场地放大系数概化模型研究

A large number of strong earthquake records show that the amplification effect of the overburden soil layers on the ground motion has obvious spatial variation characteristics, hence it is necessary to adjust the seismic design parameters according to the site types. The site factors of current seismic ground motion parameters zonation map of China are obviously lower than the relevant standards in Europe and America, In addition, available domestic and foreign codes only give a set of determined adjustment coefficients for different site classes, which cannot reflect the local site effect uncertainties. Based on the theoretical solution of soil layer amplification in the frequency domain, this project intend to carry out an in-depth study on the influence of shear wave velocity and burial depth of bedrock on site effect and put forward the corresponding correction method, thus guide the analysis of KiK-net strong earthquake records and provide the site-specific code-oriented coefficients. Random numerical simulation of site seismic response will be carried out by considering the varieties of input ground motion and shear modulus and damping ratio to explain the uncertainty source of amplification effect and its transmission mechanism. Then the means and standard deviations of amplification coefficients on each site will be summarized on the combination with strong ground motion analysis results, following the correlation between them and the site characteristics to be quantified. Finally, a probabilistic model of amplification coefficients of different site types will be established and a multi-parameter prediction approach of site effect to be proposed. The objective of this project is to serve for providing more reasonable and probabilistic ground motion parameters for seismic fortification of engineering structures in the performance-based seismic design framework.

大量强震观测数据表明，覆盖土层对地震动的放大作用具有明显的空间变异特征，根据场地类别对设计地震动参数进行调整是客观需要。我国现行区划图场地系数明显低于欧美等地区相关标准，且现有国内外规范都只针对不同场地类别给出了一套确定的调整系数，无法体现局部场地效应不确定性。项目基于土层放大频域理论解答就基岩剪切波速和埋深对场地效应影响展开深入研究并提出相应的修正方法，指导KiK-net强震记录分析并给出面向规范的场地系数。考虑输入地震动以及剪切波速和模量阻尼比不确定性开展场地地震反应随机数值模拟，阐明场地放大效应不确定性来源并揭示其传递机制，结合强震记录分析结果给出各场地放大系数均值和标准差，量化其与场地特征指标之间的相关性，建立不同类别场地放大系数概化模型并提出场地效应多参数预测方法，在基于性态抗震设计理论框架内为工程结构抗震设防提供更加合理的、具有概率意义的设计地震动参数服务。

场地系数是各个国家和地区规范中考虑地震动场地效应的关键指标，决定着各类工程结构设防水准，我国现行第五代区划图第一次明确提出需要根据场地类别对地震动峰值加速度进行调整，但其中场地系数建议取值显著小于欧美等地区相关标准。采用土层-基岩理想场地模型，导出了土层场地与参考基岩场地地震动响应定量关系解析表达。提出了场地放大系数一般规律，并通过数值模拟验证了其可靠性，指出土层场地地震动响应与参考基岩场地相比，在全频域空间恒定表现为放大，我国规范中软场地系数取值严重偏于保守。基本周期是表征场地动力特性的最根本属性，项目基于一维地震响应理论解答提取阻抗比和周期比作为表征场地周期变化规律的基本特征量，提出了子层周期贡献系数的概念，建立了周期贡献系数与阻抗比和周期比一般关系。采用子层周期加权累加方式形成一套简单实用且具有明确物理意义的成层场地基本周期估算方法。与传递函数法对比结果表明传统法无法反映土层结构对场地周期的影响，其偏差可达40%，但新提出的简化算法在极端情况下最大偏差也不超过8%，可以在实际工程中推广应用，也可在此基础上进一步开展场放大效应与其地基本周期之间的相关性研究。采用一维等效线性化程序开展了土层地震反应随机数值模拟，研究了输入地震动不确定性对场地放大系数的影响。共选取了179次地震中记录到的310条加速度记录，将其峰值分别调幅至5、20、50、100、150和200gal作为输入。结果表明在不同强度地震动作用下，土体动剪应变中值分别7×10-6、3×10-5、7×10-5、1×10-4、2×10-4和2.5×10-4。随着输入地震动强度的增大，地表输出的峰值加速度标准差逐渐增大，但放大系数标准差逐渐减小。

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