Distortion - induced fatigue failure at main girders of a railway bridge - strain measurements in service and analyses based on fracture mechanics

At the main girders of a three span continuous plate girder railway bridge, built in the 1970s, through-thickness cracks were detected in the web, at the location of the transverse stiffener to main girder connections. Based on former Austrian standards for steelwork, the transverse stiffeners are not welded to the main girder flanges in order to increase their fatigue resistance. Instead, fitting pieces between the stiffeners and the bottom flanges were used.

Within the paper the geometry and the location of the cracks are presented. For the global stresses, due to moments and shear forces, numerical simulations of the train passages were done. Based on these global stresses only, the fatigue damage for the main girders was evaluated at each position of a transverse stiffener connection. Since there was no significant agreement between the distribution of the global fatigue damage along the main girder and the crack positions, the out-of-plane distortion in the web gap region was expected as the obvious cause for the cracks.

To verify the complex local stress field in the web gap region (interaction of global and local stresses), comprehensive strain measurements were done during service. The relevant results, including passages of slow and fast individual locomotives, passenger and freight trains, are presented and compared.

Finally, based on the measured stress range spectra the further crack growth behavior was analyzed and the residual fatigue life was predicted using a linear elastic fracture mechanics (LEFM) approach where also the mixed-mode conditions at the crack tips were considered.