Measurements on a large rotor dynamic test rig in the Turbomachinery Laboratory with a concrete and steel foundation have shown some critical speeds which can only be predicted if the foundation is included in the model. Mobility transfer function measurements on the foundation were made to obtain the required parameters for including the foundation in the rotordynamic computer model. These measurements eliminate the necessity for building a large finite element model of the foundation structure and allow the use of fast transfer matrix codes for modeling rotor bearing systems which have strong foundation participation. A method first described by Nicholas was used to get a set of equivalent bearing coefficients from a combination of the foundation parameters with the true bearing coefficients. Measurements made by the present authors show that the modal damping of the foundation can be obtained easily from the mobility curves, and that the improved critical speed predictions can be obtained in some cases by using only one foundation mode to calculate the equivalent bearing coefficients.

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