Reseach Of Dual-Axis Fatigue Theory And Design Method Of Steel Bridges

Fatigue behavior of structure details is one of the key problems in the field of research of the steel bridge. In recent years, the steel bridge has developed rapidly, and new structure form, structure details and manufacturing technology of steel bridge are continuously emerging. The expansion of application scope of the steel bridge results in the increases of the kinds and level of load which the structures bear, such as long span, high speed, cross-sea, heavy load. With the increasing of construction level of steel bridge, the demand of economic performance and aesthetic properties is enhanced gradually, which will lead to the complexity of the structure and its mechanical response, correspondingly improve the utilization efficiency of materials. In this situation, the probability of fatigue and fracture of steel bridge will be significantly increased.At present, the classical assessment method of steel bridge fatigue strength is the structure detail method based on nominal stress method. This method is the evidence for the categorization of the fatigue tests of structure details, thus fatigue failure mechanism of materials is not reflected reasonably. On the one hand, this method can not include all kinds of structure details, and the ability of computational estimation of classical and special structure details of steel bridge is insufficient. On the other hand, the local fatigue failure region of steel bridge is in the complex stress-strain state. According to the fatigue failure mechanism of materials, so fatigue behavior of steel bridge component will not be estimated reasonably if the effect is only considered under the uniaxial load. The fatigue failure of steel bridge component is analyzed and discussed according with multiaxial fatigue method of materials in order to establish the multiaxial assessment method of steel bridge structure details which can be used in the engineering practice. Therefore, a series of research work is carried out as follows:(1) Factors influencing the material fatigue strength, which have an effect on the fatigue failure of steel bridge component, are analyzed, and main features of fatigue failure of steel bridge are summarized. Research results indicated that in the reasonable design state, fatigue failure of steel bridge refers to the high-cycle and low frequency fatigue problem which is caused by taking the variable load as dominant factors and comprehensive factors of the natural environment in the bridge position. Assessing criterion of multiaxial material fatigue strength is reviewed, and their scopes of application are discussed. And then the stress criterion is selected to evaluate steel bridge component fatigue strength.(2) The dissertation investigated assessment methods of classical steel structure details in the codes and the fatigue failure features of the special steel structure details. The research results showed that the prediction failure of special steel structure details is related to the limitation of the uniaxial assessment method in the code. From the perspective of the anti-fatigue design, based on the mechanical factors inducing the fatigue failure for classical and special steel bridge structure details and the distribution of the fatigue cracks, the research found that the fatigue failure has the significant multiaxial feature. According to this, two classical problems of fatigue failure are put forward, therefore the first kind and the second kind multiaxial fatigue problems are established.(3) The commonly used fatigue strength assessment methods are reviewed. And the application scope of nominal stress method, hot spot stress method, and local method are analyzed and discussed. Combined with the feature of steel bridge fatigue behavior, the dissertation proposed that the application of the local method to predicating the fatigue strength of steel bridge component is an effective approach to assess the multiaxial fatigue strength of the steel bridge. Stress field intensity method is selected as the research object of the steel bridge multiaxial fatigue strength assessment method.(4) Limitations of the traditional stress field intensity method are analyzed and expounded. The model of the stress field intensity method is improved to overcome limitations. The improved stress field intensity method established the new method of calculating field diameter by using the fatigue damage area of critical distance method, and the new approach to solving weight function by using function fitting technology.(5) Analytical method and elastic-plastic finite element method of solution to local stress-strain field are compared and analyzed. According to material properties and bearing loads feature of steel bridge, proper elastic-plastic finite element constitutive relationship model is selected to solve the local stress-strain field of steel bridge component.(6) The predictability of the traditional stress field intensity method and the improved stress field intensity method are verified by employing the fatigue strength of steel bridge fatigue tests. The research results show that the prediction precision of the improved stress field intensity method is significantly higher.(7) Fatigue strength of steel bridge gusset plate is predicted by using the traditional stress field intensity method and the improved stress field intensity method. In the solving process, equivalent life load group and equivalent field diameter models are established. The results show that when fatigue life is not changed, if the cyclic load of some direction increases, the cyclic load of another one will decrease. The steel bridge gusset plate predictions of traditional stress field intensity method points out the relation of cyclic load appear linear, and the relations of improved stress field intensity method appear parabolic. Based on this, the design formula of uniaxial load of steel bridge gusset plate is established.