Slab on grade and concrete piers

I have a large building with an arena at the second level. I am retaining the slab on grade on the first level except for some small cuts to run piping. On the first level are several concrete piers which extend to the second level, all of which are to remain. In calculating the plan area of the slab on grade, should I net out the plan areas of the piers, or since they are to remain, can I just figure the slab area as everything within the exterior walls? If I net out the piers, can I treat them like walls and calculate their vertical surface areas? Any feedback will be appreciated.