A beam of hollow cross-section shown is made of steel that is assumed elastoplastic with E- 200 GPa and oy = 240 MPa. Considering bending about the z axis, determine: (a) The bending moment for which the first yield occurs. (b) The bending moment at which the plastic zones at the bottom and top of the cross section are 20 mm thick (each). (c) The corresponding radius of curvature for the above elastoplastic state of stress. (d) The residual stresses upon unloading to zero bending moment. y ZA 15 30 mm 15 20 50mm 20