Canadian Consulting Engineer

Growth Spurt – Toronto’s new Skyscrapers

The sky above Toronto is scraped by an array of instantly recognizable buildings. First Canadian Place, at 72 storeys and 335 metres, was once counted as the tallest structure outside Chicago and New York. And no local postcard would be...

December 1, 2013   By John G. Smith

The sky above Toronto is scraped by an array of instantly recognizable buildings. First Canadian Place, at 72 storeys and 335 metres, was once counted as the tallest structure outside Chicago and New York. And no local postcard would be complete without the punctuation of the 553-metre CN tower, described by the American Society of Civil Engineers as one of the Seven Wonders of the Modern World.

Now the city’s skyline is in the midst of another growth spurt.

Toronto has more tall structures under construction than any other city in the western hemisphere. These days, however, residential condominiums are the signature projects, whereas it was banking towers that dominated tall orders in the late 1960s and 70s. Recently completed near the bottom of Bay Street is the Trump International Hotel and Tower, with 65 floors and measuring 277 metres at the tip of its spire. It will be joined by the 272-metre, 75-floor Aura tower at Yonge and Gerrard streets, Tridel’s 224-metre Ten York project, and Great Gulf’s 257-metre One Bloor. The dramatic curve of the L-Tower will arch skyward by 205 metres when it’s complete. And by 2015, the surrounding skyline will boast 44 buildings taller than 150 metres, up from 13 in 2005, according to the Council on Tall Buildings and Urban Habitat (CTBUH).

Theatre impresario and developer David Mirvish hopes to reach even higher with a trio of condos along King Street West designed by Canada’s famed architectural son Frank Gehry. With levels skewed like the individual blocks in a massive game of Jenga, each tower would be more than 80 storeys high. Such high-density in the theatre district has come up against resistance from city planners, but if it is approved Gehry’s creation will be what CTBUH calls “supertall.”

Towering heights represent exciting prospects for engineering teams. “Designing skyscrapers is a positive experience because you know that once the building is complete, you will have contributed to the skyline of downtown Toronto,” says Balázs Farkas, P.Eng., principal of mechanical engineering at Hidi Rae Consulting Engineers, a firm that worked on Toronto’s Trump project.

Structural engineering for height

Advances in engineering have made such projects possible in the first place. Structural engineer Fazlur Khan focused the eyes of the world ever skyward in the 1960s when he first began to replace steel frames with his “framed tubes” – a series of interconnecting exterior columns that could stand firm against strong winds at high heights. The various tubes and buttresses which followed pushed higher yet. Concrete became stronger, and now the world has three buildings soaring more than 500 metres.

Still, the demand for ever-taller structures represents no small feat, even in a city like Toronto where there is a healthy layer of low-lying shale bedrock for the footings.

Engineers need to control the shrinkage, creep and elastic shortening of structural elements that shoulder the burden of any tall building’s added weight, says Jeff Stephenson, P.Eng., managing principal of structural engineering at Halsall Associates.

High-strength concrete and steel, along with thin post-tensioned slabs, can all help to shed unwanted structural mass while increasing strengths. A typical 10-storey office building might be made with 30 MPa concrete, but Toronto’s L-Tower (structural engineers are Jablonsky, Ast & Partners) features 55 MPa concrete from B3 up to the 16th floor, with the strengths decreasing in 5 MPa increments until the 36th floor, after which 25 MPa concrete is used.

There are seismic design requirements to consider, as well as wind loads and the tendency of tall buildings to sway. Excess amplitude will introduce stress on the structure, while acceleration can be discomforting for the occupants inside. The latter issue may be less noticeable when walking around an office, but it is decidedly unsettling for condo residents who are lying in bed.

While shorter structures can be stiffened around a central elevator and stairs, taller buildings require approaches such as a “tube-in-tube” concept with a frame extending around the perimeter. “They get even more sophisticated than that where they start to link that exterior tube to the interior tube with outriggers,” Stephenson says.

The motion that remains can be offset through the help of a tuned mass damper, such as a massive water tank that acts like the end of a pendulum. “When the building moves, it [the damper] lags behind the building, and then you’re connecting that to some sort of shock absorber or baffles to control the flow of the water,” Stephenson explains. “It effectively pulls the building back in the opposite direction.”

Ensuring a building stands tall and still is just the beginning. The valuable space inside must be maximized despite the need for added strength and features like extra elevators. “The columns, walls and everything else gets much larger at the lower levels,” Stephenson says, which means that “the amount of floor space or rentable space left behind gets smaller.”

Staying cool and quiet — mechanical design

“The biggest challenge from an HVAC perspective is dealing with the stack effect, especially with heights that reach above 60 to 65 stories,” Farkas says. “If a building enclosure is insufficiently sealed, an exchange of air will take place between the indoor conditioned space and the outdoor environment. This stack effect in combination with high winds will create havoc.” If the problem is left unaddressed, during the heating season air would surge through the lobby, up the elevator shaft and into the surrounding suites. “The lobby is a critical area where you can enable or disable the stack effect,” Farkas says.

The solution depends on what type of tower it is. Revolving exterior doors work well in commercial settings, but developers of residential buildings tend to prefer multiple doors and an elevator that is separated from the lobby. Vestibules and weather-sealed doors can help to seal the elevator machine rooms at the top of a building, containing any rising air.

Inside the suites, exhaust systems for range hoods, washrooms and clothes dryers must be strong enough to feed air to the outside. “The wind pressure at the top of these tall buildings is so high that without proper control it can actually overcome the exhaust fans,” Farkas says.

The floor-to-ceiling glass windows that are characteristic of these condominium towers bring their challenges for the cooling systems. The highest floors are less likely to be overshadowed by surrounding buildings, so the glazing is exposed to high solar gains. Window coverings help; however, the cooling systems must be designed to handle peak loads.

Then, in a large building that has mixed uses there’s the need to isolate the mechanical systems. In the Trump project, the emergency generator was installed on the 32nd floor to allow the hotel to run independently of the residential units above it. “Large silencers were installed in the generator room to provide proper acoustic isolation and silencing for the generator intake and exhaust,” Farkas explains. Features such as a floating floor, an acoustically insulated ceiling and walls, and mechanical services suspended from the slab above using spring isolators, helped to further reduce the noise level.

Farkas is confident that developers will continue to push residential towers ever higher. When they do, he expects that residential projects will have to embrace features traditionally reserved for commercial buildings. “The use of a mid-level mechanical room, which was used in the Trump tower, allows developers to consider staged occupancy, where the time lapse between the first and last occupancy dates might evolve from months to years. De
pending on market conditions, a developer with plans for a 150-storey tower could build the first 75 storeys, place a mechanical room at that level, occupy the first part of the building, and resume construction at a later date,” he says.

The Council on Tall Buildings and Urban Habitat is equally as optimistic that developers will continue to scrape the sky. “The need to create efficient, high-density districts for people to live and work is pushing skylines higher,” the group concludes in a recent annual report. “There is no evidence that those factors will subside any time soon.”cce

John G. Smith is the president of WordSmith Media in Ajax, Ont.

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Sidebar

Aura

By Tibor Kokai, Ph.D., P.Eng.

Aura is the tallest residential tower in Canada. Located at Yonge and Gerrard Streets in downtown Toronto, it has 78 residential floors and a rooftop penthouse. The complex (under construction but with its lower levels occupied) has a large mixed-use podium accommodating retail spaces, a fitness club and restaurant, over five levels of underground parking.

Designing the structure of buildings that have vertically layered different uses is a challenge as the occupancies require different column and wall spacings. Aligning the structure vertically is therefore an optimization process of balancing cost and the maximum use of architectural spaces vs. speed and simplicity of construction. A process like this is complex and time consuming.

The Aura tower’s basement is a reinforced concrete flat slab construction commonly used in the Toronto area. The tower core and eight mega columns supporting the tower structure and the podium transfer slab extend straight to the bedrock.

The podium structure under the tower footprint had to follow the large column spacing requirement of the retail areas. Therefore most of the tower columns and all shear walls, except the core, had to be transferred out; this was achieved by a 2.5-m thick post-tensioned transfer slab that was poured in two stages to reduce the shoring requirements.

The tower structure sits on the 5th-floor transfer slab and it consists of the large residential core with radiating walls that are coupled to the core with overhead coupling beams and columns supporting the typical floor slabs. The column spacing and locations were established to suit the marketing requirements and to allow for the tower setback without transfers.

The tower’s “slenderness ratio” (approximately 1 to 8) is such that at this height, dampers were not required. It is interesting to note that with special visco-elastic dampers the same wall thicknesses could have been used to build a 92-storey tower with the same wall layout – this however did not happen.

 

Developer: Canderel. Architect: Graziani + Corazzi. Structural: Halcrow-Yolles. Structural engineering peer review: Reed Jones Christoffersen.
Mechanical-electrical: Smith and Andersen. Wind: RWDI.


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