Evaluation and Improvement of the Canadian Seismic Provisions for Moderately Ductile Steel Concentrically Braced Frames with Chevron and Split-x Bracing

Steel concentrically braced frames (CBFs) are widely used as the lateral load-resisting system of multi-storey buildings. Various bracing configurations are used as CBFs, including X, diagonal, V, Inverted V (or chevron), and two-storey X (or split-x). Among these configurations, chevron and split-x CBFs have gained popularity as they provide more architectural freedom in buildings by facilitating door and window openings.

Under seismic loading, frame lateral inelastic deformation may not be distributed evenly between the storeys due to the inherent poor hysteretic response of diagonal braces. Hence, lateral inelastic deformations tend to develop in one or a few storeys but not all the storeys, despite design assumptions.

Although steel CBFs have been the subject of numerous studies in the past, little information exists on their design parameters in the framework of the Canadian design practice, in particular, column flexural demands and their stability design parameters, loading scenarios to determine the unbalanced loads applied on the beams, and how these parameters influence the lateral response of CBFs.

Numerous CBFs were generated by varying bracing configuration, frame total height, beam design method, column orientation, and column base condition. The frames are then modelled numerically and their seismic response is examined by performing nonlinear response history analysis. The results obtained from the analyses are used to examine drift concentration, investigate column flexural demands, quantify brace forces affecting beam design demands, and evaluate beam and column design parameters. The analysis results were finally used to propose improvements to beam and column design guidelines and mitigate drift concentration in CBFs in the Canadian steel design standard framework.