场地放大系数的理论解答

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,但国内外规范就此存在显著差别。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,在此基础上对国内外规范场地系数的合理性进行检验。结果表明: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,场地放大系数随场地变软整体表现出增大趋势,这与美国新规范场地系数取值和规律一致;中国规范中软场地系数取值严重偏于保守,强烈地震动作用下软场地系数小于1的现象不合理,中国规范Ⅳ类场地系数普遍小于Ⅲ类的取值方案有待商榷。

Theoretical solution of site amplification coefficient

The site coefficient is a key index for considering the seismic site effects in various national and regional standards, which determines the criterion of earthquake-resistant design, but there exist significant differences between domestic and foreign standards. Based on the ideal soil-bedrock site model, an analytical expression for the quantitative relationship between soil site and reference bedrock is derived and examined through numerical simulation. The general rules of site amplification coefficient are put forward, and the reliability of different recommended values are verified. The results indicate that the soil ground motion is always amplified in the whole frequency domain compared to the reference bedrock. The site amplification coefficients show the overall increasing trend with the softening of the site, and are consistent with the suggested values in the new NEHRP provisions of the United States of America. The coefficient of soft site in China is seriously conservative and the value is less than 1.0 under strong vibration, which is clearly unreasonable. It is debatable whether the site coefficient of class site IV should be generally less than that of class site III in China's code.