Re: [TowerTalk] exploding foundations

[Jim Lux <jimlux@earthlink.net>]

K4SAV wrote:
> 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."

The problem I have with the statement is that there are some things not 
stated:
a) did that foundation actually have a Ufer ground? or, was it just 
bolted to the bolts protruding from the concrete? And, if the latter, 
did those bolts have sufficient length in the concrete, and good 
connection to the tower.  Ufer's papers (and others since) make the 
point that just bolting structural steel to bolts sticking out of the 
foundation doesn't necessarily make a good grounding connection, hence 
the idea of the 20 feet of AWG 4 copper wire that you see in the NEC.

b) A single narrow crack doesn't really look like the kind of damage 
seen from steam explosions. It might just be the camera angle, etc., but 
pictures I've seen of actual cases where spalling and fracture occurred 
look more like large hunks blasted out of the side, and you can see the 
bolt or rebar in the bottom of the hole.  I'd want to see a good picture 
of the top of that footing, or a section cut through the footing.  And, 
of course, a surface crack does not a failure make.


I've also seen a fair amount of the kind of effects from an exploding 
wire in a surrounding material,and it doesn't look like that. However, I 
never tried concrete as a surround, so it might be different.

I've seen fractures resulting from large electromagnetic forces from 
high currents, and they don't look like this either (and in any case, 
the force between two wires carrying a current in the same direction 
tends cause them to pull together)

Upshot is that the text and picture don't provide enough information.

Jim
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