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It depends what you think should be the frequency of the external force, which will cause it to vibration. If the frequency of the external force is overlapping with any of the natural frequencies, then the structure resonates and you have high amplitude of vibration, which can catastrophically damage the structure.


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Assuming that the model is adequate and appropriate, then to answer your second question: Which frequency out of those five is right? All of those modes and frequencies are possible in the structure. Usually the frequencies are provided in increasing order (i.e. the numerical smallest is presented first etc). The frequency that is usually more important, ...


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