Structural Reliability Assessment Of Existing Concrete Girder Bridges Under Random Traffic Loads Considering Flexural Shear Coupling

Structural response analysis and reliability assessment of in-service bridges based on measured traffic load data is an important means of obtaining a true picture of the operational load state and structural safety of bridges,and this research currently focuses on the reliability analysis of single mechanical states of bridges,such as moment,shear and axial tension.However,bridges are often subject to complex mechanical behaviour under external loads,such as moment-shear,compression-moment,moment-shear-torsion,etc.,and the composite forces often become an important failure mode in controlling the safety of the structure.At present,the main focus on composite forces is on the resistance level,and the need to resist composite forces for a given structural design is investigated by means of tests;however,the internal force effects of bridges under random traffic loading also show composite characteristics,and this law has not received sufficient attention,which is particularly critical for the structural safety of bridges in service under the current situation of very serious overloading and heavy loading in China.The main research content and results of this paper include:(1)A probabilistic reliability assessment method for moment and shear composite forces on girder bridges was developed,and a basic assessment framework was formed.Firstly,based on the reliability assessment of a single failure mode,two methodological strategies are summarised in this study for the reliability assessment of composite forces:firstly,the path method based on the failure path to derive the most unfavourable reliability calculation plane,and secondly,the Lamé curve model is used to establish the functional function method for the combined moment-shear failure;subsequently,for the multivariate non-linear limit state equations corresponding to the constructed functional functions,the reliability of the moment-shear composite is solved by a The reliability index of the moment-shear composite bridge is then solved for the multivariate non-linear limit state equation based on gradient direct differentiation(i-HLRF),and the accuracy of this reliability calculation method is verified to be within 0.001 with excellent convergence by means of an example.(2)The reliability assessment of the composite force behaviour of bridges under the action of overloaded vehicles is particularly investigated for the safety of a large number of small and medium span bridges subjected to the action of overloaded vehicles.Firstly,a probabilistic model of the moment-shear coupled resistance is established based on the MCFT theory recommended by AASHTO;secondly,a model of the overloaded vehicle loads is established by combining the WIM data of vehicle loads at actual locations and the distribution law of the moment-shear composite load effects under the action of overloaded vehicles is analysed;finally,the path method and the Lamé curve functional function method are used respectively to illustrate in detail the critical control in conjunction with an engineering case Finally,the structural reliability of the moment-shear composite forces at critical control sections is illustrated in detail by using the path method and the Lamé curve functional method respectively,combined with an engineering case.The results of the study show that in these sections where both moment and shear effects are significant,the probability of failure of the moment-shear composite action is greater than that of the pure moment or shear damage modes.(3)In order to accurately reflect the characteristics of the composite force behaviour of the bridge in actual service,a probabilistic reliability assessment under the action of random traffic flow was carried out.Firstly,based on the vehicle load WIM data of a one-way two-lane motorway in China,a realistic composite load effect model was constructed with the aid of extreme value theory,and the reliability indexes were calculated and compared for the above engineering case according to the basic assessment framework of composite force behaviour established in the thesis;then,based on the available information on bridge operational loads and structural performance degradation,a modified pressure field theory was combined to establish The time-varying model of moment-shear force degradation is then combined with the modified pressure field theory to analyse the time-varying reliability of the bridge under the moment-shear composite stress condition.It is shown that in daily operation,the moment-shear composite stresses are less favourable than the single stresses and have a greater impact on the bridge as service time increases.The paper establishes a probabilistic reliability assessment system for composite force behaviour under heavy vehicles and random traffic respectively.Although the study is focused on the composite moment and shear stresses of girder bridges,the research idea can be extended to the safety assessment of bridge structures under random loads with any composite stress failure mode,which provides a reference for improving the design theory and methodology of existing codes and clarifying the probabilistic reliability assessment of bridge structures under composite stress behaviour.