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Can the Corner Bars of beam or column be made smaller than the Middle bars(Bars placed between Corner Bars)?

 
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Pramod123
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PostPosted: Thu Aug 18, 2022 12:04 pm    Post subject: Can the Corner Bars of beam or column be made smaller than the Middle bars(Bars placed between Corner Bars)? Reply with quote

Q1.I wanted to know if the size of Middle rebar taken in Beam/Column can be made larger in diameter than the Corner Rebars during design ? For Eg: In beam can we provide 2 Corner bars of 12mm in diameter and 1 Middle rebar of 16mm in diameter ?

Q2.The rebar sizes increases in 12,16,20,25,32 etc. Rather than taking adjacent rebars while choosing rebars can we take other size of rebars while design ? For Eg: In beam can we provide 2 Corner bars of 20mm in diameter and 1 Middle rebar of 12mm in diameter (Rather than taking adjacent 16mm rebar can we jump directly from 20mm rebar to 12mm rebar) ?

Note: Above situation seems to be more economical than other situations, hence i was wondering if it have any adverse impacts on overall structures if implemented.

Info: *Corner rebars = Rebars provided at corners


*Middle Rebars = Rebars provided between any 2 corner rebars
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vikram.jeet
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PostPosted: Fri Aug 19, 2022 4:02 am    Post subject: Reply with quote

IMHO:
1.0  It is very common for top reinf bars wherein Continuing  2 nos bars are of smaller dia and extra bars at support can be of larger dia. Hence 2nos 12 dia + 1 no 16 dia is very common.
2nos 16 dia + 1no 25 dia can also be prevalent to economise , but bar jumping by one step is better. This is in case of top bars.

2.0 Already replied for top bars . However sometimes for bottom bars , bars are required in second layer say 3 nos 25 dia Continuing + extra 2 nos 16 dia or even 12 dia to make up steel area can be used .

3.0  For rcc columns it is always preferable to keep corner bars of larger dia to earn maximum benefit in either direction moments . Inner bars can be of smaller dia.

Hope structure experts can throw more light on effect of bar mixing which is unavoidable.
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Pramod123
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PostPosted: Fri Aug 19, 2022 5:26 am    Post subject: Reply with quote

Thanks for the reply, Vikram sir !
1. So it's okay to provide : 2 no's-12mm Φ Continous bar + 1 no-16mm Φ Extra bar but would it be same if all 3 bars are continious ie 2 no's-12mm Φ Continous bar + 1 no-16mm Φ Continious bar.
2. Based on SP 34:1987 clause 8.1, "For bottom bars, the larger diameter bars must be placed on outside".
a.Does it stand true for both, the continious bars and Extra bars or For continious bars only ie If Extra bar size is larger than continious bar then it's not possible to provide Extra bars on corner so do we have to reduce the extra bar size until it's size is equal or lesser than continious bar size?
b.Is this clause applicable for Top bars too ?



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vikram.jeet
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PostPosted: Fri Aug 19, 2022 6:07 am    Post subject: Reply with quote

1.0 Top Continuing bars can be 3 nos ( 2 nos 12 dia + 1 no 16 dia)  although out.  No problem

2.0  - Bottom bars are always higher dia outer and inner can be smaller dia .

In top bars , tension is at support region and therefore continuing bars are planned smaller and extra bars of higher dia 👌 to make up steel area .This is prevailing practice .

Bottom bars - If 3 nos bars are provided for span + ve BM , say (2 nos 20  dia + 1 no 16 dia ) , All three bars can be continued all thru in span into support or central one no  16 mm dia can be curtailed at about 0.1L from support , I.e. it can be provided of length 0.8L

I think Sp 34 also talk about only  bottom bars to be in line as 2.0 , stated above.
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spsvasan
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PostPosted: Sun Aug 28, 2022 11:45 am    Post subject: Reply with quote

Pramod123 wrote:
Thanks for the reply, Vikram sir !
1. So it's okay to provide : 2 no's-12mm Φ Continous bar + 1 no-16mm Φ Extra bar but would it be same if all 3 bars are continious ie 2 no's-12mm Φ Continous bar + 1 no-16mm Φ Continious bar.
2.
Based on SP 34:1987 clause 8.1, "For bottom bars, the larger diameter bars must be placed on outside".
a.Does it stand true for both, the continious bars and Extra bars or For continious bars only ie If Extra bar size is larger than continious bar then it's not possible to provide Extra bars on corner so do we have to reduce the extra bar size until it's size is equal or lesser than continious bar size?
b.Is this clause applicable for Top bars too ?



Dear SEFIans

I am unable to think of any justification for providing the larger diameter beam bottom bars on the outside at the corners.

Can somebody give a convincing explanation for the above clause from SP:34?
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mjnasar
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PostPosted: Wed Sep 14, 2022 1:20 pm    Post subject: Reply with quote

Hi Eng.Pramod
Greetings

Please go through below my general view.

Explanation for bigger diameter corner bars.

The corners should have bigger diameter for resisting torsion
( see the staadpro beam results -  the Torsional moment is resisted by the corner bars only )


The Top or Bottom bars of beams should place such way that
more than 60% Area of steel should be continuous and
40% Area of steel should be additional.

( i suppose this rule is to comply the general relation of minimum steel area to max steel area in any given sections of beam is 33% )

And As the corner bars are hangers for stirrups  and continues in the whole span , it is to have bigger diameter - continuous bars in the corners and have smaller diameter bars - middle as additional reinforcement is easy way of distribution of  the reinforcement.

Best regards
mjnasar
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vikram.jeet
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PostPosted: Wed Sep 14, 2022 4:36 pm    Post subject: Reply with quote

mjnasar wrote:
Hi Eng.Pramod
Greetings

Please go through below my general view.

Explanation for bigger diameter corner bars.

The corners should have bigger diameter for resisting torsion
( see the staadpro beam results -  the Torsional moment is resisted by the corner bars only )


The Top or Bottom bars of beams should place such way that
more than 60% Area of steel should be continuous and
40% Area of steel should be additional.

( i suppose this rule is to comply the general relation of minimum steel area to max steel area in any given sections of beam is 33% )

And As the corner bars are hangers for stirrups  and continues in the whole span , it is to have bigger diameter - continuous bars in the corners and have smaller diameter bars - middle as additional reinforcement is easy way of distribution of  the reinforcement.

Best regards
mjnasar


Yes as explained by Er MJ Nasar , it is a better detailing practice for bottom bars to be of higher dia on bottom  corners and a smaller dia inside bar/s.

Apart from better torsional resistance  , corner bars also resist lateral bending ,if there .
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