Load Rating of Prestressed Concrete Girder Bridges: A Comparative Analysis of LFR and LRF

With the intention of supporting the Federal Highway Administration’s implementation of Load and Resistance Factor Design (LRFD), research efforts were made to facilitate the transition from Load Factor Rating (LFR) to Load and Resistance Factor Rating (LRFR) in the state of New Mexico. Five prestressed concrete girder bridges, courtesy of the NM bridge inventory, were rated using the BRASS-GIRDER and BRASS-GIRDER (LRFD) software. Research objectives include 1.) the evaluation of the BRASS software prior to full implementation by the NM Department of Transportation (DOT), 2.) the identification of the source of dissension between LFR and LRFR rating factors, 3.) the identification of any trends in the rating factors as affected by bridge geometry, 4.) the identification of any questionable bridges within the sample, and 5.) the use of the research findings to provide training of the LRFR method to the NMDOT. In verifying the BRASS software, all strength-based rating factors were in agreement with hand computations for LFR. The serviceability rating factor, however, differed by 16.7 percent and was therefore considered inadequate. With respect to BRASS-GIRDER (LRFD), potential errors relating to the Modified Compression Field Theory interfered with the computation of beta and theta, thus affecting the shear resistance. However, it was concluded that BRASS-GIRDER (LRFD) produces accurate results under the premise that the shear resistance is determined by means of a user defined beta and theta. The LRFR method generally yielded lower rating factors for flexure, with the longer span bridges demonstrating a larger deviation between LFR and LRFR. The live load effects were identified as the contributing parameter to the difference in rating methods. The dead load effects and flexural resistance had little impact. The LRFR rating factors for shear were generally lower than those produced by LFR. The discrepancy in rating factors was linked to the live load effects and shear resistance. The dead load effects contributed little to the variation in LFR and LRFR rating factors for shear. Overall, the shear ratings controlled over those based on flexure. Finally, a number of bridges proved inadequate for the shear ratings, while the flexure ratings were satisfactory.


  • English

Media Info

  • Media Type: Digital/other
  • Edition: Final Report
  • Features: Appendices; Figures; References; Tables;
  • Pagination: 128p

Subject/Index Terms

Filing Info

  • Accession Number: 01352639
  • Record Type: Publication
  • Report/Paper Numbers: NM02STR-01
  • Created Date: Sep 22 2011 6:31PM