Twenty-four, thin-shelled precast canopies supported on single columns will protect commuters from the elements beginning this summer at Calgary's new Shawnessy LRT Station, one of two new stops being added to the southern extension of the rail line. The unusual and challenging design and engineering of the 18- × 20-ft. canopies was made possible with 105 cu. yd. of Lafarge North America's Ductal, a high-performance, fiber-reinforced specialty concrete product that promises high compressive strength without sacrificing aesthetic possibilities.

Columns, struts and the curved shells — whose cross-sections are as thin as ¾ in. — were all fabricated using Ductal at Lafarge's Calgary precast plant. Even the rain gutters, which run along the bottom of the arches, are made of Ductal and are designed to be strong enough to hold the weight of a person. At night, the curved canopies serve a dual function, diffusing and reflecting the artificial light to the platform below while animating the façade through the louvered windows.

For the architects, CPV Group Architects & Engineers Ltd. of Calgary, the canopies were by far the most challenging aspect in engineering the design of the station, according to Vic Perry, vice president and general manager of Lafarge's North American Ductal division. “The thin-shelled canopies actually curve in two directions. The structural engineers used a finite element analysis program for the design work. They input the geometry, ran the analysis, refined the sizes and redid the geometry. To form the arc properly, they experimented with mesh sizes, thickness and overall dimension for quite some time using structural engineering software,” he says.

According to Lafarge, fiber-reinforced Ductal (which can use either metallic or poly-vinyl acetate fibers) may be used from fluid form to dry-cast. Under normal conditions or acclerated curing, the product provides a high early strength in excess of 60 MPa (9,000 psi) in less than 16 hours; after 48 hours of thermal treatment, it can reach 200 MPa (30,000 psi). The nonporous nature of Ductal provides resistence to dirt, grime or corrosive chemicals and solvents, as well as color fading, cracking and peeling, even in extreme environments, says Lafarge.

To verify its engineering results, Lafarge turned to the Massachusetts Institute of Technology to provide independent testing by means of aerospace software traditionally used to model the hulls of aircrafts. “MIT's results were nearly identical to ours,” says Perry. “We also ran scale load tests at the University of Calgary, which confirmed that the canopy would withstand a snow load and severe wind uplift. All of this testing was done before we started building for the City of Calgary.”

Perry adds that the next big test for Ductal is a bridge project, dubbed the “Bridge of the Future,” located near CIA headquarters for the Federal Highway Administration in Washington, D.C. Begun last fall and scheduled for completion in April, the structure is constructed in a test track at the Turner Fairbank Laboratory in McLean, Va. It features a 3-in. deck with no reinforcing steel and is said to be strong enough to drive tractor trailers across. “The last of the superstructure — the girders and deck together — is being shipped now,” Perry says. “The team wanted it to look futuristic, and I think we succeeded in that.”