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In the thread which led to starting this one, David B. Benson pointed out that Bazant does not disclose the form of the force function used in B&L. A couple things about that jump out immediately.
It seems like a very significant omission. After all, the entire outcome depends on this! If it didn't matter, anything could be chosen. But it is the core of the analysis, and I suspect it matters a great deal.
No such disclaimer as to the inapplicability is given, neither is there an indication of the sensitivity of the outcome to mitigating adjustments or additions. It's a simple system which happens to bolster the confidence in a simple model, not a narrative for how it went.
Major_Tom wrote:But how can anyone know F from just some building specs?
David B. Benson wrote:OneWhiteEye --- I'm willing to consider several force functions.
All --- While not considered (much) when the towers were designed, it is now required for large sturctures in earthquake prone areas to carry out a thorough major earthquake study, up to the point of collapse. So structural engineers now routinely use com[puter simulations to shake the design hard enough for inelastic behavior to occur.
What happens is that one buys a copy of a modern FEA which is capable of handling progressive collapse. For certain structures even that has to be part of the design, although that is ratther rare AFAIK.OneWhiteEye wrote:Once it gets moving, you need a physicist on staff.
That is true, in general, for all of empirical science. However, there are only a few dissipative force function which approximate reality. Certainly possible to try all of those.einsteen wrote:but the problem is that it is not possible to know the real force functions.
The Sauret video and OneWhiteEye's treatment of it is of excellent quality. I could wish for a longer interval of observation, but if wishes were horses beggars would ride.We measure y(t) which is full of errors due to limited data.
Turns out it depends upon mass and speed, not distance.If we find the fn that fits best we cannot determine f as function of distance, ...
I compute, on the side, energy expended in resistance, kinetic energy and potential energy made available by the drop. The sum of the first two is always less than the energy available....but we can only guess energy drops per distance.
Dr. G wrote:Elastic and inelastic strain buildup in steel frame structures such as WTC 1 & 2 can absorb a lot of energy, but only up to a fracture limit.
Dr. G wrote:Heiwa:
The verinage technique of demolition PROVES without doubt that a smaller part of an isotropic or composite 3-D structure, when dropped on and impacting a greater part of same structure by gravity, can indeed one-way crush down the greater part of the structure.
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