Dynamic Eigenmodes#

The SSD task task_dynamic_eigenmodes_small Dynamic Eigenmodes provides the calculation of eigenmodes, based on the new solver technology, including mode filtering and missing mass residual modes. A variety of tools, e.g., load-to-mass-convertion, modal damping or shear and bending stiffness reduction can account for specific boundary conditions.


Define settings for the calculation of eigenmodes and seismic actions.

GUI Task *Dynamic Eigenmodes* - Eigenmodes Tab

GUI Task Dynamic Eigenmodes - Eigenmodes Tab (click on image to enlarge)#


General, Solver Settings#

Specify the maximum number of eigenmodes and the storage load case number, as well as the computational method used for the calculation.

See also

For more information about the computational methods please see the FEABENCH manual.

Damping, Stiffness and Inertial Effects#

Adjust modal damping and shear and bending stiffness globally or individually by group. In addition, utilise Inertial Effects to consider or ignore acceleration and mass effects for specific groups.

It is possible to leave cells empty in the secondary groups table, thus leaving the properties unchanged from the settings made in the primary group table.

GUI Task - Eigenmodes Tab


Primary groups

Each structural element can belong to exactly one primary group. Therefore, the order of the primary groups is unimportant because conflicting settings cannot be made. Individual group settings override the All Groups setting.

Secondary groups

Additionally, structural elements can belong to multiple secondary groups. Therefore, the group settings will be handled from top to bottom, meaning that the bottom-most setting overrides earlier ones, including primary group settings.


Filter options to minimize the number of eigenmodes and hence calculation time, are available. The filter for total mass participation is set to 90% and activated by default. Alternatively, it is possible to filter out all eigenmodes with a mass participation smaller than a specified percentage. In addition, vertical eigenmodes can be eliminated.

Furthermore, 3 additional load cases will be calculated by activating the missing mass residual modes. They correspond to the mass which is not activated by the periodic modes and is subjected to the unit acceleration in the global X, Y and Z direction. The subsequent Response spectrum analysis will automatically consider these modes and the peak modal responses will be calculated by multiplying their responses with the spectral rigid response Sa-ZPA (T= T-ZPA) and the ground acceleration.

Only the eigenmodes with the period higher than the rigid cut-off period input T-ZPA will be considered as periodic modes with the activated missing mass option.

GUI Task *Dynamic Eigenmodes* - Eigenmodes Tab

GUI Task Dynamic Eigenmodes - Eigenmodes Tab - Filtering#

Load-to-Mass Conversion#

Choose which load cases to consider for the load to mass conversion. This will initiate the recalculation of the centre of mass.

GUI Task - Eigenmodes Tab

GUI Task Dynamic Eigenmodes - Eigenmodes Tab - Load-to-Mass Conversion#


Select the extent of the generated report.

GUI Task *Dynamic Eigenmodes* - Output Tab

GUI Task Dynamic Eigenmodes - Output Tab (click on image to enlarge)#