A machine element weighing 800 lb is supported by two springs, each having a constant of 200 lb/in. A periodic force of maximum value 30 lb is applied to the element with a frequency of 2.5 cycles per second. Knowing that the coefficient of damping is 8 lb s/in., determine the required value of the coefficient of damping if the amplitude of the steady-state vibration of the element is to be 0.15 in.