The Grandview Heights Aquatic Centre’s roof system wasn’t supposed to be wood. The cables that support the gently undulating system topping the Surrey, British Columbia, building were designed as steel, to be anchored into concrete slabs topping V-shaped concrete pylons to form a catenary structure, with wood infill. Then Vancouver-based structural engineering firm Fast + Epp noted that wood alone could support the roof’s tensile loads. This “aha” moment, says HCMA managing partner Darryl Condon, also in Vancouver, prompted the designers to ask, “Why don’t we just use wood? We were interested in wood as the primary element rather than a secondary element. To us, this treated the wood as having more authenticity and strength rather than it being subservient to steel.”
The result is the world’s longest-span timber-catenary roof, supported by what look like ribbons of Douglas fir stretching as long as 188 feet. The cables enable a remarkably thin roof that defines the 95,000-square-foot structure, which opened earlier this year and includes a 50-meter competition pool, waterslide, and a digital rain curtain.
In fact, the thinness of the roof system was more critical than its clear span. A deeper structural system, Condon says, increases the potential for corrosion and condensation buildup due to high humidity of natatoriums. For several years, HCMA—which has designed about a half-dozen of these difficult building types—experimented with thinner roof systems.
The center’s roof is just 18 inches deep, which includes the wood roof deck. (For comparison, the roof system of the nearby Richmond Oval, completed in 2008 and designed by CannonDesign, is about 8 feet deep.) Its slim profile reduces the building’s volume, eases maintenance, and eliminates the need for fire sprinklers in the bays between the glulam cables. Besides its maintenance benefits, a thinner roof is easier to sculpt, says Condon, who employed the catenary to express “a fluid form, something that would be reflective of the idea of water in motion.”
Having never used glulam as cables, HCMA worked closely with Vancouver-based structural engineering firm Fast + Epp, a longtime collaborator. The architects modeled the structure in Autodesk Revit, varying the height of the roof according to the building program: higher for the diving platforms and waterslide, lower for the entrance and changing areas. (The resulting cross slope also allowed the architects to channel rainwater efficiently to galvanized steel leaders along the north side of the building.)
Dozens of papier-mâché models followed, with HCMA settling on a sculpted roof form 380 feet long and 150 feet wide, interrupted only by the central concrete tower that divides the roof into a 188-foot span to the east, and a 120-foot span to the west. (The concrete supports at the building's ends and center make up the difference in total length.)
Theoretically, that central pylon wasn’t necessary. According to Fast + Epp’s calculations, the timber cables could span the entire 300-foot building. The problem was vertical deflections. In the winter, heavy snow—though rare for the coastal city—might collect in the roof’s two concave bowls, weighing them down in the middle but pushing them up at the ends (similar to what happens when a person sits in a hammock or hops up on a slackline.) By adding the mid-span concrete support, deflections were limited to 8 inches, allowing HCMA to specify a glass curtainwall perimeter.
The final roof system is supported by more than 100 glulam cables, each 5 inches wide and 10 inches deep, coupled and anchored into the post-tensioned towers. Completing the roof are two roughly 0.5-inch plywood sheets, a vapor barrier, 4 inches of polyisocyanurate insulation, a 0.5-inch of gypsum board, and a single ply of TPO membrane.
Inside, pearl-colored Tectum panels are hung between the glulam cables, flush with the wood. By altering the length of the cables and the height of the bearing points (which range from more than 70 feet above finished floor to less than 30 feet), HCMA achieved its desired sculptural form using glulam cables with a single radius of 32 feet (down from more than a dozen in earlier iterations), lowering the cost of production significantly, Condon says.
Despite its unusual form, the roof was erected in just 12 days. The glulam cables, split into roughly 60-foot sections (for transportation purposes) were joined with a series of steel pins, whose holes were plugged with wood pegs, and lifted into place.
The real challenge, Condon says, was getting the contractors and local code officials “to accept [an] unconventional structure,” even in progressive Surrey, the province’s second-largest city. “Whenever you try to do something different, you get a lot of resistance,” he says. “There’s a lot of inertia in conventional systems.”
But for a region still largely covered in forest, and where one in every 16 jobs is tied to the forest-products industry, Condon sees great potential in timber structures. He hopes the center will further demonstrate their potential. Architects “need to be more daring with wood,” he says. “We need to challenge ourselves and challenge the industry to push its limits.”