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 ofεin 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.Mis the surface wave magnitude and Ris 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,andεthat may generate a PGAgreater 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,andε.The mean and mode of M,R,andεare 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 PGAof the mean and mode of M,R,andεwere 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 natural frequencies.For the computed response spectrum of the scenario earthquake fitted to the target PGAand consistent spectrum,the sample space was adapted so that the PGAof M,R,andεcomputed from the attenuation relationship was approximately the same as the target PGA.Strong ground motion records were obtained from the NGA database based on the scenario earthquake for use in simulating aleatory uncertainty in rock ground motion.Stochastically generated soil profiles were used to investigate the uncertainty of the dynamic characteristics of soil and shear-wave velocity testing results.The strong ground motion records were combined with the soil profiles to create input files that were used to perform an equivalent linear site response analysis,which included an assessment of uncertainty in the amplification factor.Here,the amplification factor is the ratio between the response spectrum of soil surface acceleration and that of rock acceleration time histories.Thus,the distribution of the amplification factor of the spectrum was obtained.The response spectrum of the scenario earthquake was multiplied by the estimated amplification factor to act as the soil surface acceleration response spectrum.Although the scenario earthquake is neither the expected nor most probable event,its seismic influence field at the site exceeds the target PGA.By taking into account both the target PGAand consistent spectrum,in which all of the events in the fifth potential seismic source will generate ground motion at the site,the safety of important structures can be achieved.