Experimental validation of locally annealed braces towards ductile and efficient concentrically braced frames

Abstract

In special concentrically braced frames (SCBFs), brace yielding and buckling are the preferred mechanisms of seismic energy dissipation, and mid-length brace fracture is the preferred failure mode. However, due in part to material overstrength and limited ductility typical of cold formed square steel braces commonly used in the USA, achieving the desired seismic performance requires careful attention to detailing which affects the perceived efficiency of this system. This paper presents the results of experimental testing to validate a recently proposed simple ductile fuse to improve the efficiency and ductility of SCBFs. The fuse is created by selectively reducing the brace material strength via annealing the steel. This in turn reduces connection force demands, increases brace material work hardening rate leading to improved brace ductility, and improves normalized brace compressive strength due to residual stress relief. Detailed material testing on ASTM A500 steel from square HSS tubing was conducted to characterize the mechanical property changes from the annealing process and FE parametric studies were conducted to study how changes in material properties affect brace ductility. Finally, the results of 8 full-scale brace tests and detailed post-test FE analysis using calibrated constitutive models are presented to validate the proposed concept. Based on the results of the study, preliminary design recommendations are proposed, and future research needs are identified and discussed.