How to Make a More Tornado-Resistant Home

Tornadoes this year have ravaged Alabama, Georgia, Indiana, Kentucky, and Ohio, killing at least 39 people and injuring hundreds more. The stories are incredible: In one horrific incident on Tuesday, a 36-year-old Indiana woman who huddled in her basement with her two children had both of her legs severed by collapsing steel beams. She survived, and her children?whom she had wrapped in a heavy blanket?escaped serious injury.

This dramatic example raises a fundamental question that is surprisingly difficult to answer: Can a home be built to withstand a tornado?

The answer is yes?if the home were constructed like a bomb-proof bunker, with thick walls, a reinforced concrete roof, and a super-strong internal support system to prevent structural collapse. Window and door openings would require fortification with impact-resistant shutters that could be swung shut and locked at a moment?s notice.

None of this is practical for the average home, which is built out of ordinary construction lumber, plywood, and particle board. Even a well-built house with walls of brick or stone is vulnerable to a tornado?s wrath. But wood-frame structures are particularly at risk: Siding and roofing blow off or are pulled away by suction; subassemblies of rafters and wall studs with plywood attached are peeled away; sheets of plywood turn into sails, multiplying the already intense force of the wind; long pieces of lumber exert tremendous leverage.

In short, a tornado turns a wood-frame house into a gigantic, wind-driven pry bar. Once the departing structural elements have left a hole in the structure, the wind enters and dismantles the home like a bomb. After watching several online videos showing homes being decimated by tornadoes, I would guess that they generate tens or even hundreds of thousands of pounds of force.

Doing Better

Given this awesome power, the question is not how to "tornado-proof" a house, but rather how to increase its resistance to damage. According to the engineers we contacted, such as Randy Shackelford, a licensed structural engineer with Simpson Strong-Tie, a company that makes high-strength metal connectors, that much is possible.

"Studies of tornadoes [that occurred in 2011] revealed weaknesses in the load path of homes built to the minimum standards of the International Residential Code, which is the most widely used building code in the United States," Shackelford says. "To resist high winds, whether they?re from tornadoes or hurricanes, homes must be built with what is called a ?continuous load path,? a series of reinforced connections that tie every element together from roof to foundation, like a chain. Important connections would include rafters to top plates, top plates to studs, studs to bottom plates and bottom plates to foundation."

But in fact, Shackelford says, stronger construction really isn?t sufficient. "For the ultimate safety from the strongest tornadoes, homeowners should consider a safe room," he says. "These are built to resist the highest winds that are anticipated. They provide life safety in the strongest storms."

The Importance of Load Path

The key thing to understand is that a house is prone to its parts being ripped off and pried apart. The nails that fasten the parts end up becoming nothing more than flimsy hinges. To make the home?s parts perform like a monolithic structure?and resist those massive prying forces?you have to either build it so it performs like a single-piece or you have to retrofit it. That?s creating the "continuous load path" that Shackelford refers to.

You do this with steel connectors that bridge the house?s framing components (rafters to top plate, for example). Each connector is made from a thick piece of steel and fastened with multiple nails, screws, or even bolts. In the event of damaging high winds, the home?s parts are not easily torn loose because the forces are not concentrated but distributed from one wood framing member to another by means of the steel connectors and their nails, screws, and bolts.

I don?t mean to suggest that retrofitting a house for high-wind resistance is easy. It?s not. Building materials, insulation, plumbing and wiring block access to the framing. And other components can be extremely vulnerable. If your garage door blows in, all the work put into fortifying the structure won?t do much good. Fortunately, there are vertical door braces that can be installed after the house is built, which we covered during last year's tornado season.

Once the individual members of a home?s framing have been fastened together, either in a massive retrofit remodeling job or in new construction, you still can?t treat a tornado as a casual event. When sheltering at home, your best bet is to follow FEMA's guidelines.

Source: http://www.popularmechanics.com/outdoors/survival/tips/how-to-make-a-more-tornado-resistant-home-7148806?src=rss

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