[TowerTalk] Calculating Antenna Wind Load

[Dana Roode]

Tower Talkans -

I wanted to follow up on this posting from last week or so.  Ive interacted 
with several folks about this including W7NI, N4ZR, K7LXC, K5IU and others. 
  I've gotten some helpful but not particularly specific replies from US 
Tower and Force 12 as well.  My own conclusions (I'm not quoting anyone) are:

* I'm going to multiply Force-12 wind load numbers by 1.5 to compare against 
Rohn and US Tower wind load ratings

* I understand that the right way to compute wind load is using the greater 
of the projected area of the boom or elements, and not the sum of the 
squares method I mentioned below (the ARRL should update its materials, I 
got that info out of their Yagi Antenna Classics book published within the 
last year or two).

Many thanks for all of the info on this topic.  Wish I had bought the 
HDX-555 instead of the TX-455, but that's life.

  Dana


Dana Roode wrote:
> TowerTalk Folk,
> 
> As a tower novice, I have been looking into wind load calculations, to 
> see what I can really afford to put on my US Tower TX-455 (rated at 12.3 
> sq ft of wind load at 70mph).  I had been using the antenna vendor's 
> specified wind load numbers, but wondered why a 6 element Force-12 6 
> meter beam was rated at 2.0 sq ft where as 5 element Cushcraft beam was 
> rated at 2.9 sq ft.  Better wind-load design on the Force-12 might 
> explain some of it, but something didn't seem to add up.
> 
> I posted some questions about this on the Force-12 reflector and was 
> told that Force-12 uses an "effective" area calculation rather than a 
> "projected" area calculation.  Sure enough, the Force-12 brochure has 
> the following explanation of their wind load calculation:
> 
> "WIND LOAD is the worst case wind resistance for the antenna. Using the 
> latest structural analysis, the wind load is either the total element 
> wind load OR the boom wind load, whichever is the larger resistance to 
> the wind. Most beams have more element than boom wind load. The figure 
> specified is the effective area, which is the projected area of the 
> elements or boom, multiplied by 2/3 for a cylindrical surface."
> 
> It was suggested that I needed to multiply the Force-12 number by 1.5 to 
> get a number that would be appropriate to compare against the Rohn (or 
> in my case US-Tower) "projected" wind load maximum.
> 
> I did my own calculations on my C-3E yagi, adding up each separate 
> element section's wind load computed by multiplying the diameter times 
> the length. I also read an article by W7NI (January 1992 NCJ & ARRL Yagi 
> Antenna Classics) that said you compute the maximum wind load on an 
> antenna as the square root of the sum of the squares of the boom area 
> and the element area.  Calcs are multiplied by 2/3 due to the round 
> shape of the elements.
> 
> My calculations were:
> 
>   Total Element Projected Area times 2/3 = 5.67 sq ft
>   Total Boom Projected Area times 2/3    = 2.0 sq ft
>   SqRoot of sum of squares times 2/3     = 6.0 sq ft
> 
> The Force-12 catalog has the C3E wind load rating at 5.8, but their 
> manual says "5.9 square feet max at 21 degrees from boom center".  So, I 
> have 6 potential C3E wind load numbers: 5.67 (total of elements), 7.67 
> (total of elements plus boom), 6.0 (square root of sum of squares), 5.8 
> (catalog), 8.7 (1.5 times the catalog) or 5.9 (instructional manual).
> 
> Question - which one to use against my US-Tower 12.3 number?  The 6.0 sq 
> ft number makes the most sense to me - seems like the real MAXIMUM wind 
> load that can be on the antenna, slightly off center from directly into 
> the elements.  Wind doesn't blow simultaneously directly into the boom 
> and directly into the elements (the 7.67 or 8.7 numbers).
> 
> I also computed the wind load for the rest of the antennas I'm currently 
> looking at.  The results, listing vendor rating, calculation using 
> square root of sum of squares, and calculation of total of boom plus 
> element projected area were:
> 
>                         Rated  Squares    Total
> Cushcraft D3W            0.9     1.75      1.75
> Cushcraft 3 element 6m   1.8     1.1       1.5
> Cushcraft 5 element 6m   2.9     1.9       2.6
> 
> Now maybe I made some errors in my calculations, I'll have to double 
> check them.  Perhaps the vendors use a more sophisticated technique to 
> determine wind load numbers.
> 
>    Dana
> 
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