[TowerTalk] New Tower Construction - Questions

[John D. Farr]

Gentlemen

I would be careful about adding rebar. Rohn does not specify any for a guyed
25G tower. Apparently, casting the base section into concrete per their
specifications is adequate. I have not read TIA/EIA-222-F, but I have read
and understood the concrete code, the ACI 318-95. This code specifies a
maximum amount of steel to use. The maximum allowed is 75% of a "balanced"
strain condition in the cross section. "A balanced strain condition exists
at a cross section when the maximum strain at the extreme compression just
reaches 0.003 simultaneously with the first yield strain f_y/E_s in the
tension reinforcement". In English, this means that you want the steel to
fail well before the concrete. This is because the steel gives lots of
warnings before it parts where the concrete gives none at all. I know of no
exceptions to this rule. One should never over reinforce concrete.

Development length is the length of embedded rebar required such that in
tension or compression, the full strength of the bars can be achieved
without the bar slipping in the concrete. If Rohn is using the base section
as rebar, you should not coat the base section prior to pouring the
concrete. The ACI allows only for uncoated bars and epoxy coated bars. I
would imagine that there is a specification for the epoxy used. The
development length is increased by a factor of 1.5 for the epoxy coated
bars. I have heard of sealing the area where the base section enters the
concrete with roofing pitch. I don't see a problem with that.

The long and the short of it is that you should build the foundation per
Rohn's specifications. A detailed knowledge of the governing codes (I'm not
sure that the ACI governs in tower bases) is required to make changes.

gl
John
KC4ZXX

> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of K7LXC@aol.com
> Sent: Thursday, August 31, 2000 10:13 AM
> To: aa4lr@radio.org; towertalk@contesting.com
> Subject: Re: [TowerTalk] New Tower Construction - Questions
>
>
>
> In a message dated 08/30/2000 2:42:27 PM Pacific Daylight Time,
> aa4lr@radio.org writes:
>
> > * I'm planning to embed a Rohn 25 tower section in the concrete
> base. I'm
> >  unsure if direct contact with the concrete will cause corrosion of the
> >  section. Should embedded tower sections be coated with some kind of
> >  protectant? Or would a protectant reduce the mechanical
> coupling between
> >  the base and the tower? Is corrosion (other than pooling of
> water in the
> >  legs due to improper drainage) not really a problem?
>
>     Most everyone just sinks the section in the concrete as is.
> OTOH W3LPL
> had serious corrosion problems by doing that. Now he coats metal surfaces
> that'll be in the concrete with building tar. That might be a
> 'belt-and-suspenders' approach but it'll sure take care of any potential
> longterm problems.
> >
> >  * I know I need a rebar cage in the base, but I'm fuzzy on the
> details of
> >  how to construct it. This is a standard 2x2x4 foot base, and I
> understand
> >  the rebar should be about 3" from the edges of the concrete. This
> >  translates into a 1.5x1.5x3.5 foot cage. I can see rebar at all the
> >  edges, how much is needed on the long faces? Are there
> horizontal pieces
> >  on the way down each face? Spaced how often? How about spacing for
> >  vertical pieces? Are there pieces on the top and bottom faces?
> Are there
> >  any pieces on the INTERIOR of the cage? Isn't there a problem
> with rebar
> >  and tower section trying to occupy the same space with the interior
> >  elements?
> >
>     You can add any rebar that you want - the specs are for the
> MINIMUM size
> and amount. If you want to add vertical pieces to hold everything
> in place or
> additional horizontals - go nuts.
>
> >  * I plan to build a small wooden form to shape the top few
> inches of the
> >  concrete base. Is there something you can put on wooden
> surfaces to make
> >  the concrete not stick to it? This would make it easier to remove the
> >  forms later.
> >
>     You can cover the parts of your wooden form that'll touch the
> concrete
> with motor oil. The forms will come off easily and the motor oil
> won't affect
> anything.
>
> Cheers & GL!   Steve    K7LXC
>
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