On the topic of structural tubing... It is my understanding that round tubing is stronger than square tubing, or conversely that, that round tubing is lighter for a tube of the same strength. Does it necessarily follow that the best possible use of material when making a tube, is to use it all for round tube wall thickness? Or, is it possible I could make a stronger tube of the same weight by using a thinner walled round tube and inscribing a square tube inside it? One thought experiment is to imagine that I have 3 different tubes. Tube 1 will be a round outer tube, tube 2 will be a round tube which fits snugly inside tube 1, tube 3 will be a square tube which fits snugly inside tube 1. I'm going to make a composite tube by sliding either tube 2 or tube 3, inside tube 1. All 3 tubes have the same weight per unit length. I really have 2 options in this scenario for a composite tube, 1+2 or 1+3. If the strength of the composite tube is simply the sum of the strengths of the individual tubes, then 1+2 would be stronger because 2 is stronger than 3 and 1 would be the same in both cases. If the strength of the composite is not a simple sum, then it is possible 1+3 may be stronger. It's easy to imagine the straight edges of the square tube acting like internal bracing for the round tube, so I really have no idea which combination would be stronger. Also: this thought experiment assumes that composite 1+2 would be equivalent to a single round tube of equivalent wall thickness. I don't know if that is true or not. Further ponderings: What about other internal structures? Suppose I could find a triangular tube to inscribe into my round tube? Or perhaps I could pack my round tube with hollow spheres? Could the answer depend on exactly what type of forces I want the finished tube to resist (bending, denting, stretching, compression, torsion, etc)? I'm not at all sure if I've explained my question well. I'll clarify if need be.