september 11, 2001 the world trade center 11 2001... · september 11, 2001 attack. 4/22/2010 6....
TRANSCRIPT
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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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