Cyclic behavior and performance of beam-column connections in concentrically braced frames

Currently it is common for steel seismic force-resisting systems, particularly concentrically braced frames ∥CBFs∥, in moderate seismic regions to be designed using a response modification coefficient, R, of three, which allows for seismic detailing to be ignored. However, this procedure has no clear basis and there is no assurance that the amount of ductility inherent in these systems justifies the selection of R = 3. This issue was studied by Hines et al. [in press] using nonlinear time-history analysis and it was determined that there may be an unacceptable probability of structural collapse under maximum considered earthquake ∥MCE∥ level seismic hazard for buildings designed with R = 3. In order to mitigate the collapse potential of R = 3 structures, it may be more appropriate to develop an alternate design philosophy for moderate seismic regions where wind typically controls the design. In this approach, CBF braces and connections are designed for wind forces, but the brace-gusset connections are permitted to fracture under seismic demands. The resulting period elongation typically reduces seismic demands, but a semi-ductile reserve system is necessary to provide collapse prevention performance under these demands. One possibility for providing this reserve system is to employ the flexural strength and stiffness of the beam-column connections in the CBF after the braces are no longer active.