John,
I wanted all the strength I could get using the adapter plates. The original tower base
plate had only 6 - 1" mounting bolts, while the new tower plate has 12. Besides, I
did not know mathematically how to figure what thickness of steel plate I would need.
1" sure seemed thick enough to me.
With the additional six thread stock bolts coming out of the concrete and going
through the adapter plates and tower base, I am confident that the tower will
stay in place.
Ray,
N6VR
----- Original Message -----
From: john@kk9a.com
To: towertalk@contesting.com
Sent: Sat, 15 Aug 2015 10:28:38 -0400 (EDT)
Subject: Re: [TowerTalk] Concrete anchor
Why wouldn't you just tap 1-8 holes in the adaptor plate, bolt this plate to
the concrete using the existing bolts and then bolt the new tower base
directly to the adaptor?
John KK9A
To: Rich Hallman - N7TR <rich@n7tr.com>
Subject: Re: [TowerTalk] Concrete anchor
From: Ray Benny <rayn6vr@cableone.net>
Date: Sat, 15 Aug 2015 03:50:13 -0400 (EDT)
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Rich,
I replaced a medium size 71ft tower with a heavy duty 71ft tower several
years
ago. None of the existing six 1" bolts matched with the new tower base
plate,
12 bolt holes. In a sense, I made adapter plates for the old and new tower
base
bolts
What I did was purchase two 1" thick by 10" flat steel plates (one was 30",
the other 36" long) and scribed the existing 6 bolts, and the new tower base
bolt pattern onto these plates. I then had the steel company drill 1 1/8"
holes
at the center of the scribe marks. Now the 1" steel plates fit over the
existing concrete bolts, and allows the new tower base plate to set on top
with
matching holes. The existing 6 bolts were then bolted onto the 2 plates with
nuts between them and the concrete. On all but six of the new base plate
bolt
holes I used short bolts, nuts and bolted the plates and new tower base
plate
to the concrete. The remaining 6 holes would have new bolts (hardened
threaded
stock) epoxied into the existing concrete base.
Next, I was able to find a local guy who specializes in concrete boring. I
had
him bore 6 - 1 1/8" holes into the concrete base to 20" depth, aligned with
the
steel plates and new tower base holes. I used the steel plates as templates.
I
then bought 1" hardened threaded rod stock, cut to 24" lengths at the steel
shop using a special blade (remember its high grade steel) and used the
epoxy
recommended by the steel company to epoxy in the new 1" thread stock. Since
the
1" plate holes and concrete holes were cut to 1 1/8", there is not much room
for miss-aligning. Make sure the thread stock goes thru everything and will
seat at the bottom of the hole. Not all mine came out properly aligned so I
had
to use a rotary rasp and elongate a few of the holes on the steel plates.
Mixing and pouring the epoxy is a messy job. The epoxy comes in two separate
tubes and you must use a special mixing applicator device that mixes the
epoxy
as it is injected into the holes. Luckily, my local concrete guy had one I
could borrow. Putting the right amount of epoxy into the hole is a hit and
miss. You want just enough in for it to ooze out of the hole once the thread
stock is pushed in all the way down, but not too much extra. The epoxy is
not
cheap. The first time I tried, the epoxy did not come out of the hole, so I
had
to pull the threaded stock and add more epoxy. This makes a real mess, the
epoxy sticks to everything! the epoxy hardens in about 10 minutes and so the
thread stock must be set right the first time! I suggest you use a small
wooden
dowel as a depth gauge once you determine the right amount epoxy that is
needed
per hole.
According to the manufactures specs, maximum pull out strength is obtained
at
just 20" depth into the concrete for a 1" threaded rod stock in a 1 1/8"
concrete bored hole. You must go to the specs and see what size hole to bore
and to what depth for your base bolts/threaded stock size.You cannot just
bore
any size hole, fill it with epoxy and shove in the thread stock.
I feel totally comfortable with what I did. And according to the specs, the
bolt will shear/break before it would pull out of its hole!
I can provide photos of the 1" plates, the concrete boring, and the final
completed job.
With the right size threaded stock and correct hole size bored and depth,
epoxy
should work on any concrete tower base, as long a the base meets the
manufacturers size and rebar requirement.
Good luck,
Ray,
N6VR
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