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Message No 65 Guaranteed min & Max strength of rebar

 
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inchak at tatasteel.com
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PostPosted: Sat Jul 16, 2005 5:52 am    Post subject: Message No 65 Guaranteed min & Max strength of rebar Reply with quote

Dear Ms Alpa Sheth
I have a few observations on the clarification provided by you (vide
Message No 65 Re: Guaranteed min & Max strength of rebar) I quote your
response and add my observations below each point.

a) Most times, fy in Fe 415 or Fe500 is not 415 or 500 but much higher.
Typically Fe 415 steel has fy between 430 to 470. Ditto for Fe500.
My response:
At present the IS 1786 stipulates only minimum YS for different grades. the
actual values can be and normally is much higher with a wide variation. In
last couple of years we have checked some 257 samples collected from
market. The highest YS we got was 788 MPa(!). 26 samples had YS 600MPa or
more, of which 5 had YS above 700 MPa.  Minimum was of course 416 MPa if we
ignore some 57 samples with YS below 415 MPa and therefore not compliant to
IS 1786. Under such circumstances " fy between 430 to 470" may not be a
valid assumption.

b) Additionally, Steel can take upto 1.25fy due to stain hardening
My response:
This is not correct to assume without a qualifier that "Steel can take upto
1.25fy due to strain hardening" . Actually the ability of the steel to
strain harden is accurately reflected by the UTS value. If the UTS is 10%
higher than YS (IS 1786 today!) then you can expect only 10% strain
hardening before fracture. This is why Mr. Neelkanth D Joshi is rightly
advocating UTS/YS ratio of 1.25  (vide Message No 66 ).

You would have noticed that in the Super Ductile bars  (vide Message No 30)
we have tried to increase the UTS/YS ratio to 1.20 and also put a cap on YS
to 550 Mpa and on UTS as 50% higher than YS. The actual numbers can be
further tailored to the actual needs of the Structural Engineers

Surely the matter is important enough to merit a threadbare discussion and
resolution. May be we have to have a fresh look at the codes. Any idea how
this point is addressed by the codes of the other countries? You may be a
better expert to lead the debate and bring it to a logical conclusion
because I am not knowledgeable enough on the finer aspects of structural
Engineering Design.
Regards,
Indranil Chakrabarti


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