The One Bloor West – Modular Composite Columns project, delivered by Walters Group in collaboration with RJC Engineers, was recognised with the constructsteel Innovation of the Year Award 2025. The project demonstrates how advanced steel solutions can address complex architectural and structural challenges in supertall construction.
In this expert Q&A, Cora Pulnicki, MEng, P.Eng., Project Engineer at RJC Engineers, shares insights into the role of steel in enabling the project’s design, the development of the modular composite column system, and the innovations that supported its delivery.
What were the key structural challenges in delivering the design vision for One Bloor West?
One of the primary challenges was the creation of the “Urban Room,” a massive column-free retail volume at the base. Because the building sits on a constrained 57m x 57m urban corner, the central reinforced concrete core could not extend to the ground. This necessitated a total “load-path transformation” where the gravity and lateral loads of the 85-storey tower had to be transferred from the core to the perimeter at the lower levels. These constraints led to the exploration of non-traditional systems capable of redistributing loads efficiently while maintaining architectural intent.
How did steel enable the transition from architectural concept to a feasible structural solution?
Steel played a critical role in enabling the transfer of loads from the upper concrete tower to the base. While the upper levels are primarily conventional reinforced concrete, the lower levels transition into a composite system where steel diagonals and mega-columns redistribute loads to the perimeter. This approach allowed the project team to balance architectural ambition with cost constraints, using steel strategically where it delivers the greatest structural value.
Steel provided the high strength-to-weight ratio required for the hybrid transfer system. While earlier design iterations featured a visible exterior exoskeleton, the final structure utilizes eight massive perimeter mega-columns and an internal network of steel braces. Between Levels 2 and 8, we engineered eight-storey-deep steel transfer trusses. These trusses act as the structural “bridge,” collecting loads from the interior residential core and redirecting them to the mega-columns, allowing the ground floor to remain completely open.
Can you explain the role of the composite mega-columns in the building’s performance?
The eight perimeter mega-columns are fundamental to the building’s structural system. They concentrate the gravity loads of the 85-storey tower, ensuring that even under significant wind-induced overturning, the structure avoids uplift and tension in the columns. These composite columns combine high-strength steel sections with reinforced concrete, achieving very high compressive capacity while maintaining relatively compact profiles.
The eight mega-columns are the primary gravity and lateral load-bearing elements. These are Modular Composite Columns (MCC) that combine high-strength steel (65 ksi) sections and high-performance concrete. By using HSS, we achieved an extremely high compressive capacity within a compact footprint of only 1.2m x 2.2m at the base. This maximized the leasable floor area while ensuring the tower remains stiff enough to resist Toronto’s significant wind-induced overturning forces without experiencing uplift or tension.
What innovations in steel design and fabrication were required for this project?
Several innovations were necessary, particularly in the design and fabrication of composite columns and complex node connections. The project used high-strength steel (65 ksi) combined with dense reinforcement to achieve the required performance. Fabrication processes were adapted to integrate rebar within steel fabrication shops, enabling prefabricated sections with tight tolerances. Additionally, solid steel nodes were developed to handle multi-directional forces, ensuring reliable load transfer at critical junctions.
How does the structural use of steel contribute to both performance and commercial value?
Beyond enabling the “Urban Room,” the architectural demand for completely unobstructed corner views prohibited the use of traditional corner columns. This requirement necessitated the use of steel hanger columns in the residential tiers to suspend the floor slabs, which allows for incredibly slender perimeter supports and provides tenants with panoramic floor-to-ceiling views. Architecturally, the building features massing breaks every 18 storeys: these are not just aesthetic because they serve to disrupt wind vortex shedding, reducing wind loads by up to 10 percent. Finally, the use of Electric Arc Furnace steel significantly lowered the project’s embodied carbon compared to buildings of this height with alternative materials.
This project demonstrates how targeted use of steel within a hybrid structural system can unlock both architectural ambition and engineering performance in supertall construction.
This project demonstrates how targeted use of steel within a hybrid structural system can unlock both architectural ambition and engineering performance in supertall construction.