Report - PEER - University of California, Berkeley

clays with PI = 30 were used for the stiff clays, whereas for the soft clays the shearmodulus reduction and damping curves for Holocene Bay Mud proposed by Sun et al.(1988) were used. Figure 3b illustrates the resulting spectral amplification factors.Observe that the effect of different average dynamic shear wave velocities over the upper100 feet is similar to the effect of changing the depth to bedrock, as observed in Figure 2;that is, the peak spectral amplification factor shifts toward higher periods. Hence, caserecords and analytical studies support a site classification scheme that captures both theimportant influences of soil stiffness and soil depth on seismic site response and resultingdamage.Seismic site response is also a function of the intensity of motion due to thenonlinear stress-strain response of soils. The effect of nonlinearity is largely a function ofsoil type (e.g., Vucetic and Dobry 1991). Factors such as cementation and geologic agemay also affect the nonlinear behavior of soils. The effect of soil nonlinearity is two-fold:(a) the site period shifts toward longer values, as illustrated in the previous example, and(b) material damping levels in the soils at a site increase. The increased damping levelsresult in lower spectral amplifications for all periods. The effect of damping, however, ismore pronounced for high frequency motion. Hence, PGA is more significantly affectedby soil damping. The consequences of the shift toward longer site periods depend on thesoil type and the input motion. For some sites, the site period may be shifted towardperiods containing high-energy input motion, resulting in large spectral amplificationfactors with an associated increase in PGA. Conversely, the site period may be shifted toperiods where the energy of the input motion is low, resulting in large spectral7