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Storm Shelter Planning© Emerson Lockwood 2007 A storm shelter can be a life saving investment for any home since tornados can happen anywhere in the United States, at virtually anytime of the year. There are numerous stories of how tornado shelters have saved people from the dangers that tornados and severe weather can bring. These shelters that we hear about include both underground shelters and ones referred to as “safe rooms”. Safe rooms are built above ground inside the home. These are particularly valuable if a handicapped person lives in the home. They are also easily accessible by children or the elderly. Underground shelters require stairs that may be difficult to use by older people or anyone that handicapped.
There are many decisions that you need to make before getting to work on a storm shelter for your home. Some of these decisions are about the size, function, and location. Will it serve strictly as a shelter from storms? If it will serve purely as a storm shelter then a 4 by 8-foot structure would accommodate space for four people to sit or lie down up to an hour waiting for a storm to pass. If you opt for a small shelter it must never be used for storage. If a portion of the shelter is to be used for storage or an office, consider building a unit at least 8 feet by 8 feet in size allowing about 10 square feet per person. A shelter this size would provide storage space for items needed to endure long hours of bad weather. Where will the shelter be located? Will it be in the house, attached to the house, or in the yard? To be effective, shelters they must be conveniently located and adequately strong. Quick and easy access is a must for any storm shelter. When planning a new home, the best location is in the center with an entry from a central hallway. For existing homes, an attached shelter with a direct entry from the home is much preferred over a shelter located in the yard. Shelters located away from the home have two inherent problems. A person can be injured on the way to it or if the warning comes too late, one may not have time to reach it. Locate under-ground shelters as close to the residence as possible to minimize problems of getting there during a storm. Provide counter balanced doors and hand rails on the stairs. The hinged door should open inward so it cannot be jammed shut by debris. Underground shelters may be impractical in areas with a high water table or bedrock near the ground surface. There are major design considerations that must be considered in the design of a tornado-resistant shelter. The first one is the pressure force created by high winds. These forces on walls and roofs can range from less than 50 pounds per square foot for a weak tornado, to over 200 pounds per square foot for a violent tornado having winds of almost 300 miles per hour. The sudden pressure drops create pressure forces on the shelter. Because of this you will need to install air vents to eliminate pressure forces. In-ground shelters must be strong enough to withstand earth pressures as well. Earth pressures vary depending upon depth of the shelter, soil type, and soil moisture content. Sudden pressure drops are not a major problem for adequately vented shelters so you will want to install appropriate ventilation. The total vent open area (square feet) should be at least .001 x the volume (cubic feet) of the shelter. For a 10 by 10 foot shelter with an 8 foot ceiling, the vent area would be 0.001 x 800 = 0.8 square feet, or 115 square inches. Vent pipes, one 10-inch diameter and one 8-inch diameter, can provide these with adequate ventilation. If heavy-screen safety covers are used on the vents, increase the vent area at least 25 percent. For the above example, screening would require increasing the vent opening to 144 square inches (1.25 times 115). Then, two 10-inch diameter vent pipes would be needed. Above ground shelters must be designed to withstand the full force of wind and debris. The primary advantage of this type of shelter is it can be located inside a new home or attached to an existing home. These can also serve as a bathroom, office, large walk-in closet, or a combination of office and storage for valuables. Above ground shelters should be designed by professional structural engineers. Wall and roof reinforcements need to be continuous, from the deep piers right up into the shelter roof, anchoring the shelter to the ground. Walls of above ground shelters are massive, made of cast-in-place concrete or sand and gravel block. With all cores filled with concrete they need to be reinforced both laterally and vertically. Planning a storm or tornado shelter is not generally a do it yourself type of project. It is wise to turn to an expert to help with the design of a tornado shelter. Be sure the contractor follows the plans or has any changes reviewed by a registered engineer or architect. A properly constructed storm shelter should last indefinitely. Therefore the time spent planning one is a very good investment. An above ground storm shelter is a good idea if you are looking for ways to add on another room anyway. Since the above ground storm shelter can double as a closet, or office, you can make the most out of that space. Just be sure that you do not find yourself cluttering your tornado or storm shelter with so much stuff that you and your family cannot get into it if the need arises during a storm. 
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