A New Kind Of Concrete Code

New footing and foundation standards work better for residential construction.

Source: TOOLS OF THE TRADE Magazine
Publication date: 2006-07-11

By James R. Baty II

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.

Photo: Rick Schwolsky

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.

Concrete Foundation Walls Reducing foundation wall thickness at the top to receive brick or stone veneers may require special reinforcement according to the new code.

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.

After 20 years of effort, the American Concrete Institute has published its first residential concrete code.

Not all design conditions are easily addressed or recognized by building codes. In the residential concrete industry, for example, are the common issues of footing discontinuity and reduced wall thickness. Extreme changes in site elevation and transitions from frost walls to basement walls (for example, a garage to a basement) result in designed wall steps at varying intervals. At these locations, vertical footings serve no structural purpose. Still other conditions exist where the designed wall must account for footings that are spanning trenches or poor soil, or that are interrupted by penetrations. ACI 332-04 provides support for these field conditions with sound design guidance. These transitions are considered discontinuities in the footing design –the foundation wall must therefore be designed to perform as a beam spanning from the end of the footing above to the beginning of the footing below. ACI 332 states:

"Where a wall footing is discontinuous due to an abrupt elevation change, the maximum horizontal discontinuity of the wall footing shall be 4 feet."

The code recognizes the need for and the historical success of footing discontinuity, such as at abrupt elevation changes in the foundation. ACI 332-04 limits the horizontal discontinuity, the distance between the footing ends, to 4 feet and requires a minimum of two No. 4 horizontal reinforcing bars, one at the top and the other at the bottom of the wall, in addition to other required wall reinforcement when the step is greater than twice the footing thickness. By providing reinforcement as stated above, the foundation wall spans the footing discontinuity and effectively serves as a structural beam, transferring the vertical load within that span to the areas of the wall directly over the two footing sections.

Equally important to today's builder and foundation contractor is the need to address brick ledges in the design of foundation walls. Brick ledges are reduced thickness regions along the tops of foundation walls deep enough to support the above-grade brick façade. The foundation wall carries not only the vertical loads of the framed home above, but the additional dead load of the entire brick façade. The code again responds to existing construction methods in a practical way, placing limitations on the depth of this reduced thickness and controlling the integrity of the foundation wall. ACI 332 states:

"The thickness of the top of a foundation wall shall be permitted to be reduced. The height of the reduced thickness section shall not exceed 24 inches."

This section of the code goes on to state that the thickness of the top of the foundation wall shall not be less than 3-1/2 inches –to allow a 4-inch recess in a nominal 8-inch wall. The code also states that when a foundation wall is 4 inches or less in thickness, it shall be reinforced vertically with No. 4 bars at 24 inches on center. By recognizing acceptable performance for reduced sections, ACI 332-04 allows greater flexibility in the economical use of 8-inch walls. Beyond 24-inch-high recesses, the walls would have to be thickened to provide a minimum 5-1/2-inch wall thickness.

Photo: Concrete Foundation Walls The new residential concrete code specifies requirements for discontinuous footings and reinforcement.

Few will argue that the design of foundation walls is a challenge for the builder, contractor, and inspector. And the change over the past few decades from concrete block foundation walls to cast-in-place concrete has over-simplified, too conservatively, the requirements for reinforcing steel in these walls.

Historically, most foundations have been successful with little or no structural reinforcement (although horizontal steel for crack control has always been a feature in quality walls). Designers and contractors, however, have been forced to sift through the ACI 318's complicated requirements or the increasingly conservative tables in general building codes (such as the simplification effort in the 2003 IRC). These simplified tables for concrete walls were created by combining the previous concrete wall tables with those for masonry walls. The result was an increase in the required steel for cast-in-place walls because previous values used for masonry were retained.

ACI 332-04 simplifies the steel requirements. ACI 332 resolves the problem of too-conservative restrictions for concrete walls by modifying the ACI 318 formulas with a 50 percent increase in the moment strength of concrete (Mn) when walls are at least 7-1/2 inches thick and not greater than 10 feet tall. Walls can be a minimum of 5-1/2 inches thick if their height is not more than 4 feet tall and the same concrete strength values are allowed. This modified strength factor allows the design to more closely represent the state-of-the-art success seen throughout the foundation construction industry.

ACI 332 doesn't stop here, though, realizing that formulas are neither user-friendly nor practical. An added appendix provides a set of 10 tables with specific minimum structural reinforcement requirements, including conditions where structural reinforcement is not necessary. These tables are the most complete reference tables available for residential foundation walls, allowing you to vary concrete compressive strength from 2,500 psi to 4,500 psi and to use either 40,000-psi or 60,000-psi steel.

Chapter 7 of ACI 332-04 and Appendix A provide significant code simplification and code improvement for residential concrete foundation walls. Recognizing that horizontal steel for shrinkage-crack control should always be present, this document validates an increase in the use of plain concrete walls (no vertical steel) for foundations.

With the adoption of ACI 332, design professionals and industry experts have worked together to improve the rational design and construction of residential foundation walls to acknowledge the growth and special requirements of residential foundations and footings.

<i> –James R. Baty II is technical director of the Concrete Foundations Association. For more information on the new ACI codes, go to www.concrete.org or visit www.cfawalls.org. This article first appeared in our sister publication Residential Concrete.</i>