On Sat, 22 Aug 2009 23:19:05 -0400, Dave Johnson wrote:
>In your post you included "guy wires" so that makes me think you are talking
>about putting up a guyed tower. If so, you don't really need any concrete.
FALSE! This statement is as silly as the 3:1 mass "rule of thumb" that the
previous poster asked about. I'm an EE, not an ME, but I went through the
University of Cincinnati at a time when we had to study some fundamental ME
courses as part of my basic EE course.
A tower must remain solidly in place in the presence of some rather
significant forces imposed by mother nature, as well as the weight of the
antennas, mast, other hardware, and a climber. Wind at the top of a tower
gets multiplied by a rather substantial bending moment at the bottom of a
tower, and forces applied at various heights create other forces at other
parts of the tower. Anchoring the tower base rigidly in place, and anchoring
multiple points along the tower with guy wires, minimizes these bending
moments and makes it more likely that the tower (and the climber) survive
those forces.
Having a tower fall is not good. Depending on what it is, what it holds, and
where it is, it can break things on the ground, hurt people on the ground,
even kill them. And at the very least, it will probably break your antenna
and the tower itself. Any one of these things can significantly lighten your
bank account, threaten your retirement, even make you homeless.
My tower is in 1 cu yd of concrete. I'm far too old and out of shape to climb
it, so I hired an experienced climber to help me build it and put the antenna
on it. We guyed it as we built it, at intervals of 30 ft (recommended by Rohn
for the 120 ft ht we built). He reported that the tower felt too unstable to
climb more than about 25 ft above the highest attached guy wires, and would
not climb above them until the next higher set was attached. And this was
with virtually no wind, the only forces on the tower being his weight on one
side of it.
I recall the calculations to assess the relative safety of a tower as being
rather complex. (In case you haven't noticed, Mother Nature causes winds to
do some rather interesting things at times, and in some parts of the world,
the earth occasionally shakes.) If you're not equipped to perform them, you
should either accept the published recommendations of the tower manufacturer
or hire a competent ME to do them for you. Some of the standards for which
these computations are done are based on local building codes, and carry the
force of law.
BTW -- someone recently asked about the cost of a tower. Since I just
finished my installation, I totalled the costs. I figure that buying a 3-el
SteppIR, securely installing it at 120 ft, feeding it, running control lines
for the antenna and rotor, and properly building the tower cost me just over
$9,000. I bought all of the Rohn 25 pieces used, as well as the hard line.
Most of the rest of it had to be bought new -- guy wire, turnbuckles, guy
hardware, guy anchors, rotor, control cable, rated coupling hardware for the
guys, etc. That cost includes $1,700 for labor to do the climbing and help me
mix and pour the concrete. Roughly $500 of that cost is the additional costs
because the tower is about 280 ft from my shack. Cost was reduced because I
did not have to pour concrete for guy anchors -- instead I used big lag
screws into the bases of giant redwoods that surround the antenna on all
sides, one screw for each guy wire (that is, 12 screws).
73,
Jim K9YC
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