TELUS Sky is a 59-storey, LEED Platinum designed, mixed-use tower located in Downtown Calgary, Alberta. Totalling 750,000 square feet, the TELUS Sky building provides open office space in the lower portion of the building and evolves to incorporate residential occupancy as the tower ascends. The mixed-use typology required a seamless structural unity between the two occupancies, while maintaining functional spaces for each typology. As the building rises, the façade steps back on opposing sides to form a skewed floor plan maximizing the glazed façade at the residential levels without compromising the column-free office spaces below. The massing of the building necessitated a parametric load bearing structure which follows the curving lines of the building facade, which also produced significant unbalanced forces generated by the complex overall shape. The radially symmetric massing and parametric columns induce a dramatic torsional force on the building, which dictated a torsionally robust core structure. Two lateral load resisting systems were utilized and stacked above each other to resolve the permanent gravity leaning and prevalent wind loads on the broad faces of the building. Below grade, the mat foundation rests below one of the deepest excavations in the city.
A unique lateral load resisting system was implemented linking the mixed-use functionality of the building. The large, centralized core walls which enclosed the elevator banks within the office spaces, transition to a hybrid core and wing wall system in the residential upper half of the structure to fit geometrically within the more narrow residential floor plates. A dedicated residential elevator bank continues up the center of the building for the full height of the tower. On the upper levels, concrete wing walls located strategically between unit partitions connect to the central residential core. The outrigger walls are supported vertically by columns beneath. Within the office levels, dedicated office elevator banks supplement the wing wall system.
Due to the nature of the massing, and to maintain functionality of the office space, the core walls within the upper half of the office massing are eccentric to the floor plan. The inflection of the parametric columns within the middle third of the tower concentrate gravity induced lateral loads within these eccentric core wall levels; this eccentricity proved particularly complex to ensure sufficient torsional stability and stiffness was maintained in the lateral system. The wing wall and core wall system together provide both capacity for the wind and unbalanced gravity lateral loading which generates a substantial leaning and torsional force on the core. The permanent torsional and leaning roof displacements were limited to less than 125mm as a function of the stiffness achieved by the dual system. The lateral loads induced by the parametric façade columns are resolved within the floor diaphragms via rebar drag lines to resolve the in plane stresses back to the core structure.
The outrigger columns supporting the wing walls resist significant axial and tensile loads at various positions over the height of the column. The tensile loads are ultimately offset by the gravity loads within the office and are in compression at the base of the tower. The entirety of the outrigger column load is transferred beneath the ground floor by a series of transfer walls 8m in depth. Beneath these walls, high strength structural steel columns carry the full weight of the building. Concrete encasing the high strength steel mega columns supports the remaining parkade slab structure down to the foundation. For the remaining non-outrigger columns, 110mPa concrete was used to limit the cross section of the columns below ground level to maximize the functionality of the parking.
The effective depth of the lateral force resisting system is maximized in the slender upper half of the tower by introducing a wing wall system in lieu of a conventional central core system. By engaging the full depth of the building, the stiffness of the system was optimized thereby reducing the wind load demands due to vortex shedding. This, coupled with the continuous load path of a uniform column grid up the full height of the tower, maximized the outrigger stiffness of the system. Below the outrigger system, within the office space, the coupled core wall system increases the torsional resistance where the torsional forces induced by the walking columns are the greatest. The continuity of the load bearing columns limited costly load transfer structure to only a few locations where dictated by the massing of the building. Utilizing high strength concrete for heavily loaded columns gained considerable usable floor area and limited the necessity to incorporate extensive hybrid structural steel and concrete elements which would impede the construction schedule significantly. The entirety of the structure, although complex in geometry, was almost entirely done with cast in place and conventionally reinforced concrete as was typical for the local market.
The most fundamental aspect of the TELUS Sky project was to optimize the functionality of a building with two distinct building typologies. The office levels on the lower half of the building necessitate an open concept and rectangular floor plans to enhance the use of the space. As the building rises, the floor plates slowly reduce in size and pixelate providing opportunities for offices with small balconies and terraces, and ultimately results in smaller residential floor plates with balconies above. The structure was designed to unify the two distinct elements – working and living – without loss of efficiency, functionality, or aesthetic continuity. By creating one structure that can flawlessly integrate the two uses, TELUS Sky creates a more varied and walkable city center in Downtown Calgary; therefore, providing relief on transportation systems and having a major impact on reducing pollution from vehicle emissions.