Formulation of Potential Energy Pi for 2d elements.

Good evening, how the potential energy 'PI' is formulated for CST and axisymmetric problems. i am much interested in the first part i.e. Pi= S.E. + W.P. How the triple integral is written in case of axisymmetric problems. How the work potential is formulated in case of both CST and Axisymmetric problems subjected to Traction force. ANY REFERENCE LINK OR VIDEOS ARE MUCH APPRECIATED. The derivation and obtaining stiffness matrix is not my importance.