ArcelorMittal's HISTAR® steel optimises The Bow's external diagrid structure

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Architectural design shaped by climate analysis

The Bow is a bold new landmark on the Calgary skyline and equally significant in urban, social, and environmental terms. Its form is shaped by climate analysis: Facing south, the tower curves towards the sun to take advantage of daylight and heat, while maximising the space for offices with views of the Rocky Mountains.

By turning the convex facade to the prevailing wind, the structural loads are minimised, which contributed to reducing the amount of steel used for the inherently efficient diagrid system. The external diagrid structure consists of triangular-shaped steel sections spanning six storeys and helps to visually break down the size of the building.

As the first major development on the east side of Centre Street, an important axis through central Calgary, the 58-storey tower also establishes a connection with its surrounding buildings. At two points it is fused with the city's system of enclosed walkways offering a retreat from the region's harsh winters. Also, the second floor is open to the public and offers shops, restaurants, and cafes.

On the outside, the building's arch shape defines a large landscaped public plaza with a landmark sculpture by Spanish artist Jaume Plensa in its centre.

The Bow is the country's tallest commercial office building outside of Toronto and provides a headquarters for the companies Encana and Cenovus, offering a total of 195 000 m2 above grade and six levels below grade (97 000 m2) with parking spaces for 1375 cars.

The office floors are punctuated by three sky gardens that provide natural ventilation, each of them is six storeys high. In the part of the building that is curved inwards, the glazed facade was pulled forward and a series of atria was created. They run the full height of the tower and act as climatic buffer zones insulating the building. Both the atria and the sky gardens help to significantly reduce the building's energy consumption.

The sky gardens also have a social significance: As each floor has been sized to accomodate a whole business unit, there was the need to promote collaboration across the companies. The three sky gardens that project into the atria at levels 24, 42, and 54 enable vertical access to the office floors and incorporate trees, seating, meeting rooms, catering facilities, and local lift cores. An auditorium can be found on the top floor.

An innovative steel structure: Smaller structural shapes thanks to HISTAR® steel

Given the design and sustainability goals, the geometry of the building, cladding design, interior and exterior aesthetics, and space planning, steel was a natural choice for the building's structure. The use of high strength structural steels offered additional advantages. Smaller vertical load-carrying members allow the use of light-weight long span floor framing, larger column-free areas for more open spaces and higher flexibility and – not to forget – more economical foundations due to its reduced weight.

The building's characteristic perimeter diagrid frame acts as one of its structural systems that make up the hybrid Lateral Force Resisting System (LFRS). It consists of three primary braced frames on the curved south elevation and two on the north elevation, combined with steel moment resisting and braced frames. Three conventional secondary bracing systems augment the lateral stiffness of the tower - one is situated around the main lift core in the centre and two along the core stairs in the building's so called 'fingers'.

Due to its location south of the Bow River, one of the urban guidelines was to keep the building low enough to avoid shadowing the river in the September equinox period. This limitation of height affected its gravity load carrying system. In order to keep the depth of the floor beams low (below 485 mm/19 in.) a network of interior columns was added.

It is Canada’s largest steel-framed building, and a total of 39 000 tonnes of steel and 84 000 m2 of glass were used. ArcelorMittal supplied 4900 tonnes of W14 x 16 x 145-730 sections, of which 3320 tonnes were provided in HISTAR® grade (ASTM A913-65).

The raft slab for the foundation was the largest single concrete pour in Canadian construction history. The foundations are 3m thick and contain 14 000 m3 of concrete. 400 000 m3 of land was excavated.

Sustainability as a main goal

Pursuing green goals was a major issue for the building's design. Its structure, as well as its service life, is oriented at reducing the building's environmental footprint.

The building’s form deflects the prevailing winds, allowing for a lighter structure and thus reducing the material employed and the foundations.

The solar heat collected in the atrium is redistributed throughout the year by means of extraction during winter and heat exchange during summer, reducing the load on the mechanical systems.

Large glazed areas reduce the need for artificial lighting. The building is provided with a heat redistribution system, and ventilation is displaced via a raised floor.