[TowerTalk] More crank-up questions
Roger (K8RI) on TT
K8RI-on-TowerTalk at tm.net
Sat Nov 30 20:26:30 EST 2013
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.
> 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
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