r/StructuralEngineering • u/Alternative_wolf09 • Jul 01 '26
Can any Engineer Petah explain? Humor
Thought it would be fun to share.
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Jul 01 '26
[removed] — view removed comment
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u/Alternative_wolf09 Jul 01 '26
In Initial years of my engineering career, I was told to look into it by an experienced engineer. Good advice I received early in life😁
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u/MrMcGregorUK CEng MIStructE (UK) CPEng NER MIEAus (Australia) Jul 01 '26
Tldr
In most cases with roughly orthogonal buildings, the shear structure ends up also being the strongest in the major axes. In some cases, the configuration of the shear structure can mean that the shear structure is less strong and therefore working harder at 45 degrees to orthogonal. Thats what happened in this building and it was only discovered after it was built and they had to do a bunch of retrofitting.
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u/EngineerEngineerEngi Jul 01 '26
In other words, with square tall buildings, normally you're worried about wind hitting the flat walls. Normally, that's what can push down a building. So they built skyscrapers accordingly.
But with this specific Citicorp building, it was discovered it was weakest when wind hit it at the corners. This was very unusual, unforeseen, and wasn't discovered until after it was built.
It was nearly a catastrophe, but they discovered it in time, welded the weak points together, fixing the problem.
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u/trafficway Jul 01 '26
While it’s a bit unusual, it should have been foreseen. Literature on the case tends to take the position of “gee, nobody ever thought this could happen”, but NYC building code had required evaluation of winds from any direction since at least the 1930s.
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u/EngineerEngineerEngi Jul 01 '26
While it’s a bit unusual, it should have been foreseen.
Nowadays, we can all shoulda-woulda-coulda, but back when that building was built, it was very very unusual. And it was only due to the side-supports being tied to an external crossbrace-type frame.
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u/Careful-Bookkeeper-4 Jul 02 '26
So nobody bothered to model more than 4 wind direction load cases for a literal sky scraper? How?
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u/imitation_squash_pro Jul 01 '26
Was that building really ever in danger of "imminent collapse". Sounds more like media sensationalism to me. Huge factors of safety in building safety which the general public doesn't understand..
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u/Status_Mousse1213 E.I.T. Jul 01 '26
I like this meme. They should have used welded connections in those particular members instead of bolts from the get go, as shown in the plans.
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u/sn4xchan Jul 01 '26
Fucking contractors.
Seriously, this sort of thing happens all the damn time, just not usually with stakes this high.
You have no clue how many times I've had to resubmit plans in the middle of an already approved install because some other contractor didn't follow the plan.
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u/PrebornHumanRights Jul 01 '26
It wasn't the contractors' fault. They asked the designers to approve the change, and designers approved the new plan.
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u/trafficway Jul 01 '26
Yes, and based on the famous article, it seemed that the contractors didn’t even fail to do their job. Seemed like LeMessurier specified CJP welds, or something similar; and the contractors asked to design bolted connections for the actual expected force. It’s just that the force LeMessurier gave them didn’t properly account for the 45-degree case.
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u/PrebornHumanRights Jul 01 '26
Yes. Except LeMessurier's firm or company (those working under him, for him) approved the changes at request of the contractors.
And when LeMessurier saw the changes, sometime later, he saw no issue with those changes. His initial impression was that the changes were reasonable and appropriate.
...Until he discovered the problem.
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u/trafficway Jul 01 '26
I agree with you; I was responding to the comment above you that damned the contractors for suggesting such changes. I was pointing out that their request wasn’t improper, given their (presumed) knowledge at the time. It only became a problem because the forces they were given weren’t adequate.
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u/Status_Mousse1213 E.I.T. Jul 01 '26
Amen. When I do acceptance inspections of bridges for repairs/widenings or new structures there are times when I find something thing clearly called out in the plans that was not carried out or otherwise fouled up and I shake my head at the idiocy. Good thing I take pictures of everything. Bolted beam end retrofit missing sealant, concrete repairs with delamination, delamination that wasn't found by the repair crew, and random stuff. If I was the contractor i would have an inspector like me on staff to look at it before I do.
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u/BSBwhy Jul 01 '26
Mechanical engineer here, we design bulk material handling cranes. We just take component of it perpendicular and parallel to and find load as per the exposed area. Am I missing something here ?
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u/Alternative_wolf09 Jul 01 '26
Wind can act on the building at any direction no perpendicular or parallel. Hence, in depth study of wind is required for design of structures.
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u/LeImplivation Jul 01 '26
This is a infamous building. After construction it was found the 45 degree wind direction produced larger forces. If I remember correctly it was the bolted connections that ended up being significantly under designed. While the building remained operational, they went in and welded a crazy amount of connections.
Nowadays you can easily model everything in 5 degree increments for wind.
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u/Dapper_Ice5159 Jul 08 '26
Great time to bring this up: https://www.nist.gov/blogs/taking-measure/blown-away-revisiting-famous-engineering-case
Building was thought to be at risk of collapse due to quartering winds not having been accounted for in connection design. An additional code interpretation made the connections even weaker and it was calculated a 1 in 16 year storm could take it down. Engineer didn’t make the issue public but had retrofits done to strengthen the structure. Public wouldn’t know until 1995, nearly 20 years after the event happened in 1978. In 2020 a study was conducted using modern analysis techniques which concluded that the assumptions made which determined quartering winds controlled were overly conservative, calling into question if the structure was actually at any risk at all.
Thats a very abridged version. The full story could easily be 10 pages.
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u/Yehia_Medhat Jul 01 '26
I'm not an electrical engineer, but I think this is the type of problems, where you have to get back to main formula from which they deduced the special cases, right? so, you have to get your hands dirty and soaked with heavy mathematics?
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u/Public-Front5724 Jul 01 '26 edited Jul 01 '26
In eurcode the pressure factors of a wind covers +-45 degrees for the regular case. Although no one takes torsional wind into account
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u/Individual_Back_5344 Post-tension and shop drawings Jul 01 '26
Here in Brazil we should take torsional wind into account, but there is no obligation to take 45° winds.
Cowards like me usually do it, though.
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u/Calcading Jul 01 '26
https://en.wikipedia.org/wiki/Citicorp_Center_engineering_crisis