Role of Concrete Confinement in Seismic Fragility of Elevated RC Water Tanks

Elevated reinforced concrete (RC) water tanks are a critical component of urban infrastructure. However, past earthquakes have exposed their vulnerability–particularly due to poor material quality and inadequate detailing. Although the benefits of concrete confinement are well recognized in structural engineering, their specific impact on the seismic performance of elevated tanks has not been adequately quantified. This study addresses that gap by evaluating the effect of confinement on the collapse fragility of a typical 100 m3 RC water tank under varying conditions. A comprehensive parametric study was conducted using 48 nonlinear finite element models incorporating two material grades (low-strength C10/S220 and moderate-strength C20/S420), four support conditions (fixed base and soil types C, D, D1), and three reservoir fill levels (empty, half full and full). Each scenario was modelled with and without confinement and subjected to non-linear time-history analysis using 30 ground motions scaled through Multiple Stripe Analysis. This resulted in 7200 simulations. Collapse fragility curves were developed based on three limit states: local (single-hinge), combined (double-hinge), and global (drift-based) collapse criteria. The results demonstrate that confinement significantly improves seismic capacity–raising median collapse accelerations by up to 40–60%, particularly in low-quality, fully filled tanks. The absence of confinement was found to increase collapse risk more than poor soil conditions or low material strength. These findings offer new insights by isolating the role of confinement and its interaction with the foundation/soil and fluid systems in tank fragility–an aspect that was overlooked in previous studies–and underscore the need to include it in seismic vulnerability assessments.