Discrete Law Recognition Of Ground Motion And Its Impact On Seismic Response Of Long-period Structures

In recent years, the various long-period structural seminars have discussed themethods of seismic design and prediction of seismic response for long-period structures.In addition, according to the accumulation of guiding seismic design work of high andsuper-high buildings, realize that China’s current prediction methods of seismicresponse of structures have the following three noteworthy problems.1) Consideringmany factors,the current design response spectra are artificially adjusted in the longperiod,it can’t represent the ground motion intensity with a clear statistical significancein the area,therefore the selected ground motion by fitting response spectra aren’trepresent the ground motion intensity with a clear statistical significance.2)Uaually anearthquake occurs,even though the magnitude,site conditions and distance are thesame,because of transmission route multiformity of seismic waves and complexity ofgeologic structure,the spectral characteristic of ground motion records are discrete,whilethe input discrete ground motion will cause the discrete seismic response. However, therecently study of long-period structure properties din’t consider the discreteness ofseismic response caused by the discrete ground motion. In order to obtain the authenticregularity of discreteness, the ground motion records used to study the discreteness musthave enough reliability in long period.3) Because the around seismic enviorment anddynamic characteristics of each structure are different, the ground motion which specificstructures on the specific site are likely to suffer from are different. To predict theseismic responses of structures, it should reflect the influence of seismic enviormentsurrounding the structure. But the method of selecting ground motion according to theChinese design code can’t reach this goal. Obviously, the seismic responses ofstructures by this way aren’t accurate or reasonable enough.Based on the problems existed in the methods of predicting seismic responses,theresearch work accomplished by this paper is as follows:①Textual research of ground motion in the long period and a discussion aboutcredibility of ground motion in the long period from measuring instrument,erroranalysis and data processing methods of ground motion records.②Collecting261ground motion records which have long enough credible periodand are the equivalent of Ⅱsite condition in the Chinese seismic code from AmericanPacific Earthquake Engineering Research Center(PEER) and New Zealand FTP strong ground motion database. Then sorting these ground motion records by magnitude anddistance, studying the character of acceleration response spectrum of ground motionrecords, finally studying the distribution regularities of discreteness of ground motionrecords.③Aimed at the deficiency of current ground motion record selection methods,coming up with a method of ground motion records selection using fitting responsespectrum which have clear statistical significance as target spectrum based on thediscreteness of ground motion,the method can also reflect the influence of seismicenviorment around the structures.④According to the proposed method of ground motion records selection,selectingthe three statistical level total21groups (each statistical level choose7groups) groundmotion records. Then using PERFORM3D to finish nonlinear dynamic analysis for along-period structure which has been designed and expert panel reviewed, comparingwith the structural response of different statistical level input ground motionrecords,studying the discreteness of structural response caused by the discretenessground motion,in order to achieve the target that predictding the structural responsereasonably.Through the above work, the main conclusions are obtained:①Compared with the early analog seismograph records, the gorund motion recordsobtained by new generation of digital seismographs can provide more accuratelong-period component,the digital seismograph records after data processding canprovide credible long-period information, it can be used for the studying oflong-period characteristic of gorund motion.②Magnitude is a decisive factor that affecting the long-period component size ofgorund motion,with the increase of earthquake magnitude, When the earthquakefault distance is small,the peak range of acceleration response spectrum tends toincrease,or forms the second peak area. The greater the magnitudes,the long-periodvalues of dynamic amplification coefficient β spectrum of near-field ground motion arelarger. Especially when the big earthquake more than M7.0, the long-period values ofdynamic amplification coefficient β spectrum of near and far field ground motion areboth larger. The ground motion records of the same magnitude, with the increase of thedistance, the long-period values and rate of decay of dynamic amplification coefficientβ spectrum have little difference, it means that the influence on the “distance” toresponse spectrum long-period of ground motion records. Regardless of the earthquake magnitude and fault distance, the peak of dynamic amplification coefficient β spectrumare near2.5, the range is2.3~3.15.③Suggested method of ground motion records seletion is: using obtained fittingresponse spectrum that have clear statistical significance (the average responsespectrum,the average plus one standard deviation response spectrum, the average plustwo times the standard deviation response spectrum) as target spectrum,and using theresult of probability seismic risk analysis do deaggregation analysis, recognizing theearthquake type controlled the seismic risk of structures, leting the magnitude anddistance of selected ground motion records coordinate the result of deaggregationanalysis. Then calculating each acceleration response spectrum of the two horizontalground motion records and geometric mean of the two acceleration response spectrum,controlling the average of demarcated geometric mean isn’t less than target spectrum inthe period range of0.2T1~1.5T1.④Getting the discreteness distribution regularities of structural response in thedifferent level of statistical significance of input ground motion records, suggestingthat,for the ordinary structures, in accordance with the traditional design of controllevel,using the average fitting response spectrum that have clear statistical significanceas target spectrum,then selecting ground motion records as input to study the seismicperformance of structures. But for the particularly important structures, it’s necessary toconsider structural response in the higher level,such as the ground motion intensity ofaverage plus one standard deviation or average plus two times the standard deviation. Itmeans that using average plus one standard deviation response spectrum or average plustwo times the standard deviation response spectrum as target spectrum, then selectingground motion records as input to study the structural response.If this condition doesn’tmeet the performance goal,engineers can consider adjusting the design plans ofstructures.