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

Post-tensioning (PT) strand systems have been employed in a number of self-centering seismic systems as part of their restoring force mechanism to eliminate residual building drifts following seismic loading. Because of their large elastic deformation capability, PT strand systems have been shown to be effective in these applications. Although typically designed to stay elastic during design basis earthquake events, PT strands can be subjected to inelastic strains during extreme seismic events. Furthermore, the yielding and fracture behaviors of PT strand systems are central to understanding and predicting the collapse behavior of self-centering systems. The loading conditions associated with prestressed/post-tensioned concrete applications are vastly different, and only limited research has explored the behavior of PT strand systems as subjected to inelastic cyclic loading. To better characterize the behavior of PT strand systems subjected to inelastic cyclic loading, a testing program was conducted to investigate important parameters related to self-centering system applications. The testing program consisted of both monotonic and cyclic tests of PT strand systems to failure. Variations in the test configuration included strand obtained from two manufactures, two types of multiple use anchorage systems, and several levels of initial post-tensioning strand stress. Characteristics of the response that were investigated included seating losses, deformation capacity prior to initial wire fracture, additional deformation capacity after initial fracture, and the overall load-deformation behavior. An equation was developed that provides the reduction in PT strand force due to seating losses. Other practical information is also summarized, including suggested strain limits to prevent PT wire fracture.