Post subject to wind

[leisure17]

If I have a round post sticking 6m up in the air, how much relative wind pressure will there be on it. I've seen a formulae for a flat surface of 1.38v2 (/m2) but I'm guessing there will be a coeficient to lower this slightly for a round section? Lamp posts seem like they must have quite a reduction over a flat face of the same dimensions.
Digressing slightly, why should there be a different value for whether wind pressure is acting in the country or near the sea, surely a gale is the same speed where ever it is.
Thanks

 

[Ron]

The factor for your application would be about 0.75 under the typical US code. Your's might be a bit different.

The roughness of the ground surface affects the wind speed greatly in the first 10m or so above the surface. Wind coming off water is not slowed by friction as much as wind blowing across land. Therefore, the coefficients applied to the wind pressure calculations, accommodate these differences.

Wind pressure computations are a bit more complex than one would guess at first glance.

 

[rowingengineer]

Just a little addition to Ron's comments, in the context of water it is assumed to be flat, rough water can induce the same drag as small trees.

Then there is a events called cyclones (hurricanes, typhoons, tornadoes) that normally start in the sea and will travel inland only a small distance (100km or so).

However to help you with your poll problem the code you are using would be helpful.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[SlideRuleEra]

Also, to put Ron's and rowingengineer's accurate description of how surface roughness affects wind speed into "mechanical" terms, consider it a large-scale "boundary layer" .

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BAretired]

For slender free-standing cylindrical structures, you should consider the dynamic effects of vortex shedding. When wind blows across slender cylindrical structures, vortices are shed alternately from one side and then the other along the length, giving rise to a fluctuating force acting at right angles to the wind direction.

Check out some of these links:

http://www.google.co.uk/search?hl=&...ff&rlz=1B3GGGL_enCA259CA260&ie=UTF-8&aq=0&oq=

BA

 

[leisure17]

Thanks Gents. it seemed easy enough till BA mentioned Vortex Shedding, although if wind is slowed by friction then I thought it would be a lower speed & not a factor of something else?.

I think I know the the natural frequency of my pole but I can't seem to make sense of that Strouhal number. I can't ssem to find a calculation for the frequency of the vortex shedding tying in with anything other than the NF which is known.
It's only a pole !!!

 

[BAretired]

Sorry to complicate your life. According to the Canadian Code, the Strouhal Number is approximately 1/6 for small diameter structures such as chimneys and 1/5 for large diameter structures such as observation towers or buildings. For non-circular cylindrical structures the Strouhal Number is approximately 1/7.

BA

 

[rb1957]

"It's only a pole" ... yes, but it can become a missile

 

[leisure17]

Thanks for all the input Gents. I will look at the whole thing in more depth now.