square girder versus cylinder

[Johan van der Kogel]

I have a beam question, that I can't seem to find a right answer for.
Consider a squere hollow profile of say aluminum, height 7 cm and width 3 cm. It can carry a certain weight as a girder. If I want to compare this profile with a hollow tube with the same wall thickness. Is there a rule of thumb that says what the radius should be to carry the same weight?
So how much does the cylindrical shape contribute to it's bearing capacity

[jboggs]

Study area moments of inertia. You will get the answer to your question.