Hurricane straps keep roofs on houses and can improve safety during tornadoes
Many people think of a well-built house as a house whose walls are strong enough to support the roof so that it cannot fall on it. This is reinforced by children’s stories such as the stories Three little pigs, where the brick house is strongest when the Big Bad Wolf comes to town.
When a strong tornado sweeps through a neighborhood, it results in total chaos. Debris is everywhere. Shingles and siding and bricks are thrown. Entire roofs are thrown away and often end up on neighboring houses. Walls collapse, cars are rolled and flipped. Insulation sticks to any surface like a strange snowfall.
On July 15, a tornado struck Barrie, Ontario, destroying several homes: Could anything have been done to minimize damage in Barrie?
Hold on to the roof
With any wind storm, including tornadoes, the roof must be secured – this is due to the buoyancy force, the same physics that allows an airplane to fly. This goes against our intuition, as we tend to think of collapsing roofs, not flying. The abolition is the main vulnerability of houses. The structure of residential roofs in Canada is usually strong because they are designed to handle the heavy weight of snow in winter.
For the wind, which works in the opposite direction to the snow, it is the nails that connect the rafters to the top of the wall that become crucial in ensuring that the roof stays in place during a severe storm.
The National Building Code of Canada requires: three nails – just over three inches long – in each roof-to-wall joint. These toenails, as they are called, hold the roof down in the wind. If this is done well, the roof construction must be safe up to wind speeds of about 160 kilometers per hour. At such wind speeds, asphalt shingles can blow off, but the roof construction remains intact.
Homes are not designed for tornadoes, although the building code does discuss tornadoes. The requirements for fixing the walls to the foundations are designed to withstand tornadoes. These were included in the 1995 edition of the National Building Code after tragic deaths in 1984 then cottages in Blue Sea Lake in Québec. were swept, together with post-storm observations following a 1985 tornado in Barrie.
It is now known that toenails are the weakest links in the structure, with a greater chance of: fail for the roofing and before the walls detach from the foundation. Walls are more likely to collapse when the roof is gone, and the roof itself can become airborne. Both pose a threat to life and safety.
The Northern Tornado Project investigations in the immediate aftermath of the recent Barrie tornado found that several homes failed to meet building codes due to missing toenails.
This is nothing new.
To design for tornadoes, we need to know the wind speeds in tornadoes. These are rarely measured. Instead, tornado wind speeds are judged by the damage they cause, via the improved Fujita scale.
The recent Barrie tornado had a maximum wind speed of 210 kilometers per hour based on the damage to some homes. It was a strong tornado. However, if this strong tornado had passed through farmers’ fields and missed all the buildings and trees, it would have been rated as an EF-0 (90-130 km/h) tornado, even if its true strength was greater. Clearly, using damage to estimate tornado wind speeds is challenging, especially in sparsely populated areas.
As a result, a tornado’s reported intensity — the maximum wind speed — depends on what it hits and thus on the quality of its construction. A roof with just a single toenail would be rated as EF-1, while properly placed toenails yield an expected wind speed of approximately 195 kilometers per hour, which is EF-2.
Put another way, a wood-framed roof is expected to fail in an EF-2 tornado, while incorrect toenails would lead to roof failure in an EF-1 tornado. Due to this uncertainty, quantifying wind speeds in tornadoes is still a challenge an active subject of research and development.
Nevertheless, the combination of tornado simulators, wind tunnels that create tornado eddies to determine wind forces on buildings, and large-scale lab tests on homes to determine their strength. They have provided good estimates of failure-inducing wind speeds during tornadoes under a variety of conditions, including: of the roof-to-wall connections.
Could anything have been done to limit the damage in this EF-2 tornado? In any tornado, the highest wind speeds occur only about a relatively small part of the total damage path. That means damage mitigation measures can be quite effective in mitigating damage from the overall storm.
If all the houses in Barrie had been built to building codes, there would have been less damage overall, although there would still have been significant damage due to its intensity.
There’s a better solution: using hurricane belts instead of toenails. Developed to withstand hurricanes, this proven technology can keep the roof attached to the walls tornadoes with wind speeds up to about EF-2. They are inexpensive, cost less than $200 per home to install, and are easy to inspect for compliance.
Because EF-2 and lower-rated tornadoes represent more than 95 percent of all tornadoes, requiring lashings in the building code could reduce much of the damage from these severe storms and significantly improve safety.
While there are a few other things that need to be done to fully enable homes to… withstand EF-2 tornadoes, this modification would eliminate the weakest link, increase resilience and safety by keeping the roof on the walls, and prevent entire roofs from flying downwind and hitting other buildings.
Quote: Hurricane belts keep roofs on homes and could improve safety during tornadoes (2021, July 29) retrieved July 29, 2021 from https://techxplore.com/news/2021-07-hurricane-roofs-houses-safety-tornadoes.html
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