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Conclusions of the Analysis
Floor sagging and inward bowing of an exterior wall were necessary but not sufficient conditions to initiate collapse. In both WTC 1 and WTC 2, significant weakening of the core due to aircraft impact damage and thermal effects was also necessary.
The National Institute of Standards and Technology considered the observed performance, evidence, and analysis results for each tower, and reached two conclusions. First, in the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001 likely would not have led to the collapse of a WTC tower.
Second, the towers likely would not have collapsed under the combined effects of aircraft impact and the subsequent multi-floor fires encountered on September 11, 2001 if the insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.
The existing thermal insulation, had it not been stripped off in the impact, would have been sufficient to keep the steel temperatures low enough to minimize deformation. Also, the investigation team neither found nor invoked any extraordinary events, beyond the terrorist attack that damaged the structure and removed the insulation, that led to the collapse of the towers.
The difference in the time it took for each WTC tower to collapse was due primarily to the differences in structural damage, the time it took the fires to travel from the impact area across the floors and core to critical locations, and the time it took to weaken the core and exterior columns.
The structural damage to the WTC 2 core was asymmetric, including a corner core column that was severed. The damage to WTC 1 was more symmetrical; it was located in the center portion of the core and extended from the north side to the south side. The fires in WTC 2 reached the east side of the building more quickly (within 10 to 20 minutes) than the 50 to 60 minutes it took for the fires in WTC 1 to reach the south side.
Floor sagging and inward bowing of an exterior wall were necessary but not sufficient conditions to initiate collapse. In both WTC 1 and WTC 2, significant weakening of the core due to aircraft impact damage and thermal effects was also necessary.
The National Institute of Standards and Technology considered the observed performance, evidence, and analysis results for each tower, and reached two conclusions. First, in the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001 likely would not have led to the collapse of a WTC tower.
Second, the towers likely would not have collapsed under the combined effects of aircraft impact and the subsequent multi-floor fires encountered on September 11, 2001 if the insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.
The existing thermal insulation, had it not been stripped off in the impact, would have been sufficient to keep the steel temperatures low enough to minimize deformation. Also, the investigation team neither found nor invoked any extraordinary events, beyond the terrorist attack that damaged the structure and removed the insulation, that led to the collapse of the towers.
The difference in the time it took for each WTC tower to collapse was due primarily to the differences in structural damage, the time it took the fires to travel from the impact area across the floors and core to critical locations, and the time it took to weaken the core and exterior columns.
The structural damage to the WTC 2 core was asymmetric, including a corner core column that was severed. The damage to WTC 1 was more symmetrical; it was located in the center portion of the core and extended from the north side to the south side. The fires in WTC 2 reached the east side of the building more quickly (within 10 to 20 minutes) than the 50 to 60 minutes it took for the fires in WTC 1 to reach the south side.
