Punching shear and one-way shear for closely spaced columns

[BAGW]

Hi,

I have a scenario at the expansion joint of building where 4 columns meet. These columns are supported on spread footing (I would rather call it a mat foundation). The foundation is 2m deep and the spacing between the columns 750mm. So, the punching perimeter for individual columns over lap from the other columns. Can I assume the perimeter for punching shear and the one-way shear as shown in the image below. Is there any source which talks about such situations?

 

[JasonMcKee71]

Both Eurocode and ACI code provide good principles in their corresponding chapters regarding punching shear resistance.

Jason McKee
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[BAGW]

Do ACI provide guidelines when columns are close to one another? If yes, can you guide me where to find it?

Thanks

 

[hokie66]

Codes can't cover everything. I think your assumption is valid, provided the column loads are of the same magnitude. With a 2 m deep footing and if your plan is to scale, you probably don't have much in the way of flexural behaviour, and the design is best approached using strut and tie methods.

 

[rapt]

Actually, all codes do cover this.

As an oldish professor I know well would say, Engineering judgement is required in the application and interpretation of design codes!

It is not flexural, it is a deep beam/strut tie condition. There is no punching between the columns, only possibly for the whole group depending on the edge distance from the group. Treat it like a pile cap!

 

[BAGW]

Just for my understanding, if the footing was like 500mm deep and if the columns were still placed in such a way that the punching shear perimeter coincides, how do you tackle these situations? Is the image I have posted above is the way to go?

 

[KootK]

Further to hokie's point (I think), I'd be inclined to design the combined punching perimeter for four times the load of the most heavily loaded column applied concentrically. That way, you'll have a little extra left in the tank to cover load cases where disparate column loads would produce a moment needing to be resisted by the punching surface. If that method become too punitive for footing depth, consider a more sophisticated analysis. Lastly, I'd make sure that each column would work for punching based on it's own perimeter in the absence of the other columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

Is there any source which talks about such situations?

This is similar to the punching shear check required for a pile cap. Have a look at CRSI's design guide on pile caps. In short, you check different perimeters for different combinations of axial loads. For your case, I believe the full perimeter for all 4 columns will control.