[TowerTalk] Tower grounding connections and foundation

[Patrick Greenlee]

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.

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.

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