Research On Design Theory Of Precast 40m Simply Supported Box Girder For High Speed Railway

China's high-speed railway technology had reached the world's leading level,and accounted for more than half of the world's high-speed railway operating mileage.A remarkable feature was that bridges accounted for more than 50% of the line mileage on average,and some lines accounted for up to 90%.Among them,standard prestressed concrete simply supported girders accounted for more than 90% of the bridge mileage.The design theory,construction mode and operation performance control of standard simply supported girder was one of the major scientific problems for the high-speed railway construction,and became the key to ensure high-speed railway line smoothness and long-term stable operation.The construction technology of 32 m standard simply supported box girder had been developed.The 40 m simply supported girder had the advantage of vibration elimination for about 25 m length EMUs in China,and it could save a lot of engineering investment in the case of large topographic relief and high cost bridge foundation in rivers and lakes,and the engineering application demand was large.40 m simply supported box girder represented the development direction of railway bridge construction technology.The innovation of design theory was an urgent problem to be solved.In this paper,the 40 m prestressed concrete simply supported box girder of high-speed railway was the research object.The research included dynamic design and static design,allowable stress method design and ultimate strength theory design,deterministic analysis and reliability analysis considering the randomness of parameters.A full-scale test girder was built,and the systematic test technology was studied.The research work and innovative achievements were as follows:(1)Taking dynamic coefficient and deck acceleration as control indexes,the design limit of vertical natural frequency of 40 m simply supported girder had studied by using moving load column model.The research results demonstrated the vibration elimination effect of the 40 m girder,and the design limit of the fundamental frequency could adopt the lower limit of the code.The large mass of the concrete structure determined the deck acceleration didn't control stiffness design.However,the analyses of the fundamental frequency limit of 40 m girder with small mass provided reference for the development of light bridge structure and track structure.In addition,taking the acceleration of vehicle body as the control index and using the vehicle-bridge coupling dynamic analysis,the deflection span ratio,residual creep deformation and uneven settlement limits of 40 m girder were studied.The research principles were given respectively according to the deformation adjustability.The limit of deflection span ratio obtained in the study was larger than existing code,and didn't control the stiffness design.Based on the relationship between vehicle body acceleration and speed,the limit of residual creep deformation and uneven settlement corresponding to operation speed of train were given separately,and the combined limit of residual deformation and displacement were given.(2)Based on the current code,a 40 m simply supported box girder for high-speed railway was designed,which fully meet the engineering application conditions.The design considered the control index of the operation state,the concrete stress under the construction condition,the reinforcement stress and the crack width of the transverse frame structure.The design results meet the requirements of the code.The prefabricated quality was good.Based on the test platform and loading system,the bending performance,torsion resistance,crack safety factor,cracking load,degree of prestress and strength safety factor of 40 m girder were verified,and the results meet the design requirements.For the complex anchorage zone,the test verified the safety of the structure under the condition of prestressed tensioning.(3)The truss model was used to analyze the bending ultimate bearing capacity of40 m simply supported girders,and compared with the domestic and foreign code formulae.The bending capacity of 40 m girder designed by allowable stress method was5.9%?10.7% better when checked by limit state method.A shear design method was proposed using Mohr Compatibility Truss Model(MCTM),Rotating-Angle Softened Truss Model(RA-STM)and Fixed-Angle Softened Truss Model(FA-STM).The softened constitutive model of concrete was considered,and the yield sequence of the shear reinforcement and ultimate shear stress were obtained based on the test data and reinforcement arrangement of full-scale test girder.The whole failure process of 40 m girder in pure torsion state was analyzed by RA-STM.With the increase of torsion angle,the principal compressive strain of concrete,the thickness of shear flow zone and the yield sequence of reinforcement were obtained.The results showed that the 40 m girder had a torsional ductility ratio of 15.8,and had a good plastic deformation capacity.According to the stress characteristics of the anchorage zone,three kinds of web models and two kinds of bottom plate models were established based on the Strut-and-Tie model.The results showed that the splitting force of prestressed tendons didn't control reinforcement design of the girder end.(4)Considering the randomness of the mid-span deflection caused by the second stage dead load and the loading age of the second stage dead load,the reliability of residual creep deformation was calculated by the First Order Second Moment(FOSM)method and Monte Carlo method based on Latin Hypercube Sampling(LHS)respectively.The results showed that the development of residual creep deformation could be effectively controlled by increasing the second stage dead load loading age.The reliability of bridge deck acceleration was studied considering the randomness of bending stiffness,mass and damping ratio.The reliability of vehicle body acceleration was studied considering the randomness of vertical amplitudes of track irregularity and residual creep deformation.The acceleration response conformed to the extreme value type I distribution.The accelerations of 99% cumulative probability were obviously larger than the result of deterministic calculation,which could be used as a reference index for design.