I have a query related to Vortex shedding.
As per client's requirement in one of the project, Dynamic wind analysis is to be carried out for columns.

As per our understanding, Vortex shedding option has to be checked for columns. Please confirm that our understanding is right.

But if i check the column for vortex shedding in PVELITE, It gives very unrealistic result, even the anchor bolts are failing with twice of the bolt dia than the one which is used without selecting this option.

Please clarify,
1.when the vortex shedding option is to be used ? and
2.How to perform Dynamic wind analysis in PVELITE?

Please notice that the columns have equipment supported platforms & ladders along it's length which may resist the formation of vortex shedding.

Thank You.
Regards,
Bipin
L & T.

Hi Bipin

Vortex shedding occurs mainly in a fairly tall vertical vessel which subjects the column to cyclic pressure differences on either side of the vessel. If the period of these pressure differences coincide with the natural frequency of the free standing tower, the amplitude of the deflections can be excessive, rendering the tower unstable. These large cyclic defections can cause large longitudinal stresses to be experienced in the lower sections of the tower (such as you are experiencing the excessive bolt loads). Under these conditions, the tower is said to be excited. It is rather like pushing a child in a swing where the push is given at the natural frequency of the child in the swing, and the amplitude grows with each push. Now, the tower has natural damping from internal friction and other factors such as insulation, internal liquid and attached pipework etc. It is interesting that the platforms themselves server as a damping device.

We give the user the option of checking or un-checking the vortex shedding box. First, see if the vortex shedding frequency is near to the natural frequency of the tower. Remember, the vortex shedding frequency is associated with a wind speed. If the frequency of the vortex shedding is critical at a wind speed you expect the tower to experience, then it would be wise to check the box. I have a feeling that you can leave the vortex shedding box un-checked.

When using a wind code, PV Elite follows the procedure set out in that code. You do not have any control as to whether the analysis is static or dynamic. That is determined by the natural frequency of the column, and PV Elite detects that automatically and does the analysis accordingly.