A progressive collapse would have either continued to accelerate, or terminated.
Dave
I disagree.
A progressive collapse is merely one that continues to an amount disproportionate to the initial failure.
It doesn't have to, and usually does not, progress to total collapse. And it didn't progress to total collapse in the case of either WTC1, 2 or 7. All of them had a few columns still standing at the end, but clearly fit into the category of progressive collapse.
There is nothing in the definition that says whether or not it must progress thru the structure at an accelerating, decelerating or constant rate.
The single most common example of progressive collapse - dominoes - progresses at a fairly constant rate. I'd guess that the speed is inversely proportional to the distance between dominoes.
The upper blocks of the towers initially accelerated, but to a very high probability, reached a near constant terminal velocity that occurred when gradually increasing force required for the expulsion of air (plus the other approximately constant fracturing & disassembling forces) equilibrated with the gradually increasing weight of the falling mass.
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BTW, I don't like the term "crush" used to describe what happened to the floors during the crush down phase.
In order to crush things, you have to restrain them from both directions. For the large structural components (core columns & concrete), they were not restrained from below until the falling mass reached the ground.
On the way down, the concrete fractured. Since it is brittle, it will throw out lots of small particles in a wide range of sizes coming from the fracture planes. But between the fracture planes, the vast majority of the concrete will remain in blocks & chunks, not reduced to small particles.
Most of the outer columns got thrown out of the footprint. No crushing there. The outer columns that got entangled within the falling mass, along with the core column, generally didn't get crushed either. Bent, fractured, but not crushed. Some did in some local areas, but that was few & far between in my opinion from looking at the columns in photos of GZ.
Comparatively weak structures (desks, office contents and, alas, people, etc.) obviously did get crushed. But the energy required for this is inconsequential in the big picture.
I think "fractured" is a better term than "crushed".
It describes the failures of connections, welds, support plates & concrete better than "crushed".