Merged: Applicability of Bazant's model to the real world

[pgimeno]

This text addresses how and in what sense Bažant's model, initially stated in [B 2001] and [BZ], and refined in [BV] and [BLGB], with clarifications in [BL] (see below, References), can or can not apply to real world buildings. The author of this text is a layman in the matters in question, yet felt the need for an explanation to exist in view of recent discussions in other threads. Ideally, this analysis should have been done by someone in the structural engineering field who could write such an explanation authoritatively, but apparently noone felt compelled enough to do so, leaving the author the burden to write it despite his lack of qualification. The inaccuracies arising from it may be the result of the lack of author's understanding of engineering principles and practices, and the author welcomes any constructive criticisms aimed at addressing such inaccuracies as long as they are stated with arguments other than incredulity and disbelief, and preferably with sound engineering arguments.

The need for this text arises from the fact that, as stated by another forum member recently, many skeptics of the theory of a government conspiracy misinterpreted Bažant's papers, methods and conclusions, including the author at some point in past, plus there's a new wave of 9/11 deniers who are misinterpreting them for discrediting purposes. So it's important to know how far the scope reaches; where the model can and where it can't be applied to real buildings. Some people, in both sides of the discussion, believe that the models are intended to be applied literally to buildings as a description of what happened. Others, from both sides of the discussion, think that the model is just a theoretical limiting case with no applicability to real buildings. The author has believed both of these things at different points in time, and hopes to prove here that both are incorrect.

A model is an idealized representation of a physical phenomenon in the real world, which can be used to study its behavior and make predictions about it. An example of a model is Newtonian physics. It can describe a wide range of behaviours of our environment and that makes it useful for us.

However, most models, if not all, have limitations. Newtonian physics don't describe the behaviour of objects traveling at a speed close to that of light, for example. That led to the creation of a revised model, special relativity physics, further generalized by general relativity physics. That model still has limitations. And yet, Newtonian physics are still useful as an approximation, as long as we know where the model's limits are and don't ask more of it than what it can give.

When we are confronted with the task of modeling a building collapse, there's one thing that becomes immediately obvious. It's flat impossible to make an accurate model that copes with all possible structural failure causes and modes as they happen in the real world. The reason is simple: the process is too chaotic and even tiny variations in the pressure, inclination, density or other parameters of the structural and non-structural elements can make e.g. a column bend in different angles, with the potential of causing it to impact other columns that would otherwise not be affected by the failure of that column, or to miss them where they would be affected. That's just a very tiny example of how chaos affects a collapse. Even any attempt at a computer simulation of a collapse will be subject to uncertainties in a myriad of tiny parameters such as the exact real-world positioning of a column or girder or the exact distribution of the strength of a welded connection, which will inevitably make the simulation deviate from the real world collapse as it happens, to the point that only a handful of columns, if at all, would show a movement which has any resemblance to the real world. This aspect is clearly demonstrated by the NIST simulations of WTC7 collapse, in which, even if the collapse development exhibits some features of the actual one, the details differ substantially.

To put a bit of order in that chaos, in order to get a workable model which can be rigurously formalized, Bažant made a simplification to his modeling of the WTC collapse: the column impacts would always be axial, regardless of how they would be in a real case.

What is an axial impact:

Axial impact is the impact that is produced when the impacting surface is the base of the column when impacting vertically. Similar to how a billiards cue stick hits the ball.

This brought consequences to his formalization. One of them was that in such a situation, using the parameters for the towers, the collapse progression would happen by first crushing down the bottom part of the building, and then crushing up the top part. This is the infamous crush-down/crush-up sequence which has been in the center of most misunderstandings and criticisms to Bažant's work.

Obviously, these axial impacts between columns would be quite unlikely for many buildings and failure modes. For example, a possible outcome of an impact of a vertical column with an inclined surface is that the column is dislodged rather than bent, depending on surface angle and other factors, resulting in the column acquiring an inclination itself and losing its ability to carry loads, after using probably less energy than a plastic axial deformation would require.

What is a plastic axial deformation:

When a column impacts axially (see above), it behaves much like a spring. Just as springs, it can be compressed, and just as springs, it can recover its shape after the force that caused its deformation ceases. However, if it's compressed too much, it will suffer a plastic deformation, that is, it won't recover its shape anymore. This can't be reproduced in all springs by compressing them, but it can be reproduced in many springs by pulling them and forcing them to expand a lot. Eventually, they will be deformed and will lose the ability to recover their original shape. With columns, the principle is the same and applies to compression as well.

Despite that and other simplifications, the model was still useful to draw conclusions from it. In particular, already in [B 2001] Bažant stated (p.1):

The details of the progression to failure after the decisive initial trigger that sets the upper part of the structure in motion are of course more complicated. The upper part of the structure, for example, tilts as it falls; furthermore, because the structure is a framed tube with floor beams of large spans, the impacted floors may collapse ahead of the tube, thus depriving the tube wall of its lateral support against global buckling. Regardless of these and other details, however, we can make the following two simple and crude estimates of the overload ratio of the columns of the floor just below the critical floor that triggered the catastrophic chain of events.

From there it is immediately obvious that Bažant was perfectly aware that there were important differences between his model and the real behavior since the very beginning. That didn't prevent him from going on drawing a crude estimate from it. Had he modeled crush-up occuring simultaneously with crush-down, that would probably mean a factor of two deviation in some of his results. Yet that wouldn't affect the outcome, as he explained in an addendum of [BZ] (p.370):

Once accurate computer simulations are carried out, various details of the failure mechanism will undoubtedly be found to differ from the simplifying hypotheses made. Errors by a factor of 2 would not be terribly surprising. But that would hardly matter since the analysis in the paper reveals order-of-magnitude differences between the dynamic loads and the structural resistance. Crude order-of-magnitude estimates made easily by pencil suffice in this case to rule out various intuitive theories that were advanced to explain the collapse.

The corollary is that it is possible to draw some conclusions even from an inexact model, just not any conclusions, and we have to be careful about which ones can be extracted and which ones won't match the real world to any sufficient degree of accuracy at all.

This approach is used several times in [BLGB] to make comparisons and extract several consequences. For example, the free fall myth is finally put to rest in some fronts, including videos from the start of the collapse and seismic records. Today that myth seems to be losing some strength, fortunately.

In particular, the comparison with videos requires a whole section dedicated to compensating the model for the tilt that it does not provide, because it assumes that the top falls straight (see p.901, "Analysis of Video-Recorded Motion and Correction for Tilt").

In order to fine-tune a model so that it more closely resembles the real world whenever the parameters being considered can't be solved in the theoretical field, measurements are taken if possible and applied to the model. That is what NIST did when several severity cases were elaborated, of which the most severe one was picked in view of the results. This proceeding, which has been widely used to blame NIST for making up data, is just a standard parameter adjusting practice for the model to fit reality as closely as possible.

That proceeding is indeed described in the [BLGB] conclusions (p.905, emphasis added):

Several of the parameters of the present mathematical model have a large range of uncertainty. However, the solution exhibits small sensitivity to some of them, and the values of others can be fixed on the basis of observations or physical analysis. One and the same mathematical model, with one and the same set of parameters, is shown to be capable of matching all of the observations, including: (1) the video records of the first few seconds of motion of both towers; (2) the seismic records for both towers; (3) the mass and size distributions of the comminuted particles of concrete; (4) the energy requirement for the comminution that occurred; (5) the wide spread of the fine dust around the tower; (6) the loud booms heard during collapse; (7) the fast expansion of dust clouds during collapse; and (8) the dust content of the cloud implied by its size. At the same time, the alternative allegations of some kinds of controlled demolition are shown to be totally out of range of the present mathematical model, even if the full range of parameter uncertainties is considered.

Here's an example of such finetuning also proposed in [BV] (from the abstract, p.308, emphasis added):

The parameters are the compaction ratio of a crushed story, the fracture of mass ejected outside the tower perimeter, and the energy dissipation per unit height. The last is the most important, yet the hardest to predict theoretically. It is argued that, using inverse analysis, one could identify these parameters from a precise record of the motion of floors of a collapsing building. Due to a shroud of dust and smoke, the videos of the World Trade Center are only of limited use. It is proposed to obtain such records by monitoring with millisecond accuracy the precise time history of displacements in different modes of building demolitions.

Finally, there have been some claims that the comments in [BL], in particular points 4 ("Can Crush-Up Proceed Simultaneously with Crush Down?", p.917) and 5 ("Why Can Crush-Up Not Begin Later?", p.919) mean a direct application of the crush direction part of the model to the WTC 1 and 2 as if it was what actually happened. That is wrong. That section is dedicated to discussing the theoretical basis of the crush direction part of the model, about which [G 2008] objected, as this excerpt shows (p. 915):

Applying Newton's third law to the collapse of the Twin Towers, it is clear that the downward force imposed on Part B by the upper Part C generates an equal but opposite upward force. It logically follows that if the downward force generated when Part C impacts Part B is destructive, then the equal and opposite upward force generated in accordance with Newton's third law will be destructive. Instead of embracing this basic law of physics, the paper treats Part C as a rigid body during the crush-down phase, then allows Part C to start deforming only at the start of the crush-up phase

Since the author's always been not-so-good for writing closing words, please indulge him in his use of [BLGB] to do it:

These conclusions show the allegations of controlled demolition to be absurd and leave no doubt that the towers failed due to gravity-driven progressive collapse triggered by the effects of fire.

(Thanks to ozeco41 for raising the point that led to writing this text.)


References

• [B 2001]: Bažant, Z.P. (2001), Why Did the World Trade Center Collapse?. Siam News, Vol. 34, No. 8, pp. 1 and 3.
• [BZ]: Bažant, Z.P.; Zhou, Y. (2002), Why Did the World Trade Center Collapse?—Simple analysis. Journal of Engineering Mechanics ASCE 128 (No. 1) pp. 2-6, with addendum March (No. 3), pp. 369-370. It's an expansion of the former with corrections, appendices and an addendum published later.
• [BV]: Bažant, Z.P.; Verdure, M. (2007), Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions. Journal of Engineering Mechanics ASCE 133 (No. 3), pp. 308-319.
• [BLGB]: Bažant, Z.P.; Le, J.-L.; Greening, F.R.; Benson, D.B. (2008), What did and did not cause collapse of World Trade Center twin towers in New York. Journal of Engineering Mechanics ASCE 134 (No. 10), pp. 892-906.
• [BL]: Bažant, Z.P.; Le, J.-L. (2008), Closure to "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" by Zdeněk P. Bažant and Mathieu Verdure. Journal of Engineering Mechanics ASCE 134 (No. 10), pp. 917-923.
• [G 2008]: Gourley, J.R. (2008), Discussion of "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" by Zdeněk P. Bažant and Mathieu Verdure. Journal of Engineering Mechanics ASCE 134 (No. 10), pp. 915-916. It's the discussion which is followed up by the above closure. The PDF file in which both appear is the same.

[femr2]

Originally Posted by pgimeno

the author welcomes any constructive criticisms aimed at addressing such inaccuracies as long as they are stated with arguments other than incredulity and disbelief, and preferably with sound engineering arguments.

Firstly, it's good to see additional attempts at clarifying scope.

A couple of points, which I hope you will not take as nit-picks...

Quote:

there's a new wave of 9/11 deniers who are misinterpreting them for discrediting purposes

9/11 deniers ? Strange turn of phrase. I've commented that there are folk who apply the model too literally, but the purpose is clarity, not *discrediting purposes*.

Quote:

Since the author's always been not-so-good for writing closing words, please indulge him in his use of [BLGB] to do it:

These conclusions show the allegations of controlled demolition to be absurd and leave no doubt that the towers failed due to gravity-driven progressive collapse triggered by the effects of fire.

Ah. Not a great closure in my opinion.

The model does not include initiation, and so does not deal with what caused failure.

It proves that it is possible, post initiation, for gravity to drive progressive collapse. (ROOSD is imo a good description of that process)

It does not disprove.

To illustrate, and to illustrate only... If *evidence* arose tomorrow, which *proved* that *space aliens* *made it happen on purpose*, then the scope of the model does not change at all.

The conclusion steps outside the bounds of model applicability.

[Major_Tom]

Here is an excellent example of somebody applying the concept of crush down then crush up to the description of a 3-D building:

Originally Posted by R.Mackey

This is not the only factor, but it too is partially correct.

Think of it in terms of impulse -- the total change of momentum at a particular impact. Impulse is equal and opposite, by conservation of momentum. Impulse is equal to F delta-T (force times the time over which the force is applied), or M delta-V (the raw change of momentum in its familiar definition P = m V).

When we look at the "upper block," it's delta-V is smaller than the delta-V experienced by the newly broken part of the lower block. As you say, the upper block decelerates by an average 1/3 g, while the lower block accelerates by an average 2/3 g. This is because the participating part of the lower block masses less than the participating part of the upper block -- it really is the compacted mass and upper block versus a small number of floors at a time, not the entire lower block.

The reason only part of the lower block participates at any given time is because the lower block is still a mostly intact sparse structure of braced columns. When it's hit, the columns lose bracing, get loaded eccentrically, shear their welds and bolts, and in some cases are totally overwhelmed and fracture entirely. These pieces break at a stress much too low to actually support the descending mass. This also has nothing to do with the strength of the perfectly intact building -- the descending rubble heap isn't contacting the lower structure at its strongest points, and it's introducing brand new failure modes, so the effective opposing strength of the lower structure is far lower than its ideal carrying capacity. Furthermore, where the lower structure does resist at or near its ideal strength, it can only do so for a very brief delta-T -- until reaching its failure strain, which takes only about ten milliseconds at the speeds of collapse -- and this is not enough to amount to all that much total impulse.

The upper chunk, in contrast, is cushioned by a thick layer of rubble. This is compacted about as far as it can, thus it doesn't have those complex failure modes and it doesn't suffer much more "damage" even at much higher stresses. So the rubble pile remains, and the lower structure gives way. This is for the same reason you don't sink into the ground, even though you can push your finger easily through a cupful of soil.

The "upper block," what remains of it, rides on top of this cushion of debris. It is supported pretty well. It also only decelerates at that lower rate, thanks to the much greater inertia of the upper block + debris. So the only real force it suffers is the inertial force, i.e. its own self-weight times its deceleration, again about 1/3 g. It can be expected to survive this deceleration. It's only when the rubble pile has nowhere else to go and the upper block has to suddenly stop, dissipating all of its momentum in mere milliseconds, that it totally fails.

Again, this is slightly idealized, but you get the point. Unless you're a Truther.

He calls this discription based on a literal interpretation of the 1-D stick model introduced in BV "slightly idealized".

He clearly believes in the magic zone B rubble layer and the upper block that rides the rubble to earth like the Virgin Mother descending from the clouds.


Honestly, pgimeno, is not R Mackey taking the 1-D concept of crush down, then crush up quite literally in his description? He seriously believes that this upper block exists, just like Bazant.

[pgimeno]

Originally Posted by femr2

9/11 deniers ? Strange turn of phrase. I've commented that there are folk who apply the model too literally, but the purpose is clarity, not *discrediting purposes*.

See below your post.

Originally Posted by femr2

Ah. Not a great closure in my opinion.

The model does not include initiation, and so does not deal with what caused failure.

Ah, but that's because you're looking for the philosopher's stone, that thing which will make you rich and famous once you find any evidence of it and that you keep looking for because you know it must exist somewhere.

That paragraph addresses the theories that were into fashion by then. But belief currents wear off too, e.g. nobody is pushing for thermate or freefall of the towers these days.

[beachnut]

It is cool Bazant and company are able to do on a napkin what 911 truth can't do at all given all the answers and 8 years!

Funny, 911 truth CD delusion cult members attack Bazant's model and fail to understand their attacks prove they don't understand models, engineering, and more. Where do their idiotic CD delusions come from?

Bazant's model is like a trap to catch people with idiotic claims as they attack and comment on Bazant's work to expose their incompetence and ignorance. Why do they not have a clue. Kind of makes it clear they are not engineers, only a tiny fraction of engineers are prey to delusions like the lies 911 truth pushes.

What level do you have to be at to understand without effort why the WTC Towers were doomed by impacts 7 to 11 times greater than design, and fires set with 66,000 pounds of accelerant!

http://www.nae.edu/Publications/TheB...adeCenter.aspx Robertson, the first expert on the WTC towers structure

The answer is easy, and can be found with google! Amazing Google can be used to support reality if you are capable of critical thinking and logical skills to weed out the lies, hearsay and delusions of 911 truth. Robertson calls the CD delusions nonsense.

Google "lead structural engineer reflects, fall of the World Trade Center towers", and you find someone uniquely qualified to say the WTC towers fell due to impacts and fire. And he is proved correct by many independent studies, and he calls 911 truth claims nonsense.

How many 911 truth failed idea believers will show up to attack Bazant and try to twist things so they can back in their favorite fantasy on 911?

People should not attack Bazant's work, they need to do what it takes to complete their own work and see if it floats; publish it in a real journal. 911 truth can't do that, they can only attack NIST, Bazant's, and if they could do research I suppose they would attack the hundreds of other studies, papers and research showing impacts and fires did it. 911 truth are challenged researchers who can't find evidence or other work and understand those efforts to understand their claims are idiotic delusions.

How many loyal 911 truth cult members will show up to battle Bazant and fail faster than free-fall?

[bill smith]

Readers whould check out what Major Tom has to say in the link that pgimeno has distanced himself from. Is Bazant worthwhile ? Was Bazant ever worthwhile really ?

http://www.internationalskeptics.com...3&postcount=83

[ozeco41]

Originally Posted by pgimeno

...The need for this text arises from the fact that, as stated by another forum member recently, many skeptics of the theory of a government conspiracy misinterpreted Bažant's papers, methods and conclusions, including the author at some point in past, plus there's a new wave of 9/11 deniers who are misinterpreting them for discrediting purposes. So it's important to know how far the scope reaches; where the model can and where it can't be applied to real buildings. Some people, in both sides of the discussion, believe that the models are intended to be applied literally to buildings as a description of what happened. Others, from both sides of the discussion, think that the model is just a theoretical limiting case with no applicability to real buildings. The author has believed both of these things at different points in time, and hopes to prove here that both are incorrect...

A good summary. It states the extreme positions clearly and goes to one aspect of Bazant's work has concerned me from the start of my 9/11 involvement mid 2007.

I have no doubt as to his overall validity - the global position that there was energy to spare to ensure the global collapse. And no problem if Bazant's claims are taken no further than that global position.

But time and again I have seen people from both sides take it too far when applied to 9/11. And, sadly from my perspective, little attempt to put clear scope boundaries about either what Bazant says OR how it can actually apply to WTC 9/11.

Here is the core of my concerns:

Originally Posted by pgimeno

..To put a bit of order in that chaos, in order to get a workable model which can be rigurously formalized, Bažant made a simplification to his modeling of the WTC collapse: the column impacts would always be axial, regardless of how they would be in a real case...

So "the column impacts would always be axial"?? Could anything be further from what actually happened with the twin towers on 9/11?

Remember we are talking about the global collapse - the top block has started to fall - a point most people seem comfortable with.

But, if the top block is falling, what does that say about "end for end axial contact" of top block columns and lower tower columns.

The case of the outer tube columns is relatively easy to see - by the time global collapse was progressing the bulk of falling mass was falling inside the outer tube - shearing floor connectors. The outer tube columns falling away after the falling mass has passed by several floors. Those columns were not buckled and fell aside as sheets of varying sizes - apparently and naturally failing mostly at the construction joints.

So what "axial load transfer" are all these papers talking of when they miss this point? The only axial loads on the outer columns being some friction of falling mass plus the force resulting from the shearing impacts of successive floors. At least one order of magnitude lower forces than anything likely to cause buckling.

There are questions to answer as to how the top block ended up inside the outer tube of the lower tower. And whether or not its own columns were still attached at that stage. So what price discussion of "crush up" unless people address the fundamental of how and in what condition the top block got itself inside the outer tube of the lower tower.

The case of "top block core on lower tower core" raises similar but more complicated questions which I will leave for later explanation.

So those are a couple of points which go to my concerns about people taking Bazant too far outside his base assumptions and trying to too literally apply Bazant's "crush down/crush up" to an actual situation where those concepts are at best a strained fit. that is unless we define what actual mechanisms we are supposed to be discussing.

Originally Posted by pgimeno

...(Thanks to ozeco41 for raising the point that led to writing this text.)..

..thank you for the acknowledgement and the comprehensive post.

Lets see where discussion leads.

[ozeco41]

Originally Posted by femr2

...The model does not include initiation, and so does not deal with what caused failure.

It proves that it is possible, post initiation, for gravity to drive progressive collapse...

Good points.

In extended discussion on another forum I tried to "nail down" CTs and truthers to which stage of collapse they were describing.

For purposes of my own explanation the two critical stages were:

1) the progressive deterioration leading to the cascading failure of the "initial collapse"; AND
2) The global collapse.
..with the line between the two being the brief period when the top block first started its descent.

At that point the columns of the lower tower were carrying less load than the full weight of the top block AND there was little if any axial contact of top block columns on lower tower columns AND such axial contact as may have existed was in a process of rapid buckling failure of said columns.

Which gives a reliably defined boundary between the two stages.

Then your point restated - the "initial collapse" caused failure - once that initial collapse started the top block descending then the "global collapse" was inevitable. This latter point as claimed by NIST, demonstrated in a limiting case analysis by Bazant AND independently determined by many others of us using our own variations of explnation.

[femr2]

Originally Posted by ozeco41

Which gives a reliably defined boundary between the two stages.

The more relvant point in the context of this thread is the erronious closure...

Originally Posted by pgimeno quoting Bazant

These conclusions show the allegations of controlled demolition to be absurd and leave no doubt that the towers failed due to gravity-driven progressive collapse triggered by the effects of fire.

As I said, the model does not include initiation, and so does not deal with what caused initial failure.

It does prove that, if the simplified initial state of the model is attained, post initiation, that it is possible for gravity alone to drive global collapse. (with a few caveats)


It does not disprove.


The conclusion steps outside the bounds of model applicability.


(Clarification of bounds and scope being the entire point of this thread, this point should be made clear, and acknowledged by all.)

[ozeco41]

Originally Posted by femr2

The more relvant point in the context of this thread is the erronious closure...

As I said, the model does not include initiation, and so does not deal with what caused initial failure.

It does prove that, if the simplified initial state of the model is attained, post initiation, that it is possible for gravity alone to drive global collapse. (with a few caveats)..

Agreed all three points - sorry if it was not explicit enough in my previous.

Quote:

...The conclusion steps outside the bounds of model applicability.
...

...it sure does.

[pgimeno]

Originally Posted by ozeco41

So "the column impacts would always be axial"?? Could anything be further from what actually happened with the twin towers on 9/11?

Remember we are talking about the global collapse - the top block has started to fall - a point most people seem comfortable with.

No, we're not talking about the global collapse. We are talking about a mathematical model of the collapse which is an approximation of it. That in itself makes it irrelevant how the impacts actually happened, just as the fact that greater than light speed is not contemplated or prevented by Newtonian physics.

Despite the differences between model's behavior and real behavior which affects things like the way the columns impacted, the model still has many resemblances to real world and it can be used for that purpose, just not e.g. for how the column impacts actually happened, just as Newtonian physics can still be used to obtain very good approximations of how objects behave, just not for cases where the speed approaches light speed.

Originally Posted by ozeco41

So what "axial load transfer" are all these papers talking of when they miss this point?

I think it's you who is missing the point. The point is that it doesn't matter - the differences in the impact behavior can still make little differences in many of the results. However, actual impact behavior is very difficult to model as there's a cloud of uncertainty surrounding it, while axial impact behavior has been modeled successfully.

Granted, there's still the concern that the results making big differences and the ones making little differences need to be separated. In that sense, [BLGB] is a good reference as it compares the model against several real-world parameters.

[metamars]

Originally Posted by femr2

It does prove that, if the simplified initial state of the model is attained, post initiation, that it is possible for gravity alone to drive global collapse. (with a few caveats)

Considering that Bazant Zhou postulates an axial impact - in their theory, not in the real world collapse - you'd think that they would have referred to the literature on axial impacts which existed well before their paper was written. I'm referring to a body of technical literature, some (probably most) of which has been experimentally tested.

I still don't know if using a more realistic, purely elastic theory, for an idealized, axial impact would result in the elastic limit being exceeded, or not. I tend to think so, based on calculations I've done, but I'm still not sure, since the situations weren't good enough matches to the BZ scenario. (As for absurdities of the BZ model which predispose it towards collapse, you can search my other writings on the subject. They mixed assumptions which both favored collapse, as well as favored survival, and made no attempt to quantify the relative implications of each. Thus, their paper is illogical! It proves nothing even remotely related to a real world collapse, because it can prove nothing. Kind of interesting that self-styled debunkers would fail to notice this, huh?)

However, an axial impact on a steel rod which exceeds the elastic limit doesn't automatically result in failure. Depending on the details of the problem, you could arrest the striking weight's motion, with energy being dissipated in plastic deformation. (This is what Gordon Ross attempted to calculate, though without benefit of the knowledge of the literature which already existed. Gordon was quite open to knowledgeable input and criticism, contrary to what some of the people who have smeared him have implied. I know this from personal communication. Unfortunately, I hadn't yet run across the body of knowledge about elastic/plastic deformations which follow axial impact, when we had these communications.) The plastic deformation wave travels at about 1/10th the speed of an elastic wave, in steel.

Ari-Gur, to name one researcher in elastic/plastic collision theory, has a relatively simple theory for this, which he has tested, and got fairly good experimental confirmation of his theory.

To date, neither debunkers nor ae911truth have bothered applying the most relevant, and experimentally tested theory, to the BZ scenario (actually, a modified BZ scenario where the entire column lines would be assumed to be able to transmit both elastic energy, and undergo plastic deformation in the manner observed in experiments).

I really don't expect much of debunkers, but I had, at one time, expected the engineering contigent at ae911truth to rise to this relatively modest challenge.

Shame on both the debunkers and ae911truth.

If anybody ever does this, it'll probably be an open-minded member of the911forum.freeforums.org, not ae911truth or a serial debunker member of JREF.

The whole Bazant Zhou thing is a bit of a red herring, even at best. It's main value is probably in showing how irrational* NIST and debunkers are, and subsequently how, um, "un-engineering like" ae911truth has proven to be. (I wanted to write "lazy", but as shutting down ae911truth's forum was - presumably - Gage's doing, and quite a while ago, it's certainly possible that ae911truth's engineering contingent would have attempted this in the last few years, if they had been told about this. So, whoever shut down their forum, shame on you, too.)

If you just gently rested the top block on the bottom block, but 'off-center' (columns resting on floors, except when outside the footprint of the base), then I expect the building to collapse. You wouldn't even need to drop it by one storey height.

What such a collapse would look like.... I don't know....


* And perhaps dishonest.

[femr2]

Originally Posted by metamars

If anybody ever does this, it'll probably be an open-minded member of the911forum.freeforums.org, not ae911truth or a serial debunker member of JREF.

There has been much discussion of the model there of course, but the main problem, as far as I am aware, is the omission of the form of the force function used in B&L.

I'd like to see the function provided, and the model implemented with those handy computer things, such that it is possible to:

a) regenerate and replicate the published results
b) test parameter sensitivity

[beachnut]

Originally Posted by femr2

There has been much discussion of the model there of course, but the main problem, as far as I am aware, is the omission of the form of the force function used in B&L.

I'd like to see the function provided, and the model implemented with those handy computer things, such that it is possible to:

a) regenerate and replicate the published results
b) test parameter sensitivity

Contact Bazant. You and metamars are exposing you have zero engineering skills and don't understand models.

You and metamars as truthers should publish your fantastic engineering work and prove to all the debunkers how it is. What is stopping you? Engineering degrees and skills? Publish your claims and prove you got something. Because to this engineer you guys post nonsense based on ignorance and your need to have a delusion of CD. When you make silly comments on Bazant's model you expose your lack of knowledge on models. This is a trap for you guys with delusions on 911 - stop being so easy.

Quote:

I really don't expect much of debunkers, but I had, at one time, expected the engineering contigent at ae911truth to rise to this relatively modest challenge.

You debunk yourself, and there is no rational engineering contingent at ae911truth. They can't support the lies of Gage and your fantasy about 911. You are a debunker when you expose your lack of knowledge and attack a model. Make your own model, publish it. You are very good at posting nonsense and making it look technical; are you an engineer, or what? Where did you get the skill to makeup posts that have the facade of being technical? That is a skill you have; I have marveled on the massive expanse of posts you have made over the years. They go on and on, and you have made zero progress to push your version of 911. But you have produced a lot of mush. Send a letter to a real Journal with your critique of Bazant! Take some action, what if you are right!? Do something to make up for 8 years of failure and delusions. Heiwa did it! You can do it!

[pgimeno]

Originally Posted by metamars

I still don't know if using a more realistic, purely elastic theory, for an idealized, axial impact would result in the elastic limit being exceeded, or not. I tend to think so, based on calculations I've done, but I'm still not sure, since the situations weren't good enough matches to the BZ scenario. (As for absurdities of the BZ model which predispose it towards collapse, you can search my other writings on the subject. They mixed assumptions which both favored collapse, as well as favored survival, and made no attempt to quantify the relative implications of each. Thus, their paper is illogical! It proves nothing even remotely related to a real world collapse, because it can prove nothing. Kind of interesting that self-styled debunkers would fail to notice this, huh?)

Dude, even Björkman got a criticism of Bazant's work published in the JoEM, and I think even you will agree with me in that it had no merit at all. Why don't you publish a good, well-referenced criticism in such a publication? I'm eager to see a well reasoned discussion of his paper rather than the farces that have been seen so far.

Originally Posted by metamars

To date, neither debunkers nor ae911truth have bothered applying the most relevant, and experimentally tested theory, to the BZ scenario (actually, a modified BZ scenario where the entire column lines would be assumed to be able to transmit both elastic energy, and undergo plastic deformation in the manner observed in experiments).

I really don't expect much of debunkers, but I had, at one time, expected the engineering contigent at ae911truth to rise to this relatively modest challenge.

Shame on both the debunkers and ae911truth.

ae911truth is a well-defined collective, and that you expected more from them, who have no single paper about their ideas published in any real journal but whose founder has an agenda of attracting gullible people with flawed arguments, speaks lots.

Why don't you take your own challenge, as I took this one when publishing the above text when noone else jumped into it, instead of stating your opinions and waiting for someone else to do the job?

Besides, you may think that the matches between theoretical and real data in [BLGB] are just luck, but the burden of proof lies upon whoever thinks that. No shame for those who admit it as real. I'd rather say shame on those of ae911truth that don't believe it and are dedicated to smearing it, and shame on you for not dedicating to prove what you are openly claiming here.

Originally Posted by metamars

The whole Bazant Zhou thing is a bit of a red herring, even at best. It's main value is probably in showing how irrational* [* And perhaps dishonest] NIST and debunkers are, and subsequently how, um, "un-engineering like" ae911truth has proven to be.

Can you please explain

1. what makes NIST irrational (*and perhaps dishonest) in this respect, and
2. what makes the collective you call "debunkers" irrational (*and perhaps dishonest)?

That part of your post, however, is really interesting. What you are saying here is basically the same that most critics of ae911truth say, and indeed you are being a critic of ae911truth here. The best they have is Chandler's and Szamboti's flawed analyses, none of which bears a critical examination and none's been published in a real journal.

[Major_Tom]

The Bazant Zhao argument of 12 ft freefall before the first collision has been a total red herring.

We want to know what initiated collapse, not whether the building would bounce after a 12 ft fall.

Metamars writes in post #12: "If you just gently rested the top block on the bottom block, but 'off-center' (columns resting on floors, except when outside the footprint of the base), then I expect the building to collapse. You wouldn't even need to drop it by one storey height. "

Exactly. If this is true why would anyone argue for a big bounce after 12 ft? It's an argument for suckers.


The demo would initiate collapse and once those columns are displaced is there any reason to expect the building to fare well?

I don't care if buildings were designed to survive a good bounce. I care about what initiated collapse.

It's a total red herring as both Bazant and the NIST skip over the real issue of the initial column failure sequence.

[Major_Tom]

Pgimeno, you seem to be very impressed by the publication process. We are debating here, now, on this forum.

If you cannot tell the difference between a 1-D stick model and the real towers here, now, how will it help you if you see our arguments in print?

[Major_Tom]

Pgimeno, many of us have been critics of the scholars and AE911T for a long time. Quite vocally to the point of being kicked out of their forums.

The best research I have seen has been done by independent researchers not associated with either group.

In my opinion the standard debunker vs AE911T back and forth arguments have been mostly fake. Fake arguments and equally fake counter-arguments.

Tweedle-dee vs Tweedle-dum, while the better independent researchers go largely ignored in all the smoke and mirrors.

[TexasJack]

Originally Posted by Major_Tom

The best research I have seen has been done by independent researchers not associated with either group.

You mean by independent research those with unpublished work and a degree from Google University.

[pgimeno]

Originally Posted by Major_Tom

I don't care if buildings were designed to survive a good bounce. I care about what initiated collapse.

That's fine, but speak for yourself. Dedicating efforts to the study of collapse progression instead of your object of interest doesn't make the resulting engineering studies such as Bazant's a "red herring", no matter how much you try to smear them. That's quite an egocentric point of view. Along time, people have studied what has concerned them instead of what has concerned you. Face it.

Originally Posted by Major_Tom

Pgimeno, you seem to be very impressed by the publication process. We are debating here, now.

No, I'm confident of it as a guarantee of competence, of which you have shown quite little here so far.

Plus, that tends to be the excuse of those who know they can't get published.

But my point stands. Even Björkman's incompetence was exposed to the engineering world in an engineering journal. If you have a good point, you should have no problem to be published there as Björkman was. I just hope that, in case you don't even pass even the most basic threshold to get published there, you won't come crying about "the establishment" or a standard excuse like that.

[beachnut]

Originally Posted by Major_Tom

Pgimeno, many of us have been critics of the scholars and AE911T for a long time. Quite vocally to the point of being kicked out of their forums.

...

What independent researchers? Are they scenics or engineers? Are they better than Bazant? You have conspiracy theories on your conspiracy theories. Smoke and mirrors? Is that your work?

It is funny how you are stuck on NIST.

You are kicked out of forums because you post BS, which you think is engineering. It is funny as ae are do nothing cult members with thermite rot on the brain, it is amazing they can see your work as nonsense. It is ironic your BS CD delusion is kicked out from people with the same delusions and worse. Are you making up another CT?

If you have problems with Bazant publish them! It will be interesting to see the adjectives of nonsense and delusional used in a real journal when your junk is evaluated. Heiwa did it why can't you! You can always make up another CT after it goes bad.

[femr2]

Originally Posted by pgimeno

This text addresses how and in what sense Bažant's model, initially stated in [B 2001] and [BZ], and refined in [BV] and [BLGB], with clarifications in [BL] (see below, References), can or can not apply to real world buildings.

Quote:

The author welcomes any constructive criticisms aimed at addressing such inaccuracies as long as they are stated with arguments other than incredulity and disbelief

Quote:

Since the author's always been not-so-good for writing closing words, please indulge him in his use of [BLGB] to do it:

These conclusions show the allegations of controlled demolition to be absurd and leave no doubt that the towers failed due to gravity-driven progressive collapse triggered by the effects of fire.

I highlighted the inapplicability of your quoted closing remarks in the very first response to your OP.

Namely that...

The conclusion steps outside the bounds of model applicability.

Bearing in mind the specific purpose of your thread, I assume you are going to acknowledge this point shortly.

Not only is it a perfect example of the way that the model is used incorrectly, but it is a quote from the model authors themselves.

I also suggest you request that further posts from ALL members directly address the thread topic at all times.

[beachnut]

Originally Posted by pgimeno

Since the author's always been not-so-good for writing closing words, please indulge him in his use of [BLGB] to do it:

These conclusions show the allegations of controlled demolition to be absurd and leave no doubt that the towers failed due to gravity-driven progressive collapse triggered by the effects of fire.

Bazant knows it was not controlled demolition, as do 99.99 percent of all engineers. Nice to see it published so 911 truth has some move evidence they can ignore.

It was also cool the OP is referenced. 911 truth pseudo-wannabe-engineers never use source or references for their made up nonsense which they use for science and evidence.

When you find 911 truth work referenced and sources, you can follow the references and sources to debunk the work. It is classic 911 truth. 911 truth cult members can't comprehend the closing statement is based on evidence, not failed paranoid conspiracy theories based on lies and hearsay. They don't do evidence, or understand Bazant does.

[Furcifer]

Originally Posted by Major_Tom

We want to know what initiated collapse, not whether the building would bounce after a 12 ft fall.

Fire.

[ozeco41]

Originally Posted by pgimeno

No, we're not talking about the global collapse. We are talking about a mathematical model of the collapse which is an approximation of it. .....I think it's you who is missing the point. The point is that it doesn't matter - the differences in the impact behavior can still make little differences in many of the results. However, actual impact behavior is very difficult to model as there's a cloud of uncertainty surrounding it, while axial impact behavior has been modeled successfully....

Point taken that I need to be very explicit.

If we are talking about the Bazant model as a generic model then the model is valid as far as it goes.

BUT if we apply the generic model to a specific situation where the assumptions of the generic model are not valid - then the model ceases to be correct and may introduce errors which may be significant.

So you clarify that "we're not talking about the global collapse [of WTC1 and WTC2]"

Whereas I was attempting to relate the Bazant generic model to the specifics of the WTC twin towers global collapse stage.

So continue discussion of Bazant's model without referring to WTC global collapse and I have no concern.

My concern arises only when Bazant's model is taken into areas of analysis of WTC collapse where it does not fit. Where his assumptions, especially the assumptions about axial contact, are significant.

So your next comment "We are talking about a mathematical model of the [global] collapse which is an approximation of it."

So, like me you are relating to WTC global collapse. So your statement hinges on what you mean by "approximation"? I hold to my several times repeated statement. I have no doubt that Bazant was "globally" correct - there was more than enough energy to ensure complete collapse once it was initiated. I question the validity if the concept is taken to further detail given Bazant's assumption of axial column contacts in his model and the near absence of such contacts in the WTC global collapse.

[femr2]

Originally Posted by ozeco41

PMy concern arises only when Bazant's model is taken into areas of analysis of WTC collapse where it does not fit. Where his assumptions, especially the assumptions about axial contact, are significant.

This reminds me of discussions I've had with David Benson, which arose through discussion of the roughly linear rate ejecta streams equating to the position of the primary *crush fronts*.

The velocity of the WTC 1 south-side crush front was found to reach fairly constant rate after about 5 seconds, reaching ground in roughly 14.5 seconds.

David's input to the discussion was that such details matched outputs from the model.

This is a concern imo.

The crush fronts are essentially the result of *pancaking*, with ROOSD being a more detailed description.

Why do the outputs from the model involving axial column impacts match behaviour, velocity profile and global descent time of a completely different mechanism ?

(Especially when it is clear that actual column destruction was significantly slower in propogation rate than ROOSD)

That is why, as indicated earlier, it would be useful if the form of the force function was provided, such that the model results can be replicated and parameter sensitivity tested.

[pgimeno]

Originally Posted by femr2

I highlighted the inapplicability of your quoted closing remarks in the very first response to your OP.

Namely that...

The conclusion steps outside the bounds of model applicability.

Bearing in mind the specific purpose of your thread, I assume you are going to acknowledge this point shortly.

Rigorously speaking, that's true.

Certainly, Bazant doesn't deal with collapse initiation, where there is certainly room for a deliberate action against the towers. However, the amount of demolition theory believers that sustain that point of view is so low that they can be readily dismissed. Thus Bazant's comment as quoted is accurate in its context (the clear intention of dismissing claims on the merit of floor-by-floor explosive demolition) even if not scalpel-accurate. I must say, that making that much emphasis in that distinction does not seem neutral to me at all, but the contrary. It looks to me as giving last-term excuses for those who can be convinced by Bazant's arguments in order to sustain the unproven, purely faith-based deliberate destruction of the towers. It's precisely the lack of any kind of support from either evidence or rational thought (but I must put emphasis on lack of evidence), that should lead to dismiss it without giving it any serious consideration when objectively evaluated. It pertains to the realm of speculation and it's sad that faith is the only motivation for the search of evidence in that respect from people that mostly seem otherwise quite rational.

[bill smith]

If you want to see just how inapplicable Bazant is to the 9/11 paradigm just think about his 'upper block'.

We know that 15% or less of the columns, both core and perimeter that connected upper and lower blocks were destroyed by the plane crash. So 85% or more were intact.

So what is all this about an ' upper block ' ?

We should be talking about how those 85% apparently simultaneously ceased to exist as columns.

[femr2]

Originally Posted by pgimeno

Originally Posted by femr2

The conclusion steps outside the bounds of model applicability.

Rigorously speaking, that's true.

Agreed.

Response to the rest of your post is over at the911forum...
http://the911forum.freeforums.org/po...?hilit=#p12226

[pgimeno]

Originally Posted by ozeco41

Point taken that I need to be very explicit.

Yes, this is an area where misunderstandings have been and have the risk to be common.

Originally Posted by ozeco41

If we are talking about the Bazant model as a generic model then the model is valid as far as it goes.

BUT if we apply the generic model to a specific situation where the assumptions of the generic model are not valid - then the model ceases to be correct and may introduce errors which may be significant.

So far, so good. That's precisely why I use this sentence:

Originally Posted by pgimeno

The corollary is that it is possible to draw some conclusions even from an inexact model, just not any conclusions, and we have to be careful about which ones can be extracted and which ones won't match the real world to any sufficient degree of accuracy at all.

Originally Posted by ozeco41

So your statement hinges on what you mean by "approximation"? I hold to my several times repeated statement. I have no doubt that Bazant was "globally" correct - there was more than enough energy to ensure complete collapse once it was initiated. I question the validity if the concept is taken to further detail given Bazant's assumption of axial column contacts in his model and the near absence of such contacts in the WTC global collapse.

That's a valid point that I certainly admit to have kind of handwaved because of my lack of knowledge in the field. I have a strong feeling that some of the results will apply equally to both the real case and the model, but I can't tell you precisely which ones. That's not only my sensation, though. In another forum someone made a comment from one of my sentences:

Quote:

Quote:

Had he modeled crush-up occuring simultaneously with crush-down, that would probably mean a factor of two deviation in some of his results.

Those results, specifically, would be count of number of members failed and the associated energy in deformation and failure until crush-up concludes. Displacement of the roofline would be considerably greater over the mixed crush interval, not necessarily a factor of two. There is strong reason to believe most other dynamic quantities of interest would not be affected much, at the level of detail of the model.

(Emphasis in original).

Wish I could be any more precise. Sorry, I can't. I have ventured an assumption regarding e.g. this quote from [BV] (p.312):

Detailed finite element analysis simulating plasticity and break-up of all columns and beams, and the flight and collisions of broken pieces, would be extremely difficult, as well as unsuited for extracting the basic general trends. Thus it appears reasonable to make four simplifying hypotheses: [...]

I assume that he knows how the simplifications he makes affect the obtained results and that he knows that they won't have a very significant effect on the results he intends to obtain.

But then, I guess I would need to be an engineer to be able to actually make an evaluation about that.

[ergo]

Originally Posted by Major_Tom

The Bazant Zhao argument of 12 ft freefall before the first collision has been a total red herring.

We want to know what initiated collapse, not whether the building would bounce after a 12 ft fall.

Metamars writes in post #12: "If you just gently rested the top block on the bottom block, but 'off-center' (columns resting on floors, except when outside the footprint of the base), then I expect the building to collapse. You wouldn't even need to drop it by one storey height. "

Exactly. If this is true why would anyone argue for a big bounce after 12 ft? It's an argument for suckers.

The demo would initiate collapse and once those columns are displaced is there any reason to expect the building to fare well?

If I may be allowed to comment in this more technical discussion... Showing that a jolt or bounce back accompanies any collision demonstrates that the lower portion is sufficiently providing resistance, as would be predicted. Which in turn shows that crush down would not occur first.

As for "gently resting the upper block" off-centre on the lower, how and why would two floors of off-centre columns precipitate a global collapse of the entire rest of a building whose columns are intact? This makes no sense. And if you're further suggesting that it would be a crush down, this makes even less sense.

[pgimeno]

Originally Posted by femr2

Agreed.

Response to the rest of your post is over at the911forum...
http://the911forum.freeforums.org/po...?hilit=#p12226

Despite this clear sentence I added to the beginning and end?

Originally Posted by pgimeno

It is taking quite a good amount of time to follow one forum and I didn't have any plans to follow two.

[...]

As a final word, as outlined above, I will probably not comment much or at all to any responses.

femr2, the titles of your videos speak lots, and your attitude does too. I have already discussed some personal aspects of this with you, so I won't extend in that direction.

[metamars]

Originally Posted by femr2

There has been much discussion of the model there of course, but the main problem, as far as I am aware, is the omission of the form of the force function used in B&L.

I'd like to see the function provided, and the model implemented with those handy computer things, such that it is possible to:

a) regenerate and replicate the published results
b) test parameter sensitivity

I was referring specifically to BZ.

Subsequent papers looked more relevant to a real collapse. (Which is a good thing since, as noted, BZ is illogical, and can't prove anything about a real world collapse.) Though I vaguely recall deriving an unbelievable prediction from one of them (I think it was BV; I may have posted results at physorg), I'm not really competent to analyze the 'definitive' BLGB, as it involved theorizing of a sort that I'm not familiar with. (I can't remember the term, but it wasn't theories based on first principles, or derivations like one makes in a dynamics class, but rather a theory, such as hooke's law, that follows from observation, and has some sort of statistical validity.)

I don't trust such theorizing, because in the case of the WTC, it couldn't be tested. (Unlike Hooke's Law, which is readily tested, as much as we please.) But maybe I'm just showing my ignorance. I never had time to study the references that Benson suggested.

My general impression of the BLGB paper (for whatever it's worth), is that it shows that a collapse in the time frame observed was possible given the assumptions of the theory (many of which were reasonable; perhaps all were somewhat reasonable, though IIRC, it didn't take into account transport of elastic energy into the ground, nor any possible dampening effect of floors oscillating like membranes, nor plastic waves that exceeded the collapse speed* ).

I vaguely recall concluding that their model was probably very good for when the collapse really got going, something like halfway done. However, that's the part of a collapse where you really don't need explosives, even to make sure that there's no chance of toppling over.

Sorry, I don't remember more clearly. Again, I don't have the technical background to understand the later works, fully, anyway.


BTW, have you ever looked at the papers of Charles M. Beck, posted at arxiv? They are way over my head. His analysis suggested CD:

Quote:

We examine four WTC 7 descent curves, labeled "C," "E," "N," and "O," either anonymously published, or confidentially communicated to us. Descent curve describes apparent height of a collapsing building as a function of time. The set "C" suggests that there are three active phases of collapse. Phase I is a free fall for the first $H_1\simeq28$m or $T_1\simeq2.3$s, during which the acceleration $a$ is that of the gravity, $a=g=9.8$m/s$^2$. In Phase II, which continues until drop $H_2\simeq68$m, or $T_2\simeq3.8$s, the acceleration is $a\simeq5$m/s$^2$, while in Phase III which continues for the remaining of the data set, $a \simeq -1$m/s$^2$. We propose that the collapse of WTC 7 is initiated by a total and sudden annihilation of the base (section of the building from the ground level to $H_1$), which then allows the top section (building above $H_1$) to free fall during Phase I, and then collide with the ground in Phase II and III. The total duration of the collapse, assuming that Phase III continues to the end, is in the range $7.8-8.6$s. We derive a physical model for collision of the building with the ground, in which we correct the "crush-up" model of Ba\v{z}ant and Verdure, J. Engr. Mech. ASCE, {\bf 133} (2006) 308, and estimate the magnitude of the resistive force in the top section. We compare our findings to those of NIST investigators and find an agreement with respect to the distribution of damage in the primary zone. We conclude that the building was destroyed in a highly controlled fashion.

(emphasis mine)

His paper on WTC 1/2 is here.


* Even a plastic wave traveling at 1/10 of 5,000 m/s or so is much faster than the collapse

[pgimeno]

Originally Posted by metamars

Subsequent papers looked more relevant to a real collapse. (Which is a good thing since, as noted, BZ is illogical, and can't prove anything about a real world collapse.)

Care to explain what makes it illogical? It proves that in the most favorable scenario for collapse arrest, the building could not arrest the collapse, thus proving it was unstoppable once started, which was the point and is perfectly logical, so I'd like to see an explanation on why you think otherwise.

[femr2]

Originally Posted by femr2

it would be useful if the form of the force function was provided, such that the model results can be replicated and parameter sensitivity tested.

I have been informed that the form of the force function was provided. My bad.

[ozeco41]

Originally Posted by pgimeno

Yes, this is an area where misunderstandings have been and have the risk to be common.


So far, so good. That's precisely why I use this sentence:

That's a valid point that I certainly admit to have kind of handwaved because of my lack of knowledge in the field. I have a strong feeling that some of the results will apply equally to both the real case and the model, but I can't tell you precisely which ones. That's not only my sensation, though. In another forum someone made a comment from one of my sentences:
(Emphasis in original).

Wish I could be any more precise. Sorry, I can't. I have ventured an assumption regarding e.g. this quote from [BV] (p.312):

Detailed finite element analysis simulating plasticity and break-up of all columns and beams, and the flight and collisions of broken pieces, would be extremely difficult, as well as unsuited for extracting the basic general trends. Thus it appears reasonable to make four simplifying hypotheses: [...]

I assume that he knows how the simplifications he makes affect the obtained results and that he knows that they won't have a very significant effect on the results he intends to obtain.

But then, I guess I would need to be an engineer to be able to actually make an evaluation about that.

Comments noted - we are in broad agreement.

As for "...I would need to be an engineer to be able..." my experience as both a practising engineer and (for much longer) a manager of practising engineers is that many engineers "lose the plot". Whether you call it the "technical context" or the "big picture" they can get too close to detail. Somewhat parallel to the old adage "when you are up to your arse in alligators it's easy to forget that the objective was to drain the swamp". The need is to ask "why are we here and why are we doing this?" Another sub-set of the problem is the tendency to rush into intense maths and calcs without first working out where you are going and what you are trying to clarify by the maths.

Take the question of "crush up" of the top block. It is clear that most of the "top block" mass of both towers fell inside the outer tube of the lower tower. It's not clear how because of the dust clouds.

However think about how this falling top block landed on the lower and get past the confused tangle of bent and broken bits so we are really in the "global collapse".

Ask this question "Where did the oputer tube columns of the top block fall?"

There are three possibilities - viz:
(1) inside the lower tower tube so, in effect the top block was wedged inside the lower tower outer tube and still more or less intact;
(2) The top block outer tube columns fell outside the lower tower - which means that those top block columns were sheared of in the first stage of the global collapse; OR
(3) Some inside and some outside.

For several reasons I discount (3) and prefer (1) out of the remaining two options.

But let's pursue both a little.

Think only of one wall for simplicity. If the top block outer tube fell outside the lower tower outer tube THEN the top of the lower block outer wall of columns would act as a "knife blade" and shear the top block floors off their outer columns as almost the first stage of global collapse.

Conversely if the top block outer tube fell inside the lower tower outer tube THEN the underside of the top block outer wall of columns would act as a "knife blade" and shear the floors of the lower tower off their outer columns, again as almost the first stage of global collapse.

Now whichever of these it was the first impact of the outer floor area and outer tube columns lands on a floor and has only the shear strength of the floor joist to column connections to overcome. AND the only significant load transferred to the columns is the force resulting from that shearing.

So much for any forces of column buckling magnitude whether "crush up" or "crush down".

The third option takes a bit more explaining but the engineers should see that it changes little.

So I am dubious of any "crush up" style explanation which relies on column buckling forces. Including some papers which IIRC claim there was not sufficient upwards force to buckle columns. So what? There was no need for such high magnitude forces when the only force required was to shear one floor at a time off the columns.

(And dealing with the core is nearly as straightforward but no need to complicate this post further)

Suffice that Bazant's model is limiting case valid for the global collapse of WTC1 and WTC2.

It does not address collapse initiation.

It is not valid in detail to explain the global collapse of the twin towers.

[ozeco41]

Originally Posted by femr2

....This is a concern imo.

...a concern which I have shared for some years.

Originally Posted by femr2

....The crush fronts are essentially the result of *pancaking*, with ROOSD being a more detailed description...

..agreed!!!

Originally Posted by femr2

...Why do the outputs from the model involving axial column impacts match behaviour, velocity profile and global descent time of a completely different mechanism ?

...the world of engineering has many examples of approximations giving correct answers for wrong reasons.

Originally Posted by femr2

...(Especially when it is clear that actual column destruction was significantly slower in propogation rate than ROOSD)

...presumably column "destruction" for the core columns --- the outer columns were simply peeled off.

[femr2]

Originally Posted by ozeco41

...presumably column "destruction" for the core columns --- the outer columns were simply peeled off.

In general, yes.

[metamars]

Originally Posted by pgimeno

Care to explain what makes it illogical? It proves that in the most favorable scenario for collapse arrest, the building could not arrest the collapse, thus proving it was unstoppable once started, which was the point and is perfectly logical, so I'd like to see an explanation on why you think otherwise.

Use the search function, please. Here, at physorg, and the911forum.

It can't be "the most favorable scenario for collapse arrest" if it incorporates assumptions that both favor collapse, and that favor survival....

[femr2]

Originally Posted by metamars

It can't be "the most favorable scenario for collapse arrest" if it incorporates assumptions that both favor collapse, and that favor survival....

Must be worth noting that core column destruction did arrest for significant portions of the core of both WTC 1 & 2.