Use of Simple Model for Dynamic Foundation Design

The dynamic response of a structure can be significantly influenced by the motion of the foundation and the under lying soil, increasing both the flexibility and damping of the system. These additional degrees of freedom can be included into a simple analysis of a structure using widely available stiffness and damping coefficients. This investigation compared the prediction of the response of a structure resting on an elastic soil layer using two models; a simple three degree of freedom model and a more complex finite difference program, using transfer functions as a means of comparison. It was found that the simple model closely predicted the first mode frequency and damping found in the complex model, but did not predict the deviation in response observed in the finite difference program. These differences were more significant for softer soils were the influence of the foundation was greater on the response of the structure. The deviation between the models was attributed to frequency dependence of the foundation coefficients and to the effect of the shallow soil layer causing shear waves to reflect back to the surface from the base rock.