고체역학 (Gere) — Torsion: Problem 3.8-4

3.8-4 A hollow steel shaft $ACB$ of outside diameter $50\text{ mm}$ and inside diameter $40\text{ mm}$ is held against rotation at ends $A$ and $B$ (see figure). Horizontal forces $P$ are applied at the ends of a vertical arm that is welded to the shaft at point $C$. Determine the allowable value of the forces $P$ if the maximum permissible shear stress in the shaft is $45\text{ MPa}$.