I don't not-buy that a floor falling is going to take out the next, and when they hit the third it will fall quicker and so on. You have more and more mass and inertia adding to it, making each floor easier to take off than the next. The WTC had none of that. From the moment it went, the whole thing was in free fall. Not even a slow top floor hitting the one below it. The entire building went down as if it was dropped.