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Because people spend so muchtime at home and at work,having a safe room in their homeor small business can beimportant. According to the mostrecent National Oceanic andAtmospheric Administration(NOAA) Storm Prediction Centerdata, nine out of ten (89.6%)continental US counties haveexperienced a tornado watch

since 2003, with affectedfamilies spending a whopping 27hours per year under tornadowatches. The area goes beyondtornado alley to include 42 statesand the District of Columbia.Taking a longer view of thesituation, Figure 1 maps sixdecades of strong tornadoes.These facts make a strong casefor the need for safe rooms.

Designing and BuildingConcrete Masonry Safe Rooms

Using FEMA P-320 Jamie Farny

Continuing Education

Figure 1. US activity for EF3, EF4 andEF5 tornadoes from 1950 through 2013

This article provides a summary ofconstructing a concrete masonry saferoom based on Federal EmergencyManagement Agency (FEMA) P‐320, TakingShelter from the Storm: Building a SafeRoom for Your Home or Small Business.While the criteria apply to safe rooms foruse by up to 16 people, this article willrefer to building to mean both residentialor commercial structures.

FEMA P‐320 is primarily intended forhomeowners, builders and contractors,but can also be used by design

Tornado Activity

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MASONRY SAFE ROOMS

1 Describe the loadconditions that must beresisted to build a saferoom, i.e. wind speeds

2 Describe the properties of reinforced masonrymaterials assumed in the design

3 Tell what size of roomscan be built using theseprescriptive requirements(max L, W, H)

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Upon readingthe article youwill be able to:

professionals and local officials fordecision‐making guidance on tornado andhurricane safe rooms. Even though P‐320provides prescriptive designs, that is,engineered solutions for building saferooms, this would be a pretty significantdo‐it‐yourself project. Because theintegrity of the construction is essential inorder to achieve a safe installation, it isbest to involve professional designers andbuilders involved in any safe room project.

FEMA P‐320 was first published in 1998,and in 2014, was revised to the fourthedition. In between (in 2008), theInternational Code Council, with thesupport of the National Storm Shelter

Association, released a consensusstandard on the design and constructionof storm shelters. This standard, theICC/NSSA Standard for the Design andConstruction of Storm Shelters(ICC 500 2014), codifies many of therecommendations from FEMA documents.A safe room designed and constructed tothe prescriptive designs in P‐320 will meetor exceed the ICC 500 residential stormshelter design criteria. The NationalConcrete Masonry Association’s TEK 5‐14,Concrete Masonry Hurricane and TornadoShelters, provides details specifically forCMU safe rooms following requirementsof ICC‐500.

Tornadoes may be slow or fast movingweather systems, but they typically resultin relatively short duration winds atsignificant speeds; the strongest, EF5, havea base speed of 200 mph. On the otherhand, hurricanes are generally slowermoving systems resulting in sustainedexposures at somewhat lower wind speeds— Category 5 Hurricanes have a basespeed of 157 mph. The safe room designs inP‐320 are rated for 250‐mph winds andoffer life‐safety occupant protection during atornado or strong hurricane.

Following recommendations found inChapter 3 of P‐320, Planning Your SafeRoom, this article describes the design andconstruction of a concrete masonry saferoom. Chapter 3 should be consulted forthese and all other aspects of installing asafe room, from considering floodhazards, to designing the structure, tosizing and locating the room within abuilding, to the type of foundationrequired to ensure the proper anchorageof the room to its foundation.

Safe rooms can protect human life in highwind areas. While safe rooms protectlives, they may not provide propertyprotection. If that is desired as well,building the building’s entire structuralenvelope of wind‐resistant constructionshould be executed. While therecommendations in P‐320 only addresssafe rooms, there are plenty of resourcesfor building entire buildings usingconcrete masonry or other robust buildingsystems. The main thing is to create a solidbuilding envelope. Walls and roofs mustbe robust to resist forces created by windand impact penetration from flying debris.What’s more, the entire envelope must

maintain structural integrity, so strongconnections are needed.

Effects of Extreme

Wind on BuildingsWindows and doors should remain closedand intact to reduce the pressure actingon the interior of the building. Extremewinds can pull off roof coverings or decks,push in windows and doors and pull offsiding. Openings allow wind to enter thebuilding and push on its walls and rooffrom the inside. (Figure 2) Becausebuildings are generally not designed toresist forces acting on both the inside andthe outside of the building, these forcesoften result in failure.

New Building Safe Rooms

and RetrofitsIn new construction, it’s typically easier and less costly to add a safe room than itis to retrofit one into an existing building.For exterior walls made from concretemasonry units (CMU), exterior walls at thesafe room space are given additional steelreinforcement and fully grouted. Interiorwalls should also be constructed usinggrouted, reinforced CMU. The room iscompleted by adding a concrete roof deck over the safe room and a special saferoom door.

In an existing building, the adequacy of the foundation must be checked by aprofessional engineer or architect. Mostslab‐on‐grade foundations in homes arenot designed to transfer the loads from

WIND PRESSURE ON WALLSInternal pressure adds to wall suction

TOPVIEW

WIND PRESSURE ON ROOFInternal pressure adds to roof uplift

SIDEVIEW

FIGURE 2. When wind enters a building, the added forces on the walls and roof often lead to failure.

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Safe room designs in P-320 arerated for 250-mph winds

and offer life-safety occupantprotection during a tornado

or strong hurricane

dynamicsofmasonry.com RESILIENCE | CALL FOR ENTRIES v2.6 2016 SMART | 27

the safe room to the ground, even if theyhave some reinforcement. In that case, aportion of the slab must be cut out and anew, thicker, reinforced slab withfootings must be poured for the saferoom. With CMU walls, the weight of thesafe room may be sufficient to resist theoverturning forces without consideringthe weight of the slab, but only thedesigner can make that determination.

Safe Room MaterialsLocal building material suppliers should have everything needed for safe roomconstruction. For concrete masonry,standard ASTM C90, hollow loadbearingunits are used with either a Type M or Smortar conforming to ASTM C270. MostCMU now exceed 1900 psi or 2000 psicompressive strength. Safe room designsin P‐320 require a minimum compressivestrength of masonry of 1500 psi. Figure 3of TMS 402/602 (2011) shows the unitstrength of typical material combinations.For instance, a 1900 psi CMU with a TypeM or S mortar will achieve 1500 psicompressive strength of masonry.

The plans in FEMA P‐320 require that CMUwalls be built in running bond, whichmeans that head joint in successive coursesare offset at least 1/4 of the unit length.(Figure 5) The nominal sizes of masonryunits allow for 3/8” joint thicknesses.

Foundation Type FEMA P‐320 notes thatthe following types of foundations may besuitable for the installation of a safe room:

• Basement: Typical reinforcementtechniques used in existing residential

proper support and resistance to all(gravity and wind) loads.

• Crawlspace or pile: Generally thesefoundations provide more challenges

for safe room construction, so readersare referred to P‐320 forrecommendations. P‐320 does notinclude prescriptive solutions for pile foundations.

basement walls will not provide sufficientresistance to extreme wind loads. Fornew construction, builders reinforcewalls used for the safe room. For existingconstruction, reinforcing walls is oftencost‐prohibitive or not feasible. Verticaland horizontal surfaces with sufficientsoil cover have to be able to resistextreme wind loads, but do not have tobe tested for resistance to debris impact.

• Slab‐on‐grade with footings andreinforcement: Concrete slabs requiresteel reinforcement to prevent crackingand to resist tension resulting fromextreme wind loads acting on the saferoom. In new slab‐on‐gradeconstruction, FEMA recommends thatthe slab or foundation beneath the saferoom be adequately reinforced and bethickened or have footings to ensure

Figure 5. NCMA TEK 3‐8A Figure 1, Mortar Joint Tolerances. CMU wall section shows a running bondpattern of 1/2 unit and typical joint thickness. Typical joints are 3/8" with some tolerance, dependingon the joint.

Figure 4. Safe room under construction has fully grouted walls with vertical steel reinforcing bars from the foundation to the concrete roof deck.

* For units of less than 4" (102mm) height, 85% of the values listed. Pounds per square inch (PSI) andmegapascals (MPa).

Net area compressive strength ofconcrete masonry units, psi (MPa*) Net area compressive

strength of masonry,psi* (MPa)Type M or S mortar Type N mortar

–

1,900 (13.10)

2,800 (19.31)

3,750 (25.86)

4,800 (33.10)

1,900 (13.10)

2,150 (14.82)

3,050 (21.03)

4,050 (27.92)

5,250 (36.20)

1,350 (9.31)

1,500 (10.34)

2,000 (13.79)

2,500 (17.24)

3,000 (20.69)

INITIAL BED JOINT THICKNESS =1/4 IN MIN3/4 IN MAX

BED JOINT THICKNESS =3/8 IN œ 1/8 IN

INITIAL BED JOINT THICKNESS =1/4 IN MIN3/4 IN MAX

HEAD JOINT THICKNESS = 3/8 IN -1/4 IN, +3/8 IN

FOOTING

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 unitsand type of mortar used in construction.

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With CMU walls, the weight of thesafe room may be sufficient to resistthe overturning forces withoutconsidering the weight of the slab

Using FEMA P-320 Design

DrawingsFEMA P‐320 comes with construction plansfor various materials, but this article focuseson grouted CMU construction. Plans in P‐320allow for safe rooms from 8' 0" to 14' 0" oneach side. Wall heights are limited to 8' 0" tothe bottom side of the ceiling/roof. Theseare fully engineered solutions to buildingfoundations, walls, roofs and all theconnections. Readers should refer to P‐320for more detail, for door requirements andfor other wall types.

Basic design for CMU safe rooms in P‐320 is as follows (see References forASTM standards):

• Select the compressive strength ofmasonry, ƒ’m. 1500 psi is the minimumallowed. See Figure 3 of TMS 402/602.

• Use 6'' or 8'' hollow loadbearingconcrete masonry units conforming toASTM C90 laid in running bond. Figure 5

• Use ASTM C270 Type M or S mortar.Figure 6

• Use Grade 60 reinforcing steel (yieldstrength of 60 ksi).

• Grout all cells solid using grout thatconforms to ASTM C476, which includesself‐consolidating grout. Grout pourmaximum height is 64" unless cleanoutsare provided at the bottom of each cellcontaining reinforcement or at a maximumspacing of 32", whichever is less.

• For vertical reinforcement, use #5 barsat recommended spacing of 16" oncenter in 6" walls, and up to 40" oc for8" walls, with additional requirementsfor bars around openings. Verticalreinforcement shall be full height in thecenter of grouted cells at wallintersections, corners and door jambs.

• For horizontal reinforcement, minimumreinforcement includes a bond beam atthe top of the wall. For 6'' walls, use aminimum of one continuous #4 bararound the perimeter; for 8" walls, one#5 bar. Use bent bars at corners andwall intersections to lap horizontalreinforcement and maintain continuity.

• If below grade, provide adequatewaterproofing and drainage for walls.

• Follow manufacturer’s installationprocedures for anchorage to slabs.

Adding a safe room to a new or existinghome or small business is a lot easier since

FEMA has developed engineeredsolutions. Since the first edition of FEMAP‐320 was issued in 1998, more than 1million copies have been distributed, andnearly 25,000 safe rooms for up to 16people have been constructed with FEMAfunding assistance.

FEMA 361 publication, Design andConstruction Guidance for CommunityShelters, was also updated to the thirdedition. Community shelter requirementsare similar to those for residences andsmall businesses, but call for additionalfeatures required to accommodate largerareas and more people. It providesupdated and refined criteria for safe roomsand commentary reflecting six more yearsof post‐damage assessments and lessonslearned since the 2008 version.

ReferencesICC 500, ICC/NSSA Standard for the Design andConstruction of Storm Shelters, 2014

NCMA TEK 3‐8A, Concrete Masonry Construction

NCMA TEK 5‐14, Concrete Masonry Hurricane andTornado Shelters

TMS 402/602‐2011, Building Code Requirements andSpecification for Masonry Structures

ASTM C90, Standard Specification for LoadbearingConcrete Masonry Units

ASTM C476, Standard Specification for Grout for Masonry

ASTM C270, Standard Specification for Mortar forUnit Masonry

ASTM A615, Standard Specification for Deformed andPlain Carbon‐Steel Bars for Concrete Reinforcement

Jamie Farny,director, buildingmarketing for thePortland CementAssociation inSkokie IL, focuseson promoting theuse of cement-based

materials for low-rise buildings, includingconcrete masonry and autoclaved aeratedconcrete (AAC). Farny is also involved inpromoting white cement, plaster andarchitectural and decorative concrete.

Farny is a member of The Masonry Society’s402/602 Committee, which maintains anddevelops code requirements and specificationfor masonry structures. He is also a member ofAmerican Society for Testing and Materials’committees on plastering, mortars andmasonry 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 ofTechnology. 847.972.9172 | jfarny@cement.org

Mortar Type Cement^ Mortar Cement

M S N

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Figure 6. ASTM C270 Table 1, Proportion Specification RequirementsNote – Two air-entraining materials shall not be combined in mortar

Reprinted, with permission, from C270-14a Standard Specification for Mortar for Unit Masonry, copyright ASTM International,100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM,www.astm.org.

Learn more about resilient construction atBuildStrongBuildSafe.com

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