Assessment of seismic design provisions for multi-tiered ordinary concentrically braced frames

Multi-tiered braced frames (MT-BFs) are commonly used as lateral force resisting systems in tall single-story buildings such as performing arts and sports centers, industrial warehouses, and airplane hangars. Horizontal members (struts) are used to divide the tall single-story into several bracing panels or tiers, without intermediate floors or out-of-plane supports. Special conditions in MT-BFs during nonlinear seismic response lead to concentration of drifts in the tiers and impose additional flexural demands on the columns. These flexural demands, in combination with axial demands, can cause column instability and compromise the seismic performance of the frames. The seismic design provisions for multi-tiered ordinary concentrically braced frames (MT-OCBFs) are assessed in this study. MT-OCBFs are intended to achieve modest levels of ductility, and a relatively simple design procedure is used for them. The current design approach, contained in the 2010 AISC Seismic Provisions, requires an axial force amplification for the columns. In contrast, the newest design approach, contained in the 2016 AISC Seismic Provisions, requires an additional axial force amplification to approximately account for imposed flexural demands on the columns. This new requirement leads to larger column sizes, which in turn can dramatically modify seismic response. A set of eighteen frames, with varying total frame height, brace configurations (X, chevron, and split-X), and tier heights, are designed as per both provisions. Their seismic performance is assessed by employing nonlinear static and time history analyses on a three-dimensional, numerical model developed using the OpenSees simulation platform. The results show that the 2010 AISC Seismic Provisions severely underestimate column demands in MT-OCBFs, leading to significant inelastic drift concentration in one tier and column buckling. The 2016 AISC Seismic Provisions lead to larger columns, which improve redistribution of inelastic drift over the frame height and reduce story drifts, but do not necessarily reduce inelastic drift concentration. Potential for brace loss due to low-cycle fatigue fracture is apparent in both designs. These new provisions reduce the propensity for column buckling, but it is not necessarily prevented. Brace configuration also influences demands on the column. In general, the split-X configuration leads to larger in-plane flexural demands in taller frames and onset of column buckling at small story drift values. In contrast, the shortest frames in this study exhibited relatively better performance with the chevron bracing configuration