[TowerTalk] QST article on how to put up a tower

[Jim White, K4OJ]

...IF YOU LOOK AT REBAR I THINK IT WILL BE OBVIOUS THAT IT IS A 
FUNCTIONAL PRODUCT - ONLY!

A lot of rebar is essentially recycled steel... it has to be up to 
certain standards depending on where it is used... most residential 
rebar used in floor slabs, pools, etc. is a very low grade... rebar used 
in commercial construction of massive civil engineering projects, be 
they bridges or buildings is usually held to a higher standard!

Because of the questionable heritage of the steel (which even though it 
is recycled at times is perfectly suitable for reinforcing applications) 
there is probably concern over welding it - if you do not know fer sure 
what you are welding you will not know fer sure how to weld it!  I think 
this is why you see some negatives as welding the cage...  for 
structural reasons the cage is either welded or wired to make sure the 
steel is where it is supposed to be within the pour... concrete coming 
out of the shoot can have some force to it and a bar slammed by the 
falling concrete will of course try to move unless it is restrained.

As to the role of the cage electrically - recommend you research the 
archives on this one for all those questions about grounding systems 
within or external to the pour, etc. - it has been covered on towertalk 
a number of times!

Keep it wet!

K4OJ






Jim Lux wrote:
> At 02:56 AM 6/20/2003 +0000, Paul Playford wrote:
> 
>> I am under the impression that the rebar cage should be tied, not welded.
>>
>> Comments?
>>
>> de Paul, W8AEF
>>
> 
> Found on the web (no guarantee of validity, etc.):
> 
> Regarding my recent query as to whether it is better to weld the rebar cage
> for a tower base or to tie it together with wire:
> 
> There were 15 or 20 responses.  Many were of the "I always/never ...
> weld/tie rebar in tower base" flavor.  A few pointed out that (1) Rohn
> specifically prohibits welding rebar cages and embedments in their plan
> drawings but that on the other hand (2) Triex sells pre-welded rebar cages.
> However, the biggest revelation was a bit of serendipity, as someone 
> pointed
> out to me that there is a column from KI7NF (PE and ARRL VCE) on pg. 79 of
> 4/96 QST that addresses this exact issue.  His recommendations against
> welding rebar have to do with the uncertain metallurgical makeup of rebar
> purchased from most sources and the difficulty in making a proper strucural
> weld without a low-hydrogen welding rod and the correct type of welding rig
> (not the garden variety type apparently).  However, he does go on to state
> that if your purpose in welding a cage is _NOT_ structural, but rather
> electrical (continuity to minimize arcing/or for ufer grounds), then you 
> are
> free to go on your merry way as long as you provide some other structural
> joint, i.e. his tied wire "lap splice" method.
> 
> As far as Triex providing pre-welded cages, I'm guessing that they have a
> certified welder on staff with the proper equipment and that they are
> certain of the makeup/carbon content of the rebar that they use.
> 
> One respondent told me to cadweld the joints and another mentioned brazing
> the joints instead of welding them.
> 
> The net result is that I will definitely tie my cages, and maybe weld the
> non-Rohn cages as well this summer as I begin construction of my new
> station. Thanks to all for their responses.
> 
> de Pete, AD4TU
> 
> -------------
> 
> Pete's comments make sense...
> I've also seen some comments to the effect that rebar is not 
> manufactured to be weldable (that is, the alloy, temper, etc.), so the 
> structural quality of a welded joint is uncertain, even if low hydrogen 
> rods, etc.
> 
> Jim, W6RMK
> 
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> 
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> 
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