This paper analyzed seismic response of SDOF elastic-perfectly plastic (EPP) hysteretic systems considering P-Δ effect. It used totally 370 earthquake records belonging to four site and soil conditions to calculate the inelastic deformation ratios (Cd) with different ductility, and used the characteristic periods Tga and TgR to normalize the Cd spectra to keep the valley spectral values at these two periods. Results indicate that both P-Δ effect and damping ratio have significant effects on Cd , especially nearby the characteristic periods.
