Dimensional Analysis Of Seismic Responses Of Base-isolated Building Structures Subjected To Near-fault Ground Motions

The near-fault ground motions usually possess large amplitude and long-period velocity pulses,which can cause serious damage of building structures,casualties and economic losses.With the development of seismic isolation theory and the research and popularization of isolation technology,the seismic isolated building structures are more and more recognized and applied.However,because of the randomness,irregularity and complex movement mechanism the ground motions,the seismic responses of base-isolated structure under different earthquake ground motion records present larger difference,which is especially remarkable for the near-fault ground motions different from the far field motions.The researchers encounter a difficult problem that is to reveal the inherent law of different near fault ground motion records,and to profoundly explain the variation of seismic responses of isolated building structures.As an important and effective scientific research method,the dimensional analysis theory establishes the dimensionless expressions of related physical quantities and reveals the essential laws by normalizing the parameters of engineering problems.In view of this,this thesis introduces the theory of dimensional analysis to explore the normalized seismic responses of base-isolated structures.A series of research on the normalized displacement responses of the base isolated building is performed based on two kinds of length scale under near-fault ground motions,and the intrinsic rule of seismic responses of the base-isolated buildings is revealed.This thesis will promote the theoretical research and engineering application of base-isolated structure which is of great significance.The detailed contents are presented as follows:Firstly,a simplified model of the equivalent two-particle isolation system for a base-isolated building is established,and the normalized maximum displacement responses of different types of equivalent two-particle isolation systems are studied by inputting three kinds of different ideal pulse excitation.The capability of energy length scale and intrinsic length scale to characterize the normalized displacement response of the near fault isolation building is compared and analyzed.It is found that the normalized maximum displacement responses of the base-isolated building are affected by the different mass ratios and frequency ratios of the superstructure.When the intrinsic length scale is used,the variability and dispersion of normalized maximum displacement responses of the isolation system are effectively suppressed,and the distribution of variation curves is more uniform.Secondly,the normalized seismic response of the two-particle isolation system under actual near-fault ground motions is investigated.By introducing the average period of actual ground motion records,the self-similar response spectra of the isolation systems are established,and the coefficients of variation of response spectra based on the two length scales are compared.The results show that the curves of variation coefficients of self-similar response spectra based on the intrinsic length scale have the coincident trend,and the mean response spectra possess the complete similarity with regard to the normalized yield displacement,which indicates that the intrinsic length scale can effectively suppress the dispersion of normalized responses of the isolation system.Finally,the finite element software is utilized to establish the computational model of the base-isolated high-rise building,and the normalized maximum displacement responses of the high-rise building,the multi-particle isolation system and the two-particle isolation system are compared and examined under two kinds of ideal pulses and actual near-fault ground motions.By fitting the mean response spectrum of seismic isolation system subjected to actual ground motion records,a regression model is constructed to predict the seismic response of high-rise isolated buildings.The research results illustrate that the normalized responses of the base-isolated high-rise building under ideal pulse excitation have the consistent trend with the dimensionless responses of two kinds of isolation system.The normalized responses of tall building under real near-fault records spread on the fitted regression curve,which indicates the simplified isolation system can characterize more accurately the seismic responses of the base-isolated tall building.The proposed regression equation can effectively fit the self-similar response spectrum and predict the seismic responses of isolated buildings.