Jim / Paul / Doug and list:
I have been holding back from responding to this thread as I have responded to
this idea in the past, but I just couldn't hold back any more.
The exploding concrete concept is mostly myth not fact. If you look at the
referenced fig. 4 in the cited resource you see the vertical crack at the
surface of the concrete pier. There is no spalling or other evidence of
exploding concrete in this photo. My take is that this appears more like a
freeze-thaw crack given the mountain-top location. If indeed the water in the
concrete flashed to steam ( a big stretch) the water was there due to the crack
already being there. Concrete does not have free-water pockets incorporated in
it if properly placed and consolidated. The water is used up in the chemical
reaction of the concrete curing process. There is some free water in the
micro-pores of the concrete-sand-rock matrix, but in miniscule amounts. If the
crack is 1/8" wide then it is significant but to attribute it to a lighting
strike with lack of before / after data is bad science.
I have been engineering foundations for large power plants, manufacturing
facilities, stacks, towers, etc. for almost 40 years. We have always used Ufer
type grounding with an external ground grid and rods. All tied together with
exothermic welded connections and 2/0 or 4/0 stranded ( heaven forbid! ) bare
copper cable. I have yet to see ,or been notificed of, any evidence of a
lightning strike induced foundation failure or explosion damage.
For the sake of good engineering practice and knowledge I remain open to all
information that shows that what we have been doing is dangerous and
inappropriate. I am focused on providing my clients with the best practical
engineering solutions to their projects.
Lonberg Design Group, Ltd.
H.S. Lonberg, P.E.,S.E.
-------------- Original message --------------
From: Jim Lux <firstname.lastname@example.org>
> Paul Christensen wrote:
> >> What exploding concrete?? Back up your claim with real life validated
> >> proof. My experience has been the opposite.
> >> Doug
> > Want evidence? See Figure 4 (and the text below it) in the link below:
> > http://www.mikeholt.com/newsletters.php?action=display&letterID=407
> The text says the only grounding was through the foundation and makes no
> mention of whether it was a properly constructed CEGR (i.e. 20 feet of
> conductor, etc.)
> Also, the picture does not show whether a structural failure occurred as
> a result of the lightning damage (the tower is still standing, after
> all), or whether that's just surface damage. If it's a 1/8" deep crack
> in the surface and doesn't penetrate, who cares? For all we know from
> the limited data in the article there was a prexisting crack in the
> concrete that was enlarged as a result of steam induced spalling.
> Considering that Ufer did an awful lot of tests over the years, and
> subsequent researchers have also done a lot of tests, I think that if
> there actually was a realistic concern about spalling and exploding
> foundations it would have shown up in the reviewed literature (as
> opposed to in anecdotal reports or ground rod manufacturer sales
> literature). I have no doubt that there are cases where there has been
> lightning damage, but I would suspect that those don't fit in the
> category of a proper concrete encased grounding electrode. For instance,
> the "3 2 foot J bolts in the top of the footing" used to bolt the tower
> mounting plate is probably NOT a suitable grounding connection.
> The article also sort of contradicts itself. On the one hand it
> advocates external grounding systems, but then, it says that the
> external 20 rod grounding system showed an impedance of 150 ohms, but
> after bonding to the structural steel, the impedance dropped to 1 ohm.
> Sounds to me like the steel was a better ground than the rods.
> The article also mentions the "conduit as choke" thing, which I used to
> believe in, but subsequently have not seen a good analysis to show that
> it actually works, particularly for large surge currents, taking into
> account the magnetic saturation of the steel.
> > According to NFPA 780, Ufer grounding is generally satisfactory provided
> > that it is not the only earth grounding mechanism and that the ground is
> > brought out beyond the concrete encasement to make the earth connection.
> Which section? (just curious.. I don't have my copy of 780 here)
> > Paul, W9AC
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