Effect of beam yielding on chevron braced frames

Abstract

Special concentrically braced frames (SCBFs) are commonly used as lateral-load-resisting systems in seismic regions. Chevron (or inverted V-braced) SCBFs are used to accommodate architectural openings, such as doors, windows, mechanical openings and elevators. However, current AISC provisions for SCBFs require beams of chevron SCBFs to be designed to resist large vertical and axial demands resulting from unbalanced brace forces due to post-buckling compressive strength deterioration and full tensile yielding of the braces. As a result, very large beams are required to support the braces, and chevron SCBFs are seldom built today. Recent research showed that limiting beam response to elastic behavior may reduce the overall deformation capacity of chevron SCBFs and some beam yielding may be beneficial. A research study was initiated to investigate the advantages of and limits for beam yielding. Six large-scale, single-story chevron SCBFs were designed, built, and tested to investigate current SCBF requirements and the impact of reducing the beam strength below current requirements to permit beam yielding. Nonlinear continuum analyses were performed using the Abaqus finite-element analysis program to extend the experimental research. The research shows that chevron SCBFs with moderate beam yielding may have improved seismic performance compared to other SCBF configurations. Yielding of the beam increases its deflection, but the increased beam deflection increases inelastic deformation capacity of the braces prior to brace fracture. The maximum lateral resistance and overstrength of the frame is reduced, but the resulting resistance remains larger than the design target and is retained through relatively large story drifts.