Concrete-filled steel circular tubes subjected to constant amplitude cyclic pure bending

This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of concrete-filled tubular (CFT) beams made of cold-formed circular hollow sections and filled with normal concrete. Cyclic bending tests were performed using a constant amplitude loading history on different CFT specimens with diameter-to-thickness ratios (D/t) ranging from 20 to 162. The CFT beams exhibited stable hysteresis behaviour up to the formation of plastic ripples and then showed considerable degradation in stiffness, strength, and ductility depending on the D/t ratio. Seismic capacity parameters are presented including strength, stiffness, rotation capacity, hysteresis loops and modes of failure of the specimens. Peak moments obtained in the cyclic tests are compared with those obtained previously in monotonic tests and also with design moments predicted using the available design rules for composite beams. The deformation ductility demand was determined and used to derive new fully ductile section slenderness limits suitable for seismic design.