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Abishek_Siingh Bronze Sponsor
Joined: 18 Nov 2010 Posts: 609 Location: New Delhi

Posted: Mon Feb 20, 2017 8:14 am Post subject: Detailing of Column Reinforcement 


Dear All,
I recently had a discussion with some engineers in my circle and they advocated on the fact that no matter what the moment on the RECTANGULAR column is and no matter which direction the moment is, we should provide reinforcement equally on all four sides because SP16 says so?
I disagree on this but nowhere could I find in codes, any clause(s) which distinctly state(s) that the reinforcing steel bars CAN be unequally spaced along column faces depending upon the moment direction.
Can you help me with the answer to this please?
Best,
Abhishek 

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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5356 Location: Gaithersburg, MD, U.S.A.

Posted: Tue Feb 21, 2017 2:12 am Post subject: Re: Detailing of Column Reinforcement 


Dear Er Abhishek,
Yes, we can provide reinforcement equally on all four sides irrespective of the direction of moment. But it will not be economical.
It will be economical to provide maximum reinforcement in the direction of maximum moment.
Regards,
NS
Kumar_Abhishek_Singh wrote:  Dear All,
I recently had a discussion with some engineers in my circle and they advocated on the fact that no matter what the moment on the RECTANGULAR column is and no matter which direction the moment is, we should provide reinforcement equally on all four sides because SP16 says so?
I disagree on this but nowhere could I find in codes, any clause(s) which distinctly state(s) that the reinforcing steel bars CAN be unequally spaced along column faces depending upon the moment direction.
Can you help me with the answer to this please?
Best,
Abhishek 


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kapildingare ...
Joined: 15 May 2009 Posts: 104

Posted: Tue Feb 21, 2017 6:17 am Post subject: 


Yes Kumarji you can definitely provide column Reinforcement as per it's value and direction, which may lead to it's uneual distribution on column's faces.
1) I don't think you need any codal reference to do the same.
2) As Respected NS Sir has adviced equal distribution may lead to uneconomical solution.
3) Also direction of moments can always change with respect to considered load case direction, it can't be fixed, except if you are considering load case of DL and LL alone.
4) And most important site execution convenience, from this point also it is advisable to have column reinforcements equal on either two or four sides.
Last edited by kapildingare on Tue Feb 21, 2017 8:26 am; edited 1 time in total 

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sakumar79 ...
Joined: 18 Apr 2008 Posts: 682

Posted: Tue Feb 21, 2017 7:18 am Post subject: 


Dear Er Kumar Abhishek Singh,
Not sure if I have misunderstood you, but does not the code have charts 27 to 38 for reinforcement distributed about 2 sides only? Hence, if moment is basically about one direction only, you should be able to provide the same on 2 sides only and provide nominal reinforcement on the other side to satisfy detailing requirements such as maximum distance between reinforcement.
If you have biaxial bending and want to do unequal spacing of bars due to one bending being more predominant that the other, you can do the same and work out the PM interaction diagram from basics and design reinforcement. In fact, you can do the same in ETABS using custom Section Design.
In case I have misunderstood you, I request you to put up a sketch explaining your query.
Hope this helps,
Arun 

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zubairmeer1 General Sponsor
Joined: 27 Feb 2010 Posts: 88 Location: Hyderabad

Posted: Tue Feb 21, 2017 7:23 am Post subject: 


Dear Abhishek Sir,
Having worked with BS codes few years ago, i don't remember it had any charts for equal distribution of steel on 4sides of a column, even SP16 has charts defined for equal distribution of steel on 4 sides and equal distribution distribution on two sides. You may ask your colleagues the reason for provision of design aids in SP16 with reinforcement in columns distributed equally on two sides.
Thanks & Regards
Meer 

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knsheth123 Silver Sponsor
Joined: 26 Jan 2003 Posts: 123

Posted: Tue Feb 21, 2017 2:31 pm Post subject: 


Dear Er. Abhishek,
Rect. Column Design is carried out using Interaction Diagrams given in SP 16. Charts directly give Reinf. percentage for Axial Load + Uniaxial Bending case. It is necessary to iterate assuming percentage of steel and satisfy inequality (IS 456) for Biaxial Bending.
The charts are given for :
case 1 : Equal Reinf. on two opp. faces only
case 2 : Equal Reinf. on all four sides
Case 1 can be used for case of Uniaxial bending only as calculation of capacity in the perpendicular direction can not be obtained from such charts.
Thus for biaxial bending case 2 is the only choice.
It is evident that placing large fraction of reinf. on faces resisting larger bending moment will yield optimal design. This is not possible using SP16 design aids.
However such limitations of SP16 are overcome with formulation from first principle and devloping design aids by way of worksheets or programs. The tools by Shri N. Prabhakar are amongst the leading one. We have also attempted to develop such tools and are in use from several years by Engineers in regular design or checking.
When design internal forces 9Pu, Mux, Muz) due to all load combinations gives inequality ratio <=1, it is acceptable design. Out of all acceptable arrangement, the one with minimum percentage of steel is optimum one.
The links are
http://www.sefindia.org/forum/viewtopic.php?p=7492&highlight=#7492
http://www.sefindia.org/forum/viewtopic.php?p=70713<o:p></o:p>
(First_Col_Dsn_22102015.xls) :
With warm regards
Prof. K. N. Sheth
Dharmsinh Desai University, Nadiad. 

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JVCSNL ...
Joined: 26 Jan 2003 Posts: 159

Posted: Wed Feb 22, 2017 4:07 am Post subject: 


Dear All,
The code gives basic principles of the RC design ( i.e., strain block and stress block parameters) and subsequent to that it is up to engineer to use those principles to design the concrete elements.
SP 16 provided a set of charts to address the columns that are most likely in most situations. On 4 side equal reinforcement, it assumes that total number of bars are 20 or so.. But it also does not restrict an engineer to have interaction diagram based actual position of reinforcement and diameter of rebar at that location.
For economical design, the engineer will certainly size the column for technical reasons and during reinforcement design, engineer will use engineering judgment to place the reinforcement as per design forces.
In my understanding, there can be different reinforcement on long and short faces (both in numbers and diameter) and will be based on design considerations only.
What would you do if you have columns like triangular, pentagon or hexagon shape? The engineer has to develop separate PuMu curves based on the geometry of section and principles of RC design given in code.
Regards,
Jignesh V Chokshi 

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