Seismic-induced forces are detrimental and can significantly destroy lives and properties. Among the methods for seismic protection of buildings, base isolation is a common technique. Properly designed based isolated buildings do not lose functionally even after strong earthquakes and can be reoccupied immediately. Despite the availability of many isolators, the curved surface slider (CSS) is widely used to protect bridges and buildings from seismic damage. Lately, double concave CSS (DCCSS) bearings operating on the pendulum principle have been adapted to improve their functionality. The advantages of DCCSS include their ability to operate under extreme conditions and accommodating large displacements than single CSS.
While the functionality and performance of both CSS and DCCSS isolators have been validated under different conditions, there are limited experimental studies on the behaviors of DCCSS upon attaining the displacement limit. Generally, seismic code provisions for designing base-isolated systems are based on factor behaviors to improve safety. For instance, rigid rim bearing designed based on American standards utilizes end-stroke restraints like restraining rings that get damaged upon stronger earthquakes larger than MCE. In contrast, DCCSS isolators designed based on European standards do not have end-stroke restraints. As such, the inner slider runs along the edge of the sliding surface beyond the over-stroke regime, especially under earthquakes exceeding ultimate limits, to preserve their capacity to withstand gravity loads and re-centering ability. This necessitates more research to improve the seismic performance of isolated buildings with strong end-stroke mechanical elements and over-stroke displacement.
On this account, Professor Antonio Di Cesare, Professor Felice Carlo Ponzo and Dr. Alessio Telesca from the University of Basilicata studied the impact of over-stroke displacement of DCCSS and mechanical restrainers on the seismic response of base-isolated buildings. The case study was for a six-story residential building comprised two parts. In the first part, the bearings over-stroke behavior was numerically modeled and calibrated onto an experimental displacement-controlled test considering the information available in the literature. In the second part, static and dynamic analyses were carried out to determine the influence of seismic stop and over-stroke capacity on the overall performance of the system. A parametric analysis was finally performed to predict the annual frequency associated with exceeding the superstructure yielding limit. They aimed at enhancing the seismic resilience of DCCSS isolated buildings. Their original research work is currently published in the journal, Engineering structures.
Results showed that increasing the displacement capacity of the isolators for flat rim configuration without end-stops did not improve the seismic performance of the superstructure. This is because removing the seismic stops delays the inelastic action within the superstructure for all the cases. By comparing the fragility curves, the authors observed that the models with sufficient over-stroke displacement and displacement capacity exceeding 1.75 and 1.00, respectively, resulted in the development of the drift failure mechanism before the occurrence of base shear failure. Annual frequencies exceeding the global drift EDP were calculated, and the results were comparable to that of structures isolated with elements having over-stroke capacities with lower probabilities.
In summary, the study investigated the enhancement of earthquake resilience of buildings equipped with DCCSS based on a six-story residential building with different isolation systems. Based on the results, the over-stroke capacity could be applied to realize the resistance hierarchy for different structural failure modes. In a statement to Advances in Engineering, the authors their study findings will advance further experimental tests to improve the resilience of taller isolated buildings.
Di Cesare, A., Ponzo, F., & Telesca, A. (2021). Improving the earthquake resilience of isolated buildings with double concave curved surface sliders. Engineering Structures, 228, 111498.