The design of the Sabo Bridge in Minneapolis allowed even minor winds to compromise its suspension cables over time, according to an analysis ordered after the bicycle and pedestrian bridge had to be closed for safety reasons last winter.

A set of cables on the bridge broke loose in February because of wind-induced vibrations, a consultant said Friday, noting that the stresses generated by winds as light as 5 to 10 miles per hour might have been overlooked by the engineers who designed it. The designer, California-based URS Corp., also did work on the Interstate 35W bridge over the Mississippi River before it collapsed in 2007.

The firm is now seeking a $94 million engineering contract for the proposed Southwest Corridor light rail. The Metropolitan Council delayed action until it could see the Sabo report, and the findings could mean new trouble for URS.

"I think the finding is absolutely a cause for concern," Met Council Chair Susan Haigh said on Friday, adding that the report "raises serious questions." She said that the light-rail contract decision will be delayed until the council can see the full report, due out June 28, and perform a more extensive review.

"We want to have our engineers look at it," Haigh said. "We want to meet with Minneapolis and Hennepin County. And of course we want to meet with URS."

A URS spokesman declined to address the consultant's findings in detail. "We look forward to receiving the completed report and are continuing to work with the city and county on bridge retrofit solutions," URS spokesman Ronald Low said.

A summary of the report, prepared by Wiss, Janney, Elstner Associates Inc., concluded that "wind-induced cable vibrations" weakened the steel plates that anchored the cables to the bridge pylon until two of the plates fractured.

"Stay cable vibrations (wind induced) were not included in [URS'] original design calculations package," the summary concludes, referring to documents URS provided to the consultant.

The Martin Olav Sabo Bridge, named after Minneapolis' former congressman, opened to pedestrians and bicycles in 2007. When a set of cables broke loose Feb. 19, no one was hurt and the bridge remained standing. But workers had to shore up the bridge and didn't reopen it until June 1.

What did they consider?

Wiss, Janney, Elstner was hired by the city and Hennepin County to investigate the incident. Brian Santosuosso, a senior associate with the firm, said they plan to have further talks with URS to determine how vibrations were accounted for in the bridge design.

What they know now, he said, is that "the documentation that we were given didn't include it."

URS told the consultants that it monitored the cable vibrations for about 30 days after the bridge was built and "felt it wasn't really significant," said Wiss, Janney, Elstner unit manager Michael Koob, who also led the firm's 35W investigation.

"What they have implied to us is that they did consider it, they wanted to build the bridge and watch it for a while and see how the response to those cables occurred," Koob said.

Minneapolis Public Works director Steve Kotke wasn't ready to cast judgment on URS until the company has a chance to fully respond.

"We need to have conversations with URS," Kotke said. "So I think it's premature for us to charge one way or another at this point."

URS has continued to receive city contracts totaling more than $9 million in the past five years. Just last month, the City Council approved a $1.2 million contract with URS to study streetcars along Nicollet and Central Avenues.

Concerns about URS date to 2007, when the company was a consultant studying the state-owned 35W bridge before it collapsed, killing 13 people. Victims of that disaster alleged in a lawsuit that URS overlooked the bridge's structural flaws, and the company agreed to pay $52.4 million in 2010 to settle the suit without admitting fault.

The company's qualifications received renewed interest this spring when Gov. Mark Dayton's spokeswoman said the governor had "very strong concerns about the state doing business with URS." Dayton's office later said he would meet with the company and consider its extensive experience.

YouTube premonition

Seven months before the cables broke loose, Patrick Valdez crossed the bridge while on a bike ride with his dog and twin daughters.

The south Minneapolis resident noticed that a cable was vibrating violently, though it was not a windy day. He shot a short video and posted it on YouTube.

"At that time I thought, 'I'm surprised this cable doesn't break loose,'" Valdez said. When he saw news of the "wind-induced" vibrations Friday, he said "it kind of all clicked in my mind.'"

Eric Roper • 612-673-1732 Twitter: @StribRoper