Ground Motions and Structures - Earthquake Engineering for Structural Design - page 302

302 Earthquake Engineering for Structural Design

analyzing the soil-foundation-structure system in elastic linear field only. In all these
analyses the foundation is considered as infinitely rigid, so its response to dynamic
loading arises solely from the displacement of the supporting ground.
The known impedance solutions do not take into account the soil inelasticity, but
consider the non-linear behavior of the ensemble foundation-soil by the decreasing of
soil stiffness with the strain level. So, the method examines the behavior of the
foundation-soil ensemble from the beginning of loading until the non-linear behavior,
assuming the ultimate load as a limitation of the soil deformation at an acceptable level.
Bearing capacity method . Contrary to the Impedance Method, the Bearing Capacity
Method determines only the ultimate load corresponding to the failure mechanism.
Figure 8.6 illustrates the various failure modes of shallow foundations which may occur
during seismic events (Pappin, 2002):
- Horizontal sliding failure may occur when the lateral load exceeds the lateral
capacity of the foundation as shown in Figure 8.6a. Generally, the lateral capacity has a
minimum value when the vertical applied load is also at a minimum. It is important to
note that is refers to seismic loads only.

(a) (b)

(c)

Figure 8.6 Failure modes of shallow foundations: (a) Sliding; (b) Overturning;
(c)Shear failure (after Pappin, 2002)