Behavior of Post-Tensioning Strand Systems Subjected to Inelastic Cyclic Loading

Post-tensioning (PT) strands have been employed in a number of self-centering seismic force-resisting systems as part of the restoring force mechanism that eliminates residual building drifts following seismic loading. Unbonded PT strand systems are particularly well-suited for providing elastic restoring force because they possess large elastic strain capacity. Although typically designed to stay elastic during design basis earthquake events, strands may experience inelastic cyclic loading during extreme earthquakes. Furthermore, the yielding and fracture behavior of PT strand systems is central to the collapse behavior of self-centering systems. A testing program was conducted to characterize the cyclic inelastic behavior of monostrand anchorage systems as they might be applied in self-centering seismic force-resisting systems. The experimental program included more than 50 tests with variations in testing protocol (both monotonic and cyclic tests to failure), strand manufacturer, anchorage manufacturer, single-use versus multiple-use anchorage systems, and initial post-tensioning strand stress. Characteristics of the response that were investigated include seating losses, deformation capacity prior to initial wire fracture, additional deformation capacity after initial wire fracture, and aspects of the load-deformation behavior. For the tested monostrand anchorage systems using typical industry barrel and wedge anchorage systems, the mean first wire fracture strain was found to be 2.3% and 2.7% for multiple-use and single-use chucks, respectively, and two standard deviations below the mean (representing a relatively low probability of wire fracture) was 1.2% and 1.3%, respectively. Furthermore, these monostrand anchorage systems were shown capable of an average of 85% additional elongation after first wire fracture. It was concluded that the tested monostrand anchorage systems, because of their high strength, large elastic deformation capacity, ductility prior to wire fracture, and additional postwire fracture deformation capability, are well-suited for self-centering seismic force-resisting systems.