[Boatanchors] Tower Construction

Sun Jun 11 10:39:10 EDT 2006

I have 70 feet of Rohn 45. 4 feet in the ground and lots of loading on top
of the exposed
66 feet. The kind of stress your talking about to do damage to a piece of
tower in the ground
is pretty much off the scale. If the wind gets to the point of damaging the
tower at the point
it exits the concrete everything else around will be gone. Gone. Been there
and done that.
Another point about flexing is this, I have a small 55 foot US crank up
crank over that is properly
installed using the correct J bolts 3 feet down into the concrete. That
tower is not going to
flex. It just is not going to happen. It might break but it will never flex.
I always overkill on
the size and amount of concrete in the ground.
The one point that has been made to some degree in this thread is rebar. It
is extremely
important that the rebar be built according to specs and welded into a cage
configuration
before getting into the ground and pouring concrete on it.
For Engineers and non engineers alike Rohn used to send out catalogs with a
lot of installation
information in them. I have one and refer to it once in a while. It is a
permanent part of my
reference library. It must be 30 years old but the installations have not
changed much. Get one
if you can. They are about an inch thick.
I think you can free stand 30 feet of Rohn 25 tower within a certain wind
load on the top.
Probably 40 feet with 45 and 50 feet with 55 but the wind load you can do
this with is specific.
Not sure what it is. Maybe 10 feet or less. But the installation process has
to be followed by
the book to do this free standing.
As I mentioned before about the rebar. Do it right. One PE here mentioned
the rebar inspection
before pouring concrete. He may be able to detail that part of the project.
If the rebar is not
done properly it can allow a tower to be blown out of the ground (just like
a tree) by lightning.
So think proper grounding of the tower as well as your mechanical
installation when your planning
your tower. It really is not that hard to do and when you get a well
engineered small tower installation
you can have fun and piece of mind as well.  Good Luck!

K0AZ  Mike Sanders
18169 Highway 174
MT Vernon, Missouri 65712-9171
k0az at k0az.com <mailto:k0az at k0az.com>

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net
[mailto:boatanchors-bounces at mailman.qth.net]On Behalf Of Al Parker
Sent: Sunday, June 11, 2006 9:00 AM
To: sdaitch at ibb.gov; BAqth
Subject: Re: [Boatanchors] Tower Construction

Hi Sheldon,
    A good point.  Basically you should not put a stress concentration (
as caused by a weld or hole) at the point of maximum stress.  So, if you
were to embed a tower section in concrete you shouldn't put the point of
connection of a brace, etc. at the surface of the concrete.  I wouldn't
bury part of the tower like that, I'd use either the recommended base, or
tubular lengths of the proper diameter and wall thickness, if recommended
by the manufacturer.

    Since Dave, W3ST brought up the subject, I'll cc here a note he sent
to me last nite:
" >Our soil here is mostly shale.  I have been told that cementing in a
>section into the ground is not as good as a base mount.  Seems like you
>need some flexing in high winds or icing.  Cementing the tower in
>concrete makes the tower very rigid, of course."

     I'd add that a base mount will be rigidly attached to the concrete
via anchor bolts and grout, so you would still have a rigid mount at that
point.  Some movement could happen, not a desirable situation, at the
normal tower joints if they were not bollted tightly.  Any actual movement
of a joint would eventually cause wear and weakening of the joint.  Any
tower will flex (bend) slightly under load, but there shouldn't be any
movement between pieces.
73,
Al, W8UT
New Bern, NC
BoatAnchors appreciated here
http://www.boatanchors.org
http://www.hammarlund.info

----- Original Message -----
From: <sdaitch at ibb.gov>
To: "Al Parker" <anchor at ec.rr.com>
Cc: <boatanchors at mailman.qth.net>; <AMradio at mailman.qth.net>
Sent: Saturday, June 10, 2006 9:53 PM
Subject: Re: [Boatanchors] Tower Construction

> Al,
>
> let me ask you a question about this, from another point of view.
>
> In terms of bending moment capability at the base of the tower, does it
make any difference at what point the embedded tower section comes out of
the concrete block?
>
> My initial reaction would be it would not make any difference, since the
> metal structure continues into the block, but the bending point (which
should be close to or close to the top of the concrete) could be at a weak
point (which I suspect might be between the horizonal braces of the truss
section), thus depending on where the steel actually exits the concrete.
>
> Could you share with us a thought on this?
>
> 73
> Sheldon
> WA4MZZ
>
>
>
> ----- Original Message -----
> From: Al Parker <anchor at ec.rr.com>
> Date: Saturday, June 10, 2006 8:32 pm
> Subject: Re: [Boatanchors] Tower Construction
>
> > Hi Dave,
> >    Eddie's thoughts are a good guide, but realize that there are
> > 2 (at
> > least) things that need to be closely considered.
> >  1.  The soil and wind conditions at your QTH, and
> >  2.  What Rohn rates the tower sections for in an unguyed mode
> >  3.  How much stuff you're gonna hang on top of the tower
> >  Your intial comment of "cementing a section" in is already in
> > error, you
> > really mean "concrete".  And the amount of it needed depends upon
> > the soil
> > conditions, and the wind  load on the tower.  I'd suggest, that if you
> > don't know a licensed Professional Engineer in your area that will
> > helpyou, you should find out what Rohn suggests.  They won't be
> > specific for
> > liability reasons, but should have some general guidelines for "normal
> > soil conditions."   Then get a local professional opinion.
> >    I am a P.E., and have twice installed a 55 ft self-supported
> > tower for
> > myself, in 2 different QTH's.  At the first I needed to submit, to the
> > local building permit dept., engineering calculations (from the tower
> > mfgr) on the tower itself, and drawings of my proposed
> > installation.  An
> > inspection of the rebar in the hole for the foundation was
> > required before
> > the concrete was poured.
> >    Safety is the main concern in these things, failures can be
> > hazardousto health.
> > 73,
> > Albert W. Parker, P.E., W8UT
> > New Bern, NC
>
>

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