You don't say what will be on top of the tower, but if the weight is all
in the vicinity of 60 feet what would make the tower bow lower down? If
anything it would probably bow toward the wall, not away from it.
How is lightning that travels thousands of feet through the air going to
be inhibited from jumping across a few inches of air gap between the
ground rod and the surrounding concrete? I suspect that could be an
issue, though, since I'd bet that the concrete slab (assuming it has a
reasonable amount of rebar in it) provides a better path (either
capacitive or Ufer) to ground than the ground rods do. I'm no expert,
but I'd use a stud finder to find the rebar and try to bond the ground
rods to them in several places using heavy gauge wire and cadwelds.
73,
Dave AB7E
On 3/16/2012 12:19 PM, Kipton Moravec wrote:
> I am supervising the installation of a 60 foot Rohn 25 tower. (This
> means finding the people with knowledge and muscle.)
>
> It will be placed next to a 50 foot building with concrete walls, a flat
> roof, and a 5 foot parapet (wall) around the roof. (Top of the parapet
> is 50 feet above the ground.)
>
> It will have one wall bracket for each section of Rohn 25 along the
> wall. I know this is probably overkill, but that is what they want, and
> they have the money for it.
>
> The proposed plan is to bolt a tilt-over base to the large 6" concrete
> driveway/parking lot by using butterfly expanders.
>
> Then they want to assemble it on the ground, and tie a couple of ropes
> to it at 50 ft and have 6-8 people pull it up from the flat roof. (With
> the same number on the ground to help get it started.)
>
> First question is this a good plan?
> Do we need to also pull from the middle (25 feet) so there is not a
> bow?
>
>
>
>
> Second part. The ground is all 6" concrete driveway. They are worried
> that we can not just drill a 5/8" or 3/4" hole through the concrete and
> put ground rods in because we have to have a certain amount of air gap
> between the ground rod and the concrete, or the concrete will explode
> when lightning hits.
>
> That does not sound right to me, because the even though concrete is
> somewhat conductive, the ground rod is going 10 feet into the earth. And
> if I have three ground rods the lightning will be spread into the ground
> and not as much through the concrete and the unknown places of rebar in
> the concrete. And we already have the tilt over plate bolted to the
> concrete driveway. So it is not like there is a point connection to the
> concrete.
>
> Do we really need an large air gap between the ground rods and the
> concrete driveway? If so how much?
>
> Kip
>
>
>
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