ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/697527
C O N C R E T E B R I D G E T E C H N O L O G Y 40 | ASPIRE Summer 2016 the concrete (creep and shrinkage) and the prestressing steel (relaxation) are presented. Design provisions of the American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications that address prestress losses related to time-dependent material characteristics are presented in a later chapter. Prestressing with Post-Tensioning Two chapters of the manual deal with the mechanics of prestressing a girder with post- tensioning. Chapter 3 presents typical stress summaries that are the result of girder self- weight, superimposed loads, and post-tensioning. Equations for the summation of total stress are then rearranged in two useful ways, so that the prestressing force required at a cross section can be found directly for a given tendon eccentricity and the permissible limits of eccentricity can be found for a given prestressing force. Figure 2, taken from Chapter 3, graphically depicts the first of these two, where the internal moment resulting from the prestressing, taken about the upper kern, is equated to the externally applied bending moment. Chapter 3 also presents the geometric features of post-tensioning tendons comprised of a series of parabolic profiles, the tendon geometry most commonly used in cast-in-place concrete box-girder construction. These geometric features are then used to evaluate secondary moments due to the prestressing of continuous-span girders. Losses in prestress resulting from tendon friction, wobble, anchor set, and elastic shortening are developed and presented in Chapter 4. The calculation of tendon elongations is presented in this chapter, as well as lump-sum time-dependent losses in prestressing force as predicted by applicable AASHTO LRFD guidelines. Considerations for single-end and two-end stressing are discussed. Figure 3, taken from Chapter 4, shows a typical tendon force diagram along the length of a three- span, post-tensioning tendon after two- end stressing and anchor set. The Federal Highway Administration (FHWA) is pleased to announce the release of a new manual for the analysis and design of concrete box-girder bridges. The Post- Tensioned Box Girder Design Manual was developed as a part of the FHWA project Advancing Steel and Concrete Bridge Technology to Improve Infrastructure Performance. Brian Kozy and Reggie Holt of FHWA are providing direction to the project team led by Lehigh University and that includes Corven Engineering. The author of the manual is John Corven of Corven Engineering. The Post-Tensioned Box Girder Design Manual focuses on cast-in-place, post- tensioned concrete box-girder bridges with superstructure cross sections similar to those shown in Fig. 1. The manual serves as a resource to state departments of transportation and consulting firms that are exploring the benefits of, or are looking for guidance on, using this method of bridge construction. Introductory Chapters The FHWA Post-Tensioned Box Girder Design Manual was d eveloped for engineers who have limited exposure to the design and construction of prestressed concrete bridges. Chapter 1 presents a brief history of the use of the bridge type and describes the basic components of a box-girder superstructure. Typical geometries of post-tensioning tendons fo r c a s t - in - p l a c e c o n c r e t e b rid g e s are also presented in Chapter 1, along with descriptions and photographs of the components that comprise a post- tensioning tendon. This chapter concludes with an overview of the construction of cast-in-place concrete box-girder superstructures. Material characteristics of the concrete and prestressing steel are presented in Chapter 2 of the manual. Pertinent time- dependent characteristics in accordance with the CEB-FIP Model Code (1990) of New FHWA Post-Tensioned Box Girder Design Manual by Reggie Holt, Federal Highway Administration, and John Corven, Corven Engineering Figure 2. Equilibrium of internal and external moments. Figure 1. Typical concrete box-girder cross section. All Figures: Developed by Corven Engineering for the Federal Highway Administration.