Here are some details on containers.
"Corner castings measure 178x162mm and frequently project 12.5mm below the underside of the container"
Empty Loads
40ft Container - 3000kg
20ft Container - 2000kg
Here is a table showing loading conditions for typical fully loaded containers...
I had similar issue of having to 'design' a slab 'on grade'.
There is unfortunately little to it. I did come upon a book 'Ground Bearing Concrete Slabs' by John Knapton.
He goes through several methods; none of which I particularily liked. Based on loading his methods alter the depth of the...