Concrete-filled circular steel tubes subjected to pure bending

Current design codes and standards provide little information on the flextural behaviour of circular concrete filled tubes (CFT) as there have been few experimental studies. There are significant differences in d/t-limits recommended in various codes for CFT under bending. This paper presents an experimental investigation of the flexural behaviour of circular CFT subjected to large deformation pure bending where d/t = 12 to 110. The paper compares the behaviour of empty and void-filled, cold-formed circular hollow sections under pure plastic bending. It was found that for the range of d/t≤40, void filling prevented local buckling for very large rotations, whereas multiple plastic ripples formed in the inelastic range for specimens with 74<d/t≤1 10. In general, void filling of the steel tube enhances strength, ductility and energy absorption especially for thinner sections. Based on the measured material properties, the plastic d/t-limit was found to be 112. A simplified formula is provided to determine the ultimate flexural capacity of CFT. The existing design rules for the ultimate moment capacity of CFT may be extended conservatively to a new slenderness range of 100<α _s < 188.