Wind-induced vibrations caused the fractures in cable diaphragm plates that led to the months-long shutdown of the Martin Olav Sabo bike and pedestrian bridge in Minneapolis.
That was the conclusion of a 140-page report by consultant Wiss, Janney, Elstner Associates, which the city and Hennepin County hired to determine why the top plate cracked and sent a set of cables flying loose from the bridge on Feb. 19.
The report, released Friday, reinforced conclusions outlined in a summary earlier this month. The detailed report recommended that existing plates be "retrofitted to enhance fatigue resistance" and that lower cable anchorages and sockets be examined for susceptibility to stress and damage.
The bridge reopened June 1, after workers shored up the span temporarily, but the city has yet to replace the cables. A second set of cables was removed after another cracked plate was discovered.
The consulting firm stopped short of pointing fingers at any of the companies involved in building the 4 1/2-year-old span over Hiawatha Avenue.
But officials intend to continue conversations about the findings with URS Corp., which designed the bridge, and other firms involved, city spokesman Matt Lindstrom said in a statement. Those discussions will include who should pay the costs to repair the damage and respond to the incident, he added.
URS said in a statement that the company takes "full responsibility for our work. We remain committed to working constructively and responsibly with the city of Minneapolis and Hennepin County as they move forward with retrofit solutions for the bridge."
The company said it would review the report's findings, including factors that did not involve URS, such as misaligned connection plates and defective welds.
The report noted that the plates had "welded details that offer poor fatigue resistance." Testing of the remaining 16 plates revealed welding defects or cracks in two.
The primary bridge elements were adequately designed, but the effects of wind-induced cable vibrations were not included in the original design calculations package, the report said.
The bridge was monitored after construction to determine whether cable vibrations were "problematic," but no concerns were identified, according to the report.
Maya Rao • 612-673-4210