Design a steel hollow column of length 15-m with minimum weight for a water tank such that it can carry a water tank of 45000 kg. The tank is to

designed such that to withstand a ground vibration due toan earthquake. The earthquake excitation is idealized as undergoing a harmonic ground acceleration of amplitude of 0.5 g. The natural frequency of the water tank should exceed 20 Hz. Assume the Yield Strength for the steel to be 250 MPa. Assume a damping ratio of zeta = 0.2.