Canadian Consulting Engineer

Award of Excellence: Central City Timber Structures

Central City was built to help the City of Surrey define its city centre and revitalize an existing shopping mall. The project involved adding university space for Tech BC that includes a vaulted Galleria, a 27-storey office building, and a large...

November 1, 2004  Canadian Consulting Engineer

Central City was built to help the City of Surrey define its city centre and revitalize an existing shopping mall. The project involved adding university space for Tech BC that includes a vaulted Galleria, a 27-storey office building, and a large public connecting space called the Atrium.

Within these spaces are three structures that showcase large and small exposed timbers in a refined manner not commonly seen in North America. The components are geometrically complex, highly detailed, and visually expressive — the Galleria Roof, the Atrium Roof, and the Atrium North Facade. The three structures total 7,800 square metres in area.

Working under an affiliated company, StructureCraft Builders, Fast + Epp took the unusual role of being both the structural engineer and the builder of the three timber structures. In this combined role they had the control to meet the lofty architectural goals and also assumed the risks of working within a fixed price — this on a project that had no straight lines and had to be built over an existing and fully operational shopping centre.

Galleria Roof

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The Galleria is designed as part of the university space of Tech BC but open to students and shoppers alike. It consists of two long, curving three-storey buildings suspended over the existing mall. The high vaulted roof is a 150-m long, 2,200-m2 free-form skeletal structure consisting of 20 individual three-dimensional composite timber and steel cable trusses. Each truss has a unique geometry, but is designed with key constraints to manage the complexity. As the party jointly responsible for fabrication and erection, Fast + Epp had an interest, early in the design phase, to communicate these rules to the architect without limiting his design freedom.

The trusses are lightweight spruce glulam to minimize the crane loads. Weight was a crucial factor for assembling the trusses, which are the size of a small house and weigh up to 12,000 kilograms. Two cast ductile iron connectors were developed for this roof, one of which neatly resolves into one shape the variable geometry of the cable layouts for all the trusses.

Atrium Roof

Nestled against a 27-storey tower on one side, sliding over the Galleria Roof on another, and bounded by the Atrium Facade on the third, the 3,400-m2 Atrium Roof covers the grand entry space to the Central City complex.

The roof’s three sides are formed with both convex and concave curves and its edges are generally sloped at 25 degrees from horizontal.

The engineers pursued an unconventional design that not only allowed close approximation of the curved and sharply tapered roof edge the architect desired, but also provided the required two-way stiffness for the edge which cantilevers up to 10 metres. The design adapts naturally to the tapered log cluster “tree-branch” supports located near the edges.

The roof is a tetrahedral space truss, 2.1 m deep, consisting largely of Douglas Fir peeler cores, a by-product of the plywood industry. There are about 4,000 cores in total. They are fitted at their ends with a special type of timber connection that involves long lag screws threaded into the ends of the peeler cores and confined with a larger diameter steel ring, also inserted into the ends. Nine truss members are typically joined together at the node. Where the truss spans up to 30 metres in the centre of the roof, a cluster of tapered logs with an array of cables acts as a king post truss.

Atrium North Facade

Along the Atrium’s northeast side is a glass wall, 26 m high and 86 m long, which invites people to enter the Atrium and entire Central City development. The wall has a freeform curvature in plan and is tilted outwards 4 degrees from vertical to minimize reflections, creating unusual transparency.

The glass panels and support system behind them tend to become geometrically irregular to fit the surface. The architect also wanted to display a refined timber structure as the back-up system, something unique to a facade so large and complex.

Fast + Epp’s challenge was to create a system which would break down the complexity, yet enhance the visual appeal. But there were a number of technical hurdles. Although cut in two near mid-height by a horizontal concrete canopy, the lower facade was still 14 m tall, and the timber columns to brace it against wind forces also had to carry over 100 tonnes vertical load from the structure above. Further, the glass panels, 1.8 m high by 3.6 m wide, could not be stacked to this height, so a method which would preserve the transparency needed to be devised to carry the weight of the glass. The system also needed to allow for constructing the back-up in a reliable way to the +/- 3-mm tolerances required by the glass installer.

The back-up system for the lower facade consists of a series of 600-mm diameter Parallam mullion-columns set back about 1 m from the glazing plane, with tapered Parallam arms reaching out to brace horizontal muntins that carry the glass. Large, unique ductile iron castings connect the columns at their base and top.

To deal with the difficult layout, the typical bay width is set to 3.6 m for the full height, restricting geometrical variance to the alternating bays. This strategy simplified matters for the glazier as well. To carry the weight of the glass panels, the engineers devised a series of spring-loaded stainless steel cables every 3.6 m, tilted at 4 degrees, carefully tuned to account for the movements expected during installation of the glass and for future potential building movements. The cables also served as an accurate guide for placing the entire timber back-up system, crucial to the success of the installation.

To connect the arm and muntin system together, an entirely new connector, which was lab-tested, has small, 12-mm diameter tight-fitting pins through the wood and aluminum connector plates, creating a very strong yet almost invisible connection.

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Name of project: Central City – Three Timber Structures

Award-winning firm: Fast + Epp, Vancouver (Gerald A. Epp, P.Eng., Zelimir Anic, P.Eng., Brian Woudstra)

Owner: Surrey City Centre Mall

Client: StructureCraft Builders

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Engineering


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