Hi Ron,
You have the picture almost right. The missing part is that you need to realize
that the Rohn specs you read in the catalog are for the SPECIFIC configuration
of
guying that is shown in the diagrams. You have also noted that Rohn does not
give you any additional information about what happens to the rating of the
tower
if you add another guy set. If you want that information from Rohn, you will
need to buy their optional engineerig services (which seem to be quite
expensive). As it turns out, there is LOTS of information Rohn does not give
about their towers. At least as important as the total number of square feet of
antenna you are putting on the tower is exactly HOW these square feet are
distributed on the tower. How far above the top set of guys is the highest
antenna and how long are the booms? What sort of twisting forces are applied to
the tower during stopping and starting or during strong winds? This stuff is
VERY important but not addressed by Rohn (unless you hire them to address it for
you).
Most hams will either simply ignore these concerns or compare their proposed
installation to others that have withstood winds similar to what they expect to
see and build them like those. A few will hire a local structural engineer to
check out their proposed installation for proper design and safety. I have been
a Rohn dealer for a couple of decades and I have NEVER known any of my customers
to hire Rohn to do any engineering services for them.
The examples of tower installations that Rohn gives in their catalogs are just a
few of the thousands of safe possible configurations you might want to use. The
very best thing for you to do is get a structural engineer involved in doing the
calculations for you. If you don't want to spend the money for an engineer,
then
I suggest you study successful installations similar to what you want to do. I
would limit my study to tower configurations that have at least a ten-year track
record. The second, equally important, study I would do is on all the tower
failures I could get data on. I would get as much specific data on what,
exactly, caused the failure and learn what mistakes to avoid in my installation.
This data is usually hard to get since hardly anyone wants to talk about their
tower failures . . .
As an example of another configuration that Rohn has published, on page MS-47 of
their latest LARGE catalog (more than 1 inch thick), Rohn shows a 55G
configuration rated for 70 mph winds and 94 square feet of antenna wind load.
The load is also specified as a 10 foot diameter solid dish right at the top of
the tower . . . obviously not what you want. The point is that the 16 square
feet of antenna loading you read about is not the ultimate maximum wind load
rating for 55G tower.
Good luck and I will be very interested to read other responses you get to your
questions.
Stan
w7ni@easystreet.com
Ron Bolton wrote:
> Unless I'm missing something here, I'm confused. I see the ads for stacking
> controller boxes, rotating tower bearings, rotating rings and such for
> mounting multiple beams on a single tower. The rings systems and the
> rotation tower systems all make models for 45G and 55G. As I have some
> interest in going that direction I decided to spec out some components and
> then figure up costs to see if I could make my dream a reality. What I
> found in the process is that the specs don't add up. I started with the
> current Rohn catalog specs for both 45G and 55G. As I'm in a 75MPH rated
> location, I used the 90MPH figures for 55G. For a 100' tower, it's only
> rated for 16.0 sq. ft. which means stacking just two C-4XL's exceed the
> limits. So from what I see, unless you're lucky enough to live in an area
> where you can use the 70MPH ratings, if you want to stack, you're limited to
> something like a TA33. So what's going on with all these pictures I see of
> these nice stack installations? Is everyone ignoring the manufacturer's
> specs? Can't believe the solution is to add more guys to increase the
> ratings as then you can't fit the lower antennas it and rotate them,
> besides, there are no specs from Rohn on how adding more guys affects the
> ratings without using their design service. Is something else besides 55G
> being used? If so, what (and is reasonably priced)? How can the ring
> systems and rotating tower systems honestly market products for 45G/55G when
> the towers aren't rated for that type of installation in the first place?
> And finally, if I'd like to stack 2 or 3 decent multibanders (like a C-4XL),
> what do you use for tower?
> Thanks.
>
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> -----
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
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