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I'm looking to construct a something with hollow steel tubing (e.g. 10ga 2"x3" rectangular tubing) suspended horizontally welded to an outer frame on both sides.

I've already calculated the deflection given the loads it'll experience.

However, I know there'll be a point as I increase the load where the sidewalls will buckle/crinkle, and this point could be lower than the load where the metal bends too much.

How do I calculate this load?

I've done lots of searches, but nearly everything I can find regarding buckling loads is for metal oriented as a column and compressed, and that's not the case for this metal.

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I am not sure if I understand your question correctly but to determine whether your steel sections compression flange will buckle before it will fail by some other mechanism it might be helpful to look up Section classification in following standards:

EN 1993-1-1

AISC Steel Manual

Based on size and material of your section it will place it into one of 4 classes (EN) and that will determine if you have to worry about buckling.

It is my belief that most of standard sections are designed in a way that the section fails by some other mechanism rather than buckling.

Jakub

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It depends on your load and how concentrated it is. A distributed load will not cause local buckling.

Usually for a case like yours buckling or plastic deformation start at the fixed ends for the greatest bending moment is there.

Because of the complex geometry of the connection there there is no analytical way to calculate failure load, only tests and or manufacturer charts, if available, could help.

If, however, the ends were not welded and simply supported you could, as a first estimate, consider the plastic moment at the center and reduce it by a factor of safety.

Crushing under a point load can happen in different forms and shapes, all of which can bear in the final strength. There is a random pattern in the actual shape of the buckling.

But usually the process ends by developing a plastic groove on top which will extend beyond the cross section's sidewalls and deform them by pushing them out, creating a plastic hinge, hence a mechanism.

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