Just days after the City of Minneapolis temporarily closed one of its Mississippi River bridges to inspect a corrosion problem, the city has decided to keep the bridge closed all winter.
On Oct. 22, the city Department of Public Works ordered the Plymouth Avenue Bridge closed indefinitely after an inspection revealed corroded cables within the internal bridge structure.
Just days after that, however, the city said that the corroded cables would probably have to be removed and replaced—a project that could take until the spring.
Risk of Shifting Parts
Public works director Steve Kotke said that further investigation had found corrosion on about one-fourth of the cables and that officials were “concerned enough” that the bridge warranted a longer shutdown. He said that the bridge was not in danger of collapse, but that parts could shift if the cables weren’t fixed.
Allowing traffic on the bridge before the repairs are made could worsen the problem and increase the scope of the repair, he said. On the other hand, early detection of the problem and good overall maintenance means that the bridge can probably be salvaged, he said.
Meanwhile, Kotke said, engineers are conducting load analyses and other tests and inspections to better determine the extent of the problem and the scope of repairs.
The bridge was initially closed just days after the partial collapse of another bridge 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 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.
City public works workers found the corrosion on the bridge as they were performing routine maintenance and repair work.
Upon further inspection, the corrosion was found to be more extensive than originally thought, with “a number of tendons in very, very poor shape,” Kotke said. Signs of corrosion were found on at least half of the bridge's approximately 40 tendons, tensioned cables running through the concrete structure of the bridge. The corrosion is limited to the outside-most cables. Those on the middle of the span are not affected, Kotke said.
The cables inside the reinforced concrete girders are not visible from the exterior, and Kotke credited the city’s inspection program with discovering the problem. “Corrosion of this type is not easy to find,” Kotke said. Spalling on the bottom of the bridge tipped officials to a potentially deeper problem, which the inspection later uncovered.
Officials say the problem is water infiltration, but they haven’t yet determined how that occurred. Given the concrete-and-grout encasement of the cables, corrosion should not have happened. Faulty or inappropriately applied grout could be an issue, as could the drainage system that runs through the structure, Kotke said. "It's a little surprising that we had this level of corrosion."
A Minnesota Department of Transportation (MnDOT) maintenance engineer also said the corrosion was “a bit of a surprise,” because the concrete construction “theoretically” locked the cables “away from moisture and oxygen.”
On Aug. 1, 2007, the city’s Interstate 35W Bridge over the Mississippi River collapsed, killing 13 people and injuring 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.