I recently had dinner with an old friend that I thought was fairly level-headed. But he is convinced that there was something funny about 9/11 on the basis of how the towers collapsed on themselves rather than topple over. He said it looked ljust the collapse of the old Kingdome that was brought down by a controlled explosion. I told him that I thought it was reasonable that it collapse but he found that incredible. So I ask how are buildings like the towers expected to fall?
Really, how would you expect them to fall? They were built plumb - So all the pieces only experience a force straight down. If the damage had occurred at the base of the buildings, one might expect it to topple, but the damage was at the top, so all the debris fell straight down, as one would expect.
Also, the buildings are rigidly attached at the bottom - they aren’t like building blocks resting on the ground, they are more like a post cemented into the earth. They were designed to withstand hurricane force winds without falling.
I can’t say I disagree with your friend: the outward visual appearance of the WTC collapses certainly did look like a controlled demolition event. I’ll leave the conspiracy theories for the GD forum, though.
There are a number of still photos and videos of skyscraper collapses around the world. In many of them the building ends up lying on its side, more or less intact. Two things I think contribute to that outcome:
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a traditional steel structure, with load-bearing members more or less uniformly distributed throughout the building.
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a compromise of the foundation due to earthquake, settling, or some other issue, which leaves the main structure of the building intact/strong.
From what I’ve read, the WTC towers had a unique structural configuration that facilitated the relatively clean straight-down pancake-collapse that happened on 9/11. Rather than vertical load-bearing members distributed throughout the cross section of the building, they had vertical load-bearing members at the perimeter and at the core, and horizontal “bridge” members connecting the core to the perimeter to form each floor of the building. The configuration helped to constrain any lateral deviation as the debris fell down.
Moreover, the WTC collapses started near the top (at the impact points), allowing the deadweight of the upper parts to take out the floors below them, one by one. Note that for the south tower, the upper portion (above the impact area) does in fact start to tumble on its way down. Not at all a pure-vertical descent. It’s only the floors below the impact point that collapse one-by-one.
Plus, as I understand it (though haven’t looked into it at all) what happened was that basically (due to the fire weakening beams) one floor near the top collapsed onto the one below it. The impact caused the lower floor to collapse onto the floor below that one. Repeat until all the floors are in a pile on top of each other. No toppling at all involved, just falling straight down.
Seems plausible to me.
Things in our everyday experience topple because: 1) the center of gravity moves outside of the object, and 2) because the object is rigid enough in every direction that it maintains integrity while falling.
Buildings generally will not meet (2) unless they are massively over-engineered concrete structures. Once the forces are going the wrong way, beams sheer off or buckle and it just falls straight down due to gravity.
I find these explanations convincing, especially the fact that the collapse started from a high floor. I will try to convince my friend, but don’t hold much hope.
I did not want to start a debate over conspiracy theories, BTW, just an engineering answer to my question.
There are also some excellent documentaries which explain the science of the collapses very well. IMHO, this episode of Nova did it excellently.
It’s actually really difficult to get a building to topple over sideways. You can try it yourself on a small scale with toy building blocks: Build as high a tower as you can, and then knock it over any way you want. It might start off toppling, but it can’t hold together well enough to keep on toppling. The debris will still be spread out on the floor some, but not as much as the original height of the tower.
Exactly. Back when I was teaching physics years ago (when I first joined the SDMB), I remember working a problem that demonstrated why a building cannot just topple over. IIRC, it had to do with the fact that the angular acceleration on the building necessarily increased as you considered a higher and higher point on the building (because the top of the building has a great deal farther to move in an arc than a point near the bottom). At some point, this angular acceleration exceeds the rigidity of the structure and the structure breaks into two or more pieces.
The collapse and how it happened seems totally reasonable to me. “Came straight down” as some claim is a bit misleading also. The difference between footprint of the building vs. how scattered the fall was is huge. It just appears to be small because the building was so high.
The weight of the buidling was so imense and the only thing acting on it was gravity. Even if a portion began to break away and “tip” over it would only get so far away before gravity pulled it straight down. And a building is not a solid unit like a tree or a steel beam. You can’t take a building and tip it 10 degrees and lean up against something else. Pieces snap and fail and once they do that gravity takes over.
Ironically, I finished reading Why Buildings Stand Upby Mario Salvadori on Sept. 10, 2001. Perhaps reading it will help you convince your friend. Or maybe he should read it. Because the way the towers collapsed made perfect sense to me.
Hey, look what I found. See chapter 2 on Loads.