Dealing with accidental eccentricities

[Anonymous]

Hi friends,

I've read some posts about this topic but I'm not clear enough.

For example, in the code of my country (Venezuela) accidental eccentricity of 6% should be defined as follow: 

Due to the code is not in RSA, I need to set a Modal analysis and then a Spectral analysis. Now I've read that the eccentricity should be introduce here:

I read that selecting Add nodal masses is more accurate.

But now, I need to define eccentricities in positive and negative directions. I usually do this defining a moment Tx = Vxi x 6%By (X dir) and Ty = Vyi x 6%Bx (Y dir) and then adding these moments in 2 load cases (Tx and Ty), then I combine these cases with the seismic ones in several combinations. For example:

See that when 100%Sx is acting also act Tx and when 100%Sy is acting also act Ty, and then changing signs and so on so on. Of course I want to know if its possible to do all this work with RSA. I think the answer is YES but how? I read something about define 4 modal cases and define something like:

So I want to know if it's correct and then how I combine these 4 modal cases with the seismic ones in a spectral analysis. 

How do you deal with eccentricities?

Sorry if this post it too long but I think it will help several RSA users.

Best regards,

Cesar

[Simau]

Hi cdmatheus,

assuming that the Z earthquake also acts you will get 96 combination

Modal 1 X- will give you
Sismic x,
simic y
simsic z
 and 12 newmark combination

same for modal 2 3 and 4, so you 48 combinations in total

when you add the 48 opposite combinations you get 96.

If your regulation is not in RSA, you must made them by hand.

[mustafahesenow]

Hi @Anonymous

Sorry to say that but this is wrong.Let me explain to you why.

In modal analysis there shouldn't have any eccentricities this very rare analysis and only used in some cases .

So you should have in your Model only one modal Case  without any eccentricity.

To create the seismic cases you have to fallow this steps :

1-After you create the modal analysis without eccentricity you create the typical seismic forces which doesnt include any eccentricity 

So base on Above  you will have Sx , Sy , Sz 

Sz could be Deleted its not affect the design for the typical building.

2-Create New seismic cases for each eccentricity 

For example for ex=5% , ey = 0  See attach picture :

 As per the picture above New Sx , Sy , Sz will be created for the eccentricity assigned .

3-similar to above you will create new seismic cases for the other eccentricities 

.

[Anonymous]

Hi @mustafahesenow

Many thanks for your help in this post and in the other one as well.

The problem here is that the dialog box that you display is for EC8, But due to the regulation of my county is not included in RSA I have to define a Spectral analysis rather than Seismic. When I define a Spectral analysis i dont see any place to introduce the eccentricities. So i am thinking that is not possible if the regulation is not in RSA. See picture below

[mustafahesenow · Accepted solution]

Hi @Anonymous

Cesar I think its very good point and this option need really to be implemented in the new version to allow the seismic eccentricity .

The problem that when you define the eccentricity from modal analysis this option will allow to change the mass location and the structure behavior will be totally different plus also the mass participation maybe less related to new Geometry . This is all will make the torsional moment much more than the value obtained by multiple the base shear by the given eccentricity . 

[Simau]

Hi,

"The problem here is that the dialog box that you display is for EC8, But due to the regulation of my county is not included in RSA I have to define a Spectral analysis rather than Seismic. When I define a Spectral analysis i dont see any place to introduce the eccentricities. So i am thinking that is not possible if the regulation is not in RSA. See picture below"

No, it's possible

Excentricity definition  is not spectral module, but modal definition.

[Simau · Accepted solution]

Sorry, again

No, it's possible

Excentricity definition  is not in spectral module, but in modal definition.

[mustafahesenow]

Hi @Anonymous

Please check this thread to have better understanding for this case.

https://forums.autodesk.com/t5/robot-structural-analysis-forum/strange-stories-reduced-forces-results/m-p/7237987#M55581

[Simau]

So what is the point of having 2 possibilities in Robot?

via modal and via sismic, 

Is one of them actually wrong?

[Simau]

To see what will happen to Mz, i made a simple modele with 4 configurations:

1_WO_torsion.rtd:             no excentricity

2_W_torsion_modal.rtd:   5m excenticity by modal widow

3_W_torsion_sismic.rtd:    5m excentricity by sismic window

4_W_torsion_load.rtd:       Offset load of 5m

Result

Fy is aproximatvly the same

Mz

No excent                   0 daNm  (logic)

Exc with modal           8304 daNm

Exc with sismic           35571 daNm  !!!

Exc with offset load    5503

So Exc with modal is the closest to reality

Any comment?

[Anonymous]

Hi @Simau

You got really useful results! 

Also see the behavior of ex2 in model 2 and 3. I have no idea about it.

[mustafahesenow]

Hi @Simau

They are different .

when you assign eccentricity on modal means that Geometrical eccentricity which is going to affect all the modal result and torsion of course will increase even the mass participation will be different  and mode shapes will be different at all. Anyway this case is very advanced and its not required by the code only in very special building . in addition to that this Technics is not exist only on RSA and we have to be proud of that . The other softwares Like Etabs , Midas etc the eccentricity only on seismic force doesn't exist in Modal analysis. 

 Check this link for more information .

https://skghoshassociates.com/SKGAblog/viewpost.php?id=5

[Artur.Kosakowski]

Hi @Anonymous

In Robot there are two ways of defining eccentricities.

When you define it in the modal case you calculate it with the shift of masses so depending on the defined directions of eccentricities you will obtain different shape of vibrations. This approach requires definition of several modal cases one for each of the directions of eccentricities you want to include in the design.

When you define it in the seismic case (not available for a spectral analysis) you can have only modal case however as it is the 'simplified' approach the following points apply:

1. The modal analysis performed is not exact since it does not take into account the mass eccentricities. Therefore, the mode shapes are not the same as calculated with mass eccentricities defined for the modal analysis case.

2. The eccentricity of the mass is then considered "statically" in order to get the forces in the members. Therefore, the forces in the members are not exact.

3. This method provides the same base shear for all eccentricities. Only the values of reactions in specific supports are changed but the total base shears remain the same when changing eccentricities. So for instance for symmetrical building if the mass eccentricity is making it unsymmetrical the earthquake in X direction will still result in 0 base shear along Y and vice-versa.

I hope this helps.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[Anonymous]

Hi @Artur.Kosakowski

Thanks for such a detailed answer. Any idea for define an accidental eccentricity in a spectral analysis besides in modal analysis tab?

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

As it has already been mentioned for spectral analysis you can only define eccentricity in the modal analysis parameters 

[Anonymous]

Hi @Artur.Kosakowski

Thank you. I really appreciate your help.

[mustafahesenow]

hi @Artur.Kosakowski

Could you please forward it to development team to be added . This is really important many thanks.

[Artur.Kosakowski]

Hi @mustafahesenow

I have forwarded it.

[mustafahesenow]

Hi @Artur.Kosakowski

Many thanks for your Support.Its really appreciated .

[Artur.Kosakowski]

Hi @mustafahesenow

I wish I could do more in return for your great help for this forum