DGM
Skeptic not Atheist
Thanks, I was wondering if "crumple zones" fell into this.
BTW I though nothing interesting would come out of this thread. Figures it takes a derail.
Analysis of Bazant & Zhou (2002)
- Thread starter bofors
- Start date
BTW I though nothing interesting would come out of this thread. Figures it takes a derail.
rwguinn
Penultimate Amazing
Yepper.
Automakers have some very specialized, limited utility programs to analyze the buckling that occurs in the crumple zone. (limited to special shapes and applications)
BTW I though nothing interesting would come out of this tread. Figures it takes a derail.
I enjoyed NB's explanation. Hadn't thought of using steel members for that--I always used cables, chain, or rope for that type of thing.
Mr. Skinny
Alien Cryogenic Engineer
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rwguinn,
I understood that NB meant (at least in my own head) bending rather than buckling, as I tend to think of buckling as you said, permanent damage.
Since NB was correcting me that seismic bracing was not selective failure, I substituted bending for his use of buckling.
Perhaps he uses a different definition. And it's been 35 years since I took Strength of Materials at Uni, so I'm in no position to argue with either of you.
Mr. Skinny
Alien Cryogenic Engineer
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Mr. Skinny
Alien Cryogenic Engineer
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At the risk of showing how much I've forgotten, would bending = did not exceed the Young's modulus of elasticity, whereas buckle = did exceed YMOE?
our design (a slow-flying Boeing 707 lost in the fog and seeking a landing field
Leslie E. Robertson said:
This Robertson's work, not some hearsay you produce!!
Poor research on your part and posting hearsay over and over. Not good. My Bunk stuff are FACTS, you junk is hearsay. I win. Swing, I won this one but if you study and try to do better research you may get better. I suggest going to engineering school and learn to fly and get an instrument rating. Fly big jets.
Until you 9/11 truth can recognize hearsay, 9/11 truth will not be able to find facts. Robertson wrote the paper; 9/11 truth does not have a direct quote. 9/11 truth has hearsay, people made up the 600 mph design through ignorance. You, knowing that it is false now, will be telling a lie when you say it again.
Sorry Swing, the speed was 180 mph, lost in the fog, ready to land, low fuel, cause if you had fuel you would not be trying to land in the fog. We pilots do not try to crash like terrorist. You failed to see Robertson said this is the design. Slow speed. I present the truth, you have hearsay. No, the text you refer to is hearsay, not a quote from the Chief Engineer. Let me refresh your memory and put a stop to hearsay. Those people looked up a speed for a 707. BTW the top speed at 700 feet is not 600 mph unless you are doing it on purpose! Terrorist only stuff. You should look up the speed with flaps and gear. Lost airplane was the design, not your made up hearsay stuff. (BTW, the towers could handle the impact, the moment from the impact, because the WTC had to withstand WIND!!!!!! So if you want to say the towers can handle the MOMENT from the IMPACT, you can say that. But there was no design for a high speed aircraft. IF THERE HAD BEEN a design for a high speed impact, the stairwells would survive, and the fire control systems would also survive. Wrong because you have hearsay and not the facts, and wrong by logic, not understanding engineering, energy, or reality.
Leslie E. Robertson said:
Leslie E. Robertson said: on being hit by a commercial jet -
This Robertson's work, not some hearsay you produce!!
Poor research on your part and posting hearsay over and over. Not good. My Bunk stuff are FACTS, you junk is hearsay. I win. Swing, I won this one but if you study and try to do better research you may get better. I suggest going to engineering school and learn to fly and get an instrument rating. Fly big jets.
Until you 9/11 truth can recognize hearsay, 9/11 truth will not be able to find facts. Robertson wrote the paper; 9/11 truth does not have a direct quote. 9/11 truth has hearsay, people made up the 600 mph design through ignorance. You, knowing that it is false now, will be telling a lie when you say it again.
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Newtons Bit
Penultimate Amazing
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I live in a region of moderate seismicity. Earthquake engineering is sort of my thing. Any idiot can design a gravity system. If you're familiar with the IBC code; the buildings I design typically are seismic design category C or D. Though they frequently have serious torsional or soft-story irregularities. Dang architects
rwguinn
Penultimate Amazing
Actually, Buckling does not necessarily exceed the strength. A slender column can fail in buckling and not suffer permanent damage to the column, just as NB has said.
Think of a plate balanced on top of a slender rod. if the plate is too heavy, the rod will buckle elastically, dumping the plate to the ground, and once the load is gone, resume its original shape.
Mr. Skinny
Alien Cryogenic Engineer
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Thanks very much, rwguinn.
It appears I need to go back and re-study the difference between bend and buckle. Apparently, I need to go back to school, so to speak.
Appreciate your response and those from NB as well.
rwguinn
Penultimate Amazing
When we quit learning, we begin dying
Mr. Skinny
Alien Cryogenic Engineer
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- 7,843
Agree, but it sucks to know you knew it once, but "if you don't use it, you lose it."
(yes, getting old sucks sometimes - but just ask me about the safety hazards of carbon nanotube production or high energy lasers, and I might impress ya)
ElMondoHummus
0.25 short of being half-witted
I think I may be partially responsible for Bofors tackling Bazant, Zhou, et. al.
I mean, I did bug him to the point of cross posting in different threads to look at that as well as three other works. On the other hand, I was doing it in regards to his statement of "Entropy", not as a challenge to do a point-by-point of the entire work. But, that's still a good thing: Point-by-point rebuttals provide substance to debate. I for one am glad he's doing it because I know we have engineers here who'll be able to respond substantively in return. Everyone learns something. Or at least has the opportunity to learn (not like some fantasists here have ever taken advantage of that...).
LashL
Goddess of Legaltainment™
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It doesn't even remotely resemble an analysis.
To the extent that it purports to be an analysis, this [analysis] is a travesty. It's a travesty of a mockery of a sham of a mockery of a travesty of two mockeries of a sham.
(with apologies to the original authors)
Alferd_Packer
Philosopher
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bofor's only post on this thread that makes any sense.
Alferd_Packer
Philosopher
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Yes, the part where you wrote that Bofors is a "man of science."
He is anything but that.
Dave Rogers
Bandaged ice that stampedes inexpensively through
I'm working on a full response to every point Bofors has raised here, but this one in particular looks like a Stundie - the idea that the bottom of the tower could bend over far enough for the top to fall off is rather entertaining.
Dave
Sad you are unable to do simple research. False on most counts. Pathetic ideas at best.
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Dave Rogers
Bandaged ice that stampedes inexpensively through
Response to all the points raised so far
OK, here's what I have to say on what bofors has posted so far. Some of this probably duplicates the work of other posters, but I don't think it hurts to establish consensus on these things. Warning: Long post.
Bazant and Zhou's paper is generally taken as evidence that the collapse of the Twin Towers, once initiated, would be expected to progress total collapse. In this context (and this is clear to a reasonable reader of the paper), the details of collapse initiation included are understandably brief and speculative, and for more detail the NIST report should be consulted. It's in this context that I'm assessing the criticisms of the paper from bofors.
The first couple of points address not the content, but the standard of peer reviewing, of the paper.
Incorrect on the publication date. The paper was written and submitted in two days but was published in the January 2002 edition, leaving ample time for peer review. The paper is described as a "Rapid Communication", a generally accepted format for bringing preliminary or timely scientific results to the notice of the scientific community with predetermined acceptance and review criteria.
This is not a comment on the content of the paper and so is irrelevant.
This is an assertion without proof.
The fact that questions were raised and an addendum was included to answer them is prima facie evidence of peer review. To present it as evidence of the absence of peer review is absurd.
Rejected as being unsupported by the evidence presented.
The next group of points addresses the first section of the paper, which is
presented as a likely scenario for collapse initiation and appears to be present mainly to lend context to the analysis of the progression from initiation to global collapse that forms the core of the paper. As such, detailed criticism of this section is rather futile, as the collapse initiation has been analysed in far more detail by NIST, whose work inevitably supersedes that of Bazant and Zhou.
This is not a valid criticism of the paper, which makes no claim to be anything other than a simplified approximate analysis. The vast majority of scientific analysis proceeds by way of simplified approximations that are nevertheless sufficiently good to explain observation and predict future behaviour. To demand an exact analysis in every case is to render the vast majority of scientific and engineering problems intractable, and hence to negate the usefulness of science altogether. This is a theme that surfaces again and again in these criticisms. It is not a valid criticism to comment that a paper has made simplified assumptions, when those assumptions have been fully described and justified within the paper.
Bazant and Zhou do not in fact discuss collapse initiation in any great detail. Their analysis is aimed at understanding the question of why collapse initiation leads to global collapse. In effect, bofors is criticising Bazant and Zhou for not doing NIST's job, and NIST for not doing Bazant and Zhou's job. Neither of these criticisms has any bearing on the overall understanding of events based on the combination of both.
The use of the word "majority" in quotes is misleading as Bazant and Zhou actually say "Once more than half the columns... suffer buckling". This is presented as "the likely scenario of failure" rather than as a predictive model of collapse initiation. Nor is it stated how many more than half the columns must fail; the concept being described is simply that collapse must commence when a sufficient number of columns no longer support the weight of the building. This is an obvious conclusion that needs no further explanation except in the face of wilful obfuscation. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Note also that the final section of the addendum includes an expression where the number of failed columns is included as a parameter, so the above statement is factually incorrect.
Again, this is wilful obfuscation of the obvious observation that there must exist some level of column failure at which the upper part of the building is no longer supported. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Again, this is a criticism of Bazant and Zhou for not carrying out NIST's job. The statement that heating lowers the yield strength of steel is simply a statement of a well-known property of steel and needs no explanation. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Again, Bazant and Zhou are not analysing collapse initiation in detail. Estimates of probability for different failure scenarios are not a particularly relevant concept for discussing past events. Consideration of other failure scenarios is not relevant to the question of whether global collapse follows on from the initial failure.
NIST's subsequent detailed fire analysis and experimental work has determined a reasonable set of bounds on the temperatures of the steel columns as a function of location, and despite the attempts at misrepresentation of their results given here, they result in a prediction of collapse initiation.
Bazant and Zhou make no such claim, as a correct reading of the quote included with the original post will make clear. This is overt misrepresentation.
Again, this is an attempt to criticise Bazant and Zhou for not doing NIST's job. It is a little unclear what is the suggestion here, as NIST's fire modelling finds ample airflow for these fire temperatures, which are well within reasonable limits for an office contents fire, and there is an abundant source of airflow from the large holes in the building perimeter created by the aircraft impact.
At this point, the criticism finally moves to the important part of the paper, the analysis of the events following collapse initiation.
The use of the term "demolition waves" is itself a blatant attempt at implying the conclusion that the WTC towers were demolished. What is visible in the collapse video is the emission of large quantities of dust and debris in a roughly floor-by-floor sequence as the building collapses, as would be expected from impacts between floor slabs as they collapse.
This does not necessarily give much information on the behaviour of the support columns, which were known to have fractured largely at their connection points into three-storey lengths. This latter observation effectively proves that the collapses of the support columns were inherently multi-storey processes.
Since the column welds are expected to be substantially weaker than the column sections, the expected failure mode would be for initial elastic multi-storey buckles, followed by failure at the welds. If, in this process, the welds failed before the column sections began to buckle inelastically, then the debris columns would not be permanently deformed and would indeed be buckle-free. However, this failure mode would allow the collapse to progress much more easily than that assumed by Bazant and Zhou, who effectively assume homogeneous columns throughout the height
of the building. In other words, Bazant and Zhou have deliberately made a set of assumptions which make the building unrealistically resistant to collapse, yet find that collapse still ensues. This point strengthens the rigour of their analysis rather than invalidating it.
This point is so poorly stated as to be almost meaningless. Fracture is exlicitly discussed at several points in the paper, and it is explained that only plastic failure will be examined in detail as this is more optimistic to survival. The impossibility of the building top falling off will be discussed below.
This is simply untrue. Bazant and Zhou specifically use this observation in paragraph 2 of the section "Analysis of Inelastic Energy Dissipation" to support their identification of the failure modes of the structure. Would bofors also like to explain why "multi-column perimeter sections... peeling off the sides of tower during collapse" is consistent with the assertion in point 13 that collapse videos indicate that multi-floor failures do not occur?
This, again, is simply untrue. See fig. 1 stage 5. Although this is hardly relevant to the discussion of whether global collapse ensues, it is clear that this effect arises from a combination of glancing impacts and "the upper part possibly getting wedged inside an emptied lower part of the framed tube" as stated on the first page of the Communication.
The assertion that "all of the concrete is pulverized in dust" is neither supported by evidence nor sufficiently quantitative to be of any value. The discussion of the energy required to do so is irrelevant to the question of whether global collapse ensues, since, if the support columns are unable to resist collapse, any amount of un-pulverised concrete will not do so as long as it is only present in a series of vertically separated floor slabs.
As the authors state, "For our purpose, we may assume that all the impact forces go into the columns and are distributed among them equally. Unlikely though such a distribution may be, it is nevertheless the most optimistic hypothesis to make because the resistance of the building to the impact is, for such a distribution, the highest. If the building is found to fail under a uniform distribution of the impact forces, it would fail under any other distribution." This statement justifies the failure to consider the detailed structire of the building, as such structural details could only make global collapse easier to achieve.
This returns to the error of points 3-12, in that it attempts to criticise a paper whose aim is to show that collapse initiation leads to global collapse by stating that the paper does not model collapse initiation in sufficient detail.
Points 21 to 23 appear to be rooted in a wilful refusal to understand the discussion in progress in the paper. In general, Bazant and Zhou describe a simplification they have made to the scenario under consideration, then justify that simplification on the basis that it will make collapse either less likely or no more likely. Bofors simply ignores the justification and states that the presence of any simplification is an error.
This can be likened to the approach of any branch of science that approaches an intractably complex situation by making simplifications. This level of criticism is equivalent to suggesting that thermodynamics is inadequate to describe the operation of a simple piston engine because it fails to compute the trajectory of individual gas molecules through the entire system. It is fundamental to science that properly formulated simplifications permit meaningful analysis of complex systems, and to deny this is to reject the majority of scientific understanding.
This is an extraordinarily odd criticism. If Bazant and Zhou had carried out such simulations then it would be absurd for their paper to point out that such simulations would be required. At no point is it established that it would be possible to carry out such simulations at the present state of the art of computer modelling.
Again, this is one of the "simplifying assumptions most optimistic in regard to survival" that are made. Nonuniformity of impact forces will lead to substantial eccentric loadings on steel columns, which they are less able to resist than the symmetrical loadings predicted by a uniform failure model. Again, bofors is attacking Bazant and Zhou for making conservative assumptions concerning the likelihood of collapse.
This is simply quibbling; the semantic distinction between "collapse" and "buckle, bend and break" is of no great relevance here. A column that buckles loses a sufficiently large proportion of its strength that its ability to resist collapse is negligible, a column that bends will inevitably go on under an increasing load to buckle, and a column that breaks has collapsed. This objection is therefore absurd.
The first sentence is an assertion without proof. The remainder of the point, coming from a critic who accuses Bazant and Zhou of over-simplification and of publishing results without significant justification, is reprehensible. It should also be noted that, once the columns of one floor have been severed - which they must have been in order for the upper block to begin to fall - there is no longer any connection present between the upper and lower blocks capable of exerting a lateral force on the falling block, which must then simply rotate about its centre of mass. The
tilting referred to could not therefore lead to the top falling off, as there is no transverse force available to move it laterally. This is addressed in detail in Appendix 2, which bofors appears not to have read.
Again the error of points 3-12; the assumptions of 800ºC and loss of fireproofing play no part in the analysis of whether collapse initiation would lead to global collapse.
This indicates that bofors has made no attempt to understand the paper. Bazant and Zhou state that there is an elastic compression phase, and find that "it is obvious that the elastically calculated forces in columns caused by the vertical impact of the upper part must have exceeded the load capacity of the lower part by at least an order of magnitude". The duration of the elastic phase is irrelevant to this conclusion.
This is not a criticism but a misunderstanding by bofors. Bazant and Zhou make it clear later in the section that the printed meaning is correct. Once the elastic wave has propagated to the ground, the entire building can be taken to react as a spring (the most optimistic assumption for resisting collapse). Elsewhere they calculate the instantaneous elasic response which is appropriate for times short compared with the time for the elastic wave to propagate to the ground.
This is because such a model is not physically feasible. The idea that the bottom of the building could bend over so far as to allow the top to fall off is laughably absurd. The question of rotation of the upper block is discussed in detail in Appendix 2, which bofors still appears not to have read.
This is the most ridiculously untrue statement encountered so far. The longest section of the paper considers plastic deformation of the structure. The fact that bofors is able to make this assertion is hard to understand; overall, the central thrust of the entire paper is the plastic response of the structure to the impact from the falling block.
The reaction of columns in the upper block is discussed in the addendum. Since the elastic and plastic responses of the upper block do not affect its gravitational potential energy, which must be dissipated by deformation of the lower block, the behaviour of the upper block cannot have a significant effect on that of the lower block.
Again, the question of the top falling off is addressed in Appendix 2, which I urge bofors to read at some point in his life. The assertion that a uniform model is pessimistic is not only unjustified but is completely contrary to Bazant and Zhou's statement that this is an assumption "most optimistic in regard to survival", in that a uniform structure cannot have any weak points where collapse would be most able to propagate.
To summarise, the 35 criticisms (including sub-points and summaries) levelled so far by Bofors can be categorised as follows:
Irrelevant - 3 (do not address the paper itself or make no sense)
Irrelevant to scope - 10 (discuss collapse initiation, rather than collapse progression)
Answered in text - 7 (Claim is that the paper does not discuss something it does, in fact, discuss)
Misleading - 4 (Claim is that the paper makes a claim it does not, in fact, make, or misrepresents evidence to claim it disagrees with the paper's conclusions)
Generally invalid - 6 (Criticises the paper for making assumptions which are stated and justified within the paper)
Semantic quibbling - 1 (definition of collapse)
Self-contradictory - 1 (peer review comments disprove the existence of peer review)
Contradicts another point - 1 (plus one listed elsewhere)
Unproven assertion - 1 (The paper was not peer reviewed)
Reading error by critic - 1
Valid criticisms so far - 0
I'll await the remaining points with more resignation than anticipation.
Dave
OK, here's what I have to say on what bofors has posted so far. Some of this probably duplicates the work of other posters, but I don't think it hurts to establish consensus on these things. Warning: Long post.
Bazant and Zhou's paper is generally taken as evidence that the collapse of the Twin Towers, once initiated, would be expected to progress total collapse. In this context (and this is clear to a reasonable reader of the paper), the details of collapse initiation included are understandably brief and speculative, and for more detail the NIST report should be consulted. It's in this context that I'm assessing the criticisms of the paper from bofors.
The first couple of points address not the content, but the standard of peer reviewing, of the paper.
Incorrect on the publication date. The paper was written and submitted in two days but was published in the January 2002 edition, leaving ample time for peer review. The paper is described as a "Rapid Communication", a generally accepted format for bringing preliminary or timely scientific results to the notice of the scientific community with predetermined acceptance and review criteria.
This is not a comment on the content of the paper and so is irrelevant.
This is an assertion without proof.
The fact that questions were raised and an addendum was included to answer them is prima facie evidence of peer review. To present it as evidence of the absence of peer review is absurd.
Rejected as being unsupported by the evidence presented.
The next group of points addresses the first section of the paper, which is
presented as a likely scenario for collapse initiation and appears to be present mainly to lend context to the analysis of the progression from initiation to global collapse that forms the core of the paper. As such, detailed criticism of this section is rather futile, as the collapse initiation has been analysed in far more detail by NIST, whose work inevitably supersedes that of Bazant and Zhou.
This is not a valid criticism of the paper, which makes no claim to be anything other than a simplified approximate analysis. The vast majority of scientific analysis proceeds by way of simplified approximations that are nevertheless sufficiently good to explain observation and predict future behaviour. To demand an exact analysis in every case is to render the vast majority of scientific and engineering problems intractable, and hence to negate the usefulness of science altogether. This is a theme that surfaces again and again in these criticisms. It is not a valid criticism to comment that a paper has made simplified assumptions, when those assumptions have been fully described and justified within the paper.
Bazant and Zhou do not in fact discuss collapse initiation in any great detail. Their analysis is aimed at understanding the question of why collapse initiation leads to global collapse. In effect, bofors is criticising Bazant and Zhou for not doing NIST's job, and NIST for not doing Bazant and Zhou's job. Neither of these criticisms has any bearing on the overall understanding of events based on the combination of both.
The use of the word "majority" in quotes is misleading as Bazant and Zhou actually say "Once more than half the columns... suffer buckling". This is presented as "the likely scenario of failure" rather than as a predictive model of collapse initiation. Nor is it stated how many more than half the columns must fail; the concept being described is simply that collapse must commence when a sufficient number of columns no longer support the weight of the building. This is an obvious conclusion that needs no further explanation except in the face of wilful obfuscation. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Note also that the final section of the addendum includes an expression where the number of failed columns is included as a parameter, so the above statement is factually incorrect.
Again, this is wilful obfuscation of the obvious observation that there must exist some level of column failure at which the upper part of the building is no longer supported. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Again, this is a criticism of Bazant and Zhou for not carrying out NIST's job. The statement that heating lowers the yield strength of steel is simply a statement of a well-known property of steel and needs no explanation. This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
This point, as a detail to be included in a predictive model of collapse initiation, is not relevant to a discussion of why collapse initiation leads to global collapse.
Again, Bazant and Zhou are not analysing collapse initiation in detail. Estimates of probability for different failure scenarios are not a particularly relevant concept for discussing past events. Consideration of other failure scenarios is not relevant to the question of whether global collapse follows on from the initial failure.
NIST's subsequent detailed fire analysis and experimental work has determined a reasonable set of bounds on the temperatures of the steel columns as a function of location, and despite the attempts at misrepresentation of their results given here, they result in a prediction of collapse initiation.
Bazant and Zhou make no such claim, as a correct reading of the quote included with the original post will make clear. This is overt misrepresentation.
Again, this is an attempt to criticise Bazant and Zhou for not doing NIST's job. It is a little unclear what is the suggestion here, as NIST's fire modelling finds ample airflow for these fire temperatures, which are well within reasonable limits for an office contents fire, and there is an abundant source of airflow from the large holes in the building perimeter created by the aircraft impact.
At this point, the criticism finally moves to the important part of the paper, the analysis of the events following collapse initiation.
The use of the term "demolition waves" is itself a blatant attempt at implying the conclusion that the WTC towers were demolished. What is visible in the collapse video is the emission of large quantities of dust and debris in a roughly floor-by-floor sequence as the building collapses, as would be expected from impacts between floor slabs as they collapse.
This does not necessarily give much information on the behaviour of the support columns, which were known to have fractured largely at their connection points into three-storey lengths. This latter observation effectively proves that the collapses of the support columns were inherently multi-storey processes.
Since the column welds are expected to be substantially weaker than the column sections, the expected failure mode would be for initial elastic multi-storey buckles, followed by failure at the welds. If, in this process, the welds failed before the column sections began to buckle inelastically, then the debris columns would not be permanently deformed and would indeed be buckle-free. However, this failure mode would allow the collapse to progress much more easily than that assumed by Bazant and Zhou, who effectively assume homogeneous columns throughout the height
of the building. In other words, Bazant and Zhou have deliberately made a set of assumptions which make the building unrealistically resistant to collapse, yet find that collapse still ensues. This point strengthens the rigour of their analysis rather than invalidating it.
This point is so poorly stated as to be almost meaningless. Fracture is exlicitly discussed at several points in the paper, and it is explained that only plastic failure will be examined in detail as this is more optimistic to survival. The impossibility of the building top falling off will be discussed below.
This is simply untrue. Bazant and Zhou specifically use this observation in paragraph 2 of the section "Analysis of Inelastic Energy Dissipation" to support their identification of the failure modes of the structure. Would bofors also like to explain why "multi-column perimeter sections... peeling off the sides of tower during collapse" is consistent with the assertion in point 13 that collapse videos indicate that multi-floor failures do not occur?
This, again, is simply untrue. See fig. 1 stage 5. Although this is hardly relevant to the discussion of whether global collapse ensues, it is clear that this effect arises from a combination of glancing impacts and "the upper part possibly getting wedged inside an emptied lower part of the framed tube" as stated on the first page of the Communication.
The assertion that "all of the concrete is pulverized in dust" is neither supported by evidence nor sufficiently quantitative to be of any value. The discussion of the energy required to do so is irrelevant to the question of whether global collapse ensues, since, if the support columns are unable to resist collapse, any amount of un-pulverised concrete will not do so as long as it is only present in a series of vertically separated floor slabs.
As the authors state, "For our purpose, we may assume that all the impact forces go into the columns and are distributed among them equally. Unlikely though such a distribution may be, it is nevertheless the most optimistic hypothesis to make because the resistance of the building to the impact is, for such a distribution, the highest. If the building is found to fail under a uniform distribution of the impact forces, it would fail under any other distribution." This statement justifies the failure to consider the detailed structire of the building, as such structural details could only make global collapse easier to achieve.
This returns to the error of points 3-12, in that it attempts to criticise a paper whose aim is to show that collapse initiation leads to global collapse by stating that the paper does not model collapse initiation in sufficient detail.
Points 21 to 23 appear to be rooted in a wilful refusal to understand the discussion in progress in the paper. In general, Bazant and Zhou describe a simplification they have made to the scenario under consideration, then justify that simplification on the basis that it will make collapse either less likely or no more likely. Bofors simply ignores the justification and states that the presence of any simplification is an error.
This can be likened to the approach of any branch of science that approaches an intractably complex situation by making simplifications. This level of criticism is equivalent to suggesting that thermodynamics is inadequate to describe the operation of a simple piston engine because it fails to compute the trajectory of individual gas molecules through the entire system. It is fundamental to science that properly formulated simplifications permit meaningful analysis of complex systems, and to deny this is to reject the majority of scientific understanding.
This is an extraordinarily odd criticism. If Bazant and Zhou had carried out such simulations then it would be absurd for their paper to point out that such simulations would be required. At no point is it established that it would be possible to carry out such simulations at the present state of the art of computer modelling.
Again, this is one of the "simplifying assumptions most optimistic in regard to survival" that are made. Nonuniformity of impact forces will lead to substantial eccentric loadings on steel columns, which they are less able to resist than the symmetrical loadings predicted by a uniform failure model. Again, bofors is attacking Bazant and Zhou for making conservative assumptions concerning the likelihood of collapse.
This is simply quibbling; the semantic distinction between "collapse" and "buckle, bend and break" is of no great relevance here. A column that buckles loses a sufficiently large proportion of its strength that its ability to resist collapse is negligible, a column that bends will inevitably go on under an increasing load to buckle, and a column that breaks has collapsed. This objection is therefore absurd.
The first sentence is an assertion without proof. The remainder of the point, coming from a critic who accuses Bazant and Zhou of over-simplification and of publishing results without significant justification, is reprehensible. It should also be noted that, once the columns of one floor have been severed - which they must have been in order for the upper block to begin to fall - there is no longer any connection present between the upper and lower blocks capable of exerting a lateral force on the falling block, which must then simply rotate about its centre of mass. The
tilting referred to could not therefore lead to the top falling off, as there is no transverse force available to move it laterally. This is addressed in detail in Appendix 2, which bofors appears not to have read.
Again the error of points 3-12; the assumptions of 800ºC and loss of fireproofing play no part in the analysis of whether collapse initiation would lead to global collapse.
This indicates that bofors has made no attempt to understand the paper. Bazant and Zhou state that there is an elastic compression phase, and find that "it is obvious that the elastically calculated forces in columns caused by the vertical impact of the upper part must have exceeded the load capacity of the lower part by at least an order of magnitude". The duration of the elastic phase is irrelevant to this conclusion.
This is not a criticism but a misunderstanding by bofors. Bazant and Zhou make it clear later in the section that the printed meaning is correct. Once the elastic wave has propagated to the ground, the entire building can be taken to react as a spring (the most optimistic assumption for resisting collapse). Elsewhere they calculate the instantaneous elasic response which is appropriate for times short compared with the time for the elastic wave to propagate to the ground.
This is because such a model is not physically feasible. The idea that the bottom of the building could bend over so far as to allow the top to fall off is laughably absurd. The question of rotation of the upper block is discussed in detail in Appendix 2, which bofors still appears not to have read.
This is the most ridiculously untrue statement encountered so far. The longest section of the paper considers plastic deformation of the structure. The fact that bofors is able to make this assertion is hard to understand; overall, the central thrust of the entire paper is the plastic response of the structure to the impact from the falling block.
The reaction of columns in the upper block is discussed in the addendum. Since the elastic and plastic responses of the upper block do not affect its gravitational potential energy, which must be dissipated by deformation of the lower block, the behaviour of the upper block cannot have a significant effect on that of the lower block.
Again, the question of the top falling off is addressed in Appendix 2, which I urge bofors to read at some point in his life. The assertion that a uniform model is pessimistic is not only unjustified but is completely contrary to Bazant and Zhou's statement that this is an assumption "most optimistic in regard to survival", in that a uniform structure cannot have any weak points where collapse would be most able to propagate.
To summarise, the 35 criticisms (including sub-points and summaries) levelled so far by Bofors can be categorised as follows:
Irrelevant - 3 (do not address the paper itself or make no sense)
Irrelevant to scope - 10 (discuss collapse initiation, rather than collapse progression)
Answered in text - 7 (Claim is that the paper does not discuss something it does, in fact, discuss)
Misleading - 4 (Claim is that the paper makes a claim it does not, in fact, make, or misrepresents evidence to claim it disagrees with the paper's conclusions)
Generally invalid - 6 (Criticises the paper for making assumptions which are stated and justified within the paper)
Semantic quibbling - 1 (definition of collapse)
Self-contradictory - 1 (peer review comments disprove the existence of peer review)
Contradicts another point - 1 (plus one listed elsewhere)
Unproven assertion - 1 (The paper was not peer reviewed)
Reading error by critic - 1
Valid criticisms so far - 0
I'll await the remaining points with more resignation than anticipation.
Dave
SpaceMonkeyZero
Graduate Poster
- Joined
- Aug 7, 2007
- Messages
- 1,284
I never thought that was possible for the sound of crickets to come through a keyboard that isn't even being typed on.
bofors: You stated you'd reply to comments *yesterday*. When that time came, you said that you're going to type up a long rambling manifesto and post that and *THEN* respond to questions about THAT.
Come on. Man up and answer the questions that have been posted?
bofors: You stated you'd reply to comments *yesterday*. When that time came, you said that you're going to type up a long rambling manifesto and post that and *THEN* respond to questions about THAT.
Come on. Man up and answer the questions that have been posted?
dudalb
Penultimate Amazing
Normally I would agree but bofors is so obviously spamming . Nothing can kill off a messageboard then a constant diet of spam. IF would bother to put the information in his own words I would be a lot more tolerent.