Structural Mechanical Behavior Of Precast-assemble Concrete Girder Of Hybrid Girder Cable-stayed Bridge

Through the rational use of steel and concrete materials,the hybrid girder cable-stayed bridge has a good application prospect due to its remarkable mechanical performance,span capacity and economical efficiency.The prefabricated assembled bridges are also increasingly used in China because of their good technical rationality and durability.However,the research on the hybrid girder cable-stayed bridge using segmental prefabrication assembly technology has not been reported in the literature,and the research is not in-depth.This paper studies key issues related to the design and construction of the steel-concrete joint section and the side-span concrete main beam of the hybrid beam cable-stayed bridge based on the Shishou Yangtze River Highway Bridge.The main contents and conclusions are as follows:(1)Reasonable distribution of beam stiffness in the vicinity of steel and concrete composite section of hybrid beam cable-stayed bridge is studied.The full-bridge spatial bar model has been established for structural analysis,and the internal forces of typical sections in different types of beam sections such as concrete box girder,steel and concrete composite section and steel box girder near the steel and concrete composite section under standard load combination were obtained.By working out the nonlinear moment analysis program to calculate the bending moment-curvature-axial force relationship of the section,the bending stiffness and ultimate bearing capacity of the three different types of sections has been obtained.The results show that the elastic bending stiffness of the concrete box girder section should be designed to be 2.0~3.0 times of the standard steel box girder section considering the influence of possible cracking of the concrete box girder section during operation;the concrete box girder and steel box girder adjacent to the front and the back.Compared with the adjacent concrete box girder and steel box girder,the section stiffness and bearing capacity of steel and concrete composite section after pouring concret e into steel cells are much higher,which makes the distribution of section section stiffness,control point stress and bearing capacity of the section near the steel-concrete joint section existing obvious mutations.Adopting the pure steel cells without filling concrete of steel and concrete composite section,the stiffness of the section,the control point stress and the bearing capacity all meet the structural stress requirements and the distribution is smooth,with better engineering adaptability.(2)The mixed post-tensioning for precast segmental assembly concrete box girder is designed.The mixed post-tensioning scheme flexural capacity is smaller than the internal prestressing scheme.The mixed post-tensioning scheme arranges some of the prestressed steel outside the body to improve the pouring conditions of the beam concrete.By reasonably arranging the ratio of the inner prestressed steel and outer prestressed steel,the purpose of batch loading can be achieved,and the external prestressed steel bundle can be re-tensioned and replaceable in the bridge operation stage,thereby solving the problem of excessive long-term prestressing loss.The mixed post-tensioning scheme has been designed for the north side span concrete box girder of Shishou Yangtze River Highway Bridge.The results show that the schemes meet the requirements of stress,crack resistance and flexural capacity.The principle of in mixed post-tensioning hybridization is to minimize the use of in-situ prestressed tendons to achieve structural stress during the construction phase,and to use external prestressing tendons to bear the live load of the bridge.For the prefabricated concrete box girder of the north section of the Shishou Yangtze River Bridge,the reasonable external prestressing tendons area is 12%~25%.(3)Reasonable tensioning scheme of transverse prestress for concrete box girder in early age is presented.The mechanical properties of concrete in the early age of the cross-box girder on the north side of Shishou Yangtze River Highway Bridge has been measured.The results show that the curing conditions have a great influence on the development of concrete early age strength,and the strength of the site curing specimens is lower than the standard curing one.The compressive st rength and elastic modulus of concrete increase monotonously with age,but the growth rate gradually slows and tends to converge.Under the influence of concrete shrinkage and end-mode restraint,the maximum tensile stress of the beam section is 2.00 MPa,which may lead to structural cracking,and it is necessary to carry out tension at early age.According to the measured data of the early strength development of concrete and the influence of early age prestressed tension on the box girder,a reasonable co ncrete tension time and batch tensioning scheme has been determined.The field measurement of prestress loss and structural response,finite element analysis result show that the proposed early age prestressed batch tensioning scheme effectively reduces th e cracking risk of concrete box girder.