While the GUY WIRE 'capability' of a 50 ft (to 70 ft) R25
tower guyed with 3/16 inch EHS may exceed the rating
of a screw anchor, the actual LOAD is quite a bit LESS.
A 12 sq ft antenna in a 30 psf wind (86.6 MPH) puts only
360 lbs of horizontal load on the top of the tower which is
then resolved into somewhat (but NOT greatly) higher
loads in the guys, primarily the TOP guys. The tower
itself adds some additional load. Note that each section
of R25 represents 2 sq ft of load using the OLD formula
for cylindrical members, more using the exposed area
formula.
Here is some "annecdotal" evidence regarding 4 ft mobile home
screw anchors with 6 inch plates in Alabama CLAY soil:
80 ft TV tower, guyed at 4 levels to SINGLE 4 ft long screw anchors
with 6 inch plates, loaded with TH6 on top at 80 ft, TA32 at 60 ft,
and TH3 at 40 ft survived estimated 80 to 90 MPH Tornado winds
at N4KG.
75 ft TV tower, guyed at 3 levels to SINGLE 4ft long screw anchors
with 6 inch plates, loaded with Telrex 3L20 at 75 ft and 5L10 at 45 ft
survived estimated 80 to 90 MPH Tornado winds at N4KG
55 ft TV tower, guyed at 2 levels to SINGLE 4 ft long screw anchors
with *4* inch plates, loaded with 5L10 at 62 ft, 4L15 at 55 ft, and
5L10 at 33 ft, survived the same Tornado winds at N4KG.
40 ft Aluminum tower guyed at 32 ft to SINGLE 4 ft long screw anchors
with *4* inch plates, loaded with TH7, survived the same Tornado winds
at N4KG.
130 ft R35 tower, guyed at 4 levels to TWO sets of 4 ft long screw
anchors spaced 60 and 90 ft from the tower, loaded with 3L15 at
130 ft and 3L20 at 65 ft plus several wire antennas, survived estimated
80 to 90 MPH Tornado winds AND held up TWO 70' TREES which
fell on the TOP (1/4 inch) GUY, following the same Tornado at N4KG.
I went with Screw Anchors after hearing the story that WA4SSU
lifted the front of a CRANE off the ground trying to pull a single
screw anchor out of his Georgia Clay. ( A good reporter would
VERIFY this story before publishing :-).
Tom N4KG
On Wed, 4 Sep 2002 00:57:21 EDT K7LXC@aol.com writes:
> In a message dated 9/3/02 9:23:51 PM Pacific Daylight Time,
> na9d@speakeasy.net writes:
>
> > I am shortly going to be putting up my first real tower - 5
> sections of Rohn
> > 25.
> >
> > I plan on guying the tower with two sets of guys per Rohn specs.
> My
> > question is about anchoring them. I have purchased Rohn GA5604
> screw
> > anchors. The specs on these say that they have 2500 pounds worth
> of
> holding
> > power. I was actually surprised when I received them at how
> small the
> screw
> > portion was.
> >
> > My question is this: Are these sufficient for my tower
> installation or do
> I
> > need to think about digging a hole and putting these anchors in
> concrete?
> I
> > was originally planning on doing this but now I am wondering if I
> need to
> go
> > through all that extra work.
>
> No, they're not sufficient. Yes, do the extra work.
>
> Fifty-feet of 25G has 2 sets of guys; the maximum guywire
> strength at the
> anchor before failure is 8,000 pounds. This is SIGNIFICANTLY more
> than the
> capacity of the screw-in anchors. Even if you used one screw-in per
> guy,
> you'd still have a mismatch. I'll sure someone will mention screw-in
> creep
> due to wet soil conditions.
>
> Concrete uber alles.
>
> Cheers,
> Steve K7LXC
> TOWER TECH
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