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[Towertalk] Rohn 25G Questions

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Subject: [Towertalk] Rohn 25G Questions
From: (Steven H Sawyers (na0ia))
Date: Tue, 06 Aug 2002 22:07:37 -0500
At 08:52 PM 8/6/02 , Jim Barnett wrote:
>I recently acquired a Rohn 25G Tower and I have a few questions for the list 
>members. BTW, It will be up approx. 60 ft. and I am in an 70mph county.
>Why are the holes on the section legs 2 different sizes? One is 3/16' and the 
>other is 1/4". Can I drill out the 3/16" holes and make them 1/4" like the 
>rest or should I leave well enough alone?
>I called an engineer at Rohn yesterday and he told me that he had no idea why 
>they did it that way. He said they were engineered in the 60's.

Obviously they have had some turnover in their engineering department, and the 
engineer did not want to go look up the old calculations to see what was going 
on. Every company, which manufactures life critical equipment, keeps very 
detailed and formal copies of the structural calculation. If they did not there 
would be no defense in the event of a law suit. Once upon a time I was an 
engineer for a crane company so am very familiar with engineering and legal 

When I started to get interested in towers, I wanted to make sure I was not 
missing something, so I did an analysis of the Rohn tower line to see if I 
could come up with the same answers they publish. I spent a lot of time head 
scratching and punching numbers analyzing their design.

My original calculations showed that Rohn 45 was slightly stronger thtn the 
published data. Something was not makeing sense, so I called the engineer and 
talked to the guy that did their custom engineering (where you pay them $3000 
to analyze your tower) and asked some questions. 

At that time, and this was almost 20 years ago, he told me that the bolt areas 
are selected to match the tensile and compressive strength of the tube with the 
bearing arae of the  tube on the bolts and the shear area of the bolts to come 
up with a matched design. For Rohn 25 and 55 the bolt combination selected - 
fine threads, size of bolts and grade of bolts come into this - the bolts are 
slightly stronger than than the tower leg. For Rohn 45 the bolts are slightly 
weaker which is why I could not get my numbers back then to match - I was just 
looking at tube tension and buckling at that point and had not investigated the 
bolting yet. So I followed through with the bolt analysis and sure enough they 
were right.

The tower bolts are pretty important if you are going to get the maximum load 
on the tower. The weakest link is what you are looking to avoid. 

I generally -- except for 30 foot towers -- replace the bolts with new bolt 
kits from Rohn. I get mine from WB0W - on the web to get his phone 
number - they are cheap insurance.

>Rohn's guy wire recommendations on a 60 ft. tower are one at 28 ft. and one at 
>45 ft., 48 ft. from the base. I only have about 39 ft. of space from the base. 
>Will this cause any problems?

Depends on where you are - wind area wise - and what you are going to put up 
for antennas. Really needs a full analysis. Obviously due to legal 
considerations I cannot make a blanket reccomendation, but my previous analysis 
of similar towers indicated that I wanted to use 1/4 EHS for the guys as a 
starting point for the detailed design. This should get your local PE started 
in the right direction.

>Last but not least, where can I buy Rohn accessories for a 25G? Guy wire 
>brackets, etc.

Again - WB0W - no financial connection - just a very satisfied customer. Check 
out his gin poles - I use them.

>I would appreciate any recommendations.
>Thank You,
>Jim Barnett N3TBN

Your welcome - just remember that internet advice is sometimes only worth what 
you pay for it. Make sure you double check everything with a local PE. Towers 
are necessary for antennas, but not worth dying for.

Steven H. Sawyers PE
ARRL Volunteer Consulting Engineer

>Self Supporting Towers, Wireless Weather Stations, see web site: 
>Call 888-333-9041 to place your order, mention you saw this ad and take an 
>additional 5 percent off
>any weather station price.
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