Selection and modification of ground motion records using Newmark-Hall spectrum as target spectrum for long-period structures

Input ground motions have significant impacts on the uncertainty of structural responses in time-history analysis. In this study, records were selected and scaled for the evaluation of mean structural responses according to the target spectrum. The Newmark-Hall spectrum is closely related to seismic response of short, medium and long-period structures, so it was taken as the target spectrum here. The nonlinear time-history analyses of 9-story and 20-story steel moment-resisting frame structures were carried out as examples. They represent medium and long-period buildings, respectively. Three target spectra with risk of 50%, 10% and 2% probabilities for exceedance in 50 years were calculated by the average Newmark-Hall spectrum method for three ground motion sets developed in the SAC Steel Project. The predicted structural mean responses of these Newmark-Hall spectra were compared with those calculated by the average spectral acceleration method for the same record set. It is found that both methods have similar accuracy for estimating the structural mean response. However, the method proposed herein is more effective in reducing the variability of the structural responses. Also, the proposed method is more advantageous for the time-history analysis of long-period structures or structures with more severe nonlinear responses under strong seismic excitations.