Where are the joists in my ceiling?

I'm trying to put up a hook for planter. I need to find the joists in my ceiling. I've tried stud finders magnets and using small nails to make pilot holes. What I found out is there are metal lines running all the way across the ceiling in the room that I'm trying to put the hook in. I haven't found any evidence of nails. It seems like there's metal all the way across ceiling like there's a metal I-beam or something like that going vertically across this room in parallel lines. In some of the pilot holes I've made enough of a hole that I can put a screw in there and I can feel it tapping against something metal. Where is the wood joist so I can put in this hook? Any help would be appreciated. Thank you very much.