Variable amplitude cyclic pure bending tests to determine fully ductile section slenderness limits for cold-formed CHS

This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of cold-formed circular hollow section (CHS) beams. Controlled-rotation, symmetrical cyclic bending tests were performed using a variable amplitude loading history on different sizes of compact CHS with diameter-to-thickness (D/t)(D/t) ratios ranging from 20 to 40. The CHS beams exhibited stable hysteresis behaviour up to local buckling and then showed considerable degradation in strength and ductility depending on the D/tD/t ratio. Seismic capacity parameters are presented including strength, stiffness, hysteresis loops and modes of failure for each specimen. Peak moments obtained in the cyclic tests were compared with those obtained in monotonic and cyclic tests published previously and also with design moments predicted using a number of steel specifications. New section slenderness limits suitable for the design and construction of seismic resisting structural systems were determined. A comparison is made between these seismic slenderness limits and the limits available in the design codes. The effect of the number of cycles on ductility and energy absorption is examined.