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Why this beam rebar is cranked like this?

 
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Rifat Bin Ahmed
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PostPosted: Thu Apr 09, 2020 4:17 pm    Post subject: Why this beam rebar is cranked like this? Reply with quote

Dear Sefians,

Anyone can explained me why beam rebar bend like this?




Thanks in advanced
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rajancivildesign@gmail.co
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PostPosted: Thu Apr 09, 2020 4:49 pm    Post subject: Beam rebar cranked Reply with quote

In beam design the shear  has to be resisted  by concrete and steel. After taking shear by concrete balance shear has to resisted by stirrups. To take some portion of shear say 50%  of balance shear resisted by rebar, the rebars are cranked on both ends to take some amount of shear. The balance portion has to be designed to take up by shear reinforcement in the form of stirrups. Otherwise entire amount of balance shear has to be taken by stirrups after taking concrete portion. Now adays the bars are not cranked only straight bars are provided.
V. M. Rajan.
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Rifat Bin Ahmed
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PostPosted: Thu Apr 09, 2020 5:07 pm    Post subject: Reply with quote

Rajan would you please explain me the advantages of this detailing?
Any code guide us about this detailing?
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thirumalaichettiar
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PostPosted: Fri Apr 10, 2020 1:06 am    Post subject: Reply with quote

Dear Er. Rifat Bin Ahmed
The cranked bars are in X shape and is for the formation of hinges during EQ forces. The beams might have been  designed for EQ.
I remember this is suggested in some book  but I will find out. This shape is good for reversal of forces.

I have designed and detailed like this in one of the projects but placing  of this type is somewhat cumbersome.

You might have seen the bending of bars at column bottom like this cranked bars to form hinges so that the BM  is zero and the structure is on hinged support as per design.

Vide Table 181 -hinges and bearing of Reinforced Concrete Deswigner's HB by Reynolds. Attached the copy of the page.

Thanks,

T.RangaRajan.



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es_jayakumar
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PostPosted: Fri Apr 10, 2020 2:25 am    Post subject: Reply with quote

What I think further is that this secondary beam supported on girder may have been designed for end released condition, so that torsion is not transferred to the girder. To get this effect in reality, the detailing for a hinge may be done. I have the following doubts in this connection :
1. At this location, the longitudinal reinforcement is available only in the middle of the section. Hence, won't the section here crack excessively ?
2. This is the location of maximum shear and there is no tensile reinforcement here. Won't this reduce the shear capacity of concrete ? Is the shear to be borne predominantly by closely spaced stirrups  ?
3. If the arrangement is alright, then how is the hinge (& stirrup spacing) designed theoretically ?

E S Jayakumar
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thirumalaichettiar
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PostPosted: Fri Apr 10, 2020 3:32 am    Post subject: Reply with quote

In addition to my previous posting please find the attached sheet which is Page 158/Manual of Seismic Design by James L .Stratta.


Hope this sketch clarifies the doubts of Er.es.Jeyakumar. There is 50% bars goes straight to columns as per the details of the sketch.

T.RangaRajan.



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Rifat Bin Ahmed
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PostPosted: Fri Apr 10, 2020 9:02 am    Post subject: Reply with quote

I am speculating here, but it is likely that they had a problem with torsion in the spandrel beam and instead of dealing with it in design, they are trying keep the beam from inducing torsion in the girder.

The concerns with this detail are that there will be cracking in the slab near the beam-girder interface, unless there is a bond breaker between the top of the beam and the underside of the slab in the vicinity of the hinge.

If the construction is in earthquake country, bottom cracking is also an issue. I have seen column hinges like this, but I have never seen a beam hinge in cast-in-place construction. In a column, the compression keeps cracks from developing.
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es_jayakumar
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PostPosted: Fri Apr 10, 2020 9:31 am    Post subject: Reply with quote

Famous Author, Dr.H.J.Shah also opines that this is a bizarre detailing !

E S Jayakumar
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