Consistent Site Response Spectra for use in SSI Analysis

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Title: Consistent Site Response Spectra for use in SSI Analysis
Author: Barker, Tyler M
Advisors: Dr. Abhinav Gupta, Committee Chair
Abstract: ABSTRACT BARKER, TYLER MADISON. Maintaining Consistent Site Response Spectra for use in SSI Analysis. (Under the direction of Dr. Abhinav Gupta.) The objective of this study was to determine if the current-state-of-practice for developing in-layer motions for SSI analysis generates consistent responses, and if not, propose a correction for the current-state-of-practice. Current trends in nuclear power plant design and licensing require that free-field surface and foundation surface site-specific seismic demands are encompassed by a certified design basis. If the certified design is less than the site-specific seismic demands, then the plant must undergo soil structure interaction analysis. Common SSI programs require a foundation elevation in-layer input response, which must be developed from the surface design spectra. The current-state-of-practice deconvolves the surface design motion to the bedrock elevation. The bedrock motion is then convolved up to the foundation elevation where an in-layer motion is generated. However, if the in-layer response from the current-state-of-practice is compared to an in-layer response found directly from the same input bedrock motion, then the two responses would not match. A closed-form theoretical solution was generated for the motion of a two layer soil column over a uniform bedrock halfspace. This solution was used to demonstrate “profile-motion consistency,†where if a single soil profile is used to take a bedrock motion and generate a ground motion and then return the motion back to the bedrock, it will recover the input only if the soil profile and ground motion are unaltered. If either is changed, then the responses of the system will be modified. In the current-state-of-practice, the ground motions are determined as the mean surface response, which results in an inconsistent response. Additionally, the current-state-of-practice uses a reduced number of soil profiles to transfer ground motions, again violating the “profile-motion consistency.†The closed-form solution can again be used to transfer a harmonic bedrock motion with an amplitude of 1.0 to the free surface. The average surface motion can then be used as the input at the surface of the soil column and deconvolved to the bedrock motion. The motion transfer process was not consistent, as the original input of 1.0 is not recovered. If the surface motion was instead divided by resulting bedrock response and then deconvolved, then the bedrock response is returned to 1.0. Effectively, the residual response generated from averaging the surface responses can be used to create a correction factor, which can also be calculated directly. This frequency dependent correction factor is soil specific and soil column height specific, but can be applied at either end of the soil column, traditionally the surface or bedrock. If the current-state-of-practice is modified such that the bedrock response generated during the motion transfer is multiplied by the correction factor, then the resulting responses are more consistent than current methods.
Date: 2009-12-03
Degree: MS
Discipline: Civil Engineering

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