Seismic Directionality Effects on In-Structure Response Spectra in Seismic Probabilistic Risk Assessment

Currently, seismic probabilistic risk assessments
(SPRA) for nuclear facilities use In-Structure Response Spectra
(ISRS) in the calculation of fragilities for systems and components.
ISRS are calculated via dynamic analyses of the host building
subjected to two orthogonal components of horizontal ground
motion. Each component is defined as the median motion in any
horizontal direction. Structural engineers applied the components
along selected X and Y Cartesian axes. The ISRS at different
locations in the building are also calculated in the X and Y directions.
The choice of the directions of X and Y are not specified by the
ground motion model with respect to geographic coordinates, and are
rather arbitrarily selected by the structural engineer. Normally, X and
Y coincide with the “principal” axes of the building, in the
understanding that this practice is generally conservative. For SPRA
purposes, however, it is desirable to remove any conservatism in the
estimates of median ISRS. This paper examines the effects of the
direction of horizontal seismic motion on the ISRS on typical nuclear
structure. We also evaluate the variability of ISRS calculated along
different horizontal directions. Our results indicate that some central
measures of the ISRS provide robust estimates that are practically
independent of the selection of the directions of the horizontal
Cartesian axes.

• S. Jarernprasert
• E. Bazan-Zurita
• P. C. Rizzo

• Seismic
• Directionality
• In-Structure Response Spectra
• Probabilistic Risk Assessment.

[1] EPRI, 2004, “CEUS Ground Motion Project Final Report: TR 1009684
2004, Electric Power Research Institute, December 2004.
[2] EPRI, 2006, “Program on Technology Innovation: Truncation of the
Lognormal Distribution and Value of the Standard Deviation for Ground
Motion Models in the Central and Eastern United States,” TR 1014381,
Electric Power Research Institute, August 2006.
[3] NRC, 2012, NUREG-2115, “Central and Eastern United States Seismic
Source Characterization for Nuclear Facilities,” Nuclear Regulatory
Commission, 2012.
[4] NRC, 2006, Regulatory Guide 1.208, "A Performance-Based Approach
to Define The Site-Specific Earthquake Ground Motion," Nuclear
Regulatory Commission, March 2007.
[5] NRC, 2007, NUREG-0800 "Standard Review Plan Section 3.7.2 -
Seismic System Analysis", Nuclear Regulatory Commission, March
2007.
[6] ASCE, 2013, “Seismic Analysis of Safety-Related Nuclear Structures,”
Standards ASCE 4-13, American Society of Civil Engineers, 2013.
[7] ASCE, 2005, “Seismic Design Criteria for Structures, Systems and
Components in Nuclear Facilities,” ASCE/SEI Standard 43-05,
American Society of Civil Engineers, 2005.
[8] CSI, 2007, “Reference Manual for SAP – Version 14,” Computers and
Structures, Inc., 2007.
[9] NRC, 2006, Regulatory Guide 1.92, "Combining Modal Responses and
Spatial Components in Seismic Response Analysis," Nuclear Regulatory
Commission, July 2006.