A hollow rectangular steel beam is 18 cm deep by 10 cm wide, outside dimensions. The walls are 2 cm in thickness. From the shear and moment diagram, the maximum vertical shear on the section is 180 kN. Calculate the moment of inertia of the hollow rectangular steel (the moment of inertia of a rectangular object is I=BH3/12). Given that the width of the shear plane is 4 cm and Q = 258cm3, calculate the maximum shearing stress in the beam.