-----Original Message-----
>From: patrick jankowiak <recycler@swbell.net>
>Sent: May 27, 2008 1:25 AM
>To: towertalk@contesting.com
>Subject: [TowerTalk] new member with tower question
>
>Hello,
>
>I'm new to the list, so I hope my first action of posting a question
>here will be acceptable. first let me intro. myself a little.
>
>It looks like I cannot economically put up my 56FT tower due to
>"caliche" being 1-2FT under the soil. For those that don't know, "The
>caliche reserves in the Llano Estacado in Texas can be used in the
>manufacture of Portland cement; the caliche meets the chemical
>composition requirements and has been used as a principal raw material
>in Portland cement production in at least one Texas plant. Where the
>calcium carbonate content is over 80 %, caliche can also be fired and
>used as a source of lime in areas, which can then be used for soil
>stabilization." I suppose this means I am on concrete for ground.
>Anyway, you can't dig into it.
Why can't you put up the BX? Talk to an engineer familiar with your local
conditions, and they may agree with your assessment that caliche is comparable
to concrete. The BX wants a base that is heavy and large enough that a) the
downwind side doesn't sink into the soil and b) that the upwind side doesn't
pull up. There's lots of potential concrete pads that can be created that are
compatible with this. One common strategy is to drill 4 round holes, one for
each foot (you see this on HV Transmission towers).
>
>I would like to try putting up 40-50FT of Rohn 25 on a sturdy base and
>guy it. The space is limited, so a combination of guying and a strong
>base should work well, rather than just one or the other
"should" and "will" are sort of different, the difference being the actual
calculations of the loads.
Generally, if you have to get a building permit, or the tower is somewhere that
if it falls down, someone gets hurt or something else gets damaged, you'll need
the "will" kind of analysis.
OTOH, if it's out on your back 40, and all that gets hurt if the tower falls
down is your antennas... you can put up any old thing.
>
>So far, the plan seems to be to dig down 2 FT to the caliche making a
>4x4 hole, use a 2" or 3" rock drill to make a pattern of holes 4FT
>further deep in this hole, and set pieces of 1/2" or 3/4" rebar in the
>holes, then attach them to the cage of rebar in the 4x4x3 foundation
>area which will rest on the caliche rock. The foundation will be made 1
>FT higher than the ground around it using a form.
You need to talk to a civil engineer in your area. In general, rebar that
penetrates the boundary of the concrete and soil isn't acceptable. It provides
a path for water to get in between the rebar and concrete, leading to corrosion
and failure. Rebar can stick out of the concrete ABOVE grade level where it
stays dry.
In the middle of this
>will be set the base section of the Rohn 25. The tower+base section will
>not be grounded through this rebar, but will later be grounded
>separately.
As recently discussed on this list, the current National Electrical Code, some
flavor of which you are probably subject to, requires that the rebar be bonded
to the grounding system.
This foundation is not quite enough by itself to completely
>secure the tower for the long term. Three guy pipes with two sets of guy
>cables will be used.
>
>Since this will be done in an enclosed area of yard that is only 40x40
>FT, the three guy pipes will be placed so that they are sunk 5 FT into
>the ground and extend 5 FT above the ground. We want to use 4" oil well
>drill pipe for this. We can find the pipe because any piece with a crack
>has to be discarded and they go to scrap.
Of course, that crack indicates a failure of the pipe, no? (unless you're
talking about taking a 20 ft stick with a crack at one end and chopping a
section out of the middle?)
The loads on an elevated guy anchor are pretty darn high. Do a search of the
archives for "elevated anchor" and you'll turn up some discussions on this. It
either takes a backstay to a guy anchor or a *really big* post. If you can
plant the backstay (e.g. with a commercial auger in style anchor.. A.B. Chance
is one vendor of the anchors), you might as well do the guy to that anchor.
Elevated guys are nice if you need "walking under the guy cable" clearance, but
their engineering is substantially more complex.
They are made of chrome moly
>steel. I am not concerned about anyone wringing their neck on the guys
>inside my 8 FT fence. It is only me and my brother here. The closest guy
>pipe will be 20FT from the tower base, and the farthest one will be
>28FT. This is OK for the tower's first guy at 25FT height, since the
>angle of the lowest guys will be about 40-45 degrees. The angle will be
>quite smaller as the tower becomes taller. I may have to limit the height.
>
>These are the tenative plans. I don't know how good the plans are, or not.
>
>What experiences do you guys have with odd/compact tower installations,
>especially with guys, guy posts, and the angles of guy wires versus the
>stresses on them? Obviously I can reduce the angle if I strengthen the
>guy, up to a point, but beyond that it is of no further merit and the
>tower cannot be made higher safely.
As you increase the tension on the guy (e.g. by bringing the anchor in closer
to the tower base) you increase the compression load on the tower, perhaps
above that which the mfr recommends. You also increase the load on the anchor,
tending to pull it out of the soil. And, of course, the increased downforce on
the tower tends to sink it into the soil.
An engineer can answer all these questions, tailored for your particular soil
conditions, in about an hour or two. The couple hundred bucks for the engineer
will be well spent (and you might find someone to do it free or for the
proverbial "sixpack".. check the ARRL Volunteer Consulting Engineer lists)
>
>Is there a free downloadable calculator to help with the angles of the
>guys and design of the site? All advice will be welcome.
Not one that I would trust....
Jim, W6RMK
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