Probabilistic and deterministic soil structure interaction analysis including ground motion incoherency effects

Abstract

Soil-structure interaction (SSI) is a major step for seismic design of massive and stiff structures typical of the nuclear facilities and civil infrastructures such as tunnels, underground stations, dams and lock head structures. Currently most SSI analyses are performed deterministically, incorporating limited range of variation in soil and structural properties and without consideration of the ground motion incoherency effects. This often leads to overestimation of the seismic response particularly the In-Structure-Response Spectra (ISRS) with significant impositions of design and equipment qualification costs, especially in the case of high-frequency sensitive equipment at stiff soil or rock sites. The reluctance to incorporate a more comprehensive probabilistic approach is mainly due to the fact that the computational cost of performing probabilistic SSI analysis even without incoherency function considerations has been prohibitive. As such, bounding deterministic approaches have been preferred by the industry and accepted by the regulatory agencies. However, given the recently available and growing computing capabilities, the need for a probabilistic-based approach to the SSI analysis is becoming clear with the advances in performance-based engineering and the utilization of fragility analysis in the decision making process whether by the owners or the regulatory agencies.This paper demonstrates the use of both probabilistic and deterministic SSI analysis techniques to identify important engineering demand parameters in the structure. A typical nuclear industry structure is used as an example for this study. The system is analyzed for two different site conditions: rock and deep soil. Both deterministic and probabilistic SSI analysis approaches are performed, using the program SASSI, with and without ground motion incoherency considerations. In both approaches, the analysis begins at the hard rock level using the low frequency and high frequency hard rock input motions obtained from Probabilistic Seismic Hazard Analysis (PSHA) and the site response analysis is conducted with simulated soil profiles and accompanying soil nonlinearity curves. The deterministic approach utilizes three strain-compatible soil profiles (Lower Bound (LB), Best Estimate (BE) and Upper Bound (UB)) determined based on the variation of strain-compatible soil profiles obtained from the probabilistic site response analysis and uses SSI analysis to determine a conservative estimate of the required response as the envelope of the SSI results from LB, BE and UB soil cases. In contrast, the probabilistic SSI analysis propagates the uncertainty in the soil and structural properties and provides rigorous estimates for the statistical distribution of the response parameters of interest. The engineering demand parameters considered are the story drifts and ISRS at key locations in the example structure. The results from the deterministic and probabilistic approaches, with and without ground motion incoherency effects, are compared and discussed. Recommendations are made regarding the efficient use of statistical methods in probabilistic SSI analysis and the use of such results in Integrated Soil-Structure Fragility Analysis (ISSFA) and performance-based design.