Abstract. In seismic resistant design of moment frames, the structure behavior under earthquakes has to be examined considering various damage criteria. Damage indices can be estimated by Endurance Time (ET) method with minimum computational e ort. The quality of this estimation can be improved in di erent ways. In this paper, the graphs of a certain damage index versus natural period of the structure, called damage spectra, are produced applying intensifying ET records and scaled Ground Motions (GM). Then, the excitation duration of ET acceleration functions (target time) is modi ed in order to reach acceptable consistency between ET and GM damage spectra. Moreover, various damage indices of a steel moment frame at two hazard levels are evaluated via nonlinear time-history analyses of the structure under scaled earthquakes and ET records. It is observed that the frame damage estimation can be improved by modifying the target time of ET records based on damage spectra. In addition, in most cases, there are negligible discrepancies between values of frame damage indices corresponding to ground motions and ET acceleration functions with the damage-based target time.