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Modal analysis- no mass participation on X

14 REPLIES 14
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Message 1 of 15
dan.suciu
5030 Views, 14 Replies

Modal analysis- no mass participation on X

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

14 REPLIES 14
Message 2 of 15
Romanich
in reply to: dan.suciu

Please, show us Load to Mass conversion parameters. 

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Message 3 of 15
dan.suciu
in reply to: Romanich

load-to-mass.JPG

i converted the permanent load , dead weight and snow

Message 4 of 15
Pawel.Pulak
in reply to: dan.suciu

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.

 

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

 

Regards,

 

 


Pawel Pulak
Technical Account Specialist
Message 5 of 15
dan.suciu
in reply to: Pawel.Pulak

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

Message 6 of 15
Pawel.Pulak
in reply to: dan.suciu

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
Technical Account Specialist
Message 7 of 15
dan.suciu
in reply to: Pawel.Pulak

file is in attachements,

thank you for your help

Message 8 of 15
Pawel.Pulak
in reply to: dan.suciu

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
Technical Account Specialist
Message 9 of 15
willjvizcaya
in reply to: Pawel.Pulak

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.

MASSES PIC.jpg

 

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

Message 10 of 15
Pawel.Pulak
in reply to: willjvizcaya

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
Technical Account Specialist
Message 11 of 15
willjvizcaya
in reply to: dan.suciu

I made the changes and it seems to work. Only one doubt left..

Is it the same If i keep Case 1(selfweight) into load to mass conversion and use Disregard Density?

Message 12 of 15
Pawel.Pulak
in reply to: willjvizcaya

Yes, it is the same if load case 1 contains only the self-weight load for all structure.

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 13 of 15
joaodinis
in reply to: dan.suciu

Hello.

 

I have the same problem of mass participation less than 90% on both directions (X and Y) in a simple model. In Y direction this value is almost 0.

Some isolated bars are vibrating and I don't know how to solve this problem, because I need those bars to distribute the loads.

 

I tried to turn on and off DSC algorithm, use fixed supports, use diaphragms... and anything result.

 

Can you help me please? I attach my model without results.

 

Thank you very much.

 

João

Message 14 of 15
willjvizcaya
in reply to: joaodinis

Greetings @joaodinis.

 

I dont know if its because i have Robot2018. But when i opened the model i found this:

 

There are Pinned suports in each lvl of your structure, therefore all degrees of freedoms are "locked".. meaning you cant have any or almost nule displacement or vibration- If your model is intending to recreate a construction join or some kind, maybe you should release all X and Y restrains in your supports.

 

Captura.JPG

Message 15 of 15
joaodinis
in reply to: willjvizcaya

Thank you very much for your prompt answer.

The idea is to model a steel structure between two concrete wall
structures.
Additionally to the model sent before, If I consider springs in X
direction to simulate equivalent stiffness of those concrete structures I
obtain more than 90% of mass participation.
If I use a complete model (concrete+steel) I only obtain about 65% of mass
participation in both directions for 40 modes.

Is a good way to simulate the building considering the first approach?

Thank you.

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