This shape is used when the load is parallel with the flange. As you can see, the shape is not so good with lateral forces unless you turn it sideways. When the load will come from two directions, a square tube is used. In all these cases the idea is to remove material that is not carrying much load and concentrating the material where the load is highest.
Increasing the depth of the beam increases the bending strength by the depth cubed, so we can gain a lot of stiffness this way. However, we start to get limited by things such as buckling when the sections start getting too thin. When folded sheet metal studs are used, this is done by placing a small bend (lip) at the end to add stiffness.
These forces are greatest at the very top and very bottom. So to make the stiffest beam with the least amount of material you would want the material to be only at the top and bottom sides. However you still need to connect them together or they would just be two separate plates and would not be stiff at all. So you put a web in the middle to connect them and make them work together. The resulting shape is the traditional “I-beam” or wide flange beam.
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