Ralph Lewis <rlewis10us@yahoo.com> writes:
>
> Interesting that the Empire State Building took a direct hit of an american
> air force bomber with a much more explosive fuel (aviation gas) than jet
> fuel and damage was minimal. Makes you wonder about building standards and
> building permit enforcement today.
John Young, an architect, gave this explanation on another mailing list:
:The WTC towers had a distinctive structural system which utilized
:the exterior wall framing for lateral bracing -- a so-called lattice
:framework. This allowed minimization of internal lateral bracing
:and opened up the floor plans. You can see the effect of that when
:the buildings collapsed, with the lattice framework crumbling and
:the interior imploding. The lattice works so long as it remains
:intact as a system: if a part of it goes, then the whole system
:goes.
:
:The planes punched holes in the lattice, one tower punched
:on two sides, maybe the other too. Portions of the lattice of
:the second tower briefly remained standing after the collapse,
:then fell.
:
:The system was considered daring at the time of construction, for
:it distributed loads more efficiently than legacy column-and-beam-
:supported systems. Probably the legacy systems would not have
:totally collapsed due to damage at upper floors, although floors
:above the damage would have come down if columns were
:weakened.
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