Steel Awards - Engineering

Creating western Canada's tallest building


Owner: H&R Real Estate Investment Trust/Matthews Southwest
Architect: Foster + Partners/Zeidler Partnership Architects
CISC Engineer: Halcrow Yolles
CISC Fabricator/Detailer/Erector: Supreme Group/Walters Group Joint Venture
General Contractor: Ledcor Construction Ltd. (construction manager)

The Bow in downtown Calgary is different from other buildings in North America in its application of a triangular steel diagrid system to a curved building. Not only will it be the tallest in western Canada when completed, the diagrid-a design composed of six-storey high diagonal elements-creates a perimeter frame of linked equilateral triangles curved to match the bow of the building on the north and south faces.

The external structural system frees up more floor space than a traditional building, and the diagrid design also significantly reduces the amount of steel required compared to a conventional structure. The crescent-shaped floor plan increases the number of perimeter offices that are possible and improves access to natural light.

"The curve had its own rationale," says Stephen Carruthers, managing partner, western Canada, Zeidler Partnership Architects. "It also decreases the wind resistance that would be associated with a rectangular building of the same size. It's an aerodynamic shape that allows the wind to slip around the curve, much like the wing of an airplane."

Also, the building's curve, he says, is intended to catch the rays of the sun. "It orients the building to the southwest to capture maximum sun and gives occupants a more direct view of the mountains."

The combination of the crescent shape and diagrid system necessitated a faceted curtain wall solution, Carruthers says. "What would be a straightforward rectilinear geometry in a conventional office tower suddenly becomes very subtle geometry. Because of the curvature, each steel member of the diagrid had to connect into a node at very precise angles and with very exacting tolerances."

And did it?

"It was a very tense moment when the first node was set into place," Carruthers says. "We were all very anxious that it should fit perfectly. The first one dropped into place beautifully. Everyone heaved a sigh of relief. From that point on, we said, ‘This is going to go well.'"

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