If you wish, you can close it with a beam or with a slab edge. Of course it would be better to use beams if it is actually going to be personified. Also, if you are going to use a U polygon column as in your example, you can create a floor including the inside of the elevator bucket and empty the elevator bucket by using the floor space command. But I suggest you to do this work with "shell modeled panel" like my yellowdurmus friend.