By Jaden Singh
On Feb. 21, the National Transportation Security Board (NTSB) held a virtual meeting to determine why the Fern Hollow Bridge collapsed in 2022 and to recommend ways to prevent similar issues in the future. The 447-foot-long Fern Hollow Bridge carried Forbes Avenue through Frick Park over a 100-foot ravine below. Nine people were injured when the bridge gave out early on a Friday morning. The time of day likely contributed to the small number of injuries, as the route becomes substantially busier late in the day. A two-year federal investigation into the collapse produced over 5,000 pages of findings and analysis, including city memos and emails on bridge maintenance.
The NTSB report said there was a failure in a transverse tie plate on the southwest bridge log. The Fern Hollow Bridge was built with weathering steel, which depends on a moisture cycle to create a protective rust layer. The rust layer shields the steel from environmental damage. While other types of steel typically develop porous layers of rust that later detach, weathering steel keeps a stable layer that adheres to the underlying structure. On the Fern Hollow Bridge, however, clogged drains let water run down the bridge legs and continuously corrode the plate without allowing for a dry cycle and the formation of the rust layer.
The report findings did not cite the bridge design or use of weathering steel as factors in the collapse. Instead, the report attributed the collapse to a series of errors within the inspection and maintenance process.
From 2005 to 2021, the city did not act on several maintenance recommendations from contracted inspectors. Fern Hollow Bridge had been rated in poor condition for over 10 years. Multiple inspectors additionally failed to accurately rate bridge condition, clean corrosion while taking measurements, or to urge structural reviews of the bridge legs. As recently as September 2021, the state reviewed the bridge for safety issues and did not order closure. Vulnerable components of the Fern Hollow Bridge, such as the tie plates, were not identified or marked as “fraction-critical members.” The report noted in particular that had the tie plates been properly detected, inspection protocol would likely have triggered action within seven days.
The 2014 load rating analysis conducted on the Fern Hollow Bridge also had poor data. Pittsburgh city paving records were of insufficient quality at the time of analysis to determine that the Fern Hollow Bridge was laid with almost twice as much asphalt as was accounted for in the load calculations. Furthermore, the analysis did not use the correct k-factor to estimate axial load capacity. An accurate assessment of the bridge’s load capacity, accounting for corrosion, would have required the city to close the bridge.
The NTSB report issued recommendations to various transportation agencies regarding bridge maintenance. To the Federal Highway Administration, the report urged the development of a data-driven approach to inspecting weathering steel bridges across the country and said it should prioritize maintenance when it finds issues. The report recommended that the American Association of State Highway and Transportation Officials update its Manual for Bridge Evaluation to determine localized tension zones and apply non-destructive inspection methods for weathering steel bridges. The report also recommended the Pennsylvania Department of Transportation (DOT) conduct its own report documenting the effectiveness of the changes made by the City of Pittsburgh, which was prompted to improve its paving records and cooperate with Pennsylvania DOT infrastructure evaluations.
Prior to the NTSB investigation, officials had already begun making changes to the bridge assessment process in response to the Fern Hollow collapse. The city contracted WSP Global Inc., a Canadian engineering consultant, directly after the collapse to reassess each of Pittsburgh’s bridges. It identified 66 city bridges that needed major repairs. Pennsylvania has updated procedures that flag bridges showing signs of deterioration, and Pittsburgh has expanded its bridge maintenance division while incorporating federal safety recommendations. The state DOT amended its inspection policies to include a load review every 10 years and more detailed pictures, sketches, and measurements to determine the condition of underlying metal.
One result of the investigation local to Carnegie Mellon is the 2022 closure of Schenley Park’s Charles Anderson Memorial Bridge, which also uses weathering steel. Mayor Ed Gainey said in a December 2022 press release that “inspection results … tell us this bridge needs repair work,” crediting recent safety procedures in expediting the bridge’s closure and “preventing another Fern Hollow.” In collaboration with the Southwestern Pennsylvania Coalition, which reviews infrastructure projects for federal funding, and the Pennsylvania DOT, the city plans to move forward with construction this spring.
The Pennsylvania DOT rates bridges on a zero (failed) to nine (excellent) scale. It currently lists 164 out of the 1581 bridges in Allegheny County at a four, indicating the city needs to work to prevent continuing damage.
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