June 6, 2012

PERKINS + WILL ARCHITECTS

How the new roof over Nathan Phillips Square’s theatre is envisioned.

Steel roof for Toronto’s public square theatre

DON PROCTER

correspondent

The revitalization of Nathan Phillips Square in downtown Toronto includes a new roof for the public square’s theatre.

Small and straightforward as the project might seem, engineers pored over a series of designs before selecting a pre-engineered exposed space frame. And for good reason.

The initial structural steel castings space-frame design specified was priced at $405,100. The pre-engineered space-frame design in conventional steel, including installation, came to $208,000. 

An exposed space frame is a series of triangulated elements that span in two directions, as opposed to a linear system of joist and girders, explains Kenny Cryer, the project’s engineer, of Blackwell Bowick Partnership Ltd.

Prior to deciding on the prefab structure, structural consultant Blackwell Bowick reviewed several designs, taking into account that the roof had to meet with the theatre’s heritage designation on the highest profile square in the city. The original tender specifying structural steel castings were “fairly unusual for the structural world,” he says.

While steel is not normally cast for structures today, iron was cast in the late 19th and early 20th centuries for many buildings and bridges.

Structural steel is usually cast and then hot rolled into shapes but for the theatre the steel castings initially proposed for the job were essentially fabricated by shaping hot molten steel into casting molds, explains Cryer. That eliminates the hot rolling process.

“Because the roof design is an exposed space frame, the structural steel castings gave a very sleek uniform look to it. It was a very suitable idea.”

But it was pricey and it posed hurdles for contractors unfamiliar with the erection process, says Cryer.

Still, Blackwell Bowick retained Toronto-based subconsultant Cast Connex Corp., a world leader in structural steel castings, to collaborate on the design.

The team found a number of worldwide contractors which could do the roof in cast steel but the city’s intent was to price the job locally, says Cryer.

That’s when alternative roof designs were reviewed.

“We found a number of companies in the world that could prefabricate space frames but only one priced the job.”

That company was Ottawa-based Triodetic Canada.

“There was some tradeoff going to the selected design but it wasn’t considered esthetically detrimental,” Cryer points out.

The original design in structural steel castings had “slightly less” steel, says Cryer. “However, because it is a theatre there is a lot of rigging structure in the roof required for operations of the building.”

“The space frame is intended to take a two-direction load path to its support conditions thereby eliminating some of the heavy members you might see in a traditional concrete cast-in-place building.

“One of the more complicated aspects was just having Heritage (planning officials) approve the design,” says Cryer.

Quebec-based structural steel fabricator Beauce Atlas is supplying the column supports and box beams that form some of the major architectural features of the roof, he adds.  

As much as the design of the roof has been a challenge, so will its erection – largely because the theatre is on the busy public square atop a parking garage roof.

Large cranes strategically placed off the square will hoist the steel into place, Cryer says. “Just getting heavy equipment in there is certainly challenging because it is surrounded by the parkade roof on all sides.”

In addition, because the contract is a retrofit, rather than a new building, there is little laydown space for the big steel members and other materials, he says.

PCL Constructors Canada Inc. is the project manager. Flynn Canada is doing the roof’s glazing. The design is by Plant Architect Inc. /Perkins + Will, Architect in joint venture.

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