eigenvalues reactor

[Amine A]

Hello,
I'm doing some tutorials to learn how to simulate a seismic response of a reactor. I modeled at first the reactor and did the static and modal analysis and it gives me as eigenvalues 10Hz , 14Hz, 20Hz,... 68Hz. Then Iwanted to integrate the basemat of superstructure and eigen values become 3Hz; 3,24Hz;...;4Hz. I attached a picture of the model. Can anyone please tell me why eigenvalues has changed like that , Thank you

 

[not_a_pro]

Hi Anime A,
this depends on a lot of factors. It would be helpfull to have some further information like the boundary conditions of the besement, the mass of it and the stiffness in comparison to the reactor.
For example, when the stiffness is much lower than the reactor stiffness, which would make sense I think, the eigenvalues would decrease as seen.
Can you also upload some pictures of the first few eigenforms?

 

[Amine A]

hHi,
Thanks, I've put a bigger mass for the basemat (and tried a lot of values to see if it can change the result, but I still have the same...). Here you can find some pictures of the first few eigenforms

 

[Amine A]

2

 

[Amine A]

3

 

[Amine A]

4

 

[Amine A]

5

 

[Amine A]

6