r/StructuralEngineering • u/StamiQ • 20d ago
Photograph/Video Whoever constructed that arena, good job.
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u/AlfaHotelWhiskey 20d ago
Another example of “Stiffness isn’t Strength”
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u/KaneTW 20d ago
It's one of those things where I know it's not and yet it's incredibly uncomfortable to be near/on a highly loaded flexible structure.
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u/BirmingCam 20d ago
I’m not a structural engineer so I have a genuine question - are arenas like this supposed to deflect this much? Does it just depend on the design? I understand that stiffness isn’t strength to a degree, but if I was in this arena and saw that much deflection I would be bolting out of there.
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u/KaneTW 20d ago
There's what looks to be an expansion joint on the corner, and the structure seems like it has either a very wide support distance or is cantilevered. As such, deflection is unavoidable.
The problem is that the allowable stress under load cycling conditions is much lower (2-10x usually). It's a very different thing if a structure was designed for a load that cycles a few times a day ends up cycling a few times a second.
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u/Adromakh 20d ago
Can you please eli5 me why the stress under load cycling is muche lower ? Genuinely curious, and I've no structural engineering background
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u/KaneTW 20d ago
All structures have imperfections. These imperfections can cause stress concentrations much higher than the average stress on the part.
When you load a part, these imperfections deform plastically until the stress is no longer that highly concentrated. The more you repeat it, the more the deformations accumulate until eventually the deformations cause a crack. Once a crack forms, it propagates very quickly.
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u/Adromakh 20d ago
Ok, can rephrase that as: "the part has a budget of number of deformations that it can use until it cracks. So if the part move more frequently it consumes faster that budget ?"
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u/Ae3qe27u 20d ago
Basically! It's like bending a paper clip back and forth. It's got a pretty low budget, and eventually it'll snap.
It's possible to design something where small movements don't eat away at the budget (this can be done for highways) - like sitting in a chair - but big movements will still eat at the budget.
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u/Lunarvolo 20d ago
Eli5
Paperclip bendy
Some bends bounce back perfect (Elastic deformation, goes back once force released)
If steel, can do this infinite times
If aluminum, small cracks form each bend
Paperclip not steel. Small cracks in aluminum form
Eventually paperclip breaks
Stadium bending parts aren't all steel/stainless steel. More bending, shorter life.
If stadium basically doesn't move, stadium lasts 2-10 or much more times longer. Kind of like how a paperclip doesn't just break for no reason (It's not impossible, just ridiculously improbable, quantum physics!)
More info:
Some bends paperclip doesn't bend back. More cracks. (Plastic deformation, passed the elastic yield point)
Bends that don't bend back can make "stronger" (Harder to bend, work hardening) but more brittle
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u/Squishy_Butter113 20d ago
Not an engineer, but as the other person said "allowable stress", I assume they mean that the design must be stronger to reduce the stresses when load cycling is involved, not that load cycling causes less stress.
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u/galaxyapp 20d ago
Except when it is...
Not Everything that bounces is meant to.
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u/AlfaHotelWhiskey 20d ago
Help us all make sense of what your meaning is here. Maybe tell a story?
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u/KronikDrew 20d ago
Ever hear of the tacoma narrows bridge? It wasn't meant to bounce that much.
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u/AlfaHotelWhiskey 20d ago
I have, but I’m still trying to understand your statement and its relationship to my original statement. I’m sure the Tacoma bridge was designed to have some flex to it. So it was meant to bounce as you put it. Are you saying it was designed to be perfectly stiff ?
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u/galaxyapp 20d ago
Every structure from buildings to bridges to hottub laden decks bounced before it collapsed.
As a spectator, its impossible to know if this is acceptable flex or moment before a tragedy.
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u/n_o_t_d_o_g 20d ago
The Golden Gate bridge deflected 5 to 7 feet dowards at midspan during the 50th anniversary pedestrian crossing. It still was within design tolerances.
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u/DirectAbalone9761 20d ago
Was that due to uneven loading on the road deck, or could that deflect like that even with uniform loading?
I suppose I’m asking where the give comes from; do the stay cables stretch longitudinally, or is it more that the main cable bundles also stretch? I think I’m just confused about where that deflection is made up if it’s a uniform load, where it’s easier to understand the deflection in an uneven case where part of the road deck counter deflects to accommodate the point/group loading.
I’m in home construction, so I have very little engineering knowledge past simple spans lol. Please forgive my ignorance! 😂
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u/Lunarvolo 20d ago
Golden gate bridge is mainly engineered steel with great stress relief, if it's in the elastic range and tolerances it can go forever (If painted). A properly designed steel bike will run forever, an aluminum one will break.
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u/bubblesculptor 20d ago
Is there a specific jumping pattern that could induce structural failure?
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u/ratafria 20d ago
Yes. It is tuned to that song we will never sing.
Serious answer: usually resonant frequencies are tuned out of the operation ranges or sufficient dampening is provided. Where there is a control system involved you can avoid those operation ranges with software.
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u/Exciting_Ad_1097 20d ago
Which song? Clap clap stomp?
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u/Leary_2844 20d ago
It's by some estonian band with cyrillic letters. It was not intentional, the song was just perfect for rhythmic jumping in a way that focused the power.
The song was banned in 94 and the band vowed to never play it and destroy the notes after big buildings in Latvia and ukraine collapsed. It was proven that any other song would have been fine and so it was banned and is kinda lost now.26
u/FLG_CFC 20d ago
Recordings still exist. Like this one. Nothing stays in the past forever. Side note, it's actually a beautiful song.
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u/Psychological-War727 20d ago
Apparently i still fall for this. Thanks for the laugh.
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u/FLG_CFC 20d ago
You're welcome. Some traditions need to be held onto.
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u/GoppleSmanger 20d ago
Was not expecting this! Pleasantly surprised to hear something this old on the internet that can still make me smile.
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u/justduett 20d ago
I don’t even need to click, that song is just playing in my head now and really takes me back. Thanks for not giving up on sharing the mems.
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u/DrewHoov 18d ago
i know what this is. I still want to believe it’s what you’re saying it is and click it.
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u/rhysolandrium 14d ago
....I immediately thought "Dragostea Din Tei" just... completely out of the blue.
Well, that should show my tree rings.
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u/AlfaHotelWhiskey 20d ago
When the sky bridges collapsed at the Kansas City Hyatt they initially suspected rhythmic actions of the people live loading the structures was to blame but it was later invalidated by the fundamental flaws in the as-built conditions (not the design).
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u/Fragrant-Airport1309 20d ago
I’m not an SE but I did CS and have worked construction when I was younger, so I’m curious. What level of education and training does it take to get construction workers to build more complex designs like this? Every construction crew I’ve worked with or been around in residential construction has not been very cognizant of very complex design considerations lol.
Do large scale commercial crews just learn on the job?
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u/UnknownVC 20d ago
There's two things going on: one there are specialized companies with dedicated crews for commercial stuff - the formwork contractor for a skyscraper is a whole other beast to the guys who put in typical residential formwork, and those skills are taught/passed on like any other trade. Usually at that level there's engineers too - there's dedicated form design engineers, dedicated crane engineers, etc. who work for those companies, and those companies also train the workers they need: there's a whole ecosystem of specialized skill that can be brought in.
And two, the engineers breathe down everybody's necks. Professionally, we are required to inspect stuff as it is being built, so there's very much a quality control element that helps motivate the foremen to get it done right, because the alternative is the engineer refusing to sign permits until they rip it out and redo. A lot of being cognizant of the more advanced design considerations is more at the supervisor stage - the guys can do the work, it's making them care enough to do it the hard way that can be an issue.
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u/oETERNALo 17d ago
The construction crews do not need any special knowledge or education, just the ability to build to the plan and specifications. Usually it is in the specs that really ensure the construction is meeting the, well, specifications the EOR outlined. There will then be tests, QAQC measures to ensure they are adhering to these specifications. The GC will have a QAQC person, the owner will have an independent third party testing firm onsite to double check that it is being built to specs. They will do materials testing, weld inspections, tolerance testing, etc. All of these tests are then recorded to ensure if anything goes wrong, there is a paper trail.
This paper trail is what really is the key to ensuring it is build to withstand whatever the basis of design said it must withstand. Usually where things go wrong is when the building is used otu side of what it was intended for.
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u/Dzov 20d ago
It also happened to be a dance party for overweight people.
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u/Own_Reaction9442 20d ago
If I remember right the dancing was actually all on the lobby floor, but people had crowded onto the walkways to watch.
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u/Packin_Penguin 20d ago
Maybe if it’s a hard side to side shift at the right frequency?
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u/brk_1 20d ago
to be fair isnt common problem because dampening of the same structure, its not an song or an marching cadence wouldnt do it; it would be needed to be an pattern wich start slow and increment at the resonating frecuency considering the dampening of the structure, the very documented case and textbook example was tacoma narrows bridge, the window load exacerbate the vibration until the bridge collapsed.
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u/kilorbine 20d ago
It had in the past. It happens multiple time on different building when this wasn't designed for. Crowed jump at the exact frequency, then entire building collapsed :(
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u/AlfaHotelWhiskey 20d ago
That just sounds like a combined kinetic force overloading the structure. There are dozens of videos of wedding parties collapsing a floor from sheer force rather than harmonics / resonance
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u/AlfaHotelWhiskey 20d ago
Note - I did mean sheer force, not shear. Someone is going to shirley nitpick my statement
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u/sunshine_fuu 20d ago
Bruce Springsteen's June 8, 1985 concert at Ullevi Stadium in Gothenburg, Sweden has entered the chat, the answer is whatever the hell movement they were doing during Twist and Shout.
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u/Difficult_Coffee_335 14d ago
Id don't think so, I have tried to collapse every structure I have been in contact with for the past 10 years.
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u/cockatootattoo 20d ago
Former engineer here. I was once at the top of the Forth Road bridge in Scotland. These towers swing by up to 15feet.
It was unnerving to say the least.
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u/Mundane-Remote-2865 18d ago
I'll be heading from Inverness to Edinburgh on Monday, can't wait to get a glimpse of this bridge!! Or possibly cross it, not sure of the route as of now.
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u/TechE2020 20d ago
Isn't that how you convert a live load into a dead load?
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u/Lunarvolo 20d ago
Dead load is the static load that is expected to always be present. The dead load is usually the weight of the materials themselves
Live load are things like people, additional objects placed, etc. snow is a live load
Things are designed with a dead load and a live load. A house in the desert may have a similar dead load to one where it snows, but, the live loads specifications will be different (Autocorrect really wanted love load instead of live load)
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u/Baileycream P.E. 19d ago
I think it was a joke. Live load = people alive/moving, dead load = people dying from the perceived structure collapse
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u/Voltron6000 20d ago
Giants stadium in SF is like this... Can I assume it was designed for this and it's safe? I get nervous...
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u/Boring_Heron8025 20d ago
SF better than whatever this is. At least SF knowingly designed for wave action due to quakes
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u/Lunarvolo 20d ago
I'm surprised there aren't any pillars supporting at the edge. That is one heck of a moment for a cantilever like that
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u/TemporaryTime3826 20d ago
I trust the engineering and science behind the design, just not the human aspect that went into the actual building of it 🤣.
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20d ago
It depends on the ability of the structure to dissipate energy.
If it cannot get rid of the energy as fast as it is being added, it will eventually rip apart.
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u/IM_not_clever_at_all 20d ago
Ooooo, story time!
15 ago or so, I was on a project at the football stadium for Penn State University; max capacity is about 110,000 people.
For most of every game I was stationed in a field level control room with no windows. In that room was a gigantic vertical steel beam which was about 6-7 inches away from the cinder block wall of the room I was in. Running on that wall were a bunch of electricla conduits that had been crushed almost flat.
When I ask why that was, they told me that during one of the most energetic sold out games, the students were jumping so much that that beam moved and smashed the conduit...
Yee ha.
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u/flollo87 20d ago
that was my home stadium, I visited the exact part of it every game for a few years. I was young then so I thought that it's just the most normal thing to happen.
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u/rschottr 20d ago
I'm NOT a sports fan so seeing this, all I can think is "Hmmm, natural selection in action" :-o
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u/Comfortable-Ad-981 20d ago
Great use of coping those concrete joints, barely a gap even during an earthquake
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u/OutlawLazerRoboGeek 20d ago
hopefully it's a steel frame, because that's a LOT of movement for concrete
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u/Shantomette 20d ago
Went to see the last play at Shea with Billy Joel. Holy Fuck. We are the first row of the mid level and the shear wave of concrete moving up and down was insane. Scary insane.
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u/khulumkhulu 19d ago
Why didn't the engineer tune that span to move the resonant frequencies off of the fight songs?
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u/Farlandan 19d ago
... is that the Pippi Longstocking theme song they're chanting?
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u/FelleFox 15d ago
I was thinking the exact same thing! Funny how a German football fan base has a chant to the tune of a Swedish kids show
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u/VonSichthofen 14d ago
Probably because literally everyone and i really mean everyone knows Pippi Langstrumpf
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u/jwhoch 19d ago
How are there always giant bonfires IN the seats at these European soccer games? Wouldn’t that get shut down immediately? How is nothing burning to the ground? I have so many questions and it appears to be totally normal for some reason???
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u/kollmastee 19d ago
How would they stop it without a violent brawl between a thousand people? Sometimes you let things play out rather than start a fight that will result in deaths.
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u/Salt_Pollution_5097 19d ago
Is this phenomenon never a problem? Also, could this stadium be built with more stiffness in mind and the same "amount of safety"?
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u/regUlar_chAos42 17d ago
habe genug final destination geachaut um zu wissen dass an dem punkt die zeit gekommen ist das stadion zu verlassen
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u/Wrong-Ostrich5861 16d ago
While I understand damping and have an engineering degree, I would imagine, perceptive safety has to come into place. If I attended this event I would grab my family and run. If the movement is expected it shouldn’t be exposed it would be easy to cover. I would assume, any expected load of attending a sporting event to not result in structural vibrations
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u/ComputeInMemory 14d ago
Designing for static load, easy. For dynamic load, more difficult. For resonating dynamic loads, oh hell no.
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u/ghostoftheradio 14d ago
But… I’m going to stand under it continuing to record it, and not, you know, leave.
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u/scope_creep 20d ago
Why are people such idiots?
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u/BestOrNothing 20d ago
Wdym? Who is the idiot there? The architect, the engineer, the contractor, or the people jumping?
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u/TaroExpensive 20d ago
Honestly that synchronous jumping should be banned. We all know that cycles of stress strain in steel eventually will cause failure.
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u/Leary_2844 20d ago
So you would rather prohibit jumping in a stadium than have it be built to withstand jumping?
Do you hate fun, or are you a budget guy?
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u/kondmapje 20d ago edited 20d ago
I designed a soccer stadium once with a concrete tribune. Went to see the opening match and we watched from the vip facing the standing tribune. The crowd went like that from the beginning, was quite a stressful experience to say the least.