trevor,
once the collapse is underway (and has picked up sufficient momentum, which will have happened after a sustained drop of not very much - somewhere between several inches and several feet), the structure below will, by and by, experience dynamic loads that exceed wkat it was designed for to bear (mostly statically; and, to a lesser degree, dynamically) by an order of magnitude. Think of how you can stand on a tin can for indefinitely long stretches of time, even move upon it, but when you jump on it from a few inches up, you will crush it - and crush it all the way down to the floor.
Once the first layer of steel structure below the falling part fails, it starts falling along with the stuff on top and add to the mass as gravity keeps pulling.
Rinse and repeat, as the now acreted mass impacts a next layer of structure.
Whether or not this cascade will slow and eventually come to a halt depends basically on two variables:
- How much energy can the structure absorb per layer (or per height unit)
- How much kinetic energy does the falling mass mass pick up through the action of gravity as it falls down onto the next layer (or per height unit)
If the absorbed energy exceed the picked-up energy, there is a chance for collapse arrest.
If both are about equal, collapse will continue at about constant velocity
If picked-up energy exceeds absorbable energy, collaps will go all the way down and accelerate.
This scenario is quite independent from how damaged the structure is - doesn't require any fire effects. You see, in actual controlled demolitions, they don't pre-heat the entire building (they do weaken columns and other supports to facilitate and controll the collapse, but that is not a necessity for the method to work).
It is rather easy to come up with mathematical models that envelop the actual situation by presenting a "best case for survival" scenario and showing that even the best case predicts total collapse, once collapse is under way.
The effects of high temperatures (steel weakening and/or expanding; later, as fires move on, contraction of cooling steel; perhaps compromising of auxiliary structures like concrete floors that help stiffen the floor trusses) were important to initiate the collapse, to bring the static load-bearing capacity of the first level of structural elements below its actual load, to send it moving down.
That's collapse initiation.
After that, it's collapse progression.
(Some put a "transition" phase between the two; I guess that's supposed to be a phase where progression is mainly lateral through the failing level and up to the point where vertical progression across the entire floor area becomes dominant).
"Official" theory says that fires, possibly helped by stripped fire-proofing and plane crash damage, initiated the collapses, and that after that, progression is unavoidable without further ado.
"Conspiracy theorists" variously claim that either initiation wasn't possible/did not happen by fire alone, or that progression wasn't possible/did not happen without assistance by explosives/incendiaries/space rays; or both.
Sorry if that was too detailed and perhaps too low-level. I hope it helps anyway.
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