In this paper, we propose a procedure for predicting the modal data of a thin plate structure with a rib stiffener even when the rib is attached at a location where mode shapes are not measured. This procedure is based on the structural dynamics modification (SDM) method in which the mode shape expansion (MSE) method is incorporated to estimate the experimental mode shapes at the unmeasured points. In the MSE estimation, a submodel including the possible rib attachment points is used instead of

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... the whole structure model. Numerical examples through the plate structure with the two L-shaped ribs show that the proposed procedure accurately predicts the modal data using the estimated mode shapes at the unmeasured points. The mode shapes are estimated satisfactorily with the submodel under a free boundary condition. The optimal number of analytical modes to be combined in the MSE estimation can be determined from the condition number of the modal matrix of the submodel.