The variability of inelastic response spectra is investigated by time-history analysis using sets of artificial ground motions with different strong-motion durations. It is concluded that the Newmark-Hall inelastic response spectra of elasto-plastic systems are unconservative for 5% damping, and conservative for 2% damping. New inelastic response spectra are proposed for different ductility ratios and damping coefficients. A 10-story steel moment-resisting frame is designed and analyzed to assess the validity of the design procedure based upon modal analysis and inelastic response spectra. The results indicate that at least for this or similar frames, the method is satisfactory. However, the procedure leads to slightly unconservative design for upper-story exterior columns. The study also suggests that the maximum lateral story displacement predicted directly by the SRSS modal analysis using the inelastic displacement response spectrum is too conservative. The conservatism is more pronounced in the upper stories.