Calculation of Dynamic Impedance of Foundations Using Finite Element Procedures

30 March 2015


The analysis and design of foundations under dynamic loads due machinery is routinely conducted in industrial projects. Determination of the dynamic impedance of the foundation is required for performing such analysis and design. Different tools are available for the calculation of foundation impedances including: published closed form solutions, charts, tables and specialized computer codes.

Despite the convenience of these tools, their use for production work is cumbersome. The main reason being that, in its simplest form, a dynamic impedance is provided as a frequency dependent complex function or as a spring-dashpot system, with the spring becoming negative for certain frequencies; which cannot be directly implemented in standard structural analysis codes. The use of such impedances requires a clear understanding of the theory behind their calculation along with several principles of soil dynamic, which are not covered in the regular curriculum of structural engineering programs.

This paper aims to fill this gap by providing structural engineers with the basic tools for the understanding, calculation and use of foundation impedance functions. For this purpose, numerical examples are provided to illustrate the application of the approaches discussed in this paper for the calculation and application of dynamic impedances.

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Carlos A. Coronado, Bechtel
Neha Gidwani, Bechtel