| The design response spectra in current Chinese seismic design code are definedto represent an statistically average result of many horizontal ground accelerationresponse spectra, therefore the characteristic periods of two horizontal groundacceleration response spectra are assumed to be equal, the value is determined by sitecategory and seismic environment grouping, and the maximum horizontal seismiceffect coefficient is determined by design intensity indicated in seismic zonation.Whereas the corresponding vertical design response spectra are usually assumed thatthe horizontal design response spectra times 2/3,the characteristic period of verticaldesign response spectra is taken same as that of horizontal component. Howeveractually an statistically average ground acceleration response spectra involve a set ofrandom factors, the relation among three components of the ground motion aresignificantly complex, therefore the real state of three components cannot bereasonably well represented by only one direction design response spectra. Based onabundant acceleration records in western America, a study on difference among threecomponents is analyzed. Several parameters among three components are comparedincluding characteristic period, ratio of vertical ground acceleration response spectrato the corresponding horizontal component, maximum seismic effect coefficient anddynamic coefficient. In addition, effects of several factors such as seismicmagnitude,epicentral distance and local site conditions on these parameters arediscussed. Some conclusions are proposed to be used in modification of currentdesign response spectra. The results would be a means to improve seismic design ofstructure. Moreover, a stochastic method is introduced for simulating strong ground motion.This method is based on the random process theory. Results of analyses indicate thatartificial acceleration time history obtained by stochastic method are adapted to beused in engineering practice.
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