I not familiar with the Eurocode mentioned in your OP. The way that I would analyze the forces on the pier saddles or craddles as follows: estimate the seismic acceleration for the region where the tank is to be located and from which you can calculate the convective and impulsive loads of content within the tank. Half of these loads will transfer thru the centroid of each pier craddle in the form of shear and external moments. From these values you can calculate the pier sizes and if of concrete determine the amount of rebar. Load and moment transfer from pier to footing will be done by using bent dowels or rebar. Footing size and amount of rebar will also be based on base prep details, external loads and moments that act at the saddles of the
piers. Bednar Pressure Vessel Design" as a section but for vertical tanks.
"Simple Procedure for Seismic Analysis of Liquid-Storage Tanks" is an internet article based on the Eurocode 8 that you mentioned, however, it is an evaluation for vertical tanks.
