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Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge?

 
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kawshik9
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PostPosted: Sat Dec 28, 2019 10:43 am    Post subject: Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge? Reply with quote

Dear SEFIians,

I have a small doubt regarding  detailing of slabs.

In slabs, for top reinforcement at discontinuous edge, do we need to provide development length Ld, in order to have sufficient anchorage as we provide in Beams at  Discontinuous support?

In Fig 9.5 of SP 34 (Attached) also such detail is not shown and i have not seen else where details providing LD for this reinforcement.

If it is not required, can i know the reason for the same? and if it is to be provided, Kindly give me some reference/ picture so that i can understand how such detail is provided?

Kindly clarify my doubt , Thankyou.

Regards,

Ramakanth



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Jagadeesh K M
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PostPosted: Sun Dec 29, 2019 1:30 pm    Post subject: Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge? Reply with quote

Yes, we need to provide reinforcement bars with Ld at discontinuous edge, as it has shown in SP34 for simply supported slab

Regards,  
Jagadeesh K M
Thanks & Regards,Jagadeesh K MB.E Civil, M.Tech (Industrial Structures)+919448873518
From: "kawshik9"forum@sefindia.org
Sent:Sun, 29 Dec 2019 15:56:59 +0530
To: general@sefindia.org
Subject: [SEFI] Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge?
           Dear SEFIians,
Quote:

I have a small doubt regarding detailing of slabs.

In slabs, for top reinforcement at discontinuous edge, do we need to provide development length Ld, in order to have sufficient anchorage as we provide in Beams at Discontinuous support?

In Fig 9.5 of SP 34 (Attached) also such detail is not shown and i have not seen else where details providing LD for this reinforcement.

If it is not required, can i know the reason for the same? and if it is to be provided, Kindly give me some reference/ picture so that i can understand how such detail is provided?

Kindly clarify my doubt , Thankyou.

Regards,

Ramakanth
     
Quote:



     
Quote:
Download Attachments:
beams.pdf
SP34.pdf




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valsrajvk
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PostPosted: Sun Dec 29, 2019 4:30 pm    Post subject: Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge? Reply with quote

On Sun, Dec 29, 2019, 3:58 PM kawshik9 <forum@sefindia.org (forum@sefindia.org)> wrote:

Quote:
Dear SEFIians,

I have a small doubt regarding  detailing of slabs.

In slabs, for top reinforcement at discontinuous edge, do we need to provide development length Ld, in order to have sufficient anchorage as we provide in Beams at  Discontinuous support?

In Fig 9.5 of SP 34 (Attached) also such detail is not shown and i have not seen else where details providing LD for this reinforcement.

If it is not required, can i know the reason for the same? and if it is to be provided, Kindly give me some reference/ picture so that i can understand how such detail is provided?

Kindly clarify my doubt , Thankyou.

Regards,

Ramakanth







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es_jayakumar
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PostPosted: Sun Dec 29, 2019 4:43 pm    Post subject: Reply with quote

Dear Er.Ramakanth,
Your "small" doubt is quite genuine !
My observation on your query is like this :
At discontinuous edges of slabs, especially those supported on beams, some negative moment is likely to arise due to partial fixity experienced by the slab. In order to take care of this, the Code recommends provision of 50% of the mid span bottom steel for a length of 0.1l from the inner edge of the spandrel beam / support, as top extra bars (in other words, at twice the spacing of the main bottom steel). The negative BM at such edges can be so small, that the theoretical requirement of the steel area may be less than this 50% of main steel. But, in order to restrict the spacing within limits, twice the spacing of the main bar is proposed. Obviously, such bars need not have to develop their full tensile capacity. Hence, full Ld need not be insisted upon for the anchorage length.

E S Jayakumar
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kawshik9
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PostPosted: Mon Dec 30, 2019 6:53 am    Post subject: Reply with quote

Dear ES Jayakumar sir,

Thank you for your reply and clarifying my doubt. I could not find the reason for the same in any text book or literature and I am very much convinced with your explanation. Thank you once again.

Ramakanth
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kawshik9
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PostPosted: Mon Dec 30, 2019 11:30 am    Post subject: Re: Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge? Reply with quote

Jagadeesh sir,

As you have mentioned for simply supported slabs, SP 34 recommends to provide Ld/3 from face of the support. But most of slabs in practice are restrained and for restrained slabs no such detail is shown in SP34.

However, as at edge beam there is only partial fixity, I think the same Ld/3 rule may be followed here also.

Thanks and Regards,

Ramakanth.






Jagadeesh K M wrote:
Yes, we need to provide reinforcement bars with Ld at discontinuous edge, as it has shown in SP34 for simply supported slab

Regards,  
Jagadeesh K M
Thanks & Regards,Jagadeesh K MB.E Civil, M.Tech (Industrial Structures)+919448873518
From: "kawshik9"forum@sefindia.org
Sent:Sun, 29 Dec 2019 15:56:59 +0530
To: general@sefindia.org
Subject: [SEFI] Do we need to provide development length LD for top reinforcement of Slab at discontinuous edge?
           Dear SEFIians,
Quote:

I have a small doubt regarding detailing of slabs.

In slabs, for top reinforcement at discontinuous edge, do we need to provide development length Ld, in order to have sufficient anchorage as we provide in Beams at Discontinuous support?

In Fig 9.5 of SP 34 (Attached) also such detail is not shown and i have not seen else where details providing LD for this reinforcement.

If it is not required, can i know the reason for the same? and if it is to be provided, Kindly give me some reference/ picture so that i can understand how such detail is provided?

Kindly clarify my doubt , Thankyou.

Regards,

Ramakanth
     
Quote:



     
Quote:
Download Attachments:
beams.pdf
SP34.pdf




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es_jayakumar
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PostPosted: Tue Dec 31, 2019 5:07 am    Post subject: Reply with quote

kawshik9 wrote:
Dear ES Jayakumar sir,

Thank you for your reply and clarifying my doubt. I could not find the reason for the same in any text book or literature and I am very much convinced with your explanation. Thank you once again.

Ramakanth

You are welcome, Er.Ramakanth ! I add the following to the discussion :



"This aspect can be viewed in another angle : The edge beams are normally not designed for torsion and the slabs at discontinuous edge are designed for simply supported (hinged) condition. When only minimum steel with reduced anchorage is provided in the discontinuous edge, the restraint offered by the beam to the slab is considerably reduced after cracking, which will enable us to design the slab as simply supported at edge and the beam without torsion.On the other hand, if we provided full Ld,then we are assigning fixity for the slab to the beams, which is against our design assumptions. This is similar to the case of a secondary beam framing into a main beam. The nominal top bars of the secondary beam in the end are just continued till the main beam outer edge (leaving cover), without bending down, in order to release the moments at the support to the possible extent, unless the main beam is designed for torsion."

E S Jayakumar
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N. Prabhakar
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PostPosted: Tue Dec 31, 2019 6:36 am    Post subject: Reply with quote

Dear Er, Ramakanth and Er. Jayakumar,

Where the ultimate support for end span of slab is considered as simply supported, which is in most cases, the bottom reinforcement  is to extend beyond the centre of support by 12 x bar diameter which could be provided with a straight bar/or a U-shaped bar/ or a hook as necessary.  (Ref.: Clause 26.2.3.4  of IS 456:2000).  Where the slab is monolithic with an edge beam or wall, having negative moment at support, the top reinforcement in slab is to be anchored with Ld  from the face of support, as per clause 26.2.1 of the code.

The above requirements are in the British Code also.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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es_jayakumar
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PostPosted: Tue Dec 31, 2019 9:17 am    Post subject: Reply with quote

Sir,
Are not the referred clauses pertaining to the case of continuance of the tension bars at a support beyond the point of inflection towards the span ? Please correct me if I am wrong.

Regards,

E S Jayakumar
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N. Prabhakar
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PostPosted: Wed Jan 01, 2020 2:48 am    Post subject: Reply with quote

Dear Er. Jayakumar,

The bond length of tension bars equal to 12 x diameter of bar I have mentioned earlier, is to be provided beyond the point of contraflexure where bending moment is zero.

In the case of simply supported slabs, the bending moment is zero at the centre of support and the bottom tension bar is to be extended by 12 x diameter of bar beyond the centre of support,  within the support  portion and not towards span.

In the case of slab monolithic with an edge beam and in continuous slab where top reinforcement is provided to resist negative moment near supports, this reinforcement is to be extended  by 12 x diameter of bar beyond the point of contraflexure ( which may occur at 1/4 of span) towards span.

I trust that the above details clarify the point you have raised.

With best wishes for a Happy New Year 2020.

N. Prabhakar
Chartered Structural Engineer
Vasai (E), Pin 401 208
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