I had an experience with the screw in type this summer I'd like to share.
I bought 6 anchors at fleet farm last summer. They were the 4' variety.
The purpose was to straighten my barn. Yep. I pulled it end-to-end 6"
(it's 60' x 40' x 45' tall and solid douglas fir) What a project!
I used the anchors 2 at a time. I screwed them in at about a 45 degree
angle to earth and in line with the load. The two heads were touching each
other at the ground. I was using 1/2" galvanized cable with 15" turnbuckes
(three on each line). During the pulling process, two of us pulled the
turnbuckles so tight that we had no more strength to turn them. You could
strum the cables and get a musical note from them. I have no idea how much
tension--the 5/8" turnbuckles were in excellent shape (no problem turning
the screws) and still we couldn't turn them any tighter. These were much
tighter than any antenna guy!
During the east-west pull, the anchors failed to hold. The cable had an
enormous amount of tension. It was lifting the barn. However, further
tightening simply pulled the anchor eyes out of the ground an equal amount.
We moved to plan B and anchored that pull to a different location at the far
end of the foundation.
The north-south pull was actually two sets of cables each with a pair of
screw-in anchors. It worked flawlessly.
After straightening the barn (we did pull it up in a few of hours), the
anchors were removed. What I discovered was amazing! The anchors that
failed were not pulling out of the ground, they were stretched! The paint
was cracked off like flakes in sheets of red paint. The pair of 5/8" rod
was actually elongated (I didn't measure how much).
The other observation is that the screw in process had twisted all the rods
a bit. They were screwed into a top layer of about 2' of loam and the rest
was clay. The soil was excellent for an anchor. Each of the 6 anchors had
been twisted during installation (removal took much less effort). The shank
was no longer straight, they were kinked and coiled slightly (about 1/2" to
1").
As far as trusting them to hold up an antenna, a single anchor is not enough
in my book--even though I use the screw ins to hold my 68' rohn tiltover.
If you use the screws, use a pair at each pull. If you are going to invest
that much into the anchor, why not do it right and install cement anchors?
That's my $0.02
Ford-N0FP
ford@cmgate.com
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