Advanced solver settings : Eigensolution
Both the modal dynamic analysis and buckling analysis make use of an eigensolver. These are iterative so it is useful to be able control the solution procedure.
Definition
Eigensolver
There are two eigensolvers available in GSA: Subspace Iteration and Spectra.
Maximum number of iterations and convergence tolerance
All eigensolvers are iterative and these two fields can be used to set up the maximum number of iterations allowed and the associated convergence tolerance. It is rare that these values will need to be altered.
Number of subspace vectors
The number of subspace vectors (always higher than the number of modes requested) has an impact on the progress of the iteration. Select ‘auto’ to use GSA defaults but in case convergence is slow, try a number between 1.1 times to 2 times the number of modes requested.
Shift strategy
The shift strategy accelerates the solution in the case of dynamic analysis (but not currently in buckling) so it must be set to ‘None’ when doing a buckling analysis. For dynamic analysis, select ‘Aggressive’ for fast convergence and use ‘Conservative’ or ‘None’ only if there are issues converging with aggressive. This is only available for the Subspace Iteration option.
This option is not available if the mode calculation strategy for modal analysis is set to Specify frequency range or Specify target mass participation ratio.
Frequency Shift
The frequency shift specifies the frequency around which the modes are calculated. For example, a frequency shift of 20 will find the frequencies closest to 20Hz. The frequency shift currently only works with the subspace iteration eigensolver. If a non-zero shift is added, this will automatically select subspace iteration.
This option is not available if the mode calculation strategy for modal analysis is set to Specify frequency range or Specify target mass participation ratio.
Normalisation
The results of a modal analysis are mode shapes which can be scaled arbitrarily. By default modes shapes are normalised so that the maximum displacement is 1m, but different normalisation can be set by selecting an appropriate length unit.
When the analysis is dynamic the alternative is to normalise the mode shapes based on the modal mass. This option scales the mode shapes so that the default modal mass is 1kg, or to the mass units selected.
Ignore effects of offsets in element mass calculation
When an element is offset the mass is calculated for the flexible part of the element. The offsets are assumed to be massless. However the mass has to be associated with the degrees of freedom at the node. A transformation of the mass is applied to take account of this mass being offset from the node. Ignoring the effect of the offset means that the mass is calculated as before. This mass is then applied directly to the node without any transformation.
Start mode
Occasionally it is only higher modes that are of interest. The start mode allows the lower modes to be excluded.
This is only available for mode calculation strategy of Specify the number of modes.
Additional Restraint
For a large class of structure the horizontal and vertical modes are essentially uncoupled. In these situations it may be better to do separate modal analysis for horizontal and vertical modes. The unwanted degrees of freedom can be suppressed using the additional restraints; for example, setting ‘horizontal’ additional restraint will produce only vertical modes.
Effective Mass Ratio for Filtering Out Modes
There are often a significant number of modes which contribute little to the overall mass participation. This option allows these modes to be filtered from the results on the basis of their effective mass contribution. For each mode the magnitude of the effective mass is compared with the structure mass. Results for the modes where this contribution is below the cutoff get discarded. For example, setting the filter to 0.02 will filter out any mode whose effective mass contribution is less than 2% of the total mass.
This option is not available if the mode calculation strategy for modal analysis is set to Specify frequency range or Specify target mass participation ratio.