Study On Mechanical Property And Load-Bearing Mechanism Of Embedded Perfobond RIB Shear Connectors

With the diversified development of the new type of composite and hybrid bridges, the perfobond rib shear connector (in short, PBL), as the core component of the joint part, is divided into two kinds:the composite PBL whose perfobond rib is welded on the bare structure, and the embedded PBL whose perfobond rib and accessory structure is embedded into concrete. The research results show that there is great difference among many equivalent bearing capacity models of the PBL, and there are significant deficiencies in these theoretical models with the usual concrete constitutive models. It also shows that the push-out test, whose result is proved to be conservative for the hybrid bridges, is suitable for the composite PBL. The rigidity character of the PBL in the initial loading stage is paid more attention, while the plastic behavior during the whole loading process is ignored. Therefore, the mechanical property and load-bearing mechanism of the embedded PBL is studied based on the test results and calculation results, including the following five aspects:(1) The experimental study of the embedded PBL:Accoding to the limitations of the push-out test,24specimens, sorted into8groups, were tested in the fracture test which was designed based on the joint part experiment of the hybrid pylon of the third Nanjing Changjiang river bridge and the anchorage system experiment of the fourth Nanjing Changjiang river bridge. The rib hole diameter, the rib thickness, the transversal confinement and the interface shear force was tested in this test. The working stages of the embedded PBL were divided based on the test results, and the failure modes were summarized and classified. The perfect failure mode of the embedded PBL is the coexistence of the shear-off failure of concrete dowel near the rib surfaces and the failure of perforated rebar without the failure of the surrounding concrete.(2) The establishment of the theoretical model of the PBL based on the micro plane model: According to the difficulty of the parameter determination in the concrete constitutive models, the RBF neural network parameter identification method was established based on the genetic optimization algorithm, and the parameter determination method of the micro plane model was proposed under the uniaxial and complicated stress state. The interface effect between the rib and concrete (in short, IFE) in the embedded PBL was simulated by the three-line interface constitutive model. According to the above, the theoretical model of the PBL based on the micro plane model was established with the help of the dynamics explicit solving method. The verification results show that the total bearing process of the connector, the failure process and the mechanical property of each component can be affected by the result of this model.(3) Study on the failure mechanism of the embedded PBL:The stress state of the concrete dowels under every failure mode was studied based on the calculation results of the theoretical model and the test results, and the failure mechanism under each failure mode was revealed. The results show that the shear-off failure of concrete dowels is caused by the sound condition of the concrete in the rib hole(in short, CRH) under the three-dimension stress state, the compressive failure of the CRH is caused by the small ratio between the first principal stress and the secondary principal stress under the weak transversal confinement, and the dissever failure of the CRH is caused by the poor quality of the CRH with the thin rib. Therefore, the necessary condition of the appearance of the perfect failure mode under the ultimate carrying state is the sufficient transversal confinement and the rib thickness no less than20mm.(4) Study on the load-bearing mechanism of the embedded PBL under the perfect failure mode:The mechanical property of the CRH and perforated rebar in each working state is analyzed, and their failure process is proved according to their stress state under the perfect failure mode. The generation mechanism and developing process of the IFE is investigated, and the process of load transmission and the load-bearing mechanism of the embedded PBL is analyzed. The results show that in the plastic stage, the front part of concrete dowel near the rib surface cracks firstly, and the crack gradually develops to a shear-break plane, while the intactness of the CRH. It also shows that the bending state of perforated rebar gradually changes to tensional state after the elastic stage; and the IFE is the important part of the bearing capacity and the stiffness of the embedded PBL during the whole bearing process.(5) Study on the macro mechanical behavior of the embedded PBL under the perfect failure mode:Influence factors on the ultimate bearing state and the stiffness development of the embedded PBL is analyzed based on the test results. The uniform calculation formula of the load-slip relationship during the whole bearing process under the perfect failure mode is established, and the calculation formula of the design carrying capacity with the design slip of1.2mm is established. The calculated result shows that their calculation results are in good agreement with their test result, and the two formulas can be used in the engineering design application of the embedded PBL.