I have created a model of high rise tower and tried modal analysis in Ansys workbench Mechanical, but I am not getting frequency values and mode shape. Value of frequency is showing 0 at all modes. Can anyone suggest why this is so?
Since you've mentioned all natural frequencies are zero it also important to know how many modes are extracted from Ansys?
If you have multiple bodies which are not constrained, each one will add rigid body modes (RBMs) to the problem, and you will end up with many zero frequencies. For instance, with no constraints a body in 3D will have 6 RBMs. If you have 2 such bodies, you will have 12 zero or very close to zero natural frequencies.
You can plot the mode shapes that have zero natural frequencies to visualize if any of them is not constrained properly and moving freely with respect to other parts. Exaggerate the mode shapes to observe the motion better. If this is the case, you need to define the boundary conditions properly. This might be due to the improper definition of contact surfaces in Workbench. If that is the case, you need to bond such surfaces to each other.
I encountered this kind of issue many times, during early days, while learning FEM. I always come back to static analysis, give 1.0g or 0.1g self-weight load condition, and check static equilibrium. If the displacements are in 1E3 or more (typically), it is sure that, boundary condition is the issue. And then, I correct it, rerun the static case. I also agree with other expert answers given above. They are quite involved and analyst has to understand the software and dynamics, to some extent.
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