A simple torsion-bar spring has the shear stress in the steel [G = 80GPa] shaft is not to exceed 90 MPa, and the vertical deflection of joint D is not to exceed 12 mm when a load of P = 11kN is applied. Neglect the bending of the shaft and assume that the bearing at allows the shaft to rotate freely. Determine the minimum diameter required for the shaft. Use dimensions of a = 1400mm ,b = 600mm , and c = 195 mm.