Bending moment approximation analysis for use in fatigue life evaluation of steel railway girder bridges

The fatigue life analysis of steel railway girder bridges is not currently a formalized process. The basic concepts have been available since the discovery of stress range as the main factor in steel details, combined with standard static analysis. The current process is tedious for bending moment analysis even for simply supported structures.; The fatigue analysis for a plate girder requires determination of where the investigation needs to occur. Given typical plate girder construction before modern fatigue knowledge, multiple points are examined. For railroad loadings, moment analysis must include determination of maximum moment due to live load along with the variation in moment, or moment range, created by the passage of live load.; This research developed a method to approximate both maximum moment and moment range allowing estimation of moment at any point along a simply supported beam. Types of trains were limited to “unit train” conditions, those consisting of identical cars with all cars having identical weights evenly distributed to the axles. Locomotives are assumed to have the same characteristics. These trains are common in today's environment for transporting coal and grain. Container and trailer equipment also have similar characteristics while not always a true unit train.; The unit train represents a medium between a generalized concentrated load system and a uniform load. This loading system is called a uniformly patterned concentrated moving load system.; The results show that general formulation exists for development of approximate maximum moment and moment range envelopes for a specific loading pattern, based on a cyclical relationship between the length of the loading unit and the span length of the bridge. Additional formulation for impact and span response provide modification to the moment analysis results adding accuracy for the final results.; New insights into unit train moment behavior at points other than the centerline have been discovered. This work provides a method for performing that fatigue analysis while also providing new formulation for bending moment behavior that will have implications for further railroad bridge fatigue research.