Seismic design load for elasto-plastic structures is reduced according to the permissible ductility, in that method there are two well-known theories, i.e., displacement conservation theory and energy conservation theory. However, these theories are contradictory and they give different values for the reduction factor. This study shows that the two theories are not opposed when taking into consideration the fact that the cycle number of plastic excursions is inversely proportional to the yield strength. From this observation it is derived that the energy input rate is proportional to the yield strength, and thus the spectrum of energy input rate calculated for a particular yield strength can be easily adjusted to other systems having different yield strengths. It is also found that spectrum values of energy input rate for near-field earthquakes are larger than those of oceanic type earthquakes because of the difference in hysteretic cycles.
