Crews battled subarctic temperatures to construct the Deh Cho Bridge across the Mackenzie River near Fort Providence, Northwest Territories, Canada. Billbradenphoto, courtesy of Government of the NWT
Decades in the making, the Deh Cho Bridge is the first bridge to be constructed across the Mackenzie River, connecting the Northwest Territories, Canada, to the rest of the continent for the first time.
February 5, 2013—Much of Canada’s Northwest Territories had long been isolated from the rest of the continent. The only way to get there was to cross the Mackenzie River by ferry in the summer and ice roads in the winter. So completion of the first permanent bridge over the river, known as the Deh Cho Bridge, marks more than an engineering first. It also represents a gateway to previously unattainable possibilities for the territories.
The residents of the Northwest Territories had worked for more than 50 years to construct a bridge across the Mackenzie River, the longest river in Canada, near Fort Providence, with no success. Then in 2002 a group of interested parties formed a group known as the Deh Cho Bridge Corporation (DCBC) to see the project to fruition. But high construction costs delayed the effort until 2007, when the Government of the Northwest Territories (GNWT) committed significant financial support to advance the project. Finally, a permanent crossing to replace the unreliable Merv Hardie Ferry and the Mackenzie Ice Crossing, both of which were out of service several weeks each year, was within reach.
Construction of the 1,045 m long Deh Cho Bridge, carrying Canada’s Highway 3, commenced in the spring of 2008 under the direction of the DCBC. Phase one of the project involved the erection of a temporary construction bridge and eight permanent concrete piers. Atcon Construction, based in Miramichi, New Brunswick, served as general contractor on the first phase, while Ruskin Construction, based in Prince George, British Columbia, served as the foundation subcontractor. However, an independent review revealed potential project design issues, and the general contractor became insolvent. In 2009 Ruskin was promoted to serve as the prime contractor on the remainder of the project, and Infinity Engineering Group, of Vancouver, British Columbia, was retained to develop a new superstructure design to be compatible with the existing substructure.
Comprising prefabricated segments, the Deh Cho Bridge was
constructed using incremental launching. Chad Amiel, Infinity
Engineering Group, Ltd.
The Deh Cho Bridge’s eight piers are founded on concrete spread footings cast in the bed of the Mackenzie River in the dry, using cofferdams. Each pier comprises a lower solid concrete cone that is reinforced with an outer steel shell to protect it against ice forces, according to a paper on the project written by Matthias Schueller, Ph.D., P.Eng., a principal of Infinity Engineering Group, and Prabhjeet Raj Singh, P.Eng., P.E., a vice president of Infinity, and published in the proceedings of the 2012 Conference of the Transportation Association of Canada, held in Fredericton, New Brunswick. Each cone was topped with an upper steel head comprising a base, two inclined legs, and a tie beam. The steel heads connect to the lower concrete cones via posttensioned high-strength bars, which ensure that the critical connection remains tight and sealed under service loads, the paper says.
By the time Infinity Engineering Group joined the project, the piers were nearly completed, but the superstructure design had not been approved. “Together with the representatives of the Deh Cho Bridge Corporation, we decided to abandon the old design and to redesign the superstructure, inclusive cables, towers, and abutments,” Schueller said in written responses to questions from Civil Engineering online. Soon thereafter it became clear that the DCBC would not be able to deliver the project within budget. As a result, the DCBC signed a concession agreement with the GNWT, which took over as the project manager.
Infinity Engineering faced several challenges related to the new superstructure design: the structure had to be not only economical and compatible with the existing substructure but also had to perform well in the subarctic conditions of the Northwest Territories. The structure had to be capable of accommodating temperature changes while also sustaining longitudinal forces as the result of wind, earthquake, and braking loads. To that end, Infinity Engineering used the failure mechanism concept, a design principle specifically developed for the project to encourage designers to investigate probable failure mechanisms and intentionally define the weakest link along primary load paths, Schueller said.
The Deh Cho Bridge is the first permanent bridge across the
Mackenzie River, the longest river in Canada. Billbradenphoto,
courtesy of Government of the NWT
The concept enabled the team to eliminate two costly expansion joints from the design, leading to the design of a continuous steel truss superstructure over the entire length of the bridge despite the fact that the piers are so stiff that only one could accommodate longitudinally fixed bearings. “The superstructure is longitudinally fixed only at pier four north; [the] other piers and abutments are equipped with sliding bearings,” Schueller said. “However, lock-up devices engage up to five more piers if longitudinal forces—for example wind, braking, earthquake—are dynamically applied.” In the transverse direction, all of the bearings are equipped with guides, Schueller added.
The bridge’s two tallest piers are located on either side of the river’s navigational channel. Each of these piers is topped by a 30 m tall steel A-shaped pylon supported by two spherical bearings, which allow the pylons to move like pendulums in the longitudinal direction, the paper says. Four groups of three steel stay cables extend from the pylons to support the bridge’s 190 m long main span, allowing the depth of the superstructure to remain a consistent 4.5 m over the entire length of the bridge, the paper says.
The superstructure is an adaptation of an “open” steel box girder, comprising two vertical Warren trusses joined by chevron cross frames and wind bracing at the top and bottom cords. Much of the lightweight structure was prefabricated using fast-track methods to maintain the project schedule. Prefabrication was also necessary because of the remote location and harsh weather conditions at the site. On average, one truss segment per week was delivered to the site for a total of 55 segments, the paper says.
The Deh Cho Bridge replaces unreliable ferry service and ice
roads across the Mackenzie River. Billbradenphoto, courtesy of
Government of the NWT
Crews at the site were in a constant battle with Mother Nature. In the spring, all of the temporary construction works had to be removed to allow ice from nearby Great Slave Lake to rush down the Mackenzie River, and in the winter, crews worked as temperatures plunged well below freezing. “Ruskin worked for four and a half years on the project, constructing year-round through adverse conditions,” said Corey Ross, RPF, P.Eng., the project manager of the Deh Cho Bridge for Ruskin, in written responses to questions from Civil Engineering online. “Half of the year was spent essentially in the Arctic dark, and our coldest recorded temperature was minus fifty-nine degrees Celsius.”
To speed construction, engineers designed the superstructure to be compatible with incremental launching. Ruskin and its erection engineering partner Buckland and Taylor, an engineering firm based in North Vancouver, British Columbia, developed two asymmetrical launches to install the truss simultaneously from either side of the river. “This specialized launch hardware was fabricated on the basis of results of a stage-by-stage engineering analysis,” Ross said. The result was “a midair, midwinter connection of the superstructure over the Mackenzie River,” he said. Once the superstructure was completed, an 11.3 m wide and 235 mm thick precast concrete deck was installed.
Decades in the making, the $202-million Deh Cho Bridge opened to traffic in December 2012. For residents of the Northwest Territories, the bridge represents more than just a means of crossing the vast Mackenzie River; it also marks a new chapter filled with opportunities that were never possible before, said Kevin McLeod, the director of the Highways and Marine Services Division of the GNWT, in written responses to questions posed by Civil Engineering online. Schueller agreed. “Important bridges at magnificent locations [like this] are magnets,” he said. “They attract tourism and investment. We hope that the communities will benefit from this development in order to create a better life for generations to come.”