You've received some good advise and some not so good. Bending tower
legs will weaken them, thus reducing the tower's ratings.
If I were to "plumb" a tower, I might have 5 days out of the year where
The Plum bob would not be displaced by the wind or even a light breeze
at a 20' height and like Jim I just used a bubble level on all three
legs (cancels out any error in the level.
As I said some time ago, if the tower is within the manufacturer's
specs, it's with in its other ratings as and "good-to-go!" The
deviation will not be visible to anyone other than you.
Another thought. With 1.5" out at 20 feet, the base needs to move a tiny
fraction of an inch, Digging out to move a 0.1" or less? I calculated
way less to the point of requiring a Micrometer to measure the distance
I know it's semantics, but to me Levels, transits, or anything other
than a plum bob is not plumbing a tower even though the tower probably
is closer to being plumb with those methods than with an easily
deflected plum bob. Look up transits and check their accuracy in inches
at so many feet.
This is how I did the bottom sections of my 45G.
http://www.rogerhalstead.com/ham_files/tower13.htm While the pour is
fresh you can easily correct it with guy tension. I borrowed a transit
and found my level was a lot easier to use. Yah, I know, you just
center the bubble for normal jobs.
Actually my level also has a digital readout that should be calibrated
for each use and it's a simple procedure, but getting the tower plum to
3 places, or even 2, places to the right of the decimal seemed like a
waste of time and can be very frustrating to accomplish. I do have a
Lazer transit, but look up the cost of a surveyor's transit.
I believe they have gone to using "Augmented" GPS. They increased the
accuracy and decreased the number of measurements require over hilly
terrain, but AFAIK Two accurately placed surveying units, plus the
Augmentation unit would be required, unless you are close enough to an
existing transmitter and a little Trig "if cost is no object".
OTOH Guy tension over time (depending on the soil will SLOWLY" move
the base. BUT there is a much easier way than digging out one side of
the base. The other sides do not need digging out. However when back
filling use water to make mud out of the back fill dirt. That
essentially becomes "undisturbed" soil
At-any-rate, use a water drill to thoroughly soak that undisturbed soil
all the way around the base. use guy tension to easily and precisely
position the base.. On the down stream side the soil is easily displaced
and on the upstream side the soil will follow the base as if it were
pulled by a vacuum. No digging and no back filling for the small
distances involved. done carefully, it might take a couple of hours,
but equalize the guy tension as soon as the bottom sections are as plumb
as you want them
It won't take a lot of extra tension on the down stream side.
Be sure to let the ground cure/settle as you would concrete before
putting much load on it.However it will be ready for load far sooner
than concrete
Do this whether you dig out one side and back fill. (Seems like a lot of
work when the base is going to move only a fraction of an inch if it's
already in spec.
Jim's suggestion of the loos gauge works very well. OTOH a Pier Pin
Base would have rendered all this work unnecessary. Were I to do the 45G
over, that's what I'd use, but as I can no longer climb, I'd have a big
crank up, tilt over installed.
73 es good luck,
Roger (K8RI)
On 2/12/2016 Friday 1:02 PM, Jim Brown wrote:
Doug,
I agree with all of the advice quoted below. My two towers both have
the bottom section in the concrete pour, and were plumbed simply using
a standard bubble level, with the bottom section temporarily guyed
during the pour. Once built, guys were tensioned using a Loos gauge,
and are periodically (every few years) using that Loos gauge. During
the initial build, tensions were adjusted so that there was no visible
wiggle. The tallest of my towers is 120 ft of Rohn 25, with guys at
30, 60, 90, and 120 ft. The shorter one is 45 ft of "Rohn 35" (really
a private-labeled wider-gauge tower that's between 25 and 45 in
spacing and construction, specified by Motorola for 2-way
installations). The shorter tower is unguyed, and sits on a LOT of
concrete.
73, Jim K9YC
<snip>
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