punching shear check in presence of drop beams

[chekre]

Hi guys,

How do u treat a punching shear check when the slab - column being checked versus punching has a beam framing from one side into the column ?

Personally, what I do is that I subtract from the column reaction the shear carried by the beam (Vu) and perform the usual checking after I subtract again the beam width from the bo (critical perimeter) ??
I think this is a conservative method. How about subtracting from the carried column reaction the nominal shear Vn instead of vu ?

thanks for the help

 

[dcarr82775]

I do something similar to what you described. Not sure how punching shear failure would occur with a beam (what is the shape of the critical section). I don't think subtracting Vn is great because the load still has to get around to the beam.

 

[HermanLJ]

Personally I would just design the slab for punching shear without considering the shear resistance provided by the beam coming in from the one side.

Shear links are so inexpensive that the cost is negligible and the extra safety factor assures peace of mind.

You can never be too safe when it comes to punching shear.

 

[hokie66]

So is the column surrounded by a slab, with a beam only on one side? A plan would help. But in general, the portion of the load which goes to the column rather than first to the beam must be resisted by punching shear.

HermanLJ, by "shear links" I presume you mean stirrups, which are not generally used in slabs, unless the slab is quite thick.

 

[dcarr82775]

I assumed he was talking about studrails.

 

[chekre]

Ok, attached is a pdf.

as said before, subtracting Vu is a conservative method. I think a punching failure will happen if the surrounding beam will also fail in shear (correct me please). In this case, I think that we should subtract from the column reaction the nominal shear that can be resisted by the beam. for example, if the beam stirrups has 4 legs of 12mm diameter , then we should subtract from column reaction the ultimate shear that can be resisted by these 4 legs.

I think it will greatly depend of the mode of failure. I think, for the time being, I will deduct the Vu unless I have some good reference talking about this problem.

 

[slickdeals]

In this scenario, you could treat it as an interior column and not worry about deducting the beam width from the punching shear perimeter.

 

[chekre]

and u wont take into account the presence of the beam ?

 

[slickdeals]

I agree with your logic of deducting beam shear from column reaction, but was saying that I would use the perimeter as if it's an interior column.

 

[hokie66]

Agree with slickdeals. With that arrangement, just ignore the beam contribution and check the slab as if the beam didn't exist.

 

[chekresboss]

but it is impossible to have a punching shear failure without cutting the beam and the beam stirrups.
therefore, the depth of the beam (beam section) and stirrups will be contributing to the punching shear resistance of the slab, and ignoring that will be "playing it extra safe", but is it necessary to thicken the slab if the punching shear ratio is not ok but on the limit?

 

[hokie66]

No, it is not impossible to have a punching shear failure without failing the beam. Punching shear does not always occur symmetrically. Where unbalanced moments exist, the shear is unbalanced as well.

 

[chekre]

ok thanks guys. The same logic will apply if the column is surrounded by 2 drop beams (risk of punching in this case is unlikely to happen unless the beams aren't that stiff)

 

[hussamabslh]

It makes sense to subtract the beam shear from the column reaction but you have to take into consideration the reduced concrete slab area that is resisting shear (A_c) (sort of edge column case) this case is highly influenced by moment, and punching shear may occur regardless shear failure in beam.check the pdf below. and see ACI 318M-11 ... 11.11.7.2

 

[chekre]

will u subtract the ultimat shear (vu) that the beam is carrying or the nominal shear (max shear that is carried by the beam + stirrups action) ??

 

[hokie66]

chekre,
You should think in terms of shear stress instead of shear force. The beam on one end of the column will not affect the shear stress in the slab on the other end. A punching shear failure would be an 'unzipping' type failure. The beam might in fact arrest that unzipping to prevent collapse, but failure would still have occurred.

Yes, with a continuous beam, punching shear failure is much less of a concern.

 

[hussamabslh]

chekre
to compute the punching shear force (manually if you suspect punching shear failure, since safe does not calculate punching shear in this situation )you must use the tributary area method in the pdf file I attached (critical loading surface area), calculate the punching shear force and moment acting on the column and then divide by the Area of concrete resisting p.shear (perimeter of critical shear failure zone* effective depth of slab ) you will get the stresses then you can compare with the capacity of concrete to see if links are required use ACI 318M-11 ... 11.11.7.2.

 

[hussamabslh]

Fig. R11.11.7.2—Assumed distribution of shear stress. From Commentary ACI 318M-11

 

[chekre]

Thanks guys