Safe Room
Derek Laxton
12/8/2011
Habitat for Humanity
Abstract
This report examines the feasibility of implementing a FEMA compliant safe room design into
new homes built by the White County Affiliate of Habitat for Humanity. It takes into account factors of
safety, cost, amount of unusable space, and ease of construction techniques.
Table of Contents
Introduction
1
Objectives/Specifications
1
Conceptual Designs/Alternatives
2
Findings/Results
3
Conclusions
6
References
6
Appendixes
7
1
Introduction
With every region that people live in there are always certain associated natural disasters that
the occupants must prepare for and deal with. In the state of Arkansas the most common and
potentially dangerous natural disaster is that of the tornado, and according to the Arkansas Department
of Emergency Management, the state has experienced 1,681 tornadoes between 1950 and 2007,with
239 tornadoes that were rated F3 and higher. Arkansas is also currently ranked fourth in the nation in
tornado related deaths and fifth in injuries. There is an average of nearly 30 tornados annually and
White County has the second most tornados in the state of Arkansas. (1) Fortunately, as with any
disaster, certain precautions can be made that drastically reduce the risk of fatalities for the occupants.
The most important preparation that an occupant of Tornado Alley can do is to have an interior
safe room or shelter that they can retreat inside to protect them from the worst. While exterior
underground shelters are just as protective, they often known to fall into disrepair from their infrequent
use and are not as quickly or easily accessible as an interior safe room is during harsh weather. As many
people cannot afford to install storm shelters in their home, many rely on the use of local public
shelters. However, as anyone who lives in Tornado Alley knows, tornados can form quickly and can
strike with little or no warning time to reach these shelters.
While there are already numerous designs for interior safe rooms that can withstand the FEMA
recommended specifications, many of these do not take into account that most lower income families
cannot afford these extravagant systems. The purpose of this report is to examine the feasibility of
implementing safe rooms into new homes built by the White County Affiliate of Habitat for Humanity.
Objectives/specifications
A safe room’s primary goal should protect its occupants from a tornado’s two greatest threatshigh wind pressure loads which could cause the structure to collapse and high velocity impacts from
windborne debris which could penetrate the structure. According to FEMA (Federal Emergency
Management Agency), a proper shelter should be designed to withstand wind pressures developed from
250 mph source in 3 second gusts and withstand windborne debris impact loads created by a 15lb 2 x 4
traveling at a horizontal velocity of 100 mph and a vertical velocity of 67 mph (2).
While there are already numerous safe room designs that meet and exceed these FEMA
specifications, the task of this report is to examine these different designs and find the design that is the
most easy to implement into the White County Affiliate of Habitat for Humanity’s current house design
plans. The most important aspect (other than safety) is the cost. Because Habitat for Humanity
homeowners generally earn a lower income, the safe rooms need to be as cost effective as possible.
However, Habitat for Humanity receives significant discounts on materials such as lumber and concrete
which will help mitigate the costs. The Arkansas Department of Emergency Management also has a safe
room rebate program which could potentially pay fifty percent of construction costs or $1000 of the cost
(whichever is the lesser value). However, upon discussion of the rebate program with Bonnie Arnold of
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the Department of Emergency Management, the funds appear to be somewhat limited and sporadic in
their availability so they should not be accounted as guaranteed assets to help cover the cost of the
project.
Other key aspects of safe room design selection are the size and ease of construction. Because
Habitat for Humanity limits the size of a three bedroom home to 1150 square feet to help keep the
homes affordable, size is of the upmost importance. Designs that require extremely thick walls or chase
walls that house wiring and plumbing take up useful space that the family will miss in the already small
sized home. The safe room designs should also keep in mind that the construction will likely be carried
out by volunteers who for the most part have little professional construction experience. While there
will be experienced supervisors there, designs should stray away from the use of extremely technical
construction techniques.
Conceptual Designs/Alternatives
Interior safe room designs can divided into two main categories, prefabricated and onsite
construction. Most interior prefab safe rooms are simple steel or reinforced concrete boxes that are
simply attached to the buildings foundation and the building is constructed around it. While
prefabricated designs are known to be extremely reliable they are not the best solution for the White
County Affiliate. Compared to onsite construction designs, they are extremely expensive and the entire
house plans must be designed around it. Many of the designs are not intended for everyday use except
for closet or extra storage space so they are not be easily implanted into the small existing house plans.
And of course prefab safe rooms leave little or no work for volunteers as the manufacturers usually
install them.
Most of the existing interior onsite construction methods that meet FEMA’s safety specifications
are consolidated and standardized on the FEMA 320 document. These designs can be further divided
into wood frame and non-wood frame designs. The non-wood frame designs consisted of concrete,
cinderblock (also known as CMU), and insulated concrete form (ICF) construction techniques. After
some rough cost estimations based from the FEMA 320 materials sheets, these techniques were found
to be generally higher priced, have considerably thicker walls, require chase walls for plumbing and
electricity, and need much more technical expertise than the wood frame techniques.
The FEMA 320 document shows only two different types of wood frame designs, the steel
sheathing and CMU infill designs. However, I stumbled upon another particular wood frame design
which would have been preferable provided I could get in touch with a sales representative. This design
is known as the Stormblocker Composite System and is a product of Norplex Micarta. This system
consists of a layer of composite sheet that is 5/16’’ thick and is essentially constructed the exact same
way as the steel sheathing method except this requires only 1 layer of ¾’’ plywood and no steel
sheathing. However, it appears that this product is only sold in completed safe room design packages
which are very expensive. If Habitat for Humanity could get a discount on this product it would be a
very optimal solution to implementing a safe room into existing designs because of its thin walls and
easy construction techniques. Of the other wood frame designs, the CMU infill design is the cheaper of
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the two but has much thicker walls (approx. 7” excluding drywall and texture) and has no room for
plumbing and wiring in the walls so it would also require a chase wall. Excluding the Stormblocker
Composite system, the steel sheathing design has the thinnest walls which are open to plumbing and
wiring and ventilation.
Some of the specifications mentioned by the FEMA 320 for its designs included that the interior
safe room is independent of the house’s roof system and other walls within the house. The safe room
needs to be anchored down with J hooks or other FEMA 320 compliant connectors to a reinforced
concrete slab. The optimal selection for the placement of the safe room should be that it is a small
room (bathrooms are usually preferable) away from the exterior walls.
Findings/Results
Of the existing FEMA compliant interior safe room designs, the wood frame steel sheathing is
currently the optimal solution. It has the thinnest walls and allows for easier volunteer construction
rather than the other designs. However, this method requires drilling holes and cutting 4’x 8’ sheets of
14 Ga. Steel which could prove challenging for volunteers. A generic bathroom-safe room design from
the FEMA 320 is shown below in Figure 1.
Figure 1. A generalized
version of the wood frame
steel seething design. It
can be found in drawing
AG-06 of the FEMA 320.
While the wood frame steel sheathing design is the best overall design option, there are
numerous detailed design options for the construction of this design which could drastically change both
the cost and functionality of the safe room within the FEMA 320’s safety parameters. Compiled below is
an assortment of design options and components that need to be considered. While some of the
questions consists of simple straightforward answers, others depend on unknown factors that can arise
during the assimilation of this design into an overall house design.
The wall design can either have the protective layers of ¾ inch plywood and 14 Ga. steel on the
interior or exterior of the safe room. Since this design is most likely intended to be used on a bathroom,
the protective layers should be attached to the exterior to allow for plumbing and wiring access in the
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walls. It is important to note for whichever side that the layers are on, the 14 Ga. steel layer should
always be on the innermost side with the plywood layers on the outer sides. Figure 2 shows a diagram
of a wall section which shows the protective layers on the exterior.
Figure 2. A cut-away wall view
showing the protective layers on the
exterior of the safe room’s walls.
Note the steel sheathing is on the
innermost side of the layers and the
plywood is on the outside.
This design also requires the use of many construction connectors to strengthen the joints.
While there are different companies that make connectors that adhere to the FEMA 320’s specifications,
I chose to price and plan on using the connectors manufactured by Simpson Strong-Tie. This is largely
due to the fact that these required connectors are overall somewhat difficult to find and Simpson
Strong-Tie connectors can be purchased from Lowes. (These particular connectors must still be special
ordered.) The two images shown below in Figure 3 are the only two types of specialty connectors that
are specified as being needed for the steel sheathing design. The part dimensions and specifications can
be found in the Simpson Strong-Tie catalog which is available on their website. (3)
Figure 3. Images of the
Simpson Strong-Tie
connectors LGT2 and
SPH4.
The ceiling design has many important components that need to considered. One very
important aspect is the limited interior height. Because the safe room needs to be independent of the
roof rafter immediately above it, the safe room’s 6 inch joists with protective layerings allow a maximum
interior height of approximately 7 foot (given the rafter’s lower edge is 8 foot from the ground).
Another important component of the roof is the inclusion of the ventilation system which is required in
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some capacity for all safe rooms. Powered exhaust fans are required for bathroom designs but are not
for any other safe room designs. The simplest design should not require any additional protective
layering material to be purchased, but there are variations of design depending on the ventilation
system desired shown in the FEMA 320 drawing AG-04.
The door and its hardware is easily the largest single cost component of the design. The FEMA
320 document calls for either a 12 Ga. or 14 Ga. door which can either be modified or purchased to
meet the door specifications shown in drawing MS-02. This door must include three grade 1 commercial
deadbolts with 1 inch throws and three 4 inch heavy duty 5-knuckle hinges which can quickly add to the
already expensive door cost. However, there is an alternative door design that meets the FEMA
requirements which was given to me by Dr. Ernst Kiesling, the Executive Director of the National Storm
Shelter Association and Professor of Civil Engineering at Texas Tech University. While this door has lower
monetary costs, it has a higher cost in terms of physical labor and space. Shown below in Figure 4, this
design actually consists of two doors. The outer door is a standard interior door that is intended for
daily use of the occupants. The inner door is the door that actually provides the protection. It is simply
two layers of ¾ inch plywood combined to a layer of 14 Ga. steel that is hung from a sliding rail hidden
inside of the wall. The fact that this design requires more physical labor could potentially be viewed as
an advantage over the generic FEMA 320 design since Habitat for Humanity has a plethora of
inexperienced volunteers rather than the more experienced contractors which may be needed for the
latter. However, the second wall (necessary for hiding and providing support for the sliding door) can
take up to 8 inches of additional space into the room.
Figure 4. The sliding door alternative
design with the normal interior door
for everyday use on the outside and
protective sliding door on the interior.
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Conclusions
While this report shows most of my research for the semester, it does not contain a detailed
working model of an 8 x 8 ft design and a conclusive cost estimate because they are still in progress. I
am currently working out some discrepancies concerning the FEMA 320’s ceiling joist spacing and other
dimensional issues. As far as the cost analysis goes, I cannot currently ascertain the number of
connectors and 2 x 6’s needed to attach the ceiling until I have the dimensional details for the previously
mentioned ceiling joists as well as dimensions for the sliding door design. Attached in Appendix A is a
materials list given for the FEMA 320’s design but the quantities are not entirely accurate which I am
currently working on.
References
1. “Arkansas safe room requirements,” Arkansas Department of Emergency Management;
http://www.adem.arkansas.gov/ADEM/Divisions/Admin/Mitigation/shelter.aspx
2. FEMA 320, 3rd edition;
http://www.fema.gov/library/viewRecord.do?id=1536
3. Wood Construction Connectors Catalog 2011-2012; Simpson Strong Tie;
http://www.strongtie.com/index.html?source=topnav
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Appendix A