Number of modes of 3D model

[easap]

Hi,

I am performing a model analysis using eigenvectors.

I have a 3D structure that has all translational and rotational dof's active which requires over 90 modes to obtain a mass participation ratio of 90% in all translational directions. The structure is inclined, which means that non-lateral movement has UX and UZ components, which slows down the process of obtaining a satisfactory mass participation ratio. Further, I have many modes in a row which contribute with practically zero to the response because only a very small part of the structure is moving.

Does anyone know a trick to eliminate these insignificant modes from the eigen modal analysis? I see that advanced load parameters can be specified but I don't know if this might help me somehow..

Thanks

 

[easap]

Also, I can't find any information on the frequency shift, cutoff frequency and convergence tolerance - maybe they can help me get rid of the modes that do not contribute?

 

[Guest262653]

Don't tweak the advanced load parameters. Try to use Ritz vectors instead. In the Loads applied boxes, include the accelerations along the relevant directions and you'll reduce the computing time and the zero-mass modes significantly.

Check the following link for reference:

https://wiki.csiamerica.com/display/kb/Ritz+vs.+Eigen+vectors

and the following eng-tips discussion:

http://www.eng-tips.com/viewthread.cfm?qid=239542

 

[johnbridge231]

I would as well suggest you used the Ritz method.

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