Shear friction for punching shear 3

[azulazul]

Hi All,

Are the top matt reinforcement bars reliable for punching shear considerations such as seen below?

 

[hokie66]

azulazul,

The concept of shear friction relies on reinforcement in tension providing a clamping force to hold a crack together. Bottom bars at a support are not in tension, so they can't provide clamping forces.

 

[dik]

For narrow columns, I add Xtra bottom steel for shear friction to try to stop any initial cracking one end embedded in the column and the other in the bottom of the slab.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hokie66]

dik,

I would like to see that detail. But if it is in the bottom of the slab, it can't work for shear friction. Not saying bottom steel is not a benefit for punching shear, because it is, but shear friction...no.

 

[blihpandgeorge]

dont mess with punching shear - ever

 

[dik]

@Hokie: I'll see if I can dig up a detail. It's not for punching shear or shear friction, but with narrow columns (sort of like 12x>24, 10x>24, or something of that ilk) , there's a propensity for the slab to crack, initiating at the narrow face. The moment is also higher due to the column being stiffer in that direction. It's not a code thing, just personal practice. The bars act as a tension 'restraint' anchored in the bottom of the slab, resisting initiation of punching shear. RJC used to proportion the top mat bar distribution to reflect 'narrow' columns based on research and model testing done by Per Christoffersen, as I recall.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik