Study On The Influence Of Site Classification To Design Ground Motion Parameters In Loess Area

With the rapid development of society and economy of western region, peopleare paying closer attention to the seismic works. According to the domestic andforeign study results and the practice experiece of aseismic engineering, the sitecondition has a great influence on the ground motion parameter. Especially in thewestern region of the Loess Plateau, the earthquake diasaters in loess areas aregenerally more severe than the non-loess areas due to the specific characteristics ofloess. At present, based on Code for Seismic Design Buildings (GB50011-2010), therelationship between the the characteristic period and the site classification has beconsidered by the seismic design, while the maximum earthquake affectingcoefficient takes the same value and irrespctive of the site classification. According toa great deal of practical earthquake damage experience, the earthquake damage in thesame earthquake intensity area may differ greatly depending on the site classification.In view of the above considerations, taking the loess area as the research background,this article carried out the following research work based on the previous researchwork.1. The site characteristics in the loess areas (Site ClassⅠ, ClassⅡ and ClassⅢ)has been summerized in the light of the site distribution, the topography and thethickness of covering layer,etc. By the site class I, the covering layer is relatively thinand distributes mostly in the Loess hilly and gully region and the river terrace areas.By the site class II and III, the covering layer has large thickness and distributes in theLoess Plateau, the Loess hilly and gully region and the river terrace. Therefore, it canbe seen that the different site types differed greatly with the characteristics of the threekinds of sites.2. The characteristic of earthquake records are analysed according to the changesof the peak ground acceleration(PGA) with epicenter distance, the spectrum ratio ofPGA of horizontal component and vertical component, and the change of responsespectra. The study shows that the PGA tends to decrease with the increasing ofdistance, the value of spectrum ratio of1-10Hz were significantly lower than the value of1-10Hz on both sides, and the response spectrum are different at different siteand with different magnitude.3. On the basis of dozens of collected borehole data, additional computingmodels of the site section has been constructed, there are153calculation models intotal. The bedrock ground motion input waves has been synthesized with sevendifferent intensities, and other relevant calculation parameter has been determined.4. The selected profile has been calculated by using the one-dimensionalcalculation program, and the influence of the thickness of covering loess layer on theground motion parameters has been analyzed for different types of site. As the coverthickness increases, the PGA, the site amplification and the acceleration responsespectrum at different site types has changed in different degrees.5. Through the comparative analysis of calculation results for the site of varioustypes, the site coefficient of maximum seismic influence coefficient have been gave inloess area, which can be expected to provide reference to the development of relevantnorms and specification.