The beam ABC in Figure P5.45 is made of steel ($E = 200 \text{ GPa}$). It has a hollow circular section, with outer diameter 180 mm and inner diameter 150 mm. The spring has a constant $k = 2 \text{ MN/m}$. A moment $M_0 = 40 \text{ kNm}$ is applied at C. Determine the rotation of the cross section at C.