David B. Benson wrote:I don't think so. The fit to the data is simply too good. No detectable humps or bumps: smooth descent of antenna mast feature.
Crumple zones in cars make crashes much smoother. On a smaller and non-destructive scale, shock absorbers and springs smooth out the potholes.
Too trivially light to be bothered with. Most of the mass of each tower was associated with the actual floors, not the structural steel.3) The bigger the spire, the more is bypassed.
Oh no, you've got my intention backwards. Most (all) of the strength of the structure is in the core and perimeters, yet very little of the overall mass as you say. Bypassing a substantial portion of the core and perimeter means a considerable amount of resistance is bypassed; a similar driving mass above has much less to overcome below. All this with very little sacrifice of driving mass as all the dead load is still there. This condition is already observed so doesn't need to be postulated. It actually needs to be accounted for.
Much less strength => less Zone C mass needed to maintain collapse rate.
You were saying a drop in resistive force of about 14% is required in order to let half of the top fall off half way through. The lower half is where the perimeters peeled off in great strips, and the surviving core portions become more substantial. This is precisely the region where a lot of the structural components were bypassed. There's greater capacity lower down, but a 14% reduction doesn't seem out of the question if 40-50% of the columns are still standing, however briefly.
Remember, too, if the block falls off it doesn't happen in a flash like flipping a switch.
That's the best choice of resistive force to match the data; I now use it in preference to the BLGB formulation. Zone C is like you on the top edge of your flowing snow avalanche.Avalanche in a chute could be intrinsically faster, closer to pure momentum transfer as described by Greening.
I like the whole concept. It seems close to reality, besides being the best fit. Maybe that's why it is the best fit.
Some things don't come through so well in still images. There's a particular close-up of the north wall of WTC2 as it fails, the one I used to create the column buckling GIF. Once the collapse is underway, the camera zooms out to capture the big picture and you can see stuff flowing past the still-standing portions of core and perimeter. This is not just dust and pulverized concrete, though there's plenty of that. It's everything that was there, that was solid, just a second ago. It's largely a fluid phenomena. Or borderline, certainly.
The word 'metastable' comes to mind and, if I may coin a term - externally organized criticality. Avalanche seems the closest analogy. Rapid phase transition from a stable but locally high energy minimum to a much lower energy state nearby in phase space.
With that in mind, there are some amazing avalanche videos on this page, almost worth getting a player on that box:
What I experienced (hiking, not skiing) was tiny and slooow by comparison. Only a little scary. These are massive, terrifying slides.
And a link to some avalanche research. Physical models using fluid.