RECOMMENDED RESIDENTIAL CONSTRUCTION FOR COASTAL AREAS
Building on Strong and Safe Foundations
FEMA P-550, Second Edition / December 2009
Chapter 4. Overview of Recommended Foundation Types and Construction for Coastal
Areas
Chapters 1 through 3 discussed foundation design loads and calculations and how
these issues can be influenced by coastal natural hazards. This chapter will tie
all of these issues together with a discussion of foundation types and methods
of constructing a foundation for a residential structure.
4.1 Critical Factors Affecting Foundation Design
Foundation construction types are dependent upon the following critical factors:
- Design wind speed
- Elevation height required by the BFE and local ordinances
- Flood zone
- Soil parameters
Soil parameters, like bearing capacities, shear coefficients, and subgrade
moduli, are important in designing efficient and effective foundations. But, for
the purpose of creating the standardized foundation concepts for use in a
variety of sites, some soil parameters have been assumed (as in the case of
bearing capacity for shallow foundations) and others have been stipulated (as
those required to produce specific performance – as in the case for deep driven
piles). Assumptions used in developing the foundations are listed in Appendix C,
where stipulations on pile capacity are also listed in the individual drawings.
4.1.1 Wind Speed
The basic wind speed determines the wind velocity used in establishing wind
loads for a building. It can also have a significant influence on the size and
strength of foundations that support homes. Contemporary codes and standards
like the IRC, IBC, and ASCE 7 specify basic wind speeds as 3-second gust wind
speeds. Earlier versions of codes and standards specified wind speeds with
different averaging periods. One example is the fastest mile wind speed that was
specified in the 1988 (and earlier) versions of ASCE 7 and in pre-2000 versions
of most model building codes.
The wind speed map shown in Figure 3-1 illustrates that the basic (3-second
gust) wind speeds along most of the Gulf of Mexico, the Atlantic coast, and
coastal Alaska range between 120 and 150 mph. The basic wind speed for most of
the Pacific coast is 85 mph. Several areas in the Pacific Northwest are
designated as special wind regions and wind speeds are dictated locally. The
design wind speeds for many of the U.S. territories and protectorates are
tabulated in ASCE 7.
To determine forces on the building and foundation, the wind speed is critical.
Wind speed creates wind pressures that act upon the building. These pressures
are proportional to the square of the wind speed, so a doubling of the wind
speed increases the wind pressure by a factor of four. The pressure applied to
an area of the building will develop forces that must be resisted. To transfer
these forces from the building to the foundation, properly designed load paths
are required. For the foundation to be properly designed, all forces including
uplift, compression, and lateral must be taken into account.
Although wind loads are important in the design of a building, in coastal areas
flood loads often have a much greater effect on the design of the foundation
itself.
4.1.2 Elevation
The required height of the foundation depends on three factors: the DFE, the
site elevation, and the flood zone. The flood zone dictates whether the lowest
habitable finished floor must be placed at the DFE or, in the case of homes in
the V zone, the bottom of the lowest horizontal member must be placed at the
DFE. Figure 4-1 illustrates how the BFE, freeboard, erosion, and the ground
elevation determine the foundation height required. While not required by the
NFIP, V zone criteria are recommended for Coastal A zones. Stated
mathematically:
H = DFE – G + Erosion
or
H = BFE – G + Erosion + Freeboard
Where
H = Required foundation height (in ft)
DFE = Design Flood Elevation
BFE = Base Flood Elevation
G = Non-eroded ground elevation
Erosion = Short-term plus long-term erosion
Freeboard = 2009 IRC required in SFHAs, locally adopted or owner desired
freeboard
[Begin figure]
Figure 4-1 is illustrates that the BFE, freeboard, erosion, and ground elevation
determine the foundation height required.
[End figure]
The height to which a home should be elevated is one of the key factors in
determining which pre-engineered foundation to use. Elevation height is
dependent upon several factors, including the BFE, local ordinances requiring
freeboard, and the desire of the homeowner to elevate the lowest horizontal
structural member above the BFE (see also Chapter 2). This manual provides
designs for closed foundations up to 8 feet above ground level and open
foundations up to 15 feet above ground level. Custom designs can be developed
for open and closed foundations to position the homes above those elevation
levels. Foundations for homes that need to be elevated higher than 15 feet
should be designed by a licensed professional engineer.
4.1.3 Construction Materials
The use of flood-resistant materials below the BFE is also covered in FEMA NFIP
Technical Bulletin 2, Flood Damage-Resistant Materials Requirements for
Buildings Located in Special Flood Hazard Areas in accordance with the National
Flood Insurance Program and FEMA 499, Fact Sheet No. 8 (see Appendix F). This
manual will cover the materials used in masonry and concrete foundation
construction, and field preservative treatment for wood.
4.1.3.1 Masonry Foundation Construction
The combination of high winds, moisture, and salt-laden air creates a damaging
recipe for masonry construction. All three can penetrate the tiniest cracks or
openings in the masonry joints. This can corrode reinforcement, weaken the bond
between the mortar and the brick, and create fissures in the mortar. Moisture
resistance is highly influenced by the quality of the materials and the
workmanship.
4.1.3.2 Concrete Foundation Construction
Cast-in-place concrete elements in coastal environments should be constructed
with 3 inches or more of concrete cover over the reinforcing bars. The concrete
cover physically protects the reinforcing bars from corrosion. However, if salt
water penetrates the concrete cover and reaches the reinforcing steel, the
concrete alkalinity is reduced by the salt chloride, thereby corroding the
steel. As the corrosion forms, it expands and cracks the concrete, allowing the
additional entry of water and further corrosion. Eventually, this process
weakens the concrete structural element and its load carrying capacity.
Alternatively, epoxy-coated reinforcing steel can be used if properly handled,
stored, and placed. Epoxy-coated steel, however, requires more sophisticated
construction techniques and more highly trained contractors than are usually
involved with residential construction.
Concrete mix used in coastal areas must be designed for durability. The first
step in this process is to start with the mix design. The American Concrete
Institute (ACI) 318 manual recommends that a maximum water-cement ratio by
weight of 0.40 and a minimum compressive strength of 4,000 pounds per square
inch (psi) be used for concrete used in coastal environments. Since the amount
of water in a concrete mix largely determines the amount that concrete will
shrink and promote unwanted cracks, the water-cement ratio of the concrete mix
is a critical parameter in promoting concrete durability. Adding more water to
the mix to improve the workability increases the potential for cracking in the
concrete and can severely affect its durability.
Another way to improve the durability of a concrete mix is with ideal mix
proportions. Concrete mixes typically consist of a mixture of sand, aggregate,
and cement. How these elements are proportioned is as critical as the water-
cement ratio. The sand should be clean and free of contaminants. The aggregate
should be washed and graded. The type of aggregate is also very important.
Recent research has shown that certain types of gravel do not promote a tight
bond with the paste. The builder or contractor should consult expert advice
prior to specifying the concrete mix.
Addition of admixtures such as pozzolans (fly ash) is recommended for concrete
construction along the coast. Fly ash when introduced in concrete mix has
benefits such as better workability and increased resistance to sulfates and
chlorates, thus reducing corrosion from attacking the steel reinforcing.
4.1.3.3 Field Preservative Treatment for Wood Members
In order to properly connect the pile foundation to the floor framing system,
making field cuts, notches, and boring holes are some of the activities
associated with construction. Since pressure-preservative-treated piles,
timbers, and lumber are used for many purposes in coastal construction, the
interior, untreated parts of the wood are exposed to possible decay and
infestation. Although treatments applied in the field are much less effective
than factory treatments, the potential for decay can be minimized. The American
Wood Preservers’ Association (AWPA) AWPA M4-08 Standard for the Care of
Preservative-Treated Wood Products (AWPA 2008) describes field treatment
procedures and field cutting restrictions for poles, piles, and sawn lumber.
Field application of preservatives should always be done in accordance with
instructions on the label. When detailed instructions are not provided, dip
soaking for at least 3 minutes can be considered effective for field
applications. When this is impractical, treatment may be done by thoroughly
brushing or spraying the exposed area. It should be noted that the material is
more absorptive at the end of a member, or end grains, than it is for the sides
or side grains. To safeguard against decay in bored holes, the holes should be
poured full of preservative. If the hole passes through a check (such as a
shrinkage crack caused by drying), it will be necessary to brush the hole;
otherwise, the preservative would run into the check instead of saturating the
hole.
Waterborne arsenicals, pentachlorophenol, and creosote are unacceptable for
field applications. Copper napthenate is the most widely used field treatment.
Its deep green color may be objectionable, but the wood can be painted with
alkyd paints in dark colors after extended drying. Zinc napthenate is a clear
alternative to copper napthenate. However, it is not quite as effective in
preventing insect infestation, and it should not be painted with latex paints.
Tributyltin oxide (TBTO) is available, but should not be used in or near marine
environments, because the leachates are toxic to aquatic organisms. Sodium
borate is also available, but it does not readily penetrate dry wood and it
rapidly leaches out when water is present. Therefore, sodium borate is not
recommended.
4.1.4 Foundation Design Loads
To provide flexibility in the home designs, tension connections have been
specified between the tops of all wood piles and the grade beams. Depending on
the location of shear walls, shear wall openings, and the orientation of floor
and roof framing, some wood piles may not experience tension forces. Design
professionals can analyze the elevated structure to identify compression only
piles to reduce construction costs. For foundation design and example
calculations, see Appendix D.
Figure 4-2 illustrates design loads acting on a column. The reactions at the
base of the elevated structure used in most of the foundation designs are
presented in Tables 4-1a (one-story) and 4-1b (two-story). These reactions are
the controlling forces for the range of building weights and dimensions listed
in Appendix A and shown in Figure 2 of the Introduction. Design reactions have
been included for the various design wind speeds and various building elevations
above exterior grade. ASCE 7-05 load combination 4 (D + 0.75L + 0.75Lr) controls
for gravity loading and load combination 7 controls for uplift and lateral
loads. Load combination 7 is 0.6D + W + 0.75Fa in non-Coastal A zones and 0.6D +
W + 1.5Fa in Coastal A and V zones. Refer to Section 3.8 for the list of flood
load combinations.
[Begin figure]
Figure 4-2 illustrates design loads acting on a column.
[End figure]
Loads on the foundation elements themselves are more difficult to tabulate
because they depend on the foundation style (open or enclosed), foundation
dimensions, and foundation height. Table 4-2 provides reactions for the 18-inch
square columns used in most of the open foundation designs.
[Begin tables]
Table 4-1a. Design Perimeter Wall Reactions (lb/lf) for One-Story Elevated Homes
(Note: Reactions are taken at the base of the elevated home/top of the
foundation element.)
This table shows the break down for wind speeds of 120, 130 ,140, 150 mph, and
(All V); the height of the foundation above grade (both horizontal and vertical)
for 5, 6, 7, 8, 10, 12, 14, and 15 feet.
Table 4-1b. Design Perimeter Wall Reactions (lb/lf) for Two-Story Elevated Homes
(Note: Reactions are taken at the base of the elevated home/top of the
foundation element.)
This table shows the break down for wind speeds of 120, 130 ,140, 150 mph, and
(All V); the height of the foundation above grade (both horizontal and vertical)
for 5, 6, 7, 8, 10, 12, 14, and 15 feet.
Table 4-2. Flood Forces (in pounds) on an 18-Inch Square Column
Flood Depth: 5 ft
Hydrodynamic: 1,000
Breaking Wave: 684
Impact: 3,165
Buoyancy: 465
Flood Depth: 6 ft
Hydrodynamic: 1,440
Breaking Wave: 985
Impact: 3,476
Buoyancy: 577
Flood Depth: 7 ft
Hydrodynamic: 1,960
Breaking Wave: 1,340
Impact: 3,745
Buoyancy: 650
Flood Depth: 8 ft
Hydrodynamic: 2,560
Breaking Wave: 1,750
Impact: 4,004
Buoyancy: 743
Flood Depth: 10 ft
Hydrodynamic: 4,001
Breaking Wave: 2,735
Impact: 4,476
Buoyancy: 939
Flood Depth: 12 ft
Hydrodynamic: 5,761
Breaking Wave: 3,938
Impact: 4,903
Buoyancy: 1,115
Flood Depth: 14 ft
Hydrodynamic: 7,841
Breaking Wave: 5,360
Impact: 5,296
Buoyancy: 1,300
Flood Depth: 15 ft
Hydrodynamic: 9,002
Breaking Wave: 6,155
Impact: 5,482
Buoyancy: 1,394
[End tables]
4.1.5 Foundation Design Loads and Analyses
Load analyses used to develop Case H foundations are similar to the analyses
completed for the original FEMA 550 designs. Live loads used were those
specified by the IRC and the original and augmented foundations were developed
to support a range of dead loads. Wind and flood loads were calculated per ASCE
7, Minimum Design Loads for Buildings and Other Structures (the Case H design
loads were calculated using ASCE 7-05; loads used in the original designs were
calculated using ASCE 7-02, which are consistent with the 2005 edition). Design
assumptions are listed in Appendix C.
Some noteworthy differences exist. Wind loads used in the original FEMA 550
designs were the worst case loads for a home that varied in width from 24 feet
to 42 feet and in roof slope from 3:12 to 12:12. The foundation reactions for
the original designs are listed in Table 4-1b. In the Case H designs, separate
wind loads were determined based on the number of stories (one or two) and the
building width (14 feet for the 3-bay designs, 28 feet for the 6-bay designs,
and 42 feet for the 9-bay designs). The more precise matching of wind loads to
building widths and heights provide greater design efficiencies.
Wind loads used to develop the Case H foundations are listed in Table 4-3.
[Begin table]
Table 4-3. Wind Reactions Used to Develop Case H Foundations
This table shows 3-Bay, 6-Bay, and 9-Bay construction for vertical forces on
windward edge of foundation, vertical forces on leeward edge of foundation, and
horizontal forces on windward and leeward edges of foundation at 120, 130, 140,
and 150m mph.
Note
1. (+) loads act upward; (-) pressures act downward.
2. Lateral loads are applied to both windward and leeward foundation elements.
[End table]
To account for shear panel reactions from segmented shear walls, the analyses of
foundations supporting one-story homes included 6.72 kip quarter span point
loads for the 3-bay design (point loads were applied at mid-span for the 6- and
9-bay models, Figure 4-3). The loads correspond to 10-foot tall wood framed
shear panels constructed with 7/16-inch blocked wood structural panels fastened
with 8d common nails 6 inches on center (o.c.). Foundations supporting two-story
homes were analyzed with 13.44 kip shear panel reactions or twice that of the
one-story home. The foundations will also support homes constructed with
perforated shear walls.
[Begin figure]
Figure 4-3 illustrates shear panel reactions for the 3- and 6-bay models.
Reactions for the 9-bay model were similar to those of the 6-bay.
[End figure]
Another difference in design methodology was required due to the nature of
structural frames. In the original designs, the concrete columns were considered
statically determinant and analyzed as such. The structural frames created by
the concrete grade beams, concrete columns, and elevated beams, however, are not
statically determinant and computer modeling was warranted. To analyze the frame
action developed by those structural elements, computer models using RISA©
structural software were created. Design loads were applied to the frames and
critical shears and moments were tabulated for the grade beams, columns, and
elevated beams. Critical axial forces were also tabulated for the columns.
Tables 4-4 through 4-9 summarize the critical shears, moments, and axial loads
of the computer models used to develop the Case H foundations.
[Begin table]Table 4-4 shows the Design Moments (K-ft), Axial Loads (in kips),
and Shears (in kips) for 10-Foot Tall 3-Bay Foundations with winds at 120, 130,
140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-5. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 3-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-6. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 6-Bay Foundations with winds at 120, 130, 140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-7. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 6-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-8. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 9-Bay Foundations with winds at 120, 130, 140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-9. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 9-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top, Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment, Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
4.2 Recommended Foundation Types for Coastal Areas
Table 4-10 provides six open (deep and shallow) foundation types and two closed
foundations discussed in this manual. Appendix A provides the foundation design
drawings for the cases specified.
[Begin table]Table 4-10. Recommended Foundation Types Based on Zone
Open Foundation (deep)
Braced timber pile, Case A, acceptable in V Zones, Coastal A Zone, and A Zone
Steel pipe pile with concrete column and grade beam, Case B, acceptable in V
Zones, Coastal A Zone, and A Zone
Timber pile with concrete column and grade beam, Case C, acceptable in V Zones,
Coastal A Zone, and A Zone
Timber pile with concrete grade and elevated beams and concrete columns, Case H,
acceptable in V Zones, Coastal A Zone, and A Zone
Open Foundation (shallow)
Concrete column and grade beam, Case D, not recommended in V zones, acceptable
Coastal A Zone and A Zone
Concrete column and grade beam with integral slab, Case G, not recommended in V
zones, acceptable Coastal A Zone and A Zone
Closed Foundation (shallow)
Reinforced masonry – crawlspace, Case E, not permitted in V zones, not
recommended in Coastal A zones, acceptable in A zones
Reinforced masonry – stem wall, Case F, not permitted in V zones, not
recommended in Coastal A zones, acceptable in A zones
[End table]
The foundation designs contained in this manual are based on soils having a
bearing capacity of 1,500 pounds per square foot (psf). The 1,500-psf bearing
capacity value corresponds to the presumptive value contained in Section 1806 of
the 2009 IBC. The presumptive bearing capacity is for clay, sandy clay, silty
clay, clayey silt, and sandy silt (CL, ML, MH, and CH soils).
The size of the perimeter footings and grade beams are generally not controlled
by bearing capacity (uplift and lateral loads typically control footing size and
grade beam dimensions). Refining the designs for soils with greater bearing
capacities may not significantly reduce construction costs. However, the size of
the interior pad footings for the crawlspace foundation (Table 4-10, Case E)
depends greatly on the soil’s bearing capacity. Design refinements can reduce
footing sizes in areas where soils have greater bearing capacities. The
following discussion of the foundation designs listed in Table 4-10 is also
presented in Appendix A. Figures 4-4 through 4-10 are based on Appendix A.
4.2.1 `Open/Deep Foundation: Timber Pile (Case A)
This pre-engineered, timber pile foundation uses conventional, tapered, treated
piles and steel rod bracing to support the elevated structure. No concrete,
masonry, or reinforcing steel is needed (see Figure 4-4). Often called a “stilt”
foundation, the driven timber pile system is suitable for moderate elevations if
the homebuilder prefers to minimize the number of different construction trades
used. Once the piles are driven, the wood guides and floor system are attached
to the piles; the remainder of the home is constructed off the floor platform.
[Begin figure]
Figure 4-4 is a profile of Case A foundation type (see Appendix A for additional
drawings).
[End figure]
The recommended design for Case A that is presented in this manual accommodates
home elevations up to 10 feet above grade. With customized designs and longer
piles, the designs can be modified to achieve higher elevations. However,
elevations greater than 10 feet will likely be prevented by pile availability,
the pile strength required to resist lateral forces, and the pile embedment
required to resist erosion and scour. A construction approach that can improve
performance is to extend the piles above the first floor diaphragm to the second
floor or roof diaphragm. Doing so allows the foundation and the elevated home to
function more like a single, integrated structural frame. Extending the piles
stiffens the structure, reduces stresses in the piles, and reduces lateral
deflections. Post disaster assessments of pile supported homes indicate that
extending piles in this fashion improves survivability. Licensed professional
engineers should be consulted to analyze the pile foundations and design the
appropriate connections.
One drawback of the timber pile system is the exposure of the piles to
floodborne debris. During a hurricane event, individual piles can be damaged or
destroyed by large, floating debris. With the home in place, damaged piles are
difficult to replace. Two separate ways of addressing this potential problem is
to use piles with a diameter larger than is called for in the foundation design
or to use a greater number of piles to increase structural redundancy.
4.2.2 Open/Deep Foundation: Steel Pipe Pile with Concrete Column and Grade Beam
(Case B)
This foundation incorporates open-ended steel pipe piles; this style is somewhat
unique to the Gulf Coast region where the prevalence of steel pipe piles used to
support oil platforms has created local sources for these piles. Like treated
wood piles, steel pipe piles are driven but have the advantage of greater
bending strength and load carrying capacity (see Figure 4-5). The open steel
pipe pile foundation is resistant to the effects of erosion and scour. The grade
beam can be undermined by scour without compromising the entire foundation
system.
[Begin figure]
Figure 4-5 is a Profile of Case B foundation type (see Appendix A for additional
drawings).
[End figure]
The number of piles required depends on local soil conditions. Like other soil
dependent foundation designs, consideration should be given to performing soil
tests on the site so the foundation design can be optimized. With guidance from
engineers, the open-ended steel pipe pile foundation can be designed for higher
elevations. Additional piles can be driven for increased resistance to lateral
forces, and columns can be made larger and stronger to resist the increased
bending moments that occur where the columns join the grade beam. Because only a
certain amount of steel can be installed to a given cross-section of concrete
before the column sizes and the flood loads become unmanageable, a maximum
elevation of 15 feet exists for the use of this type of foundation.
4.2.3 Open/Deep Foundation: Timber Pile with Concrete Column and Grade Beam
(Case C)
This foundation is similar to the steel pipe pile with concrete column and grade
beam foundation (Case B). Elevations as high as 15 feet can be achieved for wind
speeds up to 150 mph for both one- and two-story structures. However, because
wood piles have a lower strength to resist the loads than steel piles,
approximately twice as many timber piles are needed to resist loads imposed on
the home and the exposed portions of the foundation (Figure 4-6).
[Begin figure]
Figure 4-6 is a profile of Case C foundation type (see Appendix A for additional
drawings).
[End figure]
While treated to resist rot and damage from insects, wood piles may become
vulnerable to damage from wood destroying organisms in areas where they are not
constantly submerged by groundwater. If constantly submerged, there is not
enough oxygen to sustain fungal growth and insect colonies; if only periodically
submerged, the piles can have moisture levels and oxygen levels sufficient to
sustain wood destroying organisms. Consultation with local design professionals
in the area familiar with the use and performance of driven treated wood piles
will help quantify this potential risk. Grade beams can be constructed at
greater depths or alternative pile materials can be selected if wood destroying
organism damage is a major concern.
4.2.4 Open/Deep Foundation: Timber Pile with Concrete Grade and Elevated Beams
and Concrete Columns (Case H)
Case H foundation designs augment designs contained in the first edition of FEMA
550. They incorporate elevated reinforced beams into the V zone timber pile
foundation design. The elevated beams provide two important benefits:
The elevated beams, columns, and grade beams function as structural frames that
resist lateral loads. The frame action allows smaller concrete columns to be
used. Smaller columns reduce the flood loads imposed on the foundation and
provide more efficient designs.
The elevated beams provide attachment points for homes constructed per
prescriptive codes and standards like ANSI/AF&PA Wood Frame Construction Manual,
American Iron and Steel Institute (AISI) Standard for Cold-Formed Steel Framing
– Prescriptive Method for One and Two Family Dwellings, SSTD10-99 Standard for
Hurricane Resistant Residential Construction, and ICC-600 Standard for
Residential Construction in High Wind Regions.
Case H designs include a 3-bay design suitable for homes as narrow as 14 feet.
Designs for foundation heights of 10 and 15 feet are provided.
As previously stated, the Case H designs are more precise and foundation
strengths more closely match design loads. In addition, the structural frame
action provided by the grade beams, columns, and elevated beams allow smaller
columns to be constructed. One drawback of the design, however, is that
constructing elevated concrete beams is more complicated than constructing grade
beams and reinforced columns; therefore, more knowledgeable and experienced
contractors would be needed (Figure 4-7).
[Begin figure]
Figure 4-7 is a profile of Case H foundation type (see Appendix A for additional
drawings).
[End figure]
Sections were designed with an emphasis on strength, ductility, and
constructability. To simplify detailing and construction, axial and shear
reinforcement were made consistent through each section. Section properties
throughout each member were selected to resist the maximum forces (positive and
negative moments, axial loads, and shears) that exist within the elements (grade
beams, columns, or elevated beams). To simplify forming, the dimensions of the
elevated beams matched the columns into which they frame. The designs were based
on a belief that the potential increase in concrete costs would be more than
offset by the savings in labor costs in constructing simple forms. Design
professionals using the guidance contained in this manual may find it beneficial
to vary from this approach.
The approach used to design and detail the Case H foundation system was to
develop easily scalable column and beam systems. The size allows for the use of
variable amounts of steel while keeping rebar congestion to a minimum. A 16-inch
wide system to allow for the economical use of structural form panels with
minimal waste was selected. The consistent column and elevated beam sizes also
allow for reuse of the concrete forms between columns and beams. The member size
and aspect ratio (member shape factor) allow for high lateral capacities.
The continuous bars in the beams provide a tie around the entire structure,
imparting redundancy should an element fail, as well as the ability for the
system to bridge over failed elements below. This redundancy improves the
foundation performance, especially with impact from floodborne debris that
exceeds design loads.
4.2.5 Open/Shallow Foundation: Concrete Column and Grade Beam with Slabs (Cases
D and G)
These open foundation types make use of a rigid mat to resist lateral forces and
overturning moments. Frictional resistance between the grade beams and the
supporting soils resist lateral loads while the weight of the grade beam and the
above grade columns resist uplift. Case G (foundation with slab) contains
additional reinforcement to tie the on-grade slab to the grade beams to provide
additional weight to resist uplift (Figure 4-8). With the integral slab,
elevations up to 15 feet above grade are achievable. Without the slab (as for
Case D), the designs as detailed are limited to 10-foot elevations (Figure 4-9).
[Begin figures]
Figure 4-8 is a profile of Case G foundation type (see Appendix A for additional
drawings).
Figure 4-9 is a profile of Case D foundation type (see Appendix A for additional
drawings).
[End figures]
Unlike the deep driven pile foundations, both shallow grade beam foundation
styles can be undermined by erosion and scour if exposed to waves and high flow
velocities. Neither style of foundation should be used where anticipated erosion
or scour would expose the grade beam.
4.2.6 Closed/Shallow Foundation: Reinforced Masonry Crawlspace (Case E)
The reinforced masonry with crawlspace type of foundation utilizes conventional
construction similar to foundations used outside of SFHAs. Footings are cast-in-
place reinforced concrete; walls are constructed with reinforced masonry (Figure
4-10). The foundation designs presented in Appendix A permit elevated homes to
be raised to 8 feet. Higher elevations are achievable with larger or more
closely spaced reinforcing steel or with walls constructed with thicker masonry.
[Begin figure]
Figure 4-10 is a profile of Case F foundation type (see Appendix A for
additional drawings).
[End figure]
The required strength of a masonry wall is determined by breaking wave loads for
wall heights 3 feet or less, by non-breaking waves and hydrodynamic loads for
taller walls, and by uplift for all walls. Perimeter footing sizes are
controlled by uplift and must be relatively large for short foundation walls.
The weight of taller walls contributes to uplift resistance and allows for
smaller perimeter footings. Solid grouting of perimeter walls is recommended for
additional weight and improved resistance to water infiltration.
Interior footing sizes are controlled by gravity loads and by the bearing
capacity of the supporting soils. Since the foundation designs are based on
relatively low bearing capacities, obtaining soils tests for the building site
may allow the interior footing sizes to be reduced.
The crawlspace foundation walls incorporate NFIP required flood vents, which
must allow floodwaters to flow into the crawlspace. In doing so, hydrostatic,
hydrodynamic, and breaking wave loads are reduced. Crawlspace foundations are
vulnerable to scour and flood forces and should not be used in Coastal A zones;
the NFIP prohibits their use in V zones.
4.2.7 Closed/Shallow Foundation: Reinforced Masonry Stem Wall (Case F)
The reinforced masonry stem walls (commonly referred to as chain walls in
portions of the Gulf Coast) type of foundation also utilizes conventional
construction to contain fill that supports the floor slab. They are constructed
with hollow masonry block with grouted and reinforced cells (Figure 4-11). Full
grouting is recommended to provide increased weight, resist uplift, and improve
longevity of the foundation.
[Begin figure]
Figure 4-11 is a profile of Case E foundation type (see Appendix A for
additional drawings).
[End figure]
The amount and size of the reinforcement are controlled primarily by the lateral
forces created by the retained soils and by surcharge loading from the floor
slab and imposed live loads. Because the retained soils can be exposed to long
duration flooding, loads from saturated soils should be considered in the
analyses. The lateral forces on stem walls can be relatively high and even short
cantilevered stem walls (those not laterally supported by the floor slab) need
to be heavily reinforced. Tying the top of the stem walls into the floor slab
provides lateral support for the walls and significantly reduces reinforcement
requirements. Because backfill needs to be placed before the slab is poured,
walls that will be tied to the floor slab need to be temporarily braced when the
foundation is backfilled until the slab is poured and cured.
[Begin text box]
NOTE: Stem wall foundations are vulnerable to scour and should not be used in
Coastal A zones without a deep footing. The NFIP prohibits the use of this
foundation type in V zones.
[End text box]