How does one find the dominant natural frequencies?
The fact is, modal analysis on its own will not tell. Because analysis does not provide an input excitation, it cannot reveal which modes will be excited. Modal analysis is really a first and vital step to understanding the response of a structure. The next step is to apply an input excitation in a response analysis, or make some assumptions about the input. Response analysis is beyond the scope of this article. In the meantime, we have to rely on engineering judgment. Suppose analysis shows that a diving board has a first natural frequency of 5 Hz with a bending mode shape. That can be a basic design input. The response for the cup holder may include bending and twisting of the attachment arms and the cradle. To see what is important may require comparing test data, or doing further response analysis. But the first few modes of a structure are vital to understanding its dynamic characteristics. Consider the two examples. The diving board shows a mesh of shell elements and The cup hold
