High-Rise Steel Construction Girds The Bow
Structural Design for Western Canada’s Tallest Building
Jan 30, 2009
The Bow, future home of energy company EnCana Corporation, is a diva of a building. Urban planners hope it will revitalize Calgary’s downtown core; EnCana employees look forward to occupying a sustainable building designed specifically to meet their needs. It is The Bow’s structural design however which sets this tall structure apart from other skyscrapers and makes it an architectural and engineering star.
Halcrow Yolles of Toronto are the structural engineers responsible for The Bow’s unique design. Three different structural systems inform the design: steel diagrid for the visual openness of the south-facing atrium, perimeter moment frames to accommodate the strong curvature of the wing tips of the building, and a braced-frame solution for the heavy loads of the north face, all working in concert.
Staff at Halcrow Yolles worked closely with Foster + Partners, the architects for the building, and mechanical engineers Cosentini Associates of New York. The Bow’s form meets three criteria: it must be efficient structurally, assure the sustainability story of the building, and establish a memorable presence in Calgary’s downtown.
Jonathan Hendricks, design principal for The Bow at Halcrow Yolles, explains: “From a structural perspective, the design team was interested in shaping the building with respect to the wind in order to reduce wind load. A curved form sheds wind load which means that the total load on the building including the wind is less than for a rectangular building. Less load means potential savings on the stability structure. Once we had generated a family of optimized forms, we undertook an engineering exercise evaluating the strengths and weaknesses of various structural diagrams.”
Six structural options were explored. After examining a variety of diagrid expressions and perimeter tube systems, a hybrid solution was selected because it yielded the most efficient diagram and addressed the specific needs of each part of the overall form.
The combination of hybrid structure and curved shape has resulted in 20 per cent less steel in the stability structure than would be normal for a steel structure of this shape. Simultaneously, the engineers have delivered a structural solution that meets the client and architects’ goal for a destination edifice.
A glorious solution visually, the hybrid has already proved itself by being adaptable. When the developers and EnCana decided to add 120,000 square feet to the original tower’s total of rentable space, in order to make the building more economic given rising construction costs and the client’s need for more space, the engineers were able to extend the floor plate without changing the structure substantially.
Hendricks explains, “In extending the floor plate, we used the structure that was already there to carry the new gravity loads. We were able to do this without completely closing off the atrium.” The sustainable story of the building has been maintained, the optimal floor plate preserved, and externally the building does not look significantly different.
The Bow is slated to be completed in 2011 at a cost of $1.4 billion. The building’s owners, H&R Reit, have had to refinance the building in light of an economic downturn, but construction continues apace. At its completion, Calgary will have acquired a new identity as an architectural destination thanks to The Bow’s unique steel structure.
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