Modified Computer Code for Rating Truss Railroad Bridges

Abstract

The Association of American Railroads (AAR) computer program for rating steel railway bridges is extensively modified to analyze and rate trusses under any type of special train loading other than the conventional Cooper E‐series. The program uses a rating procedure based on the percentage of live‐load capacity of the bridge members. The modified program computes ratings directly for all bridge members, thus eliminating the need to use the maximum moment, end shear, and floor beam reaction tables generated by a separate existing program. This allows the rating to be calculated by running the modified program once, where previously two separate programs had to be run three times. In addition, the new program gives exact member stresses based on the applicable gross or net area (i.e., for stability or connection requirements) for all the bridge members. The original program gives conservative estimates for stresses in only a few selected members. Obtaining realistic forces for all members is essential for determining the remaining fatigue life in the structural connections. In addition to the global restructuring of certain subprograms, the original code has been updated to take advantage of the enhanced FORTRAN language provided by the Microsoft FORTRAN Compiler (a superset of FORTRAN 77). The use of the modified computer code is illustrated with the rating of a truss. A comparison is made between the results of the modified program and the existing AAR rating program.