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[TowerTalk] Response to R for math

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Subject: [TowerTalk] Response to R for math
From: w7ni@teleport.com (Stan Griffiths)
Date: Thu, 15 Oct 1998 17:25:03 -0700
Hi Tom,

I think you have it right.  Concrete is one of those materials that is very
good in compression.  It really isn't worth a darn in tension.  Ceramic is
great in compression and  pretty good in tension and steel is generally pretty
good both in compression and tension.  It has to do with the SPECIFIC way the
material is used.  Steel cable, for example is great in tension and not worth a
darn in compression.  Combinations of materials are used all the time to take
advantage of the specifica characteristics of each.  If you ever examine a
bridge beam that is reinforced with rebar, you will see that the rebar is
positioned within the concrete beam where it will do the most good in tension.

If you have ever seen a sharply bent mast, boom, or beam element, it will be
kinked and one side of the tubing will be pressed in against the other side.
If the mast, boom, or element were filled with a solid substance that cannot
compress, it would help to prevent the kinking from happening.  It may still
bend, however, making it pretty useless for the orginal job.  In the case of a
mast, preventing the kink from happening, even if it bends, may actually save
an antenna it supports from total destruction which may be a good thing, even
if you eventually have to replace the bent mast with a straight one.  The trick
is to fill it with something that will not compress.  Concrete is good, but
heavy, and I wonder about synthetic foams and epoxy materials.  How good are
they in compression?  Maybe not good enough . . .

So I am sure that filling elevated guy pipes with concrete helps prevent them
from kinking and totally failing and it may even help them from bending a
little.

Here is how I would look at the situation:  I would imagine the fill material
sitting outside of the tubing and taking the shape of a solid rod the length of
the mast.  How much bending will it resist without the mast around it?
Probably not very much, so the mast will probably still bend.  It MIGHT NOT
kink and totally fail, however, so there may still be some benefit to filling
your mast with something.

My real solution would be to get a stronger mast and not mess with filling it
with anything if I thought it was marginal for the job.

So that is the opinion of an unofficial, non-degreed, layman who has put up and
used a few towers over the years with no serious failures so far . . .

I am sorry if I got a little too basic in some of my explanations but I really
hate to assume the level of  anyone's insight into this stuff and thought being
too basic was better than not being basic enough . . .

Stan  w7ni@teleport.com

w8ji.tom wrote:

> Hi Denny,
>
> Pardon my ignorance, being mechanically challenged I go by rumor, folklore,
> hear-say and rough approximations on mechanical issues.
>
> My old friend Wiley Bunn, now a SK, was a mechanical engineer at NASA. I
> know for a fact he filled the four inch pipes used to support his guy lines
> with concrete, but since Wiley is dead I can't ask him why he did that.
>
> I'm wondering why a well educated mechanical engineer bothered doing that?
> Could it be to keep the pipe from kinking? Could it be some softer or
> harder wall materials benefit from filling the core?
>
> Obviously Wiley thought it was worth doing. Anyone have an idea why?
>
> > << Question then...how much additional strength, if any, is added to a 2"
> >  mast that is filled with  epoxy cement or similar substance that is
> >  poured in and becomes solid or semi-solid  >>
>
> 73 Tom
>
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