Civil Engineering Reference
In-Depth Information
FIGURE 13.36
The top of the concrete pile separated from the concrete pile cap during the Kobe earth-
quake on January 17, 1995. (
Photograph from the Kobe Geotechnical Collection, EERC, University of
California, Berkeley.
)
Besides determining the type of foundation to resist earthquake-related effects, the geo-
technical engineer could also be involved with the retrofitting of existing structures. As previ-
ously mentioned, the raised wood floor with isolated posts is rarely used for new construction.
But there are numerous older houses that have this foundation type, and in many cases, the
wood sill plate is inadequately bolted to 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 be added to the open areas between posts to give the foundation greater
shear resistance and prevent the house from sliding off the foundations, such as shown in
Fig. 13.37.
13.4.2 Slab-on-Grade
In southern California, the concrete slab-on-grade is the most common type of foundation
for houses constructed within the past 20 years. It consists of perimeter and interior con-
tinuous footings, interconnected by a slab-on-grade. Construction of the slab-on-grade
begins with the excavation of the interior and perimeter continuous footings. Steel rein-
forcing bars are commonly centered in the footing excavations, and wire mesh or steel bars
are used as reinforcement for the slab. The concrete for both the footings and the slab is
usually placed at the same time, to create a monolithic foundation. Unlike the raised wood
floor foundation, the slab-on-grade does not have a crawl space.
In general, for those houses with a slab-on-grade, the wood sill plate is securely bolted
to the concrete foundation. In many cases, an earthquake can cause the development of an
exterior crack in the stucco at the location where the sill plate meets the concrete foundation.
