Truss designs in arms

[Max Lobovsky]

Most arms on FIRST robots use some sort of large extruded aluminum box. I have often seen some sort of truss design cut into these boxes to apparently make a more efficient shape. I am trying to design the truss to be cut into our arm member (4x4" 1/8" thick square aluminum tube), and would like to know how other teams have designed theres in the past.

I have seen three main designs. Holes, equilateral triangles (leave no struts perpendicular to arm direction), and right triangles (leave struts perpendicular to arm direction).

I currently like the equilateral triangle design simply because it "looks" more efficient, but I have no real good reason. This design would remove just a bit over 50% of the material:


I know I am opening an enormous can of worms here, but I'm looking for rules of thumb. (radii of fillets, ratios of widths of different parts of it, and of course an answer to which truss design is the most efficient).