In today’s tough market, builders are always looking for ways to control costs and maintain quality. Concrete basement foundations are one phase of a project that can offer opportunities to shave a few dollars off a home’s cost without cutting corners.
Basements haven’t always gotten the attention they deserve. As consulting engineer Brent Anderson puts it, “Thirty years ago, foundation contractors pretty much did what they felt like. Most of them didn’t put any steel in the walls at all.” And on good sites with high-quality sand or gravel soil, those “plain concrete” basement walls would generally perform well.
But less favorable soil conditions, such as silt or clay, don’t provide great bearing for wall footings. And when used for backfill against the foundation walls, silt and clay soils tend to exert inward pressure that can crack the walls. In such situations (which are becoming more and more common as land scarcity forces builders to consider less-than-ideal building sites), plain concrete foundations may not hold up, and the walls and footings need some kind of steel reinforcement.
But how much steel? And where should it be placed? In the past, basement contractors had two choices: Use rules of thumb based on their own experience or turn to the concrete industry’s comprehensive design standard, ACI 318 Building Code Requirements for Structural Concrete.
Rules of thumb, of course, could result either in over-building or in under-building for the actual conditions. But ACI 318, says Anderson, would typically result in structures that are “way over-built” for structural conditions.
In any case, says concrete contractor Buck Bartley, most contractors would be unlikely to consult ACI 318 (which runs to more than 400 pages of technical language and diagrams) to build a house foundation. “It’s a huge standard,” he says, “and it’s geared to commercial construction—high-rises and everything else.” On the other hand, as the only game in town, ACI 318 could cause trouble in the case of any callback or dispute. According to Bartley, any good lawyer could easily find some aspect of a house basement that didn’t comply with ACI 318 in every detail, and whether the discrepancy related to the house’s structural performance or not, it could spell trouble for builder and foundation contractor alike.
Builders have long had a ready reference more suited to residential loads and conditions: ACI 332 Guide to Residential Concrete Construction, a how-to with generic specifications and plain-English explanations, has been around since the early 1980s. But a guide is not a standard or a code. So in the 1990s, a trade group called the Concrete Foundation Association began to push for a code-language version of ACI 332 that would carry the same authority as code. That standard was finished in 2004 and released in May 2005 as ACI 332-04: Requirements for Residential Concrete Construction and Commentary. (An update, ACI 332-08, was released last year.) Chapter 4 of the 2006 International Residential Code (IRC) refers to the new ACI 332 as an acceptable cookbook design alternative for house foundations: When ACI 332 is used to spec out a basement, says the IRC, “project drawings, typical details, and specifications are not required to bear the seal of the architect or engineer responsible for design.”