Experimental investigation of steel frames braced with symmetrical pairs of y-shaped concentric bracings

Abstract

Concentric bracings composed of three members arranged in y shaped geometry have been traditionally used to provide openings in braced bays. However, simultanous occurance of compression in three braces leads to instability and out of plane buckling of braces accompanied by low hysteretic energy absorption. In order to study the behavior of y-braced frames, a research program including experimental tests was conducted at BHRC† structural engineering laboratory. Quasi-static cyclic loading was applied to specimens including four full-scale two-bay frames with y-bracings of different cross sections and connection types. The bays are braced symmetrically to have a combination of tensile and compressive braces at all loading stages. The results show that out-of-plane buckling with single curvature in braces can be substituted by in plane, double curvature buckling through appropriate detailing of cross sections and connections. Thus, hysteretic energy dissipation of y-bracing is remarkably improved due to spreading of plastic strains in braces. In this paper, seismic performance of y-braced frame specimens and a reference X-braced frame are also assessed by capacity spectrum method.