Currently there are three methods available for load rating bridges: allowable stress rating, load factor rating, and LRFR.
To improve the procedures, in 1989 AASHTO released the Guide Specifications for Strength Evaluation of Existing Steel and Concrete Bridges, providing the first version of the LRFR procedures.
With the advancing deadline for implementation of LRFD, AASHTO recognized the growing need to further develop LRFR specifications and update much of the Manual for Condition Evaluation of Bridges.
Although the manual emphasizes the LRFR method, it provides rating procedures for both load factor rating and LRFR to allow States the option of rating their existing inventory with either method.
As noted in NCHRP Web Document 28 (Project C12-46): Contractor's Final Report: Manual for Condition Evaluation and Load Rating of Highway Bridges Using Load and Resistance Factor Philosophy, the LRFR methods presented in the 2003 manual "contain the necessary ingredients to provide a more rational, a more flexible, and more powerful evaluation strategy for existing bridges."
As with load factor rating, bridge owners can calculate load ratings on both operating and inventory levels using the LRFR method.
As stated in the AASHTO Manual for Condition Evaluation of Bridges, the basic LRFR equations are as follows:
In LRFR calculations, permanent loads are separated into the three variables DC, DW, and P, because each is assigned an independent load factor ([gamma]) for each load rating limit state.
Load factors for permanent loads and live loads are specified in the AASHTO Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges.
As with load factor rating, LRFR live loads are temporary loads that act on a structure.
The LRFR, like the LRFD code, bases this reliability on the safety index ([beta]) system, where, as described in the National Cooperative Highway Research Program's NCHRP Report No.
The LRFR target safety index is b equals 2.5 at operating level and [beta] equals 3.5 for inventory level.