[TowerTalk] Rebar

[Jim Thomson]

Date: Wed, 22 Sep 2010 18:38:34 -0700
From: "Hank Lonberg" <kr7x1 at frontier.com>
Subject: Re: [TowerTalk] Rebar

Allen:

What happens is that the strength of the reinforcing bar in
the head affected zone of the weld is reduced by 50%. This is
why you do not weld normal A615 reinforcing bars. If you
absolutely have to/ want to weld the bars then use A706
specification reinforcing bar not A615. Happy electrodes to
you.

Regards
Lonberg Design Group, Ltd.
H. Lonberg, P.E.,S.E. / KR7X
President

##  Hank is dead on 100%.   On the last page of the UST 
"formal stress analysis document"  it says the exact same thing.
Our local re-bar co  informs me, that the only stuff they carry is 
A706,  60 ksi grade, weldable  re-bar.  In some applications,where
re-bar is placed in real long horizontal spans, welding  the large 
overlaps is  preferred  to using wire. The local re-bar co's  take on it
was, why stock some of each type, just sell  weldable A706 stuff, then
you can tie with wire, or weld it, or both. If it's required to weld..after the fact,
then you are covered.   

##  remember the Oklahoma bombing of the Fed building ?  ONE
support got blown out, and the rest of the building pancaked.  The
cause was traced to the horizontal rebar's, aprx 40'  long.  Not enough
overlap, and not welded.  When the ceiling drooped with no support below it,
the horz bars just pulled apart. The building codes changed after that.  If it had been
built to the new code, it would have survived.  

##  On the UST  re-bar spec, they specify a min  radius bend, for each size of
re-bar, much like coax.  Three corners of each horizontal 'tie'  has a nice radius bend,
The 4th corner gets  a 'stirrup'.... where the bars are brought back towards  the center
of the foundation.  IE:  the entire 'tie'  is made from one continuous piece of rebar. 
The length  of the stirrup is = 10 x the rebar diam.  My local eng insisted on 12 x rebar diam. 
There was a symposium, 3 x months ago, in Texas, with PE's in attendence. They were setting
new building code standards, and one of the  topics was stirrup length. Some wanted 10 x d,
and some wanted as much as 17 x d .  Some wanted wire, and some wanted them welded.  On
my old Trylon base, stirrups were not used on the 4th corner. They just overlapped, but parallel
to the existing bar.  The stirrups  are actually stronger, and I believe a  seismic thing. 

In Israel,  I saw pix of what looked like an apt building, that had been hit with a bomb on the 3rd floor, right
on the corner. Instead of the usual miles of rebar sticking out like a porcupine, it was pretty much all 
intact...it had been welded. [ concrete was gone, huge gaping hole]. 
The building next door was also hit, again, on a corner, not welded, and 
building was a right off, couldn't be repaired. Structural integrity was compromised.  I'm no expert, but
welding some portions of the tower base cage would be beneficial. It would certainly make it more rigid.
If I knew how to weld, I wouldn't hesitate to weld the cage.  However, I wouldn't pay huge $$  to have 
it welded... unless the local building codes changed. 

later... Jim  VE7RF