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United States Department of Agriculture

Residential Tornado Safe from Commodity Wood Products Design and Development

Robert H. Falk James J. Bridwell

Forest Forest Products General Technical Report February Service Laboratory FPL–GTR–253 2018 Abstract

In the United States, tornadoes cause significant damage and Conversion result in many injuries and deaths. Although the English unit factor SI unit development and use of tornado safe have helped mile per hour kilometer per hour 1.609 decrease the human toll associated with these events, the (mi/h) (km/h) cost of these structures is often too high for many that could inch (in.) 25.4 millimeter (mm) benefit from their use. The development of a nonproprietary residential tornado safe room constructed from commodity square inch (in2) 645.16 square millimeter (mm2) wood products, buildable by a local contractor or foot (ft) 0.3048 meter (m) do-it-yourselfer, and adaptable to existing , could square foot (ft2) 0.092903 square meter (m2) lower the cost of these structures and result in more widespread use. This report is an overview of the design and ounce (oz.) 2.957 ×10–5 cubic meter (m3) development of such a residential tornado safe room. This Nominal lumber size (in.) Standard lumber size (mm) design was tested according to the requirements of the 2 by 8 38 by 184 International Code Council Standard for the Design and Construction of Storm Shelters. It has a projected material cost of between $3,500 and $4,000, which is significantly less than a prefabricated room of similar size and built from other materials.

Keywords: tornado safe room, design, wood construction

February 2018 Falk, Robert H.; Bridwell, James J. 2018. Residential tornado safe room from commodity wood products: Design and development. FPL–GTR–253. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 6 p. A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726-2398. This publication is also available online at www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries in the United States and elsewhere. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the United States Department of Agriculture (USDA) of any product or service.

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Robert H. Falk, Research Engineer James J. Bridwell, General Engineer USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin

Introduction Objective and Scope A significant portion of the United States is susceptible to The overall objective of this research was to develop a the dangerous winds of tornadoes (Fig. 1). Every year, these tornado safe room constructed from commodity wood events cause injury, death, and billions of dollars of property products that meets established performance criteria for damage (FEMA 2014). Improved weather forecasting, such structures. Several goals were pursued in the design of increased amount of warning time, and the development of the wood tornado safe room: (1) as much as possible, reinforced shelters have helped decrease the human toll of materials should be available from local building material these events. outlets and online sources, (2) the structure should be buildable by a local contractor or an advanced do-it- The principal function of a tornado safe room is to protect yourselfer, (3) retrofitting the room into an existing occupants from high winds and debris generated by a should be possible, (4) costs should be kept down by tornado even if the structure that it is enclosed in is damaged minimizing the use of specialty materials and hardware. or destroyed. Most commercially available tornado safe rooms are constructed of steel or , materials Performance Criteria for Tornado typically requiring specialized professionals and equipment to construct. Wood products are nearly ubiquitous, require Safe Rooms simple tools to size and connect, and are familiar to most The performance requirements for a residential tornado safe building contractors. Contrary to children’s fables room have been standardized via ICC/NSSA-500 suggesting that wood is inferior to other building materials (ICC/NSSA 2014). This standard presents occupancy in high winds, wood and wood construction have strength requirements, impact testing, wind pressure testing, enough to resist the forces of wind and earthquakes and also ventilation, and other performance criteria for these possess energy absorption characteristics superior to other structures. From an engineering standpoint, impact and wind common building materials. pressure testing are the most challenging of these requirements. The results of the tests performed on the safe room described here are detailed in a companion report entitled “Residential Tornado Safe Room from Commodity Wood Products: Impact and Wind Pressure Testing” (Falk and others 2018). Safe Room Design Considerations Although nonproprietary wood safe room designs exist (FEMA 2014), they are typically based on a conventional nominal 2-in. stud design with added reinforcement, either in the form of steel plates or added infill masonry. The safe room design presented here is a departure from this convention and uses mass timber construction with solid Figure 1. Tornado activity in the United States from 1950 wood and . Because commodity lumber is to 2013 (Source: Storm Prediction Center 2014). commonly sold in 8-ft lengths and wood sheathing materials General Technical Report FPL–GTR–253

Figure 2. Nail-laminated 2 by 8 wall beam. are typically 4 by 8 ft in size, the room has been designed to be 8 by 8 ft in plan with a height of up to 8 ft. This size not only minimizes material waste but also makes this space Figure 4. Safe room with plywood nailed and glued to lumber beams. suitable for other uses (for example, , ) when not needed in an emergency. The use of 2 by 8s and plywood as the basic building blocks The design of this safe room was an iterative process in of this safe room allows easy transfer and construction into which a potential wall panel design was established on the of a home or inside a , allowing the paper, built, and then impact tested to evaluate performance. room to be retrofitted into existing homes. In addition, the The design was modified and retested until a suitable design use of rows of interlocked lumber beams allows the room to evolved. Falk and others (2015) reports this wall and roof be built with various heights, which makes the room panel testing. adaptable to height-limited spaces, such as or garages with trussed roofs. Wall and Roof Panels Safe Room Based on the wall and roof testing previously referenced, a final room design was established. The walls and roof of An important component of a tornado safe room is the entry this safe room were constructed of stacked and door, and as with the tornado safe room itself, it must be interconnected nail-laminated lumber beams sheathed with designed and constructed to meet the requirements of plywood. Three 2 by 8s were nailed and glued together to ICC/NSSA-500 (ICC/NSSA 2014). Although many tornado form a beam with a tongue and groove configuration safe room exist in the marketplace, they are usually (Fig. 2). The beams were then stacked and interlocked in log constructed solely of steel. A was conducted to cabin fashion to create the walls of the safe room (Fig. 3). determine if a tornado safe room door could be wholly or Nominal 3/4-in. plywood sheathing was then nailed and glued to the walls and roof to further reinforce the room as well as to tie the walls and roof together (Fig. 4).

Figure 3. Stacking of wall beams to form walls of Figure 5. Safe room door construction. safe room.

2 Residential Tornado Safe Room from Commodity Wood Products: Design and Development

Figure 6. bolt hook (left); Gate strap hinge (right). partially produced from commodity wood building products. This study followed the same iterative design process previously described for the safe room wall and roof panels (Falk and Bridwell 2016). The result was a door made from plywood and overlaid with 18-gauge (0.05-in.-thick) steel sheeting that was simple to construct and met the required impact resistance. The door was constructed of three sheets of nominal 3/4-in. Figure 8. 14-gauge steel angles used to reinforce door frame. plywood overlaid on both faces with 18-gauge sheet steel (Fig. 5). This construction is detailed in Falk and Bridwell was used to transfer the concentrated load of the cane bolt (2016). The door was hung with three 3/4-in. gate hinges, into the tornado safe room wall when the door was subjected similar to that which might be found on a livestock gate. to wind suction forces (Fig. 10). Figure 11 shows the three These hinges were chosen because of their low cost and cane bolts in place that secure the door from the inside, and adjustability in hanging an overlaid door (Fig. 6). Figure 12 shows the door in the latched position. The door was latched from the inside using three cane bolts (Fig. 7). These bolts are normally used in a vertical Securing the Safe Room to orientation to secure a livestock gate. They were chosen the because of their low cost, ease of installation, and simple Securing the safe room to the foundation is an important operation. aspect of safe room design, and the tie-down connector, the Based on tests described in Falk and others (2018), anchor bolts securing the tie-down connector, and the 14-gauge (0.07-in.-thick) sheet metal angle was used to concrete foundation must have adequate structural strength increase the impact resistance of the door frame (Figs. 8 to resist the forces of the winds produced by a tornado. and 9). The concrete foundation must have enough thickness, In addition, a Simpson Strong-Tie HL53 angle bracket reinforcement, and surface area to resist anchor bolt pullout, (Simpson Strong-Tie Company, Inc., Pleasanton, California) foundation sliding, and room overturning. These design considerations are addressed in FEMA P-320 (FEMA 2014), FEMA P-361 (FEMA 2015), and ICC/NSSA (2014).

Figure 7. Cane bolt used as a door latch. Figure 9. Schematic of door reinforcement angles.

3 General Technical Report FPL–GTR–253

Figure 10. Angle bracket used to reinforce the cane bolt Figure 12. Latched door. latches.

The tie-down used in the wood safe room described here from this requirement. These considerations will be detailed was a Simpson Strong-Tie HTT5 (Fig. 13). The uplift and in future design documents. shear resistance of the tie-down used in this application was evaluated in Falk and others (2018). Material Costs The Appendix indicates the material costs for the safe room Other Design Considerations discussed in this report as well as sources for these ICC/NSSA (2014) specifies other design considerations materials. As indicated in the Appendix, the cost of such as occupancy, ventilation, and fire resistance. Because materials was about $3,600. Although this total was valid in the walls of this 8- by 8-ft safe room are 6 in. thick, the 2016, this value should be considered an estimate because room has a usable area of 49 ft2. For a residential safe prices can vary. In addition, the costs listed do not include room, 5 ft2 per person are required. Therefore, this safe tax, shipping, or delivery costs, which also can vary room would provide shelter for up to nine people. by region. Ventilation is required and is based on the number of occupants. For a residential safe room, 2 in2 are required for Conclusions 2 each occupant; therefore, 18 in of ventilation are The objective of this study was to develop a nonproprietary appropriate for this safe room. Although a 2-h fire rating is residential tornado safe room constructed from commodity required for storm shelters, a residential safe room is exempt wood building products, buildable by a local contractor or

Figure 11. Cane bolts used as door latches. Figure 13. Simpson safe room tie-down.

4 Residential Tornado Safe Room from Commodity Wood Products: Design and Development do-it-yourselfer, and adaptable to existing homes. A low- FEMA. 2014. Taking shelter from the storm: building a cost design that meets the requirements of ICC/NSSA tornado safe room for your home or small business. FEMA (2014) has been developed and has a projected material cost P-320. 4th edition. Washington, DC: Federal Emergency of between $3,500 and $4,000. The results of impact and Management Agency. wind pressure testing performed on this design can be found FEMA. 2015. Tornado safe rooms for tornadoes and in a companion report (Falk and others 2018). hurricanes: guidance for community and residential tornado Literature Cited safe rooms. FEMA P-361. Washington, DC: Federal Emergency Management Agency. Falk, R.H.; Bridwell, J. 2016. Development of a tornado http://www.fema.gov/media-library/assets/documents/3140. safe room door from wood products: door design and impact testing. Res. Pap. FPL–RP–686. Madison, WI: U.S. ICC/NSSA. 2014. ICC/NSSA-500. Standard for the design Department of Agriculture, Forest Service, Forest Products and construction of storm shelters. Washington, DC: Laboratory. 17 p. International Code Council. Lubbock, TX: National Storm Shelter Association. Falk, R.H.; Bridwell, J.J.; Hermanson, J.C. 2015. Residential tornado safe rooms from commodity wood Storm Prediction Center. 2014. products: wall development and impact testing. Res. Pap. http://www.spc.noaa.gov/gis/svrgis/. Norman, OK: National FPL–RP–681. Madison, WI: U.S. Department of Oceanic and Atmospheric Administration, National Weather Agriculture, Forest Service, Forest Products Laboratory. Service, National Centers for Environmental Prediction, 15 p. Storm Prediction Center.

Falk, R.H.; Bridwell, J.J.; Senalik, C.A.; Begel, M. 2018. Residential tornado safe room from commodity wood products: impact and wind pressure testing. Gen. Tech. Rep. FPL–GTR–254. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 19 p.

5 General Technical Report FPL–GTR–253

Appendix—Safe Room Material Costs

Safe room material item Quantity Cost per item Total cost Source Notes 2 by 8 lumber, 8-ft length 190 $6 $1,140 Big box 10 extra for warping and door jamb 1 by 6 treated sill, 8-ft length 4 $7 $28 Big box Nominal 3/4-in. sheathing, 21 $21 $441 Big box 4 by 8 ft 16d nails (box of 5,000) 1 $55 $55 Big box 3-1/4 or 3-1/2 in. (longer is better) Liquid nails (10-oz. case) 10 $55 $550 Big box (PPG Industries, Pittsburgh, PA) 8-in. screws 100 $65 http://www.screwsolutions.com/YTX- 14800-5-14-x-8-Gold-Star-Extra-Long- Multi-Purpose-Star-Drive-Wood-Screws-- 5-lb-94-pc-Approx_p_240.html 10-in. screws 34 $50 http://www.screwsolutions.com/YTX- 15100-ea-15-x-10-Gold-Star-Extra-Long- Multi-Purpose-Star-Drive-Wood-Screws-- ea_p_498.html Door jamb reinforcement $250 Obtained from local steel supplier. angles (14 gauge) Assumes 4 by 8 steel sheet cut down to three angles needed. Cutting and bending cost included. Price may vary with location. Door steel (18 gauge) 2 $61 $122 Local steel Obtained from local steel supplier. supplier Assumes 4 by 8 steel sheet cut down to door size. Cutting cost included. Price may vary with location. 1/4-in. door bolts 1 $20 $20 Big box Door bolt 3 $16 $48 Big box http://www.homedepot.com/p/National- Hardware-3-4-in-x-10-in-Bolt-Hook- 293BC-3-4X10-Bolt-Hook/203616842 Door hinge 3 $22 $66 Big box http://www.homedepot.com/p/National- Hardware-24-in-Zinc-Plated-Gate-Hinge- Strap-294BC-24-STRAP-HNG- ZN/203359515?MERCH=REC-_- PIPHorizontal1_rr-_-203616842-_- 203359515-_-N Tie-down 16 $25 $400 Simpson https://www.strongtie.com/holdowns HTT5 andtensionties_coldformedsteelconstructi on/s-ltt-s-dttandhtt productgroup cfs/p/s.ltt-s.dtt- Door latch 3 $63 $189 5/8-in. cane http://www.snugcottagehardware.com/Sn bolt ug%20Product%20Pages/Latches%20for %20wood%20Traditional/5000%20Cane %20Bolts.html Miscellaneous bolts for door hinges $50 and latch Anchor bolt 5/8 in. 11 $30 See FEMA P-320 specifications (FEMA 2014) Anchor bolt epoxy 2 $100 See FEMA P-320 specifications (FEMA 2014) Total safe room cost $3,604

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