Modal analysis

[Aliyaghfoori]

Dear STKO
I want to calculate fundamental periods of the layerd soil. I used different models
1)soil with fixed base and periodic boundary
2)soil with asdabsorbing boundary condition(ASDAbsorbing Command) at the edge and bottom of the domain.
The outputs of the two models are different.
How is the calculation of mass and stiffness matrix in the software?
Thanks

[STKO Team]

The difference is due to how the absorbing boundary work:
before the dynamic analysis it's just a fix (fix Z at the bottom, fix X at the left and right sides, fix Y at the front and back sides).
So your model is constrained in shear modes!
You cannot use those boundaries for eigenvalue analysis

[Aliyaghfoori]

Thanks for your reply.
Have the size of element effect on the modal analysis results?
If i use lysmer dashpot at the base and at the vertical boundary have effect on the modal analysis output?

[STKO Team]

Have the size of element effect on the modal analysis results?

Yes as in all FEM simulation: the finer the mesh, to closer the numerical result is to the exact result

If i use lysmer dashpot at the base and at the vertical boundary have effect on the modal analysis output?

You can't. Dashpots works with velocity, a modal analysis is not a dynamic analysis

[Aliyaghfoori]

Thanks very much

[STKO Team]

You're welcome