Pairs of inclined piers will support the new bridge’s three decks over the St. Lawrence River and streamline the look of the bridge. Courtesy of the government of Canada
Designers collaborate closely to develop the reference design for a new bridge across the St. Lawrence River and the St. Lawrence Seaway.
August 12, 2014—The design of a new 3 km long bridge over the St. Lawrence River in Montreal, Canada, features a harmonious blend of aesthetic appeal and functional utility. In fact, members of the project team say that the bridge’s architecture and engineering are so intertwined that it is difficult to determine who exactly conceived specific elements of the proposal.
The new bridge will replace the existing Champlain Bridge across both the Saint Lawrence River and the Saint Lawrence Seaway—an international shipping route that extends between the Atlantic Ocean and the Great Lakes. The existing crossing is a steel truss cantilevered bridge that opened in 1962 and has since served as a vital link, carrying upward of 40 million vehicles between the island of Montreal and the South Shore each year. Now, after more than five decades in service, the bridge has reached the end of its useful life and must be replaced.
The replacement bridge is the focal point of the New Bridge for the St. Lawrence Corridor (NBSLC) project—one of the largest transportation projects under way in North America. In addition to the new St. Lawrence River bridge, the project includes a new Île des Soeurs Bridge and reconstruction and widening of the roads surrounding the two bridges. Infrastructure Canada, the agency responsible for maintaining the nation’s infrastructure, is leading the project, which is expected to cost CAN$3-5 billion (U.S.$2.7-4.5 billion) and is being procured as a public-private partnership (PPP).
In 2012, Infrastructure Canada began developing its business case for the PPP and retained the international professional services firm PricewaterhouseCoopers and the international engineering firm Arup, both of which delivered services through their offices in Montreal, as well as the Vancouver-based global transportation consultancy Steer Davies Gleave. Once this team of experts had made the case for a PPP to be used on the project, Arup was hired as the owner’s engineer, partnering with architecture firms Dissing+Weitling, based in Copenhagen, Denmark, and Provencher Roy Associes Architectes, based in Montreal, to develop a reference design for the new St. Lawrence River bridge, which will have a 125-year design life.
Downtown Montreal will be visible from the bridge’s elevated main
span, which will afford a clearance of 38.5 m over the St. Lawrence
Seaway. Courtesy of the government of Canada
The bridge will be located downstream of the existing Champlain Bridge, which will remain in operation throughout construction. Curving as it crosses the river, the bridge will be visible from downtown Montreal and the city’s famous Mount Royal observation point. Those areas will also be visible from the bridge’s elevated main span over the St. Lawrence Seaway. “You go up quite steeply from the southeastern approach, and then suddenly you have this fantastic view toward Montreal,” explains Poul Ove Jensen, an architect and the director of the bridge department for Dissing+Weitling. “You can see Mount Royal and all of the high-rises in the city’s downtown.”
In addition to affording sweeping views of the city, the 238 m long elevated main span will also serve the practical purpose of providing 38.5 m of clearance for the ocean-going vessels that travel the seaway. “We have to make sure the bridge doesn’t have any impact on shipping and doesn’t disrupt the business of the St. Lawrence Seaway,” says Matt Carter, Ing., P.E., CEng, an associate principal and lead structural engineer on the project for Arup. To further enhance the bridge’s architectural quality and functionality, the main span will be a cable-stayed structure with a single tower. Carter says that the bridge type was selected because the deck can be launched over the seaway within the three-month period that the seaway is closed during the winter. The design calls for 42 box-girder spans in addition to the main span.
Three separate vehicle corridors and a bicycle and pedestrian path will extend the entire length of the bridge. The configuration gives the 60 m wide bridge a distinct appearance and isolates the center corridor expressly for public transportation, including buses and future light-rail. Marc Brazeau, Infrastructure Canada’s director general for the NBSLC project, says the agency wanted to segregate the public transit motorway from the two vehicle decks in order to prevent road salts and other solutions from compromising the rail line’s electrical system. “We really designed it as its own separate environment,” he notes.
Rather than have a pier for each corridor, a series of inclined concrete pier couplets will support the decks. “If you have one column under each deck, you get this concept that is sometimes called a ‘forest of columns,’ and it’s hard to tell which column goes under which bridge,” Carter says. “We looked at it and said, ‘Why not have two columns and some kind of beam or structure on top to make the piers a little more compact?’”
A single tower will support the bridge’s 238 m long main span over
the St. Lawrence Seaway. The shape of the tower is designed as
a continuation of the piers below. Courtesy of the government
This solution streamlines the bridge and reduces the number of foundations from three to two for each pier set, which in turn lessens the amount of foundation work and limits the impact of the foundation construction on the environment. “Visually, the piers are unique and instantly recognizable but still very much grounded in technical achievability,” Carter says. The design calls for the piers to be founded on pad footings on rock; minipiles will prevent the footings from sliding under ice pressure when the river freezes during the winter.
The geometry of the piers will be carried through to the main span tower, which will have two legs extending to approximately 155 m above the river. While a tower with two legs is not unusual, the legs of this tower will be closer together than most—6.5 m in the reference design—to accentuate the bridge’s unconventional proportions. “The tower is shaped to have common features with the piers so that it’s one seamless design,” Carter says. “The tower and piers were never looked at in isolation.”
Once the reference design was completed, the bridge’s key architectural elements—including the cable-stayed main span and inclined piers—were laid out in what is called a definition design, which the project bidders are required to follow. “By defining the shape of the bridge, you know what you’re going to get,” says Doug Balmer, Ing., CEng, an associate principal of Arup and the firm’s project manager for the bridge. “It also takes some uncertainty out of the procurement process because the bidders know what they’re aiming for and can focus on the constructability and innovation in regard to how they will deliver the bridge on time.”
The project schedule is ambitious. Completion of the bridge is anticipated by December 1, 2018—a date established by the Canadian government. Once finished, the bridge is expected to become a defining landmark for Montreal. “Every client these days wants a bridge that when people see it on television for three seconds they immediately recognize it, like the Golden Gate in San Francisco,” Ove Jensen says. “That’s our goal: to end up with a bridge that is unique and recognizable...a bridge that people around the world will associate with Montreal.”