John, in this case the Rohn design will not work. The 40 ft Rohn design
calls for a bracket at 15ft and 30ft.
Common sense says other bracket locations would work but there is no
manufacturer's data to support that.
The safe moment of restraint for 25G has been published in the past but not
in recent documents. It was about 7,000 ft lbs back when. The old
documentation also stated a wind load of about seven pounds per lineal foot of
tower just for the tower sections alone, at 70 mph wind speed. One can take
this information and derive a rational conclusion about wind load and wind
speed but there is no supporting documentation and you are out on your own.
Since the Rohn design can't be used, it becomes imperative to find a
competent engineer to do the proper calculations to derive some credible wind
load and survival data.
Note that there has been no mention of the desired antenna wind load or
survival wind speed. Both of these are critical to determine what can be done
safely.
This is a classic example of projects that can go awry and cause property
damage or worse yet injury or even death to an individual.
73,
Gerald Williamson, K5GW
GM Texas Towers
In a message dated 5/5/2016 2:04:27 P.M. Central Daylight Time,
john@kk9a.com writes:
I would do what the manufacture says: http://www.rohnnet.com/bracketed
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] Update on 40' non-guyed tower question
From: Rob Matherly <jimrob@gmail.com>
Date: Thu, 5 May 2016 11:08:56 -0500
A couple weeks ago I asked about self-supporting a 40' Rohn 25 tower.
After input from the list and a no-vote from the Missus on guy wires strung
across the yard, I've gone back to my original plan to house bracket it. I
have a two-story with 10' ceilings, so there's plenty of house to hold it
up. (There is some concern regarding my metal roof, but less than I'd have
to deal with if I were to anchor a guy wire in a flower bed.)
Question #1 - I've read at various sources that I can go 16' above the
bracket. Is this correct? Assuming my rough estimations of the house
height are correct, my top bracket will be at about 30'. I will also have
a bracket in the 15' area. I should be safe with about 45' of total
height
including my mast f I understand the Rohn data correctly. Let me know if
this is incorrect, please! I will also be sistering the studs to help
spread the weight out a bit.
Question #2 - I'm going to homebrew the house brackets since I have the
equipment and know-how to build it correctly (and not some hobbled-together
mess of metal.) Does anyone here have dimensions and specs on the
materials used? I believe the angled pieces are 2x2x3/16 galvanized, but I
am not 100% on that. I do not have any information on the thickness of the
flat steel.
Question #3 - It is my understanding that, since it is bracketed to the
house, the concrete base does not need to be as large - 2x2x3 as opposed to
3x3x3. Is this correct? I don't want ground heave to damage my house.
Thanks again, folks!
Rob, w0jrm
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