[TowerTalk] More crank-up questions

[Roger (K8RI) on TT]

On 11/30/2013 8:10 PM, Grant Saviers wrote:
> Contrary to other advice, do ground the tower anchor bolts to the 
> rebar. Then you have a great Ufer ground, considering the area of the 
> concrete in contact with the earth.  The rebar should be tied per code,


Recent, commercial rebar cages ordered with the towers have come through 
welded instead of tied.

I have the wire, but it's a whale of a lot easier to weld it.  If it's 
good enough for the manufacturer, then it's good enough for me.

73

Roger (K8RI)


> with sufficient overlaps and inside the concrete envelope per code.  
> Depending on your site and storm patterns, additional ground rods may 
> be appropriate.
>
> For my two HDX589's we mounted the anchor bolts tightly to the base 
> plate and tack welded rebar between the six bolts to make a solid sub 
> frame so that the bolts wouldn't move when the concrete was placed and 
> vibrated.  That way the concrete can be placed and finished without 
> the interference from the base plate. This sub frame was wire tied to 
> the main rebar cage.  After the concrete hardened the base frame was 
> installed and leveled.  You can order stronger concrete (4000psi or 
> higher) than the UST spec (2500) for a very slight up-charge.  The 
> limiting factor in concrete for towers is tensile strength, not 
> compression, considering the tensile/compressive strength ratio.  A 
> free standing tower has opposite forces in the legs, 1 or 2 in tension 
> and the others in compression when the wind blows hard.
>
> Proper water content and curing is important.  You can get a slump 
> test and post cure strength report from an independent testing 
> outfit.  Code required this for my towers and I think it cost about 
> $250 per tower, as they were poured on different days.
>
> The 589 is positive pull down, but it doesn't matter vs the HD70 since 
> for either design the tower weight is always on a cable, unless down 
> and blocked for climbing.  Better to avoid that anyway and use a 
> ladder or rent a boom lift.
>
> The NF7P coax standoffs work well for me - the loop types not the 
> "holds coax off the ground" type.
>
> Grant KZ1W
>
> snip..
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