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

deirdre

Senior Member.
The site is rich for reading, learning and understanding the events (collapse of the buildings) and investigations after the fact.
He doesn't want to read that site, or Metabunk, or any of the other 9/11 forums, he wants you to write it all up in an easy to read book with diagrams or make a movie about it, that he can share with friends..
 

Thomas B

Member
He doesn't want to read that site, or Metabunk, or any of the other 9/11 forums, he wants you to write it all up in an easy to read book with diagrams or make a movie about it, that he can share with friends..
I think we're talking past each other a little. I'm happy to read sites like that for my own edification and amusement. And I have been following the discussion for years. What I'm saying is that it's odd that an ordinary layperson who takes an interest in the WTC collapses has to go to such places, and learn the science from people named Ozeco, OneWhiteEye, and femr2. No matter how plausible their explanations may be, the fact that they're not the views of actual, namable engineers who publish in mainstream science journals and teach at major engineering schools makes the exercise ultimately unsatisfying, even if its sometimes interesting.

I began by asking whether anyone has a go-to source and the response seems to be NIST + some cobbled-together web sources. I can actually understand why conspiracy theorists find that position a little weak. After all, they also have (their own reading of) NIST and the web to draw on. And so it goes.
 

Thomas B

Member
You misunderstand the significance of the legal cases. In the course of the litigations at issue, many engineers, doubtlessly chosen from the best and brightest in the world as the note I shared indicates and as was the case with the Aegis experts, were tasked with creating complex engineering reports, which were then submitted the courts as evidence, subject to very strict rules. As with the Aegis reports re WTC7, those reports pertaining to the twin towers likely rival the NIST report in terms of scope and sophistication of analysis. And, because the engineers writing them were engaged in the adversarial process of the litigations, for each report, there will likely be a response from an equally qualified engineer or group of engineers. There is thus going to be a very robust engineering discussion in the court records. I suspect that there is no where in the world where you will find a similarly exhaustive discussion of these specific engineering topics, actually.
I don't really disagree with you about this. But even if there had been no legal action there would still have been a scientific understanding of the collapses. And it's the scientific understanding that I think should be popularized much more effectively than it has been so far. When science is bound up in controversies -- whether litigation or conspiracy theorizing -- things get needlessly muddled. The science should, at least sometimes, be presented as the solution to an objective puzzle, not just as a means to resolve disputes between interested parties. It should be possible to take a purely intellectual interest it.
 

Jeffrey Orling

Senior Member
I think we're talking past each other a little. I'm happy to read sites like that for my own edification and amusement. And I have been following the discussion for years. What I'm saying is that it's odd that an ordinary layperson who takes an interest in the WTC collapses has to go to such places, and learn the science from people named Ozeco, OneWhiteEye, and femr2. No matter how plausible their explanations may be, the fact that they're not the views of actual, namable engineers who publish in mainstream science journals and teach at major engineering schools makes the exercise ultimately unsatisfying, even if its sometimes interesting.

I began by asking whether anyone has a go-to source and the response seems to be NIST + some cobbled-together web sources. I can actually understand why conspiracy theorists find that position a little weak. After all, they also have (their own reading of) NIST and the web to draw on. And so it goes.
Actually.... years ago I suggested on the 911FF that some compile and publish a basic explanation for the most plausible explanation for the collapse of each of the 3 towers. Members at 911FF are engineers and scientists by profession... Many sites seem to encourage people using nick names. Many, I suppose want anonymity because they don't want to be hounded by crazies or have the jobs or families "threatened". And let's not forget ANY discussion which is not a complete support of the USG is considered "conspiracy" related and at least anti-establishment.
If you HAVE been reading the 9/11 literature published and online... you will know that the official investigation was without a doubt flawed if only for the way "evidence" was handled (not) as it was shipped offshore and melted down.

Again, I repeat that the mysteries of how the buildings went from stable to their first "downward" movements is not a settled matter. So any explanation will for sure have to include multiple alternate "initiation" scenarios.

But post initiation, NIST punts and used the term "global collapse" ensued as if global collapse is a technical engineering term that is universally accepted. So there is no consensus on the initiation processes and no consensus (official) or detail of the post initiation process.

I feel that the ROOSD explanation is correct for the twin towers. It has several attributes... the runaway outside the core floor "collapse/destruction"... "caged" inside the facade .... the peeling of the facade column/panels.... the destruction of the floors within the core including the core lateral bracing and finally the toppling of what remained of the unbraced core columns.

7WTC is more mysterious. Did the entire interior collapse BEFORE the facade/moment frame "dropped"? What caused the kink/rotation/split in the north facade / moment frame? What caused the core to collapse? did it collapse with the rest of the building or before the rest of the building? If (as NIST claims) there was a local NE floor collapse.... how did that involve the rest of the building:... other floors on the N, W & S outside the core and the core itself? NIST punts on detailing anything that happens after the floors around col 79 on something like 8 floors collapsed by declaring "global collapse" ensued. They did some interesting work but explain nothing in the end.

There will be no "proofs" because there is a dearth of data. There will / can only be well reasoned supported by mechanics/science/engineering hypothesis of how the collapses unfolded. One DOES need to have a good background in mechanics/physics/engineering to produce a comprehensive hypothesis. As an architect I don't have this background... but I had to study structure as part of my architecture education. I am not the person to take this on. But I like my own hypotheses. ;-) I have produced scores of drawings / graphics to articulate/explain my thinking.
 

deirdre

Senior Member.
So there is no consensus on the initiation processes and no consensus (official) or detail of the post initiation process.
I don't think that is true. Consensus doesn't mean everyone on the planet has to agree.
I think the general consensus among interested engineers and the public is what NIST shows. One big plus for NIST is they did use public input to check any mistakes, maybe worked the hardest on fire models and they had access to the most footage, documentation and interviews i think.
 

NoParty

Senior Member.
I'm curious to know where members of this forum get their understanding of (the "official" version of) the WTC collapses from. Most major (engineering as well as aviation) disasters have an often substantial "popular science" literature. Even where conspiracy theories exist, there are usually several longform articles and books that ignore them altogether (or relegate them to a footnote) and just lay out the (often very interesting) story of how investigators solved the puzzle of how it happened. There's also usually a good documentary film or two.

(The narrative is usually: here's what puzzled scientists at first and here's how they finally solved it. Sometimes there's a subplot of scientists competing to be first with a solution.)

Since around 2005, when I first got interested in this question, I've been waiting for something definitive and detailed in this vein to be published. Something that requires ordinary intelligence and a grasp of highschool physics to follow. If something did come out, I missed it. (Note: I know Popular Mechanics did a debunking piece; but I haven't seen a "straight" popularization by them of the received engineering view.) I'd prefer a book on the subject, but a very good, long magazine article in a mainstream outlet would be a good start.

A related question: what's the best "textbook" account of the collapses? I.e., where can we read the explanation that engineering students are taught?

My follow-up question is: in the absence of such an account, how reasonable is it for someone to claim (after years of trying) that they don't understand (the "official" account of) how the collapses happened? (The conspiracy theories are, I would argue, easier to understand, if harder to believe.) Has the absence of a good popular science book about the WTC collapses given room for conspiracy theories to flourish?
I do not believe that what you are asking for, exists (for reasons other posters have cited).

The physics involved are complex, there's no way to address minor differences of opinion
without going deep into the weeds (in a way that will lose the casually interested), and,
even if there were enough demand to justify some respected engineer(s) creating a great teaching tool for the layman, those that enjoy/profit from conspiracy theories will continue to do what they do, regardless.

In a nutshell, most people saw the towers fall with their own eyes after being struck by
gas-filled planes. Wild, unsubstantiated conspiracies like CD have either never occurred to
most folks, or just aren't taken seriously. I see little market for a book/film...
Why would one write a book that few want, and likely makes them a target for unhinged people?
 

Thomas B

Member
I do not believe that what you are asking for, exists (for reasons other posters have cited).

The physics involved are complex, there's no way to address minor differences of opinion
without going deep into the weeds (in a way that will lose the casually interested), and,
even if there were enough demand to justify some respected engineer(s) creating a great teaching tool for the layman, those that enjoy/profit from conspiracy theories will continue to do what they do, regardless.

In a nutshell, most people saw the towers fall with their own eyes after being struck by
gas-filled planes. Wild, unsubstantiated conspiracies like CD have either never occurred to
most folks, or just aren't taken seriously. I see little market for a book/film...
Why would one write a book that few want, and likely makes them a target for unhinged people?
It's precisely the complexity of it that could make it a page turner. Consider the facts that conspiracy theorists like to (over)emphasize: These were some of the biggest, strongest structures ever built. They had been designed to survive hurricanes and earthquakes and (with the familiar qualifications) even airplane impacts. The firefighters did not expect them to collapse because buildings like these had never before collapsed from fire. And yet, tragically, they did collapse. Why? How?

As you point out, the engineering issues are so complex that there are still differences of opinion. Research continues to this day. To anyone who is interested in tall structures (or perhaps structures of any kind) the puzzle is compelling. What needed to be explained? What data needed to be collected? Who would figure it out? Surely, there are even some interesting characters among the engineers and other scientists who studied the problem. There's the FEMA investigation and the NIST investigation and a number independent efforts. They've been looking at it for almost twenty years.

It's the sort of book that could really stimulate intelligent young people to get into engineering. If fear of cranks is really a disincentive, I don't think we'd see all those books about climate science!
 

Jeffrey Orling

Senior Member
I think if such a book were assembled... or a video.... there would be an audience for it. m I would view/read. But even the outline is daunting because there are a number of "theories" out there.
++++
I disagree with Deirdre and the reason that there is not much clamor about what NIST did is that the actual mechanisms and so forth don't matter. The "design elements" which distinguished these buildings have not been repeated... a sort of de facto acknowledgement that they may have not be the great innovations they were claimed to be. The guilty parties were the terrorists and it would be virtually impossible to hold designers and developers liable even partially for the collapses... though it was attempted.... kinda. Victims families were compensated by the USG.... People want to move on... not revisit it.
 

deirdre

Senior Member.
. a sort of de facto acknowledgement that they may have not be the great innovations they were claimed to be.
i'm agreeing with your comment except the part about diagreeing with me, only because i can't really understand what you are saying. or how it relates to what i said. From what ive read through MB NIST did explain the mechanisms. but that's me.

and while i do agree with even the bit i quoted here, just want to point out that they did stand up for a long time. long enough for most people below the impacts to get out anyway. so that's pretty impressive in my opinion for a building built in the 60s.
 

Thomas B

Member
The "design elements" which distinguished these buildings have not been repeated... a sort of de facto acknowledgement that they may have not be the great innovations they were claimed to be. ... it would be virtually impossible to hold designers and developers liable even partially for the collapses...
It may be "too soon", I guess. But that's one hell of a good hook for a book. It's my understanding that Aon Center in Chicago is basically the same design, and I thought lots of buildings used similar principles/elements. But if you're right and the WTC buildings were uniquely vulnerable to progressive collapse that's an important story that needs to be told sometime.
 

Jeffrey Orling

Senior Member
i'm agreeing with your comment except the part about diagreeing with me, only because i can't really understand what you are saying. or how it relates to what i said. From what ive read through MB NIST did explain the mechanisms. but that's me.

and while i do agree with even the bit i quoted here, just want to point out that they did stand up for a long time. long enough for most people below the impacts to get out anyway. so that's pretty impressive in my opinion for a building built in the 60s.
Thousands were killed in the collapse of the twins. No one in 7wtc. Apparently the explosion in the sub station which everyone assumed was a bomb got them all hightailing out of the building. FDNY was withdrawn when they noticed warping of the steel frame.

My take away is that the official accounts were not satisfying my need to understand the even(s) of the day. I am not the only person who has looked into this and had the same reaction.
 

Jeffrey Orling

Senior Member
It may be "too soon", I guess. But that's one hell of a good hook for a book. It's my understanding that Aon Center in Chicago is basically the same design, and I thought lots of buildings used similar principles/elements. But if you're right and the WTC buildings were uniquely vulnerable to progressive collapse that's an important story that needs to be told sometime.
The Chicago tower was designed and built at the same time as the twin towers. It has some similar features and there was a effort to reduce weight and therefore cost. It's plan had equal sized spans for the the outside the core floors. It seems to have a different approach to wind shear design. As it was not post twin towers its design was independently done All the marble cladding had to be removed!

These super talls make little sense to me.

Not much twin tower innovation was carried on according to this paper:

https://global.ctbuh.org/resources/...n-process-of-supertall-buildings-in-china.pdf
 

benthamitemetric

Senior Member
I don't really disagree with you about this. But even if there had been no legal action there would still have been a scientific understanding of the collapses. And it's the scientific understanding that I think should be popularized much more effectively than it has been so far. When science is bound up in controversies -- whether litigation or conspiracy theorizing -- things get needlessly muddled. The science should, at least sometimes, be presented as the solution to an objective puzzle, not just as a means to resolve disputes between interested parties. It should be possible to take a purely intellectual interest it.
There is certain critical information about WTC1, WTC2 and WTC7 that is unknowable, e.g., their exact internal fire progressions and intensities at any given place, the distribution of fire loads and effective fire barriers on each floor (something as minor as an office door being open or shut could make a difference), the exact structural damage incurred from exogenous events (e.g., planes in the case of the towers and the collapse of tower 1 in the case of WTC7)., and the exact structural condition of the building prior to catastrophic events of the day (were there any portions of the structures that had been altered illegally or that were otherwise in disrepair and was the as constructed state of the building error free). These are not minor variables--they are critical in every single moment of analysis, and they are not known precisely for even one moment.

Given those uncertainties and how they would necessarily multiply in a path-dependent model where each moment is dependent on the model's state in the last moment, there will never be a working engineering model that presents a truly objective conclusion that definitively proves the exact collapse sequences in any of these buildings. No one knows for sure because no one can know for sure. Every engineer will have to make subjective choices about how to simplify, generalize and predict unseen phenomena. The best answer we will likely ever have is that the collapses could have been caused by a number of different scenarios, and that maybe some of those scenarios are more probable than others. And that's exactly the battlefield on which the engineering experts in the related litigations have fought over and hashed out these issues in great detail for years.

So I think it would make sense for you go and spend a few hours trying to learn from people who have already collectively spent tens of thousands of hours on gaining relevant expertise and applying it to the analysis of these questions. Or you can wait around and learn nothing while hoping for a book that has not been, and likely never will be, written about an objective truth that cannot be known. (I know which path I'd take because I already took it when I obtained, for the relative pittance of ~$70 or so, all of the Aegis Insurance litigation expert materials that have been adding to and informing the discussion on this topic for a few years now.)
 
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Jeffrey Orling

Senior Member
There is certain critical information about WTC1, WTC2 and WTC7 that is unknowable, e.g., their exact internal fire progressions and intensities at any given place, the distribution of fire loads and effective fire barriers on each floor (something as minor as an office door being open or shut could make a difference), the exact structural damage incurred from exogenous events (e.g., planes in the case of the towers and the collapse of tower 1 in the case of WTC7)., and the exact structural condition of the building prior to catastrophic events of the day (were there any portions of the structures that had been altered illegally or that were otherwise in disrepair and was the as constructed state of the building error free). These are not minor variables--they are critical in every single moment of analysis, and they are not known precisely for even one moment.

Given those uncertainties and how they would necessarily multiply in a path-dependent model where each moment is dependent on the model's state in the last moment, there will never be a working engineering model that presents a truly objective conclusion that definitively proves the exact collapse sequences in any of these buildings. No one knows for sure because no one can know for sure. Every engineer will have to make subjective choices about how to simplify, generalize and predict unseen phenomena. The best answer we will likely ever have is that the collapses could have been caused by a number of different scenarios, and that maybe some of those scenarios are more probable than others. And that's exactly the battlefield on which the engineering experts in the related litigations have fought over and hashed out these issues in great detail for years.

So I think you could go and spend a few hours trying to learn from people who have already collectively spent tens of thousands of hours on gaining relevant expertise and applying it to the analysis of this problem. Or you can wait around and learn nothing while waiting for a book that has not been, and likely never will be, written about an objective truth that cannot be known. (I know which path I'd take because I already took it when I obtained, for the relative pittance of ~$70 or so, all of the Aegis Insurance litigation expert materials that have been adding to and informing the discussion on this topic for a few years now.)
But still.... the main plausible theories assembled in one presentation could make for an interesting presentation.
 

Thomas B

Member
...there will never be a working engineering model that presents a truly objective conclusion that definitively proves the exact collapse sequences in any of these buildings...

So I think it would make sense for you go and spend a few hours trying to learn from people who have already collectively spent tens of thousands of hours on gaining relevant expertise and applying it to the analysis of these questions. Or you can wait around and learn nothing while hoping for a book that has not been, and likely never will be, written about an objective truth that cannot be known. (I know which path I'd take because I already took it when I obtained, for the relative pittance of ~$70 or so, all of the Aegis Insurance litigation expert materials that have been adding to and informing the discussion on this topic for a few years now.)
You appear to have had confidence in your ability to learn something from those materials. I'm less sure that I could bring them together to form a coherent model of the collapses in my mind. Maybe one day I'll try.

But I want to be clear that no one should be looking for a "definitively proven" model of the "exact" collapse sequence. All we need is the most probable model (or, if there are close contenders, several such models) of the approximate sequence. I think it's important to tell the public if the collapses cannot be explained without reference to "illegal alterations", "disrepair", or construction "errors"). If those need to be hypothesized for the collapses (as observed) to make sense, then that's important. (And of course grist for the CT mill.) I don't think that's the received view in the engineering community, though. I think the collapses make sense to them even if the buildings were in perfect working order.
 
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Thomas B

Member
(something as minor as an office door being open or shut could make a difference)
This sounds strange to me. Surely, given that both towers collapsed, minor things like this cannot have made a difference. The exact distribution of the fires can't have been an issue, only the rough size, extent, spread, and temperature of them. The models didn't have to get the fires exactly right and I'd be surprised if they ran them with different combinations of doors left open and closed. But it's been a while since I looked at the reports.
 

Jeffrey Orling

Senior Member
One of the problems I have with the NIST sagging trusses is that it seems to require that all the trusses (most of them) on a given floor sag "together" to exert the force to pull in all the perimeter columns... like tightening a belt.... supposedly causing buckling all around the tower (WTC1) such that the top came essentially straight down. Now how likely is the to happen... simultaneously? NOT So if it's a limited local phenomena ... then what? Supposedly the facade because of its design had the ability to transfer axial loads around a "wound" and just sit there pretty. To me the NIST sagging trusses is a pathetic explanation. One is compelled to ask... Why would they propose it and why didn't they see how "dumb" it was? And if they did know it was bunk.... what did they think the real reason(s) were and why didn't they explore or even mention them? Of course sagging trusses pins the blame on fire and fire which was suppressed because sprinklers failed. This got me thinking the reason the sprinklers failed was the water in the system didn't get to the fire and the tanks were not replenished as they drained. And that happened why?

My "theory" involves the falling top materials kicking off ROOSD and ROOSD caused the core to go which was of little use if it remained and the rest was gone. But it does demonstrate that the entire building's structure elements depended on each other. I do wonder if just the perimeter could stand alone without fl slabs for bracing. I think not. But I don't know. I suspect the core without the concrete floors inside could stand if its bracing was intact.

I don't think engineers study in depth how their towers will collapse.... they should if they don't... and why do they do about it if they DID model it?

Even the core frame design was not garden variety.

Looks like no one wanted to assume responsibility for anything...
 

benthamitemetric

Senior Member
This sounds strange to me. Surely, given that both towers collapsed, minor things like this cannot have made a difference. The exact distribution of the fires can't have been an issue, only the rough size, extent, spread, and temperature of them. The models didn't have to get the fires exactly right and I'd be surprised if they ran them with different combinations of doors left open and closed. But it's been a while since I looked at the reports.
In the big picture, you are right: those things don't matter much, if at all. The buildings were sufficiently doomed by their circumstances that a single closed door wasn't going to make the difference of whether or not they fell.

But, if you want objective truth, then you need to completely understand the small details: which exact beams buckled first, which floor areas fractured first, which bolted connections sheered off first, etc. And you will never know that with any real certainty without knowing all of those things I listed, and many more similarly minor items, all of which are unknowable. And it all adds up. Hence the conundrum. If you don't know exactly when one thing failed, you cannot say with certainty exactly when any other thing failed; they're all part of the same system.

There are a lot of interesting engineering debates that turn upon the small things. There isn't a very interesting engineering debate at all on the big things. In wanting an engineering book that deals with the big questions without the nitty gritty of all the small details that inform the interesting engineering questions, you want to have your cake and eat it too. Maybe that's a bit clearer now?

So don't sell yourself short. Start with the engineering reports. There is plenty in the major engineering reports to which I linked in the second post in this thread that is highly readable and understandable.
 
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Jeffrey Orling

Senior Member
For me.... the perimeter of the core was a "turning point" The 4 corners were doing the heavy lifting but the belt girder which surrounded it was what was the key to to the OOS floors collapse. No floor "beam" or truss was carried on a core column....Belt girder was another "transfer" structure.... eggs in one basket kinda thing.

Also... I think the core's puniest column in the dead center played an interesting role. They were made puny I believe because the huge antenna load above them was moved to other columns by .... another transfer.... the hat truss.

But the hat truss had some interesting attributes. It did not span long distances... but it was bearing on almost all the core columns so they could take up / share the load of the antenna.

But.... it may be that those puny columns were among the first to go limp... and with that and the loss of the center of row 500... the hat truss had to SPAN which it was not designed to do (I believe)... and the hat truss buckled... and with it the the antenna plunged into the center of the tower hollowing out the core of the top block. Not good. The OOS floors up there tilted and plunged into the center of the core... now destroyed by the antenna. The perimeter was relieved of the floor loads... but it was also absent lateral restraint to hold it in place. It moved. It moved enough that it was unsupported and it dropped... and all this as the ROOSD was getting going.
 

Oystein

Senior Member
I want to at least acknowledge Oystein's remarks.
Thanks :)

It might be useful to compare my imaginary book with with Levy and Salvadori's Why Buildings Fall Down, which was published by the very reputable W.W. Norton in 1992, and presumably sold pretty well. It was reissued in 2002, updated with, among things, a short section on the WTC disaster, where they say, "Other failure scenarios will undoubtably be proposed and debated as investigations into the catastrophe take place over many years. Yet we believe that the basic outline of the failure is clear" (p. 267). It's the sequel to that book, one that follows up on those investigations and debates, that I'm thinking of. Although I'm imagining a book devoted entirely to the WTC, not on building collapses generally, it's hard to imagine that the market for books on collapsing buildings got smaller after 9/11.
...
The WHY question is puzzling.
I actually asked "why NOT" - why has the book you are interested in NOT been written?
I wondered that IF there was more than a tiny market, someone might well have done it, so perhaps there is no market that any prospective writer has identified?

I'm not persuaded by those of you who say that, after the NIST report, no further writing on the subject is really necessary. When Jeffrey tells me I should just go "look up Euler", he's sort of making my point; it's to avoid having to go through a lengthy process of independent study (and "silly navel gazing") that I'd like a popular book on the subject. Similarly, to answer my questions, Deirdre has suggested I should use Google, or go through the threads here, or even take a structural engineering course! My point is that, since the answer to my questions are known to science, it would be great to have them presented in way that laypeople can access easily and confidently, as in the case of everything from microbes to black holes.
Yes, that would be great. I might even buy it (I am part of the tiny fringe of debunkers that is even tinier than the fringe of 9/11 Truthers and thus very much not representative for the market at large).
I think that if a good book, written with the same authority as Levy and Salvadori's, had come out in the years immediately after the NIST report, and the mechanism of total progressive collapse had been patiently explained to the public, the conspiracy theories would have had a much harder time taking hold. After all, the most reasonable adherents to 9/11 conspiracy theories are often the ones that begin with the WTC collapses (rather than, say, stray passports or pet goats). The collapses really are hard to understand. The absence of engineers stepping up, not to spar with conspiracy theorists, but simply to explain the science, leaves an uneasy feeling that the collapses are not as well-understood as we'd like to think. CTs can exploit that uneasiness.
I feel you are too optimistic. The people who still push funny ideas about the collapses are rarely convinced by facts, either way. They are mostly motivated by ideology (broadly speaking), by a world-view that favors the detection of "conspiracies" everywhere. After years of debating them, I have never ever come across a single one who would hear out, let alone acknowledge, scientific or engineering arguments countering their false beliefs.

Organisations like AE911Truth would have no trouble at all pretending such a book does not exist, and/or smearing it as the fraudulent work of shills. Those who believe in "controlled demolition" non-theories would not read the book, they would jump on the smearing and go with that.
 

Oystein

Senior Member
... I'm less sure that I could bring them together to form a coherent model of the collapses in my mind. Maybe one day I'll try.

But I want to be clear that no one should be looking for a "definitively proven" model of the "exact" collapse sequence. All we need is the most probable model (or, if there are close contenders, several such models) of the approximate sequence. I think it's important to tell the public if the collapses cannot be explained without reference to "illegal alterations", "disrepair", or construction "errors"). If those need to be hypothesized for the collapses (as observed) to make sense, then that's important. (And of course grist for the CT mill.) I don't think that's the received view in the engineering community, though. I think the collapses make sense to them even if the buildings were in perfect working order.
Short question:

In your mind, are you clear about distinguishing between explanations for collapse initiation and collapse progression?
I.e. why did the collapses get started in the first place - damage to which columns, trusses, connections, caused by fire, impact, explosives, death ray, platoons of midgets with saws... vs. why did the collapse, once started, not arrest and instead accelerate and go all the way to the ground?

I think the latter is easier to understand, if properly explained. One could discuss is under an auxiliary hypothesis that all/some columns/trusses/connections were broken/removed by "magic", such that a top "block" starts moving downward. What will happen with that moving block next? How and where will it meet static structure, what resistance will this incur, how will that resistance be overturned?

Once you understand that collapse progression indeed is inevitable, regardless of what initiated the collapse, you know that it would have been unnecessary for imaginary demolition villains to help the collapse progress.
Bazant, Stassorek (was that the name?) and some others only look at this - and get it (partially) wrong by focusing on column buckling, when the evidence clearly shows that hardly any column buckled.
NIST did not really look into this, except for one item in their FAQ, where they explain how the floor truss seats would necessarily shear under the dynamic load of more than 6 floors. This is a back-of-the-envelop that ignores most details of how floors truss systems actually got loaded during the collapse, but it should be convincing nevertheless.

NIST, in contrast, was concerned with collapse initiation (and may, or may not, have gotten this partially wrong, too). I think it is unrealistic, and unnecessary, to expect that the actual detailed collapse sequence could ever be found and be agreed upon by a solid consensus. What can be shown is that that several failure modes could plausibly explain the collapses as observed, be they "floor-led" (i.e. failing floor trusses cause column buckling), "perimeter-led" (i.e. perimeter columns buckle before any floor trusses detach) or "core-led" (i.e. core columns buckle before any floor trusses detach).

As a general lesson to be included in the book would speak of the facts that steel-framed buildings are destroyed by fire all the time everywhere, and that firefighters, structural engineers and authorities everywhere understand that, if and when steel frames fail in a fire, they often fail suddenly and quickly. It's just that the vast majority of steel frame structures are not supertall highrises. Most are warehouses, factories, bridges, lowrise buildings.
The second lesson to learn is that building tall is more difficult than building low, which is why only the best structural engineers are tasked to do this: There is no magic that confers invulnerability to steel frame structures once they exceed some (never explicitly stated) height. Yes, they are stronger absolutely than less tall buildings, but actually tend to get more fragile relative to their own size and load. A 415 meter skyscraper is about as sturdy as a delicate bottle made of super-thin, brittle glass - a bottle that would break immediately if you tried to grab and lift it, that would shatter if you dropped it from a height of just millimeters. The book would need to explain the perils of scale.

A chapter coming late in the book might discuss possible differences between more conventional skyscraper designs, the WTC designs, and contemporary and future designs as regards their susceptibility to plane crashes and multi-floor fires. How in conventional structures, with their grids of columns, collapses may more likely be limited in their lateral and vertical progression to just one or few bays, where the open-office-space of the WTC towers essentially creates just one huge bay that goes down all at once.
 

NoParty

Senior Member.
I think if such a book were assembled... or a video.... there would be an audience for it. m I would view/read.
I believe you would.
As would 7 other Metabunk posters who have posted hundreds/thousands of times on this topic.
Plus 14 other folks who are more the audience Thomas B envisions: People who don't know
much about the physics & engineering involved, but are curious enough to lay down $18.99
and commit 14 hours to an attempt to sum up what happened.
I'm depressing the numbers--slightly--for comedic effect...but I still believe that your interest and
understanding is not at all typical...if there were a big demand for such a book, I reckon we'd
know it by now...
 

Jeffrey Orling

Senior Member
@Oy... there were many uses of "transfer" in all the towers... not just the massive 3 transfer trusses in 7WTC.

I noted the "transfers" in the floor system of the twins of which there were several.... the 4 transfer bar trusses at the corners 2 ways sections of the floors and the belt girder. As you well know there were no bays and the floor "beams? bar trusses were not framed to columns directly. On the core side they were framed to a "transfer belt girder" which in turn transferred the floor loads to the 24 perimeter core columns via short "cantilever" beam "stubs".

And the hat truss was another transfer structure.

There were several transfer structures below/supporting the moment frame of 7wtc. These transfers "resolved (re directed) the loads from the 57 perimeter columns above these transfers to the 26 columns below which went down to foundation. When the facade came down... these 26 axial supports had failed.

Transfers effectively work the reverse of "bays" in traditional grid frame plans. While they are not unique to the WTC buildings.... they likely played a role in the collapses.
 
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Jeffrey Orling

Senior Member
I believe you would.
As would 7 other Metabunk posters who have posted hundreds/thousands of times on this topic.
Plus 14 other folks who are more the audience Thomas B envisions: People who don't know
much about the physics & engineering involved, but are curious enough to lay down $18.99
and commit 14 hours to an attempt to sum up what happened.
I'm depressing the numbers--slightly--for comedic effect...but I still believe that your interest and
understanding is not at all typical...if there were a big demand for such a book, I reckon we'd
know it by now...
I don't think there is a lot of demand... WTC is a nerdy topic. My interest was because as an architect I wanted to understand the event and didn't. It wasn't obvious as most collapses seem to be.
 

NoParty

Senior Member.
I don't think there is a lot of demand... WTC is a nerdy topic. My interest was because as an architect I wanted to understand the event and didn't. It wasn't obvious as most collapses seem to be.
:)
My areas of expertise are far from architecture, etc., but I'm intellectually curious...
so I've read thousands of WTC posts by y'all on this site (Thanks Mick!) which has deepened my
understanding somewhat...though 50% of the posts are still too technical for me...
so I almost never contribute to those threads. I now know waaaaaaaay more about
NIST's explanations than any normal person should...
 

Thomas B

Member
In your mind, are you clear about distinguishing between explanations for collapse initiation and collapse progression?
Yes, I'm very conscious of this distinction. Sometimes I have a hard time imagining how the potential energy of the upper section was released all at once by the simultaneous buckling of all the columns, allowing the free fall of the upper section through the height of about one floor. It would be easier to imagine the columns "easing" the upper section down on the lower section tilting it back and forth, crushing one side first, then the other. A nice clear explanation of of why this didn't happen would be great.

But mostly I do think about it in the terms you suggest. I assume that collapse initiated and think of this as the "magical" disappearance of an entire floor, columns and all. Until recently, following Bazant and Starossek, I imagined the impact energy going mainly into the tops of the columns of the lower section, buckling the column lengths down to the next point of lateral support (the floor pans), which basically put the building in the same situation as "magical" initiation, i.e., the buckled columns leave a floor-height of space for the top section to fall through freely. Repeat.

I understand that some people here think of it very differently. The book I'm imagining would make it clear to all of us. Or that's my hope.


A 415 meter skyscraper is about as sturdy as a delicate bottle made of super-thin, brittle glass - a bottle that would break immediately if you tried to grab and lift it, that would shatter if you dropped it from a height of just millimeters. The book would need to explain the perils of scale.
I completely agree that this scaling issue would be important in the book. Taking the reader mentally through various "scale models", where the materials and loads are changed as the model gets smaller, in each case preserving the relative static and dynamic strength, would be an interesting (and difficult) exercise. You'd need a lot of knowledge and you'd have to be a good writer. One thing you would need to maintain throughout, however, is a persuasive analog of hurricanes and earthquakes. Obviously, these forces would be much smaller on your glass model than on the full-scale building. But your model would need to yield a few millimeters to shear forces. Also, you'd of course need to be able to puncture the outer shell with a fast-moving but even more brittle glass rod without immediately destroying the whole model.

As part of the work on the book, the author might want to commission professional modellers to build these scale models to illustrate the response of the buildings or subsystems in the buildings. Some of them might be converted into "experiments you can do at home" to demonstrate these things for yourself.
 
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Thomas B

Member
... if you want objective truth, then you need to completely understand the small details: which exact beams buckled first, which floor areas fractured first, which bolted connections sheered off first, etc.
I disagree with you about this. Consider the many popular science books about evolution. (There's something like a "conspiracy theory" to compare them to, namely, "intelligent design", but most mainstream books on the theory of evolution give them short shrift or pass them over in silence altogether. Just as "my" book would.) No one demands to know the exact natural history of, say, human beings. We don't need to know exactly each "link" in the chain of evolution, each mutation, each selection. But that doesn't mean that evolution isn't "objectively true". In fact, objectivity and generality go together, so understanding something "objectively" often means ignoring a lot of specifics, which are, like the doors we agree about, incidental to the relevant explanatory mechanism. Such details might be important if you were trying to make a "realistic" movie about what it was like to be inside the towers when they collapsed, or, reversing this idea, if you were trying to explain the exact appearance (the videos) of the actual collapses. But now we're talking, precisely, about the subjective experience of the collapses.

I think the idea that "we'll never know exactly" what happened is misplaced, especially in discussions with conspiracy theorists. We don't have to know anything exactly to be able to decide the truth about whether demolition charges were necessary. I don't think the lack of an "exact" conspiracy theory, for example, discourages anyone from going down the rabbit hole. It's the perceived lack of an even approximate explanation of gravity-driven collapse that sends people that way. The question I'm raising (and which I think most of you are answering in the affirmative) is whether that perception is accurate, or at least reasonable. The book I'm thinking of would be a game changer in that sense.
 

Jeffrey Orling

Senior Member
It is confounding to figure out how say the top block of 1WTC went from static to moving down. What he do know is what it did not have the support to keep it stationary. Often the explanation is loss of column strength and buckling. However there could be several explanations all playing a part.

A number of columns were destroyed by the plane impact
A number of columns were dented/damaged/distorted by the plane impact.

Essentially post plane impact there was a loss of axial capacity in the aggregate of the columns. But because of load redistribution and/or reserve capacity of the design... the top remained static. Excess capacity is "designed in" to many things including things such as tire tread for example.

Loads were not disappearing... capacity was. But as the tower remained static adequate capacity remained. But we need to understand at not all columns had the same amount of reserve capacity. I suspect the engineers did not produce what if models for various column loss scenarios as this is simply not an expected event.

We also know that something was changing over time. And that would be whatever the fires were doing. They were spreading and probably intensifying consuming more flammable materials. What would heat do to elements of the structure? It would impact different elements differently. Beams and floor trusses would loose capacity and expand as they are heated. Columns would weaken and perhaps distort as explanding would mean lifting the structure up as they expanded. Expansion joints would close up. Bolted connections would perhaps see steel move. Bolts might shear if the steel was moving. Sheared bolted connections would become none performing. Beams might drop at the sheared failed knife connections at columns. This would mean that columns might loose bracing. Unbraced columns are weaker (Euler) and are subject to lateral movement. If a column moves laterally and mis-aligns from the column above or below bearing area is lost. So it is entirely plausible as the fire raged in the core.... the core column's capacity and their "positions" in the grid were slowly changing as beams were failing to do what they were supposed to.
If bracing was lost on one side of a column... it was unrestrained and able to be displaced to the unrestrained side... while the columns above and below were "held in place" where it was "cooler" and braces were in tact. My guess is that the columns were being displaced by heated beams. And it doesn't take much dispolacement to destroy the bearing area needed for load transfer/performance. This would lead to web and flange crippling/buckling. Column axial alignment is mission critical for load transfer.

But the core wasn't doing much structurally (kinda). Its 24 perimeter core columns supported to OOS floors via the belt girders which carried the floor trusses. Those 24 columns were the "tube" in the tube we hear about. What about the 23 interior core columns? They supported lobby floors, some occupancy uses inside the core and of course framed the elevator and mechanical shafts. The core grid made it a stiff structural spine which help resist wind / lateral loads. In 1WTC it also supported the massive 360 ton antenna which was located above the 3 smallest columns of the core. The antenna has a wide base to spread its load outside its narrow plan dimensions. The hat truss was designed to support those loads and move them with diagonal members to all the columns of the core.
Essentially, I would assert that the antenna lost its axial support as columns in the central core "failed" and with that the hat truss began to warp, deform.... and fail. The antenna dropped into the building absent support. And with it much of the core grid of steel in the upper block was severely damage and much destroyed. When the destruction reached the perimeter core columns of the upper block and at the level of the plane impact... the belt girders collapsed... and the OOS floors lost their core side support and "fell"... some of the material slid into core and continued to drop all the down to the ground gutting (parts?) the core. Much of the floors of the upper block fell down on the floors in the static part of the building undamaged by the plane or fires. This kicked off the ROOSD... runaway floor destruction outside the core.
Both the failing hat truss and the loss of OOS floors led the the shell of the upper block's loss of stability and it too translated laterally enough to break column connections...buckling at the break an then it too plunged down... breaking apart into facade panels.largely falling straight down.

++++

If this explanation doesn't satisfy you.... go with NIST which proposed sagging floor trusses pulled the facade in, causing it to buckle and plunge. But they don't explain why the antenna moved first. ;-)

++++++

Like many others on the 9/11 "discussion" sites such as this one... I try to read, watch everything I can. I've been to several "events" (AE911T dog and pony shows) and one event which was a NIST disaster response seminar... memorable because my car got towed!

But my 9/11 work and thinking was the drawing output such as the attached items to this post. I have many many others. And rather than write narrative, do engineering computations... I make sketches to express something I feel interesting or relevant. My output could be pure rubbish. Or maybe some of it has some good ideas.

For me the collapse was a visual "thing" and words may not be the best way to tell that story. But a good author can make science come alive and understandable for sure.
 

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Thomas B

Member
For me the collapse was a visual "thing" and words may not be the best way to tell that story. But a good author can make science come alive and understandable for sure.
There's a lot of information in those drawings. I've been wondering for some time: what is the simplest and smallest (and cheapest?) possible structure one could build (or just draw) that would collapse by the same mechanism as the WTC? Could we imagine a 10-storey tower, with only 8 perimeter columns and four core columns behaving in the same way? (I'm suggesting building the tower for the sole purpose of collapsing in a way that demonstrates the mechanism. So we can keep many other things simpler than we would in real life. As long as the loads and structural elements (columns, floors, beams, etc.) are then. What is the minimum limit and why? (If I'm not mistaken, Bazant essentially claimed he could model it with a single column, a dozen floors or less, and only half a meter between floors. But that was using only the column-failure model.)

For pedagogical purposes, the author would have to break the larger structure and collapse mechanism down into smaller "machines" working in sequence or vectored. Remember Newton's cannonball: a simple model of how the Moon orbits the Earth. Or consider how the Galilean Cannon is used to explain how supernovas work. When is Brian Greene going to go on the Late Show and explain how the WTC collaped?
 

Jeffrey Orling

Senior Member
There's a lot of information in those drawings. I've been wondering for some time: what is the simplest and smallest (and cheapest?) possible structure one could build (or just draw) that would collapse by the same mechanism as the WTC? Could we imagine a 10-storey tower, with only 8 perimeter columns and four core columns behaving in the same way? (I'm suggesting building the tower for the sole purpose of collapsing in a way that demonstrates the mechanism. So we can keep many other things simpler than we would in real life. As long as the loads and structural elements (columns, floors, beams, etc.) are then. What is the minimum limit and why? (If I'm not mistaken, Bazant essentially claimed he could model it with a single column, a dozen floors or less, and only half a meter between floors. But that was using only the column-failure model.)

For pedagogical purposes, the author would have to break the larger structure and collapse mechanism down into smaller "machines" working in sequence or vectored. Remember Newton's cannonball: a simple model of how the Moon orbits the Earth. Or consider how the Galilean Cannon is used to explain how supernovas work. When is Brian Greene going to go on the Late Show and explain how the WTC collaped?
I think the twins' model could be simpler. But time and forces... including heat can't really be scaled.

Some clever person could, I suppose, make a computer model... but figuring out where to place the energy inputs and their magnitude would be difficult. How DOSE fire move through a building? How to model the fuel disbursement? What about the contents? Seems like a lot of computing power/time and lots of variables to program. I think Hulsey was unable to do it and just faked it to get the result he wanted.

++++

In the top drop sink hole concept... someone could perform the calcs on the building removing one core column at a time on say the 92nd flr.... in addition to the ones destroyed by the plane. At some point the mathematical model will show a runaway failure process. Representing that graphically is a whole other problem. If TDSH is "correct" the collapse should resemble real world.... at least for the top dropping. If it doesn't... match the idea is bunk.

I think one could do the same for ROOSD.

For any model to be valid it has to match the real world event.
 

Jeffrey Orling

Senior Member
Here is the framing of the roof with the antenna being the 25x25 square in the center... above only 3 columns... which obviously were not strong enough to support the antenna. There were 44 other columns to help out. And help out they did with the hat truss. You can see if the hat truss got into trouble... so would the antenna. If the antenna "fell into the roof" what would it do to the core?
 

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benthamitemetric

Senior Member
I disagree with you about this. Consider the many popular science books about evolution. (There's something like a "conspiracy theory" to compare them to, namely, "intelligent design", but most mainstream books on the theory of evolution give them short shrift or pass them over in silence altogether. Just as "my" book would.) No one demands to know the exact natural history of, say, human beings. We don't need to know exactly each "link" in the chain of evolution, each mutation, each selection. But that doesn't mean that evolution isn't "objectively true". In fact, objectivity and generality go together, so understanding something "objectively" often means ignoring a lot of specifics, which are, like the doors we agree about, incidental to the relevant explanatory mechanism. Such details might be important if you were trying to make a "realistic" movie about what it was like to be inside the towers when they collapsed, or, reversing this idea, if you were trying to explain the exact appearance (the videos) of the actual collapses. But now we're talking, precisely, about the subjective experience of the collapses.

I think the idea that "we'll never know exactly" what happened is misplaced, especially in discussions with conspiracy theorists. We don't have to know anything exactly to be able to decide the truth about whether demolition charges were necessary. I don't think the lack of an "exact" conspiracy theory, for example, discourages anyone from going down the rabbit hole. It's the perceived lack of an even approximate explanation of gravity-driven collapse that sends people that way. The question I'm raising (and which I think most of you are answering in the affirmative) is whether that perception is accurate, or at least reasonable. The book I'm thinking of would be a game changer in that sense.
Respectfully, I think your view on this topic owes to your lack of familiarity with the engineering work that has already been done on this topic. At what level of generality would this book be, in your view? If it's "fire weakens steel," then I doubt you have a "game changer" on your hands. As drill down beyond that into probabilistic scenarios, the details and simplifications are the only things that matter, and its exactly those details that have already been discussed in the extant literature ad nauseum. What use or interest is "objective truth" at such a high level of abstract? It resolves none of the interesting open questions, adds nothing to the literature.

I'll also note that the only way you can be confident in the objective truth of fires destroying the buildings is because engineers already took the time to do enough of the detailed work to demonstrate that. And the same goes for evolution. A book that says, "animals evolve into different animals over time" is not going to be a game changer. The details matter. In this case, however, it seems like you do not know enough about the details and how they were arrived at to actual know the gap you are trying to fill with this book.

It all makes you a very odd advocate for such a book. You don't know what the engineering reports say but you know that you want a book that says something else, and that something else must not be too detailed but must still reach some objective truth, which truth you think can be established without caring too much about the details. I mean, ok. It sounds like you just want NIST's FAQs in book form.
 

Oystein

Senior Member
Sometimes I have a hard time imagining how the potential energy of the upper section was released all at once by the simultaneous buckling of all the columns, allowing the free fall of the upper section through the height of about one floor.
Not sure this is at all needed, or even an apt description of what really happened.
All you need is some minimum amount of downward momentum, which would depend on which specific resistance would have to be overcome next.
In step 1, simply skip how that initial momentum comes about.

It would be easier to imagine the columns "easing" the upper section down on the lower section tilting it back and forth, crushing one side first, then the other. A nice clear explanation of of why this didn't happen would be great.
No doubt this, too, is a matter of collapse progression, but lateral.
Best to skip this initially.

But mostly I do think about it in the terms you suggest. I assume that collapse initiated and think of this as the "magical" disappearance of an entire floor, columns and all.
But that would certainly be wrong. I do not suggest the disappearance of an entire floor. I suggest that all or some of the columns, connections or trusses involved might be considered as disappeared, to get a collapse started. You know, just "enough".

Until recently, following Bazant and Starossek, I imagined the impact energy going mainly into the tops of the columns of the lower section, buckling the column lengths down to the next point of lateral support (the floor pans), which basically put the building in the same situation as "magical" initiation, i.e., the buckled columns leave a floor-height of space for the top section to fall through freely. Repeat.

I understand that some people here think of it very differently. The book I'm imagining would make it clear to all of us. Or that's my hope.
Definitely. It's a matter of direct observation that columns by and large did not buckle.
If you look at a floor from above, vertically down in the direction that things tend to fall, you will see that 98% of the area is floor structure, only 2% is columns. SO even if there were a way for mass to impact columns (that idea has serious problems), it's just so much more likely for any bit of structure to hit a floor, not a column.

I completely agree that this scaling issue would be important in the book. Taking the reader mentally through various "scale models", where the materials and loads are changed as the model gets smaller, in each case preserving the relative static and dynamic strength, would be an interesting (and difficult) exercise. You'd need a lot of knowledge and you'd have to be a good writer. One thing you would need to maintain throughout, however, is a persuasive analog of hurricanes and earthquakes. Obviously, these forces would be much smaller on your glass model than on the full-scale building. But your model would need to yield a few millimeters to shear forces. Also, you'd of course need to be able to puncture the outer shell with a fast-moving but even more brittle glass rod without immediately destroying the whole model.

As part of the work on the book, the author might want to commission professional modellers to build these scale models to illustrate the response of the buildings or subsystems in the buildings. Some of them might be converted into "experiments you can do at home" to demonstrate these things for yourself.
Part of the perils of scaling is that a physical scale model that accurately responds to all forces and deformations true to scale is impossible. You'd need different models for demonstrating what happens when a plane crashes, when a hurricane pushes, when an earthquake shakes and when collapse has initiated.
 

Jeffrey Orling

Senior Member
Respectfully, I think your view on this topic owes to your lack of familiarity with the engineering work that has already been done on this topic. At what level of generality would this book be, in your view? If it's "fire weakens steel," then I doubt you have a "game changer" on your hands. As drill down beyond that into probabilistic scenarios, the details and simplifications are the only things that matter, and its exactly those details that have already been discussed in the extant literature ad nauseum. What use or interest is "objective truth" at such a high level of abstract? It resolves none of the interesting open questions, adds nothing to the literature.

I'll also note that the only way you can be confident in the objective truth of fires destroying the buildings is because engineers already took the time to do enough of the detailed work to demonstrate that. And the same goes for evolution. A book that says, "animals evolve into different animals over time" is not going to be a game changer. The details matter. In this case, however, it seems like you do not know enough about the details and how they were arrived at to actual know the gap you are trying to fill with this book.

It all makes you a very odd advocate for such a book. You don't know what the engineering reports say but you know that you want a book that says something else, and that something else must not be too detailed but must still reach some objective truth, which truth you think can be established without caring too much about the details. I mean, ok. It sounds like you just want NIST's FAQs in book form.
I think what he may be looking for is more a play by play.... still obviously speculative hypothesis.... of how the building went from static to a pile of debris. I don't think anyone doubts that heat is "destructive" to "materials. But I do think lay people don't understand any of the details. Did steel "fail" because it lost strength... expanded? sheared connections? sagged? warped? People are familiar with expansion joints in bridges and roads. But this is normal air temps. Fires are very different. What happens to concrete in high heat situations? What about welds and bolted connections?

You can only do a play by play when you make reasonable informed assumptions about the mechanical damage, the fire spread, temps and damage and account for how the structure was "changing" over time because of mech damage and the effect of fire.

But why not offer some play by plays?
 

Thomas B

Member
It all makes you a very odd advocate for such a book. You don't know what the engineering reports say but you know that you want a book that says something else, and that something else must not be too detailed but must still reach some objective truth, which truth you think can be established without caring too much about the details.
This is a bit unfair. You're the one who said that the relevant are ultimately "unknowable":

But, if you want objective truth, then you need to completely understand the small details: which exact beams buckled first, which floor areas fractured first, which bolted connections sheered off first, etc. And you will never know that with any real certainty without knowing all of those things I listed, and many more similarly minor items, all of which are unknowable.
I was responding to your suggestion that I was implicitly demanding an impossible level of detail. All I've been arguing is that the book can be objective without getting each smallest detail right.

I have a question about this, though:

... the details and simplifications are the only things that matter, and its exactly those details that have already been discussed in the extant literature ad nauseum...

You don't know what the engineering reports say...
Like I say, I've been looking at this for a long time, and I'm pretty sure I've looked at all the relevant literature. I don't claim to understand everything fully (I'm the layperson who would be reading -- not writing -- this book, after all.) But I am pretty sure that the play-by-play (thanks, Jeffrey!) of the progressive collapse isn't extant. If it is, I missed it. So it would be enormously helpful if you named the report or paper you're thinking of.
 

Thomas B

Member
All you need is some minimum amount of downward momentum, which would depend on which specific resistance would have to be overcome next.
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.

If you look at a floor from above, vertically down in the direction that things tend to fall, you will see that 98% of the area is floor structure, only 2% is columns. SO even if there were a way for mass to impact columns (that idea has serious problems), it's just so much more likely for any bit of structure to hit a floor, not a column.
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.

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.

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. 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.

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. 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.)
 

Jeffrey Orling

Senior Member
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'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.

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.

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. 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.

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. 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.)
So we may be getting somewhere. The notion of lateral translation is now on the table in this discussion. I have noted above and in comments over the years that the lateral displacement of the column to column ends need only be a small amount (less than an inch in most cases) to destroy the connection by rendering the bearing area insufficient to support to upper column and its loads. Post 118 imagines a magical 3' displacement in each axis. But what was probably taking place with respect to "displacement" was that this":
local not global ie did not involve the entire set of columns (at least initially)
problems would manifest with displacements of fractions of an inch.
In the drawing of col 704 at floor 88 in an earlier post I showed a displacement of 3/4" in each axis. The bearing area would be reduced to 3.7% of the static engineered configuration. Clearly this is insufficient for the connection to perform. In real world the connection would FAIL before even a 3/4" displacement... perhaps a 1/4" displacement would be enough.

So assuming a 1/4" displacement would fail the connection... this would represent the beams expanding .25". Considering the dimensions of the core... the cumulative distance from col 701 to 704 was over 61 feet .... a 1/4" displacement would represent an expansion in length of 0.034%. This "seems" reasonable.

It should be noted that the end to end connections were not simple bearing ones....but included plates bolted to the column flanges... and the beam/bracing connections were knife connections with some tolerance for movement.

The point being... the behavior (displacement) of columns from heat expanded beams could be modeled and it might likely explain how the top's columns were displaces sufficiently to buckled, leaving the top without adequate support and eventually dropping as an entire block. The sequence being individual columns loose bearing, displace/buckle then this "spreads" to more and more columns until the aggregate axial capacity is inadequate. This would be a LOCAL progression of failures... caused by displacement from heat.

2WTC was a bit different. It LOST columns asymmetrically with the NW remaining as the SE axial capacity was lost. The top block then eventually had inadequate bearing AND asymmetrical support and tip toward the unsupported SE corner. But axial erosion could have been from the same process.
 

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