Civil Engineering Reference
In-Depth Information
FIGURE 8.3
Local shear foundation failure. (
After Vesic 1963.
)
6.
Footing dimensions:
Usually the structural engineer will determine the size of the
footing by dividing the maximum footing load by the allowable bearing pressure. Typically
the structural engineer uses values of dead, live, and seismic loads that also contain factors
of safety. For example, the live load may be from the local building code, which specifies
minimum live load requirements for specific building uses (e.g., see Table 16-A,
Uniform
Building Code,
1997). Thus the load that is used to proportion the footing also contains a
factor of safety, which is in addition to the factor of safety that was used to determine the
allowable bearing pressure.
The documented cases of bearing capacity failures during earthquakes indicate that usu-
ally the following three factors (separately or in combination) are the cause of the failure:
1.
Soil shear strength:
Common problems include an overestimation of the shear
strength of the underlying soil. Another common situation leading to a bearing capacity
failure is the loss of shear strength during the earthquake, because of the liquefaction of
the soil or the loss of shear strength for sensitive clays.
2.
Structural load:
Another common problem is that the structural load at the time of the
bearing capacity failure was greater than that assumed during the design phase. This can
often occur when the earthquake causes rocking of the structure, and the resulting struc-
tural overturning moments produce significant cyclic vertical thrusts on the foundation
elements and underlying soil.
