Identifying and selecting critical connections for seismic response of steel moment resisting frames

In seismically designed steel moment resisting frames, it is well known that the system-level behavior does not lead to equal rotation demands on all beam-column connections, yet this observation from analysis has rarely been turned into a tool for design. In this regard, this paper examines the differences in seismically induced deformation demands on beam-column connections in steel Moment Resisting Frames (MRFs), with the goal of identifying critical beam-to-column connections for which changes to their local hysteretic behavior result in a significant change in the global probability of collapse of the frame. First, collapse fragility curves are developed for three-, six- and 12-story MRFs using reduced beam section (RBS) connections, representing current practice. The maximum rotation demands on the beam-to-column connections at each level are examined, and connections that exceed a specified mean annual frequency of exceeding the rotation at which their strength begins to degrade are defined as critical to the global performance of the frame. Next, to confirm the definition of these connections as critical, collapse fragility curves are developed for a set of frames that have high-performance low-damage self-centering sliding hinge joint (SCSHJ) connections at only the locations defined as critical, while having low-ductility pre-Northridge (PRENORTH) connections with lower rotational capacity at all other locations. Even with a limited number of connections with high ductility, these mixed connection frames surpass the performance of the frames with only RBS connections and achieve essentially the same system collapse performance as frames with SCSHJ connections at all locations. A simplified pushover analysis method for identifying critical connections is proposed and compared to the results obtained from the non-linear time history dynamic analyses. Finally, a process for selecting these critical connections is introduced, using a Direct Displacement-Based Design (DDBD) procedure to give a designer control over their locations. This paper is useful to both researchers and practitioners who seek to identify or select particular connections where increases in rotation capacity would have the greatest influence on the overall frame performance, whether for retrofit applications or to maximize the benefit of selectively applying emerging connection details that offer enhanced performance.