Initial stiffness of self-centering systems and application to self-centering-beam moment-frames

Abstract

There has been considerable development of self-centering seismic force resisting systems in the past few decades, many of which incorporate gap opening mechanisms. Experiments on these self-centering systems often result in an initial stiffness that is less than the predicted theoretical stiffness. While the discrepancy has often been attributed to uneven bearing surfaces at the gap opening mechanism, the cause of the low stiffness is not well understood and can have critical importance for drift-controlled systems such as moment frames.Concepts related to the initial stiffness of self-centering systems are investigated and the difference between experimental and theoretical initial stiffness is evaluated for a range of previous testing programs on different types of self-centering systems. Seven sources of added flexibility are identified. Then, the concepts for improving initial stiffness are applied to one specific system, the self-centering beam moment frame (SCB-MF). The self-centering beam consists of a two-part beam (upper and lower parts), post-tensioning strands to keep the parts aligned, and bolted friction elements to dissipate seismic energy. The initial stiffness of the SCB-MF is predicted using derived equations and finite element analyses and then the predictions are evaluated against experimental results. It is shown that if the added sources of flexibility are neglected (which is typical practice), the ratio of the average initial secant stiffness from five experiments to the predicted value is 38%, but that if the sources of flexibility are considered, the initial stiffness can be captured. The average ratio of measured to predicted stiffness for the theoretical model including pin flexibility is 59%, while the average ratio for finite element analyses including pin flexibility and beam length tolerance is 102%, demonstrating a significant improvement in accuracy. Using this new understanding about the sources of flexibility in self-centering systems, methods for increasing the stiffness of the SCB-MF are proposed and evaluated.