[TowerTalk] Tower base over ROCK (shale) experience??
Bob Shohet, KQ2M
kq2m at kq2m.com
Fri Oct 13 12:19:23 EDT 2017
Answers interspersed below in Blue...
From: bear
Sent: Friday, October 13, 2017 11:22 AM
To: Bob Shohet, KQ2M ; towertalk at contesting.com
Subject: Re: [TowerTalk] Tower base over ROCK (shale) experience??
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??
YES! And I did not use the special additive that bonds the concrete to 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...
It might, but I am just telling you my reasoning, what I did in 1998 and what the results are 19 years later.
- 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??
Correct. And those results have been borne out with the passing of 18 years and countless weather tests and the annual New England “Frost heave” event.
- Pier Pin?
Are we talking about a tapered base section that sits on top of a flat
surface, steel pin protruding for
alignment?
No. We are talking about a 18” steel pin made by Rohn (called a “Pier Pin”) that is put into the wet cement perpendicular to the base so that it protrudes about 6” from the concrete base. And then the short Rohn flat base section (with the hole in the middle for the pier pin to stick up through) is laid on the cement base. You then bolt your first normal 10’ Rohn 45 G section to that short section.
- 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?
Yes.
Frankly, had not thought much about that. But how is that handled in a
typical installation??
Most people just plant the first tower section directly into the cement base so the cement base and the bottom tower section winds up directly taking all that torque from the guys and the twisting from the wind. It is asking for trouble.
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...
You will have far more wind load than you can imagine and at the worst possible time. 100’ is a tall tower and subject to enormous forces. You WANT to use the flat base and pier pin.
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.
I can personally attest to the repeated success of the thin and wide base directly over uneven ledge, under the most extreme conditions. So can N2NT, K1RX, K3WW, N2TK, DL1IAO, W1NG, N7UN, WX0B, K3NM, W3EA, N1MM and many others who have climbed the tower and performed tower work on it. :-)
My philosophy has always been simple, put it up once and put it up right. After 18 years, so far so good. :-)
73
Bob KQ2M
_-_-
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 at bearlabs.com>
> *Sent:* Friday, October 13, 2017 9:58 AM
> *To:* towertalk at contesting.com <mailto:towertalk at 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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