Shelter from the Storm
Building for extreme weather
In January, Alabama was once again ravaged by tornadoes. This home used to stand in the Pilgrim's Rest subdivision in Trussville.
Photo by Caroline Baird Summers
The tornadoes that tore through Tuscaloosa and Birmingham last April left devastation in their wake, killing 243 and destroying thousands of homes. Although the 2011 tornado event was the worst to strike Alabama, the state is no stranger to severe weather. Less than six years before, Hurricane Katrina caused so much damage from wind and water that 22 counties were declared federal disaster areas.
Hurricanes, tornadoes and other extreme weather will continue to occur with unpredictable frequency in Alabama; however, the death and destruction they wreak is not inevitable, experts say. With proper construction techniques, buildings can be made more storm resistant and the injuries, death and property damage from severe weather can be greatly reduced.
For the ultimate in storm survival, a building’s design should include a safe room, a structure so stoutly constructed that it can withstand winds of 250 mph and the impact of a 15-pound two-by-four board travelling at 100 mph. Taking refuge in such a room, inhabitants may emerge unscathed from a tornado even if the rest of the building is destroyed around them.
Rooms may vary in size to accommodate a few individuals or more than a dozen.
The room may be a dedicated structure located in a basement or garage, or it may be incorporated into a closet, bathroom or conference room in such as way that the space can be used for other purposes.
If there’s no convenient area inside a building, a safe room may be sited outside, although it must be near enough to be reached seconds after a tornado siren is heard.
There are many ways to build a safe room—concrete walls, concrete masonry unit walls, wood frame with plywood/steel sheathing—but all safe rooms must be solidly anchored, usually to a concrete pad, and they must be independent of the main building’s walls, so they do not depend on them for support.
Although a number of Alabama companies build and install safe rooms, John Plisich, a civil engineer with the Federal Emergency Management Agency Region 4 mitigation division, recommends that a customer be diligent with any safe room they purchase.
“In the past, there have been folks that have stated that FEMA approved their safe rooms, but FEMA doesn’t approve or endorse products,” he cautions. “They can claim their product meets FEMA criteria, but that isn’t a FEMA approval. Furthermore, the actual safe room construction and installation is critical for the project—how it’s attached to the foundation, whether it’s in a floodplain, and what nearby hazards exist.
“Homeowners can compare the FEMA criteria to the product and installation being proposed and ask questions to help them make wise decisions.”
Building from the Ground Up—An Integrated Approach
To withstand extreme winds, a building must be constructed with wind stresses in mind in every aspect of its construction—from ground to roof.
“In a hurricane or a straight wind, but particularly in a tornado, you have a vertical uplift which can lift the roof off,” says University of Alabama professor John van de Lindt, who holds the Drummond Chair in Civil Engineering. “You also have a horizontal wind component, which passes over the structure and acts basically like the lift on an airplane’s wing.
To make the entire structure resistant to extreme winds, van de Lindt recommends an integrated construction plan including the following features:“These forces combine in a tornado to produce a much stronger uplift force,” says van de Lindt. “When the uplift forces hit the roof, the force is transmitted from the roof sheathing and trusses to the top of the wall and then it goes down the walls through the vertical studs into the sill plate and into the anchorage of the foundation. So it’s just like the weak link in a chain: if any one of those points is weak, then you lose everything.”
• Foundation: Anchor bolts in the sill plates
• Walls: Hurricane clips or straps fastening the vertical wall studs to the roof trusses
• Roof: Trusses that line up with wall studs for attachment of hurricane clips
Often, roof trusses are 24 inches apart while studs are spaced at 16 inches, causing weak spots. Van de Lindt recommends having a structural engineer work with the architect and building contractor to assure that a building can resist high winds.
A big price tag doesn’t necessarily mean a well-built house, he cautions.
“I’ve seen a beautiful house in the area that probably cost half a million dollars to build,” he says. “You would think it would be well constructed, but, to be quite honest, the construction was very poor. I think a straight wind of 100 miles per hour would tear it apart.”
A good example of a storm-resistant building is the visitor’s center at the Biloxi Lighthouse Park, constructed by Rod Cooke Construction Inc. of Mobile. The visitor’s center retains the look of the historic Dantzler House, which stood on the site until it was destroyed by Hurricane Katrina, but the new building was designed to survive a repeat of the 2005 storm.
“It’s built to withstand whatever you throw at it,” says Rod Cooke. “It has reinforced concrete walls, heavy-duty roofing and windows, and a moisture barrier membrane in the cavity between the concrete and the brick veneer. The foundation is built on pilings sunk deep in the ground. That building’s not going anywhere.”
Retrofitting a Building for Extreme Weather
There are many features that can be added to an existing building to better protect the inhabitants during a storm and to minimize damage to the building. Wind stresses vary over the surface of a building, being particularly high at corners and projections, so the first step should be finding the vulnerable spots. An architect or engineer could help, or another individual trained and certified by a program, such as the Fortified for Existing Homes™ course offered by the Institute for Business & Home Safety.
Some ways to strengthen a building for extreme weather include:
• Roof–wall juncture: Attach hurricane straps
- Higher quality fasteners everywhere
- Extra fasteners at high pressure points
- Higher quality shingles
- Secondary water barrier over deck panels
• Vents and soffits: Strengthen and brace
• Overhangs at gable end walls: Strengthen and brace
• Openings: Install pressure and impact resistant models
- Entry doors
- Garage doors
John Plisich, of FEMA, upgraded his own home recently when he replaced the roof and considers it a good investment.
“My house was built in 1982 and had shingles rated for 90 mph winds,” he says. “We put in stronger shingles that could withstand higher winds. The builder also put in thousands of extra nails by increasing the nailing pattern at the corners and edges where the wind stresses were highest. The cost was chump change, pennies on the dollar, compared to the entire roof job, but it will make a big difference in protection. A person who better understands the problem and does his research really puts himself in the driver’s seat when it comes to choosing products and reducing his
William Stevenson is a freelance writer for Business Alabama. He lives in Huntsville.