BEARING CAPACITY ANALYSES FOR EARTHQUAKES - Geotechnical Earthquake Engineering - page 211

FIGURE 8.1 General shear foundation failure. ( After Vesic 1963. )

8.1.2 Bearing Capacity Failures

Compared to the number of structures damaged by earthquake-induced settlement, there
are far fewer structures that have earthquake-induced bearing capacity failures. This is
because of the following factors:

1. Settlement governs: The foundation design is based on several requirements. Two
of the main considerations are that (1) settlement due to the building loads must not exceed
tolerable values and (2) there must be an adequate factor of safety against a bearing capac-
ity failure. In most cases, settlement governs and the foundation bearing pressures recom-
mended by the geotechnical engineer are based on limiting the amount of expected
settlement due to the static or seismic cases. In other cases where the settlement is too high,
the building is often constructed with a deep foundation, which also reduces the possibility
of a bearing capacity failure.
2. Extensive studies: There have been extensive studies of both static and seismic
bearing capacity failures, which have led to the development of bearing capacity equations
that are routinely used in practice to determine the ultimate bearing capacity of the foun-
dation.
3. Factor of safety: To determine the allowable bearing pressure q all , the ultimate
bearing capacity q ult is divided by a factor of safety. The normal factor of safety used for
static bearing capacity analyses is 3. For the evaluation of the bearing capacity for seismic