Per your comments, the two towers you refer to were isolated from the
concrete base. That is the same condition described by Polyphaser for
the single semi-verified claim of an exploding base they could find. If
the lightning path, which can be capricious if not channeled (and even
if it is channeled), decides to make its own unique path to ground
through the concrete, there is a good chance it can cause it to
explode. I was able to find at least a few pictures of concrete
structures (bridges, roads, etc) that had been damaged by lightning in
that manner. Most towers don't fit that category, though. The tower
base or supporting bolts for most ham towers are embedded well within
the concrete, and as you say in that case (which means in most cases in
my mind), brute force bonding everything together and providing any and
all practical low impedance shunt paths paths to ground around or
through the concrete would seem prudent. What some people don't seem to
get is that a wire through the concrete is in fact a shunt path to the
bulk of the concrete.
jim Jarvis wrote:
> Urban legends? I don't think so.
> WCTC-AM, New Brunswick, NJ. Took multiple strikes, and had
> structural cracks
> in the foundation afterward. This was in the 1965's, when I worked
> there as an engineer.
> WBRW-AM Somerville, NJ. 4 tower 190' inline array. One tower
> took a strike, during
> construction and proof of the array. A crack appeared in the
> foundation, from the insulator
> base to earth, opened to a width of 1", showing burn marks. 1972.
> Both of these were series fed sticks. i.e. insulated, with single-
> point tower bases.
> In the interest of length, I removed a paragraph from my post, which
> said something like:
> "The area of debate probably occurs when the tower mounting itself
> routes lightning into
> the foundation. i.e. J-bolts or tower legs extend into the
> concrete. In that case, an argument
> can be made for the brute force method of bonding everything together."
> Guess I shudda left it in, eh?
> You're right about cadwelds.
> As for my argument that you want to route lightning around the
> foundation as much as possible,
> I'm sticking to it. In addition to two broadcast arrays, I've seen
> enough exploded sidewalks from lightning,
> I'd rather not dispel the urban legend myth with first-hand experience.
> It is my contention that the shape and form of the re-bar does NOT
> constitute a low-impedance path to
> the complex lightning waveform. It is highly inductive. Tie it
> together? fine. But I'd bypass it with
> 4" copper strap, (or 1.5", if that's what you're using), making a
> nice, smooth 18" radius bend, and exiting
> the concrete away from the rebar.
> Wish I had digital pix of our ground treatment of the 500' WCTC-FM
> self supporting tower. I was consultant
> on that project in 1978. It gets hit in virtually EVERY storm.
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