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/575053
CONCRETE BRIDGE TECHNOLOGY External Tendons with Diabolos - Making Something Out of Nothing Using diabolos with external post-tensioning increases design effciencies, streamlines constructability, and improves durability of segmental concrete box-girder bridges by Jerry Pfuntner, Finley Engineering Group Inc. Originally introduced in the 1980s, diabolos used in conjunction with external post-tensioning have fallen in and out of favor with departments of transportation over the years. Recent research and a multitude of reallife case studies, however, have even convinced many industry stalwarts that diabolos provide significant benefits over internal post-tensioning or external post-tensioning with standard bent steel pipe deviation saddles, including simplified precasting details, rapid erection procedures, improved long-term durability, and tendon replacement. Diabolos Defned Diabolos are uniquely-shaped voids designed and formed into concrete deviator segments in a shape that accommodates the tendon angle change through the deviator. Diabolos allow for a continuous external posttensioning duct to pass through the deviator without any duct connections. The external tendons bear directly on the concrete void surface within the deviation segment. The shape of the void is similar to the top used in the early twentieth-century game of the same name - hence the origin of the name. Diabolos are used in combination with external post-tensioning in place of the bent steel pipes that have typically been incorporated into concrete deviator segments during casting and offer many advantages in the design and construction of complex post-tensioned segmental concrete box-girder bridges. The figure to the right illustrates the concept of a diabolo and compares it to a conventional bent steel pipe deviator. The figure is taken from the Federal Highway Administration (FHWA) Post- Tensioning Tendon Installation and Grouting Manual (FHWA-NHI-13-026), which addresses the use of diabolos in section 188.8.131.52. The figure also indicates a significant difference between the two details, which is the distribution of stress along the deviated tendon. It has been demonstrated through testing that the more concentrated stresses that occur in a diabolo can be accommodated in posttensioning systems. Development of Diabolos Driven by an endless quest for better results with post-tensioning techniques, Jacques Combault, Finley Engineering Group;s technical director, and other experts from around the globe convened an international summit in 2001. The Ghent Seminar was supported jointly by the International Association for Bridge and Structural Engineering (IABSE) and International Federation for Structural Concrete (fib). The experts met to further study some lingering issues about prestressing durability. As a result of the summit, experts determined that external post-tensioning, and the use of diabolos in particular, were deemed to be very effective when applicable. "Furthermore," explains Combault, "the problems that had occurred in the past were relatively easily rectified by providing proper training for designers, contractors, grout suppliers, and installers." Benefts of Diabolos and External Post-Tensioning Using diabolos with external continuity post-tensioning tendons increases design efficiencies, streamlines constructability, and improves durability of segmental bridges. Increased Design Effciencies Design and detailing of deviation segments is greatly simplified when diabolos are used for tendon routing. Diabolos can accommodate a wide range of tendon geometry, combined with the bridge horizontal and vertical curvature. This allows for a single deviation segment design for the entire project. The production of one deviation segment design that accommodates all tendon geometry and forces eliminates the effort of individually determining the bent steel pipe fabrication geometry for each deviation point on every tendon. This efficiency in design production also simplifies deviation segment shop drawing production. Streamlined Constructability The curved bearing surface of the diabolo void allows for a range of three-dimensional tendon entrance angles to be accommodated. This detail simplifies the external continuity tendon post-tensioning details by eliminating the traditional embedded bent