The study of the P-Δ effects due to the increase of horizontal displacement is being continued steadily. In the present paper, the P-Δ effects on the elasto-plastic response of lumped mass system subjected to strong-motion earthquakes are studied as follows : First, the stability coefficient (the parameter that characterizes the P-Δ effect) is estimated by the regression line for the height and the natural period of the buildings, and the elasto-plastic response spectra are examined. Second, the mechanism of the P-Δ effects is examined from the results of the energy response, and the estimate formula applicable to the amplification factor (the ratio between the maximum response displacement with and without P-Δ) is suggested.
