| Strong ground motion is a extremely complicated process, which is influenced by such factors as focal mechanisms, spread path, and site characteristics. Therefore, the observed seismic ground motion usually takes on strong non-stationary characteristics. Seeking for a non-stationary variable ground motion model that makes the simulated ground motion characteristics consistent with the time-frequency distribution characteristics of real ground motions has always been a hot topic of research on variable ground motion simulation.For spatially extended structures like large-sized bridges, the differences in site conditions, supporting points and site conditions usually lead to different characteristics in non-stationary time-frequency distribution at different points of the structure. Studies on the analysis and simulation of time-frequency non-stationary characteristics of spatially variable ground motion have been of great importance to study the variable ground motion input in anti-seismic analysis of major engineering projects.In this connection, this paper makes an analysis on the time-frequency non-stationary characteristics of variable ground motion and the realization of spatially variable ground motion in a complete non-stationary space. The main work and its innovative outcomes can be summarized in the following four areas:(1) Research on the quantitative analysis of non-stationary characteristics of a spatially variable ground motion fieldBy introducing the concept of non-stationary degree, we analyzed the time and frequency domain of ground motion employing nonlinear cross predictions and HHT method. Compared with the strength envelope and zero-crossing rate method, the non-stationary degree method can not only describe the time-frequency non-stationary characteristics of ground motion more intuitively, but also enable convenient horizontal comparison between different ground motions. Taking advantage of the non-stationary degree method in intuitive describing the time-frequency non-stationary characteristics of ground motion, we analyzed the differences of time-frequency non-stationary characteristics between the ground motions recorded by SMART1-45 array at different test points quantificationally, and demonstrate how the time-frequency non-stationary characteristics of spatially variable ground motion. in physical space change with the distance and site characterization.(2) Analysis and simulation studies on non-stationary characteristics of variable ground motion based on Fourier’s theoryThe simulation method for time-frequency non-stationary spatially variable ground motion was established through direct and quantitative analysis of the differences of Fourier amplitude and phase of muti-point ground motion between test points. Based on the understanding that the a common fundamental quantity exists in the Fourier amplitude and phase of multi-space and multi-point ground motion at each test point, we studied the fluctuant features between Fourier amplitude and phase and between the common amplitude and phase of multi-point ground motion field at each test point, and established a parameterized model for quantitative description of the amplitude and phase fluctuation of multi-point ground motion and studied how the fluctuation quantity of the amplitude spectrum and phase difference spectrum of multi-point ground motion changes with the distance and soil thickness in combination with the record on SMART1-45 array. In addition, based on the conventional ground motion simulation method using Fourier analysis, we established the simulation method which took into account the spatial change rule of amplitude and phase and satisfied the requirements of time-frequency non-stationary characteristics and code response spectrums, according to the transformation relation between existing response spectrums and Fourier amplitude spectrums and also the results of the research on the lognormal distribution model for phase difference spectrums.(3) Analysis and simulation studies on the non-stationary characteristics of variable ground motion based on evolutionary spectral methodThe simulated time-frequency non-stationarity of spatially variable ground motion was achieved by building parameterized models for the time-frequency power spectrum of ground motion. Modeling the time-frequency power spectrum of ground motion was realized by building models through non-uniform modulation functions and building models for time-frequency power spectrum of ground motion directly, which was used for the analysis of the spatial effect of multi-point ground motion. Based on the recording system of SMART1-45 array, we analyzed the degree of influence of local set’s spatial changes on the parameters of the non-uniform modulation function model and Kameda time-frequency power spectrum model, and established the simulation method, which took into account the changes of multi-point coordinate position and soil thickness of the site, for ground motion which is completely non-stationary and random in space.(4) Analysis and simulation studies on the non-stationary characteristics of the variable ground motion based on the model of separated frequency band wavelet of instantaneous spectrumThe simulated time-frequency non-stationarity of spatially variable ground motion was achieved by directly building models for ground motion time-frequency distributions. Through multi-scale analysis of wavelet transform, we disintegrate the ground motion signal into ground motion wavelets of fixed frequency band and proposed the improving method for modeling the instantaneous spectrum of ground motion based on sub-band wavelets. Based on the modeling of separated frequency band wavelet of instantaneous spectrum of variable ground motion, the spatial variation characteristics of separated frequency band wavelet of instantaneous spectrum parameters in the ground motion are analyzed by using the record of SMART1-45 array.. Moreover, a simulation method of variable ground motion is established based on the model of wavelet of instantaneous spectrum for spatially variable and completely non-stationary ground motion, which provides a new way of thinking for the application of instantaneous spectrum method in the analysis of variable ground motion field.Following the theme of simulating the non-stationary characteristics of spatially variable ground motion, this paper presents a study on the simulation of non-stationary variable ground motion based on the changing effect of time-frequency non-stationary characteristics recorded in real space array through three different ways. This is the most recent development of simulating the non-stationary variable ground motion. The research is not only conducive to a better understanding of non-stationary characteristics of the spatially variable ground motion, but also to the establishment of more artificial simulation methods of the artificial variable ground motion for anti-seismic engineering design.Finally, the weakness of the present study is briefly discussed and some recommendations for further research are provided. |
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