Extradosed Design Prioritizes Constructability and Aesthetics

Few bridges match St. Croix Crossing for beauty and uniqueness. Both attributes resulted from custom innovations driven by the needs of the Minnesota Department of Transportation and the Wisconsin Department of Transportation.

First and foremost, the agencies needed a bridge that could increase throughput along Minnesota Trunk Highway 36 and Wisconsin Highway 64, which link the Twin Cities area and Wisconsin across the picturesque St. Croix River. The bridge’s predecessor was the 80-year-old Stillwater Lift Bridge, which showed both operational and structural deficiencies. 

Next, MnDOT and WisDOT needed a constructible and aesthetically pleasing structure befitting a Wild and Scenic Riverway — and they needed all of this delivered on an accelerated schedule. These factors ultimately led to our extradosed final design, which we delivered in about 12 months by co-locating with MnDOT staff. When the bridge opened in August 2017, it was only the second of its kind in the U.S. and the fourth of its kind in North America.

The $377 million bridge extends 3,365 feet across the St. Croix River with 2,214 feet of approach structures. Its hybrid design — part-cable-stay, part-box-girder — allows for several advantages. Among them, the lowered maintenance of only two expansion joints for the main river crossing: one at the Wisconsin abutment and one at the Minnesota transition pier.

Another advantage is the minimal construction impact on environmentally sensitive areas on the river banks in both states. The extradosed design also curtailed the manmade structure’s visible profile versus a traditional cable-stay bridge, and reduced the number of piers in the river compared to a pure segmental concrete box-girder bridge. Thus, the viewshed is minimally obstructed.

Two of the bridge’s signature features are its sloped and curved surfaces and slender, twin-blade piers, which convey a visual quality that blends the bridge with its natural surroundings.

To accelerate construction, our team prepared advanced plans for the river foundations that were built in 2013. We supported MnDOT’s public outreach, provided visual-quality assistance and developed special provisions and cost estimates.

Challenges of St. Croix Crossing, such as its long, continuous length and aesthetic requirements, have been successfully met by careful consideration of the details and effective teamwork between HDR, our consultant partners and our clients.