| Concrete cable-stayed bridges with large span capacity,beautiful appearance and modern sense are widely used in the construction of long-span bridges in China.Under the influence of environment and load,the deformation,internal force,stress state and cable force of each member of the concrete cable-stayed bridge change gradually during the service,which are quite different from the initial state of the completed bridge.Ignoring the influence of time-varying characteristics may make a big difference to the safety evaluation and long-term behaviour prediction of the structure during service.At present,there is a certain gap between the research results of time-varying effect of concrete cable-stayed bridge and the actual engineering application,and there are still many problems worthy of futher study and improvement.Based on a concrete cable-stayed bridge in Jiangxi Province and a large number of measured data during its service period,this paper conducts a series of studies on the modification of finite element model and the time-varying effect of concrete cable-stayed bridge related issues during its service period.The main work and achievements are as follows:(1)The research on time-varying effect,health monitoring or safety evaluation of concrete cable-stayed bridges all require a baseline finite element model that can reflect the true state of the structure as the basis.The baseline finite element model can be obtained only by model updating.In order to make full use of static and dynamic test data and achieve better updating effect,a model updating method based on multi-objective optimization algorithm is proposed for long-span concrete cable-stayed bridge.A modified objective function is constructed by using the static and dynamic responses of structures such as static displacements and dynamic modal frequencies.The parameters to be modified are selected on the basis of sensitivity analysis.A fast non-dominated sorting genetic algorithm with elite strategy(NSGA-II)is used to optimize and correct the finite element model of the background cable-stayed bridge.The Pareto optimal solution set of the modified multi-objective optimization problem is modified by the model,and the modified finite element model is verified by the static and dynamic data.The results show that the static and dynamic finite element model updating method based on multi-objective optimization algorithm for concrete cable-stayed bridges can achieve satisfactory results,which provide a new idea for the finite element model updating of bridge structure.(2)A precise analysis method of prestressed concrete structure based on composite element of bar and beam is proposed.The bar element is used to simulate the prestressed tendons in concrete,and the beam element is used to simulate the concrete.Based on the plane section assumption and the displacement coordination principle,the degree of freedom transformation matrix of the two elements are established,and then the stiffness of the composite element including the contributions of the prestressed tendons and the concrete are derived,so that the bar element matrix and the beam element matrix are organically combined to form a kind of stiffness matrix.The new composite element provides an effective method for the accurate simulation of prestressed force,stress relaxation and concrete shrinkage and creep of prestressed tendons in prestressed concrete structures.In order to simulate the time-varying process of stress relaxation of prestressed tendons,an equivalent creep stress relaxation simulation method is proposed based on the intrinsic relaxation of prestressed tendons,and a recursive algorithm for the equivalent creep coefficient of stress relaxation of prestressed tendons based on the intrinsic relaxation function is derived.(3)Based on the equivalent creep model of beam-bar composite elements,the stress relaxation of prestressed tendons and the integral shrinkage-creep law of concrete members,a time-integration method for time-varying analysis of prestressed concrete structures with consideration of concrete shrinkage-creep and relaxation of prestressed tendons is established,and the corresponding finite element formulations are deduced,and the calculation program is compiled.A concrete continuous box girder bridge is taken as an example to verify the proposed method.On this basis,the finite element method based on pole-girder composite element considering concrete shrinkage,creep and relaxation of prestressed tendons proposed in this paper is used to simulate the relying project.Combining with the measured data of many years during the service period of the bridge,the key indexes such as deflection of main girder,cable-stayed force,displacement of pylon top and reaction force of auxiliary pier top of the bridge are studied in the operation stage of completion of the bridge.The time-varying law of concrete cable-stayed bridge verifies the validity and accuracy of the time-varying effect analysis method proposed in this paper.(4)Temperature plays an important role in the safety and long-term performance of concrete cable-stayed bridges.At present,the research on temperature field of concrete bridges mainly focuses on box girder and T-girder section,while the research on temperature field of Pi-shaped girder of cable-stayed bridges is relatively rare.Based on a large number of measured data of concrete Pi-shaped beam temperature field,combined with numerical calculation of temperature field,through regression analysis of measured data,the actual vertical temperature gradient model of the section of Pi-shaped beam is fitted.On this basis,the effects of temperature on displacement,stress and cable force of main girder of cable-stayed bridge are analyzed,and the effects of temperature on performance parameters of concrete cable-stayed bridge are revealed.(5)The tension and compression bearing of auxiliary pier is the key connecting component to ensure that the auxiliary pier played a supporting and restraining role in the operation of cable-stayed bridges,which has a significant impact on the static and dynamic performance of cable-stayed bridge during its service period.Taking the tension-compression bearing of a concrete cable-stayed bridge as the research object,the influence of temperature,shrinkage and creep,foundation displacement,automobile load and their combination on the tension-compression bearing of auxiliary pier in operation stage is calculated by the finite element model of the whole bridge after model updating.On this basis,ANSYS was used to establish the local finite element model of the "beam-solid-contact" hybrid unit of the auxiliary pier,the tension-compression bearing and the main beam.Combined with the actual situation of the operation phase,the actual contact working state of the bearing was simulated.Based on the elastic-plastic theory,the local stress of each component of the tension-compression bearing is analyzed.According to the calculation results,the real cause of the failure of the tension-compression bearing is analyzed,and the failure mechanism is revealed.At the same time,in order to effectively monitor the working status of tension and compression bearings in real time,a simple and effective monitoring and early warning method is proposed.Finally,according to the above research results,various countermeasures to solve the failure of cable-stayed bridge tension and compression bearings are given. |
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