A very robust cage you have there, David. Could you site where the 40 times
diameter is written? I learned by watching and asking questions of
contractors and was told 25 times diameter by more than one and no other
value. I will have to look for a reference as I have no alternate
experience. Thanks for mentioning 40 as that provides incentive to do some
research. Oh by the way... although I demand code or better whenever
possible in all endeavors the only inspection I have had building two houses
and installing a tower has been by the DEQ (Department of Environmental
Quality) who inspect and permit septic systems (my installer was lisc to
self inspect.)
Patrick NJ5G
-----Original Message-----
From: David Gilbert
Sent: Friday, January 03, 2014 10:44 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Tack welding rebar, need howto
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@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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