The Reliability Analysis And Lifespan Prediction Of The Prestressed Flexural Members Under The Dual Indexes Of Freeze-thaw And Fatigue

In northern China,many prestressed concrete structures such as bridges and wharfs not only have to withstand the vehicle’s fatigue load during the design reference period,but also suffer long-term freeze-thaw environment effects.Under the dual indicators of freeze-thaw and fatigue,the reliability of the prestress concrete members is continuously reduced,and they are prone to failure and even threaten the safety of life and property of users.Therefore,it is an urgent problem for the engineering community to establish and improve the fatigue reliability evaluation index and lifespan prediction of prestressed concrete flexural members subjected to freeze-thaw cycles.The paper relies on the National Natural Science Foundation of China’s funded project"Research on Durability Testing and Lifespan Prediction of Prestressed Structures under the Dual-Index Erosion Environment(Project Number:50878098)" and the Ministry of Housing and Urban-Rural Development’s project "Research on Fatigue Durability of Prestressed Concrete Flexural Members in Freeze-Thaw Environment(Project Number:2013-k4-26)”,the specific work and conclusions are as follows:1.Based on the results of alternating freeze-thaw and fatigue tests for prestressed concrete flexural members,the fatigue residual strength damage models for concrete and reinforcing bar under freeze-thaw conditions were established.In the damage model of concrete,the influence coefficient γ of freeze-thaw stress was introduced to characterize the influence of freeze-thaw cycles on the residual strength of concrete.Through the parameter estimation and hypothesis testing of the test data,it is verified that the fatigue life of reinforcing bar and concrete in the freeze-thaw environment can well satisfy the two-parameterWeibull distribution.2.The Monte Carlo method was used to study the fatigue reliability of prestressed concrete flexural members under freeze-thaw environment,and compared with the reliability calculation methods such as the percentile rank method and the probability distribution function.The results show that the freeze-thaw cycles play a significant role in accelerating the decay of the fatigue residual strength of the prestressed concrete structures and the fatigue failure of the members are fundamentally accelerated.3.Based on the BP neural network theory,a fatigue lifespan prediction model of prestressed concrete under freeze-thaw environment is established.The input parameters of the model are:fatigue stress level,prestressed degree and freeze-thaw cycle times,and output index is: fatigue lifespan.4.Based on genetic algorithm(GA)and particle swarm algorithm(PSO),a PSO-GA hybrid algorithm was established.The BP neural network optimized by the above three algorithms predicts the fatigue lifespan of prestressed concrete in freeze-thaw environmen.The results show that the neural network prediction model optimized by the algorithm has greatly improved the prediction accuracy.Among them,the result of prediction model based on the hybrid algorithm optimization is closest to the real value.In this paper,the durability of prestressed concrete flexural members subjected to dual indexes of freezing and thawing and fatigue has been further researched and compared.The results of this study could be significant for related personnel to evaluate and determine the fatigue durability of prestressed concrete structures in freeze-thaw environments in cold regions of China.