Is Shear force for Punching check in a raft foundation same as column axial force? 1

[T2ioTD]

Etabs has given me 'safe' punching ratios, but upon checking the shear value, I noticed it was not equal to the column load of the same combination.
My slab is a raft resting on area spring, for that matter.

Is there is something wrong?

 

[le99]

The punching shear is the net difference between the total column load/reaction less the resistance pressure times the area bounded by the failure planes. You can see it clearly by drawing the structure up-side-down with the resulting soil pressure on top of the mat.

 

[KootK]

I agree with le99's assessment of what the situation should be but a drop from 638 kN to 98 kN is pretty big for a 465 mm slab at an edge column. Also:

1) The graphical representation of the punching perimeter does not look like what I'd expect to see at an edge condition.

2) The effective punching shear perimeter is about twice what I'd expect to see at an edge condition.

 

[jayrod12]

I've been told by our in-house Etabs and Safe modeller that they don't tend to trust the punching shear designs coming out of those programs and will generally check it in a separate program or by hand.

 

[HTURKAK]

The picture is fuzzy and not sure if the shear force 96 kN .. Probably correct when you check as per EC-2.

Having worked with different standards, my opinion is that, EC- is less conservative and does not reflect the real behavior of RC mat for punching.

The punching check for EC-2 assumes that the critical perimeter is the distance (d) apart from the column face and the failure surface is between column face to 2d with rounded corners .
I will suggest you to check with hand ;

- Calculate the total load outside the perimeter =VEd
- VEd = Column load – p × [2c1xr + c2xr + π × r2x0.5 + c1 × c2] ( c1 is parallel dim. of column to the free side for edge column)

However ACI approach is different and assumes that the punching crack originate and propagate at 45 degr. from the column corners and the area of concrete which resists shear is calculated at a distance of d/2 from column face and my personnel observation ,reflects the real case.

 

[hussien Ahmed]

Generally speaking, you can obtain the punching shear force by taking the reaction of the axial force on the top of the column and reducing the pressure acting on the area enclosed by the critical perimeter. Note that if you take the reaction on the bottom of the column you have to subtract the self-weight of the column. All that have been clarified in SAFE's manual.

 

[ggcdn]

I've been told by our in-house Etabs and Safe modeller that they don't tend to trust the punching shear designs coming out of those programs and will generally check it in a separate program or by hand.

Its generally a good idea. For regular interior locations away from anything funny, I find that ETABS/SAFE tend to match up with hand calcs / spreadsheet / decon. But edge and corner conditions are often not accounted for correctly.

-JA
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