Parametric Study of Low-Ductility Concentrically Braced Frames under Cyclic Static Loading

In moderate seismic regions, engineers are permitted to use a response modification coefficient, R, of 3 in the design of steel systems without any specific seismic detailing or proportioning requirements. Recent research, however, has raised questions regarding the reliability of concentrically braced frame (CBF) systems designed using the R=3 provision because their performance under the maximum considered earthquake seismic hazard is inherently dependent on reserve capacity - lateral force-resisting capacity outside the primary seismic force-resisting system (SFRS). In a previous study by the authors, two full-scale two-story CBF tests were conducted to better understand seismic behavior and associated reserve capacity after significant damage in the primary SFRS. These tests were (1) an R=3 chevron; and (2) an R=3.25 split-x ordinary concentrically braced frame (OCBF). To complement the experimental program, a numerical parametric study was conducted to further distinguish the influence that two design parameters - system type and system configuration - have on overall system behavior including non-ductile limit states and reserve capacity. Although the prototype frames studied were designed for the same building and location, they exhibited a wide variety of limit states. The occurrence of particular classes of mechanisms and their associated reserve capacities was influenced by the system type and system configuration. Specific reserve capacity mechanisms depended on the location of damage within the primary SFRS. Predicting these damage locations is difficult because they depend on as-built weld overstrength. This paper explores the classes of reserve capacity mechanisms that were observed both experimentally and in the companion numerical simulations.