Further Study Of Two Issues On Site Coefficent In Seismic Code

In order to improve stipulation of the design response spectra in seismic design codes, this dissertation selected appropriate seismograms form NGA-West2 database as the basis of data, and analyze the site coefficients recommended by the new seismic zoning map(GB18306-2015) through descriptive statistic of the horizontal average spectra for every site. And then, on the basis of statistical analysis of the vertical average spectra for every site, with aid of the characteristics of vertical-to-horizontal spectral ratio, some suggestion are made for regulation of vertical design spectra in seismic codes.As the data base of this dissertation, the shear-wave velocity profiles of 732 boreholes are selected from Ki K-net and the databases of strong-motion observation sites in California, while 8870 sets of records containing both vertical and horizontal response spectra are adopted from NGA-West2 database.In total, the earthquake magnitude of all the selected records ranges from 4.2 to 7.9 while epicentral distances from 0.44 to 1162.5 km, the average shear wave velocity of 30 m depth(Vs30) from 99.9 to 2016.1 m/s, horizontal PGA from 0.02 to 1.77 g, and vertical PGA from 0.003 to 3.84 g.Based on the velocity structure, Vs30 and Vs30 are calculated for each site and then used for site classification according to Chinese and American codes. Then, on the basis of a recently published method that uses the average shear wave velocities of two arbitrary depths to estimate Vs30, accompanying with sites data and classification results, this paper puts forward an approach to evaluate Vs20 and the overburden thickness by Vs30 and then establish the congruent relationship between Vs30 and site classification method in Chinese code. Furthermore, the feasibility of this approach is researched by comparison of site classification of 732 sites according to this method and the actual velocity data, more than 80%. By using this method, the site classification of strong ground motion data is converted into the category of site classification according to Chinese seismic design code.Magnitudes and distances are grouped according to the CB13 ground motion predication equation built in NGA program, for better analyzing the horizontal average spectra of each group for each site classification, in order to reveal the rule of variation of response spectra and spectral ratio with PGA, period and site classification. Further,site coefficients regulated in the new seismic zoning map are within the ranges of spectral ratio discussed in this study, but show two differences, namely, monotone variation with PGA and the relative value between site Ⅳ and site Ⅲ.For the vertical direction, the variation of average spectra is investigated in the same way as the horizontal ones. Then, this paper compares the horizontal and vertical design spectra varying with amplitude and eigenperiod with the calculated spectral ratio, for argument of the capacity of vertical spectra having different eigenperiods from the horizontal counterpart. Afterwards, by comparing the characteristics of vertical spectra between each site classifications, the requirements of the maximum spectral ratio and eigenperiod are made to propose a suggestion on stipulation of vertical design spectra. Finally, further validation is made to demonstrate the reliability of this suggestion, by comparison of the new constructed spectra with the average spectra with corresponding group and site classification.