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Jones' Critique of Bazant and Bazant's reply

cmcaulif

Critical Thinker
cmcaulif
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I don't know if this has come up before but I happened across a formal critique of Bazant's papers by Steven Jones with a formal reply by Bazant.

link

This deals with a lot of questions and comments that have come up on the board such as simultaneous crush up crush down, alleged violation of newtons third law, elastic waves, the assumption of treating the upper block as rigid, and even presents a new model where crush up and crush down occur simultaneously.
 
cmcaulif said:
I don't know if this has come up before but I happened across a formal critique of Bazant's papers by Steven Jones with a formal reply by Bazant.

link

This deals with a lot of questions and comments that have come up on the board such as simultaneous crush up crush down, alleged violation of newtons third law, elastic waves, the assumption of treating the upper block as rigid, and even presents a new model where crush up and crush down occur simultaneously.
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I had not read this one. Thanks!
 
Closing Comments:
Although everyone is certainly entitled to express his or her opinion on any issue of concern, interested lay critics should realize that, in order to help discern the truth about an engineering problem such as the WTC collapse, it is necessary to expend the effort to become acquainted with the relevant material from an appropriate textbook on structural mechanics.

Adherence to this principle would lead to more intelligent science-based discourse, and it would preclude the need for everyone to waste time on baseless critiques and unproductive discussions. Of even greater importance, it would avoid misleading and wrongly influencing the public with incorrect information.​
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Double ouch!​
 
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I hadn't seen that one either. Great find!

Useful information there. Two points I particularly liked:

1. The explanation of why "crush down" really does only affect one floor at a time, rather than compressing the entire lower structure like a spring (listening, Mr. Ross?). The reason, clearly stated, is that the stress wave exceeds the plastic limit, so it causes fractures before it can possibly propagate to the rest of the structure. Stress waves at the plastic limit are the strongest that can possibly be sent further. Simple.

2. Dr. Bazant spends a lot of time talking about "creep," or "solid flow" as he clarifies for the condensed matter physicists in the room. He makes an excellent point about how at even relatively low temperatures, over time this will still lead to structural failures.

I'm tempted, based on this simple argument, to make a comparative case for WTC 1 vs. WTC 7. Let us make the very rough approximation that WTC 1 and 7 were of similar structure (i.e. similar demand-to-capacity ratio, which isn't as dumb as it sounds), suffered similar weakening from impact (shaky, but bear with me), and that WTC 1 lost fireproofing in the initation region but WTC 7 did not. Then use NIST's heating results in NCSTAR1-5B for fireproofed vs. unfireproofed steel as a function of time. Then feed this temperature into the creeping flow relationship to estimate, for that temperature, the time to critical load for each structure. Compare to the actual times the buildings stayed standing, and nod in satisfaction.

Rough as heck, as first principles models always are, but simple to follow.
 
Yes, The discussion of creep was certainly helpful. I recall NIST's FE models showing the role of creep but a discussion from a more analytical standpoint always helps.

Bazant does a bit of work in probabilistic mechanics, so I wonder if he will modify his model to consider randomness in the structure's resistance due to eccentric impacts. Such a model obviously won't change the overall outcome as compared with the deterministic model, but I think if anyone is going to asses if it is possible to design a structure to resist this type of collapse, it is a necessary exercise.
 
Bazant is very impressive. It's the mark of an expert that his intuitions on a subject this complex turn out to be valid when they're fully analysed. Jones, on the other hand, yet again turns out not to have a clue what he's talking about. I'm glad I'm not him.

The maths of crush down / crush up is a bit of a handful here - solving simultaneous differential equations is something I like to avoid doing - but having gone over Bazant's reasoning, it seems quite clear to me why simultaneous crush up / crush down is not a stable situation. Once a crushed zone has developed, the upper block is resting on it. If collapse is progressing, the force between the lower block and the crush zone is necessarily less than the downward force on it due to gravity. The crush zone is therefore accelerating downwards at some acceleration A, which is positive and less than G. The force between the upper block and the crush zone is therefore M(G-A). The supports of the upper block are necessarily designed to resist a force of at least MG, because otherwise they would have collapsed in service. The upper block will not therefore crush until the crush zone decelerates, i.e. when it hits the ground, because M(G-A) < MG. It's interesting that this is a perfectly general solution that doesn't depend on anything other than the building being able to support its own weight.

A fascinating piece of work. Thanks, cmcaulif, for posting the link.

Dave
 
Same here... the link is much appreciated. I'm not fine tuned with the math but his overall explanation of the load distribution was a good bit of information. The effect of the impacts reducing load capacity bringing remaining columns closer to their load limits (at room temperature), and then losing more of that capacity to fire.

The work is impressive.
 
Dave Rogers said:
Bazant is very impressive. It's the mark of an expert that his intuitions on a subject this complex turn out to be valid when they're fully analysed. Jones, on the other hand, yet again turns out not to have a clue what he's talking about. I'm glad I'm not him.

The maths of crush down / crush up is a bit of a handful here - solving simultaneous differential equations is something I like to avoid doing - but having gone over Bazant's reasoning, it seems quite clear to me why simultaneous crush up / crush down is not a stable situation. Once a crushed zone has developed, the upper block is resting on it. If collapse is progressing, the force between the lower block and the crush zone is necessarily less than the downward force on it due to gravity. The crush zone is therefore accelerating downwards at some acceleration A, which is positive and less than G. The force between the upper block and the crush zone is therefore M(G-A). The supports of the upper block are necessarily designed to resist a force of at least MG, because otherwise they would have collapsed in service. The upper block will not therefore crush until the crush zone decelerates, i.e. when it hits the ground, because M(G-A) < MG. It's interesting that this is a perfectly general solution that doesn't depend on anything other than the building being able to support its own weight.

A fascinating piece of work. Thanks, cmcaulif, for posting the link.

Dave
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It's a fairly decent baseline assumption, however I can see two problems with it. The first is that it assumes that the upper block still has the ability to resist forces of M(G-A) after the massive damage it sustains from the intial collapse before the crush zone develops. The second is that the upper block still exists much at all after the crush zone develops.
 
Links don't work for me either.. stupid interwebnetzzz.
 
Here's a Google conversion of the PDF to HTML. Some of the formatting is a bit screwed up, but it's still mostly readable.
 
cmcaulif said:
thats weird, the link is from Bazant's personal webpage at Northwestern:

http://www.civil.northwestern.edu/people/bazant.html

the link to the paper is at the bottom of the page. Maybe there are problems with the site.
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I'm told by a reliable source that after this thread was started here, someone posted it over at Jones' site and that several days later, Steven Jones got his knickers in a knot about it, claiming, among other things, that Bazant posted a 2007 draft of Jones' and Gourley's "paper" that Bazant didn't have permission to post. Apparently, Jones also distanced himself from the Jones and Gourley "paper", saying that he removed his name from it because of timing issues with publication.

It was subsequently removed from Bazant's Northwestern page.

So, its removal might well have been a result of complaints by Jones and friends.
 
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I went to the source and emailed him. His response;

To clarify some points, it is temporarily withdrawn. But there is
nothing wrong in the existing text - attached.
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2 pdfs were attached. He uses 'temporarily withdrawn', inferring it will be back up.
 
Anyone have an update, if/when this paper will be available. Despite searching I am still unable to find it. Gotta a feeling it makes some interesting reading.

Woof!
 
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