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From:
http://en.citizendium.org/wiki/9-11_Attack_in_New_York
After all these years there remains controversy about what initiated the total collapse of the three World Trade Center buildings, WTC1, WTC2 and WTC7. Concerning WTC1 and WTC2, the 110-story buildings that were impacted by commercial airliners piloted by terrorists, the controversy is mainly over technical structural details. About an hour after initial impact, were the edges of the truss-supported floors first stripped from their supports over several stories in the hot region in the neighborhoods of the initial impacts, followed by buckling of the consequently locally laterally unsupported perimeter walls? Or did sagging hot floor trusses pull the perimeter walls inward, giving rise to plastic hinge buckling of the inwardly bowed perimeter walls while the edges of the hot floors remained attached to these walls? Concerning WTC7, which was not impacted by an airliner: was there a conspiracy to demolish WTC7 by set charges, or was the fire within it, presumably caused by hot debris flying from WTC1 and/or WTC2, enough to cause global collapse about 6 hours after the collapse of WTC1 and WTC2?
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