Re-centring capability of flag-shaped seismic isolation systems

The re-centring capability is identified by the current design codes as a fundamental feature of seismic isolation systems. In this paper, the re-centring capability of seismic isolation systems characterised by a flag-shaped hysteretic cyclic behaviour is investigated through an extensive parametric study of single-degree-of-freedom hysteretic systems subjected to different natural records. A remarkable example of this kind of isolation system derives from the combination of flat steel-PTFE sliding bearings with auxiliary re-centring devices based on the superelastic properties of Shape Memory Alloys. The results of the parametric analyses are processed statistically and regression analysis relations are derived that show the dependence of the residual displacement after the earthquake on the governing parameters of the isolation system. Based on the analysis results, the features of the flag-shaped system that guarantee sufficient re-centring capability are identified.