[TowerTalk] ridge top derating factors

[K7LXC@aol.com]

In a message dated 04/12/2000 1:15:07 PM Pacific Daylight Time, 
david.b.curtis@intel.com writes:

> EIA-222-F lists Mariposa county California as an 85 mph county, 85 is lowest
>  possible.
>  
>  But... I have a ridge-top location. Therefore, I need to derate for wind
>  acceleration over the ridge. I've read Leeson's book and done some other
>  research, but it is not straight forward and I couldn't convince myself 
that
>  I know how to calculate the correct derating factor. So, my question to the
>  group: How can I arrive at the right derating factor?

      There is a note on many county windspeeds in the TIA-222 that states 
"Site may be within a special wind region indicated on ASCE 7-88 wind map. 
Check with local authorities before specifying basic wind speed". You might 
check with your local building department and see if this applies to your 
situation. 

     Exposed ridges are particularly exposed not only to direct horizontal 
winds but also venturi winds coming UP the side of the hill and hitting the 
antennas from underneath. This is where you'll need 120 degree trusses on 
your antenna booms to keep them from breaking.

Cheers,    Steve    K7LXC
Tower Tech 

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