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1 CHAPTER 1 Introduction At one time, bridges on curved alignments were rare; required for their design. Such guidelines are the purpose of however, modern highway bridges and traffic separation this project. structures are commonly built on a horizontal curve. This A significant body of research and development exists rel- change has come about because of higher traffic volumes and ative to the design and analysis of curved prestressed concrete speeds, the geometric constraints of the urban environment, box-girder bridges. Some of this information has found its and improved structural forms that lend themselves to curved way into design specifications, but much of it has not. There construction. is a need to collect and analyze this information in order to The concrete box-girder, particularly post-tensioned pre- evaluate its merit for nationwide design rules. Although stressed concrete that can span large distances, is one such much of this work has been conducted domestically, a signif- structural form. The cross section of these structures is in- icant body of work has been conducted by other countries herently strong in torsion. This is important because curva- and this work also needs to be considered. ture induces high torsion forces. Also, because concrete can Although most issues relative to design of curved concrete be easily molded into the required shape, it is ideal for curved box-girders have been studied to some degree, gaps in our construction. For these reasons, prestressed concrete box- understanding need to be filled. With modern computer pro- girders have become the structure type of choice in many grams and analytical models calibrated to existing physical jurisdictions. A common application of curved structures is in freeway interchanges where connector ramps or "fly- overs" 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 Single-cell Box Girder 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 Multi-cell Box Girder 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 Spread Box Beams degree of curvature, span lengths and depths, the amount of required prestressing force, and so forth, better guidelines are Figure 1-1. Types of cross sections.
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2 and experimental results, most of this can be done through box-girder bridges. In addition, detailed results from both analytical studies. Additional physical testing of existing global and local response analysis studies are presented. Final structures or laboratory experimentation, although impor- recommendations are presented in the final chapter and are tant in and of themselves, are beyond the scope of this project implemented in the form of recommended changes to the and are not necessary to accomplish this project's intended AASHTO LRFD Bridge Design Specifications and Commentary goals. and in analysis guidelines for these types of bridges. Example This report presents the results of a review of the litera- problems are also presented that illustrate the application of ture and the state of practice with respect to curved concrete these recommendations.