The Blanchette Bridge carries Interstate 70 across the Missouri River, connecting St. Louis and St. Charles counties in eastern Missouri. The westbound bridge was constructed in 1958. In 1979, the original reinforced concrete roadway deck on the bridge was replaced with a steel grid deck system, welded to supporting girders and stringers. In 1999 and 2005, cracks were discovered in stringers on the bridge approach spans. The stringers were repaired with bolted splices.

The investigation of the cracking included preliminary analyses, review of historical documents, detailed finite element analyses, and field testing of the bridge. The cracking was determined to result from the field weld detail used combined with the load distribution effects of the stiff grid deck. Recommended mitigation measures are included.