[TowerTalk] Rebar ?

[Chuck Lewis]

Dan, et al,

Just to muddy the water a little (but to agree against rebar J-bolts):
 
Hy-Gain uses rebar for their concrete base cage supporting their
crank-ups. The rebar is welded to 1/4" steel tabs (about 2 1/2 x 8)
which match tabs on the tower legs and are fastened with 3/4" bolts (2
each leg) in shear. They use one #8 and one #5 bar per leg. Note that
they are not threaded like J-bolts, but are welded along the long edge
of the tabs. From Hy-Gain's stress analysis, it appears that they use
the standard grade rebar (grade 40) which has a yield strength of 40
Kps. They also use a E70 weld spec. for the welding rod...not
particularly special.

Hy-Gain calculates their allowable loads assuming BOTH bars are #5 (a
big derating). For example: For the HG52SS, each leg has about 8,000
lbs. load in tension or compression (for 50MPH winds). A #5 (5/8"),
grade 40 rebar will have a yield strength of 40,000 lb./sq.in. x
((5/8)/2)^2 x 3.14)sq. in., or about 12,400 lbs. per bar.  Two bars per
leg means each bar must withstand 8000/2 = 4000 lbs. Their factor of
safety is therefore 12,400/4000 = 3.1 . But wait...they actually use one
#5 and one #8 bar per leg...#8s are not only thicker, but are usually
grade 60 (60,000psi), so the safety factor improves some more, but wind
speed over 50 MPH will cut into that: e.g., at 70MPH,  leg load
increases by 70MPH/50MPH = 1.4 times.

The fillet weld between rebar & tab is good for 91,300 lbs. (an 11-to-1
margin at 50MPH), according to Hy-Gain's calcs.
   
The point is that rebar, although less strong, pound for pound, than
many other materials, is viable when used in a very conservative design,
even when welded. Should you use it? It depends on the design!!!! If you
follow K7LXC's prime directive, do what the manufacturer says; but if
you don't know the manufacturer, and are on you own, do the stress
calcs. or get someone to do it for you with some credentials. By the
way, don't assume my numbers are right...this is too important to rely
on "some guy on the reflector" who said "It's OK to use rebar"! 

As for rebar J-bolts, I'd be leery. The threads are probably cut rather
than rolled, and the resulting sharp thread roots not only reduce the
section, but act as stress raisers...SCARY!

Chuck, N4NM
 

Dan Arney wrote:
> 
> Go to any steel fabricating yard or Home depot and buy REAL J Bolts and good nuts.
> With rebar J bolts you will come out after a big blow and see where the CHEAP J bolts
> let go. What is that mess of twisted steel and look at that pile of fancy Yagis look
> like corkscrews.
> Hank KN6DI
> 
> Bruce Richards wrote:
> 
> > Rebar is made up of leftover "other stuff". This is not what you want to use as J
> > bolts. It does what it is designed for well, but is not suited for other aspects of
> > construction.
> >
> > Pat Masterson wrote:
> >
> > > I was wondering if anybody has "specs" on rebar. I am going to use some
> > > j-bolts made out of 1 inch rebar to anchor a 60' self supporting tower.
> > > Is one per leg enough, or do I need 2? Whats the tensile strength of this
> > > material?


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