# What's the best popular account of the WTC collapses?

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Here's my amateur interpretation of what you're saying. (Which the book I'm hoping for would get right in whatever way I'm getting wrong.)

that sounds too complicated for a laymen to follow in a book, and too simplistic to be of any use to an engineering student.

Agreed. But you do need the downward momentum to apply to whole upper block, right? Otherwise you don't release its potential energy. Until recently, I was sure the dynamic load of the moving block (at some minimum speed on impact with the lower section) was essential to the explanation. But the model of collapse you seem to be working with (see also next quote) doesn't seem to need any initial downward momentum, only a (lateral) shift of the (static) weight of the upper section from the columns to the floor connections.
Here you are conflating collapse initiation and collapse progression.
Initiation necessarily starts out without any, or negligible downward momentum: Collapse initiates when the load bearing capacity goes below static load and no more path for successful load redistribution is available. From that moment on, the top block accelerates down. This obviously changes the geometry of the structure, and what comes next depends critically on which failures occurred and accumulated where and which parts of the structure are moving towards what direction. It may happen that a structure that has just started to accelerate downward soon runs into a configuration that manages to arrest collapse. Or it may not. This would require capacities at least twice as large as static load, as static load is now applied dynamically.
At some point (and it comes soon), momentum (or rather kinetic energy) has grown so large, that at no level the structure can absorb and dissipate that kinetic energy plus the potential energy differential associated with the vertical distance over which that energy is dissipated - that's the condition for collapse progression. This minimum kinetic energy or momentum of course is associated with a downward velocity, which in turn can be thought of, conceptually, as the result of "freefall through a height differential dh". dh does not have to be a full story. And it needs not literally be freefall: You can reach the same velocity with half g in twice the time, for example.

Anyway, what you need to get the "upper block" moving initially, all you need is gravity, and the weakening of the supporting structure to the point where capacity < load. -> Collapse initiation.
Once it moves, you are dealing with dynamically applied loads, which multiplies forces -> Collapse progression.
You may argue that immediately after initiation, while there is conceptually a chance for arrest, you have a third phase, which we might call "collapse transition". Not sure if that helps.

Here's my amateur interpretation of what you're saying. (Which the book I'm hoping for would get right in whatever way I'm getting wrong.)

Let's imagine that all the columns are cut cleanly and horizontally at floor level (say, floor 80) and that the footprint of the upper section is somehow shifted about 1 meter north-west (i.e., along the diagonal) so that all the columns (above and below the cut) are offset from each other. That would of course be catastrophic. The perimeter columns of the north and west face would now be supported by nothing at all, most of the core columns would be suspended over elevator shafts, and the south and east faces would be resting (momentarily) on the floor of the 80th storey. That floor would immediately collapse.
Exactly - and you would not even need to shift by that far. even if you shift by only half the thickness of the steel plates that build up the columns, that would reduce their capacity by a factor near 2, and be catastrophic.

After falling the height of one floor, the moving mass would impact the 79th floor. Or, at least, that's what would happen along the south and east faces. On the north and west sides, the ceiling of the 80th storey (i.e., the underside of the 81st storey) would come down on the top of the columns, where the 80th floor is still attached (since there is no weight on those connections yet). Here the weak spot is also the floor connections, but not those of the 80th floor. Rather, it's the 81st floor that would be destroyed, upwards, by the strength of the undamaged columns. While it would look more or less symmetrical from the outside, the destruction is passing asymmetrically through the building internally. To the SE the floors of the lower section are collapsing as the columns of the top section press down. But the NW the floors of the upper section are pancaking onto the tops of the columns of the lower section.
Yep, good mental model.

I'm sure the result is the same as what we saw. And I'm sure I'm oversimplifying it -- in reality it would be much more chaotic.
Of course.

But it's basically that process of floor connections being broken and columns being impacted (probably in some cases buckling) that I'd like to see described by someone more qualified than me.
Ok.

Conspiracy theorists following along no doubt imagine that a lot of the mass of the falling block dissipates as it disintegrates and falls off to the side.
It would be their job to explain how that much mass moves laterally from within the footprint to outside the footprint within the short time that a single floor collapses.

So it would be good to explain how sufficient mass remains within the footprint of the tower to keep the process going. (The collapse front of course gathers mass from the lower sections it destroys. And also some momentum as the speed increases.)
Things fall mostly straight down, don't they?

With each floor that the top block moves down, a floor slab is added to a growing layer of compacted concrete, steel and office contents. Some perimeter panels, core columns and core beams also get mixed into that growing debris layer.

Some perimeter panels get cut off and fall freely outside the lower part of the tower - this can be seen in many collapse videos, large pieces of steel falling ahead of the collapse front. Those are obviously lost to the growing debris layer - but a high percentage of the mass remains inside the footprint and gets heavier and heavier, such that the collapse gets driven by that layer punching out the floor slabs it falls in in sequence.

It can be computed fairly easily, an Excel spreadsheet would do, how such a collapse accelerate if we picture it simply as just floor slabs falling on floor slabs, just considering Conservation of Momentum and Energy, plus gravity, but ignoring structural resistance: It turns out that after just a few floors down, acceleration closes in on 2/3 of g. Which is what David Chandler measured.
So does this mean there was no structural resistance?
No.
When mere Conservation of Momentum dictates that fall rate is diminished from g to 2/3 of g, this means that through the collisions of debris layer with fresh floors, 1/3 of the kinetic energy is dissipated by "destruction": Smashing concrete to pieces, grinding gypsum to dust - and bending and breaking steel. This 1/3 of available energy can be computed - it works out to the equivalent of more than 40 tons of TNT.
This should be more than enough to cause the amount of destruction seen.

If a Conspiracy Theorist wants to argue that the equivalent of 40 tons of TNT is not sufficient to destroy the towers as seen, then they need to show work and specify how much more energy they claim is needed. Since it is highly doubtful that they have worked this out to a precision like "40 tons of TNT is too little, but 45 tons of TNT would suffice, so we need to add only 5 tons of explosives", we'll see quickly that they would have to theorize the use of at least another 40 tons worth of explosives - and then they would have to explain why we don't hear them, how they would be employed, and why no other evidence thereof has been found. You see, in actual explosive demolitions of highrises, you only need "hundreds" of pounds of explosives, in the case of the WTC towers perhaps a couple of tons. But >40 tons?

Here you are conflating collapse initiation and collapse progression.
Initiation necessarily starts out without any, or negligible downward momentum: Collapse initiates when the load bearing capacity goes below static load and no more path for successful load redistribution is available. From that moment on, the top block accelerates down. This obviously changes the geometry of the structure, and what comes next depends critically on which failures occurred and accumulated where and which parts of the structure are moving towards what direction. It may happen that a structure that has just started to accelerate downward soon runs into a configuration that manages to arrest collapse. Or it may not. This would require capacities at least twice as large as static load, as static load is now applied dynamically.
At some point (and it comes soon), momentum (or rather kinetic energy) has grown so large, that at no level the structure can absorb and dissipate that kinetic energy plus the potential energy differential associated with the vertical distance over which that energy is dissipated - that's the condition for collapse progression. This minimum kinetic energy or momentum of course is associated with a downward velocity, which in turn can be thought of, conceptually, as the result of "freefall through a height differential dh". dh does not have to be a full story. And it needs not literally be freefall: You can reach the same velocity with half g in twice the time, for example.

Anyway, what you need to get the "upper block" moving initially, all you need is gravity, and the weakening of the supporting structure to the point where capacity < load. -> Collapse initiation.
Once it moves, you are dealing with dynamically applied loads, which multiplies forces -> Collapse progression.
You may argue that immediately after initiation, while there is conceptually a chance for arrest, you have a third phase, which we might call "collapse transition". Not sure if that helps.

Exactly - and you would not even need to shift by that far. even if you shift by only half the thickness of the steel plates that build up the columns, that would reduce their capacity by a factor near 2, and be catastrophic.

Yep, good mental model.

Of course.

Ok.

It would be their job to explain how that much mass moves laterally from within the footprint to outside the footprint within the short time that a single floor collapses.

Things fall mostly straight down, don't they?

With each floor that the top block moves down, a floor slab is added to a growing layer of compacted concrete, steel and office contents. Some perimeter panels, core columns and core beams also get mixed into that growing debris layer.

Some perimeter panels get cut off and fall freely outside the lower part of the tower - this can be seen in many collapse videos, large pieces of steel falling ahead of the collapse front. Those are obviously lost to the growing debris layer - but a high percentage of the mass remains inside the footprint and gets heavier and heavier, such that the collapse gets driven by that layer punching out the floor slabs it falls in in sequence.

It can be computed fairly easily, an Excel spreadsheet would do, how such a collapse accelerate if we picture it simply as just floor slabs falling on floor slabs, just considering Conservation of Momentum and Energy, plus gravity, but ignoring structural resistance: It turns out that after just a few floors down, acceleration closes in on 2/3 of g. Which is what David Chandler measured.
So does this mean there was no structural resistance?
No.
When mere Conservation of Momentum dictates that fall rate is diminished from g to 2/3 of g, this means that through the collisions of debris layer with fresh floors, 1/3 of the kinetic energy is dissipated by "destruction": Smashing concrete to pieces, grinding gypsum to dust - and bending and breaking steel. This 1/3 of available energy can be computed - it works out to the equivalent of more than 40 tons of TNT.
This should be more than enough to cause the amount of destruction seen.

If a Conspiracy Theorist wants to argue that the equivalent of 40 tons of TNT is not sufficient to destroy the towers as seen, then they need to show work and specify how much more energy they claim is needed. Since it is highly doubtful that they have worked this out to a precision like "40 tons of TNT is too little, but 45 tons of TNT would suffice, so we need to add only 5 tons of explosives", we'll see quickly that they would have to theorize the use of at least another 40 tons worth of explosives - and then they would have to explain why we don't hear them, how they would be employed, and why no other evidence thereof has been found. You see, in actual explosive demolitions of highrises, you only need "hundreds" of pounds of explosives, in the case of the WTC towers perhaps a couple of tons. But >40 tons?
This is a very comprehensive summary explanation! Thank you!

This is a very comprehensive summary explanation! Thank you!
I agree. Thanks, everyone who pitched in. Now we just need to get an MIT engineer to sign off on it! And a science journalist to tell the story of how it was actual engineers, not some anonymous contributors to on an online forum, who came up with it. With all due respect, mind you. I think a few things fell into place for me.

I think most of us agree that something like this is what happened. The remaining mystery is why no major publication (like Popular Mechanics) or publisher (like W.W. Norton) has presented it to the public. And why are the engineers who know this stuff not semi-famous public figures?

I agree. Thanks, everyone who pitched in. Now we just need to get an MIT engineer to sign off on it! And a science journalist to tell the story of how it was actual engineers, not some anonymous contributors to on an online forum, who came up with it. With all due respect, mind you. I think a few things fell into place for me.

I think most of us agree that something like this is what happened. The remaining mystery is why no major publication (like Popular Mechanics) or publisher (like W.W. Norton) has presented it to the public. And why are the engineers who know this stuff not semi-famous public figures?

I have to somewhat disagree of your post. As sensible as the OY explanation is.... it describes a CORE "initiation" which is NOT what NIST "concluded". Their explanation is the floor trusses pulled at the exterior columns/spandrels causing a buckling which (apparently) involved the entire perimeter more or less simultaneously. Then all the facade columns, we are led to believe buckled at once... support was lost for the upper block... which then collapsed down and became a "global collapse". While this is "conceivable" it is virtually impossible (especially the uniformity and simultaneity) and completely ignores (does not account for the antenna which dropped before the upper blocked moved a centimetre downward. NIST does not represent what we saw... observations of building movement.

The mystery remains as to how DID the reasonable "core led" collapse lose axial support?

Did "heat" cause the post plane strike remaining columns to loose so much capacity while supporting acquired redistributed loads that they all buckled in a rapid sequence?

Was there some sort of lateral displacement inside the core as I outlined above... or a combination of loss of capacity and lateral displacement? Which was the main "driver"? For sure heat was making all the exposed steel weaker... and that had to be a factor.

I believe the tell is the antenna dropping before the top block moves downward. That seems to indicate that the core lost capacity under the antenna FIRST and one might logically conclude that the "failures" radiated from the core center toward the perimeter.

See if you can find such a discussion in "the literature" about the collapses.

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I really think one problem is that most of the work is the NIST explanations. I don't believe you will find developed presentations of "collapse scenarios" anywhere except 9/11 discussion forums. And of course NIST punts and doesn't describe the "global collapse" processes.

In the end there is consensus that the collapses were not CDs or false flags and it seems the technical collapse details have little currency. NIST made some fire protection recommendations which I believe have been adopted. The play by play and details have become esoterica for the obsessed.... hardly something or interest to Jan or John Doe.

.. details have become esoterica for the obsessed ...

I think ignorance of mundane details easily become esoteric obsessions. There's no reason the details shouldn't be available to the few thousand interested people that do exist.

Keep in mind that the Warren Commission at least proposed a theory to explain seven wounds with a single bullet. People can disagree with it, obsess over it even, but it's out there ... in detail. It's absence here is frustrating. I guess the rejoinder is: who wants to be the Arlen Specter of 9/11?

I think ignorance of mundane details easily become esoteric obsessions. There's no reason the details shouldn't be available to the few thousand interested people that do exist.

Keep in mind that the Warren Commission at least proposed a theory to explain seven wounds with a single bullet. People can disagree with it, obsess over it even, but it's out there ... in detail. It's absence here is frustrating. I guess the rejoinder is: who wants to be the Arlen Specter of 9/11?
NIST published the "official" reports... and they are hardly complete and seem to have some notable errors.

The understanding of the technical issues which are the main focus of truth movement concern has progressed - the number of "genuinely ignorant but seeking to learn" is these days far less than it was in (say) the historical era of 2006-2010.

Remember that the main focus is on technical claims - most prominently the AE911 strategy which is based on claims for CD at WTC. In 2006 few persons understood the WTC collapse physics in lay person understandable terms. Most active members of both the emerging and polarising "two sides" did not understand the physics. 2010 was near enough the time when correct and lay person understandable explanations were presented into active debate. Increasingly since about that time it has become not viable to honestly claim there was CD at WTC.
This suprised me. When I raised this question here not long ago I was referred back mainly to the NIST report. I wasn't expecting someone here to claim that up to 2010 it was reasonable to be in doubt about CD but after 2010 it's only plausible to conspiracy-predisposed minds.

For my part, I don't see what has changed in the last ten years for the "lay person". That is, there are no better resources available now to someone who is "genuinely ignorant but seeking to learn" than there were 10 years ago. The only thing they can do is jump into one or another forum where truthers and debunkers clash over the usual claims. In fact, that's basically been the case since the beginning. (It's hard to claim that the NIST report is easily accessible to the curious layperson.)

Finally, it seems to me that you're forgetting the birthrate of "would-be truthers". About one a minute, I'm told. And the birth-rate of ignoramuses with a genuine interest in learning is somewhere between that and the 250 babies/minute that are keeping the human race growing. And anyway, I still don't understand the mechanics of the WTC collapses. And that's not for lack of trying to find an authority who can explain it to me.

there are no better resources available now to someone who is "genuinely ignorant but seeking to learn" than there were 10 years ago.

I've heard this a bunch of times. I'm not sure what such resources would look like. The deep explanations for the collapses of the Twin Towers and 7 World Trade Center are highly technical. PBS Nova did a pretty good job covering the high points for the Twin Towers in "Building on Ground Zero" (2006), and my own popular YouTube video "Building 7 Explained: The Tube That Crumpled" (2016) attempted to do something similar for WTC7. Beyond that, things quickly get difficult, and the maths/physics required goes beyond what most lay persons can bring along. It's a bit like asking, I don't know a thing about biology or chemistry, but I seek to learn how succinate is converted to fumarate in the Krebs cycle, and there are no resources to help me. Who does that?

In most science and engineering endeavors, lay persons are content to leave technical details to folks who have studied their specialized fields for years or decades. With 9/11, though, for some reason people expect to be hand-held to a deep technical understanding without having the necessary foundations of knowledge. It's not really possible.

I'm not sure what such resources would look like ... It's a bit like asking, I don't know a thing about biology or chemistry, but I seek to learn how succinate is converted to fumarate in the Krebs cycle, and there are no resources to help me.
I feel more like this: I was pretty good at math and physics in high school, and I'm trying to build (at least) a (mental) model of WTC that shows how the the potential energy of the upper section was converted into the destruction of the lower section. My intuition tells me I should be able to do it with 4-8 columns and maybe 10 or 20 floors. (And that I should, in principle, be able to build it in Mick West's garage.) I'm told that this is impossible due to problems of "scaling". But I recall Newton was able to model the orbit of the moon with an imaginary cannonball and I've seen Brian Greene model a supernova for Stephen Colbert with five balls stacked on top of each other. That seems pretty well scaled down and was entirely illuminating. And I guess I feel like that's what such a resource might look like. But it just doesn't exist. Other than the WTC, I've never seen a structure crush itself like that.

I feel more like this: I was pretty good at math and physics in high school, and I'm trying to build (at least) a (mental) model of WTC that shows how the the potential energy of the upper section was converted into the destruction of the lower section. My intuition tells me I should be able to do it with 4-8 columns and maybe 10 or 20 floors. (And that I should, in principle, be able to build it in Mick West's garage.)
What do you feel is lacking in my garage model? Can you not mentally extend that?

What do you feel is lacking in my garage model? Can you not mentally extend that?
(1) The only weight in the structure is structural (there is nothing like the concrete pans)
(2) column sections along the height of the tower aren't "welded" and
(3) it's a 2D model.

Overall, anyone looking at it would predict (without any math) that it would collapse as it does. (I've told truthers the same thing about their "experiments", by the way.) It behaves as our intuitions tell us it would. So it doesn't explain why we're confused when we look at the WTC. What we need is a model that clearly identifies the points where our intuitions lead us astray.

(1) The only weight in the structure is structural (there is nothing like the concrete pans)
It's made of wood, steel plates, and magnets. How is this point at all relevant?

(2) column sections along the height of the tower aren't "welded" and
They actually are on the exterior columns. There are splice plates between the lower columns. The "top" is not welded, because it's assumed to have failed there. You see a splice place fall off and hit the wheel at the lower right here.

(3) it's a 2D model.
Well it's really 3D, but yes, not the same topology.

But really, you can't mentally extend that? Why not?

It behaves as our intuitions tell us it would. So it doesn't explain why we're confused when we look at the WTC.

I dunno — to me, both structures behave as my intuition tells me they should. I've never felt confused by the progression of the Twin Towers collapses, based only on knowing that static forces (e.g. a brick resting on one's head) and dynamic forces (e.g., the same brick dropped onto my head) can have remarkably different effects. For me, the only thing perhaps counterintuitive about the Twins collapses is the immensity of the dust clouds and the debris pattern, but that is explainable via the scale of the gravitational potential energy released.

What we need is a model that clearly identifies the points where our intuitions lead us astray.

Can you help us by identifying the points where your intuition is led astray? Put another way, what does your intuition say should have happened when a 10- or 20-story section was allowed to fall by one story?

It's made of wood, steel plates, and magnets. How is this point at all relevant?
There is nothing in your model to represent a weight that doesn't also provide structural support. The structure weighs what it's made out of. It doesn't carry anything.
They actually are on the exterior columns. There are splice plates between the lower columns. The "top" is not welded, because it's assumed to have failed there. You see a splice place fall off and hit the wheel at the lower right here.
I may be wrong about this, but it was my impression that the WTC was built in such a way that the welds were not (at least designed to be) weak points along the column length. In your model the joints between the boards are obviously the failure points; and, under the conditions of your experiment, they are the *only* point where the columns could buckle (as the middle column appears to at 0:02). The boards are ludicrously strong compared to the joints between them (and relative to the load they're carrying -- see point 1).
Well it's really 3D, but yes, not the same topology.
Obviously, my point. The *model* is 2D (it only models forces and resistance in two dimensions). All real things are 3D, of course.
But really, you can't mentally extend that? Why not?
My intuition tells me that if you extended your model into 3D and bound it together with spandrels on the outside in a reasonably realistic manner, it wouldn't collapse.

My intuition tells me that if you extended your model into 3D and bound it together with spandrels on the outside in a reasonably realistic manner, it wouldn't collapse.
Aha, then perhaps that should be the point to focus on.

I may be wrong about this, but it was my impression that the WTC was built in such a way that the welds were not (at least designed to be) weak points along the column length. In your model the joints between the boards are obviously the failure points; and, under the conditions of your experiment, they are the *only* point where the columns could buckle (as the middle column appears to at 0:02).
Your impression is wrong. The welds were the weak point. You might want to review this thread.

In particular, this from a structural engineer who worked on the site.

I had misgivings about the core columns I was seeing. I was sure the dunnage design would work—Kyle and Chris know their stuff—but I was unhappy that the columns I saw lying on West Street seemed to be in too-good condition. These huge columns—the largest weighed more than one ton per running foot—were almost all straight, with clean edges at both ends. There were some dents here and there, but I expected a piece of steel that had been wrenched out of a building to be bent. I examined the ends of the columns every chance I got. Every welded splice at the column ends I saw had failed the same way: by ripping out of the steel. The plates that had been assembled into boxes for the core columns varied from a couple of inches at the top to five inches at the bottom. The top and bottom ends of each column were flat and had been spliced with a partial-penetration groove weld: the upper column’s four sides were beveled about an inch and a half. When the upper column was erected over the already in-place column below, the bevel and the flat top surface of the lower column formed a lopsided “V” shaped groove, which was then filled with weld. Partial penetration welds are not as strong as full-penetration welds, where the groove is the same depth as the steel is thick, but they are far stronger than is needed for most purposes. Under the extraordinary loads imposed during the collapse, the columns were free to buckle after the welds ripped off of the flat surface of the groove. Like a lot of the structural damage I saw, this was not a normal phenomenon and it was hard to accept. I spent a lot of time noting such issues and trying not to learn too much from them. It would be easy to stop trusting my knowledge of building design, and weld performance, and steel strength, and so on. I felt that by understanding what had physically happened on September 11, I could contrast it with the ordinary engineering problems I dealt with on my projects.]
Content from External Source
Source: After 9-11, An Engineer's Work at the World Trade Center: pp 47-48

I've never felt confused by the progression of the Twin Towers collapses.
This is more important than you perhaps realize. There *was* originally some confusion, even among engineers. What I'm always asking for (and not often getting) is someone who will explain what the "puzzle" was that required any investigation at all. And then to connect that description of the puzzle with the solution that was eventually found. I have gotten so far as to realize that what puzzles *me* may not have been the same as what puzzled the engineers, but I don't think it's plausible that there was no head-scratching going on at all. A core theme of science popularizations is the solving of a puzzle.
Can you help us by identifying the points where your intuition is led astray? Put another way, what does your intuition say should have happened when a 10- or 20-story section was allowed to fall by one story?
I basically imagine the towers as a continuous, homogenous structure from top to bottom. Loaded about evenly (i.e., roughtly the same weight on each floor) and so the columns getting stronger as you get near the bottom. I imagine it's tightly bound together to resist wind and earthquake as a single unit. So I would have expected that if you drop the top part of it on the lower part of it it would, eventually (depending on from how high), stop. That's the intuition that the truthers exploit, of course.

So, all I need is a structure that I can build myself (or at least imagine building -- i.e., with fewer elements than the real WTC -- so that I can keep track of them mentally) that "feels" just as strong (tightly bound together) before getting damaged and then is able to destroy itself using only the energy stored as potential in the structure itself. Like how Brian Greene shows that, whatever intuitions (and even experiences) we have about how high the individual balls bounce, the combination of the five balls sends the little ball shooting up through Colbert's mouth.

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Aha, then perhaps that should be the point to focus on.
As I mentioned in that other thread. I know we could talk about this for hours. I was simply pointing out that, once again, we'd be a bunch of amateurs with no special resources working it out on an online forum. I was responding to the idea that somehow the situation changed in 2010. As we're now proving, it hasn't.

Your impression is wrong. The welds were the weak point. You might want to review this thread.

In particular, this from a structural engineer who worked on the site.
If this engineer's impressions were part of a major, vetted, popular account of the collapses I'd be totally satisfied. But it's in a 19-year-old first person (self published?) account.

It's got all the "puzzle" stuff I want -- he had "misgivings", was "unhappy"; the steel wasn't what he "expected". "This was not a normal phenomenon and it was hard to accept." "It would be easy to stop trusting my knowledge of building design, and weld performance, and steel strength, and so on." Great story. Now, what happened? How did he convince his peers that it was the welds that failed? How did he become the "hero" that solved the mystery of the collapses? Well, that obviously didn't happen.

The "official story" simply isn't that the welds failed and that, if only they had been stronger, the buildings would still be standing. If that *were* the story, I would be in a different situation. We all would be. It would be the story of a tragic design flaw.

How did he convince his peers that it was the welds that failed?
Because it was obvious to everyone. It's not complicated. You can see the columns failed at the welds.

I didn't realize you had moved this thread. I thought I was engaging with econ41's idea that something changed in 2010.

@Thomas B -- It's interesting that you seem to be just now learning about Donald Friedman's book After 9-11: An Engineer's Work at the World Trade Center from the quote Mick posted above. I note that I cited that book, among others, for your benefit in the second post in this very thread. Given the way you then moved past those citations and continued to harp about the deficiencies you see in the extant literature, I was under the impression that you had already read it and the other books I cited. It seems I was likely mistaken.

Given your curiosity about this topic, I thus highly recommend that you read that book and the others I cited. If you cannot find them in your local library, you should be able to purchase them relatively cheaply via google books or some other online bookstore.

It's interesting that you seem to be just now learning about Donald Friedman's book After 9-11: An Engineer's Work at the World Trade Center from the quote Mick posted above.
No, as I just said, I hadn't realized we had shifted into this other thread. Friedman's book is from 2002, and has no impact (as far as I can tell, or you or anyone has said) on either the professional or popular literature on the collapses. Econ41 suggested that something decisive had happened around 2010. A book published in 2002 (with no awareness of the NIST investigation) clearly can't have been that event.

The problem here is that we lack a Brian Greene who's willing to explain the collapses to the public. That's really the only point I've been making. Friedman just doesn't fit the bill.

This is more important than you perhaps realize. There *was* originally some confusion, even among engineers. What I'm always asking for (and not often getting) is someone who will explain what the "puzzle" was that required any investigation at all. And then to connect that description of the puzzle with the solution that was eventually found.
If you stick to the broad overview, the nice thing is that 20 years of all sorts of people with all sorts of background looking at this left two "solutions" that were eventually found: 1) Two planes crashed in the building, and the consequences of that brought it down, like the NIST report said; 2) there was a conspiracy and a controlled demolition with explosive charges.

That means you're no longer left to puzzle anything out by yourself. All you need to do nowadays is look at the evidence, and then decide which idea has the better evidence going for it. The accepted solution (1) doesn't need to explain every little detail, it just needs to have better evidence than the conspiracy solution (2), and it does.

To see that for yourself, you only need to question the idea (2) with the same vigour and doubt that these guys are questioning (1), and if you apply the same measure, you'll find them lacking.

The problem here is that we lack a Brian Greene who's willing to explain the collapses to the public. That's really the only point I've been making. Friedman just doesn't fit the bill.
Would Brian Greene fit the bill? If he gave, essentially, the same explanation I'd given, with a more 3d model, then would you be happy?

I've never felt confused by the progression of the Twin Towers collapses,
Few have been confused by the reality that collapse did progress. BUT understanding how it did was a major source of confusion for many years. And that was the issue which changed around 2010 as per my previous comments which so far Thomas B has not understood.

The consequence of false assumptions arising initially out of misunderstanding Bazant & Zhou 2002.

This being one of them:
.... what does your intuition say should have happened when a 10- or 20-story section was allowed to fall by one story?
The Top Blocks did not "fall" by one story. That was an intial assumption for a back of envelope calculation of available energy. Valid for that purpose but it led to many misunderstandings including T Szamboti's "Missing Jolt" which then confused attempts to explain the actual collapse mechnaism.

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If you stick to the broad overview,............... The accepted solution (1) doesn't need to explain every little detail, it just needs to have better evidence than the conspiracy solution (2), and it does.
True enough but it wont satisfy those of us who want to know why. Nor those who need persuading why their own ideas are wrong.

This suprised me. When I raised this question here not long ago I was referred back mainly to the NIST report. I wasn't expecting someone here to claim that up to 2010 it was reasonable to be in doubt about CD but after 2010 it's only plausible to conspiracy-predisposed minds.
There is a whole debate topic in those aspects of history. And experience tells me that many members here will disagree. Plus discussion can wait until we deal with your primary need which is this:
And anyway, I still don't understand the mechanics of the WTC collapses. And that's not for lack of trying to find an authority who can explain it to me.
As we - you and I - know from discussions in another medium - those are two separate issues and as far as I am aware there is no single solution to both.

You want to overcome your lack of understanding AND you want it to happen via a formally published work from an authority. I don't think there is any such formally published work - whether book or academic/profesional paper. And I don't think anyone is likely to write one. I suggest the best option to gain understanding is do your own research. Ask questions. Follow up. I see that several members have made essentially the same recommendation.

The Top Blocks did not "fall" by one story. That was an intial assumption for a back of envelope calculation of available energy.

Point noted, but we are trying to come up with an intuitive mental model or thought experiment, so I think this is a fair "spherical cow" assumption to make for that purpose.

This is one persistent problem causing debate to keep circling:
Thomas B is looking for "true explanations" incorporated on formally published documents - books or papers by "authorities":
I was simply pointing out that, once again, we'd be a bunch of amateurs with no special resources working it out on an online forum......

...If this engineer's impressions were part of a major, vetted, popular account of the collapses I'd be totally satisfied... But it's in a 19-year-old first person (self published?) account.
If we mere bunch of amateurs claim "the daytime cloudless sky is blue" would that be false because we are "amateurs"?
I'll overlook that I am in fact experienced professional - qualified to make the professional level coments I make. And I disclaim any where I don't have the expertise. But I am retired - does that disqualify me? Make me yet another mere "amateur"?? Why the assumption that putting something in a book by some sort of magic makes it truer than if stated online?

This forum has members with a wide range of expertise. Individually impressive. Cumulatively very significant. And arguably in sum total better informed and more credible than any single person academic who could attempt to write the book that Thomas B seeks.

Then this side track:
I was responding to the idea that somehow the situation changed in 2010. As we're now proving, it hasn't.
Not true. I briefly advised of a significant milestone in growth of understanding od WTC collapses sutable for the audience Thomas B represents. An assertion of historic fact which has NOT even been discussed. The assertion "As we're now proving, it hasn't" refers to some ill defined moving goal posts.

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Point noted, but we are trying to come up with an intuitive mental model or thought experiment, so I think this is a fair "spherical cow" assumption to make for that purpose.
I comprehend the context. Agree your point.

However the main goal of the thread seems to be to satisfy Thomas B's dual need for (a) his own understanding (b) based on an authoritative "published" book or paper.

"we" have been encouraging him to explore a range of sources. My personal offer was to explain the collapse mechanisms by discussion with him. Others have made other contributions. Hence my alerting him about the early derail of debates consequent on Bazant & Zhou. The issue of misaligned columns being central to comprehension of Twin Towers collapses. So - sooner of later - Thomas B and most of "us" will need to form an overal "intuitive mental model or thought experiment" that accurately explains the Twin Towers collapses. And "overall" is a big factor. There are lots of "bits". Mick's physical model is very good example of the genre but it is only one part of the overall picture. It is only one part of three for the "progression stage". And even putting that one bit into an overall "mental visual model" is a bit of a challenge.

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I would have expected that if you drop the top part of it on the lower part of it it would, eventually (depending on from how high), stop. That's the intuition that the truthers exploit, of course.

I suggest one thing that might lead people's intuition astray, and you've already alluded to it: scale.

The Twin Towers were, for their size and weight, more fragile than champagne glasses. You've seen a stack of champagne glasses at Ikea or the mall perhaps? Well, at full scale, that would not be possible with Twin Towers. You couldn't stack even one Twin Tower on top of another. Or part of a tower. I cannot cite a source, but I don't think any of the columns on Floor 109 of either tower had a safety factor anywhere close to 110.

This is a result of the cube–square law, in which structural strength scales by the square of distance, but mass scales by the cube of distance...which is what creates the problems of building a small model of this disaster. From a practical standpoint, it's pretty much impossible to build something at a small scale with commensurate fragility.

What this means is, if you're going to imagine for example what would happen to an exact 1:100 scale, 11-foot model of a Twin Tower, you also have to imagine that gravity is 100 times stronger. Then each member will have the correct safety factor. It'll stand and it'll be very stable...but if there's a major failure and things start moving, that stable structure will fail and fail disastrously.

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Both towers... final "phase" of their destruction was essentially identical... interior floors had collapsed in a runaway vertical avalanche inside the cage of the facade... then the facade "peels" outward and the remaining (by passed) core columns topple from Euler bucking forces.

The "beginning" of final phase seems to be the "release" and downward movement of the top block. But there is mystery there. We can't see that the top blocks ARE intact dropping blocks. We can't see that they are hollowed out "cages/facades" either. It DOES appear that there MAY have been a mutual crush down and crush up as the bottom of the top block engages to top of the bottom block. And this WOULD produced massive and growing amount of debris which was completely capable of destroying each and every WTC truss supported floor.

Can we know if the debris impact ripped the entire slab from the facade and core at the truss seats? Or did it shatter the slabs to varying size bits of slabs down to gravel size? AND also rip the trusses from the seats?

I believe there WAS evidence found in the debris of bent over and even ripped truss seats. Also found was bolt tear out on these supports. We know there were no intact slabs at the end... all slabs were pretty much ground to mostly gravel and dust and a few larger chunks of slab.

We do know that the bracing that seemed to hang on the longest was at the elevator shafts. This can be seen in videos of the "spire" before it succumbed to Euler forces.

We can safely conjecture that axial support INSIDE and supporting the upper blocks had failed in order for them to drop. Did the columns lose their capacity? If so why? Did the column connections become mis aligned so that inadequate bearing attended leading to both buckling and descent of the upper columns even partially loaded. Columns without support will surely drop downward.

The damage of the SE corner axial structures can be implied in 2wtc.... and the fact that the block fell in the direction where the support was lost seems to confirm this.

Does the drop of the antenna of 1wtc tell us that the axial structures beneath it had failed. Seems a very good assumption. Does the antenna drop signal a failure of the hat truss which "knit" all the core columns together to share the antenna load and even move some of that to the 16 facade column/locations (the "outriggers")?

What was the heat from the fires doing? Weakening columns? Weakening lateral bracing steel which supported slabs? Was heat elongated lateral steel? And or causing it to sag? Did elongated lateral steel push or pull the columns it braced and was connected to. Perhaps both depending on "factors".

After the plane smashed steel and slabs... and the towers stood... it was the fires which kicked off the second phases - top drops.

It appears to me that all the mechanisms in play were understood for more than a decade now... if not even earlier. They all supported the observed movements of the buildings as they transitioned from static and plane struck to collapsing into a debris pile.

I can only guess that it would need to ask... did the collapse have anything to do with the structural design? Would any high rise structural design collapse as they did after losing sprinklers? Do long span open office structural designs lend themselves to runaway collapses and standard grided column designs do not? Could some sort of arrest mechanism be designed into column free open office space designs?

My conclusion is that the unstoppable runaway collapse was embedded in the column free open office space design.... and it didn't matter how strong the columns were! Could this be a factor why there is so much "beating around the bush" hunting after details and missing the forest for the trees?

Food for thought.

If we mere bunch of amateurs claim "the daytime cloudless sky is blue" would that be false because we are "amateurs"?
except you "old" metabunk members who have been writing in the 911 forum for years (the one Thomas has no interest in reading, or he would understand the mechanisms like i do), don't make "the sky is blue" type claims. Y'all back it up in excruciating detail with plenty of engineering sources.

Would Brian Greene fit the bill? If he gave, essentially, the same explanation I'd given, with a more 3d model, then would you be happy?
He's not ideal, but, yes, I could imagine him (and the people around him) coming up with an interesting explanation, including models and demonstrations, that would be a game changer. It would be better if the expert/popularizer was an engineer. Since I have the issues I mentioned with your model, I imagine it wouldn't be "essentially the same", but something like, yes, might work. Obviously, it depends on exactly what he came up with.

This forum has members with a wide range of expertise. Individually impressive. Cumulatively very significant. And arguably in sum total better informed and more credible than any single person academic who could attempt to write the book that Thomas B seeks.
I don't doubt this, and I really don't mean to offend, but it's very hard for laypeople to evaluate the expertise of people posting on forums like this, even if they're using their real names. Also, even in this thread, you're disagreeing with each other, and clarifying things for each other, and reminding each other that, say, the "fall of one story," maybe isn't the most accurate way of putting it. To say that

you "old" metabunk members who have been writing in the 911 forum for years (the one Thomas has no interest in reading, or he would understand the mechanisms like i do)

somewhat overstates the kind of clarity that is available to laypeople by combing through online forums. I've read and understood Econ41's theory (and discussed it with him by DM) and we seemed to agree that the model I'm asking for doesn't yet exist. (I'll address Mendel's Edward's scale issue in a separate post.)

I briefly advised of a significant milestone in growth of understanding od WTC collapses sutable for the audience Thomas B represents. An assertion of historic fact which has NOT even been discussed. The assertion "As we're now proving, it hasn't" refers to some ill defined moving goal posts.
What was the milestone? You gave a date, but didn't say what new resource became available. I don't think anything happened outside forums like this (and no one in this thread has mentioned anything like that; that was the main takeway I got from starting it, and for which I'm grateful -- I'm not alone in not being able to find such a book.) Which means that totally fresh minds (who are born every day, like I say) come to this question in 2020 as they did in 2005 (and even 2002). If they have questions, they have to find answers in forums like this one.

I suggest one thing that might lead people's intuition astray, and you've already alluded to it: scale.

The Twin Towers were, for their size and weight, more fragile than champagne glasses. You've seen a stack of champagne glasses at Ikea or the mall perhaps? Well, at full scale, that would not be possible with Twin Towers. You couldn't stack even one Twin Tower on top of another. Or part of a tower. I cannot cite a source, but I don't think any of the columns on Floor 109 of either tower had a safety factor anywhere close to 110.
These comparisons and thought experiments are really good and they are precisely the ones I hope to see an engineer or popularizer work through in detail. Since the columns taper, it's easy to see that you can't put one WTC on top of another (like one wine glass on top of another). The top floors are much weaker than the bottom floors. But you *can* (perhaps?) interlace the weight of one tower with another, thus only *doubling* the load on each story. That's a good way of making the strength of the towers clear. And whatever model you build would need to have similar relative strength.

This is a result of the cube–square law, in which structural strength scales by the square of distance, but mass scales by the cube of distance...which is what creates the problems of building a small model of this disaster. From a practical standpoint, it's pretty much impossible to build something at a small scale with commensurate fragility.
Yes, but we can approach this scaling down in increments. First, we point out that the mechanism would work on an arbitrary amount of stories, so a 10 or 20 story building (and probably even less) would be sufficient. Then we reduce the amount of columns. I think 8 and perhaps even 4 would be fine. (There may be a limit. I'm not sure where it would be.) Next, the height of each story was probably not at a lower limit to create the necessary fragility. (Though he proposes a different mechanism, Bazant worked it out to under a 1 meter, as I recall.) At this point we have a simpler building but, in the sense we're considering here, it's "full scale" and built out of metal and concrete. (It might be only 5 meters tall and have only four columns.)

To make it even smaller, you'd have to work with "unrealistic" materials -- ones you'd never actually build a skyscraper out of. (Like Mick's wooden model.) The trick would be to reproduce the strength I mentioned at the outset in the "initial state". That is, you should be able to "interlace" the load of the tower floor-by-floor so that it has a safety margin of at least (or around) 2 and it should be easily crushable by its own total mass if placed as a single bulk on the top.

What this means is, if you're going to imagine for example what would happen to an exact 1:100 scale, 11-foot model of a Twin Tower, you also have to imagine that gravity is 100 times stronger. Then each member will have the correct safety factor. It'll stand and it'll be very stable...but if there's a major failure and things start moving, that stable structure will fail and fail disastrously.

I'm no materials engineer, but, like I say, I'm pretty sure we can imagine materials that are 100 times weaker than steel, and that we can build a 11 foot tower with 4 square foot footprint out of them in our garages.

Again, it's the non-existence of even a thought experiment like this in the mainstream (popular media) conversation that disappoints me. Until someone (with authority) "builds" this model, the truthers will continue to be born out of sheer incomprehension (not a predisposed conspiracy mindset.)

I suggest the best option to gain understanding is do your own research. Ask questions. Follow up. I see that several members have made essentially the same recommendation.
Like I say, this is the option that has been available all along (and didn't, as far as I can tell, become easier in 2010). It could conceivably solve my problem but only after a lot of effort (by me and the rest of you -- some of whom seem pretty tired of me already, actually).

We might say that this option doesn't "scale" well. So it explains why a lot of people still don't understand the collapses and are therefore open to the arguments for CD, at least long enough to begin down the "inside job" rabbit hole. Even if I don't technically need it, a good book would save me and a lot of other people (even you) a lot of time.

Just wanted to point out why it's hard to learn anything definite from forums like this:
My conclusion is that the unstoppable runaway collapse was embedded in the column free open office space design.... and it didn't matter how strong the columns were!

It's not complicated. You can see the columns failed at the welds.
Notice that these are two different accounts of what the tragic flaw of the buildings were: the open floor space vs. the welds. And neither of them has been acknowledged as the explanation for the total progressive collapse by anyone with the expertise and authority to make such judgments. Indeed, it's not acknowledged that there was any design flaw, tragic or otherwise. But both seem to suggest that if a particular detail of the design had been different, the buildings would not have collapsed.

If one of these were the official story, all would be well. But there is no "official story" of total progressive collapse. There's only what we can cobble together as laypeople from disparate sources of varying quality. I know many of you don't think that justifies broader skepticism about what happened, but I do have some sympathy for the people who in any case have a hard time getting their minds around it.

If one of these were the official story, all would be well. But there is no "official story" of total progressive collapse.
Make your mind up. Do you want to understand the collapses OR do you want an "official" or "authoritative and published" explantion.

Your call - it has been explained many times why you will not get both.
There's only what we can cobble together as laypeople from disparate sources of varying quality.
Please desist from insulting the people posting here. I for one am professionally qualified to publish the book or academic paper you seek. I am not interested in going down that path of academic and professional status games. If you dont believe me here in one to one discussion why should I expect more success if I publish a paper? Which by its very nature CANNOT address your concerns as directly and specifically focused as a simple on-line discussion? Other members here also have relevant qualifications. And I have high regard for all of those members active in this debate. Several are at the top of their aspect of the topic.
but I do have some sympathy for the people who in any case have a hard time getting their minds around it.
So do I - it was and still is the primary reason for my activity on-line.

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