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Re: [TowerTalk] exploding foundations

Subject: Re: [TowerTalk] exploding foundations
From: K4SAV <>
Date: Fri, 14 Sep 2007 11:44:15 -0500
List-post: <>
The article does say what the cause of this crack was. Here is what they said:

"A word of caution: A Ufer ground consisting solely of the tower foundation is a bad idea. Lightning surges passing through the foundation can vaporize water in the concrete and damage the foundation through rapid expansion of steam. An example of this unfortunate event is shown in Figure 4. This particular tower had no earth electrode system, other than the leg foundations."

Jerry, K4SAV

Jim Lux wrote:

Paul Christensen wrote:

What exploding concrete??  Back up your claim with real life validated
proof.  My experience has been the opposite.


Want evidence? See Figure 4 (and the text below it) in the link below:

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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