National Transportation Safety Board: Clean bridge drains to prevent deterioration found at Fern Hollow

As part of its investigation into the collapse of Pittsburgh’s Fern Hollow Bridge, the National Transportation Safety Board recommended Thursday that agencies across the country check bridges with uncoated steel to make sure they have proper drainage and don’t deteriorate like Fern Hollow did before it collapsed last year.

In an interim report, the NTSB said uncoated steel needs periods of dryness to form a protective layer to prevent deterioration. At Fern Hollow, investigators found that clogged drains and other debris didn’t allow that to happen and encouraged deterioration.

The bridge that connects Squirrel Hill and Point Breeze collapsed on Jan. 28, 2022, into Frick Park below it, injuring 10 people who were in six vehicles.

The agency stressed it hasn’t determined the cause of the Fern Hollow collapse but issued the interim report to warn other bridge operators about the importance of cleaning drainage systems. Uncoated steel will form its own protective oxide coating, known as patina, that resists corrosion — as long as it has dry periods created by good drainage.

Photos and recommendations from previous inspection reports on Fern Hollow, which had been rated in poor condition for more than 10 years, show drains frequently clogged with debris. Inspectors recommended those problems be corrected, but they weren’t — keeping the steel wet and preventing it from forming the patina.

The agency said investigators found “corrosion, deterioration and section loss” on all four of the bridge’s support legs. Some of the deterioration was so severe that it created holes in the structure.

A number of other Pennsylvania bridges have had inspection reports identifying the same problem, the agency said. As a result, the NTSB is asking the Federal Highway Administration to develop a process for bridge owners across the country to review their spans and inspection reports to make sure they aren’t developing the same problems. 

FHWA should also have recommendations for what to do if bridge owners identify those problems. The NTSB estimates more than 10,000 bridges have been built using uncoated steel.

“While the Fern Hollow Bridge investigation is not yet complete, the NTSB is making this early recommendation due to the immediate implications for bridge safety nationwide,” the agency said in a news release. “The final report with a probable cause, other findings and recommendations will be issued in the coming months.”

Kent Harries, a professor of civil and environmental engineering at the University of Pittsburgh, said he doesn’t anticipate that the NTSB has uncovered a major problem across the country. Agencies that use weathering steel, which can be less expensive to maintain because it doesn’t need to be painted, generally are aware that it needs to stay relatively dry to form a protective coating.

“Bridge professionals know this,” he said. “This is a reminder, a nudge to say, ‘Hey, guys, weathered steel is potentially good as long as it develops that coating.’ [The need for good drainage] is a true statement for any bridge.”

In its 12-page report, the NTSB found inspections at Fern Hollow highlighted deterioration from drainage problems in almost every year from 2011 until September 2021, five months before the bridge collapsed.

“The steel section loss was severe to the point that there were holes in numerous structural elements on all four legs, including the longitudinal and transverse stiffeners …,” the report said. “The NTSB concludes that the legs of the Fern Hollow Bridge experienced significant deterioration and section loss that were documented in inspection reports.”

“The deterioration and section loss resulted from the continual accumulation of water and debris, which prevented the development of the protective patina that would resist such corrosion on uncoated weathering steel.”

The report said the NTSB examined 10 other bridges of similar construction in Pennsylvania, and concluded the lack of drainage maintenance that led to the steel deterioration was “not unique” to Fern Hollow. It also noted the Pennsylvania Department of Transportation issued a technical bulletin in November to its district maintenance offices and other jurisdictions about the potential corrosion problem, and admitted it had not included such information in previous guidance about the use of weathering steel.

Because other Pennsylvania bridges had similar but less serious concerns, the NTSB believes it’s “critical” that bridge operators across the country examine bridges that used weathering steel to make sure the drainage systems are maintained properly.

“If used under appropriate conditions and properly maintained, weathering steel bridges can last decades,” the report said. “However, as with any other steel, failure to properly maintain uncoated weathering steel can lead to corrosion damage, deterioration and section loss in critical components, thus reducing the safety and service life of the bridge.”

To replace Fern Hollow, PennDOT received a $25.3 million federal grant. Some work is continuing, but the new bridge reopened to limited traffic just before the end of last year.