Collapse of the Fern Hollow Bridge

​​​​This information is preliminary and subject to change.

Investigative Update, January 26, 2023

​This report provides an update on the National Transportation Safety Board (NTSB) investigation into the January 28, 2022, collapse of the Fern Hollow Bridge in Pittsburgh, Pennsylvania.[1] Since the last update, NTSB investigators have carried out 3-dimensional (3D) scans of select portions of the bridge’s superstructure, performed mechanical and metallurgical testing of bridge components that were collected during the postcollapse examination, conducted interviews, examined bridges that have similar construction or material features, and collected and reviewed a wide range of documentation from local, state, and federal sources.

Documentation and Materials Testing

Engineers from the NTSB Materials Laboratory have examined multiple fractures that were observed on the legs of the bridge following the collapse. They scanned portions of each of the bridge’s four legs with a 3D scanner to enable further analysis. Extensive mechanical and metallurgical testing of the weathering steel comprising the bridge (figure 1), conducted at the Federal Highway Administration (FHWA) Turner Fairbank Highway Research Center in McLean, Virginia, is nearing completion (figure 2). The results of this testing will be compared to the material specifications referenced in the bridge’s original design plans.

​Figure 1. Photo showing material from the southwest leg (B1R) of the Fern Hollow Bridge. The photo, taken while investigators were on scene, focuses on a portion of the leg prior to sectioning. 

​​​Figure 2. Two photos showing, on the left, a tensile specimen machined from a girder flange necking (yielding) during testing (outlined by the red oval); the photo on the right is a close-up view of the same specimen, showing the necking in greater detail. 

Interviews

NTSB investigators interviewed personnel from the City of Pittsburgh, the Pennsylvania Department of Transportation (PennDOT), and consultant firms involved in the maintenance, inspection, and oversight of the Fern Hollow Bridge. Information from these interviews will be included in the docket at a later date.

Examination of Similar Bridges

NTSB investigators, with FHWA engineers, conducted limited inspections of 10 steel frame bridges located across the Commonwealth of Pennsylvania (figure 3). These bridges had design elements or aspects similar to the K-frame design and uncoated weathering steel construction of the Fern Hollow Bridge.

​Figure 3. An NTSB investigator and an FHWA engineer examining a bridge similar to the Fern Hollow Bridge. 

​Documents

The NTSB continues to identify and collect documents from the City of Pittsburgh, PennDOT, and various contractors. The documents provide information about the Fern Hollow Bridge design, maintenance, and inspections.

Docket

In conjunction with this update, the NTSB is opening the docket for the investigation. The docket, which can be found here, is a repository for the factual reports, other documents, and exhibits that the agency has determined are pertinent to the investigation. The items in the docket at the time of this update represent only a portion of the final docket; more items will be added as the investigation continues.​​

All aspects of the collapse remain under investigation while the NTSB determines the probable cause, with the intent of issuing safety recommendations to prevent similar events.​​​

​​[1] A preliminary report was issued on February 7, 2022, and an investigative update was issued on May 5, 2022. These are listed below.

Preliminary Report, February 7, 2022

On Friday, January 28, 2022, about 6:40 a.m. eastern standard time, the Fern Hollow bridge, which carried Forbes Avenue over the north side of Frick Park, in Pittsburgh, Allegheny County, Pennsylvania, experienced a structural failure. As a result, the 447-foot-long bridge fell approximately 100 feet into the park below. At the time of the collapse, a 2013 Ne​w Flyer articulated bus, operated by the Port Authority of Allegheny County, and four passenger vehicles were on the bridge. A fifth passenger vehicle drove off the east bridge abutment following the collapse and came to rest on its roof on the exposed ground below. As a result, 10 vehicle occupants sustained injuries.

In the area of the collapse, Forbes Avenue is a four-lane, non-divided roadway consisting of two travel lanes each in the eastbound and westbound directions. Sidewalks flanked the travel lanes on both sides. The posted speed limit on the bridge was 35 mph. The posted weight limit on the bridge was 26 tons. At the time of the collapse, it was snowing, and some snow had accumulated on the roadway and bridge surfaces.

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. [1] 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. Although certain areas of the welded steel girders were identified as being fracture critical, no primary fractures were found in these areas. [2]

The collapsed Fern Hollow bridge, as viewed from the east approach, showing the transit bus and four of the five passenger vehicles.​

Initial assessment of bridge components indicates that the collapse initiated at the west end of the structure. Further examination will be performed as debris from the bridge is removed and unobstructed access becomes available.

The Port Authority bus was traveling eastbound at the time of collapse. The bus was equipped with seven cameras: one forward-facing camera, one right side aft facing camera, and five interior cameras. Video data from these cameras have been recovered, and the initial assessment of the video data is consistent with the initial assessment of the bridge components. The National Transportation Safety Board (NTSB) will convene a group of specialists to analyze the video for further information useful to the investigation.

Assisting the NTSB investigation are the following parties:

• The Federal Highway Administration
• The Pennsylvania Department of Transportation
• The City of Pittsburgh, Pennsylvania
• The Port Authority of Allegheny County, Pennsylvania

The NTSB is evaluating the design of the bridge, its condition at the time of the collapse, its maintenance and rehabilitation history, and its inspection and load rating history. As the investigation progresses, the NTSB plans to conduct forensic examination of several of the bridge’s structural components. The recovery of evidence, including extraction and documentation, is expected to be a lengthy process. All aspects of the collapse remain under investigation while the NTSB determines the probable cause, with the intent of issuing safety recommendations to prevent similar events.

Investigative Up​date, M​ay 5, 2022

​Updated Information

Based on initial information, the preliminary report listed ten vehicle occupants as being injured during the bridge collapse. Through further investigation, the NTSB confirmed a total of nine occupants in six vehicles, including a transit bus operated by the Port Authority of Allegheny County. Two of the vehicle occupants sustained serious injuries, two sustained minor injuries, four were u​ninjured, and the injury status of one is unknown. 

Recovery of Structural Components 

NTSB investigators were on-scene during the demolition process and gained greater access to structural bridge components identified as having evidence critical to the investigation. Some of the components were initially examined underneath the collapsed structure. As these components were uncovered, investigators documented them in place, before removing them from the wreckage. The structural components were then removed from the wreckage and the NTSB examined them further, including using 3D laser scanning technology. Areas of each component were subsequently identified for additional analysis, and the bulk of the retained sections are being housed at the Federal Highway Administration’s (FHWA’s) Turner-Fairbank Highway Research Center in McLean, Virginia for future testing. Other pieces will be analyzed at the NTSB’s Materials Laboratory. 

Tests Conducted by the Turner-Fairbank Highway Research Center 

As a party to the NTSB investigation, the FHWA has provided resources and expertise to evaluate the materials used in constructing the bridge. NTSB and FHWA investigators plan to conduct both mechanical and chemical testing on material samples prepared from portions of each bridge leg, as well as several girder sections. Investigators also plan to examine plate dimensions and weld quality. 

Video Review 

On March 16, 2022, investigative party members convened at NTSB headquarters in Washington, DC, where they reviewed video footage from the time of collapse, captured by the transit bus’s camera system. Footage from two cameras, one forward-facing and one rear-facing along the curbside of the bus, was determined to have investigative value, and the group members collaboratively selected the frames depicting events of interest. Two frames, captured by the cameras and shown as figures 1 and 2, provide some information on the sequence of the collapse. The first frame (figure 1) is from the forward-facing camera and shows the bridge deck separating at the east expansion joint (the red oval highlights the expansion joint). The second frame (figure 2) is from the curbside camera and shows that, at nearly the same time, the west end of the bridge has already fallen off the west abutment (the red arrow points to the abutment).

​  Figure 1. Video frame from the forward-facing camera, showing the opening expansion joint. 

  Figure 2. Video frame from the rear-facing curbside camera, showing the west end of the bridge already fallen off the west abutment. ​

A factual report of the video examination will be completed by the group members and included in the docket for this investigation.

Thus far, the investigation has not found any evidence of widespread deficiencies with rigid K-frame superstructure types. All aspects of the collapse remain under investigation while the NTSB determines the probable cause, with the intent of issuing safety recommendations to prevent similar events.   

[1] Uncoated weathering steel refers to a group of steel alloys that are designed to, over time and with exposure to weather, form a protective patina that negates the need for painting.

[2] Per 23 Code of Federal Regulations 650.305, a fracture critical member is a “steel member in tension, or with a tension element, whose failure would probably cause a portion of or the entire bridge to collapse.”

Watch the media briefing on the NTSB YouTube Channel:

B-roll footage of ​​NTSB investigators examine collapsed Forbes Avenue bridge in Pittsburgh, PA on the NTSB YouTube Channel:

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See additional photos on the NTSB Flickr album​.​