Preliminary information from the National Transportation Safety Board says that the Fern Hollow Bridge “experienced a structural failure,” on the morning of Friday, Jan. 28.
The federal agency released the information on Monday and noted that the information is subject to change and could include errors, as it is part of an ongoing investigation into why the bridge carrying Forbes Avenue over Frick park collapsed.
The report describes the structure of the bridge — a “K-frame” bridge with no back-up support which relied solely on its design for stability — as well as its materials: the Fern Hollow Bridge was made of “uncoated weathering steel,” which develops a kind of protective skin when exposed to weather, which means it doesn’t need to be painted.
Parts of the steel girders, long beams that ran underneath the bridge, were “identified as being fracture critical,” the report said, which means failure there would probably cause part or all of the bridge to collapse. However, the NTSB found “no primary fractures” in those areas.
Officials believe the collapse began at the western end of the bridge.
The NTSB stressed that all parts of the collapse remain under investigation and that the agency will be evaluating the bridge’s design and condition when it collapsed as well as the bridge’s inspection, maintenance, and load rating history. Since 2014 the bridge had carried a 26-ton weight limit.
The goal is to “determine the probable cause, with the intent of issuing safety recommendations to prevent similar events.” However, that work is expected to be “a lengthy process.”
