Last edited Fri Jun 20, 2014, 11:51 AM - Edit history (1)
What it destroyed was the structural integrity. NIST's simulations found that the "probable cause" of WTC7's collapse was that thermal expansion pushed a girder off its seat because it had only been designed to withstand the gravity load, with only a few bolts resisting lateral movement. That floor collapsed onto the floor below, and the impact caused that floor to break away from the column, too. The progressive floor collapse that followed left column 79 with insufficient lateral restraint to resist buckling. When 79 buckled, the collapse propagated through the building because almost all of the interior column connections were also designed to carry only the gravity loads, not the bending "moment" caused by the other end of the beams falling.
Comparing WTC7 to other building fires is fallacious because that "probable cause" is based on both the asymmetric layout of the floor girders around column 79 and the specific details of the connections at that column.
Although we don't have enough data to say with certainty that that was the exact sequence of events, that explanation is widely accepted as being plausible by structural engineers who understand that when a structure loses integrity, all bets are off. With the floor beams gone, the columns couldn't stand on their own.
On the other side of the argument, we have "I don't understand that" and "I don't believe that could happen." That is far, far short of what it would take to replace NIST's "probable cause" with a better explanation. I doubt that many engineers would bet their next paycheck that NIST was 100% correct about the exact sequence, but any number of possible sequences are still far, far more plausible than silent explosives or precisely timed thermite cutting.
Structural engineers do know how to design a structure that can withstand very high fire temperatures, if that is a design requirement (as it would be for a self-cleaning oven). They don't do that however, because it is not a typical design requirement, and going beyond building code requirements would increase the cost of the structure beyond what the typical owner is willing to pay. That is the issue addressed by NIST's proposed code changes.
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