9/11 conspiracy reality: How would/could a "diabolical structural engineer" demolish the WTC?

[md2000]

@Isilder

…

It wasn’t brought down by an explosion, the plane caused a large fire inside and that fire soaked heat into the structure over the half hour that raised the steel up through 400 Celcius… which was just warm enough to soften it, and very approximately, halve its strength… so if the safety margin became lower than 0%, which means structure broke.

… The WTC floors are insignificant compares to the columns so the collapse pile only shows large sections of wall and column sticking out of the pile of unrecognisable mess.

The floors don’t do any serious structural support. AFAIK (Not an Engineer) the strength was in the steel columns around the perimeter. The floors just braced them from bending under the weight. Taking out one or two floors would not have done much. So you’d want to take out a number of support columns, right?

But wait, some Saudis already demonstrated that just taking out a large number of support columns on one wall did nothing. The building was very well overengineered. **Islider **has it right - the coup de grace was heating the support columns until they buckled after that. The standard whacko comeback is that steel melts well above the temperature of a jet fuel fire - true. But… Metal becomes soft like taffy well before it melts (the principle behind beating swords into ploughshares). After half an hour, the remaining metal side supports became soft enough to finally buckle.

Plus, despite extensive video surveillance from all angles, there is no evidence of explosions above or below, floors or support columns, when the towers finally failed. But then, the logical conclusion is that the explosives were hidden on the very floors the planes hit, which would indicate both remarkable aim by the “pilots” and a massive amount of *fire-proof *explosives. There’s always the possibility that with the elevators out and rescue personnel running all over, a dedicated team of explosive engineers carried up the buildings and planted the necessary explosives.

Or… maybe the towers fell because of the heat from the jet fuel fires.

[EdelweissPirate]

@md2000

The floors don’t do any serious structural support. AFAIK (Not an Engineer) the strength was in the steel columns around the perimeter.

Well, I am an engineer, and I’d say your post is spot-on.

For what it’s worth, my specialty is mechanical and structural simulation. After grad school, I took a course on explicit dynamics, which is basically the simulation method used for highly nonlinear and very-short-time-duration events like car crashes and detonations.

As part of that course, we studied not only this paper on the WTC collapse but also the full model the authors describe within it.

The paper and the model both strongly support the scenario MD2000 describes.

After reviewing both the setup/boundary conditions and the authors’ results, I found their work extremely compelling. The article seems to be paywalled, but if you have the means, I’d highly recommend reading it. And “the means” in this case would be either strong Google-fu or access to a university library.

[EdelweissPirate]

An addendum: Osama Bin Laden was also an engineer—specifically, a civil engineer. I don’t have a cite, but I read shortly after 9/11 that he was “pleasantly” surprised by the towers’ collapse.

IIRC, he thought collapse was possible but not probable. And if I were him (ew!) I would definitely have anticipated that possibility. So maybe OBL is the “diabolical engineer” the OP was asking about.

[Chronos]

Quoth Suburban Plankton:

And I think it’s worth noting here that even in the case of the very much uncontrolled demolition, those buildings came down pretty much perfectly. That in itself is a testament to the engineering that went into their construction.

If you mean that they fell straight down, rather than toppling over sideways, that’s down to physics, not engineering. Gravity points down, not sideways. When something topples, that requires shear strength, and the amount of shear strength you’d need for something as big as the WTC towers to topple would be immense. In other words, you’d need to insanely over-engineer a building to get it to do anything but fall straight down.

[EdelweissPirate]

I take your point, but I’d argue that the “toppling mode” is closer to cantilever bending than pure shear. Cantilever bending necessarily includes some shear, of course, but the construction of the WTC (nested tubes tied to each other by perpendicular plates) should make it quite strong in bending.

Besides, wind loads are bending loads for a skyscraper, and those are obviously an important criterion in the design process. And even unreinforced masonry structures (like old factory chimneys) topple during demolition all the time. Unreinforced masonry has almost zero strength in tension, so those structures tend to tear themselves apart as they topple, but topple they do.

On the other hand, maybe this is a question of semantics rather than substance.
ETA: I fully agree that the near-vertical collapse of those towers wasn’t the result of “the engineering that went into their construction.”

[Chronos]

Well, yes, skyscrapers do need some amount of whatever-you-call that type of strength in order to resist wind, but the forces from wind that they’re built to resist are much, much smaller than the weight of the building.

[TimfromNapa]

@Exapno_Mapcase

Isn’t this exactly the plot of Star Wars? That’s how the Death Star was blown up.

There’s no refutation possible for this nonsense. There’s never any one place to strike that will send giant complexes to their doom. It’s idiotic fantasy. It can’t be refuted because there is no substance there to refute.

I think that story goes back even further. Think Achilles and his famous heel. True, he was not an inanimate structure, but the idea of something great being brought down by some tiny, exploitable, flaw is a common thread.

[Just_Asking_Questions]

@Chronos

In other words, you’d need to insanely over-engineer a building to get it to do anything but fall straight down.

That’s not true. There have been several multi-story buildings that have topples like toys. Nothing as big as the WTC, but it has happened.

[Melbourne]

@Just_Asking_Questions

That’s not true. There have been several multi-story buildings that have topples like toys. Nothing as big as the WTC, but it has happened.

Foundation failures. Not structural failures.

[am77494]

@Chronos

Well, yes, skyscrapers do need some amount of whatever-you-call that type of strength in order to resist wind, but the forces from wind that they’re built to resist are much, much smaller than the weight of the building.

Here is my take on this.

A tall building maybe visualized as an inverted pendulum and yes it is in stable equilibrium but it will have modes of vibration / oscillations. Structural engineers indeed calculate the natural frequencies of vibrations and keep them high enough.
The problem with wind is that it creates eddies which results in vibrations. Similar vibrations are created by seismic loadings (earthquakes) too.

Steel is susceptible to fatigue (When you bend a steel clip a few times, it breaks ). So structural engineers try to avoid that. This mode of failure is different than steel failing in high static load.

So if the wind design is incorrect, the building may sway side to side and then “topple” over similar to bridges which are well known for similar resonant failures. Using topple over in the loosest or sense.

[MarvinKitFox]

@Leo_Bloom

2. So, could anyone supply a back-or-the-envelope sketch of such a diabolical plan using conventional modern techniques to bring the WTC down? Keep it general enough so you don’t blow your cover to the FBI.

The only way to do this is to liberally apply fairy dust to your unicorns, allowing them to rapidly drill holed that you then fill with troll dung.

As everyone knows, troll dung burns much hotter than jet fuel.

[CurtC]

To answer the OP’s question, what if this diabolical person installed demolition charges that simultaneously blew all the supports for one of the floors? The floor trusses were connected at each end to the perimeter columns and to the core columns. If all those were removed at once, that floor would come crashing down onto the floor below it. Would the floor below it be able to support the falling weight of the floor above it? And if not, then the one two floors down would soon have the weight of two floors falling onto it.

Now the actual mode of failure was different - the perimeter columns were bent and failed, allowing the entire top of the building to fall onto the part below. But would the scenario above be possible?

[md2000]

In fact, you can see in videos the one tower (or the section above the failure) does start to tilt but then straightens out a bit. Remember, the towers were essentially held up by the perimeter columns of thick steel. Once the columns around the fire gave way, the building started to drop and this momentum carried it down, one floor at a time.

We can speculate why the fall levelled out, but my non-engineer guess is that the columns failed, floor by floor, when the floor beams that prevented the columns from buckling were disconnected by the impact from above. So, the top tilts a bit, the taller side has more disconnected column exposure - i.e. the perimeter columns on the taller side are more susceptible to buckling and so give easier, having two or three extra stories of unbraced column, thus levelling out the top piece. Straight down is the path of least resistance.

The basic problem with any conspiracy theory is that the buildings visibly collapsed starting at the point where the planes hit, and then continued due to momentum. They supports were designed to handle a static load, not a pounding with 10 or more floors of weight and momentum. Each floor failed in succession, then the collision with the ground pancaked the top section.

Similarly, the WTC 7 had large diesel tanks in the middle where there were generators to maintain power for data facilities. Falling debris set these off and also gouged out the lobby, the most marginal structural element of the building. That building finally failed from the central south lobby support, pulling the sides in with it. (There is video footage of thick smoke pouring from 7’s windows)

There a re several documentaries illustrating the process.

The problem is - most engineered building collapses involve taking out the bottom supports and letting the building pancake, since those are the supports with the most weight, most likely to fail - the twin towers visibly did not pancake from the ground level but from the fire. Engineering building implosions is wrapped in safety after safety margins. One misfire could topple the building into the wrong place. It’s certainly not something planned and executed coordinated with a world-class level of distraction like a dual plane crash.

Plus, if you watch the French documentary about the firefighters, where they were filming in the lobby… they graciously did not show bodies plunging or the people coming off the elevators with massive burns. It appears flaming fuel poured down the elevator shafts at first; that’s predictable, not the environment any explosive expert would choose to work in.

I don’t know if bin Laden expected the buildings to fall or not. Imagine if they did not; still, hundreds possibly thousands dead, and then massive and probably unstable hulks looming over the business district for months with no easy means to demolish them safely. It was probably a convenience for the cleanup crew that the buildings actually came down.

[ftg]

@md2000

In fact, you can see in videos the one tower (or the section above the failure) does start to tilt but then straightens out a bit.

When I noticed the tilt is when I turned off the TV. I knew that the people trapped in the floors above were doomed and the buildings were coming down. No need for further magic explosives.

(I had just turned on the TV to the local morning show when they started to announce they were going to the network feed for a breaking story. I thought at first the initial incident might have been an accident. Later I heard “There’s another plane.”, looked up, and OMG. Given the wobbling around the plane was doing it’s ridiculous to claim that the plane deliberately hit where the magic explosives were.)

[md2000]

@CurtC

To answer the OP’s question, what if this diabolical person installed demolition charges that simultaneously blew all the supports for one of the floors? The floor trusses were connected at each end to the perimeter columns and to the core columns. If all those were removed at once, that floor would come crashing down onto the floor below it. Would the floor below it be able to support the falling weight of the floor above it? And if not, then the one two floors down would soon have the weight of two floors falling onto it.

Now the actual mode of failure was different - the perimeter columns were bent and failed, allowing the entire top of the building to fall onto the part below. But would the scenario above be possible?

As I recall there isn’t much in the core. One group of survivors recounted escaping a stalled elevator by hacking through the wall of the shaft - only double layers of drywall - using a window washer’s tool and ending up in the men’s washroom.

If you took out the floor trusses, all they do is brace the exterior vertical columns from buckling. It seems unlikely - based on the towers surviving the initial damage done by the crashes - that simply removing one floor’s worth of trusses would let the colums buckle. Even removing much of a floor and the supports on much of one wall - was not enough to collapse the building. It appears to have been seriously over-engineered.

[CurtC]

@md2000

As I recall there isn’t much in the core. One group of survivors recounted escaping a stalled elevator by hacking through the wall of the shaft - only double layers of drywall - using a window washer’s tool and ending up in the men’s washroom.

The core had massive steel columns. In-between those columns was drywall that the people hacked through, but the core itself was incredibly beefy. In fact, after each tower collapsed, there was a significant height of core sticking up after the rest had collapsed around them. Then the core fell because it relied on the perimeter columns and floor joists to keep it vertical. But I think it was one of those late-falling giant sections of core that hit WTC7 and put a huge gouge down its face.

If you took out the floor trusses, all they do is brace the exterior vertical columns from buckling. It seems unlikely - based on the towers surviving the initial damage done by the crashes - that simply removing one floor’s worth of trusses would let the colums buckle.

I wasn’t asking whether the perimeter columns would buckle if you removed one floor - I was asking, if one floor was suddenly cut loose and in free-fall, if the floor below it would be able to withstand the impact, or whether the floor below would also fail, and then the floor two levels down would suddenly be hit with two floors’ worth of debris, which would basically be an avalanche.

I was going with the spirit of the OP and imagining a scenario where the towers could have been rigged to fall with the least effort.