Some anecdotal experience with SCREW ANCHORS de N4KG
On Tue, 30 Jun 1998 22:20:18 -0400 LYN <designserv@ipass.net> writes:
>
>K7LXC@aol.com wrote:
>
>> If you're talking about using 2 screw-
>> ins per anchor, that should give you enough capacity.
>
>OH? Under what conditions? The load on a screw anchor is
>distributed through a volume of soil. Just how far apart in which
>direction were you going to spread these so the affected volumes
>do not overlap? How were you going to share the load and what
>does that do to the axes of the screw anchors?
>
>I think screw anchors are alright if they are properly imbedded in
>enough concrete. Have seen too many fail when soil got wet and they
>were not in concrete. Don't like them. Don't trust them. Period.
>
>Lyn, W4WDN
>
WARNING: As K7LXC so frequently reminds us,
ALWAYS DO WHAT THE MANUFACTURER SAYS
1 - determine your soil condition.
2 - determine your tower and antenna load requirements
3 - determine if screw anchors can meet the above needs
with your given soil conditions.
To be safe, you would need a professional determination.
That said, I will relate my personal experience with screw anchors.
When I was building my present station on 4 acres of Alabama
clay, I heard of a test conducted by WA4SSU in Georgia. He
reportedly made a test with a Rohn screw anchor and a large
crane. He turned the screw anchor straight into his Georgia clay
and tried to pull it out with a crane (lifting capacity long forgotten).
It was reported that he stopped pulling when the front of the
crane started to lift off the ground.
On that basis, I used screw anchors for all of my present towers.
These towers all survived a small tornado that come down in my
woods, about 100 yards east of the N/S row that includes 5 of them.
Estimated winds were 80 to 90 MPH. All of the beams above tree
top level survived with no damage. Lower beams suffered bent
elements from branches of arched over trees.
>From North to South, these include
1 - 40 ft of Heights aluminum tower with a TH7 guyed at 32 ft
to 4 ft utility screw anchors with a 4 inch plate, spaced 30 ft.
out.
2 - 55 ft of TV tower (same size as Rohn 25 but no diagonal bracing)
guyed at 25 and 50 ft, also to 4 ft anchors with 4 inch plates,
loaded with 5L10's at 62 & 31 ft. and a 4L15 at 55 ft.
3 - 80 ft of TV tower guyed at 20, 40, 60, 75 ft to 4 ft anchors
w/ 6 in. plates spaced 64 ft from the tower and loaded
with a TH6 at 80 ft, TA32 at 60 ft, and TH3 at 40 ft.
4 - 75 ft of TV tower guyed at 25, 45 and 70 ft to 4 ft anchors
w/ 6 in. plates spaced 60 ft from the tower and loaded
with a Telrex 3L20 at 75 ft and 5L10 at 45 ft.
5 - 95 ft of Rohn 45 guyed at 30, 60, 90 ft to two anchors spaced
60 and 80 ft from the tower and loaded (?) with a CC 2L40.
(OK, I just could not motivate myself to refurbish a KLM 4L40)
6 - 130 ft of Rohn 35 (15 inch spaced legs, identical to R25 legs)
guyed at 31, 62, 93, and 125 ft to two 4 ft anchors w/ 6 in.
plates
spaced 60 and 90+ ft from the tower, loaded with a 3L15 at 135 ft
3L20 at 62 ft., & 2L40 Delta Loop suspended from 20 ft boom
(Full compliment of top beams never installed).
7 - 140 ft of Rohn 25 was added after the tornado, guyed to two
anchors at 60 and 90+ ft from the tower to 5 ft anchors with
6 in. plates. Supports 80M dipoles and is fed as 160M GP.
Tower 6 is on the edge of my woods and during that tornado, TWO
70 ft poplar trees fell on the top guy (!/4 inch EHS, all others 3/16
in.)
THE 1/4 INCH GUY WIRE, SCREW ANCHOR and GUY GRIPS
HELD UP the TWO TREES !
W4WDN is correct: The cone of SOIL above the anchor holds
it in place. The more surface area of the anchor (plate), the greater
the holding power. The longer the anchor, the greater the holding
power. I especially like the utility anchors I found with 5 ft shafts.
The concrete anchors specified by Rohn have much larger surface
area than screw anchors which allows them to be closer to the
surface. The WEIGHT of the concrete has little to do with holding
power.
Concrete poured in a 6 inch hole with a screw anchor probably has
little if any benefit over a screw anchor simply screwed into the ground.
K5RC once showed me a picture of such an anchor which pulled
through his rain soaked soil causing a tower failure.
REMEMBER, your SOIL is ultimately what holds your anchor in place.
Determine it's holding capacity before planning your tower project.
de Tom N4KG
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