Study On Static Mechanical Performance Of Steel-concrete Joint In High-speed Railway Hybrid Box Girder Cable-stayed Bridge

Hybrid box girder cable-stayed bridge has widely used in long-span highway bridge duo to its advantages such as strong spanning capacity,small girder depth,good economy and so on.However,this type of bridge is rarely applied to railway bridge because of particular characteristics such as great live load of train,strong impact effect and so on.Taking the first high-speed railway hybrid box girder cable-stayed bridge with design speed up to 350 km/h—Bianyuzhou Yangtze River Bridge as background,the force transmitting performance and deformation performance of an innovative railway steel-concrete joint(SCJ)with end bearing plate and without steel cabin were investigated.The main research content and results are as follows:(1)The influence factors of force transmitting of the SCJ in railway hybrid box girder cable-stayed bridge were investigated.Equations for calculating the reasonable length of the side span considering the position of the SCJ and the anchoring position of the pylon were proposed and verified.On basis of analyzing the force transmitting mechanism of the SCJ,the force transmitting method of railway SCJ based on the presume of rigid bearing plate and considering the local deformation of the bearing plate and uneven stress lateral distribution were derived,respectively.Then,the suitability of the proposed calculation method of force transmitting were evaluated based on finite element analysis results.The results indicated that the calculation result considering the local deformation of the bearing plate and uneven stress lateral distribution was more suitable for railway SCJ and the stiffness of bearing plate and rule of stress lateral distribution have significant influence on accuracy of the force transmitting method of railway SCJ.(2)A finite element model of the SCJ in original bridge was established to investigate the longitudinal and transverse stress distribution,deformation characteristics,force transmitting characteristics and mechanical performance of the shear connectors.The results indicated that the stress of both steel and concrete is unevenly distributed along the transverse direction and the uneven level of the SCJ is more obvious than steel girder and concrete girder at two sides.The stress is gradually decrease along the longitudinal direction and has sudden stress change near the end bearing plate,indicating the end bearing plate play an important role in force transmitting process.When live load act alone,the minimum vertical curve radius of the SCJ is 24315 m,and the maximum longitudinal gradient is 2.1‰.The finite element results also indicated the end bearing plate,PBL connectors and shear studs transmit 59.9%,15.9%,10.2% of axial force,respectively.The force of each layer shear stud and PBL connector in railway SCJ is uneven distributed,while the stress distribution rule of the variable height perforated plate connectors(VHPPC)and the rectangular perforated plate connectors is similar.(3)The failure mode,ultimate bearing capacity,load-slip relationship and load-stress relationship of the variable height perforated plate connectors(VHPPC)and shear stud was investigated by push-out test.Results of VHPPC push-out test indicated the end bearing effect of the VHPPC is not obvious and the bonding effect has a significant influence on bearing capacity which could provide about 9.3% of ultimate bearing capacity.The results also revealed the load is unevenly distributed to each PBL connector.The main failure reason of the specimens was concrete splited along the section of the 1th column of transverse rebar.There are almost no small cracks on the surface of the specimen.This failure mode is different from the test results of the rectangular perforated plate connectors commonly used in the SCJ of highway hybrid box girder cable-stayed bridge.The crack load was recommended as the upper limit of design bearing capacity of the VHPPC and the calculation formulas of design bearing capacity of the VHPPC in railway SCJ were proposed.The results of shear stud pushout test indicated that the failure mode was shear handle fracture when the diameter of shear stud is relatively small and the failure mode was concrete breaking when the diameter of shear stud is relatively large.The results also indicated that the bearing capacity calculation formula in Chinese and American specifications were suitable for specimen that shear stud sheared off while these two specifications would overestimate the bearing capacity of specimen that concrete crushed.The bearing capacity calculation formula in Japanese specification and Eurocode would underestimate the bearing capacity of specimens.In addition,the results of stiffness prediction formula in Chinese specification is accurate while Eurocode would underestimate the stiffness performance of the specimen.(4)The bearing capacity,longitudinal and transverse stress distribution,deflection performance and slip characteristics of this innovative railway SCJ with end bearing plate and without steel cabin were investigated by large segment model test.The results indicated the bearing capacity of SCJ and concrete girder is higher than steel girder.The overall safety factor is not less than 1.2 if take no cracking on the top surface of concrete as the control index.The effect caused by sagging moment and hogging moment were about 1.32～1.74 and 1.41～2.98 times of that caused by axial force,respectively,indicating the moment effect is more unfavorable.Regression equations were proposed to evaluated the deformation performance of the innovative railway SCJ,and the results indicated that the deformation of the SCJ is smooth and the minimum radius of the vertical curve was 15457 m and the maximum rotation of the SCJ was 0.442/1000 under the most unfavorable conditions.After comparing with the push-out test results,it is found that the end bearing plate play an important role in decreasing the occurrence of slip.(5)The sectional characteristics and force transmit characteristics of two types of highspeed railway SCJ are compared.The results indicated that the additional bending moment of the steel-concrete interface of the new type of SCJ with rear bearing plate and without cell accounts for a smaller proportion and the force transmission ratio of the end bearing plate is higher.However,the additional bending moment of the steel-concrete interface of the SCJ with steel cells and double bearing plates accounts for a higher proportion and the load uneven level in each layer of shear connector is more obvious.The inertia moment of the SCJ with steel cells and double bearing plates changes more smoothly along the longitudinal direction while the force transmission ratio of the bearing plate is smaller.The influence of structural parameters on this innovative high-speed railway SCJ was investigated based on nonlinear finite element model.The results indicated that the length of SCJ only have slightly influence on force transmitting and deflection,while have obvious influence on the load borne by the shear connectors.The anchorage position of prestressed tendons have a significant influence on the stress state of the bearing plate and the deformation performance of the main girder.The stiffness of the shear connector has a significant impact on the occurrence of slip.The greater the stiffness,the smaller the slip.The thickness of the bearing plate has a great influence on the occurrence of slip in SCJ.In summary,no less than 27 mm-thick of the end bearing plate is recommended for the actual design,and the diameter of transverse rebar and shear stud should not be less than 8 mm and 14 mm were recommended for the actual design,respectively.