While the Bow, an iconic new office tower being erected in the eastern portion of downtown Calgary, has turned heads throughout its construction, its unique form has now garnered professional acclaim as well.
It was recently awarded the Canadian Institute of Steel Construction’s National Steel Design Award for engineering. The biennial awards recognize engineers, architects and other building professionals who use steel in innovative ways in their works.
The Bow is the first building in North America to use a triangular diagrid system in the construction of a curved building. The crescent-shaped tower is being built around a series of six-storey-tall equilateral triangles, which are linked to create a perimeter. The triangles have been positioned in such a way as to match the building’s curves on its north and south facing sides.
By building in this way, architects Foster & Partners and Zeidler Partnership Architects and structural engineer Halcrow Yolles are enabling the building to meet two key objectives.
First, by creating an external structural system, the team has managed to maximize the interior space, effectively creating more floor space than in a more conventionally-constructed site. The crescent-shaped building also allows for more offices along the building’s perimeter, which allows for more natural light throughout more of the building.
Second, the system used actually requires considerably less steel than buildings that have been built in a more traditional manner.
Zeidler Partnership Architects managing partner Stephen Carruthers noted that there are other benefits to the building’s curved form.
“The curve had its own rationale,” he said. “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.”
Given its placement in the east side of Calgary’s downtown core, the building will also make use of the natural surroundings to ensure it provides as much of a scenic view for those working inside the building as it does to passersby who take in its uncommon shape.
“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,” Carruthers said.
The correct orientation of the triangles used in the diagrid system was a necessity, with Carruthers noting that each steel piece had to align perfectly at very specific angles in order to make it work properly.
“What would be a straightforward rectilinear geometry in a conventional office tower suddenly becomes very subtle geometry,” he said. “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.’”
The Bow will include a trio of ‘sky gardens,’ multi-storey common spaces with amenities such as snack shops and copious seating. Because the sky gardens will stretch multiple storeys, they will even be able to accommodate full-sized trees, which will provide an indoor oasis for those inside the Bow and will be visible from the exterior.
“All of these features combined will ensure that The Bow will become Calgary’s premier landmark,” Carruthers said. “It is going to be spectacular.”
