| The field investigation results of actual earthquake disasters show that the abnormal points or areas of earthquake disasters are often the result of the combined action of various factors including topography and overlying layer.Nowadays,more and more large buildings are inevitable to be built on the mountain with topographic relief or the islands and reefs with complex sedimentary environment.Studying the seismic site effect of local site conditions is helpful for more reasonable earthquake fortification.Firstly,related research is carried out on the finite element numerical simulation method of the seismic response of the site based on the viscous-spring boundary,and the automatic application of viscous-spring boundary and input of ground motion are realized in the finite element software ABAQUS.Then,the influence degree of the insitu stress field generated by the self-weight on the seismic response of the site is studied.Finally,based on the above research work,the study on the influence of the platform topography and the overlying layer conditions on the site strong ground motion characteristics of the site is carried out.The main research results are as follows:1.Main research results related to finite element numerical simulation methods(1)The research on the application of the viscous-spring boundary in the finite element software ABAQUS.Two different assumptions of rigid foundation and flexible foundation,as well as the treatment of dynamic boundary condition in the analysis of site seismic response are expounded and summarized.Aiming at the seismic response analysis model of the site under the condition of lying flexible bedrock,the calculation formula of equivalent nodal force combined with free field motion is derived,and the seismic wave input formula without calculation of free field is derived according to the wave field separation theory.The program VBEA and VBEA20 for automatic setting of viscous-spring boundary and seismic wave input are realized and verified after the secondary development of ABAQUS,suiting for homogeneous media and layered media,respectively.The reasonable setting of viscous-spring boundary and the correct input of ground motions in ABAQUS provide useful tools for subsequent site seismic response analysis.(2)Research on the determination of reasonable ground motion input based on viscous-spring boundary.On the above basis,a two-dimensional soil model under the action of vertical incident shear wave(SV)is established to compare 12 different combinations of boundary conditions and seismic wave input methods.Based on the principle of transmission and reflection of seismic waves on discontinuous interfaces,the mechanism of interaction between viscous-spring boundary condition and several seismic wave input methods is revealed,which provides a basis for the correct input method of seismic wave in the subsequent finite element model.(3)Research on static-dynamic coupling effect in the seismic dynamic response of the site.The transformation method of static-dynamic boundary condition is introduced,and a method to verify the rationality,and correctness of the transformation of the static-dynamic boundary transformation is proposed.Based on ABAQUS and the self-programmed VBEA20 for applying viscous-spring boundary and equivalent nodal force,the seismic response analysis of the static-dynamic coupling model of multi-soil layer free field and the static-dynamic coupling model of soil-underground structure are carried out.Taking the peak acceleration and maximum relative displacement of the site,the horizontal relative displacement of the center column,and the maximum principal stress of important positions of the center structure as the evaluation indicators,the effects of static-dynamic coupling on the seismic response of layered sites and the seismic dynamic response of underground structures under both elastic and nonlinear conditions are studied.2.The main research results of the joint influence of topography and overlying soil conditions on the ground motion characteristics of the site.Combined with the actual mountain topography data,the finite element method calculation and analysis were carried out for a total of 24 conditions of three different height platform topography,four covering soil conditions and two kinds of ground motion input combinations.Firstly,the ground motion site effects of topography and overlying soil conditions are studied from the time history,fourier spectral ratio and spectral difference of seismic response.Then,the characteristics and laws of strong ground motion response of the site under the joint effect of the two factors are analyzed.(1)The characteristics of co-effectThe law of influence of various influence factors(such as site topography height and equivalent shear wave velocity of the overlying layer)on the ground motion characteristics of the site still applicable to the joint effect of the influence factors.The degree of influence of various influencing factors on different ground motion components can be superimposed,which is the superimposed effect of the influence of different factors on the amplitude of ground motions of the same frequency.Topography and overlying soil layers can have different degrees of influence on different ground motion components at the same time.Due to the differences in the factors and rules that affect the spectral characteristics of ground motions,the ground motion characteristics of the site under the combined influence eventually show complex characteristics and rules.(2)The law of co-effect(1)The frequency corresponding to the peak spectral ratio and the frequency corresponding to the peak spectral difference are not necessarily the same.When the two corresponding frequencies are the same,the peak spectral difference increases significantly,and this frequency also corresponds to the dominant frequency of the site seismic response.At this time,it is the strongest site earthquake response under this ground motion input condition,which is most likely to cause damage to the building structure and form an abnormal area of earthquake disaster.This is the most significant coupling effect between ground motion input and site conditions.(2)Both topography condition and overlying soil condition have an impact on the natural frequency of the site,which affect the number of predominant frequency(bandwidth)and amplitude in the site seismic response.When the natural frequency of the topography condition is consistent with that of the overlying soil condition,the joint effect of the topography and the overlying soil shows a very significant amplification of the ground motion magnitude corresponding to the predominant frequency band and the decrease of number of predominant frequencies(bandwidth).When the natural frequency of the topography condition is different from that of the overlying soil condition,the joint effect of the topography and the overlying soil shows the relatively insignificant amplification of seismic amplitude and the increase of the number of predominant frequencies(bandwidth).As the natural frequency of the topography condition is gradually close to that of overlying soil condition,the number of predominant frequencies(bandwidth)gradually reduces,and the amplification effect of the ground motion corresponding to the predominant frequency band is gradually significant.(3)There are differences in the degree of influence of different topography conditions and covering soil conditions on the ground motion of the site.When the degree of influence of the two is almost equal,the joint influence effect of the site is the most significant.When a certain factor has a greater degree of influence,the joint effects of the site mainly show the characteristics and laws of the influence of this influencing factor on the ground motion of the site. |
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