The paper deals with fatigue of grider ridges. Most of the research effort has been directed towards ultimate limit states. These limit states were considered in the recent code calibration projects in the USA, Canada, and Europe. Yet the actual bridge live load is a cyclic load, with repeated application of forces in form of truck axles. The live load model for fatigue is described by two parameters: magnitude and frequency of occurence. In generla, the load effect is a varying amplitude strain/stress, which makes the analysis more difficult. The actual live load is strongly site-specific. Therefore, it is recommended to base the load model on weigh-in-motion measurmeents, or other form of field observation. The fatigue resistance model is considered i nterms of S-N curves. Traditionally, stress vs. number of cycles to failure was considered for steel girders. The paper also includes ismilar curves obtained for concrete. Finally, the reliability indices are calculated with regard to fatigue for steel girders and they are presented as a function of time.
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