Base Shear and Overturning moment

Hi

     I am doing seismic analysis of Fluid filled cylindrical steel tanks using FEM. In this analysis I m supposed to find Base Shear and Overturning moment. Steel tank is thin shell and is flexible. For the complete loading of the shell I have its stress field i.e I have Nxx, Nxy, Nyy, Mxx, Mxy, Myy components of the thin shells middle surface. They are per unit length. Using these components how I can find Base Shear and OTM?

      Any reference/suggestion for this study....

Thanks in advance

Srinivas