On 11/30/2013 11:32 PM, Grant Saviers wrote:
Interesting, welding is clearly better as it won't loosen in shipping
or handling. There is a weldable grade of rebar which should be
used. For small tack welds, maybe not so important. One PE told me
the rebar design for towers is primarily to prevent cracking of the
base concrete. Considering the huge amount of rebar I've seen in
building columns and bridges, that makes sense to me.
When we look at rebar and tying instead of welding, a local ham
mentioned a practical reason for the tying. Remember that in industry,
roads, and where most concrete is used what it would take to weld all
the junctions in acres of concrete, the "heavy" cables that have to
reach the welding, and the huge number of welds required by certified
welders. Then realize, any kid off the street, or any green card holder
can be taught in minutes how to tie rerod, and they cost a fraction of
the certified welder while requiring no equipment other than gloves and
a pair of pliers.
It's likely that economics and not physics brought about tying rebar.
73
Roger (K8RI)
Grant
KZ1W
On 11/30/2013 5:26 PM, Roger (K8RI) on TT wrote:
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 tid 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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