By ANNE SCHIEBER
GRAND RAPIDS -- There are nearly 600,000 bridges in the United States, and many times when you travel you don't even know you're on one.
In September 2005, the bridge going over US-131 at Franklin Street in Grand Rapids had chunks of concrete fall onto the highway. No one was hurt, but there was a massive traffic backup.
At the time, the Michigan Department of Transportation blamed climate, moisture and age. Many of the bridges on US-131 were built in the 1950s, and federal laws require that states inspect their bridges every other year. Some bridges, such as the Mackinac Bridge, the Ambassador Bridge and the International Bridge, are inspected yearly.
In the mid-1980s, a major bridge collapsed in Oceana County. No one was hurt when a bridge on US-31 over the Pentwater River collapsed, taken out by a rush of flood water. It took more than a year to reopen that section of roadway.
The Zilwaukee Bridge, an eight-lane bridge spanning the Saginaw River, also experienced a failure during its construction in 1982.
In that case, a 150-foot long segment was not properly balanced and sank several feet. The opposite end of that segment rose into the air, badly damaging a foot pier. That held up the completion of the Zilwaukee Bridge by several years and added $48 million to the price.
Brian Slagter runs a company that inspects and repairs bridges in Michigan and Florida. He points to federal data that finds one-quarter of the nation's bridges on US highways are labeled "structurally deficient" - as was the Minneapolis bridge.
But transportation officials say just because a bridge is structurally deficient doesn't mean it's unsafe.
"A bridge can be labeled structurally deficient and it means that it can have potholes and may fall into that category," said MDOT's Dawn Garner.
MDOT maps all the bridges that are structurally deficient for their internal database. There are 736 bridges in an eight-county area of West Michigan, 311 in Kent County. Their condition frequently changes.
The bridge that collapsed in Minneapolis is a continuous deck truss built in 1967. Michigan has four such bridges, all located in the northern part of the state. They go over four different rivers.
MDOT said they are now inspecting each of those bridges.