Effects of far-field and near-fault cyclic loadings on seismic performance of naturally buckling braces in pairs

Abstract

Naturally buckling braces (NBBs) have been previously developed through combining low-yield and high-strength steel channel segments with an intended eccentricity along the brace length. The developed NBB formed a novel mechanism and was experimentally verified to provide a ductile and stable seismic performance with the features of early yielding, large post-yield stiffness in tension and well-postponed local buckling in compression compared to conventional buckling braces. In this study, a series of paired NBB specimens have been tested to clarify the seismic performance of NBBs in pairs in frame systems, such as the effects of brace angles, member and local slenderness, and adopted steel grades of the low-yield segment. Two types of cyclic loading protocols, including far-field and near-fault cyclic loadings, were applied to investigate near-fault effects on the seismic performance of the paired NBBs compared to that under far-field cyclic loading. The test results verified that the paired NBBs provided a comparable, ductile, and stable hysteretic performance in tension and compression under both far-field and near-fault loading protocols. A procedure to transfer the nonlinear measured strength backbone curves to bi-linear strength curves was proposed to present the major hysteretic properties of the paired NBBs, and several general equations were established to accurately estimate the major hysteretic properties. Strength degradation was observed among the specimens and was more significant under near-fault loading. An equation was developed to accurately quantify the strength degradation according to the member and local slenderness ratios of the paired NBBs, and the relationship of the strength degradation between far-field and near-fault loadings was established. Cumulative plastic deformation and total energy dissipation of specimens in the study were evaluated to quantify the fatigue life of the paired NBBs. Finally, an improved modelling approach of the paired NBBs was suggested and experimentally validated to more accurately represent the seismic performance of the paired NBBs in the frames.