One Bloor West: Engineering Canada’s first supertall structure, with HISTAR®

Outstanding on Toronto’s skyline

Located at 1 Bloor Street West in Toronto, Canada, One Bloor West stands at one of the country’s most prominent intersections, near Toronto’s busiest underground station. For these reasons, the building’s design brief called for an exceptional retail space free of columns within it and at the corners, to capitalise on the high-traffic location. The development team envisioned creating Canada’s tallest building to complement this iconic site.

The distinctive structure is an outstanding addition to Toronto’s skyline, elevating the city’s architectural portfolio. This supertall building (defined as more than 300m high) is primarily a residential tower but also incorporates hotel levels above premium retail and event spaces.

Design optimisation with steel

Designed by internationally renowned architecture firm Foster + Partners, the structure was initially conceived as a nearly 380-metre-tall building with an 18:1 aspect ratio and a steel diagrid exoskeleton. However, certain challenges necessitated modifications.

Various heights, shapes, aspect ratios, and structural systems were studied. While the steel diagrid was initially favoured, it was deemed cost-prohibitive for Toronto’s market. However, a fully concrete system could not achieve the column-free retail space at the base, leading to the adoption of a composite structure.

The final structural design features an 85-storey square tower optimised to reduce wind loads. While most towers in Toronto use conventional concrete systems, including slabs and central cores, One Bloor West’s design transfers loads above the retail levels to large, inclined steel struts resting on composite mega columns composed of concrete and steel at the ground level. This eliminated the need for interior columns or a concrete core in the retail space.

Specially designed steel nodes for tower transfer

A major consideration for the tower transfer was how to detail nodes for members that would have large tension forces in multiple directions as these forces are difficult to achieve with a typical built-up plate due to the combined compression, local bending, and bidirectional tension forces. The deceptively simple solution was to connect all elements to large, solid steel nodes. However, it is challenging to create these sections while ensuring full through-section properties, weldability, and high strength (450MPa).

Engineering experts were able to design nodes that would meet all of the design criteria, yet be practical for fabrication and installation on site. These blocks, hot forged and specially designed and cast, were combined with ArcelorMittal’s HISTAR® ASTM A913 high strength steel (65 ksi, 450 MPa) wide flange sections to create nodes and sections that could be erected fully finished at site. Final nodes and struts weighed between 30-50 tonnes.

ArcelorMittal’s HISTAR® A913 steel satisfied the strength requirements, and the lighter section sizes also reduced the loads on the crane. This grade is a high-strength, low-alloy steel widely used in tall building construction for its excellent mechanical properties and economic benefits. Its exceptional yield strength combined with high ductility allows for thinner and lighter structural elements without compromising safety or performance, reducing material use and increasing efficiency.

Composite mega columns

From the P2 level to level 9, the mega columns utilise a composite design to optimise strength and save space, while simultaneously facilitating connections to the structural steel mega struts. Composite mega columns are defined as vertical structural systems with more than one hot rolled steel section, longitudinal rebar, and ties embedded in concrete. They are subject to significant vertical loads and secondary bending moments. Composite mega columns are a convenient solution in terms of structural behaviour, cost, and constructability for the design of tall buildings - including supertall towers.

Due to the height of One Bloor West, the struts and mega columns needed to be very stiff and also strong enough to resist large loads. Steel and rebar were used to achieve this strength, in addition to high-performance concrete. This included grade 500 rebar, grade 450 structural steel, and 75 MPa self-consolidating concrete. The use of both the HISTAR® hot rolled steel sections and composite mega column design was novel at the time of design. 

HISTAR®’s unique production process ensures consistent quality and better weldability, making it ideal for complex designs such as composite mega columns. Use of these sections allowed for the creation of a light and airy space – a significant feature of the architectural design.

One Bloor West exemplifies what can be achieved when strong engineering design supports and complements bold architecture rooted in a philosophy of form following function.

Project completion is expected in 2028.

Project information

• Toronto
• Canada
• 2028
• Design architect:
  Foster + Partners
• Executive architect:
  CORE Architects
• Original developer:
  Mizrahi
• Structural engineer:
  RJC Engineers
• Steel detailer, fabricator, and erector:
  Walters Group
• Text:
  Constructalia
  ArcelorMittal Europe Communications
  Daniel Snodgrass, RJC Engineers; Marion Charlier, ArcelorMittal; Joe Dardis, ArcelorMittal; Tim Verhey, Walters
• Images:
  © RJC Engineers
  © Christopher Urtz/ArcelorMittal
  © Walters Inc.