Elevated

Table of Contents
ACKNOWLEDGMENTS PREFACE	ii v
ENVIRONMENTAL AND REGULATORY FACTORS	1 1
_ FLOODING AND THE BUILT ENVIRONMENT	1 1
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	8 9 13
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.

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.

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.