Repairs Begin on Wind-Damaged Sabo Pedestrian Bridge
The city of Minneapolis began repairs on Sunday to the Sabo pedestrian bridge, six months after winds as light as five miles per hour were found to have caused vibrations of the stay-cables that then put stress on the bridge's diaphragm plates, leading to fractures in two plates and defects in another two.
"We're going to do our best to try and move this forward as quickly as possible," explained Steve Kotke, city engineer and the director of Public Works, during an interview at the bridge site Sunday afternoon.
Crews spent much of the day high above closed Hiawatha Avenue in two cherry-pickers inspecting and beginning repairs on a test plate. Kotke described the proposed installation like a sandwich - with the original 3/4" steel plate in the middle, and a thicker 1 3/4" plate on the top and another identical plate underneath to "sandwich" the diaphragm plate and strengthen and stiffen it against future wind vibrations.
The initial plate - the third highest on the bridge pylon - will act as a trial run for proposed repairs to the other 17 on the bridge. The first reinforcement should be complete on Thursday, Kotke predicted, leading to an observation period of approximately three weeks. If the fix works as anticipated, then work can begin on shoring up the bridge's other plates, and hopefully be completed by January.
"Hopefully, when that's done, this is good for another 30, 40, 50 years," Kotke said.
Two plates fractured in February, leading the bridge to be shut down to pedestrian and bicycle traffic just five years after it was built. A subsequent investigation by consultants Wiss, Janney, Elstner Associates from Illinois concluded that bridge designer URS did take wind vibrations into account in the designing the bridge, but "deferred a final decision," on whether to install a damping system to deal with the vibrations "until after construction."
A month after monitoring the completed bridge, the consultants found, URS had decided the system was "not necessary."
Now, Kotke acknowledged that "we probably would have liked to taken a closer look," at the dampening system option and may still consider adding it after the current plates are stiffened.
"There's probably a number of things in hindsight that we, I think, all parties probably would have done a bit differently."
The city is in settlement talks with URS to resolve the issue of how the repair costs will be paid for. So far, Kotke said, about $1 million has been spent since February related to the bridge failure and another $1 million could be spent to complete repairs.
The $2 million repair bill would be two-thirds of the entire $3 million cost of constructing the cable-stayed portion of the bridge.
Watch our story above to see the crews repairing the bridge.