Is it possible to display natural frequencies separately for each direction?

Answer

Spatial models with several directions selected in the RF‑/DYNAM Pro - Natural Vibrations add-on module display no separate results of mode shapes for the individual directions. It may happen that one mode shape of the vibrations is dominant in one direction (thus, the mass is only excited in one direction, such as in the X-direction only). However, it may also happen that one mode shape has vibrations in several directions at the same time (that is, the mass is excited in two or more directions, for example in the X- and Y-direction at the same time). Therefore, the mode shapes are not dependent on the global coordinate system, but on the stiffnesses of the structure in the individual directions.

You can check the direction, in which the dominant vibration of a mode shape acts, by displaying the "Effective Modal Mass Factors" table and checking whether the mass was excited in the individual directions. Figure 01 shows on the effective modal masses that the first mode shape acts solely in the Y-direction, the second one in the X-direction, and the third one is the torsional vibration.

Figure 01 - Table "5.7 Effective Modal Mass Factors"

It is possible to define several natural vibration cases and to only activate the masses in one direction. Thus, the mode shapes for each direction are obtained separately in each case.

Figure 02 - Separate Natural Vibration Cases for Individual Directions

Keywords

Effective modal mass Modal mass factor Mode shape Dynamic analysis Mass Natural vibration case

Reference

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