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13 the flexural compression zone may be reduced by an amount content is based on Swiss standards and practices, the mate- equal to rial stresses fundamental principles. This book covers straight, skewed, and curved bridges consisting of both open sections Mu/(0.9defy) (Equation 18.104.22.168-4) and closed box sections. The book addresses issues applicable to horizontally curved box-girder bridges. AASHTO (2003a) Guide Specifications for Horizontally Article 4.6.4, "Detailing," discusses the deviation forces Curved Steel Girder Highway Bridges, American Associ- generated by curved post-tensioning tendons. The deviation ation of State Highway and Transportation Officials, force per unit length is determined as the tendon force divided Washington, D.C. by the radius of curvature of the tendon. An example of the regional transverse bending moment in the web of a horizon- This is a recently published AASHTO Guide Specification for tally curved member is presented. curved steel bridges, including box-girders. It has been sug- Article 5.1.4, "Structural Models for Bridge Superstruc- gested that the design specification contained therein be used as tures," provides guidance on developing analytical models a model for NCHRP Project 12-71. This specification discusses that can be applied to curved box-girder bridges. For example, design philosophy and limit states and includes provisions for techniques for developing grillage models of multi-cell box- loads; structural analysis; design of flanges, webs, shear connec- girders are presented. tors, bearings, splices and connections; deflections; and con- Article 5.3.2(b), "Web Design for Shear and Transverse structability. It also includes a construction specification and Bending," presents a rational method for the design of webs design examples for both an I girder and box-girder bridge. subject to combined shear and regional transverse bending, AASHTO (1999) Guide Specifications for Design and Con- a condition that occurs in horizontally curved post-tensioned struction of Segmental Concrete Bridges, 2nd Edition with box-girder bridges. The method is based on Swiss practice and Interims, American Association of State Highway and neglects the concrete contribution to the shear capacity. Shear- Transportation Officials, Washington, D.C. regional transverse bending interaction diagrams are presented. Article 5.3.4, "Diaphragms," discusses the function, neces- This second edition of the guide specifications for segmen- sity, and design of internal diaphragms. Diaphragms are rec- tal concrete bridges was prepared for use in conjunction with ommended at abutments and piers. The use of intermediate the Standard Specifications for Highway Bridges (non-LRFD), diaphragms is usually not necessary in straight and lightly and subsequent interim revisions to these specifications. This curved box-girder bridges. publication, which was developed by a broad-based commit- Article 6.1.3(b) "Influence of Girder Curvature," discusses tee organized by the American Segmental Bridge Institute, the qualitative difference in superstructure displacements due embodies several concepts, which are significant departures to temperature and shrinkage versus longitudinal prestress- from previous design and construction provisions. It is based ing in horizontally curved bridges. on recent research in the United States and abroad. The com- Article 7.6, "Curved Girder Bridges," is devoted entirely to mittee included representatives of state DOTs, the FHWA, horizontally curved bridges. The article includes subsections academicians, consulting engineers, contractors, and suppliers. on Conceptual Design, Analysis, Transformation of Torque Some of the details of this specification were discussed above. into Torsional Sectional Forces, Prestressing, and Prestress- ing Concept and Tendon Layout. The discussion of the conceptual design of curved box- Design Philosophy girders points out the role of torsion in design and how, at A number of books and papers have been written about the ultimate loads, torsion and bending moment can be redistrib- design of concrete box-girder bridges. Many of these discuss uted. The effect of torsion on bearing forces may require the the effect of horizontal curvature on the behavior of these bearings to be placed away from the webs. Expansion bearings bridges. A few of these publications are discussed below. must be able to accommodate both temperature and prestress shortening displacements, which will be in different directions. Menn, C. (1990) Prestressed Concrete Bridges, ISBN 3-7643- The book presents a simplified method for analyzing 2414-7, Birkhauser Verlag, Basel curved bridges iteratively. The method does not satisfy com- patibility equations exactly, but greatly simplifies the compu- This book provides engineers with a comprehensive tational effort. An example is given. Simple vector diagrams overview of the fundamental principles governing the design are presented to illustrate how torsional section forces are and construction of concrete bridges. The content is based on developed by a variation in the direction of longitudinal bend- the author's direct experience gained from the design and ing moments due to the curvature of the superstructure and construction of bridges in Switzerland. Although much of the resisted by shear flow around the perimeter of the box section.