Estimating the Wind Load
Joachim Köppen Strasbourg 2007
One important issue with a semi-permanent installation of such a large structure such as a 1.2 m parabolic dish is to provide it with a mounting sufficient to withstand winds during normal days. The mounting is held down in place with some counterweights in the form of concrete slabs. In our first installation we had available only two slabs, and soon after, a brisk breeze toppled over our dish, fortunately with some cosmetic damage only ...
We can estimate the wind load by computing the force that the wind of given speed V exerts on the dish:
A useful tool for estimation of the wind load on parabolic dish antennas can be downloaded from Andrew Corporation.
However, this force is not the one that must be counterbalanced, since the antenna is attached to a tube, about 1 m above ground, and the baseplate has a width of 50 cm. Supposing that a uniform distribution of the weights act at the middle of the baseplate, the counterweights are thus attached on a short arm of 25 cm length. Hence, the lever action will transform the force acting on the antenna into four times the force acting on the baseplate.
|Beaufort force||Wind speed||Force||4*Force|
|light air||1||1 ... 6||0 ... 0.004||0 ... 0.015|
|light breeze||2||7 ... 11||0.005 ... 0.013||0.02 ... 0.05|
|gentle breeze||3||12 ... 19||0.015 0.04||0.06 0.15|
|moderate breeze||4||20 29||0.04 0.09||0.17 0.35|
|fresh breeze||5||30 39||0.09 0.16||0.38 0.64|
|strong breeze||6||40 50||0.17 0.26||0.7 1.0|
|near gale||7||51 62||0.27 0.4||1.1 1.6|
|gale||8||63 75||0.4 0.6||1.7 2.4|
|strong gale||9||76 87||0.6 0.8||2.4 3.2|
|storm||10||88 102||0.8 1.1||3.2 4.4|
|violent storm||11||103 117||1.1 1.4||4.4 5.7|
|hurricane||12||117 ...||1.4 ...||5.7 ...|
The effects of our counterweights (slabs of concrete 50 by 50 cm; the buckets are filled with concrete and have 17 kg each):
|mass [kg]||weight [kN]||max.wind|
|Orig.design||4 slabs (6 cm thick)||110||1.1||6|
|Our first config.||2 slabs (4 cm thick)||50||0.5||4 ... 5|
|Present config.||2 slabs + 6 buckets||150||1.5||7|
Thus, it was our procedure for some time to remove the dish reflector whenever strong winds and storms were predicted or expected, and during longer spells when the telescope was not in use, such as during vacations.
After about two years, a number of concrete blocks from another roof installation become available. Two of them, each weighing about 200 kg, are now placed on the concrete slabs. These 450 kg have withstood every wind experienced so far! Only once or twice it was noticed that the antenna had slipped in elevation ... which was corrected and the bolts of the elevation axis tightened a bit further.
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last update: Apr. 2013 J.Köppen