COWI Engineers Complex Launch of Alaska’s Juneau Creek Bridge

Engineering solution moves nearly 7 million lb of structural steel across remote canyon in one of the longest steel plate-girder launches of its kind

ALASKA: Global engineering consultancy COWI has completed the erection engineering behind the launch of the Juneau Creek Bridge across one of Alaska’s most challenging bridge sites, using specialized temporary works, staged structural analysis and wind engineering to move nearly seven million lb of structural steel across the canyon.

The bridge is the signature structure on the Alaska Department of Transportation and Public Facilities’ (AKDOT&PF) Sterling Highway MP 45–60 project near Cooper Landing. Its 440-ft main span crosses a steep, environmentally sensitive canyon where conventional erection from below was severely constrained.

Rather than constructing the superstructure piece by piece over the canyon, the project team assembled the full-width, five-girder steel superstructure on one side before launching it across the canyon as a single system.

COWI, acting as erection engineer for the QAP/Traylor Joint Venture, developed the launch arrangement and carried out the staged erection analysis needed to control the structural behavior of the bridge throughout the operation.

At maximum cantilever, the structure extended 440 ft across the canyon before reaching its next permanent support. COWI’s solution incorporated a lightweight 180-ft launching nose together with temporary king posts and a stay system to redistribute loads and control deflection as the bridge advanced.

The engineering challenge went beyond the length of the span. Launching all five girder lines simultaneously created a wide, indeterminate structural system in which relatively small differences in girder camber or roller elevation could redistribute substantial loads between supports.

COWI therefore developed measures that allowed controlled movement between elements of the structure during launch, while survey control, staged jacking, roller adjustments and reaction monitoring allowed the field team to check its behavior against the erection analysis as the bridge moved across the canyon.

Wind was another critical consideration. A site-specific study examined the effects of the surrounding terrain and the bridge’s susceptibility to aerodynamic effects during different launch stages. The analysis identified vortex-induced oscillation as a potential risk under certain conditions, including the maximum cantilever position.

The launch was consequently engineered for defined operational and out-of-service wind conditions, with the bridge capable of being secured at intermediate stages if conditions exceeded the launch limit. Wind effects ultimately governed the design of several components and required significant strengthening of plan bracing and cross-frames in the cantilevered portion of the structure.

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

The permanent bridge comprises three steel plate-girder spans, with a 440-ft main span and two 240-ft side spans. The girders are approximately 13 ft 3 in. deep, and the permanent steelwork was itself detailed to accommodate the temporary demands generated during launch.

AKDOT&PF designed the permanent bridge, while COWI’s erection engineering included analysis and refinement of the girders, cross-frames, plan bracing and splices as well as the temporary launching nose, king posts, stay system and roll-off tail. Temporary erection demands identified through this process were incorporated into the permanent steel design through member upsizing, splice-capacity changes and bracing modifications.

Mohamad Ramlawi, Area Manager, Taylor, said: “For Traylor, the innovation on Juneau Creek was turning an extremely complex engineering concept into a safe, controlled field operation. The team assembled the full-width, five-girder superstructure in a controlled work area, then used detailed planning, real-time monitoring, and disciplined communication to move nearly seven million pounds of steel across a remote canyon where conventional construction access was not practical.”

The bridge is now progressing into the next phase of construction, with deck work scheduled for 2027.

The Juneau Creek Bridge forms part of the wider Sterling Highway MP 45–60 project, which will reroute approximately 15 miles of highway and includes 14 new structures, four wildlife under crossings and Alaska’s first wildlife overcrossing.

source https://zweiglist.com/cowi-engineers-complex-launch-of-alaskas-juneau-creek-bridge/