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Clause 6.3.4 of IS 13920

 
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ajaynkadam
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PostPosted: Thu Nov 23, 2017 11:40 am    Post subject: Clause 6.3.4 of IS 13920 Reply with quote

Clause 6.3.4 of IS 13920 says - In the calculations of design shear force capacity of RC beams, contributions of the following shall not be considered: a) bent up bars, b) Inclined links, and c) Concrete in the RC section.


So shear force capacity of vertical stirrups can be considered.

Can somebody say something about shear force capacity of main reinforcement.


Regards,

Ajay Kadam
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ashishphadke
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PostPosted: Fri Nov 24, 2017 2:34 pm    Post subject: Reply with quote

Hello Kadam sir,

But IS 456:2000, 40.4 allows,

a. vertical stirrups,
b. Bent-up bars along with stirrups, and
c. Inclined stirrups.

But 13920:2016, does not allow

a. Bent up bars,
b. Inclined links and
c. Concrete in RC section

So out of these 2 codes, what remains with us is

1. Vertical stirrups
&
2. Bent up bars along with stirrups (both working together)

As you said, mostly vertical stirrups only is ideal to use, as per me. (Group, Please correct me if wrong).

As far as main reinforcement is concerned in a flexural member, the direction of the main steel is perpendicular to the shear developed. Whereas to resist shear, the steel is needed in the same direction as that of the shear. (Eg : vertical stirrups). So the main reinforcement will not help in resisting shear at all. So the main reinforcement does not have any shear capacity. It is only to resist the BM. (Please correct me if wrong.)
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ajaynkadam
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PostPosted: Sat Nov 25, 2017 1:26 am    Post subject: Reply with quote

Ashish ji,

As you have mentioned, IS 13920 does not allow bent up bars. So we are left with vertical stirrups only. What if we have significant amount of main steel say 20 bars of 32mm dia. Does that have no shear force carrying capacity. IS 800 says that steel has got shear force carrying capacity.

Can we have an expert's opinion on this.

Regards,

Ajay Kadam
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tosifsayyed
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PostPosted: Thu Nov 30, 2017 11:18 am    Post subject: Reply with quote

Dear Sir,
I haven't seen IS 13920-2016 yet. But as per IS 13920-1993 clause 6.3.4, it says, "The contribution of bent up bars and inclined hoops to shear resistance of the section shall not be considered." As per my knowledge it doesn't say not to consider shear capacity of RC section.( may be I have misunderstood it, please correct if I am wrong.)
If so, while calculating TauC, we are considering %pt which deals with main reinforcement. So we are already considering main steel while calculating shear reinforcement.
And while going through draft of IS13920-code and commentry "http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ11.pdf", it says, "C6.3.4 –Due to cyclic nature of seismic loads, shear force can change direction. The inclined hoops and bent up bars, effective in one direction for resisting shear force, will not be effective for opposite direction of shear force".
(requesting correct me, may be I have mistaken the code)
Ashish Sir, are you from MME Pune?
Thanking everyone in advance.

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ajaynkadam
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PostPosted: Fri Dec 01, 2017 2:05 am    Post subject: Reply with quote

IS 13920 2016 clause 6.3.4


Strength of concrete is not to be considered which is calculated based on percentage of steel. The point is justified when a plastic hinge is formed. The question is can we still consider the shear strength of the main reinforcement independently.



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tosifsayyed
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PostPosted: Fri Dec 01, 2017 5:37 am    Post subject: Reply with quote

Thank you Kadam Sir for the information.
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abhishek_ecs
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PostPosted: Wed Dec 06, 2017 10:43 am    Post subject: Re: Clause 6.3.4 of IS 13920 Reply with quote

ajaynkadam wrote:
Clause 6.3.4 of IS 13920 says - In the calculations of design shear force capacity of RC beams, contributions of the following shall not be considered: a) bent up bars, b) Inclined links, and c) Concrete in the RC section.


So shear force capacity of vertical stirrups can be considered.

Can somebody say something about shear force capacity of main reinforcement.


Regards,

Ajay Kadam

Sir,
If we assume that main reinforcement has deformation reaching the yield point in case of an earthquake,its shear capacity will be negligible.Also even if vertical stirrups reach yield point strain, they can provide enough ductility to structure since they are in the direction of shear.In other words in extreme case vertical stirrups may provide enough warning against shear failure to the occupants before the structure actually fails.
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ajaynkadam
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PostPosted: Thu Dec 07, 2017 12:42 am    Post subject: Reply with quote

Dear Sir,

While designing steel Structures as per IS 800 using limit state method, shear strength of steel is not ignored at yielded state in bending stress. In fact there are formulas given to calculate shear strength in limit state method even when plastic hinges are formed.

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Ajay Kadam
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abhishek_ecs
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PostPosted: Thu Dec 07, 2017 7:30 am    Post subject: Reply with quote

Sir,
Yes plastic shear depends on yield strength of steel web but i doubt that will hold true in case of RCC structures during an earthquake .When steel reaches plastic stage strains along with earthquake acceleration,the contribution of concrete or reinforcement to shear in such a condition would not be significant. And in such a condition ,it makes sense to only consider vertical stirrups,as they are directly resisting the shear force. Also I believe that in normal conditions , shear is resisted by
1.uncracked concrete,
2.aggregate interlocking and
3.main reinforcment by dowel action.
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ajaynkadam
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PostPosted: Fri Dec 08, 2017 2:44 am    Post subject: Reply with quote

Sir,


The question is, what if we have significant amount of main reinforcement, say 20 bars of 32mm diameter. There should be a provision for considering shear strength of main steel independently. We are expecting an expert opinion. We all are aware of the basic stuff.

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