Dave, Heiwa should defend himself. But as he already said he thinks about a kind of cd to explain the telescoping. I think that is consistent with a compaction above the weakest zone and an antenna failing first. Of course robust math and good observation is needed to prove that and that’s one of the reasons that I’m an agnost concerning this. One thing that I remember from the past is that the zone with the fire is not the impact zone, seen from the north side there is a failure left down and one at the right and more up. And as been mentioned in this thread a model is a model and an approach of the real situation. David B. Benson’s example of compacted layer of floors works well within that model, which means there is a crush-up and crush-down and that zone is in fact responsible for collapsing the next story and so on, the intact stories above follow that avalanche.
One of the things we see in the south towers collapse is that the top section indeed looks destroyed. The problem is that what we see is not always what is, it is hard to measure because of the smoke and dust. Of course a destroyed section’s mass is not lost, there is no difference in kinetic energy, the only difference is that if small pieces are responsible to crush a story the energy is not applied at the same time, for an easy example if n floors apply their kinetic energy at the same time it is (1/2)nmv^2, if this is just sufficient to collapse a next story then if it is applied per floor (remember I’m talking about the model and not about funnelling etc, which should split the energies and makes it much more complex) then that floor will be arrested. In the extreme situation that you drop 100% powder on the building it cannot transfer all its momentum at the same time and there will be no global collapse (it is a perfect damper that’s why far jumpers don’t land on concrete!), but that’s not what happened. And I disagree with M*Hans that the solid block is a worst case situation; it’s the most favourable situation.
