荷载频率对动模量阻尼比影响的试验研究

目前荷载频率对动模量阻尼比影响规律尚无较为统一认识,定量结果尚少。采用新型高精度动三轴仪试验,研究不同荷载频率对典型砂土和黏土动模量阻尼比影响问题,并以此为基础从对地震动影响角度讨论考虑荷载频率相关动模量阻尼比必要性。结果表明:对砂土而言,荷载频率对其动剪切模量影响不大,随着加载频率的增大,阻尼比略有减小,但差别基本可以忽略;对黏土而言,荷载频率对其动剪切模量和阻尼比有重要影响,随着振动频率的增加,动剪切模量增大,阻尼比减小;黏土参考剪应变随荷载频率增大而增大,二者呈递增的指数函数,当f≤1 Hz时,影响十分明显,当1 Hzf≤3 Hz时,有一定影响,当f3Hz时,影响不明显;黏土最大阻尼比随频率的增大而减小,二者呈递减的指数函数,当f≤10 Hz时,影响较为明显,当f10 Hz时,影响程度显著减弱;采用两组频率下模量阻尼比曲线计算黏土层地震动,地表加速度峰值和反应谱差别很大,且随着震动增强而显著增大,说明考虑黏土层动模量阻尼比的荷载频率相关性是十分必要的。

Experimental study on influences of different loading frequencies on dynamic modulus and damping ratio

There is not a consistent conclusion on the influences of loading frequency on dynamic modulus and damping ratio, and the quantitative results are few. Through a series of tests using the new cyclic triaxial (CT) divice with high precision, the main objective of which is to evaluate the influences of excitation frequency on the shear modulus and damping ratio of typical sand and clay samples are studied. The necessity of considering the frequency-dependent modulus and damping ratio is discussed from the perspective of ground motion on the basis of the tests. The results show that: (1) For the sand, the shear modulus is almost independent of the loading frequency. The damping ratio slightly decreases due to the increase of vibration frequency, but the deviations can be ignored. (2) For the clay soil, the loading frequency has significant influences on its shear modulus and damping ratio. The shear modulus increases, but the damping ratio decreases with the increase of the loading frequency. (3) The CT tests indicate measurable growth in the reference shear strain γ_r with the loading frequency in the form of an increasing exponential function. The dynamic properties are very sensitive to the excitation frequency when f ≤ 1 Hz. In the range of 1 Hz max decreases with the vibration frequency in a form of exponential function. When the frequency is lower than 10 Hz, the influences are obvious. If f>10 Hz, the influences gradually vanish. (5) The ground motion of a simplified ideal clay site is calculated using two sets of shear modulus and damping ratio curves under two different test loading frequencies, 0.1 Hz and 3 Hz. The peak ground accelerations and the response spectra show serious dissimilarities, which are increasingly severe as the intensity of ground shaking is enhanced. This study therefore demonstrates that considering the frequency correlation of shear modulus and damping ratio is of extreme necessity.