Seismic Performance Factors of Special Moment Resisting Steel Frames Subjected to Far- and Near-Field Ground Motions

Different and to some extent poor seismic performance of structural systems under various types of near-field earthquake excitation, made re-assess and re-evaluation of “seismic performance factors” used in building codes, an inevitable important task. In this paper, seismic performance of special moment resisting steel frame system (SMRSF) under near-Field (with and without pulse) and far-field record excitation is investigated through FEMA P695 methodology. In order to cover the “design space” of the selected structural system, archetypes consisted of 1, 2, 3, 5, 8 and 15 story buildings with 4 and 8 meters bay are selected and designed based on Iran’s national building codes for a “very high seismic” region. Corresponding non-linear models are built based on most recent advances in structural components modeling using OpenSees software. At first by performing non-linear static analysis, overstrength factor and period-based ductility are evaluated and quality of non-linear models is controlled. Afterwards, incremental dynamic analyses (IDA) are performed using far-field, near-field pulse like and non pulse like records. Finally, by using IDA results, “adjusted collapse margin ratio (ACMR)” of the models are calculated and compared to “allowable collapse margin ratio (ACMRallowable)”. Therefore, seismic performance of the models are evaluated and “response modification coefficient” (R) for the system is investigated and compared with this factor under different types of ground motion records. Results indicated that except for 15 story buildings, proposed “response modification coefficient” and “over-strength factor “for SMRSF system are adequate under far-field records. However under pulse like near field ground motions, it was observed that short period structures are to some extent vulnerable. For long period structures, in contrast with far-field records, seismic performance of structures designed by prescriptive provisions has adequate performance under near-field pulse like motions.