Whoa... a lot of information there, and many questions.
Please keep in mind I am on the lower end of the learning curve, so some
of what I say or ask
may be total "newbie".
- Does your "base" merely rest on the bare rock??
My thought was that a much smaller base would work, especially if the
rock was drilled and fitted
with reybar to some depth, with the concrete (reinforced) over that...
- Stresses in the base. My limited understanding says that the majority
of force by a large margin is vertical
(downward) force, other forces being very limited in a guyed tower. So a
strong concrete base of modest size directly
to rock ought to be able to support anything in the 100ft +/1 class with
no real concerns??
- Pier Pin?
Are we talking about a tapered base section that sits on top of a flat
surface, steel pin protruding for
alignment?
- torque, just double checking, you are talking about the rotational
torque that will come as the result
of slight (or larger) variances in the angle of the guy anchor points on
the ground vs. the position up the tower?
Frankly, had not thought much about that. But how is that handled in a
typical installation??
Given that I am considering doing Rohn 45 @~100ft with a modest top load
of likely only a tribander yagi or similar
and maybe a 2m beam (maybe), so I won't have the wind load some have...
of course looking to the future and
more ambitions is not a bad thing to do either... I am ONLY going to go
for this IF I can get the tower up and down
with one or two people. As of now, that points directly to the "falling
derrick" method. At least that is what I am
considering. Still formulating the full complexity and so the cost, will
have to see IF it saves anything at all vs. some
sort of crank up affair or not. However the base on rock is most likely
to be required, and has held me back for
some time, not knowing much about the success of such things.
_-_-
On 10/13/2017 10:49 AM, Bob Shohet, KQ2M wrote:
Yes, A LOT of experience at my qth!
My 130’ Rohn 45G tower rests on a pier pin on a Rohn 45 flat base on a
very wide and long concrete base ranging from only 9” – 18” deep
MAXIMUM! To spread out the stresses, I made the concrete base overly
large ~ 6’ long by 4’ wide and I used LOTS of rebar in the concrete in
the base which was in a short wire cage also constructed of rebar, and
all inside a wooden form that we constructed. We could not go down
any deeper because of all the ledge on the top of my hill. Blasting
would have been the only other option and it was not an option for me.
So, to minimize the tower rotation stresses of the extremely high
winds at my qth, I decided to use a flat base and pier pin so that the
tower guys could “self-equalize” the torque as necessary. The first
day of tower building we were quickly alerted to just how much stress
can quickly build up when, shortly after the first 40’ was put up and
the tower guys were being tightened, we heard a ear-splitting
incredibly loud metallic BANG!!, which apparently was just the tower
“equalizing” the guy torque by rotating about 1inch !!! And that was
just from the first 40’ of tower with nothing on it!
If we ever had any questions about how valuable a flat base and pier
pin installation is, they were immediately and emphatically answered!
Now to answer your question, my 130’ tower holds 3 5L Hygain
HG205CA’s (top rotatable with Orion 2800PX and 14’ chrommoly mast) and
an 40-2CD fixed SSE. These antennas and tower has been up through
Hurricane Sandy (110+ mph wind gusts), Hurricane Irene 85 mph wind
gusts, an F0 and F1 tornado (direct hits) and more Nor-easters and
T-storms with 70 – 90+ mph wind gusts than I can count. Not to
mention the stresses from 100+ icing events since 1998 when I put up
the tower. No problems with the tower ever. I must add though that
in 2009, I made the decision to switch from regular guys to star guys
at the 80’ and 120’ levels. It was after the star guys were put up
that we had the two hurricanes and the F1 tornado. I can’t say for
sure whether the tower would have survived them with just regular
guys. But I do know that the tower base has never cracked despite all
violence. I would suggest that you use oversize guy anchors and
equalizer plates as I did and make your guy anchor holes longer and
deeper as well and use a high grade concrete from a concrete company
rather than making your own in a mixer. The higher the quality and
smoother the concrete the better. And, the bigger and stronger the
guys, guy anchors, equalizer plates and turnbuckles, the better;
especially if you are concerned about the tower base.
73
Bob KQ2M
*From:* bear <mailto:bear@bearlabs.com>
*Sent:* Friday, October 13, 2017 9:58 AM
*To:* towertalk@contesting.com <mailto:towertalk@contesting.com>
*Subject:* [TowerTalk] Tower base over ROCK (shale) experience??
Anyone have advice/experience with placing a tower over/on a rock surface?
Short of blasting a hole, that is...
My QTH is very thin soil over solid shale surface, it's a hill mostly.
_-_-WBear2GCR
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