[TowerTalk] lightening Strikes

[Jim Lux]

On 7/18/13 5:31 AM, Skip K3CC wrote:
> It's good to have humor in the thread.
>
> Many years ago our company looked into the disposal and reuse of
> concrete. I had done
> lab test inducing high voltage and high current into concrete.  The
> theory was that the
> electro-chemical reaction would separate the concrete into some of it's
> components.
> Some test were done at a Colorado facility with streamers from ground to
> cloud lightening.
> The concrete did fracture and in some cases exploded.  The cases that
> exploded were concrete
> that had rebar through the broken concrete.  There can be many answers
> why it happened .
> Possible reaction of water into Hydrogen and Oxygen, or other explosive
> gases produced.
> The bottom line is that the amount of energy produced from a lightening
> strike is enormous.

I'm aware of a company (Maxwell Labs, now part of General Atomics) that 
was proposing using electrical discharges for mining and rock 
processing. However, the mechanism there was shock waves from very fast 
rise time sparks near the face of the concrete/rock which would then 
cause the material to spall.  Essentially like repeatedly hitting the 
material with a hammer.  Same scheme as used in lithotripsy to break up 
kidney stones, just on a different scale.

The rise time on those sparks was MUCH faster than the rise time on 
lightning, though.  And it wasn't coupling the energy into the material 
electrically.


>
> Fast forward from the 1975 to today. On going  research a few years ago
> repeated our experiments.
> I think a German company now has a process were they reuse, separate,
> the concrete by an electricity.

Fritz Fruengel has a whole book (4 volumes actually) on this kind of 
thing.  If you can do it with a fast electical pulse from an inductor or 
capacitor, Fruengel wrote about it and probably filed a patent on it.



>
> A direct strike on the tower would discharge the energy to ground
> through the base to ground.
> Even if the concrete base would not explode, it would be weaken enough
> to fail.

I don't think so. The energy dissipation, while high, isn't enough, 
especially if the concrete is sufficiently "monolithic" that the energy 
is distributed throughout.

Lightning has very high peak power, but the energy isn't all that huge 
compared to something like a jackhammer, which isn't high power, but 
keeps hammering away.  A stroke might be 100kA, and you'd dissipate 1E10 
watts in a 1 ohm load, but you're only doing that for 10-20 
microseconds, so you deposit a few hundred kiloJoules into the load. 
For comparison, a pound of most explosives is around 2 MegaJoules.

You'd have to have some way to concentrate that energy so you can get a 
thermal gradient or similar to cause mechanical effects.  Crumbling old 
concrete with cracks, sure.. Solidly cast cube with rebar, I'm not so 
sure. Using the explosives analogy, you can set off that 4 oz of HE 
sitting on the surface of the slab and not much happens (maybe a bit of 
surface spalling), but drill a hole and put the HE inside, and properly 
tamp it, and sure, you can split the concrete.

My contention (and borne out by a whole lot of testing and literature 
over the years) is that well made concrete with rebar isn't going to 
have any internal structure that concentrates the lightning energy 
dissipation to result in structural failure.

In fact, I might think that dissipating a lightning stroke might be a 
good diagnostic test of the concrete quality.  Old cracked concrete with 
the rebar rusting out could well fail in a lightning stroke, but that 
just makes the lightning that straw on the camel.