Mechanical Behavior Of Multi-planar Tubular Joints And Gusset Plate Joints Of Concrete-filled Steel Tubular Truss

Concrete-filled steel tubular(CFST)truss structure is widely used in modern engineering because of its advantages of excellent mechanical properties,good integrity,economic and beautiful appearance and so on.The most common joints in CFST truss structures are tubular joints and gusset plate joints.However,there is no corresponding design guide for these joints in our country,which makes the design of these joints lack of theoretical support.Therefore,the mechanical properties of tubular KK-joints and gusset plate T-joints used in CFST truss structures were systematically studied in this thesis.Firstly,based on the axial load test of CFST K-joints,the refined finite element models of the joints were established.The correctness and accuracy of the numerical simulation method were verified by comparing with the test results.On this basis,finite element models of CFST KK-joints were established,and parametric analysis was conducted.The research results show that the wall thickness of the brace have influence on failure modes and ultimate strength of CFST tubular joints,while have little influence on stiffness of the joints,the spatial angle between the brace have little influence on failure modes,ultimate strength and stiffness of CFST joints,and filling concrete in the chord can effectively improve ultimate strength and stiffness of tubular joints.Finally,the applicability of the ultimate strength equations of steel tubular joints in design guides to CFST KK-joints were discussed,and some design suggestions on the ultimate strength of the joints were given.The stress concentration factor(SCF)is an important parameter to evaluate the fatigue life of joints.The SCF of CFST KK-joints under axial load were studied by using the finite element parametric analysis method in this thesis.The results show that filling concrete in the chord can effectively reduce the SCF of the joint and improve the fatigue life of the joint,the influence of the spatial angle between the brace on the chord is negligible,while the influence on the brace is obvious,the wall thickness of the brace has an influence on the SCF of the joint,but it does not change the distribution of the SCF along the intersection line.Finally,it is suggested that the influence of the spatial angle between the brace can be ignored in the fatigue design of the chord,but its influence should still be considered in the design of the brace.Based on the bending test of CFST gusset plate T-joints,refined finite element models of the joints were established.The details of the joints,such as the gusset plate,the connecting plate,the cross insert plate,the bolt and the weld,were considered in the models,and parametric analysis was conducted.The results show that the failure modes of CFST gusset plate joints are different from those of steel tubular gusset plate joints.For CFST gusset plate joints,with the increase of the wall thickness of the chord,the ultimate strength of the joints increases and the failure modes change,while with the increase of the wall thickness of the brace,the ultimate strength of the joints increases and the failure modes also change.By comparing the results of finite element method with the design guides domestic and overseas for steel tubular gusset plate joints,the applicability of the design guide formulas were discussed,and the design suggestions on bending ultimate strength of CFST gusset plate T-joints were given.