A major highway bridge in Minneapolis buckled during rush hour on Wednesday night, forcing dozens of cars to plummet into the Mississippi River. The Minnesota governor said a 2006 inspection of the bridge found no immediate structural problems. How do you check the structure of a bridge?
With your eyeballs. For the most part, inspectors rely on visual signs of damage to the main elements of a bridge. Teams of two or four look at the deck (i.e., the road), the superstructure (the steel support beams that hold up the deck), and the substructure (the piers and columns that make up the foundation). It's important to check the deck, which gets battered by traffic each day. Potholes and bubbles in the asphalt aren't just annoyances to drivers; they could damage the bridge support by allowing water and salt to leak down to the beams. Inspectors check the beams for cracks of any kind and measure their size and depth. These might form if the steel became brittle from bearing weight, like a paper clip that's been twisted too many times. The teams also measure the thickness of the beams to see whether rust has eaten away at the structure. As for the foundation, inspectors watch for soil erosion at the base of the bridge. Scuba divers also check for erosion underwater. If currents or storms have washed away the soil, engineers might try to bulk up the foundation by putting "riprap," or huge stones, around the pier.
By plugging the data from their inspections into computer models, engineers can get a good idea of how much load a bridge can handle. Sometimes they actually test the bridge to see how it responds to stress. (The government ran a test like this for the Minneapolis bridge in 2005.) This means fitting the steel beams of the bridge with, say, 40 to 60 sensors, then driving a truck of a known weight across. The sensors measure how much the beams bend, and the data can be used to predict what would happen under even heavier loads.
If a bridge has suffered some damage, officials will impose a load limit on the structure, saying, for instance, that no trucks heavier than 12 tons can enter. A bridge in satisfactory condition should support 45-ton vehicles, heavier than most 18-wheelers. A bridge in bad shape, on the other hand, might be able to handle only vehicles under three tons. Many new bridges today have a life expectancy of 100 years, with a few occasional repairs, like a deck replacement, over that time.
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Explainer thanks Stephen Banzaert of the Massachusetts Institute of Technology, George Goble of Bridge Diagnostic, and Charlie Metzger and Henry Berman of the Pennsylvania Department of Transportation.