TOWER - WIND LOAD

[orlenteam]

Dear Sirs,
We have designed column of 70 meters height acc. to AD S3 codes.
We have a problem with interpretation of wind load calculation results because all critical speeds are less than 25 m/s.
Microprotol software has generated following message:
The calculation due to perpendicular actions of wind is not foreseen because it correspond to Karman vortex action,
- at least one critical speed is less than 25 m/s
- effects due to Karman vortex may be induced. Further verification necessary
- appropriate solution required, for example, helicoidal plates or similar.
On the basis of our experience, we make an assumption, that Karman vortex action will be replaced by platforms and ladders (23 pcs of platforms). In this case we increased shape factor (cf ) from 0.7 (acc. to AD codes) to 1.2 (acc. to DIN 4133 for helicoidal plates).
We wait for your advise if our reasoning is right or not.
If not, please indicate what shall we do, because we do not want to use Karman or similar vortex breakers .
Best regards
JSP&LS

 

[carthago]

As I understand there is no way to change the wind excitating frequency = f = Strouhal no * Diameter / Viscosity ???
I suggest the following:
Why not "play" on the column modes , by for instance:
1/- increasing the skirt thickness
2/- or/and: make the skirt larger at its bottom ( i.e conical thickness) = this would change the skirt moment of inertia
3/-or/and: if they are different design temperatures along the column make use of the real Young modulus ( which is also a function of the temperature ) ??

I believe also ( but it is not easy to quantify ) the platforms, the ladders , piping,.. would introduce a Vortex effect and wil act lik a spoiler...

Anyhow , god luck and keep us informed