Behavior and Design of Anchorages for Unbonded Post-Tensioning Strands in Seismic Regions

This paper is on the behavior and design of anchorages for unbonded post-tensioning (PT) strands in seismic regions. While current design guidelines for unbonded post-tensioned structures require that the strands remain essentially linear-elastic under the "Design Basis Earthquake," significant post-yield strand stresses can develop under the "Maximum Considered Earthquake." Thus, the use of unbonded post-tensioning for seismic resistance can put the anchorage regions under extreme demands where premature strand wire fractures can occur. This paper describes an experimental study on the behavior of single strand/anchorage systems under monotonic tension loading, considering various design and construction parameters that can affect the performance of a strand inside an anchor. The parameters investigated include: (1) strand size (0.5-inch and 0.6-inch nominal strand diameters); (2) anchor type (cast type and barrel type anchors); (3) number of anchor wedges (2-piece and 3-piece wedges); and (4) presence of a binding ring around the wedges. The test results are used to evaluate strand/anchor configurations that may provide better probability of reaching higher strand ductility.