| Because of its superior mechanical properties and seismic behaviors, the composite structure consisting of concrete filled steel tubular columns and steel beams is considered to be an ideal structure for modern buildings. And it is widely used in high-rise buildings. By now there have been extensive theoretical analysis and engineering application researches carried out in china and abroad, which improve the theoretical system of concrete filled steel tubular column and the calculation methods gradually. And The Technical Specification for Structures with Concrete-filled Rectangular Steel Tube Members (CECS159:2004) has further promoted the development of this structure. As far as we all know that the joint is the key part of the composite structure. Not only the internal force transmission, but also the mechanical properties and seismic behaviors are all determined by the joint connection, which also restricts the development of the excellent mechanical properties of the steel beams and concrete filled steel tubular column.In recent years, a lot of new joints have been proposed by researches, but few of them are practical to be used in engineering. Although there are some joints such as the connections with outer diaphragms, connections with interior diaphragms and connections with overhang interior diaphragms recommended by The Technical Specification for Structures with Concrete-filled Rectangular Steel Tube Members, it is far from satisfying the engineering needs in practice. Therefore, it is necessary for us to do more researches on the joints with simple, reasonable load transfer and superior seismic behaviors in order to promote the applications and development of the composite structure. The main works of this paper are as follows:1. Based on extensive reviews of the previous literatures, the advantages and disadvantages of various joints are proposed for design reference and the further study.2. The effects of the types and details of the existing joints on the mechanical properties of the joint are analysed here. The effects of the welding process with hole defects on the joint strength and seismic behaviors are discussed here, as well as the reasons of the brittle fracture of the vicinity of the weld caused by steel beam flange butt welds. The new type joint-dual vertical through diaphragms connection is put forward based on the previous improved methods and those discussions as above.3.17crossing joint specimens are designed and manufactured. The strain distribution of the joint core zone, the joint failure types and the ultimate bearing capacity are reviewed by the static tests. The positions of plastic hinges, the expanding regulations of plastic zones and the seismic behaviors of the joint are analyzed in the Quasi-static tests. This paper probes into the effects of various factors of the through diaphragms on the seismic behaviors of the joint. The results show that the through diaphragms effectively reduce the stress and strain gradients at the connections of the steel beam flange and steel tubular column, load-displacement hysteresis loops of the beam are of full shape similar to the spindle, not like knead shrinkage shape. There are not too much beam deformations and destructions within the hrough diaphragms, and the yield and failure of the joint as a kind of stable ductile fracture take place in the flange within0.5hb outside of the through diaphragms.4. The joint strain rules and seismic behaviors of static and quasi-static tests are simulated by establishing finite element model involving the contact nonlinear, geometric nonlinear and material nonlinear by using software named ABAQUS. And the simulation results and test results are compared. This paper further analyses the effect of axial compression ratio, concrete strength and the parameters of the through diaphragms, reveals the transfer mechanism of joint, and analyses the strain of concrete of the core zone as supplementary. The results show that the dual diaphragms through connection, which is a kind of asymmetric joint and changes the stress transfer path of the traditional joints, makes full use of the mechanical properties of the steel tube column. The stress neutral axis and the geometric centroid axis of beam end is misalignment. The steel beam flange tension transfers along the diaphragms in a "Z" glyph, while the pressure of steel beam flange transfers along the short column of the concrete in the diagonal core, and the joint stress transfer path is spatial system. The shape and the parameters of the through diaphragm have little effect on the seismic behavior of joint, but have a great influence on the stress distribution of diaphragm through.5. Based on the test researches and finite element analysis, the stress transfer path of the core zone is defined, and the transfer mechanism and the yield mechical of joint are revealed. And the formula of bending and shearing capacity of the joint are also derived by using the principle of virtual work and superposition method.Finally, the suggestions of further study on the subject are proposed on the basis of summarizing the paper. |
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