基于黏弹性边界的地震动输入方法和边界条件选择研究

针对地下工程抗震分析中边界条件和地震动输入方式选择问题,对一地下半无限体模型在一剪切波垂直入射条件下进行不同边界条件和不同地震动输入方法的算例试算。结果发现:两侧采用竖直向位移约束、底部采用黏弹性边界、并在模型底部用等效节点力输入地震动的方法,与三边均采用黏弹性边界条件和三边均采用等效节点荷载输入地震动的方法可以获得同样的结果。该方法易于在ANSYS中通过APDL编程实现,可为地下工程抗震分析中黏弹性边界的使用提供便利。

Earthquake Input Method and Selection of Boundary Conditions Based on Viscoelastic Boundaries

In the seismic design and analysis of underground structures, in order to select the appropriate boundary conditions and earthquake input types, we conduct simulation experiments on an underground semi-infinite body model, which is vertically incident by a shear wave. The main results are as follows: (1) Considering three viscoelastic artificial boundaries (two lateral boundaries and the bottom boundary) and using stress as the seismic motion input, the model can not only simulate the absorption of the scattered waves but also the elastic recovery of the semi-infinite foundation. However, these model conditions are unrealistic with regard to the equivalent nodal loads on both the two lateral boundaries and the bottom boundary. (2) If the two lateral boundaries are viscoelastic artificial boundaries, the bottom boundary is fixed, and the acceleration time history is used as the seismic motion input, the displacements and stresses are overestimated by the model. In addition, the model cannot simulate the propagation effect of the wave in the medium. If we use the displacement time history as the seismic motion input, although the propagation effect of the wave can be simulated in the medium, the displacements and stresses are underestimated and the reflection wave can only be completely absorbed after several shocks. (3) If the bottom boundary is a viscoelastic artificial boundary and the two lateral boundaries are vertical displacement constraint boundaries, exerting nodal loads on the bottom boundary can be obtained with great accuracy. Moreover, the model is simple to use in ANSYS by APDL programming because only the equivalent nodal loads on the bottom boundary must be input. Therefore, it is a simple method for determining viscoelastic artificial boundaries in the anti-seismic analysis of underground engineering.