Seismic Margin Assessment for earthquake beyond design basis – Simplified practical approach

Abstract

Accentuated by recent Fukushima accident, and earthquake events that affected Kashiwazaki Kariwa and North Anna Nuclear power plants, vulnerability assessment of Structures Systems and Components (SSCs) of nuclear facilities for earthquake ground motion exceeding the design basis has become an important exercise to ensure safety in case of highly improbable but possible extreme earthquake event beyond design basis.For seismic performance assessment of structures, three methods viz. response reduction factor based approach, nonlinear response history analysis, and nonlinear static response analysis are available (IAEA, 2011). While the first two methods are either approximate or complex, Nonlinear Static Procedure (NSP) has ability for offering a practical and accurate solution to the problem of Seismic Margin Assessment (SMA) and is simple to adopt at design office.In the present work, step by step approach is presented for Seismic Margin Assessment of structures using nonlinear static analysis procedures. In this approach, at first, pushover analysis is performed on the structure with the appropriate load pattern (e.g. proportional to fundamental mode shape) and effect of higher modes is evaluated using an elastic response analysis. Response of higher mode is then combined to the response from pushover analysis using Modified Modal Pushover Analysis (MMPA) method (Chopra and Goel, 2004). In the second step, demand spectra for different performance objectives is evaluated by augmenting the design spectra. Hence, seismic margin of the structure is evaluated in terms of scaling factor for design spectra for different performance objectives. Material models used in this approach for pushover analysis is validated using structural response data from tests conducted on different types of structures. Application of the proposed approach is also demonstrated for Seismic Margin Assessment of a typical NPP structure.