# Eigenvalues¶

The SSD task ‘’Eigenvalues’’ contains the following tabs:

General

Additional Masses

Damping

Text Output

Graphical Output

## General¶

General settings for the eigenvalue analysis include:

Basic Settings,

Module,

and Solver Settings.

### Basic Settings¶

In the basic settings, you can first define the number of eigenvalues that should be calculated. The entered value for the Storage Loadcase Number will be used for the smallest eigen mode shape. The other vibration mode shapes are then stored as load cases with ascending load case numbers. Additionally, you can select one loadcase from the database from the dropdown menu, if you want to start of the analysis from a primary loadcase.

### Module¶

Two different SOFiSTiK modules can be used for the eigenvalue analysis - ASE or DYNA.

### Solver Settings¶

You can select one of three computational methods for the analysis. The choice of method for the eigenvalue analysis depends on the number of the sought eigenvalues.

The simultaneous vector iteration can be used in the case of few eigenvalues. The number of iterations may be reduced, if a somewhat expanded subspace for the eigenvalue iteration is used. Therefore the default value for the Number of Vectors is here the minimum between the Number of Eigenvalues + 2 and the number of the unknowns. The iteration is interrupted, if the number of the maximum iterations (default max (15.2 × Number of Vectors)) is reached or if the maximum eigenvalue has changed only by the factor less than 0.00001 opposite to the previous iteration.

The method according to Lanczos is significantly quicker than the vector iteration, if a large number of eigenvalues is sought. A good accuracy is achieved, if the Number of Vectors is at least the double one of the sought eigenvalues (default). Unlike the vector iteration the larger eigenvalues are usually worthless for Number of Vectors = Number of eigenvalues.

Using the input LMIN (Unit 1/sec^{2} for dynamic eigenvalues, factor for buckling eigenvalues, taken from
the result table Eigenfrequencies) the results can be shifted. The number of excluded
Eigenvalues are shown in the printout.

See also

For more information please see the ASE and DYNA manuals.

## Additional Masses¶

The Loadcase Selection Table at the top shows all loadcases of the database available for load-to-mass conversion. Loadcases can be selected in the first column. The structures self-weight will be taken into account automatically. For selected loads you can choose the X, Y and Z direction (Per default, only the dead weight direction is selected). Additionally, the mass effect in each direction can be defined by entering the desired percentage.

Warning

In cases where self weight and dead loads are saved in the same load case, only the self weight will be converted to masses. For that reason we recommend to use a separate load case for self weight only and another for dead loads.

Further, lumped masses can be added. The masses are additional to the primary masses defined in the system definition. They are not effective as dead load in static load cases as do the primary masses. A new lumped mass can be added using the on button on the top left of the table. To define the location of the mass, a node number must be entered. Then the amount of the translational and/or rotational mass can be defined.

For the formulation of the mass matrix, there are three options to choose from:

diagonal mass matrix,

consistent mass matrix

and consistent with rotational masses.

For most cases, the default settings should be sufficient. For more information regarding the mass matrix, please see the DYNA manual.

In the advanced options, check the box when the analysis should try to eliminate local vertical eigenforms.

## Damping¶

Per default, no damping is considered in the eigenvalue analysis. If you change the basic settings to Rayleigh Damping, the Mass Proportional Damping Factor and the Stiffness Proportional Damping Factor have to be defined. For both factors you can use the calculation tabs to determine the values based on the target damping and the lower and upper interval frequency.

In the *expert settings*, individual Rayleigh Damping Factors can be defined for each group.

## Text Output¶

In the text output task, the extent of the Report Browser can be controlled. You can choose between:

no text output,

the full text output or

you can manually control the output.

For the manually controlled output you can select the output extend for each output part individually in the control table. On the left, you can see the different output parts. On the right, you can select one of the following options:

default output,

no output,

normal output,

full output or

extensive output.

## Graphical Output¶

The tab “Graphical Output” is a common tab that appears in many SSD tasks. It is used to plot the predefined results by using the program Graphic.

Depending on the type of the task, different predefined results will be provided for the selection. To deactivate the plotting of graphics, just uncheck all checkboxes .