This paper deals with a method to estimate unmeasured rotational degrees of freedom mode shapes of a test structure as a linear combination of those of the corresponding Finite Element (FE) model. In this method, weighting coefficients for each mode shape combined are determined by comparing experimentally measured transnational degrees of freedom mode shapes with analytical mode shapes of the FE model. The accuracy of estimates is strongly dependent on both the number of mode shapes combined and measuring points for transnational degrees of freedom. First, the number of mode shapes combined is discussed. It is found that the number is different for each mode shape to be estimated and is predicted by the Modal Assurance Criterion (MAC) value between measured and analytical transnational degrees of freedom mode shapes. Second, a technique is proposed for overcoming a problem with regard to measuring points, which often causes difficulty in estimation because of the ill condition of the modal matrix of the FE model in an inverse problem. This technique incorporates the above method with the estimation method using the Modal Scale Factor (MSF) to compensate for the ill condition. A numerical example using a cantilever beam model is presented and discussed.
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