Tests of cold-formed circular tubular braces under cyclic axial loading

This paper describes a series of tests to failure of fix-ended tubular braces subjected to cyclic concentric axial loading. The braces were made from cold-formed steel grade C350L0 (350 MPa nominal yield stress) circular hollow sections (CHS). Nine different diameter-to-thickness ratios in the range of 19<D/t<56 that have a moderate member stenderness in the range of 25<KL/r<41 were tested. The effects of three loading protocols on the inelastic hysteresis behavior of the CHS braces were examined. The CHS braces exhibited stable hysteresis behavior up to local buckling, and then showed considerable degradation in strength and ductility depending on KL/r and D/t ratios. First-cycle buckling loads were compared with design loads predicted using a number of steel specifications. The effects of section and member slenderness on strength, ductility, and energy absorption capacity of the braces were examined. The structure response factor (ductility index) was determined and used to derive new section slenderness limits suitable for seismic design.