Jim,
If I understand you correctly, there was a large shunt path to ground
around the foundation (the 4 straps). Yet the lighting still apparently
entered the foundation and generated cracks when it left the concrete,
since even lightning isn't going to crack concrete by simply going
around it. Unless, of course, the lightning hitting the tower generated
mechanical forces that propagated down the tower and fractured the
concrete ... probably unlikely.
So ... if the electric field entered the foundation in spite of the
shunt, why would it be counterintuitive to provide a low impedance path
for it to exit without forcing it to pass through the bulk concrete
(where it presumably would generate fracture stresses)?
Dave AB7E
p.s. I'm pretty sure the theory behind Ufer grounds does not in any way
accept periodically losing concrete.
Jim Jarvis wrote:
>
> To which I will append a personal experience. At WBRW, in Somerville,NJ,
> we put up a 4 tower array. The re-bar cage was tied to ground, and there
> were 4 straps outside the foundation to the perimeter strap and radial
> system,
> at ground level.
>
> Within a week of installation, and while we were doing the proof of
> performance, a line squall blew through. All towers took direct hits.
>
> After the storm, as we were at the tuning units, I observed cracks in
> all the concrete bases. Not huge...but enough that water incursion and
> freeze/thaw cycles would be of concern.
>
> A hurricane two weeks later caused the river to rise and put 10kt current
> across the river bottom pasture where our antenna was. All foundations were
> undercut by the current...and two had large hunks of concrete fall off,
> where the lightning cracks were. We had to straighten them up, and build
> supporting foundations around them, to stabilize the structures.
>
> -0-
>
> I suspect the Ufer ground concept is simple: If we lose some concrete, so
> what?
> At least we don't light off the ordnance! I still don't know what's right
> for an amateur installation, but if preservation of the structure is
> paramount,
> getting the strike current off of it cleanly would seem to be paramount.
> Running it THROUGH the rebar seems counter-intuitive.
>
> n2ea
>
> Jim Jarvis
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