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