August 27, 2007

Six bridges are classified as being 'structurally deficient' in Wright Co.

But engineers say bridges are made with solid designs; are very different structurally than I-35W bridge

By Lynda Jensen

A total of six bridges in Wright County – three maintained by the county andthree by the state – are defined as “structurally deficient” according to county and state engineers.

The bridges include the Highway 12 river bridge in Delano, which is a favorite of commuters, a bridge north of Cokato, and a third near Clearwater, with the remaining three located near Monticello by I-94.

However, this designation doesn’t mean the bridges are unsafe or shouldn’t be used, according to County Engineer Wayne Fingalson and state engineer Terry Humbert of the Minnesota Department of Transportation.

“That doesn’t mean they are ready to collapse,” Fingalson assured, saying that a bridge being “structurally deficient” means it is usually scheduled for replacement.

The criteria for replacement includes measures for bridge superstructure (beams) and substructure (piers that go into the water or abutments, which hold up the superstructure), as well as non-structure based points, such as access and traffic flow (how wide lanes are, etc.).

Most bridges in Wright County are made of pre-stressed concrete, which is a simple and stable bridge design that lasts for decades, Humbert said.

Bridges are inspected and then assigned a sufficiency rating, which is based on a scale of 0 to 100, and notes a bridge’s overall capability and is used by states to determine eligibility for federal funding for repairs.

If the rate is 50 or less, this qualifies the bridge for federal funding.

The condition rating, which is based on a scale of 0 to 9, is used to rate a bridge’s decks, the superstructure that holds up those decks, and the substructure or piers that support a bridge from underneath.

A bridge is classified as structurally deficient when one or more of those components gets a rating of four or lower.

The six bridges in Wright County

The six bridges classified as structurally deficient in Wright County are:

• Highway 12 river bridge in Delano, maintained by the state. This bridge, built in 1922, has a sufficiency rating of 43.4.

This bridge is scheduled for replacement next year.

• The bridge north of Cokato on CSAH 35, east of Highway 3, maintained by the county. “It just dropped into that category,” Fingalson said. This bridge, built in 1940, has a sufficiency rating of 49.3.

• The bridge on CSAH 40, south of Clearwater and west of Trunk Highway 4, maintained by the county. This bridge, built in 1957, has a sufficiency rating of 38.9. It has been scheduled for replacement in 2011-12.

• The bridge adjacent to the nuclear power plant in Monticello along CSAH 75, maintained by the county. This bridge, built in 1936, has a sufficiency rating of 31.8. It will be replaced in 2010.

• The eastbound I-94 bridge that spans the railroad east of Monticello maintained by the state. This bridge, which was built in 1972, has a sufficiency rating of 81.7.

• The westbound I-94 bridge that spans railroad east of Monticello maintained by the state. This bridge, which was built in 1971, has a sufficiency rating of 81.3.

In general, bridges are made up of three components, the deck (the top surface that cars drive upon), superstructure (supporting beams) and substructure (piers that go into the water or abutments).

Inspectors look for a number of factors when they inspect, such as corrosion on steel or if the abutments in the soil have shifted. Usually bridges are inspected every two years unless there is a problem or as the bridge gets older. If so, they are inspected yearly until replaced.

The Delano river bridge currently is assigned a four for condition in all three areas of the rating system, that is, it received a relatively poor rating in deck geometry (traffic flow access), a four in urban waterway adequacy, and a four in structure, which includes the channel waterway.

The Delano river bridge was built in 1922 and then remodeled in 1928.

A favorite of commuters and Delano-ites

The Highway 12 river bridge, spanning the Crow River, has one of the lower ratings of the structurally deficient bridges and is also the oldest bridge in that set.

However, it’s built with a cast-in-place concrete, which is built to last. In fact, most bridges in Minnesota have a lifespan of 75 to 100 years, according to MnDOT.

The I-35W bridge, which was designed as a deck steel truss bridge, was built to last for 50 years, and originally constructed in 1965.

That bridge design was based on an interlinking set of struts and beams that couldn’t absorb stresses, compressions and tension inside and out very well – in other words, if one part failed, it all failed, like a set of dominoes, he said. There were only three built in Minnesota and Wisconsin, the I-35W bridge, another in St. Cloud on Division Street that spans the Mississippi River, and a third in Osceola, Wisc.

The river bridge was made by poured-in-place concrete. The following are newspaper clips about the Delano river bridge’s opening 85 years ago:

“Minnesota Highway bridge No. 3622, across Crow River at the South end of ourtown, is nearly ready for use by the public and a decided acquisition to this section of country. The cement work is all finished and the “filling in” is progressing rapidly.

– The Delano Eagle Thursday, Aug. 31, 1922

The new bridge was opened to the public Wednesday morning and the crew areshipping the last of their machinery, timbers etc. this (Thursday) morning.The view of the town from the bridge is very attractive and it is a prettydrive ‘round the park and to and across the bridge and back on the “WestSide.” Try it.

– The Delano Eagle Thursday, Sept. 7, 1922

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