Investigators still do not know exactly why the I-35W bridge collapsed into the Mississippi River in Minneapolis on August 1, killing 13 people and injuring about 100. But a succession of less spectacular failures over the years has raised concern among the country’s bridge engineers. New materials promise to make routine repairs less costly and intrusive, an important consideration in an era in which money for infrastructure is tight. “The days of letting a bridge deteriorate and then simply replacing it are going away,” says Mark Hirota, a consultant with Parsons Brinckerhoff and a former bridge engineer for the state of Oregon.
According to a 2005 study by the American Society of Civil Engineers, 27.1 percent of the nation’s bridges are either structurally deficient or functionally obsolete—a total of more than 160,000 bridges. The estimated cost to repair these bridges is almost $10 billion a year over the next 20 years. Built in 1967, the Minneapolis structure was a deck truss bridge with multiple steel girders and had been a concern among the state’s transportation officials for years. After the collapse, U.S. Secretary of Transportation Mary Peters quickly called for an inspection of all the nation’s steel truss bridges similar to the I-35W span. A truss bridge, one of the oldest bridge types, is fairly simple in design and relatively cheap to build as compared with suspension and cable-stayed bridges, but engineers make no generalizations about types especially prone to sudden collapse.