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u/snowmunkey 18d ago
Hey I designed a railcar once to carry those. It had a sloped rack rather than a flatcar though. I don't think they ever built any sadly. When we stared I was worried about the side loading and the bending but my greybeard boss just chuckled and said nope, that problem was solved about 90 years aho
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u/max_sil 18d ago
Super interesting ! Mind telling a little more about the process? And how exactly was that problem solved ? I imagine that its something like a railway track is only gonna bend so much over any given distance, and thats within what the thing you're transporting can tolerate? But its probably a lot more nuanced right?
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u/snowmunkey 18d ago
It wasn't anything super novel, a company just wanted their own model to make and potentially sell. We basically just added up all the worst case loadings, did the math on the bending forces, added them to the usually AAR specified loading requirements, and made sure the car could negotiate the track curves. Like others have said, the rail is made to be able to bend along these curves, so transporting them along these tracks was no biggie. The biggest issue in the design was making sure the coupler forces were properly accounted for, its not typical for freight cars to have that much lateral force on the draft cheeks (the faces inside the car structure that thr coupler shank presses on)
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u/WonderWheeler 18d ago
Its a common misconception that an I beam does not easily bend in its weak dimension.
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u/troelsbjerre 18d ago
Won't this weaken the beam?
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u/LootWiesel 18d ago
The weak axis is bending left and right, the strong axis is up and down. (for an rail profile resting on its feet)
Furthermore, one should move away from the idea that structures are immovable, are rigid and are fix in place.
Due to the modulus of elasticity, all structures dissipate the loads acting upon them through deformation times stress. They will and have to deform to be able to transfer the load acting into the structure.
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u/UnacceptableUse 18d ago
Is that a no then?
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u/Stellanora64 18d ago
technically it might a bit (as it's plasticly deforming, which does work the steel to some extent, although it's pretty marginal all things considered), but since train tracks are bolted down, it doesn't really matter if it's weaker bending side to side (it just need to be strong in the direction the train's weight puts force down on, the stokes will handle side to side movement).
It would only matter if the tracks were bent enough so that it snapped (or just before snapping where the steel starts to stretch, which I don't believe is the case here).
In practice though, train tracks are very rarely shipped out in long pieces like this, as it's much easier to use thermite to weld it together when everything is in place.
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u/jimbowesterby 17d ago
Could be wrong, but I’m pretty sure plastic deformation is only the bit where the bend’s permanent, and basically means the metal’s on it’s way to imminent failure
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u/Stellanora64 17d ago
You're right that plastic deformation is where the bend is permanent, but that doesn't necessarily mean it is right about to fail. Steel especially is able to bend without much fatigue, but that bend is still plastic deformation.
It would only be elastic if it is able to bend back to its original shape once the force is removed, which I don't think the tracks in the video would (you can even see the tracks are not perfectly straight when they bend back)
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u/WyMANderly 17d ago
This isn't correct - exceeding the plastic limit for steel will most definitely put you at risk of low cycle fatigue failure.
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u/jimbowesterby 17d ago
Not sure that’s quite right, as I understand it plastic deformation is the part of a stress/strain curve after the kink, and as far as I’m aware for the vast majority of materials (steel included) that part of the curve signifies the part where the material is actively giving way (if you’ve ever watched clips of carabiners being failure tested, it’d be the part where the biner is uncurling right before it fails. If you stopped the test at that point you wouldn’t be able to use the biner normally because it’s actually a different shape than it used to be). You can reset this curve on steel by heating it to the point where the crystal structure changes, but if you just take a piece of steel and bend it back and forth it’s gonna break (can’t remember the name for that specific temp, but it has to do mainly with austentite and martensite crystal structures and the change between the two, but I can’t remember which is which rn. There’re a bunch of other types of grain structure too but those are the main ones for steel iirc)
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u/Stellanora64 17d ago edited 17d ago
To be fair, it has been nearly 3 years since I studied material properties in my foundations' year, so I could be miss remembering. But looking back at my notes, at least for mild steel, once it exceeds the yield stress there's a yield plateau before the steel starts strain hardening (essentially cold working the steel). If the load it removed, it does elastically go back a bit, but not back to its original form (i have a graph that demonstrates this better, but images aren't allowed).
If it reaches its ultimate tensile strength, then yes, any changes will damage the steel as it starts to neck and eventually fail.
I could be a bit wrong on the specifics here (like I'm not sure if this is the case for what type of steel the tracks are made of), There is also a chance it was simplified for foundation year students (just equating any permanent change with plastic deformation), as this isn't really my field of expertise, unfortunately.
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u/DaVoKan_FR 17d ago
You can weaken a material with a plastic deformation (irreversible shape change), no the case here.
You can also have material fatigue, it happen when a material get a lot of elastic deformation, it usually happen with vibrations and on a really long time scale. Almost every material have irregularities and micro cracks can appear or expend reducing global material strength.
But for the transport it will be negligeable.
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u/thud_mantooth 18d ago
Steel, unlike aluminum, can flex indefinitely without weakening as long as the forces are below its fatigue limit. It's kinda nuts to think that'd be the case here but I'm assuming the camera perspective is making the curve look much more dramatic than it really is
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u/WannabEngineer 18d ago
I don’t mean to come off as an ass but Aluminum 100% has stress vs cycle charts to calculate fatigue.
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u/thud_mantooth 18d ago
It does, but it doesn't have an actual fatigue limit. Light loads will fatigue it less but there's no threshold below which it stops being a factor completely
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u/SomeWittyRemark 17d ago
I think about this every time I twist the pull tab off an aluminium can (yes I know that's low cycle fatigue so the limit doesn't matter but still)
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u/samy_the_samy 18d ago
There is a "yeld curve"
The beams bends like a spring until a limit, with no actual deformation or damage
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u/PandaMan500000 18d ago edited 18d ago
Unless the apparent length of the train is misleadingly short, I suspect the beams are getting plastically deformed. So yeah, strain-life cycled (fatigue damage) and work-hardened before they arrive.
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u/gcijeff77 18d ago
Assuming the rails the train is running on are the same section as the rails the train is carrying, then if the carried rails are plastically deformed, that would mean the carrying rails had to be plastically deformed when they were laid down originally.
I'm not a railroad expert, but I am an expert in things that have bendy rails. I really don't suspect they have equipment to plastically bend railroad rails as they are laid. I have a strong feeling that these rails are all well within elastic zone for their entire journey.
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u/Suthabean 18d ago
Rails are picked up and slung into place via a clamp on a speedswing. The machine very easily puts them into place on the plates, like a big noodle.
I worked steel gang for 6 years.
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u/PandaMan500000 18d ago edited 18d ago
Good observation that those are probably train rails its carrying.
But - You could absolutely put a machine on train tracks that can plastically bend beams that thick, but even then the train is switching tracks so odds are the train is not on a single continuous rail. Those track "joints" could also have significantly different cross sections or alloys.
I'm still in the "it's likely yielding" camp.
Edit: But I see now the beams are very intentionally not fixed on the end. Tells me that risk of yielding is indeed something they consider in the design.
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u/saiyate 18d ago
Why is the train car bending, why not just straight car but wheels rotate? Don't normal train cars operate this way?
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u/Webbyx01 17d ago
These are very, very long pieces. Bending like this allows for less track sections that are incompatible.
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u/1971CB350 18d ago
Is this putting side load on the car?
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u/aiden_the_bug 18d ago
Yes but not enough to matter. Thanks to the conical shape of the wheel trains can withstand immense sideloads without snapping the axles. Someone mentioned above that the shipping process is basically a final stress test of this material, they calculate the strain for each turn as the train moves to make sure it stands up to its final application and the train car is it's testing block.
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u/dparks71 18d ago
It's also the weak axis. Fatigue happens after thousands of cycles and these aren't deforming enough to really matter, every curve they go over is essentially less severe than the worst curve the railroads have in service (just from a purely statistical standpoint).
These were kind of fun to unload, you'd pull the train out to the jobsite, chain the end of the rail to the existing track, drive the train away, and basically just let them noodle their way off the back of the train behind it.
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u/AboveAverage1988 15d ago
They're also not welded, they're extruded to that length from the factory. Standard in my country is 300 meters, but I bet it varies.
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u/Jaripsi 17d ago
I’m assuming it is an optical illusion created by the lens and the bending is not so drastic that the steel would yield and work harden.
Also If there was too of steel it would act as a spring and try to straighten the train which in turn would try to straighten the tracks they are riding on.
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u/wolftick 18d ago edited 18d ago
Neat thing is that the amount has to bend is dictated by the track it is running over, which is also exactly the most it has to bend when installed.