Effects of loading conditions on the behavior of unbonded post-tensioning strand-anchorage systems

This paper presents an experimental investigation on the strand wire fracture stresses and strains in unbonded post-tensioning strand-anchorage systems subjected to a variety of loading conditions that can affect the performance of the strand inside the anchor. While unbonded post-tensioned gravity load systems (for example, floor and roof slabs) are common throughout the United States, the use of this construction technique for seismic-resistant structures can put the strands under significantly greater strain demands. Strand wire fractures can occur inside the anchorages, limiting the seismic performance of the structure. The research focuses on the following loading parameters: loading rate, eccentricity between the strand ends, postyield cyclic loading, and initial strand stress (that is, prestress). Eight representative anchor-wedge configurations with 0.5 in. (13 mm) or 0.6 in. (15 mm) nominal strand diameters were included in the investigation. While the effects of the loading parameters were found to greatly depend on the anchor-wedge configuration, the test results show that extreme 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 acceptance testing of post-tensioning strand-anchorage systems were assessed based on the investigation. In addition, as a corollary to the strand-anchorage experiments, stress-strain relationships are proposed for prestressing strand based on experiments that achieve free-length fracture of the test specimens.