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Collapse of the World Trade Center

On September 11, 2001, the two main towers of the World Trade Center complex were each hit by hijacked commercial aircraft as part of the September 11, 2001 attacks. 2 WTC collapsed to the ground at 9:59 am, less than an hour after being hit, and 1 WTC followed at 10:28 am, causing massive damage to the rest of the complex and nearby buildings.

In all, 2,595 people inside and near the towers were killed, along with the 157 people who were aboard the flights.

Owing to differences in the initial impacts, the collapses of the two towers were found to differ in some respects, but in both cases, the same sequence of events apply.

After the impacts had severed exterior columns and damaged core columns, the loads on these columns were redistributed.

The hat trusses at the top of buildings played a significant role in this redistribution of the loads in the structure.

The impacts also dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires.

This turned out to be crucial in bringing about the collapses.

In the 56 and 102 minutes before the collapse of, respectively, 2 WTC and 1 WTC, the fires, and events associated with them, weakened the core, until it was unable to carry loads.

Note:   The above text is excerpted from the Wikipedia article "Collapse of the World Trade Center", which has been released under the GNU Free Documentation License.
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August 29, 2015

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