This paper describes work that is being carried out to develop an approach for assessing the effects of dynamic loading on ageing metal bridges. The overall project involves work in a number of different areas: the development of bridge and rolling stock models and their interaction; the development of loading data to model variable track alignment and wheel profiles; the measurement of bridge responses to rolling loads; the calculation of cumulative fatigue damage and the evaluation of remaining fatigue life. This paper illustrates the approach taken in assessing the fatigue loading applied to the bridge. It discusses the methodology and mechanisms of fatigue analysis when applied. The procedure presented involves the implementation of the rain-flow algorithm. This method involves decomposing the stress history of each location on the structure into a series of stress cycles. This approach enables the application of traditional methods such as the Palmgren-Miner hypothesis. The method and its implementation are described in detail. Considerations such as material properties, stress concentrations and previous loading history are also considered and addressed. The importance of the dynamic interaction between the rolling stock, track and bridge systems, when considering fatigue, is dealt with in depth. Distinctions are made between different forms of structure and the difference in the response of short and long span bridges. The paper also addresses the different methods of approaching fatigue problems including: allowable stresses, the calculation of safe working life, or the calculation of safe working loads. The paper also discusses the ideas of low and high cycle fatigue.
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