[en] The purpose of this work is to demonstrate that the errors introduced by the assumption that the container is rigid could be avoided in practical design, since they may not be on the conservative side. Indeed, the flexibility of the container should not be ignored in cases where the structural period (say 0.10 s) corresponds to a 'peak' in the response spectra. On the other hand, these errors may yield excessively conservative results. In the method described herein the effects of liquid damping, in the order of 1/2%, is neglected. Effects of sloshing (non-horizontal free surface) and liquid compressibility are accounted for. A practical analysis procedure is presented based on the solution of the two dimensional wave equation for the liquid, with time dependent boundary conditions, imposed by the dynamic response of the flexible container walls. The liquid and the walls are allowed to iteratively interact until the final common frequencies and hydrodynamic pressures are obtained. The method while still approximate is superior to other such methods where accounting for flexibility is important. It is also generally applicable to containers of any practical shape and structure, as standard dynamic analysis procedures can be applied iteratively. The procedure begins by finding the dynamic response in the absence of the liquid. The frequency and displacement of each mode are used in the time dependent boundary condition in solving the wave equation for the liquid. A pressure function is then obtained and converted to an equivalent mass, which is used to correct the displacement shape and frequency. The process is repeated until no significant correction of the frequency is found. The implementation is done on an open rectangular tank, containing water. Four different cases of analysis are presented, accounting for sloshing and compressibility