Error 2809 - Modal Analysis

Gabriela.Bercot
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Enthusiast

Error 2809 - Modal Analysis

Gabriela.Bercot
Enthusiast
Enthusiast

Hi,

 

I am working a model and I am getting error 2809, it seems to be something linked to the Modal analysis as when I delete this, there's no issue. However, I need this one the analysis. I checked everything I thought.

Could someone help me with this? Not sure how to fix it.

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Krzysztof_Wasik
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Autodesk Support

Hi Model is not supported. It causes static instability. For such model program cannot calculate normal modes.



Krzysztof Wasik
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Gabriela.Bercot
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How could I have seismic loads in this condition? I quite don't understand, I produced a model before and had Modal Analysis/Seismic before and I haven't got the same error - something I may be missing perhaps
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Krzysztof_Wasik
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HI @Gabriela.Bercot 

In my opinion supports are missing

I have deleted all load cases except load self-weight and run calculation. Refer to displacement presentation.

Krzysztof_Wasik_0-1707828415282.png

You have to define supports in the model. Otherwise all results will be incorrect.



Krzysztof Wasik
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Gabriela.Bercot
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Understood, I'll try that. I supposed to have a raft foundation, how do I define this on the model as the support?
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Krzysztof_Wasik
Autodesk Support
Autodesk Support

Hi @Gabriela.Bercot 

I would recommend defining elastic support within slab 400 thk thickness label.

Refer to

 How to apply soil elasticity on a raft foundation created as a panel in Robot Structural Analysis

 

Definition procedure is presented also in the following video

 

 



Krzysztof Wasik
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Gabriela.Bercot
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Hi @Krzysztof_Wasik, thanks for your response, I added the spring values to the raft slab. However, I am still getting weird values on my bending diagrams. Any idea why that could be happening?

GabrielaBercot_0-1707839293548.png

 

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Krzysztof_Wasik
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HI @Gabriela.Bercot 

Which is member number with weird results. Could you explain what you expect. 



Krzysztof Wasik
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Gabriela.Bercot
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Hi @Krzysztof_Wasik.

 

I was expecting diagrams looking like the picture below as I didn't set up releases for my beams - the picture it is just an indicative from another model.

 

GabrielaBercot_0-1707899616108.png

 

However I am getting bending (My) diagrams like this, it seems something is off with the model

GabrielaBercot_1-1707899913147.png

 

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Krzysztof_Wasik
Autodesk Support
Autodesk Support
Accepted solution

Hi @Gabriela.Bercot 

Members 44 to 48 are connected to "wall" members. Moment shows interaction with wall finite elements.

Krzysztof_Wasik_0-1707904767539.png

 



Krzysztof Wasik
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