Difference & application of constrain modes , attachment modes, normal/real modes 2

[Rhyder88]

Usually i used to work with normal modes(real modes) . I observed there are few other modes in solution form called attachment modes , constrain modes, rigid body modes etc. What are they? Where we can apply them?

 

[GregLocock]

I haven't heard of the other two, but a rigid body mode is where a body is not completely constrained and therefore can move as a mechanism or a free body. As such it will have anywhere between 1 and 6 very low frequency modes. For some systems they do actually exist, but often they are indicative of modelling errors.

If you do want to model a structure 'free-free' you can add very soft springs to tie it to earth, which is roughly what we do when we test free-free.





Cheers

Greg Locock


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[namklof]

Fixed interface modes and constraint modes are used in component mode synthesis (Super elements in Nastran). These two types of modes are the Ritz vectors for the Craig Bampton method. Other types of modes can also be used. Guyan reduction is based on constraint modes.

 

[GMarsh]

You can find more on this in Fundamentals of Structural Dynamics by Craig and Kurdila textbook.

 

[Helrezer]

Attachement and constraint modes are calculated when doing a normal modes analysis with the newer calculation methods.
The attachement mode is used in response simulation (i.e. random vibration analysis) for the base excitation location.
The constraint mode is used in response simulation to apply distributes loads.

I can look for some papers for you to provide more details.

Marilyn Tomlin

 

[BlasMolero]

Dear Helrezer,
When running the Response Simulation (RS) module with NX NASTRAN and NX Advanced Simulation the SEMODES-RS (SOL103) solution generates the following types of mode shapes for the structure:

• "Normal modes", representing the deformed shapes (eigenvectors) of the structure at specific natural frequencies (eigenvalues).
• "Attachment modes", representing the static deformation shape due to a unit static load applied at the location where an excitation force will be applied. Attachment modes are necessary for using the Mode Acceleration data recovery method.
• "Constraint modes", representing the static deformation due to a unit static displacement of a boundary degree of freedom (DOF). Constraint modes are necessary for using the Enforced Motion excitation type.
• "Distributed attachment modes", representing the structural behavior due to a dynamic load. They are required for calculating mode acceleration responses to distributed dynamic excitations.

NX Response Simulation (RS) is an Advanced Simulation solution process for use with NX Nastran. It allows you to evaluate the static or dynamic responses of a structural model subjected to various loading conditions. The software calculates these responses using modal approaches.

Best regards,
Blas.

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Blas Molero Hidalgo
Ingeniero Industrial
Director

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[Rhyder88]

Thanks BlasMolero. The information is uesful