Either you can opt for (a) an un-equal quadrilateral meshing of each slab so that you can apply pressure load on the entire area of the selected elements, OR (b) an equal quadrilateral meshing of each slab to apply individual patch loads to each elements. Both the methods are to some extent tedious, for option (a) it is the meshing part and for option (b) it is the loading part.
You can refer the attached input file in which option (b) is used.
(Please visit the site to view this file)