If the wall thickness is too thin, issues such as plate buckling or local buckling could limit the load capabilities of the structure.

Concentrating all the material close to the center, like in the case of a solid circular rod will increase the critical buckling load of the member.

A thin wall circular hollow section (CHS) is expected to have a high critical buckling load.

A square hollow section (SHS) is inferior in performace in terms of critical buckling load compared to a circular hollow section (CHS) of the same thickness and cross-section area.