Timber bridges outstate are a factor behind the low ratings in the latest Minnesota analysis.
A new look at state bridge conditions shows that 11 percent of state bridges are either “structurally deficient” or “functionally obsolete,” up from 7.7 percent a decade ago.
The analysis, from the Minnesota Compass group that tracks transportation trends and other topics, scoured Minnesota Department of Transportation data and found that five of the seven worst-rated counties are outstate, where more than one in five spans fall into those two categories.
While Ramsey County had the highest percentage of either deficient or obsolete bridges at 25 percent, Pipestone, Lincoln and Mower counties in the southern swath of the state and St. Louis County in the northeast all cracked the bottom five.
Many of the troubled bridges are made of timber, dating to the mid-1900s, and fall under the control of smaller counties or townships. MnDOT estimates there are 1,528 timber bridges statewide, more than one-fifth of which are structurally deficient.
Experts point out that neither the deficient nor obsolete descriptions mean the bridges are unsafe. “Structurally deficient” means that a bridge component — such as its deck or superstructure — has fallen to four or lower on a 10-point scale. “Obsolete” bridges were built to earlier standards and are often narrow and in need of eventual replacement.
“It doesn’t surprise me because, all the way across southern Minnesota, there are a lot of timber bridges built in the 1940s and ’50s,” engineer Dave Halbersma said. “Those are the ones we’re having a lot of problems with because the timber pilings are rotting out.”
Halbersma has been Pipestone County’s engineer for 24 years, and this month he added the same responsibility for adjacent Lincoln County on the South Dakota border. He’s now in charge of bridges in two of the three lowest-ranked counties in the report. He said half of Pipestone County’s 55 deficient bridges are made with treated timber.
“There’s no need to panic,” he said. “But we’re encouraging township and county administrators to start looking at this one or that one.”
Finding public dollars to replace and repair older bridges is tricky, according to Ron Chicka, the director of the Duluth-Superior Metropolitan Interstate Council.
“It’s the same old story of dwindling funds,” he said. “There are just less federal and state funds trickling down and coming into this area.”
Enhanced scrutiny since the Interstate 35W bridge collapsed in 2007 is a factor in the analysis.
“The standards haven’t changed, but we’re taking a little bit more of a conservative approach than we did,” St. Louis County bridge engineer Matt Hemmila said. “We have more sets of eyes and better tools for gathering data.”
Researchers found that northeastern Minnesota has had a particularly rough decade in the bridge quality department. Between 2002 and ’12, its obsolete and deficient bridges have gone from 9 percent to 17 percent when the categories are combined.
Duluth bridge engineer Jim Benning said he’s not sure it’s fair to combine the two descriptions. His city’s famous Aerial Lift Bridge, for example, will always be considered obsolete because it was built narrowly. But it can still support as much weight as ever.
Yet sprawling St. Louis County has roughly 110 bridges with either timber decking or pilings crossing streams and other chasms on smaller roads.
“Driving treated timbers in the ground was the economical way to build them in the ’50s,” Hemmila said. “They’re all getting to the end of their lives and are coming to the forefront as the next chunk of bridges that need to be tackled.”
Curt Brown • 612-673-4767