基于设定地震确定非基岩场地弹塑性验算输入地震动探讨
Determination of Input Ground Motion at a Non-rock Site for Elastoplastic Analysis Based on a Scenario Earthquake

对非基岩场地的甲类建筑弹塑性验算输入地震动中存在的问题进行讨论,探讨基于设定地震确定弹塑性验算输入地震动的方法。以某设施厂址为例,采用修改后的概率地震危险性公式计算潜源对工程厂址的影响。按照震级(M)-距离(R)-衰减关系标准差系数(ε)组合,对概率地震危险性分析结果进行分解,将三元变量(M,R,ε)的均值或众值计算的反应谱定义为设定地震动,并根据设定地震及其反应谱,选取实际地震动记录近似模拟地震动的离散;采用随机生成的土层模型进行地震响应分析,最终给出土层地表设定地震动的期望值作为输入地震动。
Acceleration time histories have a significant impact on the safety evaluation of key structures because earthquake duration and loading process contribute significantly to uncertainty in structural analysis. Thus, determining acceleration time histories for time history response analysis is a significant practical problem, particularly for non-rock sites. Epsilon (ε) is the number of standard deviations by which the ground motion is above or below the median-predicted motion for the attenuation relationship. This study clarified the role of ε in determining a scenario earthquake to obtain acceleration time histories. As an example, the seismic hazard at a real site is disaggregated into its contributions from discrete variables (M, R, ε) to determine a scenario earthquake. M is the surface wave magnitude and R is the projected epicentral distance along the minor axis of the equivalent ellipse. The target peak ground acceleration (PGA) and a consistent spectrum for the rock site with a 2% probability of exceedance in 50 years using probabilistic seismic hazard analysis were obtained. As the earthquake ground motion at the site increased, the number of potential seismic sources contributing to the probability of exceedance decreased. The fifth potential seismic source dominated the seismic hazard at the real site, given that the target probability of exceedance in 50 years is 2%, so the scenario earthquake was located in this potential seismic source. A sample space formed of M, R, and ε that may generate a PGA greater than or equal to the target PGA at the site was constructed. Thus, the normalized probability of the exceedance of the target PGA is the joint distribution of M, R, and ε. The mean and mode of M, R, and ε are the expected and the most likely event in the sample space, respectively. As the site is located near the geometrical center of the fifth potential seismic source, high-magnitude, near-field seismic events are a major contribution to the seismic hazard at the site. The predictive PGA of the mean and mode of M, R, and ε were computed using an attenuation relationship:the values are significantly larger than those of the target PGA. The difference between the mean/mode response spectra of M, R, and the target spectrum is obvious, especially for the acceleration response at low