地震动时-频特性对土层场地地震反应的影响

地震动非平稳特性对土层反应有重要影响。为了能够近似定量控制基岩地震动输入特性，文章首先基于时-频包线函数建立能够表征地震动频率非平稳特性的地震动拟合方法；然后选择3条主频率变化不同的加速度记录，引入两种不同的包线函数分别模拟地震动强度或频率非平稳特性，设计了用于土层地震反应分析的三种输入方案，拟合完成42条加速度时程；最后，采用非线性场地反应计算方法对三类土体模型进行计算，分析基岩地震动频率非平稳性对地表PGA和加速度反应谱的影响。结果表明：①文章基于时-频包线函数建立的地震动拟合方法能够近似定量控制目标地震动强度与频率非平稳特性，可用于基岩地震动输入的拟合；②当基岩地震动主频率变化显著时，不考虑频率非平稳则存在低估地表地震反应的风险，且场地越硬、基岩峰值越大，低估程度越显著；③地震动频率时变特征对土层地表反应谱高频部分(2～5Hz)的影响与地震动高频成分集中的时间和土层性质相关：当基岩地震动高频成分集中在强震段时，采用仅考虑强度非平稳的拟合方法可在较硬场地得到相对保守的结果；④地震动频率随时间的变化对反应谱放大系数的影响与场地类型和反应谱周期有关，当基岩峰值较大时，对较硬场地5Hz左右的频率成分影响更大，但对三、四类场地条件，则对1Hz左右的土层反应影响更大。因此在拟合基岩地震动时，特别是输入地震动的峰值较大时，应结合结构自振周期、场地条件、工程需要等因素，考虑基岩地震动输入的频率随时间的变化特征，采用全非平稳方法进行人工地震动模拟。文章的研究成果对工程实践中合理设置基岩地震动模拟参数具有重要的指导意义。

Study of effects of time-frequency characteristics of bedrock acceleration on site response analysis

The non-stationarities of ground motion may significantly affect the results of site response analysis. To approximately and quantitatively control the ground motion characteristics of input bedrock-acceleration, a ground-motion simulation method representing the target non-stationarities is firstly established based on the time-frequency modulating function. Then, three bedrock-motion records with different predominant-frequency characteristics are selected, and two forms of modulating function are introduced for simulating temporal and spectral non-stationarities, respectively. Based on those, three input bedrock-motion scenarios with different time-frequency characteristics are designed for site response analysis, 42 artificial ground-motions are simulated. Finally, nonlinear site response analysis is performed on three types of sites to investigate the effects of spectral non-stationarity of bedrock motion on the surface peak ground acceleration (PGA_surf) and spectral acceleration (SA_surf). The results show: ①The proposed ground-motion simulation method based on the time-frequency modulating function is competent for simulating ground motion with well-controlled temporal and spectral non-stationarities simultaneously. ②For bedrock motion with significantly time-varying frequencies, ignoring spectral non-stationarities in ground-motion simulation may underestimate the hazard on ground surface. The harder thesoil, the higher PGArock is, and the more remarkable the underestimation shall be. ③The influence of time-varying frequency on the high-frequency parts of SAsurf correlates with the concentrated time of high-frequency contents and characteristics of site: if the high-frequency contents of bedrock motion concentrates at the strong-motion phase, SAsurf obtained from the temporal-nonstationary-only artificial motions shall be more conservative for on relatively hard site. ④The influence of time-varying frequency on surface/bedrock response spectral ratio (RSR) correlates with the site classification and natural vibration frequency (f) of structure. If PGArock is high, the influence is more distinct at about 5Hz for relatively hard site; while, on relatively soft sites of classification III or IV, the influence is more distinct at about 1Hz. Therefore, when simulating bedrock motions, especially those with high peak values, one shall use fully nonstationary method in combination with the factors such as natural vibration frequency of structure, site condition, and requirement of specific project, to consider the spectral nonstationary characteristics. The research results proposed here are constructive for setting reasonable parameters to simulate bedrock motion.