The Federal Highway Administration is advising state transportation departments that oversee fracture-critical bridges constructed of T1 steel to inspect butt welds, just in case they have cracks similar to those recently discovered on the Interstate 64 Sherman Minton Bridge between Kentucky and Indiana. On Sept. 9, the Indiana Dept. of Transportation closed the 49-year-old bridge—a double-deck structure spanning the Ohio River with two 800-ft main spans—after an inspection revealed a 2.5-in. crack in the butt weld of a tension tie—a lateral component crucial to reinforcing tied-arch structures.
T-1 steel is a high-strength material that was commonly used to construct bridges in the 1950s and 1960s. The September advisory specifies tension components constructed of T-1 steel prior to adoption of the so-called fracture control plan of welding codes recommended by the American Association of State Highway and Transportation Officials. Bridges constructed prior to adoption of the 1995 code may develop cracks due to a lack of hydrogen control during welding, according to FHWA.
However, FHWA does not intend to mandate more frequent inspections of such bridges, which undergo biennial inspections as a provision of the National Bridge Program. “The types of cracks found on Sherman Minton can and should be addressed during routine inspection,” says FHWA spokeswoman Nancy Singer.
FHWA defines Sherman Minton and other fracture-critical bridges as structures with non-redundant components, meaning that if one part of a bridge fails, the entire structure could be at risk.
Not Another I-35 Scenario
Although FHWA ordered inspection of gusset plates following the August 2007 collapse of the I-35 Bridge in Minneapolis, another fracture-critical bridge, “that was an issue concerning a design flaw,” says Singer. FHWA indicated that the cracks on Sherman Minton were due to the capture of hydrogen in molten metal rather than fatigue.
FHWA has classified 18,000 U.S. bridges as fracture-critical, but data compiled since the September advisory indicate that only a few of these bridges contain T1 steel, adds Singer.
The material is no longer used to construct bridges. “Welding can introduce hydrogen to T1 and embrittle the material,” says W. Mark McGinley, a professor of infrastructure research with the University of Louisville, Ky. “Welding also can lead to fatigue. The steel doesn't contract to the degree it expanded during heating, which can result in residual stresses.”
Crews discovered the Sherman Minton crack while replacing welded connections with bolted connections after a biennial inspection indicated the butt welds could result in cracks.
“The orientation of the crack was of particular concern,” says Anne Rearick, INDOT director of bridges. “It was perpendicular to the tie, potentially subjecting it to stresses that could have pulled it apart.”