Punching shear check - question

[Akeee]

Hello everybody,
I have a question regard to the punching shear check for slabs. In EC2 (and i assume in other codes too) they say that you have to check first on "u0" perimeter (perimeter at the face of the column edge) that the punching stress is lower then a compression-like concrete strength "vRd,max" (and i said compression-like because the basic formula is used for determine the compression strength for a biaxial loading (compression on the bottom and the top face of the cube and tensile on the lateral faces); on the formula here, for punching shear, is added an 0.5 factor, dont know why). So my question is, why should we check the compression strut if we will put specific reinforcement for this ? I mean what will be happening if this doesnt check and the concrete will crack/fail? We have reinforcement anyway so.. maybe i dont understand very well how shear stress/forces are flowing/transfer though the concrete element.
Thank you !

 

[rapt]

Maximum shear allowed on a section in most codes is controlled by limiting the compression stress in the compression strut. Eurocode has defined a stress limit based on the direct shear stress on the face of the column to satisfy this requirement. This is a maximum stress allowed no matter what you do to make the shear design work. How are you going to reinforce to change this requirement?

 

[Akeee]

I didnt made my self clear, yes i know what you said, but my question is : why is this limit ? why they limit the compression stress in the strut ? what happens if this fail ? especially in the punching shear case. Because the way i see it if the strut fails we have the reinforcement that will carry the stress.

PS: I draw a picture :

 

[KootK]

why is this limit ? why they limit the compression stress in the strut ? what happens if this fail ? especially in the punching shear case. Because the way i see it if the strut fails we have the reinforcement that will carry the stress.

There appears to be a fundamental misunderstanding in your thinking here. Concrete reinforcement generally does not resist load by itself. Rather, it resists load in concert with the concrete struts that restrain the ties. And those struts still have practical strength limits even when the ties are present.

The situation is analogous to that with stirrups in a simple beam. Imagine what would happen to the tie/stirrup reinforced beam shown below if the concrete struts (dashed lines) gave way. All of the rectangular panels between stirrups would rack in shear and the beam would collapse.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Yea i guess i have some trouble understanding shear in concrete beams... Can you detail more please "Rather, it resists load in concert with the concrete struts that restrain the ties." ?

 

[KootK]

You bet. Concrete shear is pretty mysterious stuff.

In the sketch that I posted above, the steel ties and concrete struts work together, in concert, to form a truss that resists load. If you removed the struts from that truss, it would no longer be stable/viable.

If that doesn't help, let me know. We'll get it cleared up if takes 100 sketches.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Ok i get the idea that if the struts are gone the hole system fails but i cant picture in mind the physic think, how the concrete transfer load tough struts etc etc. but never mind i will read more about this subject, thank you

 

[KootK]

but i cant picture in mind the physic think, how the concrete transfer load tough struts etc etc

Let's give this one last try. Imagine a one dimensional version of punching shear.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Some more from a more reputable source (Jorg Schlaich).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Akeee]

Hehe, thank you very much !!!! Now i have a better understanding, thank you KootK