Pretty Rocks Bridge

The Pretty Rocks Bridge is one of the most complex and technically demanding steel bridge projects undertaken in recent U.S. infrastructure. Located deep within Denali National Park, the site sits directly atop the Pretty Rocks rock glacier, an ice-rich permafrost body whose movement accelerated from inches per year to over 12 inches per day due to climate-driven thaw. This rapid ground deformation rendered the existing road unsafe and cut off public access to the western half of the park. A long-span bridging solution became the only viable way to restore access.

The bridge is a flagship example of how Integrated Design and Detailing (IDD) can compress schedule, de-risk construction and raise the quality bar on a complex steel truss bridge fabrication and erection. For DBM Vircon, it was the ultimate test of its integrated digital engineering capability: taking a 475-foot single-span weathering steel Warren truss, to be launched over an actively moving rock glacier in remote Alaska, from concept to fully detailed, fabrication-ready model in just five months.

PROJECT CHALLENGES

The Pretty Rocks Bridge was a particularly challenging project for several reasons:

• A Live, Climate-Driven Landslide: Pretty Rocks is not a static slope failure but a rock glacier: a mass of debris and ice with movement occurring both along a basal shear plane and internally through creep. Borings identified ice-rich permafrost with ice seams up to 15 feet thick beneath the existing road and a thick, unstable clay unit dipping steeply toward the valley, making conventional cut-and-fill reconstruction impossible. Movement accelerated from minor cracking to more than 12 inches per day in 2021. Any solution had to bypass the moving mass entirely and be tolerant of long-term permafrost degradation.
• Extremely Constrained Site and Limited Season: The bridge sits on a narrow mountain bench above a 600-foot drop, with virtually no level ground available for staging and an active slide beneath the entire span. Access is via a single-lane gravel road with strict environmental, wildlife, and operating restrictions inside Denali National Park. Construction is limited to roughly May to September each year due to snow, cold, and darkness.
• Compressed Schedule and High Stakeholder Risk: The initial procurement contemplated a pre-engineered truss with substantial completion by late 2023, but this risk profile was unacceptable to industry. The Federal Highway Administration and the National Park Service targeted reopening as quickly as possible, which meant pushing design, detailing, steel procurement, and temporary works design in parallel rather than sequentially. The team needed to advance the bridge from 30% concept design to 100% Issued for Construction (IFC) in five months while also finalizing the erection strategy.
• Complex Foundations and Permafrost Management: Both abutments sit as close as possible to the slide margins on relatively competent rock, but investigations revealed fractured rock, clay seams, and permafrost with significant ice content. The foundation solution combines precast modular abutment blocks, ground anchors, battered micropiles, soil nails, rock dowels, and a field-installed array of 23 thermosiphons to maintain frozen conditions beneath the east abutment for the long term.
• Launch-Controlled Structural Behavior: Given the moving ground beneath and the inability to use falsework in the landslide zone, the team selected an incremental launch scheme: building the permanent truss on the east side with a temporary “launching truss within the truss” and then pushing the entire 475-foot span across to a receiving tower on the west abutment before jacking it down into final bearing. This meant many members and connections had to be designed for both cantilever “diving board” and final simple-span states, with full stress reversal in some diagonals and chords. Launch engineering and permanent design were tightly coupled, and the model had to reflect both states.

THE SOLUTION

The selected design is a 475-ft single-span weathering steel Warren truss constructed on the east side of the landslide and launched across the unstable slope in a single controlled operation. The project faced extraordinary challenges, including an active landslide incapable of supporting falsework; a narrow mountain bench with virtually no laydown space; severe environmental restrictions; a remote supply chain; and an urgent need to compress the delivery schedule to reopen the park as quickly as possible.

To meet these constraints, the Federal Highway Administration developed a General Contractor delivery model with a highly integrated technical team. The project was delivered under a General Contractor / Construction Manager model, with Jacobs as lead designer, BGC Engineering as geotechnical engineer, Granite Construction as general contractor, KWH Constructors and Somerset Engineering as the steel erector and erection engineer, and DBM Vircon as the IDD partner and steel bridge detailer.

DBM Vircon’s fabrication-level Tekla model captured permanent works, temporary launching truss components, cambered and launched geometries, floorbeams, deck interfaces, and connection details to LOD 400. This early constructible detail enabled the project to issue Advance Bills of Materials and place steel procurement orders months earlier than possible under traditional workflows. The model also served as the primary coordination tool, enabling rapid Virtual Model Approval (VMA), real-time clash detection, and collaborative resolution of constructibility challenges.

The IDD approach proved critical to the project’s success. It reduced detailing and approval cycles by an estimated seven months, supported a highly complex incremental launch strategy, and delivered exceptional accuracy. This resulted in near-zero field rework on a 475-ft truss installed in one of the most inaccessible locations in North America. The digital model also guided fabrication, trial assembly, transport logistics, and field erection, ensuring the multidisciplinary team remained aligned throughout delivery.

OUTCOMES AND FUTURE IMPACT

In late 2025, the Pretty Rocks truss was successfully launched from the east abutment, landed on the west receiving tower and jacked into its final position. Deck installation and approach works were completed in 2026 and full public access to Denali’s interior is expected in 2027.

From DBM Vircon’s perspective, the project has delivered several clear outcomes:

• Schedule Compression with No Compromise in Quality: IDD and Virtual Model Approval enabled the team to move from 30% to 100% design while simultaneously producing fabrication-grade models, bringing steel procurement forward and shaving months off the critical path.
• Exceptionally Accurate Steelwork: Model-driven CNC drilling and assembly allowed a 475-foot truss to be assembled, trial-fit, shipped, reassembled, and launched with virtually no field rework. This is an outcome that would be  remarkable on any project, let alone one in the middle of an active landslide in rural Alaska.
• A Proven Template for Climate-Resilient Infrastructure: Pretty Rocks is one of more than 140 unstable slopes along the Denali road. The combined geotechnical, structural, and digital strategies used here (including bridging over active movement, leveraging thermosiphons, and using IDD for rapid, high-confidence project delivery) now form a reference model for future climate-adaptation projects in permafrost and landslide-prone environments.

For DBM Vircon, Pretty Rocks demonstrates that for the next generation of complex, high-risk steel bridge projects, Integrated Design and Detailing is not a “nice-to-have” but the critical delivery mechanism. By embedding BrIM experts directly into the design and construction team, and by elevating the 3D model to the central contractual and technical reference, DBM Vircon helped turn an almost unbuildable project into a constructible, climate-resilient bridge that will reconnect visitors to the heart of Denali for decades to come.