Experimental Study On Hysteretic Behavior Of Concrete-filled Double Skin Steel Tube Overlapped K-joints

Concrete-filled double skin steel tube overlapped K-joints is a reinforced joints in which the inner casing is added to the chord and then the ordinary concrete and fly ash concrete are poured between the chord and the inner casing.Concrete-filled double skin steel tube overlapped K-joints can significantly improve the stiffness and bearing capacity of joints and effectively reduce the plastic buckling damage of the chord surface due to the effect of concrete,and the resulting overall mechanics of the reinforced joints is better than the effect of using the two materials respectively.Based on the project of the National Natural Science Foundation of China(Project No.:51468054),this paper studied the hysteretic behavior of Concrete-filled double skin steel tube overlapped K-joints using numerical analysis and experiments.The failure modes,hysteresis curves,skeleton curves,displacement ductility coefficients and energy dissipation coefficients of the joints were obtained,and the load capacity,ductility and energy dissipation performance of the joints were quantified,and then the seismic performance of the joints was evaluated.The main research work is as follows:(1)Experimental study on the hysteretic behavior of Concrete-filled double skin steel tube overlapped K-joints:Six overlapped K-joints were designed and manufactured according to two forms,K-joints with circular chord and circular braces and K-joints with square chord and circular braces,and the type of the specimens includes Unreinforced K-joints,Overlapped K-joints with the chord is concrete-filled double skin steel tube and Overlapped K-joints with the chord is fly ash concrete-filled double skin steel tube.The experimental results show that the failure mode of the empty joints are the plastic deformation of the chord surface,and the strengthened joints with concrete-filled double skin are the tensile damage of the braces.The chord strengthened by the concrete-filled double skin enhances the stiffness and bearing capacity of the joints,especially for the joints with square chord and circular braces,However,there is little difference between the strengthening effect of the joints strengthened by the concrete-filled double skin and the joints strengthened by the fly ash concrete-filled double skin.The chord strengthened by the concrete-filled double skin enhances the energy dissipation ability and the ductility performance of the joints.(2)Finite element analysis of hysteretic behavior of Concrete-filled double skin steel tube overlapped K-joints:Based on the finite element analysis software ANSYS and combined with material parameters obtained from the material test,a constitutive model of the material was established,and a reasonable calculation model was selected to simulate the overlapped K-joints.The failure mode,hysteresis curve,skeleton curve and bearing capacity of the finite element analysis are compared with the test results to verify the reliability of the finite element analysis model.The results show that the ANSYS software can reflect the bearing capacity and hysteretic behavior of Overlapped K-joints with the chord is concrete-filled double skin steel tube.Therefore,in the absence of test conditions,the ANSYS finite element analysis software for numerical simulation of the specimen is a good choice.(3)Parametric analysis of hysteretic behavior of Concrete-filled double skin steel tube overlapped K-joints:Based on the verification of the applicability of the finite element model,by changing the concrete grade,the steel grade of the chord and inner casing and the hollow ratio and,a total of 15 finite element models were designed to further study the hysteretic behavior of Concrete-filled double skin steel tube overlapped K-joints.The parametric analysis results show that the main failure mode of the joints under the four different operating conditions is the "drum" of the pressure brace and the "diameter shrinkage" of the tension brace.With the increase of the concrete grade,the bearing capacity of the joint does not change much,and the energy consumption capacity increases first and then remains stable.When the chord uses Q345B and the inner casing adopts Q235B,the bearing performance and energy dissipation performance of the joints are the best.With the increase of the chord hollow rate,the bearing capacity of the joint increases first and then decrease,but this trend does not change significantly,and the energy dissipation performance of the joint increases first and then decreases.