| How to ensure the fatigue use safe has been an important research topic concerned by bridge engneering. Steel bridge constructed ealier, because of low level of welding at that time, designers didn't understand and consider fatigue sufficiently, thus fatigue cracking in such steel bridges is a serious problem. American researchers estimated that 90% of all fatigue cracking is the result of out-of-plane distortion at fatigue sensitive details of existing steel bridges. And the fatigue cracking caused by web gap out-of-distortion is the large part of out-of-distortion fatigue in steel bridge.Reviewd the documents about web gap out-of-plane distortional fatigue in steel bridges at home and abroad, this paper analysed the fatigue details of out-of-plane distortional fatigue in straight bridge, curved bridge and skewed bridge, and the parametric analysis aimed at different type bridge is developed to study key parameters which take control of web gap out-of-distortion, at the same time, study the maintenance and strengthening strategy for web gap out-of-distortion. In this thesis, the following aspects are investigated:1. Research result and method of web gap out-of-distortional fatigue at home and abroad was summarized.2. Analysis with 3-D finite element models according to some typical bridges was developed to discuss the difference among the web gap out-of-distortional fatigue behavior under different bridge structures.3. Parametric analysis for different bridge structures were carried out to study key structural parameters, such as web gap size, web thickness, stiffness of cross frame, type of cross frame, bridge span, radius of curvature of curved bridge, skew angle and so on, so as to make sure the key parameters which take control of web gap out-of-distortion.4. Combine the out of distortional fatigue behavior and parametric analysis results, maintenance method on web gap out-of-distortional fatigue was investigate, and the further maintenance strategy was proposed.
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