Suppose we have a ruled Shell like the one above.
You can trim it, or any Shell geometry, to the desired shape by defining its contour.
1.Go to the Floor Plan or Section window.
2.Select the Shell.
3.From the context menu, choose Define Shell Contour.
4.Draw the desired polygon: in this case, an ellipse. (Or use the Magic Wand on an existing polygon.)
5.Click to complete the polygon. The Shell is trimmed.
See also Edit Shell Contour Polygon.