Structural Solutions Behind TD Terrace

To view the figures and tables associated with this article, please refer to the flipbook above.

TD Terrace is a defining feature of Toronto’s skyline, and its distinct visual identity was brought to life through the collaboration among its designers and constructors. Rising 787 feet (240 meters) above Front Street West in downtown Toronto, the 48-story office tower features a tapered profile, sloping perimeter columns, inlet terrace levels, and a curved steel crown that distinguishes it from the surrounding skyline.

Designed by Adrian Smith + Gordon Gill Architecture (AS+GG), the tower’s sculpted form responds directly to its environment. The building’s main axis orientation is parallel to prevailing winds, and the tapering geometry reduces wind loading on the structure and improves pedestrian comfort at street level. Translating that architectural vision into a buildable structure required a structural system capable of accommodating complex geometry, significant force transfers, and demanding constructability requirements.

The solution was a hybrid system consisting of a reinforced concrete core, approximately 9,500 metric tons of structural steel framing, and steel outriggers. Though the system offers benefits in terms of construction speed and floorplate efficiency, its greatest advantage is flexibility. Structural steel provided the versatility required to realize three of the project’s most challenging architectural features: the continuously sloped perimeter columns, the inlet terrace levels, and the curved crown.

Throughout the project, many of the most complex structural challenges were resolved through close collaboration between RJC Engineers (RJC), steel fabricator Walters Inc., the design team, and the contractor, PCL Construction. The resulting solutions balance architectural intent, fabrication requirements, erection constraints, and long-term structural performance.

The tower’s tapered profile is not simply an architectural gesture. Its form is directly tied to structural performance. The tapered geometry at the top and bottom of the structure helps shed the downward force of the wind away from the ground. By mitigating the wind acceleration at street level, the building’s unique form helps increase pedestrian comfort at grade. The reduced building footprint also provides more spacious landscaped walkways.

To achieve the sculpted silhouette envisioned by AS+GG, perimeter steel columns slope continuously along the full height of the building. Column geometry changes occur approximately every three floors, creating a complex structural condition that required careful coordination among the architect, engineer, and fabricator.

Rather than locating column splices at floor level, the project team positioned them approximately 3.2 feet(1 meter) above the floor plate to improve erection accessibility while maintaining structural efficiency. At each change in column inclination, pass through forces due to the change in geometry are transferred through the steel floor diaphragm and resolved back into the concrete core. The floor beams along these column lines are heavier axial members, and in some locations high-strength steel, with more robust embed plate connections at the core.

Determining the locations of these transitions required extensive coordination early in the design process. Working points were evaluated collaboratively to minimize structural impacts. Splice locations were extensively studied and analyzed, to ensure that any construction tolerances were accounted for in change in inclination to capture the resulting forces. The typical three-tier length of columns were maintained to follow crane lift limitations and optimize fabrication and erection sequencing while preserving the architectural form. The result was a structural system that supports the tower’s distinctive appearance without compromising constructability.

Among the most technically demanding elements of TD Terrace are the inlet floors on the east and west faces of the tower. At these levels, the building steps back to create mid-tower terraces, requiring a 8.2 -foot (2.5-meter) column offset over two floor levels. This geometry introduces significant axial forces into the steel floor diaphragm and ultimately the concrete core.
Two connection details were developed to address these demands. High axial forces of nearly 2250 kips (10 MN) between the floor diaphragm and the concrete core required high strength W-column sections within the floor framing to transfer the axial load to pre-stressed Dywidag rods embedded within the core walls. To transfer force of this magnitude into the core wall, the wall plate connection had to be robust, and initial detailing from Walters required top and bottom flange connections. This resulted in unwanted fixity at the core, and with differential movement as the core shortens and creeps over time, large moments would be induced within the axial members. To overcome this and achieve the same rotational assumptions assumed analytically, the design team developed a true-pin connection.

Detailing the pin and sequencing the Dywidag stressing was an iterative process between RJC and Walters Inc. The pin connection consisted of a post-tensioned Dywidag embed plate with a temporary seat for erection of the floor beam, a three-pronged bracket that interlocked with the beam end, and the 2 inches (50mm) diameter solid steel pin that feed through the assembly. The team worked through connection clearances, the positioning of dead and live stressing ends, and the optimal configuration of the pin assembly. Resolving these details required close coordination between RJC and Walters throughout design, detailing, and fabrication.

Topping the tower is a 148-foot (45-meter) curved architecturally exposed structural steel (AESS) crown fabricated from round hollow structural sections joined with Cast Connex Diablo Bolted Splice connections. The crown houses a 75.5-foot (23-meter) glass atrium, a building maintenance unit with a 39-foot (12-meter) wide operable sliding door, and the mechanical penthouse.

The geometry of this element alone would have made structural steel the only viable material choice. The crown is also home to the lateral outrigger members at the top of the core that help control building motion. These four inclined outriggers engage the perimeter columns to control the lateral drift of the building, a critical function for a tower of this height and slenderness. To prevent precompression of the columns due to core shortening, the outrigger connections were designed to be axially released until they were locked in approximately six months after the core topped out.

Determining where to locate the release points required careful coordination: the top portion of the outriggers is exterior and exposed, while the bottom portion passes through a tight mechanical level with equipment already in place. PCL Construction, Walters Inc., and RJC worked through sequencing options together to identify the approach that was both structurally sound and constructible given the site conditions. BIM modeling by Walters, detailed down to the coupler level, was essential to resolving clashes between the embedded steel sections in the core walls, the reinforcement layout, and the climbing formwork jack locations.

A recurring theme throughout TD Terrace is the degree to which engineering solutions emerged through collaboration. The sloping perimeter columns, inlet floor connections, and outrigger system each required close coordination among architects, engineers, fabricators, and contractors throughout design and construction.

Many of the project’s most complex details were not resolved through conventional design development alone. They evolved through ongoing dialogue between RJC Engineers and Walters Inc., balancing structural performance with fabrication realities and construction sequencing. Whether refining column splice locations, developing true-pin connections, or coordinating outrigger installation, the project team worked collectively to solve challenges that no single discipline could address independently.

The tower’s most recognizable features, such as its sloping perimeter to its inlet terraces and curved crown, are ultimately the result of engineering decisions shaped by technical innovation, constructability considerations, and collaboration across the project team. TD Terrace demonstrates how these factors can work together to transform an ambitious architectural vision into a buildable reality. ■

Cait Mancuso, P.Eng., is a Project Engineer with RJC Engineers in Toronto. She was closely involved in the design and construction of 160 Front Street, helping advance the project’s complex structural steel systems through coordination, analysis and construction. She can be reached at [email protected].