The segment AB of the steel torsion bar is a cylindrical tube of constant 2-mm wall thickness. Segment BC is a square tube with a constant wall thickness of 3 mm. The outer dimensions of the cross sections are shown in the figure. The tubes are attached to a rigid bracket at B, which is loaded by a couple formed by the forces P. Determine the largest value of P if the shear stress in either tube is limited to 60 MPa.
