Strength Index Of Far-Field Long Period Ground Motion And Evaluation Of Seismic Performance Of Isolated Structures

Isolation technology has been proved to be an effective engineering application technology and has been widely used in practical engineering.However,seismic damage examples show that isolation structures will be damaged in different degrees under long period ground motion.Therefore,the seismic performance of isolated structures under long period ground motion naturally attracts great attention of researchers.However,at present,most of the relevant researches focus on the influence of near-field long period ground motion on isolation structure,while the research on the influence of far-field long period ground motion on isolation structure is scarce.In addition,reasonable ground motion intensity index is directly related to the reliability of seismic performance evaluation of structures.But up to now,there are relatively few ground motion intensity indexes applicable to isolated structures,and no unified indexes have been formed,which seriously restricts the development of seismic performance evaluation of isolated structures.To sum up,this paper will focus on the ground motion intensity index suitable for isolated structures under the action of far-field long-period ground motion,and further evaluate the seismic performance of isolated structures.The main research contents are as follows:（1）Combined with the division of near and far site vibration areas and the characteristics of long-period ground motion,40 far-field long-period ground motions satisfying the requirements were selected from the PEER,and 20 ordinary ground motions were selected as a comparative study.The amplitude and time-holding characteristics of two kinds of ground motions are systematically studied,and the time-frequency and energy characteristics of two kinds of ground motions are analyzed based on Hilbert-Huang transform（HHT）.It is found that the peak values of Hilbert spectrum of ordinary ground motions are mainly distributed in the early and middle high frequency regions of time domain,while the peak values of Hilbert spectrum of long period ground motions are mainly distributed in the middle and late period and low frequency regions of time domain.The frequency and time domain distributions of the two kinds of ground motion energies correspond to the Hilbert spectrum,but the far-field long-period ground motion has a larger input energy,which may produce a larger seismic response to the structure.（2）Taking selected far-field long-period ground motion as input and 3,5 and 7 story isolation structures as research objects,30 typical ground motion intensity indicators and 2 key engineering demand parameters of structures are selected to evaluate the above 30 ground motion intensity indicators from three aspects of effectiveness,usefulness and practicability.On this basis,the indexγ_yof far-field long-period ground motion intensity suitable for isolation structure is constructed.The key parameters ofγ_yare determined based on partial least squares regression,and the rationality of this index is discussed.Compared with 30 existing ground motion intensity indexes,the far-field long-period ground motion intensity indexγ_yconstructed in this paper has the best evaluation results of effectiveness,practicability and benefit,and can be used as the preferred index for seismic analysis of isolation structures.（3）Based on the constructed seismic intensity indexγ_y,the incremental dynamic analysis（IDA）was carried out on the isolated structure with the selected two types of ground motion as input,and the IDA curve of the structure was obtained.Through the statistical analysis of IDA curve,the analysis effect of the seismic intensity indexγ_ywas obtained,and the seismic vulnerability of the isolated structure was further analyzed.The seismic performance of the structure is evaluated from the Angle of probability.It is found that the failure probability of the isolated structure increases obviously and its seismic performance decreases obviously under the long period of far-field ground motion,but the seismic response of the superstructure is effectively reduced due to the existence of the isolated layer,and its risk is mainly concentrated in the isolated layer.