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I think I have earned a reputation for criticizing and correcting debunkers when criticsm and corrections are due.
Do you criticise MT? Do you hold him accountably to your standards of nitty-gritty detailed precision that you torture everybody else with?
I remember one modus tollens debate in which WDC also participated. Not sure if it was the same you just referred to. If so then, IIRC, it was I who introduced the observation that Modus Tollens was violated.
Also, if you studied enough to understand what the NIST WTC1 collapse initiation scenario is, you would see for yourself that it contradicts observables.
Your only advantage is you will not take the time to learn any of these things, and you will imagine your views are correct without checking them against observables.
My views? I can imagine that NIST engineers are human and thus subject to mistakes, especially in the tiny irrelevant details that don't merit much attention. But that still doesn't affect the big picture a single bit: they showed a viable mechanism that would make the towers fall under the conditions they were exposed to, making the unsupported claims of controlled demolition moot. To me, there's no need for their explanation to exactly match what actually happened. Did they differ in insignificant details? So what? It was a chaotic event, it's impossible to get every tiny detail right, you have to stop the detailing somewhere. And if you are going to contend the significance of the details, you NEED to explain how that *proves* that something other than fire and damage resulted ultimately in collapse; otherwise it's still just looking at the trees and missing the forest.
Now that you mention it, speaking of maps... Why not to look and map the parts of the NIST conclusions that would be affected by your findings, should they be true?
Already done and explained many times. I am confident you will not understand that either. It is like a memory that can always be reset to blank. Every time you post it is as if nothing was retained.
I swear, we were having the same exchanges 9 months ago. Zero acquired in all that time.
Don't make things up. I have brought up the NIST conclusions (chapters 8 and 9 of NCSTAR 1) much more recently than that.
But you can still save your honor and ridicule me, by pointing me to any 9 months old post of yours where you state which of NIST's Principal Findings (that's the title of chapter 8, remember) would change. Or even which of NIST's Recommendations (chapter 9) would be affected.
That shock movement is the cause of the light grey ejection. That happens across the whole east wall with a pretty amazing symmetry, always at the lowest points.
Again the purpose was to highlight the actual nature of the break. I accept the behaviour could be stated as a form of buckling, but certainly not the more intuitive form as suggested by others, such as the three point hinge form suggested earlier in the thread.
As I've already stated, that three point hinge is from BZ and it is acknowledged as a limiting case within the paper. Designing a column splice for the full yield capacity of the column was not standard practice in the 60's or 70's. So stating that the splices failed will not come as a surprise to any engineer.
Whilst that doesn't quite capture the actual behaviour (every panel was involved, but with staggered break points) it at least affirms the spring-back behaviour of the panel columns.
As long as everyone is aware that the break ocurred at the bolted seams along the staggered path highlighted, it's all good.
But you've been going on about how the wall didn't fail by buckling. It did. It is the forces developed by excessive p-delta that caused the splices to fail. Just because the members were still elastic and sprung back to their original positions doesn't mean it wasn't buckling.
I'm fine with acknowledging that the building fails because the connections lacked sufficient strength to develop yield level stresses. It's something of a no-brainer for myself, or any structural engineer.
Do you agree that at least three hinge points developed ?
Do you think that diagram is a good way of expressing the actual behaviour of the bolt seam fracturing ?
But you've been going on about how the wall didn't fail by buckling. It did.
I accept the behaviour could be stated as a form of buckling.
(Most people visualise buckling in a specific, and quite different, form. It is far from intuitive to suggest that if I glue two bits of wire together and stress them until the glue breaks, leaving both bits of wire straight, that the wire has buckled. Suggestion of buckling generally implies that the wire itself has bent, rather than the glue betwen two bits of wire has fractured/failed.)
And...
As long as everyone is aware that the break ocurred at the bolted seams along the staggered path highlighted, it's all good.
You did indeed, and let us hope that pgimeno agrees...
Do you agree that at least three hinge points developed ?
Do you think that diagram is a good way of expressing the actual behaviour of the bolt seam fracturing ?
It's good for a limiting case that favors collapse prevention and nothing more. That was its original purpose and it shouldn't be used for anything other than that.
As I said...
I accept the behaviour could be stated as a form of buckling.
And...
As long as everyone is aware that the break ocurred at the bolted seams along the staggered path highlighted, it's all good.
Do you agree that at least three hinge points developed ?
Do you think that diagram is a good way of expressing the actual behaviour of the bolt seam fracturing ?
It seems to me that the diagram, while useful for understanding the general case of inelastic buckling followed by fracture, is no less useful for the case of external columns in the WTC given the proviso that the three or more hinge points must have developed at bolted connections between column sections. In fact, since the diagram shows the hinges as evenly spaced, it's even more accurate in the more specific case. And it's also perfectly clear that the strength of the column is limited in this case by the connection strength, and that therefore any analysis of the collapse that uses the column dimensions rather than the bolt dimensions as a measure of fracture energy is very strongly biased in favour of structural resistance. Therefore, any calculation, based on the column dimensions alone, that gives a higher structural resistance than is suggested by any stage of the collapse, is necessarily inconclusive.
In simple terms, this means that any analysis that ignores the connections is biased in terms of survival or slower collapse. So if any such analysis suggests a greater collapse time or larger variations in acceleration than were observed, no safe conclusions can be drawn from it.
I'm surprised to see that femr agrees with what Bazant wrote nearly 10 years ago:
"The basic question to answer is: Can the fall of the upper part be arrested by energy dissipation during plastic buckling which follows the initial elastic deformation? Many plastic failure mechanisms could be considered, for example:
(a) the columns of the underlying floor buckle locally (Fig. 1, stage 2); (b) the floor-supporting trusses are sheared off at the connections to the framed tube and the core columns and fall down within the tube, depriving the core columns and the framed tube of lateral support, and thus promoting buckling of the core columns and the framed tube under vertical compression (Fig. 1, stage 4, Fig. 2c); or (c) the upper part is partly wedged within the emptied framed tube of thelower part, pushing the walls of the framed tube apart (Fig. 1, stage 5).
Although each of these mechanism can be shown to lead to total collapse, a combination of the last two seems more realistic (the reason: multi-story pieces of the framed tube, with nearly straight boundaries apparently corresponding to plastic hinge lines causing buckles on the framed tube wall, were photographed falling down; see, e.g., Engineering 2001, American 2001)."
It seems to me that the diagram, while useful for understanding the general case of inelastic buckling followed by fracture, is no less useful for the case of external columns in the WTC given the proviso that the three or more hinge points must have developed at bolted connections between column sections.
I disagree. See above. If all the bolted connections were at the same vertical position, you'd have a point. As they are staggered, and yet the inward bowing is not, then your assertion cannot be true.
Rather off on a tangent....although...there are photographs of sections of WTC1 (iirc) which show totally straight (non-staggered) perimeter break lines during peeling.
If the floor assemblies is pulling the section inwards then it is no longer providing lateral bracing. This is why you see the columns failing elastically, not plastically.
Also, the height of the region of IB must be taken into account.
It's unlikely that they could have developed co-linear hinge points as suggested, something I omitted to state because it seems too obvious to mention. That indicates that the lateral connections between panels must also have failed so as to give staggered hinge lines. For a single column (or a single vertical array of panels) the diagram appears perfectly reasonable. In fact, as the strength of the connections was significantly less than that of the column sections, it seems impossible that any other kind of buckling failure could have occurred.
If the floor assemblies is pulling the section inwards then it is no longer providing lateral bracing. This is why you see the columns failing elastically, not plastically.
What I'm saying is that, because of the staggered position of the bolted connections, and the non-staggered nature of the inward bowing (IB), up to two thirds of the panels will not be hingeing around bolted connection splices (or we'd see varying IB behaviour curvature for each panel).
I'm aware of this behaviour...
...during the initiation process, but IB was present for a long time prior to the above, without such per-panel behaviour being apparent.
In fact, as the strength of the connections was significantly less than that of the column sections, it seems impossible that any other kind of buckling failure could have occurred.
What I'm saying is that, because of the staggered position of the bolted connections, and the non-staggered nature of the inward bowing (IB), up to two thirds of the panels will not be hingeing around bolted connection splices (or we'd see varying IB behaviour curvature for each panel).
You're confusing the inelastic buckling and hinge formation that happened during collapse initiation with the elastic buckling and inward bowing that preceded it. The inward bowing was caused by tension through the floor connections, and we know it varied across the wall, so there's a variable which is already giving varying IB curvature. Do you honestly think you can deconvolute this effect from the varying stiffness, when you don't have any idea of the forces on each individual panel at each individual connection? I think it's pretty much impossible by definition.
So are you trying to argue that the columns could only have failed at the connections, therefore any failure anywhere else indicates deliberate weakening of column trees?
You're confusing the inelastic buckling and hinge formation that happened during collapse initiation with the elastic buckling and inward bowing that preceded it.
So are you trying to argue that the columns could only have failed at the connections, therefore any failure anywhere else indicates deliberate weakening of column trees?
You're confusing the inelastic buckling and hinge formation that happened during collapse initiation with the elastic buckling and inward bowing that preceded it. The inward bowing was caused by tension through the floor connections, and we know it varied across the wall, so there's a variable which is already giving varying IB curvature. Do you honestly think you can deconvolute this effect from the varying stiffness, when you don't have any idea of the forces on each individual panel at each individual connection? I think it's pretty much impossible by definition.
It is virtually impossible to determine whether the observed plastic buckling occurred prior to or after collapse initiation.
Nobody disagrees that elastic buckling could result in failure at the splices. By your own admission, ad many as 33% of the columns could fail in the middle (per your diagram) tat way. An additional 33% had splices at the floor, and the last 33% had splices at the floor above.
The splices could also fail due to the column buckling elastically.
Dance, dance, dance...
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