Ground Motions and Structures - Earthquake Engineering for Structural Design - page 362

362 Earthquake Engineering for Structural Design

However, there is the possibility that the soil-foundation-structure system could go into
a resonance field with exciting frequency, leading to much larger inertial forces acting
on the structure. In this case the soil-foundation-structure interaction is not beneficial,
but rather detrimental for the seismic response of the structures. The slender frames,
which collapsed in Mexico City during the 1985 earthquake, are examples of this
detrimental effect. At the same time, this interaction could decrease the system
ductility. Therefore, the effects of soil-foundation-structure interaction must be
carefully considered in practical design, because in some cases it may lead to
erroneous conclusions in the prediction of the structural behavior.
Currently, there are basically two approaches to deal with this interaction:
-S ubstructure approach, in which the system is considered as a system formed by
far-field soil, near-field soil, foundation and structure, each being analyzed separately
and the superposition of the results assumes that these substructures work together.
- Direct approach, in which soil, foundation and structure are included within the
same model and analyzed in a single step. The soil is discretized with solid finite
elements and the structure with beam elements.

8.6.2 SubstructureApproach

In the substructure approach, the analysis is separated in three distinct parts which are
combined together to formulate the complete solution. The superposition of these steps
requires the assumption of linear behavior both for soil and structure (Kramer and
Stewart, 2004). The three steps are presented in Figure 8.58.
Determining the foundation input motion (Fig. 8.58b) , which is the motion of the
ground which would occur on the structure base, considering both structure and
foundation without mass. The deviation of these characteristics from the free-field
motions is dependent on the type and geometry of foundation and soil and the
modifications of these free-field motions are determined in the frequency (period)
domain. For shallow foundations , the modifications result from incoherent incident
waves (see Fig.7.35). In presence of these wave passage effects, translational base-
foundation displacements are reduced relative to the free-field motions, but rotational
motions are introduced. These effects tend to become more significant with increasing
frequency, depending on the ratio between the sizes of foundation relative to the
wavelengths at higher frequencies. For embedded shallow foundations , these effects
reduce the translational movements relative to the free-fields motions and the rocking
motion in comparison with the shallow foundations.