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Prototype Testing for Distortion-Induced Fatigue Retrofit near Cairo, IL

Fatigue crack exposed after opening core
Two-hole retrofit
Field testing two-hole retrofit prototype
FEM of retrofit prototype
Researchers at the S-BRITE Center worked with the Illinois DOT to test a retrofit intended to mitigate further distortion-induced fatigue on a multi-girder bridge near Cairo, IL.

Bridge inspectors for IDOT had previously identified over 1900 locations of weld toe cracking at the weld terminations of cross bracing connection plates and the State was considering implementation of a doubler plate style retrofit.  The S-BRITE DEN was asked to determine if this would be a good retrofit approach. 

With support from IDOT crews, several simple retrofits of two different types were installed on the bridge and a fatigue analysis was performed by collecting triggered time-history stress and differential displacement data of random traffic over the course of a couple weeks.  The field data was also used by researchers to calibrate finite element models used to evaluate the performance of similar retrofit designs. Additionally, several cores were removed from the bridge confirming the presence of fatigue cracks in most of the samples removed.

The field testing and finite element analysis concluded that a 2-hole retrofit with at least 2" diameter holes could be implemented, providing infinite fatigue life for the bridge at the cross-brace connection details, while also eliminating any possible concern for unstable crack growth into the girder web plates.  This retrofit simply entails drilling two holes at each location, which would be a significant cost savings over installing a doubler-plate, stiffening type retrofit, while also maintaining the detail exposed and easy to inspect.