| The concept of redundancy is critically important to structural engineers. It provides confidence in the safety of our designs and safeguards to prevent collapse from deterioration or damage. However, the lack of a clear and quantifiable definition of redundancy in bridge design and evaluation standards illustrates just how misunderstood the concept is in current practice. This dissertation reviews past research into quantifying structural redundancy of bridges, more recent research that has gained increasing acceptance in current practice and focuses on evaluating the redundancy of twin steel box girder bridges, which have historically been considered nonredundant. A definition of "collapse" suitable for use in the measurement of bridge redundancy is proposed. Collapse is evaluated in terms of the formation of a failure mechanism and loss of function of the bridge.;When evaluating the structural redundancy of twin steel box girder bridges, the load carrying capacity of the damaged and undamaged bridge components must be determined. Because of the significant role that the concrete deck plays in transferring load from the damaged girder to the undamaged girder, proper finite element modeling of the concrete deck is critical to evaluating redundancy. An effective thickness shell element approach is developed that will replicate the nonlinear behavior of a concrete deck without requiring a rigorous solid element modeling approach. The calculated effective thickness of the shell element successfully replicates the two primary structural responses of the deck that are of interest in this study: (1) stiffness behavior of the deck in the nonlinear range (or near nominal capacity), and (2) ultimate load carrying capacity of the deck. This technique reduced the number of deck elements in a two-span bridge finite element model by 34,990 elements, or more than 50%.;Parametric non-linear finite element analyses are used to determine the role of different bridge components in transferring load 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 research indicate that twin steel tub girder bridges can be classified as redundant if the bridge is designed in accordance with the AASHTO LRFD Design Code and permanent external braces, preferably solid diaphragms, are provided Minimum design criteria are proposed to allow for a redundant classification of twin steel box girder bridges, thus reducing fabrication and maintenance/inspection costs for this increasingly popular bridge type.;KEYWORDS: Box girders; steel bridges, curved; bridge failure; nonlinear response; finite element method; diaphragms; redundancy; fracture critical. |
