By Ethan Davis
A foundation transfers the load of a structure to the earth and resists loads imposed by the earth. A foundation in residential construction may consist of a footing, wall, slab, pier, pile, or a combination of these elements. In the next few blog articles, we'll discuss the following foundation types:
crawlspace;
basement;
slab-on-grade with stem wall;
monolithic slab;
pilespa
piers; and
alternative methods.
The most common residential foundation materials are concrete masonry (i.e., concrete block) and cast-in-place concrete. Preservative-treated wood, precast concrete, and other methods may also be used. The concrete slab on grade is the most popular foundation type in the Southeast; basements are the most common type Utah and most of the East and Midwest. Crawlspaces are common in the Northwest and Southeast. Pile foundations are commonly used in coastal flood zones to elevate structures above flood levels, in weak or expansive soils to reach a stable stratum, and on steeply sloped sites.
A crawlspace is a building foundation that uses a perimeter foundation wall to create an under-floor space that is not habitable; the interior crawlspace elevation may or may not be below the exterior finish grade. A basement is typically defined as a portion of a building that is partly or completely below the exterior grade and that may be used as habitable or storage space.
A slab on grade with an independent stem wall is a concrete floor supported by the soil independently of the rest of the building. The stem wall supports the building loads and, in turn, is supported directly by the soil or a footing. A monolithic or thickened-edge slab is a ground-supported slab on grade with an integral footing (i.e., thickened edge); it is normally used in warmer regions with little or no frost depth but is also used in colder climates when adequate frost protection is provided.
When necessary, piles are used to transmit the load to a deeper soil stratum with a higher bearing capacity to prevent failure due to undercutting of the foundation by scour from floodwater flow at high velocities, and to elevate the building above required flood elevations. Piles are also used to isolate the structure from expansive soil movements.
Post-and-pier foundations can provide an economical alternative to crawlspace perimeter wall construction. It is common practice to use a brick curtain wall between piers for appearance and bracing purposes.
The design procedures and information includes the following:
foundation materials and properties;
soil-bearing capacity and footing size;
concrete or gravel footings;
concrete and masonry foundation walls;
preservative-treated wood walls;
insulating concrete foundations;
concrete slabs on grade;
pile foundations; and
frost protection.
Concrete design procedures generally follow the strength design method contained in ACI (American Concrete Institute)-318, although certain aspects of the procedures may be considered conservative relative to conventional residential foundation applications. For this reason, some supplemental design guidance is provided when practical and technically justified. Masonry design procedures follow the allowable stress design method of ACI-530. Wood design procedures are used to design the connections between the foundation system and the structure above and follow the allowable stress design method for wood construction. In addition, the designer is referred to the applicable design standards for symbol definitions and additional guidance.
In our next blog, we'll cover some information on residential concrete materials.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)