\begin{equation*} PE=\int\! \overline{U}\,dx - W=\int_0^L\! \frac{EA}{2}\left(\frac{\mathrm{d}u_1}{\mathrm{d}X_1}\right)^2\,dX_1 - \int_0^L\!pu_1\,dX_1 - \sum_{i=1}^nF_i(u_1)_i \end{equation*}