> I have gotten a bunch of things from structural engineers, who are
> arguing about exactly what happened. But none of them seems (after
> the fact) very surprised that the heat weakened the steel enough to
> start the collapse...
Here's one, linked from the University of Sydney civil engineering pages (cited in the jmcmichael at care2.com message):
http://www.hera.org.nz/PDF%20Files/World%20Trade%20Centre.pdf
This guy seems to say that the fire helped, but it was the initial impact that did most of the damage that ultimately led to the collapse. Some of it is pretty technical, but still quite readable.
/jordan