By Ethan Davis
The objectives of foundation wall design are:
to transfer the load of the building to the footing or directly to the earth;
to provide adequate strength, in combination with the footing (when required) to prevent differential settlement;
to provide adequate resistance to shear and bending stresses resulting from lateral soil pressure;
to provide anchorage for the above-grade structure to resist wind or seismic forces;
to provide a moisture-resistant barrier to below-ground habitable space in accordance with the building code; and
to isolate non-moisture-resistant building materials from the ground.
In some cases, masonry or concrete foundation walls incorporate a nominal amount of steel reinforcement to control cracking. Engineering specifications generally require reinforcement of concrete or masonry foundation walls because of somewhat arbitrary limits on minimum steel-to-concrete ratios, even for “plain” concrete walls. However, residential foundation walls are generally constructed of unreinforced or nominally reinforced concrete or masonry or of preservative-treated wood. The nominal reinforcement approach has provided many serviceable structures. This section discusses the issue of reinforcement and presents rational design approach for residential concrete and masonry foundation walls.
In most cases, a design for concrete or concrete masonry walls can be selected from the prescriptive tables in the applicable residential building code or the International One- and Two-Family Dwelling Code. Sometimes, a specific design applied with reasonable engineering judgment results in a more efficient and economical solution than that prescribed by the codes. The designer may elect to design the wall as either a reinforced or a plain concrete wall. The following sections detail design methods for both wall types.
Concrete Foundation Walls
Regardless of the type of concrete foundation wall selected, the designer needs to determine the nominal and factored loads that, in turn, govern the type of wall (reinforced or unreinforced) that may be appropriate for a given application. In light-frame homes, a lower load combination typically governs foundation wall design. Axial load increases moment capacity of concrete walls when they are not appreciably eccentric, as is the case in typical residential construction.
To simplify the calculations further, the designer may conservatively assume that the foundation wall acts as a simple span beam with pinned ends, although such an assumption will tend to over-predict the stresses in the wall. In any event, the simple span model requires the wall to be adequately supported at its top by the connection to the floor framing, and at its base by the connection to the footing or bearing against a basement floor slab.
Once the loads are known, the designer can perform design checks for various stresses by following ACI-318 and the recommendations contained therein.
As a practical consideration, residential designers need to keep in mind that concrete foundation walls are typically 6, 8 or 10 inches thick (nominal). The typical concrete compressive strength used in residential construction is 2,500 or 3,000 psi, although other strengths are available. Typical reinforcement tensile yield strength is 60,000 psi (Grade 60) and is primarily a matter of market supply.
Plain Concrete Wall Design
ACI-318 allows the design of plain concrete walls with some limits and recommends the incorporation of contraction and isolation joints to control cracking; however, this is not a typical practice for residential foundation walls, and temperature and shrinkage cracking is practically unavoidable. It is considered to have a negligible impact on the structural integrity of a residential wall. However, cracking may be controlled (minimize potential crack widening) by reasonable use of horizontal reinforcement.
ACI-318 limits plain concrete wall thickness to a minimum of 7-1/2 inches; however, the International One- Two-Family Dwelling Code permits nominal 6-inch-thick foundation walls when the height of unbalanced fill is less than a prescribed maximum. The 7-1/2-inch-minimum thickness requirement is obviously impractical for a short concrete stem wall, as in a crawlspace foundation.
Adequate strength needs to be provided and should be demonstrated by analysis in accordance with the ACI-318 design equations and the recommendations in this section. Depending on soil loads, analysis should confirm conventional residential foundation wall practice in typical conditions.
Reinforced Concrete Design
ACI-318 allows two approaches to the design of reinforced concrete with some limits on wall thickness and the minimum amount of steel reinforcement; however, ACI-318 also permits these requirements to be waived in the event that structural analysis demonstrates adequate strength and stability.
Reinforced concrete walls should be designed by using the strength design method. The following checks for shear and combined flexure and axial load determine if a wall is adequate to resist the applied loads.
Combined Flexural and Axial Load Capacity
ACI-318 prescribes reinforcement requirements for concrete walls. Foundation walls commonly resist both an applied axial load from the structure above and an applied lateral soil load from backfill. To ensure that the wall’s strength is sufficient, the designer must first determine slenderness effects (Euler buckling) in the wall. ACI-318 provides an approximation method to account for slenderness effects in the wall; however, the slenderness ratio must not be greater than 100. The slenderness ratio is defined in the following section as the ratio between unsupported length and the radius of gyration. In residential construction, the approximation method, more commonly known as the moment magnifier method, is usually adequate because slenderness ratios are typically less than 100 in foundation walls.
Minimum Concrete Wall Reinforcement
Plain concrete foundation walls provide serviceable structures when they are adequately designed. However, when reinforcement is used to provide additional strength in thinner walls or to address more heavily loaded conditions, tests have shown that horizontal and vertical wall reinforcement spacing limited to a maximum of 48 inches on center results in performance that agrees reasonably well with design expectations (Roller, 1996).
ACI-318•126.96.36.199 requires two No. 5 bars around all wall openings. As an alternative more suitable to residential construction, a minimum of one rebar should be placed on each side of openings between 2 and 4 feet wide, and two rebars on each side and one on the bottom of openings greater than 4 feet wide. The rebar should be the same size required by the design of the reinforced wall or a minimum No. 4 for plain concrete walls. In addition, a lintel (concrete beam) is required at the top of wall openings.
Concrete Wall Deflection
ACI-318 does not specifically limit wall deflection. Therefore, deflection is usually not analyzed in residential foundation wall design. Regardless, a deflection limit of L/240 for unfactored soil loads is not unreasonable for below-grade walls.
Concrete Wall Lintels
Openings in concrete walls are constructed with concrete, steel, precast concrete, cast stone, or reinforced masonry wall lintels. Wood headers are also used when not supporting concrete construction above and when continuity at the top of the wall (i.e., bond beam) is not critical, as in high-hazard seismic or hurricane coastal zones, or is maintained sufficiently by a wood sill plate and other construction above.
The concrete lintel is often assumed to act as a simple span with each end pinned. However, the assumption implies no top reinforcement to transfer the moment developed at the end of the lintel. Under that condition, the lintel is assumed to be cracked at the ends such that the end moment is zero and the shear must be transferred from the lintel to the wall through the bottom reinforcement.
If the lintel is assumed to act as a fixed-end beam, sufficient embedment of the top and bottom reinforcement beyond each side of the opening should be provided to fully develop a moment-resisting end in the lintel. Though more complicated to design and construct, a fixed-end beam reduces the maximum bending moment on the lintel and allows increased spans. A concrete lintel cast in a concrete wall acts somewhere between a true simple span beam and a fixed-end beam. Thus, a designer may design the bottom bar for a simple span condition and the top bar reinforcement for a fixed-end condition (conservative). Often, a No. 4 bar is placed at the top of each wall story to help tie the walls together (bond beam) which can also serve as the top reinforcement for concrete lintels. Figure 4.6 depicts the cross-section and dimensions for analysis of concrete lintels.
In our next blog, we will be discussing Insulated Concrete Foundation Walls.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)