On the number of required response history analyses

This study investigates the number of real ground motions (GMs) required to accurately capture the structural responses when using code-, intensity-, and risk-based seismic assessment frameworks. A key consideration is the use of as-recorded hazard consistent GMs, which appropriately characterize the seismic hazard of the site, for performing response history analysis. A comparison of the structural responses of 3 steel moment resisting buildings using different sets of GMs is made with results obtained using a large set of GMs. The empirical results demonstrate that, in most cases, a risk-based assessment needs a larger number of response history analyses than an intensity-based assessment method to accurately capture the seismic demand on the structural systems. Also, given the same number of GMs, the structural responses from a risk-based assessment are more reliable and stable than those for an intensity-based method. Moreover, the results help to quantify the uncertainty in the structural responses due to using different sample sets of GMs given the applied number of GMs. In addition, the results indicate that a significant bias in structural responses is likely when following the recommendations from several seismic design codes, which suggest utilizing the maximum structural responses if 3 GM pairs are used for response history analysis. A detailed statistical analyses show that the application of 7 GM pairs are sufficient to accurately and reliably estimate structural responses.