Seismic performance assessment of steel multi-tiered ordinary concentrically-braced frames

Multi-tiered braced frames (MT-BFs) are created when a tall single-story bay is divided into multiple bracing panels over the height, with no diaphragms or out-of-plane column supports between the base and roof. The unique conditions in MT-BFs make the column susceptible to instability from combined axial and bending moment demands during nonlinear seismic response. The present research is investigating the seismic behavior of multi-tiered ordinary concentrically-braced frames (MT-OCBFs), which are designed with a relatively simple procedure and are expected to provide limited inelastic deformation capacity. The baseline for the study is the 2010 AISC Seismic Provisions, which require column design for an amplified axial demand. The new 2016 AISC Seismic Provisions, which are based on a limited initial evaluation, stipulate an additional amplified axial demand to approximately account for moment in the MT-OCBF column. This approach is being more comprehensively studied, and the interaction effects of axial force, in-plane and out-of-plane moments are being thoroughly assessed. This paper presents the results from nonlinear static (pushover) and response history (dynamic) analyses for a set of prototype MT-OCBFs with X bracing. Significant drift concentration and column buckling is observed in all the baseline frames. The larger column sections resulting from the new design provisions possess adequate strength to delay column buckling in most cases and tend to improve the inelastic drift distribution. However, the columns do not necessarily have sufficient stiffness to eliminate the potential for brace fracture. For OCBFs, a simple but effective design approach, which does not require rigorous capacity-based calculations, is desired to control drift concentration in a single tier and maintain column stability.