Elevated
Residential
Structures
The
American Institute of Architects Foundation 1735 New York
Avenue, N.W. Washington, D.C. 20006
1984
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Table of
Contents |
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ACKNOWLEDGMENTS PREFACE |
ii v |
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ENVIRONMENTAL AND REGULATORY FACTORS |
1 1 |
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_ FLOODING AND THE BUILT
ENVIRONMENT |
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Riverine Flooding ∎ Coastal
Flooding |
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FLOODPLAIN MANAGEMENT |
4 |
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National Flood Insurance Program
∎ Base Flood Elevations ∎ A and V Zones |
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SITE ANALYSIS AND DESIGN |
8 9 13 |
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SITE SELECTION AND ANALYSIS SITE DESIGN |
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Site Flooding Characteristics
∎ Access and Egress ∎ Vegetation ∎ Flood Water |
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Drainage and Storage ∎ Dune
Protection |
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ARCHITECTURAL DESIGN EXAMPLES |
18 |
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DESIGN STUDIES |
22 |
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Bridgeport ∎ Charleston and
Newport 0 San Francisco ∎ Chicago |
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AESTHETIC CONSIDERATIONS |
35 |
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RECENT
DESIGN EXAMPLES |
45 |
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Logan House ∎ Summerwood on
the Sound ∎ Breakers Condominium ∎ |
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Campus-by-the-Sea Facility ∎
Starboard Village ∎ Gull Point Condominiums |
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DESIGN AND CONSTRUCTION GUIDELINES |
64 |
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FOUNDATIONS |
65 |
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Fill ∎ Elevated Foundations
∎ Shear Walls ∎ Posts ∎ Piles ∎ Piers ∎ Bracing |
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FRAMING CONSTRUCTION AND
CONNECTIONS |
80 |
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Framing Methods ∎ Floor Beams
∎ Cantilevers ∎ Concrete Flooring Systems 0 |
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Floor Joists ∎ Subflooring
∎ Wall Sheathing and Bracing ∎ Roof Connections |
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RELATED DESIGN CONSIDERATIONS |
92 |
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Glass Protection ∎ Utilities
and Mechanical Equipment ∎ Building Materials ∎ |
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Insulation ∎ Breakaway Walls
∎ Retrofitting Existing Structures |
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COST ANALYSIS |
98 |
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RESOURCE MATERIALS |
112 |
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GLOSSARY |
113 |
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SOURCES OF DESIGN INFORMATION |
116 |
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FEMA REGIONAL OFFICES |
118 |
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STATE COORDINATING OFFICES
FOR THE NFIP |
120 |
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PERFORMANCE CRITERIA |
125 |
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REFERENCES |
136 |
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iii |
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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.
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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.
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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.
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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. |
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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.
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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.
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- 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).
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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.
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