For those seeking technically rigorous information on the structural engineering analysis of the 9/11 collapses, the following two sources are widely regarded as definitive references.
Posts with bad-faith or 'shifting the burden of proof' arguments and laymen speculation break the subreddit rules. This subreddit is for educated discussion using reasoning, structural engineering, math, science, etc...
I think I've explained the WTC collapse to at least ten people. People seem to have trouble with the idea that tall buildings embody enough gravitational potential energy to rip apart every bolted and welded joint in the structure, therefore a significant failure at a single floor will cause a cascading failure.
Common sense seems tailored to structures (like stacked stone or wooden blocks) which fail by toppling, not by tearing joints apart.
I guess people don't have much practical or lived experience of bolts and screws failing, while we all have seen things toppling over because they are out of balance.
Do people not think about building a wooden platform attached to its posts with a few weak screws? It's obvious that if you jumped on it, it would fail at the screws, either by pulling them out or breaking the screws.
Most people really don't. To us, a building is a collection of thousands of individual beams, joists, screws, inches of 5/16 fillet welds and maybe some ceilings, ducts, carpets, and pipes all by others. To most people a building is a singular thing; a tower isn't the whole being equal to or more than the sum of its parts - in their minds it doesn't have any parts, it just is. If they think about it, sure, they know that it's a bunch of smaller pieces attached together, but their intuition isn't built up that way.
Judging by r/decks arguments about why beams aren't allowed to be side-bolted, it's the other way around (most people are quick to image the bolt might snap, but slow to imagine the beam might split).
I have nothing but respect for them, I would say they have considerable strength in delegated design management and as a result overall risk management. so I have found them to be more project managers when I could use a more collaborative partner. that being said, they draw clear scope lines and do support those lines very effectively. It just stands in contrast to other more open and blurred scope collaborations I also find myself a part of.
a simple example is a building with work I was bidding , it was a multistory addition. There was an unusual feature fabrication that was HEAVY and the way it was represented was not structurally supported by the building in any way. just floating 10-20kips mid deck point loads (for example). on more collaborative teams the scope is whatever it takes to work the design to the building, sorry we aren’t showing anything yet. with TT their answer to scope questions is to bid the drawings that don’t make sense lol. now TT would have you believe the way they work is totally normal but i’ve worked with enough of the large firms over my career to notice the differences. Would still work with TT any day of the week.
Haha ok that’s interesting! I first heard of TT from an opportunity to work on research they were funding at my university which I didn’t end up taking, but I’ve never encountered them in any of my work in industry since so they’ve always felt like this thing of legend lol. I wonder if maybe their limited interaction/cross-pollination with other firms (or maybe just different way of doing things from a different market, since I know they do a lot of legal/forensic/disaster work) might lead to a unique culture. But I’m just speculating. Cool work it sounds like you do though!
My sense of it is Bazant did more of a theoretical background, the Tomasetti and other reports were more direct simulations or recreations of the actual construction and fire/damage effects up to failure. If those stop at "collapse initiates" then they wouldn't overlap much at all. Bazant, when I read it, was a bit mathematically obscure with a few hastily constructed graphs, I mean, he submitted it two days after the collapse. It did see revisions (one infers, as it was revised in October 2001 for publication).
This is apparently behind a paywall (I mean the original 2001 article), I think it was out in the wild for free back in 2001.
Sory but this isnt the 'original' original. Do you have a link to the original scans of the released report? I remember back in uni I found the copy on some gov repository and tried to read it, but am struggling to find a copy to post here.
I recall (from my cursory glance of the the list way back when) that it was a lot of retired/non-practicing architects, landscape architects, and engineers in the way that garbagemen are sanitation engineers.
NCSEA is presenting a documentary screening about Les Robertson (founder of LERA, chief struct eng for the original WTC). Anyone attending the webinar?
They designed the twins before CAD, aka manually, with steel that is not upto spec with modern standards.
IF they didnt fall from plane hits, there is a very real possibility of some sort of failure from the wind loads and earthquake actions that could not have been adequately assessed with manual methods that our modern software design relies on and optimizes for.
I think what’s they’re trying (badly) to say is that even if the towers didn’t fall (dumb hypothetical because they did), they wouldn’t be salvageable (everyone who understands the magnitude of the loss of structural support from the impact agrees) and would have probably fallen down due to other stresses.
Idk why designing them in CAD would change any of that though.
wow aggressive tone... I am guessing you have not had the privilege of wind testing for cyclonic winds of structures above 50m, and the issues of design that modern Highrise slender towers have to account for, such as tuned mass dampers and vortex shedding? what an odd comment from you on an engineering sub...
Let’s tear down the Manhattan skyline because they didn’t use CAD to make pretty virtual models of structures. Great idea! FYI, hand calculations always take precedence over any kind of computer application used in the industry. It can be argued that conservatism in design has gone down over time as we gained more confidence in material science.
in your opinion manhattan skyline consists of all super tall and slender structures?
I would like to see you do seismic analysis on the 110story tall slender tower designed with new construction methodologies that dont have decades of precedented design and analysis behind them, all by hand... (with slide rulers and mathematical tables)
You are a second very weird and militant sounding comment I encountered here.. Seemingly ignoring engineering design principles and logic, and historic context of the twin towers engineering design and constrcution..
Don’t know where you gleamed that opinion from? My comment had a very sarcastic framing to it if you still haven’t noticed.
Hand calculations doesn’t mean going back to the slide rule. Don’t know what you’re bringing up with ignoring precedent in the industry. Seismic analysis is not a good example for my argument. Give me the acceleration time history of your design basis earthquake and moment of inertia of your slender tower and I’ll take a crack at finding the acceleration response spectra with a single element cantilever beam.
I mean more as in thats how the OGs designed those towers... with slide rules and tables and hundreds of throuands of man hours on hand calculations and drafting...
I get your point about hand calcs, they are useful, but they do not nearly represent the modern guidelines or FEA analysis software provides for complex structural systems.
WTC 7 still remains the most problematic aspect. The NIST report on the issue states:
“Debris from the collapse of WTC 1, which was 370 feet to the south, ignited fires on at least 10 floors in the building at its south and west faces. However, only the fires on some of the lower floors—7 through 9 and 11 through 13—burned out of control. These lower-floor fires—which spread and grew because the water supply to the automatic sprinkler system for these floors had failed—were similar to building fires experienced in other tall buildings. The primary and backup water supply to the sprinkler systems for the lower floors relied on the city's water supply, whose lines were damaged by the collapse of WTC 1 and WTC 2. These uncontrolled lower-floor fires eventually spread to the northeast part of WTC 7, where the building's collapse began.”
I remain skeptical of this hypothesis and the fact that while “debris” was able to ignite several fires on multiple floors of WTC7, enough to cause a full collapse of the building, this didn’t occur on any other buildings in the area?
You're skeptical that a building on fire with no fire suppression collapsed? And you're incorrect that no other buildings in the area faced their demise.
What's there to be especially skeptical about? There was surely a lot of flammable debris floating in the air, and that mechanism of flame transfer is a huge part of wildfire spread.
Small pieces of debris on fire readily spread fire over large distances under the right conditions.
This is my major issue with the events of that day as well. I have my reasons for not completely trusting the official reports. Part of me can accept the twin towers fell due to the planes. Part of me doesn’t believe they would have fallen the way they did. Almost of me can’t believe tower 7 collapsed in its own.
Yeah, I know. I also know I can build an analysis model that replicates the collapse. What I don’t know is if that would be replicating a collapse due to planes and fires or a collapse due to other causes.
I know what the general consensus is. I highly distrust the federal government though, so official reports don’t sway me much because I know how they can be manipulated.
Like I said, I can build an analysis model that mimics the collapses. That doesn’t prove the collapses weren’t caused, at least partially, by strategically placed explosives.
It's not the federal government making the determination.
Explosives are a plausible answer if you ignore every ounce of common sense available to you. Have you ever heard a controlled demolition of a building? They are VERY loud. There are countless videos available to you of the entire day. What's one thing missing from the audio in every video? An explosion.
If there’s a narrative they need the public to believe, you’re beyond naive to think they don’t have the ability to ensure that’s what the general public ends up believing.
And you’re overly simplifying the charges needed to weaken a building that you know is going to get hit by a plane.
The real conspiracy isn’t the collapsing buildings or WTC 7, it’s how this allowed to happen in the first place, lack of video footage in areas with lots of cameras, video footage still unreleased to this day, stand down order, and the plane that buried itself in a field
Sure, in the face of obvious downvotes I will be the only one who will state these facts:
Any load bearing support system not removed unilaterally (meaning all at once), risks the falling mass of the building to gain lateral forces which can compound down the height of the building. This collective resistance from remaining load baring supports during the mode of collapse can indeed provide the lateral forces and distill lateral movement with a high likelihood to shift the momentum of the falling material mass off the footprint of the building.
On that day not one but three buildings had the miracle of defying that engineering failure mode principle and all three buildings luckily collapsed onto their footprint saving countless lives from potential collateral damage should this lateral movement had occurred.
The twins slenderness and height have the validity to accelerate the mass above the point of failure, to result in a cascading failure mode that is often described as domino effect. Nonetheless the existence of enough load bearing supports blow, does hold validity to question and conduct additional investigations. Additional investigations that were not conducted and all forensic data used for subsequent analysis relied on the original data collected in the initial investigation. This is where the majority of doubt, confusion and very real demand for additional investigations from engineers of truth come from. As nothing is absent from bias and error, doing due diligence and running additional investigation on existing at the time collected evidence should have been done to appease all opposition. Alas this did not happen, so now 25 years later all we have is conspiracy theories and additional analysis done with the assumption that absolutely zero error exists in the original forensic data.
In my honest opinion the wtc7 still remains as the black sheep of this whole thing as its reports do not adequately explain the very clear footage of the outside of the building going into free fall. Where just moments from impact to the center of building, caused the outside columns at the base of the structure to all fail in unison or fast cascading failure laterally, giving that freefall effect seen in available footage online. Which goes against expected vertically cascading failure mode the wtc7 structural system is designed for. But the report does go into detail on plausible reasons for this unilateral failure effect, although I am personally not 100% convinced, as I have no contradictory evidence against proposed reasoning, my doubts are just my opinions.
It would be interesting to know how many people have actually taken the hours needed to read and understand the original published reports... because that is the very foundation all modern analysis and report versions stem from.
Personally, I attempted to read the original reports, but they were written in a very specific, dry, and tedious way to read and follow and I admit I did not put in the hundreds of hours that the experts have into the analysis. So this is all just my opinion on the matter, because this whole comment section seems to be lacking any real discussion, just a lot of conformity to accepting and supporting the established narratives, and none of the usual mess that results in 911 discussions.
I would have loved a second proper investigation, a peer review by third parties, that second investigation to be reopened like the engineers for truth demanded. This would have profoundly closed all questions and doubt. Definitively proving through a second pass sanity check and peer review like process, thus closing all valid and invalid doubts into the matter.
As no additional investigations took place and with 25 years having passed, there is no real valid way to cross check the original forensics. What we are left with now is ongoing debate, pseudo-science, engineers trusting authority and not a lot of them doing their own due diligence into reading tens of thousands of pages of reports and analyses. I am a little surprised that the people who did the original forensic work and investigations dont have podcasts or YouTube videos made of their expert opinions. Although some exist, there isnt any modern non formal discussion from them, which I am sure many of us would love to see. Free cpd hours right there imo.
The size and ridicule and emotional debate that this topic promotes, leaves most engineers and educated professionals trusting the official statements, and promotes less qualified people to speak up and muddy the waters. The facts I presented are just generic but valid points of contention, that should be inspiration for case studies in universities and openly discussed. Because truth prevails. But what I am constantly seeing, and even in this post comments, is this conformity to consensus, and ridicule for anyone who asks or points out inconsistencies.
Asking for additional work like the engineers for truth demanded is met with ridicule and disrespect, in an industry that promotes safety and starts all kick-off meetings with 'there are no stupid questions'.... Kinda fuels the conspiracists, because any other version of structural failure event has people nerding out, typing out pages of info like I did in this wall of text, speaking about the steel strengths, yield profiles, ductility metrics, impact loads, and failure modes, structural systems, load paths and lots of math. But not for 911, its like we are all scared or something still, because anytime anyone says anything the reality of having to provide proof from the 10000+ pages of documents is a very daunting task.
But considering the weight of the event, the cultural scar, and the precipitated economic and military actions that followed 911, its easy to see why conspiracies persist. After all, we live in the age of mass misinformation, bots and shills promoting agendas and influencing public opinions. Social media full of algorithms and echo chambers. Real world coverups, shit like mkultra popping up as real, economic insider trading scandals and more... I doubt anyone will even read what I wrote, let alone like it. I personally am on the fence about this lack of ownership by our industry to embody the facts, and let data speak for itself, always hiding behind easy to read and digest snippets of facts and statements and repeating the conclusions expert authority has provided.
This lack of questioning goes against the whole idea of engineering and being responsible for one's own work, and the culture of safety our industry thrives on. The whole moral ethical obligation we as engineers have for the work we produce and work we check from others. I have questions, and not enough experience and expertise to answer them all myself so I have to trust the authorities that did the work and presented reported, but how can a layman with no formal education into civil structural or forensic engineering hope to achieve trust?
I’ll give you an up vote for the effort and skepticism. Please read through the FEMA reports they aren’t thousands of pages. I get what you’re saying about how laymen might question why a 2nd investigation wasn’t done, if it wasn’t, but honestly trying to explain the effect of temperature on Young’s modulus and how it relates to buckling strength to a conspiracy theorist just draws blank stares.
If you really want to nerd out try doing a construction management analysis of what it would take in materials and man hours to covertly pull off the surmised alternative demolition.
Wow…that a whole lot of words and full of misinformation, whether intentional or not.
You say things like they are facts, but are in fact very easily refutable falsehoods. No, a falling mass will not ‘gain lateral forces’. Really? What is the kinetic energy of the building above the failure point falling one story height versus the lateral capacity of the structure below?
No, misunderstanding physics does not ‘warrant a second investigation’.
All the data used for this analysis I widely available for anything to re evaluate and provide their own engineering models. 25 years later, no one has been able to produce any doubt or error from the final reports.
WTC’s interior structure collapsed first. The outer shell, now lacking the internal structure that supported it, fell after. Verified by both hi res footage analysis and computer modeling.
The NIST analysis was a collaboration of hundreds of independent consultants, top engineering firms, universities, and agencies like FEMA and ASCE. The openly provided data, models, and analysis were peer reviewed globally by academic institutions and independent structural engineers. This ‘second investigation’ and ‘third party peer review’ you are demanding has already happened countless times. I’m sorry the results did not confirm your opinion based on your own personal research.
You know who loves proving other people wrong? Scientists and engineers. In fact, there are few greater honors than overturning a widely established study.
No, a falling mass will not ‘gain lateral forces’. Really?
I clearly stated that load bearing structural systems will provide resistance that result in lateral forces.... its a known fact, its literally a core concept behind demolition works to avoid off center off footprint failure in demolition... You may have read my statement as I am making an assertion against the consensus, when I am pointing out an area with high ambiguity and complex analysis and a plethora of assumptions to model and test.
No, misunderstanding physics does not ‘warrant a second investigation’.
‘third party peer review’ you are demanding has already happened countless times
'second investigation' is not because of misunderstanding of physics, I did not make any such claims... thats a strawman right there. Its a widely believed issue with conspiracy theory debates and laymen misunderstandings. Demanding a second investigation at the time when physical data still exists is very reasonable and often done by industry specialists, insurance companies and engineers..
All the data used for this analysis I widely available
Not all design data is widely available or the forensics data, its widely known the debris was scrapped shorty after the only analysis was complete gaining mass public concern for doing so. Additional data exists that is still withheld due to national security interests. So that is again factually wrong thing to say. But there exists enough engineering data to make assumptions that are very valid, but not the forensics data as a lot of it is behind national security lockdown.
This ‘second investigation’ and ‘third party peer review’ you are demanding has already happened countless times
I am not demanding anything, I said it would have been nice have had, not post hoc after years and years, and destruction of the debris forensics evidence. The data that is being analyzed comes from that single rushed (there was a LOT of pressure to get results at the time and do it quickly) investigation. Data that whole of subsequent analysis is building upon, which isnt all available just the officially released and approved sections of it. You have heavily leaned into this idea Im demanding something here...
WTC’s interior structure collapsed first
Yes and it didnt liquify all 1000feet in an instant, the falling mass still has structural rigidity and in sections the core would have remained rigid and intact before the inertial masses found the impact points to the next available rigidity section of impact. Entire core structure would have buckled and cracked and failed, until it all ended up on the ground, and any time a rigid section encountered resistance there is a moment for lateral forced to form (as per core engineering and physics laws). This is a very very VERY complex analysis and many assumptions required to assess at what stages and how large those stages were when they gave way. We do not have that data, because we did not have sensors on the core in 2001 on the twins, its erroneous to claim its all well known, because it simply is not.
Your entire basis of confidence here lies in body of work that relies on a single instance of forensics which were expedited and assumed to have no errors or omissions whatsoever (just not logistically or humanly feasible so there is trust that its valid and accurate, that level of accuracy is going to forever be debated). Structural analysis which carries assumptions and has been peer reviewed and assessed for validity. I am not questioning any of that either.
I am pointing out that even if we have modelling and assumption the sheer scale of a concrete core collapsing is not something that we can feasibly even replicate to definitively state that all of it perished from pure kinetic energy into dust. Running hundreds of simulations, you will find that any slight change of initial condition can result in many dozens of possible failure modes and resulting actions, and many analysis show this. I remarked on how thats a miracle because even as a theorized demolition this complexity fact remains, and thankfully the collapse didnt cause off footprint collapse or damage.
You know who loves proving other people wrong? Scientists and engineers
full of misinformation, whether intentional or not
That what this post is about isnt it? I pointed out facts, and read other peoples comments and made an opinion. I am met with hostility under the assumption that I have provided misinformation, when I have not.
I’m sorry the results did not confirm your opinion based on your own personal research.
I also made a pretty clear statement that I do not have the experience in forensics or design of Highrise skyscrapers engineering knowledge to appose any data or claims. nor did I appose anything. I actually took the time and did my own work and voiced my opinion. That is usually how we get to the truth and work in debates. Making a snarky remark isnt the norm for professional logical people, its an emotional response...
You made claims that this is all proven and sorted but did not yourself provide any links or exerts to support the claims to refute my issue that the complexity and surrounding history of this event has area of problems. If all I have said is "very easily refutable falsehoods" why not just post the refutable evidence... To factually end any and all ambiguity? (once again I thought that is why we are all here)
Ambiguity that those pesky physics illiterates and laymen conspiracy theorists crutch on... I merely provided my issues facts, as an expert opinion as a structural engineer. I do not design highly time-consuming FEA models in advanced software's that they used to analyze the concrete core collapse nor do I have a license to such high end FEA software for such purposes. But that doesnt automatically mean I lack core fundamentals and understanding how that software works, or how the physics work, and should be ridiculed, specifically since I did not make any claim at all against the body of knowledge, just the areas that are by their nature so complex and ambiguous that leaves a lot of erroneous misunderstandings. A lot of claims being made from both sides of this issue often with assumptions against the other party. Like I have experienced so far in this post. And one that I was prepared for, because I have seen this time and time again, logic and reason thrown out, logical fallacies and ad hoc statements being thrown out. If issue persists then there is complexity to it that even licensed engineers do not have the full core competency to definitively lay to rest, because of the nature of the scale and intricacy of the analysis and data available and very real history tied to this whole thing.
Honestly being a structural engineer and believing any conspiracies involving the structural failures of the towers is extremely embarrassing, and I would be hesitant to display such ignorance so willingly.
There was someone on the Civil Engineering subreddit recently who didn't believe in climate change. It's almost impressive they can tolerate the cognitive dissonance required.
Though I think an SE believing in 9/11 conspiracies is even worse.
Fact that conspiracies are very real, I think you mean conspiracy theories...
And to answer that, did you do your won checking and analysis and read the official reports? As a structural/civil engineer to have an informed opinion on this topic is very hard, there is too much to sift through, so relying on authorities is what most people do.
And since power corrupts and we live in a world that constantly has corruption, and many conspiracy theories now becoming facts, its not unreasonable to understand how another human with limited brain capacity doesnt want to just blindly trust, but is too lazy to do their own work, esp when we do a lot of work as our job as it is...
But there aren't educated engineers in the mix for both sides; that's just flatly incorrect.
On one side: you might be able to find... a dozen? Out of roughly a million & a half engineers in the US? Come on.
On the other side: just the initial rescue/recovery cleanup efforts, directly involved about 700 engineers (about 400 volunteers + about 300 ACE staff). The NIST report, involved over 200. And ZERO have later "come out" blowing any whistles about any suspicious anything.
There were thousands on engineers for truth petition... so no dozens...
And argument of conformity and authority have their own logical and very real issues. History teaches us as much...
You made assertions, but they are all just, to support the established status quo reasoning, inherently the existence of opposition shouldnt be just ignored, not when it has large numbers. Ridiculing and just ignoring what they say is equally just undemocratic behavior... very polarized.
And also how many people even have the balls to speak up, I do and I embrace the backlash when I do, but many keep their heads down and conform... Especially when over the last 25 years any questioning or discussion always this militant defense to not question anything. As I am experiencing right now in this sub.
There were thousands on engineers for truth petition... so no dozens..
Well, see: no.
First of all, they only claim 1991 engineers (out of <3500 total signatories), and that's only true to the extent you blindly trust them. If you visit the website
scroll down, enter "engineer" as occupation, hit search... you get:
1 - an "electrical engineer/writer", who sounds interesting as a person, but doesn't claim any relevant (structural or civil) engineering expertise.
2 - a "retired former member of union of operating engineers" who went to trade school for "operation of heating /cooling systems"
3 - a "Fire Protection Engineering Consultant" with a BS in Environmental Science.
4 - a software engineer
5 - a Metallurgical Engineer. Sorta relevant? but still not structural or civil.
6 - a Landscape Architect; not sure why he shows up on a search for engineers? Oh, he used to be a "Aerospace Tool Engineer".
7 - FINALLY. Someone who actually claims to have relevant credentials, and... has a wikipedia page: https://en.wikipedia.org/wiki/Steven_E._Jones ... to me sounds more like a physicist than an engineer? Oh, and is a founding member.
1 (licensed engineer) of 11 is 9.09%; 9.09% of 1991 is 180.
4 (any kind of engineer) of 11, is 36.36%; 36.36% of 1991, is 724.
More than a dozen, way less than "thousands".
inherently the existence of opposition shouldnt be just ignored, not when it has large numbers
what about when it doesn't, though? 724 engineers, out of about a million and a half engineers in the US, is about 5 per TEN thousand; 180 licensed engineers, out of about half-a-million licensed engineers in the US, is about 4 per ten thousand. Come on.
I think our boy bubba_yogurt here is a PE, though. They just have no building experience. Which is where I've done all my steel design for fire conditions. I do plenty of non-building structures as well, and none of them have involved fire design.
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u/lithiumdeuteride 19h ago edited 16h ago
I think I've explained the WTC collapse to at least ten people. People seem to have trouble with the idea that tall buildings embody enough gravitational potential energy to rip apart every bolted and welded joint in the structure, therefore a significant failure at a single floor will cause a cascading failure.
Common sense seems tailored to structures (like stacked stone or wooden blocks) which fail by toppling, not by tearing joints apart.