Seismic response of liquid-containing tanks with emphasis on the hydrodynamic response and near-fault phenomena

The present research studies the hydrodynamic response of cylindrical liquid-containing tanks with stiff walls under seismic excitations. Starting from standard hydrodynamic assumptions, the fluid oscillatory modes are separated to an impulsive mode and convective modes through the introduction of a reference frame co-moving with the base of the tank. The response of the fluid's normal modes to a lateral excitation and the role of Housner's oscillators is elucidated. Fast Fourier Transform techniques are applied to samples of earthquake records with near- and far-fault characteristics that have been appropriately scaled to match the Eurocode 8 design spectrum. Critical response quantities including the base shear and the height of the sloshing wave are computed analytically as functions of time and results are compared for near- and far-fault conditions. Particular emphasis is given on the contribution to the above quantities of the second convective mode which is systematically neglected according to current design practices for liquid-containing tanks. The results suggest that under near-fault conditions, when the directivity pulse has substantial content near the frequency of the second convective mode, current provisions may lead to a significant underestimation of the maximum height of the sloshing wave. This observation may provide an explanation of the extensive post-earthquake damage observed at many tank roofs located in the proximity of active faults. The results for near-fault records are compared with those obtained from a simplified representation of the velocity pulse proposed in the literature. The simplified wavelet leads to acceptable accuracy compared with a corresponding real record when the maximum height of the sloshing wave is examined; however, significant underestimation is detected for the calculation of the base shear.