Hydroelastic Analyses Of Floating Bridges And Fatigue Behavior Of Connectors

The floating bridge, as one important application form of the floating structures, is of great significance in the construction of military work and national defense. However, it differs from the traditional ships and offshore structures in: (1) the large ratio of length to height and the equivalent magnitudes between flexible deformation and rigid body displacement, i.e. the flexible deformation of the structure cannot to be neglected; (2) the connected slender configuration by several bridge rafts, which will make the connection force and fatigue behavior of the connectors to be critical factors that influence the safety performance of the integrated bridge.The present study is a post-research of"Hydroelastic Response of the Floating Bridge Subjected to Moving Loads in High Speed Flow", which is a national defense pre-research project in the 10th and 11th five-year plan, and mainly consists of the following aspects:(1) A literature review of the related research works, including the theory of hydroelasticity, the hydroelastic response analysis of the floating bridges, the development of the connectors, and the theory of fatigue analysis, is presented, in which a description of the existing approaches, development procedure and applied conditions is introduced.(2) Three dimensional hydroelasticity theory of the floating structures. Based on the finite element method, the equation of motion for the floating structures is given. Consequently, three dimensional hydroelastic governing equation is obtained via the analyses of the external loads and the Price-Wu boundary condition at the fluid- structure interface, which can be regarded as the theoretical foundation of the hydroelastic analyses for the floating bridge subjected to waves or a moving load.(3) Hydroelastic response analysis theory of the floating bridge subjected to a moving load. On the basis of nonlinear finite element method, a hydroelastic equation of motion for the floating bridges considering the nonlinearities between rafts, and the inertial effects of the loads is established. To ensure the implementation of the hydroelastic analysis, the super element method is applied to condense the degrees of freedom (DOFs) of the motion equation. (4) Hydroelastic analyses of the floating bridges under wave or a moving load action. Based on the above-mentioned two aspects and accuracy of the approach validated by the hydroelastic analysis of a free floating body, the hydroelastic analysis of a ribbon bridge in waves is first presented and compared with the corresponding experimental results, which indicates that the hydroelasticity theory can be properly applied in the prediction of the floating bridge. In addition, as an extension of the previous work"Hydroelastic Analysis of a Nonlinearly Connected Floating Bridge"(Fu, 2005), an investigation on a ribbon bridge and a pontoon-separated floating bridge subjected to a moving load is performed, with the focus on the connection forces of the bottom connector, which provides the related loading information for the following fatigue analysis of the connector.(5) Fatigue analysis theory. Considering the fatigue analysis of the connectors, the basic concept of low cycle fatigue and different fatigue analyses approaches are discussed and compared, with the concentration on the local stress-strain method and the definition of the corresponding fatigue properties parameters.(6) Low cycle fatigue analysis of the connectors for the floating bridges. Based on the load history of the connectors obtained from the hydroelastic analyses of the floating bridges, the fatigue life prediction is conducted on the complicated connector model constructed by the solid elements via employing the local stress-strain method. Thus, the relation between the passing speed and fatigue life of the connector can be set up, which implies that the passing speed plays a crucial role in the design of the floating bridge.(7) Summary and forecast. Summarizes all the work mentioned in the dissertation and proposes the future development trends of the related fields.The innovative research works in the dissertation are primarily represented as the hydroelastic analysis of the ribbon bridge subjected to wave loadings, the dynamic analysis of the connectors for the floating bridge subjected to a moving load based on the theory of hydroelasticity, and the fatigue life prediction of the connectors, with the associated conclusions being drawn as follows:(1) As for the ribbon bridge in waves, the hydroelastic response amplitude along the centerline of the floating bridge increases with the increase of the incident wave angle. Similarly, for the same incident wave angle, the response amplitude still goes up with the increasing wavelength, however, the accuracy of the prediction method decreases.(2) For the floating bridges, either the ribbon bridge or the pontoon-separated one, subjected to a moving load, the peak connection force rises remarkably with the increasing passing speed, and also the stress amplitude increases sharply due to the increase of the varied frequencies of the alternating loads on the connector. However, considering the heavy self-weight and large loading capacity of the pontoon-separated bridge, the vertical displacement and influential scope keep constantly and do not increase conspicuously as the ribbon bridge does.(3) In the design process of the floating bridge, the sequence of the dynamic loads acting on the connectors, the magnitude of various fatigue properties parameters and the ultimate tensile strength should be sufficiently considered, especially the passing speed of a moving load, so that it can inevitably diminish the fatigue damage of the connectors. Moreover, the welding technology should be further improved to avoid the occurrence that the fatigue damage in the vicinity of the welding position happens prior to that of the connectors due to the higher fatigue strength that the parent metal has, otherwise it will shorten the service life of the connectors.