4. (20 points) The thin-walled structure shown below is loaded with a couple at plate B and at plate C and is restrained rigidly at A. Use P = 1 lb, L = 4 in, L2 = 3 in, h = 0.5 in, and t = 0.02 in. Treat the plates A, B, and C as rigid, and neglect their thicknesses. The material is aluminum (E = 107 psi, v = 0.3). Calculate (a) the maximum shear stress Tmax according to thin-walled torsion theory, (b) the angle (in degrees) of rotation oc of the end plate C. A 21 B P 2t P