February 3, 2012

Steel hurdles for Aga Khan Museum build

DON PROCTER

correspondent

Fabricating and erecting 740 tons or so of structural steel required for the Aga Khan Museum in Toronto posed a series of unusual hurdles.

One of those hurdles was how to attach the steel frame for the second floor to the first floor — an inward-sloping poured-in-place concrete wall.

“Our connections to the concrete wall had to be fairly substantial to transfer the forces to make the building stand up,” says Rob Morrison, project manager, Benson Steel, supplier/erector for the job.

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 Another challenge for the big steel fabricator/erector was the installation of six steel-framed hexagonal skylights. Made of an HSS steel frame, the offsite-assembled 4.7 by 4-metre shaped skylights had to be carefully erected and placed on sloped sections of the roof, says Morrison.

The museum’s new auditorium features a steel “dome-like” roof. The 18 by 18-metre frame was assembled in segments on site and hoisted by crane into place.

The steel engineer says the most challenging aspect of the project was Benson’s second contract — the fabrication and erection of the 16 by 16-metre glazed interior courtyard comprised of an architectural steel framing system.

Unlike most steel-framed courtyards which are clad in non-transparent cladding such as aluminum, the frame for this museum courtyard was specified to sit flush against the glazing. To produce an appropriate steel assembly for the courtyard, Benson started by making a mockup consisting of hot-rolled steel bars with a standard finish. That model was rejected by the architects. Benson’s accepted alternative was a model with an architectural finish.

 The 12.8 metre-high steel assembly consists of slender vertical steel bars spaced two metres apart and connected to seven horizontal steels members, Morrison says. “These are made in sections like ladders. We made two ladders, and then filled in between them to make the courtyard shape.”

The steel was machined to meet precision tolerances (less than a centimetre). Detailing the assembly was tricky because the architects specified that connections between the horizontals and verticals go unseen.

 To meet the specs, Benson machined a notch at the ends of the horizontal bars to allow for welding inside the bars where they connected to the horizontal elements. Steel cover plates were ground smooth to hide the connections, says Morrison.

“We’ve never done anything quite like this. Usually the steel bars are clad in aluminum (which hides the connection details).”

 He says once the framework was installed, a survey of the frame indicated that there were “some small distortions” in the dimensions.

“That happened as a result of the heat we applied welding the connections together.”

 Benson spent about two weeks on site applying heat to the bars to bring them back to the specified tolerances which were “much tighter than any projects we’ve been on.”

The museum is a design by Maki and Associates and Moriyama & Teshima Architects. Structural designer is Yolles, a CH2M HILL company.

Benson is scheduled to wrap its work up at the museum in Don Mills this month.

Scheduled to open in 2013, Aga Khan will be dedicated to the acquisition, preservation and display of artifacts relating to the intellectual, cultural, artistic and religious heritage of Islamic communities.

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