Federal Building and Fire Safety Inves

of the World Trade Center Disast

Aircraft Impact Damage Ana

Comparisons and Findin

September 14, 2005

Fahim Sadek

Building and Fire Research Labo

National Institute of Standards and Tec

U.S. Department of Commerce

fahim.sadek@nist.gov
Motion Analysis of WTC 2 after Impact

Periods of Oscillation (s)

Video Analysis Model (

Fundamental Mode (N-S) 11.4 11.2Higher Mode (N-S)

3.9 4.0

Torsion

5.3 5.2
Motion Analysis of WTC 2 after Impact

 The estimated period of oscillation was nearly equ

first mode period of the undamaged structure, indlateral stiffness of the tower was not affected apprdamage.

 The maximum deflection at the top of the tower w

more than 1/3 of the drift resulting from the origina(about 65 in. in the N–S direction). Since the latebuilding before and after impact was essentially thconcluded that the additional stresses in the columoscillation were roughly 1/3 of the column stresseoriginal design wind loads. The building demonstcarry this additional load and therefore, still had re
Comparison between Impact Response 

Comparison between Impact Response 

WTC 2

 Impact Speed: 443 mph A downward impact trajectory angle of 

10.6 degrees

 Long span trusses between the 

impact point and the core

 Impact close to centered and 

perpendicular on the face of the tower

 Almost all of the aircraft debris passed 

through the core 

 Impact at floors 93-99

WTC 1

 Impact Speed A downward 

6 degrees

 Short span tr

impact point a

 Impact off cen

from the core

 Significant fra

outside of cor

 Impact at floo

sizes)

WTC 2
Comparison between WTC 1 and W

Comparison between WTC 1 and W

Exterior Damage Exterior Damage

W

WTC 2
Comparison between WTC 1 and W

Comparison between WTC 1 and W

Core Columns Core Columns

WTC 2
Comparisons with Observables: W

Comparisons with Observables: W

O

b

se

r

vable I

m

p

o

r

t

a

n

ce 

D

a

mage t

o t

he north exterior w

all V

ery signific

ant

D

a

mage t

o t

he south exterior w

all Slightly signific

ant

Lan

ding gear trajectory

Slightly signific

ant

Stairwell di

sruption 

Signific

ant

Floor dam

age 

visible o

n the north 

fa

c

e

Signific

ant 
Comparisons with Observables: W

Comparisons with Observables: W

Exterior and floor damage
Comparisons with Observables: W

Comparisons with Observables: W

Comparison of observed and calculated base cas

to the north wall of WTC

1
Comparisons with Observables: W

Comparisons with Observables: W

Calculated damage to the south wall of WTC 1
Comparisons with Observables: W

Comparisons with Observables: W

Landing gears were stopped in the core in 

WTC 1 simulations
Comparisons with Observables: W

Comparisons with Observables: W

Stairwell Disruption in W

TC 1
Comparisons with Observables: W

Comparisons with Observables: W

O

b

servable I

m

p

orta

n

ce 

dam

age t

o the s

outh exterior w

all V

ery signific

ant

dam

age t

o the n

orth exterior w

all Slightly significant

Stairwell di

sruption 

Signific

ant

Landing gear trajectory

N

ot significant

Engi

n

e trajectory

Slightly significant

Floor dam

age 

visible o

n the south fa

c

e

Signific

ant

The ‘

cold spot’ o

n the north f

a

c

e

Slightly significant
Comparisons with Observables: W

Comparisons with Observables: W

Comparison of observed and calculated base cas

to the south wall of WTC

2
Comparisons with Observables: W

Comparisons with Observables: W

Calculated response to the north wall of WTC

2
Comparisons with Observables: W

Comparisons with Observables: W

Stairwell Disruption in W

TC 2
Comparison with Previous Studies Comparison with Previous Studies

 MIT:

Used an energy approach to estimate damage to co

Ekinetic = Eplane + Eext. Col. + Efloo

 Weidlinger Associates, 

Inc.

Used a finite element analysis to estimate damage to both towers

Source: Levy, M., and N. Abboud. 

W

TC Structural 

Engineering Investigatio

n. Weidlinger Associates, Inc., 

Hart-Weidlinger, August 2002.
Comparison with Previous Studies Comparison with Previous Studies

Summary Comparison of Damage to Core Columns from 

WT

C I

mpact Study WTC 1 Cor

e Col

umn 

D

amag

e WTC 2 C

MIT

Im

p

a

ct A

n

aly

sis

4–1

2 Failed 

WAI

Im

p

a

ct A

n

aly

sis

23 f

ailed & signific

antly damaged 

Plus 5 D

a

m

a

g

ed 

14 f

ailed a

P

WAI

Colla

p

se A

nal

y

sis 

20 F

ailed 

NIST Base C

a

s

e

Im

p

a

ct A

n

aly

sis

3 F

aile

d

Plus 4 H

e

a

vily Dam

aged 

Plus 4 

NIST Mor

e Sev

ere

Im

p

a

ct A

n

aly

sis

6 F

aile

d

Plus 3 H

e

a

vily Dam

aged 

Plus 

NIST Less Sev

ere

Im

p

a

ct A

n

aly

sis

1 F

aile

d

Plus 2 H

e

a

vily Dam

aged 

Plus 2 
Key Findings: Aircraft Impact on W

Key Findings: Aircraft Impact on W

 The aircraft had a speed of 443±30 mph with a r

25±2 degrees (port wing downward). The verticdownward angle was 10.6±3 degrees, and the laangle was close to being normal to the north wa

 The aircraft impact resulted in extensive damage 

the tower, which failed in the regions of the fusefuel-filled wing section impacts. Damage to the extended to the wing tips, but the exterior columcompletely severed in the outer wing and verticaregions. 34 columns were completely severed, heavily damaged, and 2 columns were moderateresults of the impact analy

ses matched well with 

to the north wall. An exterior panel on the south 94 to 96 was dislodged. 
Key Findings: Aircraft Impact on W

Key Findings: Aircraft Impact on W

 Subject to the uncertainties inherent in the models, th

simulations indicated that:

• 3 core columns were severed and 4 columns were heav

case

• 6 core columns were severed and 3 columns heavily da

severe case

• 1 core column was severed and 2 columns heavily dam

case.

Floor trusses, core beams, and floor slabs experienceinduced damage on floors 94 to 96, particularly in the 

 The wing structures were completely fragmented due 

the exterior wall and as a result, aircraft fuel was dispfloors. The bulk of the fuel was deposited in floors 93 largest concentration on floor 94.

 Aircraft debris resulted in substantial damage to the n

contents (partitions and workstations) and also in dislThe bulk of the aircraft debris was deposited in floors largest concentration on floor 94.
Key Findings: Aircraft Impact on W

Key Findings: Aircraft Impact on W

 The impacting aircraft had a speed of 542±24 m

of 38±2 degrees (port wing downward). The verdownward angle was 6±2 degrees, and the laterwas 13±2 degrees clock

wise from the south wal

 The aircraft impact resulted in extensive damage 

the tower, which failed in the regions of the fusefuel-filled wing section impacts. Damage to the extended to the wing tips, but the exterior columcompletely failed in the outer wing and vertical sregions. 29 columns were completely severed, heavily damaged. The 2 columns were obscured analysis results showed that these columns werdamaged. results of the impact analys

es match

pattern to the south wall well. 4 columns on the failed. 
Key Findings: Aircraft Impact on W

Key Findings: Aircraft Impact on W

 Subject to the uncertainties inherent in the models, th

simulations indicated that

• 5 core columns were severed, and 4 columns were hea

case

• 10 core columns were severed and 1 column heavily da

severe case 

• 3 core columns were severed and 2 columns heavily da

case. 

Failure of the column splices located on floors 77, 80, significantly to the failure of the core columns. Floor trand floor slabs experienced significant impact-induceto 81, particularly in the path of the fuselage. 

 The wing structures were completely fragmented due 

the exterior wall and as a result, aircraft fuel was dispfloors. The bulk of the fuel was concentrated on floor

 Aircraft debris resulted in substantial damage to the n

contents (partitions and workstations) and also in dislThe bulk of the aircraft debris was deposited in floors largest concentration on floor 80.
Key Findings: Aircraft Impact Key Findings: Aircraft Impact

 The towers sustain

ed sig

nificant structural dam

walls, core columns, and floor systems due to This structural damage contributed to the weastructures, but did not, by itself, initiate buildinHowever, the aircraft impact damage contribusubsequent fires and the thermal response of structures that led ultimately to the collapse of 

• dispersing jet fuel and igniting building conte• creating large accumulations of combustible 

aircraft and building contents,

• increasing the air supply into the damaged b

permitted significantly higher energy release normally be seen in ventilation building fires,

• dislodging of fireproofing.
Key Findings: Aircraft Impact Key Findings: Aircraft Impact

 In general, the results of the simulations matche

reasonably well. Not all of the observables, howmatched by the impact simulations due to the unimpact conditions, the imperfect knowledge of thcontents, the chaotic behavior of the aircraft bresubsequent debris motion, and the limitations of 

 The less severe damage case did not meet two 

• No aircraft debris was calculated to exit the side o

most of the debris was stopped prior to reaching contradiction to what was observed in photographimpact event.

• The fire-structural and collapse initiation analyses 

towers (NIST NCSTAR 1-6) indicated that the towcollapsed had the less severe damage results be