Reliability Assessment And Maintenance Strategy Optimization Of Long-Span PC Box Girder Bridge

As a major component of the long-span bridge system,the long-span prestressed concrete(PC)box girder bridge has the advantages of strong spanning ability,excellent performance and beautiful shape.Its proportion in China's bridge system is steadily increasing,which is essential for ensuring the smooth flow of China's road network.However,under the joint influence of material performance degradation,corrosion and complex loads,the reliability and durability of long-span PC box girder bridges are gradually decreasing.Therefore,make a reasonable assessment of reliability and service condition of such bridges,and implement the multi-objective optimization of maintenance strategies based on the above assessment results are essential for achieving sustainable development of long-span PC box girder bridges.The present study focused on the reliability assessment and maintenance strategy optimization of long-span PC box girder bridge,and carries out following research work:(1)An estimation method for spatial correlation of random variables is proposed.The spatial correlation of random variables is characterized by the scale of fluctuation,and the existing spatial correlation estimation method is introduced.Based on the Bayesian information criterion,a new method to estimate the scale of fluctuation of material and geometrical parameters is proposed,and the scale of fluctuation of concrete compressive strength and steel pitting factor are estimated according to the measured data of the service components and accelerated corrosion specimens.The validity and reasonableness of the method are verified based on the static load test results of service components.At the same time,the estimation results of different methods are compared and analyzed.(2)A reliability evaluation model of crack resistance of box girder considering the random uncertainty of calculation parameter is established.Based on the step-by-step method,a time-dependent model of prestress loss considering the shrinkage and creep of concrete,the relaxation of prestressed tendons and the effect of corrosion is established,and the time-dependent regularity and uncertainty of prestressed long-term loss and the residual prestress force are analyzed.Then,the sources of aleatoric and epistemic uncertainty and the corresponding reliability analysis methods are introduced.Finally,considering the influence of long-term prestress loss and corrosion of prestressing steel,the time-varying reliability evaluation model of PC box girder considering the effect of random uncertainty of calculation parameters is proposed,and the anti-cracking reliability of a bridge in service is analyzed(3)A structural reliability analysis method is developed based on the random field theory and non-stationary processes.The simulation method of random field of material properties,structural size and environmental parameters is introduced,and the spatial variability of structural corrosion time is studied based on Karhunen-Loeve series expansion method.The actual resistance degradation process is described by the Gamma stochastic process,and the scale function of the Gamma distribution is simulated as a one-dimensional random field to consider the time-dependent and spatial variability of the resistance degradation.Thus,a random degradation model of structural resistance is established,and a non-stationary vehicle load effect model is also established based on the existing measured vehicle effect data.Based on the above analysis,the reliability evaluation method of box-gieder bridge considering random field and non-stationary process is developed.(4)A condition assessment method of long-span PC box-girder bridges considering uncertainty assessment information is proposed.The original interval evidence theory is improved through the conflict test of original evidence and the correction of conflict evidence based on overall credibility,then a new comprehensive condition assessment method considering the interval uncertainty of measured data and the impact of measured conflict data is proposed for long-span PC box girder bridges through the natural integration of the improved interval evidence theory(IIET)and the fuzzy analytic hierarchy process(AHP).Considering the structural stress,structural damage characteristics and common damage detection classification methods,the corresponding condition index system and assessment criteria are established.A three-span PC box-girder bridge is assessed to illustrate the implementation process of the proposed method,and the validity and superiority of the proposed method are verified by the actual test results and the specification-based calculation results.(5)An optimization framework of maintenance strategy of deteriorating bridges considering the sustainable criteria is established.First,the safety performance assessment method considering corrosion effect,spatial variability and non-stationary load effect is proposed,and a comprehensive assessment method of social impact induced by maintenances is also developed.Then,based on the assessment mothod of safety performance,maintenance cost,social and environmental impact,as well as the proposed decision-making algorithm considering the effect of different optimization objective weights,a multi-objective maintenance strategy optimization framework for deteriorating bridges considering the requirements of sustainable criterias is proposed.Finally,the effectiveness of the optimization framework is verified based on the optimization results of the maintenance strategy of an existing bridge.It is found that the final compromise maintenance strategy,which has desirable structural safety and economic levels with balanced social and environmental impacts,can be obtained by using the proposed optimization method.And the final optimal strategy obtained by the decision-making algorithm based on fuzzy AHP can reflect the will of experts well.