Thanks, Jeffrey. I was able to make the floor with your suggestions. In side view, I created a 2D rectangle. The top aligned with the bottoms of the middle chairs in the first and last row, and extended the full length of the house from back wall to the stage. I then made a 180 degree sweep. I tried to make the origin of the sweep the locus of the concentric seating radius, but I kept getting an error message that the locus was not on the same plane, and I couldn't figure out how to correct that. Anyway, I let the sweep be created in the default mode, which has the origin from the left-most point, and then did "Edit Sweep," which allowed me to place a new locus at the concentric seating radius. I then created 3D shapes on all sides of the sweep and did a Subtract Solids, which made the edges of the sweep to be the same as the auditorium walls.