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September 11, 2001The World Trade Center

New York City

David T. Biggs, P.E. Principal

Biggs Consulting Engineering, PLLC

Building Performance Assessment Team

f / b ildwww.fema.gov/rebuild/mat/wtcstudy.shtm

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Presentation

• The site before the attack

• The attack and tower collapses

• Tower construction

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N

World TradeCenter

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September 11, 2001 Attack

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Fireballs

300

350

r (f

t)

50

100

150

200

250

300

Fir

ebal

l Dia

met

er

90m fireball ≈ 11,400 liters

40000

0 1000 2000 3000

Fuel Quantity (gal)

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WTC 2 Collapse

• Initial collapse to south and west (towards zone of impact)

• Progressive collapsecollapse downward and to north west

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Ground Zero

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Underground

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FEMA - USAR

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WTC 1 & 2 • 110 stories• WTC1 – 527 m ( 1.368 ft+ 360 ft Radio

Tower)

• WTC2 – 415 m (1,362 ft)

• Floor plate of 63m x 63 m (207’ x 207’)

• Core 26m x 42m (87’ x 137’)

• 3,716 m2 (40,000 square ft) per floor

• Constructed 1968-73

• 99 Elevators (each tower)

• 3 Egress Stairwells (each tower)

Tower Construction

• Lightweight concrete slabs on metal g gdeck

• Composite trusses (bar joists)

• Exterior frame

• Interior columns in core

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WTC 1 & 2

Innovations:– use of dampers touse of dampers to

reduce wind deflection

– use of shaft wall system to protect openings

– use of automateduse of automated shop fabrication

Tubular Design• Exterior columns carry

wind plus DL and LLI t i l i il• Interior columns primarily for DL and LL

• Truss system at the top

• Exterior columns stressed about 15% under DL and LL only. Without wind, a lot of redundancy.

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Exterior Framing

• Built up columns of plates at 1m (3’-4”) p p ( )oc

• Spandrels of plates

• Trusses doubled up at 2m (6’-8”) apart

• Two – way action due to bridging y g gtrusses

• Myth: No water in the columns

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Myth: bearing seat

weakness0.8m

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Effects of Aircraft Impact

• Impact shattered and fractured 2/3 of columns on one face

• Partial collapse of floors occurred at impacted columns

• Debris penetrated building core– damaged core columns

– damaged stair shafts and elevatorsg

• Impact caused failure of fireproofing in affected area

• Initiated fire

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Egress

• First use of shaft wall enclosures

• Overpressures from fire ball blew out the walls and ceilings

• Skyways at 44th and 78th floors

• Myth: Extra stairwell..a function of width

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Myth

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1993 Bombing

• Smoke was a problem in the towersp

• Structural damage in basement

• Led to improved evacuation practices

• Fluorescent paint in stairwellsFluorescent paint in stairwells

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Fireproofing

• Asbestos Tower 1 up to 39th floor was pabated previously

• Doubled up thickness in Tower 1 at levels of impact

• Tower 2 only 78th floor

• Sprinklers added; not in original• Sprinklers disabled by plane damage

Fire Characteristics

600

800

1000

1200

ratu

re (

°C) Jet Fuel

Stress-Strain Curve0

200

400

0 10 20 30 40 50 60Time (minutes)

Tem

per

• Fuel burns at the rate of approximately 1,000

Office Equipment

Strength Reduction Factor

gallons per minute for each 10,000 sq. ft. of fire area.

• With fire involved in extensive portions of at least 4 floors, jet fuel was consumed within the first 3 or 4 minutes.

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