pic: REV Robotics building system

[GeeTwo]

Loading in what direction?

For most deflection and torsional loading (including buckling), the stiffness depends on the moment of inertia of the cross section, which, for a given amount of material, increases as the square of the distance from the center. For a given amount of material and diameter, a hollow round tube will have the greatest moment of inertia (be strongest), with a hollow square tube being a little bit behind. Shapes like I-beams and rectangular tubes are designed to have a larger moment of inertia in one dimension (the longer dimension) than another to accommodate a larger load in that direction.

For tension loading, I understand that shape makes less difference unless your material forms fibers.

Shapes like the REV and 80/20 extrusion require more weight for the same strength; their purpose is in easy fabrication using tee nuts and similar fasteners.