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.

Eigenmodes#

Define settings for the calculation of eigenmodes and seismic actions.

Eigenmodes#

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.

Damping, Stiffness and Inertial Effects#

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

Clicking the Stiffness input opens an additional dialog, where you can select from the following options:

Shear and Bending Stiffness - Specify a factor to adjust the shear and bending stiffness of all beam elements, as well as the plate bending and plate shear stiffness components of all quad elements in the selected group.
All Stiffness Components - Specify a factor to adjust all stiffness components for all elements in the selected group.
Advanced Stiffness Modifiers - Modify individual stiffness factors for membrane, plate bending, and plate shear properties of all quad elements in the selected group. If the selected group contains no quad elements, the Advanced Stiffness Modifiers option will be unavailable.

Filtering#

Mode filtering allows for selecting a subset of computed eigenmodes to be included in the modal superposition, ensuring that only the most relevant modes are considered. This approach can significantly reduce computational time while preserving the accuracy of the results. Two filtering options are available for the translational acceleration in global directions and in the case of rotational acceleration about global axes through the center of mass:

Total target mass participation per direction - where the considered effective modal mass amounts to at least a selected percentage of the total effective modal mass.
Minimum threshold of mass participation per direciton - where all computed modes with effective modal masses greater that the input threshold value are taken into account.

Additionally, there is an option to eliminate vertical eigenmodes, which can be used, for example, to suppress local eigenmodes associated with slabs in high-rise buildings.

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.

Load-to-Mass Conversion#

Choose which load cases to consider for the load to mass conversion. Only calculated load cases without a dead weight factor are available for selection in the table. The conversion will also initiate the recalculation of the centre of mass.

Output#

Select the extent of the generated report.