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punching capacity of unreinforced slab

punching capacity of unreinforced slab

(OP)
does anyone know how to prove punching of a column on a unreinforced or 'lightly'  RC slab. ground supported?  

RE: punching capacity of unreinforced slab

Use the Plain Structural Concrete chapter in ACI 318.

DaveAtkins

RE: punching capacity of unreinforced slab

(OP)
not US based sorry.  

RE: punching capacity of unreinforced slab

Unless it is too early and I am not thinking clearly, isn't this just the punching shear capacity of the concrete?
This will be laid out in any concrete design textbook.  

RE: punching capacity of unreinforced slab

(OP)
my code deals with RC conc. i.e combines the dowelling action of the bars with the concrete. As is included in this relationship.  

RE: punching capacity of unreinforced slab

Are you dealing with RC concrete or just plain?
In doing a new spread footing design I usually select the depth of the footing based on punching or wide beam shear. When doing the initial design, I only use the plain concrete to calculate the shear capacity.  

RE: punching capacity of unreinforced slab

Agree with ToadJones.  Typical method in North American codes of dealing with punching shear is to consider the concrete only unless shear reinforcement (studs, vertical ties, etc.) are provided.  What country are you in?   

RE: punching capacity of unreinforced slab

(OP)
it is plain, or only has very nominal. It is ground supported. The BS codes do not calculate just concrete, the allowable figures take into account the dowelling action of the steel. In the equation is an AS value. If this is 0 the answer is zero. You could size so that the entire footing is in compression but that is not the case. Can you give me your equation for calculating capacity of just concrete and I'll compare to what I've come up with? Or teh theory behind it?  

RE: punching capacity of unreinforced slab

...in short, it is basically the strength of a 4-side shear plain through the concrete at some distance around the column.
But....what you would use for you distance and concrete shear strength is up to you or your code.  

RE: punching capacity of unreinforced slab

Here in the US, the punching shear capacity of reinforced concrete is much higher than for unreinforced concrete--even though you are only considering the concrete.

DaveAtkins

RE: punching capacity of unreinforced slab

Dave is right.  

RE: punching capacity of unreinforced slab

(OP)
of course would have thought that is obvious. You are not considering the steel per say, just the confining effect of the bars.  

RE: punching capacity of unreinforced slab

Like Dave says use the Plain Structural Concrete chapter in ACI 318.  Concrete outside of the US acts the the same as it does here.   

RE: punching capacity of unreinforced slab

(OP)
DWHA I absolutly agree.(with conditions) but I dont have access to ACI being an american code.  

RE: punching capacity of unreinforced slab

In Canada, for plain concrete, the distance from the column to the shear plain can be a minimum of h/2 where h is the thickness of the slab.  

RE: punching capacity of unreinforced slab

The ACI318 for punching shear do not ignore the normal reinforcing. It is an empirical method that requires that the flexural reinforcing required for ultimate strength be supplied and detailed as per the detailing rules of the code.

The BS8110 method is more closely dependant on the area of steel and uses a beam shear type flexural shear strength calculation.

But both require the tension face reinforcement.

Personally, I have never designed unreinforced sections for punching shear, never having believed in the concept but some codes do allow it. Make sure it cannot fall anywhere!

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