At 03:33 PM 6/23/2005, K2EK@aol.com wrote:
>Hello all...
>
>Within the next few months, construction begins on my new hilltop QTH.
>
>Obviously, I am already planning for the installation of a pair of 45G
>towers, which
>for the heights I expect, Rhon specs (4A or 4B) call for guy anchors at a
>depth of 4 feet.
><snip>
>I imagine I could have the rock hammered out, but intuitively that sounds
>expensive.
>Rather than removing a lot of very hard rock, to just turn around and re-fill
>with concrete wouldn't it be possible to drill a suitable hole and set some
>kind of anchor rod in it?
You might be well advised to find a local engineer who can tell you how to
solve the problem. As you say, if you could just drill an appropriate hole
in suitably competent bedrock (or even, a big enough boulder), you could
use a chemical anchor (the fancy name for a bolt and epoxy) or an expansion
bolt. There's no question that you can do it, there are chemical anchors
and expansion bolts that have multi-tens of thousands of pound pull out
forces, but it might be cost prohibitive.
However, it's one of those specialty items, and is going to require some
analysis and local knowledge of the geology and rock properties, who to
hire to drill the holes (for expansion bolts and chemical anchors, they
don't have to be very big (if it's more than 1" in diameter, I'd be
surprised, or necessarily very deep), etc. This isn't a matter of whipping
on down to home depot and buying some 3/8" masonry anchors.
If it does work out, it's probably a matter of a few hundred dollars for
the engineer, maybe a hundred bucks for the hammer drill, and a few tens of
dollars for the bolts themselves. If you need a serious drill:like you
need to drive a borehole 10 feet deep and 2" in diameter, you're going to
want a crawler drill (like they use for blasting), and it's going to be a
bit more, as will the bolts. Either way, done right, the anchor will be
wicked strong, and the tower will collapse long before the anchor pulls out.
Just as a point of reference, Simpson Strongtie Epoxy based anchors that
are 3/4"x6.75" inches deep have a allowable tension in 2000 psi concrete of
10,500 lbs (with an ultimate failure of over 40k lbs) 3/8" anchors 3 1/2"
long have about 1/4 that strength (2500 lb allowable tension, 10,000 lb
ultimate) (assuming the steel doesn't fail first)
The bond strength is usually more than the strength of the steel, but, of
course, this depends a LOT on the rock you're setting them into. That's
why you need the engineer with local experience.
You might start looking at how they guy things like power poles, which have
fairly high pullout force requirements.
Jim, W6RMK
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