[TowerTalk] Exploding Foundations

[David Gilbert]

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