Problem 2.13 A vibration generator with two weights each of 30 lb with an eccentricity of 10 in rotating about vertical axis in opposite directions is mounted on the roof of a one- story builsing with a roof that weighs 300 kips. It is observed that the maximum lateral acceleration of 0.05 g occurs when the vibrator generator is rotating at 400 rpm. Determine the equivalent viscous damping in the structure.