Dr. S. Shyam Sunder

Acting Deputy Director and Lead In

Building and Fire Research Lab

National Institute of Standards and T

U.S. Department of Comme

NIST Response to the World Trade Ce

Federal Building and Fire Safety Inv

of 

the World Trade Center Disas

Media and Public Briefing

April 5, 2005
Outline Outline

• WTC Investigation Status and Schedule• Probable Collapse Sequences for the WTC t• Life Safety

• Occupant Behavior, Egress, and Emerge

Communications

• Fire Service Technologies and Guideline

• Procedures and Practices

• Analysis of Building and Fire Codes and 
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 technic

al as

protection,

occupant behavior, evacuation, and e

•

Determine the procedures and practices that wer

design, construction, operation, and maintenance buildings

• Identify, as specifically as possible, areas in c

building and fire model codes, standards, and warrant revision


Some Specific Questions Some Specific Questions

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

(102 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, coul

prevented the collapse of the W

TC towers?

 Would the undamaged WTC towers have remained stand

large building fire scenario?

 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 confor

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 t

conditions similar to those caused by the terrorist attacks of Septem

 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 w

everyday use; instead, that experience suggests that major tall build

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 buildinlocal, state, or federal jurisdiction. The PAN

Y

N

J’s longstanding state

where appropriate, exceed requirements of local building and fire co
WTC Investigation Status WTC Investigation Status

• Investigation nearing completion; drew talent from NIST, outside

contractors; $16 million investigation; $5.5 million awarded in co

•

Two public updates issued (December 2002, December 2003); 

reports issued (

May 2003, June 2004). Full text available on W

•

Three public meetings held:

• June 24, 2002 (NYC) to present draft WTC investigation plan and s

• February 12, 2004 (NYC) to solicit comments on (1) technical aspe

additional information that NIST might consider, (3) areas to be conrecommendations.

• August 24, 2004 (Chicago) to observe fire resistance test of WTC f

Laboratories

•

Six media/public briefings on investigation progress, extensive b

of the National Construction Safety Team Advisory Committee, 2003) to solicit public inp

ut for first-person interviews of occupa

•

Current findings may be revised and additional findings may be 

• NIS

T is not making any recommendations at this time; all re

made in the final report.
Schedule for Completion of Invest

Schedule for Completion of Invest

•

Major focus is on writing the Investigati

on reports; technical work is 

•

The time required t

o write and review t

he comprehensive set of dra

constitute final WTC investigation findings and recommendations ithe report.

•

The NIST reports include the overall investigation report for the WT

project reports, and 34 supporting technical rep

orts, totaling s

ome 

enormous task has taken NIST longer to accomplish than ori

ginally 

•

NIST is committed to putting accuracy, quality, and completeness

whatever time is req

uire

d to d

o the job rig

ht.

•

NIST plans to release a draft of the final report for public comment 

comment period of about 6 weeks after rel

ease of the draft report; 

final Investigation report i

n September 2005.

•

WTC 7 report will be issued as a supplement to the mai

n re

p

ort: dr

2005; final for D

ecember 2005.

• Decoupling of WTC 7 report necessary to accommodate overlapp

on WTC towers.

• This change affects mainly the collapse analysis; other WTC 7 wo

other Investigation reports.

•

WTC Conference: Putting Recommendations into Practice, Se
WTC Investigation Reports WTC Investigation Reports

7A Acc7B Inst

4A FireHistory4B FireSprinklers4C FireAlarms4D SmokeManagement

WTC To

w

ers

Investigation Rep

ort

Building and 

Fire Codes

Baseline

Performance & 

Aircraft Impact

Steel

Analysis

Active Fire 

Protection

Thermal 

Reconstruction

Occu

Beha

Egr

WTC Towers: 

Structural 

Collapse

1A Design andConstruction1B StructuralCode Comparison1C StructuralModifications1D Fire Protection 

&

Life Safety Provisions1E Fire CodeComparison 1F Comparisonof NYC Codes1G Fire Protection& Life Safety A

mend.

1H Post-Const.Modifications WTC1&21I Post-Const.Modifications WTC 71J WTC 7 FuelSystem 

9 

reports

totaling 

about 

1,500 

pages

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

2A BaselinePerformance2B AircraftImpact

6A Passive FireProtection6B FireResistance Test6C ComponentAnalysis6D GlobalAnalysis

5A 

Visual

Evidence5B SteelExperiments5C SingleWorkstation5D CeilingTile5E MultipleWorkstation 5F FireSimulation5G FireStructure Interface

34

reports

totaling 

about 

8,000 

pages

43 reports for WTC Tow

ers; 5 r

eports for WTC 
Analysis of Probable Collapse Seq

Analysis of Probable Collapse Seq

NIST developed and used a series of rigorous and comdetermine the probable collapse sequence for the WTC impact to collapse initiation. The approach:

 Combined mathematical modeling, well-established statistical 

analysis methods, laboratory experiments, and analysis of visevidence—significantly advancing the current state-of-thelimits o

f current computational capabilities.

 Analyzed the complete sequence of events from aircraft impa

ignited multi-floor fires, thermal weakening of str

uctural compo

progression of local structural failures that ultimately initiated c

 Allowed for evaluation and comparison of possible collapse se

different damage states, fire paths, and structural load redistri

 Accounted for v

ariations in models, input parameters, analyse



R

equired use of advanced strategies for managing comput

unprecedented analysis complexity and sophistication; ad

physics of phenomena essential to determining the proba
High Fidelity Aircraft Models High Fidelity Aircraft Models
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.
Effect of Engine Impact Location Effect of Engine Impact Location
WTC 1 Tower Model for Aircraft Im

WTC 1 Tower Model for Aircraft Im
AA 11 (WTC 1) Aircraft Impac AA 11 (WTC 1) Aircraft Impact Initial Conditions t Initial Conditions

FEMA

HartWeidlinger MIT

S

Best Estimate Speed (mph)

4

7

0 50

0

4

2

9 

Speed Error Estimate (mph) + 30 / -

5

0

± 51

Lateral Approach Angle (clockwise) 4.3o Vertical A

pproach Angle 

(downw

ard) 6.2o 

Aircraft R

oll (left wing dow

n) 20.7o 

UAL 175 (WTC 2) Aircraft Impact Initial Conditions UAL 175 (WTC 2) Aircraft Impact Initial Conditions

FEMA

HartWeidlinger MIT

Best Estimate Speed (mph)

5

9

0 55

0

5

0

3 

Speed Error Estimate (mph)

± 38

Lateral Approach Angle (clockwise) 11.7o 15o Vertical A

pproach Angle 

(downward) 2.7o 0oAircraft R

oll (left wing dow

n) 30.1o 
WTC 1
WTC 1 Damage: Composite Su

WTC 1 Damage: Composite Su

for Floors 93 to 98 for Floors 93 to 98

Col

umn Damage

Severe

d

Heav

y Damage

Moderate DamageLight DamageSevere Fl

o

or Damage

Fireproofing and partitionsFloor s

ystem structural d

amage 

Floor s

ystem 

removed
WTC 1 Damage by Floor WTC 1 Damage by Floor

Floor 94 

Floor 96 Floor 97 

Floors 93 to 98 Floor 93

Cumulative Damage

Col

umn Damage

Severe

d

Heav

y Damage

Moderate DamageLight DamageSevere Fl

o

or Damage

Fireproofing and partitionsFloor s

ystem structural d

amage 

Floor s

ystem 

removed
WTC 2
WTC 2 Damage: Composite Summ

WTC 2 Damage: Composite Summ

78 to 83 78 to 83

Col

umn Damage

Severe

d

Heav

y Damage

Moderate DamageLight DamageFloor DamageFireproofing and partitionsFloor s

ystem structural d

amage 

Floor s

ystem 

removed
Enhancements added by NIST.
Validation of Aircraft Impact Mode

Validation of Aircraft Impact Mode

With Observations for WTC 1 With Observations for WTC 1

Aircraft Impact Damage to WTC Tow

Aircraft Impact Damage to WTC Tow

• The two WTC towers withstood the initial impact of virtually identical aircraft (B

terrorist attacks of September 11, 2001. 

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

buildings withstand the aircraft impact. 

• The WTC towers displayed significant reserve cap

acity, vibrating immediately 

amplitudes that were more than about 1/3 of the design wind sway from the baoscillation period nearly equal to the average mea

sured for the undamaged bu

• Calculations of demand to capacity ratios due to aircraft impact

d

a

m

age 

affected b

y the aircraft impacts, the majority of the core and perimeter co

continued to carry their loads after the impact. The loads from da

m

aged 

redistributed to nearby undam

aged columns.

• The above finding supports the fact that the WTC tow

ers withstood the i

finding that they would have continued to remain standing indefinitely bevent such as the subsequent fires.
Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a

• Fires played a major r

ole in f

urther r

educing the str

uctural 

buildings, initiating collapse. While aircraft impact damage 

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

•

Compromising the sprinkler and water supply systems;

•

Dispersing jet fuel and igniting building contents over large a

•

Creating large accumulations of combustible matter contain

building contents;

• Increasing the air supply into the damaged buildings that pe

higher energy release rates than would normally be seen in building fires, allowing the fires to spread rapidly on multiple 

•

Damaging and dislodging fireproofing from structural compo

of the debris and due to the strong vibrations generated by 

•

Damaging ceilings that enabled “unabated” heat transport o

partition walls and to structural components.
Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a

• The jet fuel, which ignited the fires, was mostly con

first few minutes after impact. The fires that burned entire time that the buildings remained standing weburning building contents and, to a lesser extent, aijet fuel.

• Typical office furnishings were able to sustain inten

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

• 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 com

on floors. Mass of aircraft solid combustibles was signimmediate impact region of both WTC towers.
Initial Fire and Smoke Simulations

Initial Fire and Smoke Simulations
Reconstruction of the WTC Fires Reconstruction of the WTC Fires
Upper Layer Temperatures (WTC 1
Results of Thermal Analysis

WTC 1 WT

Shows maximum temperature reached by each co
Structural Analysis Progression Structural Analysis Progression

Subsystem Analyses WTC 1

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

 WTC 2

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

Component Analyses Knuckle Truss seat connections Single truss and 

concrete slab

 Full floor Column splice 

c

onnection

 Single story column Nine story column Nine story-nine column 

exterior wall p

a

n

el

Nonlinear analy

ses with 

component simplifications and failure mechanism simplifications to determine major subsy

stem behavior 

and sequential failure mechanisms.

Detailed nonlinear analy

ses to 

determine compo

nent 

behaviors and failure mechanisms


Progress in Determining Collapse 

Progress in Determining Collapse 

• Possible Collapse Hypotheses (May 2003) –

n

specific; key events not identified

• Working Collapse Hypothesis (June 2004) –

s

for both WTC towers; identifies chronological seqevents

• Leading Collapse Hypotheses (October 2004) 

hypothesis for each WTC tower; identifies buildinredistribution paths and damage scenarios in adchronological sequence of major events

• Probable Collapse Sequences (April 2005) –

re

specific collapse sequences with chronological sevents, load redistribution paths, and damage sc
Probable Collapse Sequence for W

Probable Collapse Sequence for W

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the North w

the wall section above the impact zone moved downward. 

• After breaching the building’s perimeter, the aircraft continued to pe

severing floor framing and core col

u

m

ns at the North side of the

cor

damaged toward the center of the core and, to a limited extent on thFireproofing was damaged from the impact area to the South perim

the center of WTC 1 and at l

east over a third to a half of the core wi

• Aircraft impact severed a single exterior panel at the center of the S

and 96.

• The impact damage to the exterior walls and to the core resulted in 

col

u

m

n loads, m

ostly t

o the columns adjacent to the impact zones. 

downward movement of the North wall, and rotated about the East-

• As a result of the aircraft impact damage, the North and South walls 

percent less gravity loads after impact, and the East and West walls percent more loads. The core carried about 1 percent more gravity 
Probable Collapse Sequence for W

Probable Collapse Sequence for W

2. Effects of Subsequent Fires and Impact Damaged Fireproo

A. Thermal Weakening of the Core:

• The undamaged core columns developed high plastic and creep 

building stood, since both temperatures and stresses were high 

and creep strains exceeded thermal expansion in the core colum

• The shortening of the core columns (due to plasticity and creep) 

truss which unloaded the core over time and redistributed loads 

• As a result of the thermal weakening (and subsequent to impact 

of the South wall), the North and South walls each carried about loads, and the East and West walls each carried about 25 perce

carried about 20 percent less gravity loads after thermal weaken

B. Thermal Weakening of the Floors:

• Floors 95 to 99 weakened with increasing temperatures over tim

and sagged. The floors sagged first and then contracted due to 

fires reached the South side later, the floors sagged, and the se

• Floor sagging induced inward pull forces on the South wall col

u

m

• About 20 percent of the connections to the South perimeter wall 

due to thermal weakening of the vertical supports.

C. Thermal Weakening of the South Wall: 

• South wall columns bowed inward as they were subjected to hig

pull forces in addition to axial loads.

• Inward bowing of the South wall columns increased with time.
Probable Collapse Sequence for W

Probable Collapse Sequence for W

3. Collapse Initiation

• The inward bowing of the South wall induced column instability, wh

horizontally across the entire South face.

• The South wall unloaded and tried to redistribute the loads via the 

weakened core and via the spandrels to the adjacent East and We

• The entire section of the building above the impact zone began tilt

faces; not only the bowed and buckled South face) to the South (ainstability progressed rapidly from the South wall along the adjacen

• The change in potential energy due to downward movement of bui

buckled columns exceeded the strain energy that could be absorbecollapse then ensued.
Probable Collapse Sequence for W

Probable Collapse Sequence for W

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the South 

the wall section above the impact zone moved downward. 

• After breaching the building’s perimeter, the aircraft continued to pe

severing floor framing and core columns at the Southeast corner of damaged from the impact area through the East half of the core up 

perimeter walls. The floor truss seat connections over about 1/4 to 1core were severed on floors 80 and 81 and over about 1/3 of the Ea

• Aircraft impact severed a few columns near the East corner of the N

and 82.

• The impact damage to the exterior walls resulted in redistribution of 

mostly to the columns adjacent to the impact zones. The impact daresulted in redistribution of severed column loads mostly to other int

East exterior wall. The hat truss resisted the downward movement oabout the East-West axis. 

• As a result of the aircraft impact damage, the core carried 6 percent 

impact and the North face carried 10 percent less loads. The East f

gravity load, w

hile the West face and the South face carried 3 perce

gravity load, respectively.

• After impact, the core was leaning toward the East and South perim

walls acted to restrain the core structure.
Probable Collapse Sequence for W

Probable Collapse Sequence for W

2. Effects of Subsequent Fires and Impact Damaged Fireproo

A. Thermal Weakening of the Core:

• Several of the undamaged core columns near the damaged and 

developed high plastic and creep strains over the duration the btemperatures and stresses were high in the core area. The plas

exceeded thermal expansion in the core columns. 

• The core continued to tilt toward the East and South due to the c

shortening (due to plasticity, creep, and buckling) and the failure truss in the Southeast corner.

• As a result of thermal weakening (and subsequent to impact), th

percent more gravity loads and the core carried about 2 percent 

walls carried between 0 and 3 percent less loads.

B. Thermal Weakening of the Floors:

• Floors 79 to 83 weakened with increasing temperatures over tim

the East side and sagged. 

• Floor sagging induced inward pull forces on the East wall col

u

m

• About an additional 1/3 of the connections to the East perimeter 

thermal weakening of the vertical supports.

C. Thermal Weakening of the East Wall: 

• East wall columns bowed inward as they were subjected to high 

pull forces in addition to axial loads.

• Inward bowing of the East wall colum

ns increased with time.
Probable Collapse Sequence for W

Probable Collapse Sequence for W

3. Collapse Initiation

• The inward bowing of the East wall induced column instability, whi

horizontally across the entire East face.

• The East wall unloaded and tried to redistribute the loads via the h

core and via the spandrels to the adjacent North and South walls.

• The entire section of the building above the impact zone began tilt

faces; not only the bowed and buckled East face) to the East (abo

3o to 4o) as colum

n instability progressed rapidly from the East wall 

and South walls. The building section above impact continued to ro

to fall downward, and rotated to at least 20 to 25 degrees.

• The change in potential energy due to downward movement of bui

buckled columns exceeded the strain energy that could be absorbecollapse then ensued.
Validation of Probable Collapse Se

Validation of Probable Collapse Se

• NIST evaluated the key factors related to:

• Innovative structural system•

Aircraft impact and subsequent fires: How safe wa

immediately after aircraft impact but before fire weastructures?

•

Post-impact condition of fireproofing

•

Quality and properties of structural steel

•

Relative roles of the perimeter and core columns a

floor system, including connections

•

Role of compartmentation (i.e. areas divided by fire

• NIST made concerted efforts to validate results with ke

obtained from its extensive collection of over 7,000 pho150 hours of videotape documenting the events at the on September 11, 2001

• The probable collapse sequences are supported by the 

NIST, including photographs and videos, recovered steaccounts, and emergency communication records
Collection and Analysis of Photog

Collection and Analysis of Photog

Video Images Video Images

• Visual database contains:

• Well in excess of 7,000 

photographs taken by more than 185 photographers

• 150 hours of videotape from 

maj

or media outlets and more 

than 20 individual

s

• From the analysis of the visual 

images to date, NIST has identified significant events forWTC 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 tow

er; wor

k is 

nearing completion for WTC 7
Time to Collapse Initiation Time to Collapse Initiation 

WTC 1

WT

Actual Time 102 min 56 mEstimated Time* 100 min 42 m

* The exact times are sensitive to the factors that con

inward bowing of the exterior columns. The sequencevents leading to collapse is not sensitive to these fa
South Face of WTC1

•

Time: 10:22 AM

•

Measurements of inward

bowing (inches)

• Maximum = 55 inches

(uncertainty ~ +/-

6 inches)

•

Floor locations approximate

•

Blue tinted region digitally

enhanced
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 2 

Minutes Prior to Collapse: WTC 2 
WTC2: East Face

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

Maximum inward bowing of columns approximately 10 inches
Map of Inward Bowing: East Fac

Time: 9:52:54 AM to 9:53:0

Pull-in (inches)

Estimated uncertainty: +/-

1 inch

Empty regions have no data (smoke, damaged aluminum, could not es

159

10

95

07

131613

6

0

6

111412

7

059

1114

10

6

0

1620

17

9

38

14

058

14

0

5

9

12

06

13

6

08

151313

7

07

1213

13

5

0

5

1517

13

7

5

14141316

6

4

12

4

7

12

0

0

15

2

6

1014

1

8

13

5

076

0

86

067

078

0

12

11

0

6

9

70

40

7

0

0

1616

12

840

Obscured in Photos
© 2001 Dean Rivier

Tilting of Building Tilting of Building Sections Sections

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

Initiatwas ftilting above of bo
WTC 1: First Responder Communi

WTC 1: First Responder Communi

• 10:06 am NYPD aviation unit advises everybody to evac

vicinity of Battery Park City and states that, abtop, it is totally glowing red on the inside and cNYPD officer advises that it is isn’t going to tathe North tower comes down and to pull emerfrom the building.

• 10:21 am NYPD aviation unit first reports that the top of 

leaning, then confirms that it is buckling and le

NYPD aviation unit reports that the North toweSouthwest and appears to be buckling in the SNYPD officer advises that all per

sonnel close 

three blocks in every direction.

• 10:28 am NYPD aviation unit reports that the roof is goi

shortly.NYPD officer reports that the tower is collapsi
Inward Bowing of the WTC 1 South Face 

Inward Bowing of the WTC 1 South Face 

1

MN

MX

Y

X

Z

WTC1 Severe Temp at 6000s w/5kip p

ull - South Face (5X) 

-.701059

4.135

8.972

13.808

18.644

23.481

28.317

33.153

NODAL SOLUTIONSTEP=33SUB =1437TIME=150UY (AVG)RSYS=0DMX =42.979SMN =-.701059SMX =42.826

Maximum Inward Displacement = 43 in.

CO

COL359

Looking from the outside of the b

uilding

Inward displacement is shown as positive displacem
Visual Evidence of Fires in WTC 1 Visual Evidence of Fires in WTC 1

Dark blue line i

ndicates extent of inward bowing observed in visual i

mages.

Light blue line is estimated region of inward bowing where visual i

mages w

ere obscu
Inward Bowing of the WTC 2 East Face a

Inward Bowing of the WTC 2 East Face a

1

MN

MX

Y

X

Z

WTC2 Reduced Model A

t 2540s Temp UnCn - East Face 

-4.143 3.179 10.5 17.821 25.142 32.463 39.785 47.106 

NODAL SOLUTIO

N

STEP=17SUB =523TIME=63.523UY (AVG)RSYS=0DMX =61.911SMN =-4.143SMX =61.748

Maximum inward displacement = 60 in.
Visual Evidence of Fires in WTC 2 Visual Evidence of Fires in WTC 2

Dark blue line i

ndicates extent of inward bowing observed in visual i

mages.

Light blue line is estimated region of inward bowing where visual i

mages w

ere obsc
Fireproofing Conditions Fireproofing Conditions

• In general, the affected floor systems in WTC 1 had upgra

fireproofing (1.

5 in. s

pecifi

ed); aff

ected floors in WTC

2 h

fireproofing (0.5 in. specified).

• Structural response is sensitive to variability in firepro

along the length of components; it is possible to deteequivalent uniform thickness that should be greater ththickness. The thermally equivalent thicknesses were 

• In the a

nalysis under the original condition, the firepro

the bridging trusses was considered to be one-half thprimary trusses from interpretation of photographic evide

upgraded condition, the fireproofing thickness on the con

sidered to be equ

al to the thickness for the primar

Status As-Applied Avg. (COV*) Thermally Equivalent S

Original

Upgraded

0.75 (0.4)

0.6 

2.5 (0.24) 2.2 

* Coefficient of Variati

o

n
Analysis of Fireproofing Damage Analysis of Fireproofing Damage

• NIST developed and used a rigorous technical approac

role of the post-impact condition of the fireproofing in WTC towers.

• The technical approach was based on a comprehensive a

analysis that predicted in detail (1) the damage to strucand building partitions and furnishings (the partitions workstations were modeled explicitly in the impact regthe debri

s field that was generated by aircraft impact, a

of jet fuel.

• NIST determined the most influential parameters that gove

aircraft impact damage analysis based on formal statisticalNIST then conducted analyses for two sets of values foinfluential parameters for each WTC tower to estimate caused by aircraft impact.

• NIST determined conservative estimates for the extent 

fireproofing by considering fireproofing damage only tcompo

nents i

n the direct p

ath of debri

s. 
Analysis of Fireproofing Damage (

Analysis of Fireproofing Damage (

• Consistent with a conservative a

pproach, NIST estimate

s ign

fireproofing on structural components i

n a much larger regio

direct path of t

he debris was dislodged by shock or strong vi

•

The WTC towers shook vigorously during the 0.5-0.7 second

Video analysis showed that W

TC 2 vibrated for over 4 minut

with amplitudes i

n excess of 20 i

nches at the roof top.

•

Considerable photographic evidence shows fireproofing dislo

columns not directly impacted by debris.

•

First-person i

nterviews of building occupants indicate that bu

aircraft impact were strong enough t

o

dislodge c

eilin

g tile

s an

throughout the height of bot

h WTC t

owers and t

o cause near

functioning.

• Difficult to establi

sh robust criteri

a to generate a c

oherent p

a

dislodging.

•

The variation of influential parameters used in det

ermining t

h

sequence included some variation in the extent of dislodged 
Examples of Types of Core Column 

SFRM + 

Wallboard

SFRM Only

SFRM –

Sprayed fire resistive material

Wallboard –

Gypsum wallboard enclosure
Types of Fireproofing on Core Col

Affected Floors

WTC 1 W

Floor

%

SFRM

only

%

SFRM +

Wallboard

% 

Wallboard 

only

94 13%

49% 38%

95 9% 49%

43%

96 11%

45% 45%

97 11%

45% 45%

98 11%

45% 45%

All 11% 46% 43%

Floor

%

SFRM

only

78 38%79 19%80 17%81 17%82 23%83 15%

Information from original design drawings All 22% 

SFRM –

Sprayed fire resistive material

Wallboard –

Gypsum wallboard enclosure
Damaged SFRM -

North Face of W

Removed from flanges (red)Intact (green)

Removed from outer 

Covered by weatherproo
Damaged SFRM -

East Face of WT
Damaged SFRM -

North Face of W

Missing from Trusses
Factors that Enhanced Building Pe

Factors that Enhanced Building Pe

on September 11, 2001 on September 11, 2001

• The unusually dense spacing of perimeter columns, coupled w

that was an inherent part of both the architectural and structuwalls, resulted in a robust building that was able to redistributperimeter columns to adjacent intact columns.

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

frame-tube system, significantly exceeded the prescriptive reqYork City building code and selected other building codes of tYork State), including the relevant national model building cod

• The robustness of the perimeter frame-tube system and the la

the WTC towers helped the buildings withstand the aircraft im

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

way flat plate action, enabled the floors to redistribute loads wplaces of aircraft impact damage to other locations, avoiding 

upon impact.
Factors that Enhanced Building Pe

Factors that Enhanced Building Pe

on September 11, 2001 (2) on September 11, 2001 (2)

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

damage and subsequent thermal effects, by redistributing loacore columns to adjacent intact columns and, ultimately, by rethe perimeter walls from the thermally weakened core columnto support the buildings’ weight.

• As a result of t

he above factors, the buildings would likel

under t

he combined effects of aircraft impact and the sub

ignited multi-floor fires, if the fireproofing had not been donly minimally dislodged by aircraft impact. The existing fireproofing prior to air

craft impact and the fir

eproofing t

floor system did not play a significant role in initiating co11, 2001.
Future Technologies and Practices 

Future Technologies and Practices 

Potentially Could Have Improved P

Potentially Could Have Improved P

on September 11, 2001 (Requires A

on September 11, 2001 (Requires A

• Fireproofing not dislodged or only minimally dislodged • Perimeter columns and floor framing with greater mass 

and buckling performance. 

• Other passive and active fire protection features (e.g., 

retard spread of building fires; thermally-resistant windair supply and retard the spread of fires; fire-protected hardened elevators for firefighter access with continuosupply for standpipes).

• Steels with improved high-temperature properties (e.g.

stiffness) and creep behavior.________________________There is far greater knowledge of how fires influence structurecase in the 1960s. The analysis tools available to calculate thto fires are also far better now than they were when the WTC 
Analysis of Life Safety Analysis of Life Safety 

• Nearly 1,200 first person interviews of occupants and e

plus 700 published media accounts

• Review of emergency communication records (radio co

calls, 500 plus post-9/11 FDNY interviews)

• Evacuation and occupant behavior:

•

Building population demographics; analysis of dece

•

Evacuation rates; roof evacuation; communications 

impaired occupants; building damage observations

• Emergency response:

•

First responder roles; situational awareness; access

radio communications; command and control

• Active fire protection systems:

•

Significant pre-9/11 fires; sprinkler and standpipe sy

system; smoke management system
Evacuation and Emergency Respo

Evacuation and Emergency Respo

Based on 1,056 interviews of surviving WTC occupants aemergency responders.

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

morning of September 11, 2001. The initial population of each tower 

in WTC 1 and 8,500 (± 900) in WTC

2. Of those present on Septembe

also present during the 1993 bombing. 

• About 6 percent of the surviving occupants reported a pre-e

mobility. These limitations included obesity, heart condition, ne

pregnancy, asthma, being elderly, chronic condition, recent surg

• About 7 percent of the surviving occupants reported having 

the building. These included fire safety staff, floor wardens, seamaintenance, and security staff. Searchers assist the floor ward

• Approxi

mately 

8

7 perc

e

nt of the W

TC tower occ

upants, inclu

ding m

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

• Rough estimates indicate that about 20 percent or more of the 2,567 

emergen

c

y resp

onders who were in the WTC towers and lost thei

in the building

s just prior to their collapse. This estimate includes 

floors of impact who died but not the large but unknown number of occimpact who may have been alive prior to collapse.
Likely Location at Time o

World Trade Center 1 Occu

At or 

World Trade Center 2 Occu

At or 

Confirmed Below Impact in WTC 2Unknown Location Inside WWTC 2First Responders (Total) 

Hosp

Volunte

Bystander/Nearby Building American Flight 11 United Flight 175 No Information Total 

Decedent Analysis Decedent Analysis

September11Victims.com: This site is dedicated to 

the victims of September 11, 2001 tragedy.

Portraits: 9/11/01: Published by the New York Times 

in 2003, this book includes short interviews with family members of many decedents.

CNN.com In-Depth Special 

(http://www.cnn.com/SPECIALS/2001/memorial/index.html): Tribute site for people to write remembrances of decedents.

Badge List maintained by Port Authority of New York 

and New Jersey: Includes name, employer, building, and floor for all occupants with badgeaccess to WTC 1 or WTC 2.

Numerous memorial sites maintained by companies 

which lost employees: Includes names and remembrances of decedents. Examples include the Port Authority, Fire and Police Departments, Marsh & McLennan Companies, EuroBrokers, Fiduciary Trust, and others.

Newsday.com: Includes short stories written about 

specific decedents.

NIST Interviews with occupants and family members* Where possible, eyewitness accounts were used to place individuals.

Where no specific accounts existed, employer and floor information was used to place individuals.
Evacuation Rates in the WTC Towe

Evacuation Rates in the WTC Tow

• The overall evacuation rate in WTC 2 (108 survivors per min) was

than that in WTC 1 (73 survivors p

er min). Overall, about 7,900 sur

73 min (i.e., from the instant the WTC 1 was struck by aircraft until WT7,500 survivors evacuated WTC 1 in 103 min.

• After the first airplan

e struck WTC 1 and before the second airpla

survivors in WTC 2 were twice as likely as those in WTC 1 to have alre

percent versus 21 percent). The rate of evacuation completion in W

WTC 1 during that same period.

• Approximately 75 percent of WTC 2 occupants above the 78th floo

descended below the 78th floor prior to the aircraft strike at 9:03 a

• Functioning elevators allowed many (roughly 3,000) survivor

during the 16 minutes prior to aircraft impact. All but one of th

were not functioning, and survivors could only use the stairways.

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

survivors in WTC 2 and WTC 1 exited at about the same rate (the pri

o

• During the l

ast 20 min before each building collapsed, the evacuation r

to about one-fifth the immediately prior evacuation rate. This suggests 

a

ble t

o rea

ch and use undamaged exits and stairway

s, the egress 

width of exits an

d stairway

s) w

as adeq

u

ate to accommodate surv
Evacuation Rates in the WTC Towe

Evacuation Rates in the WTC Tow

• Even though a percentage of evacuees reported that they perceived co

ingress) to be problem, it was found not to be a significant factor in 

occupants in WTC 1 when compared to other factors, includi

ng eva

evacuation interruption, and encountering obstacles in the evacuation p

such as smoke, water, or debris.

• Based on use of existing egress models and actual evacuation time on 

estimated that a full capacity evacuation of each WTC tower with 25the number present on September 11, 2001—would have required 

buildings been full, it is possible that as man

y as 14,000 people ma

based on rough estimates using existing models.

T

o achieve a sign

evacuation at full capacity would have required increases in egress cap

exits and stairways). The egress capacity required by c

urrent buildin

based on a “phased” evacuation strategy, not “full” evacuation. 

• The average surviving occupants moved slower down stairs and th

previousl

y reported for non-emergen

c

y evacu

ations.

• In WTC 1, the average surviving occupant spent 48 seconds per fl

This translates to approximately 0.2 m/s (0.65 ft/s), which is about 

measurement presented in the SFPE Handbook of Fire Protection 

emergency evacuations.

• In WTC 1, each stairwell door exited approximately 37 people per

minutes, which is comparable to the slowest measurement presen

Fire Protection Engineering for non-emergency evacuations.
Condition of Stairwells Condition of Stairwells

• The stairwells, with partition wall enclosures that provided a 2

h fire-ratin

integrity, were da

m

aged in the regi

on of the aircraft impacted floors. 

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

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 

the regi

on of aircraft impact.
Findings on Stairwell Remoteness Findings on Stairwell Remoteness

• The 1968 NYC Building Code required stairwells to be as ‘far 

• Stairwell separation in WTC 1 and WTC 2 ranged from 70 f

above, including the impact region in WTC 1) to 200 ft (Flthe impact region in WTC 2), demonstrating t

hat large stai

the WTC towers were practicable.

• The maximum travel distance for the W

TC towers was about 1

separation was consistent with the 2001 NYC building codthird the travel distance or 60 ft.

• On some WTC floors (e.g., the floors of impact in WTC 2), 

stairwells A and C exceeded the typical model code requirsprinklered; 147 ft unsprinklered); on other floors (e.g., thWTC 1), the stair separation 

was not consistent

with these 

• The advantages of moving stairwell locations on the floor plan 

core space for occupant use above terminated elevator shafts 

obstructions posed by equipment installed on mechanical floor

• A walking path measurement may allow two stairwells to be ph

have a significant walking path distance between doors (i.e., sthe IBC credits scissor stairs as a single stairwell.
Occupant Preparedness Occupant Preparedness

• Two-thirds of s

urviving occupants

reported having participated 

12 months pri

or to Septe

mber

11, 2001, while 17 percent reporte

training during that sa

m

e period.

• Of those participating in fire drills, 93 percent were instructed a

nearest stairwell.

•

Overall, slightly over half of the survivors, however, had neve

WTC pri

or to September 11, 2001 (NYC Local Law 5 prohi

bits r

practice stairwell evacuation.)

• Occupants were often unprepared for the physical challenge of 

Numerous occupants required one or more periods of rest durin

g sta

elevators after finding the stairwell

s strenuous.

• Occupants were often unprepared to encounter transfe

r hallway

descent. Groups of evacuees occasionally hesitated or debated a cencountering a transfer hallway. 

• Mobility challenged occupants were not univers

ally identified o

building evacuation. One occupant, for example, reported b

eing ‘le

colleagues, called authorities for assistance, and w

as eventually ass

(occupants). 
Roof Evacuation Roof Evacuation

• The PANYNJ’s standard occupant evacuation procedures and 

stairways to exit at the bottom of the WTC towers. The standa

keep the doors to the roof locked. The PANYNJ reports that to evacuate upward. Roof access required use of an electro

through the first two doors and a security officer watching a clo22nd floor of WTC 1 to open the third door via a buzzer. 

• The 1968 BCNYC required access to roofs with slopes less th

least one stair in buildings greater than 3 stories (or 40 ft) in he

• The Code does not state the purpose of this access but, since it is 

Construction and not Means of Egress it does not necessarily imp

providing fire department access to flat roofs.

• The current code (2003) permits such access from a stair, ladder o

not intended for rescue. There is no prohibition of locking this acc

fire department use since they have the means to open locked doo

• There were at least two decedents who had tried to get to the 

access locked to both the WTC towers. In addition, a PANYNFloor 105 of WTC 2 was unable to walk down the stairs, or go on radio by another PANYNJ employee (PANYNJ Channel Y).
Roof Evacuation (2) Roof Evacuation (2)

• The NYPD and FDNY policies for roof operations wer

on providing emergency responders with acce

s

s into 

the fire floors for firefighting, conventional rescue, anoccupants. Roof rescue was considered a m

easu

re o

used, for example, to assist occupants with medical e

• The NYPD aviation unit arrived at the WTC

site soon afte

attacked. Despite repeated attempts to examine the posaccess/rescue, smoke and heat conditions at the top of tprevented the conduct of safe roof operations. 

• Due to the limited capacity of a typical helicopter and 

rescue was never considered as a viable strategy for evacuation; even if it had been possible for a helicop

to the roof, possibly only a very small fraction of the people trapped above the impact zone could have beSeptember 11, 2001.
Public Address System Announce

Public Address System Announce

• Damage to the 22nd floor communication closet likely disabled th

announcement capability in WTC 1. The closet was located in a elevator shaft in the core of the building. Many announcement athe lobby command station.

• Announcements in WTC 2 were heard by occupants building-wid

aircraft struck at 9:03 am. Announcements were also heard in a(including above the impact area) after the second aircraft struck 

•

At 9:00 am an announcement stated “There is a fire condition 

secure. Please return t

o your offices.”

•

At 9:02 am an announcement stated “

May I have your attentio

is in Building 1. However, if conditions on your floor warrant, an orderly evacuation.”

•

At 9:20 am an announcement was made updating occupants 

building and progress of the evacuation and infor

ming occupa

to leave, they could then use t

he concourse.

•

Prior to 9:37 am an announcement instr

ucted occupants to “g
Emergency Responder Operations Emergency Responder Operation

Situational Awareness:

• Emergency responders working outside the WTC build

building conditions and communicate over radios had aawareness.

• Situational awareness for personnel who observed the 

fires from outside the buildings before entering experie

maintaining their awareness after entering the building

• Emergency responders working inside of the WTC buil

see what was happening outside and had poor radio copoor situational awareness. 

• Emergency responders working inside of the WTC buil

see what was happening outside and had good radio cbetter situational awareness than those with poor radio 
FDNY Access to the WTC Towers FDNY Access to the WTC Towers

• After aircraft impact, only two elevators out 

operating inside the two WTC towers. WTC lobby to the 16th floor. WTC 2, from the lobby t

• The stairways were filled with occupants evacu

buildings. 

FDNY personnel and other emerg

responders reported difficulty attempting to stairs due to this counterflow.

• Counter flow in the staircases made it difficult fo

responders to carry equipment up the stairways

• Counter flow in the staircases caused teams of 

responders to become separated, causing deladisrupting team operations.
Emergency Responders & High Emergency Responders & High

-Rise B

Rise B

• First responding FDNY units took from 4 to 10 minutes 

complex. They then got their equipment and received aanother 3 to 5 minutes. Time to begin operations 7 to 

• Of the 27 emergency responders interviewed that were 

maximum floor height achieved before WTC 2 collapsehour 13 minutes.

1 –

A police officer carrying no extra equipment and in a patr

uniform climbed to the 44th floor.

8 –

Emergency responders (FDNY, PAPD, NYPD) climbed to 

Two FDNY took an elevator to the 16th floor.

16 –

Emergency responders (mostly FD

N

Y) climbed to the 20

2 –

Emergency responders (NYPD) climbed to the teens.

• Estimated climbing rate based on a 60 minute climb

maximum height: 1.4 to 2 minutes/floor
High-Rise Buildings & Emergency 

Rise Buildings & Emergency 

Example: Fire department response to a 60 story high-ris

trapped above fires on the 58th floor and no opera

L

F

Firefighters carrying equipment and wearing PPE ~ 125 minutes

Firefighters carrying no equipment and not

wearing PPE ~ 90 minutes

Firefighters carrying equipment and

wearing PPE ~ 70 minutes

Firefighters carrying no equipment and not

wearing PPE ~ 50 minutes

Firefighters begin to climb 10 minutes

Fire department arrival 4 minutes 
Radio Communications in High Radio Communications in High

-Ri

Ri

•

Challengi

propagatiand reinfobuildings.

• Large sca

•

Numb

•

Comm

and pr

•

Surge 

• Interopera

communicamong difresponde

• Identificat

Schemat first respo

ic of W

TC Radio Repeater System
Analysis of Emergency Communic

Analysis of Emergency Communic

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

increase in traf

fic level over the normal level of emergen

communications, 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 nor u

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

the WTC

site was operating, although communications p

perceived in W

TC 1.

• NYPD aviation unit personnel reported critical infor

mation ab

collapse of the WTC towers several minutes prior to their cohas been found to suggest that the information was furthall emergency responders at t

he scene.
WTC High WTC High-Rise Radio Repeater Sy

Rise Radio Repeater Sy

• Analysis of the FDNY City-wide, high-rise, channel 7 (PAP

repeater recording indicates that the World Trade Center was operating.

• At approximately 9:05 a.m. the repeater’s recording system re

Lobby Command Post attempts to check repeater operationstalkie radio communications were recorded.

• It is possible that one or both of the following conditions comp

that took place at the WTC 1 Lobby Command Post:

• The radio repeater handset earpiece was broken.• The radio repeater handset volume was not turned up.

• It is unlikely that the repeater’s antenna was broken or m

since radio signals were received during the radio check and the communications that followed from inside WTC 2was functioning, it is possible the quality of communicat

• The repeater’s system recorded radio communications that to

several different firefighters and several different FDNY officeinside WTC 2.
Command and Control Command and Control

• Emergency responders— including key incid

ent comma

adequate information (voice, video, and data) on, nor an of, the conditions in the WTC buildings and what was haat the WTC

site. Inter

agency information sharing was in

• FDNY command and control was seriously affected by th

communications.

• A preponderance of evidence indicates that lack of timel

sharing and inadequate communication capabilities likeloss of emergency responder lives. Statement extracted fresponder interview: If communications were better, more have been saved.

• Large numbers of fire fighters were 

dispatched to the W

adequate command posts and staf

f could be assembled 

• Self-dispatch complicated command and control at the s

command and control was affected by many self-dispatched ambulance units that contributed to clogging the street

s so th

assigned to the WTC had difficulty getting through.

• FDNY apparatus had to be moved to allow some ambulance

exit the site with victims.
Command and Control (2) Command and Control (2)

• FDNY’s system for maintaining records of u

nit assignments at each 

capable of managing the numbers of units a

nd personnel assign

ed 

• FDN

Y, NYPD, and PAPD: there was no means to back-up the unit as

generated at the com

mand posts.

• Interagency operations were detrimentally affected with the loss of 

that was located inside WTC 7 due to the decision made to evacuatam before WTC 2 collapsed. First person interview data and photogra

functions became dispersed, the computer systems and other supporting 

unified operations structure was diminished. OEM personnel were work

responder departments and were located at the various department com

• A significant amount of evidence (first person interviews, reports, and ph

• In general, all departments attempted to work together to save 

and protect the citizens of New Yor

k City on the mor

ning of Se

• At times some issues related to a given department’s operational re

competitive nature of departments did exist during the WTC operatiexperienced were due to personnel not understanding the operating 

• Emergency responder interviews suggest that inter-agency compet

operatio

ns at the WTC complex b

efore the towers collapsed.
Mobility Impaired Occupants Mobility Impaired Occupants

• As the emergency responders started evacuating WTC 

of WTC 2, they found mobility impaired occupants still idown.

• Ambulatory mobility impaired occupants typically walke

one hand on each hand rail and took one step at a time addition, they were typically accompanied by one persoor an emergency responder. This blocked others behinmore rapidly down the stairs.

• FDNY and PAPD personnel found 40 to 60 mobility 

on the 12th floor of WTC 1 as they went down and at

each floor on their way out. These impaired individuaon this floor in an attempt to clear the stairways.

• Emergency responders were assisting approximate

mobility impaired people down the staircase just prof WTC 1. It is unknown how many fatalities occurred w
Factors that Enhanced Life Safety 

Factors that Enhanced Life Safety 

September 11, 2001 September 11, 2001

• Since the buildings were occupied by only about 1/3 of the bui

25,000 occupants, the egress capacity (number and width of eadequate for those survivors seeking and able to reach and us

stairwa

ys.

• Functioning elevators in WTC 2 enabled nearly 3,000 occupan

to aircraft impact.

• Greater remoteness of stairwells in the impact areas of W

TC 2 

stairwells to remain marginally passable after air

craft impact.

• Participation of a large number (two-thirds) of surviving occ

the prior 12 months, with almost all of those (93 percent) inlocation of the nearest stairwell. 

• Upgrades made to the life safety system components after the 

• Evacuation assistance provided by emergency responders to b

• As a result of t

he above factors, approximately 87 percent 

occupants, including more than 99 p

ercent below t

he floo

to evacuate successfully.
Future Practices and Technologies 

Future Practices and Technologies 

Potentially Could Have Improved L

Potentially Could Have Improved L

on September 11, 2001 (Requires A

on September 11, 2001 (Requires A

• Improved performance to delay or prevent building colla• Improved stairwell integrity via increased remoteness o

enhanced structural integrity of stairwell enclosures.

• Better communications to occupants and among first re

improved systems and timely information sharing.

• Better command and control for large-scale incident ma

location of command posts and physical assets; interag

• Better evacuation training (e.g., practice stairwell evacu

presently feasible as a standard option, existence of tra

• Other life safety features (e.g., fire protected and structu

elevators available for occupant use during emergencieelevators such as those used in seismic regions; self-evfor mobility impaired occupants; operational smoke and 
Procedures and Practices Procedures and Practices

• Applicable building codes and policies

•

NYC codes; PANYNJ policy and rationale; PANYNJ 

•

Comparison with contemporaneous codes

• Building design and construction

•

Structure; fire safety; egress; elevators

•

Design criteria and requirements

•

Comparison with code requirements

• Innovative features; code variances•

Fabrication and construction inspections

• Maintenance and modifications

• Local laws; modifications after 1993 bombing•

Tenant alteration review manuals

•

Condition surveys and structural inspections

• WTC 7 Fuel System
Condition Surveys and Structural 

Condition Surveys and Structural 

• PANYNJ developed the Standards for Structural Integrity 

Trade Center Towers A & B in 1986.

• Beginning in 1990, PANYNJ implemented a systematic fa

program for the WTC Towers. Prior to 1990, both WTC 1 inspected occasionally.

• WTC 7, which was not owned by PANYNJ, was also inspe

criteria in the Standards.

• The condition survey program included:

•

WTC 2 condition survey, 1990, PANYNJ Engineering Q

ualit

•

WTC 1 con

dition survey, 1

9

91, Office of Irwin G. Cantor, Con

•

WTC 7 survey, 1997, Ammann & Whitney

•

Due diligence physical condition survey of WTC 1 and WTC 

• Periodic inspections under the Structural Integrity Inspect

conducted by LERA and other engineering firms.
Structural Integrity Inspection Pro

Structural Integrity Inspection Pro

• In gener

al, the structural integrity inspections (SII) found that th

WTC 1, 2, and 7 wer

e in good condition. The inspection consul

routi

ne and s

ome priority recommendations for repairs to the P

• The SII reports identify some of the same deficiencies from rep

missing fireproofing on structural steel members. 

• Accordi

ng to the PANYNJ, all of the construction records on repairs 

were lost on September 11, 2001. Thus, it cannot be determined wrecommended repairs were performed.

• Fireproofing w

as reported to be missing or dama

ged in the flo

ors

b

e

B1 to B6 of WTC 1 and WTC 2) and in the hat-truss zone. 

• Fireproofing on beams and col

umns was also found missing or dam

The causes for missing and damaged fireproofing were attributed to elevator cabs and elevator cables hitting fireproofing on structural mthat a l

arge portion of missing or damaged fireproofing in elevator co

levels of the tow

ers.
Structural Integrity of Means of Eg

Structural Integrity of Means of Eg

• Building codes lack minimum stru

ctural integrity pro

visions for th

(stairwells and elevator shafts) in the building core that are critica

• In most tall buildings the core is designed to be part of the vertical grav

the structure. However, in many of those buildings, especially in regio

not dominant, the core may not be part of the lateral load carrying systthe core may be designed to carry only vertical gravity loads with no caloads, i.e., overturning moment and shear loads. In such situations, thprefer the use of partition walls over structural walls in the core area to 

• The decision to have the core carry a specified fraction of the lateral de

of a dual system to carry lateral loads, each of which would enhance th

core if structural walls were used, is left to the discretion of the structur

• Alternatively, stairway/elevator cores built with concrete or reinforced c

not part of the lateral load carrying system, may be able to provide suff

they meet, for example, ASTM E1996-03, or other more appropriate te

• In the case of the WTC towers, the core had 2 h fire-rated partition wa

integrity, and the core framing was required to carry only gravity loads. minimum structural integrity requirement to satisfy normal buildin

con

sideratio

n

s, it is con

ceivable that the damage to stairway

s in t

especially above the floors of impact, ma

y have been less extensi
Selected Findings on Codes and P

Selected Findings on Codes and P

• The 1968 New York City Building Code was comparable t

era (1964 New York State, 1967 Chicago, and 1965 BOCcode).

• Docum

ents suggest that the WTC towers generally we

maintaine

d consistent with the req

uirements of the 19

Building Code. Areas of concern include

d fireproofin

system, height of tenant separation walls, and egress assembly use space for the Windows of the World in WWorld observation deck in WTC 2.

• The PANYNJ developed and periodically updated a Tena

Manual that contained the technical criteria, standards, anguide modifications and alterations over the life of the buil

• The PANYNJ developed and implemented a formal facility 

structural integrity inspection program. The buildings genein good condition. Frequent recommendations were madmissing fireproofing at different locations, particularly in elbelow the plaza level.
Disclaimer No. 1

Certain commercial entities, equipment, products, or materials adocument in order to describe a procedure or concept adequateof the procedures and practices used. Such identification is not recommendation, endorsement, or implication that the entities, pequipment are necessarily the best available for the purpose. Nidentification imply a finding of fault or negligence by the Nationaand Technology.Disclaimer No. 2The policy of NIST is to use the International System of Units (mpublications. In this document, however, units are presented in pound system, whichever is prevalent in the discipline. Disclaimer No. 3:Pursuant to section 7 of the National Construction Safety Team Ahas determined that certain evidence received by NIST in the coInvestigation is “voluntarily provided safety-related information” t

related to the building failure being investigated” and that “disclo

would inhibit the voluntary provision of that type of information” [

In addition, a substantial portion of the evidence collected by NIS

Investigation has been provided to NIST under nondisclosure ag
Disclaimer No. 4:

NIST takes no position as to whether the design or construction compliant with any code since, due to the destruction of the WTC 

not verify the actual (or as-built) construction, the properties and 

materials used, or changes to the original construction made ovebuildings. In addition, NIST could not verify the interpretations o

applicable authorities in determining compliance when implemenWhere an Investigation report states whether a system was desirequired by a code provision, NIST has documentary or anecdotwhether the requirement was met, or NIST has independently coanalyses indicating whether the requirement was met.Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a st

an investigation under the National Construction Safety Team Acsuit or action for damages arising out of any matter mentioned in 281a, as amended by P.L. 107-231).
Web site Web site http:// http://wtc.nist.go

Email to Email to wtc@nist.gov

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

Regular mail: Regular mail:

WTC Technical Information Repository

Technical Information Repository

100 Bureau Drive, Gaithersburg, MD 2

Bureau Drive, Gaithersburg, MD 2
Background Slides Background Slides
Challenges Addressed Challenges Addressed

• Strong, conflicting public perspectives• Competing collapse hypotheses• Need for significant advances in state-of-the-art

• Need for developing NCST rules, human subjects 

• Coordination with 9/11 Commission and local auth• Blending “investigation” needs with “research” cult• Leading and managing an extended inv

estigation 

(85 staff from 3 NIST Laboratories; 124 contractorexperts; 27 secretariat/institutional support staff)
The Extended WTC Investigation T

The Extended WTC Investigation T

•

WTC Investigation Team 

•

Other NIST Technical Staff 

•

Secretariat/Institutional Support 

•

Contractor Staff 

•

Experts (Contract/Employee) 

=

Total 
NCST Advisory Committee NCST Advisory Committee

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, and draft recommendations at 6 meetings. 

z Reviewed all WTC progress reports and 

final reports

z Membership balances broad scope of 

disciplines and interests

MembersDr. Charles 

Thorn

Dr. Robert 

Dr. Robert 

Unive

Unive

Mr. Philip D

Mr. Philip D

Assoc

Assoc

Professor G

Colleg

Colleg

Dr. Kathlee

Dr. Kathle

e

Colora

Col

or

Mr. Paul Fi

Mr. Paul F

(retire

Mr. David C

Mr. David C

Professo Professor F

r F

of Ca

of Ca

Dr. Joh Dr. J

ohn B

Consu

Consu
Examples of Extensive Media Cove

Examples of Extensive Media Cov

• Wire Serv

• Assoc• Reuter• United 

• TV and Ra

• CBS E• CNN In• WABC• WNBC• Fox Ne• NY1• C-Spa• NPR• WNYC 

• Metro 

• Newspapers

• The New York Times• The Washington Post• The Wall Street Journal• The International Herald Tribune

• The Guardian (UK)• New York City newspapers• Major U.S. newspapers• International newspapers

• Magazines and Books

• The 9/11 Commission Report• City in the Sky (Gl

anz & Lipton)

• 102 Minutes (Dwyer & Flynn)

• Engineering News-Record• Popular Mechanics
Critical Analysis Inter Critical Analysis Inter

-Dependencie

Dependencie

Colla

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

Fireproofing

Damage

Structural

Damage

Ga

T

Tem

Resolution1-4 i

n.

10-6 s

Aircraft Impact 

Damage Analysis

LS-DYNA

Fir

An

The

A

Struc

and F

A

Baseline Performance Analysis

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude
Photographic Evidence of Hanging 

Photographic Evidence of Hanging 

Amount and extent of floor sag increased over the 51 
Application of SFRM to External C

•

By design, unifor

m thickness

•

As applied, region between flange tips filled

(for example, see column 246 at right)

•

Missing SFRM from outer flange indicated by

shadowing and exposed red paint
Damaged SFRM -

North Face of W

Left Side of Impact Hole
Findings on Probable Collapse Se

Findings on Probable Collapse Se

• Inward bowing and primary tilting direction at collapse initiation 

• Not observed on face where aircraft impact occurred.• Observed on face parallel to the longer dimension of the building c• Observed on face associated with less stiff (longer period) directio

• Occurred on face associated with long span direction of floor syste

capacity higher in long span direction.

• Inward bowing on South face and Southward tilting for W

TC 1

• Extent of fires similar on all faces; somewhat more extensive on E

in extent on North and South faces.

• Although North face had extensive impact damage, inward bowing 

buildi

ng tilted to South at collapse initiation.

• Thermal weakening of perimeter columns with floor sagging (which 

occurred on South side) caused inward bowing on South face and 

• Inward bowing on East face and primar

y tilting towards East fo

• Fires more extensive on East face; less extensive on North face a

significant on East side of both faces; no observed fires on West fa

• Although South face had extensive impact damage, inward bowing 

building tilted more to the East and less to the South at collapse in

• Thermal weakening of perimeter columns with floor sagging cause

face and primary tilting in that direction (with additional Southward 

impact damage).
Findings on Probable Collapse Seq

Findings on Probable Collapse Seq

• The time it took for each WTC tower to collapse was due 

•

Asymmetric structural damage resulting from air

craft impact i

WTC 1; and greater damage (severed or heavy damage) to cthan in WTC 1; higher aircraft speed/impact ener

gy and impa

span resistance and off center position) caused greater WTC 

•

Time it took for the fires, in combination with aircraft impact d

core.

•

Time it took for fires to traverse from their initial location to th

towers, and:

• time it took for heat to weaken and sag floor system, resulting in in

• time it took for heat to weaken perimeter columns observed to be b

collapse of each tower (traverse time of fires to South face of WTC East face of WTC 2 where fires already existed).

•

Time it took for heat to weaken and buckle those perimeter c

simultaneously subject to inward pull forces and, to a lesser eadditional vertical loads redistributed from core.
Findings on Probable Collapse Se

Findings on Probable Collapse Se

• The time to destructive heating was determined by the fir

intensity was determined by the large mass and wide distributnature and (rather low) loading of combustibles, 

t

he sparsen

surviving building partitions, and the ease wit

h which win

allowing oxygen to feed the fires.

• Separate analyses showed that heating of str

uctural mem

sensitive to effect of dislodged fireproofing due to debris than to episodic regions of missing fireproofing or thinnefire-affected region. As-built fireproofing conditions elserole in fire-induced collapse. 

• Debris field generated by aircraft impact r

emoved significant f

board enclosures, as well as some of the walls. Structural co

thermally weakened were generally determined by impact of t

fireproofing not been dislodged by debris field, temperatucomponents would likely have been insufficient to induce 
Findings on Probable Collapse Se

Findings on Probable Collapse Se

• Role of the Building Core

•

Core weakened significantly due to aircraft impact damage a

(thermal effects dominated WTC 1; aircraft impact damage d

• Loads redistributed to perimeter faces; additional axial loads 

not significant (only about 20-25 percent on average)

• Role of the Building Floors

•

Primary role was to provide inward pull forces that induced in

on exterior face (

South face of 

W

TC 1; East face of WTC 2).

•

Sagging floors continued to support floor loads despite exten

fireproofing; there would be no inward pull forces if floors had 

• Role of Exterior Frame-Tube

•

Column instability over an extended region of the exterior fac

system failure (loads could not be redistributed via hat truss tbuilding core; load transfer via spandrels propagated column faces) causing initiation of building collapse.

•

Column instability induced by thermal weakening of columns

sagging floors, and to a much lesser degree, additional axial 
Findings on Probable Collapse Se

Findings on Probable Collapse Se

• Performance of WTC floor system with intact firepro

• WTC 1 did not collapse during the major 1975 fire which engulf

Southeast quadrant of the 11th flo

or and spread mostly via utility 

time, office spaces in the WTC towers were unsprinklered. The fire 

to the flo

or system, and at no time was the load carrying cap

ac

compromised. The fire “did not damage a single primary, fireproof

chord members (not needed for structural integrity), some bridging floor tremor and the like) and some deck support angles (used only 

were buckled in the fire—all were unfireproofed steel.” (SCHR Lette

• The load carrying cap

acity of the flo

or system was n

ot compro

temperatures in an

y of the four fire resistance (ASTM E 119) tes

2004 up until the time they were stopped which was approximately were about twice those on September 11, 2001. The high tempewere at least as severe and lasting as the WTC fires (although the theated)

• A detailed thermal-structural analy

sis, whi

ch did not include sl

effects,

showed that a full collapse of the WTC flo

or system wo

number of failed trusses or connections.

• The horizontal and vertical capacity of the floor connections to the p

exceeded the demand under design load conditions.
Role of Fire Resistance Tests Role of Fire Resistance Tests

• The fire resistance tests cannot be used to det

ermine the 

the floor systems in t

he collapse of t

he WTC towers, nor c

whether or not t

he actual floor systems as built met code 

the PANYNJ could have taken the highly unusual step of rstructure to Class 1-C, with a 1-1/2 hour required rating forating f

or columns, w

hen installation of the sprinkler syste

prior to September 11, 2001.

• The fire resistance tests provided valuable insights into the be

for use in analyzing the thermal r

esponse and collapse of the W

• The occurrence and spread of jet-fuel due to the terrorist attac

2001 ignited multi-floor fir

es in the WTC towers. These fires w

from the fires to which floor systems in standard U.S. fire ratinConsider, for example:

•

Combustible fuel l

oad of the hij

acked jets.

•

Extent and number of floors involved i

n fires.

•

Rate of fire spread across and between floors.

•

Ventilation conditions i

n the fire-affected floors.

• The probable collapse sequence for the WTC towers are base

thermally weakened structural components that had extensive gypsum board fir

e protection induced by the debris field gener
Limitations and Uncertainties in D

Limitations and Uncertainties in D

Probable Collapse Sequences Probable Collapse Sequences

• NIST recognized the inherent limitations and uncertaint

performed for determining the probable collapse seque

• As-built condition of the WTC towers, as well as occupancy and us

• Estimated aircraft impact damage to WTC towers (structure, partit

fuel dispersion, m

aterial failure criteria, model size limitations and 

from exterior of buildings

• Estimated growth and spread of building fires (fuel load from build

ventilati

on within the core, compartmentation, fire dynamics model 

uncertainties), especially interior building fires

• Estimated response of the aircraft-impact damaged WTC tower str

fireproofing damage, material and structural failure criteria, therma

limitations and uncertainties)

• NIST validated the probable collapse sequences wit

evidence, carefully considering the sensitivity of its uncertainties.
• New York City Local Law 16 (1984) amended the 1968 

require the minimum distance between exit door openin30 feet or one-third the maximum travel distance of the greater.

• IBC 2003, NFPA 5000, & the NFPA Life Safety Code re

stairwells on any floor shall be located at least:

• One-third of the diagonal of the area served, if fully • One-half of the diagonal of the area served, if not f

measured from door edge to door edge along a straight walking path between the doors, if the corridors have wleast one hour.

• Tenant floors in WTC 1 and WTC 2 had a diagonal dista

Therefore, one-third of the diagonal distance would havhalf of the diagonal distance would have been 147 ft. TWTC Towers were two hour rated; the 1996 due diligenthat some of the corridor separation walls did not run slahave been considered unrated since they terminated at 

Findings on Stairwell Remoteness Findings on Stairwell Remoteness
Uncertainties and Limitations Asso

Uncertainties and Limitations Ass

Evacuation and Emergency Respo

Evacuation and Emergency Respo

• NIST recognized the inherent limitations and uncertain

of the evacuation and emergency response:

• Human factors associated with building occupants (training; situa

evacuation initiation tim

e)

• Human factors associated with emergency response (situational 

communications; command and control; interagency coordinatio

• Effect of building design on evacuation and emergency response 

elevators; sprinkler and standpipe systems; self-evacuation systemergency com

m

unication systems)

• In developing its findings and results, NIST carefu

these uncertainties within the context of available from documents, recordings, first-person interviewvisual data.
Applicable Building Codes and Po

Applicable Building Codes and Po

• Although not required to conform to NYC codes, the PAN

the provisions of the proposed 1968 edition of the NYC Bthan three years before it went into effect. 

• The proposed 1968 edition allowed the PANYNJ to take 

restrictive provisions and of technological advances 1938 edition that was in effect when design began for th1962. The 1968 code:

•

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 fir

e rating of the shaft walls in the building cor

•

Changed partition loads from 20 psf to loads based on wei

length (that reduced such loads for many buildings includin

•

Permitted a 1 h reduction in fire r

ating for all structural com

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

•

Permitted wind tunnel testing using models to establish des

load.
Applicable Building Codes and Po

Applicable Building Codes and Po

• The NYC Department of B

uildings reviewed the WT

1968 and provided comments to the PANYNJ concernrelation to the 1938 NYC Building Code. The architectto the PANYNJ responses to those comments, noting hconformed to the 1968 NYC Building Code. All of the the NYC review appear to deal with egress issues, not innovative features of the buildings.

• In 1993, the PANYNJ and the NYC Department of Buildings e

memorandum of understanding that:

• Restated the PANYNJ’s longstanding policy to assur

the City of New York meet and, where appropriate, exrequirements of the NYC Building Code.

•

Provided specific commitments to the NYC Department o

procedures to be undertaken by the PANYNJ to assure toperated by the PANYNJ are in conformance with the Bucontained in the NYC Building Code. 
Applicable Building Codes and Po

Applicable Building Codes and Po

• In 1993, the PANYNJ adopted a policy providing for implemen

recommendations made by local government fir

e departments 

inspection of a PANYNJ facility, and for the prior review by loc

fire safety systems to be introduced or added to a facility. Late

entered into an agreement with FDNY which reiterated the polPANYNJ and set forth procedures to assure that new or modifare in compliance with local codes and regulations.

• While the PANYNJ entered into agreements with the NYC Dep

the 1990s with regard to conformance of PANYNJ buildings co

City to the NYC Building Code and sought review and concurragreements, the PANYNJ was not required to yield, and ap

approval authority to New York City.

The PANYNJ was cre

entity, a “body corporate and politic”, under its charter, pur

sua

of the U.S. Constitution permitting compacts between states, anongovernmental and quasi-governmental entities in the Unitebuilding and fire safety code requirements of any governmenta
Standards for Structural Integrity I

Standards for Structural Integrity I

• Statistical inspections:

•

Periodic visual inspection of selected structural compone

trouble prone areas” supplemented by occasional visual structure was exposed during tenant remodeling or gene

• Review of maintenance and tenant complaint reports:

•

Examination of various reports to shed light on underlying 

maintenance reports of non-structural repairs; water leakcomplaints about unusual building movements, vibration, 

• Building movement and deformation measurements:

•

Perfor

mance of systems within buildings evaluated throu

movement or deformation using appropriate tests and insMeasurements were to be perfor

med on individual compo

well as on the entire towers themselves.
Retention of Building Documents Retention of Building Documents

• State and local jurisdictions do not require retention of 

the design, construction, operation, maintenance, and buildings, with few exceptions. These documents are building owners, contractors, architects, engineers, and 

• Building documents are not archived for more than 

and there are no requirements that they be kept in physically remote from the building throughout its 

• In the case of the WTC towers, the PANYNJ and its 

consultants maintained an unusually comprehensidocuments, a significant portion of which had not the collapse of the buildings but could be assemblthe NIST investigation.

• In the case of WTC 7, several key documents could no

they were lost in the collapse of the building. 
Roles of Architects and Engineers Roles of Architects and Engineers

• Consistent with the practice at the time, the code architect was respon

protection and designing the egress system for the WTC Towers in acprescriptive provisions of the N

YC building code. 

• The architect and owner engaged the services of structural engin

structural design and to ensure that his/her design was properly i

• At that time the fire protection engineering profession was not suf

the same standard of care employed with the structural design. 

• There is no reason to believe that the involvement of a fire protec

would have resulted in any differences in the design or performansystems.

• Today, particularly when designing a building emplo

ying innovati

involvement of a fire protection engineer in a role similar to the st

under the overall coordin

ation of the Design Professional in Resp

to the standard of care.

• The technical base and sophistication of the practice of fire protec

well advanced of where it was during the design and construction 

• When designing the structure of selected tall buildings or selecte

fires, or evaluating the fire resistance of such structures, it is essengineer and the fire protection engineer to jointly provide the

ne
Standard Fire Standard Fire-Resistance Tests Resistance Tests

• Code provisions with detailed procedures do not exist to a

data from fire resistance tests of other building componenqualify an untested building element. 

• Based on available data and records, no technical basis w

the spray-applied fire resistive material (SFRM) used (two were under evaluation) or its thickness for the large-span of the WTC towers. 

• The assessment of the fireproofing thickness needed to meet 

requirement for the untested WTC floor system evolved over t

• In October 1969, the PANYNJ directed the fireproofing contra

fireproofing to the floor trusses. 

• In 1999, the PANYNJ issued guidelines requiring that fireproo

for full floors undergoing alterations.

• Unrelated to the WTC buildings, an International Conference 

Evaluation Service report (ER-1244), re-issued June 1, 2001, recommends a minimum thickness of 2 in. for “unrestrained sconcrete” slab.
Standard Fire Standard Fire-Resistance Tests (2) Resistance Tests (2

• Code provisions are needed to require the conduct 

test if adequate data do not exist from other buildinassemblies to qualify an untested building element

• Instead, several alternate methods based on other fire

calculations or alternative protection methods are permguidance on detailed procedures to be followed. 

• Both the architect-of-record (in 1966) and the struc

record (in 1975) stated that the fire rating of the flooWTC towers could not be determined without testin

• NIST has not found evidence indicating that such a 

to determine the fire rating of the WTC floor systeminformed NIST that there are no such test records i
Fire-Resistance Ratings (1) Resistance Ratings (1)

• Use of the “structural frame” approach, in conjunction with the 

would have required the floor trusses, the core floor framing, athe WTC towers to be 3 h fire-rated, like the columns for Class 1968 NYC Building Code. 

• Neither the 1968 edition of the NYC Building Code which was u

WTC towers, nor the 2001 edition of the code, adopted the “strrequirement. 

• The “structural frame” approach to fire resistance ratings requires s

than columns, that are essential to the stability of the building as a wthe same rating as columns. 

• This approach, which appeared in the Uniform Building Code (a mo

as 1953, was carried into the 2000 International Building Code (one codes) which states: 

“The structural frame shall be considered to be the columns antrusses and spandrels having direct connections to the columndesigned to carry gravity loads.” 

• The WTC floor system was essential to the stability of the building a

lateral stability to the columns and diaphragm action to distribute withe frame-tube system. 
Fire-Resistance Ratings (2) Resistance Ratings (2)

• A technical basis is needed to establish whether the construction class

requirements in modern building codes are risk-consistent with respecand the consequences of that hazard.

• The fire rating requirements, which were originally developed based on expe

about 20 stories in height, have generally decreased over the past 80 years buildings suggested considerable conservatism in those requirements. 

• For tall buildings, the likely consequences of a given threat to an occup

more severe than the consequences to an occupant, say, on the first fl

• It is not apparent how the current height and area tables in buildin

technical basis for the progressively increasing risk to an occupa

buildings, that are much greater than about 20 stories in height, w

without the availability of firefighter elevators is limited by physio

• The maximum required fire rating in current codes applies to any buildi

in height. There are no additional categories for buildings above, for ex

stories, where different building classification and fire ratings requiremerecognizing factors such as the time required for stairwell evacuation w(e.g., due to power failure or major water leakage), the time required fo

without functioning elevators, the presence of sky lobbies and/or refuge 

height of elevator shafts. 

• The 110-story WTC towers, initially classified as Class IA based on the 1938 

classified as Class 1B before being built to take advantage of the provisions 

This re-classification permitted a reduction of 1 h in the fire rating of the comh and floor framing members from 3 h to 2 h).
Fire-Resistance of Structural Conn

Resistance of Structural Conn

• Code provisions are needed to ensure tha

connections are provided the same degreprotection as the more restrictive protecticonnected elements.

• The provisions that were used for the WTC t

7 did not require specification of a fire rating connections separate from those for the conn

• It is not clear what the fire rating of the co

the WTC towers were when the connectindifferent fire ratings and whether the applachieved that rating. 
Fireproofing: Field Application and 

Fireproofing: Field Application and 

• Rigorous field application and inspection provision

requirements are needed to assure that the as-built passive fire protection, such as SFRM, conforms to fire-resistance tests of building components and as

•

Provisions are not available to ensure that the as-ap

fireproofing thickness and variability (reflecting the qis thermally equivalent to the specified minimum fire

•

Requirements are not available for in-service inspec

protection during the life of the building. 

• The adequacy of the fireproofing of the WTC towers po

concern to the PANYNJ over the life of the buildings, anaccepted requirements and procedures for conducting iwould have provided useful guidance.
Fireproofing: Requirements for In Fireproofing: Requirements for In

-

Performance Performance

• A technical basis is needed to establish wheth

mechanical and durability related properties osufficient to ensure acceptable in-service perfbuildings. 

• While minim

um bond strength requirements e

serviceability requirements for such materials typical shock, impact, vibration, or abrasion eof a building. 

• There are existing testing standards for determini

properties, but the technic

al basis is insufficient to 

serviceability requirements. Knowledge of such srequirements would have assisted in assessing thfireproofing condition of the WTC towers. 
Fire Protection Hierarchy Fire Protection Hierarchy

• Building fire protection is based on a four-level hierarchical strate

suppression (spri

nklers and firefighting), compartme

ntation, and 

structure. 

•

Detectors are typically us

e

d to activ

ate fire alarms and notify 

emergency services. 

•

Sprinklers are designed to control small and medium fires a

beyond the typical water supply design area of about 1,500 

•

Compartmentation

mitigates the horizontal spread of more s

fires a

n

d typically req

uires fire-rate

d partitions f

or areas of a

firefighti

ng measures also cover up to about 5,000 ft

2 to 7,50

•

Passive protection of the structure seeks to ensure that a m

scenario, with sprinklers compromised or overw

helmed and 

results in burnout, not overall building collapse. The intent ofor the building to withstand local structural collapse until octhe fire service can compl

ete search and rescue operations
Compartmentation Compartmentation to Mitigate Fire 

to Mitigate Fire 

• The NYC Building Code and PANYNJ practice required partit

spaces from each other and from common spaces such as served the elevators, stairs and other common space

s in the b

• Local Law 5 (1973) required compartmentation of unsprinklere

office buildings over 100 ft in height “having air-conditioning anventilation systems that serve more than the floor on whi

ch the 

to be subdivided by 1 hour fire separations int

o spaces or 

exceed 7,500 f

t

2. Floor areas could be increased up t

o 15,

hour fire resistive construction and smoke detectors. 

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

compartmentation as a part of new tenant layouts as evidence

alteration contracts at the time. 

• Following the 1975 fire a fire safety consultant report recomm

the buildings be retrofit with sprinklers to address possible the PANYNJ realized that this would also obviate the need foand permit the unobstructed views for which the buildings were sprinkler the buildings left the arrangement again with only patenant spaces from each other and from exit access corridspaces in the core, and with shaft enclosures.
Compartmentation Compartmentation to Mitigate Fire 

to Mitigate Fire 

• Building codes typically require 1 h fire-rated tenant separations b

compartmentatio

n req

uirements (e.g., 12,000 ft2) for buildings with 

mitig

ate the horizontal spread of fire. This was the cas

e with both th

which did not require sprinklers in occupied spaces on or above the groBuilding Code, which requires sprinklers in Group E (Business) building

• The sprinkler option was chosen for the WTC towers in preferenc

e to th

in meeting the subsequent requirements of Local Law 5, adopted by Ne

• If there w

as o

nly one tenant on a WTC floor, there would b

e n

o hor

requirement. Con

versely, if there w

ere a large number of ten

ants o

highly compartmented with sep

aration w

alls. 

• The fire-affected floors in the WTC towers were mostl

y open—with 

perimeter offices and conference rooms and an occasional speciafloors had two tenants, and those spaces, like the core areas, were part

• Photographic and videographic

evidence confirms that even non-t

(such as those that divided spaces to provide corner conference rosubstantial resistance to fire spread in the affected floors. 

• For the duration of about 50 to 100 min prior to collapse of th

e

WT

were active, the presence of undamaged 1 h fire-rated compartmen

mitigating fire spread and consequent thermal weakening of struct
Sprinkler Systems Sprinkler Systems

• State and local b

uilding regulatio

ns are n

eeded that require insta

existing buildings on a reasonable time schedule, not as an optiocompartmentatio

n.

• Functioni

ng sprinkl

ers can provide significant improvement in safety f

fires and can prevent them from becoming large fires. 

• N

YC promulgated local laws in 1973 and 1984 to encourage insta

new buildings and is now considering a law to require spri

n

klers 

• The WTC towers were fully sprinklered by 2001, about 30 years after 

Sprinklering of the tenant floors in the WTC towers was completed by sprinklering of the sky lobbies was still underway at that time. 

• The sprinkler system in the WTC towers was installed in three phases

• Phase 1 was completed during initial building construction and in

areas. 

• Phase 2 was completed in 1976, consistent with Local Law 5, an

corridors, storage rooms, lobbies, and certain tenant spaces. 

• Phase 3 was begun in 1983 and completed in 2001 and resulted 

buildings.
Use of Elevators in Emergencies Use of Elevators in Emergencies

• With a few special exceptions, building codes in the United States 

protected elevators for routine emergency access by first respondmethod (after stairwells) for emergency evacuation of building occ

by emergen

c

y respo

n

d

ers would additionally mitigate cou

nterflow

• While the United States conducted research on specially protected 

the United Kingd

o

m along with several oth

er co

u

ntries that typical

have req

uired such “firefighter lifts,” located in protected shafts, fo

• Without functioning elevators, emergenc

y r

esponders carr

ying bet

gear req

uired about 1.4 minutes to 2 minutes per flo

or when using 

• While it is difficult to maintain this pace for more than about t

take an emergency responder between 1-1/2 to 2 hours to rea

floor of a tall building if that pace could be maintained. 

• Such a delay, combined with the resulting fatigue and phy

sica

responders that were reported on September 11, 2001, would rescue efforts difficult even in tall building emergencies not in

• Eac

h of the WTC towers had 106 elevators, and WTC 7 had 38 elevato

could not be used for fire service access or occupant egress during an 

not fire-protected, nor were they located in protected shafts. The elevat

normal modernization with fire service recall. All but one of the elevato

WTC tower by the aircraft impacts; though prior to the impact in WTC 2 functioning and contributed greatly to the much faster initial evacuation