Eli5- Purpose of having different structural sections, for example, I, C, Square Hollow. Could they be used interchangeably? Do they bear load differently?
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Different shapes for different purposes. Yes, at the least an I beam or a hollow square stock will absolutely have different load and tensor properties.
Different sections resist load from different directions differently. If you assume you’ve got the geometry right so that the cross-section area is all the same, they all take tension in basically the same way and are interchangeable for tension purposes.
However…most structural sections aren’t in pure tension. They’re usually in some combination of bending and compression and that’s where the differences start to show up…the stiffness of the different sections is quite different. I-beams are much stronger when the bending axis is perpendicular to the web (the “stem” of the I) because that puts most of the load into the flanges (the “top” and “bottom” of the I). They’re far weaker with the bending axis parallel to the web because you don’t have nearly as much material at what’s now the “top” and “bottom. So I-beams need to be oriented correction for bending load, and because they’re stiffer in one direction than the other you need to be careful about buckling loads (they’ll buckle “out-of-plane, in the weak direction, much more easily than in-plane). A C-beam is basically just half an I-beam, all the same logic applies
A square beam, since it’s symmetric, has the same bending/buckling resistance in both directions so it’s good for pure compression (equal buckling resistance) or bending from multiple directions.
If torsion is involved (not super common in most structures but sometimes necessary) then it’s an even bigger difference…closed shapes, like a tube or square, have *enormously* higher torsion stiffness than open shapes like a C or I. This is why you “never” see I-beams used as drive-shafts and why torque-tubes are so common.
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