Here you are conflating collapse initiation and collapse progression.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.
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.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.
Yep, good mental model.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.
Of course.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.
Ok.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.
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.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.
Things fall mostly straight down, don't they?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.)
This is a very comprehensive summary explanation! Thank you!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.
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?
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?
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.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?
.. details have become esoterica for the obsessed ...
NIST published the "official" reports... and they are hardly complete and seem to have some notable errors.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?
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.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.
there are no better resources available now to someone who is "genuinely ignorant but seeking to learn" than there were 10 years ago.
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'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.
What do you feel is lacking in my garage model? Can you not mentally extend 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.)
(1) The only weight in the structure is structural (there is nothing like the concrete pans)What do you feel is lacking in my garage model? Can you not mentally extend that?
It's made of wood, steel plates, and magnets. How is this point at all relevant?(1) The only weight in the structure is structural (there is nothing like the concrete pans)
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.(2) column sections along the height of the tower aren't "welded" and
Well it's really 3D, but yes, not the same topology.(3) it's a 2D model.
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.
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.It's made of wood, steel plates, and magnets. How is this point at all relevant?
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).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.
Obviously, my point. The *model* is 2D (it only models forces and resistance in two dimensions). All real things are 3D, of course.Well it's really 3D, but yes, not the same topology.
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.But really, you can't mentally extend that? Why not?
Aha, then perhaps that should be the point to focus on.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.
Your impression is wrong. The welds were the weak point. You might want to review this thread.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).
Source: After 9-11, An Engineer's Work at the World Trade Center: pp 47-48I 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.]
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.I've never felt confused by the progression of the Twin Towers collapses.
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.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?
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.Aha, then perhaps that should be the point to focus on.
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.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.
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.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.
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.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.
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?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.
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.I've never felt confused by the progression of the Twin Towers collapses,
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..... what does your intuition say should have happened when a 10- or 20-story section was allowed to fall by one story?
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.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.
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:
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.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.
The Top Blocks did not "fall" by one story. That was an intial assumption for a back of envelope calculation of available energy.
If we mere bunch of amateurs claim "the daytime cloudless sky is blue" would that be false because we are "amateurs"?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.
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.I was responding to the idea that somehow the situation changed in 2010. As we're now proving, it hasn't.
I comprehend the context. Agree your point.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 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.
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.If we mere bunch of amateurs claim "the daytime cloudless sky is blue" would that be false because we are "amateurs"?
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.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 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 thatThis 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.
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)
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 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.
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.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.
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.)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.
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).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.
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!
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.It's not complicated. You can see the columns failed at the welds.
Make your mind up. Do you want to understand the collapses OR do you want an "official" or "authoritative and published" explantion.If one of these were the official story, all would be well. But there is no "official story" of total progressive collapse.
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.There's only what we can cobble together as laypeople from disparate sources of varying quality.
So do I - it was and still is the primary reason for my activity on-line.but I do have some sympathy for the people who in any case have a hard time getting their minds around it.