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Modulus of Rupture

 
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jc
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Joined: 07 May 2010
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PostPosted: Mon Dec 23, 2013 11:58 pm    Post subject: Modulus of Rupture Reply with quote

Dear Members,

In our RCC design, we normally ignore the bending capacity of concrete itself. My question is why don't we make use of it so that rebar in the member will not be too congested?

If a member is subjected to a bending moment which is less than the cracking moment, then why don't we just provide the minimum steel, that's it.
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es_jayakumar
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PostPosted: Tue Dec 24, 2013 2:31 am    Post subject: Reply with quote

Dear Sri. JC,

For simple & small structures like slabs of short span, slab of a grid floor, wall of a small & shallow water tank etc.,  this can be adopted. See the attached example.

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E S Jayakumar



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jc
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PostPosted: Tue Dec 24, 2013 3:13 am    Post subject: Reply with quote

ya. this is exactly what i mean. however, you are saying that for small and simple structure. my next question is since small structure works then why not we use it in big and complicated structure like transfer beam or transfer slab and even retaining structure in order to reduce the congestion of rebar.

with all those heavily reinforced members, it might not good it will be very congested and hence concreting works i s harder. in additional, we don waste the material.
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es_jayakumar
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PostPosted: Tue Dec 24, 2013 6:12 am    Post subject: Reply with quote

For big structures with large bending moments, you may have to use very large sections for the bending element, if go by this theory. Further, I don't know, how to estimate deflection in this case.

E S Jayakumar
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jc
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PostPosted: Tue Dec 24, 2013 2:40 pm    Post subject: Reply with quote

Not necessary to use a very large section to cater a large bending moment. For instance, if my section is 1000x2200, the cracking moment is about +-3600KN.m for G45 concrete. If this section has to withstand a bending moment of 8000KN.m, in this case i will have to provide a lot of reinforcement for that.

Since the concrete itself can take 3600KN.m, then why can't i make use of it and design the rebar to take only 4400KN.m?

If the attached calculation sheet works then in the above example also should works.
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es_jayakumar
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PostPosted: Wed Dec 25, 2013 1:11 am    Post subject: Reply with quote

If you use one of the approaches, the concrete in compression will be already stressed. It may not sustain the additional stress to resist the balance moment, if the other approach is invoked.
Further, the reduction in tensile steel for a beam will reduce the shear capacity of it.

E S Jayakumar
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umeshrao
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PostPosted: Wed Dec 25, 2013 7:41 am    Post subject: Re: Modulus of Rupture Reply with quote

jc wrote:
Dear Members,

In our RCC design, we normally ignore the bending capacity of concrete itself. My question is why don't we make use of it so that rebar in the member will not be too congested?

If a member is subjected to a bending moment which is less than the cracking moment, then why don't we just provide the minimum steel, that's it.

IS 456 takes an approach that any failure shall not be a brittle failure. Considering Es/Ec = 8, stress in steel will be only 8 times flexural tensile strength of concrete, that is only 28 N/Sqmm, for M25 grade concrete.
Only after concrete is cracked steel will yield and hence permissible stress in steel is 0.55*fy.
There fore even in IS 3370 considering uncracked condition stress in steel is restricted to 150N/Sqmm.
Umesh Rao
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es_jayakumar
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PostPosted: Thu Jan 09, 2014 1:18 am    Post subject: Reply with quote

A bit of useful discussion on this topic is available in the book : "Reinforced Concrete Vol-I" by Dr. H J Shah, 2012 edition, page No.198.

E S Jayakumar
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