Page 163 - Dynamic Loading and Design of Structures

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Table 4.3 in line (though not in full compliance) with the provisions of the new US codes,
briefly discussed in the following.
Data from recent earthquakes, in particular the 1985 Mexico earthquake and the 1989
Loma Prieta (North California) earthquake, have indicated that accelerations on soft soils are
larger (sometimes much larger) than on nearby rock sites; this is related to the high level of
strain at which soft clay nonlinearity occurs, as discussed in Section 4.2.4. Moreover, soil-to-
rock amplification factors for Spa at long periods can be significantly higher than those
adopted by EC8 and previous American codes (Borcherdt, 1994; FEMA, 1995). As expected
(due to soil nonlinearity effects), the spectral amplifications are higher for motions with low
PGA and lower for higher PGA.
The 1997 UBC adopts the recommendations initially included in the 1994 NEHRP
Provisions (FEMA, 1995), that are based on the foregoing considerations. The seismic
coefficients Ca and Cv used for the definition of the response spectrum depend both on soil
conditions and on the level of the design PGA. The site classification scheme adopted by
NEHRP and UBC is quite simple, as only the shear wave velocity in the uppermost 30 m (the
typical maximum depth of boring in geotechnical investigations) of the soil are used. As an
alternative to vs, geotechnical parameters such as the standard penetration resistance (for co-
hesionless soils) or the untrained shear strength (for cohesive soils) can be used, but this will
usually lead to more conservative results (FEMA 1995, 1997a). The site dependent seismic
coefficients of the 1997 UBC are given in Table 4.4, where the definition of each soil profile
type is also included; note that the Z-factor in the Table is the seismic zone coefficient, which
for practical purposes can be seen as a PGA value (expressed in terms of g). The paramount
effect of soil conditions on the C-values (particularly on C v, which defines the response
spectrum at longer periods), is clear from Table 4.4. Note that the maximum soil to rock
amplification factors for S (calculated as the ratio of C-values corresponding to soils S and
pa
E
SA) range from 4.1 to 1.3 (corresponding to Z=0.075 and 0.40, respectively) for the short-
period coefficient Ca, and from 4.3 to 3.0 for the long period coefficient Cv. Note also the
upper bound of 0.36Na imposed on Ca in the highest seismic zone, for the case of soft soils;
this should be interpreted as the maximum acceleration that such soils are deemed to be able
to transmit (due to non-linear effects).
Comparisons between UBC and EC8 response spectra for various site conditions show that
for both ‘intermediate’ (S C and SD) and ‘soft’ (SE and SF) soils the UBC spectra result in
higher Spa-values than EC8 for PGA’s up to 0.2 g, whereas this is not generally the case for
0.3 g. On the other hand, EC8 appears to be more conservative for rock sites.
Finally, with respect to vertical motion response spectra, it appears that they are less
influenced by site conditions than horizontal spectra. Nevertheless, for short periods both
horizontal and vertical spectra for soft sites are characterized by smaller amplification than for
stiff sites; the opposite trend appears at intermediate and long periods (Ambraseys and
Simpson, 1996).