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Politics : Bush-The Mastermind behind 9/11?
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To: David Howe who wrote (16027)9/20/2006 12:14:36 AM
From: TigerPaw  Read Replies (1) of 20039
 
the joists pulled away from the core columns and started the collapse.

Actually that was an early hypothesis that turned out would not happen. What NIST says did happen is that the steel truss that held the floor heated, weakened, and expanded. Moreover, they expanded across their whole length since the fire covered pretty much the whole floor.

When the truss got to red-hot and it's expansion was limited by the attachment to the wall/core, then it began to sag in the middle since this was the only direction left for it to expand in.

When the truss sagged it no longer acted as a brace for the side walls, but instead actually began pulling the walls inward. The walls were already damaged and parts of them were getting hot too. This eventually led to the outer walls being pulled in as a kink at some point, and then the upper part of the building was no longer supported.

TP
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