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oneintheeye (Structural) (OP)
24 Jun 10 7:19
does anyone know how to prove punching of a column on a unreinforced or 'lightly'  RC slab. ground supported?  
DaveAtkins (Structural)
24 Jun 10 8:24
Use the Plain Structural Concrete chapter in ACI 318.

DaveAtkins

oneintheeye (Structural) (OP)
24 Jun 10 8:48
not US based sorry.  
ToadJones (Structural)
24 Jun 10 8:57
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.  
oneintheeye (Structural) (OP)
24 Jun 10 9:07
my code deals with RC conc. i.e combines the dowelling action of the bars with the concrete. As is included in this relationship.  
ToadJones (Structural)
24 Jun 10 9:10
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.  
cancmm (Structural)
24 Jun 10 9:17
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?   
oneintheeye (Structural) (OP)
24 Jun 10 9:17
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?  
ToadJones (Structural)
24 Jun 10 9:29
Since you are assuming an un-reinforced this generic unverified example I found with a quick Google search seems to make sense....usually we calculate punching at a distance that is related to the depth of the reinforcing "d", but with no steel you must use plain concrete provisions.

http://www.associatedcontent.com/article/1356244/punching_shear_example_1.html?cat=6
 
ToadJones (Structural)
24 Jun 10 9:34
...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.  
DaveAtkins (Structural)
24 Jun 10 10:19
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

ToadJones (Structural)
24 Jun 10 10:28
Dave is right.  
oneintheeye (Structural) (OP)
24 Jun 10 10:33
of course would have thought that is obvious. You are not considering the steel per say, just the confining effect of the bars.  
DWHA (Structural)
24 Jun 10 10:39
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.   
oneintheeye (Structural) (OP)
24 Jun 10 10:50
DWHA I absolutly agree.(with conditions) but I dont have access to ACI being an american code.  
amecENG (Structural)
24 Jun 10 13:53
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.  
rapt (Structural)
27 Jun 10 23:44
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!
hokie66 (Structural)
28 Jun 10 6:31

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