Elevated

Residential

Structures

The American Institute of Architects Foundation 1735 New York Avenue, N.W. Washington, D.C. 20006

1984

Table of Contents
ACKNOWLEDGMENTS PREFACE	ii v
ENVIRONMENTAL AND REGULATORY FACTORS	1 1
_ FLOODING AND THE BUILT ENVIRONMENT
Riverine Flooding ∎ Coastal Flooding
FLOODPLAIN MANAGEMENT	4
National Flood Insurance Program ∎ Base Flood Elevations ∎ A and V Zones
SITE ANALYSIS AND DESIGN	8 9 13
SITE SELECTION AND ANALYSIS SITE DESIGN
Site Flooding Characteristics ∎ Access and Egress ∎ Vegetation ∎ Flood Water
Drainage and Storage ∎ Dune Protection
ARCHITECTURAL DESIGN EXAMPLES	18
DESIGN STUDIES	22
Bridgeport ∎ Charleston and Newport 0 San Francisco ∎ Chicago
AESTHETIC CONSIDERATIONS	35
RECENT DESIGN EXAMPLES	45
Logan House ∎ Summerwood on the Sound ∎ Breakers Condominium ∎
Campus-by-the-Sea Facility ∎ Starboard Village ∎ Gull Point Condominiums
DESIGN AND CONSTRUCTION GUIDELINES	64
FOUNDATIONS	65
Fill ∎ Elevated Foundations ∎ Shear Walls ∎ Posts ∎ Piles ∎ Piers ∎ Bracing
FRAMING CONSTRUCTION AND CONNECTIONS	80
Framing Methods ∎ Floor Beams ∎ Cantilevers ∎ Concrete Flooring Systems 0
Floor Joists ∎ Subflooring ∎ Wall Sheathing and Bracing ∎ Roof Connections
RELATED DESIGN CONSIDERATIONS	92
Glass Protection ∎ Utilities and Mechanical Equipment ∎ Building Materials ∎
Insulation ∎ Breakaway Walls ∎ Retrofitting Existing Structures
COST ANALYSIS	98
RESOURCE MATERIALS	112
GLOSSARY	113
SOURCES OF DESIGN INFORMATION	116
FEMA REGIONAL OFFICES	118
STATE COORDINATING OFFICES FOR THE NFIP	120
PERFORMANCE CRITERIA	125
REFERENCES	136
	iii

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 occupants. 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 published in 1976 by the Federal Insurance Administration. This revision reflects changes since 1976 in floodplain management techniques and regulations, 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 techniques 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, supplements this manual's discussion of elevated residential 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 Development. It provides structural engineering guidelines 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

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 transportation system. Recreational opportunities and aesthetic enjoyment further stimulate waterside development.

This development pattern, however, leads to a conflict 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 environment 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.

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 hurricanes, 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 protection system, exacerbating the impact of storm surges and high winds.

NATIONAL FLOOD INSURANCE PROGRAM

The National Flood Insurance Program (NFIP) is the federal government's principal administrative mechanism for reducing flood damage. Established by Congress in 1968, the NFIP is administered by the Federal Emergency Management Agency (FEMA). The NFIP insures buildings and their contents in flood-prone areas, where conventional insurance had, prior to the NFIP, been generally unavailable.

The NFIP provides this insurance only in communities that agree to implement comprehensive land-use planning and management to reduce the likelihood of flood damage in their jurisdictions. Community response to this incentive generally involves the adoption of zoning, building code, and development regulations that place various requirements and restrictions on new construction and on substantial improvements to existing construction.

Note that some local governments have adopted codes and zoning ordinances that are considerably more restrictive than the minimums required by FEMA. The result is that familiarity with design requirements in one community cannot be relied on elsewhere.

The rate structure of the NFIP's insurance premiums reinforces the intent of these regulations by charging higher insurance rates for buildings subject to greater hazard. These insurance rates are set primarily on the basis of designated hazard zones and the elevation of the building or structure in relation to the level of flooding likely to occur in each zone. This differential rate structure provides a significant financial incentive to locate buildings in less hazardous zones or to increase buildings' flood safety by elevating them higher than the NFIP's minimum elevations.

habitation and must be free of obstructions.

NFIP requirements for A and V Zones as of January 1984 are summarized in Figure 1.4.

Note that FIRMs are based on a variety of assumptions about expected flood severity, development patterns, etc. The actual level of flooding from a 100-year flood may be significantly greater. In addition, the "500-year" flood level, which would be significantly greater than the 100-year flood's, could conceivably occur once or even more often during a building's lifetime. These uncertainties are further reasons for locating buildings in less hazardous zones or elevating them higher than the NFIP's minimum elevations.

ON SLAB FOUNDATION A Zones

BOTH A AND V ZONES (Numbered and Unnumbered)

- All structural components must be adequately connected and anchored to prevent flotation, collapse, or permanent lateral movement of the building during floods.

- Building materials and utility equipment must be resistant to flood damage. All machinery and equipment servicing the building must be elevated to or above the Base Flood Elevation (BFE), including furnaces, heat pumps, hot water heaters, air-conditioners, washers, dryers, refrigerators and similar appliances, elevator lift machinery, and electrical junction and circuit breaker boxes.

- Any space designed for human habitation must be elevated to or above the BFE, including bedroom, bathroom, kitchen,

dining, living, family, and recreation room; and office, professional studio, and commercial occupancy.

- Uses permitted in spaces below the BFE are vehicular parking, limited storage, and building access (stairs, stairwells,

and elevator shafts only, subject to design requirements described below for walls).

A ZONES (A1-A30)

- Buildings must be elevated such that the lowest floor (including basement) is elevated to or above the BFE on fill, posts, piers, columns, or extended walls.

- Where fully enclosed space exists below the BFE, walls must be designed to minimize buildup of flood loads by allowing water to automatically enter, flow through (in higher velocity flooding), and drain from the enclosed area. For low velocity conditions, vents, louvers, or valves can be used to equalize flood levels inside and outside enclosed spaces. For high velocity conditions, breakaway walls (see below) or permanent openings should be used.

V ZONES (V1-V30)

- Buildings must be elevated on pilings or columns such that the bottom of the structural member supporting the lowest floor is elevated to or above the BFE.

- Buildings must be certified by a registered professional architect or engineer to be securely fastened to adequately anchored pilings or columns to withstand velocity flow and wave wash.

- Space below the lowest floor must be free of obstruction or enclosed with breakaway walls (i.e., walls designed and constructed to collapse under velocity flow conditions without jeopardizing the building's structural support. - Fill may not be used for structural support.

- No construction is allowed seaward of the mean high tide line.

Figure 1.4. Key Floodplain Requirements of the National Flood Insurance Program as of January 1984.

Site Selection and Analysis

SITE SELECTION

Whenever possible, site selection should avoid flood-prone areas. If this is not possible it should be recognized that the risk and severity of flooding generally decreases with the distance from the

river channel or from coastal waters. However, this is not always the case, so it is important to check the level of expected floods in relation to the proposed site. If the base flood elevation (BFE) has not been determined, it would be wise to consult local flood history data before making a final site selection.

The regulations of the National Flood Insurance Program (NFIP) specifically prohibit building or landfill in a floodway, if such has been designated, if the results would obstruct the flow of floodwaters and thereby increase flood heights. Similarly, building in a coastal high hazard area is also not permitted unless the structure is landward of the mean high tide level.

Development should be diverted away from identified mudslide or erosion-prone areas. Only where site and soil investigation and proposed construction standards assure complete safety for future residents should such sites be considered.

Overall, customary site selection criteria should be used to evaluate the suitability of a site. Drainage, height of the water table, soil and rock formations, topography, water supply, and sewage disposal capability should be considered along with economic and planning criteria such as cost, access, and compatible land use.

SITE ANALYSIS

The site elements of primary importance for analyzing an elevated residential project are flooding, soil, and wind characteristics.

- Rate of rise, which indicates how rapidly water depth increases during flooding. This determines warning time before a flood, which will influence the need for access and egress routes elevated above floodwaters and whether valuable possessions can be kept underneath the structure and moved only when flooding is imminent. Flash flood areas often receive little or no warning of flooding.

Another hydrologic factor is ice, which in northern climates can cause serious damage to structures if flooding should occur during the spring before the ice melts. In some cases wind driven ice or ice jams have completely demolished bridges, homes, and businesses, snapping large trees and pushing buildings completely off their foundations. Floating debris can be equally dangerous in this regard. There is little that can be done to avoid these phenomena short of avoiding sites where they are especially likely to occur.

Hydrologic data concerning a site, including both technical studies and historical records, can often be provided by the local or state government and federal agencies such as the Federal Emergency Management Agency, the U.S. Army Corps of Engineers, and the U.S. Geological Survey. If needed information is not available from these sources, engineers familiar with hydrologic and hydraulic techniques can analyze the flooding potential.

Soil Characteristics

The characteristics of the soil in a flood area-soil bearing capacity, for example-can be important in determining an appropriate design. Highly erodable soil would not be desirable for use as fill in elevating a structure in a high velocity area unless the fill is properly protected. When erosion removes soils supporting building foundations, the foundations can fail (see Figure 2.3).

Site Design

Site design for elevated structures should follow standard planning criteria applicable to any site work. Typical factors to consider include slopes, natural grades, drainage, vegetation, orientation, zoning, and location of surrounding buildings, as well as expected direction of flood flow.

SITE FLOODING CHARACTERISTICS

Buildings should be positioned in the area of the site that will experience the lowest flood levels and velocities. In coastal areas, this means as far back from the beach as possible and, if feasible, behind dunes. Buildings should be oriented to present their smallest cross-sections to the flow of floodwater. This reduces the surface area on which flood and storm forces can act.

When multiple buildings are to be placed on the same site, the objective of site design is the same as for an individual building. One approach is to disperse buildings throughout the site, applying the criteria discussed above to each building. An alternative to such dispersal, when local zoning ordinances allow (e.g., a planned unit development ordinance), is to group buildings in clusters on the safest parts of the site, leaving the more vulnerable areas open. This approach not only reduces flood damage but can also allow greater flexibility in protecting the natural features on the site (see Figure 2.5).

Adjacent buildings, bulkheads, or other structures should also be considered in site layout, both for their potential to screen and divert floodwaters and water-borne debris and for their potential to become floating debris themselves. Bulkheads also tend to divert floodwaters around their ends, adversely affecting adjacent sites.

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 2.7). 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 floodwaters 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 precipitation 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

DUNE PROTECTION

Dunes provide a natural shoreline defense against storm surges and waves. Most coastal communities 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. Crossover 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.

Many of the twentieth century's most important buildings have been elevated residential structures. The rise of modern 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 3.2). 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 offers us the columns. The house is in the air, above the ground; the garden passes under the house.

exposures. Inventive landscaping also helps to control erosion and protect the 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 are 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.

For numerous functional and aesthetic reasons, earth fill with heavy stone revetment was chosen as the method for elevating residential structures in Charlestown (Figure 3.8). The homes were clustered to keep down the cost of fill and because the land available for safe building in the floodplain was limited (Figure 3.9). A small-scale,

Newport

Development in the wharf area in Newport, Rhode Island, is structured by a combination of natural and cultural conditions. Although separated from the older historic areas of Newport by a highway, its proximity to them requires special consideration of height, materials, and size. It is in a special flood hazard zone, yet its water's edge location makes it visually attractive. Changes in the use of the wharf area and its new relationship with neighboring areas have resulted in an expansion of commercial and residential development. The low height above sea level means that new structures would have to be raised approximately to the level of the highway to comply with local flood regulations. For the restoration of historic buildings, however, there is no need to elevate the first floor as long as a variance is obtained.

Analysis indicated that the optimal solution would be a combination of elevation techniques, because different zones in the wharf area are suited to different elevation strategies (Figures 3.11 to 3.13).

In the area farthest from the water, earth fill offers flood protection and a gradual level change from that of the highway. A transitional middle section could combine berming with raised structures. Level changes can be integrated by linking extended decks with ramps and stairs. In the area closest to the water, raised structures would not alter the water-to-land relationship or block views. Commercial uses are most likely to locate in the filled area, where first floor spaces are usable. Residential, restaurant, and small office uses are more suitable to the raised structures, which afford increased privacy and better views.

Spaces under and between the new buildings can be used for pedestrian malls and thus reinforce the tourist and commercial uses of the area. Decks, balconies and trellises can connect different building levels. Utilities for the raised structures could be run beneath these raised decks and trellises and then into the fill, being protected from flood damage. This manipulation of the spaces and level changes created by flood protection enhances the visual intricacy and human scale of the wharf. 29

Single-Family Residential Concept

A two-way wood post structural grid supports the living units at levels above the base flood and serves to organize and unify the various units with minimal impact on the ecology of the area (Figures 3.14 to 3.16). A seven-foot clearance beneath the horizontal structural members allows for parking, storage, and sheltered recreation space separated from and below the living units. The reduced land coverage of this design is in keeping with the architect's concern for efficient land use. Shared facilities, clustering buildings, etc., further give these houses a unique identity and sense of community. Within the prescribed vernacular of poles, decks, railings, and fences, architectural variety with continuity is achieved. The fences are strapped together to prevent pieces from floating away if damaged during a flood. Water heater and furnace and air conditioning equipment are located 18 inches above base flood level with all ductwork in second floor or attic space.

Multi-Family Residential Concept

To reduce costs, the architects have designed a conventional wood frame structure built upon a wood post platform (Figures 3.17 and 3.18). Raising the first floor to at least eight feet above grade provides an opportunity to put parking under the building. This reduces the area of the site that has to be built upon and places cars closer to apartments. However, parking under the structure requires fire separation. Exposed entrance stairs and fencing minimize the elevated appearance of the structure while providing visual variety and privacy.

Aesthetic Considerations

There is a common misconception than an elevated residential structure will be inherently unattractive-a box on stilts (Figure 3.21). This is not true. Elevated structures offer challenging design opportunities to be aesthetically appealing as well as functionally sound.

Residential development requires a significant financial investment, and if it is aesthetically appealing it contributes to the economic value of the area, both for the owner and for the community as a whole. All communities have both positive and negative examples of this. Good quality tends to foster better quality, and poor conditions lead to even poorer conditions. Appealing design can thus be an important element of making the most of our limited development resources.

BUILDING DESIGN

The integration of the foundation with the site and the building is perhaps the most important aesthetic challenge when designing elevated structures. Many elevated structures give the impression that the support foundations are treated separately from the building and the site, giving the impression of a building set on spindly legs (figure 3.24). It is essential to recognize that the foundation is an integral part of a building, rather than only "something to set the building on." A well-designed elevated residence should provide a smooth transition from ground to dwelling, with the foundation integrated with and complementary to the building itself.

Other special considerations when designing elevated residences include the design of any needed stairs and the use of the areas under the structure. More general considerations include the shape and form of the building (configuration, shape of roof, etc.), textures and color of building materials, the use and treatment of balconies, terraces, railings, windows, shutters, screens, and entries, and the arrangement of interior spaces.

Figures 3.28 to 3.29. This is an excellent example of cluster-type elevated residential development. The development is well integrated with the site; the various levels seem to roll over and blend with the dune. The vegetation and simple fencing add much to this marriage. The individual units also relate very well to each other, providing a good example of an overall development's being "more than the sum of its parts." The individual units provide the individual amenities-privacy, plan layout, etc.-while still being a part of a comprehensive whole with a strong sense of community. The form, scale and character of the development are also excellent. The sloped roofs, the balcony treatment, use of levels, and the articulation of the other elements add variety and a character that complements the site and overall development. The use of materials-color, texture, scale-also contributes to the design's appeal (see also Figures 3.63 to 3.70).

Recent Design Examples

The projects in this section are some of the best design examples discovered in a state-of-the-art survey conducted as part of the development of this manual. While these examples range from a single-family detached unit to a multi-family high rise, there appears to be a clear trend toward higher density, cluster-type development. This is probably due to higher land values and the experience gained from major floods over the last couple of decades. This is a promising trend that encourages professional design involvement in residential structures and leads to a more comprehensive approach to elevated residential and other development in flood-prone areas.

Virtually all the recent design examples that were submitted in response to our survey were coastal, as opposed to riverine, projects. This suggests that the state of the art is being set for the most part in coastal areas, especially in the higher-use resort areas. It should be noted, however, that what is being done in coastal areas can often be applied successfully in riverine, lake, and other flood prone areas as well.

45

SUMMERWOOD ON THE SOUND

Old Saybrook, Connecticut

Architects: Zane Yost & Associates, Inc.,

Bridgeport, Connecticut

Summerwood on the Sound (Figures 3.46 to 3.50), a 76-unit cluster development, won a 1979 design award for architects Zane Yost & Associates, Inc. The development is built on a peninsula tidal estuary protected by a barrier beach.

Equal in importance to protecting the buildings from flooding was the preservation of the salt marsh ecological environment. For this reason, the architects chose to locate the units only along the natural contours of the 30-acre site. For further protection of land as well as buildings, the structures are elevated above flood level, topping crawl spaces with internal drains to permit floodwater to pass in and out. The wood frame structures are covered with horizontal siding and use picket fences to soften the effect of the raised structures. Redwood stairs and decks adorn the water side of the units.

Although the overall density on the site is low (2.5 units/acre), the clustering of the units makes for a comfortable neighborhood scale.

GULL POINT CONDOMINIUMS Perdido Key, Florida

Architects: H. Shelby Dean-Richard H. Fox, Architects, Anniston, Alabama

The design of this 16-unit condominium (Figures 3.63 to 3.70) on the Gulf of Mexico successfully integrates storm protection, energy conservation, function, and economics.

The architects used pile construction to elevate the units several feet above the minimum required by the National Flood Insurance Program. This was done because analysis of the flood insurance premium rate structure showed that the added margin of safety from the additional elevation would qualify the units for significant savings in annual insurance costs.

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, physical conditions at the site, and cost. The determination 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 methods. For example, a building raised on fill at one end and piers or posts at the other could provide ground floor access at the end of the 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 construction of elevated residential structures is based on accepted building practice. Generally, a conservative 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 response 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 performance.

Note that foundations in V Zones should be designed in accordance with Design and Construction Manual for Residential Buildings in Coastal High Hazard Areas, cited in the Preface.

65

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 foundation can be bermed with earth fill to provide easy access and a conventional appearance.

Elevated foundations must be designed to withstand both hydrodynamic forces caused by velocity waters and hydrostatic forces caused by standing water. This may require added reinforcement 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 floodwaters through the foundation. This can help minimize both hydrodynamic and hydrostatic 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 floodwaters. 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.

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 undisturbed 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 4.7). The pad should be approximately as thick as half its diameter, with a minimum 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.

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.

Round timber piles are also frequently used. Generally, round piles are available in longer lengths than square timbers, and for lengths greater than about 25 feet round piles are frequently 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 constructed 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 uneven 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

an underlying layer of several feet of clay. Generally, 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' pullout resistance.

PIERS

Pier foundations (Figure 4.15) are suitable in areas away from a river or coastline where floodwaters move with low velocity and erosion will be 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 environmental 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 concrete masonry units should be filled with concrete.

Poured-in-Place Concrete Piers

Poured-in-place concrete piers are essentially reinforced 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 1¹/2 to 12 feet or more. The dimensions, reinforcement, and spacing of concrete 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

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 anticipated, bracing can provide added assurance that the structure will withstand the impact of floating debris or greater-than expected flood or storm forces. Although bracing 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.

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 horizontal 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 3/4inch 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 Construction Manual for Residential Buildings in Coastal High Hazard Areas, cited in the Preface.

toe nailing, provides little resistance to flood forces, partially because of the tendency to split the wood in the toenailed 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-floorbeam 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).

Another connection method is to eliminate the curve of the post or pile by dapping and then connecting with bolts, gusset plates, or other devices. As Figures 4.32 and 4.33 show, a dapped post will form seats that assist the beams in carrying vertical loads. Posts that are small in section, however, should not be dapped 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 common 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.

The bolts should be at least 1 inch in diameter and embedded at least 12 inches in concrete piers and 16 inches in masonry piers. If two floor beams abut on a pier, each must be anchored separately (Figure 4.38).

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 floodwater and debris. Residences supported in this manner have the additional advantage of having the first row of piles set back, reducing the visual impact of elevating the structure. A cantilever design may use longer spans for the main floor beam and thus may require larger beams.

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 accessible 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 maximum

- 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-andgroove 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 blocking along these edges. This produces a more level floor and offers a stronger diaphragm action to resist horizontal flood forces.

Structures elevated more than 10 feet should be sheathed with 3/4-inch exterior grade plywood, nailed with eight penny 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 10's or 2 x 12's or roof trusses built up of 2 x 4's or 2 x 6's. 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 connections 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.

of floodwater. This can minimize damage from velocity water or floating debris. A more secure method is to place all utility lines coming from underground within a protective, floodproofed shaft under the elevated 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 connection should be made within a waterproof enclosure. All distribution panels or other major electrical equipment should also be located above expected floodwaters. Branch circuit wiring should be fed from the first floor ceiling downward to minimize 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.

BUILDING MATERIALS

One way to increase the safety of building materials is to elevate the building higher than the minimum floodplain management requirements. Even then, however, floodwaters 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

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 are 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

be jacked with conventional house moving equipment, 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 structural 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 involves the following steps:

- Disconnect all plumbing, wiring, and utilities

that cannot be raised with the structure.

- Place steel beams and hydraulic jacks beneath

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 granular

material to support basement walls.

- Lower the structure onto the extended or new

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 insulation

from possible water damage.

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 construction 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 building methods. (Note that standards adapted by local jurisdictions are often more stringent than the NFIP's.)

Repeated studies have shown that the savings that can be realized over the lifetime of a structure by building on a raised foundation are

usually considerable when compared with the one-time increase in construction costs for an elevated foundation. This is largely because the one-time foundation costs are generally only five or six percent of the total cost of a residential structure, while the flood insurance savings that can be achieved over the life of a structure by elevating 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 government through low-interest loans and tax deductions for losses incurred. In communities participating in the NFIP, the owner of a new home can purchase flood insurance. Essentially, flood insurance 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 payments. These are more manageable than unexpected flood losses, especially if more than one large flood happens to occur in a very short time.

99

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 prepared 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.)

4. Figure 5.5 graphically compares the cost of constructing the different types of foundations 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).

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 fluctuate 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 acceptance (buyers and banks), architectural design integration, climatic conditions, site conditions, and anticipated flood hazards should also be considered.

ESTIMATING FORMS

The forms on the following pages can be used for making cost estimates for conventional and elevated foundations.

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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 floodwater. 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 reasonable 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 accumulation 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, which 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; determined by approximate methods with no base flood elevation shown.

Zones A1-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 100year 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 outside the limits of the 500-year flood.

Zone V (unnumbered) - area subject to wave action, without base flood elevation shown.

Zones V1-V30 - special flood hazard area of 100-year coastal flooding with velocity (wave action); base flood elevations shown.

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 (a) before the improvement is started or (b) 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 contributing 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.

Region V Illinois, Indiana, Michigan, Minnesota, Ohio & Wisconsin

300 South Wacker Drive 24th Floor

Chicago, Illinois 60606 (312) 353-1500

Region VI Arkansas, Louisiana, New Mexico, Oklahoma & Texas

Federal Regional Center Rm. 206

Denton, Texas 76201 (817) 387-5811

Region VII Iowa, Kansas, Missouri & Nebraska

Federal Office Building

Rm. 405

Kansas City, Missouri 64106 (816) 374-2161

Region VIII Colorado, Montana, North Dakota, South Dakota, Utah & Wyoming

Federal Regional Center Building 710

Denver, Colorado 80225 (303) 234-6582

Region IX Arizona, California, Hawaii & Nevada

Building 105

Presidio of San Francisco San Francisco, California 94129

(415) 556-8795

Region X Alaska, Idaho, Oregon & Washington

Federal Regional Center Bothell, Washington 98011 (206) 486-0721

Florida State Flood Insurance Program

Coordinator

Department of Veteran &

Community Affairs

2571 Executive Ctr. Circle East Tallahassee, Florida 32301 (904) 488-9210

Georgia Georgia Department of Natural

Resources, Environmental

Protection Division

19 Martin Luther King, Jr. Dr. S.W.

Atlanta, Georgia 30334 (404) 656-3214

Guam Office of Civil Defense Post Office Box 2877 Agana, Guam 96910 011-671-477-9841

Hawaii Hawaii Board of Land and

Natural Resources

P.O. Box 373

Honolulu, Hawaii 96809 (808) 548-6550

Idaho Department of Water Resources State House Boise, Idaho 83720 (208) 334-4440

Illinois Local Flood Plain Office Illinois Department of

Transportation

Division of Water Resources Local Flood Plain Programs 300 North State Street,

Room 1010

Chicago, Illinois 60610 (312) 793-3864

Indiana Department of Natural

Resources

608 State Office Building Indianapolis, Indiana 46204 (317) 633-5267

Iowa Iowa Natural Resources Council Wallace State Office Building Des Moines, Iowa 50319 (515) 281-5913

Kansas Chief Engineer & Director Division of Water Resources Kansas State Board of

Agriculture

109 Southwest North Street Topeka, Kansas 66612 (913) 296-3717

Kentucky Department of Natural

Resources

Division of Water 18 Reilly Road Fort Boone Plaza

Frankfort, Kentucky 40601 (502) 564-3410

Louisiana Louisiana Department of Urban

& Community Affairs

P.O. Box 44455

Baton Rouge, Louisiana 70804 (504) 925-3706

Maine Bureau of Civil Emergency

Preparedness

State House

Augusta, Maine 04330 (207) 622-6201

Maryland Maryland Water Resources

Administration

Flood Management Section Tawes State Office Building D-2 Annapolis, Maryland 21401 (301) 269-3826

Massachusetts Massachusetts Water Resources

Commission

State Office Building 100 Cambridge Street Boston, Massachusetts 02202 (617) 727-3267

121

North Dakota State Water Commission

900 E. Boulevard

Bismark, North Dakota 58501 (701) 224-2750

Ohio Ohio Department of Natural

Resources

Flood Plain Planning Unit Fountan Square Columbus, Ohio 43224 (614) 265-6755

Oklahoma Oklahoma Water Resources

Board

12th Floor Northeast 10th & Stonewall Oklahoma City, OK 73105 (405) 271-2555

Oregon Oregon Water Resources

Department Millcreek Office Park 555 13th St., N.E. Salem, Oregon 97310 (503) 378 -3671

Pennsylvania Department of Community

Affairs

551 Forum Building Harrisburg, PA 17120 (717) 787-7400

Puerto Rico Puerto Rico Planning Board P.O. box 4119, Minillas Station D-Diego Avenue Santurce, Puerto Rico 00940 (809) 726-7110

Rhode Island Statewide Planning Program Rhode Island Office of

State Planning

265 Melrose Street Providence, RI 02907 (401) 277-2656

South Carolina South Carolina Water Resources

Commission

1001 Harden Street, Suite 250 P.O. Box 50506 Columbia, SC 29250 (803) 758-2514

South Dakota Planning Bureau

State Capitol

Pierre, South Dakota 57501 (605) 773-3661

Tennessee Tennessee State Planning Office 1800 James K. Polk

Office Building

505 Dead Erick Street Nashville, Tennessee 37219 (615) 741-2211

Texas Texas Dept. of Water Resources 1700 North Congress Avenue Austin, Texas 78711 (512) 475-2171

Utah Office of Comprehensive

Emergency Management 1543 Sunnyside Avenue Salt Lake City, Utah 84108 (801) 533-5271

Performance Criteria

The following performance requirements and criteria identify a range of considerations that should be addressed during the design of residential structures for flood hazard areas. These performance criteria do not represent the entire range of items applicable to each requirement. Instead, a selective number of criteria have beenpresented.

The performance requirements and criteria are applicable to all structural materials and all construction methods used in flood hazard areas. Traditional or conventional construction solutions, as well as innovative techniques, are acceptable so long as the performance requirements and criteria are satisfied.

DEFINITIONS

Terms important to proper interpretation of the performance requirements and criteria are defined as follows:

Applicable Codes

The system of legal regulations adopted by a community setting forth standards for the construction, addition, modification, and repair of buildings and other structures for the purpose of protecting the health, safety and general welfare of the public.

Community

Any state or political subdivision thereof with authority to adopt and enforce floodplain management regulations for areas within its jurisdiction.

Design Flood (Base Flood)

The design flood is the base or 100-year flood used for purposes of compliance with the National Flood Insurance Program (NFIP).

In coastal high hazard zones the 100-year flood includes wave height above the stillwater level.

Design Loads

The design load is the minimum loading condition that the building should be designed to resist. Some loading conditions most likely will be defined in the applicable codes while other load conditions (e.g., flood impact loads) will have to be determined. The following loads constitute the design load and should be considered as minimum loading conditions as defined in Criterion A.1 (see below):

Dead Load (D)

The weight of all permanent construction. The dead load includes a) the weight of the structure itself, b) the weight of all materials of construction incorporated into the building that are to be permanently supported by the structure, including built-in partitions, c) the weight of permanent equipment, and d) forces due to prestressing.

Gravity Live Load (L)

Gravity live loads result from both the occupancy (floor) and the environment (roof) of the building, as stipulated in the applicable code. These include, where applicable, loads caused by soil and hydrostatic pressures.

Wind Loads (W)

Wind loads stipulated in the applicable code. Restraint Loads (R)

Loads, forces, and effects due to contraction or expansion resulting from temperature changes, shrinkage, moisture changes, creep in component materials, movement due to differential settlement or combinations thereof.

Sec. 602.4.3 Uplift

Full intensity of hydrostatic pressures caused by a depth of water between the design level and the RFD acting on all surfaces involved ....

Sec. 602.4.4 Hydrodynamic Loads

Hydrodynamic loads, regardless of method of evaluation, shall be applied at full intensity over all above ground surfaces between the ground level and the RFD.

Sec 602.5 Applicability

Hydrostatic loads shall be used in the design of buildings and structures exposed to water loads from stagnant floodwaters, for conditions when water velocities do not exceed five (5) feet per second, and for buildings and structures or parts thereof not exposed or subject to flowing water. For buildings and structures, or parts thereof, which are exposed and subject to flowing water having velocities greater than five (5) feet per second, hydrostatic and hydrodynamic loads shall apply.

The equivalent surcharge depth, dh, shall be added to the depth measured between the design level and the RFD and the resultant pressures applied to, and uniformly distributed across, the vertical projected area of the building or structure which is perpendicular to the flow. Surfaces parallel to the flow or surfaces wetted by the tailwater shall be considered subject to hydrostatic pressures for depths to the RFD only.

Sec. 602.4 Intensity of Loads Sec 602.4.1 Vertical Loads

Full intensity of hydrostatic pressure caused by a depth of water between the design elevation(s) and the RFD applied over all surfaces involved, both above and below ground.

Sec. 602.4.2 Lateral Loads

Full intensity of hydrostatic pressure caused by a depth of water between the design elevation(s) and the RFD applied over all surfaces involved, both above and below ground level, except that for surfaces exposed to free water, the design depth shall be increased by one foot.

- the overflow of inland or tidal waters

- the unusual and rapid accumulation or run
off of surface waters from any source

- mudslides (i.e., mudflows), which are proximately caused or precipitated by accumulations of water on or under the ground.

- The collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels or suddenly caused by an unusually high water level in a natural body of water, accompanied by a severe storm, or by an unanticipated force of nature, such as a flash flood or an abnormal tidal surge, or by some similarly unusual and unforeseeable event which results in flooding as defined above.

PERFORMANCE REQUIREMENTS AND CRITERIA FOR RESIDENTIAL STRUCTURES IN FLOOD HAZARD AREAS

PERFORMANCE REQUIREMENT A

The building, its contiguous structure(s), and its service systems shall be designed to withstand the design flood without causing unacceptable risks to its occupants or to adjacent property owners.

The building complies with Performance Requirement A if the following conditions are satisfied:

Criterion A. 1: Strength

The building is designed to resist the following loads, acting simultaneously:

1.1 D, L, R, and F

1.2 D, L, R, F, and Fl 1.3 D, L, R, W, F, and Fl 1.4 D, R, and F

1.5 D, R, W, F, and Fl

Where the working stress method of design is used the following provisions apply:

2.1 In load combinations 1.1 through 1.5 all loads are applied as listed or as required by the applicable codes for the same load combinations with loads F and Fl.

2.2 Allowable (working) stresses cannot be exceeded for loading conditions 1.1 and 1.4. For all other loading conditions the allowable stresses can be increased by the amount permitted in applicable codes for design against load combinations including wind or earthquake load.

Where ultimate-load design is used (such as instances where the American Concrete Institute, Building Code Requirements for Reinforced Concrete [AC1 378, ACI, Detroit, current edition], is applicable) load factors are applied as recommended in the applicable standard, and F will be combined with L, or factored as if it were a live load for loading conditions 1.1 and 1.4. For all other loading conditions loads F + Fl will be combined with W, or considered to be equivalent to a wind load.

Test

Structural analysis and/or physical simulation. Commentary

The criterion provides a suitable margin of safety against structural collapse when the building is subjected to the base flood. The intent of the criterion is that the margin of safety for these buildings, when subjected to the base flood, be no less than the margin required for other buildings not subjected to flooding. It is assumed that loads F may act on the building over a long period of time, while loads Fl are short-term loads. Thus the margin of safety against load combinations containing Fl need not exceed that provided against wind or seismic loads.

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Criterion A.4: Disruption of Service Systems

The service systems shall be designed to resist the loads stipulated in Criterion A. 1 with safety margins as stipulated in A.1 against disruptions which may endanger human lives.

Test

Engineering analysis and/or physical simulation. Evaluation of data and documentation for design, tests, and installation; evaluation of plans and specifications.

Commentary

This criterion only applies to disruption that may cause fatal accidents, such as rupture of gas lines. Lesser load levels are stipulated in B.1 for disruptions that constitute a health hazard.

Criterion A.5: Execution of Rescue Operations

The building is designed to permit the execution of rescue operations.

During the duration and at heights of the design flood the building shall:

1.1 Allow the safe evacuation of the occupants out of the building

1.2 Allow the safe transfer of occupants from the building to rescue vehicles

1.3 Provide means of access or adjacency for rescue vehicles.

Test

Evaluation of data and documentation for design, tests, and installation; evaluation of plans and specifications.

Commentary

Criterion A. 5 is designed to prevent the entrapment of building occupants by rising water levels. Part of the provision is designed to provide means to evacuate the building (e.g., windows, roof trap door). The other parts provide for the accommodation and execution of rescue operations (e.g., by boat, helicopter).

Test

Evaluation of data and documentation for design, tests, and installation; evaluation of plans and specifications.

Geological analysis of site. Commentary

Criterion B.3 is designed to prevent the contamination of water wells used as a source for potable water. Part of the provision provides against the contamination of the water supply source. The other part provides against the contamination of the water removal system. In any case, local health codes should be consulted.

Criterion B.3: Provision Against Contamination of Potable Water Wells

Private potable water wells shall not be contaminated by toxic substances or impurities caused by the design flood.

Criterion B.3 is deemed satisfied if the following provisions are satisfied.

1.1 Private potable well water is not supplied from a water table located less than 25 feet below grade, nor from any deeper supply that may be polluted by contamination entering fissure or crevice formations.

1.2 Each well is provided with a watertight casing to a distance of at least 25 feet below the ground surface that extends at least one foot above the well platform.

1.4.2 • Plumbing below the design flood level will not suffer loss of stability or loss of tightness that will permit leakage or physical damage to fixtures and joints and connections that will permanently impair functioning.

1.4.3 Utility connections designed to disconnect during the design flood are easily reconnected. (See Criterion

B.1.)

Test

Evaluation of data and documentation for design, tests, and installation; evaluation of plans and specifications.

Commentary

Criterion C.2 is designed to prevent unnecessary damage of living areas, major utilities, furnaces, and air-conditioning units by the design flood. Part of the provision is designed to elevate living areas and equipment above the design flood. Other parts are designed to prevent the damage of utilities and mechanical/electrical connections below the design flood.

U.S. Army Corps of Engineers. Flood-Proofing Regulations. Washington, D.C.: U.S. Army, Office of the Chief of Engineers, 1972.

U.S. Army Corps of Engineers. Low Cost Shore Protection. Washington, D.C.: U.S. Army, Office of the Chief of Engineers, 1981.

U.S. President, Executive Order 11296, Floodplain Management, Code of Federal Regulations, 1970 ed., Title 3, p. 181, USC 701.

U.S. President. Executive Order 11988, Floodplain Management. Federal Register, 42/101, 25 May 1977.

U.S. President, Executive Order 11990, Protection of Wetlands. Federal Register, 42/101, 25 May 1977.

U. S. Water Resources Council. Regulation of Flood Hazard Areas to Reduce Losses. Washington, D.C.: Water Resources Council, 1971-1972.

U. S. Water Resources Council. Flood Hazard Evaluation Guidelines for Federal Agencies. Washington, D.C.: Water Resources Council, 1972

U. S. Water Resources Council. Unified National Program for Flood Plain Management. Washington, D.C.: Water Resources Council, 1976.

Waananan, A.O., et al. Flood-Prone Areas and Land-Use Planning—Selected Examples from the San Francisco Bay Region, California. U. S. Geological Survey Professional Paper 942. Washington, D.C.: U.S. Government Printing Office, 1977.

Johnson, William U. Physical and Economic Feasibility of Nonstructural Flood Plain Management Measures. Davis, Calif.: U.S. Army Corps of Engineers Hydrologic Engineering Center, 1978.

Kusler, Jon A., and Lee, Thomas M. Regulations for Flood Plains. Chicago: American Society of Planning Officials, 1972.

Leopold, Luna B. Water: A Primer. San Francisco: W.H. Freeman and Co., 1974.

National Flood Insurers Association. National

Flood Insurance Program-Flood Insurance

Manual. New York: NFIA, current edition.

National Science Foundation. A Report on Flood Hazard Mitigation. Washington, D.C.: NSF, September, 1980.

Owen H. James. Annotations of Selected Literature on Nonstructural Flood Plain Management Measures. Davis, Calif.: U.S. Army Corps of

Engineers Hydrologic Engineering Center, March 1977.

Pilkey, Orrin H. Jr., Pilkey, O.H. Sr., and Turner, Robb. How to Live with an Island. Raleigh, N.C.: Science & Technology Section, North Carolina Department of Natural & Economic Resources, 1975.

Phippen, George R., "A New Course to Ararat," Water Spectrum, Summer 1971, pp. 9-15.

Sheaffer, John R. Introduction to Flood Proofing. Chicago: Center for Urban Studies, University of Chicago, 1967.

Tennessee Valley Authority. Guide for the Use of Technical Information and Data for Floodplain Management in the Tennessee River Basin.

Knoxville, Tenn.: TVA, October 1980.

Texas Coastal and Marine Council. Model Minimum Hurricane-Resistant Building Standards for the Texas Gulf Coast. Austin: September 1976.

* U.S. GOVERNMENT PRINTING OFFICE: 1984 0 - 438-116