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Disaster Protection with Concrete Masonry Safe Cells

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General
Wind can be devastating, whether it is the unpredictable fury of a tornado or the wide spread damage of a hurricane. Wind and the associated debris missiles can kill or injure persons that are not protected. Fortunately, properly constructed reinforced concrete masonry structures with concrete roofs can provide a proven protective structure.

A concrete masonry Safe Cell can be constructed in existing structures or incorporated with new construction. The walls of the shelter must be completely separate from the surrounding structure. Keeping the shelter separate makes it possible for the Safe Cell to remain standing even if portions of the house around or above it are destroyed.

The concept of "life safety" is used to develop the recommendations for shelters. The "life safety" intent of the design for Safe Cells is to produce a shelter that will enable its occupants to survive an extreme windstorm with little or no injury. The critical requirements for a shelter are good accessibility, resistance to wind forces, adequate fasteners, adequate anchorage and resistance to missile penetration.

Tornadoes
Tornadoes are unpredictable. Winds can be extremely high. The duration of the storm in a single location can be measured in minutes. Table 1 shows the tornado classifications and associated levels of damage. Over 99% of all tornadoes have top wind speeds greater than 250 mph (402 kph).

The short duration of tornadoes means that the Safe Cell can be modest in size since it only needs to contain a small amount of supplies.

Table 1: Tornado Classification and Fujita Scale

Hurricanes
Hurricanes are tracked for days and have a damage area measuring many miles across. The force of the wind is less than in a typical tornado, but is still strong enough to cause severe damage and devastation. The long duration of the storm requires that shelters be large and equipped with many supplies and utilities.

Near large bodies of water, the storm surge can reach many feet and preclude some locations or types of shelters because of the possibility of drowning.

Some coastal areas are subject to both tornadoes and hurricanes. Figure 1 shows the wind zones in the United States and the design wind speeds for 3-second gusts.

Destructive Nature of Tornadoes and Hurricanes
Debris is propelled when sustained winds and gusts combine. The flying debris is the most dangerous and destructive part of a tornado or hurricane. Experience has shown that walls, ceilings and doors normally used in home construction cannot withstand the impact of missiles (debris) carried by the severe winds.

To measure the suitability of a wall to resist penetration of debris, a test simulating possible conditions has been developed. In tests at the Wind Engineering Research Center (WERC) at Texas Tech University, a 10 foot (3 meter) long 2 x 4-inch (50 mm x 100 mm) board is shot at a wall at 100 mph (160 kph). The projectile can penetrate hollow concrete masonry walls and partially grouted walls. Reinforced solidly grouted walls as thin as six inches (150 mm) have demonstrated the ability to prevent penetration. The proven performance of reinforced concrete masonry makes it the ideal method of construction.

Safe Cell Location
The best place for a Safe Cell is a central location that can be accessed without leaving the structure; going outside could expose a person to flying debris. History has shown that people are often injured by debris early in a storm before they can get into an exterior shelter.

A basement is usually the best place to build a Safe Cell. Locate the cell in a corner or along an exterior wall, if the exterior walls are reinforced. If the cell is built in an existing structure, the floor may have to be removed and replaced with a thicker floor to provide adequate anchorage and stability.

If the Safe Cell is an independent structure inside the basement, it must have its own concrete roof which does not serve as the floor for the first living level. In this case the floor may have to be removed and replaced with a thicker floor to provide adequate anchorage.

Safe Cells may also be built in slab on grade homes. Possible locations are storage rooms, in a garage or in a bathroom. A bathroom has the added advantage of including a water supply. In these locations the existing floor may have to be replaced with a thicker slab for anchoring purposes.

In homes built with a crawl space, a garage may be the preferred location for a Safe Cell since it must be totally separate from the structure of the home.

Figure 1: Wind Zones in the United States
(select image for larger view)

Design and Construction
The Federal Emergency Management Agency (FEMA) has made recommendations on the size and construction of a Safe Cell. The Safe Cell should only be as large as necessary to insure its strength and stability.

Tornado
In a tornado, the duration is short so few supplies are needed. As a guide, five square feet (.46 meters) per person is recommended.

Hurricanes
During hurricanes, which have a much longer duration than tornadoes, 10 square feet (.92 sq m) is recommended for each person. The FEMA recommends a maximum floor area of 64 square feet (5.9 sq m) and a maximum wall length of 8 feet (2.4 m). Larger shelters may be built, but a licensed professional must design them.

Safe Cell Construction
Regardless of the location, the basic construction should be solidly grouted reinforced concrete masonry anchored to a concrete foundation. The FEMA, in cooperation with WERC at Texas Tech University, has developed the "National Performance Criteria for Tornado Shelters." This details the criteria for all types of shelters.

The following is a summary of the suggestions for concrete masonry shelters:

Foundation
Foundation anchorage #4 (13 mm) reinforcing steel at eight inches (200 mm) on center.

Wall type and reinforcement
Solidly grouted 8 inch (200 mm) concrete masonry.

Vertical - #4 bars (13 mm) eight inches (200 mm) on center.

Horizontal - #8 gage truss type wall reinforcement eight inches (200 mm) on center.

Around doors - Extra #4 bar (13 mm) each side and a bond beam with two - #4 (13 mm) bars above.

Roof and anchorage
Four inch (100 mm) thick concrete with #4 bars 12 inches (13 mm) on center each wall.

Anchor to walls with #3 bar (10 mm) every eight inches (200 mm) embedded in the wall 12 inches (300 mm).

Door
Metal door frame.

Outward swinging 16 gage metal door - 1-3/4 inch (44 mm) thick.

Cover outside of door with 11 gage cover on outside. Provide two-slide bolts for locking.

Ventilation
Vent with two six-inch (150 mm) vents in roof or in wall 7"-0" (2.1 meters) above the floor.

Vents to be steel pipe elbow designed to prevent debris passage.

Summary
The construction of Safe Cells using concrete masonry is possible for most homes. Using proper planning, construction costs can be minimized. The inherent properties of concrete masonry can be used to resist the fury of violent storm and save lives.