Abstract: A method for modeling the behavior of an underground structure such as an underground storage tank under seismic loads includes the steps of constructing separate finite element models for the ribs, tank and fill material. The fill material model comprises a plurality of three-dimensional solid “brick” elements. The tank model comprises a plurality of three dimensional plate/shell elements. The rib model comprises a plurality of beam elements. The nodal points for the beam elements correspond to nodes defining one edge of the tank shell. The tank/rib model is preferably joined to the soil model through a plurality of radial link elements between the brick and shell elements. Spring elements may also be added for stability. A dynamic vertical or horizontal forcing function from an earth quake is then applied to the model. Stress and/or displacement results from horizontal and vertical forcing functions may then be combined if desired.