NCHRP Report 454, Calibration of Load Factors for LRFR Bridge Evaluation, contains the findings of a study to determine load factors for use in evaluating the load capacity of existing bridges. The report includes recommended values for load factors and presents the methodology and data used to calibrate the factors to provide appropriate safety margins. The material in this report will be of immediate interest to bridge engineers involved in bridge load rating and to engineers interested in the development of load and resistance factor rating procedures.
During design, load capacity can be added to a bridge easily, and uncertainties in the magnitude of loads (and the resulting conservatism of design estimates) have only a small impact on construction costs. In contrast, the cost to strengthen an existing bridge can be very large and, to avoid unnecessary expenditures, accurate estimates of loads are needed. In order to reduce the uncertainty of load estimates, a greater knowledge of the type, size, and frequency of vehicles using a particular bridge is needed. As a consequence, the application of reliability theory to bridge load rating is more complex and varied than the application of these principals to design, and rating engineers can benefit from a greater understanding of the basis for the load factors specified.
NCHRP Project 12-46, "Manual for Condition Evaluation and Load Rating of Highway Bridges Using Load and Resistance Factor Philosophy," was initiated in 1997 with the objective of developing a manual for the condition evaluation of highway bridges that is consistent with the design and construction provisions of the AASHTO LRFD Bridge Design Specifications, but with calibrated load factors appropriate for bridge evaluation and rating. The research was performed by Lichtenstein Consulting Engineers, Inc., of Paramus, New Jersey, with Dr. Fred Moses serving as a consultant for the development of load factors. The final research report documenting the development of the recommended manual is available as NCHRP Web Document 28.