Reliability Calibration of Bridge Design Code

Abstract

The current AASHTO design loads were developed more than 40 yrs ago based on truck traffic in operation in that period. The code, however, has produced safe designs despite the vastly increased legal loads and reduced levels of maintenance. New structural analysis procedures and continuous pressure to increase the legal load limits justify a code review to maintain the current safety levels for all design applications. This is accomplished by using a reliability analysis with data from large scale field measurements on actual truck loadings and bridge responses. The data obtained from these measurements is projected to calculate the maximum expected live load in the lifetime of the structure and used to calculate a safety index. Safety indices for typical spans are first calculated for designs that satisfy the current AASHTO code. A target safety index is extracted from these values and a new design procedure is proposed to achieve this target index uniformly for all the spans considered. A parametric analysis shows that the proposed changes are insensitive to small variations in the data base if the average current safety level is acceptable and can be used as basis for future code changes.