Learning Forum

[aaronpap01]

I have run modal analysis on bridge deck. I get results for the first mode shape at around 1x10^-2 Hz. When comparing this to data on the same deck with the same material properties, the results are 3 Hz. For the sake of updating the model, I have altered the densities and Young's Modulus of the materials, and no change to the frequency occurs.

Are there any suggestions on what else I can do to increase the natural frequency results?

(The mode shape of the image is exaggerated via the scale factor, just to show the shape).

 

[Sai Deogekar]

Hi, 

Are you sure all the Rigid body modes (Rigid body translation and rigid body rotation) are properly constrained before doing the modal analysis? A zero (or close to zero) natural frequency indicates that some rigid body mode hasn't been constrained.

If you are not sure, could you share images (using "Insert Image") showing the boundary conditions you have applied?

Hope this helps,

Sai

[aaronpap01]

Hi thank you for your reply.

These points are all fixed:

 

These points are all bonded connections:

 

 

 

[Sai Deogekar]

Hi,

I am not sure what the regions C, D, E look like. Could you clarify that? Do you think you have rigid body modes anywhere?

Other thing I was wondering about was the stress in the steel cables. A stress in the cables (i.e. the presence of prestress) will alter the natural frequency of the bridge. Does the data that reports a natural frequency of 3 Hz consider any prestress that you may not have considered here?

[aaronpap01]

All three are fixed supports. For C and D two lines run through each of the towers and acts as the fixed points to the ground, rather than the tower itself. Both towers have rigid structures set on them so shouldn’t interfere with frequency.

E is another fixed support to represent the road to the tower blocks. I put this in here after running a simulation and getting rigid body modes in the X direction.

As for pre stress, there are no details of it in the report. I wouldn’t know how to apply / alter pre-stress if that was a factor.