Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 411

Advances in Seismic Design Methodologies 411

Effects of soil conditions. Earthquakes of the last decades, especially those of
Mexico City, Armenia and San Francisco Bay regions, have reemphasized the
importance of local geologic deposits on the amount of damage and the resultant loss
of life. In general, damage and loss of life in each of these earthquakes were
concentrated in areas with deposits of soft soils. These concentrations of damage have
emphasized the need to modify the response spectra patterns to better account for the
amplification effects of local geologic deposits (Borcherdt, 1994). Therefore, the
influence of local geological conditions on the features of response spectra is widely
recognized and most codes provide specifications for these relationships, usually by
categorizing soil profiles into soil classes. The ground motion experienced on the
surface results from the transmission of energy waves released from the bedrock
source, which is transmitted first through the bedrock itself and then undergoes
significant modifications in the soil layers, as soon as the energy waves come close to
the Earth's surface. Typically rock sites experience short period response, but with a
rapid decay. Thus, short duration high intensity motion may be expected in such
locations. Conversely, soft soils, particularly when they extend to moderate depth, are
likely to filter out some of the short period motion and usually amplify longer period
responses, particularly when the soil mass has a natural period similar to the high
energy component of the earthquake. Soft soil response spectra have a flatter and
broader plateau.
Considering these aspects, Eurocode 8 introduced five soil classes, defined by the
average shear wave velocity (V s ) in the upper meters of soil. Soil class A corresponds
to the rock or other rock-like geological formations; class B to deposits of very dense
sand, gravel or very stiff clay; class C to deep deposits of dense or medium sand,
gravel or stiff clay; class D to deposits of loose-to-medium cohesionless soil; and class
E to soil consisting of surface alluvium The effects of the soil conditions depend on the
earthquake type, being different for interplate (Type 1) and intraplate (Type 2)
earthquakes (Fig. 9.17). One must mention that Eurocode 8 (2002) does not explicitly
specify the framing of the two magnitude earthquake types (Type 1 and Type 2) in the
two typical earthquakes (interplate and intraplate). The spectral amplification factors,
for interplate (Type 1, M > 5.5) and intraplate (Type 2, M < 5.5) earthquakes are
presented in Figure 9.18, where the influence of soil classes is identified. Examining
the spectrum for interplate earthquakes (M >5.5) and the corresponding characteristics
given in Figures 9.17a and 9.18a, one can see that the weak soils produce an increasing
of acceleration amplification and corner periods.