S. Shyam Sunder & William L. Grossha

Building and Fire Research Laborato

National Institute of Standards and Techno

NIST's Findings On The World T

Center Fire and Collapse

NFPA World Safety Conference & Exposit

Las Vegas, NV

June 8, 2005
National Context

• The collapse of the World Trade Center structures followi

terrorist attacks of September 11, 2001 was one of the wobuilding disasters in recorded history – killing 2749 people

• More than 400 fire and emergency responders were amo

killed, the largest loss of life for this group in a single incid

• Strong private sector, public, and Congressional demand 

comprehensive response to the World Trade Center disas

• Congress passed and the President signed into law on O

2002, the National Construction Safety Team (NCST) Act

• Gives NIST authorities to investigate building failure• Modeled after the NTSB, with some differences.
NCST Advisory Committee Members

Dr. Charles Thornton, 

Thornton-Tomase

Dr. Robert Hanson, Pr

Emeritus, Univers

Mr. Philip DiNenno, Pr

Associates.

Professor Glenn Corbe

College, NYC.

Dr. Kathleen Tierney, 

Colorado, Boulder

Mr. Paul Fitzgerald, FM Mr. David Collins, The Professor Forman Will

of California at Sa

Dr. John Barsom, Pres

Consulting.

z Appointed by the NIST Director.z Functions...

• Review procedures and reports• Evaluate activities of teams• Assess implementation of 

recommendations

• Annual report to Congress

z Reviewed WTC Investigation plan, 

progress, findings, draft recommendations at 6 meetings. 

z Reviewed all WTC progress & final reportsz Membership balances broad scope of 

disciplines and interests
Dissemination

and Technical

Assistance Program

Dissemination

and Technical

Assistance Program

BPAT

Recommendations

BPAT

Recommendations

Govt.

Industry

Professional

Academic

Inputs/Actions

Govt.

Industry

Professional

Academic

Inputs/Actions

Public Inputs/

Efforts

Public Inputs/

Efforts

Technical Basis f

Building and F

and Stand

Technical Basis f

Building and F

and Stand

Standard

Developme

Owners, Contractors, Designers, Emergency Responders and Regulatory Authorities

Guidance and Tools for

Improved Practices

Guidance and Tools for

Improved Practices

Research &

Development

Research &

Development

WTC

Investigation

WTC

Investigation

WT

Respons
NIST WTC Investigation Objec

• Determine:

• why and how the WTC Towers collapsed following the 

impact of the aircraft, and 

• why and how the 47-story WTC 7 collapsed

• Determine why the numbers of injuries and fatalities were 

high depending on location, including technical aspects of 

protection, occupant behavior, evacuation, and emergenc

• Determine the procedures and practices that were used in 

design, construction, operation, and maintenance of the Wbuildings

• Identify, as specifically as possible, areas in current n

building and fire model codes, standards, and practicwarrant revision


Some Specific Questions

• How and why did WTC 1 stand nearly twice as long as WTC 2 b

collapsing (102 min. vs. 56 min.) though they were hit by virtuallaircraft?

• What factors related to normal building and fire safety considera

unique to the terrorist attacks of September 11, 2001, if any, coudelayed or prevented the collapse of the WTC towers?

• Would the undamaged WTC towers have remained standing in 

conventional large building fire scenario?

• What factors related to normal building and fire safety considera

could have saved additional WTC occupant lives or could have loss of life among the ranks of first responders?

• How well did the procedures and practices used in the design, c

operation, and maintenance of the WTC buildings conform to a

national practices, standards, and codes?
System Design Concept

Available Information on Safety of W

Towers in Aircraft Collision (1)

Type of Aircraft: Boeing 707 (largest jet aircraft in the air at th

DC-8

Speed of Aircraft: 600 mph (Port Authority, February 1964)

180 mph (FEMA 403, 2002)

Location of Impact: 80th floor (Port Authority, March 1964)

Structural design: It appears that the design of the WTC towers 

considered the impact of 707 aircraft and anaindicated that such collision would result in odamage which could not cause collapse or sdamage to the building

NIST found no documentary evidence of any analysis supporting this concl
Available Information on Safety of W

Towers in Aircraft Collision (2)

Fire safety: There are two views on whether the effect of jet fuel a

contents was a consideration in the original building d

• One view suggests that an analysis was done indicating th

problem would be the fact that all the fuel would dump into 

and there would be a horrendous fire.

• Another view suggests that the fuel load, and the fire dama

would cause, may not have been considered.

Life safety: There are two views on what would be the effect of a

impact on occupant life safety.

• One view, which did not consider the fires, suggests that th

impact would not have endangered the lives and safety of in the immediate area of impact

• Another view, which considered the fires, recognized that 

would not survive even though the building structure would 
Context of Findings • Buildings are not specifically designed to withstand the impact of fuel-laden 

airliners. While documents from the Port Authority of New York and New Jersey (

indicate that the impact of a Boeing 707 flying at 600 mph, possibly crashing into th

was analyzed during the design of the WTC towers in February/March 1964, the e

subsequent fires was not considered. Building codes do not require building d

consider aircraft impact.

• Buildings are not designed for fire protection and evacuation under the magn

scale of conditions similar to those caused by the terrorist attacks of Septem

• The load conditions induced by aircraft impacts and the extensive fires on Se

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

considered in building codes.

• Prior evacuation and emergency response experience in major events did no

total collapse of tall buildings such as the WTC Towers and WTC 7 that were oceveryday use; instead, that experience suggests that major tall building fires result 

conditions, not overall building collapse. 

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

permitting compacts between states, and is not bound by the building and fire codestate, or federal jurisdiction. The PANYNJ’s longstanding stated policy is to meet aappropriate, exceed requirements of local building and fire codes.
Applicable Building Codes and Po

• Although not required to conform to NYC codes, the PANYNJ elected to a

provisions of the proposed 1968 edition of the NYC Building Code, more t

years before it went into effect. 

• The 1968 edition had less restrictive provisions compared with the 19

was in effect when design began for the WTC towers in 1962. The 1968 c

• Eliminated a fire tower as a required means of egress;

• Reduced the number of required stairwells from 6 to 3 and the size of 

to the stairs from 44 in. to 36 in.;

• Reduced the fire rating of the shaft walls in the building core from 3 h 

• Changed partition loads from 20 psf to loads based on weight of parti

length (that reduced such loads for many buildings including the WTC 

• Permitted a 1 h reduction in fire rating for all structural components (c

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

• The NYC Department of Buildings reviewed the WTC tower drawings 

provided comments to the PANYNJ concerning the plans in relation to the Building Code. The architect-of-record submitted to the PANYNJ responscomments, noting how the drawings conformed to the 1968 NYC Building 

the issues identified in the NYC review appear to deal with egress issues, 

of the innovative features of the buildings.
Preliminary Aircraft Impact Dam

Analysis

• The impact of the exterior 

wall by an empty wing segment produces significant damage to the perimeter columns, not necessarily complete failure.

• The impact of a fuel-filled 

wing section results in extensive damage to the exterior wall panel, including complete failure of the perimeter columns.
WTC 1 Tower Model for Airc

Impact Analysis
WTC 1
WTC 1 Damage: Composite Summary 

Floors 93 to 98

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Damage by Floor

Floor 94 

Floor 96 Floor 97 

Floors 93 to 98 Floor 93

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2
WTC 2 Damage: Composite Summ

Floors 78 to 83

Column DamageSeveredHeavy DamageModerate DamageLight DamageFloor DamageFireproofing and partitionsFloor system structural damage Floor system removed
Enhancements added by NIST.

Broken 

Column Longitu

Panel JuObscure

Relative Roles of Aircraft Impact and 

• Fires played a major role in further reducing the structural capacity of 

buildings, initiating collapse. While aircraft impact damage did not, b

initiate building collapse, it contributed greatly to the subsequent firethermal response of the structures by:

• Compromising the sprinkler and water supply systems;• Dispersing jet fuel and igniting building contents over large areas;

• Creating large accumulations of combustible matter containing aircraft debris and 

contents;

• Increasing the air supply into the damaged buildings that permitted significantly h

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

spread rapidly on multiple floors; 

• Damaging and dislodging fireproofing from structural components in the direct pa

and due to the strong vibrations generated by aircraft impact; and

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

and to structural components.
Relative Roles of Aircraft Impact and Fi

• The jet fuel, which ignited the fires, was mostly consumed w

first few minutes after impact. The fires that burned for almentire time that the buildings remained standing were due mburning building contents and, to a lesser extent, aircraft cojet fuel.

• Typical office furnishings were able to sustain intense fires 

an hour on a given WTC floor. No structural component, howesubject to intense fires for the entire period of burning. The duraintense burning impacting any specific component was controlled 

• The availability of combustible materials• Fuel gases released by those combustibles• Combustion air in the specific area

• The typical floor had on average about 4 psf of combustible 

on floors. Mass of aircraft solid combustibles was significant in immediate impact region of both WTC towers.
Initial Fire and Smoke Simulati

Fall 2001
Spread of Jet-Fuel Ignited 

Multi-Floor Fires

• Consistent with available photographic and videographic evidence, NIST compute

capture the broad patterns of fire movement around the floors, with flames in a giv

lasting for about 20 min before spreading to adjacent, yet unburned combust

observed instances where fires persisted longer in regions with accumulated comb

other instances of sudden or interrupted fire spread. 

• The affected floors in the WTC towers had an open floor plan—with a modest 

perimeter offices and conference rooms and an occasional special purpose area. had two tenants, and those spaces, like the core areas, were partitioned (slab to 

slab). Photographic and videographic evidence confirms that even non-tenant spa

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

resistance to fire spread in the affected floors. 

• For the time that the fires were active prior to building collapse, the presence of u

fire-rated compartments may have assisted in mitigating fire spread and conthermal weakening of structural components.

• The 1968 NYC Building Code required buildings like the WTC towers to have 1 h f

separations, but the code did not impose any minimum compartmentation requirem

12,000 ft2) to mitigate the spread of fire in large open floor plan buildings. 
Compartmentation Requirem

• The NYC Building Code and PANYNJ practice required partitions to se

spaces from each other and from common spaces such as the corridserved the elevators, stairs and other common spaces in the building cor

• Local Law 5 (1973) required compartmentation of unsprinklered spaces i

office buildings over 100 ft in height “having air-conditioning and/or mech

ventilation systems that serve more than the floor on which the equipmen

to be subdivided by 1 hour fire separations into spaces or compartmexceed 7,500 ft

2. Floor areas could be increased up to 15,000 ft

2 if p

hour fire resistive construction and smoke detectors. 

• Shortly after the adoption of LL 5 (1973), the PANYNJ began to add the 

compartmentation as a part of new tenant layouts as evidenced by sever

alteration contracts at this time. 

• Following the 1975 fire a fire safety consultant report recommended to 

the buildings be retrofit with sprinklers to address possible smoke pr

the PANYNJ realized that this would also obviate the need for compartand permit the unobstructed views for which the buildings were known. Tsprinkler the buildings left the arrangement again with only partitions setenant spaces from each other and from exit access corridors or cospaces in the core, and with shaft enclosures.
Structural Analysis Progress

Component Analyses Knuckle Truss seat connections Single truss and 

concrete slab

 Full floor Column splice 

connection

 Single story column Nine story column Nine story-nine column 

exterior wall panel

Subsystem Analyses WTC 1

• Isolated Core• South Exterior Face• Floors 93 to 99

 WTC 2

• Isolated Core• East Exterior Face• Floors 79 to 83

Glob WT WT

Detailed nonlinear analyses to determine component behaviors and failure mechanisms

Nonlinear analyses with component simplifications and failure mechanism simplifications to determine major subsystem behavior and sequential failure mechanisms.

Nonlindetermbehavsequemecha


Critical Analysis Inter-Dependencies

Collapse Seq

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

SFRM Damage

Structural 

Damage

Gas Temperat

Time-Historie

(FSI)

Structural

Temperature T

Histories

Resolution1-4 in.10-6 s

Aircraft Impact 

Damage

LS-DYNA

Fire Dynam

(FDS)

Thermal Ana

ANSYS v.8

Structural Res

and Failure An

ANSYS v.8

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude

Baseline Performance Analysis
South Face 

of WTC1

• Maximum = 55 inches

(uncertainty ~ +/- 6 inches)

• Time: 10:22 AM• Measurements of inward

bowing (inches)

• Floor locations approximate• Blue tinted region digitally

enhanced
Inward Bowing of Perimeter Columns S

Minutes Prior to Collapse: WTC 2 East 
Tilting of Building 

Sections

Initiation of glcollapse was observed by building sectithe impact reboth WTC tow

WTC 1 tilted to the south; WTC 2 tilted to the east and south and twisted in a counterclockwise motion

© 2001 Dean Riviere
Factors that Enhanced Build

Performance on September 11, 

• The unusually dense spacing of perimeter columns, coupled with deep s

that was an inherent part of both the architectural and structural design 

walls, resulted in a robust building that was able to redistribute loads froperimeter columns to adjacent intact columns.

• The wind loads used for the WTC towers, which governed the design of 

frame-tube system, significantly exceeded the prescriptive requirements 

York City building code and selected other building codes of the era (Ch

York State), including the relevant national model building code (BOCA)

• The robustness of the perimeter frame-tube system and the large dimen

the WTC towers helped the buildings withstand the aircraft impact. 

• The composite floor system with open-web bar joist elements, framed to 

way flat plate action, enabled the floors to redistribute loads without collaplaces of aircraft impact damage to other locations, avoiding larger scale 

upon impact.
Factors that Enhanced Buildin

Performance on September 11, 20

• The hat truss resisted the significant weakening of the core, due 

impact damage and subsequent thermal effects, by redistributinfrom the damaged core columns to adjacent intact columns andby redistributing loads to the perimeter walls from the thermally wcore columns that lost their ability to support the buildings’ weigh

• As a result of the above factors, the buildings would likely n

collapsed under the combined effects of aircraft impact and subsequent jet-fuel ignited multi-floor fires, if the fireproofinbeen dislodged or had been only minimally dislodged by aiimpact. The existing condition of the fireproofing prior to aimpact and the fireproofing thickness on the WTC floor sysnot play a significant role in initiating collapse on Septembe
Innovative WTC Tower Structural Sys

• Innovative structura

when built; incorpornew and unusual fe

• Two features requir

consideration:

• Composite flo

system using open-web baspray-applied 

• Design for wi

and control of induced vibra
Fire Performance of Compos

Floor System

• Fire-protection of a truss-supported floor system with spray-

fireproofing was innovative and not consistent with then-prepractice.

• No evidence found of technical basis in the selection of 

fireproofing thickness to meet 2 h fire rating:

• 1/2 in. specified when WTC towers were built to maintain Class 1-A (not 1-

requirement of the NYC Building Code 

• 1-1/2 in. specified for upgrades some years prior to 2001• 2 in. for similar floor system in an unrestrained test (model code evaluation 

recommendation in June 2001, unrelated to WTC buildings)

• No evidence that full-scale fire resistance test of the WT

system was conducted to determine the required fireproofthickness; in 1966, the Architect of Record and, in 1975, the Engineer of Record stated that the fire rating of the WTC flocould not be determined without testing.
Results From NIST-Sponsored Tests 

___________________________________________________(1) Imminent collapse(2) Vertical displacement exceeded capability to measure accurately(3) Did not occurThe end-point criterion that determined the rating is shown in matching color.

--- 58 66 58 (3) 120(1)

3⁄4

17 ft, restrained, 1⁄2 in fireproofing

4

210

2

(2) 180 157 86 76 (3) 17 ft, restrained, 3⁄4 in fireproofing

3

146

2 

(2) --- --- 76 62 (3)

35 ft, unrestrained,

3⁄4 in fireproofing

2

116 11⁄2 

(1) --- 111 66 62 (3) 35 ft, restrained, 3⁄4 in fireproofing 1

Rating

(hr)

Maximum

(1300oF)

Average

(1100oF)

Maximum

(Ambient

+325oF)

Average

(Ambient

+250oF)

ASTM

E 119-

Failure 61

to

Support 

Load

Steel Temperatures Temperature on Unexposed Surface

Standar

Test

Terminated

(min)

Times to Reach End-Point Criteria (min)

Test Description
Wind Load Estimates for WTC 2

1960’s 13.1 10.1 16.5 8.8 12

Original WTC Design (Clarified by designer in July 2004)

2004 12.2 14.0 15.6 12.8 11

NIST / third-party SOM review / ASCE 7-02

CPP / ASCE 7-98 2002 15.1 15.3 17.1 15.5 14

*

2002 NA NA NA NA N

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 10.6 12.2 13.5 11.1 10

2002 9.7 11.1 12.3 10.1 9

RWDI / NYC Building Code

9.3 9.3 7.6 7

1968 - 2001 NYC Building Code

5.3 5.3 4.2 4

Prior to 1968 NYC Building Code

Ab

E

About NS N-S E-W Resultant

Base Mome

Base Shear 10 

3 kips

Source Year

* Using ASCE 7-98 sections 6.5.4.1 and 6.6
Results and Findings of Drift Ana

70.6 H/244 83.9 H/205 75.6 H/227 71

Refined NIST case

SOP case 56.8 H/303 68.1 H/253 59.7 H/287 56

56.6 H/304 55.7 H/309 51.2 H/335 65

Original design case

Tot

Dr

(in

Drift 

Ratio

Total 

Drift 

(in.)

Drift 

Ratio

Total 

Drift 

(in.)

Drift 

Ratio

Total 

Drift 

(in.)

E–W N–S E–W 

WTC 1 WTC 2

Loading Case

• The calculated drift ratios correspond to a damping ratio of 2.5% in es

loads.

• Typical drift ratios considered in practice (not required by building co

• H/500 (~ 32.9 in.)

• Under the original design wind loads, the WTC towers would need to h

between 1.5 to 1.9 times stiffer to achieve a H/500 drift limit; this can bachieved by increasing exterior column areas in the lower stories andadditional damping.
Summary of Evacuation Findi

• It is estimated that 17,400 occupants (± 1,200) were present in the W

on the morning of September 11, 2001; about 6 percent of the survoccupants reported a pre-existing limitation to their mobility.

• Approximately 87 percent of the WTC tower occupants, including m

percent of those below the floors of impact, were able to evacuate 

successfully. 

• Functioning elevators allowed many (roughly 3,000) survivors to se

WTC 2 during the 16 minutes prior to aircraft impact.

• The egress capacity (number and width of exits and stairways) was 

accommodate survivors who were seeking and able to reach and uundamaged exits and stairways.

• A full capacity evacuation of each WTC tower with 25,000 people—

the number present on September 11, 2001—would have required ahours and likely resulted in loss of 14,000 lives.

• The average surviving occupants moved down stairs at about half 

speed previously reported for non-emergency evacuations.

• Occupants were often unprepared for the physical challenge of ful

evacuation.
Condition of Stairwells

• The stairwells, with partition wall enclosures that provided a 2 h fire-rating but little s

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

• One of the stairwells in WTC 2 (Stairwell A on the Northwest side) was passab

of aircraft impact for some period of time after WTC 2 was attacked.

• All three stairwells in WTC 1 and the two other stairwells in WTC 2 were rendered im

the region of aircraft impact.
Selected Emergency Response Fin

• Radio communications in high-rise buildings• Emergency response operations in high-rise buildings• Evacuation of mobility impaired occupants• Feasibility of roof evacuation• Situational awareness• Command and control for large-scale incidents
Issues Related to Practice, Standards, 

• Based on the Investigation findings, NIST identified a broad set of issues 

practice, standards, and codes that will provide the basis for formulating trecommendations.

• Issues arising from the investigation are grouped under the following majo

• Increased Structural Integrity• Enhanced Fire Resistance of Structures• New Methods for Designing Structures to Resist Fires

• Enhanced Active Fire Protection• Improved Building Evacuation • Improved Emergency Response• Improved Procedures and Practices• Education and Training Programs
Categories of Issues

Category: Responsible Community

• Professional Practices• Provisions in standards, codes and regulations

• Adoption and enforcement of the provisions

• R&D or requiring further study• Education and training

Category: Affected Building Population

• All tall buildings (buildings over 20 stories in height)• Selected tall buildings (buildings over 20 stories in height that are 

design, location, use, iconic status, nature of occupancy, etc.)

• Selected other buildings (buildings less than 20 stories in height t

due to design, location, use, historic/iconic status, nature of occup

Category: Relation to the outcome on 9/11

• If in place, could have changed the outcome on 9/11

• Would not have changed the outcome on 9/11, yet is an importan

fire safety issue that was identified during the course of the invest
7A Published

Accounts7B SurveyInstruments

4A FireHistory4B FireSprinklers4C FireAlarms4D SmokeManagement

WTC Investigation Reports

WTC Towers 

Investigation Report

Building and 

Fire Codes

Baseline 

Performance 

& Aircraft 

Impact

Steel

Analysis

Active Fire 

Protection

Thermal 

Reconstructio

n

Occupant 

Behavior & 

Egress

WTC Towers: 

Structural 

Collapse

Emerge

Respon

1A Design andConstruction1B StructuralCode Comparison1C StructuralModifications1D Fire Protection &Life Safety Provisions1E Fire CodeComparison 1F Comparisonof NYC Codes1G Fire Protection& Life Safety Amend.1H Post-Const.Modifications WTC1&21I Post-Const.Modifications WTC 71J WTC 7 FuelSystem 

9 

report

s 

totalin

g 

about 

1,500 

pages

34 

reports 

totaling 

about 

8,000 

pages

43 reports for WTC Towers; 5 reports for WTC 7

3A Contemp.Steel Specs.3B SteelInventory3C SteelDamage3D MechanicalSteel3E PhysicalProperties

2A BaselinePerformance2B AircraftImpact

5A VisualEvidence5B SteelExperiments5C SingleWorkstation5D CeilingTile5E MultipleWorkstation 5F FireSimulation5G FireStructure Interface

6A Passive FireProtection6B FireResistance Test6C ComponentAnalysis6D GlobalAnalysis
Summary

• Draft WTC reports to be released June 23, 200

media and public briefings in New York City.

• Six-week public comment period closes 5 pm E

August 4, 2005.

• WTC Conference: Putting Recommendations in

Practice, September 13-15, 2005, to be held at 

• Details available at http://wtc.nist.gov
I. ACTIVE FIRE PROTECTION SYSTEMS

II. RECREATION OF BUILDING FIRES LEADIN

STRUCTURAL HEATING

III. EGRESS ISSUES AND EVACUATION MODIV. EMERGENCY RESPONSEV. RESEARCH, IMPLEMENTATION OF 

RECOMMENDATIONS

WTC INVESTIGATION DETAI
A. Sprinklers and Standpipe 

Systems

B. Fire Alarm SystemsC. Smoke Management Systems

I. ACTIVE FIRE PROTECTIO

SYSTEMS
A. Sprinkler and Standpipe Syste

• Sprinklers were installed throughout WTC 1 & 2 on Sept. 1

(except where exempted).

• Storage tanks directly connected to NYC water distribution 

supplied water to towers; sprinklers supplied automatically 

• Supply piping from the 100th floor resulted in restricted wate

several floors; however, this inconsistency with current besno impact on outcome.

• Manual initiation of electrical fire pumps was required to pro

supplemental water; automatic initiation is required by NFPhowever, unlikely that it made a difference.

• Water supply risers were configured to provide redundancy

floor level controls were vulnerable to single point failures aconnection.
Sprinkler and Standpipe Systems 

• Sprinkler systems capable of controlling 

typical fire with coverage area up to 4,500 ft

2, three times design area specified but 

less than 15 % of single floor.

• Primary and backup power were provided; 

system operability could have been affected by lack of redundancy of power lines to emergency fire pumps once power was lost.

• Due to magnitude of initial fires and likely 

aircraft impact damage, suppression systems in WTC towers could not have been expected to control the fires.
B. Fire Alarm Systems

• Provided for automatic fire detection, but required manual ac

notification devices. 

• Capable of determining and displaying (1) areas that had at 

reached alarm point conditions, and (2) areas that had not.

• System in WTC 1 & 2 had extensive back-up command cap

hardware that provided multiple places where some alarm h

were stored; up to 13 storage locations were identified.

• Monitored by PANYNJ; in WTC 1, overwhelming number of 

registered and displayed at FCS, however, information at FCused to manage evacuation; there was no means at FCS to 

whether or not announcements could be heard on intended 
Fire Alarm Systems (2)

• Manual activation of system was required in WTC towers to 

building occupants, delaying alarm for 12 minutes in WTC 1

• Notification appliance circuit and warden/standpipe telepho

were not required to have the same high level of performan

signaling line circuits.

• Firefighter telephone systems in WTC towers were not used 

not uncommon since firefighters are trained to use their radpreferred means of communications.

• Although fire alarm systems in WTC towers used multiple 

communication path risers, systems experienced performandegradation, especially in WTC 1 where all notification and communication functions were lost above impact floors.
C. Smoke Management System

• Smoke purge systems in WTC 1 & 2 were designed to rem

from fire area after suppression operations, to be activated direction FDNY.

• Systems were not initiated on September 11; if they had be

unlikely that system would have functioned as designed duelectrical power and damage to the HVAC shafts and other elements in the impact zone.

• WTC towers not required to have active smoke manageme

combination fire/smoke dampers because buildings were sthroughout.

• PANYNJ commissioned a study in 1996 which cited lack of 

stairs that were required in lieu of stair pressurization by Lo(1973), but that, due to height of stair shafts, required vents ineffective. PANYNJ pressurized corridors with outside air a
Example of Vertical Smoke Spre
Smoke Management Systems (

• Alternative smoke management systems were evaluated: 

 Core pressurization  Building pressurization Sandwich pressurization Zoned smoke control with stair pressurization

• None would have prevented smoke spread for aircraft imp

scenarios

• Stair pressurization would have been ineffective in improvi

conditions for occupants trying to exit the buildings.

• Installation of combination fire/smoke dampers in HVAC d

would have acted to slow development of hazardous conduppermost floors of the building, but would likely not have significant effect on ability of occupants to egress buildings 

impassability of the exit stairways.
II. RECREATION OF BUILDING 

& STRUCTURE TEMPERATUR

A. Collection and analysis of photograph

video evidence

B. Identification of suitable computer sim

software

C. Supporting experimental effortD. Predicted fire spread and structure 

temperatures
A. Collection and Analysis of 

Photographic and Video Image

Visual database contains:

• Well in excess of 7,000 photographs taken by more than 185 • 150 hours of videotape from major media outlets, more than 

From analysis of images, NIST has identified significant events for WTC 1 and 2 related to aircraft impact, fire development, and building damageNIST has developed detailed mappings for fire, smoke, and condition of windows at several specific times for each tower.
Visual Evidence of Fires in WTC 
Visual Evidence of Fires in WTC 
B. Computer Simulation 

Software

Collapse Seq

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

SFRM Damage

Structural 

Damage

Gas Temperat

Time-Histori

(FSI)

Structural

Temperature T

Histories

Resolution1-4 in.10-6 s

Aircraft Impact 

Damage

LS-DYNA

Fire Dynam

(FDS)

Thermal Ana

ANSYS v.8

Structural Res

and Failure An

ANSYS v.8

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude

Baseline Performance Analysis
C. Supporting Experimental Eff

Need to demonstrate capability to predict:• Heat transfer from controlled fire to steel structural member

spray-on fire resistant material (SFRM) and gypsum board

• Heat release rate of representative work station• Fire spread from one work station to another• The influence of jet fuel on carpet and furnishings• The impact of ruble on heat release rateASTM E119 testing of composite floor system• To evaluate impact of scale, restraint, and SFRM thickness• To characterize thermal environment and temperatures of f
SFRM used in Towers

Structural Member 

Component size Location Material 

Floor Trusses All All Cafco DC/F Interior columns < 14WF228 All Cafco DC/F 

≥ 14WF228 All Cafco DC/F 

Exterior Columns “heavy” Ext. Faces Cafco DC/F 

“heavy” Int. Faces Vermiculite Aggreg

Spandrel Beams All Ext. Face Cafco DC/F 

All Int. Face Vermiculite Aggreg
Heat Transfer Experiments

1.2 SECTION A-A

7.04 m 

Fire Pan

Trusses 

Baffle 

Access panels

Column Suspended bars

PLAN VIEW

(i

Truss support

3.82 m 

3.68 m 

Baffle 

 Trusses 

Inlet Vents 

Column

Truss support 

(

Fire Pan 

 3.68 m 

 Inlets  WEST 

Figure 2.4: Compartment Content Layout (not to scale). 

 Outlets 

Suspended bar 

Column support 

X 

Y 

(0,0) 

(0,0) 

NORTH 

NORTH 
Heat Release Rate, Single Workst
Workstation-to-workstation 

Fire Spread
D. Predicted Fire Spread and Stru

Temperatures

FDS Computational Domain
Upper Layer Temperatures 

(WTC 1, Floor 97)
Predicted Column Temperatur

WTC 1

Shows maximum temperature reached by each c

WTC 2
A. Methods and Data Sources

B. Evacuation ProcessC. Key Findings 

III. EGRESS ISSUES AND 

EVACUATION MODELING*

*Condensed from presentation by KATHLEEN TIERNEY at the AIA Annual Convention, Las Vegas, May 20, 2005
A. Methods and Data Sources

• 745 published “first-person” accounts, reduced to 

435 separate accounts of occupant experiences

• Systematically selected random sample of 803 

survivors, interviewed by telephone: both towers, lower, mid-level, and higher floors

• More than 200 face-to-face interviews designed to 

elicit accurate recall of occupant experiences on September 11

• Six focus group discussions• Egress modeling using alternative scenarios 

St

WTC 1

94 – 98

Zone 3Zone 2Zone 1
B. Evacuation Process

• Evacuations initiated by building occupants themselves.• 91% of survivors in WTC 1 had begun evacuating before 

plane hit tower 2.

• After attack on WTC 1, WTC 2 occupants began evacuati

spontaneously.

• Nearly 20% of tower 2 survivors used elevators.• 87% of WTC2 occupants started evacuating before secon• 75% of WTC2 occupants above floor 78 had gotten to tha

below before the plane struck.

• Evacuation already under way at 9 a.m. when announcem

that WTC2 occupants should return to their offices.

• Most of those evacuating WTC2 did not hear that announ• 41% of WTC2 survivors had already left before second pl• Self-evacuation and use of elevators saved 3,000 lives in • Most occupants moved quickly to leave, but many delaye
Evacuation Process (2)

• Delayed departure proved fatal for those on/ above impact 

2. 

• “Milling”: Normal response to uncertain but urgent circumst

involving information-seeking, information exchange, obser

• “Preparatory Behaviors”: Gathering materials, phone calls, 

back-up and shut-down, etc.

• Variations in behavior 

attributable to experience with/knowledge of 1993 bombing:

--Leave Immediately--Wait for Instructions--Don’t rely on Port 

Authority

0 5 10 15 20 25 30 35 40 45 50 55 60 65 7

0

2

4

6

8

10

12

14

16

18

20

Percentage of Survivors Exiting

Time Since WTC 1 Attack (min)
Quote from Tower 2

“I was there during the 1993 bombing. I did evac

with a group of people who had no clue as to wwe were going. What I learned from that experwas not to trust the Port Authority’s announcemHad I not experienced that, I might have listenthe Port Authority announcement and stayed p

Information

Factors

Social

Factors

Psychological

Attributes

Physiological

Factors

Perceived

Risk Confirmation Action
Evacuation Process (3)

• Average evacuation time for WTC1 survivors was 42 minu• Average times ranged from 29 to 70 minutes, depending o

height.

• Influential factors:

- Encountering debris, damage, dangerous conditions- Evacuating from a higher floor- Interrupting evacuation process (helping others, leav

stairwells, reversing direction

- Encountering transfer hallways

• Factors that did not have a statistically significant impact:

- Perceptions regarding counterflow- Perceptions regarding crowding

• Simulex, buildingEXODUS, EXIT89 used to model phased 

building evacuations under different conditions.
Evacuation Process (4) 

• Under full occupant and visitor loads 

(25,000 in each tower) and with no delays, models indicate evacuation 

would have taken close to 2-1/2 hours (compared to ~ 4 hours based upon observed egress rate on Sept. 11).

• In Tower 2, 7700 people would have 

been trapped above the impact floors.

• 14,577 people would still have been in 

the towers when they collapsed.
Preparedness and Training Before Sept. 11:

• Egress Systems and Preparedness Measures Upgraded A

Bombing

• 66% of Survey Respondents Had Taken Part in a Fire Dril

Months Before 9-11

• Training Only at Most Basic Level—Important Guidance La• Survivors Considered Training Useful

C. Key Egress Findings
Key Egress Findings (2)

Mobility Impaired Occupants:• Ambulatory mobility impaired occupants typically walked do

with one hand on each hand rail, took one step at a time, aaccompanied by another person. This blocked others behi

moving more rapidly.

• FDNY and PAPD personnel found 40 to 60 mobility impaire

on 12th floor of WTC 1 as they went down and attempted to 

floor on their way out. These impaired individuals had beethis floor in an attempt to clear stairways.

• Emergency responders were assisting approximately 20 of 

mobility impaired people down staircase just prior to the coWTC 1. It is unknown how many fatalities occurred with th
Key Egress Findings (3)

• Stairwell separation ranged from 70 ft (including the impa

WTC 1) to 200 ft (including the impact region in WTC 2).

• Building egress systems are not designed to accommoda

building evacuation.

• There is a lack of egress models and performance-based 

methodology accounting for human behavior during evacu

• Remarkably successful evacuation, given conditions Tow

occupants faced on September 11

• Spontaneous evacuation, use of elevators saved many liv• But what if there had been 50,000 people in the Towers, i

17,000?
IV. EMERGENCY RESPONS

A. BackgroundB. Operations and Situation AwarenesC. FDNY Access to TowersD. CommunicationsE. Command and Control
Background

Changes after 1993 Bombing:• FDNY worked with PANYNJ to upgrade WTC fire prot• FDNY high-rise radio repeater was installed• Fire Command Desks installed in WTC 1 and 2• Elevator intercom system was upgraded• New Operations Control Center was added to comple

level of WTC 2

• Multiple power sources installed for emergency lightin

• New decentralized fire alarm system was installed at W• Various fire drills were conducted at WTC and some in

FDNY participation
Background (2)

Role of Emergency Responders: FDNY - Established operational control and Incident Comm

for WTC operations, conducted evacuation and rescue o

and fought fires.

PAPD - Established security at WTC and conducted evacu

rescue operations.

NYPD - Established traffic control, perimeter security at sit

security for command posts, and conducted evacuation arescue operations inside. Aviation units supplied observcapabilities and assessed potential for roof rescue.

OEM - Functioned as multi-agency command resource cen

provided support for all agencies and departments worki

disaster.
FDNY’s Initial “Size-up” of WTC Conditions:

• A large aircraft had hit the WTC 1 building. • Large fires were burning on multiple floors at and above im• Elevators were not working and people were trapped insid

the elevators.

• Sprinkler system and standpipe systems were likely comp

• Likely that no water supply was available 

to fight fires at and above impact zone.

• Likely that many occupants trapped at

and above impact zone were already dead or would die before help could get to them.

B. Operations
3

26 35

66

121

171

0 0 6

23 30

74

0

50

100150200250

8:46 8:48 8:50 9:00 9:15 9:59 

time (h:m)

Number of Units 

Dispatched Signal Arrival

Dispatch and Arrival of FDNY Units 
Operations (2)

Outside Command Posts, and Inside Command Commu

with Outside Command Posts:

• Fires in buildings were too large, located too high, to 

fire fighting activities that could save lives of occupanthe fires. 

• Objective: evacuate and rescue all below the fire

Command Officers for Inside Operations:

• Fires were too large to extinguish. • Objective: get enough personnel and equipment 

to cut path through fire to rescue occupants abovalso evacuate and rescue all below fires.
Operations (3)

Company Level Command:

• Conventional but large high-rise fire.• Objective: get up to fire floors and extinguish fires

In some cases, firefighters were persuaded by higher ranking 

officers to switch from fire fighting to evacuation and rescuoperations. 

No first responder interviewed by NIST thought that the WTC towers would collapse.
• After aircraft impact, only two elevators out of 

198 were operating: from lobby to 16th floor 

(WTC1), and from lobby to 40th floor (WTC2).

• Stairways were filled with occupants evacuating buildings. • Counterflow in stairs for FDNY personnel and other emerge

responders caused:

• difficulty for carrying equipment up stairs;• teams of emergency responders to become separated, leadin

delays and disrupting team operations.

• Time to begin operations for first responding FDNY units: 7 

15 minutes.

• Estimated climbing rate based on a 60 minute climbing per

to their maximum height: 1.4 to 2 minutes/floor

C. FDNY Access to the 

WTC Towers
D. Communications

Schematic 

Repeat

• FDNY, NYPD & PAPD all experienced difficulties with radio 

communications.

• Each department was aware of shortfalls associated with th

communications systems as they related to operations in hi

buildings.

• Two basic issues with radio communications:

• Normal function of the radio equipment

in high-rise environments. (Radio 

signal attenuation in steel and 

concrete buildings).

• The volume of radio traffic
Impact of unsatisfactory radio communications:

• Emergency responders who could view building from outs

communicate over radios -- adequate situational awarenes

• Personnel who observed building damage and fires from o

experienced -- difficulty maintaining awareness after enter

• Emergency responders working inside who could not see w

happening outside and had poor 

radio communications --situational awareness was poor. 

• Emergency responders working 

inside of buildings, who could not see what was happening outside and had good radio 

communications -- better 

situational awareness.

Communications (2)

Example: Handie-Talkie Radio 

Talkie Radio 

FDNY Command P

Bat. Chief Bat. Chief 

3 to 5 Engine

Companies

3 Ladder

Companies

3 Radios/

Company 4 Radios/ Company

3 to 5 Engine

Companies

3 Ladde

Compani

3 Radios/

Company 4 Radio

Compa

Potentially 90 radios on

one frequency at one time 

H

P

Only one radio transmissio
Communication from NYPD Aviatio

10:06 amAdvises everybody to evacuate area in vicinity of Battery Parstates that, about 15 floors from top, it is totally glowing red oand collapse was inevitable. Advises that it is isn’t going to talonger before North tower comes down and to pull emergencback from building.10:21 amFirst reports that top of tower might be leaning, then confirms buckling and leaning to the South. Reports that North tower to the Southwest and appears to be buckling in Southwest corner. Advises that all personnel close to building pull back three blocks in every direction.10:28 amReports that roof is going to come down very shortly. Reports that tower is collapsing.
E. Command and Control

• Three FDNY su

magnetic Commwere set up at thsite.

• FDNY command 

was seriously aflack of good communications

• Large numbers of fire fighters and ambulances officially dis

(or self-dispatched) to WTC site before adequate commanstaff could be assembled.

• Interagency operations detrimentally affected with loss of O

command center in WTC 7. 
• A significant amount of evidence (first person interviews, re

photographic data) suggests that:

• Departments attempted to work together to save as ma

possible and protect citizens on September 11. 

• Some issues related to a given 

department’s operational responsibility and competitive nature of departments did exist during operations; some problems experienced were due to personnel not understanding operating practices of other agencies.

• Inter-agency competition had 

minimal effect on operations atWTC complex before towers collapsed.

Command and Control (2)
V. RESEARCH, IMPLEMENTATIO

RECOMMENDATIONS

A. WTC R&D -- Safety of Threatened BuProgram • Objective: provide technical foundation 

that supports improvements to codes, standards,and practices that reduce impact of extreme threats to safety of buildings, their occupants & emergency responders. 

• Outcomes:

• Increased Structural Integrity • Enhanced Fire Resistance• Improved Emergency Egress & Access • Building & Emergency Equip. Standards 

& Guidelines
B. DTAP -- Dissemination and Technical 

Assistance Program

• Complement and support parallel efforts of engineering 

societies and codes/standards organizations to improve 

technology, codes, and standards 

• Provide advice on best practices, guidance on 

vulnerability assessment, guidance on standards and codes needs

• Disseminate and implement R&D outputs
ACKNOWLEDGEMENTS

3⁄4 The people of New York City and th

many businesses associated with thWTC

3⁄4 Multiple government agencies and 

departments in New York City

3⁄4 Dozens of NIST investigation team 

members

3⁄4 Numerous private sector contractor3⁄4 NCST Advisory Committee
Web site http://wtc.nist.gov

Email to wtc@nist.gov

Facsimile to (301) 975-6122

Regular mail:

WTC Technical Information Repos

Stop 8610, 100 Bureau Drive, 

Gaithersburg, MD 20899-8610