Seismic Evaluation of Steel Moment Frame Buildings Designed Using Different R -Values

Current seismic design procedures, which permit the estimation of inelastic deformation capacity of lateral force resisting systems, have been questioned since no rationality exists for determining the values of R tabulated in seismic design codes. This study focuses on the seismic behavior of three-, nine-, and 20-story steel moment resisting frame (MRF) structures designed using different R factors (8, 9, 10, 11, and 12). A total of 30 different structures were subjected to 20 ground motions representing the hazard level equal to a 2% probability of being exceeded in 50 years (2% in 50 years) for a detailed case study. Then, a reliability-based performance evaluation procedure, which provides a degree of confidence for satisfying the collapse prevention (CP) performance objective, was applied to model buildings to assess the effect of the R factor variations. The results show that the current R factors provide conservative designs for the three- and nine-story buildings for achieving the CP performance objective. However, the 20-story buildings designed with different R factors without the minimum spectral acceleration bound on CS prescribed in the 2000 International Building Code show a low level of confidence for achieving CP performance.