pic: Beam Bending Example

[sanddrag]

The question on holes got me curious. Here is a summary of my test and results

1"x1"x1/16" wall 6061 Aluminum Box Tube, 18" long. 0.625" holes down opposite sides, spaced at 1" centers
End faces fixed constraint
500 psi pressure applied to a 2" long patch, on the top side center of the tube.

For the given loading and fixed constraints, the part exhibited a 29% higher maximum stress with the holes than without. The part with holes had approximately 15% less mass than the part with holes.

Anyhow, if your holes are on the faces 90 degrees to the one the force is being applied, you don't lose all that much strength. This is the principal behind I-beams. The more cross sectional area you put further from the center, the stronger the beam will be, because it will have a higher moment of inertia. This is why beams with a taller cross section are much stronger in bending, and why I beams have so much material so far away from the center.