A total of 11 L-shaped multi-cell concrete-filled steel tubular stub columns were fabricated and researched in axial compression test. The key factors of width-to-thickness ratio D/ t of steel plates in column limb and prism compressive strength of concrete f _ck were investigated to obtain influence on failure mode, bearing capacity, and ductility of the specimens. The test results show that the constraint effect for concrete provided by multi-cell steel tube cannot be ignored. The ductility decreases with the increase of width-to-thickness ratio D/ t of steel plates in column limb. The bearing capacity increases and the ductility decreases with the increase in prism compressive strength of concrete f _ck . A finite element program to calculate concentric load–displacement curves of L-shaped multi-cell concrete-filled steel tubular stub columns was proposed and verified by the test results. A parametric analysis with the finite element program was carried out to study the influence of the steel ratio α, steel yield strength f _y , prism compressive strength of concrete f _ck , and width-to-thickness ratio D/ t of steel plates in column limb on the stiffness, bearing capacity and ductility. Furthermore, the design method of bearing capacity was determined based on mainstream concrete-filled steel tubular codes.

A numerical analysis based on previous experiment has been carried out on T-shaped concrete-filled steel tubular columns subjected to constant axial compressive load and cyclic lateral loads. Tensile bar stiffeners were introduced to be welded on inside surfaces of steel tube to postpone its local buckling and to enhance the confinement of steel tube for concrete. A modified fiber-based method was developed to establish numerical modeling program of specimens’ cyclic behavior, incorporating the effect of stiffeners on postponing steel tube’s local buckling and the confinement for concrete. The reciprocating movement of inflection point along frame column is also considered in the numerical program. A simplified arc-length method was employed as iterative control algorithm of the numerical model. Horizontal load–displacement hysteretic curves of specimens were calculated with the numerical model and verified with test results. A restoring force model based on experimental investigation was proposed as simplified method for engineering practice.

This paper discussed the stiffness and cracking behavior of a new kind of reinforcement-stiffened cold-formed U-shaped steel and concrete composite beam (RCUCB) under negative bending. The static behavior of the RCUCB was discussed based on a negative bending experiment of 10 specimens. The reinforcement system consisting of the inverted U-shaped reinforcement, reinforcement truss, and bottom reinforcement was shown to maintain the integral action of the RCUCB, especially after the yield load. The RCUCB was found to behave well in deformability, ductility, and plastic rotation ability under negative bending: (1) the failure deflection and ductility factor reached $L_0/33$ (where $L_0$ is the effective span) and 17.7 respectively; (2) the ductility factor increased with decreasing steel ratio and decreasing reinforcement ratio; and (3) approximately 84.7%–94.0% of the web height yielded at the peak load, resulting in a plastification coefficient of 1.3–1.9. The damage process was defined with the cracking, yielding, and buckling loads approximately equal to $0.16P_u$ , $0.61P_u$ , and $0.90P_u$ , respectively. During the experiment, the degree of shear connection in the negative bending area showed little effect on the negative bending behavior before yield load. Calculation methods for initial stiffness, second stiffness, and cracking moment were proposed, and the calculation results agreed well with the test results.