Risk-targeted importance factors for prescriptive seismic design of critical buildings

The post-earthquake recovery of a community depends on the ability of important buildings to perform their functions. Lifeline and other critical buildings that meet a predictable enhanced seismic performance increase the community's disaster risk management capacity. In prescriptive force-based design standards, the seismic design force for these buildings is typically enhanced using an importance factor depending on the risk category. The present paper proposes an innovative framework based on performance-based seismic design (PBSD) principles using risk-targeted importance factors suitable for use with prescriptive standards. The framework decouples probabilistic assessments of building typologies to experts, while the structural designers can continue the conventional design approach. The framework explicitly considers probabilistic seismic demand, uncertainty in the performance of the building, and the inter-building variation within a particular building typology in seismic performance. Six special reinforced concrete (RC) frame buildings conforming to Indian standards are selected to illustrate the framework. Sensitivity studies on parameter selection are used to establish the framework's robustness. An expression for the importance factors is also proposed. The use of the proposed importance factor expression is shown to meet a wide range of risk targets corresponding to different performance levels. The paper also proposes design factors for enhanced performance objectives suitable for the risk-targeted prescriptive design. Further, the framework is implemented to estimate the seismic risk associated with existing building typology conforming to prevalent design standards. The assembly- and critical-category RC buildings are found to require higher importance factors for achieving enhanced performance objectives than currently prescribed.