Study On Shear Behavior Of CFST Column-RC Beam Light-dark Double Corbels Of Joints In Top-down Covered Excavation Metro Station

The method has been successfully applied in Xinhua Square transfer station,West University Street station,ZhongShan Road station of Hohhot Metro Line 1 and line 2.In the process of Covered top-down method,the structure is not completed in the same time.The joints of the CFST column and RC beam are very important.In the design of joints,in order to meet the needs of shear resistance,we designed a new type of joint which has two Annular corbels.It is used for later connection of CFST column with floor and longitudinal beam.Therefore,this new type of joint,the 1:4 scale model test is used to study the shear bearing capacity of the joint,the cooperative working mechanism and failure mode of the light-dark corbel,and then establish the three-dimensional numerical nodes by using ABAQUS.Based on the comparison between the numerical model and the experimental results,the shear capacity and key design parameters of the joint are studied.The expected research results are the basis for the application of this kind of node in the excavation and reverse metro station.Firstly,according to the analysis of the model test results,the failure process of the joint is that the bending cracks on the upper edge of the concrete beam in the core area of the joint extend to the dark bracket,and connect with the bending shear cracks in the core area of the joint to form the shear failure.The joints are all shear failure of concrete in the core area of the joint,and the ring bracket is not damaged;Secondly,according to the model test and ABAQUS finite element analysis,it can be seen that the cooperative working mechanism of the light and dark double corbel: at the initial stage of loading,the bonding force between the steel tube wall and the concrete beam bears the initial shear force,when this part of the bonding effect fails,the slip of the concrete beam is hindered by the light and dark corbel,because the force path of the dark corbel is short,the dark corbel plays a greater role in the loading process,when the dark corbel bends At the end of loading,the forces on the exposed and concealed corbel are uniform and bear the same shear force,which shows that they can work together;Thirdly,the ultimate shear capacity of the light-dark double corbel joint is about the sum of the bearing capacity of the single light and dark corbel joint under the same structural conditions,which proves that the light and dark double corbel joint can effectively improve the shear capacity of the joint under the same beam parameters;Fourthly,based on the study of the key parameters of the core area of the joint,such as the height and quantity of the vertical rib of the ring bracket,the amount of the upper ring plate through the center and the number of the bracket,it is concluded that increasing the number and height of the ring bracket rib and the number of the ring bracket can effectively improve the shear bearing capacity of the joint.