City of Bridges: The following report is the first is a three-part series examining the status of Pittsburgh bridges as the Pennsylvania Legislature considers funding for transportation infrastructure.
The Pittsburgh area is home to hundreds of bridges — by some counts, more than any city in the world.
It also has a higher percentage of structurally deficient bridges than any other U.S. city. If your goal is to highlight the problem, one in particular makes a pretty good backdrop.
"All you have to say is 'Liberty Bridge' and everybody knows what you're talking about," Gov. Tom Corbett said last summer while standing underneath it at a news conference. "Unfortunately, it is an example of what we're talking about when we're talking about structurally deficient bridges."
Flanked by state and local transportation officials, Corbett posed for photographs in the shadow of the rusted, 2,600-foot span as he urged House lawmakers to approve a $2.5 billion funding package for roads, bridges, and mass transit.
That didn’t happen in the spring session. But with transportation infrastructure again on the agenda in Harrisburg this fall, the iconic bridge remains something of a political football.
It’s not hard to understand why. The bridge, which towers above the Monongahela River, carries an estimated 54,000 vehicles a day between downtown and the South Hills. And, of some 4,500 Pennsylvania bridges rated structurally deficient, it’s among the most visible.
"This is the most glamorous example of a bridge that needs to be inspected regularly, needs upkeep and repairs," said Army Corps civil engineer Katie Bates.
Its condition shouldn't come as a surprise.
"Steel won’t deteriorate in a heartbeat," Bates said. "Steel won't be good one day and breaking the next day. There will be signs of stress and rust."
Those signs are plainly visible on the 85-year-old steel span, which is one reason why the state Department of Transportation lowered its weight limit in August. The Liberty is one of a thousand bridges statewide that are now subject to new, or tighter, limits on use by heavy vehicles.
It could have been worse – for now, all but the heaviest trucks and construction vehicles are still allowed on the Liberty Bridge. But there’s another reason why its design raises special concerns: It’s a truss bridge, supported entirely by structural elements underneath the deck.
Patrick Miner is a bridge designer with a Pittsburgh engineering firm that does some work for PennDOT, but has no involvement with contracts on the Liberty Bridge.
"Engineers like redundancy, multiple ways for the load to get from the deck down to the foundation," Miner said. "Truss bridges are inherently non-redundant, meaning if any one element fails, the entire structure will likely collapse."
That’s what happened in Minneapolis in August of 2007, when a section of Interstate 35 collapsed into the Mississippi River, killing 13 people and injuring many others.
The I-35 bridge was also a steel-truss design. An investigation by the National Transportation Safety Board determined the cause was a single failed gusset plate – one of hundreds of fasteners holding the structure together, any one of which could have triggered the collapse.
Could a similar disaster happen in Pittsburgh? For the moment, it's not likely. But the experts say it wouldn't take much.
"Every span would probably fail independently, so if one truss member or gusset plate were to fail it would (happen) pretty quickly," Bates said. "You’d see a large chunk of the bridge fail but probably not the entire bridge. It would be a big collapse, but I wouldn’t expect the whole bridge to go under, just a very large section."
Federal investigators blamed the Minneapolis collapse, in part, on inadequate review by transportation officials.
But unlike the I-35 bridge, the Liberty Bridge has undergone careful and continuous inspection. The weight restrictions are not about preventing a collapse so much as trying to extend its lifespan in the absence of a steady funding stream.
"Whether a bridge is safe or not is a very objective, yes or no, life or death question," Miner said. "Public officials don’t have anything to hide behind when it comes to whether these bridges are safe."
PennDOT estimates a clean bill of health for the Liberty Bridge will take between $40-60 million in repairs. But until the Legislature comes through with funding, all they can do is watch.