>
>Greetings!
>
>Every amateur tower I have seen around here so far uses screw anchors.
>Certainly they are easier to use than constructing the concrete types.
>However, being the curious, cautious type, especially after reading this
>reflector, I began to wonder about their effectiveness.
> I am planning 110' of R25 with at least 8 sq feet of antennas at the
>top, anchors spaced 50 ft from base. I have estimated the maximum anchor
>load to peak around 4,500 lb with 110 mph winds (I live in hurricane
>country). I could sure use some help confirming this. I have a Quattro Pro
>spreadsheet set up to analyze forces and would like to compare notes with
>someone more experienced with tower load analysis. The preload alone is
>about 1,800 lb per anchor (15% of breaking strength per guy).
> Well, someone mentioned that the Rohn screw anchor is good for 2,500 lb
>in 'normal soil' (a local utility company rep confirmed that the soil in
>my area is 'normal'). That makes it clearly unsuitable for my tower. I
>think it has a 4 inch diameter screw, and goes down about 3 feet. I can
>get some 6 inch diameter units having 60 inch shafts from a local utility,
>but they didn't know the rating (see below). The person I spoke with
>thought that they might be made by AB Chance.
>
> I contacted the AB Chance company, a large maker of earth anchors (see
>their website @ http://www.hubbell.com/abchance). They offer an
>"Encyclopedia of Earth Anchoring" which, I assume, features AB Chance
>products, and some software which helps you select the appropriate anchor
>for your application and soil.
> The Virginia distributor, J.A. Walder (http://www.walder.com,
>email@walder.com) was very nice and will be sending me these. They faxed
>me some screw anchor model numbers, pullout strengths, and prices, which
>follow. They don't accept credit cards, so I don't think they are used to
>selling direct to individuals.
>
>STRENGTHS ARE FOR 'NORMAL SOIL'. I don't know how to adjust the ratings
>for other types.....(* indicates galvanized)
>
>MODEL # SCREW DIA SHAFT LENGTH PULLOUT PRICE
>315SA 3 IN 15 IN 200 LB $5.25
>330SA 3 IN 30 IN 1400 LB $6.00
>430SA 4 IN 30 IN 2500 LB $7.50
>404 4 IN 40 IN 3000 LB $12.90
>604 6 IN 48 IN 4000 LB $15.24
>*4345 4 IN 54 IN 3000 LB $26.76
>*6346 6 IN 66 IN 4500 LB $34.08
>*816 8 IN 66 IN 10000 LB $52.86
>
>Clearly there is a relationship between the screw diameter, shaft length,
>and the pullout strength. Can anyone quantify this relationship?
>Apparently, it has to do with the weight of an upside-down cone of earth
>starting at ground level and pointing down to the screw.
>
>It would appear that I should go with nothing less that a 6" screw, down 5
>feet. The 8" screw gives lots of reserve, almost to the full breaking
>strength of 3 3/16 EHS guys. Since I have trees around the tower, the
>threat of one falling on a wire is real.
>
>Here's the real kicker. The Walder salesman told me that AB Chance would
>provide free engineering to determine the type of anchor required, and
>that they have dedicated full time engineers to this. This could be a
>valuable resource if they are willing to share some other data, too,
>especially a method for rating the screw anchors in differing soils.
>
>Thoughts, comments, suggestions, please.
>
>--...MARK_N1LO...--
Hi Mark,
There is another option you did not mention in your original post, above.
You can get earth anchors that need no concrete and are not installed by
screwing them in. This is what I use on my three 100+ foot towers and I
have never had a failure. Two of the towers were installed in 1973 and the
other in 1975.
I found all of this kind of hardware I could ever want at a local supplier
to our power and telephone utility companies. You could probably find out
from Chance who in your area stocks untility pole hardware. The costs were
very reasonable, too.
You generally buy these anchors in two pieces and you have several options
for each piece. I selected anchor rods that were 9 feet long, had a
"knuckle" on the end that could handle three guy wires, were about 3/4 inch
in diameter, and were threaded on the bottom end to take a large square nut
to hold the anchor plate on. I selected an anchor plate that was made of
reinforced thick steel welded into a very heavy "x" shape about 16" on a
side. The "X" has a hole in the center that the rod fits through and the
nut is installed on the other side.
Installation consisted of determining where the bottom of the anchor would
be underground when fully installed. I then hand dug a hole about 2 feet in
diameter (since I had to stand in it to dig it) and about 6 feet deep right
over that spot. I then stood on a rather tall step ladder and with the help
of another ham, drove the anchor rod in the ground with a heavy hammer such
that it entered at an angle and emerged from the side of the hole near the
bottom and at the correct angle for the guys that would attach to it. I
then installed the anchor plate at the bottom of the hole with the square
nut and filled the hole with the earth I removed and any other thing I
wanted to get rid of like broken concrete and rocks. Virtually all of the
earth the anchor plate was pulling against was undisturbed ground.
This process has some advantages and disadvantages.
It is one Hell of a lot of manual labor, especially for three towers each
requiring 3 anchor holes and another one for the base. The good part about
that is it was good exercise and I needed it . . .
I think the cost was about $15 per anchor. About $10 for the rod and $5 for
the plate. They say you can unscrew the anchor rod and leave the plate and
nut there if you ever want to move. The nut is held captive in the plate so
it will not turn as you unscrew the rod from it.
If you have ever installed a screw-in anchor, you know how much work THAT
is. What do you do if you run into a rock . . . ?
So you might consider what I did. I would do it again, just like I did before.
Stan w7ni@teleport.com
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