9/11 Structural Engineer's poll 19

[WARose]

A former colleague of mine who is a structural engineer (now retired and with apparently too much time on his hands ) discusses various conspiracy theories on the net. One of them is the 9/11 theory that holds that the buildings that fell on that day did so due to a controlled demolition. (A theory he feels is “nonsense”.)

You've probably heard followers of this theory called “Truthers” (or worse). In any case, aside from telling my friend that he is/was apparently a member of the CIA (and trying to say he wasn't even a structural engineer), apparently another angle of argument is to say that the profession is somehow afraid to talk about it (noting a lack of support in journals, etc) or that my friend somehow doesn't know what the profession talks about. That got me wondering how much this is discussed beyond my (and his) experience. So the question I have as sort of a poll for structural engineers:

Is the controlled demo theory something that is a regular topic of conversation with your colleagues?

AND (if the answer to the above question is “yes”) Do you feel compelled to view it one way or the other?

Speaking for myself, I cannot think of a single time it has come up (except in conversations with my retired friend mentioned earlier). I've certainly discussed the failures on that day with other structural engineers as an example of progressive collapse. But not much beyond that.

However, public awareness on the topic (I would call it misinformation) seems to have done nothing but grow over the years......and will likely grow more in the years to come.....so I thought I'd ask fellow structural engineers for their thoughts and ideas as to be better equipped to answer the public’s concerns in the future.

 

[WARose]

[blue]BadgerPE[/blue]
I have never been a believer of the controlled demo argument, but I paused a bit with this theory because I never considered that possibility before.

It is somewhat intriguing when you first start looking at it (i.e. why it failed). In my case, it came up right at a time when I was working on several dynamic loading problems.

 

[rb1957]

is it at all "odd" that it pancaked so nicely ? wouldn't one side of the building (obviously) receive more damage than the other ? wouldn't the fuel fire be concentrated on that side too ? maybe the 767s penetrated deep into the building, and so distributed the damage. if so that was a lucky day for us; can you imagine the additional damage if the towers had toppled ?

another day in paradise, or is paradise one day closer ?

 

[MacGruber22]

rb1957: your answer "may" be in here somewhere...Link



"It is imperative Cunth doesn't get his hands on those codes."

 

[IRstuff]

Most videos show that the first tower to collapse had its upper section severely tilted. However, I suspect the floorings tended to collapse vertically. One probable error the terrorists made was that they impacted the buildings too high on the structure; in both cases, the planes impacted in the middle of the upper half of the buildings. Had they come in from a different direction, they could hit below the 40th floors, which probably would have resulted in a toppling. I think we dodged a much worst catastrophe that could have happened.

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[SwinnyGG]

The alternative to that is that 40+ stories lower, the structure of those buildings was much more robust and the fires would have been easier to fight... so perhaps the collapse would have been avoided entirely.

 

[hokie66]

jgKRI,
The tower which collapsed first was the second to be impacted, and at a lower level. The additional load of the building above accelerated the collapse.

 

[IRstuff]

"so perhaps the collapse would have been avoided entirely"

rumor had it that Bin Laden had some structural engineering, so may be those impact points were specifically chosen...

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[SwinnyGG]

The tower which collapsed first was the second to be impacted, and at a lower level. The additional load of the building above accelerated the collapse.

I don't know enough about the buildings (and I'm also not an SE!) to state either way what could or would have happened had a plane impacted that much lower- my post was intended as food for thought only.

My understanding is that a major contributor to the collapse was how difficult it was for firefighters to work effectively that high- if the fires had been on the 30th floor instead of the 90th floor, it seems to me the fires would have been significantly easier to fight. Firefighters could have possibly used the roofs of adjoining buildings, the floors with active fires would have been easier to reach, etc.

Again, just food for though. I'm not an expert and don't claim to be. It is certainly possible that a lower impact would have caused the building(s) to topple which could have caused much more damage.

 

[IRstuff]

Fuel fires require CO2, halon, or chemicals to put out.

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[Lomarandil]

IR, I've studied a lot of structural engineering.

I wouldn't have the slightest idea where to select impact points for maximum effect-- at least not without drawings and a lot of time.

I'd imagine that the instructions given were incredibly rudimentary.

 

[SwinnyGG]

I wouldn't have the slightest idea where to select impact points for maximum effect-- at least not without drawings and a lot of time.

There's also the simple fact that flying a jet airliner at 500 mph is not a precision act.

If you instructed someone to hit a building at a specific floor, to do so in the real world would be very difficult.

 

[hokie66]

Especially in a place like Manhattan, with lots of other tall buildings around.

 

[IRstuff]

But, there are a couple of pathways; one in the exact direction the planes came from, they could have hit considerably lower without hitting anything else.

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[Trenno]

From what I understand, the fire heated up the angle seat brackets that supported the steel floor trusses and thus deformed enough for the trusses to 'fall off.'

Floor pancaking initiated and thus the effective length for the vertical elements became ever longer. Without intermediate restraints (floors) the vertical elements buckled, which initiated the total collapse of the building.

It is interesting however, that it did fall within it's own footprint. I would have imagined that it be very unlikely when global buckling is involved?

 

[IRstuff]

But, it wasn't "global" buckling; it was more like a few floors that gave way. In the first tower to collapse, its upper section did lean over, but it fas falling intact.

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[SwinnyGG]

It is interesting however, that it did fall within it's own footprint.

I see this thought a lot in conspiracy videos and discussion..

Does anyone really think that phrase is accurate? I certainly don't. Buildings as far away as 500+ feet from either tower were damaged by debris, and not just shards of glass; ton-scale sections of the facade, floor sections, and other parts of the structure were thrown that far.

'Within it's own footprint' to me implies that the debris fell entirely within a 208 ft. x 208 ft. square centered on the original foundation. It certainly doesn't fit with the fact that big pieces of the building fell in places that were offset from the building by 35%-40% of its total height.

As a disclaimer, I most certainly do not support claims that there were bombs or other nonsense involved. A 30 story section of that building falling 800 or 900 feet contains a LOT of energy, which had to go somewhere.

 

[IRstuff]

What the CTs are not talking about random debris, but that the bulk of the debris is contained almost completely within the plaza, so there was no toppling. Essentially, the roofs of the two towers fell within the confines of the plaza, as even the adjacent streets were relatively easily cleared for the rescue and salvage operations.

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[WARose]

Good pics IRstuff. I think when conspiracy theorists go on about "falling in its own footprint", it is a clumsy way of noting the fact it fell straight down (even if some of the debris fell outside the footprint).

I guess they thought it should have come down like a tree. (Which would not have made much sense.)

 

[IRstuff]

Well, I have to admit that I was puzzled by the North tower, since its top was leaning over quite a bit, so I had expected some sort of off-axis impulse train that would have caused the north tower to collapse and tilt to the south east, but the rest of the tower pretty much collapsed vertically in reality.

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[hokie66]

There was some tilting of the top of both towers, toward the side which was impacted. The South Tower, 2WTC, collapsed first because although it was impacted last, the damaged area was lower.