Seismic Behavior of Low-Ductility Concentrically-Braced Frames

Current codes allow engineers in moderate seismic regions to ignore seismic detailing as long as they design the structure using R = 3. However, recent research has raised questions regarding the reliability of such systems. When subjected to the maximum considered earthquake seismic hazard, the collapse of these systems becomes inherently dependent on their reserve lateral load-resisting capacity. Several sources of reserve capacity in these structures have been identified: connections in the gravity framing system, connections in the braced frame system, column continuity, base fixity, and brace re-engagement. In this paper, the results of several parametric OpenSees studies are presented in order to evaluate the effect of these sources of reserve capacity in traditional R = 3 systems, with focus on a three-story prototype chevron concentrically-braced frame. Nonlinear, inelastic, static analysis, as well as nonlinear, inelastic, incremental dynamic analysis for a suite of earthquake ground motions was performed. Various limit states for these structures were identified, and failure of the welded connection between the brace and gusset plate was identified as the prominent event affecting collapse performance.