Figure 2-1. Concrete Masonry Basement Wall with Exterior Insulation
STRUCTURAL DESIGN
The major structural components of a basement are the wall, the footing, and the floor (see figure 2-2). Basement walls are typically constructed of cast-in-place concrete or concrete masonry units. Basement walls must be designed to resist lateral loads from the soil and vertical loads from the structure above. The lateral loads on the wall depend on the height of the fill, the soil type, soil moisture content, and seismic activity. Because of the large number of variables involved in foundation structural design, final determination of wall thickness, concrete strength, footing dimensions, and reinforcing should be made after consultation of locally-enforced building codes or design by a licensed structural engineer.
Figure 2-2. Structural System Components of a Basement
Concrete spread footings provide support beneath basement concrete and masonry walls and columns. Footings must be designed with adequate size to distribute the load to the soil. Freezing water beneath footings can heave, causing cracking and other structural problems. Unless founded on bedrock or proven non-frost-susceptible soils, footings must be placed beneath the maximum frost penetration depth or be insulated to prevent frost penetration.
Concrete slab floors are generally designed to have sufficient strength to support floor loads without reinforcing when poured on undisturbed or compacted soil. The use of welded wire fabric and concrete with a low water/cement ratio can reduce shrinkage cracking, which is an important concern for appearance and for reducing potential radon infiltration. The slab should be poured against a control joint material so that it can move independently of the foundation wall. Where expansive soils are present or in areas of high seismic activity, special foundation construction techniques may be necessary. In these cases, consultation with local building officials and a structural engineer is recommended.
