9 Vibration-Based Approach for Identifying Closely Spaced Modes in Space. . . 87 Fig . 9. 4 Stabilization diagrams of the identified modal parameters: (a ) using 300 number of block rows and model order of 800, (b) 50 block rows is used and a model order of 100 Torsion (93.30 Hz, num.: 92.40 Hz) Lateral bending (126.42 Hz, num.: 126.12 Hz) Vertical bending (139.41 Hz, num.: 143.59 Hz) Vertical bending (185.04 Hz, num.: 191.20 Hz) Fig . 9. 5 Identified global mode shapes and corresponding frequencies Figure 9.5 shows the first four identified global mode shapes and respective natural frequencies that show good agreement with the numerical results. These mode shapes were obtained by using data that was exclusively measured at the structural nodes. From the tests with instrumented truss members, also the local behavior related to these modes could be identified well as indicated in Figs. 9.6 and 9.7. In this study, the data acquired with sensors installed at the connection nodes and those that were placed on selected truss elements were analyzed separately. This procedure made the modal identification in this case by far more feasible compared to a combined analysis including a mixed instrumentation both on the connection nodes and on truss members. Close natural frequencies of modes related to global and local vibrations made this approach necessary. 9.6 Conclusion For identifying global and local modal parameters of a space frame structure, numerical modeling is vital to have information about the expected frequencies and mode shapes. Additionally, the calculation of modal strain energies helps in the distinction between global and local vibrations. Moreover, to select relevant degrees of freedom for identifying the mode shapes of
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