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Short Description: Division of Agricultural Sciences and Natural Resources • Oklahoma State University. BAE-1010. George W. A. Mahoney ...

Content Inside: Oklahoma Cooperative Extension Service


Selecting Tornado Shelters
George W. A. Mahoney Raymond L. Huhnke
Associate Professor Agricultural Engineering

Oklahoma Cooperative Extension Fact Sheets are also available on our website at: http://osufacts.okstate.edu

Extension Agricultural Engineer Agricultural Engineering

"If you know a better `ole, go to it!" This famous saying of Old Bill, a British cartoon character of World War I and II, is appropriate advice for Oklahomans in tornado season. Even better advice would be to build a "better hole" where to use in storms. To be effective, a shelter has to be conveniently located and adequately strong. Storm shelters don't have to be damp holes in the ground. With planning, shelters can be livable and stocked with necessities for long periodsof stay. A better hole may have lights, electric outlets for a TV or radio, games for children, food, beverages, and a place for youngsters to sleep while waiting out the storm.

The result is very little, if any, room. Do not lock the shelter when storms are imminent! A key may not be available.

Design Considerations
There are three major damaging rnechanisms that must be considered in the design of a tornado-resistant shelter. First: Pressure forces created by high winds (Figure 1). Pressure 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. Second: Pressure forces created by sudden pressure drop (Figure 2). Pressure differences can range as high as 200 pounds per square foot. Sudden pressure drops are not a major problem for adequately vented shelters. The total vent open area (square feet) should be at least .001 x 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. Two vent pipes, one 10-inch diameter and one 8-inch diameter, would provide adequate ventilation. If heavy-screen safety covers are used on the vents, increase vent area at

Three decisions should be made before looking for shelter plans or hiring an engineer to design the shelter: size, function, and location. Will it be strictly a shelter from storms? If so, a 4 by 8 foot structure would accommodate several adults and older children up to an hour waiting for the storm to pass. A shelter this size would not provide space for more than four people to sit or lie down. Small shelters must never be used for storage! If a portion of the shelter will be used as a storage or as an office, consider building a unit at least 8 feet by 8 feet in size allowing about 10 square feet per person. If carefully planned, this size would provide storage space for many items and still leave room for an average family of five. Installing shelving along one wall adds storage space and room for a TV set or radio for weather reports. Foldaway bunk beds and a storage unit with padded seat can be installed in the opposite wall. To endure long hours of bad weather, stock necessities such as food, water, and blankets. Provide storage space for valuables such as legal documents, irreplaceable pictures, and the family silver. Where will shelter be located? Will it be located in the house, attached to the house, or will it be located in the back yard? Quick and easy access is a must. When planning a new home, the best location for a shelter 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 back yard. Shelters located away from the house have two inherent problems. First, one can be injured on the way to the shelter. If the warning comes too late, one may not have time to reach it. Second, the shelter may be full of neighbors and strangers.

Figure 1. Pressure forces created by 240 mph wind.

Division of Agricultural Sciences and Natural Resources Oklahoma State University


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