Design concepts for avoiding story mechanisms in steel MRFs under seismic loading

Abstract This paper proposes alternative seismic design approaches for steel moment‐resisting frames (MRFs) that improve their performance and reduce the number of undesirable one‐story and two‐story yield mechanisms that form under seismic loading, compared to MRFs designed using the conventional strong‐column‐weak‐beam (SCWB) design approach. The alternative design approaches are capacity‐design approaches that directly compare story shear force demands from nonlinear time‐history analyses (NTHA) with corresponding shear force capacities in design iterations to avoid one‐story and two‐story mechanisms. To implement the capacity‐design approach, the NTHA use special capacity‐controlled numerical models for design, in which the one‐story and two‐story shear force design demands are limited by the intended yield mechanisms of the MRF. Unlike the SCWB approach, the alternative design approaches directly address the cause of undesirable one‐story and two‐story mechanisms. The paper presents NTHA results for a set of MRFs, where some were designed using a conventional approach (including SCWB) and some were designed using the alternative approaches. The increases in nominal steel weight of the MRFs designed using the alternative approaches are relatively small compared to the weight of the MRF designed using the conventional SCWB approach. Compared to the MRF designed using the SCWB approach, the MRFs designed using the alternative design approaches have improved seismic performance with reduced tendency to form one‐story and two‐story mechanisms, more uniform story drifts, reduced maximum story drift, and improved collapse performance.