No one knows exactly why the Fern Hollow Bridge collapsed, but it’s natural to want to
In the days after the loss of the Fern Hollow Bridge, questions and speculation about what could have caused its collapse have abounded, and for understandable reasons: There are more than 400 bridges in the City of Pittsburgh alone.
It will be a while before a clear picture emerges of what happened. Officials from the National Transportation Safety Board said Saturday they would issue a preliminary report with some “factual information” in 10 days, but will not address the cause of the bridge’s collapse. That will be the subject of a final report that could take 12 to 18 months to produce.
But that report may not find a single culprit for the collapse. University of Pittsburgh professor of structural engineering Kent Harries said that his field has a theory that “one thing will never cause failure,” he said. Instead, he said, usually a collapse like Pittsburgh saw last week “is a synthesis of multiple issues.”
Some of those issues may prove to be:
A rare design
The Fern Hollow Bridge was a “K-frame” bridge, one of only six such bridges in the state.
“This bridge’s design relies on the quality of frame elements alone without any back-up support,” PennDOT District 11 spokesperson Steve Cowan wrote in an email. (The district encompasses Allegheny, Beaver, and Lawrence counties.)
That’s what engineers call a “non-redundant” bridge, said Harries. Such designs feature limited paths to disperse the stress placed on the bridge. If those paths give way, so does the bridge.
When the Fern Hollow Bridge was built in 1970, rigid-frame bridges — a category that includes K-frame bridges, Harries said — were considered a “fairly efficient use of steel,” even though they tend to be more complicated and more expensive to build.
But such designs are less popular with government agencies and engineers today, he said. “There’s a mentality today of ‘Why would you do that if you don’t have to?’” That’s because the potential cost of any problem with the bridge is higher.
PennDOT takes a never-say-never approach on such designs, said Cheryl Moon-Sirianni, who leads PennDOT’s District 11. For instance, there could be a smaller pedestrian bridge for which a non-redundant design may make the most sense.
But in general, she said, “The goal is to design redundant bridges.”
Non-redundant bridges have collapsed in the U.S. before the Fern Hollow failure, so you could be forgiven for thinking that such structures are more prone to disaster. Harries said that’s not the case, but adds the design does present challenges.
In the last 30-plus years, the nation’s engineering codes began to penalize designers for non-redundant bridges, “to push them to a better solution,” Harries said. Consequently, non-redundant bridges face restrictions on traffic flow, more stringent inspection guidelines, and more extensive inspection, Harries said and PennDOT confirmed.
“You have to take more care, arguably, of a non-redundant structure,” Harries said.
PennDOT will conduct visual inspections of the state’s five other K-frame bridges, Moon-Sirianni said, as well as review inspection reports.
“At first blush, it appears most of them are in pretty fair condition,” she said.
What kind of care was Fern Hollow getting?
The condition of Fern Hollow itself prior to its collapse will be closely examined in the coming months.
Federal law requires bridges to be inspected every two years, but bridges with more deterioration are inspected more frequently: Fern Hollow was inspected every year. PennDOT told WESA on Monday that the agency would not release Fern Hollow’s most recent report, citing state and federal laws, and the fact that it is now part of an ongoing investigation.
Any piece of infrastructure begins to deteriorate the day it’s finished, said Moon-Sirianni. The point of inspections is to make sure that as the state’s assets age and deteriorate that they get the maintenance they need.
However, “when funding is difficult to find, the maintenance doesn’t get addressed as quickly as we’d like to.”
A lack of maintenance on the city-owned and city-maintained Fern Hollow Bridge may have played a role in its collapse, reporting from the Pittsburgh Post-Gazette suggests. In 2018, a rusted brace on the bridge was removed and replaced with steel cables; other necessary repairs were delayed because officials decided they did not present an immediate danger. “Exactly what other repairs were identified is not known,” the Post-Gazette reported.
Those actions are separate from the inspection itself, which is a complicated, technical process: The 2021 edition of PennDOT’s Bridge Safety Inspection Manual runs to 558 pages. And bridge inspectors must earn a national certification.
An inspector carefully examines all elements of a bridge, and rates each on a 0-9 scale, with 9 being the best. A bridge’s overall rating is determined by the bit of it that’s taken the most wear and tear. For example, if the bridge’s surface is rated a 4 — “poor” — the entire bridge will be labeled that way.
There are 175 state- or locally-owned bridges in Allegheny County in “poor” condition, but “that doesn’t really tell you the story of the bridge,” said Jason Zang, assistant executive for PennDOT District 11, who previously worked as the district’s structure control engineer. Instead, it’s an overall description that mainly helps PennDOT to prioritize “what bridge needs preservation, what needs rehabbed, what needs replaced.”
That weight restriction.
In 2014 the Fern Hollow Bridge was posted with a weight limit of 26 tons. It’s 1 of only 9 bridges in PennDOT District 11 to carry any kind of weight restriction, said Shane Szalankiewicz, a district bridge engineer.
The process of determining whether a weight restriction is needed, and then what it should be, is another complicated process. A series of analyses takes into account the number of lanes on a bridge, the bridge’s own weight, and what’s traveling over it. Using models, engineers roll out various combinations of vehicles to set a limit they believe to be safe and meets code requirements.
But the posted restriction means what it says: If a bridge is posted for 30 tons, a 30-ton vehicle can safely cross, regardless of what else is traveling over the bridge at the same time, but a 31-ton vehicle may not.
“We’re not making this up as we go, this is all standardized across the country,” Zang said.
The analyses used to set a weight limit focus on two-axle, three-axle, and combination vehicles, which are essentially tractor-trailers. Cars typically aren’t even discussed “because cars are like feathers on the back of an elephant,” Zang said. “It’s all about the trucks, it’s all about the buses.”
Fern Hollow was a critical link in two of the Port Authority’s most-used routes, the 61A and 61B, which were often served by 60-foot articulated buses. Port Authority spokesperson Adam Brandolph said that, when empty, those buses weigh 22 tons. Forty passengers weighing 200 pounds each would add another four tons to that weight, bringing the tonnage to 26.
When asked how the agency deals with posted weight limits on its routes, Brandolph said the Port Authority could not comment, citing the pending investigation.
The posted weight limit on any bridge applies to individual vehicles and their weight at the time they cross, “because ultimately, that is the weight the bridge must carry,” Szalankiewicz said. It is the responsibility of the vehicle owner to know that weight, he said.
“Public transit agencies are responsible to know the fully loaded weight of each vehicle in their fleet,” Szalankiewicz. “When posting [a weight limit on] a bridge, part of the posting process is to notify emergency services and public transit agencies.”
Officials may not know the full story of what happened in Fern Hollow until sometime in 2022 or even 2023. But they are already trying to ensure it doesn't happen again
Mayor Ed Gainey and City Councilor Corey O’Connor, whose district includes the area around Fern Hollow Bridge, introduced legislation to create an infrastructure commission. That body will recommend how best to maintain and improve city-owned assets.