ABSTRACT
Five braces were selected from the pool of 21 coldform large-size brace specimens made from ASTM A500 grade C square tubing and tested under inelastic quasi-static cyclic loading. As shown in Table 1, the specimens were selected such that the global slenderness (KL/r), the width-to-thickness aspect ratio (b0/t, where b0 = b − 4t), and the loading protocol (symmetric or asymmetric) vary one at a time from one specimen to the other. Crosssection aspect ratios b0/t are close to or greater than the limit of 17.6 prescribed for braces with KL/r 100 in the CSA S16 steel design standard
1 INTRODUCTION
Ductile braces are designed to dissipate energy during yielding and inelastic deformations at extreme events like earthquakes. Steel braces made of Hollow Structural Sections (HSS) are prone to inelastic local buckling when globally buckle under reversed cyclic earthquake actions, which triggers stress concentrations at the plastic hinge locations that may eventually lead to cross-section fracture. Hence, brace fracture resistance is of major concern as bracing members are expected to withstand such actions without jeopardizing the seismic safety of the lateral load resisting system. Recent developments on quantification of ultra-low cycle fatigue (ULCF) life, simulation of ULCF, and prediction of the instance of earthquake-induced ductile crack in steel components under inelastic loading have provided researchers with new tools to investigate the parameters that are expected to affect ULCF life of steel braces (Kanvinde and Deierlein, 2007; Myers, 2009). Equipped with experimental testing and advanced Finite Element Methods (FEM), these so-called micromechanical models have been utilized in this study to evaluate
(CSA, 2009). The specimens were designed with welded slotted gusset plate end connections. The brace specimens selected were sized to buckle outof-plane with gusset plate connections detailed with a free length equal to twice the gusset plate thickness to allow ductile inelastic end rotations (see Figure 1). Cover plates were provided at the brace ends to reinforce the net section at those locations (Haddad and Tremblay, 2006).
