When city officials in Calgary, Alberta, Canada, autonomously selected Santiago Calatrava, FAIA, to design a footbridge in 2008, controversy brewed among residents who lamented both the absence of a design competition and the project’s roughly $25 million price tag. As the first visible investment in pedestrian and bicyclist infrastructure of this magnitude in an “auto-oriented city,” says local ward alderman Druh Farrell, the Peace Bridge and its design “deviated from people’s ideas.”
Though the city council later revised its procurement process to prohibit the design of future projects from being “sole sourced,” she says, the council maintained that “it would be important to have someone of Calatrava’s notoriety” to design the Peace Bridge, which is the first of three new footbridges planned downtown.
Despite continued protests, few residents could deny that the bridge would bring fame to Calgary and the communities—Eau Claire and Hillhurst-Sunnyside—it connected. This past March, an estimated 7,000 people, including Calatrava’s son Micael, gathered to celebrate the opening of the architectural landmark.
Just 5.85 meters high and 8 meters wide, the compact, ruby-red helical bridge stretching across the Bow River stands out from the landscape as well as from Calatrava’s portfolio of soaring, white-concrete, cable-stayed bridges. Site constraints limited the design team to a 7-meter-tall envelope bounded by the river’s flood level below and the flight path for a nearby heliport above. Ecological concerns barred intermediate supports in the water, requiring a bridge type able to span the full 126 meters (413 feet). “A tube was a clear contender,” says Calatrava, the Switzerland-based founder of his eponymous firm, “but there was a danger of creating a tunnel-like bridge.”
Working with technical consultant Stantec, the firm designed a partially enclosed helical bridge structure that acts as a single-span, simply supported truss. The high-strength, welded-steel frame forms two intertwined helixes wrapping an elliptical cylinder. To simplify manufacturing, the purity of the geometry had to be altered: The form of the cylinder was modified to have a curvature based on several circular radii instead of an ellipse’s continuously changing curvature.
The stiff, helical frame serves as the truss web that transfers loads between the truss chords—linear steel members along the bridge’s top and bottom. Transverse strength is provided by two edge beams and the bridge deck—an approximately 110-millimeter-thick concrete deck cast in place on top of a steel deck, which contains a diagrid of 150- to 200-millimeter-tall stiffeners.
Although the helical frame’s welded-steel box sections maintain a constant 300-millimeter-wide-by-250-millimeter-tall cross section, “to keep the bridge as light as possible, there is no ‘typical’ helix component,” Calatrava says. Each box is made from plates of different thicknesses based on the particular stresses experienced at its location. Even with the material optimization, the steel helixes and deck still weigh about 700 tons.
The custom-fabricated steelwork—made by Augescon in Spain using a series of jigs or templates—was shipped in 15-meter lengths to the site. Workers from Norfab Mfg, based in Edmonton, Alberta, fitted, spliced, and welded the entire helical structure on the riverbanks.
In the winter of 2011, hydraulic rams nudged the bridge across the Bow River on temporary platforms. Once the structure cleared the span, construction crews lifted precast-concrete abutments into place. Austria’s GIG Fassaden fabricated what Calatrava calls the glazed “leaves” that fit into the steel frame, forming its “open crystalline skin.” The glass roof partially encloses the bridge and provides some weather protection for users.
While many locals have yet to embrace the Peace Bridge, its worldwide reception has been generally positive. The structure has become a favorite performance space for musicians, who revel in its acoustics.
In the first week after the bridge’s opening, the city recorded 4,400 crossings—89 percent pedestrians, 11 percent bicyclists—in one day; it expects the numbers to increase as the weather warms. Given that Calgary’s downtown road infrastructure has reached capacity, Farrell says, the Peace Bridge provides another pipeline that encourages the practice of “active transportation”—a bridge we all should cross soon.