[TowerTalk] Tack welding rebar, need howto

Fri Jan 3 23:44:16 EST 2014

That's not really true.  The "standard" building code (which varies of 
course from by application and jurisdiction) requirement is for splices 
to be a length 40 times the diameter of the rebar (I looked it up since 
my last post), which means a minimum 20 inches lap for 1/2 inch rebar.  
A "slight" overlap doesn't cut it no matter what you tie it with.  
Inline overlaps need to bite into enough concrete to hold both pieces 
together ... otherwise it's just wasted steel.

More to the point of what I was trying to describe earlier, Tee 
connections for rebar are probably the weakest possible joints if not 
welded or hooked.   For a 2-dimensional example, picture the perimeter 
of concrete slab.  You have a length of rebar running around the 
perimeter of the slab roughly three inches from the edge of the slab, 
with some other pieces of rebar from side to side running across the 
interior.  Those cross pieces have virtually zero contact area to the 
perimeter runs, and they can't extend very far beyond the perimeter 
rebar because there is only three inches to the edge of the slab.  Yet 
those cross laps are the only thing other than the internal tensile 
strength of the concrete itself (which if that were worth a damn we 
wouldn't need rebar anyway) keeping the edge of the slab from breaking 
off in the vertical plane of the perimeter rebar.  Since I don't trust 
welds made to uncertain compositions of steel (you can also research 
that topic online if you want), I suggested bends to make hooks ... 
which are in fact required by code when using concrete and rebar to make 
a beam. There is more to making rebar connections than just having them 
hold together during the pour.

Lastly, someone also mentioned that welding rebar joints was desirable 
to hold the shape of the cage during the pour.  I don't think that's at 
all necessary either.  The rebar cage for the foundation of my tower was 
9 feet across and five feet deep (certainly large enough to be very 
floppy), and I never used anything except wire to hold it together.  I 
did, however, use horizontal bends at all the corners of my perimeter 
runs (also for the vertical pieces at the corners).   A twenty foot long 
stick of rebar bent in the middle gave me two sides plus one foot at 
each end to overlap with the opposing two sides ... thus a total of 24 
inches of bent overlap at those corners as well.  I weigh 180 pounds and 
crawled throughout the cage repeatedly with virtually zero flex or 
shift, and there was no displacement at all when the concrete was dumped 
into the hole directly from the truck.  It could be a much different 
story if the corers were merely crossed and tied.  A section of bent 
rebar is a bracket ... a crossed and tied joint is a hinge.

73,
Dave   AB7E

On 1/3/2014 7:55 PM, Patrick Greenlee wrote:
> When welding the ends of pieces of rebar as an alternative to a slight 
> overlap and wire tying it just doesn't matter what the alloy is if you 
> can stick it together good enough to not fall apart when pouring the 
> concrete you have met the requirement.  This is not a big deal 
> structural issue. Crazy glue (alpha cyanoacrylate anaerobic adhesive) 
> would be fine if it would hold during the pour. Also kite string, 
> dental floss etc.
>
> Patrick NJ5G
>
>
> -----Original Message----- From: David Gilbert
> Sent: Friday, January 03, 2014 6:34 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Tack welding rebar, need howto
>
>
> Well, my point with the cheap stuff is that you don't really know what
> you're welding to.  It might be part cast iron from an old engine block
> for all I know, and that doesn't weld well at all.  You wouldn't
> necessarily be able to tell if you had a good joint or not.   I used a
> LOT of 60,000 PSI rebar when I built my house (ICF walls and 4,500 sq ft
> of slab) and I was surprised how much variation there was in brittleness
> when I bent it even for the supposedly good stuff.
>
> But no, I'm not really hung up on the strength of the steel itself. All
> of it is stronger in tension than the concrete is, and if I'm in doubt I
> just use more of it.   ;)  I only mentioned it because I thought I saw
> someone earlier in the thread question whether a welded cage was needed
> for strength.
>
> 73,
> Dave   AB7E
>
>
>
>
> On 1/3/2014 3:13 PM, Jim Lux wrote:
>> On 1/3/14 12:05 PM, David Gilbert wrote:
>>>
>>> By the way, while I totally understand why you'd want to tack weld the
>>> bolts to the rebar, I've never thought much of welded rebar cages in
>>> general.  True weldable rebar (the stuff that is supposed to hold it's
>>> strength after welding) is expensive and harder to get.  The 40,000 PSI
>>> stuff you can get at Lowes and Home Depot comes from indeterminate
>>> source material (a mixture of washing machine chassis, old bed springs,
>>> car frames, salvaged rebar, etc) and almost certainly will not weld 
>>> with
>>> consistent strength, although you can buy better 60,000 PSI stuff from
>>> construction supply places.
>>>
>>
>> Yeah, but if you're using rebar from who knows what kind of metal, are
>> you really concerned about whether it retains the strength when you
>> weld it?
>>
>> As I recall, about as weak as steel gets is 30-40 ksi.. Unless your
>> welding physically damages the steel, I don't know how much lower you
>> can go.
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