In a simplified approach to the rigorous elastic half-space approach for the behavior of a rigid circular surface footing acted upon by a dynamic excitation force, the elastic half-space model is replaced by an equivalent lumped parameter system. The equivalent spring constant (K) and the equivalent damping factor (D) are expressed in terms of dimensionless mass ratio (B). A chance in the value of B thus affects the dynamic response of the footing. The paper explains a procedure to quantify this variation £or vertical mode of vibration
