Two percent. Not a big number if you're talking about, say, the rain chances on your average Tuesday. A really big number, however, if you're talking about the number of cables that could fail on a suspension bridge.
If they aren't repaired or replaced, about 2 percent of the cables on the Margaret McDermott Bridge could fail over the next two to five years, Sarah Standifer, Dallas' director of Trinity Watershed Management said Monday. There are 196 cables attached to the two bike and pedestrian bridges along Interstate 30 at the Trinity River. Two of the cables have failed so far, but if city staff's estimates are correct, three or four more can be expected fail over the next couple of years.
While a few failed cables won't lead to a collapse, scary stuff can happen when they snap.
According to information provided to the City Council's transportation committee by the engineers who built the bridge, knowing in advance whether a cable will break is impossible.
“Fracture of an anchor rod would be classified as a 'brittle failure' which means there is no appreciable plastic deformation prior to fracture. These types of failures typically occur without any prior evidence of a potential break, thus they are characterized by rapid crack propagation without significant plastic deformation. In plain language, this means there are no visual or other indications of pending cable anchor failure, so this could happen without warning."
After a cable snaps, it could swing into the pedestrian and bike path, leading to all sorts of ugly results. That's why, Standifer said, Dallas needs to decide whether it wants to retrofit the bridge's existing cables to make them less vulnerable to failure or replace them before it can open the pedestrian portions of the bridge.
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Whatever the city ends up deciding to do, the work will take 12-18 months to allow the bike and pedestrian lanes to open, Standifer said.
The project, city staff estimates, could cost anywhere between $2 million and $10 million. Then there's the issue of who's going to pay for the repairs.
"That issue has been under discussion, and we've brought in legal counsel along the way," Standifer said. "At this point in time, our recommendation is that we get the bridge open with some funding that is here at the city, and/or private [funds] and then go through the legal process later."
Any legal action the city takes, Standifer said, would likely be against Calatrava LLC, the design firm behind both the McDermott Bridge and its sister structure, the Margaret Hunt Hill Bridge.