Slenderness Effects in Naturally Buckling Braces under Seismic Loads

Concentrically braced frames (CBFs) are commonly used in steel building structures to rapidly increase structural stiffness, especially in high seismic regions. However, conventional buckling braces have been verified to typically provide low ductility due to premature failures of local buckling and fracture. An innovative steel brace with a novel mechanism was previously developed to improve the seismic performance of steel braces. This study performed an additional experimental investigation on a series of subassemblage naturally buckling brace (NBB) specimens along with knife-plate end connections to evaluate the slenderness effects, fabrication details of the channel segments, and batten dimensions on the hysteretic properties of NBBs. It was found that stocky NBBs provided more comparable strength backbone curves between tension and compression and later local buckling and fracture initiation compared to slender ones. A general numerical modeling approach was proposed and verified to accurately capture all yield mechanism as well as measured responses. An analytical parametric study covering a wide variation of features, dimensions, and steel grades was then performed to establish an estimation of the strength backbone curve of NBBs. The proposed model equations were found to accurately estimate the yield strength and initial and postyield stiffness of NBBs in tension and compression, respectively.