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Free-Form Station

Curved roof, walls establish iconic transit stop

York University Station Sept18 1
Photo: Shai Gil

From above, York University Station’s roof resembles a boomerang with upturned tips. At ground level, the building undulates with bubbles of glazing in the middle and at its ends. On its concave side, the structure is sunk into the landscape in an amphitheater-shaped bowl where a double-height curtainwall opens the interior to western daylight.

The subway station’s curvaceous massing, free-form roof and undulating exterior in Toronto are the vision of London-based Foster + Partners, design architect for the project. It is one of six new stations built on a 5.3-mile elongation of Line 1 on Toronto Transit Commission’s subway system, the Toronto-York Spadina Subway Extension.

Layer by Layer

To make the vision a reality, Mississauga, Ontario, Canada-based Bothwell Accurate Co. Inc., roofing contractor for the project, installed five of the six (1-6) primary layers of materials in the roof assembly. On top of (1) segmented steel decking, Bothwell Accurate installed (2) gypsum board and a (3) two-ply bitumen system. On top of that, the company attached a (4) metal sub-framing system of pre-curved hat channels, tracks and studs, which were connected with angle clips at various heights. Minneapolis-based Radius Track Corp. supplied the metal framing and clips. Under and around the metal framing, (5) 3-inch-thick, semi-rigid, mineral wool insulation was installed. On top of the metal studs, Bothwell Accurate installed (6) thermally broken clips, supplied by Kalzip Ltd., St. Helens, United Kingdom, and Kalzip’s Kz65/333 aluminum standing seam roof system.

Photo courtesy of Toronto Transit Commission

Adjusting for Tolerance

The metal decking was out of tolerance for the completed roof assembly at some locations. At the same time, the standing seam panels were fixed sizes and shapes. Trevor McGrath, estimating manager for cladding at Bothwell Accurate, says all height differences between the decking and standing seam panels needed to be worked out by adjusting the framing.

“We scanned the as-built roof structure, and we overlaid that with the skin model, which is what the architect did as a 3-D model,” McGrath says. “Radius Track [framing manufacturer] took the 3-D scan and did a color coded model showing how much the roof was out of tolerance at different locations, how much adjustability we had to do. And then we took that drawing and fabricated the roof structure to make up any differences.”

Radius Track used the 3-D model to design, engineer and fabricate a metal sub-framing system. In all, Bothwell Accurate installed 4,323 feet of 16-gauge G120 galvanized steel 1 5/8-inch wide, curved hat-channel, 2 1/2-inch wide, curved track, and 1/2-inch wide, curved C-stud and stand-off clips.

Bothwell Accurate started installing the roof at the midway point with a double-female panel, and then worked outwards in both directions, McGrath says. The company installed 12,271 square feet in 374 panels of Kalzip’s curved and tapering standing seam roof system. Due to the tapering roof design, the panels varied in width from 16 inches to 8 inches, and in lengths to 35 feet. The metal panels protect the two-ply bitumen system from UV degradation and resist solar heat gain.

The panels were manufactured in Koblenz, Germany, and shipped to site. Some were more than one piece. “We welded a joint just off the center of the ridge on-site,” McGrath says.

To drain stormwater, the roof has a perimeter gutter that runs into the building and exits via the sewer system. At ground level, the perimeter gutter is hidden from view by fascia. McGrath says, “The gutter started off as a steel angle frame, which we then put tapered insulation in, so the insulation thickness changes to cause sloping to the drains, which are evenly spaced apart.”

Photo courtesy of Radius Track

Column of Mullions

York University Station’s structure was built with poured-in-place concrete and structural steel. Supporting the free-form roof, a ring beam extends around the entire perimeter. On the building’s concave side, where an egg-shaped curtainwall extends the highest, the concrete base, partially clad with curved metal panels, swoops up on both sides and gets thinner as it elevates. Estelito So, job captain at Adamson Associates Architects, Toronto, architect of record, says, at the highest point of the curtainwall, the concrete no longer supports the ring beam and roof. Instead, curtainwall mullions act as a column. “We needed to use some of the mullions on that curtainwall to support that portion of the ring beam so that it will hold in place,” So says.

Burr Ridge, Ill.-based Josef Gartner USA supplied the curtainwall, and Windsor, Ontario-based Riverside Group supplied the metal wall panels. Riverside also supplied metal ceiling panels, installed at the two entrances at opposite ends of the station.

Photo courtesy of Toronto Transit Commission

Model Collaboration

Adamson has completed several projects with free-form roofs. So says the materials and applications are familiar, but because this project had a completely free-formed roof and curved walls, the 3-D models were complex and extensive. “All the materials we have here, they’re all just simple, and most experts know how they are layered. It’s just that how it is done in collaboration with the other manufacturers to form those 3-D models, to me, that’s the most exciting part.”

McGrath also pointed to collaboration with project team members with the 3-D models as central to the project’s outcome. “We engaged our fabrication partners in the beginning, and we worked hand-in-hand over the years, as the project went on through design, fabrication and engineering, and it allowed us to carry the project through successfully,” McGrath says. “We were able to pull off the project on time, on budget. So it was a good reminder of how important it is to have the right people, that you’re able to work together.”

Photo courtesy of Toronto Transit Commission

Photo courtesy of Radius Track