Structural models are well-established for the governing operators in solid mechanics,
yet the reduction of loads (data) is often performed in an
ad hoc manner, which may
be inadequate for the complex load distributions that often arise in modern
applications. In the present work we consistently convert three-dimensional
data to the form required by Kirchhoff thin-plate theory, in a variational
framework. We provide formulas for all types of resultant structural loads and
boundary conditions in terms of the original three-dimensional data, including
proper specification of corner forces, in forms that are readily incorporated
into computational tools. In particular, we find that in-plane components of
three-dimensional loads engender distributed couples, contributing to an
effective distributed transverse force and boundary shear force, the latter
generalizing the notion of the celebrated Kirchhoff equivalent force. However, in
virtual work we advocate a representation of the twisting moment in a form
that involves neither the Kirchhoff equivalent force nor corner forces. An
interpretation of the structural deflections as through-the-thickness averages of the
continuum displacements, rather than their values on the midplane, yields
explicit formulas for the thin-plate essential boundary data. The formulation
facilitates the solution of problems that would otherwise pose formidable
challenges. Numerical results confirm that appropriate use of the thin-plate model
economizes computation and provides insight into the mechanical behavior, while
preserving a level of accuracy comparable with the full three-dimensional
solution.