The Optimization Design Method Of Longitudinal Prestress In Prestressed Concrete Bridge

As a typical one of the long-span bridge structure in China ,the prestressedconcrete continuous box-girder bridge with variable cross-section has had aserious problem of girder crack and the deformation of main beam in its serviceperiod, and the longitudinal girder prestressed system directly influence thestress level of the structure, which can improve related diseases. In view of theprestressed concrete continuous box-girder with variable cross-section of thestructure, this article bases on prestressed method, considers the change ofinertia moment along the beam, and studies the design method of the longitudinalprestressing tendon. Based on analytical method and finite element method andcombined with on 7 different span prestressed concrete continuous box-girderbridges in different area highway, this paper discusses the main girder reasonablestress level. The main research work includes:(1) The common prestressed definition and calculation expression areanalysed, and emphatically the advantages of prestressed are emphaticallyintroduced. The paper also expounds calculation method of the reinforcement ofthe prestressed concrete structure, and studies the design process of differentmethods(2) Through statistical analysis of PC continuous box-girder bridge ,internal force, and stress state of different span range of continuous box-girderbridge has been mastered. By calculating load effect of representative girdersection under dead load,pre-stress, creep and shrinkage ,temperature andautomobile load ,this paper analyses the different span prestressed concretebox-girder bridge the internal force redistribution of the scope and load effect ofthe ratio of the total design load in bearing cross section area and across the L / 2section place. By Analysis of prestressed box girder bridge reasonable bridgestress state, prestressed concrete box girder bridge at the bearing section willgenerally reserved 5 ~ 8 Mpa pressure stress, at cross L / 2 section in the span of75 m or so, inferior section need 6 ~ 8 Mpa pressure stress reserves.When span ismore than 150 m, more than 10 Mpa needs to be reserved.(3) The paper bases on the stress ratio of prestressed method designconcept, combines analytical method with finite element method, comprehensively considers the effect of dead load and live load, and sets upoptimization design method of PC continuous box-girder bridge longitudinalprestressing tendon .On the basis of average bending moment method, it buildsthree blending moment equation of section-varying continuous box-girderbridge to analyse internal force under dead load. Using load equivalent methodand unit load method, it builds the effective prestressed preloading formulas.Through the force method, the creep effect of calculation formulas isestablished, and through the finite element method, the effect of live load iscalculated.Finally, the calculation formulas of variable cross-sectionprestressed reinforcement is obtained. Through the typical examples, comparedto the finite element results, this method has high accuracy.(4) The longitudinal prestress optimization design method successfullyapplies to the PC continuous girder bridge for preliminary design to study thePC continuous beam section typical within reasonable stress range. Through thePC continuous girder bridge for typical span interval, analysis size values range,adjusting the longitudinal prestressed, reasonable girder stress interval isobtained.Given 50m、100m、150m three continuous box-girder bridges withvariable cross-section, to the same span bridge choose different section size, itsbiggest prestressed value range is calculated. Further, combining the presentdesign experience, with the short-term effects portfolio tensile stress sectionedge reserve 1 ~ 3 Mpa as optimized object, the internal force and stress state isobtained under different stages of the girder.(5) The continuous girder bridge for bridge finite element model of detianbridge in sichuan province，Yangtze river Highway Bridge in jingzhou，hubeiprovince and 38 states bridge are built by BDCMS software. And someinfluencing factors such as shrinkage and creep，temperature，automobileloading effect are analyzed，the main research is the law of PC continuous beamstructure prestressed girder stress which is effected by concrete shrinkage andcreep effect and temperature effect on automobile loading.