Load distribution for slab on grade

[spockman]

I am frequently asked to review structural slabs for equipment loads and, if overloaded, design steel plates to distribute the load down to the required capacity. I have tried several different methods but am convinced I can do a more efficient design. Does anyone have any good ideas and/or references?

Thanks

 

[jayrod12]

Structural slabs and slabs on grade are two different animals.

For slabs on grade you need to ensure you are spreading the load out over enough area that the allowable bearing stress is not exceeded. Remember the concrete will spread the load at approximately a 45 degree angle (i.e. a 4"x4" baseplate on 4" thick concrete will have a bearing area at the bottom of the slab of 12"x12" provided the concrete can withstand the bending stresses applied by the soil).

Then once I know how much area I require at the top of the slab I use the standard steel baseplate design methods (I'm in Canada so it's from the 10th edition of CISC but I'm positive the AISC has the same type of baseplate calcs). This makes sure the baseplate is adequately thick enough to spread the load to it's edges.

The concrete must be checked for punching shear as well.

For structural slabs, punching shear at the point load is an important check, as well as one way shear and bending of the structural slab.