Any time. Actually, Jeffrey and I have "discussed" the same issues many times on several forums. We mostly agree but have one big difference in our approach to reasoned argument. One simple example: I can legitimately argue the proof of the sequenced cascading failure of columns driven by load redistribution. My argument based on "the first column to fail was followed by the second column >> third>> fourth...etc WITHOUT ever being able to define which specific column was "first"..... We can never know the specific sequence of column failures. I say we don't need to. Stated more generically you can often progress a generic argument validly without needing specific details. Many engineers and applied science professionals are not comfortable with that process.I'm all for an "Econ & Orling" 9/11 thread.
I'm pretty sure any student with first-year knowledge can do that. After all, I've done it above.
You don't need to be able to re-do all of the analysis to understand the nature of the fallacy: to understand that the mathematical model in the paper doesn't match what actually happened.
It's not a rookie mistake, it's an "ivory tower" mistake: he's devised his own theoretical model and uses it without having ascertained that it works in practice.
I don't know why you mention finite element methods. Whether he used finite element, finite difference or whatever method is irrelevant. He entered the fields of physics and structual engineering assuming he knew the fields without knowing the fields. He made sweeping conclusions in the empirical domain not based on real empirical evidence. Thus he's guilty of speaking of stuff he didn't know shit about. Or, alternatively, he's just being incompetent.Finite Element Analysis is so powerful because we know where it works, and what its limitations are - something that can't be said for Schneider's method. Schneider is a 42-year-old research mathematician and not an engineer, and that's the reason.
@arsyn - I've already challenged @Jeffrey Orling to take his off-topic comments and speculations to a separately OPed Thread. He seems to have done so in this thread.Tl;dr:
1. Speak for yourself.
2. That's not the subject here.
That's not relevant in this thread.
What is your point?
Doesn't Schneider at least pose an interesting question that we can actually answer?In any case, the model is one-dimensional, it assumes a progressive collapse of one floor at a time. With the one-dimensional approach, he attempts to calculate the energy dissipation of the collapse, and argues that the extra inserted "resistance" force should be sufficient to arrest the collapse.
This gave me an idea:... Schneider suggests that there will be some "algebraic relation" between the fall of the roofline and the crushing front. This may be debated, but it doesn't matter for my point.
Whatever Schneider is arguing, it should be possible to draw two curves for each of four scenarios. (1) Free fall of both the roofline and the crushing front. (2) Roofline and crushing front according to Schneider's "observations". (3) Roofline and crushing front as would be expected under demolition. And, finally, (4) Roofline and crushing front as it actually happened.
The last scenario will not be based on observations but on the predictions of the ROOSD model. Surely, these eight curves can actually be plotted and compared?
Yes, that's the question that the one-dimensional model might help us discuss with truthers in simple terms.How does the steel cage above interact with the building below?
Yes, the next step would be to translate this into closer qualitative approximations of the actual WTC ROOSD at each second.The ROOSD model is, afaik, worked out only as a qualitative mental model, with some real world observations identified as part of that mechanism.
I simply do not understand how anything can be gained by attempting to mislead truthers further with a false model.Yes, that's the question that the one-dimensional model might help us discuss with truthers in simple terms.
Which is utter nonsense. Why not simply explain what really happenedIn the model, the building below is represented as resistance to the descent of the crushing front, i.e., the bottom edge of the "cage". So we now have a falling cage with a top edge and a bottom edge that are falling at different rates. If the top is falling (i.e., accelerating) faster than the bottom, as you suggest, then the cage is getting shorter, i.e., it is being crushed as it falls.
The one-dimensional model is not false, except in the sense that all models are simplifications. It is simply true that the crushing front advanced and the roofline fell (both along the same vertical dimension). Some truthers like Schneider's model, so it's a good place to find common ground. ROOSD, I assume, arrives at different rates of fall & advance than Schneider (ROOSD implies different curves on his coordinate system). Starting with those differences and then explainining how the actual structure produced them makes good sense to me.I simply do not understand how anything can be gained by attempting to mislead truthers further with a false model.
I confirm that in Schneider's paper, equation (1) makes the crushing front faster than the roofline by a constant factor based on the crush-down.Because of this, the roofline, early on, would be moving downwards faster than the crushing front - Schneider probably believes it's the other way round.
Yes. Schneider's theory departs from reality at that point, because the floor collapse happened differently from the column collapse. (We've both talked at length about the floors and NIST FAQ 18, so I know you're aware of this.)It gets crushed. It gathers speed. It loses mass and, as Schneider reminds us, runs into gradually stronger columns.
Could you please elaborate on the first part of that quote?Schneider is surprised that, in his data, the collapse appears to have slowed a lot just as he switches from measuring roofline to measuring crushing front - but the error could well be that the motion of the roofline is NOT a good proxy for the motion of the crushing front.
I would expect a bright graduate student in physics or applied mathematics to see through the fluff and dissicate Schneider's paper
I'm pretty sure any student with first-year knowledge can do that. After all, I've done it above.
I'd like to see this line of thinking continue. As I "see" it, the following should be be possible:Let me offer a word of caution.... Temper your optimism as to what other people "see" or don't "see" when it is bleeding obvious to you.
I'm working on it, actually. It's not easy.Please go ahead and cite the data from Schneider's paper (and maybe other sources), and draw the curves.
This is a tough one. But the lower sections of the towers were of course ultimately completely destroyed, so the destruction must have "passed through" them at some (perhaps average) rate (I'm not sure, maybe the average of the lowest destroyed point and the highest intact point.)it assumes a homogeneous "crush down"-front that completely destroys previously undamaged storeys one by one. After seeing posts #119 and #120 in another thread , it occured to me that the basic assumptions Schneider - and also Bažant et al. (see references 2,3,4 in ref. ) - have made with the crushing front are false,
You still haven't played any civil engineering/bridge buildung games, have you?It's hard to imagine what would tear it apart* before it hits the ground.
I presume you're suggesting something like the forces that broke the Titanic in half when the stern rose above the water. If something like that happened to the hat truss after the collapse started, it could be known quite precisely.Tilt and irregular fall introduce forces into the truss it was not designed to withstand, so it's not predictable whether it withstood them or not.
You still haven't played any civil engineering/bridge buildung games, have you?
Do you see the contradiction here? The games you keep suggesting that I get my engineering education from are based on the predictability of the effects of forces on structures....forces into the truss it was not designed to withstand, so it's not predictable whether it withstood them or not.
You still haven't played it?Do you see the contradiction here? The games you keep suggesting that I get my engineering education from are based on the predictability of the effects of forces on structures.
I always design the bridge to withstand the forces, but it usually fails in a way I did not predict.If you did not design it to withstand the forces that the problem the game poses implies then your bridge will (predictably!) fail.
No it wouldn't. @Mendel's point of principle is valid. We don't know precisely for the real event. We have no need to know for a false analogy to "sinking the Titanic".If something like that happened to the hat truss after the collapse started, it could be known quite precisely.
In the as designed for scenario. It was NOT designed with fire-induced collapse as the scenario for analysis. And the fire collapse scenario cannot be defined sufficiently to allow such post hoc analysis. (It may be plausible with a full-scale investigation of the scale of the NIST work. BUT given that you have no defined reason for such analysis... We don't need to go there.)The load paths in the hat truss (and the upper three stories as a whole) are exceedingly well understood. After all, it was designed to redistribute loads.
Not so either for the actual collapse scenario or for any other scenario your ambiguous claim is open to.The effect of the application of irregular loads could be predicted with great accuracy.
Once again you propose alternate goalposts. We cannot know which supports were actually removed in the real event. And what is the point of speculating about other scenarios which are not the "real event"?If you could tear it appart by removing supports in the structure below, an engineer could tell you exactly which supports would need to be removed.
Strawman unless you explain why you are discussing the hat truss otherwise it is "off topic".If merely tilting and joltling it would do the trick, this too could be described.
My point is that the computer can predict the failure.I always design the bridge to withstand the forces, but it usually fails in a way I did not predict.
If we imagine the truss like a bridge in the game, it's clear that the computer can calculate the stresses that would cause it break at particular points.your assumption that the hat truss can withstand these forces as easily as the different forces it was designed to withstand when in place at the top of an intact building is simply not warranted.
It doesn't. It simulates a failure. The bridge doesn't exist in the real world, so it's not actually a prediction. The computer can do the simulation because it has complete information on the state of the simulated bridge. There's also an element of randomness involved, e.g. in one game, outcomes could differ depending on how long you let the bridge "settle" before dispatching the vehicles.My point is that the computer can predict the failure.
Yes! There's no video of it because it disappeared in a huge dust cloud, and there's not a lot of forensic evidence because search and rescue were priorities. We don't know enough about the exact collapse sequence to accurately model the forces acting on the hat truss.It sounds like you're now saying that you have no idea what happened to the hat truss because it is in principle impossible to know.
As far as I've been able to tell, these simulations do have a certain amount of realism (which, I assume, is why you suggest them as learning tools). The point is just that, given the initial state of the structure (which is known in the case of the hat truss) we can compute the effects (on that structure) of loading it in various ways.It doesn't. It simulates a failure. The bridge doesn't exist in the real world, so it's not actually a prediction.
But we know the range of possible forces (the available gravity loads) and we know, in broad outlines, how the buildings were affected. We know that the hat truss, in whole or in pieces, hit the ground in less than 20 seconds, not several minutes, for example. (And we certainly know that it didn't stop altogether at some point.) So it is possible to imagine some different scenarios. It's possible to sketch what the structure might have look liked at 2,4,6,8,10, ... etc., seconds.We don't know enough about the exact collapse sequence to accurately model the forces acting on the hat truss.
I'm happy to concede this point. That's why I stated it as an assumption. I'm not sure that calling an openly stated assumption an assumption amounts to "calling it out".Concede that you don't know when the hat truss broke apart (or give evidence), and I'm happy.
Yes. This was my understanding as well.but [the hat truss] also function to maintain the geometry of the top of the top block... a sort of "end plate" to the tube structure.
I don't understand this part. Wouldn't the hat truss always be separated from the lower block by some of the mass of the upper block? As I understand ROOSD, the crushing front was really a chaotic (almost fluid) mass of falling rubble with intact structure above and below being gradually destroyed and added to rubble (and some being ejected).I suspect the hat truss largely maintained integrity until it came down upon the lower block more intact structure.
I think your idea, Jeffrey, is a little more plausible: at some point the hat truss encounters the lower portion of the undestroyed core directly and is skewered by it. SinceI agree with [Mendel] that the fact that the core columns remained standing a little longer than the rest suggests that the hat truss must have suffered some damage by that point. It's possible that it had completely disintregrated by the time it reached the top of "the spire". I just don't get how.
we can imagine the inertia of the perimeter columns (with some floors still attached, "hanging") continuing unsupported downwars and the sudden slowing of the core as it impacts the lower core finally doing the hat truss in. But this could happen even with quite a lot of intact structure (to transfer forces) between the hat truss and lower section. So long as the perimeter is being destroyed faster than the core, this irregularity (with the perimeter "hanging" off the truss and the core resisting its fall will be present.the columns and their loads above the interruption would then hang from the truss rather than support it
I think I've found a way to represent my take on this visually. I'd love to hear what you all think. I have incorporated Mendel's suggestion that the hat truss translated and disintegrated on the way down. And I've represented the lagging collapse of the core as as well.Please go ahead and cite the data from Schneider's paper (and maybe other sources), and draw the curves.
Other than the height of the building and the height of the spire, what data from the actual collapse is represented in that drawing?I think I've found a way to represent my take on this visually.
There were a few more things I looked up.Other than the height of the building and the height of the spire, what data from the actual collapse is represented in that drawing?
All of this is of course approximate. And the next step would be to look at time-indexed photographs to adjust each snapshot to make it more realistic.
Yes, I wanted to see what that possibility could look like. The alternative, I guess, is that it tilted so far that it was essentially perpindicular to the ground when it reached the top of the spire and finally landed on its side. This would affect the curves, just the distribution of those blue bits.(My position is not that the hat truss disintegrated on the way down, my position is that we don't know whether it did.)
Interesting. But were elements able to fall faster within the ROOSD mass than the crushing front itself progressed to the ground? That is, can we imagine pieces of the hat truss moving to the bottom (or even middle) of the ROOSD mass before the crushing front (the bottom edge of the mass) gets to the ground? And can we imagine the hat truss outpacing (and outmaneuvering) pieces of the floor trusses of the, e.g., 101st floor?It broke apart like all the rest of the beams and bracing... and fell down WITHIN the ROOSD mass.
Legit question: why the focus on the hat truss? Why would we expect it to ride the collapse downward without breaking apart, when very clearly no other major assembly did? Consider the antenna. I don't recall seeing it lying in (or atop) the rubble pile in aerial shots. At least one of the very topmost floors of each tower was an equipment floor, with a reinforced floor pan. Those were completely shattered, weren't they? The building cores were of solid I-beam construction - and they were completely ripped apart above the 50th floor or thereabouts.What's the best theory of how it got there?