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Shear Strength of Concrete

 
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Dr. N. Subramanian
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PostPosted: Fri Jul 01, 2022 3:09 pm    Post subject: Shear Strength of Concrete Reply with quote

Dear All,

In another Structural Engineering Forum in Telegram, one engineer made a mistake of estimating the loads in the foundation and provided a foundation, which is now not safe in shear as per IS 456. He raised a question, why should not we take the cover concrete in shear strength and also asked a absurd question of whether we can take the PCC thickness to make the foundation safe in shear! one of my replies is given below, which may be of interest to others in this forum also

My reply:

The formula given in IS 456 for shear strength is based on the experimental research done by Prof. Vijay Rangan in the 1960s. In fact most of the codes adopt formulae based on experiments only. Shear failure, unlike bending failure is sudden and will not show any warning. Hence all the codes adopt a conservative approach. Shear strength of concrete is the most researched area in RCC. More than 10,000 papers have been published all over the world. You may read more about it in my Design of  RC Structures book in the Chapter on Shear.Shear failure is associated with bending. It will be near the support or near any heavy concentrated load. Hence there will be cracks below the neutral axis. In this regard, I like the formula developed by Prof. Robert J. Frosch et al.( Prof. Frosch is the current chief editor of ACI Structural Journal) and published in the Sept. 2017 issue of the ACI Concrete International (A Unified Approach to Shear design) in which they developed the following formula, which is applicable to both RC and prestressed concrete beams. It is given asShear strength of beam= (5/12) Lambda sq.root (fc) bw c (MPa)Where lambda is modification factor for light weight concrete=0.85 as per ACI, fc =cylinder compressive strength, bw = web width of beam, and the last term, for which I like the formula, is c which is the cracked section neutral axis depth. We normally find out c, during the flexural design of concrete. Note that the concrete section below the neutral axis is cracked and this formula considers only the un-cracked section for the shear strength calculation.  If such a formula is adopted in the codes, we will not have any doubt about the cracking behaviour and Shear strength of concrete.RegardsSubramanian
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vikram.jeet
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PostPosted: Fri Jul 01, 2022 3:52 pm    Post subject: Reply with quote

Thanks Dr NS Sir for enlightment  regarding shear strength of cracked section.

In this case :

If designer has not taken following in calculations   , may provide some relief to designer in shear check with revised loads

(i) Shear check at deff from face.

(ii) In case of tapered footings   ,  SF can be reduced by (M/d) Tan beta

(iii) Increase of concrete strength with age (past 28 days) , till time full loading come on columns

(iv) Live Load reduction depending on nos of stories as per IS 456

(V) Any compatibility moment effect on footing can be released  by redistribution. However Equilibrium moments must be taken .

Above view is assuming that design is done on manual basis , though analysis is thru softwares.
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vikram.jeet
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PostPosted: Sun Jul 03, 2022 7:16 am    Post subject: Reply with quote

In continuation to previous post , First and foremost in shear check is TWO WAY SHEAR ( PUNCHING SHESR ); Unless punching shear is ok , the one way shear and bending may not exist . Once punching stress is satisfied , bending action prevail.

For Punching shear , the designer can take advantage of age factor  on concrete strengthh.
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vikram.jeet
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PostPosted: Sun Jul 03, 2022 7:52 am    Post subject: Reply with quote

Also apart from Structural design fulfillment, the GEOTECH issue regarding Net SBC within limits is further more important. If column revised loads are increased after casting of footings , in ca such situation advantage of levelling concrete as load transferring medium can be taken in consultation with proof consultant/approving authority.
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