NO. 7 BRIDGE: Officially recognized

Nov. 2, 2018

Extradosed marvel was Minn.’s largest public works bridge project

This one mattered so much, it required an Act of Congress and a presidential signature to bring it home.

The St. Croix Crossing Bridge spans the St. Croix National Scenic Waterway, a federally protected waterway. In order for the work to proceed, the project required an exemption from the Wild and Scenic Rivers Act. It was a sticky wicket, to be sure. A new crossing was deemed to have adverse effects according to the Wild and Scenic Rivers Act. However, the existing lift bridge, built in 1931, was protected by the Historic Preservation Act and replacing it or making significant changes to it also was considered to have an adverse effect on the waterway. These conflicting laws left the communities near the bridge with no options to address daily congestion.

An environmental impact statement limited the number of piers and their height above the roadway, and mandated a minimum and maximum structure depth.

“[The project] was more difficult because of everything around it,” said Terry Zoller, project construction manager.

With the extradosed towers rising only 65 ft above the bridge surface, the resulting bridge fits into the river valley as naturally as possible. The mile-long bridge is more than five times longer than the only other extradosed bridge in the U.S and features five towers, whereas the other extradosed bridge has only two. It includes slender, reed-like piers, with pier blades that resemble cattails. The spans were extended in the design phase from 480 ft to 600 ft—eliminating two towers and minimizing the footprint of the bridge in the river.

The 5,079-ft-long bridge is a post-tensioned concrete box girder structure with curved box girder faces, superelevation transfers in the gore, and variable structure depths. The gore section ranges from 14 to 18 ft, and the on and off ramps range 10 to 14 ft. The main river spans include stay cable extradosed spans 600 ft long comprised of 650 twin precast concrete segmental box segments.

Construction operations needed to be contained within a 400-ft zone in the river, and the National Park Service imposed strict timelines on barges transporting the precast segments from the casting yard through the locks, allowing only 72 hours per run, limiting the exposure of these vessels to the invasive zebra mussel. If any barge exceeded that time limit, it would have to go through a complete inspection and wash-down to prevent the mussels from passing between the Mississippi and St. Croix Rivers.

“This was a great project to work on,” Craig Lenning, senior VP and project manager for HDR, told Roads & Bridges. “The finished bridge is proof that unique solutions can solve environmental concerns while meeting community needs.”

Project: St. Croix Crossing

Location: Oak Park Heights, Minn., to St. Joseph, Wis.

Owners: Minnesota DOT; Wisconsin DOT

Designers: HDR/COWI

Contractors: Lunda/Ames J.V.; Ed Kraemer (substructures)

Cost: $417 million

Length: 6,355 ft (incl. on/off ramps)

Completion Date: Aug. 1, 2017

About The Author: Budzynski is managing editor of Roads & Bridges.

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