Enlarging Column Necks To Overcome Punching Of Mat

[mhdhamood]

Dears;

I had a problem in a MAT foundation which is; in a part of the Mat there are two high loaded columns , that create a problem of Punching.

But I cant go in depth of the Mat, so I suggest to enlarge the column neck( The column between the slab on grade and the Mat) which is 0.5 meter in height. That solved the problem.The following questions now I have :

a. Do u agree with me????

b. I didn't design this part of column instead I reinforced it as the steel ratio of the above column. The reason that I didn't design it , is that I enlarge it around the center of the above column so no eccentricity . Do u agree??

c. And how about if it was enlarged eccentrically, Shall I add the Moment from eccentricity to the moment from analysis of the bottom of the above column and design this neck??

[WR1]

a. yes I agree with you

 

b. It might be SAFE, it might not. depends on the moment diagram which depends on the base fixity (pin or fixed). You must design it properly. If this is an interior column then it should be okay (only P and no M).

 

c. If it was enlarged eccentrically, yes you have to account for the moment M = P.e 

[mhdhamood]

Very Nice Engineer ....thank you so so so so much.
You are right that depends on the fixity of the column I will try what is the new results and share..

[Muneeb Badar]

Just one thing I want to add that if this is highrise building then you need to check the axial forces in the adjacent beams if you increase it eccentrically and if there are some grade beams. It will not effect in all cases but better to check. ;

 

Secondly if you increase it eccentrically then  take care how you will continue the reinforcement from this column to above column as everything depends upon the reinforcement. and in this case you need to model slab on grade and check the inplane stresses if there is soil outside because your total burried area will act as one unit

 

One last important thing, you said the column creating problem in punching and when you increase size it is solved. you can not suddenly increase the size in that small length too much. You need some curtailment for column size. For example if the column above slab on grade is 300mm in diameter then you can not provide 600 mm dia below slab on grade. So please take care of this.

 

These are just my opinions anybody can comment of not agree......

 

Thanks

 

Muneeb

[WR1]

Yes Muneeb is right about sudden geometry irregularity in column size. Please make sure you take this effect.

[EngrUzair]

mhdhamood,

 

IMO, by increasing column dimension in-between the slab-on-grade and the mat foundation, you are actually increasing the mat thickness, which is otherwise needed to account for larger punching. 

 

I would (in case mat foundation is yet to be poured), treat the 'enlarged column neck' and the mat portion exactly beneath it, as a single mat (of thickness equal to sum of normal mat thickness and 0.5 m i.e thickness of 'enlarged column neck') and will provide reinforcement as per requirements of larger mat thickness. This concept is similar to what has been discussed in following thread: (http://www.sepakistan.com/topic/1689-drop-in-mat-foundation/ )

 

BTW... (1) How are you analyzing & designing the mat foundation? Using some software (CSI SAFE etc.) or manually?

           (2) Has the mat foundation already been poured, or is yet to be poured?

[mhdhamood]

Thanks a lot to all;
Exactly I enlarged the section from 1.5*0.4 to 1.5*1.5 meter at the neck.
According to the development length of the steel in the column neck to be achieved I will make a hook entering into the slab on grade so I think no problem here 

Muneeb Badar

You said I cant increse it too much please can you interpret why ??


 

EngrUzair
1. Using SAFE.
2. Not executed .
Is your solution valid for my case .?

[Muneeb Badar]

Dear

 

The reason is when we suddenly increase the size of column for example from 600 to 300, the forces accumulated at these locations. Actually at each joint the lateral forces normally distributed to adjacent members and also to the below columns. If you suddenly reduce the column size there are much chances of improper stress/forces distribution. So a gradual change is required.

 

Secondly in your case you are suddenly increasing the size at Slab on Grade level. This situation is much more complex. Your structure in soil (structure from slab on grade to foundation level) act as one unit. It means there will be much inplane shear force at slab on grade level. Your complete structure will vibrate starting from slab on grade not from foundation as you provided much thicker columns below slab on grade and made this portion rigid. If you increase the size and do not model slab on grade, I think it will not be good. You can model slab on grade and check the inplane forces distribution at slab on grade level. In this case you can be sure that increasing the column size is affecting or not.

 

This is my concept, anybody can disagree........

[mhdhamood]

Dear ;
Exactly I checked on Etabs the increased 2 columns.The results where that:
At these 2 columns they attract more moment than befor Just only the difference there and I designed them.
 

[EngrUzair]

mhdhamood,

 

Since you have increased the column thickness in one direction only, my proposed solution may or may not work in the present case. You will have to verify it, by treating the 'enlarged column neck' as the part of mat footing.

 

In order to deal with punching shear, you normally need to increase shear perimeter 'bo'. This may be done either by increasing thickness of footing (more common and simpler), or by increasing the column size.

 

I was talking about the first option (increasing mat thickness locally), whereas you are tackling the problem using second option (increasing column size).

 

If you are interested in using locally increased footing thickness (instead of increased column size concept),  you may do it  easily in SAFE along with proportioning of required reinforcement.

 

Regards.

[mhdhamood]

Thanks a lot dear friends It was very useful notes .
When we share our opinions Together we get to the highest point. we don't hide our experience at the contrary we will increase it.