Determination Method And Engineering Characteristics Of Rotational Component In Earthquake Ground Motion

Both theoretical analysis and earthquake damage experience have shown thatearthquake ground motion is a complex multi-dimensional motion, which consist ofthree translational components, three rotational components (two rocking componentsaround the horizontal axes and one torsional component around the vertical axis)andmany strain factors. In the past century, with the development of modern seismology,people have accumulated lots of experience on resisting this terrible natural disaster.Among various measures, seismic design of structures is the basic one, which has beenwidely proved to be effective and many countries have promulgated laws and codes toguarantee it to become effective. However, with the accumulation of ground motionrecords as well as our deepening knowledge on earthquake, more and more unknownfactors which may influence the security of structures have drawn considerable attention.Among them, rotational component of ground motion is a key factor which cannot beignored.To further recognize and understand properties of rotational ground motions,torsional component (rotational component around the vertical axis) is studied in thisthesis. A comprehensive and thorough analysis on torsional component is performedregarding the determination method, engineering characteristics and their influences onstructural seismic response. Main contents and results are listed as follows:(1) Present methods for determining torsional component are compared anddiscussed. It is found that the method based on the theory of elastic-plasticmechanics and wave propagation and using translational components recordedat a station to calculate the torsional component is most effective and reliable atpresent. Therefore, an improved frequency domain method is proposed inwhich the effect of S wave and Love wave on torsional component isconsidered. In this method, the effect local site condition is also considered tofurther improve the precision and reliability.(2) Strong motion records from the Tohoku Earthquake are taken as data sourceand336set of torsional components are obtained using the method proposed inthis paper. Engineering characteristics of these records are systematicallystudied and compared with translational components. Study work focuses ontwo aspects:1) traditional characteristics in intensity, frequency and duration and2) non-stationary characteristics which include nonstationarity in intensityand frequency. From results about traditional characteristics, it is found that:1)for peak values, PGAA and PGAV are one magnitude lower than PGA andPGV, but their attenuation pattern are similar. Ratio of PGA and PGV (PGAAand PGAV) shows that RC component has richer distribution at short period.Arias intensity of RC components is much lower than translational components.Parameters of RC and translational components show bad correlation;2) forfrequency, influence of rupture distance and site situation are considered andratios of RC and translational components’ response spectrum parameters arestudied. Results show that peak value of magnified coefficient spectrum ismore sensitive to rupture distance, while attenuation coefficient γ is moreinfluenced by site situation;3) for duration, studies on90%energy duration T90and70%energy duration T70show that parameters and attenuation patterns ofeach component are similar, this similarity is more apparent between RC andOUT component. From results about non-stationary characteristics, it is foundthat:1) for nonstationarity in intensity, analysis results are consistent withgeneral recognition, energy distribution of each components are similar.Meanwhile, due to unique characteristics such as large magnitude, large rupturezone, complex rupture process and long duration, many records of thisearthquake have double peaks. The single-plateau envelope model generallyused doesn’t work very well in this situation;2) instantaneous frequency isused to study the nonstationarity in frequency of each component. RCcomponent shows much more high frequency contents. As well, instantaneousfrequency model get same problem as single-plateau envelope model.(3) Influence of torsional component on seismic response of frame structures withdifferent height is analyzed preliminarily. Internal force of corner column, sidecolumn and inner column is analyzed and compared. It is found that:1) RCcomponent shows highest influence on shear force of side column, then cornercolumn, and lowest on inner column;2) the influence of RC component onmoment of side column is largest and that on inner column is smallest;3) RCcomponent has remarkably influence on axial force, it even cause bigger axialforce than translational components in some situation. As well, influence of RCcomponent on axial force is more remarkable for side and inner columns;4)compared with multi-story (short period) structure, the influence of torsional component is quite larger for high-rise (long period) structures. In some cases,the inner force cause by torsional component is equal or larger than that causedby translation component. It can be concluded that in case of larger magnitudeof earthquake, the corresponding strong ground motions are always rich in highfrequency contents and the amplitude torsional component is relatively large.In this case, it is necessary to consider the effect of torsional component inseismic design of structures.A preliminary study on torsional component is performed in this paper fromaspects of determination method, engineering characteristics and influences onstructural seismic response. The target of this paper is to draw the attention ofresearchers on torsional component. This paper is only the starting point of a seriesof works. With further understanding on torsional component in future study, asystem of theory and seismic method for torsional component is hoped to beproposed.