Impact of column-to-beam strength ratio on the seismic response of steel MRFs

Abstract

The strong-column/weak-beam seismic design concept in moment resisting frames is perhaps one of the least well-understood design provisions. This study is aimed at improving the understanding of the effect of column-to-beam strength ratio (CBSR) on several seismic performance measures. Through nonlinear analyses of 3-, 9-, and 20-story moment resisting frame, the impacts of CBSR on member ductility demands, maximum inter-story drifts, and floor acceleration amplifications are investigated. For each frame, the value of CBSR is varied by changing the yield strength of the material and/or by altering sizes of the columns. The probabilities of exceeding certain performance limits are investigated through fragility analyses. The single curvature bending of the columns within a story is found to be inevitable due to the participation of higher modes of vibration. Consequently, under large ground motions, the yielding of the columns is expected even for CBSRs larger than 2.0. The fragility relationships were used to calculate the design force modification factors needed for achieving a comparable probability of column yielding for different values of CBSR. The values of the yield base shear and the inter-story drifts were found to depend more on the strength of the beams than the value of CBSR. The floor acceleration amplification was found to be the least sensitive demand parameter to the CBSR.