A structure which is dynamically modeled or tested in an unconstrained configuration contains up to six rigid body modes of vibration. Finite Element Analysis methods can easily compute these modes. However, when using experimental Modal Analysis to characterize the dynamic properties of a structure, the rigid body modes of vibration are typically not measured. Since these modes contain the inertial information of the structure, they are essential when joining two unconstrained substructures together, or to accurately reconstruct a Frequency Response Function (FRF). This paper describes a method for estimating these rigid body modes using only the structural geometry and an estimate of the lumped mass distribution of the structure. The results of a computer program which was developed to estimate the rigid body mode shapes will be presented along with test cases to demonstrate the use of the method.
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