9/11 Physics: "You Can't Use Common Sense"

[William Seger]

30. I made no such claim

> So now we've move past (by abandoning it) the earlier claim that the kinetic energy of the upper part had to collapse the lower part no matter how it was delivered. That claim was clearly incorrect

You persist in arguing against straw-men. What I've said, repeatedly, is that all the energy must go somewhere in order to halt the collapse, which is quite true even if you spread it over any amount of time. Yes, if you do it in pieces such that no single impact is large enough to push the columns into plastic deformation and then allow them to rebound before dropping more, then yes collapse would be avoided. My claim is, then, so what? That has nothing to do with what actually happened: the whole top block fell at the same time.

> Now the claim (which isn't Bazant's claim) is that we can assume that all the kinetic energy of the upper part was delivered in less time than it would take the lower part columns to rebound.

That's two straw-men: (1) It isn't just a matter of spreading the energy transfer over time, because rebounding would also require periods when there was no force to resist rebounding; and (2), once again, there is no assumption that "all the kinetic energy of the upper part was delivered." Despite your repeated assertions that Bazant's analysis "assumes" all of it was delivered, what it actually shows is that only 1/8th of the total energy would be enough to crush all the columns, so that's all that needs to be transferred to justify the conclusion. So, at least you are correct that it isn't Bazant's claim either.

> I don't see how anyone can justify such an assumption.

In a word: momentum. If a column could not completely absorb or dissipate the energy transferred in the initial collision, then momentum would carry the falling mass right through the failing column, giving it no opportunity to rebound.

> In some cases the structures of the upper part would have failed first so that they would have delivered some fractional part of the overall kinetic energy during this new time interval that we're considering.

And again, "some fractional part of the overall kinetic energy" would be all that's necessary to crush all the columns.

> So if in some cases in real life it wasn't the case then what justification can we find for assuming it in the idealized proxy?

Because you haven't proposed any realistic way that all of the kinetic energy could possibly have been absorbed or dissipated, which is the crux of Bazant's argument. It's one thing to imagine (without evidence) that there were localized cases where the initial impact force was less than the failure load and then a column had some time to rebound before being hit again, but you have not even begun to demonstrate a realistic scenario where that mechanism could dissipate enough energy to make a difference in Bazant's conclusion. Assuming that not all of the available energy was transferred in the initial collision doesn't doesn't really do the trick, because any energy that wasn't transferred in that initial collision was still available for the next collision.

Any time that you base a conclusion on a simplified model, you should ask if the conclusion might be changed by a more precise model. If the collapse/no-collapse decision here was a close call, then we should indeed make the model more comprehensive and realistic before deciding. But it's nowhere near being a close call, which is the basis of Bazant's conclusion that total collapse was unavoidable -- not any assumption that "the top piece imparts all its force onto the lower part at one single instant, as a rigid monolithic object," as you misrepresented the argument. To make a more realistic model, you would need to include cases where floors and beams were ripped from columns -- failure modes that happened precisely because they required less energy than buckling the column that supported them. That is Bazant's justification for taking full column buckling as the upper-bound limit for how much energy could be dissipated. If you think you can dissipate any significant amount of energy by rebounding, then I'm afraid you'll need to show your work.