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Re: aci 318 app d rod v rebar

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Paul,

The first thing to realize is that you are comparing two different types of analysis developed by two completely independent sources.  There is no reason that the results should match exactly.  The next thing to realize is that the failure mode for rebar is different than a headed anchor bolt which causes a completely different stress distribution within the concrete which effects edge distance requirement, group action, etc.  When rebar is tested the concrete starts cracking at the top and works it way down however a headed anchor bolt will pull out in one large divot.  Since the failure mechanism and stress distribution are significantly different and the fact that  both are based on extensive testing I would not get all hung up on the differences.

Thomas Hunt, S.E.
Fluor



pffei(--nospam--at)aol.com
01/22/2010 09:08 AM
Please respond to seaint
To
seaint(--nospam--at)seaint.org
cc
Subject
aci 318 app d rod v rebar





Harold and Richard
 
Thanks for your discussion on this matter.  There is still something about the difference in the models that I am finding confusing.  I am thinking specifically of cast in place anchors - threaded rod with end nut versus rebar.  
 
I get the idea that rebar is bonding along its length versus rod achieving end bearing on a nut or deformation and that these are different mechanisms for development.  However, they both ultimately rely on engaging a block of concrete.  
1.    If a hooked #6 rebar can develop its capacity in 16" in 3000 psi concrete with side cover of 2.5" and clear spacing of 2 bar diameters why wouldn't that same block of concrete be able to develop a rod with end nut?  
2.    why are issues like cracked concrete, anchor groups,  and the seismic consideration of cbc 1908.1.16 not considered for rebar?  
3.    if that same rod is confined along its full development with stirrups or ties I would think that would also greatly increase capacity (based on rebar development calc's) but is seemingly not given consideration in rod tension development.  Even if I went up to full development length of a straight rebar for equivalent embed of rod (enclosed along its length with ties) it would seem I should be able to develop strength with similar edge distance and spacing requirements to rebar because the block of concrete engaged is the same.
4.    What about development of anchor rods for light posts and street signs?  I have seen older designs with long threaded anchor rods that developed strength in a concrete pedestal.   Seems like a similar model.
 
Thanks for any discussion on this mater.
 
Sincerely,
 
Paul Franceschi
 
 
From: pffei(--nospam--at)aol.com
Date: Tue, 19 Jan 2010 12:06:22 -0500
Subject: aci318 app d rods vs. rebar
To:
seaint(--nospam--at)seaint.org
 

Can anyone clarify why the tension and shear development of threaded anchor=
bolts (aci appendix d)  treated so differently than rebar?  the calculatio=
ns for rebar development for shear friction and tension are much simpler th=
an threaded anchors.  It is seemingly much easier to develop the capacity o=
f rebar in concrete than an anchor rod and the impacts of minimal edge dist=
ance are much less.  also, section 1908.1.16 of the cbc has a further reduc=
tion on capacity of threaded anchors if subject to seismic loads.  there do=
es not see to be a similar provision in the code if rebar is used tor the a=
nchor element instead of a threaded rod.

 
I have found little literature on deep embedment of threaded anchors, but w=
hy couldn't depth to develop tension capacity of a f1554 grade 36 rod be tr=
eated similarly to the same diameter of grade 60 rebar?

 
Thanks in advance for any discussion on the matter.
 
Paul Franceschi
From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
To: <
seaint(--nospam--at)seaint.org>
Subject: RE: aci318 app d rods vs. rebar

 
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No.  A headed stud is different.  It develops its strength at the head in c=
onfined concrete bearing at the head of the stud.  A rebar or threaded rod =
need a defined length to develop strength along the length of the embedded =
portion. =20

 
=20
 
If you really want to shorten the embedded portion of an anchor=2C consider=
a Dywidag bar with a Lenton Terminator at the end. =20

 
Regards=2C Harold Sprague
 

From:
RichardC(--nospam--at)lbbe.com
To:
seaint(--nospam--at)seaint.org
Date: Tue=2C 19 Jan 2010 14:00:02 -0500
Subject: RE: aci318 app d rods vs. rebar

 
So then you would agree that welding rebar to an embed plate DOES yield a h=
igher capacity than using headed studs of similar make-up?
=20
I=92ve actually had another engineer recommend this to me.  On checking I f=
ound=2C oddly enough=2C his assessment to be apparently valid! not that I t=
ook that route in the end=2C however (it seemed a little like cheating=85)
=20

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