Surrey Central City - Atrium Roof
The roof over the Central City Atrium, a large open grand entry to the facility, presented an out of the ordinary set of design and construction challenges.
The design called for a highly irregular curved edge bound by a curved office tower on one side, the Galleria on a second, and the 260-foot long curve of the North Façade on a third. Support conditions were also irregular. Given these constraints, a normal framed structure, though considered, simply wasn't practical.
Instead, we decided to build a seven-foot deep tetrahedral space truss roof consisting of 3,700 Douglas fir five-inch and six and a half inch diameter peeler cores, all the same length. The idea was to use peeler cores, an inexpensive by-product of plywood peeler logs.
We chose a fine-grid space truss because it efficiently dealt with the irregular geometry by approximating the curved edges. It further provided two-way action, and consistent, continuous support for the curved roof edge cantilever that projects up to 30 feet beyond the supports.
We designed light steel framing to adapt the edge nodes of the space truss to the tapered curving contour of the roof. Support is provided at numerous locations using “tree branches", varying clusters of Douglas fir logs turned and tapered on our lathe, then mounted so they branch from reinforced concrete columns.
To make the roof structurally efficient and the connections elegant, we custom-designed and extensively tested a series of confined lag screw connectors, hidden from view, for the peeler core truss members. The testing program allowed us to use the cores for high tension as well as for compression. For the nodes, a custom-cast connector that joins nine peeler cores was designed for optimal weight and ease of use on site.
Where the span is longer, the roof is reinforced by a chandelier-like “king post" truss consisting of a log cluster and a series of pre-stressed cables.
Optimizing Structure – While not immediately apparent, the space truss roof is a finely-tuned structure, with numerous connection types load-tested and calibrated to the wide range of forces present throughout the truss. The “king post" and cables are carefully positioned to undergird the truss in the maximum span and pre-stressed to optimize forces in the peeler cores.