Pooh, sorry, I’ve only just noticed your April 21, 03:51 comment #52251; don’t know how I missed it.
Yes, I ignored mechanical resistance because I couldn’t do those calculations. So I went a different route.
I had a look at the truss seats (the connection of horizontal floor assemblies to the vertical frame), and they seemed pretty insubstantial, I thought they wouldn’t really slow things down much. But momentum transfer might, so I checked that manually for the first floor assembly below the damaged zone, and that didn’t slow things much either (~9%, and less each subsequent floor). Then came the discussion with Nikko and the spreadsheet, the one floor / ten floor correction, then eliminating Nikko’s arbitrary ejections, and it came out within the measured / estimated* collapse time (*we can’t quite see the end because the big dust cloud is welling up).
Then I calculated the energy available for deformation of materials when the falling stuff entrained another floor assembly. I chose a high collision for that because there’s more mass and velocity the lower you go. You know; you calculate the final velocity from the momentum equation, and then from the initial V and the final V, and the mass, you work out the kinetic energy before and after, subtract one from the other, and you’re left with the energy dissipated into deformation of materials, sound etc.. Convert that to TNT equivalent, and it looked plenty to break the truss seats. It doesn’t matter whether it breaks the floor assembly or the truss seat because this is a collapse, a failure; either will do.
I still didn’t have a conclusive collapse time, but then I realised, so what? Who’d run the risk of getting caught by lacing the towers with thermite or TNT or anything, just to make a tower collapse four seconds or whatever faster than it would have done anyway, without? They’d have to be stupid beyond comprehension.
So I put myself in their position and thought, what am I trying to achieve? An atrocity; loads of dead people. Well structural failure across a cross section would achieve that; the buildings will be a write-off anyway, everything’s well fucked, that’ll do. I don’t really care how big a stump of unusable building is left after, it’s going to be a massive problem to clear up no matter what. No, I don’t feel any need to lace the building with explosives or thermite or whatever, and all the hassle and risk that would entail. Just ensure structural failure across a complete cross-section; job done.
And then I found this: