Return to index: [Subject] [Thread] [Date] [Author]

Re: ductility requirements for rebars

[Subject Prev][Subject Next][Thread Prev][Thread Next]
ACI 318-99 actually does not really mention elongation either.  It
basically specifies similar criteria for A 615 bars to what Drew mentioned
(variation of specified yield strength from actual yield strength and
ratio of actual ultimate tensile strength to actual yield tensile
strength). The ASTM A 706 specification DOES include maximum limits 
to the yield strength while ASTM A 615 does not.  Section 21.2.5 of ACI
318-99 states:

"21.2.5  Reinforcement in members resisting earthquake-induced forces
Reinforcement resisting earthquake-induced flexural and axial forces in
frame members and in structural wall boundary elements shall comply with
ASTM A 706. ASTM A 615 Grades 40 and 60 reinforcement shall be permitted
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 (retests shall not exceed
this value by more than an additional 3000 psi); and
(b) The ratio of the actual ultimate tensile strength to the actual
tensile yield strength is not less than 1.25."

And the comentary for this section states:

"R21.2.5  Reinforcement in members resisting earthquake-induced forces
Use of longitudinal reinforcement with strength substantially higher than
that assumed in design will lead to higher shear and bond stresses at the
time of development of yield moments. These conditions may lead to brittle
failures in shear or bond and should be avoided even if such failures may
occur at higher loads than those anticipated in design. Therefore, a
ceiling is placed on the actual yield strength of the steel [see
21.2.5(a)].

The requirement for an ultimate tensile strength larger than the yield
strength of the reinforcement [21.2.5(b)] is based on the assumption that
the capability of a structural member to develop inelastic rotation
capacity is a function of the length of the yield region along the axis of
the member. In interpreting experimental results, the length of the yield 
region has been related to the relative magnitudes of ultimate and yield
moments. (ref 21.7) According to this interpretation, the larger the ratio
of ultimate to yield moment, the longer the yield region. Chapter 21
requires that the ratio of actual tensile strength to actual yield
strength is not less than 1.25. Members with reinforcement not satisfying
this condition can also develop inelastic rotation, but their behavior is
sufficiently different to exclude them from direct consideration on the
basis of rules derived from experience with members reinforced with
strain-hardening steel."

These requirements are very similar to requirements in the AISC Seismic
Provisions to "insure" that calculated design strength (which is based
upon the specified yield strength) is "reasonably" close to the actual
design strength (which will result from the actual yield strength).  Thus,
the "push" to use A 706 for seismic applications is similar to the "push"
to use ASTM A 992 for structural steel in seismic application (or the A572 
enhanced specification).

HTH,

Scott

On Wed, 18 Apr 2001, Drew A. Norman, SE wrote:

> Dr. Chowdhury,
> 
> 
> While we don't directly control elongation, we do worry about overstrength
> (which typically translates into reduced elongation in the test specimen)
> and require minimum ductility by calling for tensile strength at least 125%
> of yield.  Here are City of Los Angeles special requirements as they
> translate into general notes my office uses on structural design drawings
> for all projects in areas of seismic risk:
> 
> 2.  ALL REINFORCING STEEL SHALL MEET THE FOLLOWING SPECIAL DUCTILITY
> REQUIREMENTS (UNO):
> 
>    a. ACTUAL TENSILE YIELD STRENGTH SHALL BE NO MORE THAN THIRTY PERCENT
> (30%) GREATER THAN SPECIFIED.
> 
>    b. ACTUAL ULTIMATE TENSILE STRENGTH SHALL BE NO LESS THAN ONE HUNDRED AND
> TWENTY-FIVE PERCENT (125%) OF ACTUAL TENSILE YIELD STRENGTH.
> 
> STEEL CERTIFIED AS ASTM A706 PER UBC STANDARD 26-4 MAY BE ASSUMED TO COMPLY
> WITH THESE REQUIREMENTS.  OTHER GRADES AND TYPES OF STEEL SHALL BE DELIVERED
> TO THE SITE WITH EITHER MILL CERTIFICATION OR REPORTS OF INDEPENDENT
> LABORATORY TESTS VERIFYING COMPLIANCE.  SUCH CERTIFICATION OR TESTING SHALL
> BE AT THE CONTRACTOR'S EXPENSE.
> 
>  3.  REINFORCEMENT DELIVERED TO THE SITE SHALL BE ACCOMPANIED BY APPROPRIATE
> TESTING REPORTS AND CERTIFICATION, INCLUDING EVIDENCE OF CONFORMANCE WITH
> SPECIAL DUCTILITY REQUIREMENTS SPECIFIED ABOVE.
> 
>  4.  LACK OF CERTIFICATION OR INADEQUATE CERTIFICATION SHALL BE SUFFICIENT
> CAUSE FOR REJECTION OF MATERIAL.  UNCERTIFIED OR INADEQUATELY CERTIFIED
> MATERIAL SHALL NOT BE STORED AT THE SITE OR USED IN THE WORK AND IF
> DELIVERED TO THE SITE SHALL BE IMMEDIATELY REMOVED.
> 
>  5. SUBJECT TO <my firm's> WRITTEN APPROVAL, REINFORCEMENT NOT MEETING THE
> SPECIAL DUCTILITY REQUIREMENTS SPECIFIED ABOVE (NOTE 2) MAY BE PERMITTED IN
> LOCATIONS NOT SUBJECT TO YIELDING UNDER SEISMIC LOAD.
> 
> Hope that helps.
> 
> Drew A. Norman, S.E.
> Drew A. Norman and Associates
> Pasadena, California
> 
> ----- Original Message -----
> From: "Prakash C. Chowdhury" <pcchowdhury(--nospam--at)yahoo.com>
> To: <seaint(--nospam--at)seaint.org>
> Cc: <SpragueHO(--nospam--at)bv.com>
> Sent: Wednesday, April 18, 2001 6:57 AM
> Subject: ductility requirements for rebars
> 
> 
> > This query may be too basic for members but I am not a civil engineer and
> > am more concerned here with requirements for material specifications.
> >
> > Steel rebar specs around the world specify widely varying requirements
> > for rebar 'elongation %'. One reason is that the steels vary from the
> > high-carbon steels in some ASTM specs to more ductile low-alloy steels.
> > As for service requirements, the steel may not be normally required to
> > sustain a strain more than the 3 to 4% that the concrete can take. But
> > what is the situation when designing for earthquake resistance ? Is it
> > sufficient to rely on detailing for ductile design or is there a minimum
> > elongation % mandated for the steel rebar ?
> >
> > When the CEB Model Code was being formulated in Europe in the 80's, there
> > was a proposal to test the steel so that it undergoes 3 strain reversals
> > (in the plastic region) but it may have been dropped because of the
> > difficulty of regular testing for such low-cycle fatigue.
> >
> > I will welcome any responses off-list if they are considered of marginal
> > interest to list members. Thanks in advance.
> >
> >
> > =====
> > Prakash C.Chowdhury, B.Sc(Surrey), Ph.D.(N'cle),
> > Regional Resident Representative, Torsteel Research Foundation in
> India,H-1A, Hauz Khas,New Delhi - 110 016, INDIA.
> > E-Mail : pcchowdhury(--nospam--at)yahoo.com
> >
> > __________________________________________________
> > Do You Yahoo!?
> > Yahoo! Auctions - buy the things you want at great prices
> > http://auctions.yahoo.com/
> >
> > *
> > *   This email was sent to you via Structural Engineers
> > *   Association of Southern California (SEAOSC) server. To
> > *   subscribe (no fee) or UnSubscribe, please go to:
> > *
> > *   http://www.seaint.org/sealist1.asp
> > *
> > *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> > *   send to the list is public domain and may be re-posted
> > *   without your permission. Make sure you visit our web
> > *   site at: http://www.seaint.org
> 
> 
> * 
> *   This email was sent to you via Structural Engineers 
> *   Association of Southern California (SEAOSC) server. To 
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
> *   send to the list is public domain and may be re-posted 
> *   without your permission. Make sure you visit our web 
> *   site at: http://www.seaint.org 
> 


* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org