Dr. S. Shyam Sunder

Deputy Director and Lead Investigator

Building and Fire Research Laboratory

National Institute of Standards and Techno

U.S. Department of Commerce

NIST Response to the World Trade Center Di

Federal Building and Fire Safety Investiga

of 

the World Trade Center Disaster

NIBS/MMC 

WTC Recommendations Committe

October 24, 2005
Investigation Objectives Investigation Objectives

• 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 numbers of injuries and fatalities were so 

depending on location, including technical aspects of fire occupant behavior, evacuation, and emergency response

• Determine procedures and practices used in the design, 

construction, operation, and maintenance of the WTC bui

• Identify specific areas in current national building and 

model codes, standards, and practices that warrant re

Analysis of Probable Collapse Sequence Analysis of Probable Collapse Sequence

NIST developed and used a series of rigorous and comprehensto determine the probable collapse sequence for the WTC towe

 Analyzed complete sequence of events:

• aircraft impact damage to the buildings• spread of jet-fuel-ignited multi-floor fires• thermal weakening of structural components• progression of local structural failures until collapse initiat

 Combined:

• mathematical modeling• well-established statistical and probability-based analysis • laboratory experiments• analysis of visual and physical evidence

Significantly advanced current stat

tested the limits of current computational 
WTC 1 Damage: Composite Summa

WTC 1 Damage: Composite Summa

for Floors 93 to 98 for Floors 93 to 98

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

WTC 2 Damage: Composite Summary fo

78 to 83 78 to 83

Column DamageSeveredHeavy DamageModerate DamageLight DamageFloor DamageFireproofing and partitionsFloor system structural damage Floor system removed
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
WTC2: East Face

Time: 9:21:29 AM ~18 minutes post impact

Maximum inward bowing of columns approximately 10 inches
Inward Bowing of Perimeter Columns A

Inward Bowing of Perimeter Columns A

Minutes Prior to Collapse: WTC 2 East 

Minutes Prior to Collapse: WTC 2 East 
Tilting of Building Tilting of Building 

Sections Sections

Initiation of glowas first obsertilting of buildinabove the impaof both WTC to

WTC 1 tilted to the south; WTC 2 tilted to the east and south.

© 2001 Dean Riviere
Fire Performance of Composite Floor Sy

Fire Performance of Composite Floor Sy

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

was innovative and not consistent with then-prevailing practice.

• No evidence found of technical basis in the selection of fire

thickness to meet 2 h fire rating for Class 1-B construction:

 1/2 in. specified when WTC towers were built to maintain C

(not 1-B) fire rating requirement of NYC Building Code 

 1-1/2 in. specified for upgrades some years prior to 2001

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

system was conducted to determine the required fireproofing 

• in 1966, the Architect of Record and, • in 1975, the Structural Engineer of Record 

stated that the fire rating could not be determined without testin
Results From NIST Sponsored Tests at 

Results From NIST Sponsored Tests at 

___________________________________________________(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 Analysis Results and Findings of Drift Analysis

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 estimated w

 Typical drift ratios considered in practice (not required by building codes)

• H/500 (~ 32.9 in.)

 Limiting drift enhances structural stability and safety in tall buildings; this 

required to control damage in seismic regions. 

 Acceleration limits used in current practice for occupant comfort (e.g., 15-20 mg

control of drift.
Evacuation and Emergency Response Evacuation and Emergency Response

Based on 1,056 interviews of surviving WTC occupants and 116 of emergency responders.

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

towers on the morning of September 11, 2001.

• The initial population of each tower was similar: 8,900 (± 750

and 8,500 (± 900) in WTC 2.

• About 6 percent of the surviving occupants (about 1,000 

reported a pre-existing limitation to their mobility.

• Approximately 87 percent of the WTC tower occupants were a

evacuate successfully, including more than 99 percent of ththe floors of impact. 
Evacuation Rates in the WTC Towers Evacuation Rates in the WTC Towers

• Roughly 3000 survivors self-evacuated WTC 2 using the func

elevators and stairwells during the 16 minutes prior to aircra

• The egress capacity was adequate to accommodate survivors 

seeking and able to reach the exits and stairways.

• A full capacity evacuation with 20,000 people in each WTC t

to three times the occupancy on September 11, 2001—would hrequired about 3 hours.

• Under this scenario, it is estimated that roughly 14,000 peo

have lost their lives and the egress capacity would have 

inadequate for the time available. 

• The egress capacity required by current building codes is ba

evacuating a single floor.
Occupant Preparedness Occupant Preparedness

• Occupants were often unprepared for the physical challenge o

building evacuation.

• The average surviving occupant moved slower down stairs a

stairwell exits than previously reported for non-emergency eva

• In WTC 1, the average surviving occupant spent 48 second

descending the stairwell.

• After the collapse of WTC 2, emergency responders found mobili

occupants of WTC 1 still in the staircases going down.

• Ambulatory mobility impaired occupants blocked others behin

from moving more rapidly down the stairs.

• FDNY and PAPD personnel found 40 to 60 mobility impaire

occupants on the 12th floor of WTC 1. They had been place

floor in an attempt to clear the stairways.
Emergency Response in High Emergency Response in High

-Rise Build

Rise Buil

Example: Response to a 60 story high-rise building, occupants tra

fires on the 58th floor and no operating elevators.

Lobby

Firefighters begin to climb 7-15 minutes

Fire department arrival 4-10 minutes 

Estimated firefighter climbing rate

(60 minute climbing period to max height) 

2 min/floor with equipment 

1.4 min/floor without equipment

Firefighters: Fires

With equipment ~ 125 minutes

Without equipment ~ 90 minutes
Radio Communications in High Radio Communications in High

-Rise Bu

Rise Bu

• Intra-operability w

emergency respondorganization:

• Buildings pose a 

challenging radioenvironment.

• Capacity, protoc

training for largeoperations.

• Interoperability am

different emergency organizations.

Schematic of WTC Radio Repeater System
Context for Recommendations Context for Recommendations

• The tragic consequences of the September 11, 2001, attacks we

attributable to the fact that terrorists flew large jet-fuel laden comairliners into the WTC towers. 

• Buildings for use by the general population are not designed 

withstand aircraft attacks; building codes do not require considaircraft impact. 

• In our cities, there has been no experience with evacuation an

emergency response in a building disaster of this magnitudethere been any in which the total collapse of a high-rise building orapidly and with little warning.

• Documents suggest that the WTC towers generally were desig

maintained consistent with the requirements of the 1968 New Building Code.
Context for Recommendations (2) Context for Recommendations (2)

• NIST’s recommendations for improvements to codes, standards

practices are applicable under the hazards and conditions recin normal building design. 

• Public officials and building owners will need to determine appr

performance requirements for buildings that are at higher ris

their iconic status, critical function, or design.

• The recommendations are performance-oriented. They do 

prescribe specific: 

• Systems, materials, or technologies. NIST encourages c

among alternatives that can meet performance requiremen

• Threshold levels. This responsibility falls within the purview 

public policy and standards and codes development proces
WTC Recommendations WTC Recommendations

• NIST has made 30 recommendations for improvements to codes, sta

practices which fall into 8 major groups:

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

 Improved Active Fire Protection Improved Building Evacuation  Improved Emergency Response Improved Procedures and Practices Education and Training Programs

StrDe

LifSaSyPraDe
Increased Structural Integrity Increased Structural Integrity 

Consensus standards and code provisions for preventing progressive codeveloped and adopted nationwide – along with tools and guidelines for thea standard methodology be developed to reliably predict the potential for 

failures in structural systems subjected to multiple hazards. Rec. #1

Nationally accepted performance standards be developed for: 

 wind tunnel testing of prototype structures based on sound techn

that result in repeatable and reproducible results; and 

 estimating wind loads and their effects on tall buildings, based on 

testing data and directional wind speed data. Rec. #2

Appropriate criterion be developed and implemented to enhance performan

buildings by limiting how much they sway under lateral load design cond

winds and earthquakes). Rec. #3
Enhanced Fire Resistance of Structures

Evaluate -- and where needed improve -- the technical basis for determining construction classification and fire rating requirements (especially for buildings)...and make related code changes now as much as possible by econsidering: 

 timely access by emergency responders and full evacuation of occu

time required for burnout without partial collapse;

 extent to which redundancy in active fire protection (sprinkler and stan

alarm, and smoke management) systems should be credited for occupsafety;

 need for redundancy in fire protection systems critical to structural 

 ability of the structure and local floor systems to withstand maximum 

scenario without collapse -- recognizing that sprinklers could be com

not operational, or non-existent; (continued)
Enhanced Fire Resistance of Structures 

 compartmentation requirements to protect the structure, including:

 fire rated doors and automatic enclosures, and 

 limiting air supply (e.g., thermally resistant window assemblies) 

to retard fire spread in buildings with large, open floor plans,

 effect of spaces containing unusually large fuel concentrations for 

occupancy of the building; and

 extent to which fire control systems -- including suppression by autom

manual means -- should be credited as part of the prevention of fire s
Enhanced Fire Resistance of Structures 

Improve the technical basis for century-old standard for fire resistanof components, assemblies, and systems and... develop guidance fo

extrapolating results of tested assemblies to prototypical building sy

key step in fulfilling this recommendation is to establish a capability f

and testing components, assemblies, and systems under realistic f

conditions. Rec. #5

Develop criteria, test methods, and standards:

 for in-service performance of fireproofing to protect structura

components; and

 to ensure that as-installed fireproofing conforms to conditions 

used to establish the fire resistance rating. Rec. #6 

Adopt and use “structural frame” approach (structural members cothe columns carry the high fire resistance rating of the columns). Rec. 
New Methods for Fire Resistance Desig

Structures

Require that uncontrolled building fires result in burnout without partial (total) collapse. Rec. #8Develop:

 performance-based standards and code provisions -- as an alte

current prescriptive design methods -- to enable design and retrof

structures to resist real fire conditions

 tools, guidelines, and test methods to evaluate fire performance o

structure as a whole system. Rec. #9
New Methods for Fire Resistance Desig

Structures (2)

Develop and evaluate new fire resistive coating materials, systems, atechnologies with significantly enhanced performance and durability to p

protection following major events. Rec. #10

Evaluate performance and suitability of advanced structural steel, reinfand pre-stressed concrete, and other high-performance material sysuse under conditions expected in building fires. Rec. #11
Improved Active Fire Protection

Enhance performance and possibly the redundancy of active fire protesystems to accommodate higher risks associated with tall buildings. Develop advanced fire alarm and communication systems that provid

continuous, reliable, and accurate information on life safety conditio

manage the evacuation process; all communication and control paths in bneed to be designed and installed to have the same resistance to failure a

increased survivability above that specified in present standards. Rec. #13

Adapt advanced fire/emergency control panels to accept and interpret mmore reliable -- information from the active fire protection systems to prov

decision aids. Rec. #14

Develop and require systems for improved transmission to emergency responders, and off-site or black-box storage, of information from bu

monitoring systems. Rec. #15
Improved Building Evacuation

Develop and carry out public education and training campaigns to imp

occupants’ preparedness for evacuation in case of building emergencie

Design tall buildings to accommodate timely full building evacuation of when required in building-specific or large-scale emergencies such as wi

power outages, major earthquakes, tornadoes, hurricanes, fires, explosion

terrorist attack. 

 Building size, population, function, and iconic status should be tak

account in designing the egress system. 

 Stairwell capacity and stair discharge door width should be adequ

accommodate counterflow due to emergency access by responde
Improved Building Evacuation (2)

Design egress systems:

 to maximize remoteness of egress components (i.e., stairs, elev

exits) without negatively impacting the average travel distance;

 to maintain their functional integrity and survivability under forese

building-specific or large-scale emergencies; and

 with consistent layouts, standard signage, and guidance so that sys

become intuitive and obvious to building occupants during evacua

Rec. #18
Improved Building Evacuation (3)

Building owners, managers, and emergency responders should develop and ensure accurate emergency information is communicated in a t

manner to enhance awareness of occupants and emergency responthrough:

 better coordination of information among different emergency resp

groups,

 efficient sharing of that information among building occupants and 

responders, 

 more robust design of emergency public address systems, 

 improved emergency responder communication systems, and  use of the Emergency Broadcast System (the Integrated Public Ale

Warning System) and Community Emergency Alert Networks. Rec
Improved Building Evacuation (4)

Evaluate the full range of current and next generation evacuation techfor future use, including:

 protected/hardened elevators, 

 exterior escape devices, and  stairwell descent devices, 

which may allow all occupants an equal opportunity for evacuation an

emergency response access. Rec. #20
Improved Emergency Response Techno

Techno

and Procedures and Procedures

Install fire-protected and structurally hardened elevators in tall buildin

provide timely emergency access to responders and allow evacuation of mimpaired building occupants. 

 Such elevators should be for exclusive use by emergency responde

emergencies. 

 In tall buildings, consideration also should be given to installing such 

for use by all occupants. Rec. #21

Install, inspect, and test emergency communications systems, radio communications, and associated operating protocols to ensure that thand protocols:

 are effective for large-scale emergencies in buildings with challeng

frequency propagation environments, and 

 can be used to identify, locate, and track emergency responders 

indoor building environments and in the field. Rec. #22
Improved Emergency Response Techno

and Procedures (2)

Establish and implement detailed procedures and methods for gathering, 

processing, and delivering critical information through integration of rele

voice, video, graphical, and written data to enhance situational awarenemergency responders. Establish an information intelligence sector to coordinate each incident. Rec. #23

Establish and implement codes and protocols for ensuring effective and 

uninterrupted operation of the command and control system for largbuilding emergencies. Rec. #24
Improved Procedures and Practices

Nongovernmental and quasi-governmental entities that own or lease and are not subject to building and fire safety code requirements of any governmental jurisdiction -- should be encouraged to provide a level of s

equals or exceeds the level of safety that would be provided by strict co

with the code requirements of an appropriate governmental jurisdic

 As-designed and as-built safety should be certified by a qualified 

party, independent of the building owner(s). 

 The process should not use self-approval for code enforcement i

including:

 interpretation of code provisions,  design approval,  product acceptance,  certification of the final construction, and 

 post-occupancy inspections over the life of the buildings. #25
Improved Procedures and Practices (2)

State and local jurisdictions adopt and aggressively enforce building coensure that egress and sprinkler requirements are met by existing buiFurther, occupancy requirements should be modified where needed (such 

there are assembly use spaces within an office building) to meet the modecodes. Rec. #26

Building codes should require building owners to retain documents relatebuilding design, construction, maintenance and modifications over the en

the building. Means should be developed for offsite storage and maint

the documents. Relevant information should be easily accessible by re

during emergencies. Rec. #27
Improved Procedures and Practices (3)

The role of the “Design Professional in Responsible Charge” be clarified to 

all appropriate design professionals (including the fire protection and strengineers) are part of the team designing buildings that employ innova

unusual structural and fire safety systems. Rec. #28
Education and Training 

Continuing education curricula should be developed and progimplemented for: 

 training fire protection engineers and architects in structural en

principles and design, and 

 training structural engineers, architects, fire protection engineers

enforcement officials in modern fire protection principles and technologies, including fire-resistance design of structures.

 training building regulatory and fire service personnel to upgrade 

understanding and skills to conduct the review, inspection, and tasks for which they are responsible. Rec. #29

Academic, professional short-course, and web-based training 

in the use of computational fire dynamics and thermostructuanalysis tools be developed. Rec. #30
Achieving “Real” Safety Improvements Achieving “Real” Safety Improvements

• NIST believes that the recommendations are realistic, appropriate, a

achievable within a reasonable period of time.

• Aim of design is to anticipate rare design events in a rational manner

• Unanticipated events have surprised the design community (e.g., Northridge 

• Historical statistics do not adequately capture rare design events (tails of dis

• Only a few of the recommendations call for new requirements in s

and codes. Most of the recommendations deal with:

• Improving an existing standard or code requirement

• Establishing a standard for an existing practice without one

• Establishing the technical basis for an existing requirement• Making a current requirement risk-consistent• Adopting or enforcing a current requirement• Establishing a performance-based alternative to a current prescriptive requir

 Implementation of the recommendations would make buildings, o

and emergency responders safer in future emergencies.
Call for Action Call for Action

 NIST is assigning top priority to work vigorously with the building a

communities to assure that there is a complete understanding of the 

recommendations and to provide needed technical assistance.

 NIST has:

 identified specific codes, standards, and practices affected by each of the 30 

recommendations in its final report, and

 reached out to the responsible organizations to pave the way for a timely, ex

consideration of the recommendations.

• NIST held a conference September 13-15, 2005 attended by over 200 

including all major standards and codes developers.

• NIST has awarded a contract to the National Institute of Building Scienc

Congressionally authorized non-profit, non-governmental organization—appropriate recommendations into code language suitable for subm

code change proposals to the two national model code developers. drawing upon building code experts to carry out this task.

• NIST will implement a web-based system so that the public can track pr

implementing the recommendations.
Next Steps Next Steps

• NIST plans to release final versions of the 43 reports o

WTC towers, totaling some 10,000 pages, on October 2005. 

• The summary WTC towers report is about 250 pages 

contains the principal findings and recommendations finvestigation.

• Next spring, NIST plans to release an additional five re

on the investigation of WTC 7 as drafts for pubic comm
Web site Web site http:// http://wtc.nist.gov wtc.nist.gov

Email to Email to wtc@nist.gov wtc@nist.gov

Facsimile to (301) 975 Facsimile to (301) 975-6122

Regular mail: Regular mail:

WTC Technical Information Repository, Stop 8

Technical Information Repository, Stop 8

100 Bureau Drive, Gaithersburg, MD 20899 Bureau Drive, Gaithersburg, MD 20899-86