Several steps are required to solve this problem:

1. Consider equilibrium to determine the beam reactions at the pin and at the roller.

2. Draw the shear-force and bending-moment diagrams to determine the shear force and bending moment at $H$.

3. Compute the moment of inertia for the rectangular tube and compute the first moment of area $Q$ for point $H$.

4. Use the flexure formula to compute the normal stress at $H$.

5. Use the transverse shear stress formula to compute the shear stress at $H$.

6. Show these stresses on a stress element and use stress transformations to determine the principal and maximum in-plane shear stresses.