Tornado Shelter: Before You Begin

Photo Credit

USDA Forest Service

This 8 foot by 8 foot tornado shelter was developed by engineers at the US Department of Agriculture’s Forest Products Laboratory to provide a design that is adaptable for existing construction and that is easily constructed.

Type of shelter: Residential tornado

The design meets the impact and wind pressure requirements of ICC 500-2014: Standards for the Design and Construction of Storm Shelters.

Design Wind Speed: 250 mph

Wind Exposure Category: C

Internal pressure coefficient, GCpi: +-0.55

Topographic factor, Kzt: 1.0

Directionality factor, Kd: 1.0

A statement must be provided that the floor of the shelter has been constructed above the highest recorded flood elevation if a flood hazard study has been conducted for the area or the minimum elevation required by authority having jurisdiction.

Extensive laboratory testing was performed to assure that this 8’x8’ tornado shelter meets the impact and wind pressure requirements of Chapter 3 and Chapter 8 of ICC 500-2014: Standards for the Design and Construction of Storm Shelters.

The door shown in this construction guide was laboratory tested for debris impact for a 3'-0" width.

Location of the tornado shelter within a host building and a drawing of the entire building.

A shelter section or elevation indicating the height of the shelter relative to the finished grade, finished floor and the host building, where applicable.

Hold down design capacity: 4,000lb

Assumed friction coefficient for concrete cast on soil, f: 0.50

Occupant Load: 9 people

Usable floor area: 49 square feet

Venting area: 19.8 in²

Concrete foundation minimum requirements:

• 4" thick concrete slab
• 4000 psi minimum strength
• 6x6 - W1.4 X W1.4 welded wire reinforcement over the entire area or No. 4 bars, at a maximum spacing of 18 in. on center in two perpendicular directions.
• Minimum 5' slab extension on each side of shelter to prevent foundation sliding and overturning.

This shelter design meets the missile impact and wind load requirements of national standards for tornado shelters.

As part of your planning process it is important to contact your local building department to determine if there are other requirements for tornado shelters in your area. For example, some jurisdictions have requirements on door swing direction, which may affect the suitability of this design to your location.

Because the door design presented here is self-built, it may not meet the door labeling requirements (Section 108.2) of the ICC-500 standard. However, extensive laboratory testing confirms that the door meets the impact and wind pressure requirements of the standard.

Before beginning construction of the shelter, ensure that the installation site meets the following requirements:

• Proximity. If not located in the home, the shelter must be located a maximum distance of 150 ft. from the home.
• Protection from Rain and Snow. This shelter is not designed to be water-resistant and must be built inside the home, in a garage, or other structure that protects the shelter from the elements.
• Concrete Foundation. If the foundation does not meet the requirements stated earlier, FEMA P-320 provides foundation construction details.
• Detachment from Host Building. Do not attach any portion of the shelter to an adjoining wall or to the floor or roof framing of the enclosing structure (or vice versa).

The designers of this tornado shelter estimate a material cost of between $3500 - $4000 (2016 dollars). See the Appendix of the following document for a material list and costs.

Photo Credit

USDA Forest Service

This 8 foot by 8 foot tornado shelter was developed by engineers at the US Department of Agriculture’s Forest Products Laboratory to provide a design that is adaptable for existing construction and that is easily constructed.

• #2 grade stamped softwood lumber with engineering properties at least equivalent to the Spruce Pine Fir (SPF) species group.
• 22/32 softwood plywood sheathing.

The construction of this shelter design can be broken down into the following 8 phases:

1. Cutting the lumber. The lumber is cut to length. Some pieces will also be rip cut to the correct width. The installer must keep careful track of which pieces have been cut to which length.
2. Constructing the beams. The triple beams that make up the walls and ceiling of the shelter are made from three same-length 2x8s. The middle 2x8 is offset by 1-1/2” to give each beam a tongue-and-groove cross section. The 2x8s are joined with construction adhesive and nails to form a laminated beam.
3. Building the walls. The walls are constructed by laying one course of horizontal beams at a time for the entire perimeter of the shelter. The next course’s tongue will fit into the lower course’s groove and is joined using construction adhesive. The beams are laid up such that the walls lock together in a log cabin fashion. The beams are joined at the corners with 8” long wood screws and construction adhesive.
4. Installing the ceiling beams. Twelve ceiling beams made from 2x8s are installed individually in place. Once the last 2x8 beam is installed, a final beam is custom cut from three 2x10s to complete the 8 ft. x 8 ft. ceiling. All thirteen beams are attached to the walls using 8” long wood screws and to each other using construction adhesive. The screws are driven into the exterior of a wall at a 45° angle and into the ceiling beam above. This allows the beams to be attached in sites where a low ceiling exists, such as a basement.
5. Attaching the sheathing. The 23/32 plywood sheathing is cut to size and attached with nails and construction adhesive to the interior and exterior faces of all four walls and the interior of the ceiling (as well as the top of the ceiling if it is accessible).
6. Building and hanging the door. The door is made up of three sheets of 22/32 plywood sandwiched between two sheets of 18 gauge cold-rolled steel, a material that must be sourced locally. The door is hung on bolt hooks installed on the exterior of the shelter wall, and has latches on its interior surface for locking the shelter when occupied. Sheet metal angles are used to reinforce the door opening. These need custom fabrication and are sourced locally. Most cities have a steel fabricator that can supply, cut, punch, and bend sheet metal to custom sizes.
7. Ventilation. Ventilation holes are drilled through the wall at specified locations to provide fresh air to the interior of the shelter.
8. Anchoring the shelter. The shelter is secured to the concrete slab using Simpson Strong-Tie (or equivalent) hold-downs nailed to the shelter’s exterior. 5/8” dia. anchor bolts secure the hold-downs to the slab with an epoxy adhesive per anchor manufacturer’s requirements.

Note 1: Actual dimensions for lumber differ from the nominal dimensions. A nominal 2x8 board actually measures 1-1/2” x 7-1/4”. The actual thickness of 23/32 plywood can vary from 0.69” to 0.75”.

Note 2: The lumber quantities above are the minimum required to make each component. It is advised that actual lumber orders include a waste factor (i.e., additional boards) to account for warpage, waste, loss or mistakes.

1. Measuring tape
2.  
3. Hammer
4. Circular saw
5. Sawhorses
6. Hand saw or reciprocating saw (Sawzall)
7. 12” woodworking clamps
8. Pneumatic framing nail gun with air compressor
9. Socket set / wrenches (various sizes)
10. Drill capable of driving 8” screws through 6” of wood
11. Drill bits:
    1. 1” auger drill bit for ventilation holes
    2. 1/2” drill bit for bolt holes in door
    3. 3/4” auger drill bit for door hinges
    4. Step drill (3/4” minimum)
    5. Drill bit for concrete anchors
12. Anchor installation tools per manufacturer
13. Caulking gun
14. Drywall lift (optional)
15. Table saw (optional)
16. Miter saw (optional)