Effects of Loading Parameters on the Behavior of Unbonded Post-Tensioning Strand/Anchorage Systems in Seismic Regions

This paper presents an experimental study on the ultimate behavior of unbonded post-tensioning (PT) strand/anchorage systems subjected to a variety of loading parameters that can affect the performance of the strand inside the anchor. While unbonded post-tensioned gravity load systems (e.g., floor and roof slabs) are common throughout the United States, the use of this construction technique for seismic resistance can put the anchorages (where all of the PT forces are transferred from the strands to the structure) under extreme demands. Strand wire fractures can occur inside the anchorages, limiting the lateral strength, stiffness, ductility, and self-centering capability of the entire structure. The research described in this paper focuses on the strand stresses and strains at wire fracture considering the following parameters: (1) loading rate; (2) eccentricity between the strand ends; (3) post-yield cyclic loading; and (4) initial stress in the strands. Six representative cast and barrel anchor configurations with 0.5-in. and 0.6-in. nominal strand diameters are included in the investigation. While the effects of the loading parameters greatly depend on the strand/anchor configuration, the test results show that seismic loading conditions can cause a significant reduction in the strand strain at fracture as well as an increase in the standard deviation of the fracture strains. Current industry requirements for the testing and acceptance of PT strand/anchorage systems are assessed based on the investigation.