Three years after one of its Mississippi River bridges collapsed, killing 13 people, the City of Minneapolis has closed another bridge over the river due to corrosion.
The city’s Plymouth Avenue Bridge was ordered closed indefinitely on Friday (Oct. 22) after an inspection revealed corroded cables within the internal bridge structure, according to a statement issued by the city.
The bridge will remain closed until further investigation can be completed and repairs made, the statement said.
The bridge closure came just days after the partial collapse of a bridge over Elk Creek near the town of Brewster, southwest of Minneapolis. That bridge was being prepared for bituminous overlay Oct. 19 when a section gave way under an 80,000-pound milling machine, reports said. No injuries were reported.
The Plymouth Avenue Bridge connects north and northeast Minneapolis. The span is a post-tensioned box girder bridge, “a type of bridge design that includes multiple redundancies in the structure, and which is not a fracture-critical design,” the city’s statement said.
The four-span bridge, built in 1983, was the first segmental concrete girder bridge built in Minnesota. The bridge—943 feet long, 55 feet wide and 25 feet over the Mississippi—carries about 10,000 vehicles daily.
The concrete is engineered to be salt-resistant by the use of post-tensioning. Tubes run through the concrete structure, carrying strands of cable. With tension on the cables, the structure is designed to be under compression. This prevents cracks and hinders the intrusion of salt water.
In a statement, the city noted that it inspected all bridges annually. Workers found the corrosion on the Plymouth Avenue Bridge as they were performing routine maintenance and repair work.
“Additional investigation led engineers to conduct an examination of the interior of the bridge structure to determine the extent of the corrosion,” the statement said.
“That examination was done [Friday] morning, and due to the level of corrosion, Public Works officials decided that as a safety measure, the bridge should be closed while further evaluation is being done.”
City officials said that the bridge was not in danger of collapsing, but that they wanted to “err on the side of safety.”
‘It Had to Be Water’
Maintenance workers found signs of corrosion on at least five of the bridge's approximately 40 tendons, tensioned cables running through the concrete structure of the bridge, the Minneapolis Star Tribune reported. The cables inside the reinforced concrete girders are not visible from the exterior.
"Obviously, it had to be water coming in," City Engineer Steve Kotke told the newspaper. "It's a little surprising that we had this level of corrosion."
Tom Styrbicki, a bridge construction and maintenance engineer at the Minnesota Department of Transportation (MnDOT), noted that the cables were encased in concrete, with grout filling any gaps.
“[T]heoretically, that locks it away from moisture and oxygen,” Styrbicki told the news site financeandcommerce.com. “So, in theory, there’s no corrosion, so it’s a bit of a surprise to see the strands corroded like that.”
I-35W Bridge Collapse
On Aug. 1, 2007, the Interstate 35W Bridge over the Mississippi River collapsed, sending dozens of vehicles and a school bus carrying about 60 children into the water.
The accident killed 13 people and injured 145. The National Transportation Safety Board cited a design flaw as the likely cause, exacerbated by too much weight on the bridge at the time. The bridge was rebuilt and reopened 13 months later.
A video of the bridge collapse is available at http://www.youtube.com/watch?v=C31IlOHNzbM.