Probability Density Evolution Of Fatigue Resistance Of Key Components Of An Existing Old Steel Bridge Considering The Randomness Of Corrosion

There are a large number of old steel bridges on Chinese highway and railway lines,and most of them have been in service for more than half a century.Due to the limitations of theoretical methods,manufacturing level and economic conditions in the era of bridge construction,poor steel anti-corrosion technology and bad atmospheric environment make the corrosion of these bridges serious,and the problem of structural fatigue resistance deterioration is becoming more and more serious.The complex electrochemical corrosion process makes the key parameters such as corrosion degree and distribution have significant randomness,which makes the fatigue resistance degradation of old steel bridges in service is not deterministic,but a time-varying evolution process in continuous probability space.How to clarify the propagation law of the randomness of corrosion in the fatigue performance of old steel bridges,and then reveal the probability density evolution characteristics of fatigue resistance of key components is the basis to accurately evaluate the residual fatigue life of old steel bridges and ensure their service safety.In this thesis,a typical old steel bridge is taken as the research object,and a series of studies are carried out to solve the above problems.The main contents are as follows:(1)The history and developments of the research on the mathematical model of corrosion development,the randomness of corrosion and the effect of corrosion on the fatigue performance of steel bridges have been introduced,and the disadvantages of the present studies on fatigue performance evaluation of old steel bridges considering corrosion effect have been clarified.The urgency of studying the probability density evolution of fatigue resistance considering corrosion randomness have been expounded.(2)Based on the corrosion investigation of the structural service state of a typical old steel bridge,the corrosion type and corrosion degree of the structure during long service have been carried out,and the probability distribution of corrosion depth of key components has been obtained.The occurrence probability and time-varying equation of corrosion of key components have been deduced,on the basis of which the probability density evolution model of corrosion depth of key components of the old steel bridge has been established from the point of view of probability theory.The time-varying probability density function of corrosion depth of key components during the whole life service period have been calculated.(3)The deterioration effects of pitting corrosion and uniform corrosion on the fatigue resistance of key components of the old steel bridge have been clarified.Based on the quantitative analysis of the relationship between corrosion depth and fatigue resistance deterioration degree,the fatigue resistance deterioration equation has been proposed,and the physical relation model of corrosion depth,fatigue life and stress amplitude level has been established.On this basis,the time-varying physical relations among corrosion depth,fatigue life and stress amplitude level of key components of the typical old steel bridge has been determined.(4)Based on the physical phenomenon that fatigue resistance decreases with the increase of corrosion depth,the probability density evolution model of fatigue resistance of key components of the old steel bridge has been established by using probability density evolution method.On this basis,the time-varying probability density function of fatigue resistance of key components in the whole life cycle has been calculated,and the transfer relationship between random source(corrosion)and target physical quantity(fatigue resistance)probability density in resistance system has been clarified.The time-varying evolution mechanism of the probability density of fatigue resistance of key components of the typical old steel bridge has been revealed.The research results can provide theoretical and methodological support for the performance evaluation and maintenance decision-making of in-service old steel bridges,which are of great significance for ensuring their service safety,improving their service quality,and promoting their sustainable development.