Forum Index Register Members List Events Mark Forums Read Help

 International Skeptics Forum Bazant's crush-down/crush-up model

 Welcome to the International Skeptics Forum, where we discuss skepticism, critical thinking, the paranormal and science in a friendly but lively way. You are currently viewing the forum as a guest, which means you are missing out on discussing matters that are of interest to you. Please consider registering so you can gain full use of the forum features and interact with other Members. Registration is simple, fast and free! Click here to register today.
 Tags Zdenek Bazant

 22nd August 2010, 02:35 PM #1 pgimeno Illuminator     Join Date: Feb 2009 Location: Spain Posts: 3,647 Bazant's crush-down/crush-up model I'm trying to understand Bazant's crush-down/crush-up model, which I know is widely accepted. I am not questioning it, I merely want to understand it, and hopefully the explanation will be useful for someone else too. In order to explain where my doubts arise, I propose the following thought experiment. Consider a "WTC top" composed of the top 12 WTC1 floors, just floating freely in space. No gravity there. Consider also a "WTC bottom" made of the bottom 98 floors of WTC1, attached to a rigid, indestructible platform which has some rockets mounted. These rockets are assumed to be able to put a thrust to the "WTC bottom + platform" mount and are regulated so that once started, in the platform an accelerometer always measures 9.81 m/sē. The "WTC bottom" is aligned with the "WTC top" just like the real building was, and is separated from it just the distance of two floors, which IIRC was Bazant's assumption. Here's a simple diagram of the setup: At a given point of time, the rockets are started. From within a camera in the platform, the crushing should look just like the collapse predicted by Bazant (this thought experiment is assumed to be set up exactly like in Bazant's model, including the column-to-column collisions). What I'm doing with this setup is merely to change the reference frame with respect to Bazant's description, defining the "WTC top" as having zero velocity. If it has a theoretical pitfall which causes it not to be equivalent to Bazant's model, I'd like to know. But if it is, can someone please explain why the "WTC top" is not significantly crushed by the impact and successive force exerted on it until it reaches the platform, just as the "WTC bottom" is? That's the part that I'm not getting, and I believe it's the part that most people have trouble with. Obviously, in this experiment the "WTC top" will accelerate at the first impact, reaching about 1/3 of g as it crushes the "WTC bottom", since the top's downward acceleration was about 2/3 of g. The only explanation I can find for the top not being crushed is that the crush front will "lag" with respect to the accelerating platform's reference frame, thus its global acceleration will be less than g, possibly allowing for the "WTC top" to resist the force, because it's less force given that the acceleration of the crush front is reduced due to that "lag". Is that the cause? If so, why doesn't the same happen to the "WTC bottom"?
 22nd August 2010, 02:47 PM #2 R.Mackey Philosopher   Join Date: Apr 2006 Posts: 7,854 Interesting way to frame the problem. There is a slight difference between your schematic and the real problem, however -- it's only equivalent if you assume the rockets are firing for a long time, and there's no remaining dynamic response in the intact tower, before you yank away the supports between the lower and upper portions leaving the upper portion to be at constant velocity while the rest continues to accelerate. But it's a minor detail. There is a second important detail: Your rockets will have to throttle down as the collapse continues. They're providing a constant acceleration only on the intact portion of the lower block, i.e. a lower force as the mass of that block decreases with time. You have it drawn correctly, specifying a constant acceleration rather than a constant force, but it's a bit counter-intuitive for rocket engines. Ideally you will get the same kind of "crush down / crush up" behavior. What happens is that at impact, the accelerating lower block hits the nonaccelerating (and thus slower) upper block, damage occurs to both. But the remainder of the upper block has been accelerated, being pushed by the mass below, at a rate lower than 1 g. And -- this is the key point -- the rubble interface layer between is at the same velocity as the upper block, and is not accelerating at 1 g. As the lower structure continues to accelerate through the rubble layer and upper block, it suffers increasingly more damage at the top of the lower block. It has the same force pushing up, but it hits an increasingly larger mass and thus is opposed by more and more inertia. The upper block, on the other hand, never feels any worse acceleration than it did at initial contact. At least, that is, until it hits the rocket platform itself, which we have declared to be ideal and indestructible, at which point the upper block begins to be destroyed in the "crush down" phase. So, one more time: As the collapse progresses, the rocket-accelerated lower block is being driven at constant acceleration into a growing, larger and larger mass. Correspondingly, the resisting force goes up, and the lower block continues to suffer damage. The upper block, however, becomes a smaller and smaller fraction of the total impacted mass, and the force upon it actually decreases -- it suffers its worst damage at first contact, and afterwards will tend to survive. Have you read Dr. Bazant's papers? In particular, have you read where he clarified this issue to Frank Gourley? Last edited by R.Mackey; 22nd August 2010 at 02:57 PM. Reason: Added need for throttle-down and summary
 22nd August 2010, 03:19 PM #3 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Even with the ideal and simplified Bazantian abstracted model, this portion stands out at this point... Originally Posted by R.Mackey [WTC Top] suffers its worst damage at first contact, and afterwards will tend to survive To me this clearly implies simultaneous crush-up and crush-down at all times, even if you accept one side crushing more than t'other. Of course these simplified abstractions don't directly apply to real world behaviour and are, as is made clear, a limiting case. Real world behaviour of, say, WTC 1 was clearly quite different. ETA... Originally Posted by R.Mackey at which point the upper block begins to be destroyed in the "crush down" phase. Did you mean "Crush up" ? __________________ http://the911forum.freeforums.org Last edited by femr2; 22nd August 2010 at 03:22 PM.
 22nd August 2010, 03:31 PM #4 ergo Illuminator     Join Date: Aug 2010 Posts: 4,339 Originally Posted by pgimeno Consider a "WTC top" composed of the top 12 WTC1 floors, just floating freely in space. No gravity there... What I'm doing with this setup is merely to change the reference frame with respect to Bazant's description, defining the "WTC top" as having zero velocity. If it has a theoretical pitfall which causes it not to be equivalent to Bazant's model, I'd like to know. Well, if it's floating in space then it will ride with the acceleration upward of the lower rocket block indefinitely. If it's hitting against some immutable resistance or force then yes, it will be crushed between the lower rocket block and the force. So your intuition about it is correct, and you are not to be blamed for having "trouble" with it or "not getting" it.
 22nd August 2010, 04:23 PM #5 pgimeno Illuminator     Join Date: Feb 2009 Location: Spain Posts: 3,647 Thanks for your answer. "Putting the E in the JREF", as I've seen some say Originally Posted by R.Mackey Interesting way to frame the problem. There is a slight difference between your schematic and the real problem, however -- it's only equivalent if you assume the rockets are firing for a long time, and there's no remaining dynamic response in the intact tower, before you yank away the supports between the lower and upper portions leaving the upper portion to be at constant velocity while the rest continues to accelerate. But it's a minor detail. There is a second important detail: Your rockets will have to throttle down as the collapse continues. They're providing a constant acceleration only on the intact portion of the lower block, i.e. a lower force as the mass of that block decreases with time. You have it drawn correctly, specifying a constant acceleration rather than a constant force, but it's a bit counter-intuitive for rocket engines. Sorry for any real-world inaccuracies. The experiment was designed for the intuition to make it easier to shift the reference frame. The pretended effect was that the situation was the same exact one, just as an aid in understanding. Seems I got it "right enough". Originally Posted by R.Mackey Ideally you will get the same kind of "crush down / crush up" behavior. What happens is that at impact, the accelerating lower block hits the nonaccelerating (and thus slower) upper block, damage occurs to both. But the remainder of the upper block has been accelerated, being pushed by the mass below, at a rate lower than 1 g. And -- this is the key point -- the rubble interface layer between is at the same velocity as the upper block, and is not accelerating at 1 g. Indeed; however the rubble part is accelerated as well and pushes the upper block, which in itself also opposes an inertia which results in a force in the interface between itself and the rubble layer. If (emphasis on the if) the force is enough to crush it, then as it is crushed there will be less mass of it that opposes the movement and its crush-up will eventually be arrested, because of lack of mass, until the platform comes (unless the platform comes before that happens). That's how I see it. So the problem is, in my view: is there something that ensures that the force between the rubble and the top will always be less than needed for crushing it? Or is the no-crush-up just a specific case studied for the situation with the towers (and maybe applicable to some other buildings), but not valid in general? I'm starting to think that this is the explanation. Originally Posted by R.Mackey As the lower structure continues to accelerate through the rubble layer and upper block, it suffers increasingly more damage at the top of the lower block. It has the same force pushing up, but it hits an increasingly larger mass and thus is opposed by more and more inertia. This part is perfectly clear. That's the obvious reason for the crushing to be produced at all. Originally Posted by R.Mackey Have you read Dr. Bazant's papers? In particular, have you read where he clarified this issue to Frank Gourley? Seems that that will be my next reading I have only read the first article (BZ) and have been waiting for an excuse to read the rest. Major Tom almost got me to read more of them, pity he didn't focus on one of them.
 22nd August 2010, 04:28 PM #6 pgimeno Illuminator     Join Date: Feb 2009 Location: Spain Posts: 3,647 Originally Posted by femr2 To me this clearly implies simultaneous crush-up and crush-down at all times, even if you accept one side crushing more than t'other. Pending R.Mackey's reply and a reading of the explanation to Gourley on my side, will you at least concede my point that even if it is crushed, it would be eventually arrested as the top loses mass?
 22nd August 2010, 04:52 PM #8 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by pgimeno will you at least concede my point that even if it is crushed, it would be eventually arrested as the top loses mass? I don't understand what you are asking. If what is crushed ? What would be arrested ? If you are suggesting that in a purely mathematical sense, not directly applicable to the real world, that the relative amount of crush-up as opposed to crush-down would be equal to zero, then, as Ryan has said it could tend towards zero. But of course, that's just maths. Bazant never intended on modelling the real-world behaviour. Even the terms crush-down and crush-up can only be loosely associated with the towers destruction when describing the real world behaviour. __________________ http://the911forum.freeforums.org
 22nd August 2010, 05:00 PM #9 ergo Illuminator     Join Date: Aug 2010 Posts: 4,339 Originally Posted by R.Mackey What the Truthers do wrong is neglect the rubble layer. How thick is this rubble layer? Where do we see evidence of it? I might believe in it if I could see it. Anywhere. Quote: If the rubble layer didn't exist, somehow swept away as soon as it was produced, then the impact of upper and lower block would result in relatively symmetric damage. However, after the upper block has shrunk by only four or six floors, the rubble layer outmasses the surviving upper block, and you absolutely cannot neglect its behavior. You mean this behaviour? : Quote: The rubble ... are free objects. They are not reliably carried by the lower structure's columns, but are instead impacting all over the place -- columns, floors, eccentrically, and so on. Thus, the rubble counts as part of the detached mass, which includes the upper block. The rubble's eccentricity means it must therefore be coupled with the upper block? Why? And why in both cases?
 22nd August 2010, 05:03 PM #10 R.Mackey Philosopher   Join Date: Apr 2006 Posts: 7,854 Let me make a slight correction to the "throttling down" comment -- The rocket thrust would throttle down, but the actual thrust profile would have to match the timing of the overall collapse. My observation above that the thrust should be equal to 1 g times the mass of the surviving lower block is not quite right; there is an additional thrust component opposing the "downward" force exerted by the impacted pieces, which will be some fraction of their mass and difficult to predict. The idea is simply to move with the reference frame of the interface, whatever thrust profile that may be. That interface will not accelerate at 1 g, but rather will have a varying acceleration over time. This is what I mean by "throttling down."
 22nd August 2010, 05:11 PM #11 Loss Leader I would save the receptionist.Moderator     Join Date: Jul 2006 Location: Florida Posts: 27,741 Originally Posted by pgimeno But if it is, can someone please explain why the "WTC top" is not significantly crushed by the impact and successive force exerted on it until it reaches the platform, just as the "WTC bottom" is? That's the part that I'm not getting, and I believe it's the part that most people have trouble with. As a lay person with no engineering background or knowledge of the mechanics of the tower collapses: I think the top will be crushed. It will be completely ruined. You won't be able to hold meetings there or, really, get any work done. However, that doesn't look like it matters. The mass of the top won't change whether it is crushed or not. It will weigh the same. So the damage that it's going to do to your thought experiment rocket will stay the same (adding to it the mass of each new crushed section) so long as the rocket's acceleration into the top is 32 ft/sec2 or whatever gravity is these days. ETA: Having now read Mackey's response, I realize that I have absolutely no idea what I am talking about and should, in all probability, just be ignored. __________________ I have the honor to be Your Obdt. St L. Leader Last edited by Loss Leader; 22nd August 2010 at 05:18 PM.
 22nd August 2010, 06:10 PM #12 pgimeno Illuminator     Join Date: Feb 2009 Location: Spain Posts: 3,647 Originally Posted by femr2 I don't understand what you are asking. If what is crushed ? What would be arrested ? Sorry, let me rephrase: Will you at least concede my point that even if the top is crushed-up as the collapse progresses, its crushing would be eventually arrested(*) as the top loses mass (because it's converted in rubble), since the total force would be decreasing and thus the columns will eventually be able to support the force? (*) "Stopped" might be a better word? Sorry, English is not my first language. (note that this question was previous to Mackey's clarification, which I am still in the process to digest)
 22nd August 2010, 06:24 PM #13 Loss Leader I would save the receptionist.Moderator     Join Date: Jul 2006 Location: Florida Posts: 27,741 Originally Posted by pgimeno Sorry, let me rephrase: Will you at least concede my point that even if the top is crushed-up as the collapse progresses, its crushing would be eventually arrested(*) as the top loses mass (because it's converted in rubble), since the total force would be decreasing and thus the columns will eventually be able to support the force? See, this one I think I know. It wouldn't be arrested because the top is not losing mass. So long as you're still accelerating a static body into it, you're going to get the full force. Force=MA is not Force=M(that's still all in one piece)A. The M never changes. In fact, as Mackey has said, the M grows as lower floors are crushed because they stop being part of the static mass and become part of the thing it's hitting. __________________ I have the honor to be Your Obdt. St L. Leader
 22nd August 2010, 06:30 PM #14 R.Mackey Philosopher   Join Date: Apr 2006 Posts: 7,854 Originally Posted by Loss Leader See, this one I think I know. It wouldn't be arrested because the top is not losing mass. So long as you're still accelerating a static body into it, you're going to get the full force. Force=MA is not Force=M(that's still all in one piece)A. The M never changes. In fact, as Mackey has said, the M grows as lower floors are crushed because they stop being part of the static mass and become part of the thing it's hitting. Yup. Think about it this way, if it helps: What about the very top floor? When does it fail? The force on the top floor is only equal to its own mass times its deceleration. That deceleration will be some fraction of 1 g, thus it never collapses -- its load is actually less than it was when it was free-standing and static -- until it suddenly hits an immovable object, and has to dissipate all of its built-up momentum in a hurry. As a result it's easy to see that the very top of the upper block will outlast the entire lower block, no matter what, in the ideal case. The situation for the floor just below the very top is similar, except it's also loaded with the inertial force of the floor above. And so on. In short, the load on the upper block stays bounded, regardless of how much the debris layer in between grows. The same cannot be said for the lower block. Last edited by R.Mackey; 22nd August 2010 at 06:32 PM. Reason: Added quote
 22nd August 2010, 06:31 PM #15 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by Loss Leader It wouldn't be arrested because the top is not losing mass. pgimeno is not referring to *collapse arrest*, but rather to *crush-up arrest*. __________________ http://the911forum.freeforums.org
 22nd August 2010, 06:33 PM #16 Sam.I.Am Illuminator     Join Date: Apr 2009 Posts: 4,627 Quote: as the top loses mass (because it's converted in rubble) The mass is still there, just in a different form. 1 pound weighs 1 pound regardless of the form it's in. The old adage "The straw that broke the Camels back" comes to mind. That one straw weighing a single gram was "The final straw". Truthers seem to forget this basic principal when discussing the falling rubble. It's not the individual pieces that count, it's all of the individual pieces almost simultaneously piling onto the lower floors one or two floors at a time in one chaotic event that counts. Even if those pieces could be carefully placed, one after the other over a week or more per floor the end result would be the same. The floor will fail and it will take out all of the floors below it in sequence. __________________ "Swift, silent and deadly" was a part of my job description Upon hearing me say that my friend asked me "So you're a fart?"... About my avatar.
 22nd August 2010, 06:51 PM #17 Loss Leader I would save the receptionist.Moderator     Join Date: Jul 2006 Location: Florida Posts: 27,741 Originally Posted by R.Mackey Yup. I'm so proud. I printed out your post and showed it to my wife. Originally Posted by femr2 pgimeno is not referring to *collapse arrest*, but rather to *crush-up arrest*. Maybe. But, then, why does his question include this phrase: Originally Posted by pgimeno and thus the columns will eventually be able to support the force? That sounds like he means that the crush-down should eventually be arrested when the top floor of the top section is finally destroyed. __________________ I have the honor to be Your Obdt. St L. Leader
 22nd August 2010, 06:59 PM #18 pgimeno Illuminator     Join Date: Feb 2009 Location: Spain Posts: 3,647 Originally Posted by Sam.I.Am The mass is still there, just in a different form. 1 pound weighs 1 pound regardless of the form it's in. Under the "rubble is able to crush the top" assumption, the mass of the top is converted into rubble as the crushing progresses, thus the top loses mass which now forms part of the rubble layer. As the top has less and less mass, it has less inertia too, which is what causes the force between the rubble and the top in this assumption. Eventually, the force will be less than the structure can resist and the crush-up will be arrested. That's what I meant. You are right and my assertion that the top loses mass was correct. I neglected to specify that that mass is converted to rubble, which apparently has caused confusion. --- Originally Posted by femr2 However it's important to ensure there is clear understanding that the Bazant model does not, and is not intended to, reflect the real world WTC behaviour. I don't lose track of it at any moment. But it proved that the gravity collapse was unstoppable once started, something that many people still contend. --- Sorry, I badly need some sleep, almost 4am here, not in a good condition for this kind of brain frying Hopefully I will get it tomorrow with a fresher mind.
 22nd August 2010, 07:06 PM #19 Sam.I.Am Illuminator     Join Date: Apr 2009 Posts: 4,627 Quote: I neglected to specify that that mass is converted to rubble, which apparently has caused confusion. Ok, that makes much more sense. After rereading what you wrote I admit that I misunderstood what you were getting at. __________________ "Swift, silent and deadly" was a part of my job description Upon hearing me say that my friend asked me "So you're a fart?"... About my avatar.
 22nd August 2010, 10:02 PM #20 Major_Tom Graduate Poster   Join Date: Oct 2007 Posts: 1,960 Can the Bazant concept of crush down, then crush up be applied to WTC1? In this thread people tend to agree that it cannot. But Bazant applies it to WTC1 in BLGB. From the paper(BLGB)What Did and Did not Cause Collapse of WTC Twin Towers in New York Zdenek P. Bazant, Jia-Liang Le, Frank R. Greening and David B. Benson http://www.civil.northwestern.edu/pe...20collapse.pdf "Generalization of Differential Equation of Progressive Collapse The gravity-driven progressive collapse of a tower consists of two phases—the crush-down, followed by crush-up (Fig. 2 bottom), each of which is governed by a different differential equation (Ba?zant and Verdure 2007, pp. 312-313). During the crush-down, the falling upper part of tower (C in Fig. 2 bottom), having a compacted layer of debris at its bottom (zone B), is crushing the lower part (zone A) with negligible damage to itself. During the crush-up, the moving upper part C of tower is being crushed at bottom by the compacted debris B resting on the ground. The fact that the crush-up of entire stories cannot occur simultaneously with the crush-down is demonstrated by the condition of dynamic equilibrium of compacted layer B, along with an estimate of the inertia force of this layer due to vertical deceleration or acceleration; see Eq. 10 and Fig. 2(f) of Bazant and Verdure (2007). This previous demonstration, however, was only approximate since it did not take into account the variation of crushing forces Fc and F0c during the collapse of a story. An accurate analysis of simultaneous (deterministic) crush-up and crush-down is reported in Ba?zant and Le (2008) and is reviewed in the Appendix, where the differetial equations and the initial conditions for a two-way crush are formulated. It is found that, immediately after the first critical story collapses, crush fronts will propagate both downwards and upwards. However, the crush-up front will advance into the overlying story only by about 1% of its original height h and then stop. Consequently, the effect of the initial two-way crush is imperceptible and the hypothesis that the crush-down and crush-up cannot occur simultaneously is almost exact." Does Bazant seem to believe that the concept of crush down, then crush up can be applied to WTC1? From the above quote, clearly he does. We see him applying crush down, then crush up to WTC1 in a paper entitled "What Did and Did not Cause Collapse of WTC Twin Towers in New York" yet the posters here tend to agree that we cannot apply this concept to WTC1. Well, you should let Dr Bazant know.
 22nd August 2010, 11:46 PM #21 Major_Tom Graduate Poster   Join Date: Oct 2007 Posts: 1,960 In the paper BL there is no doubt that Bazant believes that the concept of crush down, then crush up applies to WTC1 and WTC2. Bazant & Le Closure to "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" http://www.civil.northwestern.edu/pe...%20Replies.pdf In the section titled "Reply to R Gourley", points #4 and 5 clearly show that Dr Bazant believes the concept of crush down, then crush up can be applied quite literally to the cases of WTC1 and 2. Point 4 entitled "Why cannot crush up proceed simultaneously with crush down?" Point 5 entiled "Why cannot crush up begin later?" There is no doubt that Dr Bazant is applying his idea of crush down, then crush up literally to the case of WTC1 and 2. It is also clear from the quote from BLGB in my last post. Last edited by Major_Tom; 22nd August 2010 at 11:49 PM.
 22nd August 2010, 11:58 PM #22 Major_Tom Graduate Poster   Join Date: Oct 2007 Posts: 1,960 SO who is correct, the posters who believe that crush down, then crush up cannot be literally applied to WTC1 or 2 , or Dr Bazant? Ironically, you are right and your expert is wrong in this case.
 23rd August 2010, 01:19 AM #23 Oystein Penultimate Amazing     Join Date: Dec 2009 Posts: 17,094 Originally Posted by Major_Tom SO who is correct, the posters who believe that crush down, then crush up cannot be literally applied to WTC1 or 2 , or Dr Bazant? ... Would I go out on a limb if I answered "both, to some extent"? The Bazant model is, of course, a model: It assumes some ideal condictions, like a homogeneous, horizontal crush front. It neglects some mass loss due to lateral ejections. It ignores tilt. It ignores the fact that the core is stronger than the rest of the structure and actually managed to penetrate the rubble front. Etc. As a result, one would expect the crush-up-crush-down scenario, only dirtier due to many deviations of the real world from model.
23rd August 2010, 02:03 AM   #24
Cuddles
Penultimate Amazing

Join Date: Jul 2006
Posts: 18,591
 Remain on topic and avoid making personal attacks. Posted By:Cuddles

 23rd August 2010, 02:29 AM #25 Oystein Penultimate Amazing     Join Date: Dec 2009 Posts: 17,094 By the way: Interesting approach to changing the point of view, and excellent arguments from R.Mackey! Thanks all! I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction.
 23rd August 2010, 02:31 AM #26 leftysergeant Penultimate Amazing     Join Date: Jul 2007 Posts: 18,863 I think Bazant went badly astray when he referred to "crush-up." This does not invalidate his assertion that there was potential energy enough acvailable to cause total collapse. All I see occuring is crush-down. The top block is constantly crushing its own lower regions ab initio. As I understand such things, it does, however, remain a "geometric solid." It is just not a homogenous solid. Whether discombobulated rubble and steel elements, or a nicely-constructed box, it still has mass and volumn. That mass and volumn continue to increase as collapse continues. And it still hammers the structure below it. My problem with the rocket anology is that the rocket must be constantly accelerating, as was the top block in each of the towers. __________________ No civilization ever collapsed because the poor had too much to eat.
 23rd August 2010, 02:41 AM #27 Oystein Penultimate Amazing     Join Date: Dec 2009 Posts: 17,094 Originally Posted by leftysergeant I think Bazant went badly astray when he referred to "crush-up." This does not invalidate his assertion that there was potential energy enough acvailable to cause total collapse. All I see occuring is crush-down. The top block is constantly crushing its own lower regions ab initio. As I understand such things, it does, however, remain a "geometric solid." It is just not a homogenous solid. Whether discombobulated rubble and steel elements, or a nicely-constructed box, it still has mass and volumn. That mass and volumn continue to increase as collapse continues. And it still hammers the structure below it. I guess the point of the theory, besides being purely interesting (because counter-intuitive) on its own merits, is that it shows that not much kinetic energy was "wasted" on structural deformatin in the top block, and thus more was available to crush the lower portion. Even though that is not necessary in order to explain why total collapse occurred at all (there was enough energy around to make that inevitable), but it might add a little velocity to the event. Originally Posted by leftysergeant My problem with the rocket anology is that the rocket must be constantly accelerating, as was the top block in each of the towers. Uhm, that's what rockets do as long as they fire... Last edited by Oystein; 23rd August 2010 at 02:43 AM. Reason: ETA "in order to explain", plus spelling
 23rd August 2010, 02:54 AM #28 Furcifer Guest   Join Date: Apr 2007 Posts: 13,796 Originally Posted by Oystein I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction. At some point in the collapse the core pierced the upper section and it was "crushed up". Bazant's just shown mathematically it doesn't matter. It's amusing how this frustrates the hell out of the truthers.
 23rd August 2010, 03:39 AM #29 leftysergeant Penultimate Amazing     Join Date: Jul 2007 Posts: 18,863 Originally Posted by Oystein I did read (well, skim) the Bazant papers, including the annotations, but didn't really get it until now why the top part should experience less stress and destruction. The top part had only to resist the downward acceleration of its own mass (which mass was decreasing as it disintegrated) while the bottom had to resist the mass of the top part, plus the increasing amount of its own mass that was being accelerated downward. __________________ No civilization ever collapsed because the poor had too much to eat.
 23rd August 2010, 06:16 AM #30 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by 3bodyproblem At some point in the collapse the core pierced the upper section and it was "crushed up". Bazant's just shown mathematically it doesn't matter. It's amusing how this frustrates the hell out of the truthers. Bazants model assumes crush-up does not occur until after full-cycle crush-down. It is clear that no such behaviour actually ocurred. Even the terms crush-up and crush-down can only be loosely applied to the towers... Originally Posted by R.Mackey In the real WTC situation, we don't have a true "crush down / crush up" anyway. What actually happens is the core and perimeter structure of the lower block funnels falling material onto the floors. The truss floors preferentially fail downward, whereas the beam-framed floors in the core preferentially fail upward. In the scenario above, shortly after initiation there is no real semblance of upper block remaining. There are several zoned debris avalanches causing essentially internal pancaking, with a tangled mess of core lagging behind and sections of perimeter either being ejected or peeling even further behind. In addition, whilst floor destruction progressed to ground, core destruction arrested. Could you expand on what you mean by *it doesn't matter* ? __________________ http://the911forum.freeforums.org
 23rd August 2010, 06:18 AM #31 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by Oystein it shows that not much kinetic energy was "wasted" on structural deformatin in the top block, and thus more was available to crush the lower portion. Even though that is not necessary in order to explain why total collapse occurred at all (there was enough energy around to make that inevitable), but it might add a little velocity to the event. It sounds like you are applying the Bazant model behaviour to the real-world events. As we have agreed the Bazant model behaviour does not and was not intended to match real world behaviour. Could you clarify your intent ? __________________ http://the911forum.freeforums.org
 23rd August 2010, 06:24 AM #32 Oystein Penultimate Amazing     Join Date: Dec 2009 Posts: 17,094 Originally Posted by femr2 It sounds like you are applying the Bazant model behaviour to the real-world events. As we have agreed the Bazant model behaviour does not and was not intended to match real world behaviour. Could you clarify your intent ? I did not participate in any such agreement. See my post 23.
 23rd August 2010, 06:40 AM #33 Furcifer Guest   Join Date: Apr 2007 Posts: 13,796 Originally Posted by femr2 Bazants model assumes crush-up does not occur until after full-cycle crush-down. It is clear that no such behaviour actually ocurred. Actually that's not technically correct. With the mathematical proof it's no longer an "assumption" per se, but a validated simplification. It's clear that it adequately describes the actual behaviour.
 23rd August 2010, 06:41 AM #34 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by Oystein I did not participate in any such agreement. See my post 23. Okay, then... Quote: it shows that not much kinetic energy was "wasted" on structural deformatin in the top block Do you agree that in the real world there was no real semblance of *top block* shortly after initiation ? Quote: and thus more was available to crush the lower portion. Do you agree that core destruction and floor *pancaking* propogation was separated by considerable distance ? Quote: there was enough energy around Agreed, and imo the primary valid result of the model. Other behavioural properties of the model simply don't match the real world events. Quote: but it might add a little velocity to the event. Floor destruction propogation (ROOSD) attained a fairly constant velocity after about 4s. Core destruction arrested. Are you sure you're not inadvertently applying model behaviour to the real world inappropriately ? __________________ http://the911forum.freeforums.org
 23rd August 2010, 06:49 AM #35 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by 3bodyproblem Actually that's not technically correct. With the mathematical proof it's no longer an "assumption" per se, but a validated simplification. No. Floor destruction propogation did not involve an *upper block* at all, and was basically driven entirely by a *rubble layer*. Core destruction arrested. Perimeter essentially peeled. Quote: It's clear that it adequately describes the actual behaviour. As the limiting case, it shows there was enough energy available and given it's simplfying assumptions that propogation to ground would ensue in those conditions, but it does not describe the actual behaviour at all. __________________ http://the911forum.freeforums.org
 23rd August 2010, 07:46 AM #36 Furcifer Guest   Join Date: Apr 2007 Posts: 13,796 Originally Posted by femr2 No. Floor destruction propogation did not involve an *upper block* at all, and was basically driven entirely by a *rubble layer*. Core destruction arrested. Perimeter essentially peeled. As the limiting case, it shows there was enough energy available and given it's simplfying assumptions that propogation to ground would ensue in those conditions, but it does not describe the actual behaviour at all. I'm not sure how you figure breaking it up makes any difference? Bazant has already shown it doesn't matter. You have a variety of ways to get from A to B. Either way you end up at B.
 23rd August 2010, 08:09 AM #37 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by 3bodyproblem I'm not sure how you figure breaking it up makes any difference? I'm highlighting to you that stating *it adequately describes the actual behaviour* is not correct. The real world behaviour and the behaviour of the Bazant model are very different, obviously. Quote: Bazant has already shown it doesn't matter. What doesn't matter ? Please cite where Bazant uses a model without the *top block*, and also includes early *crush up*. You can infer from the energetics results that there was enough energy available, sure. *Showing it doesn't matter* in this context is your inference, not something Bazant has shown. The energetics inferences are totally different to stating that the model behaviour matches real world behaviour. That's just not true. Quote: You have a variety of ways to get from A to B. Sure, and my point is that it is erronious to apply crush down/up behaviour of Bazants model to the real world events literally. Again, as I'm sure you'll listen to Ryans' words more than mine... Originally Posted by R.Mackey In the real WTC situation, we don't have a true "crush down / crush up" anyway. What actually happens is the core and perimeter structure of the lower block funnels falling material onto the floors. The truss floors preferentially fail downward, whereas the beam-framed floors in the core preferentially fail upward. This description does not match the Bazant model behaviour well at all. The vast majority of floor destruction does not involve a top block at all, and simply a *rubble layer*. Quote: Either way you end up at B. Bazant does not mention tipping cores. IF the large core remnant of WTC 1 that remained after core destruction arrest had not dropped, would you still describe the outcome as *B* ? That core portion was not destroyed by *crush-down*, and so is not accounted for by the model. __________________ http://the911forum.freeforums.org
 23rd August 2010, 08:45 AM #38 Furcifer Guest   Join Date: Apr 2007 Posts: 13,796 Originally Posted by femr2 I'm highlighting to you that stating *it adequately describes the actual behaviour* is not correct. Sure, and my point is that it is erronious to apply crush down/up behaviour of Bazants model to the real world events literally. It's pretty clear it's not meant to describe the events literally, but it's adequate. It seems to me you're just playing a game of semantics?
 23rd August 2010, 09:55 AM #39 femr2 Illuminator   Join Date: Apr 2009 Posts: 3,859 Originally Posted by 3bodyproblem It's pretty clear it's not meant to describe the events literally Of course. That is why I asked you to clarify *It's clear that it adequately describes the actual behaviour.*, and pointed out the actual behaviour was very different. Quote: but it's adequate. For it's intended purpose, limiting case energy availablility, it's okay. Not a whole lot different in that sense to other simplified models kicking around. Applying it to actual behaviour is, again, wrong. Minimal axial column impacts. Minimal column buckling. No true crush-down or crush-up in the Bazantian sense. No *upper block* shortly after initiation. Separate and different floor, core and perimeter modes of destruction. Arrest of core destruction for a significant proportion of the core. etc. Quote: It seems to me you're just playing a game of semantics? Not at all. I'm simply making sure that the model is not used incorrectly. As there is enough energy in the limiting case for complete destruction, does that mean the model can be used to state there is enough energy for alternate modes of destruction...sure, but does it mean that the behaviour matches alternate modes of destruction (such as the actual real world event) ? No. Perhaps you could highlight a number of real world behaviours that do match the behaviour of the Bazant model ? __________________ http://the911forum.freeforums.org

International Skeptics Forum