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
consequences.
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 - www.wb0w.com 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
>_______________________________________________
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>http://www.mscomputer.com
>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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