[TowerTalk] Tower grounding connections and foundation

[Patrick Greenlee]

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


> Thanks for your reply.  Why drive rods? Because I didn't know any 
> better.  I read that ground rods were to be spaced their length apart 
> and with the Ufer grounds on 14 ft centers an 8 ft rod between each 
> would fit that advice OK.  If the additional functionality is marginal 
> I can skip those three rods. The piers constituting the Ufer grounds 
> are about  3 1/2 ft deep in the ground and extend into the cage in the 
> rectangular solid LUMP above them.. I haven't driven the "redundant" 
> rods yet and can use them elsewhere.

> 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.
   I will be able to apply this to the central rebar cage from the tower 
company.

>
> 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.
>
Good to know but time late in this instance as the Ufer grounds are 
finished and cured.  They have steel tubing (bonded to the rebar) coming 
out of their tops.  So, should I encircle the tower with copper 
connected to the three Ufer grounds?
I have a 315 ft roll of #6 I bought for this sort of thing and have some 
repurposeable copper tubing as well. note the three Ufer grounds will be 
connected by rebar to the central foundation and it rebar cage. So would 
circling the Ufers be redundant?
>>
>> 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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