February 6, 2012

National Research Council web wind tool helps with roof design

GREG MECKBACH

digital media editor

The National Research Council is offering a computer tool on its Internet site, which is designed to make it simple for construction engineers to use the National Building Code of Canada to design roof cladding that can withstand winds in a given area.

Wind-RCI, or the Wind Roof Calculator on the Internet, is available on the website of the NRC’s Institute for Research in Construction.

“The Wind Roof Calculator is a duplication of the building code, so whatever is specified in the building code you can do it in Wind-RCI,” said Bas Baskaran, group leader for the NRC Institute for Research in Construction.

When calculating the wind load for a roof covering, the design pressure depends on several factors, including the type of roof, slope, wind speed, building height, roof area, building terrain, building type and openings, according to a paper co-authored by Baskaran.

The National Building Code includes an equation designed to calculate the design wind pressure, though the Wind-RCI calculator has some limitations.

Although it does provide calculations for various cities, depending on their wind speed, it does not include calculations for buildings situated on hills and escarpments. It cannot calculate the design pressures for hipped roofs, post-disaster building configurations and for roofs with overhangs.

“The building code doesn’t cover all the configurations,” Baskaran said.

“As well, the building code is the minimum requirement because people have the view in their mind that if I meet the building code I am doing excellent,” he said.

“Whereas the building code specifications are for minimum design requirements, so there are no provisions on the Wind-RCI to go beyond code scenarios.”

Baskaran said in an interview that there is a three-step process in roof design.

Wind-RCI, he said, is designed only for the first step, which is to calculate the load.

The second step, he said, is to evaluate the resistance of the building assembly and the third stage is to ensure that the resistance given by the roof assembly is greater than the design load specified by the National Building Code of Canada.

“We are fortunate to have a model code for Canada, so we manage to produce tools of this nature which are useful coast to coast,” he said.

“We tried to do the same thing for the U.S. Since there are so many codes, we couldn’t manage to proceed for that.”

Baskaran said Wind-RCI was recently changed to reflect the 2010 building code, rather than the 2005 code.

These changes included the addition of new cities, which have been added to the wind roof calculator.

He said Wind-RCI has been used about 5,000 times but this does not necessarily reflect the total number of users, because some may have used it more than once.

Users of Wind-RCI are taken through several screens and encounter pull-down menus where they select factors.

For example, under “exposure,” designers can pick open (which includes level terrain with only scattered buildings, trees or other obstructions) or rough (which includes urban, suburban or wooded terrain).

Under type of roof, users could choose, for example, a low slope where the angle is less than or equal to seven degrees.

Under “openings,” the code has three categories. For example, Category One includes buildings without any large or significant openings but having small uniformly distributed openings amounting to less than 0.1 per cent of the total area. Category Three includes buildings with large openings through which gusts are transmitted to the interior.

Baskaran said his group is working with a consortium known as Special Interest Group for Dynamic Evaluation of Roofing Systems, or SIGDERS, to monitor more modern roofs.

The aim, he said, is to propose code changes and, if required, update the code.

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