Photo Courtesy of Minneapolis Public Works
Bridge design did not account for wind-induced cable vibrations, report shows.

Wind-induced cable vibrations caused steel diaphragm plates to fracture and fail on a 2,200-ft cable-stayed suspension bridge in Minneapolis, according to a preliminary report prepared by Northbrook, Ill.-based engineer Wiss, Janney, Elstner Associates, the firm the city hired to investigate the cause of the failure.

According to the report, "stay cable vibrations (wind induced) were not included in original design calculations package," prepared by San Francisco-based URS Corp., designer of the five-year-old Martin Olav Sabo pedestrian and bicycle bridge.

"The documentation provided by URS didn't include those calculations," says Wiss, Janney, Elstner's project engineer, Mark Chauvin.

"We know what caused the fractures but are stopping short of indicating what the designer did or did not do," says the Minneapolis deputy director of public works, Heidi Hamilton. "We're still engaged in conversations with the firm."

"We look forward to receiving the completed report and are continuing to work with the city and county on bridge retrofit solutions," URS stated.

The report indicates that "low wind speeds, approximately five to 10 mph … induced damaging stress range cycles at fatigue-sensitive details in diaphragm plates" anchoring the bridge's 18 pairs of cables to a 100-ft-high pylon.

On. Feb. 19, a diaphragm plate and a pair of the bridge's longest support cables were discovered on the bridge's deck, prompting closure of the structure. Days later, the city directed crews to remove another pair of cables. Following the erection of temporary shoring, the bridge re-opened on June 1.

The city of Minneapolis and Hennepin County retained Wiss, Janney, Elstner to retrofit 16 remaining in-service diaphragm plates at a cost of $1 million, says Hamilton. All retrofit options "are designed to provide increased strength and fatigue resistance for the cable connections," according to the summary report.

Work is expected to be completed later this year, Hamilton says.

Because the bridge continues to be supported by cables, "we're inspecting the diaphragm plates several times a week to ensure they are securely in place and functioning as intended," Hamilton says.

In the meantime, the Metropolitan Council, a regional planning agency serving the Twin Cities, has delayed the award of a $94-million engineering contract for a proposed Southwest Corridor light-rail line, a project on which URS and one other firm bid.

In 2003, the Minnesota Dept. of Transportation retained URS to perform a fatigue evaluation and redundancy analysis of the I-35W bridge in Minneapolis. The bridge collapsed in 2007, killing 13 people and injuring 145.