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. Why is it, then, that codes can also be so complex that the very implementation of them causes such struggles to occur that work often is delayed, or worse yet, halted indefinitely?
Residential construction is no different in this way than any other construction market today. Recognizing this, the American Concrete Institute created a committee specific to the residential industry (332- Residential Concrete) with the goal of creating guides and standards to help shape the quality and efficiency of this market. Now, nearly 20 years after the formation of this committee, work has been completed on the first ACI Residential Concrete Code, set to be published in March 2005. Over the coming months, this article will digest some of the more substantive areas altering or improving the requirements for residential concrete as the industry prepares to shift from design and construction by ACI 318, chapter 22 to ACI 332.
The ACI 332 Standard, developed by this committee, provides the minimum cast-in-place design and construction requirements for one- and two-family dwellings and their accessory structures. The current version of the code (332-04) specifically addresses those requirements for concrete footings (wall, thickened slab and isolated), basement or foundation walls constructed only with removable forms and slabs-on-ground. Other elements and methods for residential construction do exist but are beyond the scope of this current document. Those not included remain under the provisions of ACI 318 and other code references.
Designing the structural wall
Without much debate, nearly everyone involved in the residential construction industry will agree that the design of the foundation wall is one of the most challenging aspects of the relationship between the builder, contractor and inspector. For decades, successful wall construction has changed the shape of residential foundations from concrete block walls to cast-in-place concrete. The vast majority of these foundations have been constructed with little or no reinforcement for structural loads. Where steel has been required, complicated references to ACI 318 using Chapter 22 in combination with tables in the International Residential Code (IRC) have left many projects using increased conservatism and more steel than potentially required. Most recently, in an effort to simplify the code, the IRC took steps to combine the required reinforcement tables for concrete walls with those for masonry walls while keeping the values previously used for masonry. This step only increased conservatism in the design of concrete walls.
One of the most significant effects the new 332 Standard will have on the design and construction of foundation walls is simplifying the reference to and requirements for steel in residential foundation walls. Chapter 7 of this Standard begins by identifying the premise for the provisions specific to the structural integrity of walls covered:
Foundation wall design shall be based on analyzing the wall as a simply supported vertical flexural member with the top and bottom laterally supported. Walls shall be designed as either plain concrete…or reinforced concrete….
ACI 332 recognizes the conservative restrictions on poured concrete walls in the formulas used in ACI 318 and makes modifications to those formulas by increasing the moment strength of the concrete by 50 percent. Providing that the wall being designed has a minimum wall thickness of 7.5 inches (5.5 inches if not over 4 feet tall) and a height that does not exceed 10 feet, the formulas of this document improve the designed response of foundation walls to more closely represent the current and successful state-of-the-art in the foundation construction industry.
Formulas and criteria, however, are not user-friendly to the vast majority that turn to residential codes. ACI 332 has taken this into consideration from Chapter 7 by adding 10 tables to Appendix A that provide the 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 the user to vary concrete strength from 2,500 psi to 4,500 psi using both 40,000- and 60,000-psi steel.
Together, the use of Chapter 7 and Appendix A provides a significant step in the development of true code simplification and code improvement for residential concrete foundation walls. Although a minimum amount of steel is always required for shrinkage crack control, the use of this document validates a much larger section of the industry that has been successful in integrating plain concrete walls for foundations. In section 7.2 of this chapter, the commentary states:
The minimum area of vertical wall reinforcement amounts to No. 4 bars at 36 inches on center. This minimum reinforcement and the maximum bar spacing of 48 inches correspond to the extensive history of satisfactory performance.
Together in ACI 332, the design professionals and industry experts have combined efforts to begin improving the rationalization and the construction of residential foundation walls in order to respond to the market's growth and constant pursuit of the mantra "bigger, better, faster and more economy."