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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?

### 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.

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