Elastic Foundation
Elastic Foundation
(OP)
Hello.
Can anyone help me with my problem, please? I am working on my project and the task is modal analysis of tank on elastic foundation in Ansys APDL. Elastic foundation is represented by Winkler's model.
For elastic stiffness foundation I used element type SURF154 and add real constant.
This element I surfed to the bottom of the tank. Then I set boundary condition to the base of the tank - fixed displacement to all base without displacement in direction UZ.
Eigenfrequencies of this tank are much lower (1st freq. = 11 Hz) than the tank resting on the rigid foundation (1st freq.=120 Hz). I think that is not correct because I think that eigenfrequencies of the tank on elastic foundation should be higher than tanks on rigid foudnation. I think that I set wrong boundary conditions which should represent Winkler's model.
Could anyone help me?
Thanks.
Can anyone help me with my problem, please? I am working on my project and the task is modal analysis of tank on elastic foundation in Ansys APDL. Elastic foundation is represented by Winkler's model.
For elastic stiffness foundation I used element type SURF154 and add real constant.
This element I surfed to the bottom of the tank. Then I set boundary condition to the base of the tank - fixed displacement to all base without displacement in direction UZ.
Eigenfrequencies of this tank are much lower (1st freq. = 11 Hz) than the tank resting on the rigid foundation (1st freq.=120 Hz). I think that is not correct because I think that eigenfrequencies of the tank on elastic foundation should be higher than tanks on rigid foudnation. I think that I set wrong boundary conditions which should represent Winkler's model.
Could anyone help me?
Thanks.





RE: Elastic Foundation
Frequency = (k/m)^(0.5)
And when you use a finite (less than infinite) stiffness, you have reduced k, and therefore the Frequency is reduced.
As a check of sensible solution magnitude, you should see a simple global vertical "bounce" mode, with a frequency which correlates to (k/m)^(0.5) where k is the total vertical stiffness of the foundation, and m is the total mass of the tank and contents. This mode would not be evident in the rigid foundation model, because k is infinite for this case.
Conversely, if you use the Winkler model and set the coefficient of subgrade reaction to a very high value, it should trend back to the rigid foundation solution.
You might want to think about the other base constraints while you're looking at it - fully fixing all the other DOF at every node might over-restrain the base of the tank.
Hope this helps!
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