I sometimes receive requests for help designing barns, sheds and other structures. It is a bit out of my scope of practice, but there are loads of designs available from the Midwest Plan Service (at Iowa State University) including their free building plans section.
There are also other compilations of plans available from the Canadian Plan Service and North Dakota State University.
Two other, specific publications may be very useful for many smaller farms. These are scans of publications from the Northeast Regional Agricultural Engineering Service (NRAES) which no longer exists.
Pole and Post Buildings – Design and Construction Handbook – NRAES-1
Small Farms – Livestock Buildings and Equipment – NRAES 6 – NRAES 27
Post-Frame Building Handbook – NRAES 1