At 09:16 PM 12/29/2006, AB4D@aol.com wrote:
>Hi All and Happy Holidays,
>
>I'm in the process of trying to put together an installation of Rohn 25g
>somewhere between 39 to 49 feet high as the first of several towers
>for my QTH.
>However, I have been unable to find enough sufficient information in the
>Rohn published specs to put together what I believe is a suitable
>engineering
>package to request a building permit.
What level of analysis is needed for your permit folks? If you need
signed and sealed drawings and calcs, then you'll need to find an
engineer, and he or she will be able to do the design and the calcs.
>
>My proposed installation is as follows: Concrete base 3.5'x3.5'x3.5'
>reinforced with a rebar cage and utilizing a Rohn SBH25G (short
>tilt tower base
>section) encased in the concrete base, tower bracketed to the house
>at the 21
>foot level, bracket (Rohn HB25CG) bolted to the house with 4 threaded rods
>extending 6 - 8 feet into the attic, double bolted through multiple
>arrays of 2 x
>4 wood joist stud supports, and extending the tower another 18 to 28 feet
>above the bracket without any guys. The antennas (VHF/UHF yagis) will be
>mounted to a Glen Martin Hazer H-4 with a total windload of around
>10 square with
>the hazer only extended during times of station operation.
From a permitting standpoint, whether you claim you're going to
retract the thing or not is probably a non-issue. You've got to
analyze for the extended case (that is, you can't control whether a
90 mi/hr gust happens to arrive out of the blue while you're operating).
>
>What are your thoughts concerning my proposed installation and what is the
>best way to convert the data provided by Rohn? The specs provided
>by Rohn for
>bracketed towers don't cover anything bracketed below 30 feet and extending
>to 39 feet, or bracketed with one bracket. I can use two brackets, but the
>lower bracket would not provide much support, because I can only
>attach it to
>the 4 inch wall studs or use anchors behind 3/4 inch plywood without
>performing modification to the wall framing of the house. I know
>using less/lower
>bracket support devalues the tower load rating, but by how much? The tower
>would probably never need to be climbed above 30 feet as the hazer can be
>lowered down to 21 feet which allows the antenna system to be
>worked on at or just
>above the roof top level, and if necessary the tower could also be tilted
>over.
I think a bigger issue than the tower itself is the loads on all that
bracketing and your house. Your design might just wind up being a
very effective way to rip the studs out of the wall, for
instance. You need to find someone who is familiar with how your
house is built and what kind of loads it can tolerate.
>
>I previously performed a similar installation at a prior location with the
>bracket at 18 feet, tower to 39 feet, with a windload of about 16-18 square
>(stacked Cushcraft X-7 with X740, under a 13B2 and a A50-5S, H-4
>Hazer, single
>12 foot Hy-gain mast, Yaesu SDX 800 rotor). The installation was
>very secure
>without any problems even during an ice storm with 3/4 inch ice loading, but
> I installed it without obtaining any permits, therefore I don't have any
>engineering calculations from the last time.
But that's just one anecdotal case, with a different house with a
different tower with different loads, so it's not necessarily
directly comparable. You might have "gotten lucky" with that installation.
There's a fairly large difference between "probably won't fail" and
"certain it won't fail", and the difference is NOT in the particular
materials, design, or components, but in the calculations and
engineering: the knowledge.
Building permit folks, if it's not a rule for "aesthetics", rather
for "safety", tend to want "certain" not "probably".
Jim, W6RMK
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|