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RE: ACI 318 paragraph 21.2.5

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Warren:

The intent behind the requirement for the use of A706 or the additioanl
requirements placed on A615 for use in seismic zones was explain very well
by Harold.  He did not comment, however, of the goal/purpose of the
requirement in the "grand scheme" of concrete design in seismic area
(although I am sure that he would be willing to add).

The goal behind this requirement is to insure a more predictable response
from your concrete structure due to a seismic response.  As you may or
may noy be aware, one of the main ideas of seismic design is to "try"
(that is too simplistic, but I will leave) to pick the mechanism that will
result due to the seismic event in your structure.  One example of this is
the strong column-weak beam philosophy used in seismic design.

The reason that the reinforcement strength is "controlled" is to make sure
the actual strength of the members that your designed is close to what you
calculated them to be.

For example, if you use A615 rebar, then there is no upper limit imposed
on the yield strength of the bar.  Let's say that you get A615 bar that is
supposed to be Gr 60 bar, but has a really high yield strength (say 75+
ksi) that is used for the column bars.  The steel for the beam though only
has a yield of 61 ksi.  Both would met the A615 spec, but this result
would potentially cause a problem with the strong column-weak beam
philosophy used in the code.  You could end up with your column failing
before the beam does.

This provision is very similar to the R (or is it Ry) factor used in the
AISC seismic provisions for steel design.  In steel design, One uses an
Fy(design)=R*Fy(minimum).  This allows the code to "increase" the Fy used
for A36 steel since there are less controls on the yield strength of A36
steel and many A36 steels are dual-certified as A572 or A992 steels (i.e.
have a yield strength of 50 ksi or more).

The point is that in seismic design knowledge of the actual material
properties is much more important that for gravity load design.

So to answer your questions:

1. The dymanic properties of A706 and A615 are going to be very similar
(i.e. how the steel will actually behave/respond with stressed).  The
concrete with the different bar types could behave drastically different
under dynamic loading.  When we design concrete, we want the R/C concrete
to behave in a ductile manner.  This typically means that we want the
steel bars to yield before the concrete crushes.  If you design the
concrete section assuming a yield strength of 60 ksi, but get bars with a
yield strength of 75 ksi, then your calculations would be inaccurate and
the section could fail in a non-ductile manner.  Thus, with the A706 bars
in a concrete section, there is a much better assurance that the section
will fail in the ductile manner.  The section with A615 steel could fail
ductily or not depending on what yield strength you actually get.  So,
while the steel bars themselves will likely have similar dynamic
properties, the concrete with the bars may not.

This is not to say that the dymanic properties are identical.  Part of the
different between the two is the ratio of ultimate to yield.  The higher
this ratio the more ductile the steel will behave.  So a A615 bar that has
a much high yield than the minimum (60 ksi) will likely have a lower ratio
of ultimate to yield, which means that it can "absorb" less energy and
will behave is a less ductile manner.

2. Don't know.  I am not currently too in touch with the cost of rebar.  I
am sure others can comment, including Harold.

3. I can't say for sure, but I can speculate.  Harold might be able to
provide some better insight since he has been involved with the BSSC which
produces the NEHRP provisions (FEMA 302 and 303).

One possible reason is that concrete and masonry are handled by different
subcommittees.  While they do coordinate efforts and communicate (and
there are many people that look at the whole document), it is possible
that this particular issue was not address in the same manner for both
materials.  That is, it was an issue that the concrete subcommittee dealt
with, but that the masonry subcommittee did not yet have time to deal
with.

Keep in mind that (concrete) masonry design, similar in many ways to
concrete design, is a little behind where concrete design is today.  The
masonry code (MSJC) is just introducing strength design into the 2002 code
(even though is has been present in the UBC and IBC).

HTH,

Scott
Ypsilanti, MI


On Sat, 9 Feb 2002, Foy, Warren wrote:

> Harold,
>
>
> Thanks for the reply.  Immediately after I posted the original question, I
> found the answer in the footnote.  I am still curious about a couple of
> issues though.  The project in question is a facility designed to resist
> internal blasts and is constructed of substantial reinforced concrete walls
> and roof located in a low seismic hazard area.  There is no doubt in my mind
> that ASTM A 615 reinforcing, if designed to resist the blast loading, will
> be adequate to resist the seismic loadings, however, the reviewer wants to
> conform to the code and require A706 material.  Because of the dynamic blast
> loading, the design criteria allows for a design yield strength of 81,180
> psi (considers an average yield of 66,000 psi with a dynamic increase factor
> of 1.23).  The design criteria (COE TM 5-1300) does not address the use of
> A706 material. The following questions come to mind.
>
> 1. Do you think that the dynamic material properties of A615 and A706
> materials are the same?
>
> 2. Is there a premium on the material costs?
>
> 3.  Why does FEMA 302, which by reference to ACI 318 chapter 21 requires
> A706 for special reinforced concrete shear walls, but does not have such a
> requirement for special reinforced masonry shear walls?
>
>
> -----Original Message-----
> From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
> Sent: Friday, February 08, 2002 5:27 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: ACI 318 paragraph 21.2.5
>
>
> Warren,
>
> There were 2 problems with A615 bar that ACI was trying to address.  One was
> establishing an upper yield strength limit so that inelastic rotation would
> occur at a predicable level.     The second problem was to have a yield
> strength greater than the tensile strength to get greater inelastic
> rotation.
>
> A706 has established a minimum and a maximum yield strength directly listed
> in Table 2.  Table 2 also has a footnote that is almost identical to
> 21.2.5(b).
>
> Regards,
> Harold O. Sprague
>
> > -----Original Message-----
> > From:	Foy, Warren [SMTP:Warren.Foy(--nospam--at)mhgrp.com]
> > Sent:	Friday, February 08, 2002 12:44 PM
> > To:	seaint(--nospam--at)seaint.org
> > Subject:	ACI 318 paragraph 21.2.5
> >
> > Can anyone explain the intent of this paragraphs' requirement that if ASTM
> > A615 material is used, "the ratio of the actual ultimate tensile strength
> > to
> > the actual tensile yield strength is not less than 1.25"?  It seems that
> > the
> > intent is to establish an upper limit on yield strength similar to the
> > requirements of ASTM A706 but I do not see the 1.25 ratio requirements in
> > ASTM A706.
> >
> > Warren S. Foy
> > Structural Design Manager
> > The Mason & Hanger Group Inc.
> >
> >
> > 300 West Vine Street, Suite 1300
> > Lexington, KY   40507-1814
> > voice: 859.252.9980
> > fax: 859.389.8870
> > web: http://www.mhgrp.com
> >
>
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