Just read back a bit and missed a conversation on the pancaking and lack thereof.
As I understand it, the failure mechanism was the collapse of the exterior columns in the impact zone, not the floors. I've always understood the "pancaking" to refer to what happened AFTER collapse started - that is when the lower floors crashed down on each other.
However, from my own layman's observations, I feel the collapse, at least in the North Tower, was more about the disintegration of the exterior columns.
Certainly, from day one, I have described the collapse of WTC1 as looking like a banana being peeled. Hence my amusement at claims of it coming down "in its own footprint" like a "textbook controlled demolition". Massive sections of the exterior columns - 5, 10, 20 floors high - peeled away from the structure.
I am a bit firmer about this scenario, given the core remained standing for some time.
My hypothesis:
Due to the inward bowing at the impact zone, the collapse debris was contained almost exclusively inside the boundaries of the structure created by the exterior columns.
As they fell they "peeled" the exterior columns (which acted like a skin due to their componant structure of multiple floors and columns together in sections) away from the building.
Essentially you are left with a "naked" structure - the core and floors intact, with no exterior walls.
Except, of course, that's not going to happen. With nothing holding up the outer edges of the floors, and with the debris from above crashing down, the lower floors were shredded from the core one after the other, crashing down upon each other.
This left a severly damaged core standing on its own. Of course, it could not possibly remain standing like that, and within seconds it too collapses.
Once the floors started their "stacking" down the core, I can see this progression rapidly accelerating ahead of the "peeling" of the exterior columns - afterall the only thing keeping the floors up is the bolted points along the core and exterior.
That, balanced up against the collapse force crashing down across an acre of floor space. That's a lot of leverage.
Thoughts?
-Gumboot