Structural redundancy evaluation of steel tub girder bridges

As a result of construction problems with curved I-girders, increased tub girder research, and improved fabrication techniques, twin steel tub girder superstructures are increasingly more common for curved highway ramps. Historically, however, choosing a twin tub girder bridge instead of a bridge composed of three or four I-girders has resulted in a nonredundant or "fracture critical" designation for the bridge. This paper presents results from an analytic investigation of the redundancy of twin steel tub girder bridges. Parametric nonlinear finite-element analyses are used to determine the role of different bridge components in developing load transfer from a damaged girder to an undamaged girder. The key parameters of span length, bridge continuity, curvature, location of girder damage, and type and spacing of external bracing are investigated. The results of this study indicate that twin steel tub girder bridges can be classified as redundant if the bridge is designed in accordance with the AASHTO Load and Resistance Factor Design (LRFD) design code and additional design and proportioning criteria are incorporated. Minimum design criteria are proposed to allow for a redundant classification, thus reducing fabrication and maintenance/inspection costs for this increasingly popular bridge type.