On Thu, 4 Mar 2004 17:13:36 +0000 (UTC),
wrote:

wrote:
wrote in message ...

Beaten to death many times in this news group.

Do a google search of this group with the terms concrete lightning explosion
and see LOTS of discussion.

And I did, but I do not think 20 hits are a LOT of discussion. All of
them are 'what if' and other theoretical stuf. None of them however
claim they really had an explosion and tower collapse....

Together with Jason, I am also very curious about 'real experiences'.

Arie

About 20 threads, but a lot of discussion.

A quote from a book by Polyphaser can be found at:
http://groups.google.com/groups?hl=e...k.n et&rnum=8


"It is a common misconception to think that a lightning strike will blow up
a concrete pad. However, consider first, a myth-perpetuating case of an
improperly designed system where the tower leg "J"-bolts are imbedded
directly into the concrete pad. In this case, due to the poor nature of the
tower ground system, each of these Jbolts will actually share a significant
amount of strike current which in turn will flow through the concrete. Since
the surface area interface between the J-bolts and the concrete is small,
the surge current density is very large. The corresponding heat generated
by the energy transfer can turn the concrete moisture into steam and possibly
crack the pad. We have only seen this happen once on a mountain top in the
Nevada desert. However, a few poorly implemented occurrences can give a
valuable technique a bad reputation. If during construction, all of the
rebar in the concrete pad becomes an integral part of your ground system,
the overall surge current density will be several orders of magnitude lower
than the myth-perpetuating case above. With the surge current distributed
over all of the rebar there will be little to no opportunity to develop the
temperatures necessary to vaporize the imbedded moisture. The pad will not
crack."

In other words, do it right, weld it all together and there is no problem.

I suppose someone could do a really horrid installation and...


If you read a little farther they also recommend that the tower
concrete rebar not be the only ground for the tower. Other ground rods
and radials are also recommended.

I have seen a couple of tower pads partially exploded from lightning
strikes. These were on towers that the J bolts were just in the
concrete and not attached to the rebar and no other ground connection
to the tower legs. It blew a chunk of concrete out exposing one of the
J bolts almost completely.

For the handbook to recommend not having contact with metal objects in
the tower base, I wonder how they think the tower should be attached
to the concrete base?

73
Gary K4FMX