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Canada sees a lot of extreme weather, and many Canadian experts are exploring how to help the construction industry protect homes and other buildings from the ravages of wind. Below are some highlights of Canada's wind expertise.
Predicting a hurricane's force
Since roofs are constantly exposed to the elements, they are one of the first parts of a building to suffer damage during a hurricane.
The low-slope roofs of critical buildings such as hospitals, emergency shelters and fire stations must be able to withstand persistent, high winds as well as the destructive rains that accompany hurricanes.
The National Research Council in Ottawa recreates high wind suction and wind-driven rain conditions on roofing membranes in a special chamber called the Dynamic Roofing Facility, which uses a gust simulator and fans to simulate wind fluctuations.
“We can test pressures up to 300 pounds per square foot, equivalent to 250-kilometre-per-hour winds, which covers most of the wind conditions in Canada,” says NRC’s Dr. Bas Baskaran. “Using our testing facility, we can predict what will happen if a hurricane passes over a roof.”
NRC’s protocol for testing the effects of wind suction on roofs was adopted by the Canadian Standards Association and is now being used by the construction industry.
Tracing the path of wind
Engineers know that when wind strikes a building, the dynamic force or “load” that reaches the foundation is much less than the force striking the building’s exterior.
“It loses strength,” says Dr. Ted Stathopoulos of Concordia University in Montréal. “Because of damping and what is happening inside the building, the wind force becomes smaller.”
But without knowing how much smaller the force becomes, engineers have to design foundations to withstand the worst-case scenario. This means buildings can cost more than they need to.
Using windy New Brunswick as a test site, researchers from Concordia and the University of New Brunswick built a specially equipped test house that would allow them to trace the path taken by the wind force through the building’s skeleton to its foundation.
Their goal was to find out exactly what loads were reaching the foundation, so that engineers can design buildings to withstand the actual wind force. The results could lead to buildings that are more economical without sacrificing safety.
Wind tunnel testing of tall buildings
Guelph firm RWDI has tested scale models of some of the world’s highest structures, including the Burj Khalifa in Dubai, which at 828 metres is the tallest building in the world.
While Canada’s skyscrapers may not reach such exceptional heights, our tall buildings still present their own challenges. For buildings such as the 236-metre (58-storey) BOW in Calgary, RWDI’s tests include, among others, the following three wind tunnel studies.
The first involves testing of wind loads on the overall structure. This includes predicting how much the building is going to sway.
“We then compare that information with what people might feel,” says Peter Irwin, chairman of RWDI. While not necessarily dangerous, a detectable sway can be disturbing for the building’s occupants.
Second, they test wind pressure and suction at hundreds of points on the exterior of the building so that external components and secondary support systems can be designed to withstand the highest expected loads.
Third, they look at the comfort of pedestrians walking around the building’s base. “A tall building like that can redirect strong winds to ground level,” says Irwin. “So people may feel much higher winds than if the building wasn’t there.”
Once factors such as the building’s overall shape and orientation have been adjusted to reduce wind, additional improvements can come from measures such as landscaping, wind breaks, trellises or the addition of colonnades to create alternative routes for pedestrians.
Learn more on the RWDI website.
ISSN 1927-0275 = Dimensions (Ottawa. Online)