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In order to effectively assess large fluid storage tank foundations under seismic loading, it is necessary to run a series of Dynamic Soil Structure Interaction (DSSI) analyses. Traditionally, the behaviour of the enclosed liquid is simulated using a lumped mass analogue representing the impulsive and convective masses. Although this method captures the overall response of the structure, the structural demand in the tank shell cannot be assessed as the analogue is dependent on the tank shell being considered as rigid. An alternative method has been developed that considers Fluid Structure Interaction (FSI) with the fluid explicitly simulated and in contact with the steel shell of the tank.
A comparison between the two analysis methods has been completed and the distribution of pile forces was found to be particularly sensitive to the approach used. The lumped mass model with a rigid base plate predicted significantly higher axial forces in the outer ring of piles. With the more representative stiffness in the FSI model, the tank base plate and concrete raft were allowed to dish. This mobilised the central piles to generate a more even distribution of forces, allowing the foundation to be optimised with potential for reducing material quantities whilst still achieving adequate behaviour.