Comparative Seismic Response Evaluation of Steel Multistory and Multitiered Ordinary Concentrically Braced Frames

Concentrically braced frames (CBFs) are widely used to resist lateral loads in buildings because they are relatively simple and economical to design and construct. In tall, single-story construction, it is often practical to replace a single brace or brace pair within the story with multiple bracing panels or tiers. This leads to a multitiered braced frame (MT-BF), which in contrast to the more traditional multistory braced frame (MS-BF) lacks intermediate out-of-plane supports between the ground and the roof levels. While the primary seismic energy dissipation mechanism in both systems is through brace inelastic axial response, in MT-BFs, the majority of the mass is concentrated at the roof level, which has important implications for seismic design and response. The unique conditions that arise in the inelastic response of a MT-BF during a seismic event are known to cause drift concentration and increase the propensity for column instability due to combined axial and flexural demands. This study employs detailed models that consider geometric and material nonlinearity to rigorously quantify the behavioral differences between the two system types. For both multistory and multitiered frames, the models can capture column buckling and story-sway mechanisms. Results from nonlinear static pushover and response history analyses for two pairs of frames are used to evaluate the demands that develop during an earthquake event, and new insights are provided on the effect of column orientation and bracing conditions. The paper shows that the potential for column buckling is not exclusive to MT-BFs; considerable in-plane flexural demands, combined with axial loads, are shown to cause in-span plastic hinges that can also lead to buckling of MS-BF columns. Out-of-plane moment demands are larger in MT-BFs due to the lack of restraint at the tier levels. Drift distribution tends to be improved in MS-BFs compared to the corresponding MT-BFs, but the potential for brace fracture under maximum considered earthquake seismic input remains high regardless of system configuration.