Corrosion closes Plymouth Av. bridge

City closed the bridge for at least a week after inspectors found the problem on multiple cables. Traffic was diverted to Broadway Av. bridge.

The Plymouth Avenue Bridge was closed Friday as a precaution after a routine inspection discovered corrosion on multiple cables tying together the 27-year-old structure.

The closing, expected to last at least a week, diverted eastbound traffic to the Broadway Avenue Bridge, which in its first rush-hour test Friday afternoon appeared to handle the increased flow without significant delays.

For now, joggers and cyclists who have made the bridge part of a loop that allows them to take in both sides of the Mississippi River will have to devise new running and cycling routes.

Still to be seen is whether the closing hurts business at the Northeast Yacht Club, a bar at Marshall Street and 8th Avenue NE., where the happy hour crowd was thinner than usual about 5 p.m. Friday. But, then, the bartender said, the weather was beautiful.

The timing of a bridge reopening will depend on further inspections and needed repairs. About 10,000 vehicles a day will be diverted to the Broadway Avenue Bridge, which is the recommended detour six blocks upstream, or the Hennepin Avenue Bridge downstream.

The 944-foot Plymouth bridge connects north and northeast Minneapolis.

On Friday, signs directed motorists away from the bridge. Sidewalks, too, were closed to pedestrian traffic. At 4:39 p.m., and again at 4:57 p.m., vehicles covered the entire length of the Broadway bridge. But motorists heading east into the northeast neighborhoods were able to cross within two to three minutes.

On Friday, the city said that maintenance workers had found signs of corrosion in the bridge's "tendons," cables under tension in three sections that squeeze pieces of the box-girder bridge into a whole. The dozens of cables inside the reinforced concrete girders are not visible from the exterior.

Further checking led the Department of Public Works to close the bridge until it is better evaluated. City Engineer Steve Kotke said it will take at least a week to know how long the closing will last. Kotke said the bridge is designed to be redundant so the failure of a single component doesn't augur a collapse. He also said that the large hollow box girders showed no signs of the shifting that would indicate more advanced problems.

"I'm not concerned about a collapse," Kotke said. But, he added, he prefers to err on the side of public safety.

The 1983 bridge is the third on that site and was the first in the state built as a post-tensioned box girder bridge, a technique that now is more common and was used on the new Interstate 35W bridge.

"Obviously it had to be water coming in," Kotke said, referring to the corrosion found on at least five of the roughly 40 cables that tie together the five-span bridge. "It's a little surprising that we had this level of corrosion."

According to a riverfront district website, the bridge was built from a series of cantilevered concrete box girders. That allowed faster construction without wooden false-work to hold the bridge until it could support itself. The stranded cables in the concrete strengthen the material and are supposed to resist salt corrosion.

sbrandt@startribune.com • 612-673-4438 alonetree@startribune.com • 612-673-4109

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