March 12, 2010

GREAT LAKES DOME COMPANY

Monolithic dome design starts with an airform, then foam, rebar and finally shotcrete to create a structure 15 per cent less expensive.

FEATURE | Concrete/Masonry

Dome, sweet dome: going back to the future

PETER KENTER

correspondent

Dome-shaped structures have permeated the North American landscape for millennia, from igloos in the north to adobe huts in the American southwest and now a Canadian company is translating them into monolithic concrete structures.

The Great Lakes Dome Company, with headquarters in Southampton, Ont., was founded in 2004 by Collin MacLeod and Sunny Cushnie, tradesmen in search of a more efficient way to erect buildings. The company has more than a dozen dome projects under its belt, most of them residential. However, the technology used to build the domes allows for the construction of buildings up to 1,000 feet in diameter.

“We’ve built domes from Halifax to Lake Erie,” says Cushnie. “It’s the best building technology we’ve found and we’ve enjoyed perfecting the art, building quality small structures first, then gradually moving up in size.”

Cushnie lives in a double-dome structure comprised of two overlapping sections built in 2004. The larger dome is 40 feet in diameter and the smaller is 24 feet across.

The company’s first commercial project was the Toronto EcoShell, a concrete egg built for Urbancorp that same year.

The construction process for a monolithic dome is virtually identical, regardless of dome diameter.

“We begin by creating a concrete ring foundation, slab on-grade, reinforced with steel rebar,” says Cushnie. “At the same time we embed vertical steel bars in the ring beam footing and these are later attached to the steel reinforcing of the dome itself.”

The shape of the dome is determined by an inflatable air-form made of PVC and nylon, which is custom-sewn to architect’s specifications. While primarily dome-shaped, the airform can be designed with rectilinear planes for doors, windows or dormers. The airform may include one or more connecting domes.

The airform is attached to the concrete base with Tapcon screws, then inflated using fans to create the shape of the dome. The inflated structure is left in place for about a day, and inspected for flaws.

“After this, all construction takes place inside,” says Cushnie. “The first layer is insulation, a skin of polyurethane foam, which we spray on the interior of the airform. It’s a seamless layer of insulation without any thermal bridging that can achieve R-60 or higher. At this point we imbed specially designed rebar hangers in the foam, then apply a second layer of foam. Once the foam has hardened, we hang the steel rebar in a vertical and horizontal lay-out pattern specified by the structural engineer.”

At this stage wiring, conduits and mechanical systems are installed.

“We then spray the inside of the structure with shotcrete, a special spray mix of concrete, which has the highest load capacity of any concrete mixture without additives,” says Cushnie. “We apply shotcrete from three to six inches thick — anywhere up to a foot — embedding the rebar and then we allow it to cure.”

The airform is cut away so windows and doors can be installed. The exterior of the dome can then be finished with a number of materials, including stucco.

“The result is a monolithic, steel-reinforced, concrete dome, built without any posts or pillars at a construction cost ranging from five to 15 per cent cheaper than similar-sized conventional construction,” Cushnie says. “Once erected, a dome will last centuries with minimal maintenance.”

While the construction schedule can vary, depending on dome size, Cushnie says a dome 55 feet in diameter can be completed in about six weeks, with a maximum crew of five.

The company is now setting its sights on larger structures like garages, storage buildings, churches, schools and ice rinks. The company has made presentations to various municipal councils, including that of Port Colborne, in hopes of landing a major municipal contract.

“We’re currently working with the Bluewater District School Board hoping to promote the use of a monolithic dome for a kindergarten class,” says Cushnie. “With monolithic domes, there are no supports, so the teachers can enjoy clear sight lines to keep track of students at all times.”

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