>Steve, Here are my unanswered questions. These are related to Rohn 45.
>
>Since Rohn's wind loads are substantially higher for a house bracketed 70'
>tower (at 56' and 28') (32 sq. ft. at 70 mph) than a tower with their two
>recommended guys at 35' and 65' (9.5 sq. ft. at 70 mph assuming tower has 8 sq.
>ft. for sidearms and 6 transmission lines coming up the sides), how much extra
>wind load, if any, would be gained by using 3 guys at 23' increments?
>
>Concerning subtracting 8 sq. ft. for the side arms, is that 8 sq. ft. a round
>member spec or a flat member spec? (If it is the former that would be
>equivalent to 4.8 sq. ft. of antenna wind load.)
>
>The Rohn notes say that the tower design includes a 1/2" and 7/8" transmission
>line on each of the three faces. Ours will only have two (probably .5 inch) so
>it appears that I can also subtract the surface area of the missing 4 lines (1
>1/2" line and 3 7/8" lines), i.e. ( 70' * (1/2 + 3 * 7/8) / 12 ) * .6 ) = 10.9
>sq.ft. But that assumes that wind load of the coax along the side is the same
>as the wind load for an antenna at the top, which probably isn't equivalent.
>How much is the right amount, if any, to subtract for the unused coax?
>
>Thanks, Pete ws4g
Hi Pete,
I think Rohn has covered your situation very well on the last page in my
catalog. This is the green page titled "Rohn Product Services Available".
They offer a service called "Product review of non-catalog towers with
simaltaneous product review for optional loads". Your tower is clearly
"non-catalog" since you plan to put guys in places not shown in the Rohn
catalog. The price for this service is $3,000 (minimum) per tower plus $300
per hour. It is all right there in the Rohn catalog.
I suspect they will take a rather dim view of a request from you to get this
information for free. It would be cheaper for them to give you a free tower
. . . With Rohn wanting this kind of cash for the answers you crave, how
good can free answers you get on the tower reflector be? What I am saying
is, anyone who has the skill to really answer you with the kind of expertise
I would trust is out there selling that skill for big bucks and not giving
it away for free here. However, if you are willing to receive some educated
guesses with no guarentees, you will get some good advice. You will also
get some bad advice. Knowing the difference is the real trick . . . and
really requires an engineering degree plus a lot of experience working in
the field.
The average person is simply not equipped to design a safe system like the
one you describe. Most of us try to over-build it and still take some
chances . . . Sometimes our towers crash. Sometimes we get hurt. Usually,
we are lucky. Minimizing the risk is expensive.
I believe the reason you did not get any answers to your original questions
is that you posed a very complex engineering problem no one here is equipped
to deal with thoroughly (for free at least).
Seriously, I would visit as many sites as I could with towers similar to the
one you propose, including antenna loads. I would ask LOTS of questions of
the owners of those towers like the following:
1. How long have these towers been up with these loads on them?
2. Who installed it and can I ask questions of him?
3. Was the system designed by a professionsal engineer?
4. Can I obtain a copy of the engineering documents and calculations?
5. Ask dozens of detailed questions about the guys anchors, guy wire
material, turnbuckles, etc.
6. Have these tower endured any heavy winds? How heavy? How many times?
You will then have a good idea of what works in your area. If you design
stronger (in all respects) than what you have seen, you should be OK.
Good luck on your project. Be careful.
Stan w7ni@teleport.com
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