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# Punching shear v Beam shear capacity comparison

## Punching shear v Beam shear capacity comparison

(OP)
Hi,

I have always been under the impression that the AS3600 punching shear equations would be more conservative than the beam shear equations. However recently I have found this isn't the case (?).

CL9.2 merely calculates a concrete shear stress, then applies that to the shear perimeter for the shear force capacity, this capacity may be reduced due to a) column geometry and b) applied moment.

However, if you use the CL8.2 beam shear equation for say, one side of the same critical shear perimeter, I get almost 2.5 times less capacity, even if I have a higher fcv value. This is primarily due to the (Ast/bd)^(1/3).

I assume this is something to do with the nature of the two failure mechanisms however I don't think I fully understand the details of it. It would seem to me that the punching shear equations can often given better results due to the Ast/bd being ignored.

On another note, can someone please clarify why in beam shear, we subtract the width of PT ducts from the effective width, however in the punching shear equations we only account for the benefit of the compression? again, this feels like a bit of a contradiction to me.

I see the commentary talks about monolithic slab floor system providing considerable resistance compared to an isolated beam. Is this the simple explanation to the above?
Replies continue below

### RE: Punching shear v Beam shear capacity comparison

what beta3 value did you use for the beam shear equations? If you use 2.0 instead of 1.0 for the perimeter at d0/2 as it is the final strut before a support that will take the difference you have between the 2 equations from 2.5 to 1.25.

### RE: Punching shear v Beam shear capacity comparison

In most codes beams shear is given a higher factor of safety than slab shear I believe the theory is that you are unlikely to have poor concrete across the full shear width of a slab whereas it is more likely on a narrow beam.

### RE: Punching shear v Beam shear capacity comparison

I was under the impression that the beta 3 factor is generally reserved for more concentrated loads within the proximity of a column?

Regarding the PT ducts, could you not argue that if the ducts are fully grouted, any shear cracks would need to shear through the ducts/grout as well? Seems overly conservative to reduce the effective width of the entire section, especially in a deep transfer element.

### RE: Punching shear v Beam shear capacity comparison

Yes, Beta3 is to allow for loads applied at the top face near the support. When thios is used, shear needs to be checked to the face of the support, not D from the face. A better method in this case would be to use strut tie.

"Seems overly conservative to reduce the effective width of the entire section"

In that case, either every design code in the world is overly conservative requiring this reduction, or you are wrong. I will go for the later.

### RE: Punching shear v Beam shear capacity comparison

Bent, the commentary has the following: "At failure, the measured strength in combined torsion and shear of the torsion strip at the side face that contained closed ties is about four times that of a similar isolated beam. these beneficial effects of slab restraint are included in the strength equations."

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