Document Columbiacountyga_doc_a4940e985b

Elevated Residential Structures FEMA 54 I March 1984 Federal Emergency Management Agency ---PAGE BREAK--- Elevated Residential Structures ---PAGE BREAK--- Acknowledgments Many people contributed valuable assistance to the prepara- tion of this manual. We wish to acknowlege especially the guidance provided by Melita Rodeck and, later, John Gam- bel, the Federal Emergency Management Agency's technical representatives in this work. John Gambel's advice was particularly valuable in determining the final content and form of this manual. In addition, this project would not have been possible without the help of Richard W. Krimm, Assistant Associate Director of the Federal Emergency Management Agency's Office of Natural and Technological Hazards, who saw the importance of increasing architects' involvement in flood damage mitigation efforts. Finally, Ray Fox provided a wealth of useful advice in additio~ to his technical services throughout the course of the project. Prepared by The American Institute of Architects Foundation 1735 New York Avenue, N.W. Washington, D.C. 20006 Charles R. Ince, Jr., President Earle W. Kennett, Administrator, Research Donald E. Geis, Program Director and Project Manager Karen N. Smith, Administrative Manager Paul K. McClure, Editor Technical Consultants Raymond R. Fox Professor of Civil Engineering The George Washington University Washington, D. C. Mark Riebau Assistant Chief of Floodplain and Shoreland Management Wisconsin Bureau of Water Regulation and Zoning Madison, Wisconsin Cost Consultant Daniel Mann Johnson & Mendenhall, Architects-Engineers Washington, D.C. Paul Brott, Vice President Ernest Posch, Estimator The Design Studies section of the manual was developed on the basis of background data and design concepts submitted for the 1976 version of this manual by Zane Yost and Associates, Bridgeport, Connecticut; KEF Corporation, Metairie, Louisiana; Keck and Keck Architects, Chicago; Duval/Johlic Architects-Planners, San Francisco; Louisiana State University, Department of Architecture; Rhode Island School of Design; University of California at Los Angeles, School of Architecture and Urban Planning; and University of Miami, School of Architecture. ii Graphics and Book Design Assarsson Design Company Washington, D.C. Allan G. Assarsson, President Mark P. Jarvinen, Layout and Graphic Design Jeffery Banner, Graphic Design Photographs Raymond R. Fox, Dames & Moore, pp. 64 and 124 and Figures 4.23 and 4.26; Federal Emergency Management Agency, p. 1 and Figure 4.1; U.S. Geological Survey, pp. vi, 122, and 123; U.S. Department of Housing and Urban Development, p. 118 and Figure 2.3; Philip Schmidt, U.S. Department of Housing and Urban Devel- opment, Figure 2.7; National Park Service, pp. iv and 112; Spencer Rogers, p. 18; Rosenthal Art Slides, Figures 3.1 and 3.2; AIA Library, Figures 3.3 and 3.4; U.S. Army Corps of Engineers, pp. 3 and 115; Davis and Associates, Figure 4.49; Pittsburgh City Planning Depart- ment, p. 4; PARNG Photo, p. 2; and James K. M. Cheng, p. 98. Most of the photographs and design data in the Recent Design Examples section were supplied by the designers of the buildings shown there. All other photographs were taken by Donald E. Geis of The American Institute of Architects Foundation. Disclaimer The statements contained in this manual are those of The American Institute of Architects Foundation and do not necessarily reflect the views of the U.S. Government in general or the Federal Emergency Management Agency in particular. The U.S. Government, FEMA, and The American Institute of Architects Foundation make no warranty, express or implied, and assume no responsibility for the accuracy or completeness of the information herein. This manual was prepared under Contract No. EMC-C-0579 with the Federal Emergency Management Agency. ---PAGE BREAK--- Table of Contents ACKNOWLEDGMENTS PREFACE ENVIRONMENTAL AND REGULATORY FACTORS FLOODING AND THE BUILT ENVIRONMENT Riverine Flooding D Coastal Flooding FLOODPLAIN MANAGEMENT National Flood Insurance Program D Base Flood Elevations DA and V Zones SITE ANALYSIS AND DESIGN SITE SELECTION AND ANALYSIS SITE DESIGN Site Flooding Characteristics D Access and Egress D Vegetation D Flood Water Drainage and Storage D Dune Protection ARCHITECTURAL DESIGN EXAMPLES DESIGN STUDIES Bridgeport D Charleston and Newport D San Francisco D Chicago AESTHETIC CONSIDERATIONS RECENT DESIGN EXAMPLES Logan House D Summerwood on the Sound D Breakers Condominium D Campus-by-the-Sea Facility D Starboard Village D Gull Point Condominiums DESIGN AND CONSTRUCTION GUIDELINES FOUNDATIONS Fill D Elevated Foundations D Shear Walls D Posts D Piles D Piers D Bracing FRAMING CONSTRUCTION AND CONNECTIONS Framing Methods D Floor Beams D Cantilevers D Concrete Flooring Systems D Floor Joists D Sub.flooring D Wall Sheathing and Bracing D Roof Connections RELATED DESIGN CONSIDERATIONS Glass Protection D Utilities and Mechanical Equipment D Building Materials D Insulation D Breakaway Walls D Retrofitting Existing Structures COST ANALYSIS RESOURCE MATERIALS GLOSSARY SOURCES OF DESIGN INFORMATION FEMA REGIONAL OFFICES STATE COORDINATING OFFICES FOR THE NFIP PERFORMANCE CRITERIA REFERENCES ii v 4 8 9 13 18 22 35 45 64 65 80 92 98 112 113 116 118 120 125 136 iii ---PAGE BREAK--- ---PAGE BREAK--- Preface Whenever possible, residential structures should not be located in flood-prone areas. Flooding in these areas is virtually assured at some point in the future, bringing with it the potential for property damage-no matter how well a structure is designed-as well as danger to building occu- pants. However, it is not always possible to avoid flood-prone areas. This manual is for designers, developers, builders, and others who wish to build elevated residential structures in flood-prone areas prudently. The readers of this manual are assumed to have knowledge of conventional residential construction practice; the manual is limited to the special design issues confronted in elevated construction. This is a revision of a manual of the same title pub- lished in 1976 by the Federal Insurance Admini- stration. This revision reflects changes since 1976 in floodplain management techniques and regu- lations, improvements in construction materials and practice, increases in construction costs, and additions to the relevant literature. This revision also contains increased information on elevating structures in coastal areas, although all the tech- niques described here apply to both coastal and riverine areas unless otherwise stated. A second document, published by the Federal Emergency Management Agency (FEMA), Design Guidelines for Flood Damage Reduction, supple- ments this manual's discussion of elevated residen- tial structures with information on the full range of other floodplain management strategies. A third document, Design and Construction Manual for Residential Buildings in Coastal High Hazard Areas, is published jointly by FEMA and the U.S. Department of Housing and Urban Devel- opment. It provides structural engineering guide- lines and other information on designing structures in coastal areas subject to severe wind and velocity wave forces. Structures in such areas should not be designed without consulting it. v ---PAGE BREAK--- ENVIRONMEN~fAL AND REGULATORY FACTORS ---PAGE BREAK--- Flooding and the Built Environment Rivers and seacoasts have always been focal points for development. Access to water has provided drinking supplies and sanitation, an important source of energy, and a valuable part of the trans- portation system. Recreational opportunities and aesthetic enjoyment further stimulate waterside development. This development pattern, however, leads to a con- flict between the natural and built environments. The need for direct access to water places human settlements in low-lying areas that are subject to periodic flooding by rivers and the sea. In the United States, more than six million dwellings and a large number of nonresidential buildings are currently located in the nation's 160 million acres of floodplains. Flooding of these floodplains is responsible for more damage to the built environ- ment than any other type of natural disaster. The total flood damage in 1978, for example, was an estimated $3.8 billion. The following year, Hurricane Frederic alone caused $1.8 billion in damages. ---PAGE BREAK--- 2 RIVERINE FLOODING Floods are part of the natural hydrologic process. Riverine flooding is associated with a river's watershed, which is the natural drainage basin that conveys water runoff from rain and melting snow. Water that is not absorbed by soil or vegetation seeks surface drainage lines, following local topo- graphy and creating rivers and other streams. Flooding results when flow of runoff is greater than the carrying capacity of watershed streams. Riverine flooding usually involves a slow buildup of water and a gradual inundation of surrounding land. However, flash flooding, a quick and intense overflow with high water velocities, can result from a combination of steep slopes, a short drainage basin, and a high proportion of surfaces impervious to water and unable to absorb runoff. In addition to the direct threat to buildings, development in riverine floodplains alters natural topography, modifying drainage patterns and usually increasing storm water runoff. Develop- ment also displaces much of the natural vegetation that formerly absorbed water and decreases the permeability of the soil by covering it with build- ings or with nonporous surfaces for roads, side- - walks, and parking. The effect of these changes is to increase the severity of flooding throughout the riverine environment. COASTAL FLOODING Coastal flooding is generally due to occan- based storm systems. Hurricanes, tropical s torm and extratropical storms such as "northeasters" arr the principal causes, with flooding occurring when storm tides are higher than the normal high tidr, and arc accompanied by water moving at relatiwly high velocity and velocity wave action. The maximum intensity of a storm tide occurs at high tide, so storms that persist through srveral tides are the most severe. ---PAGE BREAK--- The velocity and range of coastal floods vary in part with the severity of the storm that induces them. The damaging effects of coastal flooding are caused by a combination of the higher water levels of the storm tide and the rain, winds, waves, erosion, and battering by debris. The extent and nature of coastal flooding is also related to physiographic features of the terrain and the characteristics of the adjoining body of water. Pacific coastal areas are vulnerable principally to earthquakes, tsunamis (seismically induced tidal waves) and other natural forces that can trigger excessive erosion, mud slides, and flash flooding. Great Lakes coastal areas are subject to erosion and severe winter storms. The Atlantic and Gulf Coasts are consistently exposed to the forces of hurri- canes, lesser tropical storms, and northeasters. Coastal flooding is most frequent on the Atlantic and Gulf Coasts, which are made up of a succession of barrier islands, beaches, and dunes. These physiographic elements are maintained in dynamic balance as sand is moved by wind, waves, and ocean currents. This self-replenishing beach-dune system takes the brunt of the force of storm surges and helps buffer inland areas. In coastal areas the removal of beach sand and the leveling of dunes, along with the construction of seawalls, jetties and piers, are common practice. These can help destroy the shoreline's natural protec- tion system, exacerbating the impact of storm surges and high winds. 3 ---PAGE BREAK--- 4 Floodplain Management There have long been attempts to moderate the impact of riverine flooding, with major federal efforts in the United States since 1936. Until recently, these efforts have been concentrated on flood control measures devised to reduce or eliminate flooding itself-chiefly dams, levees and similar structural works. Despite a number of positive results, these measures have not succeeded in reducing flood damage significantly. Since the mid-l 960s, therefore, federal policies have reflected a recognition that structural works need to be complemented by nonstructural mea- sures. Rather than trying solely to prevent floods, current floodplain management programs address the need to reduce the losses incurred when inevitable flooding does happen. Elevating residential structures above the reach of flood waters, the subject of this manual, is only one of several floodplain management techniques currently used to reduce flood damage. For example, construction is prohibited in critical floodplain areas (termed floodways) unless it has been determined that construction will not in- crease flood levels elsewhere. Where buildings are already located in these critical areas, they can either be relocated out of the flood area, elevated, or floodproofed to reduce the damage they will suffer in a flood. Buildings that are badly damaged by flooding can be razed or floodproofed rather than being restored to their original, vulnerable condition. Vacant land in flood-prone areas can be purchased by the local community and reserved for recreation, farming, or other safe uses. These and other floodplain management techni- ques (discussed in DPsign Guidelines for Flood Dumag<> R<>durtion, cited in the Preface) can be used in a coordinated way to respond to each community's various needs, resources, and flood hazards. Elevated residential structures, if used at sites appropriate for them, can be useful com- ponents of effective floodplain management. ---PAGE BREAK--- NATIONAL FLOOD INSURANCE PROGRAM The National Flood Insurance Program (NFIP) is the federal government's principal administrative mechanism for reducing flood damage. Estab- lished by Congress in 1968, the NFIP is adminis- tered by the Federal Emergency Management Agency (FE1\1A). The NFIP insures buildings and their contents in flood-prone areas, where conven- tional insurance had, prior to the NFIP, been generally unavailable. The NFIP provides this insurance only in com- munities that agree to implement comprehensive land-use planning and management to reduce the likelihood of flooY IMPA.Cf Of !7?0~ 15 ANI? 1uesuL.f:Nl~ lKANGif SHm 10 ~!:DUlf: tl':S!S1ANG!' 10 fl.000- WAIE?f. Figure 2.6. Site Design to Reduce Flood Hazards 14 ACCESS AND EGRESS Access to and egress from a building can be facili- tated by locating parking and driveways- as well as the building-in the area of a site least likely to be flooded. Access and egress are important during flooding to ensure that building occupants can evacuate and that police and fire protection and other critical services can continue to be provided. 12.f"-Of.lff\Ji 61f.UCfU~E- 10 ut:'Ult:- IURBU!..ffile- LANP5lAl'IN6o Hf'U'5 iO DE'fi.fCT !/Efl~ IS ---PAGE BREAK--- In new developments, roads should be located to approach buildings from the direction away from the floodplain, so that access roads will be less likely to be blocked by flood waters and debris (Figure 2. To reduce potential erosion, siltation, and runoff problems, roads should not disrupt drainage patterns, and road crossings should have adequate bridge openings and culverts to permit the unimpeded flow of water. If roads are to be raised, the slope of embankments should be minimized and open faces stabilized with ground cover or terracing. VEGETATION Vegetation aids in slowing the rate of storm water runoff by holding water, thus allowing it to filter into the ground or evaporate gradually. In addition, vegetation helps prevent erosion and sedimentation from flooding. Natural vegetation should be retained wherever practical, and new plantings should be introduced in locations that will be most affected by runoff. Crushed stone can be used to control erosion under low-lying elevated structures and other locations where vegetation is difficult to maintain. Larger bushes and trees can be sited to deflect floating debris away from elevated foundations. Landscaping can also be used to screen elevated foundations from view. Trees, plantings, fencing, etc., can all provide this dual function of utility and aesthetics. FLOOD WATER DRAINAGE AND STORAGE Good site drainage in riverine areas allows flood waters to recede from a site without eroding it or leaving standing water that causes damage to structural elements or health hazards from stagnant water. Water enters a riverine site either from precipita- tion or as surface runoff from upstream portions of the watershed. What happens to this water can be a major determinant of the degree of flooding and Figure 2.7. Improperly Sited Streets Can Block Emergency Egress and Access 15 ---PAGE BREAK--- 16 the amount of flood damage. Site development that increases the volume of storm water runoff can increase flooding levels. Ideally, runoff rates after development should not exceed the rates before development. Site design should work to protect the individual site as well as to minimize increased flood levels elsewhere. A number of key factors such as the amount of nonporous surface and the amount of on-site surface water storage can in part determine the ability of a site to absorb water. Land-use regulations in some communities require devel- opers to defray part of the cost of developing regional water retention sites to offset the effects of development. On the site, open channels can be used both to divert water away from erodable areas, such as short steep slopes, and to collect and transport water runoff to larger drainage courses. Channels with grass cover are appropriate where the channel gradient and consequent water velocity are low; they then serve as percolation trenches by allowing gradual infiltration while water is being trans- ported. Where vegetation cannot be established, concrete and asphalt paving or riprap can be used as channel linings. However, such linings can increase the velocity of runoff, and consideration should be given to velocity checks to control the rate of flow. On some sites it may be possible to use fill material-from either on-site or off-site-to improve drainage and control runoff. Special con- sideration should be given to soil conditions and slope stability, as well as flood water velocities and duration, to avoid erosion during flooding. When restructuring topography, exposed cut and fill slopes, as well as borrow and stockpile areas, should be protected. Runoff should be diverted from the face of slopes, and slopes should be stabilized with ground cover or retaining walls. ---PAGE BREAK--- DUNE PROTECTION Dunes provide a natural shoreline defense against storm surges and waves. Most coastal communi- ties require that construction be behind the primary dune and that dunes not be cut or breached by site features such as walkways or beach access roads. Cross-over walkways should be provided (see Figure 2.8). Existing dunes should be maintained through vegetation and sand fencing, which limit wind losses and promote further dune growth. If no dunes exist and the beach is sufficiently wide, successive tiers of sand fencing can induce dune formation; some communities require this before a residence can be built. I I I 1 I ~ , I I I I I I Figure 2.8. Dune Access 17 ---PAGE BREAK--- ARCHITECTURAL DESIGN EXAMPLES - ---PAGE BREAK--- Many of the twentieth century's most important buildings have been elevated residential structures. The rise of modem architecture, inspired by the raised houses of Le Corbusier in the 1920s, was made possible by structural innovations. The Villa Savoie at Poissy (1929), for example, is lifted above the ground on pilotis, freeing the lower level for parking and affording a spatial continuity with the landscape (Figures 3.1 and 3. In his Towards A New Architecture Le Corbusier was exultant about the possibilities of elevated design: The house on columns! The house used to be sunk in the ground : dark and often humid rooms. Reinforced concrete offe rs us the columns. The house is in the air, above the ground ; the garden passes under the house. Figure 3.2 Figure 3.1 19 ---PAGE BREAK--- Figure 3.3 20 Since the Villa Savoie was centered on a dome-like rise in a large pasture, Corbusier did not need to concern himself with the problem of flooding. Other masters of modern architecture, however, have used the principles of elevated residential design to create aesthetically satisfying and func- tionally sound responses to hazardous flood condi- tions. Mies van der Rohe's Farnsworth House (1950), considered one of the great icons of modern archi- tecture, owes at least some of its appearance to its flood-prone site (Figures 3.3 and 3.4). Built along the Fox River in rural Illinois, the house was designed to accommodate a body of water that overflows its banks each spring. Mies' solution to the problem was to raise the plane of the first floor above the flood level, creating his first clear-span building. The resulting structure seems to float above its site. Good design and good flood protection must con- tinue to be treated together. Good design entails effective use of the site and careful consideration of the needs of the surrounding neighborhood and community. The best houses provide a clear transition from ground to dwelling, integrating the foundation with the rest of the structure. Creative landscaping with trees, shrubs, and fences can enhance the appearance of elevated structures by softening the effect of potentially harsh or barren ---PAGE BREAK--- exposures. Inventive landscaping also helps to control erosion and protect Lhe dwelling from the impact of debris and high velocity flooding. Effective use of terracing and level changes can help achieve continuity with the surrounding areas and, equally important, provide a sense of variety by indicating the different functions that occur simultaneously on a single site. Such site considerations arc but one part of a total elevated design scheme. The following examples are concerned with some of the many other important factors involved in floodproof design. Figure 3.4 21 ---PAGE BREAK--- Figure 3.5 22 Design Studies The f'ollm\ ing design studies wnc dn-clopcd L) a number of archiLccLural l'irrns and architectural schools using th<' information presented in this manual. LJRIOGEPORT, CONNECTICUT iLh an ation ff<' d csi~nt ' d tlw:-c lu:-..ur: to\\ nlww;cs around a raised central social dcch. (Fi~urcs . .>and Parh.i111! is local ed IH'11caL11 the deck. lo tlw deck and lo the tow11 - hou s1':- pro' idcd b) ::-lair,., and a timber ramp. T!w ra111p pro' ides acl'css for . the handi- cappt·d and th C' cf dnl:. D11ri11µ, lime::- of flooding. Lh(' ramp can abo used for dri' inµ, au lornohiks and rcsc111· \chicles up tlic deck lc\C'l. Steel girders resting on corH·rcl<' piers support boLh Lhc social deck and the townho11 se::; (Figure' 3. The _J : J_ . · 1 I 6 I I I i I 3:1 kl - :1 I I I ~ UJ I zz o- u I x _ N rl ~ N - rl z x _ o Q FLOOR PLAN 1 10 15 = 61 ---PAGE BREAK--- 62 ---PAGE BREAK--- " Figure 3.70 63 ---PAGE BREAK--- DESIGN AND CONSTRUCTION GUIDELINES ---PAGE BREAK--- Foundations The common methods of elevating residential structures are earth fill, elevated foundations, shear walls, posts, piles, and piers. The selection of an elevation technique depends on a number of variables, including hydrologic factors, physi- cal conditions at the site, and cost. The deter- mination of the appropriate technique requires analysis of these factors in the context of federal, state, and local regulatory requirements. In some cases it can be advantageous to use a combination of elevation method~. For example, a building raised on fill at one end and piers or posts at the other coulcl J irovide ground floor access at the ern I of llte building away from the floodplain while minimizing obstruction of flood waters at the end nearer the stream channel. The following discussion of the design and con- struction of elevated residential structures is based on accepted building practice. Generally, a con- servative approach has been taken in order to ensure compliance with the building codes most widely used in the United States. In addition, the performance criteria presented later in this manual can be used to review a building's expected re- sponse to flooding. Analysis of flood-induced loads and soil conditions, as well as normal loads, stresses, and deflection of structural members, is required to ensure satisfactory building perform- ance. Note that foundations in V Zones should be designed in accordance with Design and Con- struction Manual for R<>sidential Buildings in Coastal High Hazard Areas, cited in the Preface. 65 ---PAGE BREAK--- Figure 4.1. Elevation by Earth Fill 66 FILL At many A-Zone sites with low-velocity flooding it is feasible to elevate structures on earth fill (Figure 4.1 Earth fill is a widely used elevation technique that with proper construction practices and materials can be the most economical means of elevating a building two or three feet above grade or in some locations even higher. Fill should not be used in V Zones, where high-velocity flooding occurs, or at sites where fill would constrict the flow of flood waters and cause increased flooding heights or velocities. The advantages of fill (as opposed to piles or similar elevated foundations) include its generally traditional appearance, ease of access to the lowest floor no stairs are required), the ability at many sites to connect the filled area to higher ground for emergency evacuation in a flood, the safety of building elements from deterioration caused by exposure to flood waters, and th~ thermal insulation the earth provides the bottom of a house. In cold climates, furthermore, spring flood water under a house elevated on piles can freeze, with the danger of uplifting the structure. A site's topography and soil conditions may pre. elude use of fill. Before fill is put in place existing vegetation and any unstable topsoil must be removed. The fill should then be placed in layers not exceeding 12 inches deep, with layer com- pacted with pneumatic or sheepsfoot rollers or vibrating compacting equipment. For most residential applications, compaction to 95 percent of the maximum density obtainable with the Standard Proctor Test Method issued by the American Society for Testing and Materials (ASTM Standard D-698) is usually sufficient. Provision must be made for adequate surface drainage and erosion protection. Riprapping may be required for critical exposed slopes of a fill pad. ---PAGE BREAK--- ELEVATED FOUNDATIONS In some situations site topography, poor soil conditions, aesthetics, or cost considerations may make it desirable to use an extended masonry or reinforced concrete foundation to elevate a house up to three or four feet above grade. Such a foun- dation can be bermed with earth fill to provide easy access and a conventional appearance. Elevated foundations must be designed to with- stand both hydrodynamic forces caused by velocity waters and hydrostatic forces caused by standing water. This may require added reinforce- ment in the walls. Where the foundation is not bermed with fill, a further design consideration would be the provision of sufficient openings in the foundation to allow the unimpeded flow of flood waters through the foundation. This can help minimize both hydrodynamic and hydro- static forces without affecting the strength of the foundation if designed properly. SHEAR WALLS Shear walls, although more commonly used for motels, apartments, and other more massive structures, can also be used to elevate smaller residential structures (Figure 4.2). A shear wall acts as a deep beam in resisting forces in the plane of the wall. Structurally, the most critical design consideration is the low resistance of a shear wall to lateral forces. Shear walls should thus be used only in areas subject to low- to moderate-velocity flooding and should be placed parallel to the expected flow of flood waters. It is important that load and impact forces be determined for the entire range of flow directions. In addition, a shear wall's vulnerability to lateral forces makes it critical that connections between the wall and the foundation elements below grade be well designed. Figure 4.2. Elevation by Shear Walls 67 ---PAGE BREAK--- Figure 4.3. Elevation by Posts ' 1"'-olf------wooo POGI 1"REA1fD Will-I PRE'5E'RVA1 1VS- --¥-f'INrORL!i-D lONl. lOU..AR Pt:RM115 S\-VJ.WWH'. f;'M&D- 1-iff.JI OF f'Oi.ES Figure 4.5. Reinforced Concrete Collar 68 POSTS Post foundations (Figures 4.3 and 4.4) use long, slender wood, concrete, or steel posts set in pre- dug holes. Posts can be round, square, or rec- tangular in section, though square and rectangular posts are easier to frame into than round ones. With steel pusts, wide flange shapes or pipe or square tube sections are usually used. Figure 4.4. Elevation by Posts Post foundation holes are dug by hand or machine. Posts longer than 16 feet generally require machine assistance for safe handling. Posts are generally less resistant to lateral forces from flood waters than piles or reinforced concrete masonry piers. Bearing capacity and stability of posts can be improved by pouring a concrete bearing pad at the bottom of the hole and/or pouring a concrete collar around the post after it has been partially backfilled (Figure 4.5). Post Embedment The depth to which posts should be embedded depends on soil conditions, including the depth of the frost line; vertical loads; lateral loads from flood waters, debris impact, and wind forces; the anticipated erosion and uplift; and the spacing and size of the posts. The following comments and sketches indicate embedment techniques for wood posts; steel and concrete posts' requirements are similar. ---PAGE BREAK--- Hole Depth and Post End Bearing. Wood posts are generally embedded 4 to 8 feet. Hole excavations beyond 8 feet become uneconomical, so piles are used. If design loads are small and the allowable soil bearing capacity is adequate, i.e., dense sand or medium-stiff clay, the post can be set on undis- turbed earth at the bottom of the hole (Figure 4.6). For larger loads and/or poorer soil conditions, a concrete pad should be poured into the bottom of the hole (Figure 4. The pad should be approx- imately as thick as half its diameter, with a mini- mum thickness of 8 inches. If extremely poor soil conditions are encountered it may be necessary to use concrete backfilling or piers, as discussed below, or to drive a group of piles and cast a pile cap for each post to bear on, as shown in Figure 4.8, anchoring the posts securely to the caps. This can be more expensive than other foundation types. Figure 4.7. Post on Concrete Bearing Pad 0 -+---eAC.KflU... 0 . ..------fOUNDAilON Bt:-ARIN6r AR.f:A Figure 4.6. Earth Bearing Figure 4.8. Post/Pile Foundation 69 ---PAGE BREAK--- ANCHOR 6\ RAP \~~~t----HOOKE-D ROO Plf~ \0 FOO'llN&t 0 , · 0 • . Figure 4.9. Post/Pier Foundation POUR:l::O lONlRE'rf UC,l(.FIU... FOUNDAflON ~AR I NGi . Figure 4.10. Concrete Backfill 70 Wood posts can also be supported entirely out of the ground on concrete piers (Figure 4.9). More thorough maintenance is possible with this ap- proach, but additional bracing may be required for lateral stability. Hole Size. In post construction the hole should be a minimum of 8 inches larger in diameter than the greatest dimension of a post section. This allows for alignment and backfilling. Backfilling. Clean, well-compacted backfill is necessary to ensure a structure with good lateral stability and resistance against wind and water uplift. Common backfill materials are sand, gravel, crushed rock, pea gravel, soil cement, concrete, and earth. Granular fills that provide good drainage are generally considered the best. Drainage around the posts at grade level should be positive to keep water from collecting and deteriorating the posts. Backfill materials should be mechanically tamped to adequately compact them. Wetting such back- fill materials as earth or gravel will aid compaction. Backfilling the hole with concrete rather than gravel or sand, as shown in Figure 4.10, adds stabil- ity to the structure and increases the bearing area. Shallower embedment may be possible with this method. Soil cement is an economical alternative to con- crete and attains strength nearly equal to it. Soil cement is made by mixing the earth removed from the dug hole with cement in a ratio of 1 part cement to 5 parts earth (plus water as directed by the manufacturer). To achieve the best results all organic matter should be removed from the earth, and it should be sifted to remove all parti- cles larger than 1 inch. ---PAGE BREAK--- Anchorage Lateral forces and flood forces are less likely to overturn or uplift posts if the posts are anchored to a foundation. Two ways to anchor posts are to embed them in concrete or to fasten them to metal straps, angles, plates, etc., that are themselves anchored in concrete footings, piers, or pile caps. Figure 4.11 shows one method of anchoring wood posts in concrete. Large (5/8- to 3/4-inch in diameter) spikes or lag bolts are driven into the post around its base. The post is placed into the hole and secured to bracing restraints to prevent movement through the footing while the concrete sets. The metal fastening method of anchorage can be used above or below ground. Figure 4.12 shows a square wood post lag bolted to a metal shoe that is anchored in a pier. In Figure 4.13, heavy gauge galvanized steel straps are used to anchor the wood post to a concrete pad. WIXJO POST ANCHOR GHOf f?.flNfOU!?t? CONCRHi:- 0 t! ~ c; I> 0 b , . · . Plf:R Figure 4.12. Metal Angle Anchorage Detail ~----wroo !'OS1 1 R£1'1TE9 WllH P'Ri:OSC:-RVA1 1VE: Figure 4.11 . Spike Anchorage of Post f'OS'f 1REAff:t? C,.RAOE - ec> & ~ ~-~jo Wl1H ~AL-VAJJIZ.E'O S°fl<.AP Figure 4.13. Galvanized Strap Anchorage Detail 71 ---PAGE BREAK--- Figure 4.14. Pile Foundations 72 PILES Pile foundations (Figure 4.14) use long, slender wood, steel, or reinforced concrete piles that are driven or jetted into the ground. Vertical loads can be carried by driving piles to a load-bearing layer, such as rock (end-bearing piles), or by driving the piles deep enough into the earth to develop enough friction between the surface of the piles and the surrounding soil to carry the load (friction piles). Friction piles, which can also have an end-bearing component, are most often used for typical light residential loads. Piles are structurally stronger than posts and are therefore more suitable for the extreme wind and water forces and erosion in coastal V Zones. Piles in V Zones should be designed in accordance with Design and Construction Manual for Residen- tial Buildings in Coastal High Hazard Areas, cited in the Preface. Pile Materials Piles can be concrete, steel, or wood. In coastal areas, where steel piles are not desirable because of corrosion problems, concrete piles can be particu- larly good when combined with precast concret~ floor beams; such structural systems can be efficient, economical, and flood resistant. Concrete piles can be particularly suitable for buildings of more than two stories. The vulnerabilities of different pile materials to environmental conditions are discussed in the materials section later in this manual. Wood piles are probably the most widely used foundation for elevated residential structures. In some locations, square timbers are preferred over round piles because of cost, availability, and ease of framing and connecting the structural floor beams to the piles. The most popular suitable sizes (in inches) are 10 x 10 and 8 x 8 square roughsawn members. ---PAGE BREAK--- Round timber piles are also frequently used. Generally, round piles are available in longer than square timbers, and for greater than about 25 feet round piles are fre- quently the only piles available. Round piles are often preferred because they can provide greater cross-sectional area, peripheral area, and stiffness than square sections, particularly the 8 x 8 timbers. A minimum tip diameter of about 8 inches, and a butt or top diameter (at the floor beam level) of about 11 inches or more are recommended for round piles. Pile Embedment Methods A major consideration in the effectiveness of pile foundations is the method of inserting piles into the ground. This can determine the amount of the piles' load resistance. It is best to use a pile driver, which uses leads to hold the pile in position while a single- or double-acting hammer (delivering about 10,000 to 15,000 foot-pounds of energy) drives piles into the ground. A pile driver should be used for precast concrete piles and steel piles. The pile driver method, while cost-effective for a development with a number of houses being con- structed at one time, can be expensive for a single residence. An economical alternative, the drop hammer, consists of a heavy weight (several hundred pounds) that is raised by a cable attached to a power-driven winch. The weight is then dropped 5 to 15 feet onto the end of the pile. Drop hammers must be used with care because they can damage wood piles. Disadvantages of pile driving include difficulties with alignment and with setting a driver up on un- even terrain. The advantage is that the driving operation forces soil outward from around the pile, compacting the soil and causing increased friction along the sides of the pile, which provides greater pile load resistance. A much less desirable but frequently used method of inserting piles into sandy coastal soil is "jetting." Jetting involves passing a high pressure stream of water through a pipe advanced alongside the pile. The water blows 73 ---PAGE BREAK--- Figure 4.15. Pier Foundations 74 a hole in the sand into which the pile is continu- ously pushed or dropped until the required depth is reached. Sand is then tamped into the cavity around the pile and the end of the pile pounded with the heaviest sledge hammer or other weight available. Unfortunately, jetting loosens not only the soil around the pile but also the soil below the tip. Therefore, only low end and side friction load capacity is attained, and the piles must be inserted deeper into the ground than if they were driven. If the soil is sufficiently clayey or silty, a hole can be excavated by an auger or other means. The hole will stay open long enough to drop in a pile. Some sands have enough clay or silt to also permit the digging of a hole. Then sand or pea gravel can be poured and tamped into the cavity around the pile. Again, this does not provide as good load resistance as driving the pile into the ground, and longer piles are necessary. With short wood piles, some final driving with a sledge hammer can be helpful. Soil Conditions and Embedment Depth Local building codes often specify the required cmbedment depths of piles, e.g., to al least 6 feet below grade. Such codes often do not take into account the conditions al specific sites; a soils r ngineer should be consulted in doubtful situ- ations. In addition, DPsign and Construction Manual f or R esidential Buildings in Coastal Iligh Hazard A reas, cited in the Preface, provides use- ful information on this subject. The required depth of pile embedment depends primarily on the number of piles used, the size and weight of the structure, and the type of soil at the building site. The pile depth is also influenced by the lateral forces from flooding and wind and debris impact, the manner in which the piles are inserted into the soil, and the need to allow for erosion of the soil that supports the piles. In riverine environments the soil types and the anchorage provided by the frictional force of the soil against the sides of the pile vary widely. Sand is the dominant soil component in most coastal areas, but in some areas there may be ---PAGE BREAK--- an underlying layer of several feet of clay. General- ly, clay soils provide greater load-bearing capacity with less penetration than sandy soils. Clay soils are also less susceptible to erosion. The depth of erosion of sandy soils caused by wave action is virtually impossible to predict. Piles supporting residential structures on sandy coastal shorelines should penetrate the ground deeply enough to provide resistance to wind and water loads even after extensive erosion has occurred. Posts are often backfilled partly with concrete to improve their resistance to lateral forces. The same technique can be used with piles. After piles are driven, the area around each pile is dug out and a thick concrete collar is poured, extending several feet below grade. Such collars provide protection from minor erosion, add some deadweight to the structure, and increase piles' pull-out resistance. PIERS Pier foundations (Figure 4.15) are suitable in areas away from a river or coastline where flood waters \ ~ l\ HE.M7EK /JO\ST I ANlHOK 5TRA!' I I II ii II " IZ.'x ft' ~I N FDRL:l'P " EiR.llK F\EK " ~ " " II " II " cA'OE. \711"\EHSION . • • . • move with low velocity and erosion will be Figure 4.16. Reinforced Brick Pier minimal. Pier foundations use brick, concrete masonry blocks, or poured-in-place concrete to elevate structures. To resist horizontal wind and water forces, piers should rest on substantial spread footings or a grade beam, with reinforcing steel rods extending from these elements through the full height of the piers to resist tensile stresses. Pier Materials The vulnerability of pier materials to environ- mental conditions is discussed in the materials section later in this manual. Brick and Concrete Masonry Piers Brick piers and concrete masonry piers should be a minimum of 12" x 12" and reinforced with steel rods (Figures 4.16 and 4.17). Hollow con- crete masonry units should be filled with concrete. · : 11"'· • . ----f'O~E.r7 c.Af' r1t==~:;1,----AN?~DR J ~ ~ " " " 1: rv>-----rni'\fl'Kll/>b-~ 4- · '1 H/ 4 " . 1, 'jr. . . . • . ~ . • II , ' . ~ • - • . - 1 - · · 11 -+---Z't"x2-'t'xt>'MIN. : • · ,f " · ' • ~At€. \?IMl'.N510N Figure 4.1 7. Reinfo rced Concrete Masonry Pier 75 ---PAGE BREAK--- Figure 4.18. Wal I Foundation 76 Reinforced brick piers can be used to elevate structures 1 Yz to 6 feet off the ground. Concrete masonry piers are effective for elevations of 1 Yz to 8 feet. In general, the height of reinforced con- crete masonry piers should be limited to a maxi- mum of ten times their least dimension. Square piers are preferable. If the piers are rectangular the longer dimension should not exceed the shorter dimension by more than 50 percent. According to the National Concrete Masonry Association, the allowable working stresses for concrete masonry piers are the same as those for the design of concrete masonry walls. The pier masonry should be laid with type Mor S mortar. The association also recommends that the spacing between piers supporting floor joists not exceed 8 feet in the direction perpendicular to the joists, nor 12 feet in the direction parallel to joists. These minimum requirements apply whether the pier is free standing or laterally braced. In cases where exceptionally large loading conditions may exist, the pier cross-section should be increased and/ or additional reinforcement added. A larger cross-section can be obtained by using piers several feet in length. The long dimension should be placed parallel to anticipated flood flow, as in Figure 4.18. In coastal areas, however, flood _ forces may come in at an angle, loading such a pier adversely, so alternatives should be considered. ---PAGE BREAK--- Poured-in-Place Concrete Piers Poured-in-place concrete piers are essentially re- inforced concrete columns. They are cast in forms set in machine- or hand-dug holes. The holes can be widened or belled at the base to form a footing integral with the pier, or, as shown in Figure 4.19, a separate footing can be poured. If soil conditions are appropriate the footing can be eliminated and loads left to end bearing and friction between the soil and pier (Figure 4.20). Poured-in-place piers of the latter type can be particularly effective for larger homes or developments of single-family homes and townhouses. Poured-in-place concrete piers can be used to elevate a structure lYz to 12 feet or more. The dimensions, reinforcement, and spacing of con- crete piers depend on the type of building framing used and on building and environmental loads; structural analysis is required. Pier Footings Pier footing sizes are a direct function of soil bearing capacity and loading, and can be computed on the basis of local codes. Depth of pier footings depends on local frost penetration levels and expected flooding, wind, and erosion levels. Footings in areas with soils of high volume change potential can be unstable, and should be designed with the guidance of a soils engineer. BRACING ELEVATED FOUNDATIONS • · 11· . r i 1' I LJ D 1: ' ·1 I ~ . I. : A no<- 0 l . 0 j I . ~ 1 , . ' 0 , I : o · ~ • 1 · o ·1 · o. · · 0 ' ~ . • 0 0 • • • • • 0 . . . . . . . , Rf'.:1~1'::171"00[1~. Z4' X Z4'XB' t.'\11-J • Figure 4.19. Reinforced Concrete Pier - - c:iALVANIZfP efAM A.NC.HOR W/ -%'Ji . : . G;AJ...V. f\0\...fS ~ ~ 11 o :4 • . \NfO o. 111 . o 'i . 111 l:::J c~ I 11 . I 1 • . I I. · I -I o 0 1 ° soNOw&=: I I C.OIJC Pl~ l> I o I o 1 I " :z :i C> . . Elevated foundation elements must be braced when analysis indicates that their size, number, spacing, and embedment will not be sufficient to resist lateral forces. Even in areas where low- velocity flooding is anfo:ipated, bracing can pro- vide added assurance that the structure will with- stand the impact of floating debris or greater-than- expected flood or storm forces. Although bracing Figure 4.20. Drilled Pier Foundation placed underneath a structure may be struck by floating debris, the effects of this on a structure's survivability are generally outweighed by bracing's beneficial effects. 77 ---PAGE BREAK--- Figure 4.21. Knee Brace Figure 4.22. Diagonal Bracing 78 Knee Braces and Diagonal Bracing Knee braces (Figure 4.21) and diagonal bracing can be effective in providing lateral strength. Lumber more than 2 inches thick is usually recommended. Bolts are preferred over nails for connecting bracing, because of bolts' greater resistance to pullout forces. Knee bracing is usually bolted between the floor joist and post or pile. Diagonal bracing (Figures 4.22 and 4.23) is bolted at the base of one post or pile and fastened in a like manner to the adjacent post or pile just below the floor beams. Although diagonal bracing is more likely than knee bracing to be struck by floating debris, this is generally outweighed by the greater lateral stability with diagonal bracing, especially in higher elevated structures. Steel rods can sometimes be used to diagonally brace wood posts or piles. The rods are fitted through drilled holes flooded with wood preservative and fastened with nuts and cast beveled washers. Welded connections or drill holes can be used to provide rod bracing in steel post or pile foundations. Such rods are usually 5/8 to 3/4 inches in diameter. Steel diagonal ties, while effective, require con- siderably more monitoring and maintenance than wood because of steel's susceptibility to corrosion. Figure 4.23. Diagonal Bracing ---PAGE BREAK--- Shear Walls and Floor Diaphragms In areas with low- to moderate-velocity flooding, shear walls placed parallel to the flow of flood waters and firmly attached to piles or posts can help brace them (Figure 4.24). With wood shear walls, the plywood sizes, the strength of wall edges, and the walls' anchorage are all important to effective bracing. A shear wall can be used in conjunction with a floor diaphragm (Figure 4.25) to transfer hori- zontal forces or reduce embedment depth when, for example, solid rock is reached when digging foundation holes. A floor diaphragm can be used with either pole frame or platform construction. Floor diaphragms usually call for 1/2- or 3/4- inch plywood. The severe lateral forces encountered in coastal V Zones can require the use of trusses, grade beams, or slabs to provide adequate support. These are discussed in Design and Construe lion Manual for Residential Buildings in Coastal High Hazard Areas, cited in the Preface. Figure 4.24. Shear Wall Bracing fl.a:J!1. D1Af'HICA6il-1 HO!'.IZON1J..\.- f>fJ,L:ll.JG, SHEA~ WAU- Vf:~1 1c.AI.- ~~CllJ6i Figure 4.25. Floor Diaphragm Bracing 79 ---PAGE BREAK--- Figure 4.26. Toe Nailing Provides Limited Pull-Out Resistance 80 Framing Construction and Connections The framing construction and framing connections in an elevated home can be critical to its ability to withstand flood forces with minimal damage. Construction in most non-flood areas must support loads imposed by the weight of the building materials (dead load), weight of people and objects (live load), and modest loads imposed by wind. Under normal conditions and with typical methods of framing construction and framing attachment, these loads act downward through gravity to hold the building's structure together. However, these loads represent only a portion of the loads imposed on any structural system in flood-prone areas, particularly in coastal V Zones. Additional forces can be applied to these structures by floating debris, velocity flooding, extreme winds, and wave action. These buildings' structural system must be capable of withstanding these loads and still support the structure and its contents. Coastal V Zones are virtually certain to be subjected to the extremes of these forces, and homes there should be designed in accordance with Design and Construction Manual for Residen- tial Buildings in Coastal High Hazard Areas, cited in the Preface. Even in riverine and coastal A Zones, however, prudence suggests that homes be built with a margin of safety beyond that needed in non-flood areas. Consideration should also be given to the possibility that flood forces may be greater than those anticipated on the basis of past floods or hydrologic analyses. Coastal areas pose the addi- tional danger that shifting dunes or other storm- induced topographic changes can transform rela- tively safe A Zones into V Zones, which experi- ence the full force of ocean storms. Measures to provide a home with an extra margin of safety to resist these forces are not expensive, e.g., having floor joists 12 inches on center instead of 16 inches on center, or using deformed shank or annular ring nails because of their greater holding ability. Nor are the needed craftsmanship and anchorage methods uncommon to the carpentry trade. Simple nailing, for example, especially end or ---PAGE BREAK--- toe nailing, provides little resistance to flood forces, partially because of the tendency to split the wood in the toe-nailed member (Figure 4.26 Bolts, lag bolts, or nails in metal anchors at right angles to the direction of force (Figure 4.27) are well-known methods of increasing structural strength. The following paragraphs discuss prudent framing construction and connections practice from the bottom up, starting with the foundation-to-floor- beam connections and floor beam construction and ending with wall-to-roof connections. FOUNDATION-TO-FLOOR-BEAM CONNECTIONS Post and Pile Foundations The connection of a post or pile foundation to the framing system of a structure is influenced by the method of framing used and the cross-sectional shape of the post or pile. Framing Methods. Two different methods for framing into post or pile foundations are in common use today: platform construction and pole frame construction. Platform construction entails simply cutting posts or piles off at the desired elevation and framing them with beams to support floor joists and deck. The platform thus formed serves as the first habitable floor and construction platform for any type of conventional framing structure desired (Figure 4.28). Figure 4.27. Metal Framing Anchors Figure 4.28. Platform Construction 81 ---PAGE BREAK--- Figure 4.29. Pole Frame Construction Figure 4.30. Exterior Pole Framing 8'2 In what is termed pole frame construction, the posts or piles are extended up to or through the roof, with beams framing around them as supports for floor joists and roof rafters (Figure 4.29). This method securely ties the entire structure together and is excellent for sites where lateral forces may be strong. A basic problem with piles is their alignment. Posts can be plumbed and aligned easily before they are backfilled, but piles must be jacked and pulled into position. This can be more of a problem with pole framing than platform construction. A solution is to locate piles either on the interior or exterior of a structure, not in the walls. Then, as shown in Figure 4.30, allowance can be made for alignment variations. Cross-Sectional Shape. Square posts or piles usually require only conventional framing techniques. With round posts or piles, however, the framing is some- what more complicated, and it is generally best to frame the posts with a pair of beams, girders, or rafters-one on each side. The roundness of wood posts is not a problem when using bolted or spiked connections as shown in Figure 4.31. The framing is then the same as for any other timber member. Figure 4.31. Bolted Connection to Round Pole ---PAGE BREAK--- Another connection method is to eliminate the curve of the post or pile by clapping and then con- necting with bolts, gusset plates, or other devices. As Figures 4.32 and 4.33 show, a clapped post will form seats that assist the beams in carrying vertical loads. Posts that are small in section, however, should not be clapped or they will be weakened. Generally, there should be a thickness of post or pile for the bolts to bear on equal to the total thickness of the floor beam. Two bolts should be used to connect beams to each post or pile. Spike grid connections (Figure 4.34), standard in bridge and warehouse construction, are less com- mon in residential practice. A single curved grid inserted between the post or pile and the beam substantially increases the strength of the bolted connection. With the curved side of the grid against the pole and over predrilled holes, a high-strength threaded rod is used to squeeze the two wood surfaces together, forcing the tooth of the spike grid into the grain of both members. The high-strength rod is then replaced with a conventional bolt of the proper size. A flat spiked grid is used to connect two flat surfaces, and a double curved spiked grid to connect two rounded surfaces. Figure 4.34. Spiked Grid Figure 4.32. Dapped Gusset Plate Connection Figure 4.33. Dapped Pole Connection 83 ---PAGE BREAK--- 4 t7A R? WRL>f'Pet7 t\f(!?!.HJt7 TOf OF ~M • TIE: 1HRU f'IE:R 4lt7 1~.rro r00fl1'.1£1 - - . . -W..OW r, fro'.'.:JT U/JE:: Figure 4.35. Concrete Masonry Unit Pier Y4° qBE:J., 6TR.Af' • E:MeeP IC." MIW. f'IE:R 6.l'--Jt7 "BefJt7 E:IJP l/JTO J?ilJT• B?L.1 THl<'.Ll eeAM C.1Jt7~~f" AfJCHoR- T I e- "5"f l<-AP TO Vf1\Y I CL>. L. Re l /J~IJ<=i I~ !"IE;fZ. Figure 4.36. Masonry Pier- Strap Anchor 84 Pier Foundations Pier foundations are generally used for platform framing construction rather than pole framing construction. Piers can be connected to floor beams in several ways. A pier's reinforcing steel rods can be ex- tended from the pier and bent over or into the floor beam (Figure 4.35). A metal strap well- anchored in the pier can be bolted through the beam (Figure 4.36). Or (Figure 4.37) steel anchor bolts can be embedded in the pier and bolted through the beams with nuts and large-diameter washers . ' " " ' I 11?1f>l-..1E:1E:R MllJ ALL TH1".J:W fi'CD WITH ~HER AfJP Lil.IT f>.T TOP OF E£l>.M • EMe>EV lL T'J AT ?Pl-ICE: SOL-IV' lv1tl'SONIZY 1~ OR COi.JC. ~ ~ Figure 4.38. Beam Splice on Pier 85 ---PAGE BREAK--- I 0f'AN I 5f'Af.l Figure 4.39. Floor Beam with Cantilever Overhang 86 FLOOR BEAMS The floor beams attached to foundation elements in tum carry the floor joists and subflooring. Since floor beams that are as long as the width or length of residential structures are often difficult to find and hard to handle, it is common to use splices. Splices may occur in several places and need not always be located directly over supports. Floor beams are often 4 x lO's or up to 6 x 12's, but they may be built up using standard framing lumber, such as two, three, or four 2 x lO's or 2 x 12's, spiked or bolted together. Where beams are built up using a good grade of lumber for the laminated members, the strength of the built- up beam can equal that of a solid member. All members of the built-up beam should be continu- ous between supports, because splices materially reduce strength. Built-up members should include only one splice at any one location. The ends and tops of built-up members should not be directly exposed to the weather. The primary floor beams spanning between supports should span in the direction parallel to the flow of potential floodwater. This orientation allows the first transverse member perpendicular to flow to be the floor joist. Thus, in the case of an extreme flood the beams would not be subjected to the full force of floodwater along their more exposed surfaces. This also reduces the potential for floating debris to damage the structure, and places the lowest obstacle to flow above the floor beam. CANTILEVERS A cantilever is a projecting beam that extends beyond its support. The beam must be continu- ous (not spliced) over the last support prior to the cantilevered section, and depends on the vertical load applied for counteracting reactions (Figure 4.39). The practical limit recommended for a cantilever is normally one-third the length of the beam span prior to the cantilever. ---PAGE BREAK--- The advantage of this method is that it can reduce the number of piles, poles, or piers required for a given area, as illustrated in Figure 4.40. Reducing the number of piles can result in potentially lower cost and fewer obstructions to the flow of flood- water and debris. Residences supported in this manner have the additional advantage of hav- ing the first row of piles set back, reducing the visual impact of elevating the structure. A canti- lever design may use longer spans for the main floor beam and thus may require larger beams. - - - - - - - - - - - - I I I I I I I I I I I ' I ~ • • • ~ ~ I I I I I ' ' I I ~ - - - - - - - _ - _ - - - - l l 10' l 10' l 10' l " If 1 1YPICAL Pit.<' ~~IW!C..8-IP~f lo PIU?S ~fQU11<'£~ - - - - - - - - - - I I I I I I i ~ ~ : I : ; • • r ~ ! ~ 5j I I I ' - l l ~A 11· l 11· l 11· l I\ Ir Figure 4.40. Cantilever Used to Reduce Number of Foundation Elements 87 ---PAGE BREAK--- ZX.0H JOI Figure 4.41. Wood Joist Anchors ME:CHANICA\.... FM1f:;f-JE:.!i:.'? (HUli:.RiCM.11:: GLIF":>) !'X:;1WE:E:;f-..l ~M AND Jo1-:i1~. Figure 4.42. Metal Hurricane Clips 88 CONCRETE FLOORING SYSTEMS Recently developed flooring systems using precast, prestressed concrete for floor beams, joists, and/or subflooring can often be useful in elevated struc- tures. Construction and connection techniques for these systems are beyond the scope of this manual. FLOOR-BEAM-TO-FLOOR-JOIST CONNECTIONS A positive connection is also required beneath the first floor level between the floor joists and floor beams (Figure 4.41). Metal connectors now available provide strong positive connection (Figure 4.42). Metal straps can also be used provided proper nailing is done and a sufficient number of straps is installed. At the minimum, every other joist and wall stud should be anchored with a strap, and even more for more severe loads (Figure 4.43). A good wood connector has also been developed. The capacity of these connections depends directly on the number of nails and their individual capa- city to resist loads transverse to their axis. Pullout resistance along the axis is not used; rather, the nails are placed at right angles (perpendicular) to the loads being transferred between the wood members. The number of nails counted in figuring the total connection capacity of a given joint is the lower number that exists on either side of the joint. For example, in the connection of a floor beam to a floor joist, if five nails are in the beam and four are in the joist, the capacity of the connection is limited by the four nails on the joist. ---PAGE BREAK--- FLOOR JOISTS Cross-bridging of all floor joists is recommended to stiffen the floor system. The elevation makes the floors (particularly the first floor) more acces- sible to uplift wind forces, as well as to the forces of moving water and floating debris. Effective cross-bridging requires: nominal 1 x 3 's 8 feet on center maxi- mum solid bridging same depth as joist 8 feet on center maximum. SUBFLOORING Two methods are commonly used for subfloor construction: nominal 1 x 4 or 1 x 6 boards placed diagonally over the floor joists (either tongue-and- groove or square-edge with expansion space between boards) and plywood subflooring used to create a floor diaphragm. When a plywood subfloor is planned, guidelines for thickness and methods of attachment in relation to joist spacing can be obtained from the Plywood Construction Guide published annually by the American Plywood Association. A well-constructed, firmly attached subfloor can be an important asset in resisting lateral forces. Subflooring is typically nailed directly to the floor joists. Nailing with annular ring nails or deformed shank nails is recommended. These nails provide extra strength against pulling out when the floor system is exposed to loads other than gravity. A system of nailing and adhesive application of plywood with tongue-and-groove joints along the long edges of the sheet avoids the need for block- ing along these edges. This produces a more level floor and offers a stronger diaphragm action to resist horizontal flood forces. 5TR.ArF\NG. WRAPPE:t? AROLJt{ t7 F~ooRB~ PIER Figure 4.43. Metal Strapping 89 ---PAGE BREAK--- GbLVA~ I Z.cD ME:TAL '?TRAP - 0TUD L 6Uf>F1.DO~ 7 Figure 4.44. Stud-to-Stud Connections '50UD 5~~/..1HIN<1 NA\LliOfO AU- M&M8{;!t5 Figure 4.45. Plywood Anchorage 90 FLOOR-JOIST-TO-WALL CONNECTIONS Elevated structures experience increased wind forces because wind speeds increase with elevation. Exterior walls are used as tension members to transfer wind uplift forces at the roof down to resistance provided by the foundation. It is usually necessary to use galvanized metal strap connections from alternate exterior wall studs to the floor joists or floor beams and from first floor studs to second floor studs (Figure 4.44). The capacity of these connections depends on the number of nails used. Manufacturers' brochures can be used to ascertain connectors' capacity and thus the spacing required. WALL SHEATHING Plywood is the most common sheathing in use for exterior walls (Figures 4.45 and 4.46). The major advantages of plywood are that it braces the wall framing to resist racking stresses and it forms a continuous tie from floor beam to top plate when properly installed. Plywood used for sheathing structures elevated up to 10 feet above the ground should be exterior grade and not less than 1/2-inch thick. Nailing should be with sixpenny nails, spaced 6 inches along the edges of the panel and 12 inches on intermediate studs. ---PAGE BREAK--- Structures elevated more than 10 feet should be sheathed with 3/4-inch exterior grade plywood, nailed with eightpenny nails, spaced as before. Deformed shank or annular ring nails and plywood with exterior glue are recommended. WALL BRACING Bracing vertical walls against racking is a common building practice, especially for weak materials such as some of the newer insulated sheathing. Wind forces and lateral forces from moving water are also significant factors in determining whether and to what extent to brace vertical walls. Common wall bracing methods are a let-in diagonal wood brace, diagonal boards and plywood. A common method similar to the let-in diagonal brace is a light-gauge galvanized steel strap nailed diagonally to each stud at the outside corners and framed walls. WALL-TO-ROOF CONNECTIONS Probably the most critical structural connections for wind resistance are those between walls and the roof. For single-family residences, the roof structure is usually roof rafters of 2 x lO's or 2 x 12's or roof trusses built up of 2 x 4 's or 2 x 6 Whether rafters or trusses are used, they should be spaced at about 16 inches or 24 inches on center (16 inches is the more common spacing). Roof con- nections are critical because these connections are limited in number-at most they can occur at every roof rafter or truss. A number of available galvanized metal connectors place the nails in an orientation to best resist uplift and lateral forces. Manufacturers' brochures provide the necessary design information. llt"--1t--- fX'ffflOR f'L-.YWOOC? SHr:~'IHINGi lON11NUOUS FW~ '\OP Pl..A'ft;; MAIN fl-DOR B~AM Figure 4.46. Wall Sheathing Tie from Roof to Ceiling 91 ---PAGE BREAK--- Figure 4.47. Shutters for Window Protection Sewage Water Electric Figure 4.48. Protective Utility Shaft 92 5H\JTTE.R IN CLO~E~ f05\TION Related Design Considerations GLASS PROTECTION Even moderate storms or routine high winds can cause large losses of glass in buildings, particularly along a coast. Broken glass may allow rain and floodwaters and high winds to enter the structure. Water damage can ruin furnishings and eventually damage structural members. Wind allowed into an elevated structure increases the uplift load on the structure as it applies pressure to the ceiling and wall surfaces. Exterior shutters can be used to protect glass. For small openings the traditional louvered shutter offers some protection. Additional protection is possible using 1/2-inch plywood attached to the back of the shutter, which will take the direct forces from the storm (Figure 4.4 This method allows coverage of fairly large areas of glass. UTILITIES AND MECHANICAL EQUIPMENT Structures in flood-prone areas are commonly served by combinations of electricity, water, sanitary sewer, gas (both natural and bottled), and telephone. Typical installations for these utilities expose them to potential damage from flooding and storm action. In the case of an elevated first floor, the connection from an under- ground utility line to the floor above further exposes the line to possible damage and/or con- tamination by flooding and storm action. Under- ground services are also susceptible to damage when erosion of the protective soil cover leaves them exposed during flooding. Damage to utility lines can lead to contamination of drinking water, discharge of effluent from sewer lines, gas explosions, and fires and/or shock from damaged electrical systems. The most vulnerable section of any underground utility line is the portion between the ground and the place it enters the elevated first floor. A mini- mum amount of protection can be obtained by locating these utility risers on the sides of interior elevated foundation elements opposite the direction ---PAGE BREAK--- of flood water. This can minimiu damage from velocity water or (1oating debris. A more secure method is to place all utility lines coming from underground within a protective, floodproofcd shaft under the elevatt>d first floor (Figure 4.48). If electrical and telephone lines are supplied from overhead service lines, they should be connected through the utility company's meter system above the expected reach of flood waters. However, this requirement is often in conflict with the power company's policy regarding the reading of meters and their location. If this is not possible, the con- nection should be made within a waterproof Figure 4.49. Elevated Condenser Units enclosure. All distribution panels or other major electrical equipment should also be located above expected flood waters. Branch circuit wiring should be fed from the first floor ceiling downward to mini- mize wiring on the first floor. All mechanical equipment (furnaces, hot water heaters, air-conditioners, water softeners) should also be elevated above expected flood waters (Figure 4.49). An attic location, if available, would provide the equipment maximum safety. Heating and/or cooling systems using ductwork to carry tempered air should be provided with emergency openings at their lowest elevations and a minimum slope on horizontal duct runs in order to allow the system to drain in case it becomes submerged. Figure 4.50 illustrates some of these concepts. Septic tanks should be floodproofed to ensure that flooding does not cause the tank to rise out of the ground if the tank is partially empty, as well as to ensure against discharge of effluent. OVERHEAD UTILITY Lit-JES BEAL..t::t7 WE:L.L HEAD - - - BUILDING MATERIALS Figure 4.50. Locating Utilities One way to increase the safety of building materials is to elevate the building higher than the minimum floodplain management requirements. Even then, however, flood waters may still reach building materials, so they should be protected. A building elevated above grade has the underside of its floor area exposed to climatic and flood 93 ---PAGE BREAK--- 94 conditions, and will require special attention to protecting building materials. The climate and the desired appearance will determine whether the exposed underside of a floor should be sealed. Sealing exposed floors can protect subfloors and joists from the elements, improve insulation, and help conceal utilities. The material used to enclose floor spaces should be resistant to water damage or inexpensive to replace if it is not resistant to damage. Exterior grade plywood treated with preservatives is water- resistant and can be effective. Gypsum products should not be used unless an acceptable level of performance is assured. Regardless of the material used, some provision must be made to allow water that may find its way into the floor sandwich during storms and flooding to drain out, and for the joist spaces to dry out. Wood Wood exposed to the elements should be protected by treatment with any one of a number of chemical preservatives to make the wood resistant to fungi attack, insects, bacteria, and rot. Connections should be designed so that water will not collect on or in them. They can be protected with protective flashing, by treating saw cuts and drill holes with preservatives, and by painting connections. The American Wood Preservers Institute, Tyson's Inter- national Building, 1945 Gallows Road, Vienna, Virginia 22180, can provide specific guidelines. Steel In riverine areas steel framing and foundation members exposed to the elements should be pro- tected by galvanization or by painting with rust- retardant paints. The need for painting can be eliminated through the use of surface oxidizing steels (high strength low alloy). In saltwater environments, exposed structural steel shapes, beams, pipes, channels, angles, etc., undergo very rapid corrosion, and their use should be avoided. Small connecting devices such as bolts, angles, bars, and straps should be hot-dipped galva- ---PAGE BREAK--- nized after fabrication and coated with a protective paint after installation. Standard galvanized sheet metal joist hangers and other connecting devices deteriorate rapidly despite their galvanized coating and also require additional protective coatings. Small anchoring devices, nails, spikes, bolts, and lag screws should, whenever possible, be hot-dipped galvanized. With sheet metal clips and hangers, the special nails used should also be galvanized. Regular inspection, maintenance, and replacement of corroded metal parts is necessary when steel is used in the coastal environment. Steel rods used to reinforce concrete or masonry piles or piers require special precautions to prevent saltwater from reaching the steel through hairline cracks in concrete or through masonry joints. This is discussed below. The American Iron and Steel Institute, 1000 Sixteenth Street, Washington, D.C. 20036, can provide specific guidelines. Concrete and Masonry The durability of reinforced concrete and masonry block can be improved by the use of chemical additives mixed with the concrete and mortar and by special treatments and coatings. Additives are numerous and vary from those that will prevent spalling due to freezing to those that will improve strength. Surface treatments and coatings, such as silicone and epoxy paints, can be used to reduce water absorption and penetration and to prevent damage by airborne pollutants. Guidance in the use of concrete and masonry can be obtained from the Portland Cement Association, Old Orchard Road, Skokie, Illinois 60076, and the National Concrete Masonry Association, P.O. Box 781, Herndon, Virginia 22070. INSULATION Like exposed walls of conventional structures, the exposed floor of elevated residences must be insulated against heat losses and heat gains. Depending on the climate, two factors should be considered. First, elevating a building will expose plumbing; such plumbing must be insulated against 95 ---PAGE BREAK--- IN5Ul.A110N IHi"'~f--- !2X1. C,RAPI' P\..YWOOD 1 11~ WA.1~ ~\5IA.N1' C,Y~UM BO. SPfJ..Y l"OAM \NSUL.AflON Figure 4.51. Insulated Floor Section, Wood Post Foundation l!iii2'~l---- WA.1ff R~ IS1A.t-Jf C,YPSUK NJ. - - IN6UL.A'flON liitl"'91't---- !'-Xl GiRAW f'L-YWOOO Rlt;ID INSULA110N SfR..AY fOA.M IN'?UL-A 1'10N GON11NUOU5 ~W VE:-1-!i/ DF1.J...IN -8<1. C,~!Jf' PL-YW~t1 Figure 4.52. Insulated Floor Section, Foundation Wall 96 freezing. In extremely cold climates, heating cables may be necessary with the insulation. Second, insulated floor decks may be subject to floodwaters and should therefore have either impermeable, closed-pore insulation able to with- stand water submersion or insulation that can be replaced economically (Figures 4.51 through 4.53). BREAKAWAY WALLS As indicated in Design and Construction Manual for Residential Buildings in Coastal High Hazard Areas, cited in the Preface, the area under an elevated structure in a V Zone must be free of obstructions or be constructed with breakaway walls latticework) designed to collapse under stress without jeopardizing the structural support of the building (Figure 4.54). Loads from flood waters and waterborne debris are critical considera- tions in designing breakaway walls. RETROFITTING EXISTING STRUCTURES Existing residential structures in flood hazard areas can often be raised in-place to a higher elevation to reduce their susceptibility to flood damage. The principal consideration in raising existing structu-res is often the cost; generally, the technology exists to raise almost any structure, even multistory buildings, but the cost increases as the difficulty increases. Residential structures have been satisfactorily raised up to nine feet. Aesthetics, intended use, needed flood elevation, and structural stability influence the height selected. Generally, the additional cost to raise a structure an additional foot or so is small compared to the initial set-up cost. The new foundation for an ex1shng structure should be selected and designed as discussed earlier. Raising in-place is generally feasible for structures that are 1) accessible below the first floor for placement of jacks and beams, 2) light enough to ---PAGE BREAK--- be jacked with conventional house moving equip- ment, 3) small enough that they can be raised in one piece, and 4) strong enough to withstand the stress of the raising process. Wood frame residential and light commercial structures with first floors above the ground (normally with an 18-inch crawl space beneath the first floor) are particularly suited for raising. Wood frame structures with basements below the first floor are also accessible and lightweight; however, raising the superstructure does not protect the basement, and the basement should be filled with a granular material to provide struc- tural stability for the walls. Brick, brick veneer, and masonry structures, while heavier and more difficult to handle, can also be raised. Utility equipment located in a basement can often be moved to a higher room, such as an upstairs closet, or an attic. It is important to ensure that the closet or attic floor can support the weight of the equipment. If necessary, an elevated addition can be built to house a furnace, hot water heater, and other equipment formerly housed in a basement. Protecting utility equipment in this way can be useful even if the house itself cannot or need not be raised. Raising a structure usually invoh·cs the following steps: - Disconnect all plumbing, wiring, and utilities that cannot be raised with the structure. - Place steel beams and hydraulic jacks b<'neath the structure and raise to desired elevation. - Extend existing foundation walls and piers or construct new foundation. - If a basement exists, remove water heater, furnace, etc., and fill basement with granu- lar material to support basemen t walls. - Lower the structure onto the ex tended or nf'w foundation. - Adjust walks, steps, ramps, plumbing, and utilities and regrade site as desired. - Reconnect all plumbing, wiring, and utilities. - Insulate exposed floor to reduce heat loss and protect plumbing, wiring, utilities and insula- tion from possible water damage. l !-JSUL.A110~ l'U?NUM Figure 4.53. Double-Insulated Floor Plenum, Pier Foundation Figure 4.54. Breakaway Walls 97 ---PAGE BREAK--- COST ANALYSIS ---PAGE BREAK--- Once a community decides that the economic risk and environmental impact of developing floodplain land for residential use is acceptable, the dollar cost of that development must be evaluated. Two factors bear significantly on any such evaluation: first, the net cost of con- struction that meets the standards of the National Flood Insurance Program (NFIP) in light of the potential and unpredictable hazard of flooding and the losses that may ensue; second, the cost differentials between construction on elevated foundations and conventional build- ing methods. (Note that standards adapted by local jurisdictions are often more stringent than the NFIP's.) RPpeated studies have shown that the savi ngs that can be realized oYer the lifetime of a struc- ture by building on a raised foundation are usually considerable when com pared with the one-time increase in construction costs for an elevated foundation. This is largely becau th t> the one-time foundation costs arc generally only fiye or six percen t of the total cost of a residrntial structure, while the flood insurancP savings that can be achieved over the life of a slructu re elernting it can be considerable. The economic cost to the individual of building a home in the floodplain consists of both flood damages that will occur and the costs of whatever measures are taken to mitigate such damages. The cost of flood damages to the homeowner may be partially shifted to federal, state, and local govern- ment through low-interest loans and tax deduc- tions for losses incurred. In communities parti- cipating in the NFIP, the owner of a new home can purchase flood insurance. Essentially, flood insur- ance allows the homeowner to spread the flood risk to others facing the same hazards and, more importantly, permits one to pay for expected flood losses, which are unpredictable as to size and time of occurrence, in predictable annual pay- ments. These are more manageable than un- expected flood losses, especially if more than one large flood happens to occur in a very short time. 99 ---PAGE BREAK--- r . i ~ I I ' r, 26'-0" ,r ' _ _J 24'-0" j 50'-0" FOUNDATION PLAN ~ 1 • . I - L L _ FOUNDATION PLAN ~ I 26'-0" 24'-0 " FOUNDATION PLAN " 11•~ "'"chm tl"h1 Emn.enct t 1S" h4• 16 81oct. 8•8•16 8h o(t. Frou 4" Conc:Sl• b 8•6- 6/8 W.tded Wir• R9infon:inti 4 mlt f'lut k: V•por r 4" Comp.cted Grawl Of Crulhtd Ston• FOUNDATION SECTI ON SLAB-ON-GRADE $4.61 per square foot % Dia. Ancl'lor Both Em09dcMd 1~ " Min - 8' 0,C. MH Bx8x16 Block Fouodetlon Continuou1 Fooling . FOUNDATION SECTION 16" CRAWL SPACE $5.13 per square foot 2x10H...:l•r - 11." Plywood Dack Ancl'lo• Boin 8'0,C. 2x 10Joifl1 18"0.C, Gird9< 4)1,Mx11Y'll1'" to Colurnn •nd Uggtd OfSpikedtt)G~ 6"10 8"C.,,nrt.«1 Gr....,.IOf Siona Over 0<11n T1I• Contil'luou• BASEMENT $11.01 per square foot 4 " Cone, Floor SI.ti 6x6-616W91ded Wit'•Rtinforcing 4 mU Pl tlc V"PO' 20"•20"1l8" Footing Figure 5.1. Conventional Foundat ions (Estimates are spring 1983.) 100 ---PAGE BREAK--- COST COMPARISON APPROACH The costs of post, pile, and pier foundations are compared here to each other and to the costs of conventional slab, crawl space, and basement foundations. Cost data and estimating forms are provided for roughly estimating one's particular foundation costs. 1. Slab-on-grade, crawl space, and basement foundations were selected as three of the most common types of residential foundations, and detailed drawings of them were prepared (Figure 5.1). Detailed drawings were also pre- pared for the three most typical elevation foundation types. These are post, pile, and pier foundations (Figure 5.2). (Regarding use of earth fill, see below.) FOUNDATION SECTION CONCRETE PIER $7.08 per square foot Double 2~12 Gorde" WOOD POST $6.96 per square foot WOOD Pl LE $6.58 per square foot Figure 5.2. Elevated Foundations (Estimates are spring 1983.) FOUNDATION SECTION 101 ---PAGE BREAK--- Conventional Foundations Slab-on-Grade $4.61 per sq. ft. $5.13 per sq. ft. $11.01 per sq. ft. Crawl Space Basement Elevated Foundations Wood Post Wood Pile Concrete Pier Estimates-Spring 1983 $6.96 per sq. ft. $6.58 per sq. ft. $7 .08 per sq. ft. Figure 5.3. Foundation Cost Estimates % Increase of Total House Cost Dollar Increase, Foundation Cost Only Elevated Foundations Wood Post Wood Pile, Concrete Pier Wood Post Wood Pile Concrete Pier 2. The estimates are summarized in Figure 5.3. They are based on the foundation and deck of a 1,500-square-foot house, 28'x50', with a small offset. The total cost of this house is approximately $60,000, excluding land. All estimates were based on FHA construc- tion practices. 3. Using data from this cost sampling, the average cost of each conventional foundation type is compared to the average cost of each elevated foundation type. This comparison is done in two ways : first, each foundation as a percentage of the cost of the entire house (conventional foundations were established as base 100) and, second the dollar increase in the cost of the foundati on above. Conventional Foundations Slab on Grade Crawl Spaces Basement +5.9 +4.6 -10.1 +4.9 +3.6 -11.1 +6.2 +4.9 -9.8 $3,525 $2,745 -$6,075 $2,955 175 -$6,645 $3,707 $2,925 -$5,895 Figure 5.4 Cost Differentials, Conventional Vs. Elevated Foundations, for House Costing $60,000, Excluding Land. 102 ---PAGE BREAK--- 4. Figure 5.5 graphically compares the cost of constructing the different types of foun- dations at various elevations. Note that increasing the elevation increases costs at a substantial rate only in the case of the fill option (which is based on the availability of usable fill material on the site). 15 14 13 12 en 1 1 -52 0 10 9 0 en 8 'C c: 7 «S en 6 0 5 c: 4 ~ 3 8 2 1 0 Slab+ Site fill (not permitted in zone V) J' • * Cone. piers / * I I I ~ * _ . Wood posts * # * •r , • • • • • e Wood piles ~ • • • 0 • - - • ~ - 411• • • - - - Slab on grade 0 1 2 3 4 5 6 7 8 9 101112131415 ELEVATION (in feet) Figure 5.5. Relative Costs of Foundations Elevated to Different Heights 103 ---PAGE BREAK--- 104 Fill Fill can often be used in A Zones to elevate con- ventional foundations such as slab-on-grade. The cost of this approach varies widely, depending on the availbility, quality, and unit cost of fill as well as the height and compaction necessary. Local building officials or soils engineers should be consulted to evaluate local conditions. COST COMPARISON CAVEATS The comparative cost data given above do not take into account a number of factors that can affect either basic construction costs or long-term insur- ance costs. Insurance Costs Insurance rates under the NFIP vary greatly de- pending on the elevation of a building and other features related to flood safety. Differences in these rates can overshadow the construction cost differentials discussed in this chapter, and should be considered carefully in making design deci- sions. Design Assumptions Each house elevated on piles, posts, and piers was assumed to have 21 foundation elements. In addi- tion, each element was assumed to be an average length that included the length below grade and the length between grade and the structure. These are 16 feet for piles, 14 feet for posts, and 15 feet for piers. In practice, both the number and length of foundation elements will vary depending on soil conditions, expected flood levels, etc. Earthquakes Constructing elevated foundations in earthquake areas may require additional structural expendi- tures that should be noted in cost estimates. Local building officials or a structural engineer should be consulted to evaluate local conditions. ---PAGE BREAK--- Stairs and Utilities Elevating a residence may result in increased cost for stairs and for utilities that must be elevated above grade. These costs were not considered in the estimates presented here since they vary with height of elevation, cost assignment, i.e., who pays for installation of utilities, and elevation method. Regional Cost Variations The cost data presented above are based on national averages, and do not take into account regional cost variations. Cost Inflation Building costs are difficult to predict because of the tendency for the cost of basic construction commodities-lumber, concrete, and steel- to fluc- tuate and to vary relative to each other. The costs here are estimated using data for the spring of 1983. Non-Cost Considerations Cost is not the only determinant for selecting the material and method for elevating. Market accept- ance (buyers and banks), architectural design inte- gration, climatic conditions, site conditions, and anticipated flood hazards should also be con- sidered. ESTIMATING FORMS The forms on the following pages can be used for making cost estimates for conventional and ele- vated foundations. 105 ---PAGE BREAK--- SLAB-ON-GRADE ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Perimeter Square Footage of Foundation Wall Enter you costs (combine labor and material) and extend: Layout house on lot Trench for footing x LF Place footings x LF Lay-up or form & pour foundation wall x SF Fill & grade for slab x SF Place vapor barrier, wire mesh & insulation x SF Place & finish slab x SF Grand Total $ _ 106 ---PAGE BREAK--- CRAWL SPACE ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Perimeter Square Footage of Foundation Wall Number of Piers Enter your costs (combine labor and material) and extend: Layout house on lot Trench for footing x LF Place footings x LF Lay-up or form and pour foundation wall x SF Place pier footings x Ea. Lay-up or form and pour piers x Ea. Backfill x CY=$ Floor Girder x LF Floor Framing x SF Insulation & sealer x SF Subfloor x SF Place floor slab x SF Grand Total $ _ 107 ---PAGE BREAK--- 108 BASEMENT ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Perimeter Square Footage of Basement Wall Area Number of Basement Support Columns Enter your costs (combine labor and materials) and extend: Layout house on lot Excavation & spoil removal x Place footings x Place pier footings x Lay-up or form & pour foundation wall x Parge wall x Set drain tile x Backfill x Place vapor barrier and wire mesh x Place and finish floor slab x Place girder x Frame Floor x Place subf/oor x Grand Total SF LF Ea. SF SF LF CY SF SF LF SF SF =S $ _ ---PAGE BREAK--- WOOD POST ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Girders Number of Posts Enter your costs (combine labor and material) and extend: Layout house on lot Auger or dig post holes and remove spoil x Oty Place concrete punching pad x Oty Place poles x Oty Backfill poles and plumb x Oty Set girder x LF Frame floor x SF Place insulation & sealer x SF Place subfloor x SF Grand Total $ _ 109 ---PAGE BREAK--- WOOD PILE ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Girders Number of Piles Total Lineal Footage of Piles Enter your costs (combine labor and material) and extend: Layout house on lot Bring pile-driving equip- ment to site x Furnish and drive piles x LF Set girder x LF Frame floor x SF Place insulation and sealer x SF Place sub floor x SF Grand Total $ 110 ---PAGE BREAK--- CONCRETE PIER ESTIMATING FORM TO DETERMINE LOCAL COSTS Compute the following and enter: Square Footage of Floor Area Lineal Footage of Girder Number of Piers Enter you costs (combine labor and material) and extend: Layout house on lot Auger or dig pier holes and remove spoil Place concrete footing Form & pour piers Backfill Set girder Frame floor Place insulation and sealer Place subfloor Grand Total x Oty=$ x Oty x Oty x Oty x LF x SF x SF x SF $ _ 111 ---PAGE BREAK--- RESOURCE MATERIALS ---PAGE BREAK--- Glossary Base Flood Elevation (BFE) The elevation for which there is a one-percent chance in any given year that flood levels will equal or exceed it (see Special Flood Hazard Areas). The BFE is determined by statistical analysis of stream- flow records for the watershed and rainfall and runoff characteristics in the general region of the watershed. Coastal High Hazard Area The portion of a coastal floodplain that is subject to high velocity waters caused by tropical storms, hurricanes, northeasters, or tsunamis. Labeled V Zones on Flood Insurance Rate Maps, these areas experience breaking waves of three feet or more. Debris Impact Loads Loads induced on a structure by solid objects carried by flood water. Debris can include trees, lumber, displaced sections of structures, tanks, runaway boats, and chunks of ice. Debris impact loads are difficult to predict accurately, yet rea- sonable allowances must be made for them in the design of potentially affected structures. Encroachment Any physical object placed in a floodplain that hinders the passage of water or otherwise affects flood flows. Existing Construction Those structures already existing or on which construction or substantial improvement was started prior to the effective date of a community's floodplain management regulations. Flood or Flooding A general and temporary condition of partial or complete inundation of normally dry land areas. Flooding results from the overflow of inland or tidal waters or the unusual and rapid accumula- tion of surface water runoff from any source. Flood Insurance Rate Map (FIRM) An official map of a community, issued or approved by the Federal Emergency Management Agency, that delineates both the special hazard areas and the risk premium zones applicable to the community. Zones are as follows: Zone A (unnumbered) - special flood hazard area inundated by the 100-year flood; deter- mined by approximate methods with no base flood elevation shown. Zones Al-A30 - special flood hazard area inundated by the 100-year flood; determined by detailed methods with base flood elevations shown. Zone B - area between the limits of the 100- year flood and the 500-year flood, or certain areas subject to 100-year flooding with average depths less than 1 foot, or areas protected by levees from the base flood. Zone C - area of minimal flooding; located out- side the limits of the 500-year flood. Zone V (unnumbered) - area subject to wave action, without base flood elevation shown. Zones V 1-V30 - special flood hazard area of 100-year coastal flooding with velocity (wave action); base flood elevations shown. 113 ---PAGE BREAK--- Floodplain Any normally dry land area that is susceptible to being inundated by water from any natural source. This area is usually low land adjacent to a river, stream, watercourse, ocean, or lake. Floodplain Management The operation of a program of corrective and preventive measures for reducing flood damage, including but not limited to flood control pro- jects, floodplain land-use regulations, flood- proofing of buildings, and emergency prepared- ness plans. Flood way The channel of a river or watercourse and the adjacent land areas that must be reserved to discharge the one-percent-probability flood with- out cumulatively increasing the water surface elevation more than a designated height, generally one foot. Hydrology The science of the behavior of water in the atmos- phere, on the earth's surface, and underground. Hydrodynamic Loads As flood water flows around a structure it imposes loads on the structure. These loads consist of frontal impact by the mass of moving water against the structure, drag effect along the sides of the structure, and eddies or negative pressure on the structure's side. Hydrostatic Loads Those loads or pressures resulting from the static mass of water at any point of flood water contact with a structure. They are equal in all directions and always act perpendicular to the surface on which they are applied. Hydrostatic loads can act vertically on structural members such as 114 floors, decks, and roofs, and can act laterally on upright structural members such as walls, piers, and foundations. Mean Sea Level The average height of the sea for all stages of the tide, usually determined from hourly height ob- servations over a nineteen-year period on an open coast or in adjacent waters having free access to the sea. New Construction Structures on which construction or substantial improvement was started after the effective date of a community's floodplain management regu- lations. One-Hundred Year Flood (See Special Flood Hazard Areas). Permeability The property of soil or rock that allows passage of water through it. Regulatory Floodway Any floodway referenced in a floodplain ordinance for the purpose of applying floodway regulations. Special Flood Hazard Areas Areas in a community that have been identified as susceptible to a one-percent or greater chance of flooding in any given year. A one-percent probability flood is also known as the 100-year flood or the base flood. Stillwater Elevations The elevation that the surface of the water would assume if all wave action were absent. ---PAGE BREAK--- Storm Surge A rise above normal water level on the open coast due to the action of wind stress and atmospheric pressure on reduction on the water surface. Substantial Improvement Any repair, reconstruction, or improvement of a structure, the cost of which equals or exceeds 50 percent of the market value of the structure either before the improvement is started or if the structure has been damaged, and is being restored, before the damage occurred. Watershed An area from which water drains to a single point; in a natural basin, the watershed is the area contri- buting flow to a given place or stream. Wave Height The vertical distance between a wave crest and the preceding trough. Wave Crest Elevation The elevation of the 100-year storm surge plus wave height. 115 ---PAGE BREAK--- Sources of Design Information Information Required Purpose or Implications of Data Possible Forms of Data Potential Sources of Data • National Flood Insurance Pro- • Requires local communities to • Program regulations • Federal Insurance Admm1stration gram (NFIP) implement floodplain regulations. • Insurance rate 1nformat1on and • Federal Emergency Management • Sets minimum standards for tables Agency floodplain regulations. • Flood Insurance Studies • State Floodplain Management • Prohibits federal funding tor • Flood Maps Coordinating Agency pro1ects 1n v1olat1on of floodplain Section 1362 Guidelines • Local Government Planning regulations Agency • Prohibits federal loan guaran- tees for proiects 1n v1olat1on of floodplain regulations. • Establishes flood insurance rate differentials for properties 1n flood-prone areas • Local Government Planning • Implements floodplain regulations. • Planning and Zonmg Ord1 nances • Local Government Planning Programs • Determines local floodplain • Zoning Maps Agency regulations based on NFIP • Building Codes • Local Government Enqineer guidelines (includes zoning and • Building Code Ott1c1als subd1v1s1on regulations. per- formance standards. Planned Unit Development ordinances. building codes, etc.) Note Local regulations can be set at a higher standard than NFIP m1n1mum standards. de- pending on local needs and circumstances • State Floodplain and Coastal • Provides statewide floodplain • State program regulations • State Floodplain Management Zone Programs development regulations and • State development guidelines Coord1nat1ng Agency guidelines • State Office of Coastal Zone • Regulates development 1n Management coastal zones • State Office of Natural or Water • Coordinates 1mplementat1on of Resources NFIP in !ocal 1ur1sd1cttons and 1n areas where multiple state agencies have an mterest 1n flooding • Clearinghouse tor Floodplain Management Information • Regional Planning Restrictions • Can provide additional regula- • Program regulations • Regional Authorities Ten- or Guidelines t1ons and guidelines for regional • Development guidelines nessee Valley Authority. Appa- 1ur1sd1ct1ons lach1an Regional Commission. etc.) • Coordinates act1v1t1es of differ- • Regional Planning ent agencies within the region • River Basin Comm1ss1ons • Source of mformat1on and , tn some cases. technical assistance • Federal Agency Requirements • May include regulations relating • Program regulations • U S Army Corps of Engineers and Gu1del1nes (other than NFIP) to development 1n flood-prone • Environmental Protection Agency areas (e g , Corps of Engineers • Federal Emergency Manage- permits for development on ment Agency navigable rivers) • State Floodplain Management • May involve federal funding , the Coord1nat1ng Agency use of which is restricted 1n • Local Planning Agency flood-prone areas • Proiects may require federal approval for development 1n flood-prone areas (e g En- vironmental Impact Statements) Information Required Purpose or Implications of Data Possible Forms of Data Potential Sources of Information • Flood Hazard Boundaries • Determines where floodplain • Flood Hazard Boundary Maps • Local Government Planning regulations. insurance. and fed- • Flood Insurance Rate Maps Agency eral f1nancmg restrictions apply • Flood Boundary and Floodway • Determines sp_ec1 f1c flood haz- Maps • Local Government Municipal ard zones • Hydrolog1c Atlases Engineer • Determines variable flood 1nsur- • Local Zoning Maps ance rate zones • Flood Insurance Studies • State Floodplain Coord1nat1ng • Flood Depths • Indicates elevations at which • Flood Elevations Agency flood damage is likely to occur • Water Surface Profiles • State Office of Natural Re- • Determines approPnate butld- • Stream and Coast Cross-sections sources 1ng elevations for meeting • Flood Insurance Studies floodplain regulations and flood • Federal Insurance insurance restrictions and rates tion • Indicates hydrostatic loads 1n flood-prone areas • Federal Emergency Manage- • Flood Water Velocity • Determines hydrodynamic • Floodplain Technical Studies ment Agency loads 1n flood-prone areas • Hydrolog1c Studies • us Army Corps of Engineers • Determines debris-impact loads • Flood Insurance Studies in flood-prone areas • Indicates potential for erosion • us Geologic Survey and slope detenorat1on 116 ---PAGE BREAK--- • Warning Time • Indicates importance of emergency evacuation as part of the design program • Influences design of floodproof1ng techniques such as flood shields • Influences design of drainage systems • Influences design of wet flood- proofing techniques • Duration of Flooding • Affects seepage into buildings and saturation of soils and building materials • Affects the length of time fac1h- ties might be inaccessible or inoperable. • Affects building design relative to orientation configuration. and choice of floodproofing techniques. • Frequency of Flooding • Influences site choice. • Affects choice of floodproof1ng techniques. especially those that require 1nstallat1on before every flood • Indicates need for special access • Climate and Weather • Indicates frequency and type of precipitation and, 1n turn. the type and magnitude of flooding that is hkely • Ground Water Level • Influences potential waler pressure on footings . founda- lions. and floors. • Affects site design techniques for controlhng water runoff • Structural Flood Control Meas- • Existing measures can affect ures (e g dams. levees. chan- site 1f the hm1ts of the flood con- nel improvements) trol device are exceeded • Proposed measures can. when implemented alter basic flood data Information Required Purpose or Implications of Data • Phys1og1 aph1c Features • Affects location and magnitude of flooding on the site • ldent1f1es areas of the site that should be avoided or protected • Affects onentat1on. d1stnbut1on. and density of built elements on the site • Identifies physical constraints and advantages for site devel- opment • Topography • Influences s1t1ng of buildings • Indicates erosion potential • Indicates need for. and feas1b1I- 1ty of using, hll material on the site • Indicates appropriate site de- sign techniques for controlling water runoff • Soil Characteristics • Soil porosity influences the rate of water runoff and flooding po- tent1al • Determines the feas1b11ity and design spec1f1cat1ons for use of ftll material to elevate buildings. the use of backfill around foun- dat1ons. and construction of earth berms • Indicates required depth for footings, p1hngs. or columns • Slope Stability • Affects ch0