Study On Mechanical Properties And Calculation Method Of Confined Perfobond Connectors

Perfobond connectors are widely used in composite bridges and hybrid bridges owing to their high shear stiffness,high shear capacity,good ductility,excellent fatigue resistance and ease of installation.Existing studies indicate that the confined perfobond connectors used in steel–concrete joints have a higher shear capacity and a better deformation capacity than the superposed perfobond connectors used in steel-concrete composite beams and steel–concrete composite plates.Because there are many factors affecting their mechanical properties,the bearing mechanism is still unclear and the results of different shear capacity equations are quite different.Therefore,funded by the National Key R&D Program of China(2016YFC0701202),this paper analyz and summariz the related research on perfobond connectors,and proposed the confinement coefficient ? in order to quantify the confinement effect,and systematically studies the bearing mechanism and slip deformation behavior of confined perfobond connectors through thirteen sets of push-out tests and the fine finite element method.The bearing model and the slip deformation model of the confined perfobond connector are proposed and the mechanical behavior of the confined perfobond connector in the steel–concrete joint is analyzed.Finally,the calculation methods of the ultimate shear capacity,ultimate slip,load-slip curve and secant stiffness are proposed based on the bearing capacity model and deformation model,and a simplified method for calculating slip distribution in the steel–concrete joint is proposed based on the linear elastic partial interaction theory.The main work and conclusions of this study are as follows:Based on push-out tests of confined perfobond connectors,the effects of circular,wide and long shapes were considered in the tests and two sizes of each shape were designed to analyze the influence of the size of the hole.The test results show that: the difference in constraints is the main reason for the difference in mechanical behavior between the superposed perfobond connector and the confined perfobond connector.Increasing the hole size of the confined perfobond connector significantly improved its bearing capacity without weakening its ductility for its strong confinement.The hole shape does not significantly affect the shear capacity although it significantly affects the shear stiffness and failure mode;the representative secant stiffness of the long-hole connector is far less than that of the wide-hole connector with the same size.Confined perfobond connectors exhibit a high shear capacity and good ductility even without the perforating rebar.Based on the refined finite element model,the failure process and influence factor on the mechanical properties of perfobond connectors are analyzed.The finite element analysis shows that: The confinement along the direction of the perforating rebar is the primary factor affecting the shear capacity of perfobond connectors.The ratio of height to thickness of the perfobond plate determines the strength relationship between the shear and local compression of the concrete dowel,and has a decisive influence on the elastic stiffness,plastic stiffness and ultimate slip of perfobond connectors.The shear strength of the concrete dowel decreases with the increase of the hole area and is less affected by the height width ratio of the concrete dowel.The perforating rebar subjected to pure shear when confinement along the direction of the perforating rebar is strong.In the ultimate state,the normal displacement of the steel-concrete interface is obvious due to the dilatancy of the concrete dowel,so the contribution of interface bonding and friction to the shear capacity of the connectors is limited.Therefore,the shear capacity of the confined perfobond connectors is mainly provided by the concrete dowel and the perforating rebar.Based on the refined finite element model of the steel–concrete joint and steel-concrete composite anchor plate,the confinement and mechanical properties of the connectors in the structure are studied,and the effects of the layout of the connectors on the mechanical properties are analyzed.The finite element analysis shows that: In the elastic stage,the shear force of the connectors in the steel–concrete joint varies greatly along the loading direction,and tends to be uniform in the plastic stage.The spacing and location of the joints have little effect on the bearing capacity.If the confinement coefficient is consistent,the shear capacity of the perfobond connectors in the steel–concrete joint is in good agreement with that of the push-out tests.In the steel-concrete composite anchor plate,the dilatancy of the concrete dowel is balanced by the reinforcement at the upper and lower sides of the steel anchor plate anchor plate.The perfobond connectors on the upper and lower sides of the steel anchor plate are more confined,so its shear capacity is higher than that of the connector in the middle.Because the tensile effect of concrete is not considered in the calculation of the confinement coefficient,the calculation method is conservative.The analytical solution of slip distribution is obtained based on the linear elastic partial interaction theory and is in good agreement with the results of the test and finite element analysis.According to the analytical solution of slip distribution,the influence of stiffness of steel members,concrete members and shear stiffness of connectors on the distribution of slip is analyzed.The parameter analysis shows that the increase of interface stiffness and the length of the steel–concrete joint can reduce the maximum slip value and increase the load ratio of the connector,but also reduce the effective utilization of the whole connector group.Finally,simplified formulas for calculating the maximum slip,the effective utilization ratio of the connectors and the load ratio of the connectors are proposed.Based on the test results,the formulas for calculating the shear capacity and ultimate slip of the concrete dowel and of the perforating rebar are proposed considering the effects of hole size,hole shape,perforating rebar,concrete strength,and lateral confinement.The power function is introduced into the calculation formula to consider the nonlinear relationship between the influencing factors and the shear capacity,so as to ensure the calculation accuracy of the formula when the parameters vary in a large range.Because the influence of compression deformation and shear deformation is considered in the stiffness calculation formula,the stiffness characteristics of perfobond connectors with different shapes and sizes can be accurately reflected.Based on the shear capacity and ultimate slip,the dimensionless load-slip curves are obtained.According to the characteristics of the curve,an appropriate load-slip curve expression is constructed and then the specific parameters are obtained by regression analysis.The final expression can reflect the main characteristics of the load-slip curves of the confined perfobond connectors.