Warp & Twist FAST + EPP

While tilt-up construction methods have mostly been used for standardized, industrial type buildings, this economical building technique is now starting to be used in more architecturally innovative ways.

The Sunset Community Centre in Burnaby, B.C. is a striking example. Here, tilt-up construction was combined with an undulating, free form steel roof to create a playful and expressive architectural design.

Bing Thom Architects designed the unique form of the 3,000-m2 community centre to reflect its open park setting. As structural engineers, Fast + Epp had to design a structure that needed to warp and twist to match the playful geometrical architecture, yet which also had to be left exposed for reasons of durability and economy.

Roof Structure

The undulating oval roof involves five independent steel roof structures. Each varied in plan, size, slope and elevation, and every piece of steel is a custom piece.

The documents were shared with all the construction subtrades, and the steel fabricator kept a detailer on site with a laptop to coordinate the 3D model.

The free-form roofs were created by varying the vertical bearing points, and thus the slopes of conventional open web steel joists, while maintaining a fixed horizontal spacing. The horizontal spacing aligned the supporting columns with the wall glazing grid.

A key challenge was finding a relatively simple way for the contractor to attach the straight roof deck to the warped roof truss. Fast + Epp designed a special con-continued nection detail with unique, site-rotatable clamps (see figure). These were added to the top of the OWSJs (open web steel joists) to allow the deck-bearing surface to rotate freely to match the angle of the deck, while providing the necessary even bearing surface for the deck-to-joist connection.

While one end of each OWSJ is supported by the tilt-up structure, the other end is supported by elliptical HSS (hollow steel section) columns. These columns also provide horizontal support for the curtain wall system.

Tilt-up concrete

Because concrete is poured horizontally for tilt-up panels, the tilt-up technique made it is easier to create the necessary reveals and contours at the top of the walls where they meet the undulating roof.

Forty-six uniquely shaped doublewythe concrete tilt-up panels were used. Each consisted of a structural wythe, insulation and an exterior veneer wythe. Among the panels are some large spandrel units, one of which is 25 m long and 3.6 m high, weighing 60 tonnes. This panel serves as a major load-bearing span in the open main lobby, and it is likely the largest panel of its type in North America.

The tilt-up precast construction required the structural engineer to provide details for all the wall elevations. As well, the engineers had to take into account potential stresses on the panels caused when they were lifted up off the ground by a crane.

Pouring the concrete horizon-tally into one flat form allows more control over the concrete finish. The walls at the community centre were left exposed and hand-ground smooth, then sealed to preserve the natural colour of the concrete.

The end result, completed in 2007, is a building of poetic simplicity that has walls with no form lines, cold joints, or tie holes that are hallmarks of standard cast-in-place walls, all lending to the sculptural feeling of the architecture. The project won the 2009 Award for Excellence from the U.S. Tilt-Up Concrete Association.

Owner: City of Burnaby, Board of Parks & Recreation

Structural engineer: Fast + Epp (Gerald Epp., P. Eng. , John Miller, P. Eng.)

Architect: Bing Thom Architects

Other key players: Haebler Construction (contractor), Solid Rock Steel (steel fabricator), Mardina Construction (tilt-up contractor)