Okay, just for clarity, I've had a ticket for over 30 years and this
isn't the first tower I've ever put up or helped put up or been up.
And it is also important to me to do it right, but I am not looking to
do thousands in the base and do this in a realistic, practical manner.
The circumstances are different here because of the rock, and that's
why I asked this knowledgeable group.
We were in the Houston area in the past, which is mostly gumbo soil.
Here it is rock, on the side of a hill up 1800 feet, and the hole is
in solid rock - not shale, not limestone, a boulder. A solid rock
shelf. The specified hole, by the manufacturer, is 42" X 42" X 5.5
feet. We are now at about 3.5 feet, making the hole a couple of feet
shallow at the moment. It is SOLID rock, nothing more, nothing less -
no cracks, no splits, solid.
It is going to be where we will stay, likely, forever. I am a quad and
homebrew antenna guy, so it will never have multi-stacks or whatever.
A quad now, a monobander (if that) in the future. A tied-in pad at the
top which is say, 5X5 feet would have to be pushed into the soil from
the opposite side to move, and that will not happen as the area is
rock and less than an inch under the soil. That is why I thought a top
pad might help, even though I know it needs to be deeper or at least
epoxied into the stone shelf. I have to have a squared up pad at the
top anyway, so 4-5 feet square would accomplish both tasks.
The tower base doesn't have holes - it is a HG52SS and I have the
factory heavy-duty rebar base with the ears on it that double bolt to
the tower legs. The base sits in the hole, is cross-braced, and
concretes in with only the ear part sticking out to attach the tower
to.
I may cut the lowest brace off the factory base and drill 3 holes for
the three base legs to drop into and then epoxy them in with the
special rock epoxy. The factory base will then lack the lowest brace,
but I'd add one up higher that would be in the concrete pour and the
three 1" rebar factory legs would be epoxied in about 18" into the
boulder.
Still looking for ideas so this is a right as I can make it, and, for
the record, I won't lower the tower all the time, but will when I know
there is weather coming or when away for any period of time. The rest
of the time it will have a 2-el quad on it.
Mike, k5wmg
Pipe Creek, Texas
Fast cars, slow boats, big dogs, and summers off to write
On Sat, Feb 13, 2010 at 3:44 PM, Wilson Lamb <infomet@embarqmail.com> wrote:
> In answer to Kipton, below.
>
> The slug of concrete sitting in the rock bowl can't be expected to hold
> anything, unless the sides of the hole are near vertical. The point of the
> rods drilled into the rock is to take all the load in tension, like the
> nails someone mentioned and as they will do in all the base designs with
> concrete. He would have been as well off to just drill into the top of the
> rock.
>
> The unknown, of course, is the quality of the rock, which he can determine
> with deeper borings and horizontal excavation to see if there are cracks
> coming to the surface. In other words, he doesn't want to bore into a
> separated block of rock that can move.
>
> Do not depend upon letting the tower down in strong winds! Sure, you can do
> it sometimes, but what about the time you're away, or asleep?
>
> Deep expansion anchors, with their threaded rods grouted or epoxied, seem a
> good way to go.
> It would be a good idea to have an engineer sign off on it though, at least
> under the assumption that the rock is good. He would not have to come out
> to do it.
>
> A good first cut would be to use threaded rod as large as will go through
> the holes in the tower base and set the anchors to develop the working
> strength of the rod. No design can do better than that, if you use the
> right rod material.
>
> If we had a drawing of the base and the expected moment, we could actually
> talk about the loads with some accuracy.
>
> Of course one set of "insurance" guywires at the top would make things
> really nice!
>
> Wilson
>
>
>
> Not to hijack the thread or anything, but I have been following this
> discussion, and the comments suggest drilling holes into the rock base
> and attaching the rebar into the holes with something like epoxy. That I
> can understand.
>
> He is 3 1/2 feet down. Would it be a good idea to also drill
> horizontally into the sides and epoxy rebar into the sides also? It
> seems to me (uneducated in his stuff) that would have a big effect
> preventing the forces from moving the hunk of concrete. With all the
> rebar cemented vertically you are depending on the strength of the epoxy
> (plus the weight of the concrete) to prevent the tower from falling
> over. But if you also went in the sides, then you would also have to
> shear the rebar to get it to tip over.
>
> Of course you would have to be low so you do not have a chance of
> breaking out the top of the stone. Am I thinking correctly or is there
> something I am missing?
>
> --
> Kipton Moravec AE5IB .- . ..... .. -...
>
> _______________________________________________
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>
>
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