Investigation Summary
The destruction of the World Trade Center was the result of a coordinated terrorist attack involving four hijacked aircraft, two of which targeted the New York landmark. American Airlines Flight 11, a Boeing 767, struck the North Tower between floors 93 and 99. Seventeen minutes later, United Airlines Flight 175, also a Boeing 767, struck the South Tower between floors 77 and 85. The impacts severed numerous perimeter columns and damaged the buildings' central cores. While the towers initially remained standing, the aircraft impacts dislodged spray-on fireproofing from the remaining steel structure. The subsequent fires, though not hot enough to melt the steel, reached temperatures sufficient to weaken it significantly. The NIST investigation concluded that the sagging of floor trusses pulled the perimeter columns inward, leading to their buckling. Once the support structure in the impact zones failed, the upper sections of the buildings fell onto the floors below, initiating a progressive collapse that the remaining structure could not arrest.
Final Conclusions
The National Institute of Standards and Technology (NIST) released its final reports on the Twin Towers on October 26, 2005. The investigation concluded that the collapses were caused by the combination of structural damage from the aircraft impacts and the weakening of the steel structure by intense, multi-floor fires. The loss of fireproofing during the initial impact was cited as a critical factor that allowed the steel to reach debilitating temperatures. The South Tower collapsed first at 09:59, followed by the North Tower at 10:28.
Photographic Evidence (1)
Video Analysis
9/11, 2001: The Technical Reconstruction of the Twin Towers Collapse — A technical overview of the structural failures and fire-induced weakening that led to the collapse of the World Trade Center towers following the terrorist attacks.
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Systems & Failure Modes
Tube-frame Structure
The towers used a 'tube-frame' design with closely spaced perimeter columns to resist wind loads, which initially allowed the buildings to redistribute loads after many columns were severed by the aircraft.
Passive Fire Protection
Spray-on fireproofing was intended to protect the steel from heat, but much of it was stripped off by the debris and shockwaves of the aircraft impacts.
Floor Trusses
Long-span floor trusses connected the core to the perimeter; their sagging due to heat pulled the perimeter columns inward, contributing to the global collapse.
Interesting Facts
- 01The attacks killed 2,749 people at the World Trade Center site, including 400 emergency responders.
- 02NIST analyzed 236 individual pieces of recovered steel during its multi-year investigation.
- 03Approximately 87 percent of the estimated 17,400 people in the towers at the time of the attacks were successfully evacuated.
- 04The fires at the site burned for three months until the majority of the rubble was removed.
Safety Actions & Advisories
Building and Fire Code Updates
Source ↗NIST made 31 recommendations leading to updates in U.S. codes regarding structural integrity, fire resistance, and occupant evacuation for high-rise buildings.
Fire Resistive Steel Standards
Development and validation of performance criteria for fire resistive steel to provide occupants more time to escape during high-rise fires.
Known Controversies & Unanswered Questions
Necessity of Fireproofing Loss
Reference ↗Some researchers, including Barbara Lane of Arup, argued that computer simulations showed the towers could have collapsed from fire alone even if the fireproofing had remained intact, challenging the NIST conclusion that its removal was a necessary factor.