Building tall buildings

At 61 storeys each, M City’s twin towers will take a lot of know-how and experience to execute. Site Magazine reached out to some of the key players to get their tricks of the trade. By Steven Barr, with contributions from CORE Architects, RJC, RWDI, Smith + Andersen, Soberman Engineering, and Ellis Don

Increasing concentrations of population around large cities is underlying the development of high-density living at an unprecedented scale. Not just evident in downtown cores, but peripheral urban centres around the GTA and other major Canadian markets are seeing a new wave of mega-towers to meet this demand.

Earlier this year, Urban Capital and Rogers Real Estate Investments Limited launched the first two phases of M City in Mississauga, Ontario. A 10-tower master-planned community on a 15-acre un-serviced “greenfield” site, M City will see the construction of over 6,000 units across 4.3 million square feet in towers that reach sky-high at 60+ storeys.

Unlike development in Toronto’s 19th century urban core, this new territory affords the exciting opportunity to make architecture that is unapologetically big. But with big buildings come big engineering. Indeed, the project meetings at M City tend to reach capacity, filling boardrooms with the design and construction professionals needed to pull this off. As we gear up for construction on the twin 61-storey buildings of Phases 1 & 2, we asked the team to highlight some key challenges in bringing this vision to life.

Sculptural Exterior, Livable Interior
Architects: CORE Architects

The towers’ unique appearance results from the rotation of the floor plates. In each building there are seven typical floor plate shapes (A, B, C, D, E, F and G), beginning with a rectangle (D) and skewing first towards one extreme (G), then skewing back through D to the other extreme (A). The corners of each floor plate shift over 1 m from the plate below.  This is done with short shear walls that ‘walk’ along with the skewing and overlap above and below.

Unlike other sculptural towers that sacrifice the livability of the units to achieve unique shapes, the precise geometric procedures here do not compromise the layouts of the rooms.

Twisted By Gravity
Structural Engineers: Read Jones Christoffersen

The dramatic articulation of M City’s building massing was achieved by tilting end columns back and forth up the height of the tower. This tilting creates a tendency for the building to twist back and forth under its own self-weight (gravity).  

With this in mind, we employed a novel configuration of coupled shear walls, where only two lines of shear walls were used. These wall lines were located one bay in from each end of the tower to maximize the distance between them, which dramatically reduced the building’s tendency to twist under gravity loads. By forcing the lateral stiffness onto two wall lines (as far apart as possible), this twisting effect is mitigated.

As a building’s height and slenderness increase, innovative structural framing is required. Tall slender structures can also become susceptible to dynamic interaction with the wind. In addition to the wind pushing and pulling on the broad faces, it also peels off the corners of the building, creating a swirling wind current known as a “vortex”, which causes the building to sway ever so slightly from side to side. As a result, tall buildings require structural engineering that focuses not only on the strength and stability of the structure, but also on the motions that occupants feel from the interaction with wind.

Slosh Tanks and Building Sway
Wind Engineers: Rowan Williams Davies & Irwin

While this swaying effect would be imperceptible from street-level, occupants of tall buildings may actually feel the motion, which can cause comfort issues. To combat this at M-City, each tower will have two large sloshing damper tanks placed at the upper level mechanical penthouses. These water-filled slosh tanks effectively counteract the dynamic motion created by wind, whereby the building sways in one direction and the water in the tank sloshes in the opposite direction. The energy of the sloshing water is dissipated by drag created as the water passes through a series of screens within the tank.  The dissipated energy has a counteracting effect on the motion created by wind. We conducted scale wind tunnel tests in order to optimize a slosh tank design specific to the M City buildings and their microclimate.

During the detailed design stage we studied not only the impact of wind on the building, but the impact of the building on wind. Tall buildings tend to intercept winds at higher elevations and redirect them to the ground level – referred to as “downwashing flow.” A pedestrian-level wind study will quantify the resultant wind environment against established comfort and safety criteria. For M City, we built a full scale model of Phases 1 & 2, as well as the future build-out of the entire M City master plan, and placed it in the context of the surrounding neighbourhood, all of which was constructed building-by-building in perfect scale size. The model was then fitted with multiple sensors and placed in a wind tunnel, where we used simulated wind scenarios to get a precise picture of the wind behaviour at street-level and at the outdoor amenity areas. With each round of testing, the team tried different configurations to mitigate wind impacts, from changes to the actual building façades to wind screens at grade-level, to ensure the area is comfortable for its intended use.

Stopping Stack Effect
Mechanical & Electrical Engineers: Smith + Andersen

One of the central mechanical concerns in high-rise
developments is stack effect. Stack effect is the movement of air into and out of buildings as a result of air buoyancy due to differences in indoor and outdoor temperatures. In winter, warm air rises to the top of the building, sucking cold are in at the bottom. This effectively pressurizes the top of the building. In summer the reverse occurs: cool air falls and gets pushed out at the bottom of the building, which draws in hot outside air at the top. 

Stack effect is a vicious cycle: in winter, as the warm air rises, residents at the upper levels may feel overheated so they open a window, which pulls in cold air at the lower levels. In turn, residents at the lower levels feel cold and turn up their thermostats, thus forcing more hot air upwards. It’s a condo version of the age-old thermostat battle, and it’s a terribly inefficient use of energy too. 

As buildings get taller, the stack effect gets worse. Residents can physically feel the effects of the pressurized airflow: building entrance doors become impossibly heavy to open; elevator doors struggle to close and stay fully closed; a wind whistles beneath suite entry doors.

To combat stack effect, you must impede airflow from the lower levels up through vertical paths such as elevators, stairs, and mechanical penetrations, and out of openings in the upper half of the building. At M City, we addressed the biggest stack effect contributors first: openings to the outdoors. Often referred to as “pressurizing the lobby,” the design challenge is to make up for air that is escaping further up the building – in part this is handled by designing an HVAC system that supplies more outside air than it exhausts. The M City lobbies impede airflow through the introduction of main entry vestibules with high-performance automatic door hardware, and elevator vestibules that are sealed off from the main lobby. Further, careful attention has been given to the various penetrations in the building envelope – exit stairs, mechanical shafts for ducts and piping, outside-air intakes – and how they should be sealed against air leakage.

Elevators
Elevator Consultant: Soberman Engineering

Tall buildings exist by the grace of elevators. Building access plays a crucial role in the development and feasibility of these structures. There are many factors considered when determining the best solution for vertical traffic flow.  Some factors considered at M City were:

• Separating elevator banks for high rise and low rise floors
• Size (capacity) of the elevator cabs
• Speeds of cars
• Number of cars
• Door sizes and opening speeds.

Naturally these decisions come at a cost – financial costs, lost saleable area – and ultimately will have a major impact on how residents experience life at M City. With buildings that accommodate nearly 800 units each, our key objective was to minimize elevator wait times, and ensure ride comfort. As such, we decided it would be most prudent to split the elevator banks in separate stacked towers (zones) to minimize continuous shafts over the tower’s total height – with dedicated elevators for the ground floor retail, podium levels, lower tower levels, and upper tower levels. We also opted for high-speed elevators and larger capacity cabs. Taken together, the result is reduced wait times that out-perform industry standards typical of other buildings around Toronto and the GTA.

Cranes and Concrete Pumps
Construction Managers: Ellis Don

A project of this magnitude and design complexity will have several key stages that will allow for a successful execution. The general idea is to create a flow state for labour and material so that each trade that works on a typical floor knows exactly what they are doing when they start their day. This building is quite different than a standard high rise due to the changing floor plates; with seven different layouts it has to be ingrained in the trades that they are effectively building seven separate nine-story buildings folded together like a deck of cards. Usually it would take a good crew around three to five floors to gain a full understanding of what they are doing on each given day. With the complexity of M City it will require a greater attention to detail and increased focus on the daily plan in order to develop that flow state.

With all projects higher than 20 floors, the cycling of material to feed the construction can create issues. Getting material to the floor takes time, and the higher the building the more time is needed for material staging. There are ways to counteract this scheduling challenge by fine-tuning the process. One of the ways is to use a concrete pump to feed all of the concrete pours. This frees up the crane to complete lifts around the project and prepare for work on the following day. The pump is set up at street-level and is piped up the building to a placing boom, which can be utilized without occupying the crane. In situations like this the pours can be staggered allowing the crane and the crews to remain busy throughout the day, even during times when the concrete is being placed. Staggering the pours would mean that you would be pouring each floor in two halves. For example, on the same day we can pour the north half slab of the 30th floor and the south half columns of the 29th floor. The forming would then be rotated in between these pours with the column formwork going up to the 30th floor slab and the tables going to the 29th floor. This allows for the formwork trade to work in a way where they do not have to remove all of the walls and fly them down to a staging area on the ground only to bring them back up three days later.

Each tower will have two cranes and a concrete placing boom. Hammerhead cranes and concrete placing booms will climb up the towers to facilitate the top floor; a luffing crane will complete the podium; and a derrick on the roof will remove the hammerhead crane and placing boom when construction is complete. Furthermore, we will utilize concrete pump trucks to complete the podium pours while the tower concrete placing boom can feed the tower levels. This creates more efficiency with the crane time without sacrificing schedule.

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