[TowerTalk] New Tower Construction - Questions
R. Otto
R. Otto" <N8NGA@one.net
Wed, 30 Aug 2000 18:36:52 -0400
Hello Bill,
Couple of things for you to consider. First, most communities that
have building inspection departments will want to see the cage
requirements spelled out on a drawing -- signed by an engineer.
Normally, this is readily obtainable from the tower company, even if
you didn't buy the tower directly from them (as in used towers). Just
look in their catalog, or call Rohn. You should follow their
recommendation. The questions you asked don't get quite to the detail
you need. For instance, you need to know what dia. of rebar is
required, the spacing, the method of tie, and how the rebar will be
integrated with the tower. I normally do NOT recommend placing the
tower itself in the cement. Rohn sells a tower base, and it's used
with "J" bolts in the concrete. By doing this, you have a way to
repair the tower without abandoning the base. The cage should be
suspended in the hole with wire to insure it's positioned properly
when the concrete is poured. In the finished product, the cage
usually isn't any closer than 3" from an outside concrete surface, and
the cage is uniformly laid within the space, often on 12" centers.
Lastly, you should read Polyphaser's report on tower grounding. Some
time ago I posted an E-Mail here saying I'd been taught not to allow
any rebar to extend beyond the concrete because it would rust
abnormally, and not to use the rebar as a grounding mechanism as the
concrete had moisture in it and when struck by lightning, the moisture
superheats and could cause the concrete to explode. I was
subsequently told I was nuts!! <G> In reading the Polyphaser
publication on grounding I found:
A) It is advisable to use the rebar cage as PART (not all) of a grounding
system. Lightning will in fact go through the tower to the rebar and
the concrete for a ground. (interesting note, most building
departments now require the rebar in concrete swimming pools to be
grounded).
B) The concrete, under certain conditions can, in fact, explode
exactly as I described and for the reasons I described. When a
proper system is done, though, the chance of this happening is very
slim.
C) Part of the grounding system should be a Ufer ground, which is
made by placing copper wire in a pattern about 3" from the base of the
concrete and tieing it to the tower legs. More on that in the
Polyphaser publication. (see http://www.polyphaser.com/ )
D) Lastly, go to that publication as well to find out how to place
your ground rods for the tower. I read many comments recently on this
list that are in conflict with what Polyphaser says. A general
guideline might be that if you use an 8' ground rod, you should have
three rods, one on each leg of the tower - 8' from the tower, and each ground
rod should be no less than 16' (double the length) from each other.
They give a very detailed explanation on how to calculate what you
need and how to set it up correctly.
Now having said all the above (which is the RIGHT way to build your
tower & base), I'm sure there will be hundreds of HAM's who tell you the
following:
"I done this a bunch of times and it werks fine"
A) Dig a hole
B) Dig it bigger
C) Place the tower base in the hole
D) Throw rebar in the hole as you pour the concrete in
E) Don't bother to call your building inspector to inspect the
base..........who needs that headache.
F) Attach one ground line to a tower leg and put three ground rods on
that line.
G) Fire up your radio and enjoy!! <G>
All kidding aside, there are many more good, constructive things that
can/might be done, and I'm sure many of the Elmers here will suggest
lots of them. It's up to you how much effort you want to invest
in doing your tower right. Hope this helps.
73's from.......
R. Otto
N8NGA@one.net
Cincinnati, Ohio
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Wednesday, August 30, 2000, 5:44:38 PM, you wrote:
BCA> I just got my building permit today, so construction begins this weekend.
BCA> I do have some questions, and thought that the TowerTalk community might
BCA> have some answers:
BCA> * I'm planning to embed a Rohn 25 tower section in the concrete base. I'm
BCA> unsure if direct contact with the concrete will cause corrosion of the
BCA> section. Should embedded tower sections be coated with some kind of
BCA> protectant? Or would a protectant reduce the mechanical coupling between
BCA> the base and the tower? Is corrosion (other than pooling of water in the
BCA> legs due to improper drainage) not really a problem?
BCA> * I know I need a rebar cage in the base, but I'm fuzzy on the details of
BCA> how to construct it. This is a standard 2x2x4 foot base, and I understand
BCA> the rebar should be about 3" from the edges of the concrete. This
BCA> translates into a 1.5x1.5x3.5 foot cage. I can see rebar at all the
BCA> edges, how much is needed on the long faces? Are there horizontal pieces
BCA> on the way down each face? Spaced how often? How about spacing for
BCA> vertical pieces? Are there pieces on the top and bottom faces? Are there
BCA> any pieces on the INTERIOR of the cage? Isn't there a problem with rebar
BCA> and tower section trying to occupy the same space with the interior
BCA> elements?
BCA> * I plan to build a small wooden form to shape the top few inches of the
BCA> concrete base. Is there something you can put on wooden surfaces to make
BCA> the concrete not stick to it? This would make it easier to remove the
BCA> forms later.
BCA> That's all for now, other questions I got answered by searching the
BCA> archives. Great resource, TowerTalk.
BCA> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
BCA> Quote: "Boot, you transistorized tormentor! Boot!"
BCA> -- Archibald Asparagus, VeggieTales
BCA> --
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