Reply #159: More on the columns

[freedom fighter jh]

I am a civil engineer by training but not experience. (I am licensed.) As I said in my original post, I lack (as most of us do) specific information about the kinds of beams used in the columns. I also lack a steel reference manual, which, if I knew the kinds of beams used, would tell me the force needed to cause the beams to fail. It would also tell me the beams' dimensions, which would tell me the distance through which that force would act. That would be the energy to cause one beam to fail in one place. (But one would have to check all possible modes of failure and find the one that used the least energy). I hope that answers your question. If you have a clue to this information, I'd like to hear it.

If each column failed in more than one place then you would have to figure in multiple failures for each. Each column *must* have failed in more than one place, since there were no pieces of columns remaining that were several hundred feet long.

Here's how I got 20%: I calculated how long it would take the buildings to collapse in near free fall. By "near free fall," I mean the scenario that seems to me most consistent with the pancake theory: everything above the point of impact falls to the next floor, which is sitting unsupported but unmoving, waiting to be hit. Then gravity plus the momentum of the moving mass cause that floor to slam into the next one, etc, all the way down. I assumed all the energy of motion went into accelerating the next floor down. Realistically, of course, you would not expect a perfectly elastic collision, but I figured this was a good baseline assumption for later comparison. It came out to 9.2 seconds for the tower that was hit on the 85th floor. If the actual time of collapse was 10 seconds, that means the tower's average speed of collapse was (rounding off) 9/10 of what it would have been unimpeded. So (squaring and rounding off) its kinetic energy was 8/10 of what it would have been in freefall.

That 80% became kinetic energy and brought the towers to the ground. What happened to it after that I don't know.

I'm new to DU, as you can see, so I don't know how to post a spreadsheet, if such a thing is possible. But I'm sure you can replicate my calculations, and anyway my 20% is not far from the 25% you assumed in one of your posts.

Those columns had to be strong enough to slow the collapse by more than a hiccup. But to me the more important question is what happened to the columns. Only small pieces were left. How did they get destroyed? What could account for the force needed to make that happen?