Experimental investigation of curved steel knee braces with adjustable yield displacements

Abstract

The performance-based seismic design suggests multiple performance objectives for building structures. A preselected yield displacement may be chosen as a design target to control the inelastic deformation threshold of structural elements. The yield displacements of knee-braced steel frames are related to the yield displacements of the main lateral-force-resisting elements, i.e., knee braces. However, the yield displacements of the commonly used straight steel knee braces (SSKBs) are difficult to adjust because they are inherently proportional to the steel yield strain and brace length. A special type of steel knee braces, called curved steel knee braces (CSKBs), is proposed in this paper. An eccentricity is introduced into the CSKBs by a circular arc-shaped axis to adjust the yield displacement. A simplified physical model is proposed to approximate the backbone curve of the CSKBs. An experimental study is conducted on three CSKB specimens, and the adjustability of the yield displacement of the CSKBs is verified. The test results also show that all specimens exhibit an asymmetric hysteretic behavior. High equivalent viscous damping ratios (0.29–0.44) and high tensile fully plastic stiffness ratios (0.059–0.298) are obtained with a maximum strain of approximately 0.02. The ultimate compressive loading tests of two specimens indicate that the strength degradation is more moderate than that in the SSKBs that are not buckling-restrained.