Plastic and yield slenderness limits for circular concrete filled tubes subjected to static pure bending

The current slenderness limits in international design codes are often based on certain rotation capacities obtained from plastic bending tests of Concrete Filled Tubes (CFT). In the past, a plastic slenderness limit of λ_s=188 was obtained by the first author based on a fracture rotation limit of the steel tube. However, such limit may be questionable being brittle and insufficient for plastic design of CFT members, sub-assemblies and frames where adequate strain/deformation ductility is required. The main aims of this paper are to present (i) a new method to determine new ductile slenderness limits suitable for plastic design of structures based on the measured strains in plastic bending tests on CFT; (ii) a closed-form solution for the elastic and inelastic buckling strains of CFT under pure bending using a new simplified energy approach employing the well-known Ritz method. The critical strains obtained from such analysis were used to derive new slenderness limits for CFT; and (iii) finite element modelling of CFT and compare the experimental and numerical moment-rotation responses. The effect of concrete filling on the post-buckling strength of restrained tubes is quantified. The current design rules for unrestrained Circular Hollow Sections (CHS) in steel specifications are also compared with the restrained strength obtained from the tests. Two new compact and yield slenderness limits were derived based on the strength corresponds to the appearance of the plastic ripples during the test. The experimentally obtained and the theoretically derived slenderness limits are compared against the available limits in the design codes and standards. The newly derived compact limit of λ_p=79 was found in a good agreement with λ_p=72 specified for CFT in the ANSI/AISC 360-10 specification. However, the new yield limit of λ_y=150 was found considerably lower than λ_y=254 for CFT specified in the ANSI/AISC 360-10.