Crews Launch Alaskan Wilderness Truss With Help From Advanced Digital Modeling

Intolerable risk. That was what came to Todd Keller’s mind when Granite Construction looked at the design-build solicitation from the Federal Highway Administration in 2022 to rebuild a section of Denali Park Road, in the Alaska national park, which had been washed out by a landslide.

The agency’s request for proposals suggested using a pre-designed and prefabricated steel truss bridge, with substantial completion needed by November 2023. But risks such as the remoteness of the site, its complex geotechnical conditions, the state’s limited construction season, strict protocols of the park and preserve and the length needed for the Polychrome Bridge replacement structure to span the gap gave Granite officials pause.

“It was a tough piece of work,” says Keller, the contractor’s group business development manager. “So we put a qualifier into our statement of qualifications and said, ‘consider collaborative delivery for this one, because the risk profile is not something we can take.’ That opened a discussion with the [agency].”

Federal Highway Administration officials agreed, pivoting to the construction manager-general contract delivery method for the roughly $200-million project that “provided flexibilities for [the agency], the National Park Service and the contractor to work together to address inherent challenges and complexities,” a spokesperson told ENR. “Restoring access with the Polychrome Bridge project is crucial because without the bridge, 47 miles of roadway and at least five major attractions in Denali, and in the town of Kantishna, would be inaccessible by motor vehicle.”

Milepost 45 of the 92-mile road was shut down in 2022. The Pretty Rocks landslide, which has been present for decades, began accelerating due to rising temperatures that melt permafrost. By 2021, it was moving at a rate of more than 1 ft per day and had cut off that section of the road.

In January 2023, the highway agency and National Park Service, with Jacobs as project lead designer and BGC Engineering as geotechnical firm, convened with Granite’s team, which includes KWH Constructors Inc. and its Somerset Engineering division for truss structural detailing, construction engineering, preassembly, on-site erection and launching. The team also included steel supplier and fabricator Gunderson-Greenbrier; DBM Vircon for the 3D integrated design-detailing; Hamilton for bridge substructure, foundations and crane operational support; DBM for underground elements; Arctic Foundations providing thermosiphons; and Advanced Blasting for rock demolition.

Doug Johnson, Granite preconstruction manager, recalls that kickoff meeting. The two federal agencies wanted the road reopened as soon as possible. “To do that, we had to come up with a process that allows us all to make critical decisions literally hourly,” he says. The team used 3D integrated design-detailing (IDD), which enabled a new design and erection scheme to take shape in five months.

Photos Courtesy Granite Construction, original Denali national Park map courtesy of the National Park Service, original Alaska map by Getty Images/Milena Krakowa

Supermodel

Developing the final design with Jacobs, KWH engaged the bridge detailer “from day one,” says Justin Sieg, its project director. “The first thing they did was create the 3D wireframe geometry. There were iterations back and forth” with Jacobs, he adds. “Jacobs figured out the main member sizing, and we confirmed that the temporary works would support that.”

The 3D modeling informed pricing from the fabricator and subsequent purchases. “It allowed the engineer of record to provide information … for maximum bolt spacing, edge distances, parameters, general shapes,” Sieg says. “We were creating design and shop drawings at the same time, then the engineer of record produced design drawings from that. Normally it’s the reverse.”

DBM Vircon had been working with 3D bridge modeling for decades, including the east span of the San Francisco-Oakland Bay Bridge, notes President Gino Pezzente. “Traditionally, in a bid-build job, the fabricator would send us an RFP, and we’d price the work.” Approached by KWH about the Polychrome Bridge, the firm was challenged to model truss elements in parallel with fabrication, erection, site planning and real-time decision-making on site.

“[3D integrated design-detailing] allowed us to do shop drawings concurrently with the bridge design,” says Keller, estimating that the modeling cut about seven months from the detailing schedule.

After working through various options from summer 2023 to February 2024, the team settled on a game plan. Bridging the gap on the existing road would be less impactful to the surrounding wilderness than creating a new alignment.

A 475-ft-long steel Warren truss could be broken down into components less than 50 ft long for shipping to the remote location. With limited space for equipment, the truss has a steel sandwich plate system of deck panels, precast abutments that would be post-tensioned on site, and micropiles and ground anchors requiring less space for drilling equipment.

Modeling of the new truss gets down to specific details such as where to place bolts.
Graphics courtesy of KWH

The team also decided on a single-sided launch of the truss. It proved to be fortuitous; the team then found a block of ice at least 90 ft wide on the west side of the landslide where the anchorage would have been located, notes Keller.

“The critical path flowed from owner to designer to fabricator to supplier to erector to final design, almost hourly,” says Johnson. “As Jacobs was designing the main truss span, we were creating shop drawings to order the material.” The model informed details down to what sizes the elements had to be broken down into to fit into shipping containers, as well as sizes and placements of bolts on the truss, and sizes for the crane and the openings for workers’ hands to reach into to screw bolts in place.

“It allowed us to know intimately the erection method to be used so that we could design for that process to the lowest-level detail,” says Gary Conner, Jacobs senior bridge engineer.

The one-sided inside-out bolts, a proprietary product of LeJeune Bolt Co., were new enough that the FHWA did not have a standard specification for them, notes Keller. Wrenches were designed for them.

“Typically, the nubs would break off; these do not,” adds Pezzente. “Due to the confines of the truss box sections, there were clearance concerns. So we were mocking up early connection details and seeing whether there was enough access into those boxes.” The team adjusted bolt spacing, access openings, diagonal members and other design details accordingly.

Micropiles, tie-down anchors, soil nails and thermosiphons form the base of the bridge.
Graphic courtesy of Jacobs/BGC Engineering

Modeling Payoff

Modeling details down to fractions of inches paid off. Out of 20,000 modeled bolts, only one ended up not fitting in real time, notes Tyler Vander Linden, KWH general manager.

Even aesthetics decisions were informed by 3D integrated design-detailing. Upon seeing a model of the bridge in a bright white color, the Park Service opted for a more golden “Jersey milk” color instead, recalls Johnson.

During the construction season, workers live in a camp at around mile 27, saving nearly 90 minutes of driving from the park entrance alone. The team also set up staging yards along the narrow route to the site to store materials as they came in.

As stunning as the setting was, it also posed extra challenges. Granite had done construction in Denali before, so it knew the rules, such as a strict speed limit, staying in construction vehicles at a distance behind the frequent tour buses that stop to view wildlife and waiting at designated animal crossing times. Crews had to be mindful of bears wandering into staging yards and the worker camp.

“We have six months maximum for the construction season, around May to September,” notes Keller. In 2023, that season was spent setting up the yards and camp. In 2024, the foundations and substructure work were completed.

Even rougher than weather and wildlife were the geologic conditions, with variables including highly fractured rock, permafrost and the landslide creeping steadily toward the valley floor 600 ft below the site. Ground conditions are uneven, steep and in motion.

To combat differential movement caused by the freeze-thaw nature of the environment, 23 thermosiphons are installed about 100 ft deep into the bridge abutment. “There is a six-inch layer of ice eighty feet below the abutment, and we want it to stay frozen,” says Conner.

The two-phase thermosiphons provided by Arctic Foundations are passive refrigeration devices that transfer heat against gravity. Closed-ended tubular vessels are charged with a two-phase working fluid. The fluid’s vapor phase fills the majority of the vessel interior, with the liquid phase filling the rest. During the cold season, air cools and condenses the fluid. Once the air gravitates underground, it warms, vaporizes and rises upward to repeat the cycle.

While the technology is tried and true, “this was a complicated installation with everything else going on,” says Edward Yarmak Jr., the firm’s president. “On a typical project, everything is manufactured to size and assembled in the field,” he says. “At Pretty Rocks, because they were drilling and we didn’t know what would be where, things were cut and fit in the field. It’s just tight out there.”

Crews grouted and post-tensioned 13 precast concrete blocks, each 4-ft x 4.5-ft x 15-ft and connected by steel to vertical ground anchors and micropiles nearly 40-ft deep, to form the east abutment. A soil nail wall stabilizes the fractured rock there.

“We needed to locate the abutments in areas that were not moving,” notes Johnson. The team constantly monitored the relentless progression of the landslide.

All that work, done in the short construction seasons of 2023 and 2024, set things up for the launch of the truss itself this past August.

Working in constrained conditions above a valley, crews placed and post-tensioned precast blocks for bridge abutments and assembled the steel truss that would be cantilevered out over the active landslide.
Photos courtesy Granite Construction

Truss and Teamwork

Some 1.6 million lb of steel for the truss and nearly an equivalent amount for temporary supports traveled from steel mills in the Midwest to Portland, Ore., where KWH performed a trial assembly of the bridge at the facility of fabricator Gunderson Marine & Iron. The steel components then made a long trek to the Port of Tacoma, Wash., and onward to Whittier, Alaska, by barge and north to Fairbanks by train. Trucks then carried components to an offsite storage yard and eventually to one of the Denali staging sites. “We could only haul [materials] from 10 p.m. to 6 a.m. in the park,” notes Keller.

Even in summer, each day the incremental launch was conditional upon fickle weather. Crews began assembling the main truss on the constrained site, cantilevering it out from a temporary launch beam on the east abutment.

A temporary launch truss was built inside the permanent truss as it cantilevered out and then incrementally pushed westward over the landslide, heading for a receiving tower placed on the west abutment, which is composed of 13 precast concrete blocks.

Once the launch truss was connected to the receiving tower, the permanent truss pushed westward and eventually jacked down about 10 ft into its final position on the west abutment, says Keller. The process took approximately two weeks.

Next year’s final construction season will consist of installing the deck plates and finishing the approaches—precast concrete slabs and walls, and deck coatings.

“What collaborative contracting allowed us to do was take a potentially unbuildable project and create a buildable project out of it,” says Keller.