From the study of the nonlinear behavior of single degree of freedom (SDOF) oscillators, response modification factor spectra corresponding to failure conditions of structures are assessed by means of damage indices. Two different damage indices are used: one that neglects and one that takes into account the way in which the hysteretic energy is dissipated. Ground motion records obtained from several earthquakes and grouped as a function of magnitude of the event, source-to-site conditions and site-soil conditions are used as input for nonlinear dynamic analysis of inelastic SDOF systems in order to evaluate constant damage spectra of the strength reduction factors. Simplified relations depending on a limited number of significant parameters are then proposed to approximate the computed mean reduction factors, in order to derive inelastic response spectra directly from the elastic ones.