| [Building Damage] [Basis of Shelter Design] [Shelter Size] [New vs. Existing Homes] |
| [Foundation Types] [Shelter Location] [Shelter Cost] |
| Building Damage [TOP] |
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To understand what happens when extreme winds strike, you must first understand that tornado and hurricane winds are not constant. Wind speeds, even in these extreme wind events, rapidly increase and decrease. An obstruction, such as a house, in the path of the wind causes the wind to change direction. This change in wind direction increases pressure on parts of the house. The combination of increased pressures and fluctuating wind speeds creates stress on the house that frequently cause connected building components to fail. For example, the roof or siding can be pulled off or the windows can be pushed in. Buildings that fail under the effects of extreme winds often appear to have exploded, giving rise to the misconception that the damage is caused by unequal wind pressures inside and outside the building. This misconception has led to the myth that during an extreme wind event, the windows and doors in a building should be opened to equalize the pressure. In fact, opening a window or door allows wind to enter a building and increases the risk of building failure. Damage can also be caused by flying debris. If wind speeds are high enough, missiles can be thrown at a building with enough force to penetrate windows, wall, or the roof. For example, an object such as a 2" x 4" wood stud weighing 15 pounds, when carried by a 250-mph wind, can have a horizontal speed of 100 mph and enough force to penetrate most common building materials used in houses today. Even a reinforced masonry wall will be penetrated unless it has been designed and constructed to resist debris impact during extreme winds. Because missiles can severely damage and even penetrate walls and roofs, they threaten not only buildings but the occupants as well. |
| Basis of Shelter Design [TOP] |
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The purpose of a shelter is to provide a space where you and your family can survive a tornado or hurricane with little or no injury. In hurricane-prone areas, the shelter cannot be built where it can be flooded during a hurricane. Your shelter should be readily accessible from all parts of your house, and it should be free of clutter. To protect the occupants during extreme windstorms, the shelter must be adequately anchored to the house foundation to resist overturning and uplift. The connections between all parts of the shelter must be strong enough to resist failure; the walls, roof, and door must resist penetrations by wind borne missiles. Extensive testing by Texas Tech University and other wind engineering research facilities has shown that walls, ceilings, and doors commonly used in house construction cannot withstand the impact of missiles carried by extreme winds. The shelter designs discussed account for these findings by specifying building materials and combinations of building materials that will resist penetration by missiles in extreme winds. The shelter designs, including both materials and connections, are based on wind speeds that are rarely exceeded in the United States. Therefore, a shelter built according to these designs is expected to withstand the forces imposed on it by extreme winds without failing. Those forces may cause cracks or other signs of stress in the materials or connections used in the shelter, and they may cause materials or connections to yield. However, the intent of the design is not to produce a shelter that will always remain completely undamaged, but rather a shelter that will enable its occupants to survive an extreme windstorm with little or no injury. It is very important to note that predicting the exact strength of tornadoes and hurricanes is impossible. That is another reason why the shelter designs mentioned are based on extreme wind speeds and why the primary consideration is life safety. Designing a building to resist damage from more than one natural hazard requires different, sometimes competing, approaches. For example, building a structure on an elevated foundation to raise it above expected flood levels can increase its vulnerability to wind and seismic damage. These design approaches need to be thoroughly considered. In flood prone areas, careful attention should be given to the warning time, velocity, depth, and duration of flood water. These flooding characteristics can have a significant bearing on the design and possibly even the viability of a shelter. B&L Homes can provide you with information about other natural hazards that affect your area and can recommend appropriate designs. |
| Shelter Size [TOP] |
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The amount of floor area per person that your shelter must provide depends partly on the type of windstorm the shelter is intended to protect you from. Tornadoes are not long-lasting storms, so if you are relying on your shelter only for tornado protection, you will not need to stay in the shelter for a long time. As a result, comfort is not of great concern, and a shelter that provides about 5 square feet of floor area per person will be big enough. When the shelter is intended to provide protection from storms such as hurricanes, which can last up to 12 hours, the comfort of the occupants should be considered. For this type of shelter, the recommended amount of floor area per person is about 10 square feet. Necessities, such as water and toilet facilities, should be provided. The shelter designs offered are based on a maximum floor area of 64 square feet and a maximum wall length of 8 feet. A shelter of that size used for hurricane protection can accommodate up to six people in reasonable comfort. If you plan to build a shelter with any wall longer than 8 feet, consult B&L Homes. |
| New vs. Existing Houses [TOP] |
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The shelter designs discussed were developed primarily for use in new house, but some can be used in existing houses. When a new house is being built, the builder/contractor can construct walls, foundations, and other parts of the house as required to accommodate the shelter. Modifying the walls or foundation of an existing house as necessary for the construction of a shelter is more difficult. As a result, some of the shelter discussed are not practical for existing houses. The shelter designs are applicable for any on site construction. However, in a modular house, the shelter location would be limited to the basement or the belowground module. A modular house is a house constructed of modular units that have been built elsewhere, brought to the site, and installed on a permanent foundation. |
| Foundation Types [TOP] |
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Houses on the following types of foundations are suitable for the installation of a shelter: |
| Shelter Location [TOP] |
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There are several possible locations in your house for a shelter. Perhaps the most convenient and safest is below ground or in your basement. If your house does not have a basement, you can install an in-ground shelter beneath a concrete slab-on-grade foundation or a concrete garage floor. Basement shelters and in-ground shelters provide the highest level of protection against missiles and falling debris. Another alternative shelter location is an interior room on the first floor of the house. Researchers, emergency response personnel, and people cleaning up after a tornado have often found an interior room of a house still standing when all other above ground parts of the house have been destroyed. Closets, bathrooms, and small storage rooms offer the advantage of having a function other than providing occasional storm protection. Typically, these rooms have only one door and no windows, which makes them well suited for conversion to a shelter. Bathrooms have the added advantage of including a water supply and toilet. Regardless of where in your house you build your shelter, the walls and ceiling of the shelter must be built so that they will protect you from missiles and falling debris, and so that they will remain standing if your house is severely damaged by extreme winds. If sections of your house walls are used as shelter walls, those sections must be separated from the structure of the house. This is true regardless of whether you use interior or exterior walls. The following are possible floor plan options for shelter locations: |
| Shelter Cost [TOP] |
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The cost of your shelter will vary according to the following:
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