Hurricane and Tornado Resistance

Shandy Heil, Reichel, Claudette Hanks, Skinner, Patricia  |  1/6/2007 2:48:22 AM

Preventing wind damage involves strengthening areas where things could come apart. The walls, roof and foundation must be strong. Also, the attachments between them must be secure. For a home to resist hurricane and weak tornadic winds, it must have a continuous load path -- connections that tie all structural parts together and can resist all the types of wind loads -- from the roof to the foundation.

Imagine turning your house upside down and shaking it. That is essentially the kind of stress hurricane-force winds put on a house. The weak link in the load path is what’s most likely to fail. Wind exerts three types of forces on your home:

  • Uplift load - Wind flow pressures that create a strong lifting effect, much like the effect on airplane wings. Wind flow under a roof pushes upward; wind flow over a roof pulls upward.

  • Horizontal wind pressure that could cause racking of walls, making a house tilt.

  • Lateral load – Horizontal pushing and pulling pressure on walls that could make a house slide off the foundation or overturn.

The actual effects of these wind forces on houses depend on the design, construction and surroundings of the house. Among other things, high wind pressures tend to:

  • Collapse garage doors, window units and patio doors.

  • Rip off roofing and roof decking.

  • Destroy gable end walls.

  • Damage roof overhangs, awnings, porches and other features that trap air beneath them.

In addition, broken windows and doors can expose your home to serious damage from internal wind pressures and water entry.

Check the wind zone of your location and use the protections that will help your home resist the wind speed of your region or a neighboring region closer to the coast. Louisiana’s coastline is receding, which means many south Louisiana homes are getting closer to open water.

Code-Plus Programs: It’s a good investment to go beyond minimum code requirements. Doing so adds additional damage resistance, peace of mind and market value. Enhancements also may qualify homeowners for current or future incentives, such as property insurance discounts. Two code-plus programs are widely recognized and included in the Louisiana building code legislation as alternatives for code compliance.

  • Fortified…for safer living is a home certification program of the Institute for Business and Home Safety, a research and educational organization of property insurance companies. A Fortified…for safer living designation means that a qualified inspector has confirmed that the house is designed and built to withstand the perils commonly experienced in the area. Some insurance companies offer premium discounts on wind coverage for certified Fortified homes.

Its guidelines provide a more standardized set of requirements that offer greater safety, damage resistance and simplicity than minimum code requirements. They also include best practices to reduce water damage that are not addressed in the codes. For more information and videos on property protection techniques, and to download the Fortified building guidelines, visit www.ibhs.org.

  • Blueprint for Safety is an educational program of the Federal Alliance for Safe Homes (FLASH), a non-profit, charitable education organization dedicated to promoting home safety. The program offers guidelines and builder training designed to provide current and reliable information about disaster-safety building techniques and features for floods, hurricanes, wildfires and windstorms.

Its guidelines are also more standardized than code and apply to houses that fit within a given set of design criteria. They are based upon a design wind speed of 140 mph. For more information and the building guidelines, visit www.blueprintforsafety.org. For information on ways to protect existing homes and to see animated “how-to” clips, visit www.flash.org

For a more thorough description of how wind affects buildings, visit the Web sites of the Wind Science and Engineering Research Center and FEMA.


General Wind-resistance Features 

Building dimensions: It is easier and less expensive to satisfy the target wind load if the house falls within some design and dimension guidelines. Designing to fit these parameters may also make it possible to comply with one of the prescriptive wind standards (and not have to hire an engineer to calculate loads and specify details) or a code-plus certification program such as Fortified…for safer living. In general, the simpler the house design, the less it will take to meet the desired wind resistance.

It is recommended that the house be no more than 60 feet long, and the length not more than twice the width. The house should have no more than two stories, and the ceiling (wall height) of each story should be not more than 10 feet high. If the home does not meet these criteria, it can still be considered for a Fortified…for safer living designation if a registered professional engineer or architect certifies that the structure was designed for wind loads corresponding to the wind design speed required for certification. At this time, the program requires 130 mph or higher, but criteria could change in the future.

Reinforced safe room within house: Safe rooms are typically designed to provide near absolute protection to you and your family from the high winds expected during tornadoes and associated flying debris, such as pieces of destroyed buildings. A master bedroom closet or bathroom with no windows is a common location for a safe room. In areas with little exposure to strong tornados but at risk of hurricanes or high wind storms, an in-home storm shelter can be built inexpensively to a lower standard than the FEMA guidelines while still providing greater protection (primarily from flying debris) than the rest of the house.

Storm storage: Include in your home design storage for outdoor items that may become flying debris in high wind. Outside structures and accessories, such as air conditioning condensers, light fixtures and sheds, which can’t be moved indoors for protection, should be anchored securely to a foundation or to structural parts of the house.

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