Footings & Foundations
- Types of Footings
- Types of Foundations
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.
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.
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 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.
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)|
|CONVENTIONAL LIGHT-FRAME CONSTRUCTION|
|4-INCH BRICK VENEER OVER LIGHT FRAME OR 8-INCH HOLLOW CONCRETE MASONRY|
|8-INCH SOLID OR FULLY GROUTED MASONRY|
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
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.
1 Coastal 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.
For complete design and construction details refer to the SFPA publication Permanent Wood Foundations — Design and Construction Guide. Additional technical and design information on the PWF can be obtained from the American Wood Council at www.awc.org.