Unconfirmed information from a recent email:
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.
- Actually, that appears to be verifiable: The L.A. Times says [this]
- In his 2000 book "Building Big," architect David MaCaulay? described the towers' engineering as "a series of load bearing exterior columns spaced 3 feet apart and tied together at every floor by a deep horizontal beam, creating a strong lattice of square tubing around each tower."
And ABC news says
[this]:
- Built by the Port Authority of New York and New Jersey in 1973, the World Trade Center towers were the best examples of tube buildings of their time. Tube buildings are reinforced by closely spaced columns and beams in their outer walls, forming a steel tube. A series of glass windows fill in the space between the beams. And an internal core beam adds to the stability of tube structures. ... The steel beam-lined buildings rely on their tube network of beams to sustain hurricanes and seismic events.
There is also speculation at those sites that the fire from the planes' fuel caused the steel to melt, leaving the cement floors unsupported.
There are other World Trade Centers besides the one in New York. See e.g.
http://www.wtc.se/eng/index.html (World Trade Center Stockholm), and
http://www.wtca.org/ (World Trace Centers Association Online)
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