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Fw: 60,000 psi limit on Reinforceing bars

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Thought I would throw in my 2 cents:

ACI 318-89 (revised 1992):

Section 3.5.3.2 - Deformed reinforcing bars with a specified yield strength
fy exceeding 60,000 psi shall be permitted, provided fy shall be the stress
corresponding to a strain of 0.35 percent and the bars otherwise conform to
one of the ASTM specifications listed in 3.5.3.1.

Section 21.2.5.1 - Reinforcement resisting earthquake-induced flexural and
axial forces in frame members and in wall boudary elements shall comply
with ASTM A 706.  ASTM A 615 Grades 40 and 60 reinforcement area allowed in
these members if (a) the actual yield strength based on mill tests does not
exceed the specified yield strength by more than 18,000 psi...(b) the ratio
of the actual ultimate tensile stress to the actual tensile yield strength
is not less than 1.25.


>From these two sections, it would appear that the ACI does NOT limit the
bar yield strength to 60ksi.  In the course of evaluating a number of
existing structures, and reading a lot of literature on evaluation, most
reinforcement I have seen yields at around 65ksi, and ultimate tensile
strength is somewhere around 95ksi.  When this is the case, it becomes a
matter of using the actual (average) yield and ultimate tensile strengths
for any calculations, including bond strength.  I agree that the
stress-strain curves for the steel will need to be checked in order to
lessen the possibility of fracture.  My point is that you are unlikely to
get rebar with a yield strength of EXACTLY 60ksi, so don't sweat it if it
is higher.  Just account for it in any checks you are making. 

On the other hand, there ARE some requirements in the ACI that limit the
yield strength when running check equations (e.g. shear friction).  This
would appear to me to be added safety factor, and in these cases it would
be appropriate to assume 60ksi even if you have 65ksi.  But for seismic
calculations, I try to always use the mill certificates to determine my
assumed fy and fu.

T. Eric Gillham PE
 
----------
> From: ALEX C. NACIONALES <anacio(--nospam--at)skyinet.net>
> To: seaoc(--nospam--at)seaoc.org
> Subject: Re: 60,000 psi limit on Reinforceing bars
> Date: Tuesday, June 23, 1998 6:56 AM
> 
> Dear Michael,
> 
> Thank you for your response. I have a follow up question.
> 1. Using higher strength concrete would increse bond srength would it
not?
> 2. Is not posssible or practical to manufacture 100,000 psi or more
> reinforcing steel with  adequte ductility as required by code.
> 
> I agree with you on the possible brittle failure of high strength steel.
> 
> Thanks again. I hope you have a nice week a head
> 
> Alex C. Nacionales, C.E.
> A.C. Nacionales Construction
> Iloilo City, Philippines
> 
> -----Original Message-----
> From: Michael Valley <mtv(--nospam--at)skilling.com>
> To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
> Date: Tuesday, June 23, 1998 3:06 PM
> Subject: Re: 60,000 psi limit on Reinforceing bars
> 
> 
> The Code imposes the same limit on yield strength for reinforcement
> (in seismic applications) regardless of the concrete compression
> strength.  Again, the concern is assuring ductile response.  There
> are at least two potential ductility problems with higher strength
> reinforcement:
> 1) The wrong limit state (bond failure) controls.
> 2) The reinforcing steel lacks adequate ductility at the material
> level.  This is why material satifying ASTM A706 or A615 (with
> additional requirements) is specified in the Code.
> 
> Internationally, some have used LOWER strength reinforcement to
> acheive higher levels of ductility in R/C response.
> 
> On Sat, 20 Jun 1998 13:04:50, Alex Nacionales wrote:
> > What if  I  use 5000 psi or even 7000 psi
> > concrete?
> >
> > Alex C. Nacionales, C.E.
> >
> >
> > Michael Valley wrote:
> >
> > > On Thu, 18 Jun 1998 04:47:18, Alex C.
> > > Nacionales wrote:
> > > > I do not understand the 60,000 psi  yield
> > > point
> > > > limit on the concrete  reinforcing bars.
> > > > Can some please give me a reason for this.
> > > > Some bars tested on my projects reached more
> > >
> > > > than 60,000 psi  before it yielded.
> > >
> > > ACI 318-95 allows higher yield reinforcement
> > > (for non-seismic
> > > applications) per 3.5.3.2.
> > >
> > > The commentary to ACI 318-95 section 21.2.5
> > > explains why the yield
> > > stress is limited for seismic applications.
> > > The concerns center on
> > > the potential for limit states controlled by
> > > shear or bond failure.
> > >
> > > - - - - - - - - - - - - - - - - - - - - - - -
> > > - - - - - - - - - - - - - -
> > > Michael
> > > Valley
> > > E-mail: mtv(--nospam--at)skilling.com
> > > Skilling Ward Magnusson Barkshire
> > > Inc.                  Tel:(206)292-1200
> > > 1301 Fifth Ave, #3200,  Seattle  WA
> > > 98101-2699          Fax:        -1201
> > >
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> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
> Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
> 1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201
> 
> 
> 
> 
> 
>