OOS Collapse Propagation Model

[Major_Tom]

Myriad, when Bazant makes the following statement, what does he mean:

"So it must be concluded that the simplifying hypothesis of
one-way crushing (i.e., of absence of simultaneous crush-up),
made in the original paper, was perfectly justified and caused
only an imperceptible difference in the results. The crush-up
simultaneous with the crush down is found to have advanced
into the overlying story by only 37 mm for the North Tower
and 26 mm for the South Tower. This means that the initial
crush-up phase terminates when the axial displacement of
columns is only about 10 times larger than their maximum
elastic deformation. Hence, simplifying the analysis by neglecting
the initial two-way crushing phase was correct and
accurate."

Is he applying this to WTC1, or to just a hypothetical extreme case?

 

[DavidJames]

I'm very glad other posters have came out to defend the BV and BL papers.

Dave, In BL, can you explain why Dr Bazant insists that crush down must be complete before crush up occurs. Does he mean this literally?

R Mackey seems to be saying he doesn't mean this to apply to WTC1 literally.

What do you think?



That is in BL and I suggest all of you who wish to participate reread it carefully. You'd save me some time if you do.

Why the obsession with Bazant? It was written within a few days doesn't represent a full analysis, Why not critique the NIST report?

 

[Dave Rogers]

Dave, In BL, can you explain why Dr Bazant insists that crush down must be complete before crush up occurs. Does he mean this literally?

Bazant showed that initial crush-up would arrest very quickly, and that in effect the upper block would ride down on the intervening rubble block as it destroyed the lower structure. This is for the specific case of a perfectly level collapse with axial column-on-column impacts of a previously undamaged structure, which is the limiting case Bazant used to determine whether it was possible for the structure to resist collapse.

R Mackey seems to be saying he doesn't mean this to apply to WTC1 literally.

What do you think?

That's correct. As everyone keeps pointing out ad nauseam, Bazant is considering the limiting hypothetical case in which the structure is best able to resist collapse, and finding that it cannot.

A big part of the upper block was smashed into rubble for sure, then comes the dust cloud. What is rational to assume?
- that the rest of the upper block stayed intact and smashed the block underneath it.
- that the rest of the upper block was destroyed, too.

Neither. Without some kind of modelling the only rational assumption would be that we don't know whether the crush-up of the upper block continued, and anything more would be a guess. Since the only case that's been analysed indicates that crush-up is expected to arrest, then it's reasonable to assume that that was the case in reality.

Let me point out, though, that your two alternatives are highly misleading. The two possible assumptions are in fact:
- that the rest of the upper block stayed intact and smashed the block underneath it.
- that the rest of the upper block was destroyedsmashed to rubble, too, which was then able to smash the block underneath it.

I'm sure you didn't mean to suggest that the upper block ceased to exist and that the rubble couldn't have destroyed the lower structure, but it's best to clear these things up in advance.

Dave

 

[Dave Rogers]

Is he applying this to WTC1, or to just a hypothetical extreme case?

Since he doesn't account for the extensive damage to the structure preceding the collapse, it's obvious that this is a hypothetical case, isn't it?

Dave

 

[R.Mackey]

Why the obsession with Bazant? It was written within a few months doesn't represent a full analysis, Why not critique the NIST report?

It's much worse than that. Bazant and Le Zhou was written within 48 HOURS of the incident.

I talked about this in my discussions on Hardfire. Basically, Bazant and Le Zhou present a simplified model to illustrate certain features and phenomena relative to the actual events. The Truthers, casting about in desperation for a windmill to tilt at, seize not upon the phenomena nor on providing an alternate conclusion, but instead bitch about the model.

We know the model isn't precise. That's the whole point.

It's like watching reviewers in another country critique Shakespeare despite not understanding the language, going off what they get from the Google translator. The Truthers simply do not understand what they're talking about, and wind up criticising their own misapprehensions. That, plus being totally ignorant of the many other journal papers on the subject. Is it any wonder there's no point talking to them?

I also don't think the simple model is a perfect representation of reality. In fact, going back to Hardfire, I presented my own model (a cartoon, really, but it could be expanded on) of the collapse initiation and progression. It's in these slides. Reading this, the Truthers will probably crow that I've "refuted" or I "discount" Dr. Bazant, but of course that's not true at all.

 

[Dave Rogers]

It's much worse than that. Bazant and Le was written within 48 HOURS of the incident.

Don't you mean Bazant and Zhou?

Dave

 

[R.Mackey]

Oops. You are correct. Fixing now...

Too many acronyms!

ETA: I wonder what Major_Tom meant by "BL" then? There is no relevant article with only Dr. Bazant and Jia-Liang Le. I just assumed he meant Bazant & Zhou.

 

[bio]

Bazant showed that initial crush-up would arrest very quickly, and that in effect the upper block would ride down on the intervening rubble block as it destroyed the lower structure. (...)

Since the only case that's been analysed indicates that crush-up is expected to arrest, then it's reasonable to assume that that was the case in reality.

okay, I understand your statement so, that the rest of the intact upper block surfed down the tower.

Let me point out, though, that your two alternatives are highly misleading. The two possible assumptions are in fact:
- that the rest of the upper block stayed intact and smashed the block underneath it.
- that the rest of the upper block was destroyedsmashed to rubble, too, which was then able to smash the block underneath it.

I'm sure you didn't mean to suggest that the upper block ceased to exist and that the rubble couldn't have destroyed the lower structure, but it's best to clear these things up in advance.

Dave

Why you revise your statement above? Are you claiming that, the rest of the upper block was destroyed into rubble, but was nevertheless able to smash the block underneath it? If yes, then the question:

what do you / bazant mean with ...

""So it must be concluded that the simplifying hypothesis of
one-way crushing (i.e., of absence of simultaneous crush-up),
made in the original paper, was perfectly justified and caused
only an imperceptible difference in the results. The crush-up
simultaneous with the crush down is found to have advanced
into the overlying story by only 37 mm for the North Tower
and 26 mm for the South Tower. This means that the initial
crush-up phase terminates when the axial displacement of
columns is only about 10 times larger than their maximum
elastic deformation. Hence, simplifying the analysis by neglecting
the initial two-way crushing phase was correct and
accurate."

Perhaps there were two crush-ups?

The first crush-up of the upper section at the beginning of collapse, then the rubble of the upper block destroyed the block underneath it, then the second crush-up destroyed the rubble of the upper block?

 

[bio]

It's much worse than that. Bazant and Le Zhou was written within 48 HOURS of the incident.

I talked about this in my discussions on Hardfire. Basically, Bazant and Le Zhou present a simplified model to illustrate certain features and phenomena relative to the actual events. The Truthers, casting about in desperation for a windmill to tilt at, seize not upon the phenomena nor on providing an alternate conclusion, but instead bitch about the model.

We know the model isn't precise. That's the whole point.

It's like watching reviewers in another country critique Shakespeare despite not understanding the language, going off what they get from the Google translator. The Truthers simply do not understand what they're talking about, and wind up criticising their own misapprehensions. That, plus being totally ignorant of the many other journal papers on the subject. Is it any wonder there's no point talking to them?

I also don't think the simple model is a perfect representation of reality. In fact, going back to Hardfire, I presented my own model (a cartoon, really, but it could be expanded on) of the collapse initiation and progression. It's in these slides. Reading this, the Truthers will probably crow that I've "refuted" or I "discount" Dr. Bazant, but of course that's not true at all.

Please answer the straight forward questions of Major_Tom.
It would be interesting.

 

[Newtons Bit]

Please answer the straight forward questions of Major_Tom.
It would be interesting.

Why? I already did. He ignored the responses.

 

[Myriad]

Myriad, when Bazant makes the following statement, what does he mean:

"So it must be concluded that the simplifying hypothesis of
one-way crushing (i.e., of absence of simultaneous crush-up),
made in the original paper, was perfectly justified and caused
only an imperceptible difference in the results. The crush-up
simultaneous with the crush down is found to have advanced
into the overlying story by only 37 mm for the North Tower
and 26 mm for the South Tower. This means that the initial
crush-up phase terminates when the axial displacement of
columns is only about 10 times larger than their maximum
elastic deformation. Hence, simplifying the analysis by neglecting
the initial two-way crushing phase was correct and
accurate."

I'm pretty sure he means that it must be concluded that the simplifying hypothesis of one-way crushing (i.e., of absence of simultaneous crush-up), made in the original paper, was perfectly justified and caused only an imperceptible difference in the results. This, he explains, is because the crush-up simultaneous with the crush down is found to have advanced into the overlying story by only 37 mm for the North Tower and 26 mm for the South Tower. Which, he goes on to explain, means that the initial crush-up phase terminates when the axial displacement of columns is only about 10 times larger than their maximum elastic deformation. Hence, he concludes, simplifying the analysis by neglecting the initial two-way crushing phase was correct and accurate.

Is he applying this to WTC1, or to just a hypothetical extreme case?

He is applying it to his model of a WTC tower, which incorporates best-case assumptions for the possibility of collapse arrest. That he is analyzing a model is indicated by the methodology used -- which is performing mathematics on the model, as opposed to, say, running up and down the tower with a tape measure as it was collapsing.

In other words, complete crush-down preceding significant crush-up is not an arbitrary assumption imposed on the best-case model to simplify the calculations, it is also a result derivable from analysis of the best-case model.

If it could be shown with actual evidence that crush-down did not in fact precede crush-up in the real event, all that would mean is that the real case was far worse, in terms of the likelihood of collapse arrest, than the assumed best case. (Which we already knew, because for most of the collapse, the load sheared floors from the columns instead of buckling the columns, reducing the actual amount of energy absorbed in inelastic strain to only a small fraction of the maximum theoretically possible).

Again: if your goal is to show that there should have been a better than best-case outcome, showing that there was a worse than best-case event won't get you there, or even move you in the right direction. You're basically pushing on a rope here.

Respectfully,
Myriad

 

[bio]

(...)

If it could be shown with actual evidence that crush-down did not in fact precede crush-up in the real event, all that would mean is that the real case was far worse, in terms of the likelihood of collapse arrest, than the assumed best case. (...)

See video evidence of WTC-1 collapse, crush up precedes crush down in the first seconds. Why continued crush up would not had lead to collapse arrest?

 

[femr2]

I don't really see the point of spending too much time on Bazant et al. It is what it is. It was always a limiting case, not intended to be used as a direct description of the actual behaviour, and where is is used in direct context of describing the actual motion of WTC 1/2 it's, in all fairness, being mis-applied.

I do see fair reason to produce a model which does apply directly to observable mechanisms, and can be applied directly to actual behaviour.

I don't see any argument that the primary destructive mechanism involved the separated *crush fronts* as described, and a model which takes account of energy requirements to do so is, if even just for academic purposes, worth progressing.

Development of the model which also predicts the observed terminal/linear velocity of the *crush fronts* would also be useful.

Determining the requirements for initiation of the process, such that it is self-sustaining, will also provide a lot of information about the behaviour during initiation, some of which cannot be seen visually.

Instead of arguing about Bazant et al, why not develop the equations of motion which directly apply to *ROOSD*, which should, for WTC 1, result in around a 14.5s timing to ground, including propogation through the mechanical floor regions.

 

[Newtons Bit]

I don't really see the point of spending too much time on Bazant et al. It is what it is. It was always a limiting case, not intended to be used as a direct description of the actual behaviour, and where is is used in direct context of describing the actual motion of WTC 1/2 it's, in all fairness, being mis-applied.

Ok.

I do see fair reason to produce a model which does apply directly to observable mechanisms, and can be applied directly to actual behaviour.

Why?

Development of the model which also predicts the observed terminal/linear velocity of the *crush fronts* would also be useful.

Why?

Determining the requirements for initiation of the process, such that it is self-sustaining, will also provide a lot of information about the behaviour during initiation, some of which cannot be seen visually.

How?

Instead of arguing about Bazant et al, why not develop the equations of motion which directly apply to *ROOSD*, which should, for WTC 1, result in around a 14.5s timing to ground, including propogation through the mechanical floor regions.

Good luck.

 

[Major_Tom]

A word of advice for all posters:

The papers Bazant & Zhao and Bazant & Verdure present different arguments with different purposes.

BZ describes a best case scenario for survival.

BV derives actual equations of motion which are intended to be applicable actual buildings, the most famous case being WTC1.

If you keep mixing the concept "best case scenario for survival" with the actual equations of motion derived in BV intended to describe the movement of actual buildings, you will argue yourselves into a corner.


................

Myriad, in BL, can you explain why Dr Bazant insists that crush down must be complete before crush up occurs. Does he mean this literally?

 

[Dave Rogers]

Why you revise your statement above? Are you claiming that, the rest of the upper block was destroyed into rubble, but was nevertheless able to smash the block underneath it?

No, nor would anyone honestly read that sense into what I said. I'm pointing out that, even if your unsubstantiated assumption were true in that the upper block was pulverised first, the rubble would still have been available to destroy the lower structure, in that a very large mass of falling debris would still have been present which the lower structure did not have the structural capacity to arrest.

Perhaps there were two crush-ups?

The first crush-up of the upper section at the beginning of collapse, then the rubble of the upper block destroyed the block underneath it, then the second crush-up destroyed the rubble of the upper block?

Yes, that's exactly what Bazant's case predicted. However, the first crush-up was small enough not to make any significant difference to the overall result. I see Myriad's also rephrased that for you; do you need a third synonymous paragraph or will two be enough?

Dave

 

[Dave Rogers]

A word of advice for all posters:

The papers Bazant & Zhao and Bazant & Verdure present different arguments with different purposes.

BZ describes a best case scenario for survival.

BV derives actual equations of motion which are intended to be applicable actual buildings, the most famous case being WTC1.

If you keep mixing the concept "best case scenario for survival" with the actual equations of motion derived in BV intended to describe the movement of actual buildings, you will argue yourselves into a corner.

Another cautionary word, though: Bazant and Verdure's model is a simplified one-dimensional homogeneous approximation to the behaviour of a real building, which is intended to reproduce gross features of collapse behaviour. Any attempt to apply it to the detailed features of collapses of specific, three-dimensional, inhomogeneous and (most importantly) already locally damaged buildings is absolutely certain to fail.

Having said that, would you like to repeat your enquiry about the model's predictions as applied to the detailed features of the collapse of a specific, three-dimensional, inhomogeneous and already locally damaged building, if you think it's a topic worth pursuing?

Dave

ETA: And also, since you've stressed the importance of not confusing Bazant & Zhou with Bazant & Verdure, could you please clarify which one you're referring to as BL?

 

[Myriad]

Myriad, in BL, can you explain why Dr Bazant insists that crush down must be complete before crush up occurs.

Dr. Bazant provided a rigorous explanation of that, in the analysis just before he stated the conclusion resulting from that analysis. The conclusion was: "So it must be concluded that the simplifying hypothesis of one-way crushing (i.e., of absence of simultaneous crush-up), made in the original paper, was perfectly justified and caused only an imperceptible difference in the results." If you read the part before that conclusion, you'll see the reasoning leading to that conclusion.

Does he mean this literally?

He is applying it to his model of a WTC tower, which incorporates best-case assumptions for the possibility of collapse arrest. That he is analyzing a model is indicated by the methodology used -- which is performing mathematics on the model, as opposed to, say, running up and down the tower with a tape measure as it was collapsing.

In other words, complete crush-down preceding significant crush-up is not an arbitrary assumption imposed on the best-case model to simplify the calculations, it is also a result derivable from analysis of the best-case model.

If it could be shown with actual evidence that crush-down did not in fact precede crush-up in the real event, all that would mean is that the real case was far worse, in terms of the likelihood of collapse arrest, than the assumed best case. (Which we already knew, because for most of the collapse, the load sheared floors from the columns instead of buckling the columns, reducing the actual amount of energy absorbed in inelastic strain to only a small fraction of the maximum theoretically possible).

Again: if your goal is to show that there should have been a better than best-case outcome, showing that there was a worse than best-case event won't get you there, or even move you in the right direction. You're basically pushing on a rope here.

Respectfully,
Myriad

 

[Major_Tom]

BL is Bazant and Le, at totally different paper. We are dealing with 3 different papers, Bazat and Zhao (BZ), Bazant and Verdure (BV) and Bazant and Le (BL).

My beef is with BV and BL, in which crush up and crush down equations of motion are derived and justified.

Another cautionary word, though: Bazant and Verdure's model is a simplified one-dimensional homogeneous approximation to the behaviour of a real building, which is intended to reproduce gross features of collapse behaviour.

Let's ask B and V.

From the introduction:

"attention will be focused on the progressive collapse,
triggered in the WTC by fire and previously experienced
in many tall buildings as a result of earthquake or explosions
(including terrorist attack). A simplified one-dimensional analytical
solution of the collapse front propagation will be presented. It
will be shown how this solution can be used to determine the
energy absorption capability of individual stories if the motion
history is precisely recorded. Because of the shroud of dust and
smoke, these histories can be identified from the videos of the
collapsing WTC towers only for the first few seconds of collapse,
and so little can be learned in this regard from that collapse.
However, monitoring of tall building demolitions, which represent
one kind of progressive collapse, could provide such
histories. Development of a simple theory amenable to inverse
analysis of these histories is the key. It would permit extracting
valuable information on the energy absorption capability of various
types of structural systems in various collapse modes, and is,
therefore, the main objective of this paper."

I think they state it clearly.

The product of the paper, which are the equations of motion, is equation 12 (crush-down) and equation 17 (crush-up).

Current knowledge of features of the WTC1 collapse show that BV cannot be applied to WTC1. In fact, BV has nothing to do with WTC1 at all.

Why? Because the 4 simplifying assumptions used to derive the equations contradict what we now know about the collapse mechanics.

 

[bio]

(...) even if your unsubstantiated assumption were true in that the upper block was pulverised first, the rubble would still have been available to destroy the lower structure, in that a very large mass of falling debris would still have been present which the lower structure did not have the structural capacity to arrest.

(...)

I am really not an expert, but my common sense says that:

When the upper block is smashed by the intact block underneath, then the upper block had lost its destructive energy.