Launching seven million pounds of steel across an Alaskan canyon

The Juneau Creek Bridge will increase safety and mobility along one of Alaska’s most important transportation routes between Anchorage and the Kenai Peninsula. The bridge is the signature structure of the Sterling Highway MP 45–60. For the communities, businesses, workers and visitors who rely on Alaska's Sterling Highway, the bridge will improve a vital connection while helping protect the area's unique natural environment.

The bridge and the rerouting of traffic away from the riverbank of Juneau Creek will also reduce travel time and the risk of accidents as well as decrease bottlenecks, traffic noise and air pollution in Cooper Landing. Furthermore, the project will minimise the risks of hazardous fuel spills from the existing roads into unique salmon habitats.

Launching seven million pounds of steel across a remote Alaskan canyon

The Juneau Creek Bridge crosses a deep, environmentally sensitive canyon, making conventional erection from below extremely challenging. The remote location and demanding Alaskan climate, which limits construction activities to a short season between April and October, added further complexity to the project.

As erection engineer, COWI developed the engineering solutions that enabled the bridge's 7-million-pound (3,175-tonne) steel superstructure to be assembled on one side of the canyon before being launched across the span as a single unit. Instead of erecting the superstructure piece by piece over the canyon, the project team assembled the full-width, five-girder steel superstructure in a controlled work area before advancing it into position.

The construction methodology reduced the need for work within the canyon itself while allowing a large proportion of fabrication and assembly activities to take place away from the canyon edge, thereby minimising environmental impacts and improving constructability on a highly constrained site.

A week-long launch across the canyon

The launch itself required nerves as steely as the bridge itself. It took an entire week to carefully slide the whole structure into place at a pace of 2 feet per hour while maintaining a tight 1-inch (2.5-cm) alignment tolerance. At maximum cantilever, the bridge extended approximately 440 feet (134 metres) before reaching its next permanent support.

Believed to be among the world's longest launches for a steel plate girder bridge of its type, the operation demonstrates how innovative construction engineering can deliver major infrastructure in locations where conventional construction methods are not feasible.

The engineering task extended far beyond simply moving a large structure. Throughout the project, the bridge’s structural behaviour changed continuously as girders moved between support points, creating a highly complex structural system.

“The challenge wasn’t simply getting a very large bridge across a canyon,” says Claus Fredriksen, Head of Department, Roads and Major Crossings, COWI.

“At every stage of the launch, the structural system changed as the girders moved from one support condition to another. We had to understand how five girder lines, the temporary stays, launching nose, rollers and permanent steel would behave together throughout that process while accounting for wind, seismic demands, camber and the tolerances inherent in a structure of this scale.”

“The result was an erection system that allowed an exceptionally large bridge superstructure to be assembled in a controlled area and then moved across a site where conventional construction access was extremely difficult,” he says.

Managing wind, seismic and construction risks

Wind effects were a critical design consideration throughout the launch operation. A site-specific aerodynamic assessment evaluated the influence of the surrounding terrain and the structure's behaviour during each launch stage.

The analysis identified vortex-induced oscillation as a governing condition during certain launch configurations, particularly at maximum cantilever. The launch system was therefore designed to withstand defined operational and out-of-service wind conditions, allowing the bridge to be safely secured at intermediate positions if required.

Temporary seismic demands were also accounted for throughout all launch stages, ensuring the structure remained safe and controllable under both construction and environmental loading scenarios.

The integrated launch solution incorporated a lightweight 180-foot (55-metre) launching nose, together with temporary king posts and a stay system to redistribute loads and control deflection as the bridge advanced. The permanent steelwork was refined to accommodate launch-induced forces, while temporary works were designed to maintain stability and performance throughout the operation. 

Improving access while protecting the environment

The Juneau Creek Bridge is a key component of the Sterling Highway MP 45-60 programme, which reroutes approximately 24 kilometres of highway and includes 14 new structures, four wildlife undercrossings and Alaska's first wildlife overcrossing.

The highway serves one of Alaska's most important tourism, recreation and fishing regions. The corridor will improve traffic flow, reduce congestion and enhance safety along a route that experiences significant seasonal demand.

The project demonstrates how innovative construction engineering can unlock infrastructure solutions in some of the world's most challenging locations while also helping minimise the environmental impact.

The bridge deck is expected to be completed in 2027, with final bridge items following in 2028, and the full roadway corridor is anticipated to open to the public in 2030.