Steel plate girder bridges have details that are often vulnerable to fatigue cracking caused by localized secondary stresses. The objective of the study is to first identify the driving mechanism of fatigue cracking at the bottom flange-to-gusset connection in a two-girder bridge and then the retrofit to mitigate the crack driving source. These objectives are achieved by performing a finite element study to characterize the global superstructure behavior under the moving truck load and to reveal the local stress distribution around the flange-to-gusset plate connection, designing a suitable retrofit, and verifying its performance with pre- and post-retrofit field testing. Analysis revealed that cracks developed due to the prying and the racking action of the cross-frame members and the lower horizontal bracing members on the gusset plate. The strains caused by this secondary action are small and therefore only a qualitative interpretation of the results is feasible. The study correctly identified the hot-spots, which was corroborated by the observed crack initiation locations. Structural repairs were performed to firmly attach the gusset plate to the bottom flange, and post-repair analysis showed that the procedures were successful in reducing the prying and racking action and eliminating the hot-spots. The field test data validated the finite element models and allowed repairs to be put in place that were effective. The field tests and the finite element analysis showed that the post-repair stresses were below the fatigue limit and therefore fatigue cracking should not recur at this detail.
Abstract