concrete blinding dimensions

[sim85]

hello
for the concrete blinding dimensions under isolated footings, does it need to be same size of the footing or bigger. in our case we have a 30cm blinding since the hard rock is at 60cm below but the foundation top level is fixed. do i need to have the blinding size bigger than the foundation by 30cm (to follow the thickness)and is there anything in any code obliging me to do it.

 

[MotorCity]

Can you tell us what you mean by a "blinding" dimension and perhaps provide a sketch of what you are trying to do?

 

[hokie66]

As I understand it, you are founding isolated footings on rock, and the depth to rock is variable. But you want to use your footing design, and column length, for a constant elevation. So digging the hole to rock of the size of your footing, filling with blinding concrete to bottom of footing elevation, then constructing your footing, is acceptable practice. If the hole is too deep, it may need shoring for worker protection.

MotorCity, blinding is just a working mat.