DON'T DO IT ! ! !
Don't even think of pulling up 90 ft of tower on a simple hinged base.
The forces at the BASE are ENORMOUS. The SAFEST solution is
the old fashioned gin pole and rope assembly method of stacking
one section at a time, starting with a temporary set of safety guys
at 10 to 20 ft and permanent guys (3/16 inch EHS) at 30, 60, and 90 ft.
The (unapproved) method I use is to dig a 3 ft hole with a post hole
digger, then use a square bladed shovel to make the hole triangular.
I widen the bottom out, put down some bricks (pressure treated 4X4's
might be better) and pour in several inches of gravel to allow for
drainage from the legs (due to condensation). Then I start filling with
dirt and add some water as I go. Let this harden for at least a WEEK.
W1EVT has 19 Rohn 25 towers on a hilltop using flat base plates
on top of cinderblocks for leveling of the bases. For your purposes,
4X4's would probably suffice. Of course you will need temporary
guys for the first section until the first set of permanent guys is
secured.
Again, these towers were installed one section at a time and NOT
pulled up. Note that the base forces are almost entirely VERTICAL
for an ERECT tower.
My guys are secured to screw anchors which can be unscrewed
when you leave. Be sure to use Preformed Guy Grips (not cable
clamps) to attach your guys. My 60 and 80 ft towers use one
screw in anchor per guy direction. My 95 and 130 ft towers
use two anchors per direction spaced 60 and 90 ft respectively,
but these towers have Yagi's on top. Personally, I would feel
comfortable using one anchor per direction for a 90 ft Rohn 25
tower with only a light on top assuming *moderate* winds.
Of course Two anchors per direction adds an extra safety factor.
NO TRACE. NO PROBLEMS in 25 years.
Contact W7NI for Rohn products and advice.
de Tom N4KG
On Sat, 05 May 2001 "Eric Schurman" <eric_schurman@hotmail.com> writes:
> I am going to be putting up about 90' of Rohn 25G tower and had a
> couple of questions about guying. This is going to be a temporary
installation
> in a location which has a leave no trace policy.
>
> Things that make my project different include:
>
> No concrete can be used.
> The soil is very dry and fairly crumbly (it's a dried alkaline lake
> bed) There will be no antenna at the top, simply a set of lights, so
less
> wind load.
>
> My plan was to use a Rohn hinged base, nailed into the ground with
> 3' long
> steel nails, assemble the tower on the ground, attach the guys and
> lights,
> then pivot the tower up on the hinge (I've got lots of manpower) and
> attach
> the guys to the anchors, for which I'm considering the Rohn GAS604
> 4' long
> screw anchors.
>
> The anchors are the part that worries me a bit. The ground is not
> terribly
> strong and I'm somewhat worried about the anchors pulling loose. To
> spread
> out the force a bit, I'm considering using an extra anchor on each
> side, as
> shown in the 3k gif here: http://byz.org/~eric/tower.gif. I would
> guy the
> top and middle points to the outer anchor and the middle and bottom
> to the
> inner anchor, meaning that the middle point is anchored twice. Guy
> AZ is
> parallel to guy BY, so the inner anchor is placed about 80% of the
> height of
> the middle anchor point.
>
> Does this seem like it would increase the overall strength of the
> tower and
> reduce chance of failure through anchor pullout? Does anyone have
> other
> ideas or approaches?
>
> Finally, can anyone recommend a good place in the Seattle area to
> actually
> purchase Rohn 25G components?
>
> Thanks
> -Eric
>
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