[TowerTalk] Guy Anchor Advise

[Jim Lux]

At 03:33 PM 6/23/2005, K2EK at 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