Figure 1. US activity for EF3, EF4 and EF5 tornadoes from 1950 through 2013
Because people spend so much time at home and at work, having a safe room in their home or small business can be important. According to the most recent National Oceanic and Atmospheric Administration (NOAA) Storm Prediction Center data, nine out of ten (89.6%) continental US counties have experienced a tornado watch since 2003, with affected families spending a whopping 27 hours per year under tornado watches. The area goes beyond tornado alley to include 42 states and the District of Columbia. Taking a longer view of the situation, Figure 1 maps six decades of strong tornadoes. These facts make a strong case for the need for safe rooms.
This article provides a summary of constructing a concrete masonry safe room based on Federal Emergency Management Agency (FEMA) P-320, Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business. While the criteria apply to safe rooms for use by up to 16 people, this article will refer to building to mean both residential or commercial structures.
FEMA P-320 is primarily intended for homeowners, builders and contractors, but can also be used by design professionals and local officials for decision-making guidance on tornado and hurricane safe rooms. Even though P-320 provides prescriptive designs, that is, engineered solutions for building safe rooms, this would be a pretty significant do-it-yourself project. Because the integrity of the construction is essential in order to achieve a safe installation, it is best to involve professional designers and builders involved in any safe room project.
FEMA P-320 was first published in 1998, and in 2014, was revised to the fourth edition. In between (in 2008), the International Code Council, with the support of the National Storm Shelter Association, released a consensus standard on the design and construction of storm shelters. This standard, the ICC/NSSA Standard for the Design and Construction of Storm Shelters (ICC 500 2014), codifies many of the recommendations from FEMA documents. A safe room designed and constructed to the prescriptive designs in P-320 will meet or exceed the ICC 500 residential storm shelter design criteria. The National Concrete Masonry Association’s TEK 5-14, Concrete Masonry Hurricane and Tornado Shelters, provides details specifically for CMU safe rooms following requirements of ICC-500.
Safe room designs in P-320 are rated for 250-mph winds and offer life-safety occupant protection during a tornado or strong hurricane
Tornadoes may be slow or fast moving weather systems, but they typically result in relatively short duration winds at significant speeds; the strongest, EF5, have a base speed of 200 mph. On the other hand, hurricanes are generally slower moving systems resulting in sustained exposures at somewhat lower wind speeds — Category 5 Hurricanes have a base speed of 157 mph. The safe room designs in P-320 are rated for 250-mph winds and offer life-safety occupant protection during a tornado or strong hurricane.
Following recommendations found in Chapter 3 of P-320, Planning Your Safe Room, this article describes the design and construction of a concrete masonry safe room. Chapter 3 should be consulted for these and all other aspects of installing a safe room, from considering flood hazards, to designing the structure, to sizing and locating the room within a building, to the type of foundation required to ensure the proper anchorage of the room to its foundation.
Safe rooms can protect human life in high wind areas. While safe rooms protect lives, they may not provide property protection. If that is desired as well, building the building’s entire structural envelope of wind-resistant construction should be executed. While the recommendations in P-320 only address safe rooms, there are plenty of resources for building entire buildings using concrete masonry or other robust building systems. The main thing is to create a solid building envelope. Walls and roofs must be robust to resist forces created by wind and impact penetration from flying debris. What’s more, the entire envelope must maintain structural integrity, so strong connections are needed.
Upon reading the article you will be able to:
- Describe the load conditions that must be resisted to build a safe room, i.e. wind speeds
- Describe the properties of reinforced masonry materials assumed in the design
- Tell what size of rooms can be built using these prescriptive requirements (max L, W, H)
Effects of Extreme Wind on Buildings
Windows and doors should remain closed and intact to reduce the pressure acting on the interior of the building. Extreme winds can pull off roof coverings or decks, push in windows and doors and pull off siding. Openings allow wind to enter the building and push on its walls and roof from the inside. (Figure 2) Because buildings are generally not designed to resist forces acting on both the inside and the outside of the building, these forces often result in failure.
FIGURE 2. When wind enters a building, the added forces on the walls and roof often lead to failure.
New Building Safe Rooms and Retrofits
In new construction, it’s typically easier and less costly to add a safe room than it is to retrofit one into an existing building. For exterior walls made from concrete masonry units (CMU), exterior walls at the safe room space are given additional steel reinforcement and fully grouted. Interior walls should also be constructed using grouted, reinforced CMU. The room is completed by adding a concrete roof deck over the safe room and a special safe room door.
In an existing building, the adequacy of the foundation must be checked by a professional engineer or architect. Most slab-on-grade foundations in homes are not designed to transfer the loads from the safe room to the ground, even if they have some reinforcement. In that case, a portion of the slab must be cut out and a new, thicker, reinforced slab with footings must be poured for the safe room. With CMU walls, the weight of the safe room may be sufficient to resist the overturning forces without considering the weight of the slab, but only the designer can make that determination.
Figure 3. Building Code Requirements for Masonry Structures (TMS 402/602 2011) Table 2, Compressive strength of masonry based on the compressive strength of concrete masonry units and type of mortar used in construction.
** For units of less than 4″ (102mm) height, 85% of the values listed. Pounds per square inch (PSI) and megapascals (MPa).**
Safe Room Materials
Local building material suppliers should have everything needed for safe room construction. For concrete masonry, standard ASTM C90, hollow loadbearing units are used with either a Type M or S mortar conforming to ASTM C270. Most CMU now exceed 1900 psi or 2000 psi compressive strength. Safe room designs in P-320 require a minimum compressive strength of masonry of 1500 psi. Figure 3 of TMS 402/602 (2011) shows the unit strength of typical material combinations. For instance, a 1900 psi CMU with a Type M or S mortar will achieve 1500 psi compressive strength of masonry.
The plans in FEMA P-320 require that CMU walls be built in running bond, which means that head joint in successive courses are offset at least 1/4 of the unit length. (Figure 5) The nominal sizes of masonry units allow for 3/8” joint thicknesses.
Foundation Type FEMA P-320 notes that the following types of foundations may be suitable for the installation of a safe room:
- Basement: Typical reinforcement techniques used in existing residential basement walls will not provide sufficient resistance to extreme wind loads. For new construction, builders reinforce walls used for the safe room. For existing construction, reinforcing walls is often cost-prohibitive or not feasible. Vertical and horizontal surfaces with sufficient soil cover have to be able to resist extreme wind loads, but do not have to be tested for resistance to debris impact.
- Slab-on-grade with footings and reinforcement: Concrete slabs require steel reinforcement to prevent cracking and to resist tension resulting from extreme wind loads acting on the safe room. In new slab-on-grade construction, FEMA recommends that the slab or foundation beneath the safe room be adequately reinforced and be thickened or have footings to ensure proper support and resistance to all (gravity and wind) loads.
- Crawlspace or pile: Generally these foundations provide more challenges for safe room construction, so readers are referred to P-320 for recommendations. P-320 does not include prescriptive solutions for pile foundations.
With CMU walls, the weight of the safe room may be sufficient to resist the overturning forces without considering the weight of the slab
Figure 4. Safe room under construction has fully grouted walls with vertical steel reinforcing bars from the foundation to the concrete roof deck.
Using FEMA P-320 Design Drawings
FEMA P-320 comes with construction plans for various materials, but this article focuses on grouted CMU construction. Plans in P-320 allow for safe rooms from 8′ 0″ to 14′ 0″ on each side. Wall heights are limited to 8′ 0″ to the bottom side of the ceiling/roof. These are fully engineered solutions to building foundations, walls, roofs and all the connections. Readers should refer to P-320 for more detail, for door requirements and for other wall types.
Basic design for CMU safe rooms in P-320 is as follows (see References for ASTM standards):
- Select the compressive strength of masonry, ƒ’m. 1500 psi is the minimum allowed. See Figure 3 of TMS 402/602.
- Use 6” or 8” hollow loadbearing concrete masonry units conforming to ASTM C90 laid in running bond. Figure 5
- Use ASTM C270 Type M or S mortar. Figure 6
- Use Grade 60 reinforcing steel (yield strength of 60 ksi).
- Grout all cells solid using grout that conforms to ASTM C476, which includes self-consolidating grout. Grout pour maximum height is 64″ unless cleanouts are provided at the bottom of each cell containing reinforcement or at a maximum spacing of 32″, whichever is less.
- For vertical reinforcement, use #5 bars at recommended spacing of 16″ on center in 6″ walls, and up to 40″ oc for 8″ walls, with additional requirements for bars around openings. Vertical reinforcement shall be full height in the center of grouted cells at wall intersections, corners and door jambs.
- For horizontal reinforcement, minimum reinforcement includes a bond beam at the top of the wall. For 6” walls, use a minimum of one continuous #4 bar around the perimeter; for 8″ walls, one #5 bar. Use bent bars at corners and wall intersections to lap horizontal reinforcement and maintain continuity.
- If below grade, provide adequate waterproofing and drainage for walls.
- Follow manufacturer’s installation procedures for anchorage to slabs.
Adding a safe room to a new or existing home or small business is a lot easier since FEMA has developed engineered solutions. Since the first edition of FEMA P-320 was issued in 1998, more than 1 million copies have been distributed, and nearly 25,000 safe rooms for up to 16 people have been constructed with FEMA funding assistance.
Figure 6. ASTM C270 Table 1, Proportion Specification Requirements
Note – Two air-entraining materials shall not be combined in mortar
FEMA 361 publication, Design and Construction Guidance for Community Shelters, was also updated to the third edition. Community shelter requirements are similar to those for residences and small businesses, but call for additional features required to accommodate larger areas and more people. It provides updated and refined criteria for safe rooms and commentary reflecting six more years of post-damage assessments and lessons learned since the 2008 version.
ICC 500, ICC/NSSA Standard for the Design and Construction of Storm Shelters, 2014
NCMA TEK 3-8A, Concrete Masonry Construction
NCMA TEK 5-14, Concrete Masonry Hurricane and Tornado Shelters
TMS 402/602-2011, Building Code Requirements and Specification for Masonry Structures
ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units
ASTM C476, Standard Specification for Grout for Masonry
ASTM C270, Standard Specification for Mortar for Unit Masonry
ASTM A615, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement
Learn more about resilient construction at BuildStrongBuildSafe.com
Jamie Farny, director, building marketing for the Portland Cement Association in Skokie IL, focuses on promoting the use of cement-based materials for low-rise buildings, including concrete masonry and autoclaved aerated concrete (AAC). Farny is also involved in promoting white cement, plaster and architectural and decorative concrete.
Farny is a member of The Masonry Society’s 402/602 Committee, which maintains and develops code requirements and specification for masonry structures. He is also a member of American Society for Testing and Materials’ committees on plastering, mortars and masonry units, and American Concrete Institute committees.
He has written extensively on cement, masonry, and concrete. Farny has a Bachelor of Science in Civil Engineering from the Illinois Institute of Technology. 847.972.9172 | firstname.lastname@example.org