If you only drop the top of the slab then use a void to cut it, the dropped area will become thinner. If you need to maintain the thickness of the slab then create two different slabs with different elevations and join them.
im doing also the same thing (maintaining slab thickness ). but i want to make normal slab and drooped slab in one object. is that possible to do it ???
I don't see you can do it as one slab if the drop is vertical. If you can live with a slightly slanted drop, you can edit sub elements, add split lines or points and change their elevations to create the drop.
