[TowerTalk] wind load vs Rohn specs

[Tower2sell@aol.com]

In a message dated Fri, 1 Sep 2000  6:01:39 PM Eastern Daylight Time, "Pete Brunet/Austin/IBM" <brunet@us.ibm.com> writes:

<< 
I've seen windload vs Rohn specs discussed before but never saw an answer
that made sense.  Let me try to explain my understanding of how to match
windload to a Rohn Tower spec.  Please let me know if this is right.

Projected area  = diameter x length
Windload = 0.67 x projected area
projected area = windload / 0.67

If an antenna is rated at 10 sq ft of windload, then its projected area is
10 / 0.67 = 15 sq ft.

The spec to use from the Rohn catalog is the round member antenna projected
area.  A configuration with a round member spec of 15 sq feet would handle
an antenna with a windload of 10 sq. feet and thus a projected area of 15
sq ft.

Also, since the Rohn specs have accounted for 3 runs of coax so we can
raise the round member spec by the projected area of those three coax runs.
For a 40' tower with 3 runs of 5/8 inch coax we can raise the allowable
round member projected area by 40 x 3 x (5/8)/12 = 6.25 sq ft.  Of course
we have to subtract the projected area of the coax, mast, and rotator that
we do use.

Let me know if this makes sense.

Thanks, Pete ws4g

>>
Pete,
You are in the wrong code. There is no .67

To get the ROHN assumed EPA value you multiply their are by 1.2 for rounds and 2.0 for flats.

EPA=1.2 x Area Round
EPA=2.0 x Area Flat

They should produce about the same number. 

ROHN has listed the actual projected areas without shape factor.

Compare your antenna EPA to the tower's EPA with your own shape factor. 

Ca = .8 to 1.2 for rounds
     1.4 to 2.0 for flats  per EIA-222-F

You did not say which towers you were looking at. FYI some of the guyed towers are limited by the diagonal brace capacity at the above the top guy level, so don't assume it is safe to add more wind area to the top of the tower because you don't have all the lines. The lines are seprate from the antenna load.

Tower2sell@aol.com


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