Seismic fragility and reliability of elevated water tanks as urban water-supply lifeline components under near-fault ground motions: a demand amplification framework

Urban water-supply systems depend on elevated water tanks for storage, pressure support, firefighting, and post-earthquake service continuity. This study evaluates the component-level seismic fragility and reliability of three 250 m 3 Intze-type reinforced-concrete frame-staging tanks with normal, radial, and cross bracing. Incremental dynamic analysis was performed using 44 records divided into far-fault, near-fault directivity with low and high peak ground velocity (PGV)/peak ground acceleration (PGA) ratios, and fling-step ensembles. Five structural and hydrodynamic demand parameters were examined. High-PGV/PGA directivity motions produced the greatest fragility, reducing the normal frame without diagonal bracing (WT1) median collapse PGA from 0.992 g under far-fault motions to 0.442 g. Over PGA = 0.2 g–0.8 g, the maximum inter-storey drift ratio (MIDR)-based near-fault demand amplification factor ranged from 2.486 to 2.583 across the three configurations under high-ratio directivity motions. Robustness was assessed using 2,000 bootstrap resamples, leave-one-record-out checks, and alternative formulations, confirming that the numerical factor remains configuration- and dataset-specific. The corresponding impulsive-mass-acceleration factors were 0.706, 1.200, and 1.194, indicating no systematic attenuation. At Zone V, the WT1 50-year collapse probability increased from 0.75% under far-fault motions to 14.66% under fling-step motions. The findings support near-fault-sensitive component-level assessment without implying complete hydraulic-network functionality.