Re: Structural failure of NY's WTC

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 16 Sep 2001, Ludwik Kowalski wrote:

> Without reading anything my answer would be "under the
> weight of what was falling down."

I might agree, but only if by "weight" you mean the force of
contact rather than m*(9.8 N/kg).  After all, the lower portion of
the building always supported *that* "weight."

To answer Tucker's question a little more specifically, however,
it is necessary to consider the structural details of many (most?)
modern skyscrapers which in essence consist of a stack of concrete
floors attached to a steel skeleton.  The steel skeleton amply
supports the weight of the building, but when the upper floors
collapse onto a lower floor, its attachment to the skeleton is
woefully inadequate to its newly assigned task.  The resulting
"pancaking" is a completely predictable and well understood
result.

I heard the architects of the WTC say pretty specifically that the
buildings were not--and, indeed, that no economically viable
building could be--designed to withstand the sort of heat the twin
towers were subjected to without collapsing.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm