S. Shyam Sunder

Acting Deputy Director and Lead Invest

Building and Fire Research Labo

National Institute of Standards and Tec

U.S. Department of Commerce

sunder@nist.gov

Building and Fire Safety

Responding to 

the World Trade Center Dis

Chicago Committee on High-Rise 

September 9, 2004
Background on WTC Investigation Background on WTC Investigation

• Good, solid technical progress on investigation; drawing on ta

experts, and contractors; $16 million investigation; $5.5 million 

• Two public updates issued (December 2002, December 2003

progress reports issued (May 2003, June 2004)

• NYC Public Meeting (February 12, 2004) to solicit comments 

of investigation, (2) additional information that NIST might conbe considered for recommendations

• Second technical progress report released June 18, 2004; 1,0

available on Web site http://wtc.nist.gov

• Investigation is ongoing; current findings may be revised 

will be presented in final report

• NIST has not made any recommendations at th

recommendations will be made in the final repo

• NIST expects to release the draft of the final investigation rep

in December 2004
NIST WTC Investigation Objectives NIST WTC Investigation Objectives

• Determine:

• why and how the WTC Towers collapsed follo

impact of the aircraft, and 

• why and how the 47-story WTC 7 collapsed

• Determine why the numbers of injuries and fatalit

high depending on location, including technical as

protection, occupant behavior, evacuation, and emresponse

• Determine the procedures and practices that were 

design, construction, operation, and maintenance buildings

• Identify, as specifically as possible, areas in n

and fire codes, standards, and practices that w
North tower: 

Collapsed aft

94th floor

99th floor

South tower: Hit at 9:02:59 AMCollapsed after 56 minutes

77th floor

85th floor

South Face

About 0.79 H
North tower

South tower

Point of impact: close to the corner and with an angle

Point of impact: close to the center and nearly normal to the building

Some Specific Questions Some Specific Questions

 How and why did WTC 1 stand nearly twice as long as W

(103 min. vs. 56 min.) though they were hit by virtually ide

 What factors related to normal building and fire safety con

to the terrorist attacks of September 11, 2001, if any, could 

prevented the collapse of the WTC towers?

 Would the undamaged WTC towers have remained stand

building fire?

 What factors related to normal building and fire safety con

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

 How well did the procedures and practices used in the des

operation, and maintenance of the WTC buildings conform practices, standards, and codes?
Context of Findings Context of Findings

 Buildings are not specifically designed to withstand the impact of fue

airliners. While documents from the Port Authority of New York and indicate that the impact of a Boeing 707 flying at 600 mph, possibly c

was analyzed during the design of the WTC towers in February/Marc

subsequent fires was not considered. Building codes do not require consider aircraft impact.

 Buildings are not designed for fire protection and evacuation under th

conditions similar to those caused by the terrorist attacks of Septemb

 The load conditions induced by aircraft impacts and the extensive fir

which triggered the collapse of the WTC towers, fall outside the norm considered in building codes.

 Prior evacuation and emergency response experience in major even

collapse of tall buildings such as the WTC Towers and WTC 7 that weveryday use; instead, that experience would suggest that major tall burnout conditions, not overall building collapse. 

 The PANYNJ was created as an interstate entity, under a clause of t

permitting compacts between states, and is not bound by the authori

federal jurisdiction, including local building and fire codes. The PANYpolicy is to meet and, where appropriate, exceed requirements of loc
Role of WTC Investigation Role of WTC Investigation

• Bring clarity to important technical issues related to the 

and loss of life based on objective and impartial analysfollowing areas:

• Building performance (e.g., BPAT Recommendatio• Occupant behavior and evacuation • Emergency response procedures

• Develop findings and recommendations, as appropriate

technical issues affecting the safety of buildings and cothe risk of unusual (e.g., terrorist) threats

• Develop findings and recommendations, as appropriate

technical issues affecting the general safety of building
Objective 1a: Collapse of WTC To

Objective 1a: Collapse of WTC To

 Working Hypothesis: The following chronological seq

led to the eventual collapse of the WTC towers; specific paths and damage scenarios for each building are undedetermine the most probable collapse sequence for eac

• Aircraft impact damage to perimeter columns, resulting in 

loads to adjacent perimeter columns and to the core colum

• After breaching the building’s exterior, the aircraft continu

buildings, damaging core columns with redistribution of cointact core and perimeter columns via the hat truss and flo

• The subsequent fires, influenced by the post-impact cond

weakened columns and floor systems (including those tha

aircraft impact), triggering additional local failures that ultiinstability;

• Initiation and horizontal progression of column instability r

redistributing loads could not be accommodated any furthensued. 
Working Hypothesis: Factors Bein

Working Hypothesis: Factors Bein

• Innovative structural system• Aircraft impact and subsequent fires• Post-impact condition of fireproofing• Quality and properties of structural steel• Relative roles of the perimeter and core co

composite floor system, including connectio

Working Hypothesis is consistent with all evidence cur

including photographs and videos, 

and emergency c
Photographic Evidence of Hanging 

Photographic Evidence of Hanging 

East Face of the South Tower. Image shows what appear to be a flofloor hanging across window opening over a large fraction number of 

Photo by Allen Muarabayashi, 9:47:05 am ± 5 s
Photographic Evidence of Hanging 

Photographic Evidence of Hanging 
Photographic Evidence of Hanging 

Photographic Evidence of Hanging 

North Face of the South Tower. Image shows what appear to be pseveral floor slabs hanging across window openings on Floors 80, 
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 2 

Minutes Prior to Collapse: WTC 2 
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 1 

Minutes Prior to Collapse: WTC 1 
Tilting of Building Sections Tilting of Building Sections

Initiation of global collapse was first observed by the tilting of building sections above the impact regions of both WTC towers.

WTC 1 tilted to the soto the east and south counterclockwise mot
Innovative WTC Tower Structural S

Innovative WTC Tower Structural S

Exterior Framed-Tube

Floor Slab

Central CoreBox Columns

59 columns per side

Bar Joist

Concrete Slab

Floor

Covering

Trough

Decking

Air-conditioning

Electric DuctDuct

• Innovative 

when builtnew and u

• Two featur

considerat

• Com

sysopesprafirep

• Des

and indu
Fire Performance of Composite Flo

Fire Performance of Composite Flo

• Fire-protection of a truss-supported floor system wit

fireproofing was innovative and not consistent with tpractice.

• Lack of technical basis in the selection of firepro

meet 2 h fire rating:

• 1/2 in. specified when WTC towers were built• 1-1/2 in. specified for upgrades some years prior • 2 in. for similar floor system in an unrestrained te

evaluation service recommendation in June 200buildings)

• No full-scale fire resistance test of the WTC floor 

conducted to determine the required fireproofing thArchitect of Record and, in 1975, the Structural Engstated that the fire rating of the WTC floor system codetermined without testing.
NYC Building Code Provisions NYC Building Code Provisions

(Fire Resistance in hours) (Fire Resistance in hours)

Floors

3

2 

Columns

4

3

1938 1968* 

* Building code governing original design and oc** Sprinklers required for buildings of unlimited h
Results From NIST Results From NIST

-Sponsored Tes

Sponsored Tes

___________________________________________________(1) Imminent collapse(2) Vertical displacement exceeded capability to measure accurately(3) Did not occur

--- 58 66 58 (3) 120(

17 ft, restrained, 1⁄2 in fireproofing

4

210

(

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

3

146

(

--- --- 76 62 (3)

35 ft, unrestrained,

3⁄4 in fireproofing

2

116 

(

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

Maximum

(1300oF)

Average

(1100oF)

Maximum

(Ambient

+325oF)

Average

(Ambient

+250oF)

Failure 

to

Support 

Load

Steel Temperatures Temperature on Unexposed Surface

Test

Termi

ated

(min

Times to Reach End-Point Criteria (min)

Test Description
Use of Wind Tunnel Testing to Est

Use of Wind Tunnel Testing to Est

• Wind loads were a major governing factor in the des

components that made up the WTC tower structuresperimeter frame-tube system

• Relevant to evaluating baseline performance of the 

reserve capacity of the structures to withstand events such as a major fire or impact damage, aand procedures; state-of-knowledge is evolving in tengineering

• Large differences in resultant forces, of as much as abo

results from two sets of wind tunnel tests for the WTC to2002 by independent laboratories, and voluntarily providparties to an insurance litigation

• NIST is conducting an independent analysis to establish 

performance of the WTC towers under the original desigcompare those results with then-prevailing code require

• In July 2004, the designer provided NIST with clear

information in original source documents to determloads used for the WTC towers
Wind Load Estimates for WTC 1

1960’s 9.8 10.6 14.0 1

Original WTC Design (Clarified by designer in July 2004)

2004 14.1 13.0 16.1 1

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

CPP / ASCE 7-98 2002 NA NA NA N

2002 NA NA NA N

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 12.3 11.3 14.0 1

2002 11.4 10.5 13.0 1

RWDI / NYC Building Code

9.3 9.3 7

1968 - 2001 NYC Building Code

5.3 5.3 4

Prior to 1968 NYC Building Code

Ab

N

Most 

Unfavorable 

Condition

N-S E-W

Base Shear 10 

3 kips

Source Year
Wind Load Estimates for WTC 2

1960’s 13.1 10.1 16.5 

Original WTC Design (Clarified by designer in July 2004)

2004 12.2 14.0 15.6 1

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

CPP / ASCE 7-98 2002 15.1 15.3 17.1 1

*

2002 NA NA NA 

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 10.6 12.2 13.5 1

2002 9.7 11.1 12.3 1

RWDI / NYC Building Code

9.3 9.3 

1968 - 2001 NYC Building Code

5.3 5.3 

Prior to 1968 NYC Building Code

A

N

Most 

Unfavorable 

Condition

N-S E-W

Base Shear 10 

3 kips

Source Year
Base Shears and Base Moments Du

Loads from Different Building Code

4.2 7.7 7.6 

Base Moment(106 kips-ft)

5.3 9.3 9.5 

Base Shear(103 kips)

BO

1964

NY State Code

1968-2001

NYC Code

1938

NYC Code

• The two WTC towers withstood the initial impact of virtually iden

767-200ER) during the terrorist attacks of September 11, 2001. 

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

WTC towers helped the buildings withstand the aircraft impact. 

• The WTC towers displayed significant reserve capacity, vibratin

following impact with amplitudes that were about half the amplituconditions expected by the building designers and an oscillation that measured for the undamaged building.

Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a
Preliminary Aircraft Impact Damag

Preliminary Aircraft Impact Damag

 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.
Preliminary Aircraft Impact Damag

Preliminary Aircraft Impact Damag

 Engine impact against 

an exterior wall panel results in a penetration of the exterior wall and failure of impacted perimeter columns.

 The residual velocity and 

mass of the engine after penetration is sufficient to fail a core column in the event of a direct impact
Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a

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

collapse. While aircraft impact damage did not, by itself, initiate bucontributed greatly to the subsequent fires by:

• Compromising the sprinkler and water supply systems;

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

• Creating large accumulations of combustible matter containing airc

• Increasing the air supply into the damaged buildings that permitted 

release rates than would normally be seen in ventilation limited buito spread rapidly within and between floors; and

• Damaging ceilings that enabled “unabated” heat transport over the 

walls and to structural components.

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

The fires that burned for almost the entire time that the buildings remain

mainly to burning building contents and, to a lesser extent, aircraft conte

• Typical office furnishings were able to sustain intense fires for at least a

floor. The typical floor had on average about 4 psf of combustible mateaircraft solid combustibles was significant in the immediate impact regio
Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a

• Consistent with available photographic and videographic evidence

simulations capture the broad patterns of fire movement around the given location lasting for about 20 min before spreading to adjcombustibles; some observed instances where fires persisted lonaccumulated combustible debris; other instances of sudden or inte

• The affected floors in the WTC towers were mostly open—with 

perimeter offices and conference rooms and an occasional special floors had two tenants, and those spaces, like the core areas, were slab). Photographic and videographic evidence confirms that even partitions (such as those that divided spaces to provide corner co

provided substantial resistance to fire spread in the affected f

• For the duration of about 50 min to 100 min prior to building collaps

active, the presence of undamaged 1 h fire-rated compartment

mitigating fire spread and consequent thermal weakening of s

• Applying the 1968 NYC Building Code, the WTC towers were requ

tenant separations, but the code did not impose any minimum comrequirements (e.g., 7,500 ft

2) to mitigate the spread of fire in large o
Reco

Rec

of th

of th
Collection and Analysis of Photog

Collection and Analysis of Photog

Video Images Video Images

• Visual database now contains:

• Well in excess of 6,000 

photographs taken by more than 185 photographers

• 150 hours of videotape from 

major media outlets and more than 20 individuals

• From the analysis of the visual 

images to date, NIST has identified significant events for WTC 1 and 2 related to aircraft impact, fire development, and building damage

• NIST has developed detailed 

mappings for the fire, smoke, and the condition of windows at several specific times for each WTC tower; work is nearing completion for WTC 7
• NIST has developed a rigorous technical approach to evaluate the 

condition of the fireproofing, including its thickness, on the collapse 

• In general, the affected floor systems in WTC 1 had upgraded or th

specified); affected floors in WTC 2 had the original fireproofing (0

• Structural response is sensitive to variability in fireproofing th

length of components; it is possible to determine a thermally ethickness that should be greater than the specified thickness.

• Simple analytical models show that 100 g to 150 g acceleration wo

dislodge fireproofing similar to that used in the WTC towers with tyexperiments underway to verify results.

• Results of aircraft-impact damage analysis being used to estimate 

fireproofing was dislodged from structural members 

Role of Fireproofing Conditions Role of Fireproofing Conditions

Upgraded

Original

Status

2.5 (0.24) 2.2 0.75 (0.4) 0.6 

Thermally Equivalent As-Applied Avg. (COV)
Analysis of Recovered WTC Steel Analysis of Recovered WTC Steel

 NIST has a total of 236 WTC steel pieces in NIST’s

possession; collection is adequate for determining the quality and properties of steel for the investigation; 

• Impact/fire damage region emphasized• All 14 grades of steel for perimeter columns• Both grades for steel trusses• Two grades (representing 99%) for core columns

 Based on stampings on steel and mechanical tests, 

analysis indicates that the correct specified materials were provided for specified elements; 

when these data are combined with available precollapse photographs, aircraft impacted pieces from 

WTC 1 were in precise locations specified in design drawings

 Metallography and mechanical property tests indicate 

that the strength and quality of the steel used in 

the towers was adequate, typical of the era, and likely met all qualifying test requirements. 

 The room-temperature strength of steel used in 

the WTC towers met relevant standards and, in 

many instances, exceeded requirements by 5 to 10 percent.
© 

2001. Roberto Rabanne/CORBIS

Enhancements added by NIST.
Analysis of Most Probable Collaps

Analysis of Most Probable Collaps

• NIST is simulating highly-complex failure modes at the component, su

to determine the most probable collapse sequence. In many instanc

limits of current engineering software.

• The computational models developed by NIST include:

• A detailed model of a typical truss-framed floor of the WTC towe

elements and 166,000 degrees of freedom. 

• A detailed model of a typical beam-framed floor of WTC towers w

and 35,000 degrees of freedom.

• A detailed global model of WTC 1 with over 80,000 elements and 

freedom (with 17 flexible and other rigid diaphragm floors).

• A similar detailed global model of WTC 2 with over 78,000 eleme

of freedom. 

• A model of a typical turbofan engine of the Boeing 767-200ER ai

60,000 elements and 100,000 nodes.

• A comprehensive model of the Boeing 767-200ER aircraft, includ

landing gear, fuel tanks, passenger cabin, and cargo bay, with ov740,000 nodes. 

• The first four models described above are being used to evaluate the 

the WTC towers under design gravity and wind loads. They also servanalyses of aircraft impact damage and response of the thermally-insu

subsequent fires.
High Fidelity Aircraft Model High Fidelity Aircraft Model
Front SparBulkhead

Main Landing GearWheel Well

Center Wing

Rear SparBulkhead


Objective 1b: Collapse of 47 Objective 1b: Collapse of 47

-Story 

Story 

 Working Hypothesis: The working hypothesis for th

47-story WTC 7 building, if it remains viable upon furtsuggests that it was a classic progressive collapse 

• An initial local failure at the lower floors (below Floor 1

to fire and/or debris induced structural damage of a cr

initiating event), which supported a large span floor a

• Vertical progression of the initial local failure up to the 

large floor bays were unable to redistribute the loads, binterior structure below the east penthouse; and

• Horizontal progression of the failure across the lower f

Floors 5 and 7, that were much thicker than the rest of by damage due to the vertical failure, resulting in dispcollapse of the entire structure. 
Observed Sequence of Failures in 

Observed Sequence of Failures in 
Visual Observations for WTC 7 Visual Observations for WTC 7



The first exterior sign of structural failure in WTC 7 was the sinking of the east penthouse roof structure into the building.

 Witnesses reported structural 

damage to WTC 7 on its south face and southwest corner from WTC 1 debris.
Visual Observations for WTC 7 Visual Observations for WTC 7

Fires in WTC 7—which began soon after WTC 1 collapsed—were observand 11 near the middle about half an hour before collapse; Floor 12 was 

Fires were also seen on Floors 12, 13, 22, 29, and 30 at various times du
Fuel System for Emergency Power 

Fuel System for Emergency Power 

• NIST has reviewed and documented the fuel system for emer• Floor 5—which did not have any exterior windows and co

pressurized fuel distribution system on the south, west aareas—is considered a possible fire initiation location, suand/or analysis that improve knowledge of fire conditions in th

• The two 6,000 gallon tanks supplying the 5th floor generators 

piping system were always kept full for emergencies and were 

• Both tanks were found to be damaged by debris and empty se

collapse. Some fuel contamination was found in the gravel besand below the slab on which the tanks were mounted, but no found in the organic marine silt/clay layer underneath.

• This finding allows for the possibility, though not conclusively, 

contributed to a fire on Floor 5.
Objective 2: Evacuation and Emer

Objective 2: Evacuation and Emer

Response Response

Based on interviews of WTC surviving occupants:• It is estimated that 17,400 occupants (± 1,200) were prese

on the morning of September 11, 2001. The initial populatiosimilar: 8,900 (± 750) in WTC 1 and 8,500 (± 900) in WTC 2. September 11, 2001, 16 percent were also present during the 

• About 6 percent of the surviving occupants reported a pr

their mobility. These limitations included obesity, heart condassistance to walk, pregnancy, asthma, being elderly, chronic surgery or injury, and other.

• About 7 percent of the surviving occupants reported havi

about the building. These included fire safety staff, floor wabuilding maintenance, and security staff. Searchers assist the facilitating evacuation.
Background Information on Evacu

Background Information on Evacu

• Two-thirds of surviving occupants reported having participa

12 months prior to September 11, 2001, while 17 percent repo

no training during that same period. Of those participating in 

were instructed about the location of the nearest stairwell. Ov

the survivors, however, had never used a stairwell at the WTC 2001.

• Two thousand one hundred forty-six building occupants (1,466 

2, and 56 others in WTC 1 and WTC 2) and an additional 421 including security guards but not aircraft passengers and crew reported to have lost their lives on September 11, 2001.

• Approximately 87 percent of the WTC tower occupants, inc

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

• Rough initial estimates, which will be refined as data analysis 

that about 20 percent or more of the 2,567 building occupantwho were in the WTC towers and lost their lives may have bebuildings just prior to their collapse.
Evacuation Findings Based on Firs

Evacuation Findings Based on Firs

Interviews Interviews

• Overall, about 7,900 survivors evacuated WTC 2 in 7

instant WTC 1 was struck by aircraft until WTC 2 colla

7,500 survivors evacuated WTC 1 in 103 min. 

• The overall evacuation rate in WTC 2 (108 survivo

about 50 percent faster than that in WTC 1 (73 sur

• Functioning elevators allowed many survivors to 

prior to aircraft impact. Most of the elevators in WTfunctioning, and survivors could only use the stairway

• The stairwells, with partition wall enclosures that prov

but little structural integrity, were damaged in the regiimpacted floors.
Evacuation Rates in the WTC Towe

Evacuation Rates in the WTC Towe

• After the first airplane struck WTC 1 and before the second air

survivors in WTC 2 were twice as likely as those in WTC 1 to hbuilding (41 percent versus 21 percent). The rate of evacuatio

was twice the rate in WTC 1 during that same period.

• Soon after WTC 2 was struck by the airplane until about 20 mi

collapsed, the survivors in WTC 2 and WTC 1 had exited at abprior evacuation rate of WTC 1).

• During the last 20 min before each building collapsed, the evac

buildings had slowed to about one-fifth the immediately prior evsuggests that for those seeking and able to reach and use u

stairways, the egress capacity (number and width of exits adequate to accommodate survivors.

• Based on use of existing egress models and actual evacuation 

2001, it is estimated that a full capacity evacuation of each Wpeople—three times the number present on September 11required about 4 hours. To achieve a significantly faster tota

capacity would have required increases in egress capacity (nuand stairways). 
Emergency Communication Recor

Emergency Communication Recor

• NIST has reviewed audio communications tapes recorded by 

a recording of the FDNY’s city-wide high-rise Channel 7 (Port Department’s [PAPD] Channel 30) radio repeater that was loc

• NIST has reviewed audio tapes copied from original NYPD co

including NYPD internal department operations.

• FDNY communications recordings from the WTC location on 

are not available because the primary field communication trurepairs. A back-up field-communications van used in its placerecording capability—was destroyed when the WTC towers co

• The best record of radio communications reflecting fire d

came from the FDNY Channel 7/PAPD Channel 30 and firsprovided by FDNY personnel during their interviews.

• The PANYNJ installed the radio repeater system for use by F

bombing.
Preliminary Analysis of Emergency Preliminary Analysis of Emergency 

Communications Communications

• After the first aircraft struck WTC 1, there was an approximat

increase in traffic level over the normal level of emergencommunications, followed by an approximate factor of 3 ste

level of subsequent traffic.

• A surge in communications traffic volume made it more 

the flow of communications and delivery of information.

• Roughly a third to a half of the radio messages transmitted d

traffic surge conditions were not complete messages or un

• FDNY’s city-wide high-rise Channel 7 (PAPD Channel 30) 

the WTC site was operating. 

• NYPD aviation unit personnel reported critical information ab

collapse of the WTC towers several minutes prior to their cohas been found to suggest that the information was furthall emergency responders at the scene.
WTC High WTC High-Rise Radio Repeater Sy

Rise Radio Repeater Sy

• Several FDNY personnel at the incident site did not think tha

repeater was working.

• Based on tests conducted by two chief officers working 

the first command post was being set up in that lobby. 

• Following test, a chief officer involved in the test chose 

channels for command and tactical communications du

• As FDNY operations increased in WTC 2, it was determined 

that the high-rise repeater was functioning, and use of the c

• While preliminary analysis indicates that the repeater was op

appears to have been some type of malfunction with the comequipment that was detected, but not identified, by FDNY of

• NIST continues to evaluate the repeater system and its o

the handheld radios, which were used on September 11

• These findings will be updated and additional findings will be 

the investigation is complete.
Active Fire Protection Systems Active Fire Protection Systems

• The smoke management systems in the WTC towers were not 

fires on September 11, 2001. It was determined that the likelih

being functional was very low due to the damage inflicted by th

• Analysis indicates that HVAC (heating, ventilation, and air-c

was a major path for vertical smoke spread in the building

• Analysis indicates that stair pressurization systems would h

resistance to the passage of smoke in WTC 1 and WTC 2 hon September 11, 2001. 

• The fire alarm system in WTC 7 sent only one signal (at 10:00

WTC 2 collapsed) to monitoring company indicating a fire condnot contain any specific information about the location of the fir

• The resistance to failure of the fire alarm system commun

between the fire command station and occupied WTC towbeen enhanced if fiber optic cable had been used instead 

• Although the fire sprinkler system was damaged by aircraft imp

riser system lacked redundancy, and there existed the poten

failure of the water supply connection on each floor. 
Example of Vertical Smoke Spread Example of Vertical Smoke Spread
• The 110-story WTC towers were among the 

world’s tallest buildings, while the 47-story WTC 7 represented a more typical tall building. 

• These buildings provide case studies to 

document, review, and, if needed, improve the procedures and practices used in the design, construction, operation, and maintenance of tall buildings. 

• This investigation objective is independent of 

other objectives which are focused specifically on the consequences of the attack on September 11, 2001, specifically the building collapses, evacuation, and emergency response. 

• While some findings under this objective are 

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

Objective 3: Procedures and Prac

Objective 3: Procedures and Prac
Applicable Building Codes Applicable Building Codes

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

the proposed 1968 edition of the NYC Building Code, more than went into effect. 

• The 1968 edition allowed less restrictive provisions compar

that was in effect when design began for the WTC towers in 196

• Eliminated a fire tower as a required means of egress;• Reduced the number of required stairwells from 6 to 3 and 

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

• Reduced the fire rating of the shaft walls in the building cor

• Changed partition loads from 20 psf to one based on weigh

length (that reduced such loads for many buildings includin

• Permitted a 1 h reduction in fire rating for all structural com

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

• The NYC Department of Buildings reviewed the WTC tower 

provided comments to the PANYNJ concerning the plans in relaBuilding Code. The architect-of-record submitted to the PANYN

comments, noting how the drawings conformed to the 1968 NY
Standards, Codes, and Regulation

Standards, Codes, and Regulation

NIST has reviewed the then-prevailing and current standards, corelevant to assessing the procedures and practices used in the doperation, and maintenance of the WTC buildings. That review raises the following issues that merit further con

• Detailed procedures to analyze and evaluate data from 

of other building components and assemblies to qualify building element.

• Requiring the conduct of a fire resistance test if ade

exist from other building components and assemblies to building element.

• Adoption of the “structural frame” approach to fire resist

requires structural members other than columns, that ar

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

• Ensure that structural connections are provided the 

protection as the more restrictive protection of the c
Standards, Codes, and Regulation

Standards, Codes, and Regulation

• Minimum mechanical and durability related properti

fire resistive materials (SFRM) to ensure acceptable i(typical shock, impact, vibration, or abrasion effects) ove

• Assure that as-built condition of the passive fire prot

SFRM, conforms to conditions found in fire resistan

components and assemblies.

• In-service inspections of passive fire protection dur

building.

• Early installation of sprinklers in existing buildings, 

lieu of compartmentation.

• Minimum structural integrity for the means of egres

elevator shafts) in the building core which are critical to 
Standards, Codes, and Regulation

Standards, Codes, and Regulation

• Installation of fire-protected elevators and their use f

access by first responders or as a secondary method (aemergency evacuation of building occupants.

• Explicit standards and code provisions for structura

would mitigate progressive collapse.

• Conduct of wind tunnel tests and the methods used 

estimate design wind loads from test results.

• Retention of documents, over the service life of a bu

design, construction, operation, maintenance, and modiincluding retention off-site.
Fire Safety and Egress Design Met

Fire Safety and Egress Design Met

• Historical fire loss data over more than half a century, for diff

occupancies, suggests that prescriptive requirements in stanconsiderable built-in conservatism to adequately protect build

• As a result, there has been a trend in recent decades to red

egress requirements, sometimes in conjunction with addition complementary fire protection requirements (e.g., detectors a

• The lower fire rating requirements when combined with the co

building design efficiency that have been achieved, have also 

thermal mass of buildings—an indicator of how much heat eneabsorb passively without damage. 

• The empirical rules and test methods used in prescriptive des

with experience over the years, do not lend themselves readwhether the performance of building fire safety and egresconsistent, considering both the hazards and the consequen

• Performance-based methods that explicitly define the des

specific design-basis fire hazards or evacuation events a

analysis, enabling appropriate protection to be provided where 
Fire Safety and Egress Design Met

Fire Safety and Egress Design Met

The increasing use of performance-based methods, as an alternative fire safety and egress design, raises the following issues that merit 

• Considering fire as a design condition in structural desig

the fire performance of the structure as a whole system. This already being used in building design practice for earthquake a two-level design that includes a higher probability operationprobability life safety event).

• Detailed procedures to select appropriate design-basis fi

performance-based design of the sprinkler system (e.g., a frefire), compartmentation (e.g., a moderate severity but less freprotection of the structure (e.g., a maximum credible fire).

• Validated and verified tools, for use in performance-based 

analyze the dynamics of building fires and their effects on the would allow engineers to evaluate structural performance undscenarios and fire protection strategies. 
Fire Safety and Egress Design Met

Fire Safety and Egress Design Met

• The technical basis to establish whether the construct

fire rating requirements are risk-consistent. Specificalhow the current height and area tables in building codes cbasis for the progressively increasing risk to an occupant otall buildings that are much greater than 200 ft in height.

• Sprinklers improve safety in most common building fires a

becoming large fires. The technical basis to establish toff” in current codes, considering fire safety risk factors complementary functions of sprinklers and fire-protected sthe need for redundancy should one system fail to function 

available. The sprinkler trade-off provides an economic in

installation of sprinklers by allowing a lower fire rating for s

• The design of egress systems to achieve a target perf

evacuation rate or time) for a given occupant population bconsidering travel distance, remoteness requirements, and occupant size, stairwell environmental conditions, visibility
Objective 4: Approach to Recomm

Objective 4: Approach to Recomm

• In the United States, state and local governments are r

promulgating and enforcing building and fire regulation

• With some exceptions, the state and local regulations a

model building and fire codes developed by private sec

• The model codes, in turn, reference voluntary consens

developed by a large number of private sector standardorganizations (SDOs) accredited by the American Natio

Institute (ANSI).

• NIST does not set building codes and standards, but p

support to the private sector and other government agedevelopment of U.S. building and fire practices, standa

• NIST recommendations are given serious consideratio

organizations that develop national standards and mod

provide minimum requirements for public welfare and s
Approach to Recommendations (2

Approach to Recommendations (2

• The NIST building and fire safety investigation of the WTC dis

formulated recommendations. However, in formulating its rNIST will consider the following:

• Findings from the first three independent investigatio

building performance, evacuation and emergency responpractices.

• Whether findings relate to the unique circumstances 

terrorist attacks of September 11, 2001, or to normal bconsiderations, including evacuation and emergency 

• What technical solutions are needed, if any, to addres

buildings, occupants, and first responders, considerinhazards and the consequences of those hazards.

• Whether the risk is in all buildings or limited to certain 

height and area, structural system), buildings that contain 

features, iconic/signature buildings, or buildings that hous
Approach to Recommendations (3

Approach to Recommendations (3

• NIST urges organizations responsible for building and fire 

carefully consider the interim findings contained in this re

• NIST welcomes comments from technical experts and the 

interim findings. Comments can be sent by e-mail to wtc@n(301) 975-6122, or regular mail to WTC Technical Information 100 Bureau Drive, Gaithersburg, MD 20899-8610.

• In its final report, a draft which is expected to be released 

NIST will recommend appropriate improvements in the wa

designed, constructed, maintained and used. It will be im

recommendations to be thoroughly and promptly consideorganizations responsible for building and fire safety. 

• As part of NIST’s overall WTC response plan, the Institute 

out to these organizations to pave the way for timely, expof recommendations stemming from this investigation. Nexpanded its research in areas of high priority need.
http:// http://wtc.nist.gov wtc.nist.gov

Thank You Thank You