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Minimum Reinforcement at top of Pilecap.

 
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chandrakant
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PostPosted: Thu Nov 14, 2019 9:15 am    Post subject: Minimum Reinforcement at top of Pilecap. Reply with quote

what will be the relevant clause for calculation of reinforcement at top of pile cap. As according the attached "A& C slip no.4 dated 15/11/16" the temperature and shrinkage clause are removed from IRS CBC.


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es_jayakumar
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PostPosted: Sat Nov 16, 2019 11:10 am    Post subject: Reply with quote

I feel, the top reinforcement in a pile cap is not an essential structural requirement. Many text books have not considered it,though SP-34 has indicated the same. Some minimum steel say, #12 @200 mm c/c may be sufficient. Famous author of RCC books, Dr.H.J.Shah has opined like this : "It is a designers choice, I suppose.Some of my friends provide top steel even in isolated footings. In EQ zone V,for example, to avoid local cracking, we may use top bars (minimum) in isolated footings also".<o></o>
The attached drawing taken from “Manual for Detailing RCC toEC2” does not indicate any top steel.

E S Jayakumar



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spsvasan
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PostPosted: Sun Nov 17, 2019 5:08 am    Post subject: Re: Minimum Reinforcement at top of Pilecap. Reply with quote

chandrakant wrote:
what will be the relevant clause for calculation of reinforcement at top of pile cap. As according the attached "A& C slip no.4 dated 15/11/16" the temperature and shrinkage clause are removed from IRS CBC.



Dear Er.Chandrakant

There are some cases where the top reinforcement is essential in a pilecap. In the case of two-pile pilecap, eccentricity of pile will cause torsion in the pilecap and it is necessary to provide stirrups and top bars

In the case of large span industrial shed in coastal area [with side cladding instead of masonry walls] there can be net uplift. This can necessitate designed top reinforcement.

If the axial force in column is small, but bending moment at the bottom of the column is large, part of the pile cap can have tension at top

The above are unusual cases which we encounter rarely. In other normal cases I agree with Er.Jayakumar's posting

However, I feel that the bottom reinforcement should not be straight, but should be bent up as shown in SP 34

S.P.Srinivasan
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es_jayakumar
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PostPosted: Sun Nov 17, 2019 5:57 am    Post subject: Reply with quote

Srinivasan sir is an infrequent visitor of the Forum. But when appears, he will come up with gems of advises !
I have one doubt. When a column is supported by a large grid say, 5x5 / 4x4 pattern of piles, is it not possible that negative moment (that necessitates top steel) occurs at intermediate pile point in the pile cap raft, theoretically ? If we assume 100% rigid raft, then it may not.

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E S Jayakumar
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spsvasan
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PostPosted: Mon Nov 18, 2019 1:50 am    Post subject: Reply with quote

es_jayakumar wrote:
Srinivasan sir is an infrequent visitor of the Forum. But when appears, he will come up with gems of advises !
I have one doubt. When a column is supported by a large grid say, 5x5 / 4x4 pattern of piles, is it not possible that negative moment (that necessitates top steel) occurs at intermediate pile point in the pile cap raft, theoretically ? If we assume 100% rigid raft, then it may not.

Regards,
E S Jayakumar



Dear Er.Jayakumar

I agree that in the case of pilecaps supported by a large grid of piles negative moments can develop if the pilecap is flexible and will not develop when the pilecap is rigid.

I hope some engineers who have designed basement raft supported by multiple piles, with many piles in between column locations will give their opinion

Regards
S.P.Srinivasan
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