Real structures and vibrational modes

Structures of interest in the modal test laboratory are, of course, more complex than the one, two, and three degree-of-freedom structures (Vibration and DOFs, modes, and mode shapes). A diving board, for example, is really an infinite string of single lumped-mass structures. Accordingly, it can be deflected at an infinite number of points by an infinite number of natural frequencies. In other words, a real structure possesses an unlimited number of vibrational modes.

In order to simplify real structures, one must look at them as if they were made up of a finite number of points. Such points on a structure under investigation are referred to as nodes and tend to behave very much like hypothetical lumped-mass structures. Here is an example of an airplane characterized by 30 nodes.

Each node can have up to 6 DOFs:

• Displacement in X direction

• Displacement in Y direction

• Displacement in Z direction

• Rotation in X direction

• Rotation in Y direction

• Rotation in Z direction

Thus, this example has 30 X 6 = 180 DOFs and for each DOF, there is one mode shape and one natural (or resonance) frequency.

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Real structures and vibrational modes, Simcenter 3D 2021.1 Series

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