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Inspectors Look for Weaknesses in Bridges

June 24, 2008

By Tu-Uyen Tran, Grand Forks Herald, N.D.

Jun. 24–Under the Sorlie Bridge in East Grand Forks, rust patches the steel beams that support the road above, which, during most of the day and night, rumbles with passing traffic.

But for bridge inspector Jeff Moore, the rust isn’t a big deal because as long as it’s just cosmetic, it won’t have any impact on the strength of the beams. “Paint makes it look worse than it is.”

With funding from the Minnesota Department of Transportation, he and his crew are looking for things such as cracks, popped rivets and heavy rust that cause actual pitting of the steel.

Today, they’ll be checking the Kennedy Bridge, which the state has named one of 11 major bridges it wants to replace in the next decade.

The inspectors will spend about eight days on both bridges.

MnDOT’s already done inspections, but Moore, a lead inspector with Minneapolis-based PB Americas, is doing another as part of an intensive inspection plan focused on gusset plates on truss bridges.

A gusset plate is a metal plate that, by welds or rivets, connects steel beams. The failure of these plates is what likely caused the collapse of the Interstate 35W bridge in Minneapolis last summer, according to the National Transportation Safety Board.

The plates were not as strong as they needed to be because engineering calculations were off, said MnDOT spokeswoman Lucy Kender. The state now is recalculating and having inspectors check those calculations in the field, she said.

According to the state, there are 59 truss bridges in the state. The Kennedy and Sorlie are two.

Replacements

The last time the Sorlie got a checkup was October. Inspectors said at the time that it was “functionally obsolete.” This doesn’t mean it’s in bad shape, just that, built in 1929, it doesn’t meet modern standards.

The Kennedy, built in 1963, got its checkup in May 2007 and got an “adequate” bill of health, meaning repairs aren’t needed. That’s pretty good for a bridge.

It needs replacing only because, as a truss bridge, it has only two structural supports, one on each side. Should one fail, there are no other supports to keep the bridge deck from flipping into the river. Again, this doesn’t mean it’s in bad shape, just that it’s not modern.

Replacing the Kennedy would cost $40 million to $70 million, which is way more than what’s available from the regular funding channels, qualifying the project for special state funding.

The Sorlie needs replacing, too, but doesn’t qualify for special funding.

Newer bridges of similar length would have multiple beams of pre-stressed concrete, which allows one to fail without catastrophic results, Moore said. These also take less time to inspect, he said.

Checkup

For the most part, Moore and his crew will conduct a visual inspection and check with records of past inspections to see how things have changed.

They’ll focus more attention on areas where the structure bears the most weight; that includes checking the trusses above and the beams below, he said. In some areas, he said, they’ll use a probe to detect the thickness of the steel and compare that with the original plans.

If the steel isn’t as thick as the plans say, that could spell trouble.

Inspectors also will look at the concrete pier in the middle, Moore said, to check for major cracks.

Many bridges, he said, last 75 years or more with maintenance. Concrete likely will fail first, he said, but it’s not very hard to replace.

Reach Tran at (701) 780-1248; (800) 477-6572, ext. 248; or send e-mail to ttran@gfherald.com.

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Copyright (c) 2008, Grand Forks Herald, N.D.

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