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  • 5-1DESIGN AND CONSTRUCTION GUIDANCE FOR COMMUNITY SHELTERS

    5 Load Determination andStructural Design CriteriaThis chapter presents a summary of previous research and testing and outlinesthe recommended methods and criteria for use in the structural design of acommunity shelter. Other engineering factors and concepts involved in thestructural design of a shelter are also discussed in this chapter. Detailedguidance concerning performance criteria for debris impact is presented inChapter 6. The design criteria presented in this chapter are based on the bestinformation available at the time this manual was published. Commentaryintended to provide supplemental guidance to the design professional for thischapter and Chapter 6 is presented in Chapter 10.

    5.1 Summary of Previous Guidance, Research, andTesting

    To date, the majority of the research, testing, and analysis concerning aninterior hardened room has been conducted by the Department of CivilEngineering at Texas Tech University (TTU) and the Department of CivilEngineering at Clemson University (Clemson). At TTU, the WindEngineering Research Center (WERC) and the Institute for Disaster Research(IDR) managed this work. At Clemson, work was performed at the WindLoad Test Facility (WLTF). Both research universities have performed testson various combinations of construction materials to determine theirresistance to wind-induced forces and the impact of windborne and falling debris.

    5.1.1 Previous Design GuidanceDesign guidance for high-wind shelters was provided previously in thefollowing FEMA publications and informational documents. (Details aboutmissile tests and testing history are provided in the TTU report ResidentialShelter Design Criteria in the sections titled Wind-Generated Missiles andPrevious Research on Missile Impact. Excerpts from these reports areprovided in Appendixes E and F.)

    FEMA 342: Midwest Tornadoes of May 3, 1999: Observations,Recommendations, and Technical Guidance

    National Performance Criteria for Tornado Shelters

    FEMA 320: Taking Shelter From The Storm: Building a Safe Room InsideYour House

    CROSS-REFERENCESee Chapter 10 for descrip-tions of the FEMA publicationslisted here.

  • 5-2 FEDERAL EMERGENCY MANAGEMENT AGENCY

    CHAPTER 5 LOAD DETERMINATION AND STRUCTURAL DESIGN CRITERIA

    FEMA TR-83B: Tornado Protection: Selecting and Designing Safe Areasin Buildings

    FEMA TR-83A: Interim Guidelines for Building Occupant ProtectionFrom Tornadoes and Extreme Winds

    5.1.2 Previous Research and Missile TestingTTU has performed the majority of the previous research and testing ontornado shelters and the effects of tornadoes on buildings. Clemson hasconducted tests to determine the effects of hurricanes and lower-intensitytornadoes on buildings. The tests and research performed by these twoinstitutions have included investigating wind speeds and associated loads,wind speed and associated debris impact, and the ability of the buildingmaterials to resist these loads and impacts. Tested construction materials (wallsections, doors, door hardware) that meet wind and missile impact criteria ofthis manual have been summarized and are listed in Appendixes E and F.

    The following materials have been successfully tested as part of largerstructural systems in laboratory studies developed specifically for shelterdesigns to resist missile impact:

    6-inch to 12-inch concrete masonry units (CMU) with at least #4 verticalreinforcing steel, fully grouted in each cell, and horizontal jointreinforcement as required by masonry design code

    reinforced concrete (roof and wall sections at least 6 inches thick) with atleast #4 reinforcing steel at 12 inches on center (o.c.) both horizontally andvertically

    12-gauge steel sheets or heavier

    wood stud cavity walls filled with dry-stacked solid concrete block andencapsulated with plywood sheathing

    3/4-inch plywood wall panels (when used as exterior cladding incombination with other materials)

    metal doors with at least 14-gauge skin (with interior supports)

    metal doors with less than 14-gauge skin clad with metal sheeting (14gauge or heavier) attached

    Building materials and how they are combined are very important in thedesign and construction of shelters. If these materials fail, wind may enter theshelter or the shelter itself may fail. Either situation may result in death of orinjury to the shelter occupants. The design professional should select materialsthat will withstand both the design wind loads and the design impact loads.

  • 5-3DESIGN AND CONSTRUCTION GUIDANCE FOR COMMUNITY SHELTERS

    CHAPTER 5LOAD DETERMINATION AND STRUCTURAL DESIGN CRITERIA

    Many window and door systems have been tested for their ability to resistwind and impact loads associated with high winds. The test protocols usuallyfollow ASTM E 1233/E 330 and ASTM E1886/E 1996, the South FloridaBuilding Code standard, or a similar test standard. Glass products have beenproduced that may withstand extreme pressures and missile impacts. Thedesigner who wishes to incorporate windows into a shelter should pay closeattention on the connections between the glass and the frame, and between theframe and the supporting wall system.

    Although the ASTM standard defines how tests are to be performed, andsome tests have been performed in hurricane regions of the southeast UnitedStates, the impact criteria used for those tests are less than those specified inthis manual. Windows and door systems specified for use in extreme-windshelters should be designed to meet the impact criteria presented in Chapter 6.

    5.2 Determining the Loads on the ShelterThe loads that will act on a tornado or hurricane shelter will be a combinationof vertical and lateral loads. One methodology of determining these loads ispresented in Figure 5-1.

    This manual recommends the use of ASCE 7-98 for the calculation of allloads acting on the shelter. Section 5.3 of this manual presents designguidance for calculating the wind pressures and loads associated with thedesign wind speed selected from Figure 2-2. Using this design wind speed,and the parameters specified in Section 5.3 of this manual for extreme-winddesign, the designer should follow the methodology for wind design inSection 6 of ASCE 7-98. Once these loads are determined, the designershould combine all relevant loads acting on the shelter (e.g., dead, live, snow,rain, seismic) and apply them to the shelter. Guidance on load combinations isprovided in Section 5.4 of this manual.

    5.3 Determining Extreme-Wind LoadsWhen wind loads are considered in the design of a building, lateral and upliftloads (discussed in Chapter 3) must be properly applied to the buildingelements along with all other loads. The design of the shelter relies on theapproach taken in ASCE 7-98 for wind loads. For consistency, the designermay wish to use ASCE 7-98 to determine other loads that may act on theshelter. The International Building Code (IBC) 2000 and InternationalResidential Code (IRC) 2000 also reference ASCE 7-98 for determining windloads. These wind loads should then be combined with the gravity loads andthe code-prescribed loads acting on the shelter in load combinations that arepresented in Sections 5.4.1 and 5.4.2 of this manual.

    NOTEASCE 7-98 defines the MWFRSas the main wind forceresisting system in a buildingor structure. Similarly, ASCE 7-98 defines C&C as the compo-nents and cladding elementsof a building or structure.

    NOTEC&C elements include wall androof members (e.g., joists,purlins, studs), windows,doors, fascia, fasteners, siding,soffits, parapets, chimneys,and roof overhangs. C&Celements receive wind loadsdirectly and transfer the loadsto other components or to theMWFRS.

    WARNINGTests for doors and windowscommonly used in hurricane-prone areas do not meet thecriteria for extreme windpressures and debris impactsrecommended in this manual.

  • 5-4 FEDERAL EMERGENCY MANAGEMENT AGENCY

    CHAPTER 5 LOAD DETERMINATION AND STRUCTURAL DESIGN CRITERIA

    Design wind loads for buildings are generally treated separately for the designof the structural system and the design of the cladding and its attachment tothe structural system. Design loads for the structural system of a shelter startwith the basic loads from the applicable building code governing the non-refuge use of the shelter. The determination of design wind loads acting on theshelter is based on standard provisions and formulas (equations) for the MainWind Force Resisting System (MWFRS) as defined in ASCE 7-98. Thedesign of cladding and its attachment to the structural system are based onstandard provisions and formulas for the components and cladding (C&C).Wall and roof panels should also be checked for out-of-plane loadingassociated with C&C loads for the appropriate tributary areas.

    Figure 5-1Shelter design flowchart.

  • 5-5DESIGN AND CONSTRUCTION GUIDANCE FOR COMMUNITY SHELTERS

    CHAPTER 5LOAD DETERMINATION AND STRUCTURAL DESIGN CRITERIA

    5.3.1 Combination of Loads MWFRS and C&CAccording to ASCE 7-98, the MWFRS is an assemblage of structuralelements assigned to provide support and stability for the overall structureand, as a consequence, generally receives wind loading from all surfaces ofthe building. Elements of the building envelope that do not qualify as part ofthe MWFRS are identified as C&C and are designed using C&C wind loads.Some elements of low-rise buildings are considered part of the buildingenvelope (C&C) or the MWFRS, depending upon the wind load beingconsidered (e.g., the exterior walls of a masonry building). In the design ofthese masonry walls, the MWFRS provisions are used to determine the in-plane shear forces, and the C&C provisions are used to determine the out-of-plane design bending lo