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#1 2016-12-19 11:11:16

dbpatankar
Member
From: Roorkee, Uttarakhand, India
Registered: 2010-05-22
Posts: 172

[SOLVED] Modal analysis of structure with cable elements

Hi,

I am trying to do modal analysis of  a structure which has cable elements. I can get the natural frequencies using MODE_VIBR in DYNA_NON_LINE. But it saves only the first mode shape. However I need all the mode shapes.

I cannot use calc_mode as the model contains cables. So, my question is :

Is there any way in aster using which I can get all the mode shapes and natural frequencies of a structure having cable elements?

Last edited by dbpatankar (2016-12-27 19:27:41)

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#2 2016-12-19 11:47:30

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,570

Re: [SOLVED] Modal analysis of structure with cable elements

hello

did you try to run an analysis on a real case ?

jean pierre aubry

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#3 2016-12-19 14:33:39

dbpatankar
Member
From: Roorkee, Uttarakhand, India
Registered: 2010-05-22
Posts: 172

Re: [SOLVED] Modal analysis of structure with cable elements

I do not understand what do you mean by real case. The structure is a four storey building frame with cables at the base storey. I am attaching the mesh and comm files in case that helps in understanding my problem.

I have noticed that as soon as I change the cables to beam elements, the CALC_MODE command works properly as there is no nonlinearity involved due to absence of cables.


Attachments:
frame2.tar.gz, Size: 37.92 KiB, Downloads: 146

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#4 2016-12-19 15:40:28

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,570

Re: [SOLVED] Modal analysis of structure with cable elements

to be able to do a modal analysis you have to prestress the cables otherwise there is no solution
just like in true life!

how to do that is explained in my book at chapter 10.2

how to perform a modal analysis on a prestressed structure is explained at chapter 15.1.9

"
The rough guide lines to preform a Modal analysis on an pre-loaded model are as such:

make a linear elastic analysis with the pre-load, for example here, a tension in the stem;

extract the geometrical stiffness matrix 'Kg' associated to this deformed shape;

add this geometrical stiffness matrix  to the mechanical stiffness matrix, 'K + Kg';

perform the modal analysis.
"

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#5 2016-12-20 08:50:56

dbpatankar
Member
From: Roorkee, Uttarakhand, India
Registered: 2010-05-22
Posts: 172

Re: [SOLVED] Modal analysis of structure with cable elements

Thanks a lot for this information.

I could see that I can combine the matrices using COMB_MATR_ASSE. But I am unable to get hot to extract K, Kg and M from the result of MECA_STATIQUE.

Can you please point to the appropriate document to look for? Or may be a test case where it is implemented?

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#6 2016-12-20 11:03:43

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,570

Re: [SOLVED] Modal analysis of structure with cable elements

here is an example
almost the same as in chapter 15.1 of my book
but with prestress


Attachments:
example_modal_prestress.zip, Size: 3.33 KiB, Downloads: 158

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#7 2016-12-20 16:15:23

dbpatankar
Member
From: Roorkee, Uttarakhand, India
Registered: 2010-05-22
Posts: 172

Re: [SOLVED] Modal analysis of structure with cable elements

Thanks for the example. But the example is for beam elements. So there is no problem in linear elastic analysis with MECA_STATIQUE. But in my case, the structure involves cables so I cannot go for MECA_STATIQUE as suggested by following error :

! Erreur utilisateur :                                                                        !
   !   -> L'utilisation de la commande MECA_STATIQUE avec l'option RIGI_MECA pour                !
   !      les éléments du type MECABL2 n'est pas autorisée.                                      !
   !   -> Vous utilisez la commande MECA_STATIQUE il n'y a pas moyen de contourner cette erreur. !
   !                                                                                             !
   ! La maille incriminée est M249.                                                              !
   !                                                                                             !
   ! Pour information :                                                                          !
   !    Le nom du résultat         : stat                                                        !
   !    Le type de concept produit : EVOL_ELAS

Instead, can I do nonlinear analysis for the same and get the required matrices directly?

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#8 2016-12-21 08:14:28

jeanpierreaubry
Guru
From: nantes (france)
Registered: 2009-03-12
Posts: 3,570

Re: [SOLVED] Modal analysis of structure with cable elements

But I am unable to get hot to extract K, Kg and M from the result of MECA_STATIQUE.

it extracts in the same way from a STAT_NON_LINE

however i suggest you to read V5.02.138 together with test case sdnl138
which is what you want to do with prestressed cables

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#9 2016-12-27 19:27:22

dbpatankar
Member
From: Roorkee, Uttarakhand, India
Registered: 2010-05-22
Posts: 172

Re: [SOLVED] Modal analysis of structure with cable elements

Thanks! I could solve my problems with modal analysis based on above discussion.

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