An improved procedure to estimate maximum inelastic deformation demands of short period structural systems is investigated. The procedure makes use of inelastic displacement ratios that permit the estimate of the maximum inelastic deformation from the maximum elastic deformation. It is shown that the ratio of peak inelastic to peak elastic displacement is strongly correlated to the ratio of peak ground velocity and peak spectral velocity for structures with short periods of vibration. This interesting observation is then used to propose a simplified displacement modification factor that is a function of the period of the system, its lateral strength and the ratio of spectral velocity to peak ground velocity. A large set of earthquake ground motions recorded on sites having average shear wave velocities larger than 180 m/s are used to compute the error statistics on the inelastic deformation estimations of this new equation. It is shown that the new procedure leads to significant reductions in the variability of the results, and hence provides a more reliable tool for estimating inelastic deformation demands to be used in displacementbased procedures.
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