system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do engineers use I-beams when the triangle is the strongest shape.

I am confused at why engineers would use an I-beam instead of a triangle shaped beam. If you were to put a point load in the middle of a beam, intuition seems as if the triangle shaped beam would be able to withstand much more weight. Also using a triangle, the beam would be less likely to twist. The I-beam I feel has a greater potential for it to sort of collapse on itself due to welds, material imperfections, etc. The only downside I can think of to the triangle shape beam is that the top would only be a point, instead of a flat surface.
gpt
I-beams are intended to span between columns, so they specifically need to be optimised to handle *bending*. When a beam (any beam) is loaded, it bends in the middle, and its internal stress varies between max tension at the bottom and max compression at the top. It turns out that the most efficient way of handling that is to have essentially all the steel at the top and bottom, and reduce the webbing (the bit in between) to the minimum necessary to hold it together. If the webbing is overstressed, it gives a little and transfers the stress to the top or bottom anyway.