Another useful model explaining strength principles is your "Sunday Paper."
Stand it on edge, and it supports very little. Roll it up, and it supports a lot.
The difference is in the rigidity. Form it into some sort of shape, and the shape is much harder to distort. leave the sheets straight, and it doesn't take much to get that bending started. And, once the bend starts, there goes the strength!
With this model in mind, it seems pretty clear that the "important" area is the area where the bend is. The bend in a formed steel purlin, the "intersections" in the web of an I-Beam, the flanges of an I-Joist....you get the picture!
