Session: SE-07-01 Seismic Evaluation of Systems, Structures and Components

Paper Number: 105307

105307 - Uncertainties in In-Structure Response Spectra Due to Uncertainties in Input Motion Amplitude and Phase Spectra 

To estimate the mean seismic responses of linear structural systems, a common practice is to use multiple response-spectrum-matched input acceleration time histories to obtain stable estimates of the responses. However, in our PVP2020-21132 paper (Reference 1), we demonstrated that the uncertainties in phase spectra of the input motions alone, which are considered irreducible, could result in a coefficient of variation (COV) in In-Structure Response Spectra (ISRS) around 40%, with which four or five input motions in the current practice were found not able to lead to stable ISRS estimates with reasonable confidence levels.  In that work, the amplitude spectra of the input time histories remained constant and their uncertainties were not considered.  The uncertainties in the amplitude spectra are expected to increase the uncertainness in ISRS. This paper presents results that include uncertainties in both phase spectra and amplitude spectra of the input motions.

The uncertainties in amplitude spectra can be strongly influenced by the method used for response spectrum matching, and are often not separable from those of the phase spectra.  In this paper, we explore how one widely used response spectrum matching algorithm (based on Reference 2) can affect the COV in ISRS, and compare that with what is reported in the literature.  If this algorithm produces a level of COV in ISRS equal to or greater than what has been reported, it further affirms the current practice of using 4 or 5 input time histories may not be sufficient to attain stable ISRS estimates.  This study intends to show that a check of power spectral density (PSD) functions is necessary even using multiple input time histories.  Reference 1 reasons that because PSD checks ensure the input time histories have sufficient power over the entire frequency range of interest, the mean ISRS using 4 or 5 input time histories, although not as stable as necessarily required for reasonable levels of confidence, would tend to vary on the conservative side.

References:

1.       Nie, J., J. Xu, V. Graizer, and D. Seber  (2020). “Estimating Stable Mean Responses for Linear Structural Systems by Using a Limited Number of Acceleration Time Histories,” American Society of Mechanical Engineers Pressure Vessels and Piping Conference (PVP2020-21132), Virtual, Minneapolis, MN.

2.       Atik, L. and N. Abrahamson (2010). “An Improved Method for Nonstationary Spectral Matching,” Earthquake Spectra, 26(3), pp 601-617.

Presenting Author: Jinsuo R. Nie U.S. Nuclear Regulatory Commission

Presenting Author Biography: Dr. Jinsuo Nie is a structural engineer at Nuclear Regulatory Commission. He performs research and technical reviews mainly in the seismic and structural areas and his technical interests also include geotechnical engineering, computational mechanics, fragility, and probabilistic risk assessment. Before joining the NRC, he was a scientist at Brookhaven National Laboratory. He also had multiple years of consulting experience in structural analysis and design.

Dr. Nie serves on the ASCE Nuclear Standards Committee and ASCE DANS Committee. He also serves on the ANS 2.36 Code Committee.

Authors:

Jinsuo R. Nie U.S. Nuclear Regulatory Commission
Jim Xu Nuclear Regulatory Commission
Vladimir Graizer NRC
Dogan Seber Nuclear Regulatory Commission