Originally Posted by
jere Will how do they increase rigidity if they are not dimpled?
I don't recall the exact calculations, but the holes should be evenly distributed across the surface. IIRC, Mechanics of materials proves that circular holes increase the modulus of rigidity of a material, of course there are optimal hole sizes and distributions. Ideally, the holes should be as perfect as possible, without scrapes or burrs.
Take a small strip of sheet metal and twist it a little to feel how stiff it is. Then punch a series of holes in it and try twisting it again, it will be far stiffer.
I can find more specific/accurate info on it later if you want