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Reply #175: Please throw that red herring back, it's under the legal limit [View All]

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William Seger Donating Member (1000+ posts) Send PM | Profile | Ignore Tue Nov-10-09 11:18 PM
Response to Reply #130
175. Please throw that red herring back, it's under the legal limit
> Bazant asserts that only crush-down will occur during the first phase.

Correction: Bazant asserts that in his simplified model, under the clearly stated assumptions and qualifications, there would be very little crush-up during the first phase, and he provides the math for making that assertion. He also provides the math that shows an accurate prediction of the collapse time using that simplified model, which demonstrates that the model is fundamentally sound despite the simplifications. The title of your OP is "Bazant's progressive collapse math is bogus," yet you persist in a bait-and-switch tactic of arguing something completely different: that Bazant's model doesn't match what you believe you see in the videos, so therefore, as a matter of pure personal opinion, you don't see any value to his model. But you have yet to address the actual logic of Bazant's argument. In an attempt to get you back on track, let me call your attention to this, from the paper you linked to:

In broad terms, this scenario was proposed by Baant (2001), and Baant and Zhou (2002a,b) on the basis of simplified analysis relying solely on energy considerations.
...
The kinetic energy of the top part of the tower impacting the floor below was found to be about 8.4 x larger than the plastic energy absorption capability of the underlying story, and considerably higher than that if fracturing were taken into account (Baant and Zhou 2002a). This fact, along with the fact that during the progressive collapse of underlying stories the loss of gravitational potential per story is much greater than the energy dissipated per story, was sufficient for Baant and Zhou (2002a) to conclude, purely on energy grounds, that the tower was doomed once the top part of the tower dropped through the height of one story (or even 0.5 m).
{Emphasis added.}


A falling mass has kinetic energy. In order to arrest the fall, that energy must be dissipated somewhere. The fundamental logic of Bazant's argument for using a simplified model is that even if you were to do a detailed analysis of where energy went other than crushing the lower structure, there would still be much more than could possibly be absorbed by plastic deformation of that lower structure. As noted in the section I quoted, he only considered plastic deformation even though fracturing would have dissipated less energy. Elsewhere, he notes that he has assumed that the connections between columns and the floor beams were strong enough to hold while the columns were being crushed, although it's a virtual certainty that in many cases, those connections were broken and columns were merely pushed aside, again dissipating less energy than his simplifying assumptions. His analysis is of a one-dimensional, straight-down force on the columns, even though we do know from the videos that both tops tilted, and we know that columns cannot sustain as much asymmetric, off-center loading as they can symmetric, centered loading, so even Bazant's estimate of plastic deformation energy is on over-estimate.

You persist in side-stepping this critical energy argument and instead keep pointlessly arguing that his model is not a realistic representation of what you believe actually happened, completely ignoring that Bazant himself says that it is not intended to be. You repeatedly claim that his crush-down/crush-up hypothesis is a major flaw in the analysis and conclusion, yet you have failed to establish that it's even a minor flaw: First, you have not demonstrated that the hypothesis is wrong, but much worse, you have not offered even a ghost of a rebuttal for how it could possibly refute the conclusion in Bazant's energy argument, even if you were right. Neither the available kinetic energy nor the ability of the lower structure to absorb energy are affected in any substantial way by when the upper section was destroyed!

All of this has been pointed out to you before in the original thread that you dredged up and again here, and yet you just keep making the same failed arguments. Unless you intend to actually address the actual Bazant argument at some point, I do believe we are finished.
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