Fatigue Analysis Of Orthotropic Steel Decks Based On Hot-Spot Stress Approach

Orthotropic steel decks are made up of longitudinal and transverse stiffeners and bridge plates which are perpendicular and welded together. Because of the advantages like light weight, big ultimate bearing capacity, short construction period, and beautiful appearance, orthotropic steel decks are widely used in large and medium span Bridges at home and abroad. But the structure of orthotropic steel decks are complex, the welds length are long and have welding residual stress, coupled with the defects of the structure itself, poor construction quality and wheel load repeatedly function directly, orthotropic steel decks are easy to suffer from fatigue damage.At present, in lots of countries, fatigue performance of steel decks are analyzed by using nominal stress approach. This approach is suitable for relatively simple structure, but not an ideal approach for complex orthotropic steel decks. When using nominal stress to express fatigue life, the discreteness of results are very large, it is difficult to get accurate S-N curve. Relative to the nominal stress approach, hot-spot stress approach can better adapt to the complex structure. It has become one of important approach for the analysis of weld fatigue. But its application in the orthotropic steel decks is not widely. To solve the problem, this paper conducted research focused on the application of hot-spot stress approach in the weld fatigue analysis of orthotropic steel decks.ANSYS models are set up based on orthotropic steel decks of Sutong bridge, and used to analyze the most unfavorable load positions of every typical weak part in them. Stress concentration conditions of the weak part are got. This part provides the reference methods about the finite element analysis of the orthotropic steel decks, including modeling and loading method.The welds of orthotropic steel decks are complex. Four hot-spot locations are selected to analyze: RD(Rib-to-Deck) plate weld toe, RD plate weld root, RD longitudinal weld toe and RF(Rib-to-Floorbeam) weld toe. Detailed finite element calculations are carried on to study the finite element grid’s influence on their stress value and the stability of stress distribution in extrapolation areas. Appropriate finite element mesh size is advised to model in practical engineering.There are hot-spot stress calculation method about RD plate weld toe in existing specifications. But in orthotropic steel decks, the stress concentration situation of RD plate weld root is more serious, the crack is more destructive. In existing specifications, there are not hot-spot stress calculation method about RD plate weld root. Detailed superficial extrapolation method was deduced to calculate the hot-spot stress of the four hot-spot locations and compared with the existing specifications. The deduced results of three weld toe are consistent with specifications, but the applicable specifications of them are different. The relevant conclusion about RD plate weld root is similar to weld toe area, but the extrapolation points locations are different with existing specifications. So existing specifications can not be used to calculate the hot-spot stress of RD plate weld root. The change of weld elastic modulus has no effect on superficial extrapolation methods and formulas, so weld elastic modulus can be same with the parent metal when modeling in engineering practice.Parametric analysis was conducted using the deduced formulas to consider the possible change of the main basic parameters in orthotropic steel decks, indicating that plate thickness is the biggest impact of the parameters. So the fatigue life of orthotropic steel decks can be improve by increasing plate thickness.