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Here's the sequential mechanism. One floor collapses 70 or 80 floors up, The entire weight of the part of the building above that floor falls into the floor below,
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Let me stop you there, as I think this doesn't match with the video footage. The video footage shows that the part above the impact zone collapses in on itself whereby the impact zone 'stays put'. Only after the part above the impact zone has collapsed halfway to 2/3rds does the whole thing start moving down from the impact zone.
http://sumitbhatnagar.multiply.com/video/item/4