[TowerTalk] Re: Projected Area (something rotten in Denmark)

[Steven H. Sawyers n0yvy]

At 11:37 PM 8/21/98 -0400, Guy Olinger, K2AV wrote:
>
>Thank you! Where can one get the formula's, etc that generate this bit of
>wisdom and other such? Are they available for R45 & R55 as well?
>
>73, Guy
The problem is that there is no one simple formula - it is about 50 to 60 
formulas and they all relate to one another.

Generally on the shorter (less than 100 feet) you can safely increase the 
guy wire size and increase the allowable wind load by the same ratio 
as the guy wire strength increase. 

A Rohn 25 section will handle over 16,000 lbs of compressive load if 
you keep it straight. In a 90 MPH wind district the 100' tower in the 
Rohn literature only has a base reaction of 4,290. The larger guys
are not going to overstress the tower IF you use the Rohn guy bracket
to distribute the load in to all three legs instead of just looping
the guy wire around the nearest leg. Off course you will have increase 
amount of concrete in the ground to hold the guy wire.

Basically the same thing applies to the 45 and 55 tower sections, but 
then you have to complication of Rohn including some side arm area
in the foot notes.

 If you are truly interested in getting an understanding of this, then get a 
copy of the EIA-222 Rev F for wind loading, get a copy of the AISC Steel 
Design Handbook for allowable stresses,the Uniform Building Code for 
soils loading, and a little book called Design of Cable Structures that Tab 
books put out several years ago. These reference materials and about 
sophomore level Structural Analysis class, or just being good in high 
school physics with an understanding of vectors will get you started.
There is nothing magic about this, it is very straight forward when 
you follow a standard. If you don't follow an engineering standard, then 
you can get slightly different answers, but it is always safer to follow the 
engineering standards.

I am a PE and I have been doing crane and tower design for 25 years. 
I have been doing enough Rohn work for the local hams to have 
developed a set of basic designs. Unfortunately, no one's request ever 
matches my basic design or the last one I did. So I customize what 
I have done before. I have also sat with the spread sheet and done a lot 
of "what if" analysis. I have successfully replicated most of the Rohn 
tower designs. So I have developed a "feel for the iron" to quote one of 
my first engineering supervisors.

de n0yvy steve

Steven H. Sawyers PE
ARRL Volunteer Consulting Engineer



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