Sabo designer, URS, also involved in I-35 bridge

Whatever caused one of the 18 cables supporting the Martin Olav Sabo Bridge to break on Sunday night eluded bridge examiners just six months before when they conducted a major on-site review.

The city of Minneapolis owns the $3.5 million bridge; Hennepin County oversaw its construction in 2007 to serve bicyclists and pedestrians using the Midtown Greenway in south Minneapolis.

City engineers conduct annual reviews of all bridges in Minneapolis, and every five years subject structures to a more rigorous exam, said Mike Kennedy, director of transportation maintenance and repair for the city of Minneapolis.

That test was completed on the Sabo bridge in September, but the engineers discovered no signs of weakness. “There were no red flags, nothing to raise any alarms at all,” Kennedy said.

The cable that broke was the longest on the bridge, stretching from its eastern edge to the top of the pylon that stands in the center of the span, and the break occurred at the connection to the pylon. Kennedy said the adjacent cable also was compromised but wasn’t broken.

City engineers and consultants from the local office of San Francisco-based URS, the engineering company that designed the bridge, have been at the site since the single cable broke on Sunday, Kennedy said. “Our engineers are trying to identify what happened and what the next step should be. But it’s too early to speculate on what went wrong.”

URS was the engineering firm that had evaluated the safety of the Interstate 35W bridge that collapsed in 2007. In 2010, the company paid $52.4 million in a settlement to victims of that disaster, which killed 13 people and injured 145.

A spokesman for URS, which has an office in Minneapolis, did not return a phone call before publication. A spokesman for Burnsville-based Ames Construction, which built the bridge, referred questions about the bridge design and cable break to URS.

No injuries were reported in the Sabo incident, which affected traffic on Hiawatha Avenue and the Hiawatha light rail line on Monday. An advisory on Metro Transit’s website said replacement bus service would continue on the Hiawatha LRT line between 38th Street and Franklin Avenue into the morning rush hour Tuesday.

The Sabo bridge uses a cable-stayed design, which enables it to cross long spans without the regularly spaced columns or piers that support conventional bridges. Instead, the deck is suspended by cables that are stretched laterally from the top of a single central pylon to points along the deck.

The concept is hundreds of years old, but it has been used more frequently in bridges since the 1980s, said Sharon Wood, the engineering department chairwoman at the University of Texas in Austin and a bridge expert.

Using cable-stayed design on the Sabo bridge solved the logistical problem caused by needing to cross six lanes of vehicle traffic and the Hiawatha LRT line, which would have made the use of columns almost impossible.

“For a span the length of the Minneapolis bridge (about 220 feet) it’s an efficient structural system,” Wood said.

But the design has some flaws. Cables have broken on bridges in Louisiana, Florida, Texas and Virginia, sometimes from corrosion, sometimes from vibration caused by rain and wind that can weaken the cable, Wood said.

She would not speculate on what could have caused the Sabo bridge cable break.

But she also pointed out that all the previous cable failures were remedied without a major bridge failure or injury, indicating that the redundant safety features worked. “The Minneapolis bridge didn’t collapse either, and from that point it did what it was supposed to,” Wood said.

The new Lowry Avenue bridge contains elements of the cable-stayed design, too, but Kennedy said the cables are supported on that bridge by a pair of arches and not a single pylon. “It would be unfair to compare them.”

“While it’s still a cable-stayed bridge there’s not much similarity between the two,” and he said the failure on the Sabo bridge cable wasn’t cause for increased examination of the Lowry bridge.

The cable-stayed design has also gained popularity in recent years because it enables light, sweeping designs that “create a striking aesthetic statement,” said Cathy French, a professor in civil engineering and a bridge authority at the University of Minnesota.