瑞利阻尼对砂土场地非线性地震响应的影响分析

在地震荷载作用下,自由场地会产生土体侧向变形和地表响应放大现象。由于土体的高度非线性,计算自由场地地震响应时,不同的阻尼比及剪切模量取值是造成其计算结果与试验结果相差较大的原因之一。目前动力计算常采用瑞利阻尼方法,其系数取值会在一定程度上影响计算结果。选用两模态简化瑞利阻尼系数计算方法,分析土体阻尼比及控制频率的取值对计算结果的影响,对比离心机模型试验,利用开源有限元平台OpenSees,采用适合于土体动力分析的多屈服面本构模型(PDMY),建立剪切梁模型模拟三维自由场地,并分析瑞利阻尼参数对自由场地地震响应和侧向变形计算结果的影响。结果表明,针对相对密度为60%的Nevada干砂,阻尼比为4%、控制频率比为5时,场地响应计算结果与试验结果较为符合。综合分析显示场地非线性响应时域计算时,应特别注意选用的瑞利阻尼参数值。

Influence of Rayleigh Damping on Nonlinear Seismic Responses of Sand Sites

Under seismic loads, some phenomena will occur in free field, including lateral deformation of soil and ground response amplification. When evaluating the seismic response of free field, it is the highly nonlinear characteristics of soil that significantly influences the results obtained when utilizing differing damping ratio and shear modulus values and is one of the reasons that experimental results and calculated results can largely differ. Currently, the Rayleigh damping method is most commonly used for site seismic dynamic response analysis. Based on a simplified version of this method, this study analyzes the influence of different values of soil damping ratio and control frequency on calculated results. Using the finite element platform OpenSees, we have adopted the PDMY constitutive model fitted for soil dynamic analysis and established a one-dimensional shear beam model to simulate the three-dimensional free field. Compared with centrifuge model test results, this study analyzed the effects of the Rayleigh damping on the site response and lateral deformation of free field. In the case of Nevada dry sand (relative density 60%), there is good agreement between the calculated results and the test results when the damping ratio and control frequency ratio are set as 4% and 5%, respectively. Our comprehensive analysis suggests that special attention should be paid to value selection of Rayleigh damping in the time domain calculations of site nonlinear response.