Quick Answer
Above-ground shelters are not effective against strong tornadoes and can actually be more deadly than being in the open. This is because above-ground shelters often lack the necessary protection from wind-borne debris and extreme wind pressures. The National Weather Service recommends that above-ground shelters not be designed to withstand tornadoes rated EF3 or higher.
Tornado Wind Pressures
Above-ground shelters are generally designed to withstand wind pressures of up to 200-250 pounds per square foot, which is typical for weak to moderate tornadoes. However, strong tornadoes can generate wind pressures of over 1,000 pounds per square foot, easily exceeding the capacity of most above-ground shelters. This is why above-ground shelters are not recommended for areas prone to strong tornadoes.
Debris Penetration
Another significant concern with above-ground shelters is debris penetration. During a tornado, flying debris can easily penetrate the shelter, causing injuries or fatalities. This is particularly concerning for above-ground shelters that are not reinforced with debris-resistant materials. In contrast, underground shelters can provide better debris penetration resistance due to the earth’s natural protective properties.
Design and Construction Standards
Above-ground shelters must adhere to strict design and construction standards to provide any level of protection against tornadoes. These standards typically include reinforced walls and ceilings, secure doors and windows, and anchoring systems that can withstand extreme winds. However, even with strict adherence to these standards, above-ground shelters can still be vulnerable to strong tornadoes.
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