Acoustic Emission Monitoring of Fatigue Cracks in Steel Bridge Girders

This report presents results from a laboratory study and field implementation of acoustic emission monitoring of fatigue cracks in cover-plated steel bridge girders. The acoustic monitoring successfully detected growing fatigue cracks in the lab when using both source location and a state of stress criteria. Application of this methodology on three field bridges also proved successful by detecting a propagating crack in two of the bridges and an extinguished crack in a third bridge. Researchers tested a double angle retrofit, designed by the Minnesota Department of Transportation, both in the lab and in the field of girder with fatigue cracks in the top flange. This retrofit does not require removal of concrete deck, and only involves bolting the retrofit to the bridge girder web. The double angle retrofit applied to laboratory test girder resulted in a reduction of flange stresses by 42 percent. Field implementation of the retrofit had mixed success. On one bridge, stress ranges in the cracked flange was reduced by 43 percent. However, on a second test bridge, the reduction was only 8 percent, likely due to the inadequate space for proper installation of the retrofit.