However this has no bearing on the collapse since the bedrock did not fail. The steel columns failed. they have a much lesser ability to resist the force of the mass on them. They are designed to take the force due to the gravity load of the portion of the building above them. There was another force involved in the collapse though, the impulse of the falling upper section. (actually there were a lot of ways the energy of the falling mass was used, this is one) The mass came down and acheived a certain velocity and thus a certain value of momentum. It acted upon the columns below which slowed the upper section by some degree. The force exerted on the columns would be the change in momentum divided by the time it took to reduce that.
Actually, watching the collapse, I suspect very little impact force was applied directly to the columns. I think that's one of the problems with the design that resulted in such a catastrophic collapse.
By nature of gravity, most of the collapsing mass fell inside the building's footprint (it had to). This means it was
inside the exterior columns. We can see evidence of this in the fact that the exterior columns peel
outwards as the collapse occurs. That means, immediately, half the mass-supporting structure is removed from the equation.
That leaves the core columns, but as we know the building twisted as it fell. That makes it highly unlikely that the core columns lined up.
Instead, the force of collapse was left on the light weight truss systems.
Designers of the WTC talked about aircraft impacts being like a pencil pushed through a mosquito net. I think that's a good analogy for what happened in the collapse. The lower core columns would easily punch through the floor trusses in the upper section, and the upper core columns would easily punch through the floor trusses in the lower section.
Consider the below very simplified collapse model:
In this model, what is actually taking the force of the collapse? The columns?
-Gumboot
