| Seismic design code is a technique rule for seismic design. Ii is established relaying on current experiences and data and matches science and economical development level. The provision of earthquake action is the most important chapter in various versions of the seismic design code. It is the foundation for seismic verifying and seismic reinforce. And it shows the essential difference between the Seismic design code and other design codes. After evolutive process of earthquake action provision in main countries is summarized, 5 aspects of earthquake action provision area studied on the basis of abundant strong motion records, suggestions for improvements are put forward. The main achievements in this study could be summarized as follows: 1 Shear wave velocity profile data at 254 stations with strong motion records are collected in the western part of the USA. The stations are classified by site condition from the provisions of Code for Seismic Design of Buildings (GB50011-2001). 713 records are also gained on free field, in 1st floor or basement in low (no more than 3 stories) buildings. 132 digital records with longer than 10 seconds noise pre-record are collected in Taiwan ChiChi earthquake to study the reliability of long period response spectra. 402 high-quality records on C, D and E site type are collected for the study of response spectra shape in long period. The records mentioned above and 1220 strong motion records quoted from the results (Wang G. X., 2000) offer a strong foundation for the study. 2 The reliable range of response spectra in long period is studied. The records are classified as analog and digital ones. For the former, the response spectra are reliable in the period range less than 6 second. For latter, in ChiChi earthquake, the long period range is less than 10 second, the ratios between the Fourier spectra from records and the pre-recorded noise are bigger than 90, the fact shows that the effect of noise to record can be ignored. The effect of baseline offset to acceleration response spectra at long period is examined by comparing the response spectra from baseline corrections by EMD and the general ones. The result shows that the effect can be ignored. 3 On the foundation of the study of mechanism and main character about amplitude of response spectra decreasing with period increasing, the response spectra shape in the long period is studied for both analog records and digital records. The result shows that the provision on response spectrum shape in the code of GB50011-2001 is very conservative. The comparison of the response spectra character in long period between analog and digital records show it is feasible to extend response spectra from analog records to longer period. It is pointed out that the response spectrum curve in 6 second cad be extend to 10 second with descent rate of T-1 and the lower limitation must be deleted. 4 That current research and development tendency is summarized shows that it is necessary to provide the maximum amplitude of response spectrum with considering site condition too. The difficulty and emphasis are pointed by comparing the existing research. A direct comparison method in the study is developed. The statistical result from strong motion records shows that the maximum amplitudes of response spectra are varied with site category. The relation of the variety with site classifying criterion and earthquake intensity is built. Referencing provision in 1997 NEHRP and other research, the proposed site coefficients are put forward based on statistical result in the study. In the suggestion, the maximum amplitude of response spectrum is determined from the one in the code of GB50011-2001 multiplying by the proposed site coefficient. The improvement is not only in short period but also in long period. 5 The ratios of vertical to horizontal response spectra are calculated from abundant records. The essential character that ratios varied with period is emphasized. It is found that in general, there is a peak in short period on the ratio curve, then a sag, and a constant in long period. The statistical result shows that the peak is not high, and the sag is shallow on site category I, so that the ratio can be considered as a constant independent of period. The peak is near 1.0, and the period of sag is 0.5 second or so, the constant is just a little more than 0.5 on site category II, III and IV, so it is suggested that the vertical response spectra could be determined by horizontal one multiplying by a spectrum ratio function dependent of period. The simplified spectra ratio function is developed from the analysis of the effect of magnitude, epicenter distance and site category on the ratios. For the analysis of the physics essence of V/H spectra ratio, the character of Fourier spectra ratios are analyzed qualitatively when p-wave, s-wave and Rayleigh wave radiate on different site models. It is found that the soil deposits velocity profile is the control factor to Fourier spectra ratios onthe ground surface. 6 82 strong motion records in rock site are adopted to simulate the envelope curve of ground motion time history. The parameters of the envelope curve are picked up. The PSDF of every stationary random process is calculated. They are used to simulate standard spectrum density function, and pick-up the parameters of PSDF. The simplified method are developed to determine the envelop function parameters andβg,ωc value in PSDF. The attenuation functions of S0 和ωg are fitted. The response spectra parameters from the conversion from the PSDF in the study are compared with the ones directly from attenuation function. It is found that character frequencies and the amplitudes of response spectra are comparative from the two methods. So the suggestion are put forward that the PSDF can be determined from the one converted from design response spectra.
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