Answer:
Round sections are blocked in the SHAPE‑THIN cross-section library because they represent relatively solid cross-sections, and therefore cannot be calculated according to the theory for thin-walled cross-sections. A round bar can only be roughly modeled as a rectangular element. Such cross-sections should, therefore, be calculated as solid cross-sections in the SHAPE‑THIN program.
To simplify matters, it would be possible to define circular cross-sections in SHAPE‑THIN as pipes with large wall thickness.
