Why Force is not considered by software during modal and frequency response analysis 1

[ninjaz]

Is there any specific reason behind , the analysis software is not considering the force if they are applied in any specific nodes. But they consider forces in static analysis.
Is there any other way to consider that force in modal and dynamic analysis

Appreciate your comments

 

[ninjaz]

Is wish to add one more query to the discussion. I think , i can consider this force in frequency domain by creating a seperate function. If that is right, Is it possible to apply a force function at one node and acceleration function in excitation points at the same time

 

[ESPcomposites]

A modal analysis is used to determine the modes -aka natural frequencies- (the eigenvalues) and associated mode shapes (eigenvectors). The natural frequency is a function of the mass, stiffness, and boundary conditions of an object. It is not a function of applied load. You can not recover meaningful deflection magnitudes, stress, strains, or reactions forces. Relative displacements are meaningful though (the mode shape).

In a static analysis, you may apply a load to a part/assembly and determine deflections, stress, strain, reactions forces, etc.

Brian

http://www.espcomposites.com

 

[ninjaz]

Thanks for your comments brain. I understand that load will not be considered in modal analysis.

But ,It is possible to make a response analysis (Frequency response for my study) in which we can make a function( force vs frequency) and input at the desired point. Right?

In brief, my case has to be excited with a vibration at component fixing point and this force has to be applied at a specific node . Stress has to be observed as output for a limited frequency range

 

[jhardy1]

I think you're looking for Harmonic Response Analysis or Transient Analysis.

http://julianh72.blogspot.com

 

[ninjaz]

Jhardy, I am looking for harmonic response analysis.. Time domain is not involved..

 

[GregLocock]

Basically your modal analysis creates a list of frequencies and mode shapes. You then need to provide damping estimates for each mode, and you can then synthesise the FRFs. When you apply your force spectrum to the FRFs you will get vibration spectra at each node.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Sjqlund]

You can take 'pre-stress' effects into account in most FE programs. This is a way of taking into account that loads can make a structure stiffer. Think of tightening a guitar string for example - this would alter its frequencies.

 

[johnbridge231]

Modes depend only on the mass and stiffness. Loads are not considered in modal analysis. On the other hand, loads can be used to describe the mass of a model (for instance super dead loads), so in that particular case they are taken in account.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html