Study Of Fatigue Strength Evaluation Of Concrete Cable-stayed Bridge

Along with the fast developing of economy, large demands of freight transportation makes the traffic flow and vehicle capacity increased obviously. The frequent overloading is also a question couldn’t be ignored. Those factors make the bridges working under high stress states, and, the frequency and amplitude of the dynamic stresses born by the bridges come out as new characteristics to the load of bridges. In addition, inappropriate construction quality may lead to the premature failure of bridges, even result in collapse, is also an important factor to the fatigue failure of bridges. Compared to developed countries like UK and USA, the fatigue design theory of reinforced concrete bridges in our country is just in the level of study basic concept of fatigue strength, far more to develop complete code or method. Therefore, the study of fatigue design and lifespan assessment for reinforced concrete bridges is of great significance.The main works of the paper are follows in detail:(1) The factors to the fatigue of reinforced concrete bridges are summarized.(2) Considering the fatigue design of concrete, comparison in different aspects among the BS5400, AASHTO LRFD, code for design of concrete structures and code for design of steel structures is proposed, to study the advantages and shortcomings. Then based on the shortages of the terms in the concrete bridge fatigue assessment code in our country, the paper points out parts needed to be developed and parts could be used.(3) Combining with the detail design of a cable-stayed bridge, a model of a cable-stayed bridge is built by Madis /Civil. The paper processes comprehensive static analysis to the abridge and checks the deflections and stresses under twelve conditions, And then finds out the vulnerable section during the operation of the bridge.(4) Basing on the fatigue strength evaluation method in BS5400, AASHTO LRFD and code for design of concrete structures, loads are applied to the model to analyze and evaluate the fatigue strength of the prestressed reinforcement. Finally, the results of each method are compared.(5) Basing on the fatigue strength evaluation method in the three codes, loads are applied to the model to study fatigue strength of the cables and then the result are evaluated.(6) Considering the phenomenon that the dangerous sections of the bridge are different under static and fatigue conditions, the method for find out the dangerous section of fatigue is proposed.(7) At the reasonal perspective, the fatigue strength evaluation theory of concrete cable-stayed bridge in our country, should develop fatigue resistant design criterion of concrete bridge basing on the concept of finite life design. Detailed grades of different types of the welded joint of reinforcement should be processed and the related fatigue resistance be confirmed based on trails. The American code can be a reference for the assuring of weight of the fatigue vehical in our country. Values of the axle load, number of axles and spread of axles, should base on the main fatigue damage degree of the concrete bridges. The paper proposes that the reference value of the weight of fatigue vehical should be 460 kN,395 kN,343 kN and 290 kN conresponding to the traffic ratio— very large, large, normal and small, of heavy vehicals on the main line of highways. The temperature load also should be considered in the fatigue conditions. To the method to assuring the stress spectra, three step by step method in BS5400 should be a good referece.Based on the study, the BS5400 is detailed in fatigue resistance design method of concrete bridges. The AASHTO LRFD is more accurate in the equivalent method for assuring fatigue vehical weight, regarding to the realtime changing of the traffical flow. However, there is still no complete fatigue resistance design method of concrete bridge in our country. To the checks of fatigue strength of bridge, BS5400 is more conservative, then the Chinese code, the AASHTO LRFD is more dangerous. However, to make the fatigue strength evalution of concrete bridges more reasonable, the foreign method couldn’t be used directly in our country. The consideration of the heavy traffic condition, large rating loading capacity, and real construction quality of concrete bridges in our country is needed, to develop reasonable fatigue strehgth evaluation of concrete cable-stayed bridges for our country.