Learning Forum

[lleonart]

Hi everyone, I am new in Ansys software and I am not familiar with features of the software yet.

I did a spine model of a multiple span bridge. As you can see in the first and second picture, I modeled the superstructure as a single beam element with physical properties equal to the actual geometry of the superstructure and then I modeled the substructure as usual.

[1shan]

The zero frequencies are due to unconstrained rigid body motions possibly due to the beam end releases that you have you have used. You could insert a deformation result for each mode, play the animation and check if you see any unwanted motion in your structure. You will have to redefine these connections accordingly.
Regards Ishan.

[lleonart]

I'll try this.
Thank you very much for your response.

Regards

[lleonart]

I have checked the deformation at all directions and I don't see any problem or unusual behavior.

***** Transverse Deformation (along x dir.)

***** Longitudinal Deformation (along x dir.)
(I expected this behavior since my connections on that joints are expansions)
******Vertical deformation (Along y-dir.)

And then I tried to suppress all the beam end releases and this is the result of modal analysis.
The results are still unusual, I still got zero frequencies and a very small values(close to zero) for mode 9 and above.

I have similar model in other software(for checking) and it works fine. I'm not really sure what I am missing here. By the way, I am using ANSYS because I need to perform the Multiple point response spectrum for my school project.
Thank you

[lleonart]

I finally solved the problem. The problem is with the polar moment of inertia of my superstructure which is almost equal to zero. After editing the polar moment of inertia, the natural frequency is not zero anymore and the results (frequencies and mode shapes) are almost similar with the results of the other software. Anyway, thank you very much for your previous response.

[artithombre]

how do we apply polar moment of inertia in ansys workbench? I couldn't find
the option , Please suggest me steps to assign polar moment to the components.

[ErKo]

many thanks for posting your solution.

We have a follow up question from . would it be possible to expand on the added polar moment of inertia that you used (e.g., how did you do that, and where/which parts - did you use a point mass, or changed the beam section properties,...)?

Many thanks

Erik