| Concrete Filled Steel Tube (CFST) has already experienced several structural forms from the traditional ones after many years of development. As CFST has been used widely in high-rise buildings, long-span bridges etc, the columns are required to increases not only high bearing capacity but also good ductility. Therefore, one new CFST column reinforced by circular steel tube or steel column or CFRP cylinder, has arisen as heavy-loaded columns which is called composite CFST structure in this thesis. This structure can effectively reduce members' cross-section area, and also significantly improve the load capacity, anti-fire capabilities with better ductility. In recent years, the composite CFST structure has been verified its feasibility in theoretical research and engineering practice, showing a great potential for more development.To facilitate the analysis and design applications, the composite CFST was classified as four forms, i.e. inner-steel-tube-enhanced CFST, steel-column-enhanced CFST, CFRP-enhanced CFST and hollow composite CFST. For various sections of the composite CFST, this paper established a unified calculation method of the compressive stiffness and bearing capacity by theoretical analysis and numerical calculation. On the other hand, the connection is the very important and weak part in CFST system. While the reinforced concrete beam-slab structure is applied in the majority of Chinese buildings. Therefore, to solve the problem of connection between CFST and RC beams is one of the keys to apply CFST structure. This paper put forward one new connection——Ring Beam Joint with Discontinuous Outer Tube between composite CFST and RC beams. The outer tube is discontinuous in joint zone, longitudinal reinforcement of RC beam can be run through to ensure loads transferred in the joint core zone, and the construction can preserve the structural integrity, so this new joint has a good application prospects.The mayor contributions of the work presented in this thesis are listed as follows:1. Compressive stiffness and bearing capacity are the important mechanical properties of structural elements. Based on simple superposition, unified theory of CFST and method of elasticity, axial compression stiffness of composite CFST has been studied. The main factors such as coefficient of lateral deformation of concrete, Poisson's ratio of steel and steel ratio, evidently influenced the axial compression stiffness of the composite column.2. This paper presented the composite confinement factor in order to apply unified theory of CFST into the strength calculation of composite CFST, and the formula can be easily used for all the sections included. Meanwhile, every component of the composite CFST under the ultimate state was analyzed by the Twin Shear Unified Strength Theory. Then according to principle of superposition, the compressive bearing capacity of the composite columns can be calculated by the method of limit equilibrium. The both methods have achieved good agreement with the experimental results, and the influencing factors to the axial bearing capacity were also analyzed.3. Optimal design was obtained for every type composite CFST columns by Parameter Optimization according to the axial behaviors analysis. With consideration of the connection with RC beams, the section of circle inside and square outside was considered as more ideal one among the various sections of composite CFST columns by comparison of mechanical properties and steel ratio among all the sections of composite CFST. This optimal composite CFST structure can be widely applied in practice engineering.4. In order to study the seismic behavior of this new connection system, the thesis carries out the experiment of 4 specimens under the cyclic loading. Failure modes, damage mechanism, hysteresis curve, skeleton curve, displacement ductility, stiffness degradation and energy dissipation capability of the joint under fixed axial force and repeated shear force were analyzed in detail. The restoring force model of the Ring Beam Joint with Discontinuous Outer Tube was established with consideration of the better hysteretic behavior, which can be available of the elastic-plastic seismic response analysis as references.5. A three-dimensional nonlinear finite element analysis was performed to simulate the joint behavior by software ANSYS. The obtained load-displacement curve of monotonic loading simulation was compared with the skeleton curve of experimental results. At the same time the axial properties was studied in order to analyze the joint's internal mechanism. The axial behaviors were discussed under the influence of the strength of concrete, inner steel tube and reinforcement of ring beam. Coming from the results of experiment and finite element method, some suggestions were put forward for checking the bearing capacity, and some design method and construction measures were also given for the new joint.
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