[Boatanchors] Tower Construction

[Al Parker]

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
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----- 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
>
>