A firm foundation, including properly installed footings of
adequate size to support the structure and prevent excessive
settlement, is essential to the satisfactory performance of
buildings including raised floor systems.
Foundation systems are often classified as shallow or deep foundations,
depending on the depth of the load-transfer member below the
super-structure and the type of transfer load mechanism. The
required foundation system depends on the strength and compressibility
of the site soils, the proposed loading conditions, and the
project performance criteria (i.e. total settlement and differential
settlement limitations).
Foundation designs are based on the assumed bearing capacity
of the soil at the building site (see Table
7). In construction sites where settlement is not a problem,
shallow foundations provide the most economical foundation systems.
Shallow foundation construction is typically utilized for most
residential and light commercial raised floor building sites.
Where poor soil conditions are found, deep foundations may be
needed to provide the required bearing capacity and to limit
settlement. Additionally, structures in coastal high-hazard
areas are required to be elevated above the base flood elevation
(BFE), commonly on piles. Examples of deep foundation systems
include driven piles (e.g. pressure-treated timber piles, concrete,
or steel), drilled shafts, or micropiles. See Pile
Foundations.
Storm surge protection in hurricane-prone coastal areas requires an elevated floor system. Wood piles are also an option for this application.
TYPES OF FOOTINGS
Footing requirements are generally
covered in the building code and sized in accordance with the
bearing capacity of the soil and the weight of the building.
In areas subject to seasonal frost, the bottom of the footing
must be placed below the frost line to prevent damage to the
footing and structure due to frost heave. Typical footing types
include:
See Figure 9, Typical Footings — Types
and Layouts; and Figure 10, Pier or Foundation
Wall Options by Footing Type.
Spot Footings
A spot or pad footing is used to support a single point of contact,
such as under a pier or post. A spot footing is typically a
2' by 2' square pad, 10" to 12" thick, and made with reinforced
concrete rated to 3,000 to 5,000 pounds per square inch (psi)
in compression.
Continuous Spread Footing
A continuous spread footing is commonly used to provide a stable
base around the entire perimeter of a structure. Buildings with
spread footings often include interior spot footings. A spread
footing supports the weight (load) from the exterior or foundation
walls. The footing thickness provides the strength needed to
support the weight. The wider width of the footing base creates
a large area to transfer this weight to the ground and to prevent
settlement.
The dimensions of a continuous spread footing vary according
to the soil conditions under the building, the load placed on
the footing, and the construction style of the structure being
supported. It is common practice to make the footing thickness
equal to the thickness of the foundation wall, and to provide
a footing projection on each side of the foundation wall equal
to one-half the foundation wall thickness. Spread footings are
frequently 16" to 24" wide, 6" to 16" thick, and made with reinforced
concrete rated to 2,000 to 5,000 psi in compression. Table
9 lists the minimum footing widths required for a range
of allowable bearing capacities and building sizes.
Grade Beam Footing
A grade beam footing is a continuous reinforced-concrete member
used to support loads with minimal bending. Grade beams are
capable of spanning across non-load bearing areas, and are commonly
supported by soil or pilings. A continuous grade beam is frequently
constructed by digging a trench at least 8" wide to the
depth needed to span the distance between supports. Grade beam
footings differ from continuous spread footings in how they
distribute loads. The depth of a grade beam footing is designed
to distribute loads to bearing points, while the width of a
continuous spread footing is designed to transfer loads to the
ground.
TYPES OF FOUNDATIONS
The two most commonly used foundations with raised floor systems
are pier-and-beam and stem wall foundations. Regardless of the
foundation system used, the foundation and the footings must
be of adequate size and strength to support the design loads.
Pier-and-Beam Foundations
Pier foundations are commonly constructed of reinforced masonry
(brick or concrete block) supported by individual, reinforced-concrete
pad footings or by continuous, reinforced-concrete spread footings.
For pier-and-beam foundations, pier spacing will also depend
upon arrangement of floor framing, particularly the location
of bearing walls and partitions. Spacing of piers in the range
of 8' to 12' is common practice. The openness of pier foundations
creates natural venting of the crawlspace. Refer to the section
on crawlspace
ventilation.
See Figure 11, Interior Pier Detail; Figure
12, Framing Anchorage at Perimeter Pier; and Figure
13, Pier and Beam with Brick Veneer.
Stem wall foundations may also be constructed with pressure-treated
wood members, commonly referred to as a Permanent Wood
Foundation, or PWF.
Continuous Foundation Walls
(Stem Wall Foundations)
Continuous (stem wall) foundations are frequently constructed
of reinforced masonry or poured concrete, supported by a continuous,
reinforced-concrete spread footing. Refer to Figure
14 and Figure 15 for construction details,
and to Table 9 for minimum footing widths.
Stem wall foundations may include interior spot piers for support
of the raised floor system. Moisture control of the crawlspace
created by the stem wall foundation is an important issue. Refer
to moisture control, site
and building drainage, and crawlspace
design and construction.
Table
9 Minumum Width of Concrete or Masonry Footings (inches)1
Load-Bearing
Value of Soil (psf)
1,500
2,000
3,000
≥4,000
CONVENTIONAL
LIGHT-FRAME CONSTRUCTION
1-story
12
12
12
12
2-story
15
12
12
12
3-story
23
17
12
12
4-INCH
BRICK VENEER OVER LIGHT FRAME OR 8-INCH HOLLOW CONCRETE
MASONRY
1-story
12
12
12
12
2-story
21
16
12
12
3-story
32
24
16
12
8-INCH
SOLID OR FULLY GROUTED MASONRY
1-story
16
12
12
12
2-story
29
21
14
12
3-story
42
32
21
16
Source:Internationl Residential
Code for One- and Two-Family Dwellings, Table R403.1 International
Code Council (Falls Church, VA, 2003).
1 Where minimum footing width is 12", a single wythe of solid
or fully grouted 12"-nominal concrete masonry units is permitted
to be used.
psf = pounds per square foot
Pile Foundations
Where poor soil conditions are found, foundations may need to
be constructed on preservative-treated timber piles capped with
wood or concrete sills. In such buildings, support may be provided
by the end-bearing capacities of the piles or by friction between
the pile and soil. In pile-supported structures where the building
support relies upon friction between the pile and soil, two
important soil parameters must be known or determined:
angle of internal friction (for cohesionless soils)
cohesion value in pounds per square foot (for cohesive
soils)
Handling
Wood Piles
Precautions should be taken when handling and storing
pressure-treated wood piles. Piles should not be dragged
along the ground or dropped. They should be stored on
well-supported skids to ensure air space beneath the piles,
and to ensure they are not in standing water. Additional
procedures and precautions for pile handling, storage
and construction are found in Standard M4 of the American
Wood-Preservers' Association Book of Standards (www.awpa.com).
Friction piles may also be required to support standard foundations
in unstable soil.
In buildings supported by pile foundations, the layout of the
horizontal girders and beams should consider that the final
plan locations of the tops of the piles may not be precise.
Irregularities in the piles and the soil often prevent the piles
from being driven perfectly plumb. The use of thick shims or
over-notching for alignment at bolted pile-girder connections
will adversely affect connection capacity. A rule of thumb regarding
notching is to notch no more than 50% of the pile's cross-sectional
area. Notching more than 50% will require reinforcing the pile
with a steel plate or other suitable material.
Pile foundations are also used in coastal areas where the foundation
may be subject to inundation and possible wave action. Elevated
wood pile foundations enable buildings to be constructed above
the base flood elevation (BFE) as required by the National Flood
Insurance Program. For information on coastal construction,
consult the FEMA Coastal Construction Manual¹.
For more information on pile foundations, refer to the Timber
Piling Council publication Timber Pile Design and Construction
Manual, at www.timberpilingcouncil.org.
1Coastal Construction Manual: Principles and Practices of
Planning, Siting, Designing, Constructing and Maintaining Residential
Buildings in Coastal Areas, Federal Emergency Management
Agency, www.fema.gov.
Permanent Wood Foundations
Permanent Wood Foundations (PWFs) are fully engineered systems
accepted by all the major building codes, as well as by federal
agencies and lending, home warranty, and fire insurance institutions.
Stem wall foundations constructed in accordance with the system
are an increasingly popular option for houses and other wood-frame
buildings. Foundation walls are typically load-bearing, lumber-framed
walls sheathed with structural plywood panels. All lumber and
plywood components in a PWF are pressure treated with a relatively
high concentration of a waterborne preservative to withstand
decay from moisture and insect damage.
The PWF system can be utilized for both basement and crawlspace
(raised floor) foundation systems. Foundation walls are designed
to withstand both backfill (lateral) and vertical (axial) loads,
and are typically supported by foundation footings of crushed
stone. Figure 16 shows a typical PWF wall
for crawlspace (raised floor) construction.
