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Spacing of Main Reinforcement in concrete column 1

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manzoorquadri

manzoorquadri

Structural
Jul 3, 2009
3
Recently I designed a G+4 Building in Etabs, After Designing and getting all the results cross checked I had detailed the drawings accordingly.
I had a 9"X12" columns at the upper floors i.e. From first to Roof floor. However as per my load coalculations, analysis and Design results I needed 697 Sqm Steel area, I provided 4 dia 16 mm rebar i.e. 800 Sqm Steel Area and released the drawings.
Now another Engineer at site is in a technical Argument with me that, we cant provide 4 dia 16 bar in those columns and we need to Provide 2-12 dia Extra Rebars in them i.e 4-16+2-12.
He is giving the IS456 code Ref. Clause# 26.5.3.1, point # g) "Spacing of longitudinal bars measured along periphery of the column shall not exceed 300 mm".
However in my case it is not exceeding but he is telling since its at the boundary we shall opt for 2 dia 12 extra bar as "Code" Doesnt allow.
My Question is, Why shall add Extra bars as my design and loads are cent percent correct and I met the minimum Reinforcement criteria.
Coould someone help me in this regard.
 
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I'm more comfortable with british codes, but I'd hazard that the requirements are for crack control, most spacing requiremts are for that
 
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From my background (mostly ACI with a little IS/AS), I don't see anything wrong with the design you've proposed. Like you say, you meet the IS requirement for maximum spacing.

----
just call me Lo.
 
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I'm always curious about crack control for compression elements, myself... doesn't seem to make any sense.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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If the column is 9"x12", the maximum spacing of bars is 12" - 0.375" stirrup - 2(1.5") cover = 8.625". How is that "close to the boundary"?

Tell the site engineer the design stays as it is.

BA
 
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It's more about confinement of the core I believe. Closer spacing means better confinement along the sides of the member.

Most codes have a minimum number of bars in a column criteria. Unsure of Indian code so look into that as well. For comparison NZ code requires 8 minimum longitudinal bars, can relax to 4 or 6 if you meet certain criteria such as a low level of axial load or bars being spaced at less than 150mm after dropping the number of bars. In ACI I believe its simply 4 bars for rectangular ties, or 6 for circular.

So if Indian code actually says 4 is minimum and you meet the spacing, then I don't see any issue with what you've designed. However, it would be considered better detailing practice to have intermediate side bars on any column and links or ties around those to enhance the confinement afforded by the transverse reinforcement, especially if subject to seismic forces. Every code is different of course, the NZ code being far more onerous for transverse reinforcement and longitudinal reinforcement. So I look upon 4 column bar scenarios with some level of distrust, different folks, different strokes.

 
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I’d ask which code clause the person is referring to.
 
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Totally agree with all of the above comments. How can he say that the spacing is even close to 300 mm if the actual size itself is about the same.[bigsmile]
Agent666, your explanation is really good. Indian code require just 4 bars in rectangular/square columns as do ACI. Would like to read the NZ code though. Is it free to download from somewhere or has to be purchased?
 
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The following is a column section detail used on 24" x 24" column for 60' span PT multi-level parking structures in my area by one local consultant.

All longitudinal column rebar on two faces - for bending about X-X axis. Code = ACI-318

COLUMNX_hytz5w.png
 
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Ingenuity said:
All longitudinal column rebar on two faces - for bending about X-X axis. Code = ACI-318
Click to expand...
Amazing. The building officials may have accepted it as ACI 318 does not have any requirement for maximum spacing of longitudinal bars in columns.
However, I personally feel safe to limit maximum spacing to 200mm-250mm whenever I design or Review something(even to ACI code).
 
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ACI does require bars on the sides for the earthquake resistance clauses, but it's phrased more around the need to provide an additional column bar to limit the spacing of ties for confinement across the section to a max spacing of 350mm. Refer to CL18.7.5.2(e):-
image_dzifvz.png

image_qyfoi8.png


Surprisingly there is no similar provision for structures "not subject to earthquakes" if there is even such a structure. Which is pretty scary in terms of the lack of confinement the detail Ingenuity posted actually affords along the vertical sides (not to mention the 90 degree hooks ACI uses in the cover concrete, but don't get me started on those!).

Axial loads require confinement under gravity and seismic conditions, it has nothing to do with the moment only being about one axis. Like I said, very surprised to note this to be honest in ACI, thought the above applied to all structures, coming from somewhere with much more onerous confinement criteria it seems like an omission, but obviously shows some intent by the code authors that this good detailing only applies to structures subject to earthquakes.

 
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On looking a little further CL25.7.2.3(b) contains some further limitations on spacing of cross ties.

I find this quite odd, as it's saying you don't need to put in intermediate bars on the sides for example at all, but if you do you need to ensure they are confined in a fairly robust way (for confinement and anti-buckling). But they are ok with a poorly/partially confined core of concrete otherwise which seems very odd to me as an approach.

I'm curious for those designing regularly to ACI, whats stopping you putting 4 bars in any column at any size in the corners as long as it provides the strength in moment/shear....??? Seems like something is wrong if this is the intent?

These requirements are more like what is in the NZ standard, which I'm used to.

image_egefr5.png




 
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Agent666, good study but it still suggest that if you use a tie then you can still just use longitudinal bars around the corners right?
 
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@Tomfh , he is Referring to Indian code IS-456 Ref. Clause# 26.5.3.1, point # g) "Spacing of longitudinal bars measured along periphery of the column shall not exceed 300 mm"
 
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Many Thanks !! Everyone for your precious time and, in depth Explaination by @agent666. So, in simple terms it is not necessary to provide 2-dia-12 Extra bars for loading criteria, rather it would be good practice to provide the same for Cracking and Confinement criteria.
 
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Agent said:
whats stopping you putting 4 bars in any column at any size in the corners as long as it provides the strength in moment/shear....???
Click to expand...
(As I practice in a decidedly not seismic region)... Mostly the limitation that bigger bars are harder to bend manually and develop/lap.

To a lesser degree, the idea that with more small bars, I can better optimize the strong and weak axis bending strength of my columns.

But lots of my columns do end up with 4 bars, or 6. If I can avoid intermediate ties, I do.

----
just call me Lo.
 
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I'm of the opinion that
image_z10rfb.png


should not be used because you want the rebar cage to be fairly 'rigid'... not provided by dual 'cages'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Agent666 said:
Axial loads require confinement under gravity and seismic conditions, it has nothing to do with the moment only being about one axis.
Click to expand...

True, but when the said consultant was questioned why they did such a longitudinal rebar placement they stated "...90% of the total moment was about the X-X axis".
 
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