[TowerTalk] Rohn BPC25G

[Jim Lux]

At 08:18 AM 5/14/2007, Gary Schafer wrote:


> > -----Original Message-----
> > From: towertalk-bounces at contesting.com 
> [<mailto:towertalk->mailto:towertalk-
> > bounces at contesting.com] On Behalf Of James Wolf
> > Sent: Monday, May 14, 2007 12:27 AM
> > To: towertalk at contesting.com
> > Subject: Re: [TowerTalk] Rohn BPC25G
> >
> >
> > --------------------------
> > >You don't want anything sticking out of the concrete either rebar or a
> > ground rod. Not because it is a lightning path but because it will corrode
> > >(rust) and the corrosion will work its way up into the concrete and that
> > will crack the concrete.
> >
> > >It is fine to use the rebar inside the concrete as an auxiliary ground
> > for
> > the tower but it should all be tied together in the concrete so that there
> > is >no arcing from one piece to another in there.
> >
> > >It is just as important to build the concrete so that no water
> > accumulates
> > around any pier pin or j bolts sticking out of the top too. If they rust
> > down >into the concrete you will have the same problem with the concrete
> > cracking.
> >
> > >73
> > >Gary  K4FMX
> >
> > Gary,
> > Hi Jim,
>
>If I understand your setup right, you have ground rods coming out of the
>bottom of the concrete. That is not a good idea for reasons stated above. It
>is standard practice to not have any rebar or other metal that is inside of
>concrete extending out into the earth. Anything that is exposed to the soil
>will rust and that rust will move right into the concrete and as it rusts
>inside the concrete it will expand and can crack the concrete.
>A copper coated or galvanized ground rod will take longer to rust than raw
>rebar but it will eventually rust.

Another problem is that the coefficient of expansion of the bar and 
the concrete is different, so temperature cycling will eventually 
result in a very thin gap between concrete and rod.  Aside from 
providing a path for dissolving the concrete away, if there IS a 
lightning hit, the steam in that gap can flash to steam, fracturing 
the concrete.

I can't find the reference right now, but there's a fairly complete 
analysis of cases where lightning hit something anchored into 
concrete where the concrete was damaged (typically some form of 
spalling), and in most cases, the actual damage was attributed to 
steam explosions (based on the fracture characteristics).

In order to get lightning damage, there has to be some mechanical 
force created by the current.  The rebar itself or the concrete won't 
materially expand during the stroke, and the magnetic forces, while 
impressive, are hardly high enough to actually exceed the yield 
strength of either concrete or copper or steel. The conductivities of 
all of these are high enough that they are unlikely to actually melt, either.