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[nikhilbidri]

Dear Sefians,
I have a query regarding the check for one way shear in footings where the critical sections overlap with eachother due to closely spaced columns.
Assume a case that if the combined footing depth is 1200mm and the clear distance between the column faces is 2000mm, the critical section for one way shear will overlap with each other for both the columns, in such cases is it ok to ignore the one way shear check?

My understanding says that even if critical sections overlap we have to check for the one way shear at critical sections for both the columns. But some of the structural engineers are ignoring one way shear if the critical sections overlap because of the absence of any guideline regarding the same.

Is there any technical reference for such cases?

Please provide your valuable guidance.

Thanks in advance.

[vikram.jeet]

By thinking logically :-

For Two way Shear to be checked at d/2 - if Columns are closer than d,  then two way shear will overlap on this direction,, and There is need to check assuming Two columns and a perimeter al-around  both at d/2 ,,

For one way shear : If columns are closer than 2d  - shear critical  sec overlaps in closely spaced columns,, I think better to check one  way shear at mid section between the two columns ,  but if columns are closer than d,, the one way shear check between columns not required,,

My personal opinion,, could always be debated for my further distant learning.

[nikhilbidri]

Thank you for your reply.
I agree and am aligned with your logic.

For punching shear it is clear in text books as well that when critical sections for 2 way shear overlap we need to check for the combined effect.

But for one way shear it is not clearly explained for conditions where the critical sections overlap. Because of this some engineers totally ignore checking one way shear in such conditions.

Hence is it ok to ignore the one way shear even if the one way shear critical sections are overlapping just for a small distance.

Is there any technical reference in which this condition is explained.?

Can any Senior Engineer's in this forum guide on this please.

[JVCSNL]

It is imperative to perform the one-way shear strength check of the RC section.  

When the beam shear section of one column overlap with the region of other nearby columns, the shear failure will not happen at distance d, but can happen at the midpoint between two columns and shear check can be done at midpoint.

Since, the shear section now chosen is less than distance 'd' from face of column, the shear strength calculated from Table 19 shall be increased. You may refer section 40.5 IS 456 or other overseas codes for additional guidance.

Shear check cannot be ignored as you and other experts have rightly explained.  

I hope this helps.

Jignesh V Chokshi

[nikhilbidri]

Thank you for your inputs Mr. Jignesh.

My only concern is that some Structural engineers are taking disadvantage due to lack of proper explanation for such cases in codes as well as in books and are completely ignoring the one way shear check even if the one way shear sections overlap just for a small distance.

Hope so that some seniors take this into cognizance and publish some technical explanation/guideline over this issue.

[vikram.jeet]

Shear check - is a must

One way shear needs to be checked at mid section of two columns spacing distance,, in situation under discussion  

To just summarize : (Statement already given in my previous post and also seconded by Er Jignesh saheb) .

In case clear Distance between columns is more than 2* d One way Shear check at d - as usual and  clearly spelt out in Codes

In case Clear distance between columns is Less than  2*d but greater than d  - One way Shear check at mid section of column spacing

In case clear distance between columns is less than d  - No one way shear check

Rate of change of bending moment is shear force (one way) . In case BM is constant (uniform) in certain reach of span ,, The SF is NIL in that zone of span.

[rahul.leslie]

I presume, enhanced shear capacity due to shear span 'av' being less than 2d too can be utilized...

...btw, can you please attach a sketch of a section of the footing along a line connecting the two columns, with the SF diagram marked?

[vikram.jeet]

Enhanced shear capacity can be taken in above case as per IS 456 as enlightened by Er Rahul saheb.

[spsvasan]