Investigation, Replacement Ongoing for PA Bridge
Earlier this month, the National Transportation Safety Board released an update on its investigation into the collapse of the Fern Hollow Bridge in Pittsburgh, as the first concrete pours for its replacement are scheduled to begin this week.
The NTSB provided an update on the recovery of structural components and tests currently being conducted to evaluated materials used when constructing the bridge, as well as a review of new footage recovered from a transit bus camera system.
According to the update, in March, investigators reviewed video footage from the time of the collapse from two of the cameras on a Port Authority transit bus. The cameras, one forward-facing and one rear-facing the curbside, were determined to have “investigative value” with frames depicting events of interest.
The first frame, from the forward-facing camera, reportedly shows the bridge deck separating at the east expansion joint. The second frame, from the curbside camera, reportedly shows that, at nearly the same time, the west end of the bridge had already fallen off the west abutment.
The NTSB states that 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,” wrote the NTSB. “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.”
While NTSB investigators were on-scene during the demolition process, some structural bridge components were initially examined underneath the collapsed structure and, as they were uncovered, they were documented in place before removal. These components were then furthered examined using 3D laser scanning technology.
The NTSB reports that the bulk of the retained sections are being housed at the FHWA’s Turner-Fairbank Highway Research Center in McLean, Virginia for future testing, and other pieces will be analyzed at the NTSB’s Materials Laboratory. Both the NTSB and FHWA plan to conduct both mechanical and chemical testing on material samples prepared from portions of each bridge leg, as well as several girder sections. Plate dimensions and weld quality will also be examined.
The first frame, from the forward-facing camera, reportedly shows the bridge deck separating at the east expansion joint.
Additionally, while the preliminary report listed ten vehicle occupants as being injured during the bridge collapse, further investigation has confirmed a total of nine occupants in six vehicles, including the Port Authority transit bus. Of the injuries, two vehicle occupants sustained serious injuries, two injuries were minor, four were uninjured and the injury status of one is unknown.
The investigation update came the same day that Pittsburgh and community leaders hosted a virtual townhall meeting to discuss public input and the progress of the project. However, Trib Live reports that Pittsburgh Deputy Mayor Jake Pawlak cautioned residents that the urgent nature of the rebuild and the need for a swift resolution mean that public input and feedback was limited.
“We have attempted to move as expeditiously as possible in rebuilding the bridge and utility connections across the Fern Hollow valley so that our first and foremost responsibility to the public — which is the delivery of safe and healthy living conditions — is restored,” Pawlak said during the virtual meeting.
“That does come with a necessary tradeoff, which is that the normal procedures, both publicly consultative and legal, that we would normally deploy in a bridge reconstruction or restoration project had to be expedited.”
The second frame, from the curbside camera, reportedly shows that, at nearly the same time, the west end of the bridge has already fallen off the west abutment.
Eric Setzler, Chief Engineer for the Department of Mobility and Infrastructure, said that work on the bridge is progressing, and crews from PennDOT have drilled and placed caissons to anchor the bridge under the ground surface.
“Our first two months were really focusing hard on getting the structure designed,” Setzler said. “We need that designed so we can get the materials procured and start construction.”
Pittsburgh Mayor Ed Gainey also announced the creation of a Bridge Asset Management Program to address the city’s infrastructure needs. The purpose of the plan is to ensure that the city is maintaining its bridge inventory in a structurally safe and serviceable condition, including recommended maintenance activities, preservation/repair activities and rehabilitation activities.
A Request for Proposal was issued by the city regarding the program. The question submission deadline is May 20, and the proposal submission deadline is June 3.
The Pittsburgh Post-Gazette reports that construction will begin this week for the replacement bridge with the pouring of concrete for the first caissons to support two sets of columns that look like double capital Ts. The previous bridge had steel supports at each end.
The $25.3 million bridge will be constructed with precast concrete I-beams due to a shortage of steel. The Pennsylvania Department of Transportation and contractor HDR said that they hope most of the construction can be completed this year, with a final completion date being determined by how quickly those beams are available.
On Friday, Jan. 28 around 6:40 a.m., the Allegheny County Police Department was notified about a partial bridge collapse over Frick Park. Emergency crews arrived at Forbes Avenue Bridge, also referred to as the Fern Hollow Bridge, around Forbes and Braddock avenues.
An articulated Port Authority bus and four passenger vehicles were on the bridge at the time of the collapse, along with a fifth passenger vehicle that drove off the east bridge abutment following the collapse. First responder crews rappelled 100 to 150 feet to reach victims, while others formed a “human chain” to rescue people from the bus.
Initial reports from witnesses described a loud noise and hissing sound, along with a strong smell of natural gas in the area. Responders later confirmed that the collapse caused a gas line to break, prompting an evacuation of homes in the surrounding area. Authorities were eventually able to off the gas line, but a few hours later service was restored and families were permitted to return to homes, according to Pittsburgh Fire Department Chief Darryl Jones.
President Biden was briefed on the collapse and spoke with state and local officials prior to his arrival in the city. Biden visited the bridge site around 1:30 p.m. that day ahead of his previously scheduled trip to Carnegie Mellon University’s Mill 19 research and development center to talk about infrastructure, the economy and supply chain issues.
Later in the afternoon, 13 members of the National Transportation Safety Board investigation team arrived at the site of the collapse. In their short time there, investigators were reportedly already able to study some of the bridge elements. On Saturday (Jan. 29), the NTSB announced it had launched an investigation into the collapse, planning to look at the full history of the bridge, including design, construction and maintenance.
NTSB Chair Jennifer Homendy told reporters at a briefing in Pittsburgh on Jan. 29 that the investigation would be “lengthy.” Currently, the final board report on the cause of the collapse will not be issued for 12 to 18 months, but the NTSB hopes it will take less time.
Pennsylvania Gov. Tom Wolf signed a proclamation of disaster emergency for the county following the collapse, authorizing state agencies to use all available resources and personnel, as necessary, to manage this emergency situation. According to the release, time-consuming bidding and contract procedures, as well as other formalities normally prescribed by law, are also waived through the proclamation.
A similar declaration was signed by Gainey on Jan. 30. The declaration will increase the availability of federal funds, facilitate closer coordination between PennDOT and Department of Mobility and Infrastructure (DOMI) and speed up cleanup and reconstruction of the Fern Hollow Bridge.
Crews from Allegheny Crane Rental lifted the Port Authority bus from the site on Jan. 31. Utilizing a 400-ton crane, the company’s largest piece of equipment and built on site at the collapse, the bus was lifted by straps underneath it.
The 447-foot, rigid steel frame bridge with three spans was built in 1970 and owned by the city. According to reports, the posted weight limit of the bridge is 26 tons and sees an average of more than 14,000 vehicles a day.
Forbes Avenue is a four-lane, non-divided roadway with two travel lanes in both directions. Sidewalks flanked the travel lanes on both sides, and at the time of the collapse, some snow had accumulated on the roadway and bridge surfaces. The bridge goes across Frick Park, one of the city’s largest parks, over the Hot Dog Dam Dog Park and a public walking trail.
Gainey told reporters the bridge was last inspected in September 2021, with the report from Pennsylvania Department of Transportation stating that the bridge received an overall “poor” rating and has been consistently found to be in this condition in since 2011. The deck condition and superstructure condition received poor scores, while the substructure condition gained a “satisfactory” rating.
Preliminary NTSB Investigation
In its preliminary report, the NTSB reports that 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, with each girder additionally supported by two inclined, welded and uncoated weathering steel frame legs on concrete thrust blocks.
According to the release, certain areas of the welded steel girders were identified as being fracture critical, but no primary fractures were found in those areas. Initial assessment indicates the collapse began at the west end of the bridge.
As more debris are removed and areas are more accessible, further examination is expected to be performed. The NTSB also plans to conduct forensic examination of several of the bridge’s components, as well as evaluate 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.
Footage recovered from the seven cameras on the Port Authority bus will be analyzed for further information by a group of specialists. These cameras include one forward-facing camera, one right side aft facing camera and five interior cameras.
The initial assessment of these cameras is reportedly consistent with the initial assessment of the bridge components.
“The recovery of evidence, including extraction and documentation, is expected to be a lengthy process,” the NTSB wrote. “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.”
Replacement Design Plans
In March, PennDOT announced design and construction plans to replace the bridge, in collaboration with City of Pittsburgh officials and the Federal Highway Administration. PennDOT reports that it has selected HDR Inc. and Swank Construction Company to design and construct the new bridge. The Design Build partnership plans to take environmental complexities into account regarding its location with Frick Park.
According to the release, the team determined after evaluating several options that a three-span continuous composite prestressed concrete I-beam with integral abutments would be the best structure type. Material availability, cost, design concerns and delivery time, along with supply chain concerns, were factored into the decision.
The new structure will reportedly remain along the same roadway alignment and width, but plans to include four 10-foot-wide travel lanes, two-foot-wide shoulders on both sides, a five-foot-wide sidewalk and a 10-foot 5-inch-wide shared use path on the southern side of the bridge.
Aesthetic treatments planned include treatments to the concrete pier columns and bridge barriers, painting of the beams, a stream restoration plan, ornamental bridge lighting and a site restoration plan with tree plantings to restore damaged areas. PennDOT is consulting with Pittsburgh Parks Conservancy and the Pittsburgh History and Landmarks Foundation and seeking public input on these elements.
Emergency declarations issued following the collapse reportedly allowed the city and PennDOT to manage construction and design for the project immediately, while also using emergency procurement procedures through the FHWA to being work within seven days of the collapse. These emergency procedures have allowed the replacement to occur two to three years earlier than would have been possible using typical design and construction methods.