Study On Characteristics Of Seismic Response Of Soil Layers

The influence of the site condition on the seismic response is a fundamental subject in engineering seismology and has been studied for a long historical period. In this paper, it is further discussed in three aspects: non-linear characteristics of site soil, influences of soft and hard layer in site soil and of the rational classification of site conditions.The study of influences of site condition on seismic ground motion is reviewed, especially these methods which have been used in the seismic design codes, about influence of site conditions on the seismic ground motion. The study results of influences of site condition on ground motion are examined and some problems needed are pointed out. Using the numerical method for seismic response analysis, the seismic responses of different site models under all possible combinations of seismic input from the base are calculated. Based upon these calculated results, the influences of site condition on characteristics of ground motion are studied qualitatively and quantitatively. The framework and results of this study is thus summarized as follows:1. Analyzing the effect of the nonlinear parameters on seismic response. So far the G/G_max-γ(Transient dynamical shear modulus over the maximum shear modulus vs. shear strain) and theλ-γ (equivalent shear hysteretic damping ration vs. shear strain) curves are widely employed in the experimental method in studying soil dynamic property, supposing under equiamplitude cyclic load. In this paper the effects of the variation (uncertainty) of these two curves on site seismic response are studied. It is necessary to check whether the effects of these on site seismic response are needed to take into account in a real engineering or not. The result of the study indicates it is necessary to take the nonlinear parameters (uncertainty) of soil sample into consideration in seismic ground motion analysis.2. Studying the effects of soft layer and hard layer on seismic response. We further investigate the behavior of both the soft layers and the hard layers with the shear wave speed less than 500m/s effecting on the maximum acceleration and the shape of response spectrum, respectively. It shows that hard layers barely affect the ground motion and can thus be neglected in general. The soft layers, on the other hand, can influence the ground motion greatly, i.e., seismic response will decreases when the period less than 0.1s and it increases when period greater than 0.3s, but it will not change significantly for the long period over 3s. Therefore the whole effect of the soft layers on earthquake ground motion in fact depends on the characteristic frequencies input.3. Analyzing predominate period of ground motion. Using the standard design response spectrum in code for seismic design of buildings, one can estimate theseismic response according the site classification. This approach only uses to estimate the effect of the first order mode. However, it is found that the second or the third order mode shape rather than the first order, are responsible for the seismic responses on free surface under many case in this paper. The characteristics of seismic response transfer function is studied statistically. To describe the effects of soil layer on characteristics of ground motion the one new approach are suggested to characterized the predominate period of ground motion by site equivalent predominate period. The idea and method for calculation of site equivalent predominate period is given according to equivalent .shear velocity.