Combining simple materials like glass and steel into fascinating modernist silhouettes is British starchitect Norman Foster’s eloquent hallmark. His works are often an amalgam of sleek, modern, and high-tech designs. Norman Foster founded Foster + Partners, one of the world’s most prolific firms. The firm has completed projects all over the world and has continued to create innovative works of high-tech architecture that incorporate sustainable design. One such work is The Bow, located in Calgary, Canada‘s southernmost city.
The bow is a 58-story crescent-shaped office tower completed in 2012 as a shared headquarters for energy companies EnCana (now Ovintiv) and Cenovus. This building sits on the confluence of the rivers Bow and Elbow. It is a prominent feature of Calgary’s skyline. With an overall height of 236 meters, it is the city’s second-tallest structure and the third tallest structure in Canada outside of Toronto.
The project is notable from an urban, social, and environmental standpoint as the tower’s public base is filled with cafes, shops, and restaurants and extends into a bounteous landscaped plaza, while the office floors accentuate three six-story sky gardens that encourage natural ventilation and help in significantly reducing energy use.
A tall order | The Bow
In 2005, EnCana Corporation approached Foster + Partners for the design of their new headquarters in Calgary. The brief included an area of 186,000 square meters of office accommodation, along with profuse retail and public space. The local planning authorities viewed this project as a first magnanimous marker in the master plan to develop a new zone in the city along with meeting the sustainable goals.
This project would serve as a catalyst for the city’s growth into one of Canada’s largest, as well as a draw for large corporations, particularly those in the global energy industry.
Concept and design approach
Foster + Partners designed the structure in collaboration with Calgary-based Zeidler Architects. After working on hundreds of potential layouts with design reviews from Sir Foster and clients, the design team had agreed on a final concept of a crescent or the curved shape building. The crescent shape was chosen because it made the best use of the available site space, provided better perimeter accommodation, and shed wind load more effectively than a rectangular building.
The floor plate design is a fusion of the ‘outside-in’ approach which adapts the design to reflect external factors such as wind, sun, site, and views, and an ‘inside-out’ approach accounting for the demands arising from the future occupants.
The organizational structure of the companies was taken into account when designing the spaces. Encana and Cenovus typically work in divisions of 200 people, with each division housing over three floors and each floor plate sized to accommodate 70 people per division. Vertically, it is divided into six-level multiples within three segments. There are 58 stories, the top two of which are reserved for services.
The entrance to the bow is from the south-facing plaza opening up into a towering light-filled atrium. The entrance adjoins a six-story lobby containing reception, elevators, and escalators leading to the public level 2. Furthermore, level 2 includes public spaces, retail spaces, and cafes, as well as access to the Plus 15 Network from two points. Plus 15 Network is Calgary’s weather-defying enclosed network of pedestrian walkways, which covers almost 14.4 km of the city. At the end of this vital pedestrian link, sits the Bow.
The elevator system is the key to efficient movement around the building. Express elevators stop at levels 24 and 42. These levels contain two floors of restaurants, meeting, and recreational spaces, linked by a massive staircase. These spaces are housed in and around by signature sky gardens. After the express elevators drop the staff off in the sky lobbies, local elevators take people to their floors.
Every four stories of office space have a view of one of the six-story-high sky gardens. All office floors have corridors running right from the northern to the southern facades with rooms on either side. The internal hallways have natural lighting.
Structural aspects | The Bow
Nigel Dancey, a senior partner at Foster + Partners said: “Every aspect, from the raised floors to the diagrid structure, is designed to be highly efficient,” while talking about the structural integrity of the skyscraper.
The bow is the first building in North America to integrate a version of a diagrid system in its structural frame. A hybrid diagrid perimeter system has been used due to the curvatures on the floor plate. The tower was braced in this system by three different diagrid faces which are affixed together by connecting elements to form a highly functional closed-perimeter structure. The primary truss diagrid tube frames are six-story high bays with diagonal braces and vertical columns installed on the perimeter’s curved northeast and northwest sides.
A braced tubular frame connects the northeast and northwest primary diagrid frames in the north, at the central core. These braced faces join and run throughout the core of the structure with a string of secondary braced frames that secure the structural frame in place, between the elevators and stairs on the northern side.
The atrium wall diagrid is located on the south side and is similar to a six-story diagonal grid. One drawback is, it lacks vertical columns and radically spans throughout the exterior face of the atrium. Drag struts at each end connect it to the building’s floors. A chain of rigid-tube moment frames connects the three primary diagrid frames and expand from the “fingertips” to either side of the atrium wall.
When viewed from the inside, the atrium diagrid consists of refined triangular sections that lower the visual impact of the framing. Columns were added to the floor plate in the interiors to curb the depths of the beams. All in all, the tower is a trussed tube-type structure.
Form and facade of the tower
The building’s form was influenced by climate and organizational analysis. The tower faces south, curving towards the sun to maximize daylight and heat while maximizing the perimeter for cellular offices with views of the Rocky Mountains. The amount of steel required for the efficient diagrid system is minimized by making the convex facade of the building face the prevailing wind direction which would lower the structural loading. Each triangulated sectional band of the structure accounts for up to six stories which help in visually breaking down the overall scale of the building.
Wonderland sculpture: The Wonderland sculpture is a 12-meter tall public art installation piece made by the Spanish sculptor, Jaume Plensa. It is a steel wire mesh sculpture of a little girl’s head that stands at the entrance to the south plaza. The sculpture’s goal is to inspire anyone who sees it as a representation of our bodies’ architecture, the palace of our dreams. The sculpture truthfully showcases the dreams and hopes of the young people and airs the fresh youthful energy of the tenants.
Sky Gardens: The gardens in the Bow are each six stories high. They are filled with fresh air, light, plants, and mature trees. These gardens make the building more green and act as the lungs of the building. The gardens channel movement in the most efficient way, act as social spaces, and enhance interiors and exterior aesthetics along with stunning panoramic views of the Rocky Mountains. It is an unprecedented work environment – a break from the traditional office setting.
Public Space: The Bow contains a highly pervious public level with a new hub of shops, restaurants, and cafes. On the base level, the arc of the building wraps a beautiful south-facing plaza warmed by sunshine, triangular entrance canopies inviting visitors inside.
Sustainability | The Bow
Sustainability is incorporated into every aspect of the building, from its form and structure to its details. A raised floor system is built-in to allow optimum control over the heating of the building efficiently by localizing the climate control. The atria provide several sustainable strategies that help reduce energy consumption. The absorbed solar heat in the atrium space is utilized for heat redistribution throughout the year that reduces energy consumption by 30%.
Because of the large glazed areas, artificial lighting is not required. Furthermore, the sky gardens induce natural ventilation, bring oxygen-producing plants to the building, and save energy. Ergo, the various sustainable strategies work together complimentarily.
The bow has been a bold symbol for the city and one of the esteemed works of Norman Foster. The bow has been regarded as one of the world’s most spectacular corporate buildings by Emporis and has won the National Steel Design Award in 2012. It was also in the Azure Magazine’s Top 10 Architectural Projects of 2012.
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Fosterandpartners.com. n.d. The Bow | Foster + Partners. [online] Available at: <https://www.fosterandpartners.com/projects/the-bow/> [Accessed 22 May 2021].
Designbuild-network.com. n.d. The Bow, Calgary, Alberta – Verdict Designbuild. [online] Available at: <https://www.designbuild-network.com/projects/the-bow-calgary-alberta/> [Accessed 22 May 2021].