Study On Time-frequency And Energy Characteristics Of Long-period Ground Motion

With the continuous development of urbanization,more and more flexible structures have entered people’s lives,including super high-rise buildings,long-span structures,high-rise structures and so on.Since these buildings have long self-vibration periods and are prone to resonance with long-period ground motions,it is important to study the characteristics of long-period ground motions and propose effective and feasible countermeasures.At present,there is no clear delineation of the indicators for long-period ground motion definition.There is no systematic method for the classification and screening of long-period ground motion.For the above reasons,this paper intends to analyze the long-term ground motion classification and time-frequency analysis.The corresponding research is carried out.(1)A classification study of long-period ground motions was conducted,using the 1999 Taiwan Jiji earthquake,the 2015 Nepal earthquake and the 2008 Wenchuan earthquake in the Weihe basin as a sample,using a three-index classification method,followed by the epicenter distance,the shear wave velocity and the weighted average of the ground motion amplification factor βl,,which are used to classify the three strong earthquake records.(2)Related research on long-term ground motion signal processing and spectrum analysis was carried out.The Hilbert transform method is used to decompose the ground motion signal,and the energy weighted average period of the ground motion record(T:)is calculated.The system summarizes four time domain parameters for the analysis of conventional ground motion and long-period ground motion as a comparison;according to the classification study of the Fourier spectrum peak and the main frequency distribution of ground motion,the results show that in the long-period ground motion,the far-field vibration value is larger than the near-field;as the venue becomes softer,the value will gradually increase;The values corresponding to the ground motion in the direction are also different,which are sequentially decreased from the EW,NS,and UD directions.The method of solving the main energy corresponding to the ground motion signal is obtained by solving the cumulative energy of three consecutive IMF components.The peak value of the periodic ground motion acceleration is generally small and the long-field vibration is relatively long.As the low-frequency component of the ground motion gradually decreases,the peak value of the Fourier spectrum increases continuously,and the main frequency bands of the ground motion are different under different site types.(3)The research work on the correlation of the average period of ground motion was carried out.Comparing the Fourier spectrum average period(Tm),the energy weighted average period(Tz),the acceleration response spectrum average period(Tr)and the marginal spectrum(Tmh)provide a basis for the definition of long-period ground motion.The results show that:Tm is more suitable as a parameter of long-period earthquake.The dynamic definition index of long-term ground motion is when Tm>1.2 and PGV/PGA>0.2;If Tm<1.2 or Tm>1.2 and PGV/PGA<0.2,then the ground motion is conventional.(4)The research work on the method of judging the severe area of earthquake damage based on energy was carried out.According to the latitude and longitude of the station where the strong earthquake record is located and the energy-weighted average period obtained above,the three-dimensional joint time-frequency distribution map of the main energy distribution of the Taiwan earthquake is drawn.The study shows that the earthquakes in Taichung,Miaoli and Nantou are serious.