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1At one time, bridges on curved alignments were rare; however, modern highway bridges and traffic separation structures are commonly built on a horizontal curve. This change has come about because of higher traffic volumes and speeds, the geometric constraints of the urban environment, and improved structural forms that lend themselves to curved construction. The concrete box-girder, particularly post-tensioned pre- stressed concrete that can span large distances, is one such structural form. The cross section of these structures is in- herently strong in torsion. This is important because curva- ture induces high torsion forces. Also, because concrete can be easily molded into the required shape, it is ideal for curved construction. For these reasons, prestressed concrete box- girders have become the structure type of choice in many jurisdictions. A common application of curved structures is in freeway interchanges where connector ramps or âfly- oversâ carry traffic from one freeway to another at relatively high speed. Cross sections of curved box-girders may consist of single-cell, multi-cell or spread box beams as shown in Fig- ure 1-1. Because only a very few spread box beam bridges use curved beams, only the first two types were considered in this study. It has become common practice to analyze and design these structures as if they were straight. Live load distribution is often addressed using the whole-width design approach. Local problems, such as the lateral forces induced by curved prestressing ducts, are often handled using specific design rules and details that have been developed over the years. For the most part, this design approach has been used success- fully, but some recent cases of poor bridge performance have made it clear that this approach has its limitations. Because it is likely that (1) the use of curved structures is going to increase and (2) the geometries of some of these structures will continue to push the limits with respect to the degree of curvature, span lengths and depths, the amount of required prestressing force, and so forth, better guidelines are required for their design. Such guidelines are the purpose of this project. A significant body of research and development exists rel- ative to the design and analysis of curved prestressed concrete box-girder bridges. Some of this information has found its way into design specifications, but much of it has not. There is a need to collect and analyze this information in order to evaluate its merit for nationwide design rules. Although much of this work has been conducted domestically, a signif- icant body of work has been conducted by other countries and this work also needs to be considered. Although most issues relative to design of curved concrete box-girders have been studied to some degree, gaps in our understanding need to be filled. With modern computer pro- grams and analytical models calibrated to existing physical C H A P T E R 1 Introduction Single-cell Box Girder Multi-cell Box Girder Spread Box Beams Figure 1-1. Types of cross sections.
and experimental results, most of this can be done through analytical studies. Additional physical testing of existing structures or laboratory experimentation, although impor- tant in and of themselves, are beyond the scope of this project and are not necessary to accomplish this projectâs intended goals. This report presents the results of a review of the litera- ture and the state of practice with respect to curved concrete box-girder bridges. In addition, detailed results from both global and local response analysis studies are presented. Final recommendations are presented in the final chapter and are implemented in the form of recommended changes to the AASHTO LRFD Bridge Design Specifications and Commentary and in analysis guidelines for these types of bridges. Example problems are also presented that illustrate the application of these recommendations. 2
