## 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

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

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

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

## RE: Punching shear v Beam shear capacity comparison

## RE: Punching shear v Beam shear capacity comparison

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

"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."