Fictitious masses are used to improve the accuracy and efficiency of modal-based structural analyses which involve substructure synthesis, local excitation and local structural changes. New formulations, which allow easy applications in various subsequent analyses, are given for two categories of fictitious masses, "regular" and "very large". Regular masses are of the order of entire substructures, while very-large ones are several orders of magnitude larger. The regular fictitious masses are added to selected coordinates of the finite-element model for normal-mode analysis, and then removed in a way that produces modes with local deformations near the selected points, in addition to the nominal natural vibration modes. Subsequent analyses of local nature can then be performed inthe standard way. very-large masses are used to generate static constraint modes for fixed-boundary modal coupling, and "broken" modes for representing rigid-body relative motion between structural segments with application to loads analysis. The paper facilitates the inclusion of fictitious masses as optional elements in standard structurla dynamci procedures.
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