Western researchers working with builders to limit tornado damage
Ideas include better building practices to stop roofs from coming apart in high winds
When a tornado tears a building to bits, engineering student Sarah Stevenson and her colleagues at Western University's Northern Tornadoes Project get to work.
Where others only see a mind-numbing scatter pattern of broken buildings and the materials used to make them, Stevenson zooms in on key sections of the structure to pinpoint its weakest link.
"We try to understand what failed, not just what the house looks like after, but where that failure started," she said.
A common culprit when buildings are written off after a tornado strikes is the roof. Tornado-generated winds often peel off shingles, which can often be replaced. However, if the plywood roof sheathing comes free, the building is often a write-off due to the volume of water that can pour into the gap. Also, plywood pieces become potentially lethal missiles as they're swirled around by wind speeds that can exceed 200 km/h.
Another problem is the upward force a storm can exert on a roof, in some cases literally lifting it off its exterior walls.
For years, Stevenson's research supervisor, Greg Kopp, and other building engineers have called for changes to Canada's building code that would mandate alterations to how buildings are constructed, to give them a better chance of withstanding the force of a tornado.
One simple change would be the use of so-called hurricane ties. These small metal brackets secured with screws or nails create a stronger bond between roof trusses and the building's walls. The building industry has pushed back on adding these clips to the building code, arguing they are unnecessary and roof trusses properly nailed to the top plane of a framing wall would stop the roof from separating.
Stevenson said she and her colleagues are now stepping back a bit from the battle with builders over hurricane ties and are instead working with developers to find other solutions that will boost a building's strength without adding significant construction costs.
This week, she's been working with Doug Tarry Homes in a subdivision of single-family houses under construction in St. Thomas.
Instead of hurricane clips, Stevenson is asking builders to try a longer threaded screw to secure each truss to the top of the wall.
"We've found the strength is similar to a hurricane clip," said Stevenson.
Another new protocol they're trying out is the use of longer nails (and the use of more of them than the current standard) to hold down the roof sheathing.
Stevenson has been inspecting each house at the framing stage to ensure the fasteners are installed correctly and gathering input from builders about best practices.
So far, the framers she's been working with have been receptive to the changes.
"They're not being asked to do a lot more, so it doesn't add a lot of extra cost."
Earlier this month, an EF-2 hurricane struck near Barrie, damaging about 150 homes. There were injuries but, miraculously, no one was killed. Stevenson didn't survey the damage in person, but said a preliminary look at the photos taken by her colleagues suggests most of the buildings that failed in the Barrie tornado didn't use hurricane ties.
"The damage was exactly what we expected from lots these events," she said. "In some instances ,the roofs had shifted and sheathing panels were off."
Instead of continuing the contentious fight to get hurricane ties written into the building code, Stevenson said many building engineers hope to get the new roofing protocols they're trying out in St. Thomas written into a new national standard. It won't be a requirement for all new homes, but it will allow builders to tell clients their homes meet that standard.
"This way a builder can say this house was built to this standard and it's sort of a supplement to the building code," she said.