Since I volunteered to moderate the discussion, I don't want to get too involved in it, but I also want to spur discussion along when I feel I can add something, having done a fair amount of research on the subject. The following quotes are by Thomas Eagar, who is a Thomas Lord Professor of Materials Engineering and Engineering Systems at MIT. He discussed how fire could have brought down the towers in the first paragraph, and how the design of WTC 7 contributed to its eventual collapse in the second.
What I got from this is that structural steel doesn't have to be near its melting point, and doesn't have to lose much strength to cause damage, all that has to happen is for it to deform until connections to the center and the steel exoskeleton start to break, transferring loads to adjacent columns until those are also overloaded. The resulting domino effect would be what cause the collapses minutes and hours later.
In WTC1 and WTC2, the planes' impacts seem to have been sufficient to strip away fireproofing materials from columns. Damage from debris approaching the extent claimed in the NIST report in addition to seismic shock from the towers falling is likely to have caused significant damage to fireproofing material surrounding columns in WTC7. Seismic shock and debris are also likely to have damaged windows, at least in the immediate area of the fires, which is significant according to Dr. Egan.
Quote:
"... you had a hot fire, but it wasn't an absolutely uniform fire everywhere. You had a wind blowing, so the smoke was going one way more than another way, which means the heat was going one way more than another way. That caused some of the beams to distort, even at fairly low temperatures. You can permanently distort the beams with a temperature difference of only about 300°F ... If there was one part of the building in which a beam had a temperature difference of 300°F, then that beam would have become permanently distorted at relatively low temperatures. So instead of being nice and straight, it had a gentle curve ... But the steel still had plenty of strength, until it reached temperatures of 1,100°F to 1,300°F. In this range, the steel started losing a lot of strength, and the bending became greater. Eventually the steel lost 80 percent of its strength, because of this fire that consumed the whole floor.”
“trusses on the fifth and seventh floors were designed to transfer loads from one set of columns to another. With columns on the south face apparently damaged, high stresses would likely have been communicated to columns on the building's other faces, thereby exceeding their load-bearing capacities.”
Tyson, Peter.”The Collapse: an Engineer's Perspective.”Why the Towers Fell. 2002. NOVA Online. May 2002 <http://www.pbs.org/wgbh/nova/wtc/collapse.html>
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