Why are the cross-sections for round steel bars locked in the library of SHAPE-THIN?
Circular profiles are locked in the cross-section database in SHAPE-THIN because they basically represent solid sections, and thus they cannot be calculated according to the theory for thin-walled sections. A round bar, for example, can hardly be modeled by a rectangular element or a polygonal chain. Therefore, these cross-sections should be calculated as solid sections (SHAPE-MASSIVE)
For the definition of circular cross-sections in SHAPE-THIN, you can simplify it by using pipes with a high wall thickness.