NCHRP 12-78 Evaluation of Load Rating by LRFR

The FHWA released a memorandum requiring all new bridge designs to be LRFD-based after October 1, 2007. This directive was the culmination of more than a decade of effort by Modjeski and Masters (M&M) in leading a team of experts in the development of the AASHTO LRFD Bridge Design Specifications. In line with this directive, the FHWA set a date of October 1, 2010 after which all new bridges are to be rated using the LRFR method rather than the LFR method. However, the choice of load rating method for existing bridges may affect the transportation of goods and services over the nation’s highways by restricting routes that were previously unrestricted. The bridge community needed additional research and comparisons of the LRFR and LFR ratings to be sure that the rating process maintained an acceptable level of reliability without unnecessary restrictions on commerce.

M&M supported a team led by Michael Baker, Jr. to recommend refinements to the LRFR methods in the AASHTO Manual for Bridge Evaluation. Our technical experts worked with the team to explain changes in truck restrictions and to develop a comprehensive database of rating comparisons. Our problem solvers are using the database to identify differences in ratings and aid in developing proposed refinements. The team continues to refine the analysis and is developing a summary report with recommendations. M&M has worked on several assignments with NCHRP including: NCHRP 12-83, Calibration of LRFD Concrete Bridge Design Specifications for Serviceability, NCHRP 20-7, Task 270, Application of AASHTO LRFD Specifications to Design of Sound Barriers; FHWA’s Manual for the Design, Construction, and Maintenance of Orthotropic Steel Bridges; and our ongoing maintenance of the LRFD specifications, which we began in 1996.