The St. Croix Crossing is an extradosed bridge that spans the St. Croix River between Oak Park Heights, Minn., and St. Joseph, Wis., connecting Minnesota State Highway 36 and Wisconsin State Highway 64. It is five times longer than any other extradosed bridge in the U.S. and the largest public works bridge project in Minnesota.

The story of the bridge and its design and construction goes back to 1951, when Wisconsin and Minnesota made the first attempt to replace the now 90-year-old Stillwater Lift Bridge. After lawsuits, an act of Congress to create an exemption to the federal Wild and Scenic Rivers Act and a new preservation plan for bald eagle nests, a new bridge was finally realized 66 years later. The St. Croix Crossing connects to a new five-mile trail that increases traffic and trade between Wisconsin and Minnesota, while the historic Stillwater bridge is now a pedestrian crossing that the team updated and preserved. During construction, general contractor Lunda/Ames took extra care to protect nature and prevent disturbances by relocating mussels and collecting seeds from endangered plants before moving them.

HDR’s design includes slender, reed-like piers, with pier blades that resemble cattails. During the design phase, the spans of the bridge were lengthened by 120 ft, two towers were eliminated and the bridge’s footprint in the river was minimized while the team also improved constructibility. A stringent visual quality requirement resulted in the entire bridge being rounded or tapered. In fact, there are only two flat surfaces on the entire bridge.

The Lunda/Ames joint venture followed rigorous environmental rules on the project, such as using rollers to paint the bridge rather than sprayers to avoid overspray. Workers also made sure not to disturb an active bald eagle nest near the site by maintaining a 300-ft perimeter, and they later observed three eaglets hatching in the spring.