I have seen a few concrete bases cracked from lightning strikes. These were
all where there was no ground system attached to the tower at all. Just the
tower attached to the J bolts in the concrete. I can't recall seeing one
cracked where there was a ground system attached.
A few years ago where I had my boat docked a palm tree was hit by lightning.
The tree was close to the concrete sea wall and it blew out a chunk of the
sea wall and concrete shrapnel flew in many directions. I think there was a
crack in the concrete before the lightning strike so there was probably
moisture in the crack. So yes concrete can explode.
The EMP screwed up the compass and a few other things on the boat. The
compass came back to normal after a few weeks but it had quite an error for
awhile.
For anyone that disbelieves concrete can explode into shrapnel, if you have
an acetylene torch (I don't know if a regular propane torch is hot enough)
just hold the flame an inch or so from your concrete floor in the garage for
a few seconds and you will get a big bang and a chunk of concrete will
explode out sending bits everywhere. Wear safety glasses. It doesn't need to
have a crack in it either.
I am not saying this will happen in a tower foundation as I have never seen
it happen there.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of David Gilbert
> Sent: Friday, September 14, 2007 12:38 PM
> To: TowerTalk
> Subject: [TowerTalk] Exploding Foundations
>
>
> I'm firmly in the camp that says exploding foundations are a myth. I've
> spent hours and hours searching the internet and haven't been able to
> find a single documented case of a grounded tower foundation that
> exploded. I haven't been able to find anyone else who has ever
> discovered such a reference either. That includes externally grounded,
> Ufer grounded, or ground wires running out the bottom of the base. In
> addition, Polyphaser says it is a myth. Check their website.
>
> That being said, I also agree that it is always prudent to provide as
> many low an impedance paths to ground as possible. In my opinion, that
> includes ground wires connected to the tower legs that fan out radially
> from the tower, and it includes making a Ufer ground by connecting the
> tower to the rebar cage, AND in many cases it includes having ground
> wires connected to the tower base inside the concrete that exit the
> bottom of the foundation to buried ground rods.
>
> That last one seems to generate the most debate, but here's why it makes
> sense to me. The tower base is already embedded into the concrete
> foundation, in many cases quite deeply. The base of my
> soon-to-be-installed AN Wireless HD-70, for example, runs to within just
> a few inches of the bottom of the five foot deep foundation. No way
> anyone can convince me that the massive tower base itself is not already
> the lowest impedance path to that point. I can string a dozen #2 wires
> from the tower above the base to ground rods surrounding the tower, but
> they won't provide a viable lightning shunt to those three large tower
> legs (with cross braces) that are already five feet into the
> foundation. If I were worried that the Ufer ground was insufficient and
> if I wanted to prevent the bottom of the foundation from blowing out in
> a lightning strike, I'd have more confidence bringing some heavy copper
> wire from the bottom of the tower base out to ground rods buried under
> the footing than I would trying to shunt the current around it. I'd not
> want to let the ground rods penetrate the concrete, of course, but I'd
> have no worry about the copper wire doing so ... it isn't going to
> corrode either inside the concrete or in the soil, at least not enough
> to matter to me or my heirs. For the sake of argument here, I'm
> ignoring the non-trivial issue of making a reliable connection to the
> tower base inside the concrete.
>
> I've asked before on this reflector and didn't get any result, but I'll
> ask again. Can anyone identify a specific reference (book, scientific
> paper, internet link, etc) that documents a case where a tower
> foundation exploded from a lightning strike because of a conductor that
> exited it below ground? The only related instance I ever found was
> where the structure hadn't been grounded at all and the strike had no
> place to go except out the side of the foundation.
>
> Here's a second question. Why would a low impedance copper wire
> generate more heat, and therefore create more damage to a concrete
> foundation, than a higher impedance path through the concrete itself?
> Before you answer, consider that that the two paths are in parallel.
> The copper wire isn't replacing the broader impedance path of the bulk
> concrete around it ... the copper wire is only replacing the volume of
> concrete it displaces.
>
> Third question ... why would a solid copper wire that can exist forever
> within concrete without corroding, and exist virtually forever in normal
> soil without corroding, corrode when leaving the concrete to enter the
> soil?
>
> Thoughtful replies, whether supportive or contradictory, would be
> sincerely appreciated. Since I will soon be installing my tower, this
> is more than just an exercise for me.
>
> 73,
> Dave AB7E
>
>
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>
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>
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