Seismic Stability of Multi-Tiered Ordinary Concentrically-Braced Frames

Multi-tiered braced frames (MT-BFs) are created when a tall single-story braced bay is divided into multiple bracing panels over the height, with no diaphragms or out-of-plane column supports between the base and roof. Due to the unique conditions in MT-BFs, during nonlinear seismic response, they are susceptible to column instability due to combined axial force and bending moment. The present research is using numerical simulations to investigate the seismic response 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 previous version of the AISC Seismic Provisions (AISC 341-10), which require column design for an amplified axial demand. The newer AISC Seismic Provisions (AISC 341-16), which are based on a limited initial evaluation to develop the multi-tiered OCBF requirements, stipulate that MT-OCBF columns be designed for an additional amplified axial demand to approximately account for moment. This approach is now being more comprehensively studied, and the interaction effects of axial force, in-plane moment and out-of-plane moment are being thoroughly assessed. This paper presents the results from nonlinear static (pushover) analysis of a subset of the prototype frames. Concentration of inelastic deformations and column buckling were observed in some of the baseline designs, while the newer provisions allow for a more even distribution of inelastic demand over the frame height. For OCBFs, a simple but effective design approach is desired so that drift concentration in a single tier is limited and column stability is maintained, even without employing a rigorous capacity-based procedure.

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