Statistic analyses on the vertical to horizontal acceleration response spectra ratios of Sichuan-Yunnan-Gansu-Shaanxi region in the North-South Seismic Belt

Based on 802 groups of strong ground motions collected from 263 sites in Sichuan, Yunnan, Gansu and Shaanxi Provinces, this paper analyzes the shape of the vertical to horizontal acceleration response spectra ratios on the condition of different site conditions, magnitudes and epicentral distances, as well as corresponding average value of the spectra ratio in different period intervals. And then the average spectral ratio values are compared with the standard value 0.65 specified in the Code for Seismic Design of Buildings （GB50011−2010）. The results show that the ratios of vertical to horizontal acceleration response spectra are affected by the factors such as site, magnitude and epicentral distance. The spectral ratio of class Ⅰ and class Ⅱ sites in the whole period is basically greater than 0.65. Whatever the grouping situation, the mean value of spectral ratio in the period of 0.1−1.0 s is lower than 0.65. In the period larger than 1.0 s, the spectral ratio is basically much higher than 0.65. Therefore, it is worth discussing that the ratio of vertical to horizontal seismic influence coefficients is set as 0.65 in most of the codes for seismic design of building. It is suggested that the vertical seismic action of the existing vertical strong motion records should be determined according to the calibration method of the horizontal response spectrum.