Structural Hot-spot Stress Analysis And Reliability Assessment Of Orthotropic Steel Bridge Deck

As an integral bridge deck system, orthotropic steel bridge deck is widely used in suspension bridge and cable-stayed bridge. Because of it has some advantages such as light self-weight, saving material of steel, big torsional stiffness, good integrality and great ultimate bearing capacity. However, the stress concentration effect exists at the weld toe on welding connection of deck plate, longitudinal rib, and transverse bulkhead in welded joint of orthotropic steel bridge deck. Structure failure easily happened on weld toe under the action of traffic load directly. In this thesis, a comparative study is conducted to determine the structural hot-spot stress at the weld toe of the welded joint in the orthotropic steel bridge deck using different methods, and then to carry out mechanical behavior analysis and reliability assessment of steel bridge deck. The main research contents and results as follow.The finite element models of the welded joint are established using the finite element software ANSYS with the solid element and the shell element, respectively. Seven types of the element mesh size are created adjacent to the weld toe zone. The structural hot-spot stress at the weld toe of the welded joint in the orthotropic steel bridge deck is calculated by use of the direct extraction method, the surface extrapolation method and the structural stress method. The results obtained from different methods are compared in respect of the calculation accuracy and the sensitivity with the finite element mesh size and the element type. The results achieved from this study reveal that when the finite element models are constructed by use of different mesh sizes and element types, the structural hot-spot stress calculated by the direct extraction method and the surface extrapolation method has a greater discreteness; while the structural hot-spot stress derived from the structural stress method has a good consistency.Based on structural stress method is an effective numerical procedure to determine the reliable stress distribution adjacent to the weld toe. In order to achieve realistic condition reliability assessment of the welded joint, the probabilistic approach based on the structural reliability theory is adopted to derive the reliability index and the failure probability by taking into account the uncertainties inherent in the material properties and load conditions. First, the sensitivity of the element size and the element type is investigated in calculating the structural stress under different loading scenarios on the basis of both the solid element model and the shell element model. Then, the limit state function is formulated in terms of the structural resistance effect of the material strength and the load effect which is described by the structural stress obtained by the structural stress method. Finally, the reliability index and the failure probability are computed by use of the first-order reliability method, and compared with a target reliability index to facilitate the safety assessment. The results achieved from this study reveal that the calculation of the structural stress using the structural stress method is insensitive to the element size and the element type under different external loading conditions, and the obtained structural stress results serve as a reliable basis for structural reliability analysis.