Size Factor.

As per tests, shear strength of a concrete does not increase in direct proportion with member depth. As per ACI, this phenomenon is called the Size Effect. Do mind that it is the contribution of Concrete only, and not including its reinforcement. 

What this means is, a concrete block with X depth does not have twice the shear resistance of block with depth X/2. 

As per ACI 318-19,


How this acts in terms of one way and two shear can be seen from the code books. It's more dominant  in two way shear, a heads up. You can see from a simple calculation that the size effect factor of a 1000mm thick reduces the shear resistance of an element to around 63% of the original strength.

But wait, if that's the case, won't the requirement for footings which take all the loads from the structure be too much. Clause 13.2.6.2 to the rescue.

It permits the negligence of size effect factor for foundations. What a miracle. No it isn't. It's all calculated that's what I believe. 

Now, do you have to always think about this factor the next time you design a drop/footing for shear/punching check? 
NO.
You don't need to. ETABS/SAFE considers this when you assign the shell as drop or footing. And I have my experience to back it. The punching check for a 1500mm drop would be a 1000mm footing. Mind you, this value is just one out of my mind. It would differ a lot per your loading and dimensions. 

Always up for discussion.




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