This paper investigates the effects of adjusting the system matrices of a Finite Element (FE) model on the eigen-properties of the model for the purposes of dynamic model updating. It is shown that minor modifications to the connectivity of the model can cause unexpectedly significant eigenvalue perturbations of the lower modes, especially if the physical location of the modification is near an antinode of vibration of a lower mode. These modifications can be introduced either by non-parametric updating techniques or unwittingly by truncation of the FE structural matrices. It is proposed that model updating techniques specifically avoid introducing such changes to the model either directly or indirectly. A mathematical bound has been established which gives a limit on eigenvalue perturbations. From this bound it has been deduced that a definable degree of computational precision is necessary for FE analysis for the purposes of model updating.

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