Fatigue Life Assessment Of Typical Welded Joints In Steel Bridges Using Master S-N Curve Method

Steel bridges, especially which with a large amount of welded joints, are prone to fatigue damage and failure because of the repetitive application of both traffic loadings and environmental effects, while these applied loadings may be well below the structural inherent resistance present in nature. Such devastating events occur suddenly and result in heavy losses of life and property. Therefore, it is of great importance and necessity to perform the fatigue life assessment of steel bridges timely and accurately.As a common method for estimating the fatigue life of steel bridges, the nominal stress method has been widely used in most existent codes and standards, e.g. Chinese standards (GB500172003), British standards (BS54001980), European standards (Eurocode32005), American Association of State Highway and Transportation Officials (AASHTO) specification (AASHTO2012), and the International Institute of Welding (ⅡW) reports. This method is based on a global consideration focusing on the average stress in the studied cross section according to the fundamental theory of structural mechanics. In each code or standard, a variety of S-N curves relevant to specific structural detail (e.g. geometrical dimension and weld patterns etc.) and loading conditions (e.g. axial tension, in-plane bending and out-plane bending, etc.) are of great importance and necessity for fatigue life assessment of steel bridges by use of the nominal stress method. However, due to the lack of test data, the nominal stress method is not suitable for all the structural joint, especially the object detail is extraordinary complicated and incomparable to any classified joints, or the loadings are complex to make it difficult or impossible to determine the nominal stress. Therefore, as to some structural components of steel bridges, the fatigue life predicted by use of the nominal stress, largely ignoring stress concentration effects, may be unreliable. In order to seize the stress concentration effects in welded joints, researchers have proposed the hot spot stress extrapolation method, hot spot stress at weld toe could be evaluated by surface extrapolation from the specific reference distances or by linearization through the plate thickness. However, it has an obvious drawback in that it exists relative mesh sensitivity in finite element analysis. In addition, hot spot stress extrapolation formulation is not suitable for use when the mesh size exceeds1.0t, especially for relatively large and/or complicated weld joints if sub-model technology is not used. Based on above considerations, Dong presented a mesh-insensitive structural stress method combining the features of the hot spot stress methods with those of the through thickness methods. Based on the structural stress method and fracture mechanics, a master S-N curve fatigue model has been developed and applied for fatigue life assessment on offshore/marine structure, vessel and piping welded joints, etc. Considering the comprehensive effects of stress concentration, plate thickness and loading mode, a single S-N curve (master S-N curve) is available for all steel weld joints. Master S-N curve method largely facilitates or eliminates the uncertainty in stress calculation and difficulty in choosing a proper S-N curve.In the present study, taking account of comprehensive effects of stress concentration, plate thickness and loading mode, typical welded joints including longitudinal attachment with the length of60and200mm and rib-to-cross-girder connection with cut-out holes in orthotropic steel bridge decks are selected to conduct fatigue life assessment by use of master S-N curve method with FEA and corresponding post-process. Moreover, for the welded joints in orthotropic decks, the mesh insensitivity characteristic of the structural stress is also verified through a comparative study of FEA with four mesh sizes, four element types and three weld modeling technologies.The results reveal that the calculation of structural stress is mesh-insensitive, and the related fatigue assessment results are relatively robust; A single master S-N curve could be applied to the investigated joints that are classified as different categories according to traditional nominal stress method; Oblique shell element is deemed as the most proper weld modeling approach in the process of fatigue life assessment based on master S-N method.