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Guaranteed min & Max strength of rebar

 
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acpl_sefi
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Joined: 26 Jan 2003
Posts: 63

PostPosted: Wed Jul 06, 2005 6:36 pm    Post subject: Guaranteed min & Max strength of rebar Reply with quote

Dear all,

Back in the 80's in the US,a need felt among structural engineers for weldable "Tor" steel to be available to them that not only had a guaranteed minimum strength but also a maximum strength. The reason was that in order for the strong column weak beam theory to be implemented properly the beam steel could not be stronger than the column steel in terms of material strength. Also weldability avoided the usual site problems. The answer from the industry to this was ATMS A706 bars.  

Do we have an IS equivalent to this and if not is there any hope in the near future.

Best wishes

Vipul Ahuja

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u.mukesh
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Joined: 26 Jan 2003
Posts: 98
Location: Delhi

PostPosted: Thu Jul 07, 2005 4:15 am    Post subject: Guaranteed min & Max strength of rebar Reply with quote

As the ratio of the ultimate strength to yield strength of bars
available in Indian market is quite low (1.11 to 1.16 as per Dr. D.C.
Rai and it is only 1.2 even for TATA STEEL Super Ductile Rebars).
Why are we overestimating plastic hinge moment and consequently shear
as per IS 13920, as the ratio 1.4 is 1.25fy/0.87?

Regards
Mukesh Upadhyay

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alpa_sheth
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Posts: 263

PostPosted: Thu Jul 14, 2005 9:54 am    Post subject: Guaranteed min & Max strength of rebar Reply with quote

Dear Mr. Upadhyay:

You have raised a very relevant point.
The ratio of Ultimate to yield strength is only 1.1 to 1.2. And yet the
factor for Plastic Mom Capacity /Shear Capacity  is (1.25/0.87) 1.4.  Why
so?

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. This
needs to be accounted for as that is the plastic moment capacity that can be
generated.

b) Additionally, Steel can take upto 1.25fy due to stain hardening

c) Plastic Moment Capacity that can be generated is important only to
calculate the shear demand at time of formation of plastic hinge. (Note that
plastic hinge moment capacity is not used for flexural design anywhere). To
ensure that the shear capacity exceeds the flexural capacity to prevent  a
brittle shear failure,  IS 13920 requires that the shear capacity is 1.4
times the shear generated by the formation of Mu, the moment of resistance
for which one uses 0.87 fy for calculation.

Thus 1.4 factor in IS 13920 has really not to do much with  the ratio of
ultimate to yield strength of steel but has more to do with the overstrength
in the Fe415 and 500 steels and also to ensure that the shear capacity
exceeds the shear demand sufficiently at plastic hinge formation.


Regards,
Alpa Sheth

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u.mukesh
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Joined: 26 Jan 2003
Posts: 98
Location: Delhi

PostPosted: Mon Jul 18, 2005 11:01 am    Post subject: Guaranteed min & Max strength of rebar Reply with quote

Hello Ms Sheth

I agree with Mr. Indranil; strain hardening has already occurred when
a specimen fails at ultimate tensile strength.
Plastic moment capacity of a section is calculated by assuming that
the ultimate stress in tensile steel is 1.25 time the yield stress.
I think when we reach at ultimate strength; all over strength is accounted for.
So now draft code of IS 13920 prepared by IIT Kanpur; requires that
minimum ratio of actual UTS to actual YS should be 1.25.
This code also requires that actual yield strength should not exceed
the specified yield strength by 120 MPa; so that shear failure may not
occur before flexural hinge formation.

I would like to ask Mr. Indranil, what were the ultimate strengths of
those samples which you tested.

Regards, Mukesh Upadhyay


On Thu Jul 14 15:24:27 2005, alpa_sheth@vakilmehtasheth.com
<alpa_sheth@vakilmehtasheth.com> wrote:
Quote:
*************************************************
SEFI e-Conference on Steel Reinforcement from June 27 to July 17, 2005
*************************************************
Sponsored by TATA Steel
*************************************************



Dear Mr. Upadhyay:

You have raised a very relevant point.
The ratio of Ultimate to yield strength is only 1.1 to 1.2. And yet the
factor for Plastic Mom Capacity /Shear Capacity  is (1.25/0.87) 1.4.  Why
so?

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. This
needs to be accounted for as that is the plastic moment capacity that can be
generated.

b) Additionally, Steel can take upto 1.25fy due to stain hardening

c) Plastic Moment Capacity that can be generated is important only to
calculate the shear demand at time of formation of plastic hinge. (Note that
plastic hinge moment capacity is not used for flexural design anywhere). To
ensure that the shear capacity exceeds the flexural capacity to prevent  a
brittle shear failure,  IS 13920 requires that the shear capacity is 1.4
times the shear generated by the formation of Mu, the moment of resistance
for which one uses 0.87 fy for calculation.

Thus 1.4 factor in IS 13920 has really not to do much with  the ratio of
ultimate to yield strength of steel but has more to do with the overstrength
in the Fe415 and 500 steels and also to ensure that the shear capacity
exceeds the shear demand sufficiently at plastic hinge formation.


Regards,
Alpa Sheth


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