[Boatanchors] Tower Construction

[Al Parker]

Hi Mike,
    It sounds like you have the same US Towers 55 footer that I do.  They
provide good info, including engineering calculations on the stresses of
each section of the tower and mounting.  But they do flex, everything
does.  It's the nature of things, put stress on a material and it will
deflect.  Maybe un-noticeably, but it does.  IIRC the stress to strain
ratio is called Poisson's Ratio, probably for the Frenchman who first
recognized, or put numbers to, the relationship.  Brittle things have a
high ratio, they don't bend much before they snap.  It's a characteristic
of the material, not of how strong it might be.  Stand down at the bottom
of your tower and look up when the wind's blowing, you'll see it bend.
    Yes, rebar needs to be in there, that's what helps hold the concrete
block together, in a macro-sense.  Concrete has a high stress/strain
ratio, it won't deflect much before it breaks, so you add the rebar to
strengthen it and prevent cracking.  I think Eddy mentioned that you don't
want to put your ground rods thru the concrete.  The reason for that is
that if lightning does strike and the high resulting current goes thru the
ground rod it will get hot, expand, and fracture the concrete around it.
    I expect that most tower failures are from a combination of bending
stress and torsional stress.  I think the ones I've seen toppled are bent
over somewhere up, often at the point of guy wire attachment.  We usually
just loop guy wires around a tower leg, over a crossbrace.  There's a
stress concentration at the brace's attachment, and the guys will put the
maximum bending stress at that point.  At least some tower mfgrs sell a
clamp to fit around the tower for guy attachment, which will reduce the
stress concentration, and can provide more torsional stability.
73,
A. W. Parker


----- Original Message ----- 
From: "Mike Sanders K0AZ" <k0az at corpranet.net>
To: "Al Parker" <anchor at ec.rr.com>; <sdaitch at ibb.gov>; "BAqth"
<boatanchors at mailman.qth.net>
Sent: Sunday, June 11, 2006 10:39 AM
Subject: RE: [Boatanchors] Tower Construction


> 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
> >
> >
>