Damage to the Sabo bike and pedestrian bridge in Minneapolis was caused by vibrating cables, according to the assessment released today by an engineering firm hired by the city of Minneapolis and Hennepin County to study the damage.
The summary of the report says the vibrations caused stress fractures in the steel diaphragm plates that held the cables to the top of the bridge. In February, one of those plates broke in half causing a pair of cables to fall more than 100 feet to the ground.
The five-year-old Sabo bridge is completely supported by 18 pairs of metal cables. Each pair of cables is connected to the bridge deck by an anchor and they are fastened to the top of a 100-foot tall support by steel diaphragm plates.
Brian Santosuosso of Wiss, Janney, Elstner Associates, the engineering firm that studied the damage, said wind-generated vibrations stressed two steel plates to the point of fracture, leaving the bridge without two pairs of cables.
The assessment does not indicate whether the plates were insufficiently designed to handle the stress, however, Santosuosso says the bridge was designed to withstand the loss of a few cables.
"The design, provided by URS, accounted for both maintenance issues and for problems that may arise along the lifespan of the structure; that a certain amount of cables can be removed and the bridge would be stable," Santosuosso said.
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San Francisco-based URS Corporation, one of the largest engineering, design and construction firms in the world, consulted on the I-35W bridge which fell in 2007. While it didn't design or build that bridge, it did agree to pay more than $50 million to settle lawsuits stemming from the collapse.
Minneapolis Public Works Director Steve Kotke said URS is working with the city to help retrofit the Sabo bridge. He said it is too early to say how much the retrofit will cost or if the city or county will ask URS to help pay for repairs.
"We're going to have conversations with the designer and talk to them about our findings and I'm sure the conversation about responsibilities and where we go from here will come up."
A URS spokesperson declined to be interviewed but issued a statement that the company is looking forward to receiving the completed report. The company has designed other cable-stayed bridges around the country.
The Metropolitan Council is considering URS for a nearly $100-million contract for the Southwest Light Rail project stretching from Minneapolis and Eden Prairie.
Metropolitan Council President Sue Haigh said the summary on the Sabo bridge damage is concerning and said council staff will closely study the full report before making a decision about URS' bid.
"It's really too early to say what the result of the due diligence report will be other than to say we are going to take the time to look at this report very carefully — really understand what the design issue is here," Haigh said. "And then ask our staff to come back with a recommendation to us."
The summary of findings comes a week after the city temporarily shored-up the bridge and reopened it to bike and foot traffic. A full report on the bridge damage will be released at the end of this month. City officials expect repairs should be completed sometime before winter.