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

Session VI -

Structural Fire Respons

Collapse Analysis

Observations for Structural Re

Structural and Fire Protection 

Due to Aircraft Impact

September 15, 2005

Therese McAllister

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce
Observations Observations

Development of probable collapse sequence fo

was guided by evidence

 Photographs and videos Design and maintenance documents Eyewitness accounts

Changes in structural performance are not obs

significant deformation has occurred.

Observations of structural behavior from over 7

photographs and 150 hours of video records 

 Severed components Deflections and buckling (inward bowing of exterio Disconnection and sagging of floors at exterior wa Relative alignment of columns (tilting of upper buil
Observations Observations

Visual evidence was used in three ways:

 Determine input information (exterior column dam

sagging floors, and failure of floor connections at t

 Impose time related constraints on analyses (time 

bowing was observed)

 Validate analysis results (global stability after impa

structural response to fire)

Observations of structural behavior were broken into 

 Key observations - significant events used in analy

a timeline of structural events

 Noted observations – significance to structural res

conclusively identified
South Face of WTC 1 -

9:40 a.m. No in

8:46 9:40 
South Face of WTC1 - 10:23 a.m. In

• Maximum = 55 inches

(uncertainty ~ +/-

6 inches)

•

Floor locations approximate

•

Blue tinted region digitally

enhanced

8:46 10:2
South Face of WTC1 - 10:23 a.m. In

• Maximum = 55 inches

(uncertainty ~ +/-

6 inches)

•

Floor locations approximate

•

Blue tinted region digitally

enhanced

8:46 10:2
Visual Evidence of Fires in WTC 1, 

Visual Evidence of Fires in WTC 1

10:28 a.m. 10:28 a.m. 

Dark blue line indicates extent of inward bowing observed in visuaLight blue line is estimated region of inward bowing where visual im

North Face Eas

South Face Wes
WTC 1 Collapse Initiated WTC 1 Collapse Initiated 

at 10:28 a.m.

at 10:28 a.m.

© 2001 

Tim Main

and Mi

k

e Ballou

 WTC 1 tilted to the south (view from the northeast). Initiation of global collapse was first observed by the tilting of the building sections above the impact region.

8:46 
East Face of WTC 2 -

9:03 a.m. Ha

objects in east windows of Floor 8

9:03 
East Face of WTC 2 -

9:10 a.m. Ha

objects in north window of Floor 8

9:03 9:10 
East Face of WTC 2 –

9:21 a.m. Maxi

bowing of approx. 10 in. (18 min after 

9:03 9:21 
East Face of WTC 2 East Face of WTC 2 

– 9:47 a.m. Ha

9:47 a.m. Ha

83 Slab in Windows of Floor 82 83 Slab in Windows of Floor 82

© 2001 Allen Murabayashi, 9:47:05 am ± 5 s

9:03 9:47 
East Face of WTC 2 –

9:53 a.m. 

Maximum inward bowing of approx. 20 in.

9:03 9:5
Visual Evidence of Fires in WTC 2, 

Visual Evidence of Fires in WTC 2

9:58 a.m. 9:58 a.m.

North Face Eas

South Face We

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
East Face of WTC 2 East Face of WTC 2 

– 9:58 a.m. Inw

9:58 a.m. Inw

of Perimeter Columns Seconds Befo

of Perimeter Columns Seconds Bef

9:03 9:5
WTC 2 Collapse at 9:59 a.m. WTC 2 Collapse at 9:59 a.m.

© 2001 L

uigi Cazzaniga

© 2001 L

uigi Cazzaniga

WTC 2 tilted to tthis view from th

9:03 
WTC 1 Observations and Timeline 

WTC 1 Observations and Timeline 

 Inward bowing of south face first seen at 10:2

before collapse initiated.

 Maximum inward bow was 55 in. ±

6 in. at Flo

 Extent of fires on all WTC 1 exterior faces wa North face did not bow inw

ard.

 Time to collapse was 102 min. Tilt of upper building section took place near 

column buckling progressed rapidly across thwest faces.

 Upper building section tilted to the south with 

east or west component in the tilt. 

 Tilt reached 8° before view was obscured by 
WTC 2 Observations and Timeline 

WTC 2 Observations and Timeline 

 Draped object

s were observed in the windows of 

faces which appeared to be hanging floors. The with time and extended across approximately ha

 Maximum inward bow of the east face was 10 in

first seen at 9:21 a.m., 18 min after impact.

 Maximum inward bowing increased to 20 in. ±

1 

before collapse initiation.

 Fir

es wer

e more extensive on the east side of th

inward bowing of the west wall were observed on 

 Time to collapse was 56 min. Tilt of upper building section took place near Floo

column buckling progressed rapidly across the asouth fac

es.

 Upper building section tilted 3° to 4° south and 7 Tilt to the east reached 20°

before view was obs
Aircraft Impact Damage to Str

Aircraft Impact Damage to St

Systems and Fire Protect

Systems and Fire Protect
Aircraft Impact Damage for Structu

Aircraft Impact Damage for Structu

Response Analysis Response Analysis

Damage to exterior columns was determined from photogDamage to core columns, floor systems (slabs, core beam

fire protection and partitions was determined from aircresults, which were validated with evidence from photo
Core Column Damage Core Column Damage

W

S

e

v

e

r

e

d

H

e

a

v

y

M

o

d

e

r

a

t

e

Lig

h

t

Light Damage

Moderate Damage

Heavy Damage Severed 

Light Moderate H 

eavy Severed 
Structural Floor Damage Structural Floor Damage

Severely damaged or missing areas incapable of supporting loads

 Severed trusses, beams, and 

connections

 Concrete damage

Floor slab damage from aircraft impact analysis

Floor afor both in struc

Floor slab damage for 

structural response 

analysis
Damage to Passive Fire Protection 

Damage to Passive Fire Protection 

Structural Steel Structural Steel

Debris and fuel field traveled 

across multiple floors and tended to fill floor spaces.

Damage to interior structural 

framing, partitions, and furnishings was calculated by the aircraft impact analysis.

Aircraft Imp

act
Damage to Passive Fire Damage to Passive Fire Protection for Structural Protection for Structural Steel

Criteria w

for estiof disloprotectdebris 
Criteria for Dislodged Passive Fire 

Criteria for Dislodged Passive Fire 

of Floor Trusses of Floor Trusses

If the debris impact damaged or 

destroyed room furnishings (modular office workstations), then the debris field was considered to extend high enough and be strong enough to dislodge the fire protection in the same floor area.

If the debris field did not damage 

room furnishings, then the fire protection in the same floor area was considered to be intact.
Criteria for Dislodged Passive Fire 

Criteria for Dislodged Passive Fire 

of Core Columns of Core Columns

If debris impact failed partitions immediately in front of cor

fire protection was assumed to be dislodged over the fcolumns behind failed partitions.

If the partitions remained intact, then the fire protection on 

these partitions was assumed to remain intact. 

The representative strength of building partitions was assu

based on a survey of data for partition walls and moduthe representative laboratory cohesive and adhesive stprotection measured by NIST was generally less than 

Passive fire protection for core columns included sprayed 

(SFRM), gypsum wallboard enclosures, or a combinati
Criteria for Dislodged Passive Fire 

Criteria for Dislodged Passive Fire 

of Exterior Columns of Exterior Columns

If debris impact damaged or destroyed room furnishings (modular office workstations) adjacent to the exterior columns, then the fire protection on the inside face of the exterior columns in the same vicinity was assumed to be dislodged over the floor height.If furnishings remained intact adjacent to the exterior columns, then the fire protection on the inside face of the exterior columns behind the furnishings was assumed to remain intact.The other three faces of the exterior columns were protected by the windows and/or aluminum cladding and were assumed to have no fire protection damage.
Estimate of Fire Protection Damag

Estimate of Fire Protection Damag

Consistent with a conservative approach, NIST estimates ignored 

passive fire protection on structural components over a much direct path of the debris was dislodged by shock or strong vib

•

The WTC towers shook vigorously during the 0.5-0.7 sec

Video analysis showed that WTC 2 vibrated for over 4 mimpact with amplitudes in excess of 20 inches at the roof 

•

Considerable photographic evidence shows fireproofing d

columns not directly impacted by debris.

•

First-person inter

views of building occupants indicate tha

to aircraft impact were strong enough to dislodge ceiling 

throughout the height of both W

TC towers and to cause n

stop functioning.

•

Difficult to establish robust criteria to generate a coheren

induced dislodging.

•

The variation of influential parameters used in determinin

sequence included some variation in the extent of dislodg
Application of SFRM to External C

• By design, uniform 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
Thermal Analysis of Dislodged Pas

Thermal Analysis of Dislodged Pa

Fire Protection Fire Protection

For intact fire protection:

•

Floor trusses -

a thermally equivalent uniform thic

•

Columns - the specified thickness determined from 

documents. 

For dislodged fire protection:

•

Core columns and floor trusses - the fire protectio

be missing on the component. Separate analysis significant regions of missing fireproofing is essenthere being no fireproofing.

•

Exterior columns - the fire protection was conside

the inner face.
WTC 1 Damage: Composite Summ

WTC 1 Damage: Composite Summ

Floors 93 to 98 Floors 93 to 98

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

Floor 96

Floor 93 

Floor 94

Floor 97
WTC 2 Damage: Composite Summ

WTC 2 Damage: Composite Summ

78 to 83 78 to 83

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

Floor 78 Floor 79 

Floor 81 Floor 82 
Aircraft Impact Damage to Structu

Aircraft Impact Damage to Structu

and Fire Protection and Fire Protection

 In WTC 1, structural systems were damaged and f

dislodged by direct debris impact over Floors 94 to north floor areas, the core, and central regions of thextending to the south wall.

 In WTC 2, structural systems were damaged and f

dislodged by direct debris impact over Floors 78 to floor area in front of the core, central and east regiomost of the east floor areas, extending to the north 

 Fire protection damage estimates were conservativ

outside area of direct debris impact was not includeto estimate vibration-induced dislodging could not b
The End The End