), which, in turn, also increased the chances that people would 

encounter environmental cues. But it was encountering environmental cues that had a large and direct 

effect on increasing the amount of time that people spent, on average, to traverse their evacuation 

stairwell. In addition to this multi-step process with environmental cues as the key predicting variable, 

interrupting the process of evacuation for any reason also increased the amount of time, on average, that 

people used to descend their evacuation stairwell. 

10.3 ISSUES THAT IMPACTED OCCUPANT EVACUATION 

While the multivariate regression analysis of the telephone data has extraordinary power to distill the 

telephone interview data to the salient features, the analysis was necessarily limited by the constraints of 

the number and type of questions contained in the telephone interviews. Face-to-face interviews and 

focus groups provided a more detailed understanding than can be achieved by exclusive use of the 

telephone interviews. This section provides a discussion of some of the issues identified from both the 

telephone survey and face-to-face interviews that impacted the ability of occupants to successfully 

evacuate the two towers on September 11, 2001, including both those issues that aided evacuation and 

those that made evacuation more difficult. 

10.3.1 Environmental Cues and Information 

In any emergency, occupants obtain information about the event by passively receiving it and actively by 

seeking it out through various means. Information may have led occupants to evacuate or to delay 

evacuation to gather additional information or to perform additional tasks prior to beginning evacuation. 

As soon as WTC 1 was struck, most occupants’ first reaction was to look out the window for information. 

These first environmental cues from the building (hearing the explosion, feeling the building shake, etc), 

coupled with the reaction of looking out the window (and seeing the fireball and smoke in WTC 1, the 

debris falling outside of the window, or even the plane hitting the tower), was enough to signal that 
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NIST NCSTAR 1-7, WTC Investigation 139

something was wrong for many in the towers. This first set of information caused some to begin the 

evacuation process immediately. 

An occupant in WTC 1 from a floor in the 70s reported immediate 

evacuation initiation: “The building was swaying more than normal and 

I slammed into the cabinets. I got up and headed for the staircase.” 

Interview 1000574 (NIST 2004) 

Also, an occupant in WTC 2 from a floor in the 80s also noted 

evacuating immediately. “Somebody in my area jumped up and said 

there is a bomb. At this stage, everybody else took the stairs. I went 

directly to the Stairwell C. I didn’t think I was in danger at this point.” 

Interview 1000568 (NIST 2004) 

In both towers, many delayed evacuation to find out more information before entering a stairwell or 

elevator. This may have involved talking with coworkers, making and receiving phone calls, sending and 

receiving emails, searching the internet, and watching TV. 

According to the telephone interviews, 11 percent of the occupants in WTC 1 and 21 percent of the 

occupants in WTC 2 received information related to the event through various means. Of the occupants 

who received information, 57 percent in WTC 1 and 65 percent in WTC 2 received information about 

what happened. A majority of the occupants in WTC 2 (54 percent) received information about what 

happened to WTC 1 through the public address announcement. Many of them, already in the process of 

evacuating, were faced with the decision of whether to return to their offices or continue their evacuation. 

While on their floor, occupants were receiving information about what was going on in the buildings from 

each other and through a variety of technological means. Technology such as landline phones, cell 

phones, blackberries, email, internet, and TV was used inside the building to receive such information. 

An occupant from a floor in the 80s of WTC 1 received a phone call 

from a co-worker. “A co-worker was calling from home. She said that 

‘a plane hit the building and you better get out of there.’” Interview 

8000005 (NIST 2004) This information caused the occupant to begin 

their evacuation process. 

However, the situations in the towers were different before WTC 2 was hit. The information that was 

received by the occupants in both buildings was that only WTC 1 had been hit by an aircraft. This 

message produced a variety of responses from occupants in WTC 2. 

After hearing the public address system announcement, an occupant from 

a floor in the 50s in WTC 2 was calling an out-of-town office to let them 

know they were evacuating the building. “I got a hold of them and they 

told me that a plane hit the building [WTC 1] and to get out of there.” 

The respondent hung up and continued to call loved ones and answer 

other phone calls until his or her own building was hit by an aircraft. 

Interview 1001666 (NIST 2004) 

An occupant in the 40s from WTC 2 was watching TV before WTC 2 

was hit. “I saw what happened on CNN and was informed that a plane 
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140 NIST NCSTAR 1-7, WTC Investigation

hit Tower 1. I entered the emergency stairwell A and proceeded down. 

Interview 1000867 (NIST 2004) 

Even after entering the stairs, the evacuation flow continued through the use of technology and 

information sharing among the occupants inside the building. Although they had already entered the 

stairs, additional information (aside from the public address system announcement in WTC 2 telling 

occupants to return to their floor) sometimes increased the occupants’ motivation to evacuate the 

building, as shown in the following quotes from WTC 1 occupants. 

An occupant from a floor in the 50s in WTC 1 was traveling in a 

stairwell when he or she heard another occupant say “that another plane 

hit the building. [The person] got this information from a cell phone and 

we speeded up our evacuation.” This respondent went on to indicate that 

“cell phones were working and people were trying to figure out what was 

going on.” Interview 1000071 (NIST 2004) 

While in Stairwell B, an occupant from a floor in the 70s in WTC 1 

“heard a fire department radio say the words ‘. . . Structural Instability . . 

. ‘“ This radio message was relayed at the time of the collapse of 

WTC 2. The respondent went on to say that they “stopped because there 

was no movement in the staircase, but then became frightened for the 

first time. [I] began to focus more on getting out.” Interview 1000118 

(NIST 2004) 

An occupant in WTC 2 also experienced the same knowledge about his 

building during evacuation. “The phone was ringing, [so] I answered the 

phone on the 3rd floor. I listened to my daughter tell me that both 

buildings were hit [by a] commercial airliner, and that it was a terrorist 

attack. I told my boss what I learned from my daughter, so we would 

both have more determination to get out.” Interview 1000003 

(NIST 2004) 

When people were given new information about the event from outside of the building, they shared that 

information freely with those around them. People provided information about the phenomena that they 

witnessed themselves (at first awareness) or information that they had received from another source, such 

as from someone outside of the building. As evidenced in face-to-face interviews, the behavior of sharing 

information was frequent, regardless of whether the individuals involved were strangers. 

For example, there was an occupant in WTC 1, who originated on a floor 

in the 40s, whose phone rang while in the stairwell: “I answered the 

phone and my [spouse] was telling me that another plane had struck 

Tower 2. [The spouse] said to get out of the building as fast as you can 

and that it is a terrorist attack. [The spouse] also told me about the plane 

hitting the Pentagon. [The spouse] mentioned that there was another 

plane in the air but [the spouse] wasn’t sure where it was. I told others 

around me what had happened, in order to spread the available 

information.” Interview 1000572 (NIST 2004) 

In addition to passively receiving information from inside the buildings, 28 percent of the survivors in 

WTC 1 and 29 percent of the survivors in WTC 2 actively sought out information related to the event 
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NIST NCSTAR 1-7, WTC Investigation 141

through various means. In the face-to-face data, this includes nearly everyone when the occupants who 

looked out the window for information are included. However, the telephone interview data shows that 

approximately 40 percent of these who sought out information about the event in both towers were not 

successful in finding the information they were searching for. 

For example, an occupant from a floor in the 60s of WTC 1 tried making 

several phone calls after the building was hit, but was unsuccessful. 

“The building started to sway and everything started shaking. I knew 

that there was something [wrong]. I ran to my desk and made a couple 

of phone calls. I dialed about five times trying to reach my [spouse]. I 

also called my sisters to find out more information.” Interview 1000733 

(NIST 2004) 

Occupants also attempted to seek information about what was going on in the buildings through a variety 

of technological means. This technology included landline phones, cell phones, blackberries, email, 

internet, and television broadcasts inside the building. 

An occupant in the 70s performed a variety of information seeking 

activities before deciding to leave WTC 1. “I was curious and wanted to 

get information on what happened. I tried to find information on the 

internet, [because] that was the fastest way of getting information. 

[Also], I knew my mother had TV access. I called my mother to get 

information on what she had seen so far on TV.” This occupant also 

phoned a brother-in-law for more information until the occupant 

witnessed the plane impact WTC 2. Then, the occupant began to 

evacuate. Interview 1000583 (NIST 2004) 

Also, an occupant from a floor in the 100s in WTC 2 noted that “there 

were a lot of people trying to find out what was going on by using cell 

phones.” Interview 1000563 (NIST 2004) 

Emergency responders also confirmed what happened to inquiring occupants inside the stairwell, but then 

reassured them that everything would be okay. Obtaining information from emergency officials inside 

the building was not frequently reported. In return, occupants would provide information to the 

firefighters about where certain people were waiting for rescue assistance. 

Overall, the information flow and its impact on occupants’ evacuation were dependent upon many factors, 

including the tower in which the occupant was located and the time the information was received during 

the emergency. Occupants in WTC 2 were sometimes affected differently to news that only WTC 1 had 

been hit by aircraft, when compared to occupants in WTC 1. Information was received and sought out at 

various times throughout an occupants’ evacuation and from different sources. However, many 

occupants in both towers reacted to the first set of information provided by the plane crash into WTC 1, 

either by initiating evacuation or by beginning pre-evacuation activities. 

10.3.2 Building Alarm Systems 

Fire alarms have been a hallmark of fire protection for more than a century, with systems proposed in 

New York City as early as 1847 (Bukowski and Moore 2003). Typically, the systems serve dual purposes, 
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142 NIST NCSTAR 1-7, WTC Investigation

occupant notification and transmission of information about the fire to emergency responders. On 

September 11, 2001, with significant damage to the fire alarm system in the towers, only a small 

percentage of occupants in both towers (14 percent in WTC 1 and 10 percent in WTC 2) noted hearing a 

fire alarm during different stages of their evacuation. Many occupants in WTC 1 had already begun to 

evacuate their building due to a variety of building observations from the plane impact (less than 

5 percent of occupants in WTC 1 said that their decision to evacuate was based on the fire alarm going 

off). The same is true for occupants in WTC 2 who heard alarms after their tower was hit, since a large 

majority had already begun evacuation before or when their tower was hit (less than 1 percent said their 

decision to evacuate was based on hearing a fire alarm). 

Some occupants of WTC 2 believed that the emergency was limited to WTC 1 and were unclear if their 

evacuation was necessary. Also, occupants in WTC 2 were faced with a public address system 

announcement informing them to return to their offices. For example, a respondent in WTC 2 heard the 

announcement that the building was secure first, followed by the fire alarm. 

“At the 25th floor, I heard someone on the loudspeaker who said that 1 

World Trade was hit but that 2 World Trade was OK and that we should 

all go back to our office. After I heard that announcement, I heard the 

general alarm which meant to me to leave the building even quicker.” 

Interview 1000740 (NIST 2004) 

And, in response to this, the occupant kept walking down the stairs to leave the building. 

In both towers, occupants reported hearing fire alarms at various floors throughout the tower. For those 

occupants in WTC 1 and WTC 2 who heard fire alarms on their floors, the fire alarm was not frequently 

nor specifically mentioned as the reason for evacuating. Instead, the top three reasons for occupants to 

begin evacuation from either towers were the plane hitting WTC 1, being told to evacuate, or feeling 

afraid or in danger (in no particular order for each tower). 

10.3.3 Public Address Announcements 

Public Address announcements were not mentioned in the face-to-face interviews as being heard after 

each building was hit by aircraft. However, from the telephone survey, 14 percent of the survivors in 

WTC 1 reported receiving information from a public address system announcement – which suggests that 

the occupants received an announcement after their tower was struck. There is also information from the 

9-1-1 tapes that a second announcement was heard in WTC 2 by people near and above the floors of 

impact after the building was hit, although NIST found no evidence from face-to-face interviews that 

survivors below the floors of impact heard this announcement.

A building-wide public address announcement was made to the occupants in WTC 2 approximately three 

minutes before the aircraft hit their building. Based upon analysis of many face-to-face interviews and 

published accounts, the first announcement provided to occupants in WTC 2, before their tower was hit, 

stated the following: 

There is a fire condition in WTC 1, WTC 2 is secure/safe, please return 

to your offices. 
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NIST NCSTAR 1-7, WTC Investigation 143

Fifty-four percent of the occupants in WTC 2 reported receiving information from the public address 

system. Since 40 percent of WTC 2 survivors had evacuated before WTC 2 was attacked, the 

overwhelming majority of occupants still in the building heard the 9:00 a.m. announcement. While some 

occupants interpreted the first announcement as a suggested course of action, other interpreted it as an 

instruction. 

Approximately two minutes later, at 9:02 a.m., one minute before WTC 2 was attacked, a second 

announcement was made, contradicting the first announcement. Recorded in the background of an 

answering machine receiving a call from an occupant calling home from floor 98, the announcement 

indicated that it was now permissible to initiate an evacuation: 

“May I have your attention please. The situation is in Building 1. 

However, if conditions on your floor warrant, you may wish to start an 

orderly evacuation.” Interview 3000001 (NIST 2004) 

The 9:02 a.m. announcement was noted by one occupant catalogued in the collection of published 

accounts (Fahy and Proulx 2003), however, the large discrepancy in the percentage of occupants 

reporting this announcement when compared to the percentage who reported the 9:00 a.m. announcement 

remains unresolved. 

The majority of occupants in WTC 2 began their evacuation before the first announcement was made. 

Some of them never heard the announcement (due to evacuating via elevator, starting in the lobby, or 

possibly evacuating from a lower floor before the announcement was given) and some heard the 

announcement from either the stairs or a lower floor than their original starting floor. It is known that 

occupants listened to the announcement from the stairs, left the stairs to hear the announcement more 

clearly, or were led out of the stairs minutes before the announcement on skylobby floors by security 

personnel. Those who heard the announcement and had begun the evacuation process were faced with a 

decision on whether to comply with the instruction and return to their offices or continue their evacuation. 

The face-to-face interview data shows that while many people did not follow the instruction/suggestion in 

the announcement, others chose to return to their office. Each individual’s decision was affected by a 

number of factors at the time of the announcement, including: 

• their perception of danger and ability to evacuate: 

“While I was on the steps, I heard a public address system announcement 

that said the building was secure and to return to our offices. It made me 

angry to hear this announcement because I felt we were in danger.” 

Interview 1000048 (NIST 2004) This occupant, who had a medical 

disability, was helped down the stairs by a fellow occupant, until they 

took an elevator to the ground floor. 

• the actions or statements of others in the immediate area (group behavior): 

An occupant from a floor in the 70s, who used both an elevator and a 

stairwell, heard an announcement around floor 20: “[I heard] a PA 

announcement that told people to return to their offices. I shouted to 

people to continue down the steps; to keep the flow of traffic going--so 

that people wouldn't be going past me. I intentionally lied to people. I 
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144 NIST NCSTAR 1-7, WTC Investigation

heard the PA announcement tell people to return to their offices, but I 

thought it was better for them to evacuate the building and I knew that if 

the people didn't turn around and try to move past me that I could 

evacuate faster.” Interview 1000024 (NIST 2004) 

An occupant from a floor in the 60s in WTC 2: “I heard a public address 

system announcement that said to stay where we were--that our building 

was secure and the problem was with the other building, but the fire 

warden said to ignore the message and to leave the building anyway.” 

Interview 1000050 (NIST 2004) 

An occupant from a floor in the 90s heard an announcement around floor 

78 in WTC 2: “We heard an announcement that said ‘the building is safe; 

you can go back.’ I continued walking down the stairs, to keep moving, 

to not hold up the others that were behind me.” Interview 1000070 

(NIST 2004) 

• desire to follow instructions: 

An occupant from a floor in the 30s in WTC 2: “The PA announcement 

informed us, ‘. . . You may return to your office.’ I exited the stairwell 

on the [a floor in the 30s] and proceeded to the elevator lobby on that 

floor. I entered the elevator and took it to [a floor in the 30s] to return to 

my office as instructed. I [then] called my [spouse] at about 9:03 a.m. to 

tell [the spouse] that everything was ok – but my conversation was cut 

short when the second aircraft struck Tower 2.” Interview 1000049 

(NIST 2004) 

• and even serendipity: 

An occupant from a floor in the 10s in WTC 2: “I heard an 

announcement saying that the problem is in Building One and for us to 

stay where we are or to return to our floors. I stepped off of the stair into 

the 9th floor to take the elevator to return to my floor. I was distracted 

because of conversation and did not realize that the elevator was going 

down until it started moving.” Interview 1000922 (NIST 2004) This 

occupant did not return to their office, but instead, evacuated the building 

after exiting the elevator. 

Occupants who were led out onto skylobby floors also hesitated after hearing the announcement. They 

engaged in milling with other coworkers in an attempt to decide what to do. Minutes later, WTC 2 was 

struck, which caused them to initiate their evacuation. 

Other occupants who did not begin the evacuation process were still located on their floor when the PA 

announcement was made. The announcement helped to reinforce their original goal of remaining on their 

floor for some time. Most of these occupants listened to the announcement and remained on their floor. 

“I heard an announcement on the intercom that told us to stay. I decided 

to stay to avoid getting in more danger. I heard the phones ringing off 

the hook and my boss and I were going from phone to phone answering 
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NIST NCSTAR 1-7, WTC Investigation 145

them. This was to reassure the families that everyone was ok.” 

Interview 1000524 (NIST 2004) 

Regardless of where occupants were located in the building and whether they had decided to begin the 

evacuation process, their final evacuation decision was prompted by the impact of the aircraft into their 

building, WTC 2, at 9:02:59 a.m. 

10.3.4 9-1-1 System 

NIST was given access to all emergency calls to the New York City 9-1-1 system related to the World 

Trade Center attacks on September 11, 2001 from 8:45 a.m. until shortly after 10:30 a.m. The 9-1-1 

system was flooded with calls from 8:46:30 a.m., when WTC 1 was attacked, until roughly 8:55 a.m., and 

experienced a second surge in call volume at 9:03 a.m., when WTC 2 was attacked, which continued for 

several minutes. The majority of calls were eyewitnesses reporting that they had observed something 

crashing into the World Trade Center or reporting that the World Trade Center was on fire. Initial reports 

included descriptions of small planes, helicopters, large (commercial or military) aircraft, as well as a 

bomb or a missile. A few callers sought advice about whether they should evacuate their nearby 

buildings or information about what was going on. Other callers reported observations of suspicious 

activities or people. After several minutes, the first WTC occupants began to call 9-1-1 seeking advice or 

rescue assistance. Occupants continued to use the 9-1-1 system, both as a resource for information and an 

outlet for reporting their situation, until around 10:00 a.m., when WTC 2 collapsed. 

During the time period between aircraft impacts, advice from 9-1-1 operators to occupants experiencing 

smoky conditions was largely to shelter in place and await rescue from emergency personnel. 

One of the common questions asked by trapped occupants of the 9-1-1 operators was whether to break 

windows in order to obtain fresh air. The advice given to occupants varied by operator. Some operators 

encouraged the occupants to assess their own unique situation and determine whether breaking a window 

would help, sometimes warning the occupants that breaking a window may only serve to introduce more 

smoke from the outside than it would relieve from the inside. Other operators simply advised occupants 

not to break windows. The 9-1-1 operators, largely acting without complete knowledge of an evolving 

and traumatic situation, were forced to strike a balance between efficiently logging incoming calls, 

compassionately counseling sometimes desperate occupants, and disseminating relevant event 

information. 

Another common question from occupants to 9-1-1 operators was whether they should evacuate, and 

further, whether that should be upward or downward (in other words, where was the impact region 

relative to their location). Some 9-1-1 operators did not initially offer advice, routinely telling the 

occupants to defer to the instructions they were (presumably) receiving from building authorities. Other 

operators consistently advised occupants to stay in place, rescue was on the way. At least one occupant 

below the floor of impact in WTC 2 was repeatedly advised to await rescue by emergency personnel who 

were advised of the occupant’s whereabouts. Sometime after 9:02:59 a.m., some operators began to 

encourage the occupants to evacuate, if possible. Information about the location or magnitude of the 

impact in either WTC 1 or WTC 2 was not generally communicated to trapped occupants. 

Operators routinely collected from callers specific location information, which proved useful during the 

NIST investigation for establishing the time-evolving building conditions (fire, smoke, damage, number 
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146 NIST NCSTAR 1-7, WTC Investigation

of people), including: building, floor, corner (northwest corner, for example), or office number. 

Additionally, callers would sometimes relay individual or group observations about the tenability or 

status of the egress path (i.e., access to stairwells or elevators). For these reasons, the 9-1-1 recordings 

proved to be a valuable record of the conditions above the impact floors for which there would have been 

no other method to discover. 

10.3.5 Previous Evacuation Experience 

Using prior evacuation experience to guide future evacuation decisions, may or may not produce better 

outcomes. Recall from Section 4.3 that 16 percent of survivors on September 11, 2001 were also present 

during the 1993 bombing. A survivor from the 70s in WTC 1 described: 

“My response to the ’93 bombing wasn’t sufficient. I realized there was 

no real purpose in sticking around. I was going to get out as quickly as I 

could.” Interview 1000525 (NIST 2004) 

Another survivor from a floor in the 70s in WTC 1, however, used their 1993 evacuation experience to 

justify delaying their evacuation: 

“I was there in 1993, and [this time] I wanted to wait for some directions 

from someone, through the speaker system, fire alarms, etc. I stayed on 

around the general vicinity where I worked. This is the main reason why 

I stayed longer on the floor.” Interview 1000576 (NIST 2004) 

Similarly, 16 percent (n=59) of WTC 2 evacuees on September 11, 2001 also evacuated in 1993. In 

WTC 2, however, only 75 percent (n=42) felt that they made the right decision in 1993 (compared to 

95 percent in WTC 1), possibly due to the fact that many more waited to evacuate in 1993 in WTC 2 

(69 percent (n=39)) than did so in WTC 1. Only 31 percent (n=17) who reported their decision evacuated 

immediately from WTC 2 in 1993, keeping in mind that the bomb had a more significant impact on 

WTC 1 in 1993. 

Nearly every respondent who compared the 1993 bombing evacuation to the 2001 evacuation indicated 

that the 1993 evacuation was slower, more difficult (presence of smoke in the stairwells and floors), and 

more stressful. Having participated in the 1993 evacuation, those occupants generally felt much better 

about the progress and conditions during the 2001 evacuation. As a consequence of their 1993 

experience, however, several 2001 survivors who had direct 1993 experience reported having diminished 

confidence in building announcements (or the lack thereof), exemplified by this occupant from a floor in 

the 40s in WTC 2: 

“I ... was there during the 1993 bombing. I did evacuate--with a group 

of people who had no clue as to where we were going. What I learned 

from that experience was to not trust the Port Authority's 

announcements. Because of experiencing the '93 bombing, I felt a strong 

conviction to get out this time. Had I not experienced that, I might have 

listened to the Port Authority announcements and stayed put.” Interview 

1000048 (NIST 2004) 
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NIST NCSTAR 1-7, WTC Investigation 147

10.3.6 Occupant Activities 

Occupants of WTC 1 and WTC 2 performed a number of activities before and during evacuation on 

September 11, 2001. These activities included gathering personal belongings, milling with other 

occupants, seeking additional information, and calling family or friends, among others. In general, these 

activities either delayed the start of their evacuation (pre-evacuation activities) or interrupted their 

evacuation once it was in progress and are described below. 

Pre-Evacuation Activities 

Occupants performed a variety of activities prior to entering the stairwell (or elevator in WTC 2) to begin 

their evacuation. An activity such as gathering occupants or warning them to leave has a variable amount 

of time associated with it, depending upon the floor space to search and how reluctant others are to leave, 

whereas an activity such as gathering belongings usually requires under a minute to complete. Other 

activities noted by occupants were milling, seeking information (such as looking out the window, making 

phone calls, searching the internet), helping others, following emergency procedures, fire fighting, and 

working or closing up a work desk. The delay reported by survivors in starting their evacuation was 

predominately one to five minutes, while a small number delayed for more than an hour. The 

distribution of evacuation initiation delay times was discussed in detail in Chapter 6. 

The majority of the survivors, 70 percent of those in WTC 1 and 75 percent in WTC 2, engaged in the 

activity of talking to others, or milling. The behavior of milling is used by occupants to discuss what they 

witnessed and to arrive at conclusions for what happened and what to do next. 

For instance, an occupant in the 90s of WTC 2 looked for other 

coworkers after he heard a sound coming from WTC 1. “I moved back 

out of my office where I could get in sight with other people to share my 

observations, to discuss what ought to be done, and to hear what the 

consensus would be.” Interview 1000689 (NIST 2004) 

After hearing an explosion and feeling the building rock, an occupant in 

the 60s of WTC 1 turned to a coworker. “I discussed with my coworker 

what we were going to do. The noise of the explosion, the shaking of the 

building, and seeing the paper falling was what made me decide it was 

something serious. We left our conference room and went into the 

hallway to get to the stairs.” Interview 1000053 (NIST 2004) 

Near the floors of impact in WTC 1, a few survivors worked to secure an area of refuge or perform fire 

fighting activities. The floors near the impact area were much more difficult to navigate due to the smoke 

and damage. Because of this, some occupants remained in an office, surrounding the openings with wet 

towels, before being rescued by Port Authority personnel. 

In both towers, the face-to-face data captures the fact that some occupants delayed their evacuation start 

to wait to receive evacuation instructions, as part of their emergency procedures. Fourteen out of 

124 respondents in WTC 1 and 12 out of 69 respondents in WTC 2 waited for instructions. The 

percentages are higher for WTC 2 because it included those occupants who heard the public address 

system announcement while still on their floor before they began evacuation. 
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148 NIST NCSTAR 1-7, WTC Investigation

On at least one floor in the 30s in WTC 1, occupants followed the 

emergency procedures. “We knew that we were supposed to go out to 

the hallway and that there would be someone there with information or 

information would be given via loudspeaker. We went out to the 

hallway to listen for any instructions.” Interview 100009 (NIST 2004) 

Despite the number of pre-evacuation activities of some occupants, most occupants in WTC 2 chose to 

begin evacuating before their tower was attacked. Eighty-six percent to 91 percent61 of the survivors in 

WTC 2 began their evacuation before their building was hit at 9:02:59 a.m. Similar to the telephone 

interview results (which are generalizable), 12 percent of the face-to-face respondents in WTC 2 stayed 

until their tower was attacked. Of those who stayed (8 interviews out of 66 from WTC 2), five held 

positions of corporate leadership or fire warden. The remaining three made multiple decisions to leave 

before the plane hit, but were delayed by announcements, phones ringing, and the desire to gather 

belongings. 

Helping 

September 11, 2001 showed that people are willing to help out others during an emergency, even if they 

do not know the person ahead of time. Those occupants in need of help during the evacuation included 

occupants who use canes or wheelchairs, overweight, elderly, pregnant women, and people with asthma. 

Many of those who supported occupants in need throughout their evacuation were strangers. These 

helpers often remained with the occupant in need throughout their entire evacuation, even though they 

were putting themselves at risk. Short-term examples of helping behaviors mentioned in both towers 

were comforting coworkers in the stairwell, leading occupants in need to the elevators, helping an 

occupant carry belongings, and encouraging others to keep evacuating in times of stress or exhaustion. 

Occupants also exhibited major feats of heroism on September 11, 2001 by caring for an individual 

throughout their entire evacuation, sometimes assisting strangers down more than 60 stories to the 

building. 

Examples of this heroism include: 

A group of coworkers helped to rescue a wheelchair-bound occupant 

from a floor in the 60s in WTC 1. 15 minutes after the impact of their 

tower, WTC 1, they located the evacuation chair and transferred the 

disabled occupant into it. “We began our descent sliding the evacuation 

chair, step by step, [with] two helpers holding the chair on top and two 

helpers holding the chair on the bottom. [Soon] we began carrying [the 

occupant] down the staircase.” Interview 1000123 (NIST 2004) 

Two occupants assisted an overweight occupant in WTC 1 in navigating 

the stairwell from a floor in the 50s. Towards the bottom of the stair, the 

occupant’s “legs would give out and [the occupant] would fall in our 

arms. Our descent slowed dramatically as we practically had to carry 

[the occupant] down each flight of stairs.” Interview 1000093 (NIST 

2004) 

 

61 The discrepancy reflects a small difference among responses to two independent questions: ‘How long did it take you to 

evacuate?’ and ‘Did you evacuate WTC 2 prior to WTC 2 being attacked?’, as discussed previously. 
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NIST NCSTAR 1-7, WTC Investigation 149

On a floor in the 80s of WTC 1: “I heard the secretary, who was in 

flames, calling for help near the door. I extinguished the flames with my 

bare hands, along with a colleague who was using a sweater.” Interview 

1000113 (NIST 2004) 

However, an offer of help was not always accepted. One survivor from a floor in the 60s in WTC 1 

recalled: 

“There were some people that stopped in descending because they 

needed to catch their breath. We stopped about three times and offered 

help, which was not accepted.” Interview 1000878 (NIST 2004) 

Resting During Evacuation 

Some occupants felt the need to rest at certain points during their evacuation. An occupant coming down 

from the 90s by the stairs was forced to walk roughly one-half mile before eventually leaving the 

building.62 Occupants often chose to rest inside the stairway, either on the steps or, more frequently, on a 

stairwell landing. Resting during evacuation was noted by respondents beginning evacuation from either 

the high or medium strata of each building. Most often, resting was reported by occupants with 

respiratory problems, obesity, or other physical or medical conditions. 

From the face-to-face interviews, respondents’ experience of resting on the stairs is captured in the 

following quotes: 

An occupant in WTC 1 had to travel from a floor in the 70s. Around 

floor 25, the occupant’s “legs were really hurting and it was difficult to 

walk. I slowed down and stopped about three times. The first time, I let 

people get ahead of me. I stopped at a landing for approximately two 

minutes. I proceeded down but then stopped for the second time, 

somewhere between the 15th and tenth floors. I waited there for three or 

four minutes to think what was the best way to continue. There was 

water gushing from the fifth floor onto the staircase. I stopped and began 

to think about the safest way to get down and decided to proceed more 

slowly and take a firmer grip on the rail.” Interview 1000111 (NIST 

2004) 

In WTC 2, a group of coworkers found themselves also having to rest 

multiple times during their evacuation from a floor in the 90s. “On the 

78th floor, we felt tired. My coworker, who is a diabetic, hadn’t eaten 

breakfast yet. We stopped on the stairs to rest for a minute. A lot of 

people seemed to be stopping, probably due to the heat and needing to 

catch their breath. On the 64th floor, another coworker turned to me and 

said [he or she] was getting tired and didn’t feel well. I said ‘We will 

take a little break’ and I gave [the occupant] mints.” Interview 1000526 

(NIST 2004) The group stopped at least two additional times because a 

 

62 At roughly 30 degree slope, an occupant descending stairs from 1,000 vertical ft also travels over 1,500 horizontal ft, for a total 

combined distance of over 2,500 ft, or approximately one-half mile. Travel from stair exit and transfer floors increases this 

distance. 
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150 NIST NCSTAR 1-7, WTC Investigation

coworker wasn’t feeling well and didn’t think he/she could make it down 

the stairs. 

Occupants also observed other occupants in the stairs who were resting inside the stairwell. Face-to-face 

interview respondents sometimes mentioned specific categories of individuals who were resting in the 

stair, including people having trouble breathing (asthma), overweight, elderly, helpers, and firefighters. 

Leaving the Stairs 

In WTC 1 and WTC 2, occupants left the stairs for a variety of reasons. While occupants left the stairs 

onto floors throughout the buildings, the most commonly reported floor was one of the skylobbies (floor 

44 or 78). In addition, there were a number of occupants who left the stairs at the lower floors of WTC 1, 

which may be attributed to either debris from the collapse of WTC 2 or the fire department rest station for 

occupants on or about floor 20. 

In WTC 1, the most frequent reasons cited for leaving the stairs were an instruction to do so from 

firefighters, Port Authority, or building security (33 percent) and the stair condition (41 percent), 

including crowding or smoke/dust/jet fuel in the stairs. Before the collapse of WTC 2, occupants were 

directed out of certain stairs to either switch to another stair immediately or wait on the floor for a certain 

period of time. 

One occupant who originated on a low floor in WTC 1 stated that, “We 

were directed by a man (I am unsure if he was a security guard). He was 

telling people to go to the 18th floor in order to scatter the traffic from 

the stairwell. When we arrived at the 18th floor, we got out of the 

stairwell and into a vacant floor space.” This occupant later stated that, 

“He said he was instructed to pile up the people into the vacant floors to 

control the flow of traffic.” Interview 1000769 (NIST 2004) The 

occupant demanded to know why this was happening and decided to find 

another stair to take out of the building. A group then followed this 

occupant to another stairwell. 

After the collapse of WTC 2, the occupants near the lower part of WTC 1 were faced with an onslaught of 

debris from the collapse. For this reason, occupants were again instructed to leave the stairs and if not 

instructed, took it upon themselves to switch stairs at times. 

Two occupants helping an overweight colleague down Stair A in WTC 1 

after the collapse of WTC 2 “were told by firemen and rescue workers 

that [they] had to go up to the fourth floor [because] the exit was 

blocked.” Interview 1000093 (NIST 2004) 

Another frequent reason for leaving the stairs in WTC 1, which may have been unknowingly caused by 

either instruction or stair condition, was occupants following other occupants (9 percent). These people 

felt comfortable following others out of the stair, without always knowing the reason for leaving or who 

had initiated the stair move. Other reasons for leaving the stairs in WTC 1 include a jammed exit at the 

transfer floor (76) of Stair A, occupant’s being uncomfortable, the presence of a mobility impairment, 

retrieving something, helping another person, and seeking information. 
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NIST NCSTAR 1-7, WTC Investigation 151

In WTC 2, the most frequent reasons for leaving the stairs were instructions to leave the stairs by Port 

Authority or building security (10 percent) personnel, the public address system announcement made 

before WTC 2 impact (32 percent), and elevator usage (13 percent). Some occupants in WTC 2 were 

being instructed out of the stairwells at the skylobbies minutes before the announcement was made. 

When the announcement was made to occupants throughout the building, some occupants inside the stair 

walked out onto a floor to hear the announcement more clearly and/or to react to the information given by 

the announcement. Also, some occupants decided to take the elevators to a different point in the building 

(to either evacuate or return to their offices) both before and after the announcement was made. Other 

reasons for leaving the stairwell in WTC 2 included following the crowd, occupants being uncomfortable, 

and seeking information. 

10.3.7 Aids and Constraints to Evacuation 

Evacuation incentives are interpersonal interactions or physical features of the built environment that 

helped people to evacuate. Analysis of the telephone interview data reveals that evacuees that among 

occupants who received help from other people, 9 percent reported that they were assisted by floor 

wardens, 44 percent reported that they were helped by police or firefighters, and 65 percent reported that 

they were assisted by “others.” 

For occupants who were helped by building features, 33 percent of survivors in WTC 1 and 17 percent of 

those in WTC 2 reported that they were helped by photoluminescent markings. The discrepancy between 

towers may be due to the fact that lights were lost in WTC 1 after WTC 2 collapsed, thereby 

demonstrating the usefulness of the photoluminscent qualities. Additionally, occupants who used 

elevators in WTC 2 would not have observed the photoluminescent paint. 

Conversely, certain conditions presented constraints to evacuation. Table 10–1 shows the most frequently 

reported constraints to evacuation from the telephone interview data. Three areas were reported by more 

than half of the evacuees in WTC 1: crowded stairwells, emergency responders in the stairwells, and 

injured or disabled persons in the stairwells. The findings from the causal model for normalized stairwell 

evacuation time in WTC 1, however, provide a scientific basis for refuting the occupant’s perception that 

firefighters entering the building adversely affected the overall flow down the stairwells. While more 

than half (63 percent) reported that the firefighters / police in the stairwells were a constraint, an occupant 

who encountered firefighter or police did not have a significantly slower stairwell travel time than an 

occupant who did not encounter firefighter or police in the stairwell, all other factors being held constant. 

Table 10–1. Constraints to evacuation. 

 WTC 1 WTC 2 

Stairwells were too crowded 73 % (321) 69 % (206) 

Firefighters / police in stairwell 63 % (275) 27 % (80) 

Injured / disabled in stairwell 52 % (226) 33 % (99) 

Lack of direction / information 24 % (104) 29 % (106) 

Locked doors 16 % (72) 7 % (25) 

Poor lighting 11 % (48) 4 % (15) 

Bad / missing signage 5 % (23) 5 % (18) 

Source: NIST WTC telephone survey data. 
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152 NIST NCSTAR 1-7, WTC Investigation

A possible explanation for this seeming inconsistency is that while an occupant was required to stop and 

stand to the side in the stairwell as firefighters and police moved past, they were able to temporarily 

increase their speed to catch back up to where they would have been before they had met the firefighters 

or police. This explanation is contingent upon the occupants descending the stairwell at less the 

maximum speed, which was found to be the case earlier in this chapter (the travel speed was about onehalf what would be expected in a non-emergency evacuation). 

Elevator Usage in WTC 2 

At least 18 percent of the survivors from WTC 2 reported using the elevators for at least part of their 

evacuation, including those who used elevators from the basement levels. Those who used elevators in 

WTC 2 did so for various reasons. While most occupants used elevators for evacuation, some had 

decided to find a quick way to return to their office once the 9:00 a.m. announcement was given. Elevator 

usage was not necessarily dominated by people with mobility impairments, but used by all people with 

the intent to evacuate the building quickly. However, one occupant using the elevators out of need started 

out in the 90s of WTC 2. 

“I wanted to get out of the building as quickly as possible. I was taking a 

new medication and knew I should not walk down the stairs.” Interview 

1000553 (NIST 2004) This occupant took an elevator from the 95th 

floor to the 78th and then switched to another elevator at the 78th floor to 

travel to the lobby level. 

Despite the availability of elevators for occupants with mobility challenges in WTC 2, however, 

approximately the same percentage of mobility-challenged occupants chose to use elevators in WTC 2 

(19 percent), when compared to the surviving population overall (16 percent). 

Face-to-face interview respondents also refer to elevator usage after the plane hit. An occupant, injured 

on a floor in the 70s, was evacuated via elevator by a firefighter and a security guard, along with two 

other injured occupants, after WTC 2 has hit by the airplane. 

“As we were walking down, we saw a fireman coming up and told us to 

get to 40 and that someone would take us in an elevator down to the 

lobby.” Interview 1000562 (NIST 2004) 

The use of elevators in WTC 2 saved many lives due to the fact that occupants from floors 78 – 107 in 

WTC 2 used both stairwells and elevators in order to move below the impending impact region prior to 

the WTC 2 attack. In order to estimate the total number of occupants able to descend below the impact 

zone prior to the WTC 2 attack, the following assumptions were made: (a), no occupant began evacuating 

WTC 2 prior to 9:02:59 a.m.; (b), no elevators were usable; and (c), the evacuation rate of WTC 2 

mirrored the observed evacuation rate of WTC 1 in Figure 10–2 (starting at 8:46:30 a.m.). Under these 

three assumptions, over 3,000 people would have remained in the building as it collapsed, with over 

2,000 occupants remaining trapped above the 78th floor. Therefore, self evacuation (starting to evacuate 

prior to 9:02:59 a.m.) and the use of elevators during that time period is estimated to have saved roughly 

3,000 lives in WTC 2 on September 11, 2001. 
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NIST NCSTAR 1-7, WTC Investigation 153

Footwear 

Evacuation speed on the stairs can be significantly affected by the choice of footwear worn by the 

occupants, especially women. High-heeled shoes, especially heels higher than 3 in., dramatically 

decrease movement capability on the stairs (Templer 1992). People, mostly women, removed their shoes 

in the stairwell in both towers. This presented a potential hazard for other occupants in the stair, who had 

to maneuver around the pile of shoes, as well as for the occupants without shoes walking through the 

damaged portions of the building. 

In WTC 1, an occupant noticed a pile of shoes in a stair near the 28th 

floor. “There was a pile of shoes that accumulated from people kicking 

them off. Some of the people around me were tripping on them and 

warning others to watch out for them.” Interview 1000042 (NIST 2004) 

Firefighters (Interviews 1000081 and 1000540 (NIST 2004)) and Port Authority personnel (Interview 

1000071 (NIST 2004)) occasionally instructed occupants to put their shoes back on. 

Transfer Hallways 

As described in Chapter 2.2, the stairwells did not descend in a straight vertical alignment in WTC 1 and 

WTC 2. The horizontal connections, more numerous and lengthy in Stairwells A and C than Stairwell B, 

could extend more than 100 ft and require several 90 degree turns. In addition to slightly increasing 

evacuation time (compared to a design with no horizontal transfers), the transfer hallways introduced 

uncertainty in the minds of the evacuees regarding the correctness of their evacuation path. 

An occupant from a floor in the 60s in WTC1 described the transfer 

hallways: “As I descended the stairs down to the Mezzanine Level, once 

or twice I had to exit the staircase through a door and go down a corridor 

in order to reconnect to the same stairwell. I found this to be extremely 

disconcerting. Everyone who did this, stopped before they exited the 

staircase to make sure they were doing the right thing. This slowed us 

down and there was concern that the door would lock behind us.” 

Interview 1000053 (NIST 2004) 

10.3.8 Emergency Responders and Building Authorities 

Emergency Responders 

In addition to organizing the response to the attacks on the two towers and assisting occupants during 

their evacuation, building staff and emergency responders had to use the stairwells to attempt access to 

impacted floors in the buildings. This resulted in small groups of firefighters in bulky bunker gear moving 

against the flow of occupants down the stairs. This phenomena is often referred to as counterflow. Police 

and fire department involvement with building occupants was identified by survivors as both an 

evacuation aid and constraint. The police and firefighters were identified as an evacuation aid by 

44 percent of the occupants in WTC 1 and 30 percent of the occupants in WTC 2 and as a constraint to 

evacuation by 62 percent of the occupants in WTC 1 and 27 percent of the occupants in WTC 2. The 

lower numbers should be expected since before WTC 2 was hit, emergency responders naturally 
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154 NIST NCSTAR 1-7, WTC Investigation

concentrated their efforts in WTC 1. Many of those who evacuated early from WTC 2 never encountered 

police or firefighters in the building. 

Inside WTC 1 stairwells, firefighters were sighted by interviewees in all three stairwells, with a 

concentration on Stair B (from the face-to-face interviews). Also, from the face-to-face interviews, 

firefighters were sighted as high as the 60s in WTC 1. Interviews 1000576 and 1000645 (NIST 2004) For 

those meeting firefighters in the stairs, some mentioned slowing down, crowding, and even stopping 

several times. An occupant in stairwell A of WTC 1 was both slowed down and reassured by the 

firefighters. 

“We encountered firemen ascending, starting at about the 35th floor. I 

came to a dead stop numerous times in Stairwell A for about 5 minutes 

each time to aid the firemen to get up to the problem. The firemen were 

easy-going, and attempted to put people at-ease. They were also 

extremely winded and sweating profusely from their climb.” Interview 

1000103 (NIST 2004) 

Figure 10–7 shows an FDNY firefighter ascending a 44 in. (1.1 m) stairwell in WTC 1 on 

September 11, 2001. Figure 10–8 shows how an occupant and a firefighter in bunker (turnout) gear may 

not be able to pass one other in a 1.1 m (44 in.) stairwell without either the occupant or the firefighter 

moving somewhat to the side. Figure 10–8 was not taken in a stairwell from the WTC complex, but was 

intended to be a generic demonstration of the counterflow phenomena. 

Figure 10–7. Firefighter and occupants using a 44 in. stairwell in WTC 1 on 

September 11, 2001. 
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NIST NCSTAR 1-7, WTC Investigation 155

The firefighters provided a sense of 

security to some occupants in the towers. 

In some cases, occupants went so far as to 

show their appreciation. An occupant 

from WTC 1 Stairwell A recalled 

encountering firefighters near the 20th 

floor. 

“Firemen were going up and they were 

looking exhausted. People were 

cheering the firemen and some of the 

firemen said, ‘Don’t stop, you’re 

almost there.’ As we moved aside, we 

handed the firemen water.” Interview 

1000722 (NIST 2004) 

Throughout the event, emergency 

responders supported the occupants by 

performing a variety of actions, including 

directing occupants to change stairs 

(especially in WTC 1 when exits were 

blocked by debris from the collapse of 

WTC 2), providing directions on how to 

exit through the Concourse, giving out 

snacks, water, and oxygen (from air 

bottles) to occupants in need. 

Firefighters reportedly established a rest 

station on floor 20 (Interview 1000543 

[NIST 2004]). Firefighters here instructed occupants to drop certain occupants off at this floor for 

assistance, as well as suggesting that evacuees stop on this floor if they required rest or assistance. 

In addition to walking up toward the fire floors, many firefighters moved injured occupants to safety. An 

occupant inside Stairwell C of WTC 1 heard instruction from floor above to keep to the right. 

The respondent then “noticed that the firemen in uniforms were bringing at least two people” down the 

stairs. “One gentleman had his arms severely burned and one lady who had her head halfway burned and 

was screaming with pain. After the fireman came down, others came up – several of them. They had 

gear on their back and each one had an extra hose on their shoulder. I was on the right and was letting 

them up, and a particular firemen patted me on the left shoulder and said, ‘Don’t worry, you will be ok.’” 

Interview 1000697 (NIST 2004) 

Impact of Authorities on Occupant Evacuation 

In both towers, occupants followed directions from those in a position of authority. In an office 

emergency, higher rank can mean multiple things. On the floors, occupants followed instructions to leave 

Figure 10–8. Occupant and firefighter in bunker 

gear passing in a generic 44 in. stairwell. 

Source: NIST.
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156 NIST NCSTAR 1-7, WTC Investigation

from supervisors or fire wardens, even though supervisors and wardens may have had the same 

information as the employee about what was going on. Also, occupants followed directions throughout 

the building from the building and fire officials who were familiar with the building layout, such as the 

Port Authority employees, police, firefighters, and building security. Analysis of face-to-face interview 

data revealed that these instructions included when and where to evacuate the stairs, when and where to 

change stairs, when to move to the right to let firefighters go up and injured down, or where to travel upon 

leaving the stairwell. 

Workplace Authority 

In an emergency, there is usually a combination of reasons why a person begins their evacuation 

process.63 However, there is usually one significant reason that finally makes them decide to leave or that 

weighs more heavily on their decision than the others. When occupants in both towers were asked to 

name the one thing that made them decide to evacuate, 14 percent of the occupants in WTC 1 and 

21 percent of the occupants in WTC 2 said that the reason they evacuated was being told to evacuate. 

(Telephone Interviews, NIST 2004) 

At the first awareness that something was wrong, the occupants could only rely on each other (or 

themselves, if alone) to understand what had happened and to decide what to do next. Fire alarms 

sounded in certain areas of WTC 1 (and possibly WTC 2); however, occupants did not generally report 

the fire alarms as their reason to evacuate. The others on their floor were their subordinates, coworkers, 

or superiors (supervisors or fire wardens). From a total of 208 face-to-face interviews, 86 people 

mentioned being instructed to evacuate, as well as their reaction to that instruction. Most respondents 

began their evacuation when told to leave, whether by a superior or co-worker. Although the percentage 

was higher when direction came from a superior, it is difficult to draw any conclusion from the face-toface data since it is not statistically-based and cannot be generalized to the entire population of the 

buildings. Only one face-to-face interview respondent reported telling their superior to evacuate, and in 

that isolated case, the superior ignored the instruction. 

An occupant from the 40th floor in WTC 1 left the floor due to persuasion by the fire warden: 

“As I was typing the email message, I heard a loud voice say ‘Leave.’ I 

kept typing the message when a fire warden grabbed my arm and pulled 

me out of the seat.” The fire warden also pointed this occupant in the 

direction of the elevators and stairs in addition to instructing the 

occupant to leave the floor. Interview 1000802 (NIST 2004) 

After hearing an explosion from WTC 1, an occupant with workplace authority on a floor in the 60s of 

WTC 2 instructed others on the floor to leave: 

“I saw fire and burning debris and smelled jet fuel. I ran out of my office 

and yelled to staff and co-workers to order them to evacuate. We ran to 

the stairwell.” Interview 1000556 (NIST 2004) 

 

63 Nelson and MacLennan. “Emergency Movement.” In The SFPE Handbook of Fire Protection Engineering, 2nd Edition. 

NFPA, Quincy, MA. 1995. 
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NIST NCSTAR 1-7, WTC Investigation 157

And, even after WTC 2 was hit, occupants on a floor in the 50s of WTC 2 remained on their floor 

discussing what had just happened and what they should do next: 

“Our facilities manager told me it was time to go. We did a quick check 

of the floor to make sure no one was left behind. In the hallways we saw 

that everyone was heading in one direction and we followed them to the 

stair.” Interview 1000557 (NIST 2004) 

It would appear from the face-to-face interviews that occupants were likely to follow the instruction given 

by their superior in an emergency. However, even if the final decision to evacuate was prompted by the 

instruction to evacuate, other factors may have been involved in making that decision, including seeing 

the plane strike into WTC 1, past experience, or other observations of the event. 

Building Authority 

Similar to the FDNY role in the WTC towers on September 11, 2001, the building authority played an 

important role in providing guidance to occupants about where to go once they left their floors, which the 

occupants frequently followed. The Port Authority personnel were observed giving instructions and 

directions to occupants at the skylobbies, Mezzanine, and Concourse areas of the WTC towers. Their 

instructions for some occupants consisted of when and where to leave the stairs, whether to use the 

elevators, and in WTC 2, when to return to their offices. Port Authority direction consisted of how and 

where to go through the building and Concourse area in order to leave the WTC complex. In most cases, 

occupants followed the instructions and welcomed the directions given by the building authority in the 

towers. 

Many noted the tremendous help that building personnel provided at the base of the building. An 

occupant in WTC 2 noted that the building authorities were present throughout the Concourse: 

“Trade center people were directing us into the Concourse because you 

couldn’t go out Liberty street – they had all those exits closed because 

there was debris flying all over the street. Security guards were like a 

human chain telling us which direction to go. We followed the security 

guards’ direction . . . towards Borders.” Interview 1000842 (NIST 2004) 

An occupant in WTC 1 noted the Port Authority giving directions to occupants on the Mezzanine: 

“As soon as I arrived onto the Mezzanine floor, I saw a chief operating 

officer giving directions to get onto the stairs [escalator] and go down. 

He also said not to look out onto the plaza because it was unsafe and 

dangerous.” Interview 1000639 (NIST 2004) 

In addition to directing occupants throughout the buildings, there were two cases from the face-to-face 

interviews where occupants were rescued from their floors by the help of the building authority in 

WTC 1. In both cases, the occupants were faced with worsening conditions on their floor and were 

guided to the stairwells by the official. 

From a floor in the 80s in WTC 1: After preparing their office with wet 

towels around the doors to prevent the heavy smoke from seeping in, the 
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158 NIST NCSTAR 1-7, WTC Investigation

respondent “saw a beam of light through the smoke, which turned out to 

be a fireman and a building worker. They told us to drop everything and 

follow them into the hall that led to the staircase.” Interview 1000055 

(NIST 2004) 

After two unsuccessful attempts to open the jammed door from a floor in the 80s into stairwell A, the 

heavy smoke on the floor drove the group of coworkers back to their office to wait, until: 

“We heard one of our coworkers in the hall [say that] the Port Authority 

fellow had opened up the doors to Stairwell A for us.” Interview 

1000137 (NIST 2004) 

10.3.9 Occupant Experience 

Experience from the 1993 bombing and other past evacuation experience affected some occupants’ 

actions in 2001. For example, many of those involved in both 1993 and 2001 recalled long evacuation 

times in 1993, including occupant congestion, smoke in the stairs, and not being allowed to return to the 

building for weeks after the event. Because of this experience, some occupants started their evacuation as 

soon as WTC 1 was hit and noted that their experience in 1993 was the principal reason. On the other 

hand, some occupants performed specific activities that they wished they had performed in 1993, such as 

calling home so their parents/family wouldn’t worry about them and taking certain belongings in case 

they couldn’t return to the building right away. 

Even though occupants of the towers may not have been present for the 1993 bombing, the bombing 

event may have played a role in their 2001 evacuation. Many of those present in 1993 shared their 

experiences with other employees both before and during the 2001 attack: 

“I was not in the building in 1993, but a lot of people who had been there 

during the 1993 bombing were very helpful because they were exiting 

faster. They knew that every second counted based on that previous 

experience and I took my cue from them.” Interview 1000518 (NIST 

2004) 

“I wasn’t at the building in 1993, but I knew that it took over an hour to 

get down the stairs, which influenced my decision to use the elevators.” 

Interview 1000731 (NIST 2004) 

10.3.10 Mobility-Challenged Occupants 

When evacuating a high-rise building, many different physical and medical conditions can affect travel 

ability on stairs. As the total distance traveled to reach an exit increases, the number of people unable to 

successfully complete the evacuation without resting or requiring assistance increases. In the WTC, stair 

travel challenged occupants with certain conditions, such as wheelchair confinement, pregnancy, asthma, 

visual impairment, physical impairment, obesity, arthritis, and old age. The current NYC Building Code 

defines a physical disability as any one of the following: 

• Impairment requiring the use of a wheelchair; or 
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NIST NCSTAR 1-7, WTC Investigation 159

• Impairment causing difficulty or insecurity in walking or climbing stairs or requiring the use 

of braces, crutches, or other artificial supports; or 

• Impairment caused by amputation, arthritis, spastic condition or pulmonary, cardiac, or other 

ills rendering the individual semi-ambulatory; or 

• Total or partial impairment of hearing or sight causing insecurity or likelihood of exposure to 

danger in public places; or 

• Impairment due to conditions of aging. 

Evacuation of WTC 1 and WTC 2 during the 1993 bombing identified an inefficiency at the World Trade 

Center in evacuating the mobility-challenged. The Report of the World Trade Center Review Committee 

in 1995 indicated: “Evacuation of persons with disability from the World Trade Center was slow and 

arduous, the [sic] Committee proposes that methods of evacuation should be studied to provide equally 

safe egress for these building occupants.” (New York City 1995) 

Despite introduction of evacuation chairs and a buddy system for pre-identified mobility-challenged 

occupants, 51 percent of the occupants in WTC 1 and 33 percent of the occupants in WTC 2 in 2001, 

noted that injured and disabled persons in the stairwell were a constraint to evacuation. However, 

occupants were quick to aid these individuals by guiding them throughout their evacuation or simply 

moving to the side of the stairwell to let those who were injured and others in need pass by when they 

could. 

In WTC 1, “someone was being carried down in some kind of 

handicapped apparatus and was strapped in. The occupant was being 

carried by two fellow occupants around the 30s. We stopped to allow 

them access.” Interview 1000834 (NIST 2004) 

In some cases, occupants noted passing slower mobility challenged individuals in the stairs and even 

slowing or stopping behind them. 

In WTC 2, “we saw an [occupant] who was hyperventilating. [The 

occupant] was walking down the stairs with assistance. We slowed 

down and came to a stop [because] we couldn’t get around the two 

[occupants].” Interview 1000556 (NIST 2004) 

In WTC 1, two occupants were helping an overweight occupant evacuate 

the building. During one of the helper’s descent down the stairs, the 

helper noted that “we took up the entire width of the stairway and no one 

could get around us until we came to a landing.” Interview 1000093 

(NIST 2004) 

Finally, some occupants reported mobility impaired occupants waiting on the stairs and/or landings for 

others to help them or to be rescued by the fire department. Many respondents recalled two specific 

occupants from WTC 1 who weren’t able to evacuate the building in time. These included an occupant in 

a wheelchair waiting with a friend on the stairs and an occupant with arthritis waiting at a fire department 

rest stop on floor 20. 
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160 NIST NCSTAR 1-7, WTC Investigation

Mobility-Challenged Occupants and Mortality below the 92nd floor in WTC 1 

While the mobility status of every decedent known to be below-the-impact-region (107) in WTC 1 could 

not be determined, it does not appear that mobility challenged individuals were significantly 

overrepresented amongst the decedent population. However, several occupants perished assisting 

mobility-challenged colleagues (Fahy and Proulx 2003) and many more occupants and emergency 

responders risked their lives assisting mobility-challenged colleagues who successfully evacuated. Being 

trapped by debris on the starting floor, delayed evacuation initiation (of statistical outlier magnitude), or 

performing emergency response building responsibilities accounts for the majority of the below the 

impact region deaths. 

10.4 EVACUATION SIMULATIONS 

The purpose of modeling evacuation from the World Trade Center towers was to obtain evacuation times 

for a variety of scenarios in order to provide additional context with which to understand the 

September 11, 2001 evacuation of WTC 1 and WTC 2. Table 10–2 shows each of the six general egress 

simulations performed, along with details regarding the evacuation type (full building evacuation or 

phased evacuation, also known as defend-in-place), the number of evacuees included in the simulation, 

the input response delay, which models were used, and any other relevant information about the 

simulation. There were five full building evacuations and one phased evacuation simulated. The full 

building evacuation simulations explored the effect on the total evacuation time (or number of successful 

evacuees if the time was fixed) with respect to the number of simulated evacuees, the presence or absence 

of building damage (observed on September 11, 2001), and the type of model used for the simulations. 

10.4.1 Egress Simulation Results 

Phased Evacuation 

The following section is a summary of the egress simulation results. For a more complete discussion of 

egress modeling inputs, assumptions, limitations, and results, refer to Appendix E: Egress Modeling. The 

purpose of simulation 1, phased evacuation, was to understand not only the time necessary to perform a 

phased evacuation, but to compare the results using three different egress models: Simulex, EXIT89, and 

buildingEXODUS. 

Table 10–3 shows the total phased evacuation times for each model. The three models simulate a total 

phased evacuation time between 31⁄2 and 4 minutes for all 600 occupants, assuming that evacuees start 

evacuating immediately. If the evacuees were randomly assigned a delay time between 0 and 10 minutes, 

the simulated total evacuation time was between 11 and 111⁄2 min for all 600 occupants. Phased 

evacuation is an efficient strategy to quickly remove occupants most at risk quickly from ‘routine’ 

emergencies, or those that fit within the design envelope of the life safety systems. 
Draft for Public Comment Discussion and Analysis 

NIST NCSTAR 1-7, WTC Investigation 161

Table 10–2. Egress simulation matrix. 

Scenario 

Full Building or 

Phased 

Evacuation 

Number of 

Evacuees 

Simulated 

Response 

Delay 

Model(s) 

Used Notes 

1 Phased 600 Zero 

0 - 10 min 

Simulex 

EXODUS 

EXIT89 

• Occupants travel three floors 

below fire floor 

2 Full Building 19,800 Zero 

0 - 10 min 

EXODUS 

EXIT89 

• Fully-occupied, without visitors 

• No damage 

3 Full Building 25,000 Zero 

0 - 10 min 

EXODUS 

EXIT89 

• Fully-occupied, with visitors 

• No damage 

4 Full Building 8,800 Zero 

0 - 10 min 

EXODUS 

EXIT89 

• September 11, 2001 population 

• No damage 

5a Calibration 

Simulation 

7,200 6 – 30 min EXODUS • Stop and Go 

• Ground – Floor 90 

• Damage above Floor 90 

5b Full WTC 1, 

with damage 

16,000 6 – 30 min EXODUS • Stop and Go 

• Ground – Floor 90 

• Damage above Floor 90 

6a Calibration 

Simulation 

7,400 2 – 17 min 

(Above 

Floor 77) 

6 – 30 min 

(Ground to 

Floor 76) 

EXODUS • Stop and Go 

• Ground – Floor 76 

• No Damage 

6b Elevator, WTC 2 19,800 Zero ELVAC • 14 Minute Elevator Simulation 

6c Full WTC 2, 

With Damage 

17,260 2 – 17 min 

(Above 

Floor 77) 

6 – 30 min 

(Ground to 

Floor 76) 

EXODUS • Stop and Go 

• Ground to Floor 107 for first 16 

min 

• Ground to Floor 77 for time range 

17 min – 72 min 

Table 10–3. Results for phased evacuation simulations. 

Evacuation Time (s) 

Evacuation Model 

Occupant Type, 

Characteristics No Delay 0 – 10 min Delay 

Simulex All office staff 

60 % men 

40% women 

240 690 

EXODUS 5 % males, age 17 - 29 

38 % males, age 30 - 50 

21 % males, age 51 - 80 

3 % females, age 17 - 29 

22 % females, age 30 - 50 

11 % females, age 51 - 80 

243 660 

EXIT89 All medium body size 

Emergency speed 

210 690 
Chapter 10 Draft for Public Comment 

162 NIST NCSTAR 1-7, WTC Investigation

Full Building Evacuation 

The purpose of simulations 2, 3, and 4 was to explore the effect of additional building occupants on the 

total building evacuation time. The three populations selected were (1) the NIST estimate of building 

population on September 11, 2001, (2) the NIST estimate of a fully-occupied tower without visitors 

(19,800 occupants), and (3) the NIST estimate of a fully-occupied tower including visitors (25,500 

occupants). Figure 10–9 shows the model results for the three building populations using the results from 

the buildingEXODUS model. The slope of the regression was identical whether comparing the 

September 11, 2001 population evacuation time to fully-occupied building evacuation time ((112 min - 55 

min) / (19,800 – 8,800) = 5.2 min per thousand additional occupants) or compared to fully-occupied with 

visitors building evacuation time ( (142 min – 55 min) / (25,600 – 8,800) = 5.2 min per thousand 

additional occupants). Thus, for a given building geometry, and a number of other important simulation 

assumptions, adding additional occupants to a building population resulted in a linear increase in total 

building evacuation time. 

Extrapolation of Evacuation Time for a Fully-Occupied Tower on September 11, 2001 

In Chapter 4.1 of this report, NIST estimated that the number of successful evacuees from WTC 1 on 

September 11, 2001 was 7,500, the total of which required roughly 100 minutes to exit the building. As 

shown in Figure 10–9 and Table 10–4, the buildingEXODUS model estimated that a fully-occupied 

building (population 25,500) required approximately 2.6 times as long (142 / 55 = 2.6) as a building with 

a September 11, 2001 occupant load; therefore, on September 11, 2001, a fully-occupied WTC tower with 

visitors, may have required roughly (100 x 2.6) = 260 min (over 4 hours) to fully evacuate. 

Using the same logical approach, if WTC 1 had been occupied by approximately 20,000 occupants on 

September 11, 2001 (fully-occupied without visitors), the evacuation would have taken (112 min / 55 min 

= 2.0 x 100 min = 200 min) over 3 hours to complete. 

Table 10–4. Total building evacuation time (simulated) for various occupant loads. 

Evacuation Model 

Evacuation 

Initiation Delay 

Input 

Evacuation Time 

(min): 8,800 

occupants 

Evacuation time 

(min): 19,800 

occupants 

Evacuation 

time (min): 

25,500 

occupants 

EXODUS 10 Minute Delay 55 112 142 

No Delay 52 110 141 

EXIT89 10 Minute Delay 71 – 74 92 – 113 119 – 139 

No Delay 58 – 78 97 – 11764 114 – 140 

 

64 The underlying theory for people movement in EXIT89 is based upon the work of Predtechenskii and Milinskii. They 

observed an inverse relationship between density and speed for three different types of movement: emergency, normal, and 

comfortable. The effect of crowding (density) on overall evacuation speed was greater when no delay time was assumed. A 

delay time when 19,800 occupants were present, according to EXIT89, spaces the occupants out and increases overall 

evacuation efficiency. This was not found to be the case for all occupant loads (25,500). 
Draft for Public Comment Discussion and Analysis 

NIST NCSTAR 1-7, WTC Investigation 163

Full Building Evacuation of a WTC Tower

0

20

40

60

80

100

120

140

160

0 5000 10000 15000 20000 25000 30000

Number of occupants in Tower

Evacuation Time (min)

8,800 - 9/11 capacity

19,800 - full capacity

25,585 - full capacity 

w ith visitors

Figure 10–9. Simulation of full building evacuation of a WTC tower with different 

occupant loads. 

Estimated Occupant Mortality From a Fully-Occupied WTC Tower on September 11, 2001 

Scenarios 5 and 6 were simulated in order to estimate the consequences of a fully-occupied, without 

visitors (total building population: 19,800) from WTC 1 or WTC 2 on September 11. Each tower was 

simulated with a September 11, 2001 population, and model assumptions and inputs (such as evacuation 

initiation delay time) were refined in order to roughly match the actual outcome on September 11, 2001. 

After the buildingEXODUS model results were determined to resemble gross characteristics of the 

September 11 evacuation, more occupants (fully-occupied building, without visitors) were added, and the 

results are presented below.

WTC 1 was assumed to have no passable stairwell above floor 91 starting at time zero. Elevators were 

assumed to be rendered inoperable. The simulation was ended at 103 minutes and any simulated 

occupant remaining in WTC 1 was assumed to have perished. The simulation shows that 69 percent of all 

occupants (13,600 occupants out of 19,800) would have evacuated WTC 1 prior to collapse: 0 percent 

(0 out of 3,800) from above the impact zone and 85 percent (13,600 out of 16,000) from below the impact 

zone. Table 10–5 summarizes the results of scenarios 5 and 6. 
Chapter 10 Draft for Public Comment 

164 NIST NCSTAR 1-7, WTC Investigation

Table 10–5. Simulated evacuation results for fully-occupied WTC 1 and WTC 2 on 

September 11, 2001. 

Percentage of Occupants Who 

Successfully Evacuated Relative to 

Where They Started 

Building 

Total 

Number of 

Occupants 

at t = 0.0 

Potential 

Number 

of 

Evacuees 

Occupants 

Remaining 

in Building 

at Collapse 

Occupants 

Trapped 

Above 

Floors of 

Impact Total 

Below 

Impact 

Above 

Impact 

WTC 1 19,800 16,000 6,200 3,800 69 % 85 % 0 % 

WTC 2 19,800 17,260 8,377 3,900 58 % 75 % 44 % 

Total 39,600 33,260 14,577 7,700 63 % 

Occupants of WTC 2 were assumed to use elevators for a period of 16 minutes, after which all elevators 

were assumed inoperable. After 16 minutes, when WTC 2 was attacked, it was assumed that no 

occupants survived above floor 77. The simulation was ended after 72 minutes, when WTC 2 collapsed. 

The simulation showed that 11,423 of the 19,800 occupants, or 58 percent of the initial population, would 

have successfully evacuated. Of the 11,423 simulated occupants, 8,883 evacuated using the stairwells, 

while 2,540 simulated occupants evacuated using elevators. Of the 8,377 occupants who remained in the 

building at the time of collapse, 3,900 simulated occupants were trapped above floor 77. Another 4,477 

were evacuating from below the 78th floor when WTC 2 collapsed: 1,231 simulated occupants were 

initially above the 77th floor, while 3,246 did not evacuate despite starting on a floor below the 

78th floor. The initial population of occupants below the 78th floor was 12,783. Therefore, (3,246 / 

12,783) 25 percent of the occupants who started below the impact floors did not successfully evacuate. 

Similarly, 3,900 of the initial 7,017, or 56 percent of the occupants at or above the impact region at 

8:46:30 a.m. in WTC 2 did not successfully evacuate. The simulation showed that 549 occupants initially 

above floor 77 successfully evacuated using stairwells, while only 3 of the 549 simulated occupants 

originated above floor 100. 

When combining the evacuation modeling results for WTC 1 and WTC 2, assuming 19,800 occupants in 

each tower and assuming that the aircraft impacts and collapses occurred at the same times as they did on 

September 11, 2001, over 14,000 occupants may have perished, not including any emergency responders, 

aircraft passengers, or bystanders. 

10.4.2 Egress Modeling Conclusions 

The egress modeling revealed three principal insights into the evacuation of WTC 1 and WTC 2 on 

September 11, 2001. First, while not an appropriate strategy for an emergency the scale of 

September 11, 2001, phased evacuation, under certain circumstances, moves occupants most at risk to a 

place of relative safety much more quickly and with less total impact upon building tenants than full 

building evacuation. Second, additional occupant load in a WTC tower (compared to September 11, 

2001) resulted in a linear increase in total building evacuation time. Third, assuming that the ratio of 

observed to simulated evacuation time of 2.6 would extend to the fully-occupied with visitors 

(25,500 occupants), full building evacuation scenario, WTC 1 would have taken over four hours to fully 

evacuate under the conditions of September 11, 2001. 
NIST NCSTAR 1-7, WTC Investigation 165

Chapter 11

FINDINGS

The evacuations of World Trade Center (WTC) 1 and WTC 2 on September 11, 2001, were documented 

and analyzed. In order to provide context for the 2001 evacuations, relevant historical egress events were 

explored. The evolution of the egress and communication systems, as well as the emergency procedures 

within WTC 1 and WTC 2 was documented. Over 1,000 interviews, using advanced interrogatory 

methods, were conducted. Hundreds of published accounts from a variety of sources were collected and 

analyzed. Emergency call records, emergency communication transcripts, Occupational Safety and 

Health Administration complaints, and Port Authority of New York and New Jersey design records and 

plans were analyzed. Analysis and compilation resulted in the following conclusions. 

11.1 BUILDING POPULATION AND DEMOGRAPHICS 

• There were 8,900 ± 750 people in WTC 1 at 8:46:30 a.m. on September 11, 2001. Of those, 

7,470 (or 84 percent) survived the attacks, while 1,466 – 1,539 occupants were killed in 

WTC 1. The National Institute of Standards and Technology (NIST) Investigation found that 

at least 107 occupants were killed below the 92nd floor. 

• There were approximately 8,540 ± 920 people in WTC 2 at 8:46:30 a.m. on 

September 11, 2001. Of those, 7,940 (or 93 percent) survived the attacks, while 624 – 697 

occupants were killed in WTC 2. The NIST Investigation found that at least 7 occupants 

were killed below the 78th floor. 

• Sixty-seven percent of WTC 1 occupants and 51 percent of WTC 2 occupants had started 

working at the World Trade Center during the four years prior to September 11, 2001. 

• Two-thirds of WTC 1 and WTC 2 occupants participated in at least one fire drill in the twelve 

months prior to September 11, 2001. Seventeen percent of the occupants did not participate 

in a fire drill during that time period, and 17 percent did not remember whether they had 

participated in a fire drill during that time period. 

• Nearly all occupants (93 percent) who participated in a fire drill were instructed about the 

location of the nearest stairwell. 

11.2 EVACUATION 

• Approximately 87 percent of WTC occupants, and over 99 percent of those below the floors 

of impact, were able to successfully evacuate. 

− At 9:02:59 a.m., when WTC 2 was hit and 17 minutes after WTC 1 was hit, 21 percent of 

survivors had exited WTC 1 and 41 percent of survivors had exited WTC 2. 
Chapter 11 Draft for Public Comment 

166 NIST NCSTAR 1-7, WTC Investigation

− By 9:37 a.m., 22 minutes before collapse, 95 percent of survivors had exited WTC 2. 

− At 9:58:59 a.m., when WTC 2 collapsed, 88 percent of survivors had exited WTC 1. 

− By 10:12 a.m., 16 minutes before collapse, 95 percent of survivors had exited WTC 1. 

• Occupants of WTC 1 overwhelmingly initiated their own evacuation. The NIST 

Investigation found no evidence that public address system announcements were heard by 

occupants of the building, although the fire command station was attempting to make 

announcements. 

• Self-evacuation and use of elevators for 16 minutes in WTC 2 saved roughly 3,000 lives. 

• During the last 20 minutes before each building collapsed, the evacuation rate in both 

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

that for those seeking and able to reach and use the undamaged exits and stairways, the 

egress capacity (the number and width of exits and stairways) was adequate to accommodate 

survivors. 

• In WTC 1, the average surviving occupant spent approximately 48 seconds per floor in the 

stairwell. That does not include any time prior to entering the stairwell, which was often 

substantial. 

• Some occupants of WTC 1 delayed or interrupted their evacuation resulting in over 

100 deaths below the impact region. 

• The NIST Investigation found no evidence to indicate that anyone who was above the 91st 

floor in WTC 1 after the airplane impact survived. This was due to the fact that the stairwells 

and elevators were destroyed and helicopter rescue, despite several attempts by both 

occupants and aircraft, was not possible. 

• In WTC 2, approximately 75 percent of the occupants above the 78th floor at 8:46:30 a.m. 

had successfully descended below the 78th floor prior to the airplane impact at 9:02:59 a.m. 

This occurred in spite of conflicting announcements, first urging people to return to their 

offices around 9:00 a.m., and then informing them that they may initiate an evacuation if 

conditions warranted around 9:02 a.m. 

• The NIST Investigation found evidence that 18 people successfully used Stairwell A in 

WTC 2 to leave the building after being on or above the 78th floor when Flight 175 hit the 

building. One of the 18 later died from injuries sustained on September 11, 2001. 

Additionally, at least two people went to or above the 78th floor after having been below the 

78th floor at the time of impact in order to assist trapped people and did not survive the 

collapse of WTC 2. 

• Minutes or seconds prior to collapse of WTC 2, an NYPD ESU officer reported from the 20s 

that his team was having trouble ascending the stairwell because of the volume of people in 

the stairs. Two individuals (at least) were aware of the tenability of Stairwell A and were 
Draft for Public Comment Findings 

NIST NCSTAR 1-7, WTC Investigation 167

above the 78th floor at some point after 9:02:59 a.m. Several callers from above the floors of 

impact were on telephone as WTC 2 collapsed. These facts suggest that among several 

possibilities a group of occupants from above the impact floors had identified a way through 

the impact region and were making their way down the stairwell some minutes before 

building collapse. 

• Computer egress modeling indicated that a full capacity evacuation of a single WTC tower 

with 25,000 occupants and visitors required a minimum evacuation time of 2 h and 15 min. 

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

elevator use, a full capacity evacuation of the WTC towers by 25,000 people would have 

required about 4 h (2.5 times 100 min). To achieve a significantly faster total evacuation at 

full capacity would have required increases in egress capacity (number and width of exits and 

stairways). 

• Egress modeling indicates that, had WTC 1 and WTC 2 been fully-occupied on 

September 11, 2001 with approximately 20,000 occupants each, over 14,000 occupants may 

have perished in the building collapses. 

11.3 DELAYS IN EVACUATION 

• Occupants in WTC 1 delayed starting their evacuation because environmental cues 

(information from the physical environment that something was terribly wrong) and floor 

(increased distance to safety) caused people out to find additional information. Next, the act 

of seeking additional information, that is “milling” about to make sense out of the situation, 

led people to take actions to prepare to evacuate. Finally, taking those actions to prepare to 

evacuate delayed the initiation of actually evacuating. Although there were other factors that 

had lesser influence on what people did, the paths of causal influence that defined the main 

process that led people to delay in the evacuation of WTC 2 on September 11, 2001 were 

identical to those for WTC 1, with one decided difference. Perceived risk was predicted by 

environmental cues and initial floor and also contributed to seeking additional information 

and taking pre-evacuation actions in WTC 2, while the effect of perceived risk was 

substantially lower in WTC 1. 

• Starting floor (increased distance to safety) substantially increased the odds that people would 

encounter environmental cues. Floor also increased delay in starting evacuation (this 

relationship is elaborated upon in much greater detail in Chapter 10 of this report), which, in 

turn, also increased the chances that people would encounter environmental cues. 

Observation of environmental cues (information from the physical environment that 

something was terribly wrong) in the stairwell had a large and direct effect on increasing the 

amount of time that people spent, on average, in their evacuation stairwell. In addition to this 

multi-step process with environmental cues as the key predicting variable, interrupting the 

process of evacuation for any reason increased the amount of time, on average, that people 

used to descend their evacuation stairwell. 

• Contrary to the perceptions of the occupants, counterflow in WTC 1 was determined by 

causal modeling analysis not to be a significant predictor of increased total evacuation time 
Chapter 11 Draft for Public Comment 

168 NIST NCSTAR 1-7, WTC Investigation

while in the stairwells when compared to other factors, including evacuation interruption and 

environmental cues. 

• Occupants in WTC 1 delayed starting their evacuation because environmental cues 

(information from the physical environment that something was terribly wrong) and floor 

(increased distance to safety) caused people out to find additional information, most likely 

information about what was going on and what they should do about. Next, the act of seeking 

additional information, that is “milling” about to make sense out of the situation, led people 

to take actions to prepare to evacuate. Finally, taking those actions to prepare to evacuate 

delayed the initiation of actually evacuating. Although there were other factors that had 

lesser influence on what people did, the main paths of causal influence that defined the main 

process that led people to delay in the evacuation of WTC 2 on September 11th were 

identical to WTC 1 with one decided difference. This was that perceived risk was predicted 

by environmental cues and initial floor and also contributed to seeking additional information 

and taking pre-evacuation actions in WTC 2 while the effect of perceived risk was 

substantially lower in WTC 1. This was likely the case because WTC 1 was hit without 

warning, and only the people in WTC 2 had time to wonder (perceive) if their tower was 

going to be a target. 

11.4 EMERGENCY MANAGEMENT AND PLANNING 

• Building occupants, 9-1-1 operators, fire department dispatch, WTC building officials, and 

Port Authority personnel lacked adequate situational awareness despite nearly constant crosscommunications. Many opportunities to communicate important information in a timely 

manner were missed, such as telling building occupants the general location of the impact 

region or whether to evacuate or not. 

• Faced with an uncertain situation, occupants of both buildings received conflicting feedback / 

advice from a variety of sources (including 9-1-1 operators, FDNY, family and friends, and 

the Port Authority) regarding whether to evacuate, whether to break windows, and what the 

nature of their situation was. 

• World Trade Center occupants were inadequately prepared to encounter horizontal transfers 

during the evacuation process and were occasionally delayed by confusion as to whether the 

hallway led to a stairwell and confusion about whether the transfer hallway doors would open 

or be locked. 

• In addition to an announcement in WTC 2 just prior to the airplane impact, announcements 

were made from the fire command station in the lobby of WTC 2 after the aircraft impact, 

although the NIST Investigation found no evidence that any surviving occupants heard these 

announcements. 

• The decision to establish the primary evacuation route underground through the Concourse 

(mall) and out up to street level by WTC 5 (commonly recalled as being by the Borders 

Bookstore) prevented a significant number of injuries and/or deaths. 
Draft for Public Comment Findings 

NIST NCSTAR 1-7, WTC Investigation 169

• The first “first responders” were colleagues and regular building occupants. Acts of everyday 

heroism saved many people whom traditional emergency responders would have been unable 

to reach in time. 

11.5 OCCUPANTS WITH MOBILITY CHALLENGES 

• Approximately 1,000 surviving occupants (a projection of the 6 percent reported in the 

telephone interviews) of WTC 1 and WTC 2 had a limitation that impacted their ability to 

evacuate, including recent surgery or injury, obesity, heart condition, asthma, elderly or 

otherwise requiring assistance to walk, pregnancy, and others. The most frequently reported 

disabilities were recent injuries and chronic illnesses; the number of occupants requiring use 

of a wheelchair was very small, relative to the frequency of other mobility challenges. 

• While many mobility-challenged individuals were able to successfully evacuate, often with 

assistance from co-workers or emergency responders, others were temporarily removed from 

the stairwells in order to allow more able occupants to evacuate the building (such as the rest 

station low in WTC 1 [somewhere between floors 12 and 20]). It remains unclear whether all 

of the mobility-challenged occupants and the helpers were able to successfully evacuate on 

September 11, 2001. No evidence of a mobility-challenged rest station in WTC 2 was found. 

• While the mobility status of every decedent known to be below the impact region (107) in 

WTC 1 could not be determined, it does not appear that mobility-challenged individuals were 

significantly overrepresented among the decedent population. 

• Mobility-challenged occupants were not universally accounted for by existing evacuation 

procedures, as some were left by colleagues (later assisted by strangers), while others chose 

not to identify their mobility challenge to any colleagues. 
Chapter 11 Draft for Public Comment 

170 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 171

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New York 1973. Fire Safety in Office Buildings, New York City - Local Law 5, The City of New York. 

January 18. 

New York 1984. Fire Safety in Buildings, Including Hotels, New York City - Local Law 16, The City of 

New York. . 

New York 1995. The Report of the World Trade Center Review Committee. The City of New York. 

NIST 2004a. NIST WTC Face-to-Face Interview Data Set. 2004. Gaithersburg, MD, Natl. Inst. Stand. 

Technol. 

NIST 2004b. NIST WTC Emergency Responder Interview Data Set. 2004. Gaithersburg, MD, Natl. Inst. 

Stand. Technol. 
Chapter 12 Draft for Public Comment 

174 NIST NCSTAR 1-7, WTC Investigation

PANYNJ Undated. “WTC – CHANGES TO EVACUATION SYSTEMS AFTER 1993.” The Port 

Authority of New York and New Jersey. WTC-115-P 

PANYNJ 1995. The Port Authority of NY & NJ World Trade Center Fire Safety Plan (Towers One and 

Two), The Port Authority of New York and New Jersey, New York. WTCI-13-NYC. 

PANYNJ 1996. WTC Fire Safety 1996. Video. WTCI-463-P 

PANYNJ 2001a. Port Authority of New York and New Jersey Radio Communications Transcripts from 

September 11, 2001. 

PANYNJ 2001b World Trade Center Emergency Procedures Manual 2001 – Confidential. The Port 

Authority of New York and New Jersey. 

PANYNJ 2001c. World Trade Center Space Book Plan, Building 1 and Building 2, Port Authority Real 

Estate Department, The Port Authority of New York and New Jersey, New York. 

Predtechenskii, V. M. and A. I. Milinskii. 1978. Planning for Foot Traffic in Buildings. New Delhi: 

Amerind Publishing Co. Pvt. Ltd. 

Powers, W. R. 1975. One World Trade Center Fire New York, N.Y., February 13, 1975. 1-15. Boston, 

MA, National Fire Protection Association. 

Powers, W. R. 1970. One New York Plaza Fire, New York, N.Y., August 5,1970. 1-15.. New York, New 

York Board of Fire Underwriters. 

Schank, R. C. and R. Abelson. 1977. Scripts, Plans, Goals, and Understanding. Nillsdale, NJ: Erlbaum. 

Sharry, J. A. 1974. South America Burning. Fire Journal, 1974, 23-33. 

Solomon, J., 1968. Emory Roth and Sons, New York, personal communication to M. Levy, The Port of 

New York Authority, New York. January 25. WTCI-477-P 

Solomon, J., 1975. Emory Roth and Sons, New York, personal communication to M. Levy, The Port of 

New York Authority, New York. February 18. 

Templer, J. 1992. Staircase: History and Theories. MIT Press , Cambridge, MA. 

Willey, A. E. 1972. High Rise Building Fire, São Paulo, Brazil. Fire Journal, 1972, 7-108. 

Williams, F. D. 1990. SLAM: The Influence of S.L.A. Marwill on the United States Army. In TRADOC, 

(Ft. Monroe, VA: U. S. Army). 
NIST NCSTAR 1-7, WTC Investigation 175

Appendix A 

TELEPHONE INTERVIEW SCRIPT

I would like to start by getting some background information. What year did you first start working at the 

World Trade Center? RANGE: 1975 - 2001 

$E 1975 2001 

DK 9998 

RF 9999 

«YRWRK» 

 

On September 11th, 2001, were you in any of the following positions with the World Trade Center? 

Port Authority Staff 1 

Fire Safety Staff 2 

Floor Warden or Searcher 3 

Maintenance or Security Staff 4 

NONE OF THESE 0 X 

DK 8 

RF 9 

«ROLES_01» 

«ROLES_02» 

«ROLES_03» 

«ROLES_04» 

 

During the year from September 11th, 2000 to September 11th, 2001, how many fire drills did you take 

part in at the World Trade Center? 

$E 0 99 

NONE 00 => SWLOC 

DK 98 => SWLOC 

RF 99 => SWLOC 

«FIRED» 

 

During these drills, were you ever instructed about the location of the emergency stairwell nearest to your 

office? 

YES 1 
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176 NIST NCSTAR 1-7, WTC Investigation

NO 2 => SWLOC 

DK 8 => SWLOC 

RF 9 => SWLOC 

«DEXIT» 

 

How many emergency stairwells were you shown? 

ONE 1 

TWO 2 

THREE 3 => LVFSW 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«HMEXT» 

«O_HMEXT» 

 

Before September 11th, had you learned in other ways about the locations of the three emergency 

stairwells? 

YES 1 

NO 2 

DK 8 

RF 9 

«SWLOC» 

 

SKIP IF NO FIRE DRILLS 

=> USESW 

Else => +1 

if FIRED=00,98-99 

«SOUT1» 

 

During any of the fire drills, did you leave your floor using one of the stairwells? 

YES 1 

NO 2 => USESW 

DK 8 => USESW 

RF 9 => USESW 

«LVFSW» 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 177

 

Which stairwells did you use? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSW1_01» 

«WHSW1_02» 

«WHSW1_03» 

«WHSW1_04» 

«O_WHSW1» 

 

Which side of the building was the stairwell located on? 

=> +1 

if NOT WHSW1=8 

NORTH 1 

SOUTH 2 

EAST 3 

WEST 4 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSL1» 

«O_WHSL1» 

 

Had you ever used any of the emergency stairwells prior to September 11th? 

=> DHELP 

if LVFSW=1 

YES 1 

NO 2 => DHELP 

DK 8 => DHELP 

RF 9 => DHELP 

«USESW» 
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178 NIST NCSTAR 1-7, WTC Investigation

 

SKIP FOR NO DRILLS AND NO USE OF STAIRWELLS 

=> AEVOF 

Else => +1 

if FIRED=00,98,99 AND USESW>1 

«SOUT2» 

 

Which stairwell did you use? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHSW2_01» 

«WHSW2_02» 

«WHSW2_03» 

«WHSW2_04» 

«O_WHSW2» 

 

SKIP IF NO FIRE DRILLS 

=> AEVOF 

Else => +1 

if FIRED=00,98-99 

«SOUT3» 

 

When you were evacuating on September 11th, how helpful was your experience during these drills? 

=> +1 

if FIRED=00 

Very Helpful 1 

Somewhat Helpful 2 

Somewhat Unhelpful 3 

Very Unhelpful 4 

DK 8 

RF 9 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 179

«DHELP» 

 

Prior to September 11th, were you aware of the evacuation procedures for your floor? 

YES 1 

NO 2 => FLWAR 

DK 8 => FLWAR 

RF 9 => FLWAR 

«AEVOF» 

 

Prior to September 11th, what was the evacuation procedure you were told to follow? 

LEAVE BUILDING IMMEDIATELY 1 

GO TO ELEVATOR LOBBY 2 

GO TO FLOORS UP OR DOWN 3 

GO TO ROOF 4 

STAY WHERE YOU ARE 5 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«EVACP» 

«O_EVACP» 

 

Did you know that there was a Floor Warden for your floor? 

=> +1 

if ROLES=1-4 

YES 1 

NO 2 

DK 8 

RF 9 

«FLWAR» 

 

The next questions ask about 3 different time periods. The first series of questions asks about when you 

first became aware that something had happened at the World Trade Center. This is a period of just a few 

seconds. The next series of questions asks about the time from when you first became aware that 

something had happened, to the time you first entered a stairwell or elevator to exit the building. The 

third series of questions asks about what happened during your evacuation, meaning the time from when 
Appendix A Draft for Public Comment 

180 NIST NCSTAR 1-7, WTC Investigation

you first entered a stairwell or elevator until you exited the tower. At the end of the interview, I will ask 

you if there is anything else about your experience on September 11th that you would like to contribute. 

CONTINUE 1 D 

«IFAWA» 

 

Now thinking back to the morning of September 11th, how did you first become aware that something 

had happened at the World Trade Center? 

$E 1 9 

HEARD SOMETHING (BOOM, CRASH, EXPLOSION, BLAST, ROAR, RUMBLING, ALARM)

 01 

SAW SMOKE OR FLAMES 02 

SAW DEAD BODIES 03 

SAW A PLANE 04 

SAW DEBRIS 05 

FELT SOMETHING (BUILDING MOVING, IMPACT, SHAKING, SWAYING, ROCKING, JOLT, 

EARTHQUAKE) 06 

FELL DOWN/FELL OFF CHAIR 07 

WARNED BY SOMEONE AROUND ME 08 

CONTACTED VIA PHONE 09 

CONTACTED VIA EMAIL 10 

PUBLIC ADDRESS SYSTEM 11 

NEWS MEDIA (TELEVISION, RADIO) 12 

OFFICE FURNITURE OR FIXTURES FALLING 13 

FURNITURE OR OTHER ITEMS FALLING OVER/DOWN 14 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«FAWAR» 

«O_FAWAR» 

 

What were you doing when you first became aware that something had happened to the World Trade 

Center? PROBE: Anything else? 

$E 1 9 

WORKING INDEPENDENTLY 01 

IN MEETING 02 

ON PHONE 03 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 181

CHECKING/WRITING EMAIL 04 

WAITING FOR ELEVATOR 05 

RIDING IN ELEVATOR 06 

CHATTING WITH COWORKERS 07 

EATING/HAVING COFFEE 08 

ENTERING BUILDING 09 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«ACTV1_01» 

«ACTV1_02» 

«ACTV1_03» 

«ACTV1_04» 

«ACTV1_05» 

«ACTV1_06» 

«ACTV1_07» 

«ACTV1_08» 

«ACTV1_09» 

«ACTV1_10» 

«O_ACTV1» 

 

At the moment when you first became aware that something had happened at the World Trade Center, did 

you notice any of the following? FOLLOW UP: Was that in your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the 

Tower 

Smoke 

Fire or Flames 

Fireballs 

Collapsed walls 

Jet Fuel 

Severely or fatally injured people 

Sprinklers going on 

A fire alarm sounding 

Power outage or flickering lights 

Fallen ceiling tiles 

Extreme heat 
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182 NIST NCSTAR 1-7, WTC Investigation

«NOT01_01» 

«NOT01_02» 

 

TIME PERIOD: 1 

Were there any disaster related events going on around you at this time? 

=> WHTW2 

if OR[NOT01-NOT11]=2-3 

YES 1 

NO 2 => WHTW2 

DK 8 => WHTW2 

RF 9 => WHTW2 

«OEVEN» 

 

TIME PERIOD: 1 

What was going on? 

ENTER RESPONSE 1 O 

DK 8 

RF 9 

«GOING» 

«O_GOING» 

 

TIME PERIOD: 1 

Were you still in<WHTOW>at this time? IF YES, SELECT APPROPRIATE CHOICE IF NO, ASK 

WHICH TOWER THEY WERE IN 

TOWER 1 1 

TOWER 2 2 

DK 8 

RF 9 

«WHTW2» 

 

TIME PERIOD: 1 

And were you still on the<WHFLO>floor at this time? RANGE: 1st - 110th FLOOR IF YES, 

SELECT/ENTER FLOOR IF NO, ASK WHICH FLOOR THEY WERE ON AND SELECT/ENTER IT 

$E 1 110 

BASEMENT 990 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 183

CONCOURSE/LOBBY 991 

PLAZA 992 

IN ELEVATOR 993 

OTHER, SPECIFY 997 O 

DK 998 

RF 999 

«WHFL2» 

«O_WHFL2» 

 

TIME PERIOD: 1 

At the moment when you first became aware that something had happened to the World Trade Center, 

approximately how many people were with you? RANGE: 0 - 999 PEOPLE WE WANT THE 

NUMBER OF PEOPLE THAT WERE IN THE SAME LOCATION AS THE RESPONDENT. (IN 

THEIR LINE OF SIGHT) 

$E 0 999 

NONE 00 => YOUIN 

DK 98 => YOUIN 

RF 99 => YOUIN 

«PEOP1» 

 

TIME PERIOD: 1 

Were any of these people injured at that time as a result of the event? 

YES 1 

NO 2 

DK 8 

RF 9 

«PEOIN» 

 

TIME PERIOD: 1 

Were you injured at that time, as a result of the event? 

YES 1 

NO 2 => ORISK 

DK 8 => ORISK 

RF 9 => ORISK 

«YOUIN» 

 
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184 NIST NCSTAR 1-7, WTC Investigation

TIME PERIOD: 1 

Would you say your injury was a ... 

An injury that did not impact your ability to evacuate, 1 

An injury that did impact your ability to evacuate but was not life threatening, or 2 

A life threatening injury 3 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«NATIN» 

«O_NATIN» 

 

TIME PERIOD: 1 

Still thinking about the moment when you first became aware that something had happened at the World 

Trade Center, did you believe that other people were in danger of being killed? 

YES 1 

NO 2 

DK 8 

RF 9 

«ORISK» 

 

TIME PERIOD: 1 

Did you believe you were in danger of being killed? 

YES 1 

NO 2 

DK 8 

RF 9 

«YRISK» 

 

TIME PERIOD: 2 

Now please think about the time period between when you first became aware that something had 

happened and when you first entered a stairwell or elevator to leave the tower. During this entire time 

period, were you given any additional information about what was going on? AFTER BECOMING 

AWARE OF THE EVENT, BUT BEFORE EVACUATION 

YES 1 

NO 2 => SEEKI 

DK 8 => SEEKI 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 185

RF 9 => SEEKI 

«GETIN» 

 

TIME PERIOD: 2 

Who gave you this information? PROBE: Anyone else? 

MANAGER/SUPERVISOR 1 

COWORKER INSIDE BUILDING 2 

FAMILY/FRIEND OUTSIDE BUILDING 3 

POLICE/FIREFIGHTER 4 

FLOOR WARDEN 5 

MEDIA PERSON (TV/RADIO) 6 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHINF_01» 

«WHINF_02» 

«WHINF_03» 

«WHINF_04» 

«WHINF_05» 

«WHINF_06» 

«WHINF_07» 

«O_WHINF» 

 

TIME PERIOD: 2 

What information did you get? PROBE: Any other information? 

INFORMATION ABOUT WHAT HAD HAPPENED 1 

INSTRUCTIONS TO LEAVE 2 

INSTRUCTIONS TO STAY 3 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHATI_01» 

«WHATI_02» 

«WHATI_03» 

«WHATI_04» 
Appendix A Draft for Public Comment 

186 NIST NCSTAR 1-7, WTC Investigation

«O_WHATI» 

 

TIME PERIOD: 2 

How did you get this information? PROBE: Any other way? 

FACE TO FACE 1 

TELEPHONE 2 

EMAIL/BLACKBERRY 3 

PA ANNOUNCMENT 4 

TV/RADIO 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«HOWGT_01» 

«HOWGT_02» 

«HOWGT_03» 

«HOWGT_04» 

«HOWGT_05» 

«HOWGT_06» 

«O_HOWGT» 

 

TIME PERIOD: 2 

And during this same time period, did you try to get additional information about what was going on? 

AFTER BECOMING AWARE OF THE EVENT, BUT BEFORE EVACUATION 

YES 1 

NO 2 => ORIS2 

TRIED, BUT WAS UNABLE TO GET INFORMATION 3 => ORIS2 

DK 8 => ORIS2 

RF 9 => ORIS2 

«SEEKI» 

 

TIME PERIOD: 2 

Who did you go to for this information? PROBE: Anyone else? 

MANAGER/SUPERVISOR 1 

COWORKER INSIDE BUILDING 2 

FAMILY/FRIEND OUTSIDE BUILDING 3 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 187

POLICE/FIREFIGHTER 4 

FLOOR WARDEN 5 

MEDIA PERSON (TV/RADIO) 6 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«GOINF_01» 

«GOINF_02» 

«GOINF_03» 

«GOINF_04» 

«GOINF_05» 

«GOINF_06» 

«GOINF_07» 

«O_GOINF» 

 

TIME PERIOD: 2 

What type of information did you try to find? PROBE: Anything else? 

INFORMATION ABOUT WHAT HAD HAPPENED 1 

INSTRUCTIONS TO LEAVE 2 

INSTRUCTIONS TO STAY 3 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHAI2_01» 

«WHAI2_02» 

«WHAI2_03» 

«WHAI2_04» 

«O_WHAI2» 

 

TIME PERIOD: 2 

How did you get this information? PROBE: Any other way? 

FACE TO FACE 1 

TELEPHONE 2 

EMAIL/BLACKBERRY 3 

PA ANNOUNCMENT 4 
Appendix A Draft for Public Comment 

188 NIST NCSTAR 1-7, WTC Investigation

TV/RADIO 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«HOWG2_01» 

«HOWG2_02» 

«HOWG2_03» 

«HOWG2_04» 

«HOWG2_05» 

«HOWG2_06» 

«O_HOWG2» 

 

TIME PERIOD: 2 

And during the time between when you first became aware that something had happened at the World 

Trade Center and when you first entered the stairwell or elevator to leave the tower, did you believe that 

other people were in danger of being killed? AFTER BECOMING AWARE OF THE EVENT, BUT 

BEFORE EVACUATION 

=> YRIS2 

if ORISK=1 

YES 1 

NO 2 

DK 8 

RF 9 

«ORIS2» 

 

TIME PERIOD: 2 

During that time period, did you believe you were in danger of being killed? 

=> PEODO 

if YRISK=1 

YES 1 

NO 2 

DK 8 

RF 9 

«YRIS2» 

 

TIME PERIOD: 2 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 189

During this time period, what were the people around you doing? PROBE: Were they doing anything 

else? AFTER BECOMING AWARE OF THE EVENT, BUT BEFORE EVACUATION 

$E 0 10 

NOONE AROUND/WAS ALONE 00 X 

TALKING TO OTHERS 01 

GATHERING PERSONAL/WORK ITEMS 02 

SEARCHING FOR OTHERS 03 

CALLING OTHERS 04 

FIGHTING FIRE/SMOKE 05 

LOCKING UP 06 

WORKING 07 

EVACUATING THE TOWER 08 

CRYING, RUNNING AROUND, IN SHOCK 09 

HELPING OTHERS 10 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«PEODO_01» 

«PEODO_02» 

«PEODO_03» 

«PEODO_04» 

«PEODO_05» 

«PEODO_06» 

«PEODO_07» 

«PEODO_08» 

«PEODO_09» 

«PEODO_10» 

«O_PEODO» 

 

TIME PERIOD: 2 

Did the people around you start evacuating before you did? 

=> DOBEF 

if PEODO=08 

YES 1 

NO 2 
Appendix A Draft for Public Comment 

190 NIST NCSTAR 1-7, WTC Investigation

DK 8 

RF 9 

«EVACB» 

 

TIME PERIOD: 2 

Did you do any of the following before starting your evacuation? 

$E 1 9 

Talk to another person face to face 01 

Gather personal items 02 

Telephone other people 03 

Continue working 04 

Save or transfer computer files 05 

Search for others 06 

Fight fire or smoke 07 

Move to another floor 08 

Help others 09 

Logging off/shutting down computer 10 

NONE OF THESE 11 X 

«DOBEF_01» 

«DOBEF_02» 

«DOBEF_03» 

«DOBEF_04» 

«DOBEF_05» 

«DOBEF_06» 

«DOBEF_07» 

«DOBEF_08» 

 

TIME PERIOD: 2 

Did you do anything else during this time? 

ENTER RESPONSE 1 O 

NO OTHER ACTIVITIES 0 

DK 8 

RF 9 

«OACTI» 

«O_OACTI» 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 191

 

TIME PERIOD: 2 

Before you began your evacuation, was there anything you wanted to do, but couldn't? 

YES 1 

NO 2 => SEE01 

DK 8 => SEE01 

RF 9 => SEE01 

«WANTD» 

 

TIME PERIOD: 2 

What was that? PROBE: Anything else? 

$E 1 7 

GATHER WORK ITEMS 01 

GATHER PERSONAL BELONGINGS 02 

CALL FRIEND/FAMILY MEMBER 03 

FIND FRIEND/COWORKER 04 

HELP FRIEND/COWORKER 05 

LOCK UP 06 

EVACUATE IMMEDIATELY 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WANAC_01» 

«WANAC_02» 

«WANAC_03» 

«WANAC_04» 

«WANAC_05» 

«WANAC_06» 

«WANAC_07» 

«WANAC_08» 

«O_WANAC» 

 

TIME PERIOD: 2 

Why couldn't you do that/those things? 

$E 1 9 
Appendix A Draft for Public Comment 

192 NIST NCSTAR 1-7, WTC Investigation

AFRAID 01 

LOCKED DOORS 02 

PHONE LINES DEAD 03 

INJURED 04 

EXIT BLOCKED 05 

TOO CROWDED 06 

TOLD TO STAY IN BUILDING 07 

TOLD TO LEAVE 08 

FATIGUE 09 

DISABLED 10 

SMOKE 11 

DAMAGE TO FLOOR 12 

WAS HELPING OTHERS 13 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«WHYNO_01» 

«WHYNO_02» 

«WHYNO_03» 

«WHYNO_04» 

«WHYNO_05» 

«WHYNO_06» 

«WHYNO_07» 

«WHYNO_08» 

«WHYNO_09» 

«WHYNO_10» 

«WHYNO_11» 

«WHYNO_12» 

«WHYNO_13» 

«WHYNO_14» 

«O_WHYNO» 

 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 193

Still thinking about the time between when you first became aware that something had happened at the 

World Trade Center and when you entered the stairwell or elevator to leave the tower, did you notice any 

of the following? FOLLOW UP: Was that in your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the 

Tower 

Smoke 

Fire or Flames 

Fireballs 

Collapsed walls 

Jet Fuel 

Severely or fatally injured 

people 

 

Sprinklers going on 

A fire alarm sounding 

Power outage or flickering lights 

Fallen ceiling tiles 

Extreme heat 

«SEE01_01» 

«SEE01_02» 

 

TIME PERIOD: 2 

Were there any disaster related events going on around you at this time? 

=> EVACF 

if OR[SEE01-SEE11]=2-3 

YES 1 

NO 2 => HELPY 

DK 8 => HELPY 

RF 9 => HELPY 

«ODISE» 

 

TIME PERIOD: 2 

What was going on? 

ENTER RESPONSE 1 O 

DK 8 

RF 9 

«GOIN2» 

«O_GOIN2» 
Appendix A Draft for Public Comment 

194 NIST NCSTAR 1-7, WTC Investigation

 

TIME PERIOD: 2 

Were you still on the<WHFL2>floor at this time? RANGE: 1st - 110th FLOOR IF YES, 

SELECT/ENTER FLOOR IF NO, ASK WHICH FLOOR THEY WERE ON AND SELECT/ENTER IT 

$E 1 110 

=> +1 

if (AND[SEE01-SEE11]=1) AND PEODO>0 AND PEODO<98 

BASEMENT 990 

CONCOURSE/LOBBY 991 

PLAZA 992 

ELEVATOR 993 

OTHER, SPECIFY 997 O 

DK 998 

RF 999 

«EVACF» 

«O_EVACF» 

 

TIME PERIOD: 2 

Did anyone help you in any way before you started your evacuation? 

YES 1 

NO 2 => DECID 

DK 8 => DECID 

RF 9 => DECID 

«HELPY» 

 

TIME PERIOD: 2 

Who helped you? PROBE: Anyone else? WE WANT THEIR ROLE NOT THE NAME OF THE 

PERSON 

POLICE OFFICER/FIREFIGHTER 1 

COWORKER 2 

STRANGER 3 

FLOOR WARDEN 4 

MANAGER/SUPERVISOR 5 

OTHER, SPECIFY 7 O 

DK 8 X 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 195

RF 9 X 

«WHOHE_01» 

«WHOHE_02» 

«WHOHE_03» 

«WHOHE_04» 

«WHOHE_05» 

«WHOHE_06» 

«O_WHOHE» 

 

TIME PERIOD: 2 

What did they help you with? PROBE: Anything else? 

$E 1 7 

LOCATING OTHERS 01 

HELPING OTHERS 02 

FINDING EXITS 03 

TREATING YOUR INJURIES 04 

PROVIDED INFORMATION/INSTRUCTIONS 05 

GATHER BELONGINGS 06 

CALM DOWN/EMOTIONAL ASSISTANCE 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHATD_01» 

«WHATD_02» 

«WHATD_03» 

«WHATD_04» 

«WHATD_05» 

«WHATD_06» 

«WHATD_07» 

«WHATD_08» 

«O_WHATD» 

 

TIME PERIOD: 2 

What was the one thing that made you decide to evacuate? 

WAS TOLD TO EVACUATE 1 
Appendix A Draft for Public Comment 

196 NIST NCSTAR 1-7, WTC Investigation

FRIENDS CO-WORKERS EVACUATED 2 

AFRAID/FELT IN DANGER 3 

FIRE ALARM WAS GOING OFF 4 

SAW SMOKE 5 

SAW FIRE 6 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«DECID» 

«O_DECID» 

 

How many minutes had passed before you started to evacuate? IF NEEDED: How much time passed 

between when you first became aware that something had happened to the World Trade Center and when 

you entered the stairwell or elevator to leave the tower. THIS IS NOT TIME TO EVACUATE. 

PLEASE CLARIFY WITH RESPONDENT IF TIME APPEARS TOO LONG. RESPONDENT WAS 

IN<WHTW2> RANGE FOR TOWER 1: 1 - 103 MINUTES RANGE FOR TOWER 2: 1 - 75 

MINUTES 

$E 1 103 

DK 998 

RF 999 

«TIMEP» 

 

SKIP FOR TOWERS 

=> EVAC2 

Else => +1 

if WHTW2=2 

«SKIP1» 

 

Did you begin your evacuation... WE ARE INTERESTED IN WHAT THEY KNOW NOW. THEY 

MAY NOT HAVE KNOWN WHEN THEY WERE EVACUATING, BUT NOW THEY CAN TELL US 

WHEN IT WAS. 

Before the plane hit Tower 2 1 

After the plane hit Tower 2, but before the Tower 2 collaspe 2 

After the Tower 2 collaspe 3 

DK 8 

RF 9 

«EVAC1» 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 197

 

SELECT1 

$S NS=2 CO=1 IN=EVAC1<=1 ;CO=2 IN=EVAC1<=2 ; 

Before the plane hit Tower 2 1 

After the plane hit Tower 2, but before the Tower 2 collaspe 2 

After the Tower 2 collaspe 3 

DK 8 

RF 9 

«SEL1» 

 

SELECT2 

Before the plane hit Tower 2 1 

After the plane hit Tower 2, but before the Tower 2 collaspe 2 

After the Tower 2 collaspe 3 

DK 8 

RF 9 

«SEL2» 

 

Did you begin your evacuation... 

=> EVCSO 

if EVAC1>0 

Before the plane hit Tower 2 1 

After the plane hit Tower 2 2 

DK 8 

RF 9 

«EVAC2» 

 

SELECT4 

$S CO=1 IN=EVAC2<=1 ; 

Before the plane hit Tower 2 1 

After the plane hit Tower 2 2 

DK 8 

RF 9 

«SEL3» 

 
Appendix A Draft for Public Comment 

198 NIST NCSTAR 1-7, WTC Investigation

Was there anything that kept you from evacuating sooner? 

YES, RECORD RESPONSE 1 O 

NO 2 

DK 8 

RF 9 

«EVCSO» 

«O_EVCSO» 

 

TIME PERIOD: 3 

When you began your evacuation, were you alone or with other people? PEOPLE THAT THEY KNOW, 

PEOPLE THAT THEY WERE TALKING WITH 

ALONE 1 

WITH OTHER PEOPLE 2 

DK 8 

RF 9 

«ALONE» 

 

TIME PERIOD: 3 

Which stairwell did you use for your evacuation? 

STAIRWELL A 1 

STAIRWELL B 2 

STAIRWELL C 3 

USED ELEVATOR 4 => FOLA1 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«STAIR_01» 

«STAIR_02» 

«STAIR_03» 

«STAIR_04» 

«STAIR_05» 

«O_STAIR» 

 

TIME PERIOD: 3 

Which side of the building was the stairwell located on? 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 199

=> /WHYST 

if NOT STAIR=8,7 

NORTH 1 

SOUTH 2 

EAST 3 

WEST 4 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«WHISI» 

«O_WHISI» 

 

TIME PERIOD: 3 

Why did you choose that/those stairwell(s) for your evacuation? PROBE: Any other reason? 

CLOSEST ONE 1 

FOLLOWED OTHER PEOPLE TO IT 2 

OTHER EXITS WERE BLOCKED 3 

SAME AS I USED IN PREVIOUS EMERGENCY 4 

I WAS TOLD TO USE THIS STAIRWELL 5 

OTHER, SPECIFY 7 O 

DK 8 X 

RF 9 X 

«WHYST_01» 

«WHYST_02» 

«WHYST_03» 

«WHYST_04» 

«WHYST_05» 

«WHYST_06» 

«O_WHYST» 

 

TIME PERIOD: 3 

At any time during your evacuation, did you leave that/those stairwell(s)? DO NOT INCLUDE PEOPLE 

WHO FOLLOWED THE PASSAGE WHERE THE STAIRWELLS START AND END. 

YES 1 

NO 2 => FOLA1 
Appendix A Draft for Public Comment 

200 NIST NCSTAR 1-7, WTC Investigation

DK 8 => FOLA1 

RF 9 => FOLA1 

«LEVST» 

 

TIME PERIOD: 3 

Which floor were you on when you left the stairwell? IF RESPONDENT UNSURE, SELECT 997 AND 

RECORD RANGE OF FLOORS EXAMPLE: 34-40 

$R 1 110 

UNSURE, RECORD RESPONSE 997 O 

«FLLST» 

«O_FLLST» 

 

TIME PERIOD: 3 

Why did you leave the stairwell? PROBE: Any other reason? 

$E 1 9 

I GOT LOST 01 

WAS TOLD TO LEAVE STAIRWELL 02 

TO HELP SOMEONE 03 

TO GO BACK AND GET SOMETHING 04 

TOO CROWDED 05 

SMOKE IN STAIRWELL 06 

PATH OBSTRUCTED 07 

A LOCKED DOOR 08 

STAIRWELL LED TO A FLOOR 09 

OTHER, SPECIFY 97 O 

DK 98 

RF 99 

«WHYLS_01» 

«WHYLS_02» 

«WHYLS_03» 

«WHYLS_04» 

«WHYLS_05» 

«WHYLS_06» 

«WHYLS_07» 

«WHYLS_08» 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 201

«WHYLS_09» 

«WHYLS_10» 

«O_WHYLS» 

 

Screen [Template 3] -> FLOA5 

=> +1 

if FLWAR>1 

Did any of the following help you evacuate while you were in the building?

 Yes No DK RF

Instructions or assistance from your floor warden 

Instructions or assistance from Police or Firefighters 

Support and encouragement from others 

Exit signs 

Photo luminescent paint in stairwells 

«FOLA1» 

 

Screen [Template 3] -> EVCM7 

=> +1 

if NOT STAIR<4 

Did any of the following make your evacuation more difficult while you were in the building? 

 Yes No DK RF 

Crowded stairwells 

Firefighters or Police moving up stairwell 

Disabled or injured people being taken down stairwell 

Locked doors 

Poor lighting 

Confusing or missing signs 

Lack of clear instructions 

«EVCM1» 

 

Screen [Template 3] -> EXP11 
Appendix A Draft for Public Comment 

202 NIST NCSTAR 1-7, WTC Investigation

Please tell me if you noticed any of the following at any time during your evacuation. FOLLOW UP: 

Was that in your immediate area or outside the Tower? 

 Did Not Notice Noticed in Immediate Area Noticed Outside the Tower

Smoke 

Fire or Flames 

Fireballs 

Collapsed walls 

Jet Fuel 

Severely or fatally injured people 

Sprinklers going on 

A fire alarm sounding 

Power outage or flickering lights 

Fallen ceiling tiles 

Extreme heat 

«EXP01_01» 

«EXP01_02» 

 

TIME PERIOD: 3 

During your evacuation, did you turn back at any time? “TURN BACK” MEANS “GO BACK UP”. 

YES 1 

NO 2 => EXITS 

DK 8 => EXITS 

RF 9 => EXITS 

«TURNB» 

 

TIME PERIOD: 3 

Why did you turn back? PROBE: Any other reason? 

$E 1 7 

I GOT LOST 01 

I WAS TOLD TO TURN BACK 02 

TO HELP SOMEONE 03 

TO GET SOMETHING 04 

IT WAS TOO CROWDED 05 

SMOKE IN THE STAIRWELL 06 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 203

MY PATH WAS OBSTRUCTED 07 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHYTB_01» 

«WHYTB_02» 

«WHYTB_03» 

«WHYTB_04» 

«WHYTB_05» 

«WHYTB_06» 

«WHYTB_07» 

«WHYTB_08» 

«O_WHYTB» 

 

TIME PERIOD: 3 

Did you exit the stairwell or elevator to the mezzanine or to the concourse? 

MEZZANINE 1 

CONCOURSE 2 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«EXITS» 

«O_EXITS» 

 

TIME PERIOD: 3 

How much time passed between the moment you first began your evacuation to when you exited the 

Tower? PLEASE CLARIFY WITH RESPONDENT IF TIME APPEARS TOO LONG. RESPONDENT 

WAS IN<WHTW2> RANGE FOR TOWER 1: 1 - 103 MINUTES RANGE FOR TOWER 2: 1 - 75 

MINUTES 

$E 1 103 

DK 998 

RF 999 

«TIMP2» 

 

SKIP FOR TOWERS 

=> +2 
Appendix A Draft for Public Comment 

204 NIST NCSTAR 1-7, WTC Investigation

Else => +1 

if WHTW2=2 

«SKIP2» 

 

TIME PERIOD: 3 

Did you exit the tower... 

Eliminate -> 2 

According to NOT SEL1-SEL2 

Before the plane hit Tower 2 1 => GETOU 

After the plane hit Tower 2 but before the Tower 2 collapse, or 2 => GETOU 

After the Tower 2 collapse 3 => GETOU 

DK 8 => GETOU 

RF 9 => GETOU 

«EXIT1» 

 

TIME PERIOD: 3 

Did you exit the tower... 

Eliminate -> 1 

According to NOT SEL3 

Before the plane hit Tower 2, or 1 

After the plane hit Tower 2 2 

DK 8 

RF 9 

«EXIT2» 

 

Please remember that this study is intended as a fact finding mission and not a fault finding mission. It is 

crucial that we determine why some people were successful in their evacuation while others were not. 

Was there anyone on your floor that was not successful in their evacuation? 

YES 1 

NO 2 => PHYSI 

DK 8 => PHYSI 

RF 9 => PHYSI 

«GETOU» 

 

Why didn't they make it out? PROBE: Any other reason? 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 205

$E 1 8 

WAS INJURED 01 

WAS DISABLED 02 

REFUSED TO LEAVE 03 

DID NOT THINK IT WAS SERIOUS 04 

STAYED BACK TO HELP SOMEONE 05 

WAS TOLD TO STAY 06 

STRUCTURAL DAMAGE 07 

SMOKE OR FIRE 08 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«WHYNG_01» 

«WHYNG_02» 

«WHYNG_03» 

«WHYNG_04» 

«WHYNG_05» 

«WHYNG_06» 

«WHYNG_07» 

«WHYNG_08» 

«WHYNG_09» 

«O_WHYNG» 

 

On September 11th, 2001, did you have any physical problems that made it more difficult for you to leave 

the tower? Please do not include injuries caused by the incident or evacuation. 

YES 1 

NO 2 => AGE 

DK 8 => AGE 

RF 9 => AGE 

«PHYSI» 

 

What type of physical problem? PROBE: Anything else? 

$E 1 9 

BLIND/PARTIALLY BLIND 01 

DEAF 02 
Appendix A Draft for Public Comment 

206 NIST NCSTAR 1-7, WTC Investigation

IN WHEELCHAIR 03 

NEED WALKING ASSISTANCE 04 

OBESITY 05 

HEART CONDITION 06 

PREGNANT 07 

ASTHMA 08 

ELDERLY 09 

OTHER, SPECIFY 97 O 

DK 98 X 

RF 99 X 

«LIMIT_01» 

«LIMIT_02» 

«LIMIT_03» 

«LIMIT_04» 

«LIMIT_05» 

«LIMIT_06» 

«LIMIT_07» 

«LIMIT_08» 

«LIMIT_09» 

«LIMIT_10» 

«O_LIMIT» 

 

What is your age? RANGE: 1 - 98 YEARS 

$E 1 99 

RF 99 

«AGE» 

 

READ ONLY IF YOU CAN'T TELL. What is your gender? 

MALE 1 

FEMALE 2 

RF 9 

«GEND» 

 

What language do you speak best? 

ENGLISH 1 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 207

SPANISH 2 

OTHER, SPECIFY 7 O 

DK 8 

RF 9 

«PLANG» 

«O_PLANG» 

 

Were you working in Tower 1 or Tower 2 during the 1993 bombing? 

=> SAY11 

if YRWRK>1993 

YES 1 

NO 2 => CONCR 

DK 8 => CONCR 

RF 9 => CONCR 

«WBOMB» 

 

During the 1993 bombing, did you evacuate immediately or wait to evacuate? 

EVACUATE IMMEDIATELY 1 

WAIT TO EVACUATE 2 

DK 8 => +2 

RF 9 => +2 

«EVBOM» 

 

At the time of the 1993 bombing, did you feel you that your decision to<EVBOM>was the right decision? 

YES 1 

NO 2 

DK 8 

RF 9 

«DEC93» 

 

After the 1993 bombing how concerned were you that terrorists would attack the World Trade Center? 

Were you... 

Extremely Concerned 1 

Very Concerned 2 

Moderately Concerned 3 
Appendix A Draft for Public Comment 

208 NIST NCSTAR 1-7, WTC Investigation

Slightly Concerned 4 

Not at all Concerned 5 

DK 8 

RF 9 

«CONCR» 

 

Is there anything else you would like to say about your experience on September 11th? 

YES, RECORD RESPONSE 1 O 

NO 2 

DK 8 

RF 9 

«SAY11» 

«O_SAY11» 

 

IMPACT FLOOR FLAG 

=> * 

if IF(((WHTW2=1 AND WHFL2>91 AND WHFL2<99) OR (WHTW2=2 AND WHFL2>77 AND 

WHFL2<111)),1,0) 

IMPACT FLOOR FLAG 1 

«FFLAG» 

 

163: LFLAG 

 Single 

min = 1 max = 1 l = 1 

2003/09/18 15:21 

LOCATION FLAG 

=> * 

if IF((WHFL2>990 AND WHFL2<994),1,0) 

LOCATION FLAG 1 

«LFLAG» 

 

EVENT FLAG 

=> * 

if IF(((AND[NOT02-NOT06]=2-3) OR (AND[SEE02-SEE06]=2-3) OR (AND[EXP02-EXP06]=2-

3)),1,0) 
Draft for Public Comment Telephone Interview Script 

NIST NCSTAR 1-7, WTC Investigation 209

EVENT FLAG 1 

«EFLAG» 

 

DISABILITY FLAG 

=> * 

if IF((PHYSI=1),1,0) 

DISABILITY FLAG 1 

«DFLAG» 

 

ROLE FLAG 

=> * 

if IF((ROLES=1-4),1,0) 

ROLE FLAG 1 

«RFLAG» 

 

We may be interested in learning more about your experience on September 11th. Would it be okay if we 

follow up with you sometime in the future to get more detailed information on your evacuation 

experience? 

=> +1 

if FFLAG+LFLAG+EFLAG+DFLAG+RFLAG==0 

YES 1 

NO 2 

«FOLUP» 

 

PRESS ENTER TO CONTINUE 

Those are all the questions we have. The valuable information you provided will help designers and 

engineers improve building safety, and help emergency planners improve building evacuation procedures. 

Thank you so much for taking the time to talk with me, and have a good day/evening. Good-bye. 

END OF SURVEY 1 D => /INT99 

«THANK» 
Appendix A Draft for Public Comment 

210 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 211

Appendix B 

QUALITATIVE ANALYSIS CODING

The qualitative data from the face-to-face interviews was used to enrich the statistically-based telephone 

interview data by providing detailed descriptions of the experiences of occupants of the two towers on 

September 11, 2001. These data identified unknown information, evaluated technical hypotheses, and 

explored conscious and subconscious motivations for occupant behaviors, while allowing for 

comparisons to the telephone interview data. This appendix provides an annotated listing of the coding 

used by National Institute Standards and Technology (NIST) to analyze the face-to-face interview data. It 

was intended to serve the dual purposes of the project by identifying 1) building damage and 

environmental conditions before or during evacuation and 2) patterns of behavior before or during 

evacuation that may have helped or hindered occupants’ evacuation. Some of the codes would apply to 

an entire interview (for example, the tower 1, tower 2, or Building 7 code simply indicates the building 

where the respondent was located at the beginning of the event), some part of their evacuation experience 

(such as the counterflow, crowded, or elevator codes), or an observation of their surroundings (such as the 

floor damage, building damage, saw fire, or saw smoke codes). The codes are simply presented in 

alphabetical order. 

Alarm: Heard fire or other alarm (not public address system) 

Alternative Activity: Decision to suspend evacuation in favor of another activity (call, get 

drink/food/possession ...) 

Antisocial Behavior: Any directly observed behavior which presents obstacle to people getting out 

Assist: Assisted or motivated an injured / handicapped / reluctant person 

Bldg Employee: Building service employee (janitor, elevator operator, contract service employee -- not 

building security) as an information source 

Boss or higher: Boss, manager, or higher as an information source 

Building 7: Occupant was in Building 7 when Tower 1 was struck 

Building Damage: Directly observed damage to building structure after beginning evacuation (see floor 

damage for damage on floor where they began their evacuation) 

Building Security: Uniformed building security personnel (not Port Authority, NYPD, or FDNY) 

Cellular Phone: Use of a cellular phone as an information source 

Concourse: Occupant mentions being in the Concourse during their evacuation. Autocoded with phrases 

such as Concourse Borders shopping mall,... 

Confusion: Unaware of where to go because of a lack of visual certainty 

Counterflow: Movement of people or responders against the flow of egress delayed evacuation 

Coworker: Coworker as an information source 

Crowded: Egress slowed because of density, but no specific identifiable cause 

Elevator - Considered: Occupant considered using elevators but did not make use of elevators for 

evacuation. Code should also be used when occupant states they knew not to use elevators. 
Appendix B Draft for Public Comment 

212 NIST NCSTAR 1-7, WTC Investigation

Elevator - Used: Used an elevator during evacuation 

Email: Used email or PDA device as an activity 

Email/PDA: Use of email or PDA as an information source 

Evacuation Decision: Marks when respondent decided they needed to leave. Note that his may occur 

more than once if the occupant decides to interrupt their evacuation on it has begun. 

Experience Aid: Prior experience aided evacuation 

Experience Obstacle: Experience caused them to delay evacuation 

FBI: Occupant specifically mentioned seeing someone identified with the Federal Bureau of Investigation 

FDNY: Fire department personnel as an information source 

Fire Affecting Egress: Observed fire in egress path 

Firefighting by Occupants: Occupant engaged in firefighting activities prior to or during evacuation 

First Awareness: First indication that respondent became aware that something was wrong on the 

morning of September 11 

Floor Damage: Directly observed damage to building structure on floor where they began evacuation 

prior and to beginning evacuation (see building damage for damage during evacuation and not on 

floor where they began their evacuation) 

Floor Start: Starting floor for evacuation that is different from the employer assigned floor 

Floors 1 to 9: Occupant began their evacuation in the stated range of floors 

Floors 10 to 19: Occupant began their evacuation in the stated range of floors 

Floors 100 to 110: Occupant began their evacuation in the stated range of floors 

Floors 20 to 29: Occupant began their evacuation in the stated range of floors 

Floors 30 to 39: Occupant began their evacuation in the stated range of floors 

Floors 40 to 49: Occupant began their evacuation in the stated range of floors 

Floors 50 to 59: Occupant began their evacuation in the stated range of floors 

Floors 60 to 69: Occupant began their evacuation in the stated range of floors 

Floors 70 to79: Occupant began their evacuation in the stated range of floors 

Floors 80 to 89: Occupant began their evacuation in the stated range of floors 

Floors 90 to 99: Occupant began their evacuation in the stated range of floors 

Floors below ground: Occupant began their evacuation in the stated range of floors 

Followed Crowd: Respondent reports following a crowd, or else avoiding a crowd, eg a stair looks too 

crowded. 

Fruin: An indication of Fruin level of service for crowding in the stairwells 

Get Information: Looking for more information (not milling) 

Get Out Instruction: Identifying the instruction to evacuate the building - it is of interest as to who this 

instruction came from (secretary or supervisor, for instance). 

Health: Occupant reports being tired, sick, in pain ... effecting egress 
Draft for Public Comment Qualitative Analysis Coding 

NIST NCSTAR 1-7, WTC Investigation 213

Helping: Stopped or slowed by helping other who needed assistance 

High: Floors 77 to 110 in the Towers and 26 to 47 in Building 7 

Immediate Evacuation: Where occupant made an immediate decision to evacuate 

Instruction: Instruction to take action counter to continuing or commencing evacuation (any source). 

Instruction Aid: Received an instruction (any source) that aided evacuation 

Interesting behavior: This code is a chance to identify any interesting behavior to highlight for the 

report. 

Internal Aids: Any thoughts or techniques which calmed occupant during evacuation (counting, 

humming, thinking of family ...) 

Know 1993: While not there, occupant was influenced by second-hand knowledge of the 1993 

WTC bombing 

Left Stairwell: Occupant left stairwell during evacuation 

Location: Indicates a specific location within a building before or during evacuation 

Location – Concourse: Indicates the occupant was on the Concourse Level 

Locked/Jammed Door: First hand observation of a locked or jammed door effecting egress 

Low: Floors 1 to 40 in the Towers and 1 to 25 in Building 7 

Media: Use of the media (TV, radio ...) as an information source 

Medical Disability: Asthma, illness, pregnancy 

Medium: Floors 43 to 74 in the Towers 

Mill: Talked with other people inside the building as an activity 

Milling Aid: Milling as an aid to evacuation 

No alarm: Occupant specifically reports not hearing an alarm at any time during their evacuation 

Other Activity: Activities of interest to the NIST investigation not otherwise categorized 

Other Egress Aid: Aid to egress of interest to the NIST investigation not otherwise categorized 

Other Info: Other information source of interest to the NIST investigation not otherwise categorized 

Other Mobility Challenge: Mobility challenged information of interest to the NIST investigation not 

otherwise categorized 

Other Obstacle: Obstacle effecting egress not otherwise categorized 

Overweight: Overweight or out of shape 

Pager: Occupant reported using a pager at some time before or during their evacuation 

Phone: Used a phone to receive information (not cellular phone) 

Phone Call: Respondent made use of a telephone as an activity (not cell phone). Typically to inform 

someone of status -- family, 911, etc. For phone as an information source, use cellular phone or 

phone 

Photoluminscent Paint: Photoluminescent paint mentioned as an aid to evacuation 

Physical Assistance: Received physical assistance which aided evacuation 
Appendix B Draft for Public Comment 

214 NIST NCSTAR 1-7, WTC Investigation

Physical Disability: Wheelchair, crutches, cane, bad knees, ... 

Police: NYPD or other official gun-carrying officers as an information source 

Port Authority: PAPD or person with building responsibility as an information source 

Practice Drill: Participated in practice drills at WTC 

Present 1993: Occupant was present at the 1993 WTC bombing 

Prior Evac Experience: Participated in previous building evacuation (WTC or not), except for the 1993 

bombing. 

Prosocial Behavior: Letting people in line, crowd calming conversations, singing, counting out loud, 

emergent leadership or other behaviors which aid evacuation 

Public Address Aid: Information from public address system assisted evacuation 

Public Address System: Public address system as an information source 

Quote: A short quotation that may warrant inclusion in the final report because it creates, supports, or 

refutes a hypothesis or is particularly compelling account of a particular event 

Rest on stairs: This code identifies when the respondent actually rests in the stairs and/or observes this 

behavior. 

Retrieve: Retrieved a personal or business belonging 

Risk - High Level: High perception of danger faced by the occupant 

Risk - Low Level: Low perception of the danger faced by respondent 

Saw Airplane: Directly observed airplane hitting occupant’s building (saw other impact for observation 

of plane hitting other building) 

Saw FDNY: Respondent mentions the presence of firefighters during their egress whether or not they 

helped or hindered egress or gave instructions 

Saw Fire: Directly observed flames within the building before or during evacuation (does not include 

observation of fire on exterior of another building) 

Saw Injured: Directly observed an injured or dead person, not including themselves 

Saw Other: Saw something relevant to NIST investigation not otherwise categorized 

Saw Other Impact: Directly observed impact of plane into a building other that the one the respondent 

was in (for example someone in T2 seeing plane hit T1) 

Saw People: Observed someone doing something noteworthy, but it did not impact egress 

Saw Smoke: Directly observed smoke within the building before or during evacuation (does not include 

observation of smoke on exterior of another building) 

Saw Water: Directly observed water within the building before or during evacuation (not drinking water) 

Saw/Felt/Heard Impact: Indication of the impact of a plane hitting the building. (If you saw the impact 

of the plane hitting the other building and felt it in your building, it would get coded twice; once 

with this code and once with Saw other impact) 

Search: Searched for a person or people 

Shoes: Poor footwear choice or other's footwear cluttering pathway effects egress 

Signage: Signs posted in evacuation route aided evacuation 
Draft for Public Comment Qualitative Analysis Coding 

NIST NCSTAR 1-7, WTC Investigation 215

Slow Occupant: A slow occupant ahead of respondent in line impeded egress 

Smell/Saw Fuel: Direct observation or smell of jet fuel in the building 

Smoke Affecting Egress: Observed smoke in egress path 

Stair Unknown: Direct mention that respondent did not know which stairwell was used for evacuation 

Stairway from basement: Occupant used stairs from below-ground levels during evacuation 

Stairwell A: Occupant was in Stairwell A 

Stairwell A or C: Occupant was in Stairwell A or C (narrow stairwell) 

Stairwell B: Occupant was in Stairwell B 

Stairwell C: Occupant was in Stairwell C 

Strange: For something that just doesn't make sense and is worth further investigation 

Superflow: Prosocial behavior of letting higher priority evacuees (i.e., injured) pass faster delayed 

evacuation 

Superflow Aid: By helping someone injured or handicapped, was able to egress faster than they 

otherwise would have 

T2 Collapse: Note by respondent of collapse of Tower 2 

T2 hit: Note by respondent of Tower 2 being hit 

Time: Indicates a relative or absolute time 

Tower 1: Occupant was in Tower 1 when Tower 1 was struck 

Tower 2: Occupant was in Tower 2 when Tower 1 struck 

Training Aid: Prior training aided evacuation 

Transfer Floor: Occupant notes use of a transfer floor during egress by stairwell 

Trapped: Occupant was trapped prior to or during evacuation 

Victim: Mention of people who died in the event 

Visibility: low light condition or dust (but not smoke) effecting egress 

Visitor: Respondent specifically mentions being a visitor or being with a visitor at some point during 

their evacuation 

Walkie-Talkie: Respondent mentions the use of a walkie-talkie either by the respondent or by someone 

else 

Warden: Activities associated with responsibilities of the floor wardens 

Warden Info: Floor warden as a source of information 

Water: Water within the building before or during evacuation (not drinking water) was an obstacle to 

evacuation 
Appendix B Draft for Public Comment 

216 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 217

Appendix C 

CAUSAL MODELING

The telephone interview results permit rigorous statistical analysis of individual components (questions). 

In order to understand what factors impacted the overall evacuation time of the average occupant in 

World Trade Center (WTC) 1 or WTC 2, two primary dependent variables were predicted: how long an 

individual delayed initiating their evacuation, where initiation is defined as entering a stairwell or elevator 

with the intention of exiting the building; and how long an individual spent traversing the stairwells. The 

sum of these two times was the total evacuation time. Multivariate regression modeling was utilized to 

rigorously establish factors which contributed to increasing the overall evacuation time. 

C.1 PREDICTING EVACUATION DELAY IN THE WORLD TRADE CENTER 

This project analyzed the factors (variables) and social processes (the major paths of causal links between 

variables) that influenced people delaying the initiation of their evacuation out of WTC 1 and WTC 2 on 

September 11, 2001. Evacuation delay was defined as the number of minutes that passed from when a 

person first became aware that something was wrong until they began their evacuation. 

Method 

The purpose of the Telephone Interviews was to collect 800 computer assisted telephone interviews 

(CATI) of persons occupying either of the two WTC towers (WTC 1 and WTC 2) at the time of the 

terrorist attacks on September 11, 2001 in order to explore occupants’ evacuation experiences. A total 

sample size of 800, with an allocation of 400 to each tower, was determined to simultaneously maximize 

the statistical precision within each tower. Estimates of percentages from tower specific survey data at 

400 exhibit sampling errors not greater than 2.5 percent, and 98 percent confidence intervals of 

percentages are no greater than ± 5 percent. This level of precision is more than adequate for examining 

characteristics of occupants and egress attributes. Additionally, mutlivariate modeling requires the use of 

F tests to determine significance of the regression models, i.e., testing the null hypothesis that the ratio of 

explained variance to error/residual variance is equal to or less than zero. The sample size of 400 per 

tower is more than adequate for this analysis since, for example, in a model featuring 20 independent 

variables, a sample size of 400 and a .05 level of significance (Type I error), the power of the F test to 

detect an R-squared of .06 is just over 81 percent. 

Attempts were made to equally divide the respondents among WTC 1 and WTC 2 occupants (i.e., n=400 

occupant telephone interviews from each tower). Within each of the WTC buildings, independent 

proportionate stratified samples of survivors were drawn. In other words, each occupant of a particular 

tower had an equal probability of being selected. The sample was collected and weighted in accordance 

with the analysis presented in Section 3.2.2 of this report. The questions posed to the respondents are 

included in earlier in this report. 
Appendix C Draft for Public Comment 

218 NIST NCSTAR 1-7, WTC Investigation

Pre Modeling Analysis 

Two bodies of research and theory were reviewed to inform our consideration of what explanatory 

factors/variables should be used to predict occupant evacuation from the towers. 

First, we compiled a comprehensive list of the 300 publications in the research record on human 

response/evacuation to risk events/information for natural and technological hazards and disasters, for 

example, research on floods, hurricanes, earthquake predictions, tornadoes, nuclear power plant accidents, 

hazardous chemical spills and many others. These studies included not only examinations of evacuations, 

but also events in which other protective actions were warranted. This literature was reviewed, 

summarized, and synthesized for salient theoretical constructs and the relationships between them. This 

bibliography is available in both standard and annotated formats from the Natural Hazards Research and 

Applications Information Center at the University of Colorado at Boulder. 

Second, we accessed the record of research on human evacuation during high rises fires. Experts in both 

areas of research developed a list of the most salient factors to include in this segment of our research. 

A range of key possible explanatory variables on which to gather data to use to predict evacuation delay 

in towers 1 and 2 were selected. We collected telephone interview data on these factors, and then 

statistically examined their import in predicting evacuation delay time. The list of factors follows. 

• Location in Building 

• Social Context 

• Demographic Characteristics 

• Pre-event Experience 

• Roles of Responsibility 

• Preparedness and Training 

• Environmental Cues 

• Social Cues 

• Receiving Information about the Event 

• Perceived Risk 

• Seeking Additional Information 

• Injuries 

• Obtaining Help from Others 

• Pre-evacuation Activities 
Draft for Public Comment Causal Modeling 

NIST NCSTAR 1-7, WTC Investigation 219

Preliminary statistical analyses were performed to determine which of these had and did not have salience 

in predicting evacuation delay out of WTC 1 and WTC 2. Those that had no significant impact on 

evacuation delay were excluded from further consideration. The factors that had salient impacts on 

evacuation delay were carried forward into the analysis to model tower evacuation. 

The Model That Was Estimated 

The model used to predict evacuation delay in WTC 1 and WTC 2 of the World Trade Center on 

September 11, 2001 used factors that preliminary analyses suggested as salient, closely followed general 

evacuation theory from the social and fire sciences, and is diagrammatically illustrated in Figure C–1. 

Note that e4 – e7 are the unexplained variance for variables 4 – 7, respectively. The model can be 

described as follows: (1) delay in evacuation initiation is a direct consequence of environmental cues, 

floor, obtaining information without seeking it, perceived risk, seeking additional information, and taking 

pre-evacuation actions; (2) taking pre-evacuation actions is a direct consequence of environmental cues, 

floor, obtaining information without seeking it, perceived risk, and seeking additional information; (3) 

seeking additional information is a direct consequence of environmental cues, floor, obtaining 

information that was not sought, and perceived risk; and, finally, (4) perceived risk is a direct 

consequence of environmental cues, floor, and obtaining information without seeking it. This model is 

parsimonious, consistent with the evacuation theory that stems from research on existing evacuation and 

risk communication research, and it well-represented the positive findings of our preliminary analyses of 

the many variables that could have impacted evacuation delay. 

Measurement 

Environmental cues (X1) was measured by asking respondents about the number and type of 

environmental cues (severe signs of danger, and non-severe signs of danger) that they saw prior to 

initiating there evacuation. Answers to these questions were coded as dummy variables (0 or 1) and then 

added to form an environmental cues scale that could vary between 0 and 2. 

Floor (X2) was measured by asking respondents what floor they were on when the event started. 

Responses were coded as -7 (basement floors) to 105. Negative floor values were transposed into positive 

ones since this measure sought to determine how many floors people were from their building’s floor of 

exit. Missing data was coded to the mean. 

Obtaining information without seeking it (X3) was measured by asking respondents: “Now please think 

about the time period between when you first became aware that something had happened and when you 

first entered a stairwell or elevator to leave the tower. During this entire time period were you given any 

additional information about what was going on?” Answers were coded as a dummy variable where 

1 = yes and 0 = no or missing data. 

Perceived risk (X4) was measured by asking whether the occupant whether they believed that other people 

were in danger of being killed during the time when they first became aware that something had happened 

and when they first entered the stairwell or elevator to leave. Once again, answers were coded as a 

dummy variable, where 1 = yes and 0 = no or missing data. This measure of perceived risk was used 

instead of danger to self of being killed because the latter contained insufficient variance to include in the 

analysis. 
Appendix C Draft for Public Comment 

220 NIST NCSTAR 1-7, WTC Investigation

Figure C–1. Model of evacuation delay. 

Seeking additional information (X5) was measured by asking the respondent whether they tried to get 

additional information about what was going on. Answers were coded a 1 = yes (for both successful and 

unsuccessful attempts to get additional information), and 0 = no. 

Taking pre-evacuation actions (X6) was measured by asking respondents whether or not they performed 

any of the following tasks: talk to another person face-to-face, gather personal items, telephone other 

people, continue working, save or transfer computer files, search for others, fight fire or smoke, move to 

another floor, or help others. These nine items were added to create a scale of taking actions (post-first 

became aware that something was wrong but pre-evacuation initiation) that varied between 0 and 9. 

Finally, respondents were asked about evacuation delay (X7) that is how many minutes passed from the 

time when they first became aware that something was wrong until they actually began their evacuation. 

Their responses were coded as an interval scale of minutes that varied between 1 to 80 minutes for 

WTC 1, and 1 to 75 minutes for WTC 2. The means, medians, and modes for WTC 1 were 5.61 minutes, 
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NIST NCSTAR 1-7, WTC Investigation 221

3.00 minutes, and 1 minute. These same values for WTC 2, respectively, were 6.04 minutes, 4.00 

minutes, and 5 minutes. As discussed previously, however, a significant bias for a segment of the WTC 2 

population may exist: occupants above floor 78 whose evacuation delay time exceeded 16 minutes did not 

successfully evacuate WTC 2 on September 11, 2001. NIST has estimated that approximately 75 percent 

of the occupants above the 78th floor at 8:46:30 a.m. were able to evacuate on September 11, 2001, 

therefore this bias accounts for less than 10 percent of the overall WTC 2 population. 

The Structural Equations for the Model 

The theoretical model presented in Figure 1 was represented by the following structural equations: 

 X4 = β41X1 + β42X2 + β43X3 + e4 (Eq. C–1)

 X5 = β51X1 + β52X2 + β53X3 + β54X4 + e5 (Eq. C–2)

 X6 = β61X1 + β62X2 + β63X3 + β64X4 + β65X5 + e6 (Eq. C–3)

 X7 = β71X1 + β72X2 + β73X3 + β74X4 + β75X5 + β76X6 + e7 (Eq. C–4)

These equations cast perceived risk (X4) as a direct linear function of environmental cues (X1), floor (X2), 

and obtained information (X3). Seeking information (X5) is a direct linear function of environmental cues 

(X1), floor (X2), obtained information (X3), and perceived risk (X4). Pre-evacuation actions (X6) is a direct 

linear function of environmental cues (X1), floor (X2), obtained information (X3), perceived risk (X4), and 

seeking information (X5). Finally, evacuation delay (X7) was cast as a direct linear function of 

environmental cues (X1), floor (X2), obtained information (X3), perceived risk (X4), seeking information 

(X5), and pre-evacuation actions (X6). 

Estimation of the Model and Assessing for Regression Assumptions 

The model was estimated on the data from both WTC 1 and WTC 2. The estimated model parameters 

included path coefficients (β), explained variance for each equation (R2

), and other estimates. These are 

presented in Table C–1 and Table C–2, respectively, for WTC 1 and WTC 2. 

The models were assessed for specification error, multicolinearity, nonlinearity, and heteroscedasticity in 

order to determine if basic regression assumptions could be reasonably made, and if the estimated model 

parameters were unbiased. 

Specification error was not determined to be a problem. The model included only major variables of 

import suggested by evacuation theory and excluded variables shown in our preliminary analyses as nonpredictive in the data sets being analyzed. 

Multicolinearity can bias model estimates because it can increase the standard errors of estimated 

regression coefficients. The models for both WTC 1 and WTC 2 were assessed for multicolinearity in two 

ways. First multicolinearity does not impose a problem unless it is nearly perfect. The zero-order 

correlation matrices for both models (Table C–3 and Table C–4) were inspected to determine if any 
Appendix C Draft for Public Comment 

222 NIST NCSTAR 1-7, WTC Investigation

Table C–1. Estimated parameter of the model for WTC 1. 

Variablesa Path Equation 

Endogenous Exogenous Coefficient Estimate α α R2

X4 X1 β41 .38 .00 .00 .55 

 X2 β42 .37 .00 

 X3 β43 .14 .00 

 

X5 X1 β51 .21 .00 .00 .25 

 X2 β52 .21 .00 

 X3 β53 .01 N/S 

 X4 β54 .15 .02 

 

X6 X1 β61 .27 .00 .00 .68 

 X2 β62 .41 .00 

 X3 β63 .05 .06 

 X4 β64 .08 .04 

 X5 β65 .20 .00 

 

X7 X1 β71 .29 .00 .00 .49 

 X2 β72 -.17 .00 

 X3 β73 .20 .00 

 X4 β74 -.02 N/S 

 X5 β75 .10 .01 

 X6 β76 .47 .00 

a. Where X1 = environmental cues, X2 = floor, X3 = obtained information, X4 = perceived risk, X5 = sought information, 

X6 = pre-evacuation actions, and X7 = delay in evacuation initiation. 
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NIST NCSTAR 1-7, WTC Investigation 223

Table C–2. Estimated parameters of the model for WTC 2. 

Variablesa Path Equation 

Endogenous Exogenous Coefficient Estimate α α R2

X4 X1 β41 .30 .00 .00 .59 

 X2 β42 .49 .00 

 X3 β43 .12 .00 

 

X5 X1 β51 .25 .00 .00 .25 

 X2 β52 .11 N/S 

 X3 β53 .07 N/S 

 X4 β54 .17 .02 

 

X6 X1 β61 .20 .00 .00 .69 

 X2 β62 .36 .00 

 X3 β63 .07 .04 

 X4 β64 .23 .00 

 X5 β65 .17 .00 

 

X7 X1 β71 .13 .01 .00 .56 

 X2 β72 -.19 .00 

 X3 β73 .23 .00 

 X4 β74 .05 N/S 

 X5 β75 .11 .01 

 X6 β76 .51 .00 

a. Where X1 = environmental cues, X2 = floor, X3 = obtained information, X4 = perceived risk, X5 = sought information, 

X6 = pre-evacuation actions, and X7 = delay in evacuation initiation. 
Appendix C Draft for Public Comment 

224 NIST NCSTAR 1-7, WTC Investigation

Table C–3. Zero-order correlation matrix for WTC 1.a

 X1 X2 X3 X4 X5 X6 X7

X1 1 .68 .25 .67 .45 .71 .59 

X2 - 1 .26 67 .45 .76 .46 

X3 - - 1 .34 .17 .29 .37 

X4 - - - 1 .43 .64 .47 

X5 - - - - 1 .55 .44 

X6 - - - - - 1 .64 

X7 - - - - - - 1 

a. Where X1 = environmental cues, X2 = floor, X3 = obtained information, X4 = perceived risk, X5 = sought information, 

X6 = pre-evacuation actions, and X7 = delay in evacuation initiation.

Table C–4. Zero-order correlation matrix for WTC 2.a

 X1 X2 X3 X4 X5 X6 X7

X1 1 .60 .43 .64 .46 .67 .56 

X2 - 1 .44 .72 .41 .75 .48 

X3 - - 1 .46 .30 .47 .55 

X4 - - - 1 .44 .72 .55 

X5 - - - - 1 53 .47 

X6 - - - - - 1 .68 

X7 - - - - - - 1 

a. Where X1 = environmental cues, X2 = floor, X3 = obtained information, X4 = perceived risk, X5 = sought information, 

X6 = pre-evacuation actions, and X7 = delay in evacuation initiation. 

correlations between the regressors where around .80 or higher—this is a typical cutoff value below 

which multicolinearity does not seriously bias model estimates. An inspection of the zero-order 

correlations in Table C–3 and Table C–4 led to the conclusion that coefficients were not sufficiently high 

for multicolinearity to be a problem. Second, multicolinearity was assessed by regressing each exogenous 

variable in each equation on all other exogenous variables in that equation, and the explained variances 

for these regressions were inspected to see if any approached 1.00, which would indicate a biasing level 

of multicolinearity. This assessment also led to the conclusion that multicolinearity was not a source of 
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NIST NCSTAR 1-7, WTC Investigation 225

bias in the estimated parameters of the model in either the data set for WTC 1 or the data for WTC 2. 

Therefore, it was concluded that the regressors in both models were orthogonal. 

The models were then assessed to determine if the assumption of linearity could be met. Exogenous 

variables in each equation were transformed to alternative nonlinear forms, for example, the natural 

logarithm of X, the square of X, the reciprocal of X, and the square-root of X. These transformed 

variables were then correlated with each of the pre-determining and endogenous variables in both models. 

None of the correlations involving the transformed exogenous variables increased substantially beyond 

the linear correlations presented in Table C–3 and Table C–4. A visual inspection of scatter plots also 

confirmed the conclusion that relationships were linear. 

The assumption of homoscedasticity was also assessed by visual inspection of regression residuals in 

scatter plots for each relationship in both models; and it was concluded that this assumption was met. 

The observed means, standard deviations, and ranges for all of the variable included in the models for 

both towers are presented in Table C–5. 

Table C–5. Observed means, standard deviations, and ranges for variable 

measurements in WTC 1 and WTC 2. 

WTC 1 WTC 2 

Variablea Mean SD Range Mean SD Range 

X1 .84 .73 2.00 .68 .70 2.00 

X2 41.87 25.71 90.00 48.99 30.56 104.00 

X3 .11 .32 1.00 .21 .41 1.00 

X4 .63 .48 1.00 .67 .47 1.00 

X5 .28 .44 1.00 .29 .45 1.00 

X6 2.08 1.50 7.00 2.37 1.46 8.00 

X7 5.61 8.34 79.00 6.04 8.06 74.00 

a. Where X1 = environmental cues, X2 = floor, X3 = obtained information, X4 = perceived risk, X5 = sought 

information, X6 = pre-evacuation actions, and X7 = delay in evacuation initiation. 

Judging the Success of the Models 

The estimated parameters of the models for WTC 1 and WTC 2 (see Tables 1 and 2) revealed that the 

model had a very high degree of success in explaining evacuation initiation delay, pre-evacuation actions, 

seeking information, and perceived risk in both towers. The adjusted explained variance (R2

) for 

perceived risk was 55 and 60 percent in WTC 1 and WTC 2, respectively. The adjusted explained 

variance for sought information was 25 percent in both towers. Respectively, the adjusted R2

 for preevacuation actions was 68 and 69 percent for WTC 1 and WTC 2, respectively. Finally, the adjusted R2

for delay in evacuation for WTC 1 and WTC 2 was, respectively, 49 and 56 percent. These are 

extraordinarily high levels of adjusted explained variance to observe in a study of human evacuation and 

these R2

s, thereby, establish the strong predictive power of the models for both towers. All of the 

equations in the models for both towers were statistically significant at the .001 level or higher. 
Appendix C Draft for Public Comment 

226 NIST NCSTAR 1-7, WTC Investigation

Direct Effects 

With a few exceptions, similar findings emerged in both towers. This lends validity to the conclusions 

that can be drawn from the analyses. The findings are presented below. First, we consider each of the four 

equations in both models for both towers (see Figure 1), and then the models are interpreted as a whole so 

that the most significant paths of influence for each tower can be distinguished. 

Predicting perceived risk. The findings that emerged regarding predicting the risk that people perceived 

were virtually identical across the two towers. The R2

 for perceived risk was 55 percent in WTC 1 and 

60 percent in WTC 2 (see Tables 1 and 2). In WTC 1, both environmental cues and floor had strong and 

similar impacts on predicting perceived risk, respectively, β41 and β42 were .38 and .37, while obtained 

information had a weaker but statistically significant impact, β43 was .14. In WTC 2, once again, both 

environmental cues and floor had strong impacts on predicting perceived risk, respectively β41 and β42

were .30 and .49, and, once again, obtained information had a weaker but statistically significant impact, 

β43 was .12. These findings suggest that the risk that people perceived before they began their evacuation 

increased largely as a function of floor height and being exposed to environmental cues. It is most likely 

that floor had this effect due to the perceived increased time needed to evacuate as a result of being higher 

in the towers. Environmental cues likely had this effect since seeing, hearing, feeling, and so on physical 

cues that indicate danger make discounting danger—most people’s natural inclination--harder to achieve. 

Obtained information likely increased perceived risk because people learned more about the seriousness 

of the event through the information they obtained. Clearly, however, information had a lesser impact on 

risk perception than did the two more salient variables of experiencing environmental cues and floor 

height. The only difference in findings between the towers was that, in WTC 2, floor height was by far 

the strongest predictor of perceived risk. 

Predicting seeking information. Once again, the findings that emerged for predicting seeking 

information were, almost, identical across both towers. Explained variance (R2

) for seeking information 

was 25 percent in both towers. In WTC 1, environmental cues and floor both had the strongest and 

identical impacts on seeking information, β51 and β52 were both .21; obtained information had no impact 

on seeking information, β53 was not statistically significant; and perceived risk had a slight impact on 

seeking information, β54 was .15. In WTC 2, environmental cues had the strongest impact on seeking 

information, β51 was .25; the impacts of floor and obtained information, β52 and β53, were not significant; 

and perceived risk had a slight impact on seeking information, β54 was .17. Seeking information in times 

of rapid onset emergencies is a typical human response since people need to interpret and make sense out 

of an event before they act on it. The finding that environmental cues were the strongest predictor of 

seeking additional information is consistent with this theoretical finding about “milling” from past 

research. Obtained information had no impact on seeking information in either tower. This was likely 

because information to make sense out of the event had already been obtained. Perceived risk had a 

similar effect on seeking information—albeit lesser of an effect than environmental cues—in both towers, 

likely because it increased the urgency people had to interpret the situation. Interestingly, floor height (or 

distance from the exit) had a significant effect on seeking information in WTC 1, but not in WTC 2. 

Evacuation theory would predict that this effect would be present, it was in the tower that was struck first, 

and it was not present in the tower stuck second likely because the event began for the occupants of 

WTC 2 long before the second plane struck their tower. 
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NIST NCSTAR 1-7, WTC Investigation 227

Predicting pre-evacuation actions. The explained variance (R2

) for taking pre-evacuation actions in 

WTC 1 was 68 percent, and it was 69 percent in WTC 2 (See Tables 1 and 2). In both towers, the 

strongest predictor of taking pre-evacuation actions was floor, β62 in WTC 1 was .41 and it was .36 in 

WTC 2. Environmental cues was also predictive of pre-evacuation actions, β61, respectively, in WTC 1 

and WTC 2 was .27 and .20, respectively. Once again, observing clues that one is at risk and being high 

in the building with a longer path to safety emerged as strong predictors, in this case of taking actions to 

ready to leave. Obtaining information had virtually no impact in either tower; β63 was .05 in WTC 1 and 

.07 in WTC 2. Seeking information impacted pre-evacuation action, β65 was .20 in WTC 1 and it was 

.17 in WTC 2; likely because the information obtained supported the need to evacuate and, hence, related 

to getting ready to leave. Finally, the impact of perceived risk on taking pre-evacuation actions (β64) was 

.23 in WTC 2, but it was weaker in WTC 1 where is was .08. 

Predicting delay in evacuation. Explained variance (R2

) in evacuation delay was 49 percent in WTC 1 

and 56 percent in WTC 2. The impacts of environmental cues (β71), floor (β72), obtained information (β73), 

perceived risk (β74), sought information (β75), and pre-evacuation action (β76) on delay in evacuation 

initiation, respectively, were .29, -.17, .20, -0.2, .10, and .47 for WTC 1; and for WTC 2 they were 

.13, -.19, .23, .05, .11, and .51. The greatest predictor of evacuation delay in both towers was taking preevacuation actions. Obviously, doing anything before initiating evacuation—including things to ready to 

leave—delayed departure. Setting this factor aside, some clear differences emerged between the two 

towers in terms of the relative impacts of the remaining variables in the model. Perceived risk (β 74) had 

no direct effect on evacuation initiation delay. This finding is consistent with general evacuation theory 

where perceived risk’s impact on actual behavior is indirect through other factors. The three factors with 

the strongest direct effects on evacuation delay were the same in both towers. These were environmental 

cues, floor, and obtained information. In both towers, floor’s effect was negative, that is, the more floors 

one was from the exit, the quicker people were to initiate their evacuation. Environmental cues and 

information that was received passively both increased delay in the initiation of evacuation. Finally 

seeking additional information had a minimal impact on evacuation delay. 

Paths of Greatest Influence and Conclusions 

Although each of the aforementioned findings are interesting in and of themselves, perhaps the most 

important findings that we can offer are those that emerge when all of the individual findings offered 

above are brought together and viewed at the same time in the context of the entire model. 

Bias As discussed previously, any conclusions about evacuation initiation delay time in WTC 2 should 

consider the impact of disproportional decedent location, particularly as a source of the disproportionality 

may be highly correlated to the variable of interest, evacuation delay. In other words, those who 

exhibited long delay times in one region of the building were unable to be interviewed, thus artificially 

shortening the average delay time for one-third of the building. In the causal modeling, this would affect 

the relationship between ‘floor’ and ‘delay initiating evacuation,’ likely tending towards zero a slightly 

negative estimate (-0.19) of the beta value between the two variables. As floor was not a primary path 

which directly predicted evacuation initiation delay in WTC 2, the impact of this bias was considered 

secondary. The effect of this bias as it worked through other variables was not considered. 
Appendix C Draft for Public Comment 

228 NIST NCSTAR 1-7, WTC Investigation

WTC 1 Although there were other factors that had some lesser impacts on influencing what people did, 

the paths of causal influence that defined the main process that led to delay in the evacuation of WTC 1 

on September 11, 2001, follows. 

Environmental cues (information from the physical environment that something was terribly wrong) and 

floor (increased distance to safety) caused people to set out to find additional information, most likely 

information about what was going on and what they should do. Next, the act of seeking additional 

information, that is, “milling” about to make sense out of the situation, led people to take actions to 

prepare to evacuate. Finally, taking those actions to prepare to evacuate delayed the initiation of actually 

evacuating. 

In addition to this four step causal process, environmental cues and floor also had indirect impacts on 

evacuation delay as follows. Both factors increased the odds of seeking information and both factors 

increased the chances that people would take pre-evacuation actions prior to evacuating. Both factors also 

had direct impacts on actual evacuation delay. Environmental cues increased delay while floor decreased 

delay. 

WTC 2 Although there were other factors that had lesser influence on what people did, the paths of 

causal influence that defined the main process that led people to delay in the evacuation of WTC 2 on 

September 11, 2001 were identical to WTC 1 with one decided difference. 

Environmental cues (information from the physical environment that something was terribly wrong) and 

floor (increased distance to safety) predicted perceived risk. Environmental cues, floor, and perceived 

risk caused people to set out to find additional information. Next, the act of seeking additional 

information, that is “milling” about to make sense out of the situation, and perceived risk both led people 

to take actions to prepare to evacuate. Finally, taking those actions to prepare to evacuate delayed the 

initiation of actually evacuating. 

In addition to this five step causal process, environmental cues and floor also had indirect impacts on 

evacuation delay. Both factors increased the odds of seeking information and both factors increased the 

chances that people would take pre-evacuation actions prior to evacuating. Both factors also had direct 

impacts on actual evacuation delay. Environmental cues increased delay, while floor decreased delay. 

C.2 Predicting Normalized Stairwell Evacuation Time In WTC 1 On 

September 11, 2001 

This project analyzed the factors and social processes that influenced the normalized stairwell evacuation 

time per flight of stairs for the people who evacuated out of WTC 1 on September 11, 2001. WTC 2 was 

excluded from this analysis because evacuees used stairs, elevators, and/or a combination of both for their 

evacuation. Evacuation time was defined as the average number of seconds per flight of stairs that it took 

people from the time they entered a stairwell until they completed their evacuation out of the building. 

For a discussion of the data collection method, see Section 10.2.1 of this report. 
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NIST NCSTAR 1-7, WTC Investigation 229

Pre-Modeling Analysis 

The model estimated to predict normalized stairwell evacuation time was specified as follows: 

First, we accessed the record of research on human evacuation during high rises fires. A group of 

individuals selected for the expertise on human evacuation during high rise fires was assembled. The 

individual experts developed the final list of variables, and the relationships between them, to use to 

specify the model used in this analysis. 

Second, preliminary statistical analyses were performed to determine which variables thought to be 

important predictors of normalized stairwell evacuation time by the team of experts had and did not have 

predictive salience in WTC 1. Those that had no significant impact on normalized stairwell evacuation 

time were excluded from further consideration. The factors that had salient impacts on normalized 

stairwell evacuation time were carried forward into the model used in this analysis. 

The Model that was Estimated 

The model used to predict normalized stairwell evacuation time in WTC 1 of the World Trade Center on 

September 11, 2001, is diagrammatically illustrated in Figure C–2. This model can be described as 

follows: (1) normalized stairwell evacuation time is a direct consequence of floor, evacuation decision 

delay, environmental cues, emergency responders, crowding, and evacuation interruption; (2) evacuation 

interruption is a direct consequence of floor, evacuation decision delay, environmental cues, emergency 

responders, and crowding; (3) crowding is a direct consequence of floor, evacuation decision delay, 

environmental cues, and emergency responders; (4) emergency responders is a direct consequence of 

floor, evacuation decision delay, and environmental cues; (5) environmental cues is a direct consequence 

of floor and evacuation decision delay; and, finally, (6) evacuation decision delay is a direct consequence 

of floor. This model is parsimonious, consistent with the input received from high rise fire evacuation 

experts, and it well-represented the positive findings of our preliminary analyses of the many variables 

that could have impacted normalized stairwell evacuation time. 

Measurement 

Since multiple regression analysis requires that all responses be analyzed in numerical form, answers had 

to be ‘coded’ or converted to numbers. By convention, 0 = ‘no’, and 1 = ‘yes.’ Further, 2 is always a 

greater quantity of whatever the measurement is evaluating than 1. 

Floor (X1) was measured by asking respondents which floor they were on when the event started. 

Responses were coded from negative seven (basement floors) to 105. Negative floor values were 

transposed into positive ones since this measure sought to determine how many floors people were from 

their building’s floor of exit. Missing data was coded to the mean. 

Evacuation decision delay (X2) was measured by asking respondents how minutes passed from the time 

when they first became aware that something was wrong until they actually began their evacuation. Their 

responses were coded as an interval scale of minutes. 
Appendix C Draft for Public Comment 

230 NIST NCSTAR 1-7, WTC Investigation

Figure C–2. Model of normalized stairwell evacuation time. 

Environmental cues (X3) was measured by asking respondents about the number and type of 

environmental cues (severe signs of danger, and non-severe signs of danger) that they saw prior to 

initiating their evacuation. Answers to these questions were coded as dummy variables (0 or 1) and then 

added to form an environmental cues scale that could vary between 0 and 2. 

Emergency responders (X4) was measured by asking respondents whether items from a list made their 

evacuation more difficult. Affirmative answers to the “firefighters/police” choice were coded as 1, and 

“no” answers and missing data were coded as 0. 

Crowding (X5) was measured by asking respondents whether items from a list made their evacuation 

more difficult. “Yes” answers to “crowded stairwells” were coded as 1, and “no” answers and missing 

data were coded as 0. 

Evacuation interruption (X6) was measured by asking respondents whether they turned back at any time 

during their evacuation. “Yes” answers were coded as 1, and “no” answers and missing data were coded 

as 0. 

Finally, normalized stairwell evacuation time (X7) was measured by asking respondents how much time 

passed between the moment they first began their evacuation until they exited the tower. Answers were 

coded on an interval scale; “don’t know” and “refused to answer” responses were coded to the mean. 

These raw numbers were then normalized. The hydraulic model of people movement dominates the 

average evacuation time per floor, so it had to be removed from normalized stairwell evacuation time so 

as to fully explain the fluctuations (denoted by a prime) in reported evacuation time. This was 
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NIST NCSTAR 1-7, WTC Investigation 231

accomplished by a least squares analysis of reported evacuation time against floor for each tower, with 

statistical outliers removed. The final WTC 1 estimate was: 

 AETF ́j = RETj - .056i (Eq. C–5)

where: 

 AETF ́ = Average fluctuation in evacuation time per floor for occupant j, 

 i = Floor evacuation began from for occupant j, and 

 RET = Reported evacuation time for occupant j. 

The Structural Equations for the Model 

The theoretical model presented in Figure 1 was represented by the following structural equations. 

 X2 = β21 X1 + e2 (Eq. C–6)

 X3 = β31X1 + β32X2 + e3 (Eq. C–7)

 X4 = β41X1 + β42X2 + β43X3 + e4 (Eq. C–8)

 X5 = β51X1 + β52X2 + β53X3 + β54X4 + e5 (Eq. C–9)

 X6 = β61X1 + β62X2 + β63X3 + β64X4 + β65X5 + e6 (Eq. C–10)

 X7 = β71X1 + β72X2 + β73X3 + β74X4 + β75X5 + β76X6 + e7 (Eq. C–11)

These equations cast evacuation decision delay (X2) as a direct linear function of floor (X1). 

Environmental cues (X3) was set as a direct linear function of floor (X1) and evacuation decision delay 

(X2). Emergency responders was seen as a direct linear function of floor (X1), evacuation decision delay 

(X2), and environmental cues (X3). Crowding was cast as a direct linear function of floor (X1), evacuation 

decision delay (X2), environmental cues (X3), and emergency responders (X4). Evacuation interruption 

(X6) was cast as a direct linear function of floor (X1), evacuation decision delay (X2), environmental cues 

(X3), emergency responders (X4), and crowding (X6). Finally, normalized stairwell evacuation time was 

cast as a direct linear function of floor (X1), evacuation decision delay (X2), environmental cues (X3), 

emergency responders (X4), crowding (X5), and evacuation interruption (X6). 

Estimation of the Model and Assessing for Regression Assumptions 

The model was estimated on the data from WTC 1. The estimated model parameters included path 

coefficients (betas), explained variance for each equation, and other estimates. These are presented in 

Table C–6. The zero-order correlations between each of the variables are presented in Table C–7. 
Appendix C Draft for Public Comment 

232 NIST NCSTAR 1-7, WTC Investigation

Table C–6. Estimated parameters of the model for WTC 1. 

Variablesa Path Equation 

Endogenous Exogenous Coefficient Estimate α α R2

X2 X1 Β21 .58 .00 .00 .34 

 

X3 X1 Β31 .78 .00 .00 .79 

 X2 Β32 .17 .00 

 

X4 X1 β41 .23 .00 .00 .57 

 X2 β42 -.01 N/S 

 X3 β43 .56 .00 

 

X5 X1 β51 .18 .00 .00 .72 

 X2 β52 .03 N/S 

 X3 β53 .37 .00 

 X4 β54 .35 .02 

 

X6 X1 β61 .12 N/S .00 .11 

 X2 β62 .04 N/S 

 X3 β63 .15 N/S 

 X4 β64 .01 N/S 

 X5 β65 .07 N/S 

 

X7 X1 β71 .03 N/S .00 .44 

 X2 β72 -.05 N/S 

 X3 β73 .46 .00 

 X4 β74 .09 N/S 

 X5 β75 .08 N/S 

 X6 β76 .18 .00 

a. Where X1 = floor, X2 = evacuation delay, X3 = environmental cues, X4 = emergency responders, X5 = crowding, X6 = 

evacuation interruption, and X7 =normalized stairwell evacuation time. 
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NIST NCSTAR 1-7, WTC Investigation 233

Table C–7. Zero-order correlation matrix for WTC 1.a

X1 X2 X3 X4 X5 X6 X7

X1 1 .58 .88 .71 .77 .84 58 

X2 - 1 .63 .47 .53 .25 .34 

X3 - - 1 .75 .81 .35 .58 

X4 - - - 1 .77 .28 .54 

X5 - - - - 1 .31 .57 

X6 - - - - - 1 .39 

X7 - - - - - - 1 

a. Where X1 = floor, X2 = evacuation delay, X3 = environmental cues, X4 = emergency responders, X5 = crowding, X6 = 

evacuation interruption, and X7 =normalized stairwell evacuation time. 

The model was assessed for specification error, nonlinearity, and heteroscadasticity in order to determine 

if basic regression assumptions could be reasonably made, and if the estimated model parameters were 

unbiased. The multicolinearity assumption did not have to be assessed since the model contained only one 

exogenous variable. 

Specification error was determined not to be a problem. The model included only variables of import 

suggested by individual experts, and excluded variables shown in our preliminary analyses as nonpredictive in the data sets being analyzed. 

The model was then assessed to determine if the assumption of linearity could be met. Exogenous 

variables in each equation were transformed to alternative nonlinear forms, for example, the natural 

logarithm of X, the square of X, the reciprocal of X, and the square-root of X. These transformed 

variables were then correlated with each of the pre-determining and endogenous variables in the model. 

None of the correlations involving the transformed exogenous variables increased substantially beyond 

the linear correlations presented in Table C–7. A visual inspection of scatter plots also confirmed the 

conclusion that relationships were linear. 

The assumption of homoscedasticity was assessed by visual inspection of regression residuals in scatter 

plots for each relationship in both models, and it was concluded that this assumption was met. 

The observed means for each of the variables contained in the model were: 47.2 for floor, 5.36 for 

evacuation delay, 1.73 for environmental cues, 0.60 for emergency responders, 0.73 for crowding, 0.12 

for evacuation interruption, and 13.01 for normalized stairwell evacuation time. 
Appendix C Draft for Public Comment 

234 NIST NCSTAR 1-7, WTC Investigation

Judging the Success of the Models 

The estimated parameters of the model for WTC 1 (Table C–6) revealed that the model had a very high 

degree of success in explaining normalized stairwell evacuation time. The adjusted explained variance 

(R2

) for normalized stairwell evacuation time was 44 percent, 11 percent for evacuation interruption, 72 

percent for crowding, 57 percent for emergency responders, 79 percent for environmental cues, and 34 

percent for evacuation initiation delay. With the exception of evacuation interruption, these are 

extraordinarily high levels of adjusted explained variance to observe in a study of human evacuation. 

These R2

’s, thereby, establish the strong predictive power of the model. All of the equations in the model 

were statistically significant at the .001 level or better. 

Direct Effects in the Model 

We first consider each of the six equations in the model (see Figure 1), and then the model is interpreted 

as a whole so that the most significant paths of influence can be distinguished. 

Predicting evacuation initiation delay. The findings that emerged regarding predicting delay in the 

initiation of evacuation from floor were that the R2

 was 34 percent with a value of .58 for β21 significant at 

the .001 level. This relationship was elaborated earlier in this appendix. 

Predicting environmental cues. Explained variance (R2

) for observing environmental cues was 

79 percent, and the equation was statistically significant at the .001 level. Floor had a very strong direct 

impact on observing environmental cues; β31 was .78 and it was significant at the .001 level. The effect of 

delay in the initiation of evacuation (β32) was .17, and it was also statistically significant at the .001 level. 

It appears that the longer a person took to begin their evacuation, the more the physical impacts of the 

event grew and the more likely people were to experience them. 

Predicting emergency responders. The explained variance (R2

) for the third equation in the model 

predicting encountering emergency responders was 57 percent, and the equation was statistically 

significant at the .001 level. The relative effects of floor, delay in beginning evacuation, and 

environmental cues, respectively, were as follows: β41 was .23 statistically significant at the .001 level, β42 

was -.01 and was not statically significant, and β43 was .56 statistically significant at the .001 level. Floor 

height and experiencing environmental cues both predicted encountering emergency responders. This 

makes sense when one considers that emergency responders would be most likely to go to areas 

experiencing the impacts that would also yield environmental cues, and the higher one was in the tower, 

the more stairwells one had to traverse and the greater the odds of encountering emergency responders. 

Predicting crowding. The explained variance (R2

) for the fourth equation in the model that predicted 

perceived crowding on the evacuation stairwells was 72 percent. The four variables in the equation had 

the following effects on perceived crowding: floor (β51) was .18, significant at the .001 level; evacuation 

delay (β52) was .03, and it was not statistically significant; environmental cues (β53) was .37, significant at 

the .001 level; and encountering emergency responders (β54) was .35, significant at the .001 level. 

Perceived crowding largely increased as a result of environmental cues and encountering emergency 

responders. 

Predicting evacuation interruption. The explained variance (R2

) for interrupting evacuation was only 

11 percent. Even though this equation was statically significant at the .001 level, none of the 5 predictor 
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NIST NCSTAR 1-7, WTC Investigation 235

variables in the equation had a statistically significant impact on evacuation interruption. The beta 

weights for these variables (floor, evacuation delay, environmental cues, emergency responders, and 

crowding) were, respectively (β61 through β65) .12, .04, .15, .01, and .07. At best, we can interpret these 

findings to mean that there was a slight but not statistically significant tendency for people to interrupt 

their evacuation if they had more rather than fewer floors to traverse to safety, and if they encountered 

environmental cues (perhaps obstacles) in the process of evacuation. 

Predicting normalized stairwell evacuation time. Explained variance (R2

) in predicting normalized 

stairwell evacuation time was 44 percent, and the equation was statistically significant at the .001 level. 

The impacts of floor (β71), evacuation delay (β72), environmental cues (β73), emergency responders (β74), 

crowding (β75), and evacuation interruption (β76), on normalized stairwell evacuation time, respectively, 

were .03, -.05, .46, .09, .08, and .18. Of these, only two factors in the equation were statistically 

significant both at the .001 level. These were environmental cues (β73 was .46) and evacuation interruption 

(β76 was .18). Clearly, the single factor that had the biggest impact on increasing the amount of time 

people spent, on average, per stairwell was environmental cues. The only other factor that had a 

significant impact was interrupting evacuation, obviously, because stopping egress would increase the 

amount of time needed to complete evacuation. 

Conclusions 

Although each of the aforementioned findings are interesting in and of their own right, the most important 

findings that we can offer are those that emerged when all of the above findings are brought together and 

viewed at the same time in the context of the model as a whole (see Figure 1). What doing so revealed 

was that, based on this analysis, the main process that led to increased normalized stairwell evacuation 

time in the evacuation of World Trade Center WTC 1 on September 11, 2001, was straightforward and 

clear. It can be described as follows. 

Floor (increased distance to safety) substantially increased the odds that people would encounter 

environmental cues. Floor also increased delay in starting evacuation, which, in turn, also increased the 

chances that people would encounter environmental cues. But it was encountering environmental cues 

(which likely blocked egress) that had a large and direct effect on increasing the amount of time that 

people spent, on average, to traverse their evacuation stairwell. In addition to this multi-step process with 

environmental cues as the key predicting variable, interrupting the process of evacuation for any reason 

also increased the amount of time, on average, that people used to descend their evacuation stairwell. 
Appendix C Draft for Public Comment 

236 NIST NCSTAR 1-7, WTC Investigation

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NIST NCSTAR 1-7, WTC Investigation 237

Appendix D 

EGRESS MODELING

The purpose of modeling evacuation from the World Trade Center (WTC) towers was to obtain 

evacuation times for a variety of scenarios which provide context to understanding the actual evacuation 

on September 11, 2001. The following six scenarios were selected and modeled: 

Phased evacuation 

• Scenario 1: Occupants in an emergency situation have to travel 3 floors below the fire floor, 

floor 48 

Total evacuation 

• Scenario 2: Full capacity tower without visitors or damage 

• Scenario 3: Full capacity tower including visitors without damage 

• Scenario 4: Sept. 11th capacity tower without damage 

• Scenario 5: Full capacity tower with plane damage blocking floors 91 and above in WTC 1 

• Scenario 6: Full capacity with plane damage blocking floors 78 and above in WTC 2 after 

16 minutes, including estimates of elevator usage 

D.1 METHOD FOR THE SIMULATING PHASED EVACUATION (SCENARIO 1) 

The purpose of simulating a phased evacuation was to understand how the evacuation strategy would 

have worked during an emergency at the WTC towers. For a phased evacuation, the occupants of the 

emergency floor, the occupants on the floor above, and the occupants on the floor below were to evacuate 

to three floors below the emergency floor. On floors 47 through 49, 200 occupants were placed on each 

floor. These 600 occupants traveled to the 45th floor where they would be considered “safe.” Floor 48, 

the emergency floor, was a transfer floor; therefore, occupants from floor 49 were faced with moving 

through the horizontal corridor on the 48th floor. Figure D–1 shows a three-dimensional view of the 

45th – 50th floors, as entered into the buildingEXODUS model. The occupants were placed on 

floors 47 – 49 and the three stairwells connected all 6 floors together. 
Appendix D Draft for Public Comment 

238 NIST NCSTAR 1-7, WTC Investigation

Figure D–1. 3-D view of a phased evacuation showing floors 45 – 50 (Exodus). 

Figure D–2 shows occupants traveling through one of the transfer corridors on floor 48. Occupants are 

traveling on the stairs that connects floor 49 and 48, walking through the transfer corridor and eventually 

onto the stair connecting floor 48 to 47. 

Figure D–2. Transfer corridor on floor 48 (Exodus). 

D.2 METHOD FOR SIMULATING TOTAL EVACUATION (SCENARIOS 2-6) 

Total building evacuation represented an emergency in which all occupants of the building evacuated 

simultaneously. Scenarios 2 – 6 were total evacuation simulations, which varied the total occupant 

population inside a tower and the damage to the building at the time of evacuation. 

D.2.1 Population Estimates 

Scenarios 2 – 4 required population estimates. Scenario 2 required an estimate of the number of 

occupants in the building, without the inclusion of visitors. Furniture and egress floor plans were studied 

from a sampling of floors: in WTC 1 – floors 94-100 and in WTC 2 – floor 77. On each floor, the 
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NIST NCSTAR 1-7, WTC Investigation 239

number of offices, cubicles, and other workstations was counted. From these numbers, the usable floor 

area (square footage) of the floors was divided by the corresponding number of persons to obtain an 

occupant density (area per person). Each floor in the towers received an average density (14 m2

/person), 

and the number of occupants per floor changed according to the total usable area of each floor. This 

resulted in a total of 19,800 persons to be simulated in Scenario 2. 

For scenario 3, the full-capacity population with visitors, the number of occupants inside the towers was 

determined by the same method as Scenario 2, adjusted for visitor spaces. Visitor spaces included 

conference rooms, waiting areas, lunch tables, and library chairs. The same floor plans were used from 

Scenario 2 population development to count these additional spaces. Then, as in the previous calculation, 

the usable floor area of each floor in the towers was used to calculate the number of occupants, including 

visitors in each tower. With these calculations, the occupant density was 11 m2

/person when including 

visitors. This resulted in an estimate of 25,500 people. The findings of each step of the estimation 

method are summarized in Table D–1. 

Table D–1. Floor and occupant modeling parameters. 

Zone Represented Floors Approx. area (m2) 

No. of chairs 

(w/o visitors)a

No. of chairs (with 

visitors)a

1 9 - 26 

27 - 34 

35 - 40 

2,723 

2,826 

2,922 

200 260 

2 43-47 2,679 190 240 

3 49 - 54 

55 - 56 

57 - 74 

2,974 

2,935 

3,007 

210 270 

4 77 - 81 2,860 200 260 

5 83 – 95 

103 - 104 

96 – 102 

105 - 106 

3,188 

3,309 

3,245 

3,245 

230 300 

a. Averaged over variations within each zone. 

The population for Scenarios 4, 5, and 6 was based upon the National Institute of Standards and 

Technology (NIST) estimate of the number of occupants present at WTC 1 and WTC 2 on 

September 11, 2001, or approximately 8,800. The purpose of Scenario 4 was to obtain a lower bound 

evacuation time for a September 11, 2001 population in the absence of constraints to evacuation or 

elevator usage. The 8,800 estimate was used to ‘calibrate’ (refine the model output to match the gross 

evacuation characteristics from the actual evacuation of WTC 1 and WTC 2) the model for Scenarios 5 

and 6, where additional occupants were added in combination with building damage (Galea 2004). 

For the total building evacuation scenarios, occupants were placed on all floors of the building, excluding 

mechanical spaces. Floors 1 through 107 were modeled (floors 108-110 were not normally occupied). 

During each simulation, the occupants left their floors and traveled into one of the three stairwells, in 

which they remained throughout the entire evacuation. The stairwells included all transfer corridors and 

emptied into the Mezzanine or the lobby area, depending upon the stairwell. Occupants reached safety as 

soon as they exited the tower (either to the outside or into the Concourse mall area). 
Appendix D Draft for Public Comment 

240 NIST NCSTAR 1-7, WTC Investigation

Finally, the value of 8,800 occupants for the number of occupants in a WTC tower on 

September 11, 2001 was based upon the statistical projection derived from the telephone survey data 

described previously in Table 4–1. 

D.3 WTC Tower Geometry Development 

Each Tower, without including the basement floors, contained a total of 110 floors. In order to ease the 

difficulty of floor plan input in each evacuation model, 11 representative floor plans were created and 

replicated throughout the building. The core layout and stair position did not change significantly in 

between the transfer floors. Therefore, knowing the floor location of core and stair walls, the tower was 

divided into zones, the boundaries of which were transfer floors. Either tower (since they were identical 

in core layout and stair position) was divided into 5 zones, in addition to the four significant transfer 

floors (floors 42, 48, 76, and 82). The 11 representative floors were the Concourse, Mezzanine, a 

representative floor from each of the 5 zones, and the 4 unique transfer floors. Using representative floor 

plans greatly simplified floor plan input into each evacuation model used, with little loss of accuracy. 

Even though floor duplication was used, each floor had to be created individually to achieve an accurate 

tower description within the evacuation model. At and around the mechanical spaces, the floor to ceiling 

measurements increased, meaning that throughout the building, the number of stair risers between floors 

ranged from 18 to 26 risers throughout the tower. Therefore, each floor was developed individually to 

ensure it contained the correct number of stairs to connect to the floor above. Also, even though 

occupants were not placed on these floors, mechanical floors were added to each model to include the sets 

of stairs that led throughout the mechanical spaces. 

Three additional stairwell movements were not modeled, as the stairwell translation was less than 5 m: 

floors 26-41 (Stair A) and 66-68 (Stairs A and C). Note that the stair movement on floors 26-41 was 

input into the buildingEXODUS evacuation model only, as stair positioning was more important in this 

model, as discussed below. 

Table D–2 shows the major floors plans contained in each zone. In Zones 1, 3, and 5, there were minor 

changes to the core walls from floor to floor, shown in the additional floor plans. The table outlines 

which floors these changes take place. However, due to the size of the building and the relative 

insignificance of a core wall change to the evacuation (since most of an occupant’s travel time was spent 

in the stairwells), the floor plan of the majority of floors within a zone was used as the representative floor 

in each evacuation model. Table D–2 shows that floor plan Zone 1-1 was used as a representative of 

Zone 1, Zone 3-3 as a representative of Zone 3, and Zone 5-1 as representative of Zone 5. 
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NIST NCSTAR 1-7, WTC Investigation 241

Table D–2. WTC floor plan with core walls and stairwell locations. 

Zone 1: Floors 3-41 

Zone 1-1: Floors 9-26 

There is a slight change in 

Stair A at floor 25. 

**Used for all of Zone 1 

Zone 1-2: Floors 27-34 

Variations are found in the upper 

right quadrant throughout this 

zone. 

Zone 1-3: Floors 35-41 

Variations are found in the upper 

right quadrant throughout this 

zone. 

Transfer Floor 42 

Stairs A and C transfer 

 View of Stair A transfer 

Zone 2: Floors 43-47 

Escalator positions: There is an escalator on the right side of the upper 

right quadrant between floors 44-45; Escalator on left side of upper left 

quadrant between floors 43-45 – these will not be used for evacuation 

purposes. 

Transfer Floor 48 

Stairs A and C transfer View of Stair C Transfer
Appendix D Draft for Public Comment 

242 NIST NCSTAR 1-7, WTC Investigation

Zone 3: Floors 49-75 

Zone 3-1: Floors 49 – 54 Zone 3-3: Floors 55-56 Zone 3-3: Floors 57-75 

1) Variations are found in the 

upper left and right quadrants 

throughout this zone. 

2) There is a slight change in 

Stairs A, C between floors 66-68. 

*Used for all of Zone 3 

Transfer Floor 76 

Stairs A, B, and C transfer View of Stair B Transfer 

Zone 4: Floors 77-81 

Transfer Floor 82 

Stairs A and C transfer View of Stair C Transfer 
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NIST NCSTAR 1-7, WTC Investigation 243

Zone 5: Floors 83-107 

Zone 5-1: Floors 83-95 

Variations are found in the lower 

left quadrant throughout this 

zone. 

*Used for all of Zone 5 

Zone 5-2: Floors 103-104, 107 

Variations are found in the lower 

left quadrant at floor 107. 

Zone 5-3: Floors 96-102, 105-

106 

Variations are found in the lower 

left quadrant throughout this 

zone. 

D.3.1 Single-Tenant and Multi-Tenant Floor Plans 

The floor plans shown in Table D–2 (used in the evacuation modeling) were single-tenant (open) floor 

plans, which contained only the outer walls, the core walls, and the stairwells. This section evaluates the 

quality of the assumption that the subdivision of a floor space into multiple tenancies did not significantly 

change the model results when compared to modeling the space as a single open floor plan. The 

assumption was based upon the premise that in a building as large as WTC 1 or WTC 2, most of the 

average occupant’s evacuation time would be spent in the stairwell. Specific multi-tenant floor plans 

were chosen to study, developed based upon the demising wall layouts found in the WTC Space Book 

Plan (PANYNJ 2001c). The Simulex model was used for this analysis. 

An open, single-tenant floor plan from each zone (1-5) was simulated by placing the appropriate number 

of occupants on each floor, running the model, and recording the numbers of occupants using each stair. 

The percentage of occupants who used each stairwell was calculated for each zone. Then, by factoring in 

the number of floors in each zone (that the sample floor represents), the weighted percentages of occupant 

use of each stair was calculated for the entire building. These percentages are shown in on the left hand 

side, labeled as the single-tenant floor plan percentages. 

Using the Space Book Plans, at least three representative multi-tenant floor plans were selected from each 

zone. Each multi-tenant floor plan contained at least 3 corridors, and/or at least 10 different companies. 

In each of the five zones, the movement of occupants within the multi-tenant floor plans was simulated, 

and the percentages of stair use were averaged over each zone. The number of multi-tenant floors and 

single-tenant plans per zone were calculated and assigned their appropriate average percentages. As 

shown in Table D–3, the weighted percentages of occupant use of each stairwell were calculated for the 

entire building. 
Appendix D Draft for Public Comment 

244 NIST NCSTAR 1-7, WTC Investigation

Table D–3. Stair use percentages for single-tenant versus multi-tenant floor plans. 

Single-tenant Floor Plans only Single- and Multi-tenant Floor Plans 

Stairwell C Stairwell B Stairwell A Stairwell C Stairwell B Stairwell A 

32.96 % 31.71 % 35.33 % 33.50 % 32.40 % 34.10 % 

Number of people in each stair with 19,800 population 

6526 6279 6995 6633 6415 6752 

Difference (Gross) -107 -136 243 

Difference (Percent) -0.5 % -0.68 % 1.2 % 

In summary Table D–3 shows that there was approximately 1 percent difference between assuming all 

floors to be single tenant (open) floor and modeling both single- and multi-tenant floors with respect to 

the choice of stairwells. Therefore, representative single-tenant (open) floor plans were used in the 

subsequent evacuation modeling. 

D.4 EVACUATION MODELS 

Three evacuation models were used. The Simulex model (IES 2001) was used to perform the singletenant and multi-tenant floor plan analysis and the phased evacuation scenario (Scenario 1). The 

buildingEXODUS (Gwynne et al. 1998) model was used for all scenarios from the WTC towers 

(Scenarios 1-6). Finally, the EXIT89 model (Fahy 1999) was used for the phased evacuation and the total 

evacuation scenarios (without damage) from the WTC towers (Scenarios 1-4). This section provides brief 

descriptions of each of the three models used in this project as well as the reason for picking these three 

among all other available evacuation models. 

Simulex 

The Simulex model was developed by Thompson, from IES, Inc. in Scotland, and is a widely used model 

in the field of fire safety. The model has been validated and focuses on the movement of occupants 

throughout a structure (IES 2001), (IES 2000). The model allows the user to specify a distribution of 

occupant types, associated with a certain body size and unimpeded movement speed, throughout the 

building. The model includes important occupant movements, such as overtaking, side-stepping, backstepping, and others. Simulex predicts that occupant movement throughout building spaces will slow (in 

speed) as the inter-person distance between occupants and / or obstacles decreases. The model has fixed 

limitations on the number of floors and exits that the model can handle, however the developer is willing 

to expand these to meet the needs of the users. Also, Simulex provides a 2-D visualization of the 

evacuation throughout the building and allows the user to input CAD drawings of the building for ease in 

building geometry development. 

The input required for this model included the following: 

• Floor plans in the form of CAD drawings 

• Connections of floor levels by stairways or ramps (involving user input on stair width and 

length) 
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NIST NCSTAR 1-7, WTC Investigation 245

• Distance maps that can be created to block certain exits or paths from groups of occupants 

• Occupant movement characteristics for each individual or a group of individuals with a 

corresponding body size, initial horizontal speed, and percentage decrease of speed on stairs 

(the user can use default values provided by Simulex or create his / her own) 

• Occupant delay times to be assigned from a random, triangular, or normal distribution 

buildingEXODUS 

buildingEXODUS (Gwynne et al 1999; 1998; 2001) was developed by Galea at the University of 

Greenwich, UK. buildingEXODUS is a well-documented and well-validated model, claimed to be 

limited only by the storage capacity of the computer used to run the program. The model contains six 

submodels that work together to form the evacuation simulation: occupant, movement, behavior, hazard, 

geometry, and toxicity submodels. Any time occupants wish to occupy the same node, or a node that is 

already occupied, a conflict resolution time is added to their total evacuation time. The method of conflict 

resolution is how the model simulates congestion throughout the building. Individual abilities of the 

occupants and conflict resolution are probabilistic. The behavioral model makes route choices depending 

upon the environmental situation of the building and then passes this information onto the movement 

model. Familiarity and awareness variables can be assigned to exits, and drive and patience variables are 

assigned to the occupant. These variables are random and difficult for the user to specify for any 

evacuation. buildingEXODUS provides the user with 2-D and 3-D visualization capabilities of the 

building evacuation and allows for the input of CAD drawings of the building for ease in building 

geometry development. 

The input required for this model included the following: 

• Floor plans in the form of CAD drawings (other methods are available to provide building 

information) 

• Connections of floor levels via stairways, with an option for including effective width inside 

the stairwell 

• Distribution of occupant types throughout the building (user can specify information about 

each individual or group such as speed, ability, drive, patience, etc.) 

• Option of including special nodes at certain points in the building to regulate occupant paths 

through building and flow through the exits/doors 

• Option of including boundary nodes used to predict slower movement near walls, obstacles 

• A series of simulation options such as stair packing, occupant randomization throughout the 

building, response time distribution, stair edge preference, and extreme behavior 
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246 NIST NCSTAR 1-7, WTC Investigation

EXIT89 

EXIT89 (Fahy 1999), (Fahy 2001) is an evacuation model developed by Fahy at the National Fire 

Protection Association (NFPA). The model has been tested on tall buildings in the past and has the 

capability of modeling a large number of occupants in a building. The model relies on the density versus 

speed data from Predtechenskii and Milinskii (Predtechenskii and Milinskii 1969) for different building 

components, such as horizontal components, doorways, up stairs, and down stairs. It also uses 

conditional movement, depending upon the presence and density of smoke in the evacuation path. All 

occupants in the building are made up of the same body size and density versus speed distribution, both 

specified by the user. EXIT89 uses a series of nodes and arcs instead of CAD drawings to represent any 

type of structure. In this type of model, the floor plan is entered as a series of nodes (rooms, corridor, 

stair sections, etc.) and arcs (distance between nodes). When using a node/arc model for a single-tenant 

(open) floor plan such as those used for the WTC Towers, the user determines how to segment and link 

the building space, and then should check to make sure that occupants are traveling in realistic patterns to 

the exits. While EXIT89 does not provide visualization capability, the model does provide a particularly 

detailed output that notes where each occupant is at each time period throughout the evacuation. EXIT89 

contains a variety of input choices for the user pertaining to each evacuation simulation: 

• Shortest travel route or user-defined route for occupants 

• The use of CFAST smoke data, user-defined blockages, or none 

• The choice of a body size for occupants which applies to entire population of the building - 

Large (0.1458 m2

), Medium (0.113 m2), or Small (0.0906 m2) 

• The choice of speed for the entire population of the building – Emergency (horizontal 

unimpeded speed = 1.36 m/s) or Normal (horizontal unimpeded speed = 0.91 m/s) 

• A random delay time (uniform distribution) 

• The modeling of disabled occupants, including the percentage of decrease in travel speed for 

these occupants compared with the rest of the population 

The three models described above were used in different capacities for this project. Simulex was used in 

a limited capacity to provide analyses of occupant movement on individual floors and to simulate the 

phased evacuation. EXIT89 and buildingEXODUS were used to model scenarios of an entire 

WTC Tower with a large number of occupants within the building. 

The three models were chosen for this project for different reasons. The Simulex model was chosen to 

perform certain aspects of the project due to its heavy focus on understanding and accurately predicting 

occupant movement, and was used for some of the smaller aspects of the project. The following reasons 

justified buildingEXODUS and EXIT89 as reasonable choices to model WTC 1 and WTC 2: 

• Both had the capacity to simulate evacuation from a 110-story building with 25,500 people. 

• Both provided the output that was needed to answer evacuation questions, including 

visualization capabilities from buildingEXODUS 
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NIST NCSTAR 1-7, WTC Investigation 247

• Both could handle the complexity of the building, such as the transfer floors and differences 

in floor to ceiling heights in different part of the Tower 

• Both provided explanation for the underlying data used for the movement of occupants 

throughout the Tower 

• Both provided sufficient knowledge and direction to NIST, through personal contact and/or 

user manuals, on how to use the model 

• Both developers had published validation studies on the model 

• Both models had been used by NIST or outside of NIST. This way NIST could identify their 

inefficiencies ahead of time and compensate for these during this project. 

D.4.1 Phased Evacuation (Scenario 1) Inputs and Results 

All three models were used to perform the phased evacuation analysis, Scenario 1. Each model required 

building geometry, population, and behavioral input conditions. The phased evacuation scenario involved 

the simulation of floors 45-49, and the emergency floor was floor 48. All three stairs, containing 19 risers 

per floor from floors 45-49, were added to the models with the appropriate diagonal length, stair width, 

and door width into and out of the stairwell. The number of occupants used in the phased evacuation was 

200 per floor on floors 47-49, which totaled to 600 occupants. The number of six hundred occupants was 

used as a rounded estimate of the number of occupants seen on three floors within Zone 2 of the building. 

The movement response delay was set to zero for one set of simulations, and ranged between zero and ten 

minutes for the other set of simulations. 

The difference between models for the phased evacuation scenario was the population characteristics of 

the occupants. The population type used in the Simulex model was the “office staff” type which 

distributed 60 percent males and 40 percent females throughout the building. The population distribution 

input into buildingEXODUS for the phased evacuation scenario was taken from WTC 1 telephone survey 

demographic data: 5 percent males age 17-29, 38 percent Males age 30-50, 21 percent Males age 51-80, 

3 percent Females age 17-29, 22 percent Females age 30-50, and 11 percent Females age 51-80. All 600 

occupants simulated with the EXIT89 model were assigned the medium body size and emergency speed 

parameters. 

The total evacuation time for the phased evacuation, Scenario 1, was similar across all three models. 

Without a time delay, Simulex predicted that occupants would travel to floor 45 in approximately 240 

seconds, buildingEXODUS predicted 243 seconds, and EXIT89 (emergency conditions) predicted 210 

seconds. With a 0 to 10 minute time delay, Simulex predicted that occupants would travel to floor 45 in 

approximately 690 seconds, buildingEXODUS predicted 660 seconds, and EXIT89 (emergency 

conditions) predicted 690 seconds. These results are shown in Table D–4. From the three evacuation 

models, the average minimum time to complete a phased evacuation in WTC 1 or WTC 2 building was 

approximately 230 seconds, or around four minutes, assuming no evacuation delay and approximately 

680 seconds, or around eleven minutes, assuming a 0 to 10 minute time delay. 
Appendix D Draft for Public Comment 

248 NIST NCSTAR 1-7, WTC Investigation

Table D–4. Phased evacuation model input conditions. 

Evacuation Model Occupant Type/ Characteristics 

Evacuation 

Time, No 

Delay (s) 

Evacuation 

Time, 

 0 – 600 s 

Delay (s) 

Simulex All office staff; 60 % men, 40% women 240 690 

EXODUS WTC 1 distribution: 5 % Males age 17-29, 38 % Males age 

30-50, 21 % Males age 51-80, 3 % Females age 17-29, 22 % 

Females age 30-50, and 11 % Females age 51-80 

243 660 

EXIT89 All Medium body size and emergency speed 210 690 

D.4.2 Scenarios 2 and 3: Inputs and Results 

Two models, buildingEXODUS and EXIT89, were used to perform the total building evacuation analysis 

of a full-capacity tower with and without visitors, Scenarios 2 and 3. Each model required building 

geometry, population, and behavioral input conditions. 

buildingEXODUS 

The buildingEXODUS model was used to simulate Scenarios 2 and 3, a total building evacuation of a 

fully-occupied tower both with and without the inclusion of visitors. Without visitors, the population 

totaled 19,800 people and with visitors, the population totaled 25,500 people. As discussed earlier, the 

tower was input into the model using 11 

representative floor plans from CAD, including 

the four transfer floors. Mechanical floors were 

input into the model; however, no occupants 

were placed on these floors. All usable floor 

space had to be filled with nodes measuring 0.5 

m x 0.5 m on which the occupants would travel 

throughout the building. 

On each floor throughout the Tower, three 

stairs were added in their appropriate position 

on the floor and number of risers, depending 

upon the floor. These stairs were added to the 

model with the appropriate diagonal length, 

stair width, door width into and out of the 

stairwell, number of landings, and accurate stair 

riser and tread distances. 

Figure D–3 shows a representation of a stair 

configuration of a 1.1 m (44 in.), 19 riser stair 

(Stairs A or C) in the buildingEXODUS model. 

The solid blue nodes represent the landing 

space and the green patterned nodes represent 

the stair steps. The figure shows a “spiral” 

stair that is 2 nodes wide with the distances in 

0.6 m between 

centers of landing 

nodes

0.31 m on 

diagonal of 

stair nodes 

0.45 m 

on both 

sides of 

stair

0.6 m 

between 

centers of 

landing nodes

Figure D–3. 44 in stair configuration in 

buildingEXODUS. 
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 249

between the centers of the nodes. The occupants can also move diagonally between the outer and inner 

track of the stair, which is not shown in the figure to prevent confusion. Also, the dotted lines in between 

the landing nodes are used to represent that one side of the landing will be located on one floor and the 

other side of the landing will be located on the floor above. Due to the 7.5 in by 9.5 in riser and tread 

configuration (not including the nose extending the tread space), the diagonal space between steps 

measured approximately 12 in. Similar stair configuration was used for the 56 in. stair (Stair B). The 

only difference was the distances from node to node; for instance, instead of a 0.6 m spacing between the 

centers of landing nodes, there was a 0.7 m spacing input. The number of nodes specified along the width 

of the stairwell is the buildingEXODUS’s method of simulating stair width, and since not more than one 

person is allowed to occupy the same node, specifying 2 nodes across each stairwell allows for 2-person 

movement abreast in all three stairwells. NIST is aware of the narrowness of the 44 in. stair (Stair A and 

C), however, did not find it appropriate to model only 1-person abreast inside the stairwell throughout the 

entire building. Therefore, two nodes were used across each stair as an assumption made when using the 

buildingEXODUS model. Stair node connection lengths do not imply areas of the stairs, and therefore 

density of the space, but rather the distance that occupants travel inside the stair. 

For the population, either 19,800 occupants or 25,500 occupants (including visitors) were distributed 

randomly throughout the building space, depending on the scenario. Similar to the phased evacuation 

scenario, the following occupant types were used to describe the WTC tower population: 5 percent Males 

age 17-29, 38 percent Males age 30-50, 21 percent Males age 51-80, 3 percent Females age 17-29, 

22 percent Females age 30-50, and 11 percent Females age 51-80. All occupants were able-bodied 

without mobility impairments. 

The minimum evacuation time from the Tower was simulated by choosing an immediate response time 

and the stair pack option, which predicted that occupants will “pack” inside the stairs during evacuation 

(however in the simulation each occupant still tries to leave the step ahead of them empty). Also, it was 

predicted that occupants traveled to the nearest stairwell on each floor. The simulation predicted that 

occupants from a fully-occupied building without visitors, with a response delay of zero, evacuated the 

building in approximately 110 minutes. Also, when visitors were added to the simulation, 

buildingEXODUS predicted an evacuation time of 141 minutes. 

Another group of simulations provided the evacuation time from the Tower if the occupants delayed their 

evacuation by 0 to 10 minutes. All other options chosen for the group of simulations were similar to 

those described for the non delay simulations. buildingEXODUS predicted that occupants from a fullyoccupied building without visitors, with a response delay of 0 to 10 minutes, evacuated the building in 

approximately 112 minutes. Also, when visitors were added to the simulation, buildingEXODUS 

predicted an evacuation time of 142 minutes. 

EXIT89 

The EXIT89 model was also used to simulate Scenarios 2 and 3, a total building evacuation of a fullyoccupied tower both with and without the inclusion of visitors. Without visitors, the population totaled 

19,800 people and with visitors, the population totaled 25,500 people. The tower was input into the 

model by describing each of the 11 representative floor plans with a series of nodes and arcs. On the 

mechanical floors, only the stair nodes and arcs were associated with these floors, and no occupants were 
Appendix D Draft for Public Comment 

250 NIST NCSTAR 1-7, WTC Investigation

placed on these floors. All usable floor space, including stairs and transfer corridors, was filled with 

larger-sized nodes ranging from 2 m2

 to 104 m2 and connected by arcs. 

Similar to buildingEXODUS, three stairs were added in their appropriate position on each floor 

throughout the tower. These stairs were added to the model with the appropriate diagonal length, stair 

width, and door width into and out of the stairwell. 

The stair input for the EXIT89 model was different than that used by the buildingEXODUS model. 

Instead of the stair consisting of two lanes of nodes, a stairwell in EXIT89 was represented by a 

horizontal area (m2

), stair width measured where stair nodes meet, and the stair travel distance measured 

along the diagonal, which included the lengths traveled on the landings. Even though the stair input 

method varied between buildingEXODUS and EXIT89, stair distances were verified as being similar in 

both models for the Tower geometry. 

Within EXIT89, the transfer corridors were input as horizontal nodes connecting vertical stair nodes at 

each transfer floor, also part of the geometry input. At each transfer floor, the stairs leading to and 

leading away from the floor were considered completely different stairwells and named accordingly. 

However, since EXIT89 could account for only 10 different stairwells, the smallest transfer at floor 42 

was considered part of a stairwell. 

The number of occupants intentionally placed on each floor within the input file corresponded to the 

occupant numbers calculated per zone from both the 14 m2

/person and 11 m2/person densities, depending 

upon the scenario. All occupants were assigned the medium body size and both emergency and normal 

speed options (from multiple simulations as shown by the range of evacuation times in Table D–5). 

The simulation predicted that occupants from a fully-occupied building without visitors, with a response 

delay of zero, evacuated the building in approximately 97 to 117 minutes (emergency to normal speed 

input conditions). Also, when visitors were added to the simulation, EXIT89 predicted an evacuation 

time of 114 to 140 minutes. 

When a response delay of 0-10 minutes was introduced to EXIT89, the simulation predicted that 

occupants from a fully-occupied building without visitors evacuated the building in approximately 92 to 

113 minutes. Also, when visitors were added to the simulation, EXIT89 predicted an evacuation time of 

119 to 139 minutes. 
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 251

Table D–5. Total Building Evacuation Time (Simulated) for Various Occupant Loads 

D.4.3 Scenario Four (September 11th Capacity) Inputs and Results 

The buildingEXODUS and EXIT89 models were used to perform the total building evacuation analysis 

of a WTC tower, Scenario 4, with a population similar to that observed on the morning of 

September 11, 2001. All inputs for the geometry and simulation options in buildingEXODUS and 

EXIT89 were identical to those used in Scenarios 2 and 3. Also, the inputs for the population options 

were identical, except for the number of occupants simulated. For scenario four, a total building 

evacuation was simulated with 8,800 occupants (the capacity estimated for a tower on the morning of 

September 11, 2001). 

buildingEXODUS predicted that occupants from a building holding a September 11, 2001, capacity 

evacuated the building in approximately 52 minutes with no delay and 55 minutes with a 0 to 10 minute 

delay time. EXIT89 predicted that occupants from a building holding a September 11, 2001, capacity 

evacuated the building in approximately 58 to 78 minutes (depending up the speed option chosen, 

emergency to normal) with no delay and 71 to 74 minutes with a 0 to 10 minute delay time. 

D.4.4 Scenarios Five and Six (Full-Capacity with Damage) Inputs and Results 

To model how the event would have changed if the buildings were fully occupied, models were run using 

the calculated fully-occupied number of occupants in each tower, 19,800 occupants (without visitors). 

Through running these scenarios, NIST hoped to answer the following questions: 

• What would have been the consequences if WTC 1 had been fully-occupied on 

September 11, 2001, the building had been hit by aircraft, and then had collapsed 103 

minutes later? 

• What would have been the consequences if WTC 2 had been fully-occupied on 

September 11, 2001, the building had been hit by aircraft 16 minutes after WTC 1 was hit, 

and then had collapsed 72 minutes after WTC 1 was hit (56 minutes from the time that 

WTC 2 was hit)? 

 

65 The underlying theory for people movement in EXIT89 is based upon the work of Predtechenskii and Milinskii. They 

observed an inverse relationship between density and speed for three different types of movement: emergency, normal, and 

comfortable. The effect of crowding (density) on overall evacuation speed was greater when no delay time was assumed. A 

delay time when 19,800 occupants were present, according to EXIT89, spaces the occupants out and increases overall 

evacuation efficiency. This was not found to be the case for all occupant loads (25,500). 

Evacuation Model 

Evacuation 

Initiation Delay 

Input 

Evacuation Time 

(min): 8,800 

occupants 

Evacuation time 

(min): 19,800 

occupants 

Evacuation time 

(min): 25,500 

occupants 

EXODUS 0 - 10 Minute Delay 55 112 142 

No Delay 52 110 141 

EXIT89 0 - 10 Minute Delay 71-74 92-113 119-139 

No Delay 58-78 9765-117 114-140 
Appendix D Draft for Public Comment 

252 NIST NCSTAR 1-7, WTC Investigation

To model these scenarios, NIST ran a series of calibration simulations for each tower in an attempt to 

crudely simulate the occupant conditions inside the towers on September 11, 2001 with the capacity of 

8,800 occupants. Once the calibration simulations approximately achieved the goal of simulating the 

September 11, 2001 scenario, the fully-occupied population was added to each tower. From the fullyoccupied simulations of each tower, possible consequences were predicted, such as the number of 

occupants who would be trapped inside the tower at the time of collapse and the congestion points (the 

Mezzanine escalator) that developed with a larger population. 

For the calibration simulations, there were some difficult aspects of the evacuation that needed to be 

accounted for in the evacuation, such as pre-evacuation activities, occupants changing stairs for various 

reasons and the activities that they perform on the floor during evacuation, occupants resting in the stairs, 

occupants being delayed by helping behaviors, occupants giving way to injured persons (superflow), and 

firefighters counterflow. Although it was recognized that the models cannot accurately simulate all of 

these activities as of yet, an attempt was made to account for time lost to perform such activities in the 

calibration simulations. The buildingEXODUS model was the primary model used for this set of 

scenarios. Due to the way that the stairs are created in the model (lanes of occupants instead of an 

occupied space), it was difficult to model actual firefighters walking throughout the building and achieve 

the same kind of occupant response and effect that were seen on September 11, 2001. It was also difficult 

to simulate the effect of a larger bodied individual on the rest of the occupants in the stair, due to the 

method of simulating stair movement used in the buildingEXODUS model (2-lane stairways). 

The successful calibration simulation used for the WTC involved a method (to be referred to as) the “stop 

and go” method. The stop and go method involved the creation of “doors” inside the stair that would 

open and close throughout the simulation. This method attempted to account for evacuation interruptions 

and the “stop and go” reports from survivors during the evacuation at different points throughout the stair, 

such as leaving/changing stairs (sometimes occupants did this multiple times) for various reasons, resting 

on the stairs, helping, waiting behind larger or disabled occupants, superflow, firefighter counterflow, etc. 

Evacuation delays were distributed uniformly throughout the building, depending upon the Tower, to 

account for pre-evacuation delays and any actions that occupants took when leaving the stairs, such as 

short activities like moving directly to the next stairs, and longer activities such as seeking information, 

making phone calls, looking for staff/coworkers, and looking out windows. In each building, a minimum 

evacuation delay of 6 minutes was assigned to the population because even if the occupant left their floor 

immediately, they were still faced with other challenges/actions taken while outside of the stairs. 

Although the evacuation delay distribution on September 11, 2001 was not uniform, this assumption was 

made for the model. 

One must remember that the “stop and go” method was only an estimate of the reported “stop and go” 

activities observed in the WTC towers. This “stop and go” method alleviated a large crowd surrounding 

the escalator, which was a result in the simulations run for Scenarios 2-4, but was not overwhelmingly 

reported by survivors of September 11, 2001. 

D.4.5 WTC 1 Calibration Simulation 

The inputs for the calibration simulation of WTC 1 were the following: 

• 7,200 occupants distributed from floors 90-ground floor
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 253

• “Stop and go doors” located inside the stairwells at floor 76 (early in the evacuation) to 

simulate the locked doors in the transfer, floor 5 (late in the evacuation), and floors 15 and 48 

during the evacuation

• 6-30 minute delay uniformly distributed over the entire population

• Distribution of gender and age used in Scenarios 2-4

• Stair packing and local potential

In the calibration simulation, 7,200 occupants were distributed from floor 90 to the ground floor. The 

“stopping” points occurred (door closed) at every 5 minute interval for 2 minutes at floors 15 and 48 

starting after 10 minutes. Then, at floor 76, the “door closed” only three times for 2 minutes each, to 

simulate the locked doors on floor 76 in Stair A at the beginning of the simulation. Lastly, on floor 5, the 

door closed 5 times for 2 minutes each at the ending time of the evacuation. Also, a 6 - 30 minute delay 

was uniformly distributed to the entire population of the building. 

The simulation was judged to be “successful” at predicting a September 11, 2001, scenario if the 

following criteria set for the WTC 1 calibration simulation were met: 

• Evacuated more than 90 percent but less than 100 percent of the occupants from the tower 

before WTC 2 collapse (72 min) 

• Alleviated significant crowding at the entrance to the Mezzanine escalator 

For the calibration simulation, when 7,200 occupants in WTC 1 were given a 6-30 minute time delay with 

stoppage points, no significant queue developed at the escalator. Also, 7,200 occupants (under a uniform 

delay) evacuated the building in approximately 71 minutes. This is a good estimate for WTC 1, because 

90 percent of the occupants reported leaving the WTC complex before WTC 2 collapsed. Therefore, this 

geometry and scenario set up was used to simulate a fully-occupied WTC 1, which contained 

16,000 occupants below floor 92 (19,800 occupants minus those trapped above floor 91). Of the 

remaining 10 percent who were trying to evacuate, however, some were located inside the towers at the 

very base of the building (which the model falls a bit short of predicting), and some were located in the 

Concourse area trying to evacuate the complex at this time. Further, this simulation did not account for 

those who waited until WTC 2 collapsed to initiate their evacuation, (e.g., occupants on floor 64, and 

some very high in the building trapped by significant floor damage). 

D.4.6 WTC 1 Full-Capacity with Damage Simulation 

When 16,000 occupants were simulated using the calibrated input assumptions, buildingEXODUS 

calculated that there were still 2,400 occupants left inside WTC 1 as the building began to collapse. 

There were larger queues simulated at the escalator in a full-capacity scenario when compared to the 

September 11, 2001, calibration simulation. It is understood that with a larger population, occupants 

would have encountered even more stoppages in the stairs than occupants did on September 11, 2001; 

however, this is not quantifiable and only the delay times and stoppages used in the calibration simulation 

were used in this scenario. Stoppage points in the full-capacity simulation continued until 10 minutes 

after the collapse of WTC 2. 
Appendix D Draft for Public Comment 

254 NIST NCSTAR 1-7, WTC Investigation

D.4.7 WTC 2 Calibration Simulation 

The inputs for the calibration simulation of WTC 2 were the following: 

• 7,400 occupants distributed from floors 107-ground floor (after subtracting elevator users).

• “Stop and go doors” located inside the stairwells at floor 5 (late in the evacuation), and floor 

15 and 48 during the evacuation, beginning after 20 minutes

• “Door” closed inside stairs at floor 78 after 16 minutes

• 2-17 minute delay uniformly distributed over population from floors 107-78; 6-30 minute 

delay uniformly distributed over the population from floors 77 - ground

• Distribution of gender and age used in Scenarios 2-4

• Stair packing and local potential

A total of 7,400 occupants (84 percent of 8,800) were specifically placed throughout each of the three 

building sections, depending upon the percentage of occupants reporting elevator usage within that 

section. Stoppage points were placed low in the building to simulate stopping for fire officials, 

superflow, helping behaviors, etc. on floors 15 and 48 (throughout the evacuation beginning at 

20 minutes) and floor 5 (toward the end of the simulation). The reason that the stoppages began after 

20 minutes is because the fire department and others did not respond to WTC 2 until the building was hit. 

This simulation modeled evacuation from the entire building until WTC 2 was attacked. After 

16 minutes, it was assumed that everyone above the impact floor, 78, would not be able to evacuate past 

the impact point, even though it is known that 18 occupants escaped from this area. The reason for 

assigning a difference in delay times to occupants above and below the impact zone was because the time 

delay was used to account for both pre-evacuation activities and activities performed during the 

evacuation, such as waiting on a floor. Since occupants above the impact zone were not able to complete 

their evacuation, a lower delay range was assigned to them. 

The simulation was judged to be “successful” at predicting a September 11, 2001, scenario if the 

following criteria set for the WTC 2 calibration simulation were met: 

• All occupants below the 78th floor evacuated before T2 collapses (72 min) 

• A loss of approximately 600-700 occupants was predicted above 78 when WTC 2 collapsed 

• Alleviated significant crowding at the entrance to the Mezzanine escalator 

The WTC 2 calibration simulation predicted that all occupants evacuate from below floor 78 and that 

645 occupants remained above floor 78 when WTC 2 collapsed. Also, there was no significant queuing 

predicted at the entrance to the Mezzanine escalator. These modeling assumptions were then used to 

simulate the full-capacity tower containing 17,260 occupants, which accounted for occupants who used 

elevators, as described below. 
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 255

Elevators 

For WTC 2, a fully occupied building on September 11, 2001 was assumed to contain 19,800 occupants. 

It was necessary to estimate the number of occupants who would have been able to use the elevators 

within the first 16 minutes on September 11, 2001, when WTC 2 was fully occupied. Calculations were 

performed using the ELVAC model (Klote 1993). From the estimations made by the face-to-face 

interview respondents using the shuttle elevators (NIST 2004), the peak speed of the shuttle elevators was 

approximately 4.8 m/s (close to 78 floors in 60 seconds). The ELVAC model was used to calculate how 

many occupants could have evacuated to the ground floor of WTC 2 in less than 16 minutes using 

80 percent of the shuttle elevators available on floors 44 and 78. Only 80 percent of the elevators were 

modeled, under the assumption that some elevators would be out of service. Under the assumptions that 

the peak elevator speed was 4.8 m/s and had an acceleration of 1.5 m/s2

, 20 percent of the occupants 

(1,337) from the high section of the building (floors 78-110), 25 percent of the occupants (1,661) from the 

middle section of the building (floors 44-77), and 6 percent of the occupants (388) from the lower section 

of the building would have been able to evacuate via elevators before their tower was hit. 

The maximum number of occupants who would have been able to use the shuttle elevators, which had a 

capacity of 50 occupants each, to evacuate the building in under 14 minutes was estimated to be 

3,386 occupants. However, since it was discovered from face-to-face interviews (NIST 2004) that some 

elevators traveled to the ground floor without full capacity, it was assumed that only 75 percent of the 

maximum number of occupants who could use elevators (0.75 x 3,386 = 2,540) evacuated using the 

elevators. Therefore, approximately 17,260 occupants used the stairs in this simulation (19,800 – 2,540 = 

17,260). 

D.4.8 WTC 2 Full-Capacity with Damage Simulation 

When 17,260 occupants were placed within the calibration simulation, buildingEXODUS predicted that 

there were still over 8,000 occupants left inside WTC 2 after 72 minutes. Occupants were distributed 

throughout the building, above and below the floors of impact, according to predicted elevator use, and 

assigned an appropriate response delay. As was found with the WTC 1 full-capacity simulation, a larger 

amount of queuing was predicted by the buildingEXODUS model, especially at the entrance to the 

Mezzanine escalator. 

Table D–6 shows the results for the calibration and fully-occupied scenarios with building damage for 

both WTC 1 and WTC 2. 
Appendix D Draft for Public Comment 

256 NIST NCSTAR 1-7, WTC Investigation

Table D–6. WTC 1 and WTC 2 full capacity evacuation simulation results. 

Simulation 

Title 

Geometry; 

Population Simulation 

# occupants evacuated 

before tower collapses 

WTC 1 

Calibration 9/11 

Floors 90-ground; 

7,200 occupants 

Response time = 6-30 min, stoppage 

points throughout the stairs to simulate 

stopping of occupants for approx. 2 

minute intervals 

7,200 evacuate from below 

impact region within 71 

minutes 

WTC 1 fullyoccupied 

building on 9/11 

Floor 90-ground; 

16,000 occupants 

Response time = 6-30 min, stoppage 

points throughout the stairs to simulate 

stopping of occupants for approx. 2 

minute intervals 

13,600 occupants evacuated 

before WTC 1 collapsed; 

85 % evacuated from below 

impact zone 

WTC 2 

Calibration 9/11 

Floors 107-ground; 

7,400 occupants 

when removing 

occupants using 

elevators 

Response time 2-17 minutes above 78; 

6-30 minutes all other places; stoppage 

points throughout the stairs that begin 

after 20 minutes, “door” closes inside 

stair at 78 after 16 minutes 

6,755 occupants evacuated 

before WTC 2 collapsed; 

645 remained above floor 

78 

WTC 2 fullyoccupied 

building on 9/11 

Floors 107-ground; 

17,260 occupants 

when removing 

occupants using 

elevators 

Response time = 6-30 min, stoppage 

points throughout the stairs to simulate 

stopping of occupants for approx. 2 

minute intervals 

8,883 occupants evacuate 

before WTC 2 collapsed; 

3,900 remained above floor 

78; 58 % evacuated the 

WTC 2 

Differences were found between the results of the full-capacity with damage simulations for WTC 1 and 

WTC 2. The reasons for the differences in results are the following: 

• In WTC 1, all “stop and go” points within each stairwell were ceased 10 minutes after the 

collapse of WTC 2. This allowed “free flow” from 82 minutes to 103 minutes in WTC 1. In 

WTC 2, the firefighters had no prior warning to stop counterflow measures and begin to 

evacuate. Therefore, the “stop and go” points continued until the collapse of WTC 2. 

• Occupants in WTC 1 had a total of 103 minutes before their building collapsed, whereas 

occupants in WTC 2 had only 72 minutes. 

• In WTC 2, with a much higher population, there were a greater number of congestion points 

throughout the building, in addition to the “stop and go” points added to the building. 

Additionally, some occupants higher in the building were given a longer evacuation time 

delay to account for activities performed during evacuation, causing them to take longer than 

the allotted 72 minutes to reach the ground floor. These congestion points were located at the 

transfer floors. 

• Only 58 percent of the occupants in the fully-occupied building evacuated WTC 2, via the 

use of stairs or elevators. 2,540 were removed from the evacuation because they used 

elevators and 8,883 occupants evacuated via the stairs from WTC 2, for a total of 11,423 out 

of 19,800 occupants. 
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 257

D.5 CONCLUSIONS AND FINDINGS 

As shown in Table D–7, four different scenarios were run using three evacuation models, with and 

without a 0-10 minute delay time randomly distributed among the occupants. The first column of results 

shows the times for phased evacuation (with and without a delay time of 0-10 minutes) and the last three 

columns show full evacuations of occupants with different building populations. On average, the phased 

evacuation time (with initiation delays) for three floors to evacuate to three floors below the fire floor was 

11 minutes. However, for a fully-occupied building (19,800 occupants), evacuation times were calculated 

as 8 to 10 times larger for a total evacuation when compared to phased evacuation for this building. 

These scenarios assume a 0 to 10 minute delay time and do not account for other types of behavior that 

may delay evacuation and increase total evacuation time. 

Table D–7. Summary of egress simulation results. 

Evacuation 

Model 

Evacuation 

Initiation Delay 

Input 

Phased 

Evacuation: 

600 occupants 

Evacuation 

Time (min): 

8,800 occupants 

Evacuation time 

(min): 19,800 

occupants 

Evacuation time 

(min): 25,500 

occupants 

0-10 Minute Delay 11 55 112 142 

EXODUS No Delay 4 52 110 141 

0-10 Minute Delay 11.5 71-74 92-113 119-139 

EXIT89 

No Delay 3.5 58-78 9766-117 114-140 

0-10 Minute Delay 11.5 

Simulex 

No Delay 4 

Some mid-rise buildings that plan for phased evacuation can completely evacuate their building, at full 

capacity, in less than 20 minutes. However, a building such as a World Trade Center tower produces full 

evacuation times of over 100 minutes, which is a large difference in comparison to the phased evacuation 

time of 11 minutes. With buildingEXODUS, adding a 0-10 minute delay did not significantly affect the 

evacuation times from the building, adding only two additional minutes. The movement inside and from 

the base of the stairwell was the limiting factor and controlled the evacuation time. When the starting 

population roughly doubled, the evacuation time roughly doubled. As this simulation did not account for 

behavioral aspects of the evacuation (such as leaving the stairs and resting), the full impact may be 

underestimated somewhat by these evacuation model simulations. 

Scenarios 2 – 4 estimated the total evacuation time for a fully-occupied building on September 11, 2001. 

Figure D–4 shows that the evacuation with 25,585 occupants took approximately 2.5 times longer to 

evacuate than the 8,800 occupant population. Therefore, to estimate the total evacuation time from a 

fully-occupied building under the conditions observed on September 11, 2001, multiply by 2.5 the 

observed time required to evacuate WTC 1 on September 11 (approximately 100 minutes). A fully 

occupied building with visitors, but without elevator usage, would have required approximately 

 

66 The underlying theory for people movement in EXIT89 is based upon the work of Predtechenskii and Milinskii. They 

observed an inverse relationship between density and speed for three different types of movement: emergency, normal, and 

comfortable. The effect of crowding (density) on overall evacuation speed was greater when no delay time was assumed. A 

delay time when 19,800 occupants were present, according to EXIT89, spaces the occupants out and increases overall 

evacuation efficiency. This was not found to be the case for all occupant loads (25,500). 
Appendix D Draft for Public Comment 

258 NIST NCSTAR 1-7, WTC Investigation

four hours (2.5 x 100 minutes ≈ 4 hours ) to evacuate the building. Using the same method, if WTC 1 

was occupied with approximately 20,000 occupants (without visitors), the evacuation would have 

required roughly twice as long to evacuate than the 8,800 occupant simulation. Therefore, a fully 

occupied building without visitors and without the use of elevators would have required over three hours 

(2 x 100 minutes ≈ 3 hours ) to evacuate the building. 

Full Building Evacuation of a WTC Tower

0

20

40

60

80

100

120

140

160

0 5000 10000 15000 20000 25000 30000

Number of occupants in Tower

Evacuation Time (min)

8,800 - 9/11 capacity

19,800 - full capacity

25,585 - full capacity 

w ith visitors

Figure D–4. Full building evacuation simulation results with differing building 

populations. 

Scenarios 5-6 were modeled to estimate the consequences of a fully-occupied building on 

September 11, 2001 by estimating the number of lives lost at each building collapse. As the results show, 

more people would have lost their lives, particularly in WTC 2 due to its earlier collapse. Table D–8 

shows the simulated number of occupants (by starting location) who successfully evacuated WTC 1 and 

WTC 2 for a fully-occupied building subject to the conditions observed on September 11, 2001. 

• WTC 1 – 69 percent of all occupants (13,600 occupants out of 19,800) would have 

successfully evacuated the Tower, which breaks down to 0 percent (0 out of 3,800) from 

above the impact zone and 85 percent (13,600 out of 16,000) from below the impact zone in 

103 minutes. 

• WTC 2 – 58 percent of all occupants (8,883 by stairs and 2,540 by elevators) would have 

successfully evacuated the tower with the use of stairs and elevators in 72 minutes, which 

breaks down to 44 percent from above the impact zone and 75 percent from below the impact 

zone in 72 minutes. 
Draft for Public Comment Egress Modeling 

NIST NCSTAR 1-7, WTC Investigation 259

Table D–8. Simulated evacuation results for fully-occupied WTC 1 and WTC 2 on 

September 11, 2001. 

Percentage of Occupants Who 

Successfully Evacuated Relative to 

Where They Started 

Building 

Total 

Number of 

Occupants 

at t = 0.0 

Potential 

Number 

of 

Evacuees 

Occupants 

Remaining 

in Building 

at Collapse 

Occupants 

Trapped 

Above 

Floors of 

Impact Total 

Below 

Impact 

Above 

Impact 

WTC 1 19,800 16,000 6,200 3,800 69 % 85 % 0 % 

WTC 2 19,800 17,260 8,377 3,900 58 % 75 % 44 % 

Total 39,600 33,260 14,577 7,700 63 % 
Appendix D Draft for Public Comment 

260 NIST NCSTAR 1-7, WTC Investigation

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