Experiments to Understand the Impact of Local and Global Slenderness on the Inelastic Deformation Capacity of Square and Round HSS Braces

Abstract

Concentrically braced frames (CBFs) are widely used as seismic force-resisting systems in low- to mid-rise buildings. In these systems, the braces serve as the primary yielding components. Ordinary CBFs (OCBFs) are intended to achieve a moderate level of ductility, while special CBFs (SCBFs) are intended to achieve a high level of ductility. Hollow structural sections (HSS) are commonly used as the brace component in CBFs and limits on their local slenderness (i.e., width-to-thickness or diameter-to-thickness) defined in the AISC Seismic Provisions (AISC 341-22) are used to achieve a target ductility and drift range capacity for OCBF and SCBF systems. To evaluate these limits, a comprehensive experimental program on the inelastic deformation capacity of HSS was conducted at the University of Washington. The test program was comprised of HSS braces with square and circular cross-sections covering a wide range of local and global slenderness ratios. The selected brace cross-sections include those that meet the high ductility and/or moderate ductility limits of the AISC Seismic Provisions and those that do not meet the current limits. The test setup enabled testing braces with various lengths, allowing for the examination of the effect of global slenderness, and its interaction with local slenderness, on brace inelastic deformation capacity. The experimental results demonstrate that the current AISC 341-22 local slenderness limits are overly conservative for square braces at moderate ductility and for circular braces at both moderate and high ductility. Findings also suggest that local slenderness limits could be more efficiently expressed as a function of global slenderness. It is clear from the experiments that braces with larger global slenderness can have larger local slenderness and still meet deformation capacity targets for braces in SCBFs and OCBFs, and braces with small global slenderness require smaller local slenderness to meet the deformation capacity targets.