Award of Excellence: Brentwood Station

CATEGORY: TRANSPORTATION

FAST + EPP

The Brentwood Town Centre SkyTrain Station is a key transfer point in a series of 12 stations connecting the most recent expansion of the Greater Vancouver Rapid Transit network. Since the station is in a spectacular natural setting and is elevated along a major transportation corridor, the client called for a modern, signature design that would take full advantage of the views of the surrounding mountains and cityscape. From these requirements, the architect envisioned an 80-metre long station with a sleek, dynamically shaped shell roof structure.

Fast + Epp, the structural consultants for Rapid Transit Project 2000, worked closely with architects Busby and Associates to realize the station design. The station, completed and opened in 2002, incorporates several unusual structural features.

First, it incorporates timber as a primary structural element. Its use of wood lends it a distinctly West Coast architectural ambience and is a radical departure from conventional steel and concrete train station structures. Predictably, the design was met with initial skepticism. The constructability of a compound roof curve with an extremely tight radius about the longitudinal axis, along with issues of fire rating, maintenance and vandalism, represented a formidable challenge.

Seventeen pairs of steel/glue-laminated timber ribs at 5-metre spacings support a solid timber roof shell. The upper portion of the ribs consists of glue-laminated timber that is compatible with the roof shell and lends the station architectural warmth. The lower ends of the ribs, called the haunches, were constructed with custom-curved steel plates in order to achieve the tightly curved radius that would have been difficult to accomplish using wood. The steel haunches were also required to minimize vandalism on the accessible support structure.

To form the complex shape, the roof construction revisited a local, historical framing method that is now seldom used. It consists of standard 2 x 4 dimensioned lumber of random lengths spiked together and sheathed on the top side with 10-mm thick plywood. This simple framing concept provided ample tolerance to form the compound curve of the roof surface. The construction meets the 45-minute fire rating requirement, and with the inclusion of timber elements it achieves a stunning architectural effect that can be likened to an “inverted hardwood floor” ceiling.

Stability

The station is situated in a high-risk seismic zone and experiences magnified forces due to its position 15 metres above ground level. The seismic constraints on the structural design complicated the architect’s desire to have uninterrupted views at the sides of the station. A further complication was the architectural requirement for a wide central roof opening along the full length of the station. The key to solving this problem was a steel “zipper.”

The “zipper” connecting the roof flanks consists of diagonal struts using double steel plates with spacer bars to create visually light members. Longitudinal steel gutters along the edge of the roof shell also act as structural “zipper chords.” The zipper transfers snow and self-weight loads from the wood ribs to the central steel V-columns. The zipper also transfers longitudinal seismic forces into inverted V-braces at each end of the station, thereby freeing up the station sides for unobstructed views of the mountains and city. The choice of steel as the zipper material helps to avoid maintenance problems associated with exposing timbers in a rainy climate.

Budget considerations

The location of the station directly above the Lougheed Highway, coupled with the complex form of the station roof, presented serious budget challenges.

Strategic decisions were made to successfully navigate around the problem, and the station was completed for $8.1 million, on time and on budget.

First, the geometry of the station allowed all the ribs to have an identical inside curved shape. The depths and end-cut locations of the ribs were varied to achieve the compound curved roof geometry and satisfy varying strength requirements. This approach allowed for a single jig to be used for shaping all the ribs, resulting in manufacturing efficiency.

Second, eight curved concrete edge beams were developed from a segmented curve with a fixed radius. This approach allowed all eight beams to be cast from a single form.

Third, the design team developed simple connections to create the composite timber-steel rib. The connections between the wood portion of the rib and the steel haunch and steel zipper struts at either end not only had to transfer very high bending and shear forces, but also they demanded a clean aesthetic. Conventional bolted details would have rendered the ribs clumsy and unworkable. The solution was to incorporate long steel tension plates welded to the haunches and zipper, and connected to the timber with glulam rivets (a distinctly Canadian invention!). These plates seamlessly transfer forces from wood to steel and achieve the desired architectural elegance.

Fourth, the platform’s precast concrete beam cross-sections for Brentwood were kept identical with three other stations on the new line — Gilmore, Rupert and Renfrew. A single form could be used for all four station platform beams.

Fifth, the design team was granted permission from the client to transfer a portion of the budget from a sister station — Gilmore — to the Brentwood Station budget (Gilmore was designed by the same architects and engineers.) In order to free up budget amounts from Gilmore, a completely novel, yet simple and economical, modular wood-steel roof panel design was developed for that station. The panels incorporate a relatively new manufactured wood panel material called Timberstrand, which is pre-bowed through the use of light steel cables and elegant steel struts. The panel design was recently granted a 2002 North American Wood Council Citation award.

Name of project: Brentwood Town Centre Station, Burnaby, B.C.

Other key players: Busby + Associates Architects