A static analysis defines loads and determines internal forces. A dynamic analysis, on the other hand, is based on masses . The "Modal Analysis" add-on includes several options that allow you to define masses for determining natural frequencies and mode shapes.
Import Masses
You can import loads and masses from a load case with the "Modal Analysis" type assigned, and thus use them directly for the modal analysis (see the image Load Case of "Modal Analysis" Type for Direct Load Transfer). You can also import masses from another load case or a load combination (see the image Importing Masses from Load Case and Importing Masses from Load Combination). Thus, the load types, such as forces and moments, are imported and converted to masses.
You can make further specifications for masses (mass conversion type, mass matrix settings, neglecting masses) in the Modal Analysis Settings.
Forces are converted into masses using a mass conversion constant that is entered in the Base Data of the model. The default setting for g is 10 m/s² (1 kN = 100 kg).
You can combine load cases in a design situation to create a seismic/mass combination (see the image Design Situation "Seismic/Mass Combination" ).
Defining Masses
In addition to or as an alternative to the options for importing masses described above, you can manually define node, line, member, or surface masses in a modal analysis load case or define area moments of inertia. These options are described in the chapter Additional Masses.
If you have set a "multiplication factor" for the loads in the Static Analysis Settings, this is not taken into account in the modal analysis. In this case, specify the load case factor in the manually defined load combination to scale the masses.