Speaking at the March 1954 ground-breaking ceremony for the 110-mile Northeast Extension of the Pennsylvania Turnpike, then-Gov. John S. Fine summed up the looming challenge of building a superhighway through the rugged terrain of the Pocono Mountains with a reference to a planned bridge that will rise nearly 200 feet above Mud Run Gorge in Carbon County’s Hickory Run State Park.
Later named for a nearby scenic waterfall, the Hawk Falls Bridge “will triumphantly typify the pioneering strategy that carried the Pennsylvania Turnpike system across wide, turbulent streams … and through the rocky cores of mighty mountains,” said Fine, a Republican who served from 1951-55.
For nearly 60 years, the 738-foot, double-span steel deck crossing has earned more than the governor’s oratorical eloquence, both as an engineering marvel and as part of the main north-south connection between Philadelphia’s suburbs and the Scranton-Wilkes-Barre metropolitan area.
But with the Hawk Falls Bridge reaching the end of its useful life after decades of use and exposure to the often harsh mountain climate, the state Turnpike Authority launched a five-year, $88 million replacement effort, which is on track to be completed by general contractor Trumbull Corp. by the end of 2026. The project also replaces the next 3-meter-long state bridge, 3-meter state route. 534, one of the state park’s key roads.
For nearly 60 years, the original three-span, 738-foot steel deck truss crossing was a key part of the north-south connection between suburban Philadelphia and the Scranton-Wilkes-Barre metropolitan area.
Photo courtesy of Modjeski and Masters
Revisiting a legacy
Institutionally, at least, the project has been familiar territory for design consultant Modjeski and Masters Inc., which designed the original Hawk Falls and Hickory Run bridges more than 70 years ago. Tom Murphy, senior vice president, notes that while the replacement goals reflect many of the site and constructability challenges addressed by the project’s 1950s predecessors, they also incorporate many 21st century transportation concerns, such as safeguarding and harmonizing with the park’s natural and recreational features, ensuring durability and minimizing the need for rehabilitation and maintenance.
For this reason, Murphy says, the team initially considered the feasibility of less invasive rehabilitation.
“We found that this approach would result in a bridge that still did not meet modern service needs and would extend the service life only for a short period of time,” he says, adding that the choice of a new design was readily apparent.
“The rocky walls of the gorge lent themselves to a structural steel arch design.”
—Tom Murphy, Senior Vice President, Modjeski and Masters Inc.
“The rocky walls of the gorge lent themselves to a structural steel arch design, providing a solid foundation for the outward thrust that creates an arch,” explains Murphy. The design also ticked other boxes, including versatility and ease of modification. “In addition to reducing long-term maintenance, the steel superstructure allows more flexibility for future expansion with minimal disruption,” he adds.
Modjeski and Masters’ final design is a 720-foot-long, four-lane bridge with a 480-foot deck arch span flanked by two spans of 60-foot deck plate I-girders at each end. Unlike the current crossing, the new bridge will provide 12-foot-wide outer shoulders and 6-foot-wide middle shoulders.
Murphy says the bridge’s focal point is the three-rib system of two-hinged arches. Each rib section is composed of 11 segments, weighing from 60,000 to 202,000 pounds. The two-hinged design also allows the ends of the ribs to rotate slightly as loads are moved. The bridge rests on extended footings resting on the rock walls of the gorge, with the interface angle adjusted to avoid the need for shear keys.
He adds that the plinths were reduced to a small pedestal on which the arch bearings rest, “resulting in a very small portion of the plinth that will be visible after construction is complete,” Murphy says.
The key piece of the arch weighs 101,000 pounds and has a radius of 418 feet.
Photo courtesy of Modjeski and Masters
Bow arrangements
Anticipating the access and construction challenges presented by the gorge, Murphy says the team designed the arch rib segments progressively shorter in span, allowing large cranes to place segments from behind the new buttresses.
Developing the most effective construction strategy required a year of planning by Trumbull Corp., construction manager STV Inc. and assembly engineer Genesis Engineering. Jarred Musser, Trumbull’s project manager, says the team pivoted from its original plan to limit work in the gorge using tin cables anchored in the buttresses to a strategy of bracing towers.
“He owns a lot of shoring towers, gratings and cap beams from previous work that could be adapted for this project,” he explains.
Four lines of shoring towers would be constructed within the freeway-owned easement, ranging in height from 60 to 180 feet.
As the arch neared completion, the team tried to identify the ideal conditions for closing the structure, using daily surveys to measure the thermal effects on the steel.
Photo courtesy of Modjeski and Masters
Musser’s team also used the months of prep time to familiarize themselves with the two Liebherr LB11000 cranes, which would be used to pick up bridge sections approaching a quarter-million pounds in weight when full construction begins in 2023.
Counterweight management would be critical, Musser says, as would managing the steps from delivery to placement. Although the bridges were being built in a new alignment just east of the existing structures, the massive cranes would still be too close to the freeway to make a 180° turn. Instead, the bridge components would be delivered in front of each crane along a specially constructed 300-foot runway. There, the sections were lifted off the transport trailer, handled as needed during post-assembly, then lifted and placed on top of the shoring towers.
“Each of these steps required a designed selection plan,” says Musser. “Most were crane-critical lifts because of the nature of how many counterweights we had loaded on the machine at any given time and other configurations. Just understanding these cranes was a huge undertaking.”
Construction was also managed with the dynamics of mountain weather, particularly wind conditions that often exceeded the cranes’ operating limit of 28 mph. Activity was also suspended during the winter of 2023-2024 due to extended periods of below-freezing temperatures.
“We wanted to be as efficient as possible to get the cranes out of there as quickly as possible.”
—Jared Musser, project manager, Trumbull
“We wanted to be as efficient as possible to get the cranes out of there as quickly as possible,” says Musser. “This involved a lot of coordination with the steel fabricators and the delivery schedule and almost constant pre-planning to optimize the workflow and keep the crane moving.”
Using the vertical hydraulic lifting capabilities on the highest strut tower lines, each half of the arch was initially raised to the height of each strut tower. Musser estimates that more than 100 picks from both cranes were required for the operation, the largest of which, a preassembled assembly of three box columns and a floor beam, weighed 243,000 pounds with a 170-foot radius, pushing the crane to 94 percent of its capacity. The key piece of the arch, weighing 101,000 pounds with a radius of 418 feet, was the farthest selection.
As the arch neared completion, Musser’s team tried to identify the ideal conditions for closing the structure, using daily surveys to measure the thermal effects on the steel.
“We determined that the ideal ambient temperature would be around 68°, and it was best to have the entire arc as close to the same temperature as practical,” he says. The morning of June 3, 2024, was ideal for this operation, he adds. “Conditions were better before 10:00 a.m. as the sun was not high enough to warm the east face of the arch while the west side was still cool from the night.”
Each arch half was gradually lowered until the apex of the arch was closed. Musser says the apex of the arch was closed with a temporary pin until the final screw could be completed. Once all the screws were installed, the temporary pins were removed.
The rocky walls of the gorge provide a solid base for the outward thrust that creates the arch.
Photo courtesy of STV Inc.
Finishing characteristics
With the new Hawk Falls Bridge complete, Trumbull is working to complete the smaller Hickory Run Road bridge, a 120-foot-long single-span steel plate girder structure. It removes two existing concrete piers to provide a more open feel for travelers on State Route 534 and also has room for future road upgrades. Both passages are expected to be fully open by 2026, when large-scale demolition of the original structures will begin. Trumbull’s remaining work also includes excavating up to 60,000 cubic yards of material for additional erosion and sediment control features.
While the new Hawk Falls Bridge may not inspire the soaring oratory of its predecessor, the Turnpike Authority will welcome the benefits of a structure that lends itself to easier maintenance. While the patina developed by weathered steel components eliminates the need for periodic painting, Murphy says the welded rigid steel box sections are sized to allow inspectors to walk upright inside and are equipped with lighting, fans and drainage systems.
“The bow’s Vierendeel reinforcement system also reduced the number of components and will minimize debris build-up and corrosion,” he adds.
Musser says the Hawk Falls Bridge project has provided valuable insights into the construction of large-scale steel structures.
“The more you can do before the erection stage, the better,” he says. “Keeping this project going was a major undertaking.”
