Abstract

Design of chevron-configured special concentrically braced frames requires beams strong enough to remain elastic under idealized forces, where the tension brace sustains its full capacity and the compressive brace is either at 100% or 30% of its compressive capacity (Pcr). These requirements result in large, deep and stiff beams, which has resulted in a reduction in the use of SCBFs, with engineers preferring BRBs or other systems, including reinforced concrete shear walls. A recent AISC-sponsored project has investigated a new approach to designing these systems that includes a secondary yield mechanism of beam yielding. The research project tested six (6) one-story frames and one (1) three-story frame to investigate the impact of beam yielding on the seismic performance with a focus on the development of yield mechanisms and failure modes and drift-range capacity. Test variables included beam strength, beam stiffness and beam-to-columnconnection restraint. The results indicate that beam yielding increases the drift capacity of SCBFs while maintaining the design strength. However, beams must have adequate resistance to develop the full resistance of the braces prior to buckling and this requirement must be part of the design. Based on recommendations from the advisory panel, an additional nonlinear analytical study was performed to extend the experimental research studying the impacts of various design parameters outside the tested range and is reported on here. The study used experimentally validated numerical modeling to investigate parameters that were addressed in the experiments, specifically: (1) beam strength with a focus on the axial stress ratio induced by the brace unbalanced forces, (2) brace seismic compactness ratio and KL/r ratio, (3) brace angle, (4) beam-to-columnflexural strength ratio, and (5) beam-to-column connection type/flexural stiffness and strength. The results of this parameter study are combined with the experimental results to select and improve the optimal design expressions for the unbalanced load. It is expected that this new design approach will improve the economy and seismic performance of chevron-configured SCBFs. A design change proposal based on this research has been submitted for consideration by AISC Task Committee on Seismic Design.

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