how to add a rib in shell object?

I'm trying to make a plastic part. I have made a hollow box by using shell command, now i want to add rib to my object for strengthening it but I am not able to select the inner sides (shelled side) of the box.

1 Answer

You need to create a sketch on a plane perpendicular to the two faces you want the rib attached to and draw a profile for the rib.