Effect of Kinematic Hardening and Ductility Ratio on Inelastic Input Energy Spectra of Near-Fault Ground Motions

In energy-based seismic design of structures, ground motion effect is considered as an energy input to the systems. Consistent development of input energy spectra is of great importance for the energy-based seismic design since the total energy input to structural systems can be practically obtained by means of these graphs. The main purpose of the present study is to investigate the influence of post-yield stiffness ratio and ductility demand on inelastic input energy spectra of near-fault ground motions. A wide range of nonlinear single-degreed-of-freedom (SDOF) systems characterized by their natural periods ranging from 0.02 to 3.0 s and normalized lateral strength are considered. Bilinear elastoplastic (BEP) hysteresis models with six different post-yield stiffness ratios are used to generate the results for constant ductility ratios ranging from 2 to 5. Mean ± one standard deviation input energy equivalent velocity spectra of a set of 21 near-fault accelerograms exhibiting pulse-like characteristics are computed based on nonlinear time history analyses of SDOF systems with 5% damping. The analytical results have shown that the influence of post-yield stiffness ratio on inelastic input energy spectra of near-fault ground motions can be neglected practically, whereas the influence of ductility ratio is more obvious. Moreover, a transition period of approximately 0.7 s between the increasing and decreasing input energy equivalent velocity spectra based on ductility ratio is identified.