Clearly, there will be some careful investigations into the exact mechanism
for the collapse. However, I suspect that cause for the failure ultimately
will boil down to the following. First, structural engineers know that high
heat can cause loss of strength in steel beams. For that reason the steel
structural elements in large structures often are coated with fire resistant
material during construction.
In these particular structures much of the stiffness of the vertical beams
also depended on their tubular cross-section. The floors themselves were
supported by truss elements attached to the vertical beams.
The initial impacts probably caused some vertical elements to fail. As the
fires progressed strength was lost in the remaining beams on the fire
floors. When they failed the floors above the fire fell onto the floors
immediately below the fire floors. To survive both the beams and the truss
elements had to withstand not the static forces for which they were
designed, but the dynamic forces and torques that they were experiencing.
My guess is that the truss elements on the floors immediately below the fire
floor began to fail, producing additional torques on the vertical tubular
elements. The tubular elements probably were distorted far enough out of
their normal shape to cause further loss of strength. The overall result
was that the dynamic forces from the falling material just overwhelmed the
weakened structural elements below, and the building "pancaked".
Part of the lesson here is that the vertical elements in a building are
designed with mg in mind (plus a good safety factor), and with the ability
to withstand some horizontal loading from wind. However, they are not
designed with mgh in mind. No one expects that a 30 story building will be
dropped on an 80 story building.
Would any PHYS-Ler care to speculate regarding why a structural
failure (due to intense heat) at or above mid-section would lead to a
domino-like structural failure all the way down? Clearly, many
experts in structural integrity were surprised when the collapses
occurred -- some of those experts were rescue workers toiling
underneath the doomed buildings.