Select the Pick tool and set the Topology level to Segment Sequence
Hold down the Shift key to activate multi-select, then select the bottom segments of the spiral staircase to effectively create a "loop"
-If you have any extra edges outside of the loop, delete those at this time.
Use the Join tool (in the Edit toolset) to combine the segments into one closed object
-Ensure that the Reference Plane is still set to the XY axis
Use the Extrusion tool (set to Perpendicular to Surface in Tool Options) on the new shape so that teh full extrusion passes through the Reference Plane
Draw a Rectangle on the Reference Plane so that it passes through the entire extrusion
Use the Surface Split tool to split the extrusion
Delete the object created below the Reference Plane
