Normal Modal Analysis Deformation Values after Displacement Normalization

[struclearner]

Dears,
When the Normal Modes from FEA Modal analysis are normalized by displacement, the maximum displacement in a mode is used to get the unit value and the relative displacement values at the other areas of the structure.
Is this possible that the maximum deformation values after normalization by displacement be less than unity (a fraction value), if yes, what that maximum deformation value means.
Thanks for the explanation and input.

 

[NRP99]

The physical significance of displacements and other quantities are intact but not the magnitude significance for modal analysis. Check the Mode normalization technique used in your software-help documentation. May be that will provide answer.

If normalization is done using Max displacement and setting max displacement to 1, its not logical to see max value to be fractional. There is strong possibility software is not using the displacement normalization technique or the displacement normalization method calculation is different.

 

[FEA way]

If you’re asking this question because you observed such behavior in the analysis then please tell us which software you are using. Keep in mind that displacements are presented by FEA programs in different forms - not only as displacement components in each axis but also as a resultant displacement.

 

[GregLocock]

You could for example normalise by the maximum resultant displacement amplitude, which would give the effect you observe.

Cheers

Greg Locock


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[struclearner]

I was able to get correct Normalization Eigenvectors by maximum deformation by using PCG Lanczos Eigen Slover in ANSYS Workbench. The other program has only Lanczos listed in the solver option, it gives the deformation values less than unity for some modes after using Normalization by Max option.
The investigation of the cause is in progress.
Thanks for friends reply and input.

 

[BlasMolero]

Hello!,
Although the scaling of normal modes is arbitrary, for practical considerations mode shapes should be scaled (i.e., normalized) by a chosen convention. In FEMAP & Simcenter Nastran we have four normalization choices: MASS, MAX, AF, and POINT normalization.
• MASS normalization is the default method of eigenvector normalization, scaling each eigenvector to result in a unit value of generalized mass.
• MAX normalizes to the largest component of mass in analysis set, and
• POINT normalizes to mass at a specific nodal degree of freedom. When using Point, you must also specify a node ID and degree of freedom.

Numerically the MASS method results in a modal mass matrix that is an identity matrix. This normalization approach is appropriate for modal dynamic response calculations because it simplifies both computational and data storage requirements. When mass normalization is used with a model of a heavy, massive structure, the magnitude of each of the terms of the eigenvectors is very small.

Although mode shapes are relative quantities, a number of modal quantities can be helpful in predicting qualitative responses or in isolating troublesome modal frequencies. Since relative strains, internal loads, and stresses develop when a structure deforms in a mode shape, you may recover these quantities during a normal modes analyses. Basically, any quantity that you can recover for static analysis is also available for normal modes analysis.

In any case, it is important to remember that all the above output quantities are based on the relative displacements of a mode shape. The output quantities can be compared for a given mode, but not necessarily from one mode to another. However, they can still be effectively used in the analysis/design process.

Modal quantities can be used to identify problem areas by indicating the more highly stressed elements. Elements that are consistently highly stressed across many or all modes will probably be highly stressed when dynamic loads are applied.

Best regards,
Blas.

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Blas Molero Hidalgo
Ingeniero Industrial
Director

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