Thin walled box beam with end load at an angle.

A box beam made from an extruded aluminium ally having 5 414 mm wall thickness is loaded by an inclined force P=100 N, as shown in the figure.  Determine the maximum stress on section a-b.  For the box beam, use the centreline dimension for finding I. (Note that the stress is far away from the support where large local stress concentrations are unlikely.)