Project prolongs Blatnik’s lifeConstruction work on the Blatnik Bridge did more than slow commuter traffic this summer. It prolonged the life of the 52-year-old bridge and pulled it off the national list of structurally deficient bridges.
By: Maria Lockwood, Superior Telegram
Construction work on the Blatnik Bridge did more than slow commuter traffic this summer. It prolonged the life of the 52-year-old bridge and pulled it off the national list of structurally deficient bridges.
The span connecting Superior and Duluth, however, will remain on the list of bridges that are fracture critical.
“It’s always been fracture critical and it always will be,” because of the way it’s designed, said Perry Collins, district bridge engineer for Minnesota Department of Transportation District 1.
In fact, two bridges that connect the cities — the Blatnik and the newer Bong Bridge, built in 1984 — boast fracture critical designs.
“Fracture critical bridges are safe,” Collins said. “If they weren’t we would close them.”
The term means that if a key supporting element (fracture-critical member) failed, it could cause partial or full collapse of the bridge. Design options are limited when a bridge has a long, clear span, Collins said. And the Twin Ports spans are the longest (Bong) and second-longest (Blatnik) bridges in Minnesota.
Work on the Blatnik Bridge in the 1990s strengthened the approaches to the bridge so they were no longer fracture critical, according to Allan Bjorklund, structures maintenance engineer with the Wisconsin DOT-Northwest Region, but the center span is inoperable.
“It’s an entirely different design and there is really not an efficient way to remove that designation from it,” he said.
Of more concern is a structurally deficient rating. Bridges that fall into that category typically require significant maintenance and repair to remain in service, according to the national subcommittee on transportation communications. But making the list doesn’t make the bridge unsafe, according to Christine Ouellette, spokeswoman for the Wisconsin DOT-Northwest Region.
“The terms are used by engineers to determine which bridges need to be monitored more closely,” she said.
A 2011 inspection of the Blatnik Bridge revealed corroded steel on the main-span truss and led to it being classified as structurally deficient, Collins said. A two-year, $13 million construction project kicked off in May 2012 to address the problems. The work, which included painting, truss repair and replacement of the expansion joints that allow the bridge to accommodate the temperature changes in the Northland, is slated to wrap-up next month. When the final bridge report is filed for 2013, Collins said, the Blatnik Bridge should be off the structurally deficient list. The current 40-ton weight limit signs will also come down, he said.
About 30,000 vehicles travel across the Blatnik Bridge each day. The heavy traffic, coupled with extreme temperature changes and the structure’s age, mean MinnDOT crews will be keeping a close eye on it.
“We’re going to have lane closures out there every summer, if just for inspection,” Collins said. Drivers can expect a minimum of two to four weeks of lane closures each year.
When the bridge was built in 1961, the average life expectancy of bridges was about 50 years, Collins said.
“We need to keep it in good condition as long as we can,” he said.
About 18 bridges in the Wisconsin DOT Northwest Region 20-county area are listed as structurally deficient and fracture critical. About 60 bridges statewide made both lists.