[TowerTalk] Tower grounding connections and foundation

[Jim Lux]

On 8/7/15 12:31 PM, Patrick Greenlee wrote:
> Since I will be mechanically and electrically bonding three horizontal
> radial cast beams to the tower's rebar cage at one end and likewise at
> the other end to piers in the ground with rebar in them (Ufer grounds,
> see below) I may not need to put copper straps onto the tower base,
> tower, or tilt accessory.  Instead I would be driving an 8ft copper clad
> ground rod between each of  the piers. The piers are on 14 foot
> centers.  I would be connecting the rods to the (Ufer ground) piers with
> wide copper strap, heavy gauge copper wire, or I might use 1/4 inch ID
> copper tubing or thereabouts to run between The Ufer grounds and ground
> rods.

If you're talking about "lightning and electrical safety" grounds (as 
opposed to antenna counterpoise/ground screen), then there's no 
particular advantage in what kind of connection (wire, strap,pipe) other 
than mechanical.  AWG6 solid copper wire (or larger) is what the code 
says, where bonding is necessary.

Not sure why you're driving rods.. the Ufer ground is a better 
connection than any rod (more contact area, lower resistance, etc.).




>
> My Ufer grounds:  Each of the three Ufer grounds consists of three each
> 12 inch diameter rebar reinforced  piers connected to each other by  a
> 18 inch high 24 inch wide 7 ft long rectangular block of steel
> reinforced concrete.  All the rebar in each set of three piers is bonded
> together and is electrically accessible by way of a steel tube
> protruding out of the top of each group of three piers.

While bringing a piece of rebar up through the slab as a connection 
point for the Ufer ground is done, AWG 6 copper wire is easier to 
handle.. you can get 30 feet of wire, put 20 feet into the concrete and 
have 10 feet sticking out the top to route where it needs to go.

Interestingly, there's no requirement (or apparently need) for the rebar 
in the concrete to be electrically connected.  There's been studies on 
concrete encased grounding electrodes where the rebar was just "in 
proximity" in the concrete; e.g. wired together with iron wire in the 
usual way, without any special electrical connection (welding, for 
instance) and it worked as well as electrodes where the rebar was all 
welded into a unit. Probably because of the huge contact area between 
concrete and rebar in general.


>
> Additional mechanical integrity beyond that called out by the MFG:
>
> The MFG asked me to pick one of two foundation designs: 1. a 5 ft
> diameter 8 ft deep hole or 2. a 4x4x8 ft hole where the 8 is depth. The
> best estimate I got for drilling the round hole was $1030 and some
> liability for teeth on the drill etc.  I have estimates ranging from
> $300 to $500 for the  square hole.  The square hole may end up 4x? at
> the  surface to get a 4x4 dimension at the  bottom of the hole.  Some
> operators say they would have one sloping side making the top of the
> hole 4x8.  Others say maybe 4x6 at the top and yet another says he can
> do 4x4 from the surface down 8 ft.  The reality may be somewhere in
> between but at over $100 per yard of delivered mud  things could get
> expensive if the hole grows much.
>
> The concrete is to be poured against undisturbed earth so forming and
> backfilling to reduce the requirement for extra concrete at the sloping
> side is not in compliance with the  written specs from the MFG.
>
> The central foundation (MFG's rebar cage) will  be tied into three
> radial cast in place rebar reinforced beams that are each terminated at
> the three outlying Ufer grounds for additional strength holding the
> tower upright. The width and depth of these beams as well as their rebar
> schedule has not been finalized.  Also the attachment of the beams to
> the outlying piers has not been finalized but will likely be one of two
> ways.
> 1. drilll and dowel into the outlying foundations or 2. Use anchor bolts
> set into drilled holes to attach a vertical piece of steel channel with
> holes in its flanges.  Rebar with bent ends will be inserted into the
> holes and bent some more to secure them during the  pour.
>
> All of the above will be done only if passed by my mechanical
> engineering consultant, a good friend with MS Mech Eng from UCLA and 35+
> years of hands on experience with steel structures (mainly a deep diving
> research submarine and its various ancillary equipment.
>
> I am soliciting suggestions for improvements, warnings, or whatever.
> Please sanity  check the above.
>
> TIA for your consideration,
>
> Patrick    N5G
>
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