刚度和阻尼频率相关的等效线性化方法

Equivalent linear method considering frequency dependent stiffness and damping

在实际工程中发现,在强地震动作用下等效线性化方法低估了地表地震动的峰值加速度反应。通过与场地效应竖向台阵观测记录的对比分析证实了这一现象,其主要原因是等效剪应变水平的取值高于高频段实际剪应变水平,从而抑制了土层地震反应中的高频成份。若干分析表明,动力反应过程中土体的剪应变是随频率而变化的,且试验表明土体的刚度和阻尼是与剪应变相关的,据此假设土体刚度和阻尼是与频率相关的。基于这一假设,对传统等效剪应变的选取如何使得土层对基底地震动输入中的高频成分产生滤波效应的原理进行探讨,且通过频率相关的等效线性化方法对该原理进行证明。将该方法应用于PortIsland场地效应竖向台阵的分析,并与实际地震动记录进行了对比,结果表明,较之传统等效线性化方法,能更加合理地考虑地震动输入中的高频成分。

It is found that the conventional equivalent linear method underestimates the peak ground acceleration of the soil layer earthquake response under the strong earthquake input in practice. The phenomenon is proved by calculating and analyzing the record data from the site effect borehole station array. The reason is that the artificial value of the equivalent shear strain is quite larger than the real shear strain in high frequency band and it reduces the contribution of the high frequency components of the site soil layer earthquake response. Studies show that the shear strain changes with the frequency; the shear strain-dependent shear modulus and damping was proved in the experiments; so the shear modulus and damping are supposed to be frequency-dependent. Based on this assumption, how the high frequency component of the earthquake input motion is filtered by the soil layer caused by the equivalent shear strain reduction factor is discussed. Meanwhile, the frequency-dependent stiffness and damping are applied to the equivalent linear method and the feasibility is discussed by numerical calculation. The accuracy of the proposed method is examined by comparing with the record borehole data from large earthquakes, for example, the Port Island borehole record data in Hanshin Earthquake of 1995. The proposed method always gives much better results than the conventional equivalent linear method; so that the high freauencv comoonent can be reasonablv considered.