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RE: ACI 318 paragraph 21.2.5

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I've preferred A706 over the modified A615 because it has a higher specified
elongation.  Seems like this should be a consideration when designing for
inelastic action.

Eric Lehmkuhl,  S.E.
KPFF Consulting Engineers, San Diego


-----Original Message-----
From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
Sent: Sunday, February 10, 2002 4:31 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: ACI 318 paragraph 21.2.5


Warren,

Internal blasts imply that the project is a test range, but TM 5-1300 is for
accidental explosions leading me to think that we are talking internal
"improvised explosive device" (IED- terrorism 101).  If it is an accidental
explosion due to a predicable maximum explosive device, and you are
resisting the force in the linear range, you could use the plain old A 615.
A low seismic area implies bad bar (plain old A615) is dominate in the
market.

If the structure is a hardened structure, there is another document you need
to get from your friendly USACE representative.  It is also a pocket size
like the TM 5-1300, and it is a FOUO document.

Seismic performance and blast performance are similar in some respects and
different in others.  If any loading is defined well and your performance is
in the elastic range, the toughness of the rebar is pretty much irrelevant.
If you are designing for explosions, and the size of the explosive device
and stand-off are predictable, you should be able to design in the elastic
range of the materials.  TM 5-1300 was written in a manner implying elastic
response in some areas and inelastic response in other parts.

Once you assume nonlinear performance, you must provide a system that will
not collapse and will absorb energy as the structure moves in the nonlinear
range.  Whether the force is an inertial response to an earthquake or if it
is due to the over pressure from an explosive device makes no difference.
The rebar should provide for predictable nonlinear deformation.

	1. Do you think that the dynamic material properties of A615 and
A706
	materials are the same?

	If the structural performance is linear, the performance of A706 or
A615 will be the same, and the toughness is not an issue.  If the
performance is nonlinear, you should use the modified A615 bars per ACI 318
Sec. 21.2.5 or A 706.  The TM 5-1300 was written in the late 1980's.  The
seismic people first added the A706 or modified A615 requirements in ACI
318-83.

	2. Is there a premium on the material costs?

	I ran through this exercise a while back.  The premium cost is
variable depending on the bar size and the location of the plant.  For bar
sizes #3 or #4 there is virtually no difference due to the fact that many
bar suppliers are using coiled bar, and you can only coil bar in A706.  If
the bar manufacturer supplies to precast manufacturers (they weld a lot of
bar) or if they cater to the high seismic market, chances are there is
little or no difference in cost because they are generally supplying A706.
You can still find more rebar in A615 than A706 and the law of supply and
demand dictates that A615 bar is cheaper, but that cost difference virtually
disappears in the cost of the project.  It is not significant.

	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?

	Please forgive our reinforced masonry brethren.  They are just now
getting into a limit state design.  But they do have a provision for special
rebar when it is in a special moment frame of masonry.  Check out FEMA 301
(NEHRP 1997) Sect. 11.12.3.4 or better yet the new and improved FEMA 368
(NEHRP 2000) sect. 11.11.3.4.

	Also note that you can use plain old A615 bar in concrete. Sect.
21.2.5 requires A706 and the special A615 rebar in frame members and in
structural wall boundary elements only.  It is true that the definition of
frame members is a bit fuzzy, but the intent is a bit more obvious when you
look at where 21.2.5 is invoked.  In the Frame definitions, only the Special
Moment Frame is explicitly required to meet 21.2.5.  I prefer to use 21.2.5
if I perceive that any elements will go into the nonlinear range.  I will
NOT mix bar specs on a project.  It is difficult enough for the rod busters
to get the right bar in the right spot, and for the special inspectors to
get it right.

Bottom line.
If the explosion over pressure forces are resisted by elements in the linear
range, and you are in low seismic land, avoid the contractor whining, and
use regular A615 bar.

If your response is nonlinear, use the A706 or modified A615 bar.  I would
also include other elements of Chapter 21 of the ACI 318-99 that increase
toughness.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Foy, Warren [SMTP:Warren.Foy(--nospam--at)mhgrp.com]
> Sent:	Saturday, February 09, 2002 11:47 AM
> To:	'seaint(--nospam--at)seaint.org'
> Subject:	RE: ACI 318 paragraph 21.2.5
>
> 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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