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:

Force = Area * Air density * (Wind speed)ē
For simplicity, we shall assume that the dish is a solid flat plate; we may take into account its shape by an additional factor. But as an initial estimate, one obtains:
Force = 0.105 * Vē
if we measure the wind speed in km/h and the force in N.

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 speedForce4*Force
calm0 000
light air1 1 ... 60 ... 0.0040 ... 0.015
light breeze2 7 ... 110.005 ... 0.0130.02 ... 0.05
gentle breeze3 12 ... 190.015 … 0.040.06 … 0.15
moderate breeze4 20…290.04 … 0.090.17 … 0.35
fresh breeze5 30…390.09 … 0.160.38 … 0.64
strong breeze6 40…500.17 … 0.260.7 … 1.0
near gale7 51…620.27 … 0.41.1 … 1.6
gale8 63…750.4 … 0.61.7 … 2.4
strong gale9 76…870.6 … 0.82.4 … 3.2
storm10 88…1020.8 … 1.13.2 … 4.4
violent storm11 103…1171.1 … 1.44.4 … 5.7
hurricane12 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.design4 slabs (6 cm thick)1101.16
Our first config.2 slabs (4 cm thick)500.54 ... 5
Present config.2 slabs + 6 buckets1501.57

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.

| Top of the Page | Back to the MainPage | to my HomePage |

last update: Apr. 2013 J.Köppen