I dunno. Intuitively, I suppose it makes sense to provide
a robust path around the foundation, as well as a static
drain for the rebar cage. But I wouldn't want to use the
rebar cage as my primary ground path.
Now that I think about it, it would seem likely that the
high current in the shunt path induced currents within the
nearby (ungrounded) rebar cage, which exited through the
concrete, causing cracks.
Or, perhaps the inductance of the shunt path wasn't low
enough, and the lightning sought parallel paths through
the concrete & cage.
I've had other AM stations hit with some frequency, where
absolutely nothing ever happened, and I can't see any obvious
reason why not...unless it was age of the concrete.
But that, as they say, was a lifetime ago.
From: David Gilbert [mailto:firstname.lastname@example.org]
Sent: Saturday, September 15, 2007 2:56 PM
To: Jim Jarvis
Subject: Re: [TowerTalk] exploding concrete
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)?
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
> 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
> 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.
> I suspect the Ufer ground concept is simple: If we lose some concrete, so
> 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
> getting the strike current off of it cleanly would seem to be paramount.
> Running it THROUGH the rebar seems counter-intuitive.
> Jim Jarvis
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