By: Ed Sauter
Codes are developed to provide assurance that structures are designed and constructed to withstand the test of time, not only for life safety but also for long-term durability. Yet codes can also be so complex that implementation causes struggles that delay, or worse yet, halt work.
Residential construction is no different than any other construction market today. Recognizing this, the American Concrete Institute created a committee specific to the residential industry (332-Residential Concrete) to create guides and standards that help shape the quality and efficiency of this market. Nearly 20 years after the formation of this committee, the first ACI Residential Concrete Code has been published. This article continues to discuss key elements found in this new standard that alter or improve the requirements for residential concrete as the industry prepares to shift from design and construction by ACI 318, chapter 22 to ACI 332.
What is it about footings anyway?
You've got a home to construct. It will be defined by the walls, floors and roof that enclose the living space and provide the years of custom appearance that make it your own. The longevity of this project, however, does not begin with the method of construction that is selected for these elements. Before vertical expression of the design takes form, there must be a structure that will maintain the continuity for transferring the building and living loads to the selected site. This common structure, known as the footing, is often the source for much debate throughout the industry.
ACI 332 makes the following statement regarding footings:
Footings are provided under columns (also called piers) and walls when calculations show that the omission of the footing will result in soil pressures that exceed the allowable soil bearing pressures or to facilitate the placement of forms. Soil bearing pressures can be referenced in the general building code or obtained from a geotechnical report.
In essence, the footing is a required structural element because the bearing capacity of the soil (the ability to withstand an applied load) is too low to adequately handle the load transferred through the small area of the bottom of the walls.
There are generally two types of footings: continuous and isolated (or pier). Continuous footings are most common in residential construction. They can be easily seen defining the perimeter of the home in the excavated hole or just below the surrounding grade. Typically, these footings are about 12 inches thick, although the actual depth is a function of the capacity of the soil condition. ACI 332 defines the requirements for footing depth as being greater than 6 inches, or equal to one-half of the footing width minus the wall thickness. Isolated footings are most often used under the columns that support a beam spanning across a large space. In both cases, footings play two primary functions in the structural performance of the home that make them necessary for long-term performance. The first is the transfer of building loads to the weaker soil condition. The second is to provide the anchor for the perimeter walls along the base to prevent them from moving inward as pressure is applied by the backfilled site.
The latter purpose for a footing is perhaps one of the more critical positions taken by ACI 332. When an unbalanced backfill condition exceeds 4 feet in height (the distance from the basement slab to the finished grade elevation) the wall must be anchored to the footing using either a dowel connection at a minimum of 24 inches on center or by creating a continuous keyway (tapered channel) in the footing that is a minimum of 11/2 inches wide at the top and 11/2 inches deep. This physical connection of the wall to the footing prevents the base of the wall from moving due to the pressure of the backfill. It is the position of this code that the floor slab cannot be relied upon to provide enough lateral resistance or otherwise lock the base of the wall from movement due to shrinkage, cracking or expansion joints. Therefore, foundation walls that do not use a footing cannot assuredly obtain the physical resistance against the soil pressure working to move the wall inward from its perimeter location.
There are many additional design and construction requirements in ACI 332 for the footing as one of the primary structural components of the foundation system. To ensure the integrity of the footing through the construction process, the code addresses items such as formed and unformed footing constructions, the soil condition required for placement and areas of discontinuity or excavation.
Together in ACI 332, design professionals and industry experts have combined efforts to improve the rationalization and the construction of residential footings. This work comes in response to the market growth and constant pursuit of the mantra "bigger, better, faster and more economy." Contact ACI today to order your copy of ACI 332-04 Requirements for Residential Concrete Construction.
Editor's Note: As part of our partnership with the Concrete Foundations Association (CFA), the following article is the third in a series on residential construction codes. For more information, visit cfawalls.org or call (319) 895-6940.