Experimental And Theoretical Study Of Dynamic Performance Of Concrete-Filled Steel Tubular T-Joints

With the wide application of steel tubular structures, the joint loads are increasing. In order to guarantee their capacities, the joints always need to be strengthened, and filling concrete into the chord is an effective method. However, there is still a lack of research on the dynamic performance of concrete-filled steel tubular (CFST) T-joints.In this dissertation, the dynamic performance of CFST T-joints is researched by using model test method, finite element simulation method and theory analytic method.Stress concentration factor is an important parameter to assess fatigue life of joints. In this study, 5 CFST joints and 3 tubular joints are tested under axial tension, axial compression and in-plane bending. Compared with tubular joints, CFST joints have lower stress concentration factors. The effect of axial compression ratio is also studied. The test joints are simulated by accurate finite element method, and it is verified that this method can be used for predicting the stress concentration factor of CFST joints. The existing standards and theory formulas are considered not proper for CFST joints by compared with test results. Therefore a method based on equivalent chord wall thickness for predicting the stress concentration factor of the joints is given, and different equivalence principles are used in the method under different loading conditions. It shows that the method can give better prediction of the stress concentration factors of CFST T-joints.Then, the seismic performance of CFST T-joints is studied. 6 T-joints (including 1 tubular T-joint) under quasi-static axial load and 5 T-joints (including 2 tubular T-joints) under quasi-static in-plane load are tested. The capacities, stiffnesses, ductilities and abilities of energy dissipation of the joints are studied. It shows that the CFST T-joints have excellent seismic performances and higher ultimate capacities, the failure modes and the ultimate capacities of CFST T-joints are different from which of tubular joints.Accurate finite element models of CFST T-joints are established by considering the material nonlinearity, the geometry nonlinearity, the contact nonlinearity between the tube and the concrete and the real height of the welds. First of all, the finite element models are validated by simulating the entire damage and fracture processes of the joints. Then the effect of geometrical parameters of CFST T-joints is studied, in which the thin-walled CFST T-joints are specially considered. Based on the finite element analysis results, the failure modes and the ultimate capacities of CFST T-joints are influenced by the brace to chord diameter ratio, the diameter to thickness ratio of chord and the wall thickness ratio between brace and chord members. The joint capacity efficiencies are studied. It shows that, compared with tubular T-joints, CFST T-joints have higher joint capacity efficiencies in most cases, and can easily satisfy the design requirement of "strong joint, weak member". Then the failure modes and the ultimate capacities of centrifugal concrete-filled steel tubular joints are studied by finite element method, and the applicability of centrifugal concrete-filled steel tubular joints is provided.Based on the test and the finite element analysis results, theoretical methods for predicting the ultimate capacities of CFST joints are studied. Mechanical models considering the interaction between tube and concrete, the height of the welds and the accurate thickness of the punching shear plane are established. The tension capacity formula and the moment capacity formula for chord punching shear failure mode and the compression capacity formula of thin-walled CFST joints for concrete crushing failure mode are proposed. The simplified formulas for engineering design are also proposed. Then the failure mode criteria are studied.