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But let's suppose that the fire was hot enough to melt steel. What would
have happened in that case? Before it breaks, hot steel begins to bend.
This redistributes the forces in the structure and puts elastic stress
on those parts that are still cool. The process is asymmetric, so that
the structure should visibly bend before breaking. But of course, no
steel skyscraper has even bent over in a fire.

Let's suppose the structure were sufficiently weakened that it did fail
catastrophically near the point of the airplane strike. In this case,
the intact structure below would exert an upward force on the base of
the upper story portion of the building (the part that has been broken
loose), while any asymmetry would allow the force of gravity to work
uninhibited on the tip of the skyscraper. Thus, the top section of the
skyscraper would tip and fall sideways.

If it did not tip, it would have ground straight down through the
building below. The gravitational potential energy of the upper stories
would be coupled into the frame below, beginning to destroy it. The
frame below would deflect elastically, absorbing energy in the process
of deflecting. At weak points, the metal structure would break, but the
elastic energy absorbed into the entire frame would not be available to
do more destruction. Instead, it would be dissipated in vibration,
acoustic noise and heat. Eventually this process would grind to a halt,
because the gravitational potential energy of a skyscraper is nowhere
near sufficient to destroy its own frame.