## Robot Structural Analysis

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# Modal analysis- no mass participation on X

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Hi,

After I do the modal analysis and am looking to set the NMV in which the most mass participates ,

i can find the value for Y direction, but not for X

I am attaching some pics showing the settings I have done

Thank you in advance,

Dan,Suciu

Solved! Go to Solution.

# Re: Modal analysis- no mass participation on X

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# Re: Modal analysis- no mass participation on X

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i converted the permanent load , dead weight and snow

# Re: Modal analysis- no mass participation on X

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Screen capture from your first post shows that a lot of vibration modes with the same frequency of 0.43 Hz was found.

It suggests local vibrations of some members with very similar properties.

Does your model contain members with releases and DSC algorithm is active?

In such case I suggest switching off DSC to avoid/reduce the number of these local vibrations.

*---------------------------------------------**If this post answer your question please click "Accept as Solution". It will help everyone to find answer more quickly!*

Regards,

**Pawel Pulak**

# Re: Modal analysis- no mass participation on X

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even with DSC off , I have the same result and same problem.

I have releases on bars, and many similair elements(the roof is made out of trusses); releases are applied on the truss nodes so they do not transfer moment to the column

# Re: Modal analysis- no mass participation on X

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Can you attach the file?

In case of big size please use Save as with different name and without results. Then compress to ZIP.

Regards,

**Pawel Pulak**

# Re: Modal analysis- no mass participation on X

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# Re: Modal analysis- no mass participation on X

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All those local vibrations are related to lateral vibrations of roof trusses.

They are not braced in the plane of top or bottom chord so the frequencies of their lateral and torsional vibrations are rather low.

You can easily check it temporarily deleting load case 11 (selfweight) from load to mass conversion - when mass from selfweight (including selfweight of trusses) is not considered the local vibrations are strongly reduced.

Generally the model is not braced in planes of roof slopes and it should be done or using dedicated bracings or at least considering the influence of roof plates.

After adding such bracings local vibrations will be reduced.

Additional note: I have noticed that load to mass conversion was performed simultaneously for snow load cases 4to6. Are these load cases excluding between each other? If yes then only one of them should be converted to mass.

Regards,

**Pawel Pulak**

# Re: Modal analysis- no mass participation on X

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I have a Similar Problem with The attached Picture model.

1) the structure is non linear since i used the Truss option for Just Tension.

2) I checked the mass participation rate before Spectral Design and it worked >90% rate in the 4th Mode..

3) I used DSC Algorithm for moment releases in secondary bars.

4) I Used Acc/Period Spectral Analysis for X and Y.

5) Combinations are manual since non linear anayliss wont let me use automatic.

Still the problem is: I get less than 64% mass participation in X direction and wont change when increasing modes.

and the question is: the main Mode is the 1st one with more than 90% Participation rate in both directions right?

Attached the Model

# Re: Modal analysis- no mass participation on X

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Slow increase of participation mass in this model is caused mainly by local vibrations of K bracings and pinned beams.

Moreover I have noticed that load to mass conversion is made also for load case 1, which contains only the self-weight. In the same time "Disregard density" is not activated in modal analysis parameters. It results in considering self-weight in dynamic analysis twice.

After removing load to mass conversion for load case 1 and increasing the number of modes to 40 it can be observed that 90% mass participation is exceeded in X direction for 38 modes.

After changing from lumped mass matrix with rotations to lumped mass matrix without rotations 90% mass participation is exceeded in X direction for 26 modes.

Regards,

**Pawel Pulak**