[Graduates] (Quiz 03) Damping

1.
A one-story reinforced concrete building has a roof mass of 500kg, and
its natural frequency is 4Hz.
This building is excited by an exciter with two masses, each 25kg,
rotating about a vertical axis at an eccentricity of 30cm.
When the exciter runs at the natural frequency of the building,
the amplitude of roof acceleration is measured to be 0.02g.
Determine the damping of the structure.