1. Free fall and impact imply that the supports (the 280+ load bearing columns) disappear instantaneously in the fire zone. I would have expected gradual deformation of the supports due to heat/fire and no free fall/impact and that the columns, still connected to the upper block and lower structure would dampen any movement and get entangled in the lower structure, i.e. no sudden release of PE, no enormous velocity, etc.
2. See 1. I would expect the failed columns still to be connected both ends. Only IF they sheared off both ends - in order to support Bazant, Seffen - I would expect that they will never meet again.
You seem to be suggesting that while they deform, they retain their load bearing capacity. In reality, when a column buckles, once it gets past a certain point it's failure will rapidly accelerate. And your use of the word 'simultaneously' seems to imply that you don't understand progressive collapse. Generally, a progressive collapse starts slowly, with columns slowing deforming as you suggest above. But as each column loses it's load bearing capacity, the load on the remaining structure increases, so the remaining columns each fail faster than the previous ones. The result is a structure slowly losing its integrity, possibly with little visible from the outside, until it reaches a threshold where the remaining columns fail very rapidly. This was seen in all 3 WTC collapses. In 1 & 2, the columns of the outer walls could be seen slowly bowing inward until they reached the threshold and failed, at which point the rest of the structure at that level was quickly overwhelmed and failed. The fact that the upper block tilted shows that it was not 280 simultaneous failures, but a progressive failure starting on the side that buckled first and progressed towards the opposite side where the 'pivot' was.
3. An impact is a force exerted by one object when striking against another. If a 4000 m² large upper floor strikes against another 4000 m² large floor below, it is only the upper floor that strikes! Not the 15-20 other floors above. The weight of the other floors above is being transmitted to the columns ... that strike nothing.
The weight of the other 15-20 floors does not magically disapear. Like you said, it is being transmitted to the columns. If the upper block is moving, the whole thing has momentum and whatever those columns impact will have to absorb the impact of most of the structure, not just the lowest floor. It is completely impossible for them to 'strike nothing'.
4. See 3. No, it is only one floor involved.
And the rest magically levitating above?
5. ?? Evidently the upper block cannot disappear or get damaged prior local failures occur in the fire/heat zone below. Same with the structure below the fire/heat zone. Time for the upper block to free fall one storey is 0.8-0.9 seconds ... and is not seen on any video. And after these 0.8-0.9 seconds the lowest floor of the upper block is supposed to impact the uppermost floor of the structure below. Do you see that on any video? OK, smoke and dust are ejected (between intact wall columns)! Is that when the lowest floor of the upper block drops down? Nothing else has happened above?
How do you expect to see the actual impact? You can only see the outer facade of the buildings, which is immediately obscured by dust & smoke as soon as the upper block begins to move downward.
6. So why was smoke/dust ejected? Floor dropping down?
Yes, the upper block dropping causes the ejections. As the upper block moves downward, the inside volume of the building decreases. Air pressure rises, and the smoke/dust is blown out whatever openings it can find, which is the already broken facade from the plane crash, and the remaining windows that are destroyed in the initial collapse zone as soon as movement begins.
7. So a complete wall section failed first and the upper block tipped? And then the floor dropped down only on that side? OK! So the smoke and dust were mainly ejected on the side where the wall failed! On the other hand I see big amounts of smoke and dust on the opposite side.
See my answer to 1 & 2. The floor didn't only drop on that side. That side fell a fraction of a second before the other side, as proven by the tilt of the upper block. The smoke and dust were ejected wherever there were openings. Air pressure changes transmit at the speed of sound, the whole floor overpressurized at once.
8. No, Bazant/Seffen treat problem in 1-D that does not allow any elastic deformations anywhere. Everything is supposed to be rigid except at the crush front (one storey) where brittle fractures or something occur - not very clear actually from their papers - ripping columns apart like spaghetti. And when the crush front arrives at ground ... the upper block (rigid, uniform density) should just stop there. It was the gravity force of the upper block - always intact - that was driving the crush front to ground.
Again, rigidity would hamper the progression of the collapse, not aid it. The mass doesn't change if the upper block is in 1 piece or 10000, only the impact duration. But if the upper block disintegrates, you have a different problem, where all of the mass of the upper block is no longer supported by the only thing that could hold it, the columns, and instead is overloading the floors and the non load-bearing members of the structure. Had there only been a 'crush up' with no 'crush down', then the result would have been a pancake collapse once enough of the upper block piled onto the top floor of the lower block.
9. Uniform density, particularly of the upper block, is a basic assumption by Bazant/Seffen in their 1-D analysises. It suits them fine. But has nothing to do with reality. See 1.
It's a valid simplification. Reality can not be duplicated, you can only create models that approximate it.
Thanks phunk for good comments that really clarify topic.
You're welcome!