Implications of Overload Distribution Behavior on Load Rating Practices in Steel Stringer Bridges

With the ever-increasing demands for transporting goods and services, transportation officials are facing a growing challenge with the safety of in-service bridges under the passage of oversized and overweight vehicles. Current load rating practices provide the basis for evaluating the operational safety of in-service structures by using engineering judgments and simplifying assumptions. However, a true measure of the system performance under the impact of irregular loading scenarios requires knowledge of different aspects of the system-level characteristics, including the lateral load distribution behavior. In this study, nonlinear finite element analysis has been implemented to evaluate the evolution of load-distributing mechanisms in two representative in-service structures subjected to overloads in the state of Michigan. In addition, rating factors were defined for the selected structures on the basis of the load and resistance factor rating methodology. Results from this study demonstrated that current design and rating practices were overly conservative in predicting the actual distribution behavior and assessing the safety of the in-service structures under the effect of irregular loading conditions. This investigation also highlighted the importance and benefits of implementing a refined method of analysis that can help bridge engineers to support their permit and posting decisions.