How can I cut a hole in the floor with structure already above that area?

Riverarch - 1 floor.skp (1.1 MB)
I need to cut a rectangular hole in the floor for 3 side-by-side cog-wheel elevator shafts. The .skp file is the view looking up from under the floor. I traced the area with the pencil tool. For some reason I couldn’t get the rectangle tool to create a rectangle on that spot. But now I need to cut away the floor without losing the geometry on “top” of it (that is, inside the rectangle). What can I do?

See how the lines in the screenshot below fade away in one corner? This is an indication that the rectangle you drew is not planar with the face you are trying to modify (I think). Watch for the “on face” inference while drawing the rectangle.


As Shep says, when drawing the rectangle look out for the blue inference dots to be sure you are on the face you want to draw on. I think the reason you are having difficulty is that the model is off axis, making it harder to draw on this face, as the tool tries to default to the regular x,y axis on a horizontal plane. If you look at the image I am posting which was made by showing the line axes in different colors, the z axis is off, so i think the plane is off too? All the lines should be red green and blue if the construction is rectilinear. Many here are black.
If this were corrected it would make the task easier. (not to say that you can draw a rectangle on an incline, but in this case the slight discrepancy is throwing you off.