The inelastic response of a class of nonlinear oscillators, characterized by a relatively rigid superstructure on a soft ground story under base seismic excitation, is numerically investigated. Both lateral translation and rocking due to overturning of the rigid superstructure are considered in the formulation. Initially, the governing equations of motion are established in a nondimensional form in order to identify the performance parameters of the system, namely initial elastic characteristics (i.e., periods and damping), normalized resistance, and hysteretic response shape. Then, the inelastic response of the oscillator is numerically evaluated under seismic excitations at the base over a range of periods and resistance parameters (lateral and rocking) encountered in practice. Inelastic ductility demands and reduction factors are obtained and are compared to those from conventional single-degree-of-freedom lateral response used in seismic design. The results show that the presence of the rocking component increases the transverse ductility and internal force demands, when compared against the conventional spectral estimates using the single-degree-of-freedom approach.
