Fern Hollow Bridge collapsed because maintenance was ignored

Up until 6:37 a.m. on Jan. 28, 2022, when the Fern Hollow Bridge collapsed, the only priority repairs ever done on the bridge were the placement of a weight limit sign and the removal of a light pole that was badly corroded.

Four vehicles were on the bridge when it collapsed, including a Port Authority, now PRT, bus. A fifth car drove off the bridge abutment in the dark and landed on its roof. Ten people were injured.

Using video from the bus that was crossing the bridge as it fell, investigators from the National Transportation Safety Board (NTSB) reported on Wednesday, Feb. 21, that the collapse started with the failure of the southwestern leg of the bridge, which was the section closest to the Frick Park Gatehouse.

In a presentation, NTSB investigators showed photos from the inspection reports over 17 years. During that time, inspectors had documented severely corroded areas that became large holes in the legs of the bridge. The inspection performed just four months before the collapse found those holes in the steel had grown as large as 1 foot wide and 1 foot high, but still no emergency action was taken because the leg of the bridge was not considered a “fracture critical member.”

After adopting the findings of a two-year investigation into the collapse of the bridge, Jennifer Homendy, the chair of the NTSB, said engineers had looked at the bridge in 2013 and 2015 to determine what pieces of the bridge were fracture critical, in other words, what parts of the bridge were vital to its remaining standing.

The legs, Homendy said, “were not identified as fracture critical, which is astounding to us and our team.”

“Throughout the course of 15 years [the inspectors] just relied on those documents to do their inspections and did not identify that as a fracture critical member at all,” she said.

The NTSB blamed the collapse squarely on the lack of maintenance for the bridge. Inspection after inspection contained photos of drains that were clogged by leaves and debris that were not cleared out.

Inspectors also made the wrong calculations of the amount of weight the structure could bear. It was posted in 2014 with a 26-ton weight limit, but the inspectors did not realize the asphalt was 6 inches thick, instead of the 3 inches the structure had been designed to hold. The bridge engineers also incorrectly calculated the amount of torque the bridge could take as traffic traveled across it and how much the loss of steel, which in some areas had completely corroded through, would affect how much weight the bridge could hold.

“Had the correct calculations and assumptions been used, the bridge would have been closed,” Dan Walsh, an NTSB engineer, told the board during a hearing on the final recommendations.

“I think it’s just interesting that maybe they lost their priorities or didn’t understand their priorities when it came to their bridge inspections,” NTSB board member Michael Graham said. “Because this bridge didn’t collapse just by an act of God, it collapsed because of a lack of maintenance and repair, and it’s just sad for the city.”

Graham pointed out that before the collapse, the city’s budget for its 147 bridges was $300,000 a year, just a little over $2,000 for each bridge annually, which he noted was not nearly enough.

In the hearing, NTSB staff stressed that proper maintenance of the Fern Hollow Bridge was vital because the kind of steel used was designed to develop a patina over time that would protect it, much as a coat of paint would. However, that patina needed proper conditions of getting wet and drying to develop and could not be covered with debris.

The board found that the problem was not with the type of steel used or the design of the bridge.

Instead, the board adopted the statement: 

“We determined that the probable cause of the collapse of the Fern Hollow Bridge in Pittsburgh, Pennsylvania, was the failure of the transverse tie plate on the southwest leg of the bridge, a fracture-critical member (nonredundant steel tension member), due to corrosion and section loss resulting from the City of Pittsburgh’s failure to act on repeated maintenance and repair recommendations from inspection reports.

“Contributing to the collapse were the poor quality of inspections, the incomplete identification of the bridge’s fracture-critical members (nonredundant steel tension members), and the incorrect load rating calculations for the bridge. Also contributing to the collapse was insufficient oversight by PennDOT of the City’s bridge inspection program.”

Lessons from the collapse of the Fern Hollow Bridge will become part of the Federal Highway administration’s curriculum to avoid a similar tragedy in the future.

At the end of the NTSB hearing, Homendy said she wanted to end with a quote from a work of fiction, something she had never done before. She then recited a line from Kurt Vonnegut’s novel “Hocus Pocus”: 

“Another flaw in the human character is that everybody wants to build and nobody wants to do maintenance.”