Re: NITPICKING? Was failure of NY's WTC

["Paul O. Johnson" <hodgepoj@YAHOO.COM>]

> Tucker Hiatt asked :
>
> Would any PHYS-Ler care to speculate regarding why a structural
> failure (due to intense heat) at or above mid-section would lead to
> a domino-like structural failure all the way down?  ...

I had the same question as I watched the north tower collapse. The
McNeil Lehrer Report interviewed two structural engineering professors
Tuesday evening. They both knew the WTC architect and praised his
structural design.

The towers were designed to withstand a direct impact by a 707 carrying
an end-of-journey fuel load. Each building had a large steel post at
the center and a curtain of peripheral steel posts every three feet.

All the steel posts were covered with a layer of concrete of a
thickness to protect the steel against a normal office building fire --
burning wood furniture, plastics, and paper, or a jet fuel fire for 15
or 20 minutes. But the concrete wasn't thick enough to protect the
steel from the very high temperatures of burning jet fuel for an
extended time.

After an hour of being exposed to these extreme temperatures, the steel
posts at the location of the fires lost their temper and could no
longer support the weight of that part of the tower above the fire. The
top of the tower collapsed, and in the process gained enough KE to act
as a pile driver on the lower part of the tower.

Both professors said the planes impacted the towers at the worst
possible height. Their explanation makes sense to me. The cost/benefit
tradeoff obviously did not consider the possibility of an airliner full
of fuel impacting at that height.

Paul O. Johnson
Stranded in Denver

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