Experimental Research And Theoretical Analysis Of Seismic Behavior Of Concrete-filled Steel Tube Beam-columns Under Cyclic Loading

With the development of composite structures, the use of square concrete-filled steel tube (CFT) in high-rise buildings has been increasing. The seismic behavior of square CFT is being paid more and more attention to by researchers. Even though a lot of studies on the seismic behavior of square CFT have been made in the world, it is still insufficient in research on the constitutive relation for CFT material. In this paper, the constitutive relation for CFT material and the seismic behavior of square CFT beam-columns are investigated by testing nine specimens under a constant axial load and cyclically varying lateral load. The main work is summarized as follows.Nine specimens are tested for the study of the hysteretic behavior of square CFT beam-columns under cyclic loading with the main parameters being axial loading level n , slenderness ratioλand confinement indexξrespectively. The load-displacement (P-Δ) hysteretic curves of the specimens tested are plump in shapes, and have no significant pinch phenomenon, which indicate that square concrete-filled steel tube column has high energy dissipation capacity. The ductility factor of the specimens decreases with the increase of the axial compression ratio and the slenderness ratio, and increases with the increase of the confinement index.Square CFT columns are modelled under axial compression loading using nonlinear finite element programme ABAQUS. The effect of concrete compression strength f_c, section width-to-thickness ratio B/t and steel yield stress f_y on the effective stress-strain relationship for the steel tube and concrete infill is analyzed to develop an equivalent uniaxial compression stress-strain relationship mathematical equation of steel tube and concrete infill.Fiber-based beam-columns element in DRAIN-2DX, with the equivalent uniaxial compression stress-strain relationship of steel tube and concrete fibers suggested in this paper, is employed to calculate P-Δhysteretic relationship of CFT beam-columns under a constant axial load and cyclically varying lateral load. A hundred and twenty-four CFT beam-columns are analyzed with the parameters including confinement indexξ, axial loading level n and slenderness ratioλ. Based on the analysis results, the simplified model of P-Δhysteretic relationship of CFT beam-columns is developed.Absorbed hysteretic energy at each cycle is analyzed for the experimental specimens. The cumulated damage model based on the theory of energy dissipation and hysteretic behavior is employed to evaluate the damage index for square CFT beam-columns under cyclic loading. The results show that the damage indices of the specimens range between 0.6 and 0.7, as the flange of the specimen bulged, and between 0.8 and 0.9, as the specimen damaged entirely.