Seismic Performance Evaluation of Elevated Circular Water Tanks Using Incremental Dynamic Analysis with Fluid-Structure Interaction

Elevated water tanks are critical infrastructure components whose seismic performance is highly influenced by Fluid-Structure Interaction (FSI), a factor often inadequately represented in design codes. This study presents a performance-based seismic evaluation that explicitly quantifies the effects of FSI and water fill level on dynamic response. A three-dimensional finite element model was developed in ABAQUS, utilizing a Housner-based equivalent mechanical model to simulate FSI under empty, fully-filled, and half-filled conditions. The model was rigorously validated via modal analysis. Incremental Dynamic Analysis (IDA) was conducted using a suite of 11 earthquake records. The results reveal a critical paradox: contrary to conventional intuition, the half-filled condition, while exhibiting the highest apparent ductility (Response Reduction Factor, R = 4.3), was the most vulnerable due to significantly amplified seismic demands (Sa(T1) = 2.25 g). Conversely, the full-filled condition demonstrated superior stability. The study's primary contribution is the derivation of performance-based R-factors that resolve their dependence on water level, addressing a significant gap in code-based design. These findings advocate for the mandatory consideration of FSI and the establishment of the half-filled condition as a critical design scenario.