Specific design requirements for breakaway walls are included in the NFIP User’s Guide to Technical Bulletins (fema/pdf/fima/guide01.pdf) lists.
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Table of Contents Introduction .1 NFIP Regulations .. 3 Flood Insurance Considerations4 Building and Residential Code Considerations5 Wave Loads on Building Elements ..5 Performance of Breakaway Walls .. 5 Options for Enclosing Areas Below Elevated Buildings ..12 Prescriptive Design Method for Breakaway Walls .13 Simpli˜ed Design Method for Breakaway Walls 18 Performance-Based Design of Breakaway Walls .25 Impact of Breakaway Wall Provisions on Other Building Elements .26 Construction Materials .27 Existing Buildings: Repairs, Remodeling, Additions, and Retro˜tting 28 Recommendations for Coastal A Zones .28 The NFIP . 30 NFIP Technical Bulletins 30 Ordering Technical Bulletins .. 31 Further Information . 31 Glossary .32 Comments on the Technical Bulletins should be directed to: Department of Homeland Security FEMA Mitigation Directorate 500 C Street, SW. Washington, D.C. 20472 Technical Bulletin 9-08 replaces Technical Bulletin 9-99, Design and Construction Guidance for Breakaway Walls . Cover photo: Post-Hurricane Ivan photo of the underside of an elevated V zone building. The break – away walls underneath the building failed as intended during the hurricane.
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Introduction Protecting buildings that are constructed in special ˚ood hazard areas (SFHAs) from dam -age caused by ˚ood forces is an important objective of the National Flood Insurance Program (NFIP). In support of this objective, the NFIP regulations include minimum building design criteria that apply to new construction, repair of substantially damaged buildings, and substan -tial improvements of existing buildings in SFHAs. The base ˚ood is used to delineate SFHAs on Flood Insurance Rate Maps (FIRMs) prepared by the NFIP. The base ˚ood is the ˚ood that has a 1-percent chance of being equaled or exceeded in any given year (commonly called the fi100-yearfl ˚ood). Certain terms used in this Technical Bulletin are de˜ned in the Glossary. Coastal waves and ˚ooding can exert strong hydrodynamic Under the NFIP, the filow -forces on any building element that is exposed to the waves est ˜oorfl is the ˜oor of the or ˚ow of water. The NFIP requires that all new buildings, lowest enclosed area of a substantially damaged buildings, and substantially improved building. An un˚nished or buildings in Coastal High Hazard Areas (Zones V, VE, and ˜ood-resistant enclosure V1030) be elevated to or above the base ˚ood elevation (BFE) that is used solely for park -ing of vehicles, building on open foundations consisting of piles, posts, piers, or col- access, or storage is not umns. These open foundations must be designed to allow the lowest ˜oor, provided waves and water moving at high velocity to ˚ow beneath build- the enclosure is built in ings. compliance with applicable requirements. NFIP regulations require that the area below the lowest ˚oor of elevated buildings either be free of obstructions or have As used by the NFIP, an fienclosurefl is an area that any enclosed areas be constructed of non-supporting break- is enclosed on all sides by away walls, open lattice-work, or insect screening. The walls, walls. lattice, or screening are intended to collapse under wave loads without causing collapse, displacement, or other structural damage to the elevated building or the supporting founda -tion system (see Figure 1). Obstructions below an elevated building can signi˜cantly increase the potential for ˚ood damage by increasing the surface area subject to wave impact and ve -locity ˚ow. The NFIP regulations also specify that enclosures may be used only for parking of vehicles, building access, or storage; that all materials below the BFE, including materials used to con -struct enclosures, be ˚ood damage-resistant materials; and that construction methods and practices minimize the potential for ˚ood damage. Speci˜c design requirements for breakaway walls are included in the NFIP regulations. Those parameters were the subject of research on breakaway walls performed for the Federal Emer -gency Management Agency (FEMA) and the National Science Foundation by North Carolina State University and Oregon State University (Tung et al., 1999). The research evaluated fail -ure mechanisms that were demonstrated by full-scale, laboratory wave-tank tests of breakaway wall panels. TECHNICAL BULLETIN 9 Œ AUGUST 2008 1
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Figure 1. Area enclosed by breakaway walls below an elevated building This Technical Bulletin presents three design methods that are consistent with the NFIP regulations: a prescriptive design approach, a simpli˜ed design approach, and a performance- based design approach. Regardless of the approach used, breakaway walls must be designed and constructed to meet applicable building or residential codes, such as the International Building Code ® (IBC®) or the International Residential Code ® (IRC®), respectively. In many cases, design wind speeds will exceed the prescriptive limits speci˜ed in the governing residential code, which means designs must be in accordance with the governing building code or other approved standard. For example, the prescriptive design provisions of the 2006 IRC are not applicable to designs where 3-second gust design wind speeds exceed 100 mph; thus, residen -tial structures in these areas must be designed in accordance with the IBC or other standard TECHNICAL BULLETIN 9 Œ AUGUST 2008 2
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referenced in the IRC (see IRC Section R301.2.1.1). The primary reference for wind and seis -mic loading in building and residential codes is Minimum Design Loads for Buildings and Other Structures (ASCE 7-05). The prescriptive design approach for breakaway walls requires the use of typical detailing, but allows the designer to design the elevated portion of the home and the foundation system without consideration of ˚ood forces acting on the breakaway walls. This approach is only al -lowed for walls designed to have a safe loading resistance (also referred to as allowable load) of 20 pounds per square foot (psf) or less, as de˜ned in this Technical Bulletin. The simpli˜ed design approach is permitted for walls designed to have a safe loading resis -tance of more than 20 psf. The approach requires the use of typical details that are similar to those used in the prescriptive method. Although special certi˜cation is required for these walls, the process is simpli˜ed since these walls are designed to minimize ˚ood loads to the elevated structure and foundation system. The performance-based design approach allows more detailing freedom for breakaway walls, but requires the designer to consider the combined effect of wind forces acting on the elevat -ed portion of the structure, as well as wind and ˚ood loads acting on the foundation system and the breakaway walls. NFIP Regulations The NFIP regulations for breakaway walls are codi˜ed in Title 44 of the Code of Federal Regu -lations, in Section 60.3(e)(4), which states that a community shall: fiProvide that all new construction and substantial improvements in Zones V1-V30 and VE, and also Zone V if base ˜ood elevation data is available on the community™s FIRM, are elevated on pilings and columns so that (i) the bottom of the lowest horizontal struc -tural member of the lowest ˜oor (excluding the pilings or columns) is elevated to or above the base ˜ood level; and (ii) the pile or column foundation and structure attached thereto is anchored to resist ˜otation, collapse and lateral movement due to the effects of wind and water loads acting simultaneously on all building components. Water loading val -ues shall be those associated with the base ˜ood. Wind loading values used shall be those required by applicable State or local building standards. A registered professional engineer or architect shall develop or review the structural design speci˚cations and plans for the construction, and shall certify that the design and methods of construction to be used are in accordance with accepted standards of practice for meeting the provisions of paragraphs (e)(4)(i) and (ii) of this section.fl Section 60.3(e)(5) further states that a community shall require: fithat all new construction and substantial improvements within Zones V1-V-30, VE, and V on the community™s FIRM have the space below the lowest ˜oor either free of ob -struction or constructed with non-supporting breakaway walls, open wood lattice-work, TECHNICAL BULLETIN 9 Œ AUGUST 2008 3
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or insect screening intended to collapse under wind and water loads without causing col -lapse, displacement, or other structural damage to the elevated portion of the building or supporting foundation system. For the purposes of this section, a breakaway wall shall have a design safe loading resistance of not less than 10 and no more than 20 pounds per square foot. Use of breakaway walls which exceed a design safe loading resistance of 20 pounds per square foot (either by design or when so required by local or State codes) may be permitted only if a registered professional engineer or architect certi˚es that the designs proposed meet the following conditions: (i) Breakaway wall collapse shall result from a water load less than that which would occur during the base ˜ood; and (ii) The elevated portion of the building and supporting foundation system shall not be subject to collapse, displacement, or other structural damage due to the effects of wind and water loads acting simultaneously on all building components (structural and non-structural). Water load -ing values used shall be those associated with the base ˜ood. Wind loading values used shall be those required by applicable The NFIP Technical State or local building standards. Such enclosed space shall Bulletins provide guid -be useable solely for parking of vehicles, building access, or ance on the minimum storage.fl requirements of the NFIP regulations. Community or State requirements that ex- Proposals for substantial improvement of existing buildings in ceed those of the NFIP take V zones, and proposals to repair those buildings that have sus -precedence. Design profes -tained substantial damage, must comply with the requirements sionals should contact the for new construction, including requirements for breakaway community to determine walls surrounding enclosed areas below the BFE. As part of whether more restrictive provisions apply to the issuing permits, community of˜cials must review such propos -building or site in question. als to determine whether they comply with the requirements. All other applicable require- Further information on substantial improvement and substan -ments of the State or local tial damage is found in Answers to Questions About Substantially building codes must also be Damaged Buildings (FEMA 213). met for buildings in all ˜ood hazard areas. Flood Insurance Considerations Elevated buildings in V zones that do not have obstructions or enclosures below the BFE are subject to less ˚ood damage and thus lower rates are used to determine premiums for NFIP ˚ood insurance. Some considerations affecting the rates and costs of NFIP ˚ood insurance for elevated buildings in V zones include: The use of an enclosure with breakaway walls increases the premium for the entire building. An increase in the ˚ood insurance premium resulting from the presence of an enclosure depends upon the area of the enclosure; substantially higher premiums are charged for enclosures that are 300 square feet or greater in area. The presence of garage doors below an elevated building, even if designed in accordance with this Technical Bulletin, may increase the ˚ood insurance premium for the building. NFIP ˚ood insurance policies have limits on coverage of contents in enclosures under elevat -ed buildings. Designers, contractors, and owners may wish to contact a quali˜ed insurance agent or the NFIP for more information about policy coverage, coverage limits, and costs. TECHNICAL BULLETIN 9 Œ AUGUST 2008 4
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Figure 2. Impact of a breaking wave on a vertical surface Figure 3. Successful breakaway wall performance The most commonly observed problems involving breakaway wall systems are caused by poor detailing practices, inappropriately constructed additions, or other construction features. Such practices do not comply with the letter or intent of the NFIP regulations, which require structures to be ficonstructed by methods and practices that minimize ˚ood damages.fl Figures 4 through 8 illustrate some of the non-compliant construction problems that have been illustrated in MAT reports: TECHNICAL BULLETIN 9 Œ AUGUST 2008 6
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Figure 4 shows access stairs supported by component walls that were not designed and detailed to break away from the structure. To be compliant with the NFIP™s free-of-obstruc -tion requirement, stairs must be designed to either break away or to independently resist ˚ood loads and to minimize transfer of loads to the structure (for more information, see Technical Bulletin 5, Free-of-Obstruction Requirements for Buildings Located in Coastal High Haz -ard Areas ). It should also be noted that the back˜lled concrete masonry unit walls and planter may also signi˜cantly alter the ˚ow of water toward adjacent structures. Figure 5 shows damage to exterior wall covering caused by lack of a horizontal separation joint between the breakaway wall and the wall above. Figure 6 illustrates what is probably the most common problem that contributes to dam -age Œ poor detailing practices. In this example, utilities were attached to the breakaway wall. Similar damage is caused when utility lines are run through access holes, which then prevent the walls from breaking away. All utility components that must be installed below the elevated structure must be ˚ood damage-resistant, designed for ˚ood forces, and at -tached to permanent structural elements on the side opposite to the anticipated direction of ˚ow and wave approach. Figure 7 shows cross braces that were installed inside the breakaway walls and that could have prevented the breakaway walls from performing as designed. Braces, when required by the structural design, must be installed so as not to interfere with the intended perfor -mance of breakaway walls (see Technical Bulletin 5). Figure 8 shows a detailing practice where the breakaway walls spanned across vertical foun -dation elements, unnecessarily strengthening the breakaway wall and preventing it from performing as intended. Figure 4. Non- compliant stairs TECHNICAL BULLETIN 9 Œ AUGUST 2008 7
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