Project Description
Iconic Calgary office tower is unique in North America
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The Bow, the iconic crescent shaped office tower rising on the east side of Calgary’s downtown core, has provided a spectacular show for Calgarians during its construction. Spectators might not have realized they are also witnessing a North American first.
The Bow is unique on the continent in its application of a triangular steel diagrid system to a curved building. The diagrid, 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.
“The curve is also intended to be a sun catcher. 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.
“It was a very tense moment when the first node was set into place. 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.’”
Another unique aspect of The Bow is the series of three multi-storey “sky gardens” located at each of the elevator transition floors. The sky gardens provide all the common use facilities such as copy shops, coffee and snack kiosks, and generous seating areas.
“The sky gardens will become like village squares where employees can sit under trees, have coffee with their colleagues and exchange ideas,” Carruthers says. “They will be quite grand. The floor structure allowed us to put tree pits into the floors so we can plant full size trees. These ‘forests’ were enabled by the steel structure and will be visible as you look up at the building, particularly at night when the trees are illuminated.
“All of these features combined will ensure that The Bow will become Calgary’s premier landmark. It is going to be spectacular.”
Project Team
Owner: H & R REIT
Architect: Foster + Partners in collaboration with Zeidler Partnership Architects
Structural Engineer: Halcrow Yolles
Project Manager / General Contractor: Ledcor Construction Ltd.
Fabricator: Supreme Steel in joint venture with Walters Group
Detailer: Supreme Steel in joint venture with Walters Group
Erector: Supreme Steel in joint venture with Walters Group
