[Towertalk] Tower base "solution"

[A9xw@cs.com]

I made a private replay earlier.  So here is a group reply.  First I am NOT a 
PE. I simply am a broadcast engineer with over 120 stations on my built em 
list. Plus a few personal towers ranging from 56 feet to 330 feet.  

The problem with cast concrete for a base is two fold.  One, the tower base 
must be attached, ie drill holes insert bolts, cement bolts in holes. Not too 
much a problem for down thrust, but there is up thrust on the side the wind 
comes from.  The second is if you use it for a base then over pour you still 
need rebar to hold the two pieces together because it is not ahomogenous 
piece and will eventually fracture at the joint.

I have used precast, even parking lot bumpers for guy wire anchors. These 
usually were with an overpour with the anchor rod or cable going completely 
around the cast and the overpour covered the entire cast section so even if 
it did fracture, the cap would hold the pieces together.  The back fill is 
rolled to hold the soil  resistance to pull out.  

>From an engineering standpoint, as long as the center of gravity is below 
grade there shouldn't be a problem at the base, the soil bearing at the guy 
anchors should exceed 50% of the upthrust moment alone, plus the dead weight 
has to exceed by 50% the upthrust moment. alone. if that wasn't clear,  the 
dead weight is 50% more than the possible upthrust and the soil bearing is 
also 50% above the upthrust moment.  If you have 100 Kips of guy line thrust, 
you need enough weight for 150 Kips as a safety margin.  

In my local all they care about is the ground rod is attached to the tower 
and carries a peak current of 2000 amps for 1 second. I use 500 MCM copper 
stranded & cad welded to each of four rods. with interconnect to all three 
legs.  Electrical codes differ so check local code first.      

Henry AA9XW






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