RENEE MONTAGNE, host:
It will take weeks or months for experts to determine what caused the bridge there in Minneapolis to collapse. In the meantime, other cities and states will be looking at the safety of similar bridges, and there are thousands. Minnesota Governor Tim Pawlenty stressed the importance of examining all bridges at a press conference yesterday afternoon.
Governor TIM PAWLENTY (Republican, Minnesota): I think anybody who looks at the national picture, the national statistics and says we don't have a problem would be naive or misleading the situation. We do have a major problem.
MONTAGNE: As NPR's Jon Hamilton reports, many of the bridges were built before engineers discovered how vulnerable these bridges were to heavy use over many, many years.
JON HAMILTON: The central section of the Minneapolis Bridge spanned for more than 500 feet across the Mississippi River. It was supported from beneath by two arching lattice works of steel.
Mark Ketchum of OPAC Consulting Engineers in San Francisco said the design is known as a truss bridge.
Mr. MARK KETCHUM (Bridge Design Manager, OPAC Consulting Engineers, Inc): Truss bridges have been built all over the world, all over the United States for a hundred of years. And many of them far older than this have performed very well.
HAMILTON: Ketchum says truss bridges get their strength from careful geometric arrangement of hundreds or even thousands of pieces of steel.
Mr. KETCHUM: And they are connected together with riveted, bolted and/or welded joints. And it's those joints that are probably most suspect here.
HAMILTON: That's because joint are often where serious problems arise. Ketchum says stress can cause the joints to flex, which can eventually make them weak and fail abruptly. He says it's too soon to know for sure if this is what happened to the Minneapolis Bridge, but it's one of the first places investigators are likely to look.
Mr. KETCHUM: It was probably some sort of a metal fatigue leading to brittle fracture. These are very difficult conditions to detect in normal field inspections, and if they do come to a head, then they would cause this sort of a fast collapse that we saw there.
HAMILTON: Engineers have known about metal fatigue in airplanes and ships for decades. Nick Jones, dean of the Whiting School of Engineering at Johns Hopkins University in Baltimore, says the concept is pretty simple.
Dr. NICK JONES (Dean, Whiting School of Engineering, Johns Hopkins University): If you take a wire coat hanger and you bend it back a number of times, eventually you can get the wire to break.
HAMILTON: Of course, the process is much more subtle in a bridge. Instead of a few dramatic bends, the structure may flex imperceptibly with the passing of each car or truck.
Dr. JONES: With time, those many thousand or millions of cycles induce micro-damage in the structure that has to be monitored, and with time, can lead to cracking or failure of components or joints.
HAMILTON: Jones says modern bridges are engineered in withstand metal fatigue. But in the 1960s, engineers didn't know that it could affect bridges. They also didn't realize how much traffic these bridges would be carrying 40 years later.
Dr. JONES: You don't have to drive around too many freeways to feel it. In many instances, the roads are carrying more vehicles than was perhaps originally envisioned when they were designed.
HAMILTON: Before it collapsed, more than 140,000 vehicles a day drove over the Minneapolis span. And state officials were looking for damage related to metal fatigue. Two studies of the bridge looks specifically at the problem but concluded that the bridge was sound. One of the studies recommended frequent, intense inspections.
At a press conference Thursday, Dan Dorgan of the Minnesota Department of Transportation said the most recent round of inspections started this spring.
Mr. DAN DORGAN (State Bridge Engineer, Minnesota Department of Transportation): In May, we began inspections of the bridge on - had the bridge partially inspected this year. And we were going to complete that this fall. There was construction going on in the bridge so we were going to wait until construction completed to finish that inspection.
HAMILTON: That inspection will be finished by forensic experts after they raise key pieces of the bridge from the bottom of the Mississippi. Experts say there are thousands of truss bridges in the U.S. and probably hundreds similar to the one that failed. They are all likely to get a very close look in the coming months.
Jon Hamilton, NPR News.
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