Credit: Joe Nasvick
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.”
Working Smarter: With design choices that reduce the need to place and tie steel reinforcing bar, builders can avoid excess labor in foundation forming.
Credit: Ted Cushman
Dialing in Performance
The text of the ACI 332 standard spells out design specifications for concrete, reinforcing steel, formwork, footings, walls, and slabs. Simple drawings are provided for clarity. But when it comes to cost-effective design, the rubber meets the road in 10 tables at the end of the document. The tables serve as an easy road map for mixing and matching the various elements that add strength to a wall, so the builder can quickly look up the most affordable solution for the given conditions.
The main structural load on a basement wall isn’t the weight of the house—it’s the pressure of soil against the side of the wall, which becomes greater as the wall gets taller or as the backfill depth increases. To make the wall stronger, designers have many options: Use higher-strength concrete, increase the thickness of the wall, place more steel reinforcement, or use a stronger grade of steel. If you know how to read the tables, you can easily assess the results of pursuing any of these strategies. You can also assess the potential savings gained if you shorten the height of the basement wall or even just lessen the depth of backfill. And you can also see the advantage of building on a location with superior soil conditions, where the soil pressure against the wall will be relatively moderate.
“I fought like a dog against these tables at the beginning,” says Bartley, who serves on the ACI 332 standard-writing committee. “But now I think they’re brilliant.” It’s no secret where the savings are to be found, says Bartley. “Any residential contractor will tell you this: It’s a whole lot cheaper for me to pour concrete than it is for me to tie a bunch of rebar. The amount of labor that it takes for the rebar is, generally speaking, a whole lot more expensive than the cost of a yard of concrete.” So Bartley’s goal is to push the foundation design into the white space in the table that represents plain concrete (walls that don’t require vertically placed steel). “When we get a print in here,” he explains, “we might go back to the builder and say, ‘We can take this rebar out of the wall by expanding it from a 10-inch to a 12-inch wall and changing from a 2,500-psi to a 3,500-psi [pounds per square inch compressive strength] concrete.’ And generally speaking, they’ll buy that.”
Understanding the Variables
ACI 332’s tables provide a single design tool that draws together a number of well-established principles of concrete foundation wall construction. Here’s a closer look at the factors involved in basement design.
Soil Type. Sand and gravel soils are strong, stable, and freely draining, while silt and clay soils, when they get wet, tend to act more like liquids—expanding, slumping, and exerting what engineers call “equivalent fluid pressure” against walls. ACI 332’s tables list the soil types by pressure in pounds per square foot per foot of depth (psf/ft)—30 psf/ft (typical of gravel), 60 psf/ft (typical of a silty sand), up to a high of 100 psf/ft (for soils containing significant proportions of clay). Soil pressures higher than 100 psf/ft are problematic enough to take you outside ACI 332’s scope, requiring the builder to consult an engineer.
If the site has more than one kind of soil, you’re better off building where the good soils are. You can also bring in sand or gravel for backfill. Soil engineers have different opinions about how much imported fill it takes to protect a wall, says Anderson. Some say the fill should slope back 30 degrees from the footing, while others will tell you that a 60-degree slope is required. Anderson offers his own unofficial rule of thumb: “If you have an 8-foot wall and you backfill with 2 feet of sand and gravel from top to bottom, you basically will meet the requirements of a sand and gravel backfill. If you have a 9-foot wall, that zone should be about 3 feet wide; and if you have a 10-foot-high basement wall, that zone should be 4 feet wide. That is not an engineering principle, but I can say with a pretty decent degree of reliability that if you follow those rules, you’ll pretty much be where you need to be for reducing the lateral pressure.”
Wall Height and Backfill Depth. Like water pressure, soil pressure increases with increasing depth. And like a beam, a concrete wall experiences more bending force if the distance between supports is greater (basement walls are analyzed as a simple beam standing on end, loaded with sideways pressure in the middle and supported at the top and bottom ends by the floor system and the footing). The combination of these factors means that you can reduce the stress on a wall by either shortening the wall, lowering the backfill height, or both.
The relationship is not linear. Says Anderson, “The increase in lateral pressure going from an 8-foot to a 9-foot basement wall is not 15 percent. It’s about a 30 percent to 50 percent increase just by adding an extra foot.” The beauty of the ACI 332 tables is that they provide an easy shortcut to the answers, rather than requiring a higher-math analysis. As Bartley puts it, “When you make the wall shorter, you open up a lot of white space in the table.”
Wall Thickness. With or without rebar, fatter walls are better at resisting the lateral pressure of soil. Here again, engineers have complicated equations for putting numbers on the difference; but the ACI 332 tables allow anyone to look up the answer. The table for 3,500-psi concrete, for example, lets the builder omit vertical rebar in most soil conditions, as long as the wall is 11 1/2 inches thick.
Concrete Strength. Higher-psi concrete also helps give the walls greater capacity against lateral soil pressure. This effect is relatively minor, but it can make a difference in some cases: There’s just a little bit more white space in Table A.3 (3,000-psi concrete) than there is in Table A.1 (2,500-psi concrete). Higher-strength concrete may also bring other advantages—it improves water resistance, freeze-thaw resistance, and impact resistance. But when the chips are down, most builders probably will settle for 3,500-PSI concrete in a typical basement.
Rebar Size, Grade, and Placement. Reinforcing steel adds the vital property of tensile strength to a concrete wall (concrete by itself is not very strong in tension). There are two grades of steel covered in the ACI 332 tables: Grade 40 (with a yield strength of 40,000 psi) and Grade 60 (with a yield strength of 60,000 psi). If you use Grade 60 rebar, you can install fewer pieces to gain equivalent strength—“and there is virtually no cost difference between the two grades,” says Bartley. Labor is the key factor when you’re installing rebar; you save money by using fewer pieces, which you can do by increasing the diameter, the grade, or both. At diameters above 3/4 inch (#6 bar), the steel gets harder to bend around corners; but most contractors can readily bend any size or grade of bar using mechanical equipment. (Note: For all basement walls, ACI 332 requires three bars of rebar placed horizontally along the length of the wall at the top, center, and bottom to limit shrinkage cracking.)
Of course, not all home buyers want their house designs squeezed to the economic minimum. In fact, in parts of the Washington, D.C., vicinity, says Bartley, customers are insisting on basement ceilings 10 or even 12 feet high. It’s a little extreme, he admits, but he says, “It’s not my job to tell people what to do.” In cases like that, or for homes built in the very active “marine clay” soils found in parts of the Virginia lowlands, Bartley usually finds himself building to a specific engineered design—and charging accordingly.
But for the average house on good soil, Bartley’s generic solution is an 8-foot or 9-foot basement wall, 10 inches thick, poured with 3,500-psi concrete using four horizontal runs of rebar—and, in most cases, no need for vertical rebar. That’s actually pretty conservative, he says. “We’re conscientious, and we don’t want to get anyone in trouble.”
For Bartley, the new ACI 332 standard is by no means an excuse to get away with doing less. On the contrary, he says, the new rules fill a void in the residential construction industry. “We created a standard that the residential industry should be living up to—something that would represent value to the customer as well as a reasonable standard for the contractor himself to meet.”