What do bridge inspectors look for?by Bob Kelleher, Minnesota Public Radio
Two high-profile bridges span the St. Louis River and the Duluth Harbor, connecting that city with Superior, Wisc. Each year, one of those bridges gets a routine safety examination. But in the wake of the I-35W bridge collapse in Minneapolis two weeks ago, both get a look this summer.
Superior, Wisc. — Every bridge has a personality, and this one is pretty tame. It's an overpass bridge, built in 1962, to carry Hwy. 53 over a city street. It's called a "deck girder bridge," considered a strong and reliable design, with redundant support if something should fail.
Inspector Allan Bjorklund is here to look for problems. This isn't an official inspection, he's showing us how a routine inspection works.
"The visual inspection is the primary inspection," Bjorklund says. "So that's pretty much what drives the whole thing."
Primarily, Bjorklund is looking for cracks, often tiny stress cracks in the steel. It's the same thing inspectors might look for with electronic technology like ultrasound. But Bjorklund says you wouldn't normally bring in advanced testing like that until you've spotted trouble.
"You have to almost see a problem before you take it a step further to investigate the problem," he says.
We've hiked through knee-high weeds and up a steep concrete slope to get an up-and-close look at the long, parallel girders that support the road surface on a series of concrete pillars.
Bjorklund says he doesn't look at every inch of the bridge. He doesn't have to, because he's inspected this bridge every two years. He knows where stress is most likely.
"For instance, right above your head here, on these steel girders," he says. "This is called a welded cover plate. Right in here, this is called a fatigue prone detail. You look at every one of those."
It's just a small, raised portion under a girder, a long, thin steel plate that's gives the girder additional strength. But it's welded perpendicular to the traffic load -- a potential place for stress. We're looking for any hint of a crack.
"You aren't going to see it underneath the paint. So the paint's going to have to break, and basically you're going to have to see rust," Bjokrlund explains.
Bjorklund looks at problems found in past inspections, like a small manufacturing flaw in one girder. There's a pillar that was damaged this spring, when struck by a car that burst into flames.
On the top deck, Bjorklund walks the median while traffic roars by inches away. He's looking for leaks or cracks in the concrete. Finally, he'll fill out an inspection report, which includes the National Bridge Inspection Ratings, a total number up to 100 called the bridge's sufficiency rating.
Between inspections, and inspectors, those points might vary, but Bjorklund says not by much.
"Everybody's taught basically the same thing. They bring their own experiences with them. But overall, the condition is what it is," he says.
This bridge appears to be aging well. The deck is fairly new. Faded green paint has chipped off the side, but the steel beams are clean, with very little rust. Bjorklund says this bridge carries some 16,000 cars a day, a fraction of that on the Minneapolis bridge that fell into the Mississippi River two weeks ago.
"Traffic is the driving factor for the deterioration of the bridge. As a general rule, the environment contributes some, but the traffic load is going to be your driving force," Bjokrlund says.
Among other background requirements, the inspectors are federally trained. If public safety is found at risk, a single inspector can close a bridge. Bjorklund says the inspectors are well aware of the responsibility they bare.
"I drive these. My family drives these. I don't want to have an unsafe bridge out here on the system," he says. "I want to sleep at night. If we find a problem, we'll address it. And, if a bridge is open, it should be considered safe."
In the Twin Ports, inspectors are now looking under one of the two bridges between Duluth and Superior. Next week, Wisconsin inspectors look at the other one.
- Morning Edition, 08/15/2007, 7:20 a.m.