Distortion-Induced Fatigue Cracking of Bridge Girders – Design Issues

Distortion-induced fatigue cracks are common occurrences in older bridges where details were used that are now known to be fatigue prone. These details generally consist of transverse stiffeners cut short of the girder flange to which a diaphragm or cross bracing is connected. The issues related to the assessment of existing bridge with this poor fatigue detail include the evaluation of the remaining fatigue life, the susceptibility of the damaged girders to brittle fracture as the fatigue crack grows, and the repair of these fatigue cracks. A study that included both experimental work and numerical analysis was conducted to assess an existing railway bridge in which over 300 cracks had been detected before it was replaced. The experimental work, which included both field measurements on the bridge before it was taken down and controlled laboratory fatigue tests, was conducted to assess the behavior of the bridge in-situ and the behavior of the web gap. The results of the field work were used to design the laboratory experiment to replicate the field conditions. Finite element analyses provided useful information for the design of the laboratory test program and to evaluate the fatigue susceptibility of web gaps in steel plate girders. Extensive analysis of web gap areas was used to validate simple procedures for the calculation of web gap stresses. This paper presents the results of this analysis and presents an assessment of commonly used web gap stress calculation method.