Rotation Requirements for Moment Redistribution in Steel Bridge I-Girders

Abstract

One of the key assumptions in inelastic design is that members have adequate ductility to allow moments to redistribute. However, while AASHTO specifications contain moment redistribution provisions for steel I-girders, the amount of ductility that is adequate for this specific design situation has not been studied in detail. As a result, the scope of these specifications is limited to beam types known to have significant ductility, which restricts the use of these procedures and causes negative economic consequences. The goal of this work is to determine ductility requirements specifically applicable to AASHTO moment redistribution procedures that are valid for all I-girders. This is accomplished through an analytical procedure, detailed herein. The results of this work are empirical equations predicting the amount of rotation required as a function of the intended level of moment redistribution as well as the material properties and span configuration of the girder. With these requirements known, there is a basis for reducing the overly conservative nature of the existing AASHTO moment redistribution specifications.