Collapse time
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The story...

If the WTC towers tell due to fire, then why did Two WTC fall in around half the time of One WTC?  It doesn’t make sense.

Our take...

Fireproofing has been suggested as one factor.

...some experts say that fireproofing was too thin as well. Speaking at the NIST hearing, Professor James Quintiere of the Department of Fire Protection Engineering at the University of Maryland noted that the collapse times of the Twin Towers were proportional to the thickness of the fireproofing.

Specifically, One WTC, which had 1 1/2 inch fireproofing, fell in 104 minutes, while Two WTC, which had 3/4 inch fireproofing, crumbled in only 56 minutes. "The insulation used [was] too little, according to our calculations," he said.

And others suggested that even this level of fireproofing wasn't consistently applied.

Roger Morse, an architect who investigated the WTC's fireproofing from the early 1990s to June 2001, said that the towers suffered from the same sorts of deficiencies as many other high-rise office buildings in the United States and Europe. He noted that fireproofing on long-span joists was often "extremely thin" (less than the 3/4 inch specified in the FEMA/ASCE report) and that some structural elements were never fireproofed in places because ductwork prevented ready access. Moreover, he observed that fireproofing on the columns had been coming off because it had been applied over the rust that had built up on the columns, and the rust was flaking from the steel.

There's also the position at which the towers were hit.

Solomon questions whether the thickness of fireproofing is related to how fast the buildings collapsed. He notes that Two WTC, which fell first, was struck at a lower point than One WTC, and thus the damaged Two WTC had more weight to support. The relative times to collapse "probably had more to do with the additional weight that [Two WTC] was trying to support," he says.

NIST said several factors were involved, in “Finding 58” of the official report:

Finding 58: 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. WTC 2 had asymmetric structural damage to the core including the severing of a corner core column and WTC 1 had moresymmetrical damage. 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 the fires in WTC 1 to reach the south side.

Whatever the accuracy of these ideas, it’s clear that there’s more to this issue than simply the time at which the towers were hit.

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