A huge collection of agricultural buildings provided by the University of Tennessee. Check with current building codes in your area for compliance.
http://bioengr.ag.utk.edu/extension/extpubs/PlanList97.htm (http://bioengr.ag.utk.edu/extension/extpubs/PlanList97.htm)
(http://design.medeek.com/smf/images/UOT.jpg) (http://bioengr.ag.utk.edu/extension/extpubs/PlanList97.htm)