NIST tells us it was 2.25 seconds or about eight stories of the fall. Please bear in mind that irrespective of any argument about columns having a minimum resistance during buckling, that there would be eight stories of mass to be accelerated by the falling upper mass if the collapse were natural. This would cause a decrease in velocity due to conservation of momentum alone. Why don't we see this?
Oh Cripes…
No, Tony, if the supports for those 8 floors buckle, they are NOT accelerated by the upper block.
Any more than the 30th thru the 39th floor are accelerated by the 40th thru 47th.
Those 8 buckled floors, along with every floor above it, are accelerated by exactly the same thing: their proximity to that hunk of rock called "the earth". i.e., by gravity.
The buckling (more accurately, the failure of the connections near the top & bottom of that 8 floor segment), has detached those floors from BOTH 1) the floors below AND 2) the floors above.
Detachment from the floors below means that those floors immediately start to fall on their own. Again, due to gravity.
Detachment from the floors above means that the impact of those 8 floors' deceleration when they collide successively with the floors below them is NOT transmitted to the upper block.
It is only once the lowest floor of the upper block
which is still structurally connected to the rest of the upper block contacts the lower debris, that the upper block slows from free-fall. As it did after the "Stage II" fall. And this contact happens after about 8 stories of free fall.
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And I hadn't thought of this before, but this constitutes proof that the unseen, disappeared in the rubble cloud, lower floors of the upper blocks of the WTC towers were NOT "disintegrated" or "blown apart". If they had been blown apart or disintegrated, then the upper-most segment of the upper block (i.e., the roofline) would have entered free-fall, just like WTC7 did.
There MUST be intact, load bearing structure between the crush zone & the topmost visible component of the building, if that topmost element is NOT descending at free fall.
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This does clarify for me one thing that had been running around the back of my mind about the collapses:
Why did WTC7 go into free fall (for a short period) while the towers did not?
The answer seems to be: WTC7 failed at two vertical locations separated by 8 stories (i.e., an 8 story buckle), while both towers failed at essentially one vertical location (a one story buckle).