As the condition of the region’s bridges came into public focus following Friday’s collapse in Frick Park, many wondered how the widespread deterioration escaped mainstream public consciousness for so long.
The next logical question for many, after it was widely reported that 174 other bridges in Allegheny County have the same “poor” rating as the failed Fern Hollow Bridge: What’s stopping those bridges from collapsing, too?
The short answer, civil engineering experts told PublicSource, is that bridges rated “poor” are generally safe to drive on. The rating (which is a four, on a scale of one to nine) means that repair work is needed to prevent the condition from worsening further, but experts said the state inspection system is effective in identifying and closing bridges before they are in danger of failure.
“This was a catastrophe and unfortunately one that occurred in our district,” said Cheryl Moon-Sirianni, the PennDOT district executive for District 11, which includes Allegheny County. “But it fortunately is one that does not occur very often and that’s because of the due diligence of these bridge inspectors.”
Bridges are inspected at least every two years, with some in worse condition inspected more frequently. The federal government sets standards for inspections, and inspectors in Pennsylvania have to take a course and become certified by the federal government. Components are rated on a scale of one to nine, with one being “imminent failure” and nine being “excellent.”
Kent Harries, an engineering professor at the University of Pittsburgh who specializes in bridges, said the abundance of poor ratings shows that our infrastructure is, on the whole, deteriorating but “there’s no imminent danger.”
Jason Zang, the PennDOT assistant district executive for construction, said the ratings mean “that there’s some deterioration on the bridge but it’s still safe to be open.”
Harries said the rigor of bridge inspections can vary from state to state, but that Pennsylvania’s inspections are well conducted. “I would absolutely trust it,” Harries said.
On a national scale, American Society of Civil Engineers President-elect Maria Lehman pointed to statistics: She said drivers in the United States make 178 million trips over poorly rated bridges every day, “And we see bridge failure not very frequently. You’re much more apt to die in a straight-up car accident driving down a state road.”
The cause of the Fern Hollow Bridge collapse won’t be known until the National Transportation Safety Board completes its investigation, which will likely take more than a year. The most recent inspection, in September 2021, deemed the bridge safe to use.
The Fern Hollow Bridge was one of just six in Pennsylvania of its kind, known as a “K-frame” bridge. The type has no redundancy in its support design, WESA reported, and requires more stringent inspection and can be more expensive to maintain. A PennDOT spokesperson said the agency will conduct field evaluations of the five remaining bridges in light of the Fern Hollow collapse, and that each of the five are currently rated fair or better..
Some bridges, including about 30 in Allegheny County according to state data, are “posted for load,” which means there is a posted legal limit for how heavy any single vehicle crossing the bridge can be.
The Fern Hollow Bridge was one of those bridges; it had a posted limit of 26 tons. The limit applies to each vehicle on the bridge, not the cumulative weight of every vehicle on the bridge.
Lehman, the director of U.S. infrastructure at the engineering firm GHD, said load limits are determined by a mathematical formula, with engineers inputting inspection data and outputting how much weight the bridge can safely handle.
Enforcement of the limits, though, is up to local law enforcement. Harries said it is exceedingly difficult for authorities to monitor and enforce the limits, particularly in urban areas where there are no weigh stations like there are on highways.
“Largely, it’s unenforceable,” Harries said. “There’s a lot of posted bridges in Pittsburgh, and you see big trucks going on. And I personally just tap the brakes and stay back, you know.”
PennDOT officials mostly declined to comment on enforcement, saying it’s a law enforcement matter. Moon-Sirianni, asked if the limits are properly enforced, said “I would hope so, but we can’t definitely answer that.”
Pittsburgh’s Department of Public Safety did not provide information on enforcement by publication time.
Harries said the vehicles that were on the Fern Hollow Bridge when it collapsed — including a Port Authority bus — were not heavy enough to exceed the bridge’s posted limit.
“You can’t drive a really big heavy truck in Pittsburgh,” Harries said, though he noted concrete trucks and some larger fire trucks are exceptions.
Bridges are a fact of life in the Pittsburgh area. Last week’s collapse has many people questioning the safety of the infrastructure they use every day.
While information on bridge conditions has been publicly available online, it took a catastrophic collapse in Pittsburgh for many people to get up to speed on the infrastructure crisis at hand.
Should poorly rated bridges be marked with road signs so drivers can be aware before using a bridge — like health inspection ratings posted on restaurant doors?
Harries said the concept would not be beneficial because bridges rated “poor” are not considered hazardous by engineers and ones that are dangerous are closed as soon as the authorities identify that danger.
“I suspect it would not be constructive,” Harries said of such a posting system. “It strikes me that it’s not going to solve anything, and we’re not going to be repairing the bridges any faster.”
Zang said the condition ratings, while publicly posted online, are primarily for PennDOT and local authorities’ internal management and not for the public to use.
“Those condition ratings are more as a tool for the departments of transportation to really have an idea of their overall bridge condition,” Zang said. “They’re not meant to scare the public or cause concern.”
Moon-Sirianni added that inspectors go through extensive training to assign the ratings, and without that training, “I don’t think [the public] would understand what they mean.”
If the public takes anything from reading the ratings, Harries said, it should be that the region and country face a systemic problem.
“They should take it that this just doesn’t affect the folks in Regent Square,” Harries said. “It really does affect the operation of this entire region … Without resources, poor gets worse.”
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