Modal analysis and damping

[Rafacascudo]

2 identical models except for the fact that one of them has axial damping (Bar releases) on the X braces .

Help file from Bar releases says  " The damping value is considered only in the time history analysis" .

But the modal analysis results are completely different.

files are here

http://www.4shared.com/rar/a3EfBUXCce/50ANDARES_analise_modal_NR.html
http://www.4shared.com/rar/DITxjswyce/50ANDARES_analise_modal-dampin.html

thx

[t.sautierr]

How can we get your files without downloading scrapy programs or registering in any website/social network?

[Rafal.Gaweda]

Here you are:
http://we.tl/fsZadOTsvH

[Rafacascudo]

no problem here . Click on the small "download" button and in the next page wait 20 secs and click on "free download". Anyway, Rafal has kindly provided a wetransfer link. thx

[Rafacascudo]

Weirdest thing is if you just click on Ux,Cx  and it doesn´t matter if the Cx damping value is Zero or one million  ,the modal results results are the exactly the same for Zero or 1 million  and completely different if you don´t assign a damping Cx value.

[Rafacascudo]

Hi Rafal ,Artur,

Any ideas on what is happenning?

thx

[Rafal.Gaweda]

The reason : models are different:

[Rafacascudo]

Yes , but there is no way of assigning a cx damping value without assigning a ux release. In fact it is automatic . As you give a cx value ,the ux checkbox on bar tab becomes marked. If you then unmark this box , the checkbox on damping tab also becomes unmarked.

I guess that I have to use the elastic tab ,isn´t it?

[t.sautierr]

why are you expecting to change the eigen periods? it modal analysis not  time history -> see help image.

I you want to introduce damping : time history or global dampin coeffcien in modal analysis parameter.

Don't look at the values, I have deleted a lot of stories in your model to get the results faster.

However, it could be interesting to implement damping in the modal analysis Rafal (simple matrix with constant coefficient), I'dont think it is a complicated to put it in the diagonalization algorithmn for this simple way to simulate damping ?

[t.sautierr]

I guess that I have to use the elastic tab ,isn´t it?

It will change the stifness, yes. But it is not damping .... needs to be scaled I assume.

[Rafacascudo]

T. Sautier,

Thx for your considerations , but let me explain some points.

This is not a real world model. It´s part of a young enginneer MD thesis here at work. I´m helping her with Robot.

The bar axial dampers are to be used in a dynamic wind TH analysis.

I use Robot since 2001 v13.5 and had never used dampers before. So I´m almost  like rookie in dynamic analysis. It´s used only  when we have vibrating machines , or footfall analysis for pedestrian briges, or TH for  simulate vehicles over a bridge.

I´m from Brazil and there´s no earthquakes here , so our standards say nothing about seimic analysis.

Now I just realized that if you want to use bar axial damping ,you also have to use a spring(elastic release). Is that correct???

thx

[t.sautierr]

At a first sight I would not agree that you can simulate dampers by elastic releases, because one is generally linked to the speed (dampers) and the others to displacement (elastic springs), there is a "omega" between the two members due to the fact that speed is the derivate from displacement.

But I must confess that i barely use dampers in structures so my opinion is maybe the most exprerienced.

Maybe other way to bypass the problem, use damping supports (and not releases) at the ends of the bracings and suppress the bars? I have not seen in the help that modal analysis is not compatible with damping supports. 

Or maybe more realistic, use global damping factor simulating the behavior of the whole bracing system.

[Rafacascudo]

No , I didn´t mean springs instead of dampers. Since it´s not possible to use the Ux,Cx damper value without automatically fully releasing the Ux axial direction , the only way to give the axial direction some rigidity (while using dampers) was to assign a spring value to it. If you assign a very high value ,It is possible to simulate the axial rigidity as if it was fixed in the Ux direction. Using this approach I can get the modal analysis to be identical for the model without releases and the model with them(with dampers and high  spring values ,in my model 100million t/m ).

But I don´t know if this is the correct approach for this type of analysis.

[t.sautierr]

Yes it is correct approach, but how did you get rid of the limitation with dampers? if you have releases with dampers (and high elastic stiffness), the modal analysis shall not give you any change because it not taking into account dampers (with or without elastic releases). To me, if you see change, it will be only due to the stiffness of your elastic releases and if it is very high then it will not change much.

Ask Rafal or Artur to give piece of advice.

[Rafacascudo]

Some findings experimenting with bar releases/dampers

1- If you use bar dampers in a TH case without elastic coefficient , your linear static cases results are compromised because of the automatic full release on the damper direction.In my case all X-braces bars give zero value for axial force for all linear static cases

2- Interestingly , at the same time(same model run) ,for TH cases the axial release is inactive and non zero axial forces appear for the damper assigned bars!!!!!

3- Modal results are for the structure WITH axial bar releases !!!!

So my question is , what structure is used for calculating the modal results to be used during the TH analysis???

thx

[Rafacascudo]

Rafal , Artur ,

coulde please give some advice on how to use the dampers , considering what I wrote in my last post??

thx

[Pawel.Pulak]

Hello,

dampers in Robot are viscous dampers where damping depends on speed of relative movement of both ends of it. So it means: "no movement" = "no damping force"

On the screen capture below I have shown the symbols corresponding to various definitions of releases with dampers in Robot:

Type 3) corresponds to "no release" (fixed) and damper. In such case damper is not effective because if it is fixed - there is no movement so there is no damping force. That is why such definition in Robot is impossible

As concerns definitions 1) or 2) they will result in different results in:

• modal analysis: damping will not be considered but differences in results will result from free or elastic release
• time history analysis: both damping and free or elastic release will be considered

Rafascudo, your statement "linear static cases results are compromised because of the automatic full release on the damper direction" suggest that you would like to obtain that the damper would transfer the load in static conditions. In Robot it is possible only if it connected with elastic release - type 2) on the drawing above. Whether such approach is possible and how it should be modeled depends on the detailed characteristics of the dampers to be used. May be these are not viscous dampers but friction ones? Or is it some kind of tuned mass dampers where different modeling is necessary?

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Regards,

[Rafacascudo]

Hi Pawel, thx for the help!

Yes , it will be fluid viscous dampers like these ,  http://taylordevices.com/dampers-seismic-protection.html

What is the best way to model this kind of dampers??

[Tuctas]

Interesting issue indeed (fluid viscous dampers and its implementation in Robot)...

[t.sautierr]

and to poursue rafascudo's question : if one wants to obtain a damping effect with modal analysis, is it correct to modify the damping factor in the modal analysis parameter to try to simulate a global effect for a set of a viscuous dampers in a structure? (maybe you will tell me that to simulate this you need first to perform a THA so why using a modal analysis after .... 🙂 ) 

This would be only a global approach because it will not be able to simulate local effect if dampers a located at some specific areas in the structure, etc ... but maybe it a simple way in order one's able to make himself an idea of the global behavior.