Axisymmetric time-domain transmitting boundaries

Finite element analysis of dynamic foundation problems requires the use of transmitting boundaries to model the radiation of waves from the finite element mesh into the far field. Problems involving inelastic behavior of the soil in the near field are most readily solved in the time domain. The standard viscous boundary is widely used in such situations. However, in axisymmetric situations this boundary is inappropriate. This paper develops plane strain boundary equations for axisymmetric shear and dilation waves based on an approximation of the form of the outward traveling waves. These boundary equations are shown to be equivalent to mechanical systems with frequency independent components. The complex stiffnesses of the new boundaries are compared with the equivalent viscous and plane strain boundary stiffnesses, and the new boundaries are found to agree closely with the plane strain boundaries. The response of an extended axisymmetric finite element mesh subjected to a transient force of the type generated by pile hammers is computed and compared to the responses of the same mesh truncated with various transmitting boundaries.