Local vibration modes

[Anonymous]

Hi again, 

I have modeled a three-dimensional steel frame and I am currently stuck at modal analysis. I need to perform spectral analysis later and the problem is when I do not check " Disregard density" box in analysis parameters the output is one correct mode and many local modes. When I check the box the results seem correct, but the total mass is reduced by 6% (because I checked the box). So, is there a way to convert the self-weight of the bars to nodal masses automatically, or to exclude the local vibration modes from the results. I have already tried the Seismic mode without success.

[Artur.Kosakowski · Accepted solution]

The conversion of mass to nodes is governed by the mass matrix you selected (lumped vs. consistent) and you can check if changing this settings helps. My understanding is that due to local modes you have problem to reach 90% of participating mass with 'reasonable' number of modes. I can't see what these modes are but perhaps you may consider switching off the vertical direction of vibrations? For some some codes you can also use the residual mode.

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[Anonymous]

Ok, now I got it. Removing the rotational DoF of the nodes' masses did the trick,now the local vibrations are gone.

I was confused by the deformed shape of these modes and I thought that they resulted of some form of translational displacement along bar length, so I assumed that the density of the bars caused it. Obviously these modes resulted from the rotation of the nodes. This situation brought me to the question how does Robot apply masses from the density of elements? When the total weight of an element is calculated, is it applied in the mass centre of the element or it is divided between the nodes of the element? (see the scheme attached)

[Artur.Kosakowski]

Depending on the choice of the mass matrix type. If lumped then to nodes (also these defined along a bar if exist). If consistent along a bar (as linear load on a bar).

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Understood. Thanks again for the help.

Regards, 

Yordan Mitev