A Study on the Safety Assessment of the Base Isolation System Regarding the Ductile Behavior of the Superstructure

In the code-based procedures such as the ones presented in the ASCE7 standard, the use of ductility resources is not allowed in the design of the superstructure in order to prevent structural damage and provide a high level of structural performance for all types of occupancies. But these code procedures suggest the use of ductility resources for design of the fixed-base systems is allowed even for the occupancy types which need high structural performance levels. The aim of this paper is to investigate the effect of seismic isolation on the ductility reduction factor of a structural system and comparing it between a fixed-base structure and an equivalent isolated superstructure. In this regard, a simplified method based on a single-degree-of-freedom system is proposed to assess the safety of the system considering a conditional probability of collapse as the safety measure. Then, a structural model corresponding to a low-rise frame with residential occupancy is constructed and the relationship between the ductility reduction factor and the design period of the system is extracted by applying a constant level of safety and performing a regression analysis. The results indicate that under the same safety level, the isolated superstructure can be designed with a higher ductility reduction factor compared with the corresponding fixed one. Moreover, the derived relationship shows that the ductility reduction factor follows a descending trend with increasing the selected design period of the superstructure.