Construction codes often can be a source of frustration when they're so complex that implementing them causes hassles that delay work or, worse yet, halt it indefinitely and needlessly. And specifically, residential concrete construction has been handcuffed by needless complexity and over-conservative prescriptive requirements. In response, and after almost 20 years of effort, the American Concrete Institute (ACI) had its first residential concrete code published in May 2005.
ACI 332-04: Requirements for Residential Concrete Construction and Commentary is intended to become to residential construction what ACI 318 is to the general concrete industry. During 2006, ACI 332-04 will become the official building code in much of the United States through its adoption into the 2006 International Residential Code (IRC). All aspects of residential concrete construction are addressed in ACI 332-04. It balances improved design, recognition of quality, current practices, and materials performance with increased performance requirements to enhance those materials and designs.
This article focuses on three primary sections seen as substantial improvements for the industry –footing requirements, foundation wall geometry, and designing structural walls. The sections pertaining to foundation wall geometry and footing design are instrumental in supporting quality design. The section on foundation wall design improves material recognition and gives basic design guidance to substantiate the performance advantages of concrete over other systems.
Before a building reaches skyward, a quality structural element must be provided to transfer the weight and living loads into the ground. Commonly known as a footing, this element and its necessity are a source of much debate throughout the industry. ACI 332-04 not only defines the characteristics of footings, it requires the presence of footings for most soil conditions as follows:
"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."
Soil bearing capacities –the ability to withstand an applied load –are frequently too low to adequately support the house loads within the small area that is the bottom of the walls. Therefore, 332-04 provides minimum width and thickness requirements for the two footing types, continuous (strip) and isolated (pier) footings.
Load transfer, however, is not the only reason footings are required. ACI 332-04 recognizes that it is equally important to provide a base anchor for the perimeter walls, preventing them from moving inward as pressure is applied during and after backfilling. Prescriptive requirements for a physical connection between the footing and the foundation wall are given by means of a dowel or a keyway. Dowels are required to extend a minimum of 12 inches into the foundation wall and 6 inches into the footing. Keyways are required to be a minimum of 1-1/2 inches wide at the top and 1-1/2 inches deep. ACI 332-04 insists 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. Foundation walls not supported by footings cannot, therefore, guarantee physical resistance against the soil pressure that is trying to move the wall inward from its perimeter location.