If you are using the Standard Specs, Inventory Rating is based on 0.55Fy; it's considered "normal" traffic. Operating is an "occasional" overload and the rating is based on 0.75Fy.
Under the LRFD spec, same principle. The IR is based on a live load factor of 1.75 and the OR is based on a factor of 1.35.
No, for a load rating with no bridge posting requirements the inventory and operating ratings should be greater than 1.0 with a structural analyzed using a live load factor of either 1.75 or 1.35
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
A Design Inventory Rating of 1.0 is usually required for new bridges. If it's greater than or equal to 1.0 that should be OK unless the owner has specific requirements. I don't know if I would want it to be exactly 1.0. If you want to be super cool I supposed you could design for a future condition factor of 0.85 and shoot for 1.18 which would be 1/0.85. This is for LRFD, not LFD.
It's also important to remember that a Design Load Rating is not the same thing as a Legal Load Rating. The design rating is only for the HL93 loading, while the legal load rating is done for specific trucks specified in the Manual of Bridge Evaluation.
edward1: You might want to consider taking the Load rating of Superstructures course offered by National Highway Institute (NHI). It deals with all topics related to load rating such as inventory, operating ratings for HL93; legal and permit load ratings; specialized hauling vehicles; load posting etc.
