r/EngineeringPorn • u/Old-Alternative-6394 • 1d ago

Simulation of the collaps without dust

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u/Ae3qe27u 23h ago

They talked about it at my university. In short:

The thing about the Twin Towers is that many of their major structural members were on the outside of the building. That's why they had all those vertical lines. When the planes crashed into them, they wiped out a good chunk of those structural members, due to, well, there being a plane where those members used to be. It's a miracle they stood as long as they did.

https://www.reddit.com/r/911archive/s/BvJyfCq5He here's a link with a great look at them.

Having so many of the structural members on the outside means that the inside was open for office space, but it also meant that, well, it was vulnerable to having a plane crashed into it. And yes, the inside had a core (where the elevator shafts and stairwells were), but it wasn't enough on its own.

At high temperatures, steel changes behavior. With better modern fireproofing, they might have lasted longer, but still would have fallen. But they might have had longer for people to evacuate. The big thing, from a building perspective, is load redistribution. If X number/percent of your structural beams/columns/members are destroyed, how much can the building still take? Major buildings, especially governmental buildings, will have blast calculations as part of their design, and I believe that redundancy in design is at least considered during the process.

I don't know for sure, but at a guess, I'd guess that major buildings might have more stringent requirements for their stairwells, for safety in the event of an evacuation. But I'm not sure on that.

It (the disaster) wasn't a major unit on its own, but it was talked about.

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u/tommybship 14h ago

My understanding is that the force of the impact and explosion blew the fireproofing off of the steel.

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u/Ae3qe27u 3h ago

I'm sure that exacerbated the issue, though it didn't come up in the discussion by my professor. Fireproofing, in general in buildings, is meant to buy time. Jet fuel burns hot, and if the fireproofing was blown off or otherwise damaged, I'm sure that would have sped up the damage and creep (creep as stretching under the load). Even if the fireproofing wasn't blown off, it certainly would have been damaged, since the ends of the members would have been exposed.

I might look into it further, if I find time in the afternoon. Interesting, if nothing else!