Answer:
The direction action of the masses need to be defined. The masses can act in the global displacement directions X, Y, or Z by selecting the corresponding check boxes. You can also rotate the masses around the global axes X, Y, and Z by selecting the corresponding check boxes. These settings change the mass matrix and lead to different mode shapes and natural frequencies.
To perform a planar analysis of the structure, select the masses in one of the global directions only. A planar analysis is only allowed for buildings that are regular in ground plan and layout. For a three-dimensional analysis, the masses are considered in all global directions.
