like how minimal? what percent resistance?
Since it was not feasible to send anyone into the building to eyeball them, this cannot be really assessed, but, at some point, the columns below the obstructions that prevented our seeing the process in action may have offered ZERO resistance.
The back wall that we see most of was relatively undamaged. The narrower side was heavily damaged and on fire. Looking in through the windows in some of the video, the fire can be seen to be in flashover. Could be as much as 1000F, easily. Stand up in a room in that condition and you are dead.
Steel loses at 10% of its strength at 1000F, if I recall correctly. (It's late here so I am just going to wing it, and we have other fire fighters here to correct me if my memory is not correct.)
So we have the south side of the building only capable of supporting 90% of its normal load.
We have reports that fire department surveyors measured a considerable bulging of the building in that direction long before collapse.
Now let us initiate collapse. We need not quibble here about why collapse initiated. I have no background in chaos theory and it might just be a distraction at the moment.
Now you have the south face of the building accelerating downward, restrained to some degree by the still-strong north wall.
But the north face is under stress and being pulled downward and slightly southward by the collapsing south face. It leans a bit, stressing columns.
The building is a bit unconventional in design, having very few internal columns toi allow more office space without obstacles, so there are fewer than normal elements to arrest a collpase.
So we have the north face being tipped out over, effectively, a void.
Now the stressed north face columns break. There is nothing, really, to arrest them.
Can you, from what I have written here, estimate the acceleration of the north face in this condition?
I would say that there is nothing to arrest or resist the parts of the building that had swayed out over the void, once the columns broke. Think of the wall as a crane, supporting the whole of a building. When the crane's arm breaks, I see no way that it would fall any slower than g, at least until the bottom of it came to rest over a considerable pile of something.
Hope that helps.