Lets use NIST buckling model for example, they claim the outer and inner structure at the bottom pretty well "ceased to exist" which allowed a 2.25 second free-fall.
Now we have a very stiff cross braced outer structure which has apparently completely vaporized on the lower floors, (7-14) which allowed the tower to fall with no / negligible structural resistance. I understand the outer structure isn't vertically load bearing and wouldn't hold the entire weight of the building without the core, 100% paid.
I'll pay up to the fires causing the collapse of the core. (Dubious) How did we have such symmetry after the external structure was severed / buckled. According to the NIST model, and due to the varying stiffness we should not have seen such symmetry during free-fall.
I agree that the symmetry isn't a smoking gun nor is the free-fall. It's the combination of the two which I believe are impossible.
A progressive, reasonably symmetrical collapse with such a strong outer frame (leaving out how fires completely destroyed the core) after the core was removed isn't that unlikely, however at free-fall?
We still have the problem of a crossbraced outer structure collapsing into itself very close to 9.8m/s
2 even if for the sake of simplicity we assume the entire inner structure was destroyed. (In video evidence we can see the tower start it's free-fall with a lot of the west side of the interior still reasonably intact.)
I understand lift shafts arent load bearing in and of themselves always but we have say (estimating) 250mm thick concrete shafts reinforced with reebar and braced to the support structures, we then have two T 150mm by 16mm rails and two T 100mm by 8mm counterweight rails, these are pinned into the concrete every 1500mm with an average of six 12mm high tensile trubolts, the shaft is absolutely over designed for the lifts, nearly all of the static load is taken through the rails into the concrete slab, dynamic loads are taken through the shaft by the rails and rail brackets.
My point is this is a very stiff, very strong structure that has actually gained stiffness and strength from the high tensile metal.
I can't understand how the whole building fell into itself at free-fall after a single column failure.
Partial collapse slower than free-fall?
Absolutely would have bought it.
Sorry for the tl:dr.
Edit, sorry for the lame vid, got me with a good title... there are still a few views of the collapse can just watch with the sound off.
Poll