spectral analysis in Robot Structural Analysis

[Mouhammad_mazzaz]

Hi All,
Please correct me if I am wrong, I have a question regarding to the response spectrum reaction results on the shear walls it showing only a positive value and that's because that the RS(response spectrum) quantity will produce only positive value, and if you want to be able to see all the positive and the negative value.
you need to see each mode separate before combining all the modes 
which can be done using the combination sign by set the mode to the number of mode which has the highest current mass participation factor in each direction

please check the following link : https://knowledge.autodesk.com/support/robot-structural-analysis-professional/learn-explore/caas/sfd...

I have 2 questions here :
1- In the link above if we set the main mode 0 thats mean the program will combine all the modes using the combination method, but if you set the mode to a certain number then the commination it will not happened cuz there is only 1 mode ? is my understanding right ?


2-How we can check the torsion irregularity by using the response spectrum analysis?

[EduardoMeruvia · Accepted solution]

Hi @Mouhammad_mazzaz 

 To answer your questions,

1. When assigning a mode to particular direction, then the overall forces will take the sign of the assigned mode. e.g. Say mode 1 has a 80% mass participation in the X-direction, while all other modes amount to less than 10% participation in the X-direction, then it stands to reason that the sign of forces should adopt that of mode 1 for the x-direction as this would be the  main vectoral force that your structure will see. 
   In the link provided by Autodesk, not sure why there is a "0" mode shown there, as mode 0 doesn't exist! Never used a "0" mode before, so not sure what that is there. perhaps someone from Autodesk team can shed some more light on that. 
   
   
2.  What do you mean with torsion irregularity? Is this the torsion cause by the seismic base shear eccentricity to the center of mass and center of rigidity of your structure? If that is what you are after then see the link below for details on how to get all that. 
   https://forums.autodesk.com/t5/robot-structural-analysis-forum/reducing-of-seismic-forces-to-center-...

[Mouhammad_mazzaz]

Hi@EduardoMeruvia 

Thanks, The problem I am facing iny model, one of the core walls reaction results are showing like this ..

Half ot it positive and the other half negative 

 

[EduardoMeruvia]

@Mouhammad_mazzaz 

That is because you have multiple load cases or combination cases selected i.e. its showing you the MAX and MIN envelops. If you select a single load case or combination case, it should show only a single value. 

[Mouhammad_mazzaz]

I am showing only 1 load case, which is in my case the seismic one. Thats
why I'm confused

[EduardoMeruvia]

Can you upload your model? 

If not, do you get the same for ALL other seismic cases? Including the simple seismic base directions i.e. Ex, Ey or Ez separately.  

[Simau]

@Mouhammad_mazzaz 

there is nothing wrong in your screenshot if it's under sismic X