Seismic Collapse Performance of Multitiered Ordinary Concentrically Braced Frames

In tall, single-story buildings with steel concentrically braced frame (CBF) lateral force resisting systems, it is more efficient to replace a single brace or pair of braces between the base and the story (roof) level with multiple bracing panels or tiers, leading to a multitiered braced frame (MT-BF). MT-BFs lack intermediate out-of-plane supports at the tier levels, and most of the building mass is concentrated at the story level, so their seismic behavior is more complex than typical multistory CBFs. Inelastic response in MT-BFs during a seismic event can cause drift concentration in an individual frame tier and increase the propensity for column instability due to combined axial and flexural demands. These unique conditions have been the focus of studies that support the first-generation of MT-BF design requirements introduced in the 2016 AISC Seismic Provisions. Requirements for multitiered ordinary concentrically braced frames (MT-OCBFs), which is the focus of this paper, were based on a limited investigation, and the primary feature of the requirements is an axial force amplification (150% of the overstrength horizontal seismic load effect), intended to account for induced in-plane flexural demands. Prior to the introduction of design requirements specific to MT-OCBFs, they were designed as multistory frames without considering flexural demands. This study uses detailed nonlinear models to assess the seismic performance of a comprehensive set of MT-OCBF designs. The results show that potential for column instability in MT-OCBFs is reduced in designs considering the new axial force amplification, and frame collapse probabilities are within acceptable limits. The 2016 AISC Seismic Provisions also introduced an out-of-plane notional load requirement (0.6% of the vertical component of the compression brace force at each tier), which was intended to account for effects from buckling compression braces. Since this requirement does not influence column proportioning appreciably, and since performance was acceptable for MT-OCBFs, where the columns were designed without an out-of-plane notional load, this study suggests that it can be removed. MT-OCBF column instability is primarily related to axial force and in-plane moment, and the axial force amplification that accounts for this combination in a simple fashion is satisfactory.