[TowerTalk] wind load vs Rohn specs

[Pete Brunet/Austin/IBM]

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


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