Technical Conference on tBuilding and Fire Safety Inof the World Trade Center 

Session VI -

Structural Fire Respons

Collapse Analysis

Probable Collapse Sequence and K

September 15, 2005

Therese McAllister

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce
Probable Collapse Sequences Probable Collapse Sequences

NIST adopted an approach that combined mat

modeling, statistical analysis, laboratory expanalysis of photographs and videos, and accuncertainty in input parameters, analyses, aevents, to address: 

 The probable sequence of events from the m

impact until the initiation of global building co

 How and why WTC 1 stood nearly twice as lo

before collapsing (102 min vs 56 min).

 What factors, if any, could have delayed or p

collapse of the WTC towers?
Methodology for Determining the P

Methodology for Determining the P

Collapse Sequence Collapse Sequence

Identified key observables from evidence held by NIST.

 Over 7,000 photographs and 150 hours of video re

steel pieces, eyewitness accounts, and emergencrecords.

Developed hypotheses and refined through course of inveDeveloped and refined mathematical models (finite eleme

fluid dynamic analyses) for:

 Aircraft Impact  Fire spread and growth  Temperature histories  Structural response

Performed sensitivity studies to determine the influential p

component and subsystem levels to determine 3 sets influential parameters – lower, central, and upper bou
Methodology for Determining the P

Methodology for Determining the P

Collapse Sequence (cont) Collapse Sequence (cont)

Evaluated analysis results for each data set against obse

behavior, with adoption of event tree techniques, pruntree branches. 

 Perform analyses for parameter sets that provide 

damage.

 Compare results to observations. Discard (prune) branches that do not reasonably m Use observations to constrain model parameters a
Analysis Tree for All Influential Par

Analysis Tree for All Influential Par

Combinations Combinations

Tower before Impact

FDS FDS FDS FSI FSI FSI

FSI FSI FSI

FSI FSI FSI

FDS FDS FDS

FSI FSI FSI

FSI FSI FSI

FSI FSI FSI

FDS 

FSI 

FSI 

F

Base Impact Damage Combustibles, Partitions,Fire Protection,Structural

MoCoFirStr

Less Severe Damage Combustibles, Partitions,Fire Protection,Structural

(-) (0) 

(-) (0) (+) (-) 

(-) (0) (+)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

STRSTRSTR

STRSTRSTR

SSS

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STR

STRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)
Pruned Analysis Tree Pruned Analysis Tree

Tower before Impact

Influential 

Parameters 

Highly 

Correlated 

by 

Aircraft 

Impact

Results

Base Impact DamageCombustibles, Partition

s,

Fire Protection,Structural

Less Severe DamageCombustibles, Partition

s,

Fire Protection,Structural

FDS 

FSI 

Influential 

Parameters

Largely 

Independent 

of Aircraft

Impact Results

Pruned

Pruned
Variability in Sequential Analyses 

Variability in Sequential Analyses 

Imperfect Information Imperfect Information

GlobalReserveCapacityRC(t)

Time to c

initia

RC=0

Aircraft 

Impact

Cap

acity

Before 

Impact

RC(t=0)

Fire Growth and 

Structural Response

Cap

acity

After 

Impact

Events with Sig

Contribution to C

Global Cap

Variability in 

A

nalyses

0 10 20 30 ....

Observables Reduce 

Uncertainty
Limitations and Uncertainties in D

Limitations and Uncertainties in D

Probable Collapse Sequences Probable Collapse Sequences

NIST recognized the inherent limitations and uncertainties 

performed for determining the probable collapse seque

 As-built condition of the WTC towers, as well as occ Estimated aircraft impact damage to WTC towers (s

debris, fireproofing, jet fuel dispersion, material failulimitations and uncertainties) not observable from e

 Estimated growth and spread of building fires (fuel l

aircraft contents, ventilation within the core, compardynamics model size limitations and uncertainties), building fires

 Estimated response of the aircraft-impact damaged 

to the fires (extent of fireproofing damage, material criteria, thermal and structural model size limitations 

NIST validated the probable collapse sequences with avail

carefully considering the sensitivity of its results to these 
Structural Events Leading to Colla

Structural Events Leading to Colla

Four structural events were common to both towers Floor sagging

 Caused by elevated steel and concrete temp

• buckling of the truss web diagonals with dislo• disconnection of floor connections to the exte

 Substantial sagging resulted in pull-in forces 

connections, and led to inward bowing of the 

Bowing and inelastic buckling of an exterior wal

 Caused by pull-in forces from sagging floors, 

temperatures, and redistributed gravity loads
Structural Events Leading to Colla

Structural Events Leading to Colla

Core weakening 

 Caused by structural impact damage, redistributed 

temperatures, plastic and creep strains, and inelascolumns. 

 Downward displacement or shortening of the core 

hat truss.

Redistribution of gravity loads

 Resulted from impact damage, restrained thermal 

weakening, leaning of the tower section above the bowing of exterior walls. 

 Load redistribution between the core and exterior 

primarily through the hat truss. 

 Load redistribution between adjacent exterior wall

through the spandrels. 

 The hat truss restrained thermal expansion in the 

to the exterior wall and transferred loads between the core. 
WTC 1 Probable Collapse Sequenc

WTC 1 Probable Collapse Sequenc

Aircraft Impact Damage

 Exterior columns, floor sections, and core 

columns were severed through the building center. An exterior panel was severed at the south wall.

 Fire protection was damaged through the 

building center to the south exterior wall.

 Loads were redistributed mostly to 

adjacent columns and the hat truss resisted downward movement of the north wall.

 North and south walls carried 7% less 

load, east and west wall carried 7% more load, core carried 1% more load.
WTC 1 Probable Collapse Sequenc

WTC 1 Probable Collapse Sequenc

Severed or Heavily DamagedElastic + Plastic + Creep Strain

7.3

Col501 Col508

Col1001 Col1008

Severed or Heavily DamagedElastic + Plastic + Creep Strain

7.3

Col501 Col508

Col1001 Col1008

At 100 min

Thermal Weakening

 Core columns developed 

high plastic and creep strains, after initial thermal expansion. Hat truss resisted core column shortening.

 Long span south floors 

sagged and pulled inward on the south exterior wall after the fires reached the south side.

 The south wall bowed 

inward due to inward pull by intact sagging floors.

1

WTC1 FL96 

-22

NODAL SOLUSTEP=11SUB =8TIME=6000UZ (RSYS=0DMX =22.52SMN =-22.4SMX =3.108

1

MN

MX

Y X

Z

WTC1 Severe Temp at 6000s w/5kip pull - South Face (5X) 

-.701059

4.135

8.972

13.808

18.644

23.481

28.317

33.153

37.99

42.

MAR 30 2

10:23

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

COL301

COL359

FLFLFLFLFLFLFLFL

Max. = 4

2.8 
WTC 1 Probable Collapse Sequenc

WTC 1 Probable Collapse Sequenc

Collapse Initiation

 The south wall buckled and 

transferred its loads to the weakened core through the hat truss and to the adjacent exterior walls through the spandrels.

 The upper building sections 

began to tilt to the south as a rigid block. 

 Instability progressed along 

the east and west exterior walls.

 Collapse ensued as the 

released potential energy could not be resisted by the structure.
WTC 2 Probable Collapse Sequenc

WTC 2 Probable Collapse Sequenc

Aircraft Impact Damage

 Exterior columns, floor sections, and core 

columns were severed through the southeast corner and east side of the building.

 Loads were redistributed mostly to adjacent 

columns and the east exterior wall.

 Fire protection was damaged in the east 

side of the building.

 After impact, the core was leaning slightly 

toward the east and south walls. The exterior walls restrained the tendency of the core to lean through the hat truss and intact floors.

 East wall carried 24% more load, south and

west wall carried 2-3% more load, north wall carried 10% less load, and the core carried 6% less load.
WTC 2 Probable Collapse Sequenc

WTC 2 Probable Collapse Sequenc

1

MN

MX

X Y

Z

WTC2 FL82 - Maximum Damage Case Temperature at 3600 sec 

-49.44 -43.524 -37.608 -31.693 -25.777 -19.861 -13.945 -8.03 -2.114 3.

OCT 18 

13:4

NODAL SOLUTIONSTEP=7SUB =31TIME=3600UZ (AVG)RSYS=0DMX =49.572SMN =-49.44SMX =3.802

Thermal Weakening

 Several core columns on the 

east side developed high plastic and creep strains, after initial thermal expansion.

 The long-span east floors 

sagged and pulled inward on the east exterior wall shortly after impact, due to the damage and fires on the east side.

 About 1/3 of the remaining 

Floor 83 connections to the exterior wall failed.

 The east wall bowed inward 

due to inward pull by intact sagging floors.
WTC 2 Probable Collapse Sequenc

WTC 2 Probable Collapse Sequenc

1

MX

X

Y Z

 WTC-2 Severe Case Temperature Analysis

3.298

5.332

7.366

9.399

11.433

13.467

15.5

NODAL SOLUTIONSTEP=7SUB =1TIME=.007USUM (AVG)RSYS=0DMX =21.601SMN =3.298SMX =21.601

Collapse Initiation

 The east wall buckled and 

transferred its loads to the weakened core through the hat truss and to the adjacent exterior walls through the spandrels.

 The upper building sections 

began to tilt to the east and south as a rigid block.

 Instability progressed along 

the north and south exterior walls.

 Collapse ensued as the 

released potential energy could not be resisted by the structure.
Findings: Structural Response and 

Findings: Structural Response and 

Analysis Analysis

 The core columns were weakened significantly by 

damage and thermal effects. 

 Thermal effects dominated the weakening of WTC 

moved from the north to the south side of the coreweakened over time by significant creep strains on the core. 

 Aircraft impact damage dominated the weakening 

impact damage, the core subsystem leaned to the supported by the south and east perimeter walls vfloors. 

 The primary role of the floors in the collapse of the 

provide inward pull forces that induced inward bowcolumns. There would have been no inward pull foconnections had failed and disconnected.

 Column buckling over an extended region of the e

ultimately triggered the global system collapse as be redistributed through the hat truss to the alreadbuilding core. 
Principal Findings Principal Findings

Objective 1: Determine why and how WTC 1 and WTC 2 

the initial impacts of the aircraft.

 The two aircraft hit the towers at high speed and did con

principal structural components (

core columns, perimete

that were directly impacted by the aircraft or associated 

 However, the towers withstood the impacts and would h

were it not for the dislodged fire protection and the subsfires. 

 The robustness of the exterior frame-tube system and th

building

s helped the towers withstand the impact. The st

redistributed loads in places of aircraft impact, avoiding upon impact. The hat truss, which was intended to suppantenna atop each tower, prevented earlier collapse of t

 In each tower, a different combination of impact damage 

structural components contributed to the abrupt structura
Principal Findings (cont.) Principal Findings (cont.)

For W

TC 1:

 In WTC 1, the fir

es weakened the core columns and cau

south side of the building to sag. 

 The floors pulled the heated south perimeter columns in

capacity to support the building above. 

 Their neighboring columns quickly became overloaded a

buckled. 

 The top section of the building tilted to the south and beg

 The time from aircraft impact to collap

se initiation was la

how long it took for the fires to weaken the building core south side of the building and weaken the perimeter colu
Principal Findings (cont.) Principal Findings (cont.)

For W

TC 2:

 In WTC 2, the core was damaged severely at the southe

restrained by the east and south walls via the hat truss a

 The steady burning fires on the east side of the building 

there to sag. The floors pulled the heated east perimetereducing their capacity to support the building above. Thcolumns quickly became overloaded as the east wall bu

 The top section of the building tilted to the east and to th

descent. 

 The time from aircraft impact to collap

se initiation was la

the time for the fires to weaken the perimeter columns athe east and south sides of the building. 

 WTC 2 collapsed more quickly than WTC 1 because the

damage to the building core and there were early and peeast side of the building, where the aircr

aft had extensiv

insulatio

n from the structural steel.
Principal Findings (cont.) Principal Findings (cont.)

Role of thermal insulation:

 The WTC towers would likely not have collapsed u

effects of aircraft impact damage and the extensivthe thermal insulation had not been widely dislodgminimally dislodged by aircraft impact.

Alternative hypotheses:

 NIST found no corroborating evidence for alternat

suggesting that the WTC towers were brought dowdemolition using explosives planted prior to SepteNIST also did not find any evidence that missiles wthe towers. Instead, photos and videos from seveshowed that the collapse initiated at the fire and imthe collapse progressed from the initiating floors ddust clouds obscured the view.
The End