Inspectors discovered cracks on yet another plate anchoring a set of cables to the mast of the Martin Olav Sabo Bridge -- one day after they removed cables from a different fractured plate and nearly a week after the rupture of the highest anchor closed the bridge.
The fractures, found Saturday, pose no immediate threat. But crews are monitoring conditions intensely at the pedestrian and bicyclist bridge over Hiawatha Avenue as engineers work on a repair, Heidi Hamilton, deputy director for Minneapolis Public Works, told a Minneapolis City Council panel Tuesday.
Her briefing to the Transportation and Public Works Committee offered the first detailed public glimpse of the city's response to the trouble that began Feb. 19, when the bridge's longest, tallest set of cables, along with the anchoring plate, fell more than 100 feet.
Concerns about safety prompted officials to close off part of Hiawatha Avenue, cut back service at three stations on the Hiawatha light-rail line and send dozens of inspectors, engineers and consultants to shore up and evaluate the four-year-old span.
Trains and road traffic are running under the bridge again, but the city won't know what caused the cable anchors to fail for another four to eight weeks, Hamilton said.
Hennepin County, which received a similar briefing Tuesday, oversaw construction of the $5.1 million bridge. The span opened in November 2007 and ownership was transferred to Minneapolis the following year.
Both governments have asked the Illinois-based engineering firm Wiss, Janney, Elstner Associates to investigate how the cable supports, known as diaphragm plates, became compromised. Hamilton said the city and county plan to split the cost of the no-bid agreement, which was awarded last week at a cost not to exceed $100,000.
Wiss, Janney, Elstner Associates also received a $2 million, no-bid contract to investigate the cause of the Interstate 35W bridge collapse in 2007.
Sequence of events
Hamilton offered this account of what happened:
At 10:06 p.m. Feb. 19, a citizen reported finding a pair of cables lying on the bridge. City workers immediately responded, and found a partly rusted diaphragm plate that had fallen from the mast, known as a pylon. By 2 a.m., a bridge inspector had arrived and used a lift to examine the pylon. The top plate that had anchored a set of cables had broken away, while the one beneath it had significant cracks.
Workers detoured vehicle traffic away from the site, then began shoring up the bridge's eastern end.
A slew of consultants began showing up to assist, including URS, the San Francisco-based designer for the bridge.
Wiss, Janney, Elstner employees were on the scene by Wednesday, and shipped the fallen plate to Lehigh University in Pennsylvania for forensic analysis.
Hamilton said there was a "sigh of relief" after workers removed tension from the cables tied to the second cracked anchor on Thursday, then detached them entirely the following day.
Wiss, Janney, Elstner began magnetic particle testing to examine the rest of the bridge's 18 steel anchors for cracks.
They found two on the third plate Saturday. Engineers also fixed a minor defect on a fourth plate.
Council members mostly praised the swift response, with Sandra Colvin Roy noting how much the city had learned from the 35W bridge collapse.
Still unknown, according to Hamilton, is how much the response and repairs will cost the city.
Maya Rao • 612-673-4210