Shear friction for punching shear 3

[azulazul]

Hi All,

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

 

[la belle vie]

I think not. For shear friction, the movement needs to occur along a slip surface. Bars crossing the slip surface go into tension as the surface slips especially if the surface is rough. With punching shear, the failure surface is not a slip surface.

 

[dik]

top reinforcing is essential... one of the first projects I had was load testing a failed slab that was additionally reinforced... top mat ended up on the bottom and shear friction capacity was a fraction of the design load... Beautiful yield-line failure pattern. Also with 'thin' rectangular columns I add a couple of 25M bot bars in the thin direction to reduce initiation of punching shear. The pile foundation had punched through the slab and was proud by a couple of inches... total failure was prevented by the top bars going into tension

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

-Dik

 

[JoshPlumSE]

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

I'd say that the top reinforcing is essential (as dik said). After all, we base the pure concrete punching shear resistance based on the depth "d" to that top reinforcing. Right?

However, we don't consider those bars to add to this resistance at all. There are ways to do it though. Stud rails and such.

 

[azulazul]

Would it be possible to take the top rebar strength in shear friction and add it to the concrete strength calculation ?

 

[DaveAtkins]

I believe the punching shear equation already takes shear friction into account. Maybe not explicitly, but I believe the value in the code was developed through testing.

DaveAtkins

 

[hokie66]

No! Punching shear is real, and needs depth to overcome it, or effective shear reinforcement. Forget shear friction, in fact, I recommend forgetting shear friction altogether. It is a concept which doesn't make sense to me.

 

[rapt]

The empirical punching shear logic already takes advantage of the tension force in the top reinforcement which is well past yield. The concrete would have no shear capacity if there was no tension steel across the crack.

You cannot use it twice.

 

[Tomfh]

NO!

Shear friction is only for assessing a particular shear failure across a particular failure plane, most generally for assessing a perpendicular failure plane.

It is an UPPER BOUND check.

 

[azulazul]

I just find it off how the punching shear formula just asks for depth 'd' of the reinforcement, but we never define what the reinforcement content is. Surely having more reinforcement in the top matt would make a difference?

 

[la belle vie]

With shear friction, you only count on the rebar. You assume the concrete is already cracked. Calculating shear friction is useful is some cases and not necessarily an upper bound. Azulazul- the top mat of the slab is the main reinforcement. You don't include it in regular beam shear or punching shear calculations.

 

[azulazul]

As the failure plane is on an angle, wouldn’t the top reinforcement (including any other reinforcement within the failure zone) take on vertical (shear friction) and a tension component?

 

[la belle vie]

In shear friction, the bars crossing the slip plane go into tension as the surfaces on either side of the slip plane move against one another. If the surface is rough, the effect is more pronounced as the bumps on the surfaces ride up on each other and cause the plane to want to open up. This theory is not used along with diagonal tension type shear calculations, i.e. punching shear and one way beam shear. In diagonal tension, the concrete along the failure surface is in tension and the calculation takes the tensile capacity of the concrete and shear stirrups into account. I'd have to go back and review exactly why you don't count on the flexural reinforcement. Maybe because it is too close to the top or bottom.

 

[azulazul]

Thanks for your response la belle vie, I just struggle to understand why we base the pure concrete punching shear resistance based on the depth "d" to that top reinforcing in the empirical method in the code if we are only relying on concrete tensions.

 

[jayrod12]

But that's the point many people have been trying to say to you, although the code formulae only take into account the concrete, the values used in those formulae are based off testing, in which there needs to be top reinforcing. So without the top reinforcing the values obtained from testing are not valid.

Without the top reinforcing, the slab would crack and punching shear would happen immediately with almost no resistance.

If you don't have enough punching shear capacity in your situation, then provide stud rails, column capitals, bigger columns, thicker slabs, drop panels etc. I'm not sure how many times you need to hear the word no, but you can't use the slab top reinforcing as a separate resistance mechanism.

 

[rapt]

azulazul

You need to read up on shear friction. It is in the ACI Commentary.

The steel provides a clamping force across the crack, i.e. axial tension in the steel.

The method does not rely on the shearing of the steel.

 

[Tomfh]

Azulazul,

You are not simply relying on “concrete tension” when calculating punching shear, you are relying on it being reinforced.

Also, shear friction doesn’t even work at that crack angle. Shear friction effect works best for shear loads perpendicular to the crack. Once you go too far past that it doesn’t work properly. The sweet water bridge collapse is a good example of people wrongly using shear friction across an inclined joint.

 

[hetgen]

I just find it off how the punching shear formula just asks for depth 'd' of the reinforcement, but we never define what the reinforcement content is. Surely having more reinforcement in the top matt would make a difference?

Yes, it does, having more reinforcement does increase punching shear capacity. The clip below is from this ACI Tech paper by Aurelio Muttoni. As it reads ACI's punching shear formula is derived from an expression that does somehow accounts for longitudinal reinforcement. Other codes, such as the Eurocode, do account for the reinforcement ratio directly in the punching shear formula but limit how much punching shear can be gained by increasing the rebar ratio.

 

[rscassar]

punching shear is a different model to shear friction. Top steel is need to close out the truss analogy for punching shear. If relying on top steel as a dowel, it will not have sufficient distance for concrete break-out to the slab surface to provide any meaningful capacity.

 

[azulazul]

What about the bar at the bottom (or any central reo)? Would that be sufficient for shear friction ?