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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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.
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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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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
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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
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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.
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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.
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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
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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.
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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).
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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.
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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. |
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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
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O.H. Sr., and Turner, Robb. How to Live with an Island. Raleigh,
N.C.: Science
& Technology Section, North Carolina
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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
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Tennessee Valley Authority. Guide for
the Use of Technical Information and
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Tennessee River Basin.
Knoxville, Tenn.: TVA, October
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* U.S. GOVERNMENT PRINTING OFFICE: 1984 0 - 438-116