Grand Forks County Prefabricated Bridge

Any new bridge spanning the Goose River near Northwood, N.D., must remain sturdy in a flood hotspot with limited vertical clearance from the water and withstand long winters. Ideally, it’s also constructable during one of those winters.

A prefabricated steel frame ensured the refreshed 36th Street NE bridge over the river did all three. Weathering steel beams offered durability and minimal maintenance. Steel also created flexibility in the depth of the superstructure in an area prone to flooding, the ability to clear the channel with one span, and the possibility of fabricating the superstructure offsite and delivering it in minimal pieces to install quicker and easier. The substructure used steel to help facilitate cold-weather installation and expedite the construction timeline.

The design team also considered concrete, but limited availability and high cost of heating during the preferred cold weather construction window made concrete less favorable than steel or timber. A timber prefabricated superstructure option was carried out through design and included in the bid package for contractors. The structural steel prefabricated bridge was the lowest bid option at the construction bid opening.

The prefabricated bridge lent itself to completion within the sensitive construction window and the limited allowance for increasing the roadway height. Using the same size H-piles for the wingwalls and pile caps provided an economy of scale for materials.

The ability to span across the entire channel became even more crucial during construction when a near-record flood event hit toward the end of installation. The substructures were clear of the main channel area and not affected by flooded cofferdams, impact from debris, or flood cleanup once the water receded. Cleaning off steel rather than an uncured concrete pier potentially being compromised became a significant advantage when the flood hit.

The local economy is mostly agriculture-related commerce that peaks in spring and summer, and the ability to construct in the winter minimized the installation’s economic inconveniences. The bridge opened to traffic before the spring planting season.

The entire bridge substructure is also comprised of steel. HP14×73 beams provide the capacity to withstand the loadings required, and sheet piling holds the soil behind the abutment to keep the roadway in place.

The superstructure was fabricated at TrueNorth Steel’s Fargo, N.D., plant and delivered to the site in four pieces consisting of steel beams and corrugated decking. Those were placed and secured onsite to create a 32-ft-wide roadway on the bridge. All told, removing the old bridge and erecting the new one took just two months.

The project site had limited vertical clearance to carry a stream with high flow volumes during spring runoff. Steel beams allowed a slimmer superstructure depth to provide freeboard on the design flood event.

A nearby roadway with an out-of-service bridge over the river had created a higher traffic volume on 36th Street NE, making a quick replacement and shorter closure duration more significant factors in the chosen replacement alternative.

The soft soils of the Red River Valley in eastern North Dakota require deep foundations to support the superstructure. The design used 130 ft of HP14×73 to provide adequate support and galvanized sheet piling to complete the backwall of each abutment.

The bridge superstructure sections were built inside a manufacturing shop, where the weather would not impact timelines. Shop construction permitted extensive oversight as the prefabricated sections were being built, helping ensure a more uniform fit once the pieces were delivered to the site and installed. The bridge sections were placed with the same equipment that installed the substructure, meaning less mobilization of varying equipment and specialized equipment such as a concrete pump truck.

All these pieces reduced the construction timeline, thereby avoiding significant impacts on the local economy and providing a construction timeline when contractors are not typically busy, allowing them to keep staff working during the winter.

Project Team

• Owner’s representative: Grand Forks County, Grand Forks, N.D.

• General contractor: Industrial Builders Inc, West Fargo, N.D.

• Structural engineer: KLJ Engineering, Grafton, N.D.

• Fabricator/detailer/erector: TrueNorth Steel, Fargo, N.D. *AISC full member; AISC-Certified fabricator*