Engineering and Construction

In southern California, the most common types of shallow foundations used for single-family houses and other lightly loaded structures are a raised wood floor foundation and a concrete slab-on-grade, as follows:

Raised Wood Floor Foundation. The typical raised wood floor foundation consists of continuous concrete perimeter footings and interior (isolated) concrete pads. The floor beams span between the continuous perimeter footings and the isolated interior pads. The continuous concrete perimeter footings are typically constructed so that they protrude about 1 to 2 ft (0.3 to 0.6 m) above adjacent pad grade. The interior concrete pad footings are not as high as the perimeter footings, and short wood posts are used to support the floor beams. The perimeter footings and interior posts elevate the wood floor and provide for a crawl space below the floor. In general, damage caused by southern California earthquakes has been more severe to houses having a raised wood floor foundation. There may be several different reasons for this behavior:

1. Lack of shear resistance of wood posts. As previously mentioned, in the interior, the raised wood floor beams are supported by short wood posts bearing on interior concrete pads. During the earthquake, these short posts are vulnerable to collapse or tilting during the earthquake.

2. No bolts or inadequate bolted condition. Because in many cases the house is not adequately bolted to the foundation, it can slide or even fall off of the foundation during the earthquake. In other cases the bolts are spaced too far apart and the wood sill plate splits, allowing the house to slide off the foundation.

3. Age of residence. The houses having this type of raised wood floor foundation are often older.
The wood is more brittle and in some cases weakened due to rot or termite damage. In some cases, the concrete perimeter footings are nonreinforced or have been weakened due to prior soil movement, making them more susceptible to cracking during the earthquake.

4. Crawl space vents. In order to provide ventilation to the crawl space, long vents are often constructed just above the concrete foundation. Such vents provide areas of weakness just above the foundation.

All of these factors can contribute to the detachment of the house from the foundation. Besides determining the type of foundation to resist earthquake related effects, the foundation engineer could also be involved with the retrofitting of existing structures. For example, to prevent the house from sliding off of the foundation, bolts or tie-down anchors could be installed to securely attach the wood framing to the concrete foundation. Wood bracing or plywood could also be added to the open areas between posts to give the foundation more shear resistance.

Slab-on-Grade Foundation. In southern California, the concrete slab-on-grade is the most common type of foundation that is presently used for houses and other lightly loaded structures. Typical slab-on-grade foundations consist of either the conventional or the posttensioned foundation.

The conventional slab-on-grade foundation consists of perimeter and interior continuous footings, interconnected by the slab-on-grade. Construction of the conventional slab-on-grade begins with the excavation of the interior and perimeter continuous footings. Steel reinforcing bars are commonly centered in the footing excavations and wire mesh or steel bars are used as reinforcement for