Dynamic Analysis of Steel Multi-Tiered Special Concentrically Braced Frames

Steel multi-tiered concentrically braced frames (MT-CBFs) are commonly used in North America as lateral load-resisting systems of tall single-story buildings. MT-CBFs are composed of multiple tiers of diagonal braces stacked on top of each other along the height of a story. Past research showed that MT-CBF columns designed in accordance with the 2010 U.S. Seismic Provisions are prone to column buckling due to non-uniform distribution of inelastic seismic demands along the frame height. Special design provisions have been introduced in the current AISC Seismic Provisions to address limit states and ensure columns remain stable under seismic load effects. Nevertheless, the recent improvements lack full-scale experimental testing and comprehensive finite element simulations to validate the proposed design requirements further and improve design provisions. In this paper, the current seismic design provisions for multi-tiered special concentrically braced frames (MT-SCBFs) are evaluated using the finite element method. A two-tiered SCBF was first designed in accordance with the 2010 and 2016 AISC Seismic Provisions. A detailed finite element model of the frame was then created using the Abaqus program. The model was used to perform nonlinear history response analyses. The analysis results showed that the inelastic deformations in the frame, designed as per the 2010 AISC Seismic Provisions, are not uniformly distributed but rather concentrated in one of the tiers, which leads to column yielding and buckling. Whereas, the current design method led to distributing the frame's inelastic deformation along the frame height. Furthermore, it was found that column in-plane flexural demands are overestimated when the current seismic provisions are employed; however, the out-of-plane flexural demand of columns exceeded the code-specified demands.