LRFD Approach for Load Rating U.S. Army-Owned Bridges That Require Engineering Judgment

Abstract

As part of their 2020 Biennial Inspection Program, the U.S. Army is conducting a load rating effort for its bridges located at military installations throughout the United States. To date, 201 structures at 26 army bases are being evaluated for both civilian and military vehicles. Seventy-two of these structures are reinforced concrete bridges with unknown reinforcement which must therefore be rated using engineering judgment. The American Association of State Highway and Transportation Officials’ (AASHTO) Manual for Bridge Evaluation provides guidance for these structures when the live loads are civilian vehicles. For army-owned bridges, however, an allowable military load classification must also be determined. A procedure called “correlation classification” is currently used to determine the proper military load classification for a particular bridge. The current procedure, however, is based on the allowable stress method. To allow the use of load resistance factor design (LRFD), a new correlation equation and methodology using LRFD principles was developed. The methodology, however, requires the load rater to incorporate knowledge about the bridge into the load rating process. For older bridges with lighter design vehicles, and low the average daily traffic bridges, the correlation concept was also used to determine if a posting was required for state legal and emergency vehicles. This exercise demonstrated that the current correlation equation can be successfully modified to incorporate LRFD principles and applied to determine the military load classification for reinforced concrete structures that require engineering judgment. The application of the new correlation equation is illustrated with two examples: a reinforced concrete beam bridge and a box culvert.