Strength of Circular Concrete-Filled Tubes with and without Internal Reinforcement under Combined Loading

Concrete-filled tubes (CFTs) have been used in civil engineering practice as piles, caissons, columns, and bridge piers. Relative to conventional structural steel and reinforced concrete components, CFTs have several advantages. The steel tube serves as both reinforcement and formwork, eliminating the need for both, and provides large tensile and compressive capacities; the concrete fill restrains buckling of the steel tube, which increases the strength, stiffness, and deformability of the section. In some cases, internal reinforcement is used to enhance the strength and facilitate connection to adjacent members. Although these properties are well accepted, the use of CFTs in practice is awkward because design provisions among codes vary significantly and previous research has not considered internal reinforcement. An analytical research study was undertaken to evaluate and improve design provisions for CFTs with and without internal reinforcement under combined axial load and bending. A continuum model was developed to simulate prior test results subjected to combined loading and the validated model was used to investigate the strength and inelastic performance of CFTs under combined loading. Current design provisions for CFTs were evaluated using the results of these finite-element analysis and previous test results. The comparisons indicate that current design approach provides good prediction of CFT capacity subjected only to bending or axial demands, but current provisions provide conservative values for the CFTs under general combined loading. An alternative $P\text{-}M$ interaction curve for CFTs was proposed.