Effect of near-field ground motions on displacement amplification spectra in inelastic seismic performance evaluation

Abstract

Behavior of the structures under near-field ground motions is different than behavior of the structures under far-field ground motions, in terms of strength demand or inelastic displacement demand, and therefore, design processes should reflect characteristic properties of near-field ground motions. Current applications on inelastic structural performance estimation under earthquake excitations are based on constant ductility approach. However this approach is inadequate to capture inelastic displacement demand of near field earthquakes, since ductility demands are limited to certain values. In this study, nonlinear analysis of single degree of freedom structures under near-field strong ground motion excitations were performed based on constant strength reduction approach. Strength-based displacement amplification spectra of near-field ground motions were developed by utilizing 105 near-field records of 25 worldwide earthquakes. Near-field ground motions were classified according to their pulse periods. Vibration period was normalized with respect to pulse periods to eliminate the distinct pulses observed at long period regions on the amplification spectra. Effects of the stiffness degradation and post-yield stiffness ratio on the strength-based displacement amplification spectra were investigated. It was found that stiffness degradation has significant influence on the spectral displacement amplification ratios. Systems with modified-Clough stiffness degradation hysteretic model exhibit larger spectral amplification ratio than those with elasto-plastic hysteretic model. Post-yield stiffness ratios affect the spectra especially at the short normalized period regions. Finally, mean strength-based displacement amplification ratios for each pulse period intervals were proposed as regression equations as a function of normalized period and strength reduction factor. Numerical examples have revealed that for SDOF whose period larger than 1 sec., displacement amplification spectra take values between 1.1-2.6, where this value (modification factor, C1) is fixed to 1 in ASCE41-06.