Planning Your Shelter
FEMA 320, Second
edition - 7.7 MB
Now that you better understand your risk from a tornado or hurricane, you can
work with your builder/contractor to build a shelter to protect yourself and
your family from these extreme windstorms. This section describes how extreme
winds can damage a building, explains the basis of the shelter designs
presented in this booklet, and shows where you can build a shelter in your
house.
Building Damage
Extreme winds can cause several kinds of damage to a building. Figure II.1
shows how extreme winds affect a building and helps explain why these winds
cause buildings to fail.
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 causes connections between 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 (referred to as windborne missiles).
If wind speeds are high enough, missiles can be thrown at a building with
enough force to penetrate windows, walls, 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
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, and the walls, roof, and door must resist
penetration by windborne 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 in this booklet 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 designs
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 in
this booklet 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
floodwaters. These flooding characteristics can have a significant bearing on
the design and possibly even the viability of a shelter. Your local building
official or licensed professional engineer or architect can provide you with
information about other natural hazards that affect your area and can recommend
appropriate designs.
Shelter Size
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 in this booklet 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 a licensed professional engineer or architect.
New vs. Existing Houses
The shelter designs in this booklet were developed primarily for use in new
houses, 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 designs in this booklet are
not practical for existing houses. The following sections discuss this issue
further. In this booklet, the term "retrofit" refers to the process of making
changes to an existing house.
Foundation Types
Houses on the following types of foundations are suitable for the installation
of a shelter:
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basement
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slab-on-grade
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crawlspace
A house on a basement foundation is usually built on a foundation constructed
of poured concrete or concrete masonry. Most concrete foundations are
reinforced with steel bars or straps, but many concrete masonry foundation
walls have no steel reinforcement. The framing for the floor above the basement
is supported by the exterior foundation walls and sometimes by a center beam.
In a new or existing house with a basement, the shelter should be built in the
basement. You can build the shelter as an entirely separate structure with its
own walls, or you can use one or more of the basement walls as walls of the
shelter. If you use the basement walls, they will have to be specially
reinforced. Typical reinforcement techniques used in residential basement walls
will not provide sufficient protection from missiles. In new construction, your
builder/contractor can reinforce the walls near the shelter during the
construction of your house. Reinforcing the basement walls of an existing house
is not practical.
The likelihood of missiles entering the basement is lower than for above ground
areas; however, there is a significant chance that missiles or falling debris
will enter the basement through an opening left when a window, a door, or the
first floor above has been torn off by extreme wind. Therefore, your basement
shelter must have its own reinforced ceiling; the basement ceiling (the first
floor above) cannot be used as the ceiling of the shelter.
The least expensive type of shelter that can be built in a basement is a
lean-to shelter, which is built in the corner of the basement and uses two
basement walls. The lean-to shelter uses the fewest materials, requires the
least amount of labor, and can be built more quickly than other types of
basement shelters.
In general, it is easier to add a basement shelter during the construction of a
new house than to retrofit the basement of an existing house. If you plan to
add a basement shelter as a retrofitting project, keep the following points in
mind:
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You must be able to clear out an area of the basement large enough for the
shelter.
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Unless the exterior basement walls contain steel reinforcement as shown on the
design drawings provided with this booklet, these walls cannot be used as
shelter walls since they are not reinforced to resist damage from missiles and
uplift from extreme winds.
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Exterior basement walls that are used as shelter walls must not contain
windows, doors, or other openings.
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The shelter must be built with its own ceiling, so that the occupants will be
protected from missiles and falling debris.
A slab-on-grade house is built on a concrete slab that is installed on
compacted or natural soil. The concrete may be reinforced with steel that helps
prevent cracking and bending. If you are building a new slab-on-grade house and
want to install a concrete or concrete masonry shelter, your builder/contractor
must make the slab thicker where the shelter will be built. The thickened slab
will act as a footing beneath the walls of the shelter to provide structural
support. It will also help anchor the shelter so that it will stay in place
during an extreme wind event, even if the rest of the house is destroyed.
In an existing house, removing part of the slab and replacing it with a
thickened section would involve extensive effort and disruption inside the
house. Therefore, building a shelter with concrete or concrete masonry walls in
an existing slab-on-grade house is generally not practical. You can, however,
build a wood-frame shelter, because its walls are not as heavy and do not
require the support of a thickened slab. A wood- frame shelter can be created
from an existing room, such as a bathroom or closet, or built as a new room in
an open area in the house, such as a garage.
You can also build a shelter as an addition to the outside of a slab-on-grade
house. This type of shelter must have not only proper footings, but also a
watertight roof. Because a shelter built as an outside addition will be more
susceptible to the impact of missiles, it should not be built of wood framing.
Instead, it should be built of concrete or concrete masonry. Access to this
type of shelter can be provided through an existing door or window in an
exterior wall of the house.
In general, it is easier to add a shelter during the construction of a new
slab-on-grade house than to retrofit an existing slab-on-grade house. If you
plan to add a shelter to a slab-on-grade house as a retrofitting project, keep
the following points in mind:
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The walls of the shelter must be completely separate from the structure of the
house. Keeping the walls separate makes it possible for the shelter to remain
standing even if portions of the house around it are destroyed by extreme
winds.
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If you are creating your shelter by modifying a bathroom, closet, or other
interior room with wood-frame walls, the existing walls, including sheathing on
either the inside or outside of the walls, such as drywall or plaster, must be
removed and replaced with walls and a ceiling resistant to the impact of
windborne missiles and other effects of extreme winds.
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If you intend to build a shelter with concrete or concrete masonry walls, a
section of your existing slab floor will have to be removed and replaced with a
thicker slab. As noted above, this is usually not practical in an existing
house.
A house built on a crawlspace usually has a floor constructed of wood framing.
Along its perimeter, the floor is supported by the exterior foundation walls.
The interior part of the floor is supported by beams that rest on a foundation
wall or individual piers. Crawlspace foundation walls may be concrete, but are
usually constructed of concrete masonry. Crawlspace foundation walls are often
un-reinforced and therefore provide little resistance to the stresses caused by
extreme winds.
Building a shelter inside a house on a crawlspace foundation is more difficult
than building a shelter inside a house on a basement or slab-on-grade
foundation. The main reason is that the entire shelter, including its floor,
must be separate from the framing of the house. As shown in Figure II.4, a
shelter built inside the house cannot use the floor of the house. The shelter
must have a separate concrete slab floor installed on top of earth fill and
must be supported by concrete or concrete masonry foundation walls. An
alternative approach, which may be more economical, is to build an exterior
shelter on a slab-on-grade adjacent to an outside wall of the house and provide
access through a door installed in that wall.
Ventilation in the area below the floor of the house is also an important
issue. The wood-framed floor of a house on a crawlspace foundation is typically
held 18 to 30 inches above the ground by the foundation walls. The space below
the floor is designed to allow air to flow through so that the floor framing
will not become too damp. It is important that the installation of the shelter
not block this air flow. In general, it is much easier to build a shelter
inside a new crawlspace house than in an existing crawlspace house. If you plan
to add a shelter to an existing crawlspace house as a retrofitting project,
keep the following in mind:
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The shelter must have a separate foundation. Building the foundation inside the
house would require cutting out a section of the existing floor and installing
new foundation members, fill dirt, and a new slab - a complicated and expensive
operation that is often not practical.
-
A more practical and more economical approach would be to build an exterior
shelter, made of concrete or concrete masonry, on a slab-on-grade foundation
adjacent to an outside wall of the house, as described above.
Shelter Location
There are several possible locations in your house for a shelter. Perhaps the
most convenient and safest is below ground level, 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 of the house.
These are not floor plans developed specifically for houses with shelters. They
show how shelters can be added without changes to the layout of rooms.
Floor Plan 1: basement
Possible shelter locations in a basement include the following:
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in a corner of the basement, preferably where the basement walls are below the
level of the ground
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in a bathroom, closet, or other interior room in the basement
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in a freestanding addition to the basement
A space that is to be used for a shelter must be kept free of clutter so that
the shelter can be quickly and easily entered and so that the shelter occupants
will not be injured by falling objects. For this reason, a bathroom is often a
better choice for a shelter than a closet or other space used for storage.
Remember, if the basement is below the level of storm surge or the level of
flooding from any other source, it is not a suitable location for a shelter. In
this situation, a possible alternative would be to build an exterior shelter,
adjacent to your house, on a slab-on-grade above the flood level.
Floor Plan 2: house on a slab-on-grade or crawlspace foundation
Possible shelter locations in a house on a slab-on-grade or crawlspace
foundation include the following spaces on the first floor:
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bathroom
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closet
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storage room
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laundry room (provided the load-bearing wall between it and the garage can be
properly separated from the structure of the house)
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corner of the garage
Regardless of where the shelter is built, it must be equipped with a door that
will resist the impact of missiles. Remember, if the first floor of the house
is below the level of storm surge or the level of flooding from any other
source, it is not a suitable location for a shelter. In this situation, a
possible alternative would be to build an exterior shelter on a slab-on-grade
elevated on fill above the flood level.
Floor Plan 3: house on a slab-on-grade foundation
Possible locations for an in-ground shelter include the following:
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below the slab in a closet or storage room
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below the floor of the garage, in an area where cars will not be parked
Because of the difficulty of installing an in-ground shelter in an existing
house, this type of shelter is practical only for new construction. Remember,
if the first floor of the house is below the level of storm surge or the level
of flooding from any other source, it is not a suitable location for a shelter.
In this situation, a possible alternative would be to build an exterior shelter
on a slab-on-grade elevated on fill above the flood level.
Construction Materials
The materials your builder/contractor will need to build your shelter should be
available from building material suppliers in your community. These materials
have been carefully selected for their strength, durability, and/or ability to
be readily combined in ways that enable them to withstand the forces of extreme
winds and the impact of windborne missiles. Your builder/contractor should not
substitute any other material for those specified in the designs.
One of the most vulnerable parts of your shelter is the door. The materials
specified for doors in the shelter designs in this booklet were tested by the
Wind Engineering Research Center at Texas Tech University for their ability to
carry wind loads and prevent penetration by missiles. The installation of the
door is as important as the materials used in its construction. Please confirm
with your builder/contractor that the door to your shelter can be installed the
way it is shown in the designs included with this booklet.
A complete list of the shelter construction materials, with their expected
strengths or properties, is included in the shelter designs provided in this
booklet. Your builder/contractor should use it when buying the materials for
your shelter.
Shelter Cost
The cost of your shelter will vary according to the following:
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the size of the shelter
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the location of the shelter
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the number of exterior house walls used in the construction of the shelter
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the type of door used
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the type of foundation on which your house is built
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your location within the United States (because of regional variations in labor
and material costs)
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whether you are building a shelter into a new house or retrofitting an existing
house
The cost of retrofitting an existing house to add a shelter will vary with the
size of the house and its construction type. In general, shelter costs for
existing houses will be approximately 20 percent higher.
NOTE
The shelter designs in this booklet are applicable for any on-site
construction. However, in a modular house, the shelter location would be
limited to the basement or the below- ground 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
WARNING
You should not install a shelter in a house supported by piles, piers, or
columns. With building connectors commercially available, there is no
economical way to separate the shelter from the floor framing and ensure that
the shelter will withstand the forces of extreme winds.
You may be tempted to build a shelter under a house on a pile, pier, or column
foundation. However, if the house is in a storm surge area or other flood
hazard area, the area under the house would be below the flood level. A shelter
built in that area would trap its occupants in rising floodwaters.