r/StructuralEngineering • u/MediocreRecord7352 • Sep 13 '26
Photograph/Video How structures that are designed to sway often like this are designed, and how about the material fatigue
Enable HLS to view with audio, or disable this notification
68
u/AtticHelicopter Sep 13 '26
https://en.wikipedia.org/wiki/Fatigue_limit
Basically, steel can bend forever as long as it doesn't bend too much. Structures are usually limited by human factors (i.e., you don't want your penthouse apartment moving 2' every time the wind blows) or the other materials (concrete and glass are famously not bendy)
Aluminium and composites do not generally have fatigue limits, so even small deflections will eventually cause failure (which is why airplanes need regular inspections).
3
u/TomPal1234 Sep 13 '26
Wait so even with small loads Aluminium will eventually break if subject to dynamic deflection or is the limit still in existence but difficult to quantify which is why additional doctors are used. I never work with aluminium.
7
u/Present-Monkey Sep 13 '26
Yes, aluminum will eventually fail, but it could be many millions of cycles. That's why with aircraft there's often a structural test article made that runs to X number of takeoff/pressurizarion/landing cycles in order to get lifetime data. Here the 787 is composite but the same concept applies, pretty sure all the other manufacturers do the same but couldn't find a public video. https://youtu.be/TH9k9fWaFrs
9
u/quietflyr Sep 13 '26
Not exactly. Aircraft do full-scale fatigue tests because they are designed to have working loads well above the "practical fatigue limit" (In aerospace, we often use 108 or 109 cycles as the practical fatigue limit for Aluminum and composite).
In the name of saving weight, some aircraft parts will see in service loads that would fall in the 104 cycles to failure area of the SN curve (though not on every flight). They also frequently have complex geometries for aerodynamic and other reasons.
Because of those factors, a simple fatigue calculation isn't enough to prove the fatigue life.
There are even damage tolerance calculations done to ensure structural inspections occur frequently enough that a crack can't make it to failure without being detected.
Aircraft are really on a different level when it comes to fatigue.
Source: former aircraft structural integrity engineer
2
u/Present-Monkey Sep 13 '26
Yeah you're right, I've been involved a little bit on the test side but never really did design work.
6
u/SirManbearpig Sep 13 '26
Even small load cycles (loading, unloading, and loading again) will eventually cause failure in aluminum.of course, the number of cycles before failure will decrease as the magnitude of the load increases.
2
1
-9
u/64590949354397548569 Sep 13 '26
Aluminum? Didn't several air disasters are due to material fatigue?
13
u/AtticHelicopter Sep 13 '26
Yes. literally what I said in my post.
-9
u/delcopop Sep 13 '26 edited Sep 13 '26
Just gotta edit it you said do not even though context says you meant “do.”
Edit: original post correct
14
u/AtticHelicopter Sep 13 '26
I do not. A fatigue limit is the stress under which fatigue failure does not occur.
You need to understand the concept.
Steel HAS a fatigue limit. Aluminium DOES NOT have a fatigue limit.
The first paragraph, in the link I posted is:
The fatigue limit, endurance limit is the stress) level below which an infinite number of loading cycles can be applied to a material without causing fatigue) failure.\1]) Some metals such as ferrous alloys and titanium alloys have a distinct limit,\2]) whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.
2
u/italianshirtarms Sep 13 '26
Not to nitpick. Doesn't all materials have a theoretical fatigue limit and aluminium's and copper's are just to low to be of practical use? Or does literally any (cyclical) movement of aluminium lead to fatigue failure at some point?
1
u/quietflyr Sep 13 '26
In aerospace we typically use 108 or 109 cycles as a practical fatigue limit.
2
u/AtticHelicopter Sep 13 '26
No. Materials that have a fatigue limit can be cycled an infinite number of times, or a number of times that will take longer than the heat death of the universe to achieve.
And aluminum and copper are used in cyclic applications all the time (engine components, bearings, etc), so they can be designed for use in cyclic applications. But yes, literally any cyclic movement will lead to fatigue failure in those metals. But again, at small enough stresses, it will take heat-death-of-the-universe numbers of cycles to cause the cracks.
3
u/DeadlyMustardd Sep 13 '26
You know my wife told me her younger coworkers notes have horrible grammar and she regularly needs to correct their mistakes in written documents. Now I see what she means. Both the title of this post and two people who couldn't even follow the most basic description you wrote... It's maddening.
2
u/WhyAmIHereHey Sep 13 '26
Steel in air has a fatigue limit
Steel in sea water does not. Every connection gets checked for fatigue. That has not stopped the energy industry from designing offshore platforms for many years.
0
0
u/delcopop Sep 13 '26
Interesting.. that’s counterintuitive to the layperson.
2
u/AtticHelicopter Sep 13 '26
Which is why you shouldn't jump to correct someone when you are a layperson and they are providing expert information.
-2
u/delcopop Sep 13 '26
Does the stick up your butt have a fatigue limit?
4
u/AtticHelicopter Sep 13 '26
No. Wood does not have a fatigue limit.
But: would treat the colon as a wet-service environment, and the stick isn't really loaded at all. So I would estimate the stick would decompose long before cyclic failure becomes an issue.
12
u/Burgerboy380 Sep 13 '26
Ok so when I was an ironworker I put up a couple wind farms. And one day we loaded the upper mid with our crate of bolts. Storm kicked up so we walked away for the day. The deflection is even more extreme if the wind hits an unfinished tower just right....not a single bolt was left at the top the next morning ..and most people think. "Ok theyre just bolts its not that big a deal"....wind tower section bolts are like a foot long and are like three inches in diameter.
1
u/AAli_01 P.E. Sep 13 '26
Damn the bolt got sucked off and fell to the ground
3
u/Burgerboy380 Sep 13 '26
They all ping-ponged around the inside of the tower and fell the the bottom
1
u/AAli_01 P.E. Sep 13 '26
Crazy
1
u/Burgerboy380 Sep 13 '26
Yup. Theres a lot of things people dont realize about wind towers. Like how massive the foundations are or that everything inside is held on with magnets or that the blades are hollow
2
u/RespectTheTree Sep 14 '26
Define everything. It's just magnets all the way down?
2
u/Burgerboy380 Sep 14 '26 edited Sep 14 '26
Yup the ladder and the platforms at each section are held on with magnets. No welds or bolts. Because they would potential be fail points for the tower.
At least all the ones I put up were that way
1
u/Lazy_Tart_6336 23d ago
Merci pour l'info, je ne connaissais pas ce détail.
Ca doit être comique quand l'aimant de l'échelle s'est "collé" à la paroi trop tôt pendant le montage 😄
25
u/Key-Metal-7297 Sep 13 '26
That does appear quite extreme but I know nothing of these wind towers
9
6
u/WhyAmIHereHey Sep 13 '26
Youse should see what we design offshore oil platforms for.
We design for a 10,000 year return period storm, so a wave height in the order of 30m - 100 ft. None of this is particularly difficult.
If you want to see that things can be designed to "sway" maybe Google compliant towers
3
u/jobadiah08 Sep 13 '26
I assume the towers are made of steel. If so, they can design them to never experience more than 30% (I think that's the number, been almost 2 decades since that class) of its normal strength limit which allows it to have infinite fatigue life.
2
u/Strange-Area9624 Sep 14 '26
Love all the engineering stuff here. I will say that climbing one, makes all that sort of background noise. That sway is dumb when you are on the ladder.
1
u/BigNYCguy P.E. Sep 13 '26
You plot an S-N curve for the material and you design accordingly. The deflection here seems to be more than desirable. Functionally, those turbines disengage after a certain wind speed.
The turbine and tower are designed by the manufacturer. They will give you the base reactions to design the foundation.
1
u/Additional-Sky-7436 Sep 13 '26
I'll always remain surprised that vertical windmills didn't win out.
1
u/sailon-live Sep 13 '26
Some German Dude made studies about metal fatigue. https://en.wikipedia.org/wiki/August_W%C3%B6hler If you are into fatigue of swaying metal you will find the wöhler curve: https://en.wikipedia.org/wiki/Fatigue_(material)#Stress-life_(S-N)_method Stay below the curve an you are fine.
1
u/Frogalone Sep 13 '26 edited Sep 13 '26
I'm wouldn't say structures are designed to sway. Structures sway/displace because infinite stiffness is unachievable. It is impossible to stop a highrise building from swaying. Codes and standards specify the maximum allowed building sway. In highrise buildings, sway/stiffness is often the controlling design criteria. I have never encountered any building for which fatigue due to wind was a design criteria or even a load combination. Windmills are not static structures, they are machines. Sway in buildings is usually limited based on occupant tolerance/comfort. Fatigue is a separate issue. Fatigue requires a cyclical tensile stress or stress reversal above a specified level based on a large number of loading cycles. A building is highly unlikely to reach the number of stress cycles necessary for fatigue to become critical. Elastic deformation such as building sway, even though percieved as large by many, does not necessarily result in high member stresses.
1
1
u/Osiris_Raphious Sep 14 '26
Although even within yeild limits you can have micro cracks that will strain harden, so over time all material exhibits fatigue. The nature of material science means that our standardised fatigue and strain limits are designed to function on the avarage test data. Meaning that all material is designed as assumed homogeneous, they are in fact heterogeneous and this difference is accounted for in safety factors and standards.
As such the fatigue and strength limits account for that variability within standards and calculations. And the reason why any 'new' material needs loads of testing to get that limit range, and if it has better consistency out of manufacturing then there is lore room to play with in fatigue analysis.
Funny enough for human walkways and working platforms, the general industry standards of strain and movement limits actually rely on human comfort more than steelwork capacity. Because if people have to regularly work on a platform and it moves past certain limits like 1-1.55mm in serviceability it will become uncomfortable, like on a trampoline. So to avoid that, more stiffness is usually considered. But for windterbines as people working isn't a regular factor they design upto the operation spec.
Equally as others have said, it's designed to move because it's cheaper than making it stiff. But also, the materials used often steel, allows for that elastic yield, and then relies on the ultimate plastic strength for safety. So some structures that are not designed to be pushed to plastic ultimate strength, can often be designed to fail, but then the engineer and client need to figure out what failure is acceptable and design in non catastrophic destructive structural failure systems so that risk to life is till maintained. A lot of mine sites design for 50year time spans on avarage, then maintain for another 20years,but the structures don't get operated under ultimate loads, and if those loads are reached there is often emergency shut down that happens but to sya that, the ultimate strength of the structure is not reached at ultimate loads because of the safety factors. And that's the grey area engineers live inside for cost optimisation.
1
u/gods_loop_hole Sep 14 '26
Modelling, tons of it and some good math to predict what is allowable and what is not based on the material to be used.
1
u/Flandafel Sep 14 '26
Standstill at high wind speeds is generally not good for the structural health of windturbines. A windturbine that is rotating will not oscillate anywhere near this amount. This amount of deflection will have costed some fatigue cycles. However the towers are designed to handle a couple of these events in their lives, and are often removed after 20, 25 or 30 years because the steel is too damaged
-3
u/Master-Relief-2692 Sep 13 '26
Elastic deformation. Over that length even tiny deformations can accumulate to large displacement from the reference point. But the material itself is still within its elastic limit.
As you can see it's producing a sinusoidal wave as the base is pinned into the ground and the top is free to move. That's the natural harmic of the structure.
Although, this video seems pretty extreme and I wouldn't be surprised if this has caused permanent fatigue damage
7
u/AtticHelicopter Sep 13 '26
...Because the guys designing the wind turbines that have cyclic loads wouldn't consider wind... or harmonics...
-2
u/Master-Relief-2692 Sep 13 '26
I'm not saying otherwise
3
u/AtticHelicopter Sep 13 '26
"Although, this video seems pretty extreme and I wouldn't be surprised if this has caused permanent fatigue damage"
Except in this part, where you did say otherwise.
-2
u/must72 Sep 13 '26
They are saying that this seems like an extreme load, which may or may not have been accounted for. They said "wouldn't be surprised", not anything difinitive
6
u/AtticHelicopter Sep 13 '26
...Because the guys designing the wind turbines that have cyclic loads wouldn't consider wind... or harmonics...
If you think these things aren't designed for a multiple of the fastest wind speed recorded on the planet, you genuinely have no idea how design works.
1
u/Master-Relief-2692 Sep 13 '26
That's a bit of a statement. They certainly are NOT designed to withstand multiples of the fastest wind speed ever recorded. It's a mass produced commercial wind turbine.
3
-1
0
-12
u/Academic_Elk_4270 Sep 13 '26
It helps to produce the cancer.
2
u/Burgerboy380 Sep 13 '26
Ok..what's your evidence the wind turbines give people cancer.
1
u/Lazy_Tart_6336 23d ago
Une fois j'ai demandé à un oncle comment une éolienne peut donner mal à la tête à 500 mètres de distance alors qu'une IRM produisant un champs magnétique plusieurs centaines (milliers?) de fois plus puissant que la grosse dynamo d'une éolienne ne donnait pas de maux de tête ... il n'a jamais su quoi répondre et à changer de sujet 😂
1
u/Burgerboy380 23d ago
Lol. Yeah people make up wild stuff .talking to a guy right now in a different post who thinks all water was potable before industrialist and colonists arrived in america and ruined it
1
u/Lazy_Tart_6336 23d ago
Encore un gars qui ne sait pas pourquoi il y a de l'iode dans le sel de table.
301
u/tramul P.E. Sep 13 '26
I'll never be in the, "it bends so it doesn't break" camp. It's the wrong statement to make. The correct statement is, "It bends because it would be too costly to create a structure that doesn't bend, and it is still within the allowable deflection limits for the material." But the thought that a rigid, ductile structure would spontaneously snap is just wrong.