||This paper presents an elementary finite element method for geometrically nonlinear large rotation analyses of built-up plate/shell structures comprising of thin members. The tangent stiffness matrix of the element in the updated Lagrangian co-rotational reference frame is developed, based on the von Karman nonlinear theory of plates, and the Reissner variational principle, allowing for unsymmetric stresses and drilling rotations, useful in the analysis of built-up plate and shell structure. The finite rotation of the co-rotational reference frame relative to a globally fixed Cartesian frame, is simply determined from the finite displacement vectors of the nodes of the element in the global reference frame, thus allowing for an elementary transformation of the tangent stiffness matrix from the updated co-rotational reference frame to the globally fixed Cartesian frame. The element employed here is a 3-node plate element with 6 degrees of freedom per node, including 1 drilling degree of freedom and 5 degrees of freedom [3 displacements, and the derivatives of the transverse displacement around two independent axes in the plane of the plate in the co-rotational reference frame]. The (18×18) tangent stiffness matrices of the plate element in the updated Lagrangian co-rotational reference frame are derived, based on the assumptins that: (1) the inplane stress resultants Nab(unsymmetric) are constant in each element; (2) the bending moments Mab(symmetric) are linear and C0within each element; and (3) the transverse rotationsqi(including the drilling degrees ofq3) are linear and C0within each element. When compared to the primal approach wherein C1continuous trial functions for transverse displacements over each element are necessary, the trial functions for the transverse bending moments and the rotations are very simple in the current approach, and can be assumed to be linear within each element. The present (18×18) tangent stiffness matrices of the plate, based on the Reissner variational principle and the von Karman type simplified nolinear plate theory in the co-rotational reference frame, lead to analyses, which are much simpler and more physically-based, than many others in the literature for large rotation/deformation analysis of built-up plate/shell structures [such as component plates joined at an angle]. Numerical examples demonstrate the accuracy and robustness of the present method.