Analysis of the AASHTO fatigue design provisions for welded steel bridge details using reliability theory

The current fatigue design provisions in the AASHTO LRFD Specifications (2004) are based on an allowable stress approach. Provisions for most limit states, however, have been updated to the more modern reliability-based Load and Resistance Factor Design (LRFD) approach. The primary objective of the reported research was to assess the reliability present in the AASHTO fatigue design specifications for welded steel bridge details such that more uniform design provisions can eventually be implemented. To accomplish this, a reliability model was developed assuming a log-log straight line fatigue resistance model. The statistical parameters of the variables included in the model were determined based on sound engineering judgment and values used in similar past studies. The safety indices present in the current design provisions were then evaluated. The results indicate that the safety level of the current design provisions varies by span length and by detail category. Therefore, a target safety index was selected based on those currently present in design and new design fatigue resistance coefficients were proposed such that the safety level of the fatigue specifications would be uniform across all detail categories for a given set of design conditions. In addition, the safety of the design variable amplitude fatigue limits was evaluated through the use of computer simulations. New variable amplitude fatigue limits were then proposed to correspond with the proposed design fatigue resistance coefficients.