Research On Construction Mechanical Properties Of Simply Supported And Then Continuously Prestressed Concrete Box Girder Bridges

This paper takes a simple-supported and then continuously prestressed concrete box girder bridge of Fuli Expressway as the engineering background.In order to improve the shrinkage and crack resistance of concrete,concrete prism shrinkage test and concrete crack resistance test were carried out.Using numerical simulation as the main analysis method,the effects of shrinkage and creep on the structural performance of simply supported and then continuous box girder bridges were studied,and the effects of different construction sequences on the structural performance of simply supported and then continuous prestressed concrete box girder bridges were explored.The main work and related conclusions of this paper are as follows:(1)The concrete mix ratio design was carried out,and shrinkage reducer and polyoxymethylene fiber were added to the mix ratio of the test group.The 28 d compressive test of the concrete cube standard specimen was carried out,and it was found that the strength of the concrete poured according to the test mix ratio was not as expected,and it was found that the POM fiber might reduce the strength of the concrete.(2)The concrete prism shrinkage test was carried out,and it was found that the addition of shrinkage reducer and POM fiber significantly reduced the shrinkage deformation of concrete.Four different functional formulas are used to fit the experimental data,and it is found that the hyperbolic function formula and the exponential function formula have the best fitting effect.According to these two functional formulas,concrete shrinkage prediction models are proposed respectively,and the fitting effect of the two shrinkage prediction models is good.(3)The effect of shrinkage and creep on the deflection of the main beam is analyzed by using the solid finite element software.It is found that when the beam is stored for 60 days,the upper camber of the main beam will exceed 20 mm,which does not meet the design requirements.There is a certain difference in the upper camber of the main beams at different times when the beams are stored.In order to ensure the flatness of the bridge deck and the connection of the transverse wet joint reinforcement,the main beams of the same span should preferably be prefabricated in the same batch.(4)Different shrinkage and creep prediction models are used to analyze the effect of shrinkage and creep on the prestress loss of simply supported beams in the beam storage stage.It is found that the prestress loss caused by shrinkage and creep accounts for more than 55%of the prestress loss value.Shrinkage creep is one of the main reasons for the loss of prestress.Comparing the settlement results of the beam elements with the calculation results of the solid elements in the beam storage stage,it is found that the estimation of the camber and prestress loss of the main beam by the beam elements is smaller.(5)The influence of shrinkage and creep on the deflection of the main girder during the bridge completion stage is analyzed,and it is found that the camber of the mid-span and the upper span is more than 1.6 times that of not considering the shrinkage and creep,and the side span is even close to twice.The effect of shrinkage and creep on the loss of prestress in the bridge-forming stage is analyzed.Three years after the bridge was completed,the prestress loss accounted for more than 90%,and the prestress loss caused by shrinkage and creep accounted for about 50%.The effects of shrinkage and creep on the compressive stress at the top of the pier are analyzed in the bridge-forming stage,and it is found that the shrinkage and creep effect will not lead to the decrease of the compressive stress at the top of the pier.(6)The analysis of the construction sequence of the rear continuous end is carried out,and the conclusion is that it is recommended that for the 3-span,4-span,and 5-span bridges that are simply supported and then continuous,the rear continuous end can be cast at one time,and the prestressed symmetrical tensioning construction sequence can be used.It is suggested that for a 6-span simply supported and then continuous girder bridge,the construction sequence of one-time pouring at the rear continuous end and tensioning at the other end of the prestressed tendons is recommended.The dismantling of the temporary support is simulated by applying forced displacement,and the dismantling sequence of the temporary support is analyzed.It is suggested that the construction sequence of symmetrical demolition should be adopted for 3 to 6 span bridges that are simply supported first and then continuous beam bridges.