February 2, 2006

Design can buffer severe weather

Canadian researchers note roof improvements needed

W.D. LIGHTHALL

correspondent

Improving construction and design, especially in roofing, may alleviate some of the destruction caused by hurricane-force winds like those experienced during disastrous southern U.S. storms in the past two years.

Dr. Bas Baskaran, a National Research Council Canada project manager who has visited the hurricane-ravaged states of Florida and Louisiana, specializes in assessing the impact of extreme winds on roof systems, including membranes, corners, edges and parapets.

Baskaran said before his U.S.-based hurricane investigations, he was unaware of the extent to which heavy winds can cause a buildup of internal pressure within a structure.

Since, he has seen numerous cases of roof failure where the wind circulates inside after punching out windows or doors at one end of a building.

“Once the internal pressure builds up, it causes the failure on the roof,” said Baskaran, at a conference on design for a severe environment, organized by the Ontario Building Envelope Council.

“I was not a big believer that internal pressure could damage a roof to the extent I’ve now seen,” said Baskaran, adding he’s now changed his position on that technical issue.

Buildings under construction in hurricane-prone regions need to be designed strong enough to withstand severe internal pressure.

“Especially if it’s a building that may be used for disaster relief, such as schools, hotels and other public buildings.”

Baskaran said it is commonly known that roof damage often begins at the corners, where winds whipping around can create high rates of suction.

“Use of high parapets reduces the suction (rates) significantly,” he said.

But to achieve a reduction, the parapet not only has to be tall enough — Baskaran said a low parapet is worse than no parapet — it must have the design and structural strength to survive heavy winds.

“We saw another building where the failure is with the parapet, not with the roof,” he said, noting in the southern U.S. much work needs to be done with rooftop fans, mechanical boxes and air conditioning units.

“They need engineering design. We found little of that in many places,” Baskaran said.

With only the most basic fasteners — and sometimes none at all — rooftop installations tend to come off fairly quickly in a hurricane. When they do, they pose a danger not only to people and neighbouring property, but they often damage the roof as well.

“We have seen some rooftop failures caused by units blowing off that create a puncture in the roof, and once you have this puncture, it creates the (chain) effect that causes the roof failure,” Baskaran said.

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