Nonlinear Effects Analysis And Calculation Methods Improvement Of Site Earthquake Response

The great role that site conditions play in the earthquake disaster can't be neglected. First, the soil nonlinear behavior is discovered by the geotechnical engineers from the laboratory tests, and then the seismologists confirm it by analysis of the record data from the strong motion observation. Generally speaking, the site nonlinear behavior will become apparent beyond a threshold acceleration of 100~200 gal, however, to the different site types, there will be different earthquake input to cause the site nonlinear behavior. This paper will suggest the corresponding earthquake input threshold in the different sites according to the relationship between the shear wave velocity variation ratio and the earthquake excitation.The equivalent linear method is widely used in the seismic microzonation and the evaluation of seismic safety for engineering sites. In projects, it is found that the conventional equivalent linear method underestimates the peak ground acceleration under a strong earthquake input. The phenomenon is proved after the calculation and analysis of the actual borehole data with the equivalent linear method. The reason is that the artificial value of the equivalent shear strain(0.65γmax)is quite different form the real shear strain, and then, it reduces the contribution of the high frequency components in the earthquake wave to the peak ground acceleration. It is discovered that the soil shear strain becomes smaller as the frequency increases according to the statistical method. Combining with the dynamic curve of soil stiffness and damping, it is concluded that the soil stiffness and damping is frequency- dependent, although soil itself dose not show this characteristic. Following this observation, this article considers the frequency dependent stiffness and damping in the nonlinear soil response and results in a marked improvement comparing to the equivalent linear method. The accuracy of the proposed method is examined by comparing with the vertical records during large earthquakes, for example, the Port Island borehole record data. The proposed method always gives much better result than the conventional equivalent linear method.When calculating the dynamic soil response in the simplified one dimension site, this paper will adopt the idea alike the conception of the relative duration of the earthquake motion because of the disadvantage caused by the equivalent shear strain. That is, cut the earthquake input time history at the time points when it first and last exceeds some threshold, then the acceleration time history is divided into three segments. The results of these three segments can be directly added in the frequency-domain or the time- domain because of the equivalent linear theory, and then the whole soil response time history can be worked out. The way to calculate the dynamic soil response as stated above is called the earthquake segment-input method. The result proves that this method is more reasonable.