This paper presents the framework for a performance-based seismic assessment of coupled soil-structure-fluid systems via integrated 3D finite element (FE) analysis. The proposed strain-based performance approach reduces the conservatism and uncertainty associated with the stress-based approaches while better aligning with the performance requirement of the structure to maintain its functionality. By optimizing the modeling approach based on the important system responses, the integrated FE model offers a comprehensive yet efficient approach to capture the seismic-induced local and global demands of the critical structural components while delivering a reasonable computational efficiency.

The proposed framework is used to evaluate the seismic performance of a safety-class water storage tank subjected to the Performance Category (PC)-3 design basis earthquake ground motions. These flat-bottomed storage tanks are common at Department of Energy (DOE) and Nuclear Regulatory Commission (NRC) regulated nuclear facilities.

For the example tank, the relatively massive water-filled tank is anchored to a concrete ring foundation around its perimeter while its draw-off piping system is light and supported by an independent concrete slab. During a seismic event, their differing dynamic characteristics and foundation configurations result in different soil-structure interaction (SSI) responses, leading to complex seismic-induced demands at the connection that cannot be captured using decoupled SSI analysis. The local interaction of the connection and its contained water contributes to the stress concentration and plastic deformation in the connection, underlying the need for a coupled fluid-structure interaction (FSI) analysis. Moreover, the tank support configuration influences the potential uplift and rocking response due to the concurrent interactions of the soil, foundation, and water, highlighting the importance of the coupled SSI-FSI analysis.

Therefore, a 3D integrated soil-structure-fluid interaction model of the tank and its piping system is developed for the seismic performance evaluation. The integrated FE model includes the soil domain as well as a high-resolution representation of the tank-piping connection, its contained fluid, and the anchor bolts to capture their local failure due to the coupled soil-fluid-structure effects. The performance of the tank-piping connection and the anchor bolts is evaluated by comparing the governing demands from the FE analysis to the corresponding capacities defined using the strain-based performance criteria.

The proposed framework can be used to evaluate the seismic performance of similar complex structures where the coupled response of the soil, fluid and structure cannot be captured by decoupled analyses. The proposed integrated FE modeling approach combined with the strain-based acceptance criteria provide the necessary tools for realistic seismic evaluation of such complex structures.

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