Investigation On Mechanical Behavior Of High Performance Steel-concrete Joints In Hybrid Cable-stayed Bridges

The steel-concrete hybrid cable-stayed bridge,using steel girders in the middle-spans and concrete girders in the side-spans,has been widely used due to their remarkable structural and spanning advantages.Steel-concrete joint connecting the steel and concrete girders is supposed to transfer loads smoothly and has good durability and fatigue strength.However,the steel-concrete joint with traditional concrete has caused many problems,such as disengaging at steel/concrete interface,concrete cracking,and insufficient fatigue strength of the joint.Reactive Powder Concrete(RPC)is a kind of cementious material with ultra-high strength and good durability.Using RPC as the grout of steel-concrete joint may solve the problems caused by normal concrete.The recently constructed Nujiang Bridge is a hybrid cable-stayed bridge with single cable-plane and a 175m-long main-span.The steel-concrete joint in Nujiang Bridge consists of several steel cells with PBLs,headed studs,and back bearing plates.Based on the geometric dimensions of this bridge and the financial support from National Natural Science Foundation of China(Grant No.510781134),structural property and design method for steel-concrete joint with RPC were studied via model test,numerical and simplified mathematical analysis.The main contents and conclusions of this research are presented as follows:(1)The structural behavior of shear connectors was investigated through 66 push-out specimens.The results showed that the bonding effects at steel/concrete interface made a solid contribution to the shear resistant-capacity of a PBL.The PBL casted with C55,RPC(without fibers),and RPCF(with steel fibers)presented 1.22,1.16,and 1.46 times,respectively,higher resistance of their unbonded counter-parts.The PBLs with different concrete behaved discrepantly,and with RPC and RPCF replacing C55,the capacity of a PBL improved by 22%and 73%,respectively.The ductility of a PBL was mostly determined by the deformability of transverse rebar.Steel fibers and steel cells' confinement effects increased the bond-friction strength at steel/concrete interface and the shearing capacity of concrete dowel.Formulas with precise physical definitions for predicting the strength of PBL was proposed and it showed good agreement with the push-out results.The formulas can be potentially used to predict the PBL in steel-concrete joint with normal concrete or RPC grout.(2)The performance of shear connector with different numbers was studied by FEM models.Based on the assumption of equivalent mechanical springs,formulas for predicting the resistance of PBLs in steel-concrete joint were established.The results indicated that with the connector number increasing,the total resistance and stiffness of shear connector groups significantly increased,whereas the ductility,the average resistance,and the average stiffness of the connector slightly decreased,the failure of connectors also changed from PBL(stud)fracture to the yield of steel plate or the cracking of concrete.The proposed equations showed good agreements with the test results,and it can be used to predict the capacity of PBLs in steel-concrete joint.(3)A steel-concrete joint with a scale ratio of 1:3 was tested to study the structural behavior of the joint with RPC grout.The results indicated that the joint with RPC had sufficient strength and safety factor,and the steel cells of the joint perfectly bonded with the RPC.Stress in steel-concrete joint was mainly determined by the axial and bending actions,and it was barely influenced by the torque applied to the specimen.The back bearing plate and shear connectors of the steel-concrete joint separately carried 65%and 35%of the overall axial loads of the joint,and the loads transferred by back bearing plate continually increased with the increase of the thickness of back bearing plate and the height of steel cells.(4)Based on the equivalent mechanical spring model,equations for analyzing the load transferring mechanisms in steel-concrete joint,which considered the effects of steel/concrete slips and the deformation of back bearing plate,were established.The accuracy of the equations was discussed by comparing with the current and previous experimental results.A design diagram for steel-concrete joint in hybrid cable-stayed bridges was finally introduced.The results showed that the practical method derived from stiffening back bearing plate assumption could serve as the theoretical basis for the initial design of back bearing plate.The analytical method with considering the deformation of back bearing plate showed good agreements with the test results,and it could be used to determine the loads carried by components of the steel-concrete joint,and verify the efficiency and safety of the design.