Gerry, September 6 at 07:10 (#46919):
– “The North tower collapsed in about 13.8s total which equates to around 2/3 freefall, total freefall being approcx 9.2s for 1365ft. Still way too fast.”
Myself, September 6 at 16:04 (#46949)
– “Yes, the internal collapse accelerated at around 2/3 of g, so 1/3 of the entire potential energy of the structure (before collapse) was available for destruction of materials. Convert that to TNT equivalent and you can see that it was way more than enough.”
I agree that the collapse was much too fast to be consistent with Bažant’s “crush down then crush up” model. But Bažant’s model isn’t consistent with observation of either the collapse or the wreckage; it’s an interesting theoretical exercise, but it isn’t remotely realistic and it was never intended to be.
Bažant’s model is the failure mode that demands the most energy, bending every vertical column in the building. But when things break in the real world, they do so at the weakest points rather than the strongest, ie. they break in whatever way requires least energy. In the Twin Towers’ collapses, this was cascade destruction of/through the floor systems.