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

The maximum number of iterations can be set here and the associated convergence tolerance.

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.

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.