First post; I guess I'm the newbie of the newbies!

Can anyone provide guidance in making a barn door? Function matters; looks don't (the barn is isolated in the woods).

The structure is approx 18' wide and 40' long. One end is completely open. I want to build a pair of doors. They can either hinge on the sides or slide open (it would be okay if only one side at a time had access).

If I attempt to build them without plans, knowing me, the doors will weigh 2 tons! Any input would be appreciated.