Evaluating Performance in Seismic Isolated Buildings Using Performance Indexes

Abstract

The design performance objectives implicit in U.S. building codes currently differ for fixed-base and base-isolated buildings. The imposed standard for a fixed-base building is comparable to “life safety”, while the imposed standard for an isolated building is comparable to “immediate occupancy” or “operational” [SEAOC, 1995]. For example, fixed-base buildings are permitted a force reduction factor R of up to 8, while isolated buildings are limited to R no larger than 2. The base shear demands in fixed-base buildings are reduced considerably by allowing superstructure inelasticity, whereas the superstructure of an isolated building remains essentially elastic due to overstrength. Consequently, the superstructure design forces in an isolated building are often larger than in a comparable fixed-base building. Factoring in the added design, material, and testing costs; base isolation in the U.S. has become an expensive technology that is considered only when the owner is willing to pay a cost premium for very high performance. Structural systems should be evaluated or compared relative to a consistent performance objective, such as life safety or continued occupancy. In particular, relaxing the design standards for isolated buildings may lead to improved cost-competitiveness, while such systems still potentially allow for a substantial performance advantage. In recent years, a performance-based design approach has been under development in the U.S. Performance-based earthquake engineering (PBEE) encourages owners to select appropriate performance objectives for the structural and non structural building components and systems in different events or considering the composite probabilistic seismic hazard. The new approach, developed by PEER and being adapted for practice by ATC-58 [Miranda and Aslani, 2003; Krawinkler, 2005; ATC, 2007], specifies performance in terms of probabilistic losses (casualties, repair costs, downtime). When performance-based engineering matures, designers will employ the latest design and analysis techniques to create efficient designs that meet specified performance objectives, and building owners can comparatively evaluate base isolation and fixed-base design with reference to a quantitative performance objective. In a previous related study, Ryan et. al. (2006) analyzed fixed-base and base-isolated structures with identical fixed-base periods and responding with identical deformation ductility. A comparative performance measure (CPM) was developed to assess relative performance – quantified by structural drift and acceleration – of the comparable isolated and fixed-base buildings. This approach restricted comparison to structures with identical ductility demands, and did not allow identification of the best design considering several systems, performance objectives and economy. The present study presents a methodology to systematically evaluate the relative performance © 2008 ASCE 18th Analysis and Computation Specialty Conference