[TowerTalk] More crank-up questions

[Roger (K8RI) on TT]

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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