Modal Analysis

[Jade_Cereno]

I would like to ask about model analysis,w e would like to know what mode shape or what period is governing on a certain structure, how will we know if that is the right mode shape and period for the model?

I attached a screenshot of the model with modal analysis.

Parameters and defeormation tab are the ones being seen on the structure. What are those numbers represented?

[Artur.Kosakowski]

I'm not sure what you mean by the 'right' mode. Perhaps you should look at the one with largest participating mass in a given direction?

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[Anonymous]

Hi

If I understand you correctly you want to know after running the modal analysis which mode is governing at your loadcase?

I'm used to visually go through the modes and based on my judgement find the shapes that are possible in the real structure and are possibly produced by your loading you are analysing.

There might be more sophisticated methods built in robot to find governing modes but im not aware of these...

[Jade_Cereno]

If I understand you correctly you want to know after running the modal analysis which mode is governing at your loadcase?

-Exactly what I am trying to find out

I'm used to visually go through the modes and based on my judgement find the shapes that are possible in the real structure and are possibly produced by your loading you are analysing.

 -Yes I've been trying to do that too. But my colleague here wants to make the program show it. Like on this mode, where on the structure is this frequency and mode shape and fine the period on it.

There might be more sophisticated methods built in robot to find governing modes but im not aware of these...

-This means we need a help from our Autodesk Friends Out there

Let me have a follow up question on this, on modal analysis looking at the modes, why can't I display forces on members? Do I need to make load combination for this to appear?

I've attached a screen shot of my inquiry

[Mirko.Jurcevic]

Modal analysis doesn't produce forces.

It is used for getting the shapes of structure oscillations. It's showing you how your structure will act in case of, for example, horizontal forces (impulses) acting of your structure (like in the case of earthquake).

In the case of an earthquake, seismic forces are mainly result of an accelerated (dead) mass of an building (structure) + maybe some snow, etc. depending on the code, location etc.

When you do a modal analysis, for example, in your first mode, 60% of seismic mass is participated, in second 20%, in third 5% and so on.

Your goal is to find as much modes as you need to get, for example, 90% of seismic mass participated in the seismic force. You will not be able to get this in every structure. Doing 100 modes doesn't make sense.

Modal analysis depends on a disposition of the structural elements, mass, and rigidity of each element.

And of course, other forces acting on the structure.

[Mirko.Jurcevic]

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And of course, other forces acting on the structure.

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When using "Modal analysis recognizing static forces".

[t.sautierr]

To follow Mirko's post, 

the modal analysis is mathematically an eigen value matrix problem -> the solutions  gives you a frequency, a modal shape (related to an eigen vectors familly) and a participating mass, some of these data depends on mathematical definitions (for example the vectors are or are not normalized and they definition of the normalization it something arbitrary) -> the datas concerning shape, displacement, force have to be scaled to be meaningful.

that's why it is useless and wrong to look for values in a modal analysis (however the ratio between 2 displacement has a meaning).

-> look at a dynamical analysis handbook for further details.

In Robot, you will have to perform a dynamical analysis in conjunction with the modal analysis to get results (seismic, vibrations etc ...)

[Jade_Cereno]

Thanks for all youir replies. 

My thoughts on this is that once i am done with Modal Analysis, i'll be able to get the period of the structure at 90% mass participation. After that Will make a Seismic Analysis for X, Y and Z placing the obtained period from Modal Analysis.

Than After That Will make a Load Combination for this to obtain the Proper forces. Am I correct on this?

[Rafal.Gaweda]

My thoughts on this is that once i am done with Modal Analysis, i'll be able to get the period of the structure at 90% mass participation.

For sure it will not be just only 1 mode giving 90%

After that Will make a Seismic Analysis for X, Y and Z placing the obtained period from Modal Analysis.

Run Modal in the way to get ~90% mass participation factor, then seismic case(s) will automatically take all these modes for calculations.

Than After That Will make a Load Combination for this to obtain the Proper forces. Am I correct on this?

Yes.

Example attached.

[Jade_Cereno]

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@Rafal.Gaweda wrote:

Run Modal in the way to get ~90% mass participation factor, then seismic case(s) will automatically take all these modes for calculations.

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So meaning when I create the Seismic Analysis After Modal Analysis, Robot Will take the period for seismic case use?? hmmm... Can you show me how to accomplish this?

I have noticed that the modal analysis has an option to take static cases, meaning if I make a case, say seismic, robot will make use of this and incorporate it on the modal analysis?

Or i may have interpreted your explanation the wrong way??

I attach the screen shot of calculation types tweaking the parameter on Modal Analysis

[Rafal.Gaweda]

Run Modal in the way to get ~90% mass participation factor, then seismic case(s) will automatically take all these modes for calculations.

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So meaning when I create the Seismic Analysis After Modal Analysis, Robot Will take the period for seismic case use?? hmmm... Can you show me how to accomplish this?

Download zip from previous post, open doc, look at end of page 1 and page 2

[Rafal.Gaweda]

I have noticed that the modal analysis has an option to take static cases, meaning if I make a case, say seismic, robot will make use of this and incorporate it on the modal analysis?

http://docs.autodesk.com/RSAPRO/2014/ENU/filesROBOT/GUID-B3450A84-E078-484E-80B9-3AA0482F3699.htm

[Mirko.Jurcevic]

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tuklap wrote: 

My thoughts on this is that once i am done with Modal Analysis, i'll be able to get the period of the structure at 90% mass participation. After that Will make a Seismic Analysis for X, Y and Z placing the obtained period from Modal Analysis.

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Seismic forces (and modal analysis) in Z direction usually doesn't produce noticable forces, so it's often excluded from the calculation. Z direction make sense only when earthquake (epicenter) is directly under structure, and even then, if you don't have structure elements sensitive to vertical vibrations, for example, cantilevers, it's not something to be worry about. But, every structure is unique, so be careful.

Note: For Z direction I mean global Z direction / perpendicular to the ground.

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tuklap wrote:

After that Will make a Seismic Analysis for X, Y and Z placing the obtained period from Modal Analysis.

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"Periods" not "period". Seismic force is calculated from many modal modes. It combines seismic force calculated from activated mass in each mode using the CQC or SRSS method (and others). I usually use SRSS method. For more info, read this article:

http://docs.autodesk.com/RSAPRO/2014/ENU/index.html?url=filesROBOT/GUID-9A9795B9-E2B1-4F70-BC47-FF61...

[Tuctas]

What t.sautier said is perfectly true, i just want to add that the user is able to view the values of eigen vectors (i.e the values of displacements of nodes corresponding to modes) through the window of "results-diagrams for bars-deformation". These displayed values are the un-normalized total/resultan values (consisting of all components ux,uy,uz).

[Khristopherson]

I am having a problem right with this part. I want to know what is the displacement of every node, in each of the vibration cases (Each frequency or mode). I have 10 frequencies of oscillation for my structure, this means that it certainly is oscillating, but when i try to visualize the values of the displacement of each node, the table only shows minus zero and zero for every node, and this makes no sense to me.

I am tryng to simulate the behaviour of a telecomunications tower, trying to find the natural vibration frequencies, via a modal analysis, so i can make sure that this values of frequencies are different from the vibration frequency at which an aerogenerator is vibrating when spinning at 200rpms. This would assure that the system does not enter in a resonance state, what could lead to destructive interference making it to collapse.

I´d really appreciate your help and advise on this.

Thank you.

[Mirko.Jurcevic]

Of course you don't get displacement because modal anaysis doesn't produce forces that are needed to make displacements/deformations (as I said before). Look at messages No.5 and No.7 from this thread.

If you want displacements, you must somehow get forces from that periods.

For example, you'll combine activated mass from each period into seismic force (you'll first need to define seismic case).

But if you want to compare the two frequences to see if they are resonanting, why don't you just do that?

Or maybe I understood your question wrong.

[Rafal.Gaweda]

[t.sautierr]

Maybe what you want actually is to perform a FRF or an Harmonical analysis :

put as a loading a periodical force that simulation the excitation coming from your vibrating machine -> then you will be able to get acceleration, force, etc ...

Otherwise if you just want to check that there is no common period, then just perfoming the modal analysis is enought as you did.

[Khristopherson]

This is exactly what i want to visualize. But i can´t click on it, i mean, i click on the diagrams icon but no window pops up, for me to select that I want to visualize deformation. Though, i get these completely messy versions of my tower when i click on the "Displacements (deformation)" button (lower right corner). I mean, what does that mean? My model obiously is not that deformed, that is why I want to know what the deformation or displacement of the nodes is.

In the other hand, i added the eigenvectors columns, i had already donde that before asking this the first time, but as you can see in this screenshot, they all have zero and negative zero values, and i don´t know why is that.

Thanks for your answers.

[Artur.Kosakowski]

Please check message 5 from http://forums.autodesk.com/t5/Robot-Structural-Analysis/Designing-Conc-Slab-but-no-load-effects-in-r...

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.