A modal analysis is performed by setting the Modal property of the Analysis object equal to true and then calling the Analyse method. The settings for a modal analysis are contained within the ModalSettings object. This object has five properties that control the analysis which are listed in the following table.
|
Property |
Type |
Description |
|
DistributedMass |
Boolean |
Get/Set whether distributed masses are to be used in the analysis. |
|
LumpedMass |
Boolean |
Get/Set whether lumped masses are to be used in the analysis. |
|
MaxIterations |
Long |
Get/Set the maximum number of iterations to be performed in each load increment. |
|
Modes |
Long |
Get/Set the number of modes to be computed by the analysis. |
|
Scaling |
mfModalScaling |
Get/Set method used to scale modal displacements. |
|
Solver |
mfModalSolver |
Get/Set type of solver used to perform modal analysis. |
|
Tolerance |
Double |
Get/Set iteration tolerance. |
The Application and Frame properties, which are common to many objects in the Multiframe Object model, are described in Chapter 2.
The Modes property is used to control the number of mode shapes determined by the analysis. Each mode shape will be computed using a maximum number of iterations specified by the MaxIterations property. The iteration of each load increment in the analysis will be performed to the convergence tolerance set by the Tolerance property. The other two properties of the ModalSettings object control the type of mass distribution used in the analysis. If a lumped mass formulation is required, the LumpedMass property must be set true, else a distributed mass formulation can be employed by setting the DistributedMass property to true. Note that these two properties are mutually exclusive, setting one of them equal to true will automatically set the other property to false. The following script demonstrates how to perform a modal analysis.
Dim mySettings as Multiframe.ModalSettings
’Get reference to modal analysis settings
Set mySettings = myFrame.Analysis.ModalSettings
’Specify settings for analysis
mySettings.Modes = 5
mySettings.Tolerance = 0.0001
mySettings.MaxIterations = 25
mySettings.LumpedMass = true
’Perform the analysis
myFrame.Analysis.Modal = true
Call myFrame.Analysis.Analyse
As in the previous example, the script obtains a reference to the object containing the analysis settings that, in this case, is a ModalSettings object. The analysis is then set up to compute 5 modes shapes, to a tolerance of 0.0001, using a maximum of 25 iterations to compute each mode. A lumped mass formulation is selected for computing the mass matrix for each element in the structure.