Soil Behavior: Dynamic Soil-Structure Interactions - Dynamic Behavior of Concrete and Seismic Engineering - page 158

(b)

(a)

Figure 4.15. Analog model for the impedance matrix

4.4. Non-linear soil-structure interactions

In most cases, linear modeling of the soil-structure interaction represents a
reasonable approximation of the phenomena. However, extreme situations do exist,
in which taking into account the geometric or behavior non-linearities is the only
possible approach representative of the phenomenon. Taking these phenomena into
account currently represents a topic that is developing fast, and for which important
progress has been achieved in recent years. Hereafter, we restrict ourselves to
identifying the phenomena and superficially point to the way to deal with them.

4.4.1. Geometric non-linearities and uplift of the foundations

It has often been observed after major earthquakes that slender structures with
shallow foundations show good behavior and seem to have been protected from
seismic aggression by their propensity to move independently of the foundation soil;
these additional “degrees of freedom” are provided by the possible uplift at the soil-
foundation interface. This uplift is expressed by transformation of the kinetic energy
into potential energy, due to the raising of the center of gravity, and through
lengthening of the vibration periods. According to the frequency content of the
stress, the longer vibration period can shift the vibration mode towards a lower
amplification spectrum region.

4.4.2. Non-linearities of behavior

Such non-linearities come from the local plastification of soil near the foundation
element. In the case of pile foundations, plastification takes place along the upper
part of the pile shaft, at a height typically equal to 4 or 6 diameters; for superficial