Plastic theories of limit analyses are effective tools to estimate the true collapse load of a structure since the load carrying capacity is not underestimated as in case of the elastic analysis. This research aims to implement the plastic theories for the evaluation of capacity of Steel Twin-Tub Girder (STTG) bridges since a declassification of these bridges from their current fracture critical status will prevent the transportation authority from allocating funds on unwarranted maintenance and inspections. In this research, the limit theories are studied to understand the application of plastic analysis to bridge structures by validation of results from yield line analysis using experimental data. Further analysis is conducted to postulate a mechanism to analyze the bridge under specified traffic loads. The analysis guidelines are formulated to incorporate the theories as per the geometrical, loading and boundary conditions of the three major groups of bridges being analyzed: simply supported single-span bridges, exterior spans of two-span and three-span bridges and interior spans of three-span bridges. These guidelines are implemented for all the 15 bridges selected using the recommendations of the Texas Department of Transportation. The applications and ramifications of the results are discussed and the concluding remarks are drawn to assess whether the bridge remains as fracture critical or further advanced investigation is needed for a conclusive declassification.
