On 10/2/02 2:21 PM, Jon Ogden at na9d@speakeasy.net wrote:
>Sure, but like everything with Rohn, it's not completely clear why you need
>two brackets. There is no spec given for a single bracket installation.
As a practical matter, if the top bracket is 15 feet or less above the
ground, it is probably not necessary to have a second bracket, so long as
the tower is part of the concrete pour.
Given the stiffness of Rohn 25 over such a short distance, a bracket
below a 15 foot top bracket isn't going to see much, if any, force.
>Then the other funny thing in ALL their specs is why they say "Pinned bases
>must not be used." Yet they sell the BPC25G base plate and pier pins and
>they even have some drawings for pier pin bases! Now, perhaps the term
>"Pinned bases" is something different. In general what they show on the web
>is somewhat sketchy at best.
I think they mean that you can't used a pier pin in the bracketed case,
but instead have to put the tower in the concrete pour.
--
I have a bracketed tower. Mine is 44 feet of tower, with 5.5 feet of mast
above, holding a Cushcraft A3S and a few wires. Tower is bracketed to the
house at 25.5 feet and 17 feet.
I'm not a PE, but I was able to do some first-order calculations on the
strength of the bracket for the 75 mph wind zone here in Gwinnett County.
By my calculations, the tower could hold up about 7 sq ft of antenna with
JUST the lower bracket in 75 mph winds. The top bracket alone increases
this to about 9.5 sq feet. I have no qualms about the 4.4 sq ft of the
A3S.
Key is making sure the top bracket support won't pull out. I used six
3/8" carriage bolts through the top plate of the wall (2 * 2x4). This
ought to have a steel backing plate, but it doesn't. The top of the wall
is connected to a sway brace of 6x10 lumber across to the front wall of
the house, and I reinforced this junction. Theoretically, the forces of
the tower should be coupled to both the rear, side and front walls of the
house.
The lower bracket is 3/8" lag bolted into the rim joist for the joints on
the second floor. Sub-optimal. I certainly would NOT recommend lag bolts
for the TOP bracket.
The second bracket does help. It prevents the tower from bending below
the top bracket (eg, the wind blows the top part of the tower north, and
the bottom part flexes south), which gives it more strength. It also acts
as a redundant component, should part of the top bracket fail. (the ends
of the bracket are attached to the tower with 1/4" threaded U-bolts)
My tower does not creak, squeak or otherwise make noise, even in high
winds. There is a little bit of low-frequency vibration that transfers to
the house when the gusts go above about 30 mph. It's virtually
sub-audible. It doesn't keep me awake at night.
As for height above bracket, I wouldn't consider anything much above 25
feet. Mine is 18.5 feet, with about 5 more feet of mast. Bottom line --
you gotta climb it, and Rohn 25 starts to wobble a lot as you get beyond
about 15 feet. I couldn't imagine climbing 30 feet of unsupported Rohn
25.
--
Bottom line, I think you can do a bracketed tower safely, effectively and
cheaply as compared to a guyed tower. You have to be willing to accept
some compromises. The limiting factor is the height above the top
bracket, the antenna size, and the wind zone.
If you are thinking of going with some super-sized antenna in the future,
or want to be able to grow your antenna system, perhaps a bracketed tower
isn't the best choice. Go guyed, or look at other freestanding tower
systems. Using Rohn 45 will allow you a little more leeway in height,
winds and antenna size -- but it has limits as well.
Good luck.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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