[TowerTalk] Rohn tower installation specifications
wa0ttn at netdave.com
Sun Apr 1 20:01:44 PDT 2012
It looks like the comments, both on and off the reflector, have died down
for the evening so I'll recap what I've heard so far.
1. None of the arguments regarding the bracketing specs have convinced me so
far. For those of you who proposed the "wet noodle" argument, I just can't
buy that. This tower is STRONG and I think the topmost bracket is going to
take pretty much all of the lateral strain. I didn't clarify, but the eave
mount is actually attached to the roof, not an eave, so there's a lot of
surface area behind it holding that bracket.
2. The only point made about the fixed base vs. a pinned base is that the
base is intended to provide a counterweight to any sway in the tower. But
again, with the brackets taking the stress, I don't see any purpose for the
base other than to keep it from sinking.
3. And yes indeed, my current tower has a base of 2x2x4 feet, per the Rohn
spec on page 25G-4 of my Rohn Consumer Catalog. In retrospect, I see the
spec on the self-supporting tower on page 25G-2 does call for 4x4x4. But a)
the tower has survived for 8.5 years as is, solid as a rock, and b) the
relocation will eliminate this as an issue. Per page 25G-4, a 2x2x4 "drilled
pier" foundation is the recommendation for bracketed towers, which is I
suppose where I got that spec for the first installation.
The last bit of info is that the Rohn spec for a bracketed tower only goes
down to 40 feet, whereas mine will be at 35 feet. However, interpolating
their bracketing points, which correspond to where I'll be bracketing mine,
and with the windloading of my antenna, it should be good for 85 mph of
windage, which would be extremely severe in my QTH. In addition to that, the
prevailing direction of the wind is pushing the tower and antenna towards
the house, so the worst thing I can imagine would happen in a catastrophic
failure would be the brackets would collapse and the tower would fall into
the house by the few inches (if any) of standoff the brackets are providing.
I hope that gives a clearer picture of the situation. I think I'm going to
be ok with putting in a 2x2x4 concrete footing, even though our soil is like
concrete already to try to dig a hole into. But I still think that Rohn's
bracketing specs are just way to arbitrary without some corroborating math
to show how they came up with their numbers.
Thanks again for all the feedback. I really appreciate you folks' thoughts
73 Dave WA0TTN
> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of Dave Cook
> Sent: Sunday, April 01, 2012 12:25 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Rohn tower installation specifications
> Hi guys, I'm in the process of relocating my 35' freestanding Rohn 25G
> after 8.5 years on the air. I figured this time I'll bracket it to the
house, which is
> what I should have done in the first place. But I've run across a few
> with the Rohn specifications:
> 1. The tower will be bracketed to the house mid-way up and also near
> the top of the tower with an eave bracket. The specifications for the
> "must support a minimum horizontal force of 815 pounds". I can't find any
> calculations for how they came up with this number. And it seems very
> excessive for a modest 35 foot tower bracketed in two places.
> 2. The specs also state that the base must be a "fixed base", rather
> than a pinned base. Again, in this case I don't understand why a pinned
> wouldn't be sufficient for a well-bracketed tower. Why would the base
> experience any lateral loading?
> 3. The specs also state that the base has to be the same 2x2x4 feet
> concrete that was spec'ed for self-supporting and guyed towers. In this
> can understand somewhat having a solid chunk of concrete for the thing to
> on, but the entire weight of the tower and antenna will be, by my
> just a little over 200 lbs. It doesn't seem to me that it needs a 2000 lb
> concrete to keep that from sinking into the soil.
> So in short, I'm just wondering if Rohn is simply CYA'ing themselves, or
> these specs really to be taken literally?
> Thanks and 73,
> -- Dave WA0TTN
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