IP: Why Trade Center Towers Stood, Then Fell

[David Farber <dave () farber net>]

For a span of time that lasted thousands of heartbeats and spared thousands 
of lives, the World Trade Center towers withstood the crashes of two 
jetliners and the flames stoked by their fuel. But eventually, the fires 
softened the steel structures, and the twin towers collapsed in a 
terrifying avalanche.

Now engineers think they are closing in on the specific structural failures 
that, like a few loose stones on a mountain, set off the deadly sequence of 
events inside the burning, partly smashed buildings. It could all come down 
to the sagging steel underpinnings of a few floors, which then tore away 
connections held by pairs of three-quarter- inch bolts and a pattern of 
welds where the floors were bound to columns.
<snip>
http://www.nytimes.com/2001/11/11/nyregion/11COLL.html


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