Consider a sign mounted on a circular hollow steel pole of length L = 5 m, outer diameter...

Consider a sign mounted on a circular hollow steel pole of length L = 5 m, outer diameter do = 5 cm, and inner diameter di = 4 cm. Aerodynamic forces due to wind provide a harmonic torsional excitation with frequency f0 = 3 Hz and amplitude M0 = 10 N·m about the z axis. The mass center of the sign lies on the central axis z of the pole. The mass moment of inertia of the sign is Iz = 0.1 kg·m2.The torsional stiffness of the pole can be estimated as , where Gst is the shear modulus of steel, which is 79GPa. Neglecting the inertia of the pole, calculate the amplitude of vibration of the sign.

Figure P9.36