Robot Structural Analysis Forum
Welcome to Autodesk’s Robot Structural Analysis Forums. Share your knowledge, ask questions, and explore popular Robot Structural Analysis topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Modal analysis and damping

35 REPLIES 35
SOLVED
Reply
Message 1 of 36
Rafacascudo
2885 Views, 35 Replies

Modal analysis and damping

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

 

 



Rafael Medeiros
Robot Structural Analysis

35 REPLIES 35
Message 21 of 36
Tuctas
in reply to: t.sautierr

You can modify the damping (global) coefficient when you run a modal (spectral) analysis.

In the link that rafascudo attached (Taylor's devices) it writes some interesting things about it.

Message 22 of 36
Tuctas
in reply to: Tuctas

Just a comment to what i said in my previous post:

The global damping factor can be modified in case of using linear viscous damper (as the fluid viscous dampers are) and as far as i know it is the most common practice for this kind of dampers (although the dampers have to be placed properly distributed over the structure).

If someone (including the supporting team of course) has any kind of experience on the implementation of dampers in Robot (linear viscous or other kind) it will be much appreciated to tell us more. 

Message 23 of 36
Pawel.Pulak
in reply to: Tuctas

Some comments to remarks of Rafascudo, t.sautier and Tuctas - apologies that so lateSmiley Embarassed

 

The most suitable definition for fluid viscous damper is 1) from this post, i.e. "free + damper"

 

Damping in advanced parameters of modal analysis does not influence modal analysis but seismic analysis following it (somehow misleading Smiley Frustrated)  - like eta damping correction factor in EC8.

 

Tuctas also mentioned using global damping factor instead of dampers in case  of dampers properly distributed over the structure - in case of THA it can be defined in Damping parameters of this type of analysis (direct  values of relative damping for the modal decomposition method or via Alpha and Beta values for other methods)

 

I hope it helps.

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 24 of 36
mariprios
in reply to: t.sautierr

I'm working with Rafacascudo in the same project, where we're trying to use these dampers.

I'm having a problem with time-history analysis. I'm now trying to discover the structure response wihtout any damping. I have to use the non-linear analysis so I'm able to use the free dampers later and compare the results. 

The thing is, when I choose the newmark method (acceleration), the response diagram behaves as if there's some kind of damping, even when I use alpha and betha constants as 0 and also the damping constant as 0 in the release definition. Is there somewhere else where I should define the damping I want?

If I use the modal decomposition method this doesn't happen, but then I can't add dampers later.

thanks

time history.jpg

Message 25 of 36
t.sautierr
in reply to: mariprios

give your file please

Message 26 of 36
Rafacascudo
in reply to: t.sautierr

Hi all,

 

On studying this case , some doubts about the program  results led us to do some tests using a small frame model.

 

1- For TH case ,Sinodal F-T function according to 1st mode frequency is applied to VENTO1 Load case.

 

2- Brace is fully axially released at the botton and a damping coefficient is applied.

 

3- If the TH analysis is linear , everything behaves as expected

 

4- If the TH analysis is Non-linear or P-delta , the structure behaves as if there is no damping applied to the brace  and only the global damping acts.No matter what value is assigned to the brace damper , the results are the same.

 

5- Calculated Total mass(739,39kg) on modal results doesn´t match with structure total mass (1048kg , on quantity survey)

 

Model is attached

Damping -  frame.jpgDamping - linear.jpgDamping - non linear.jpg



Rafael Medeiros
Robot Structural Analysis

Message 27 of 36
mariprios
in reply to: t.sautierr

I tried to run this simple case. If I use linear analysis it works just fine, but when I try to use  non-linear analysis, the dampers don't work, and there's no effect on the structure. Is there any way I can use these dampers in a non-linear analysis?

File attahced.

Message 28 of 36
Rafacascudo
in reply to: mariprios

This can be called a "synchronized " post!!!

 

Smiley Happy



Rafael Medeiros
Robot Structural Analysis

Message 29 of 36
Rafacascudo
in reply to: Rafacascudo

Any ideas of why this is happenning??



Rafael Medeiros
Robot Structural Analysis

Message 30 of 36
Pawel.Pulak
in reply to: Rafacascudo

Confirmation. Unfortunately if Non-linear or Non-linear and P-delta analysis is activated in time history analysis the dampers existing in the model are ignored.

Reported to dev team.

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 31 of 36
Rafacascudo
in reply to: Pawel.Pulak

Thanks Pawel!!

   I did some futhers tests and this happens not only when you activate nonlinear/Pdelta on TH window checkboxes.

Having a nonlinear element like a tension only bar or an uplifted support ,is enough to deactivate the damper.

This happens even if this nonlinear element is only present on a separate structure on the same model.

 

Any chance of this being corrected in the incoming service pack??

 

thx

 

 



Rafael Medeiros
Robot Structural Analysis

Message 32 of 36
Pawel.Pulak
in reply to: Rafacascudo

It was reported with high priority but after completing development for RSAPRO 2015 Service Pack 3 which is already published.

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 33 of 36
Rafacascudo
in reply to: Pawel.Pulak

Pawel ,

    I tried to use damping through compatibles nodes or supports as a workaround for this problem , but it also didn´t work when nonlinear is choosen.It worked perfectly for Linear analysis

 

 

   Is there any other workaround to solve this problem , or  for now , any kind of damping is incompatible with any kind of nonlinearity??

 

 

   Was this always like that or it worked in any of the previous versions of the program??

 

thx!!



Rafael Medeiros
Robot Structural Analysis

Message 34 of 36
Pawel.Pulak
in reply to: Rafacascudo


@Rafacascudo wrote:
Pawel ,

    I tried to use damping through compatibles nodes or supports as a workaround for this problem , but it also didn´t work when nonlinear is choosen.It worked perfectly for Linear analysis 


Yes, any kind of viscous damper (defined in releases, compatible nodes or in supports) does not work in case of nonlinear time history.


Rafacascudo wrote:

Is there any other workaround to solve this problem , or  for now , any kind of damping is incompatible with any kind of nonlinearity?? 


In such case it is possible to use distributed damping, defined using Alpha and Beta coefficients. It was suggested some time ago by Tuctas in his post.


Rafacascudo wrote:

Was this always like that or it worked in any of the previous versions of the program??


It was always like this - since the implementation of viscous dampers in v.21 😞

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 35 of 36
t.sautierr
in reply to: Pawel.Pulak

I'm unburrying this thread : what's new about that in RSA 2016 or RSA 2017? dampers + non linear THA is working?

Thx

Message 36 of 36
Pawel.Pulak
in reply to: t.sautierr

Still not available:(

 

Regards,


Pawel Pulak
Technical Account Specialist

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report