interior or edge column for foundation punching check

[structural87]

hi all,

In the punching shear calculations for a suspended slab, a column near a discontinuous edge is considered interior if the distance from the face of column to the edge exceeds 4 x slab thickness.

Is the same applied for column resting on a foundation ? Should the foundation be equal to 8 times its thickness in order to consider the resting column as an interior ?

Thank you for the help

 

[KootK]

I'd think that the same or very similar logic would apply. I don't see a compelling reason to raise the bar for a foundation element.

 

[HTURKAK]

The footing is supported by the soil below and the column punching through the footing ( punching shear ,two way shear ) is checked at the critical perimeter , along the surface of a truncated pyramid around the column. Thus, the critical section for the punching shear is taken at a distance d/2 from the periphery of the column.

The following snap describes the concept..

 

[r13]

If you flip the foundation, it looks like a suspended slab. So the concept is the same. To your ending question, wouldn't the minimum footing edge length equals to 8h+C (C is column width), in order for the corner, or edge, column to be considered an interior column.

 

[structural87]

Thank you guys. If the column is resting on a single foundation, and the total dimension of the foundation is less than 8x foundation thickness + the column thickness in the 2 directions, how is the critical perimeter computed then ?

 

[HTURKAK]

You are expected to check punching ( two way shear ) and one way shear ..

Steps for punching shear check are;

- Calculate the punching shear perimeter = 4* ( c+ d) , c is width for square column and d is the depth of the ftg.

- Calculate the soil pressure factored load , qu

- Calculate the punching shear force for factored soil pressure , Vs = qu * ( B*L -(c+d)**2) B and L ftg dimensions..

- Calculate the punching shear stress vs= Vs / (4* ( c+ d)*d) and compare with two-way shear strength of the concrete. If you are using ACI, you may calculate the punching shear resistance of the ftg and compare with punching load..

In case of raft foundation, the flexibility of raft and soil spring stiffness will effect the calculation. It will be necessary to develop a few punching shear failure scenarios for side and corner columns and find the minimum one.

 

[structural87]

Thanks Hurtakak. However the procedure you mentioned is known. My question is the following.
If the column is centered on a single foundation and the distance from column face to the foundation edge is less than 4 x foundation thickness you can't check 2 way punching at d/2. So how do you make the verification?

 

[KootK]

Okay, I see what you're laying down now. The 4d stuff won't apply to the situation of a concentrically loaded footing. The key to this is a fundamental understanding of why we do the 4d stuff in the first place, per the sketch below.

 

[r13]

Yes, you shall check punching shear capacity, although all sides with column face to edge distance is shorter than 4*h.

 

[HTURKAK]

Let me explicitly write ; Yes ..you can check !!! Even the distance is ( d ) you are required to check..You SHALL ( not may, should or can..) check the punching shear at perimeter at a distance d/2 from the periphery of the column and one way shear at a distance (d) from the face of column. and verify that one way or two way shear is not critical..

The argument ( the punching shear calculations for a suspended slab, a column near a discontinuous edge is considered interior if the distance from the face of column to the edge exceeds 4 x slab thickness , should also valid for footing design ) is not true.

The punching shear for footing design is not as critical as in suspended flat slabs, since the footing is supported by the soil ..
The punching scenario should be based on the tendency of the column punching through the footing.

 

[r13]

The 4*h rule is to help in identifying whether a column is an edge, corner, or center column, so you can pick up the correct method and appropriate code equation to deal each condition. Link

IMO, the raft/mat foundation is similar to the flat roof construction, but up side down.

 

[structural87]

Thank you guys for your input.