The 136m-long bridge experienced a structural failure and fell approximately 30m into Frick Park on 28 January, 2022. Six vehicles were on or near the bridge when it collapsed and ten people sustained injuries.
The bridge was an uncoated weathering steel, three-span, continuous rigid K-frame structure with two welded steel girders, welded steel floor beams, and rolled steel stringers. The ends of the structure rested on reinforced concrete caps on stone masonry abutments. Each girder was additionally supported by two inclined, welded steel frame legs, also made of uncoated weathering steel, which rested atop reinforced concrete thrust blocks.
Video footage from a bus that was crossing at the time of the collapse appears to show the failure originating on the west abutment. This was followed by the bridge deck separating at the east expansion joint.
According to the update, engineers from the NTSB materials laboratory are in the process of analysing portions of the bridge’s legs with 3-D technology and metallurgical testing, focussing on multiple fractures that were observed on the legs of the bridge following the collapse. The results will be compared to the material specifications referenced in the bridge’s original design plans.
NTSB and FHWA engineers have also conducted inspections of 10 steel frame bridges in Pennsylvania that have design elements or aspects similar to the K-frame design and uncoated weathering steel construction of the Fern Hollow Bridge.
No evidence has been found of widespread deficiencies with rigid K-frame superstructure types.
Analysis, findings and recommendations will be issued by the NTSB in a final report at a later date. The NTSB update can be found here.