Interactive behavior of structures with multiple friction pendulum isolation system and unbounded foundations

In the last two decades, base isolation has been used for enhanced seismic performance to protect structures against severe earthquakes. The reduction in earthquake forces on a structure is achieved by adding horizontal soft isolation elements between the superstructure and the foundation. The efficiency of the isolator in reducing the seismic energy imparted to a structure is dependent on the flexibility of the supporting soil. The effects of radiation damping and the flexibility of the soil media are the deciding factors for structural design against earthquakes. In order to ensure the safety of isolated structures, the interactive behavior of isolated structures and the unbounded foundation during earthquakes must be considered. In this paper, an advanced analytical model based on the viscoplasticity theory and rigorous finite element derivations for the Multiple Friction Pendulum System (MFPS) is presented. To yield better design accuracy, the consistent infinitesimal finite element-cell method, accounting for the radiation damping of the unbounded soil, was adopted. Significant differences between the system with and without radiation damping were observed. It is implied from this study that to obtain better accuracy, radiation damping should be properly taken into account for structural analysis and design. Quantitative results also reveal that the interaction effects of an isolated building and the unbounded soil medium are important.